WorldWideScience

Sample records for thick cratonic lithosphere

  1. Craton Heterogeneity in the South American Lithosphere

    Science.gov (United States)

    Lloyd, S.; Van der Lee, S.; Assumpcao, M.; Feng, M.; Franca, G. S.

    2012-04-01

    We investigate structure of the lithosphere beneath South America using receiver functions, surface wave dispersion analysis, and seismic tomography. The data used include recordings from 20 temporary broadband seismic stations deployed across eastern Brazil (BLSP02) and from the Chile Ridge Subduction Project seismic array in southern Chile (CRSP). By jointly inverting Moho point constraints, Rayleigh wave group velocities, and regional S and Rayleigh wave forms we obtain a continuous map of Moho depth. The new tomographic Moho map suggests that Moho depth and Moho relief vary slightly with age within the Precambrian crust. Whether or not a correlation between crustal thickness and geologic age can be derived from the pre-interpolation point constraints depends strongly on the selected subset of receiver functions. This implies that using only pre-interpolation point constraints (receiver functions) inadequately samples the spatial variation in geologic age. We also invert for S velocity structure and estimate the depth of the lithosphere-asthenosphere boundary (LAB) in Precambrian South America. The new model reveals a relatively thin lithosphere throughout most of Precambrian South America (< 140 km). Comparing LAB depth with lithospheric age shows they are overall positively correlated, whereby the thickest lithosphere occurs in the relatively small Saõ Francisco craton (200 km). However, within the larger Amazonian craton the younger lithosphere is thicker, indicating that locally even larger cratons are not protected from erosion or reworking of the lithosphere.

  2. The electrical lithosphere in Archean cratons: examples from Southern Africa

    Science.gov (United States)

    Khoza, D. T.; Jones, A. G.; Muller, M. R.; Webb, S. J.

    2011-12-01

    The southern African tectonic fabric is made up of a number Archean cratons flanked by Proterozoic and younger mobile belts, all with distinctly different but related geological evolutions. The cratonic margins and some intra-cratonic domain boundaries have played major roles in the tectonics of Africa by focusing ascending magmas and localising cycles of extension and rifting. Of these cratons the southern extent of the Congo craton is one of the least-constrained tectonic boundaries in the African tectonic architecture and knowledge of its geometry and in particular the LAB beneath is crucial for understanding geological process of formation and deformation prevailing in the Archean and later. In this work, which forms a component of the hugely successful Southern African MagnetoTelluric Experiment (SAMTEX), we present the lithospheric electrical resistivity image of the southern boundary of the enigmatic Congo craton and the Neoproterozoic Damara-Ghanzi-Chobe (DGC) orogenic belt on its flanks. Magnetotelluric data were collected along profiles crossing all three of these tectonic blocks. The two dimensional resistivity models resulting from inverting the distortion-corrected responses along the profiles all indicate significant lateral variations in the crust and upper mantle structure along and across strike from the younger DGC orogen to the older adjacent craton. The are significant lithospheric thickness variations from each terrane. The The Moho depth in the DGC is mapped at 40 km by active seismic methods, and is also well constrained by S-wave receiver function models. The Damara belt lithosphere, although generally more conductive and significantly thinner (approximately 150 km) than the adjacent Congo and Kalahari cratons, exhibits upper crustal resistive features interpreted to be caused by igneous intrusions emplaced during the Gondwanan Pan-African magmatic event. The thinned lithosphere is consistent with a 50 mW.m-2 steady-state conductive

  3. Density heterogeneity of the cratonic lithosphere

    DEFF Research Database (Denmark)

    Cherepanova, Yulia; Artemieva, Irina

    2015-01-01

    Using free-board modeling, we examine a vertically-averaged mantle density beneath the Archean-Proterozoic Siberian craton in the layer from the Moho down to base of the chemical boundary layer (CBL). Two models are tested: in Model 1 the base of the CBL coincides with the LAB, whereas in Model 2...... the base of the CBL is at a 180 km depth. The uncertainty of density model is density structure of the Siberian lithospheric mantle with a strong...... correlation between mantle density variations and the tectonic setting. Three types of cratonic mantle are recognized from mantle density anomalies. 'Pristine' cratonic regions not sampled by kimberlites have the strongest depletion with density deficit of 1.8-3.0% (and SPT density of 3.29-3.33 t/m3...

  4. Formation of cratonic lithosphere: An integrated thermal and petrological model

    Science.gov (United States)

    Herzberg, Claude; Rudnick, Roberta

    2012-09-01

    The formation of cratonic mantle peridotite of Archean age is examined within the time frame of Earth's thermal history, and how it was expressed by temporal variations in magma and residue petrology. Peridotite residues that occupy the lithospheric mantle are rare owing to the effects of melt-rock reaction, metasomatism, and refertilization. Where they are identified, they are very similar to the predicted harzburgite residues of primary magmas of the dominant basalts in greenstone belts, which formed in a non-arc setting (referred to here as "non-arc basalts"). The compositions of these basalts indicate high temperatures of formation that are well-described by the thermal history model of Korenaga. In this model, peridotite residues of extensive ambient mantle melting had the highest Mg-numbers, lowest FeO contents, and lowest densities at ~ 2.5-3.5 Ga. These results are in good agreement with Re-Os ages of kimberlite-hosted cratonic mantle xenoliths and enclosed sulfides, and provide support for the hypothesis of Jordan that low densities of cratonic mantle are a measure of their high preservation potential. Cratonization of the Earth reached its zenith at ~ 2.5-3.5 Ga when ambient mantle was hot and extensive melting produced oceanic crust 30-45 km thick. However, there is a mass imbalance exhibited by the craton-wide distribution of harzburgite residues and the paucity of their complementary magmas that had compositions like the non-arc basalts. We suggest that the problem of the missing basaltic oceanic crust can be resolved by its hydration, cooling and partial transformation to eclogite, which caused foundering of the entire lithosphere. Some of the oceanic crust partially melted during foundering to produce continental crust composed of tonalite-trondhjemite-granodiorite (TTG). The remaining lithosphere gravitationally separated into 1) residual eclogite that continued its descent, and 2) buoyant harzburgite diapirs that rose to underplate cratonic nuclei

  5. Craton stability and continental lithosphere dynamics during plume-plate interaction

    Science.gov (United States)

    Wang, H.; Van Hunen, J.; Pearson, D.

    2013-12-01

    Survival of thick cratonic roots in a vigorously convecting mantle system for billions of years has long been studied by the geodynamical community. A high cratonic root strength is generally considered to be the most important factor. We first perform and discuss new numerical models to investigate craton stability in both Newtonian and non-Newtonian rheology in the stagnant lid regime. The results show that only a modest compositional rheological factor of Δη=10 with non-Newtonian rheology is required for the survival of cratonic roots in a stagnant lid regime. A larger rheological factor (100 or more) is needed to maintain similar craton longevity in a Newtonian rheology environment. Furthermore, chemical buoyancy plays an important role on craton stability and its evolution, but could only work with suitable compositional rheology. During their long lifespan, cratons experienced a suite of dynamic, tectonothermal events, such as nearby subduction and mantle plume activity. Cratonic nuclei are embedded in shorter-lived, more vulnerable continental areas of different thickness, composition and rheology, which would influence the lithosphere dynamic when tectonothermal events happen nearby. South Africa provides a very good example to investigate such dynamic processes as it hosts several cratons and there are many episodic thermal events since the Mesozoic as indicated by a spectrum of magmatic activity. We numerically investigate such an integrated system using the topographic evolution of cratons and surrounding lithosphere as a diagnostic observable. The post-70Ma thinning of pericratonic lithosphere by ~50km around Kaapvaal craton (Mather et al., 2011) is also investigated through our numerical models. The results show that the pericratonic lithosphere cools and grows faster than cratons do, but is also more likely to be effected by episodic thermal events. This leads to surface topography change that is significantly larger around the craton than within

  6. Lithosphere mantle density of the North China Craton based on gravity data

    Science.gov (United States)

    Xia, B.; Artemieva, I. M.; Thybo, H.

    2017-12-01

    Based on gravity, seismic and thermal data we constrained the lithospheric mantle density at in-situ and STP condition. The gravity effect of topography, sedimentary cover, Moho and Lithosphere-Asthenosphere Boundary variation were removed from free-air gravity anomaly model. The sedimentary covers with density range from 1.80 g/cm3 with soft sediments to 2.40 g/cm3 with sandstone and limestone sediments. The average crustal density with values of 2.70 - 2.78 g/cm3 which corresponds the thickness and density of the sedimentary cover. Based on the new thermal model, the surface heat flow in original the North China Craton including western block is > 60 mW/m2. Moho temperature ranges from 450 - 600 OC in the eastern block and in the western block is 550 - 650 OC. The thermal lithosphere is 100 -140 km thick where have the surface heat flow of 60 - 70 mW/m2. The gravity effect of surface topography, sedimentary cover, Moho depth are 0 to +150 mGal, - 20 to -120 mGal and +50 to -200 mGal, respectively. By driving the thermal lithosphere, the gravity effect of the lithosphere-asthenosphere boundary ranges from 20 mGal to +200 mGal which shows strong correction with the thickness of the lithosphere. The relationship between the gravity effect of the lithosphere-asthenosphere boundary and the lithosphere thickness also for the seismic lithosphere, and the value of gravity effect is 0 to +220 mGal. The lithospheric mantle residual gravity which caused by lithospheric density variation range from -200 to +50 mGal by using the thermal lithosphere and from -250 to +100 mGal by driving the seismic lithosphere. For thermal lithosphere, the lithospheric mantle density with values of 3.21- 3.26 g/cm3 at in-situ condition and 3.33 - 3.38 g/cm3 at STP condition. Using seismic lithosphere, density of lithosphere ranges from 3.20 - 3.26 g/cm3 at in-situ condition and 3.31 - 3.41 g/cm3 at STP condition. The subcontinental lithosphere of the North China Craton is highly heterogeneous

  7. Lateral heterogeneity and vertical stratification of cratonic lithospheric keels: a case study of the Siberian craton

    DEFF Research Database (Denmark)

    Artemieva, Irina; Cherepanova, Yulia; Herceg, Matija

    2014-01-01

    by regional xenolith P-T arrays,lithosphere density heterogeneity as constrained by free-board and satellite gravity data, and the non-thermalpart of upper mantle seismic velocity heterogeneity based on joint analysis of thermal and seismic tomography data.Density structure of the cratonic lithosphere...... and strongly depleted lithospheric mantle of the Archean nuclei, particularly below the Anabar shield.Since we cannot identify the depth distribution of density anomalies, we complement the approach by seismicdata. An analysis of temperature-corrected seismic velocity structure indicates strong vertical...

  8. 3D Numerical Model of Continental Breakup via Plume Lithosphere Interaction Near Cratonic Blocks: Implications for the Tanzanian Craton

    Science.gov (United States)

    Koptev, A.; Calais, E.; Burov, E. B.; Leroy, S. D.; Gerya, T.

    2014-12-01

    Although many continental rift basins and their successfully rifted counterparts at passive continental margins are magmatic, some are not. This dichotomy prompted end-member views of the mechanism driving continental rifting, deep-seated and mantle plume-driven for some, owing to shallow lithospheric stretching for others. In that regard, the East African Rift (EAR), the 3000 km-long divergent boundary between the Nubian and Somalian plates, provides a unique setting with the juxtaposition of the eastern, magma-rich, and western, magma-poor, branches on either sides of the 250-km thick Tanzanian craton. Here we implement high-resolution rheologically realistic 3D numerical model of plume-lithosphere interactions in extensional far-field settings to explain this contrasted behaviour in a unified framework starting from simple, symmetrical initial conditions with an isolated mantle plume rising beneath a craton in an east-west tensional far field stress. The upwelling mantle plume is deflected by the cratonic keel and preferentially channelled along one of its sides. This leads to the coeval development of a magma-rich branch above the plume head and a magma-poor one along the opposite side of the craton, the formation of a rotating microplate between the two rift branches, and the feeding of melt to both branches form a single mantle source. The model bears strong similarities with the evolution of the eastern and western branches of the central EAR and the geodetically observed rotation of the Victoria microplate. This result reconciles the passive (plume-activated) versus active (far-field tectonic stresses) rift models as our experiments shows both processes in action and demonstrate the possibility of developing both magmatic and amagmatic rifts in identical geotectonic environments.

  9. Global model for the lithospheric strength and effective elastic thickness

    Science.gov (United States)

    Tesauro, Magdala; Kaban, Mikhail K.; Cloetingh, Sierd A. P. L.

    2013-08-01

    Global distribution of the strength and effective elastic thickness (Te) of the lithosphere are estimated using physical parameters from recent crustal and lithospheric models. For the Te estimation we apply a new approach, which provides a possibility to take into account variations of Young modulus (E) within the lithosphere. In view of the large uncertainties affecting strength estimates, we evaluate global strength and Te distributions for possible end-member 'hard' (HRM) and a 'soft' (SRM) rheology models of the continental crust. Temperature within the lithosphere has been estimated using a recent tomography model of Ritsema et al. (2011), which has much higher horizontal resolution than previous global models. Most of the strength is localized in the crust for the HRM and in the mantle for the SRM. These results contribute to the long debates on applicability of the "crème brulée" or "jelly-sandwich" model for the lithosphere structure. Changing from the SRM to HRM turns most of the continental areas from the totally decoupled mode to the fully coupled mode of the lithospheric layers. However, in the areas characterized by a high thermal regime and thick crust, the layers remain decoupled even for the HRM. At the same time, for the inner part of the cratons the lithospheric layers are coupled in both models. Therefore, rheological variations lead to large changes in the integrated strength and Te distribution in the regions characterized by intermediate thermal conditions. In these areas temperature uncertainties have a greater effect, since this parameter principally determines rheological behavior. Comparison of the Te estimates for both models with those determined from the flexural loading and spectral analysis shows that the 'hard' rheology is likely applicable for cratonic areas, whereas the 'soft' rheology is more representative for young orogens.

  10. Rifting Thick Lithosphere - Canning Basin, Western Australia

    Science.gov (United States)

    Czarnota, Karol; White, Nicky

    2016-04-01

    The subsidence histories and architecture of most, but not all, rift basins are elegantly explained by extension of ~120 km thick lithosphere followed by thermal re-thickening of the lithospheric mantle to its pre-rift thickness. Although this well-established model underpins most basin analysis, it is unclear whether the model explains the subsidence of rift basins developed over substantially thick lithosphere (as imaged by seismic tomography beneath substantial portions of the continents). The Canning Basin of Western Australia is an example where a rift basin putatively overlies lithosphere ≥180 km thick, imaged using shear wave tomography. Subsidence modelling in this study shows that the entire subsidence history of the account for the observed subsidence, at standard crustal densities, the lithospheric mantle is required to be depleted in density by 50-70 kg m-3, which is in line with estimates derived from modelling rare-earth element concentrations of the ~20 Ma lamproites and global isostatic considerations. Together, these results suggest that thick lithosphere thinned to > 120 km is thermally stable and is not accompanied by post-rift thermal subsidence driven by thermal re-thickening of the lithospheric mantle. Our results show that variations in lithospheric thickness place a fundamental control on basin architecture. The discrepancy between estimates of lithospheric thickness derived from subsidence data for the western Canning Basin and those derived from shear wave tomography suggests that the latter technique currently is limited in its ability to resolve lithospheric thickness variations at horizontal half-wavelength scales of <300 km.

  11. Global map of lithosphere thermal thickness on a 1 deg x 1 deg grid - digitally available

    Science.gov (United States)

    Artemieva, Irina

    2014-05-01

    This presentation reports a 1 deg ×1 deg global thermal model for the continental lithosphere (TC1). The model is digitally available from the author's web-site: www.lithosphere.info. Geotherms for continental terranes of different ages (early Archean to present) are constrained by reliable data on borehole heat flow measurements (Artemieva and Mooney, 2001), checked with the original publications for data quality, and corrected for paleo-temperature effects where needed. These data are supplemented by cratonic geotherms based on xenolith data. Since heat flow measurements cover not more than half of the continents, the remaining areas (ca. 60% of the continents) are filled by the statistical numbers derived from the thermal model constrained by borehole data. Continental geotherms are statistically analyzed as a function of age and are used to estimate lithospheric temperatures in continental regions with no or low quality heat flow data. This analysis requires knowledge of lithosphere age globally. A compilation of tectono-thermal ages of lithospheric terranes on a 1 deg × 1 deg grid forms the basis for the statistical analysis. It shows that, statistically, lithospheric thermal thickness z (in km) depends on tectono-thermal age t (in Ma) as: z=0.04t+93.6. This relationship formed the basis for a global thermal model of the continental lithosphere (TC1). Statistical analysis of continental geotherms also reveals that this relationship holds for the Archean cratons in general, but not in detail. Particularly, thick (more than 250 km) lithosphere is restricted solely to young Archean terranes (3.0-2.6 Ga), while in old Archean cratons (3.6-3.0 Ga) lithospheric roots do not extend deeper than 200-220 km. The TC1 model is presented by a set of maps, which show significant thermal heterogeneity within continental upper mantle. The strongest lateral temperature variations (as large as 800 deg C) are typical of the shallow mantle (depth less than 100 km). A map of the

  12. A lithospheric perspective on structure and evolution of Precambrian cratons

    DEFF Research Database (Denmark)

    Artemieva, Irina

    2012-01-01

    The purpose of this chapter is to provide a summary of geophysical data on the structure of the stable continental lithosphere and its evolution since the Archean. Here, the term lithosphere is used to define the outer layer of the Earth which includes the crust and uppermost mantle, forms the ro...

  13. Lateral heterogeneity and vertical stratification of cratonic lithospheric keels: examples from Europe, Siberia, and North America

    DEFF Research Database (Denmark)

    Artemieva, Irina; Cherepanova, Yulia; Herceg, Matija

    of the Precambrian lithosphere based on surface heat flow data, (ii) non-thermal part of upper mantle seismic velocity heterogeneity based on a joint analysis of thermal and seismic tomography data, and (iii) lithosphere density heterogeneity as constrained by free-board and satellite gravity data. The latter...... of the Gondwanaland does not presently exceed 250 km depth. An analysis of temperature-corrected seismic velocity structure indicates strong vertical and lateral heterogeneity of the cratonic lithospheric mantle, with a pronounced stratification in many Precambrian terranes; the latter is supported by xenolith data...

  14. The Lu-Hf isotope composition of cratonic lithosphere: disequilibrium between garnet and clinopyroxene in kimberlite xenoliths

    NARCIS (Netherlands)

    Simon, N.S.C.; Carlson, R.W.; Pearson, D.G.; Davies, G.R.

    2002-01-01

    12th Annual V.M. Goldschmidt Conference Davos Switzerland, The Lu-Hf isotope composition of cratonic lithosphere: disequilibrium between garnet and clinopyroxene in kimberlite xenoliths (DTM, Carnegie Institution of Washington), Pearson, D.G. (University of Durham)

  15. Interaction Between Downwelling Flow and the Laterally-Varying Thickness of the North American Lithosphere Inferred from Seismic Anisotropy

    Science.gov (United States)

    Behn, M. D.; Conrad, C. P.; Silver, P. G.

    2005-12-01

    Shear flow in the asthenosphere tends to align olivine crystals in the direction of shear, producing a seismically anisotropic asthenosphere that can be detected using a number of seismic techniques (e.g., shear-wave splitting (SWS) and surface waves). In the ocean basins, where the asthenosphere has a relatively uniform thickness and lithospheric anisotropy appears to be small, observed azimuthal anisotropy is well fit by asthenospheric shear flow in global flow models driven by a combination of plate motions and mantle density heterogeneity. In contrast, beneath the continents both the lithospheric ceiling and asthenospheric thickness may vary considerably across cratonic regions and ocean-continent boundaries. To examine the influence of a continental lithosphere with variable thickness on predictions of continental seismic anisotropy, we impose lateral variations in lithospheric viscosity in global models of mantle flow driven by plate motions and mantle density heterogeneity. For the North American continent, the Farallon slab descends beneath a deep cratonic root, producing downwelling flow in the upper mantle and convergent flow beneath the cratonic lithosphere. We evaluate both the orientation of the predicted azimuthal anisotropy and the depth dependence of radial anisotropy for this downwelling flow and find that the inclusion of a strong continental root provides an improved fit to observed SWS observations beneath the North American craton. Thus, we hypothesize that at least some continental anisotropy is associated with sub-lithospheric viscous shear, although fossil anisotropy in the lithospheric layer may also contribute significantly. Although we do not observe significant variations in the direction of predicted anisotropy with depth, we do find that the inclusion of deep continental roots pushes the depth of the anisotropy layer deeper into the upper mantle. We test several different models of laterally-varying lithosphere and asthenosphere

  16. Continental lithospheric evolution: Constraints from the geochemistry of felsic volcanic rocks in the Dharwar Craton, India

    Science.gov (United States)

    Manikyamba, C.; Ganguly, Sohini; Saha, Abhishek; Santosh, M.; Rajanikanta Singh, M.; Subba Rao, D. V.

    2014-12-01

    Felsic magmatism associated with ocean-ocean and ocean-continent subduction processes provide important evidence for distinct episodes of crust-generation and continental lithospheric evolution. Rhyolites constitute an integral component of the tholeiitic to calc-alkaline basalt-andesite-dacite-rhyolite (BADR) association and contribute to crustal growth processes at convergent plate margins. The evolution of the Dharwar Craton of southern peninsular India during Meso- to Neoarchean times was marked by extensive development of greenstone belts. These granite-greenstone terranes have distinct volcano-sedimentary associations consistent with their geodynamic setting. The present study deals with geochemistry of rhyolites from the Chitradurga-Shimoga greenstone belts of western (WDC) and the Gadwal-Kadiri greenstone belts of eastern (EDC) sectors of Dharwar Craton to compare and evaluate their petrogenesis and geodynamic setting and their control on the continental lithospheric evolution of the Dharwar Craton. At a similar range of SiO2, Al2O3, Fe2O3, the rhyolites of WDC are more potassic, whereas the EDC rhyolites are more sodic and less magnesian with slight increase in TiO2. Minor increase in MgO content of WDC rhyolites reflects their ferromagnesian trace elements which are comparatively lower in the rhyolites of EDC. The relative enrichment in LILE (K, Rb) and depletion in HFSE (Nb, Ta, Zr, Hf) marked by negative Nb-Ta, Zr-Hf and Ti anomalies endorse the convergent margin processes for the generation of rhyolites of both the sectors of Dharwar Craton. The high silica potassic rhyolites of Shimoga and Chitradurga greenstone belts of WDC showing prominent negative Eu and Ti anomalies, flat HREE patterns correspond to Type 3 rhyolites and clearly point towards their generation and emplacement in an active continental margin environment. The geochemical characteristics of Gadwal and Kadiri rhyolites from eastern Dharwar Craton marked by aluminous compositions with

  17. Abnormal lithium isotope composition from the ancient lithospheric mantle beneath the North China Craton.

    Science.gov (United States)

    Tang, Yan-Jie; Zhang, Hong-Fu; Deloule, Etienne; Su, Ben-Xun; Ying, Ji-Feng; Santosh, M; Xiao, Yan

    2014-03-04

    Lithium elemental and isotopic compositions of olivines in peridotite xenoliths from Hebi in the North China Craton provide direct evidence for the highly variable δ(7)Li in Archean lithospheric mantle. The δ(7)Li in the cores of olivines from the Hebi high-Mg# peridotites (Fo > 91) show extreme variation from -27 to +21, in marked deviation from the δ(7)Li range of fresh MORB (+1.6 to +5.6) although the Li abundances of the olivines are within the range of normal mantle (1-2 ppm). The Li abundances and δ(7)Li characteristics of the Hebi olivines could not have been produced by recent diffusive-driven isotopic fractionation of Li and therefore the δ(7)Li in the cores of these olivines record the isotopic signature of the subcontinental lithospheric mantle. Our data demonstrate that abnormal δ(7)Li may be preserved in the ancient lithospheric mantle as observed in our study from the central North China Craton, which suggest that the subcontinental lithospheric mantle has experienced modification of fluid/melt derived from recycled oceanic crust.

  18. Regional Crustal Deformation and Lithosphere Thickness Observed with Geodetic Techniques

    Science.gov (United States)

    Vermeer, M.; Poutanen, M.; Kollo, K.; Koivula, H.; Ahola, J.

    2009-04-01

    The solid Earth, including the lithosphere, interacts in many ways with other components of the Earth system, oceans, atmosphere and climate. Geodesy is a key provider of data needed for global and environmental research. Geodesy provides methods and accurate measurements of contemporary deformation, sea level and gravity change. The importance of the decades-long stability and availability of reference frames must be stressed for such studies. In the future, the need to accurately monitor 3-D crustal motions will grow, both together with increasingly precise GNSS (Global Navigation Satellite System) positioning, demands for better follow-up of global change, and local needs for crustal motions, especially in coastal areas. These demands cannot yet be satisfied. The project described here is a part of a larger entity: Upper Mantle Dynamics and Quaternary Climate in Cratonic Areas, DynaQlim, an International Lithosphere Project (ILP) -sponsored initiative. The aims of DynaQlim are to understand the relations between upper mantle dynamics, mantle composition, physical properties, temperature and rheology, to study the postglacial uplift and ice thickness models, sea level change and isostatic response, Quaternary climate variations and Weichselian (Laurentian and other) glaciations during the late Quaternary. We aim at studying various aspects of lithospheric motion within the Finnish and Fennoscandian area, but within a global perspective, by the newest geodetic techniques in a multidisciplinary setting. The studies involve observations of three-dimensional motions and gravity change in a multidisciplinary context on a range of spatial scales: the whole of Fennoscandia, Finland, a regional test area of Satakunta, and the local test site Olkiluoto. Objectives of the research include improving our insight into the 3-D motion of a thick lithosphere, and into the gravity effect of the uplift, using novel approaches; improving the kinematic 3-D models in the

  19. Magnetotelluric investigations of the lithosphere beneath the central Rae craton, mainland Nunavut, Canada

    Science.gov (United States)

    Spratt, Jessica E.; Skulski, Thomas; Craven, James A.; Jones, Alan G.; Snyder, David B.; Kiyan, Duygu

    2014-03-01

    New magnetotelluric soundings at 64 locations throughout the central Rae craton on mainland Nunavut constrain 2-D resistivity models of the crust and lithospheric mantle beneath three regional transects. Responses determined from colocated broadband and long-period magnetotelluric recording instruments enabled resistivity imaging to depths of > 300 km. Strike analysis and distortion decomposition on all data reveal a regional trend of 45-53°, but locally the geoelectric strike angle varies laterally and with depth. The 2-D models reveal a resistive upper crust to depths of 15-35 km that is underlain by a conductive layer that appears to be discontinuous at or near major mapped geological boundaries. Surface projections of the conductive layer coincide with areas of high grade, Archean metasedimentary rocks. Tectonic burial of these rocks and thickening of the crust occurred during the Paleoproterozoic Arrowsmith (2.3 Ga) and Trans-Hudson orogenies (1.85 Ga). Overall, the uppermost mantle of the Rae craton shows resistivity values that range from 3000 Ω m in the northeast (beneath Baffin Island and the Melville Peninsula) to 10,000 Ω m beneath the central Rae craton, to >50,000 Ω m in the south near the Hearne Domain. Near-vertical zones of reduced resistivity are identified within the uppermost mantle lithosphere that may be related to areas affected by mantle melt or metasomatism associated with emplacement of Hudsonian granites. A regional decrease in resistivities to values of 500 Ω m at depths of 180-220 km, increasing to 300 km near the southern margin of the Rae craton, is interpreted as the lithosphere-asthenosphere boundary.

  20. Synthetic Analysis of the Effective Elastic Thickness of the Lithosphere in China

    Science.gov (United States)

    Lu, Z.; Li, C.

    2017-12-01

    Effective elastic thickness (Te) represents the response of the lithosphere to a long-term (larger than 105 years) geological loading and reflects the deformation mechanism of plate and its thermodynamic state. Temperature and composition of the lithosphere, coupling between crust and lithospheric mantle, and lithospheric structures affect Te. Regional geology in China is quite complex, influenced by the subduction of the Pacific and Philippine Sea plates in the east and the collision of the Eurasia plate with the India-Australia plate in the southwest. Te can help understand the evolution and strength of the lithospheres in different areas and tectonic units. Here we apply the multitaper coherence method to estimate Te in China using the topography (ETOPO1) and Bouguer gravity anomalies (WGM2012) , at different window sizes (600km*600km, 800km*800km, 1000km*1000km) and moving steps. The lateral variation of Te in China coincides well with the geology. The old stable cratons or basins always correspond to larger Te, whereas the oceanic lithosphere or active orogen blocks tend to get smaller Te. We further correlate Te to curie-point depths (Zb) and heat flow to understand how temperature influences the strength of the lithosphere. Despite of a complex correlation between Te and Zb, good positive correlations are found in the North China Block, Tarim Basin, and Lower Yangtze, showing strong influence of temperature on lithospheric strength. Conversely, the Tibetan Plateau, Upper and Middle Yangtze, and East China Sea Basin even show negative correlation, suggesting that lithospheric structures and compositions play more important roles than temperature in these blocks. We also find that earthquakes tend to occur preferably in a certain range of Te. Deeper earthquakes are more likely to occur where the lithosphere is stronger with larger Te. Crust with a larger Te may also have a deeper ductile-brittle boundary, along which deep large earthquakes tend to cluster.

  1. Global model for the lithospheric strength and effective elastic thickness

    OpenAIRE

    Magdala Tesauro; Mikhail Kaban; S. A. P. L. Cloetingh

    2013-01-01

    Global distribution of the strength and effective elastic thickness (Te) of the lithosphere are estimated using physical parameters from recent crustal and lithospheric models. For the Te estimation we apply a new approach, which provides a possibility to take into account variations of Young modulus (E) within the lithosphere. In view of the large uncertainties affecting strength estimates, we evaluate global strength and Te distributions for possible end-member ‘hard’ (HRM) and a ‘soft’ (SR...

  2. Characterizing Lithospheric Thickness in Australia using Ps and Sp Scattered Waves

    Science.gov (United States)

    Ford, H. A.; Fischer, K. M.; Rychert, C. A.

    2008-12-01

    more complicated signals at mantle depths. However, at some stations along the southern edge of the thick sub-cratonic lithosphere (previously imaged by surface waves) phases exist which may represent a lithosphere-asthenosphere boundary at depths of 110-115 km. Constraining the relationship of lithospheric thickness to the age and tectonic history of the overlying crust in Australia is important for better understanding the long term evolution of the continent.

  3. Global model for the lithospheric strength and effective elastic thickness

    NARCIS (Netherlands)

    Tesauro, M.; Kaban, M.K.; Cloetingh, S.A.P.L.

    2013-01-01

    Global distribution of the strength and effective elastic thickness (Te) of the lithosphere are estimated using physical parameters from recent crustal and lithospheric models. For the Te estimation we apply a new approach, which provides a possibility to take into account variations of Young

  4. Imaging the lithosphere-asthenosphere boundary across the transition from Phanerozoic Europe to the East-European Craton with S-receiver functions

    Science.gov (United States)

    Knapmeyer-Endrun, Brigitte; Krüger, Frank

    2013-04-01

    Cratons are characterized by their thick lithospheric roots. In the case of the Eastern European Craton, high seismic velocities have been imaged tomographically to more than 200 km depth. However, the exact depth extent of the cratonic lithosphere and especially the properties of the transition to a much thinner lithosphere beneath Phanerozoic central Europe still remain under discussion. Whereas a number of recent seismic campaigns has significantly increased the knowledge about crustal structure and Moho topography in central Europe, comparably detailed, 3-D information on upper mantle structure, e.g. the lithosphere-asthenosphere boundary (LAB), is yet missing. The international PASSEQ experiment, which was conducted from 2006 to 2008, strived to fill this gap with the deployment of 196 seismological stations, roughly a quarter of which were equipped with broad-band sensors, between eastern Germany and Lithuania. With a mean inter-station distance of 60 km, reduced to about 20 km along the central profile, PASSEQ offers the densest coverage for a passive experiment in this region yet. Here, we present first S-receiver function results for this data set, complemented by additional data from national and regional networks and other temporary deployments. This increases the number of available broad-band stations to almost 300, though mostly located to the west of the Trans-European Suture Zone (TESZ). Besides, we also process data from short-period (1 s and 5 s) sensors. The visibility of mantle-transition zone phases, even in single-station data, provides confidence in the quality of the obtained S-receiver functions. Moho conversions can be confidently identified for all stations. In case of a low-velocity sedimentary cover, as found for example in the Polish Basin, the S-receiver functions even provide clearer information on Moho depth than the P-receiver functions, which are heavily disturbed by shallow reverberations. For stations west of the TESZ, a clear

  5. Formation and temporal evolution of the Kalahari sub-cratonic lithospheric mantle: Constraints from Venetia xenoliths, South Africa

    NARCIS (Netherlands)

    Hin, R.C.; Morel, M.L.A.; Nebel, O.; Mason, P.R.D.; van Westrenen, W.; Davies, G.R.

    2009-01-01

    The ~533 Ma Venetia Diamond Mine is located between the Kaapvaal and Zimbabwe Cratons and the study of selected xenoliths provides the opportunity to investigate the temporal evolution of the sub-continental lithospheric mantle (SCLM) underneath southern Africa, as well as the extent and potentially

  6. Evolution of Meso-Cenozoic lithospheric thermal-rheological structure in the Jiyang sub-basin, Bohai Bay Basin, eastern North China Craton

    Science.gov (United States)

    Xu, Wei; Qiu, Nansheng; Wang, Ye; Chang, Jian

    2018-01-01

    The Meso-Cenozoic lithospheric thermal-rheological structure and lithospheric strength evolution of the Jiyang sub-basin were modeled using thermal history, crustal structure, and rheological parameter data. Results indicate that the thermal-rheological structure of the Jiyang sub-basin has exhibited obvious rheological stratification and changes over time. During the Early Mesozoic, the uppermost portion of the upper crust, middle crust, and the top part of the upper mantle had a thick brittle layer. During the early Early Cretaceous, the top of the middle crust's brittle layer thinned because of lithosphere thinning and temperature increase, and the uppermost portion of the upper mantle was almost occupied by a ductile layer. During the late Early Cretaceous, the brittle layer of the middle crust and the upper mantle changed to a ductile one. Then, the uppermost portion of the middle crust changed to a thin brittle layer in the late Cretaceous. During the early Paleogene, the thin brittle layer of the middle crust became even thinner and shallower under the condition of crustal extension. Currently, with the decrease in lithospheric temperature, the top of the upper crust, middle crust, and the uppermost portion of the upper mantle are of a brittle layer. The total lithospheric strength and the effective elastic thickness ( T e) in Meso-Cenozoic indicate that the Jiyang sub-basin experienced two weakened stages: during the late Early Cretaceous and the early Paleogene. The total lithospheric strength (approximately 4-5 × 1013 N m-1) and T e (approximately 50-60 km) during the Early Mesozoic was larger than that after the Late Jurassic (2-7 × 1012 N m-1 and 19-39 km, respectively). The results also reflect the subduction, and rollback of Pacific plate is the geodynamic mechanism of the destruction of the eastern North China Craton.

  7. Lu-Hf isotope constraints on plume-lithosphere interaction during emplacement of the Bushveld Large Igneous Province at 2.06 Ga: Implications for the structure and evolution of the Kaapvaal Craton's lithospheric mantle

    Science.gov (United States)

    Zirakparvar, N. A.; Mathez, E. A.; Rajesh, H.; Vervoort, J. D.; Choe, S.

    2016-12-01

    observation is that the Hf isotope signature of the plume source is only directly expressed in B-LIP bodies that intruded the suture zone, providing further evidence that the craton was already underlain by thick lithospheric mantle at the time of B-LIP magmatism.

  8. Lithospheric structure of the Northern Ordos and adjacent regions from surface wave tomography: implications to the tectonics of the North China Craton

    Science.gov (United States)

    LI, S.; Guo, Z.; Chen, Y. J.

    2017-12-01

    We present a high-resolution upper mantle S velocity model of the northern Ordos block using ambient noise tomography and two-plane-wave tomography between 8 and 143 s. The Ordos block, regarded as the nuclei of the Archean craton of North China Craton, is underlain by high velocity down to 200 km, indicating the preservation of cratonic root at the interior. However, thick lithospheric keel (≥ 200 km) is not observed outside the Ordos, suggesting craton reworking around the Ordos. The most important findings is the prominent low velocity shown beneath the Datong volcano that migrates westward with depth. At 200 km depth, the low velocity locates almost 500 km west to the leading edge of the flat-lying Pacific slab in the mantle transition zone. This observation is in conflict with the previous interpretation that the Datong volcano is fed by the deep upwelling related to the subduction of the Pacific plate. The westward tilted low velocity beneath the Datong volcano, however, is in agreement with the predominant NW-SE trending alignment of fast direction revealed by SKS splitting in this area, suggesting the Datong volcano is likely due to the asthenospheric mantle flow from west. Two possible scenarios could be related to this mantle process. First, the low velocity beneath the Datong volcano may link to the large-scale, deep-rooted mantle upwelling beneath the Mongolia, northwest to the Datong volcano at deeper depth revealed by Zhang et al. (2016). We postulate that when the raising mantle materials reaches the shallow depth, it would be forced bent by the thick lithosphere beneath the Gobi in Mongolia and flow southeastward to Datong volcano. Second, it is also worth noting that the low velocity beneath the Datong volcano connects to the low velocity zone (LVZ) beneath the Ordos block below 200km, which further links the LVZ beneath the northeastern Tibet to the west. Therefore, the Datong volcano could be fed by the mantle flow from northeastern Tibet. The

  9. Electromagnetic study of lithospheric structure in the marginal zone of East European Craton in NW Poland

    Science.gov (United States)

    Jóźwiak, Waldemar

    2013-10-01

    The marginal zone of the East European Platform, an area of key importance for our understanding of the geotectonic history of Europe, has been a challenge for geophysicists for many years. The basic research method is seismic survey, but many important data on physical properties and structure of the lithosphere may also be provided by the electromagnetic methods. In this paper, results of deep basement study by electromagnetic methods performed in Poland since the mid-1960s are presented. Over this time, several hundred long-period soundings have been executed providing an assessment of the electric conductivity distribution in the crust and upper mantle. Numerous 1D, 2D, and pseudo-3D electric conductivity models were constructed, and a new interpretation method based on Horizontal Magnetic Tensor analysis has been applied recently. The results show that the contact zone is of lithospheric discontinuity character and there are distinct differences in geoelectric structures between the Precambrian Platform, transitional zone (TESZ), and the Paleozoic Platform. The wide-spread conducting complexes in the crust with integral conductivity values reaching 10 000 S at 20-30 km depths are most spectacular. They are most likely consequences of geological processes related to Caledonian and Variscan orogenesis. The upper mantle conductivity is also variable, the thickness of high-resistive lithospheric plates ranging from 120-140 km under the Paleozoic Platform to 220-240 km under the East European Platform.

  10. Three-Dimensional Rheological Structure of North China Craton Determined by Integration of Multiple observations: Controlling Role for Lithospheric Rifting

    Science.gov (United States)

    Xiong, X.; Shan, B.; Li, Y.

    2017-12-01

    The North China Craton (NCC) has undergone significant lithospheric rejuvenation in late Mesozoic and Cenozoic, one feature of which is the widespread extension and rifting. The extension is distinct between the two parts of NCC: widespread rifting in the eastern NCC and localized narrow rifting in the west. The mechanism being responsible for this difference is uncertain and highly debated. Since lithospheric deformation can be regarded as the response of lithosphere to various dynamic actions, the rheological properties of lithosphere must have a fundamental influence on its tectonics and deformation behavior. In this study, we investigated the 3D thermal and rheological structure of NCC by developing a model integrating several geophysical observables (such as surface heatflow, regional elevation, gravity and geoid anomalies, and seismic tomography models). The results exhibit obvious lateral variation in rheological structure between the eastern and western NCC. The overall lithospheric strength is higher in the western NCC than in the east. Despite of such difference in rheology, both parts of NCC are characterized by mantle dominated strength regime, which facilitates the development of narrow rifting. Using ancient heatflow derived from mantle xenoliths studies, and taking the subduction-related dehydration reactions during Mesozoic into account, we constructed the thermal and rheological structure of NCC in Ordovician, early Cretaceous and early Cenozoic. Combining the evidence from numerical simulations, we proposed an evolution path of the rifting in NCC. The lithosphere of NCC in Ordovician was characterized by a normal craton features: low geotherm, high strength and mantle dominated regime. During Jurassic and Cretaceous, the mantle lithosphere in the eastern NCC was hydrated by fluid released by the suduction of the Pacific plate, resulting in weakening of the lithosphere and a transition from mantle dominated to crust dominated regime, which

  11. THE STRUCTURE OF THE LITHOSPHERIC MANTLE OF THE SIBERAIN CRATON AND SEISMODYNAMICS OF DEFORMATION WAVES IN THE BAIKAL SEISMIC ZONE

    Directory of Open Access Journals (Sweden)

    A. A. Stepashko

    2013-01-01

    Full Text Available  The evolution and specific features of seismogynamics of the Baikal zones are reviewed in the context of interactions between deep deformation waves and the regional structure of the lithospheric mantle. The study is based on a model of the mantle structure with reference to chemical compositions of mantle peridotites from ophiolotic series located in the south-western framing of the Siberian craton (Fig. 1. The chemical zonation of the lithospheric mantle at the regional scale is determined from results of analyses of the heterogeneity of compositions of peridotites (Fig. 2, Table 1 and variations of contents of whole rock major components, such as iron, magnesium and silica (Fig. 3. According to spatial variations of the compositions of peridotites, the mantle has the concentric zonal structure, and the content of SiO2 is regularly decreasing, while concentrations of FeO∑ and MgO are increasing towards the centre of such structure (Fig. 4. This structure belongs to the mantle of the Siberian craton, which deep edge extends beyond the surface contour of the craton and underlies the north-western segment of the Central Asian orogenic belt.Results of the studies of peridotites of the Baikal region are consistent with modern concepts [Snyder, 2002; O’Reilly, Griffin, 2006; Chen et al., 2009] that suggest that large mantle lenses underlie the Archaean cratons (Fig. 5. The lenses are distinguished by high-density ultrabasic rocks and compose high-velocity roots of cratons which have remained isolated from technic processes. Edges of the mantle lenses may extend a few hundred kilometers beyond the limits of the cratons and underlie orogenic belts that frame the cratons, and this takes place in the south-western segment of the Siberian craton.The revealed structure of the lithospheric mantle is consistent with independent results of seismic and magmatectonical studies of the region. The Angara geoblock is located above the central part of the

  12. Lithospheric diamond formation as a consequence of methane-rich volatile flooding: An example from diamondiferous eclogite xenoliths of the Karelian craton (Finland)

    Science.gov (United States)

    Smart, K. A.; Cartigny, P.; Tappe, S.; O'Brien, H.; Klemme, S.

    2017-06-01

    -rich kimberlites and olivine lamproites between 1200 and 700 Ma. We argue that this punctuated volatile-rich magmatism simultaneously metasomatised the cratonic mantle lithosphere, forming nitrogen enriched phlogopite-bearing metasomes. We propose that reduced, carbon-bearing and nitrogen-rich fluids were remobilized to form the Lahtojoki diamonds. The diamond-forming event(s) most probably occurred during or shortly prior to the entraining kimberlite magmatism as indicated by the diamond nitrogen aggregation systematics. Involvement of reduced diamond-forming fluids is supported by both the negative skewness of Lahtojoki diamond δ13C values and the more reduced nature of the diamondiferous Lahtojoki eclogites compared with their more oxidized barren counterparts. Our results from the diamondiferous eclogites derived from the deepest parts of the Karelian cratonic mantle root are in support of methane being the stable carbon volatile species at the base of thick continental lithosphere.

  13. Major element composition of the lithospheric mantle under the North Atlantic craton

    DEFF Research Database (Denmark)

    Bizzarro, Martin; Stevenson, R.K.

    2003-01-01

    nature of the Sarfartoq mantle showing comparable degrees of depletion to other cratonic roots. Modal analyses indicate that the Sarfartoq mantle is not typified by the orthopyroxene enrichment observed in the Kaapvaal root, but shows more affinity with the Canadian Arctic (Somerset Island), Tanzania...... is compositionally layered as follows: (1) an internally stratified upper layer (70 to 180 km) consisting of coarse, un-deformed, refractory garnet-bearing and garnet-free peridotites and, (2) a lower layer (180 to 225 km) characterized by fertile, CPX-bearing, porphyroclastic garnet lherzolites. The stratification...

  14. How to make a craton

    Science.gov (United States)

    Lee, C.; Chin, E. J.; Erdman, M.; Gaschnig, R. M.; Lederer, G. W.; Savage, P. S.; Zhong, S.; Zincone, S.

    2013-12-01

    Most Archean cratons are underlain by long-lived 200-300 km thick thermal boundary layers, significantly thicker than oceanic boundary layers, which eventually subduct. The longevity of cratons is perplexing because cold thermal boundary layers should be gravitationally unstable or should thermally erode with time. However, it is agreed that thermal contraction of the cratonic root is compensated by intrinsic compositional buoyancy due to extreme melt depletion. This melt depletion is also thought to have dehydrated the peridotitic residue, strengthening the cratonic mantle, making it resistant to thermo-mechanical erosion. Exactly how cratonic mantle arrives at this chemically buoyant and dehydrated state is unknown. Possible scenarios include formation by melting within a large plume head, accretion of oceanic lithosphere, and accretion of sub-arc mantle. The high degrees of melting would seem to imply formation in hot plume heads, but low Al and heavy rare earth element contents suggest formation in the spinel stability field, implying formation at shallower depths than their current equilibration pressures. We present a new thermobarometer designed to estimate the average melting pressures and temperatures of residual peridotites using whole rock major element compositions. We find that the average melting pressures and temperatures of cratonic peridotites range between 3-4 GPa and 1600 °C. If cratonic peridotites melted via adiabatic decompression, these average pressures represent maximum bounds on the final pressures of melt extraction. Currently, cratonic peridotites derive from 4-7 GPa, implying that the building blocks of peridotites experienced an increase of 1-3 GPa, equivalent to 30-90 km of overburden. Our results thus imply that cratonic mantle most likely formed by tectonic thickening of oceanic or arc lithospheres. But because both arc and oceanic lithospheres might be expected to be wet due to hydrous flux melting and serpentinization

  15. Electrical Conductivity Model of the Mantle Lithosphere of the Slave Craton (NW Canada) and its tectonic interpretation in the context of Geochemical Results

    Science.gov (United States)

    Lezaeta, P.; Chave, A.; Evans, R.; Jones, A. G.; Ferguson, I.

    2002-12-01

    The Slave Craton, northwestern Canada, contains the oldest known rocks on Earth, with exposed outcrop over an area of about 600x400 km2. The discovery of economic diamondiferous kimberlite pipes during the early 1990s motivated extensive research in the region. Over the last six years, four types of deep-probing magnetotelluric (MT) surveys were conducted within the framework of diverse geoscientific programs, aimed at determining the regional-scale electrical structures of the craton. Two of the surveys involved novel acquisition; one through frozen lake ice along ice roads during winter, and the second deploying ocean-bottom instrumentation from float planes during summer. The latter surveys required one year of recording between summers, thus allowing long period transfer functions that lead to mantle penetration depths of over 300 km. Two-dimensional modeling of the MT data from along the winter road showed the existence of a high conductivity zone at depths of 80-120 km beneath the central Slave craton. This anomalous region is spatially coincident with an ultradepleted harzburgitic layer in the upper mantle that was interpreted by others to be related to a subducted slab emplaced during the mid-Archean. A 3-D electrical conductivity model of the Slave lithosphere has been obtained, by trial and error, to fit the magnetic transfer and MT response functions from the lake experiments. This 3-D model traces the central Slave conductor as a NE-SW oriented mantle structure. Its NE-SW orientation coincides with that of a late fold belt system, with the first phase of craton-wide plutonism at ca 2630-2590 Ma, three-part subdivision of the craton based on SKS results, and with a G10 (garnet) geochemical mantle boundaries. All of these highlight a NE-SW structural grain to the lithospheric mantle of the craton, in sharp contrast to the N-S grain of the crust. Constraints on the depth range and lateral extension of the electrical conductive structure are obtained

  16. Global strength and elastic thickness of the lithosphere

    NARCIS (Netherlands)

    Tesauro, M.; Kaban, M.K.; Cloetingh, S.A.P.L.

    2012-01-01

    Thestrengthand effective elasticthickness (Te) ofthelithosphere control its response to tectonic and surface processes. Here, we present the first globalstrengthand effective elasticthickness maps, which are determined using physical properties from recent crustal and lithospheric models. Pronounced

  17. Lithospheric flexural strength and effective elastic thicknesses of the Eastern Anatolia (Turkey) and surrounding region

    Science.gov (United States)

    Oruç, Bülent; Gomez-Ortiz, David; Petit, Carole

    2017-12-01

    The Lithospheric structure of Eastern Anatolia and the surrounding region, including the northern part of the Arabian platform is investigated via the analysis and modeling of Bouguer anomalies from the Earth Gravitational Model EGM08. The effective elastic thickness of the lithosphere (EET) that corresponds to the mechanical cores of the crust and lithospheric mantle is determined from the spectral coherence between Bouguer anomalies and surface elevation data. Its average value is 18.7 km. From the logarithmic amplitude spectra of Bouguer anomalies, average depths of the lithosphere-asthenosphere boundary (LAB), Moho, Conrad and basement in the study area are constrained at 84 km, 39 km, 16 km and 7 km, respectively. The geometries of the LAB and Moho are then estimated using the Parker-Oldenburg inversion algorithm. We also present a lithospheric strength map obtained from the spatial variations of EET determined by Yield Stress Envelopes (YSE). The EET varies in the range of 12-23 km, which is in good agreement with the average value obtained from spectral analysis. Low EET values are interpreted as resulting from thermal and flexural lithospheric weakening. According to the lithospheric strength of the Eastern Anatolian region, the rheology model consists of a strong but brittle upper crust, a weak and ductile lower crust, and a weak lower part of the lithosphere. On the other hand, lithosphere strength corresponds to weak and ductile lower crust, a strong upper crust and a strong uppermost lithospheric mantle for the northern part of the Arabian platform.

  18. Integrated elemental and Sr-Nd-Pb-Hf isotopic studies of Mesozoic mafic dykes from the eastern North China Craton: implications for the dramatic transformation of lithospheric mantle

    Science.gov (United States)

    Liu, Shen; Feng, Caixia; Santosh, M.; Feng, Guangying; Coulson, Ian M.; Xu, Mengjing; Guo, Zhuang; Guo, Xiaolei; Peng, Hao; Feng, Qiang

    2018-02-01

    Evolution of the lithospheric mantle beneath the North China Craton (NCC) from its Precambrian cratonic architecture until Paleozoic, and the transformation to an oceanic realm during Mesozoic, with implications on the destruction of cratonic root have attracted global attention. Here we present geochemical and isotopic data on a suite of newly identified Mesozoic mafic dyke swarms from the Longwangmiao, Weijiazhuang, Mengjiazhuang, Jiayou, Huangmi, and Xiahonghe areas (Qianhuai Block) along the eastern NCC with an attempt to gain further insights on the lithospheric evolution of the region. The Longwangmiao dykes are alkaline with LILE (Ba and K)- and LREE-enrichment ((La/Yb) N > 4.3) and EM1-like Sr-Nd-Pb-Hf isotopic signature ((87Sr/86Sr) i > 0.706; ε Nd (t) 16.6, (207Pb/204Pb) i > 15.4, (208Pb/204Pb) i > 36.8, ε Hf (t) 3.7), and display similar EM1-like isotopic features ((87Sr/86Sr) i > 0.706; ε Nd (t) 16.7, (207Pb/204Pb) i > 15.4, (208Pb/204Pb) i > 36.9, ε Hf (t) 2.4) and EM1-like isotopic features((87Sr/86Sr) i > 0.706; ε Nd (t) 16.7, (207Pb/204Pb) i > 15.4, (208Pb/204Pb) i > 36.9, ε Hf (t) 3.7) and EM1-like Sr-Nd-Pb-Hf isotopic features ((87Sr/86Sr) i > 0.706; ε Nd(t) 16.7, (207Pb/204Pb) i > 15.4, (208Pb/204Pb) i > 36.9, ε Hf (t) 9.3) and EM1-like isotopic composition ((87Sr/86Sr) i > 0.705; ε Nd (t) 16.9, (207Pb/204Pb) i > 15.5, (208Pb/204Pb) i > 36.9, ε Hf (t) 0.705; ε Nd (t) 16.9, (207Pb/204Pb) i > 15.5, (208Pb/204Pb) i > 36.9, ε Hf (t) < -8.6). Our data from the various mafic dyke suites suggest that the magmas were derived from EM1-like lithospheric mantle, corresponding to lithospheric mantle modified by the previously foundered lower crust beneath the eastern NCC. Our results suggest contrasting lithospheric evolution from Triassic (212 Ma) to Cretaceous (123 Ma) beneath the NCC. These mafic dykes mark an important phase of lithospheric thinning in the eastern North China Craton.

  19. Comparison of mantle lithosphere beneath early Triassic kimberlite fields in Siberian craton reconstructed from deep-seated xenocrysts

    Directory of Open Access Journals (Sweden)

    I.V. Ashchepkov

    2016-07-01

    Kharamai mantle clinopyroxenes represent three geochemical types: (1 harzburgitic with inclined linear REE, HFSE troughs and elevated Th, U; (2 lherzolitic or pyroxenitic with round TRE patterns and decreasing incompatible elements; (3 eclogitic with Eu troughs, Pb peak and high LILE content. Calculated parental melts for garnets with humped REE patterns suggest dissolution of former Cpx and depression means Cpx and garnets extraction. Clinopyroxenes from Ary-Mastakh fields show less inclined REE patterns with HMREE troughs and an increase of incompatible elements. Clinopyroxenes from Kuranakh field show flatter spoon-like REE patterns and peaks in Ba, U, Pb and Sr, similar to those in ophiolitic harzburgites. The PT diagrams for the mantle sections show high temperature gradients in the uppermost SCLM accompanied by an increase of P-Fe#Ol upward and slightly reduced thickness of the mantle keel of the Siberian craton, resulting from the influence of the Permian–Triassic superplume, but with no signs of delamination.

  20. Lithospheric layering in the North American craton revealed by including Short Period Constraints in Full Waveform Tomography

    Science.gov (United States)

    Roy, C.; Calo, M.; Bodin, T.; Romanowicz, B. A.

    2017-12-01

    Recent receiver function studies of the North American craton suggest the presence of significant layering within the cratonic lithosphere, with significant lateral variations in the depth of the velocity discontinuities. These structural boundaries have been confirmed recently using a transdimensional Markov Chain Monte Carlo approach (TMCMC), inverting surface wave dispersion data and converted phases simultaneously (Calò et al., 2016; Roy and Romanowicz 2017). The lateral resolution of upper mantle structure can be improved with a high density of broadband seismic stations, or with a sparse network using full waveform inversion based on numerical wavefield computation methods such as the Spectral Element Method (SEM). However, inverting for discontinuities with strong topography such as MLDS's or LAB, presents challenges in an inversion framework, both computationally, due to the short periods required, and from the point of view of stability of the inversion. To overcome these limitations, and to improve resolution of layering in the upper mantle, we are developing a methodology that combines full waveform inversion tomography and information provided by short period seismic observables. We have extended the 30 1D radially anisotropic shear velocity profiles of Calò et al. 2016 to several other stations, for which we used a recent shear velocity model (Clouzet et al., 2017) as constraint in the modeling. These 1D profiles, including both isotropic and anisotropic discontinuities in the upper mantle (above 300 km depth) are then used to build a 3D starting model for the full waveform tomographic inversion. This model is built after 1) homogenization of the layered 1D models and 2) interpolation between the 1D smooth profiles and the model of Clouzet et al. 2017, resulting in a smooth 3D starting model. Waveforms used in the inversion are filtered at periods longer than 30s. We use the SEM code "RegSEM" for forward computations and a quasi-Newton inversion

  1. The impact of lateral variations in lithospheric thickness on glacial isostatic adjustment in West Antarctica

    Science.gov (United States)

    Nield, Grace A.; Whitehouse, Pippa L.; van der Wal, Wouter; Blank, Bas; O'Donnell, John Paul; Stuart, Graham W.

    2018-04-01

    Differences in predictions of Glacial Isostatic Adjustment (GIA) for Antarctica persist due to uncertainties in deglacial history and Earth rheology. The Earth models adopted in many GIA studies are defined by parameters that vary in the radial direction only and represent a global average Earth structure (referred to as 1D Earth models). Over-simplifying actual Earth structure leads to bias in model predictions in regions where Earth parameters differ significantly from the global average, such as West Antarctica. We investigate the impact of lateral variations in lithospheric thickness on GIA in Antarctica by carrying out two experiments that use different rheological approaches to define 3D Earth models that include spatial variations in lithospheric thickness. The first experiment defines an elastic lithosphere with spatial variations in thickness inferred from seismic studies. We compare the results from this 3D model with results derived from a 1D Earth model that has a uniform lithospheric thickness defined as the average of the 3D lithospheric thickness. Irrespective of deglacial history and sub-lithospheric mantle viscosity, we find higher gradients of present-day uplift rates (i.e. higher amplitude and shorter wavelength) in West Antarctica when using the 3D models, due to the thinner-than-1D-average lithosphere prevalent in this region. The second experiment uses seismically-inferred temperature as input to a power-law rheology thereby allowing the lithosphere to have a viscosity structure. Modelling the lithosphere with a power-law rheology results in behaviour that is equivalent to a thinner-lithosphere model, and it leads to higher amplitude and shorter wavelength deformation compared with the first experiment. We conclude that neglecting spatial variations in lithospheric thickness in GIA models will result in predictions of peak uplift and subsidence that are biased low in West Antarctica. This has important implications for ice-sheet modelling

  2. Cyclic formation and stabilization of Archean lithosphere by accretionary orogenesis: Constraints from TTG and potassic granitoids, North China Craton

    Science.gov (United States)

    Wang, Wei; Cawood, Peter A.; Liu, Shuwen; Guo, Rongrong; Bai, Xiang; Wang, Kang

    2017-09-01

    Accretionary orogens are major sites of modern continental growth, yet their role in the development of Archean continental crust remains enigmatic. Diverse granitoid suites from tonalite-trondhjemite-granodiorite (TTG) to potassic granitoids appeared during late Archean, representing a period of major continental formation and stabilization. In this study, whole-rock geochemical and zircon U-Pb and Lu-Hf isotopic data are reported for Neoarchean granitoid gneisses from the Northern Liaoning Terrane, northeastern North China Craton (NCC). Older granitoid gneisses ( 2592-2537 Ma) define three magmatic zones migrating from southeast to northwest, each showing a common magmatic evolution from high-pressure TTGs to medium-/low-pressure TTGs and potassic granitoids. They have depleted zircon ƐHf(t) of +0.5 to +8.7. Younger 2529-2503 Ma potassic granitoids and TTGs occur throughout the terrane, which are marked by variable zircon ƐHf(t) of -4.7 to +8.1, and are coeval with regional high-grade metamorphism. Petrogenetic modeling and changing Sr/Y and (La/Yb)N of the granitoids suggest that the crust experienced episodic thickening and thinning and became progressively evolved through development of potassic granitoids and sedimentary successions. The metavolcanic basement to the granitoids display tholeiitic to calc-alkaline affinities, together with the top-to-the-northwest thrusting and associated volcanogenic massive sulfide-type Cu-Zn deposits, suggesting cyclic crustal formation of Northern Liaoning within an accretionary orogen with a SE-dipping subduction polarity. Cyclic crustal thickening and thinning is related to tectonic switching from advancing to retreating relations between the downgoing and overriding plate. After 2530 Ma, this accretionary system accreted to the ancient continental nucleus of NCC (Anshan-Benxi Terrane), signifying final lithosphere stabilization.

  3. Global maps of the magnetic thickness and magnetization of the Earth’s lithosphere

    OpenAIRE

    Foteini Vervelidou; Erwan Thébault

    2015-01-01

    We have constructed global maps of the large-scale magnetic thickness and magnetization of Earth’s lithosphere. Deriving such large-scale maps based on lithospheric magnetic field measurements faces the challenge of the masking effect of the core field. In this study, the maps were obtained through analyses in the spectral domain by means of a new regional spatial power spectrum based on the Revised Spherical Cap Harmonic Analysis (R-SCHA) formalism. A series of regional spectral analyses wer...

  4. Global map of lithosphere thermal thickness on a 1 deg x 1 deg grid - digitally available

    DEFF Research Database (Denmark)

    Artemieva, Irina

    2014-01-01

    with no or low quality heat flow data. This analysis requires knowledge oflithosphere age globally.A compilation of tectono-thermal ages of lithospheric terranes on a 1 deg 1 deg grid forms the basis forthe statistical analysis. It shows that, statistically, lithospheric thermal thickness z (in km) depends......This presentation reports a 1 deg 1 deg global thermal model for the continental lithosphere (TC1). The modelis digitally available from the author’s web-site: www.lithosphere.info.Geotherms for continental terranes of different ages (early Archean to present) are constrained by reliabledata...... on borehole heat flow measurements (Artemieva and Mooney, 2001), checked with the original publicationsfor data quality, and corrected for paleo-temperature effects where needed. These data are supplemented bycratonic geotherms based on xenolith data.Since heat flow measurements cover not more than half...

  5. Seismic Structure of Southern African Cratons

    DEFF Research Database (Denmark)

    Soliman, Mohammad Youssof Ahmad; Artemieva, Irina; Levander, Alan

    2014-01-01

    functions and finite-frequency tomography based on data from the South Africa Seismic Experiment (SASE). Combining the two methods provides high vertical and lateral resolution. The main results obtained are (1) the presence of a highly heterogeneous crustal structure, in terms of thickness, composition (as......Cratons are extremely stable continental crustal areas above thick depleted lithosphere. These regions have remained largely unchanged for more than 2.5 Ga. This study presents a new seismic model of the seismic structure of the crust and lithospheric mantle constrained by seismic receiver...

  6. Lithospheric thickness jumps at the S-Atlantic continental margins from satellite gravity data and modelled isostatic anomalies

    Science.gov (United States)

    Shahraki, Meysam; Schmeling, Harro; Haas, Peter

    2018-01-01

    Isostatic equilibrium is a good approximation for passive continental margins. In these regions, geoid anomalies are proportional to the local dipole moment of density-depth distributions, which can be used to constrain the amount of oceanic to continental lithospheric thickening (lithospheric jumps). We consider a five- or three-layer 1D model for the oceanic and continental lithosphere, respectively, composed of water, a sediment layer (both for the oceanic case), the crust, the mantle lithosphere and the asthenosphere. The mantle lithosphere is defined by a mantle density, which is a function of temperature and composition, due to melt depletion. In addition, a depth-dependent sediment density associated with compaction and ocean floor variation is adopted. We analyzed satellite derived geoid data and, after filtering, extracted typical averaged profiles across the Western and Eastern passive margins of the South Atlantic. They show geoid jumps of 8.1 m and 7.0 m for the Argentinian and African sides, respectively. Together with topography data and an averaged crustal density at the conjugate margins these jumps are interpreted as isostatic geoid anomalies and yield best-fitting crustal and lithospheric thicknesses. In a grid search approach five parameters are systematically varied, namely the thicknesses of the sediment layer, the oceanic and continental crusts and the oceanic and the continental mantle lithosphere. The set of successful models reveals a clear asymmetry between the South Africa and Argentine lithospheres by 15 km. Preferred models predict a sediment layer at the Argentine margin of 3-6 km and at the South Africa margin of 1-2.5 km. Moreover, we derived a linear relationship between, oceanic lithosphere, sediment thickness and lithospheric jumps at the South Atlantic margins. It suggests that the continental lithospheres on the western and eastern South Atlantic are thicker by 45-70 and 60-80 km than the oceanic lithospheres, respectively.

  7. Comprehensive analysis of Curie-point depths and lithospheric effective elastic thickness at Arctic Region

    Science.gov (United States)

    Lu, Y.; Li, C. F.

    2017-12-01

    Arctic Ocean remains at the forefront of geological exploration. Here we investigate its deep geological structures and geodynamics on the basis of gravity, magnetic and bathymetric data. We estimate Curie-point depth and lithospheric effective elastic thickness to understand deep geothermal structures and Arctic lithospheric evolution. A fractal exponent of 3.0 for the 3D magnetization model is used in the Curie-point depth inversion. The result shows that Curie-point depths are between 5 and 50 km. Curie depths are mostly small near the active mid-ocean ridges, corresponding well to high heat flow and active shallow volcanism. Large curie depths are distributed mainly at continental marginal seas around the Arctic Ocean. We present a map of effective elastic thickness (Te) of the lithosphere using a multitaper coherence technique, and Te are between 5 and 110 km. Te primarily depends on geothermal gradient and composition, as well as structures in the lithosphere. We find that Te and Curie-point depths are often correlated. Large Te are distributed mainly at continental region and small Te are distributed at oceanic region. The Alpha-Mendeleyev Ridge (AMR) and The Svalbard Archipelago (SA) are symmetrical with the mid-ocean ridge. AMR and SA were formed before an early stage of Eurasian basin spreading, and they are considered as conjugate large igneous provinces, which show small Te and Curie-point depths. Novaya Zemlya region has large Curie-point depths and small Te. We consider that fault and fracture near the Novaya Zemlya orogenic belt cause small Te. A series of transform faults connect Arctic mid-ocean ridge with North Atlantic mid-ocean ridge. We can see large Te near transform faults, but small Curie-point depths. We consider that although temperature near transform faults is high, but mechanically the lithosphere near transform faults are strengthened.

  8. Mapping lithosphere thickness beneath the Southern Caribbean and Venezuela using body wave reflectivity and surface wave tomography

    Science.gov (United States)

    Masy, J.; Niu, F.; Levander, A.; Schmitz, M.

    2012-12-01

    The Caribbean (CAR) and South American (SA) plate boundary in Venezuela is a broad zone of diffuse deformation and faulting. GPS measurements indicate that the CAR is moving approximately 2 cm/yr respect to SA, parallel to the strike slip fault system in the east, but with an oblique convergence component in the west (Weber et al., 2001). Along the central and eastern Venezuela coast, most of the motion is accommodated by both transpression and transtension along the right lateral strike-slip San Sebastian- El Pilar fault system. The main tectonic features of the area include accretionary wedges and coastal thrust belts with their associated foreland basins (e.g. Sierra del Interior and Espino Graben). Southern of the plate boundary is located the Guayana Shield, which is part of the Amazonian Craton, and is an elevated plain consisting of Precambrian rocks. BOLIVAR (Broadband Onshore-Offshore Lithospheric Investigation of Venezuela and the Antilles Arc Region) was a multidisciplinary, international investigation to determine the evolution of the CAR-SA plate boundary (Levander et al., 2006) that included a 47 station broadband seismic array to complement the 40 station Venezuelan national array operated by FUNVISIS. The goal of this study is to map out lithosphere thickness across the region in order to understand its role for the various types of deformations observed at surface. We combined surface wave tomography and body wave reflectivity to locate the depth of the lithosphere-asthenosphere boundary (LAB). To generate a coherent 3D reflectivity volume of the study area, we used both P- and S-wave receiver-function data, as well as the ScS reverberation records of two deep earthquakes occurring in South America. We also measured Rayleigh phase velocities in the frequency range of 20-100 s using the two plane-wave method to remove multi-pathing effects (Forsyth and Li, 2005). Finite-frequency kernels were computed for a total of 63 teleseismic events to improve

  9. Spatial variations of effective elastic thickness of the Lithosphere in the Southeast Asia regions

    Science.gov (United States)

    Shi, Xiaobin; Kirby, Jon; Yu, Chuanhai; Swain, Chris; Zhao, Junfeng

    2016-04-01

    The effective elastic thickness Te corresponds to the thickness of an idealized elastic beam that would bend similarly to the actual lithosphere under the same applied loads, and could provide important insight into rheology and state of stress. Thus, it is helpful to improve our understanding of the relationship between tectonic styles, distribution of earthquakes and lithospheric rheology in various tectonic settings. The Southeast Asia, located in the southeastern part of the Eurasian Plate, comprises a complex collage of continental fragments, volcanic arcs, and suture zones and marginal oceanic basins, and is surrounded by tectonically active margins which exhibit intense seismicity and volcanism. The Cenozoic southeastward extrusion of the rigid Indochina Block due to the Indo-Asian collision resulted in the drastic surface deformation in the western area. Therefore, a high resolution spatial variation map of Te might be a useful tool for the complex Southeast Asia area to examine the relationships between surface deformation, earthquakes, lithospheric structure and mantle dynamics. In this study, we present a high-resolution map of spatial variations of Te in the Southeast Asia area using the wavelet method, which convolves a range of scaled wavelets with the two data sets of Bouguer gravity anomaly and topography. The topography and bathymetry grid data was extracted from the GEBCO_08 Grid of GEBCO digital atlas. The pattern of Te variations agrees well with the tectonic provinces in the study area. On the whole, low lithosphere strength characterizes the oceanic basins, such as the South China Sea, the Banda sea area, the Celebes Sea, the Sulu Sea and the Andaman Sea. Unlike the oceanic basins, the continental fragments show a complex pattern of Te variations. The Khorat plateau and its adjacent area show strong lithosphere characteristics with a Te range of 20-50 km, suggesting that the Khorat plateau is the strong core of the Indochina Block. The West

  10. Metasomatism and the Weakening of Cratons: A Mechanism to Rift Cratons

    Science.gov (United States)

    Wenker, Stefanie; Beaumont, Christopher

    2016-04-01

    The preservation of cratons is a demonstration of their strength and resistance to deformation. However, several cratons are rifting now (e.g. Tanzania and North China Craton) or have rifted in the past (e.g. North Atlantic Craton). To explain this paradox, we suggest that widespread metasomatism of the originally cold depleted dehydrated craton mantle lithosphere root can act as a potential weakening mechanism. This process, particularly melt metasomatism, increases root density through a melt-peridotite reaction, and reduces root viscosity by increasing the temperature and rehydrating the cratonic mantle lithosphere. Using 2D numerical models, we model silicate-melt metasomatism and rehydration of cold cratonic mantle lithosphere that is positioned beside standard Phanerozoic lithosphere. The models are designed to investigate when a craton is sufficiently weakened to undergo rifting and is no longer protected by the initially weaker adjacent standard Phanerozoic lithosphere. Melt is added to specified layers in the cratonic mantle lithosphere at a uniform volumetric rate determined by the duration of metasomatism (3 Myr, 10 Myr or 30 Myr), until a total of ~30% by volume of melt has been added. During melt addition heat and mass are properly conserved and the density and volume increase by the respective amounts required by the reaction with the peridotite. No extensional boundary conditions are applied to the models during the metasomatism process. As expected, significant refertilization leads to removal and thinning of progressively more gravitationally unstable cratonic mantle lithosphere. We show that the duration of metasomatism dictates the final temperature in the cratonic upper mantle lithosphere. Consequently, when extensional boundary conditions are applied in our rifting tests in most cases the Phanerozoic lithosphere rifts. The craton rifts only in the models with the hottest cratonic upper mantle lithosphere. Our results indicate rifting of cratons

  11. 3-D Structure of the Slave and Rae Cratons Provides Clues to Their Construction

    Science.gov (United States)

    Snyder, D. B.

    2013-12-01

    Deep geologic structures within cratons that make up continental cores were long neglected. Recently acquired geophysical data from large observational arrays and geochemical data resulting from exploration for diamond has now made possible co-registration of large-scale (400-km depth), truly 3-dimensional data sets. P-waves, surface waves and magnetotelluric observations provide 3-D wavespeed and conductivity models. Multi-azimuthal receiver functions map seismic discontinuity surfaces in 3-D. Xenolith suites erupted in kimberlites provide rock samples at key lithospheric depths, albeit at sparsely distributed locations. These multi-disciplinary models are becoming available for several key cratons worldwide; here the deep structure of the Slave and Rae cratons of the Canadian Shield is described. Lithospheric layers with tapered, wedge-shaped margins are common. Slave craton layers are sub-horizontal and indicate construction of the craton core at 2.7 Ga by underthrusting and flat stacking of lithosphere. The central Rae craton has predominantly dipping discontinuities that indicate construction at 1.9 Ga by thrusting similar to that observed in crustal ';thick-skinned' fold-and-thrust belts. 3-D mapping of conductivity and metasomatism, the latter via mineral recrystallization and resetting of isotopic ages, overprints primary structures in both cratons. Distribution of more conductivitve mantle suggests that assumed causative pervasive metasomatism occurs at 100-200 km depths with ';chimneys' reaching to shallower depths, typically in locations where kimberlites or mineralization has occurred.

  12. Using crustal thickness and subsidence history on the Iberia-Newfoundland margins to constrain lithosphere deformation modes during continental breakup

    Science.gov (United States)

    Jeanniot, Ludovic; Kusznir, Nick; Manatschal, Gianreto; Mohn, Geoffroy

    2014-05-01

    Observations at magma-poor rifted margins such as Iberia-Newfoundland show a complex lithosphere deformation history during continental breakup and seafloor spreading initiation leading to complex OCT architecture with hyper-extended continental crust and lithosphere, exhumed mantle and scattered embryonic oceanic crust and continental slivers. Initiation of seafloor spreading requires both the rupture of the continental crust and lithospheric mantle, and the onset of decompressional melting. Their relative timing controls when mantle exhumation may occur; the presence or absence of exhumed mantle provides useful information on the timing of these events and constraints on lithosphere deformation modes. A single lithosphere deformation mode leading to continental breakup and sea-floor spreading cannot explain observations. We have determined the sequence of lithosphere deformation events for two profiles across the present-day conjugate Iberia-Newfoundland margins, using forward modelling of continental breakup and seafloor spreading initiation calibrated against observations of crustal basement thickness and subsidence. Flow fields, representing a sequence of lithosphere deformation modes, are generated by a 2D finite element viscous flow model (FeMargin), and used to advect lithosphere and asthenosphere temperature and material. FeMargin is kinematically driven by divergent deformation in the upper 15-20 km of the lithosphere inducing passive upwelling beneath that layer; extensional faulting and magmatic intrusions deform the topmost upper lithosphere, consistent with observations of deformation processes occurring at slow spreading ocean ridges (Cannat, 1996). Buoyancy enhanced upwelling, as predicted by Braun et al. (2000) is also kinematically included in the lithosphere deformation model. Melt generation by decompressional melting is predicted using the parameterization and methodology of Katz et al. (2003). The distribution of lithosphere deformation, the

  13. Electromagnetic evidence of high angle convergence between the Congo and Kalahari cratons in southern Africa

    Science.gov (United States)

    Khoza, D. T.; Jones, A. G.; Muller, M. R.; Miensopust, M. P.; Webb, S. J.; Share, P.

    2010-12-01

    W.m-2 steady-state conductive lithospheric geotherm. Preferential alignment of graphite and/or interstitial sulphides along grain boundaries facilitated by deep shear movement during crustal extension and thinning could account for the high conductive region within the DGC. The Congo and Kalahari cratons are characterised by very thick and resistive lithosphere, approximately 220 km and 160 km respectively and both their cratonic roots appear to be thrusted underneath the DGC. The Archean-Proterozoic basement inliers, which are part of the Congo craton nuclei are geologically mapped within the DGC in northern Namibia, further suggesting that the craton extend far south into the orogenic belt.

  14. Contrasted continental rifting via plume-craton interaction : Applications to Central East African Rift

    NARCIS (Netherlands)

    Koptev, Alexander; Burov, Evgueni; Calais, Eric; Leroy, Sylvie; Gerya, Taras; Guillou-Frottier, Laurent; Cloetingh, Sierd

    The East African Rift system (EARS) provides a unique system with the juxtaposition of two contrasting yet simultaneously formed rift branches, the eastern, magma-rich, and the western, magma-poor, on either sides of the old thick Tanzanian craton embedded in a younger lithosphere. Data on the

  15. Crustal seismicity and the earthquake catalog maximum moment magnitudes (Mcmax) in stable continental regions (SCRs): correlation with the seismic velocity of the lithosphere

    Science.gov (United States)

    Mooney, Walter D.; Ritsema, Jeroen; Hwang, Yong Keun

    2012-01-01

    A joint analysis of global seismicity and seismic tomography indicates that the seismic potential of continental intraplate regions is correlated with the seismic properties of the lithosphere. Archean and Early Proterozoic cratons with cold, stable continental lithospheric roots have fewer crustal earthquakes and a lower maximum earthquake catalog moment magnitude (Mcmax). The geographic distribution of thick lithospheric roots is inferred from the global seismic model S40RTS that displays shear-velocity perturbations (δVS) relative to the Preliminary Reference Earth Model (PREM). We compare δVS at a depth of 175 km with the locations and moment magnitudes (Mw) of intraplate earthquakes in the crust (Schulte and Mooney, 2005). Many intraplate earthquakes concentrate around the pronounced lateral gradients in lithospheric thickness that surround the cratons and few earthquakes occur within cratonic interiors. Globally, 27% of stable continental lithosphere is underlain by δVS≥3.0%, yet only 6.5% of crustal earthquakes with Mw>4.5 occur above these regions with thick lithosphere. No earthquakes in our catalog with Mw>6 have occurred above mantle lithosphere with δVS>3.5%, although such lithosphere comprises 19% of stable continental regions. Thus, for cratonic interiors with seismically determined thick lithosphere (1) there is a significant decrease in the number of crustal earthquakes, and (2) the maximum moment magnitude found in the earthquake catalog is Mcmax=6.0. We attribute these observations to higher lithospheric strength beneath cratonic interiors due to lower temperatures and dehydration in both the lower crust and the highly depleted lithospheric root.

  16. Crustal seismicity and the earthquake catalog maximum moment magnitude (Mcmax) in stable continental regions (SCRs): Correlation with the seismic velocity of the lithosphere

    Science.gov (United States)

    Mooney, Walter D.; Ritsema, Jeroen; Hwang, Yong Keun

    2012-12-01

    A joint analysis of global seismicity and seismic tomography indicates that the seismic potential of continental intraplate regions is correlated with the seismic properties of the lithosphere. Archean and Early Proterozoic cratons with cold, stable continental lithospheric roots have fewer crustal earthquakes and a lower maximum earthquake catalog moment magnitude (Mcmax). The geographic distribution of thick lithospheric roots is inferred from the global seismic model S40RTS that displays shear-velocity perturbations (δVS) relative to the Preliminary Reference Earth Model (PREM). We compare δVS at a depth of 175 km with the locations and moment magnitudes (Mw) of intraplate earthquakes in the crust (Schulte and Mooney, 2005). Many intraplate earthquakes concentrate around the pronounced lateral gradients in lithospheric thickness that surround the cratons and few earthquakes occur within cratonic interiors. Globally, 27% of stable continental lithosphere is underlain by δVS≥3.0%, yet only 6.5% of crustal earthquakes with Mw>4.5 occur above these regions with thick lithosphere. No earthquakes in our catalog with Mw>6 have occurred above mantle lithosphere with δVS>3.5%, although such lithosphere comprises 19% of stable continental regions. Thus, for cratonic interiors with seismically determined thick lithosphere (1) there is a significant decrease in the number of crustal earthquakes, and (2) the maximum moment magnitude found in the earthquake catalog is Mcmax=6.0. We attribute these observations to higher lithospheric strength beneath cratonic interiors due to lower temperatures and dehydration in both the lower crust and the highly depleted lithospheric root.

  17. Small-scale convection at a continental back-arc to craton transition: Application to the southern Canadian Cordillera

    Science.gov (United States)

    Hardebol, N. J.; Pysklywec, R. N.; Stephenson, R.

    2012-01-01

    A step in the depth of the lithosphere base, associated with lateral variations in the upper mantle temperature structure, can trigger mantle flow that is referred to as edge-driven convection. This paper aims at outlining the implications of such edge-driven flow at a lateral temperature transition from a hot and thin to a cold and thick lithosphere of a continental back-arc. This configuration finds application in the southern Canadian Cordillera, where a hot and thin back-arc is adjacent to the cold and thick North American Craton. A series of geodynamical models tested the thermodynamical behavior of the lithosphere and upper mantle induced by a step in lithosphere thickness. The mantle flow patterns, thickness and heat flow evolution of the lithosphere, and surface topography are examined. We find that the lateral temperature transition shifts cratonward due to the vigorous edge-driven mantle flow that erodes the craton edge, unless the craton has a distinct high viscosity mantle lithosphere. The mantle lithosphere viscosity structure determines the impact of edge-driven flow on crustal deformation and surface heat flow; a dry olivine rheology for the craton prevents the edge from migrating and supports a persistent surface heat flow contrast. These phenomena are well illustrated at the transition from the hot Canadian Cordillera to craton that is supported by a rheological change and that coincides with a lateral change in surface heat flow. Fast seismic wave velocities observed in the upper mantle cratonward of the step can be explained as downwellings induced by the edge-driven flow.

  18. Global variations in gravity-derived oceanic crustal thickness: Implications on oceanic crustal accretion and hotspot-lithosphere interactions

    Science.gov (United States)

    Lin, J.; Zhu, J.

    2012-12-01

    We present a new global model of oceanic crustal thickness based on inversion of global oceanic gravity anomaly with constrains from seismic crustal thickness profiles. We first removed from the observed marine free-air gravity anomaly all gravitational effects that can be estimated and removed using independent constraints, including the effects of seafloor topography, marine sediment thickness, and the age-dependent thermal structure of the oceanic lithosphere. We then calculated models of gravity-derived crustal thickness through inversion of the residual mantle Bouguer anomaly using best-fitting gravity-modeling parameters obtained from comparison with seismically determined crustal thickness profiles. Modeling results show that about 5% of the global crustal volume (or 9% of the global oceanic surface area) is associated with model crustal thickness 8.6 km and is interpreted to have been affected by excess magmatism. The percentage of oceanic crustal volume that is associated with thick crustal thickness (>8.6 km) varies greatly among tectonic plates: Pacific (33%), Africa (50%), Antarctic (33%), Australia (30%), South America (34%), Nazca (23%), North America (47%), India (74%), Eurasia (68%), Cocos (20%), Philippine (26%), Scotia (41%), Caribbean (89%), Arabian (82%), and Juan de Fuca (21%). We also found that distribution of thickened oceanic crust (>8.6 km) seems to depend on spreading rate and lithospheric age: (1) On ocean basins younger than 5 Ma, regions of thickened crust are predominantly associated with slow and ultraslow spreading ridges. The relatively strong lithospheric plate at slow and ultraslow ridges might facilitate the loading of large magmatic emplacements on the plate. (2) In contrast, crustal thickness near fast and intermediately fast spreading ridges typically does not exceed 7-8 km. The relatively weak lithosphere at fast and intermediately fast ridges might make it harder for excess magmatism to accrete. We further speculate that

  19. Lithospheric thermal-rheological structure of the Ordos Basin and its geodynamics

    Science.gov (United States)

    Pan, J.; Huang, F.; He, L.; Wu, Q.

    2015-12-01

    The study on the destruction of the North China Craton has always been one of the hottest issues in earth sciences.Both mechanism and spatial variation are debated fiercely, still unclear.However, geothermal research on the subject is relatively few. Ordos Basin, located in the west of the North China Craton, is a typical intraplate. Based on two-dimensional thermal modeling along a profile across Ordos Basin from east to west, obtained the lithospheric thermal structure and rheology. Mantle heat flow in different regions of Ordos Basin is from 21.2 to 24.5 mW/m2. In the east mantle heat flow is higher while heat flow in western region is relatively low. But mantle heat flow is smooth and low overall, showing a stable thermal background. Ratio of crustal and mantle heat flow is between 1.51 and 1.84, indicating that thermal contribution from shallow crust is lower than that from the mantle. Rheological characteristics along the profile are almost showed as "jelly sandwich" model and stable continental lithosphere structure,which is represent by a weak crust portion but a strong lithospheric mantle portion in vertical strength profile. Based on above , both thermal structure and lithospheric rheology of Ordos Basin illustrate that tectonic dynamics environment in the west of North China Craton is relatively stable. By the study on lithospheric thermal structure, we focus on the disparity in thickness between the thermal lithosphere and seismic lithosphere.The difference in western Ordos Basin is about 140km, which decreases gradually from Fenwei graben in the eastern Ordos Basin to the Bohai Bay Basin.That is to say the difference decreases gradually from the west to the east of North China Craton.The simulation results imply that viscosity of the asthenosphere under North China Craton also decreases gradually from west to east, confirming that dehydration of the Pacific subduction is likely to have great effect on the North China Craton.

  20. Seismic imaging of Southern African cratons

    DEFF Research Database (Denmark)

    Soliman, Mohammad Youssof Ahmad

    Cratonic regions are the oldest stable parts of continents that hold most of Earth’s mineral resources. There are several open questions regarding their formation and evolution. In this PhD study, passive source seismic methods have been used to investigate the crustal and lithosphere structures...... of this research was based on Ps- and Sp- receiver functions analysis to determine crustal thickness while finite-frequency traveltime tomography is utilized to model 3D heterogeneity in the upper mantle. Combining the two methods provides high vertical and lateral resolution....

  1. Contrasted continental rifting via plume-craton interaction: Applications to Central East African Rift

    Directory of Open Access Journals (Sweden)

    Alexander Koptev

    2016-03-01

    Full Text Available The East African Rift system (EARS provides a unique system with the juxtaposition of two contrasting yet simultaneously formed rift branches, the eastern, magma-rich, and the western, magma-poor, on either sides of the old thick Tanzanian craton embedded in a younger lithosphere. Data on the pre-rift, syn-rift and post-rift far-field volcanic and tectonic activity show that the EARS formed in the context of the interaction between a deep mantle plume and a horizontally and vertically heterogeneous lithosphere under far-field tectonic extension. We bring quantitative insights into this evolution by implementing high-resolution 3D thermo-mechanical numerical deformation models of a lithosphere of realistic rheology. The models focus on the central part of the EARS. We explore scenarios of plume-lithosphere interaction with plumes of various size and initial position rising beneath a tectonically pre-stretched lithosphere. We test the impact of the inherited rheological discontinuities (suture zones along the craton borders, of the rheological structure, of lithosphere plate thickness variations, and of physical and mechanical contrasts between the craton and the embedding lithosphere. Our experiments indicate that the ascending plume material is deflected by the cratonic keel and preferentially channeled along one of its sides, leading to the formation of a large rift zone along the eastern side of the craton, with significant magmatic activity and substantial melt amount derived from the mantle plume material. We show that the observed asymmetry of the central EARS, with coeval amagmatic (western and magmatic (eastern branches, can be explained by the splitting of warm material rising from a broad plume head whose initial position is slightly shifted to the eastern side of the craton. In that case, neither a mechanical weakness of the contact between the craton and the embedding lithosphere nor the presence of second plume are required to

  2. Structure of the lithosphere below the southern margin of the East European Craton (Ukraine and Russia) from gravity and seismic data.

    NARCIS (Netherlands)

    Yegorova, T.P.; Stephenson, R.A.; Kostyuchenko, S.L.; Baranova, E.P.; Starostenko, V.I.; Popolitov, K.E.

    2004-01-01

    The present study was undertaken with the objective of deriving constraints from available geological and geophysical data for understanding the tectonic setting and processes controlling the evolution of the southern margin of the East European Craton (EEC). The study area includes the inverted

  3. Seismic and Thermal Structure of the Arctic Lithosphere, From Waveform Tomography and Thermodynamic Modelling

    Science.gov (United States)

    Lebedev, S.; Schaeffer, A. J.; Fullea, J.; Pease, V.

    2015-12-01

    Thermal structure of the lithosphere is reflected in the values of seismic velocities within it. Our new tomographic models of the crust and upper mantle of the Arctic are constrained by an unprecedentedly large global waveform dataset and provide substantially improved resolution, compared to previous models. The new tomography reveals lateral variations in the temperature and thickness of the lithosphere and defines deep boundaries between tectonic blocks with different lithospheric properties and age. The shape and evolution of the geotherm beneath a tectonic unit depends on both crustal and mantle-lithosphere structure beneath it: the lithospheric thickness and its changes with time (these determine the supply of heat from the deep Earth), the crustal thickness and heat production (the supply of heat from within the crust), and the thickness and thermal conductivity of the sedimentary cover (the insulation). Detailed thermal structure of the basins can be modelled by combining seismic velocities from tomography with data on the crustal structure and heat production, in the framework of computational petrological modelling. The most prominent lateral contrasts across the Arctic are between the cold, thick lithospheres of the cratons (in North America, Greenland and Eurasia) and the warmer, non-cratonic blocks. The lithosphere of the Canada Basin is cold and thick, similar to old oceanic lithosphere elsewhere around the world; its thermal structure offers evidence on its lithospheric age and formation mechanism. At 150-250 km depth, the central Arctic region shows a moderate low-velocity anomaly, cooler than that beneath Iceland and N Atlantic. An extension of N Atlantic low-velocity anomaly into the Arctic through the Fram Strait may indicate an influx of N Atlantic asthenosphere under the currently opening Eurasia Basin.

  4. Using crustal thickness, subsidence and P-T-t history on the Iberia-Newfoundland & Alpine Tethys margins to constrain lithosphere deformation modes during continental breakup

    Science.gov (United States)

    Jeanniot, L.; Kusznir, N. J.; Manatschal, G.; Mohn, G.; Beltrando, M.

    2013-12-01

    Observations at magma-poor rifted margins such as Iberia-Newfoundland show a complex lithosphere deformation history and OCT architecture, resulting in hyper-extended continental crust and lithosphere, exhumed mantle and scattered embryonic oceanic crust before continental breakup and seafloor spreading. Initiation of seafloor spreading requires both the rupture of the continental crust and lithospheric mantle, and the onset of decompressional melting. Their relative timing controls when mantle exhumation may occur; the presence or absence of exhumed mantle provides useful information on the timing of these events and constraints on lithosphere deformation modes. A single kinematic lithosphere deformation mode leading to continental breakup and sea-floor spreading cannot explain observations. We have determined the sequence of lithosphere deformation events, using forward modelling of crustal thickness, subsidence and P-T-t history calibrated against observations on the present-day Iberia-Newfoundland and the fossil analogue Alpine Tethys margins. Lithosphere deformation modes, represented by flow fields, are generated by a 2D finite element viscous flow model (FeMargin), and used to advect lithosphere and asthenosphere temperature and material. FeMargin is kinematically driven by divergent deformation in the topmost upper lithosphere inducing passive upwelling beneath that layer; the upper lithosphere is assumed to deform by extensional faulting and magmatic intrusions, consistent with observations of deformation processes occurring at slow spreading ocean ridges (Cannat, 1996). Buoyancy enhanced upwelling is also included in the kinematic model as predicted by Braun et al (2000). We predict melt generation by decompressional melting using the parameterization and methodology of Katz et al., 2003. We use a series of numerical experiments, tested and calibrated against crustal thicknesses and subsidence observations, to determine the distribution of lithosphere

  5. The crustal thickness and lithospheric structure of active and inactive volcanic arc terrains in Fiji and Tonga

    Science.gov (United States)

    Chen, J.; Wiens, D.; Wei, S. S.; Zha, Y.; Julià, J.; Cai, C.; Chen, Y. J.

    2015-12-01

    In order to investigate the crustal thickness and lithospheric structure beneath active and inactive volcanic arcs in Fiji and Tonga, we analyzed receiver functions from teleseismic P waves as well as Rayleigh waves from teleseismic earthquakes and ambient noise. The data were recorded by stations from three previous temporary seismic arrays deployed on the islands during 1993-1995, 2001-2002, and 2009-2010. Receiver functions were calculated with an iterative deconvolution in the time domain. We used an H-k stacking method to get preliminary Moho depth estimates under the island arcs, after assuming constant seismic average crustal P velocity. We also determined the shear wave velocity structure beneath each station from a 1-D combined inversion of receiver functions and Rayleigh wave phase velocity dispersion curves from ambient noise cross correlation at 8s - 20s and teleseismic surface waves at 20s-90s. The joint inversion models reveal that the Moho beneath the main islands of the Fiji plateau is 26-31 km deep, whereas the crust under the outer islands - including the Lau Ridge - is generally thinner, with Moho depths of 21-23.5 km. The thinnest crust (16 km) is found beneath Moala Island located between the Fiji Platform and the Lau Ridge. Crustal thickness beneath several Tonga islands is about 18-20 km. A relatively high velocity lithosphere (Vs of 4.4 - 4.5 km/s) extends to only about 60 km depth beneath the outer Fiji Islands and Lau Ridge, but to depths of 90 km underneath the main islands of the Fiji Plateau. The much thicker crust and lithosphere of the Fiji plateau relative to the Lau Ridge and Tonga Arc reflects its much longer geological history of arc crust building, going back to the early Miocene.

  6. Seismic imaging of lithospheric discontinuities and continental evolution

    Science.gov (United States)

    Bostock, M. G.

    1999-09-01

    Discontinuities in physical properties within the continental lithosphere reflect a range of processes that have contributed to craton stabilization and evolution. A survey of recent seismological studies concerning lithospheric discontinuities is made in an attempt to document their essential characteristics. Results from long-period seismology are inconsistent with the presence of continuous, laterally invariant, isotropic boundaries within the upper mantle at the global scale. At regional scales, two well-defined interfaces termed H (˜60 km depth) and L (˜200 km depth) of continental affinity are identified, with the latter boundary generally exhibiting an anisotropic character. Long-range refraction profiles are frequently characterized by subcontinental mantle that exhibits a complex stratification within the top 200 km. The shallow layering of this package can behave as an imperfect waveguide giving rise to the so-called teleseismic Pn phase, while the L-discontinuity may define its lower base as the culmination of a low velocity zone. High-resolution, seismic reflection profiling provides sufficient detail in a number of cases to document the merging of mantle interfaces into lower continental crust below former collisional sutures and magmatic arcs, thus unambiguously identifying some lithospheric discontinuities with thrust faults and subducted oceanic lithosphere. Collectively, these and other seismic observations point to a continental lithosphere whose internal structure is dominated by a laterally variable, subhorizontal layering. This stratigraphy appears to be more pronounced at shallower lithospheric levels, includes dense, anisotropic layers of order 10 km in thickness, and exhibits horizontal correlation lengths comparable to the lateral dimensions of overlying crustal blocks. A model of craton evolution which relies on shallow subduction as a principal agent of craton stabilization is shown to be broadly compatible with these characteristics.

  7. Density structure of the cratonic mantle in southern Africa

    DEFF Research Database (Denmark)

    Artemieva, Irina; Vinnik, Lev P.

    2016-01-01

    contributions of the both factors to surface topography in the cratons of southern Africa. Our analysis takes advantage of the SASE seismic experiment which provided high resolution regional models of the crustal thickness.We calculate the model of density structure of the lithospheric mantle in southern Africa...... that mantle residual (dynamic) topography may be associated with the low-density region below the depth of isostatic compensation. A possible candidate is the low velocity layer between the lithospheric base and the mantle transition zone, where a temperature anomaly of 100-200. °C in a ca. 100-150. km thick...... layer may explain the observed reduction in Vs velocity and may produce ca. 0.5-1.0. km to the regional topographic uplift....

  8. A rapid method to map the crustal and lithospheric thickness using elevation, geoid anomaly and thermal analysis. Application to the Gibraltar Arc System, Atlas Mountains and adjacent zones

    Science.gov (United States)

    Fullea, J.; Fernàndez, M.; Zeyen, H.; Vergés, J.

    2007-02-01

    We present a method based on the combination of elevation and geoid anomaly data together with thermal field to map crustal and lithospheric thickness. The main assumptions are local isostasy and a four-layered model composed of crust, lithospheric mantle, sea water and the asthenosphere. We consider a linear density gradient for the crust and a temperature dependent density for the lithospheric mantle. We perform sensitivity tests to evaluate the effect of the variation of the model parameters and the influence of RMS error of elevation and geoid anomaly databases. The application of this method to the Gibraltar Arc System, Atlas Mountains and adjacent zones reveals the presence of a lithospheric thinning zone, SW-NE oriented. This zone affects the High and Middle Atlas and extends from the Canary Islands to the eastern Alboran Basin and is probably linked with a similarly trending zone of thick lithosphere constituting the western Betics, eastern Rif, Rharb Basin, and Gulf of Cadiz. A number of different, even mutually opposite, geodynamic models have been proposed to explain the origin and evolution of the study area. Our results suggest that a plausible slab-retreating model should incorporate tear and asymmetric roll-back of the subducting slab to fit the present-day observed lithosphere geometry. In this context, the lithospheric thinning would be caused by lateral asthenospheric flow. An alternative mechanism responsible for lithospheric thinning is the presence of a hot magmatic reservoir derived from a deep ancient plume centred in the Canary Island, and extending as far as Central Europe.

  9. Mantle metasomatism in the Kaapvaal Craton lithosphere: constraints on the composition of the metasomatic agent from fluid inclusions in MARID-type xenoliths

    Science.gov (United States)

    Konzett, J.; Krenn, K.; Hauzenberger, Ch.

    2012-04-01

    The emplacement of both group I and group II kimberlites in the Kaapvaal Craton of the Kimberley region in South Africa is associated with an intense metasomatic alteration of the country rocks as evidenced by a diverse suite of xenoliths sampled by the kimberlites mainly comprising metasomatized peridotites and minor MARID-type xenoliths. These are characterized by hydrous potassic silicates and LILE-HFSE-rich titanates. Because the metasomatic agent is not preserved in these rocks its composition has to be inferred from that of the metasomatic assemblages. Here we present for the first time data on fluid inclusions from two MARID-xenoliths sampled by group-I kimberlites of the Kimberley cluster. They provide direct evidence for the nature of the metasomatic fluids involved in kimberlite-related metsomatism. The xenoliths contain phlogopite+K-richterite+diopside+ilmenite±rutile±apatite±zircon. Fluid inclusions with 4-10 µm in size were found in diopside, K-richterite and zircon and contain L+V+one-to-several daughter phases. Investigations with the freezing and heating stage indicate two different chemical systems for the fluids: (1) H2O-NaCl dominant fluids found as L+V+S inclusions in zircon together with abundant needle-like apatite, rutile and phlogopite solid inclusions. The fluid inclusions in part occur along zircon host-rutile/apatite inclusion grain boundaries which indicates that the fluids were trapped during zircon growth. They contain 30-32 mass% NaCl and show a density of 0.87-0.94 g/cm3. Halos of tiny fluid inclusions, however, indicate that most if not all zircon inclusions are decrepitated during ascent from depth and/or superheating during entrainment of the xenoliths into the kimberlite. Using EMPA, enstatite and a SiO2 polymorph were identified in opened fluid inclusions exposed at the surface of polished thin sections. Because these phases were exclusively found in the fluid inclusions, they are considered daughter crystals. The enstatite

  10. Lower Crustal Seismicity, Volatiles, and Evolving Strain Fields During the Initial Stages of Cratonic Rifting

    Science.gov (United States)

    Lambert, C.; Muirhead, J.; Ebinger, C. J.; Tiberi, C.; Roecker, S. W.; Ferdinand-Wambura, R.; Kianji, G.; Mulibo, G. D.

    2014-12-01

    The volcanically active East African rift system in southern Kenya and northern Tanzania transects thick cratonic lithosphere, and comprises several basins characterized by deep crustal seismicity. The US-French-Tanzania-Kenya CRAFTI project aims to understand the role of magma and volatile movement during the initiation and evolution of rifting in cratonic lithosphere. Our 38-station broadband network spans all or parts of fault-bounded rift segments, enabling comparison of lithospheric structure, fault kinematics, and seismogenic layer thickness with age and proximity to the deeply rooted Archaen craton. Seismicity levels are high in all basins, but we find profound differences in seismogenic layer thickness along the length of the rift. Seismicity in the Manyara basin occurs almost exclusively within the lower crust, and in spatial clusters that have been active since 1990. In contrast, seismicity in the ~ 5 My older Magadi basin is localized in the upper crust, and the long border fault bounding the west side of the basin is seismically inactive. Between these two basins lies the Natron rift segment, which shows seismicity between ~ 20 and ~2 km depth, and high concentrations at Oldoinyo Lengai and Gelai volcanoes. Older volcanoes on the uplifted western flank (e.g., Ngorongoro) experience swarms of activity, suggesting that active magmatism and degassing are widespread. Focal mechanisms of the frequent earthquakes recorded across the array are spatially variable, and indicate a stress field strongly influenced by (1) Holocene volcanoes, (2) mechanical interactions between adjacent rift basins, and (3) a far-field ESE-WNW extensional stress regime. We explore the spatial correlation between zones of intense degassing along fault systems and seismicity, and examine the influence of high gas pressures on lower and upper crustal seismicity in this youthful cratonic rift zone.

  11. Ancient Continental Lithosphere Dislocated Beneath Ocean Basins Along the Mid-Lithosphere Discontinuity: A Hypothesis

    Science.gov (United States)

    Wang, Zhensheng; Kusky, Timothy M.; Capitanio, Fabio A.

    2017-09-01

    The documented occurrence of ancient continental cratonic roots beneath several oceanic basins remains poorly explained by the plate tectonic paradigm. These roots are found beneath some ocean-continent boundaries, on the trailing sides of some continents, extending for hundreds of kilometers or farther into oceanic basins. We postulate that these cratonic roots were left behind during plate motion, by differential shearing along the seismically imaged mid-lithosphere discontinuity (MLD), and then emplaced beneath the ocean-continent boundary. Here we use numerical models of cratons with realistic crustal rheologies drifting at observed plate velocities to support the idea that the mid-lithosphere weak layer fostered the decoupling and offset of the African continent's buoyant cratonic root, which was left behind during Meso-Cenozoic continental drift and emplaced beneath the Atlantic Ocean. We show that in some cratonic areas, the MLD plays a similar role as the lithosphere-asthenosphere boundary for accommodating lateral plate tectonic displacements.

  12. Mantle Earthquakes in Thinned Proterozoic Lithosphere: Harrat Lunayyir, Saudi Arabia

    Science.gov (United States)

    Blanchette, A. R.; Klemperer, S. L.; Mooney, W. D.; Zahran, H. M.

    2017-12-01

    Harrat Lunayyir is an active volcanic field located in the western Arabian Shield 100 km outside of the Red Sea rift margin. We use common conversion point (CCP) stacking of P-wave receiver functions (PRFs) to show that the Moho is at 38 km depth, close to the 40 km crustal thickness measured in the center of the craton, whereas the lithosphere-asthenosphere boundary (LAB) is at 60 km, far shallower than the 150 km furthest in the craton. We locate 67 high-frequency earthquakes with mL ≤ 2.5 at depths of 40-50 km below the surface, located clearly within the mantle lid. The occurrence of earthquakes within the lithospheric mantle requires a geothermal temperature profile that is below equilibrium. The lithosphere cannot have thinned to its present thickness earlier than 15 Ma, either during an extended period of rifting possibly beginning 24 Ma or, more likely, as part of the second stage of rifting following collision between Arabia and Eurasia.

  13. Imaging Canary Island hotspot material beneath the lithosphere of Morocco and southern Spain

    Science.gov (United States)

    Miller, Meghan S.; O'Driscoll, Leland J.; Butcher, Amber J.; Thomas, Christine

    2015-12-01

    The westernmost Mediterranean has developed into its present day tectonic configuration as a result of complex interactions between late stage subduction of the Neo-Tethys Ocean, continental collision of Africa and Eurasia, and the Canary Island mantle plume. This study utilizes S receiver functions (SRFs) from over 360 broadband seismic stations to seismically image the lithosphere and uppermost mantle from southern Spain through Morocco and the Canary Islands. The lithospheric thickness ranges from ∼65 km beneath the Atlas Mountains and the active volcanic islands to over ∼210 km beneath the cratonic lithosphere in southern Morocco. The common conversion point (CCP) volume of the SRFs indicates that thinned lithosphere extends from beneath the Canary Islands offshore southwestern Morocco, to beneath the continental lithosphere of the Atlas Mountains, and then thickens abruptly at the West African craton. Beneath thin lithosphere between the Canary hot spot and southern Spain, including below the Atlas Mountains and the Alboran Sea, there are distinct pockets of low velocity material, as inferred from high amplitude positive, sub-lithospheric conversions in the SRFs. These regions of low seismic velocity at the base of the lithosphere extend beneath the areas of Pliocene-Quaternary magmatism, which has been linked to a Canary hotspot source via geochemical signatures. However, we find that this volume of low velocity material is discontinuous along strike and occurs only in areas of recent volcanism and where asthenospheric mantle flow is identified with shear wave splitting analyses. We propose that the low velocity structure beneath the lithosphere is material flowing sub-horizontally northeastwards beneath Morocco from the tilted Canary Island plume, and the small, localized volcanoes are the result of small-scale upwellings from this material.

  14. Finite-frequency P-wave tomography of the Western Canada Sedimentary Basin: Implications for the lithospheric evolution in Western Laurentia

    Science.gov (United States)

    Chen, Yunfeng; Gu, Yu Jeffrey; Hung, Shu-Huei

    2017-02-01

    The lithosphere beneath the Western Canada Sedimentary Basin has potentially undergone Precambrian subduction and collisional orogenesis, resulting in a complex network of crustal domains. To improve the understanding of its evolutionary history, we combine data from the USArray and three regional networks to invert for P-wave velocities of the upper mantle using finite-frequency tomography. Our model reveals distinct, vertically continuous high (> 1%) velocity perturbations at depths above 200 km beneath the Precambrian Buffalo Head Terrane, Hearne craton and Medicine Hat Block, which sharply contrasts with those beneath the Canadian Rockies (Medicine Hat Block (200 km). These findings are consistent with earlier theories of tectonic assembly in this region, which featured distinct Archean and Proterozoic plate convergences between the Hearne craton and its neighboring domains. The highly variable, bimodally distributed craton thicknesses may also reflect different lithospheric destruction processes beneath the western margin of Laurentia.

  15. Lithospheric processes

    Energy Technology Data Exchange (ETDEWEB)

    Baldridge, W. [and others

    2000-12-01

    The authors used geophysical, geochemical, and numerical modeling to study selected problems related to Earth's lithosphere. We interpreted seismic waves to better characterize the thickness and properties of the crust and lithosphere. In the southwestern US and Tien Shari, crust of high elevation is dynamically supported above buoyant mantle. In California, mineral fabric in the mantle correlate with regional strain history. Although plumes of buoyant mantle may explain surface deformation and magmatism, our geochemical work does not support this mechanism for Iberia. Generation and ascent of magmas remains puzzling. Our work in Hawaii constrains the residence of magma beneath Hualalai to be a few hundred to about 1000 years. In the crust, heat drives fluid and mass transport. Numerical modeling yielded robust and accurate predictions of these processes. This work is important fundamental science, and applies to mitigation of volcanic and earthquake hazards, Test Ban Treaties, nuclear waste storage, environmental remediation, and hydrothermal energy.

  16. Lithospheric processes

    International Nuclear Information System (INIS)

    Baldridge, W.S.

    2000-01-01

    The authors used geophysical, geochemical, and numerical modeling to study selected problems related to Earth's lithosphere. We interpreted seismic waves to better characterize the thickness and properties of the crust and lithosphere. In the southwestern US and Tien Shari, crust of high elevation is dynamically supported above buoyant mantle. In California, mineral fabric in the mantle correlate with regional strain history. Although plumes of buoyant mantle may explain surface deformation and magmatism, our geochemical work does not support this mechanism for Iberia. Generation and ascent of magmas remains puzzling. Our work in Hawaii constrains the residence of magma beneath Hualalai to be a few hundred to about 1000 years. In the crust, heat drives fluid and mass transport. Numerical modeling yielded robust and accurate predictions of these processes. This work is important fundamental science, and applies to mitigation of volcanic and earthquake hazards, Test Ban Treaties, nuclear waste storage, environmental remediation, and hydrothermal energy

  17. EVOLUTION OF SOUTHERN AFRICAN CRATONS BASED ON SEISMIC IMAGING

    DEFF Research Database (Denmark)

    Thybo, Hans; Soliman, Mohammad Youssof Ahmad; Artemieva, Irina

    2014-01-01

    present a new seismic model for the structure of the crust and lithospheric mantle of the Kalahari Craton, constrained by seismic receiver functions and finite-frequency tomography based on the seismological data from the South Africa Seismic Experiment (SASE). The combination of these two methods...... since formation of the craton, and (3) seismically fast lithospheric keels are imaged in the Kaapvaal and Zimabwe cratons to depths of 300-350 km. Relatively low velocity anomalies are imaged beneath both the paleo-orogenic Limpopo Belt and the Bushveld Complex down to depths of ~250 km and ~150 km...

  18. Unraveling African plate structure from elevation, geoid and geology data: implications for the impact of mantle flow and sediment transfers on lithospheric deformation

    Science.gov (United States)

    Bajolet, Flora; Robert, Alexandra; Chardon, Dominique; Rouby, Delphine

    2017-04-01

    The aim of our project is to simulate the long-wavelength, flexural isostatic response of the African plate to sediment transfers due to Meso-Cenozoic erosion - deposition processes in order to extract the residual topography driven by mantle dynamics. The first step of our project consists in computing crustal and lithospheric thickness maps of the African plate considering its main geological components (cratons, mobile belts, basins, rifts and passive margins of various ages and strengths). In order to consider these heterogeneities, we compute a 2D distribution of crustal densities and thermal parameters from geological data and use it as an input of our modeling. We combine elevation and geoid anomaly data using a thermal analysis, following the method of Fullea et al. (2007) in order to map crustal and lithospheric thicknesses. In this approach, we assume local isostasy and consider a four-layer model made of crust and lithospheric mantle plus seawater and asthenosphere. In addition, we compare our results with crustal and lithospheric thickness datasets compiled from bibliography and existing global models. The obtained crustal thicknesses range from 28 to 42km, with the thickest crust confined to the northern part of the West African Craton, the Kaapvaal craton, and the Congo cuvette. The crust in the East African Rift appears unrealistically thick (40-45 km) as it is not isotatically compensated, highlighting the dynamic effect of the African superswell. The thinnest crust (28-34km) follows a central East-West trend coinciding with Cretaceous rifts and the Cameroon volcanic line. The lithosphere reaches 220 km beneath the Congo craton, but remains globally thin (ca. 120-180 km) compared to tomographic models and considering the age of most geological provinces. As for the crust, the thinnest lithosphere is located in areas of Cretaceous-Jurassic rifting, suggesting that the lithosphere did not thermally recover from Mesozoic rifting. A new elastic

  19. Shear-Velocity Structure and Azimuthal and Radial Anisotropy Beneath the Kaapvaal Craton From Bayesian Inversion of Surface-Wave Data: Inferences for the Architecture and Early Evolution of Cratons

    Science.gov (United States)

    Lebedev, S.; Ravenna, M.; Adam, J.

    2017-12-01

    Seismic anisotropy provides essential information on the deformation of the lithosphere. Knowledge of anisotropy also allows us to isolate the isotropic-average seismic velocities, relatable to the lithospheric temperature and composition. We use Rayleigh and Love-wave phase velocities and their azimuthal anisotropy measured in broad period ranges across the footprint of the Southern Africa Seismic Experiment (SASE), from the Kaapvaal Craton to the Limpopo Belt. We invert the data using our recently developed, fully non-linear Markov Chain Monte Carlo method and determine, for the first time, both the isotropic-average S velocity and its radial and azimuthal anisotropy as a function of depth from the upper crust down to the asthenosphere. The probabilistic inversion provides a way to quantify non-uniqueness, using direct parameter-space sampling, and assess model uncertainties. The high-velocity anomaly indicative of the cold cratonic lithosphere bottoms at 200-250 km beneath the central and western Kaapvaal Craton, underlain by a low-velocity zone. Beneath northern Kaapvaal and Limpopo, by contrast, high velocities extend down to 300-350 km. Although this does not require a lithosphere that has maintained this thickness over a geologically long time, the data does require the mantle to be anomalously cold down to 300-350 km. Interestingly, topography correlates with the thickness of this high-velocity layer, with lower elevations where the lid is thicker. Radial shear-wave anisotropy is in the 2-5 percent range (Vsh > Vsv) from the lower crust down to 200 km, below which depth it decreases gradually. Radial variations in the amplitude of radial anisotropy show no clear relationship with those in the amplitude of azimuthal anisotropy or isotropic-average Vs anomalies. Azimuthal anisotropy changes the fast-propagation direction near the base of the lithosphere (200-300 km depth), from the laterally varying fast azimuths in the lower lithosphere to a spatially

  20. Lithospheric structure of northwest Africa: Insights into the tectonic history and influence of mantle flow on large-scale deformation

    Science.gov (United States)

    Miller, Meghan S.; Becker, Thorsten

    2014-05-01

    Northwest Africa is affected by late stage convergence of Africa with Eurasia, the Canary Island hotspot, and bounded by the Proterozoic-age West African craton. We present seismological evidence from receiver functions and shear-wave splitting along with geodynamic modeling to show how the interactions of these tectonic features resulted in dramatic deformation of the lithosphere. We interpret seismic discontinuities from the receiver functions and find evidence for localized, near vertical-offset deformation of both crust-mantle and lithosphere-asthenosphere interfaces at the flanks of the High Atlas. These offsets coincide with the locations of Jurassic-aged normal faults that have been reactivated during the Cenozoic, further suggesting that inherited, lithospheric-scale zones of weakness were involved in the formation of the Atlas. Another significant step in lithospheric thickness is inferred within the Middle Atlas. Its location corresponds to the source of regional Quaternary alkali volcanism, where the influx of melt induced by the shallow asthenosphere appears restricted to a lithospheric-scale fault on the northern side of the mountain belt. Inferred stretching axes from shear-wave splitting are aligned with the topographic grain in the High Atlas, suggesting along-strike asthenospheric shearing in a mantle channel guided by the lithospheric topography. Isostatic modeling based on our improved lithospheric constraints indicates that lithospheric thinning alone does not explain the anomalous Atlas topography. Instead, an mantle upwelling induced by a hot asthenospheric anomaly appears required, likely guided by the West African craton and perhaps sucked northward by subducted lithosphere beneath the Alboran. This dynamic support scenario for the Atlas also suggests that the timing of uplift is contemporaneous with the recent volcanismin the Middle Atlas.

  1. Sources and mobility of carbonate melts beneath cratons, with implications for deep carbon cycling, metasomatism and rift initiation

    Science.gov (United States)

    Tappe, Sebastian; Romer, Rolf L.; Stracke, Andreas; Steenfelt, Agnete; Smart, Katie A.; Muehlenbachs, Karlis; Torsvik, Trond H.

    2017-05-01

    Kimberlite and carbonatite magmas that intrude cratonic lithosphere are among the deepest probes of the terrestrial carbon cycle. Their co-existence on thick continental shields is commonly attributed to continuous partial melting sequences of carbonated peridotite at >150 km depths, possibly as deep as the mantle transition zone. At Tikiusaaq on the North Atlantic craton in West Greenland, approximately 160 Ma old ultrafresh kimberlite dykes and carbonatite sheets provide a rare opportunity to study the origin and evolution of carbonate-rich melts beneath cratons. Although their Sr-Nd-Hf-Pb-Li isotopic compositions suggest a common convecting upper mantle source that includes depleted and recycled oceanic crust components (e.g., negative ΔεHf coupled with > + 5 ‰ δ7Li), incompatible trace element modelling identifies only the kimberlites as near-primary low-degree partial melts (0.05-3%) of carbonated peridotite. In contrast, the trace element systematics of the carbonatites are difficult to reproduce by partial melting of carbonated peridotite, and the heavy carbon isotopic signatures (-3.6 to - 2.4 ‰ δ13C for carbonatites versus -5.7 to - 3.6 ‰ δ13C for kimberlites) require open-system fractionation at magmatic temperatures. Given that the oxidation state of Earth's mantle at >150 km depth is too reduced to enable larger volumes of 'pure' carbonate melt to migrate, it is reasonable to speculate that percolating near-solidus melts of carbonated peridotite must be silicate-dominated with only dilute carbonate contents, similar to the Tikiusaaq kimberlite compositions (e.g., 16-33 wt.% SiO2). This concept is supported by our findings from the North Atlantic craton where kimberlite and other deeply derived carbonated silicate melts, such as aillikites, exsolve their carbonate components within the shallow lithosphere en route to the Earth's surface, thereby producing carbonatite magmas. The relative abundances of trace elements of such highly

  2. Satellite gravity gradient views help reveal the Antarctic lithosphere

    Science.gov (United States)

    Ferraccioli, F.; Ebbing, J.; Pappa, F.; Kern, M.; Forsberg, R.

    2017-12-01

    Here we present and analyse satellite gravity gradient signatures derived from GOCE and superimpose these on tectonic and bedrock topography elements, as well as seismically-derived estimates of crustal thickness for the Antarctic continent. The GIU satellite gravity component images the contrast between the thinner crust and lithosphere underlying the West Antarctic Rift System and the Weddell Sea Rift System and the thicker lithosphere of East Antarctica. The new images also suggest that more distributed wide-mode lithospheric and crustal extension affects both the Ross Sea Embayment and the less well known Ross Ice Shelf segment of the rift system. However, this pattern is less clear towards the Bellingshousen Embayment, indicating that the rift system narrows towards the southern edge of the Antarctic Peninsula. In East Antarctica, the satellite gravity data provides new views into the Archean to Mesoproterozoic Terre Adelie Craton, and clearly shows the contrast wrt to the crust and lithosphere underlying both the Wilkes Subglacial Basin to the east and the Sabrina Subglacial Basin to the west. This finding augments recent interpretations of aeromagnetic and airborne gravity data over the region, suggesting that the Mawson Continent is a composite lithospheric-scale entity, which was affected by several Paleoproterozoic and Mesoproterozoic orogenic events. Thick crust is imaged beneath the Transantarctic Mountains, the Terre Adelie Craton, the Gamburtsev Subglacial Mountains and also Eastern Dronning Maud Land, in particular beneath the recently proposed region of the Tonian Oceanic Arc Superterrane. The GIA and GIU components help delineate the edges of several of these lithospheric provinces. One of the most prominent lithospheric-scale features discovered in East Antarctica from satellite gravity gradient imaging is the Trans East Antarctic Shear Zone that separates the Gamburtsev Province from the Eastern Dronning Maud Land Province and appears to form the

  3. Electromagnetic study of lithospheric structure in Trans-European Suture Zone in Poland

    Science.gov (United States)

    Jóźwiak, Waldemar; Ślęzak, Katarzyna; Nowożyński, Krzysztof; Neska, Anne

    2016-04-01

    The area covered by magnetotelluric surveys in Poland is mostly related to the Trans-European Suture Zone (TESZ), the largest tectonic boundary in Europe. Numerous 1D, 2D, and pseudo-3D and 3D models of the electrical resistivity distribution were constructed, and a new interpretation method based on Horizontal Magnetic Tensor analysis has been applied recently. The results indicate that the TESZ is a lithospheric discontinuity and there are noticeable differences in geoelectric structures between the East European Craton (EEC), the transitional zone (TESZ), and the Paleozoic Platform (PP). The electromagnetic sounding is a very efficient tool for recognizing the lithospheric structure especially it helps in identification of important horizontal (or lateral) inhomogeneities in the crust. Due to our study we can clearly determine the areas of the East European Craton of high resistivity, Paleozoic Platform of somewhat lower resistivity value, and transitional TESZ of complicated structure. At the East European Craton, we observe very highly resistive lithosphere, reaching 220-240 km depth. Underneath, there is distinctly greater conductivity values, most probably resulting from partial melting of rocks; this layer may represent the asthenosphere. The resistivity of the lithosphere under the Paleozoic Platform is somewhat lower, and its thickness does not exceed 150 km. The properties of the lithosphere in the transition zone, under the TESZ, differ significantly. The presented models include prominent, NW-SE striking conductive lineaments. These structures, that related with the TESZ, lie at a depth of 10-30 km. They are located in a mid-crustal level and they reach the boundary of the EEC. The structures we initially connect to the Variscan Deformation Front (VDF) and the Caledonian Deformation Front (CDF). The differentiation of conductivity visible in the crust continues in the upper mantle.

  4. Gravity anomalies and flexure of the lithosphere at the Middle Amazon Basin, Brazil

    Science.gov (United States)

    Nunn, Jeffrey A.; Aires, Jose R.

    1988-01-01

    The Middle Amazon Basin is a large Paleozoic sedimentary basin on the Amazonian craton in South America. It contains up to 7 km of mainly shallow water sediments. A chain of Bouguer gravity highs of approximately +40 to +90 mGals transects the basin roughly coincident with the axis of maximum thickness of sediment. The gravity highs are flanked on either side by gravity lows of approximately -40 mGals. The observed gravity anomalies can be explained by a steeply sided zone of high density in the lower crust varying in width from 100 to 200 km wide. Within this region, the continental crust has been intruded/replaced by more dense material to more than half its original thickness of 45-50 km. The much wider sedimentary basin results from regional compensation of the subsurface load and the subsequent load of accumulated sediments by flexure of the lithosphere. The observed geometry of the basin is consistent with an elastic lithosphere model with a mechanical thickness of 15-20 km. Although this value is lower than expected for a stable cratonic region of Early Proterozoic age, it is within the accepted range of effective elastic thicknesses for the earth. Rapid subsidence during the late Paleozoic may be evidence of a second tectonic event or lithospheric relaxation which could lower the effective mechanical thickness of the lithosphere. The high-density zone in the lower crust, as delineated by gravity and flexural modeling, has a complex sinuous geometry which is narrow and south of the axis of maximum sediment thickness on the east and west margins and wide and offset to the north in the center of the basin. The linear trough geometry of the basin itself is a result of smoothing by regional compensation of the load in the lower crust.

  5. Water in the Cratonic Mantle: Insights from FTIR Data on Lac De Gras Xenoliths (Slave Craton, Canada)

    Science.gov (United States)

    Peslier, Anne H.; Brandon, Alan D.; Schaffer, Lillian Aurora; O'Reilly, Suzanne Yvette; Griffin, William L.; Morris, Richard V.; Graff, Trevor G.; Agresti, David G.

    2014-01-01

    The mantle lithosphere beneath the cratonic part of continents is the deepest (> 200 km) and oldest (>2-3 Ga) on Earth, remaining a conundrum as to how these cratonic roots could have resisted delamination by asthenospheric convection over time. Water, or trace H incorporated in mineral defects, could be a key player in the evolution of continental lithosphere because it influences melting and rheology of the mantle. Mantle xenoliths from the Lac de Gras kimberlite in the Slave craton were analyzed by FTIR. The cratonic mantle beneath Lac de Gras is stratified with shallow (water contents extending to higher values than those from the shallow ones. The FTIR spectra of olivines from the shallow samples have more prominent Group II OH bands compared to the olivines from the deep samples, consistent with a more oxidized mantle environment. The range of olivine water content is similar to that observed in Kaapvaal craton peridotites at the same depths (129-184 km) but does not extend to as high values as those from Udachnaya (Siberian craton). The Slave, Kaapvaal and Siberian cratons will be compared in terms of water content distribution, controls and role in cratonic root longevity.

  6. Fertile lithospheric mantle beneath the northwestern North China and its implication for the subduction of the Paleo-Asian Ocean

    Science.gov (United States)

    Dai, H. K.; Zheng, J.; Su, Y. P.; Xiong, Q.; Pan, S. K.

    2017-12-01

    The nature of the sub-continental lithospheric mantle (SCLM) beneath the western North China Craton (NCC) is poorly known, which hinders understanding the cratonic response to the southward subduction of the Paleo-Asian Ocean. Mineral chemical data of spinel lherzolite xenoliths from newly discovered Cenozoic Langshan basalts in the northwestern part of the craton have been integrated with data from other localities across the western NCC, to put constrains on the SCLM nature and to explore the reworking processes involved. Compositions of mineral cores (i.e., Mg# in olivine = 88 91) and P-T estimates ( 1.2 GPa, 950 oC) suggest the Langshan xenoliths/xenocrysts represent fragments of the uppermost SCLM and experienced ancient continental crust, and 2) the sharp decrease in lithospheric thickness from the inner part to the northern margin of the western NCC, the SCLM beneath the northwestern part should have been strongly rejuvenated or replaced by fertile and non-cratonic mantle. Combined with other geological evidence on the northwestern margin, the mantle replacement and metasomatism were likely triggered by southward subduction of the Paleo-Asian Ocean.

  7. Lithospheric structure of southern Indian shield and adjoining oceans: integrated modelling of topography, gravity, geoid and heat flow data

    Science.gov (United States)

    Kumar, Niraj; Zeyen, H.; Singh, A. P.; Singh, B.

    2013-07-01

    For the present 2-D lithospheric density modelling, we selected three geotransects of more than 1000 km in length each crossing the southern Indian shield, south of 16°N, in N-S and E-W directions. The model is based on the assumption of local isostatic equilibrium and is constrained by the topography, gravity and geoid anomalies, by geothermal data, and where available by seismic data. Our integrated modelling approach reveals a crustal configuration with the Moho depth varying from ˜40 km beneath the Dharwar Craton, and ˜39 km beneath the Southern Granulite Terrane to about 15-20 km beneath the adjoining oceans. The lithospheric thickness varies significantly along the three profiles from ˜70-100 km under the adjoining oceans to ˜130-135 km under the southern block of Southern Granulite Terrane including Sri Lanka and increasing gradually to ˜165-180 km beneath the northern block of Southern Granulite Terrane and the Dharwar Craton. This step-like lithosphere-asthenosphere boundary (LAB) structure indicates a normal lithospheric thickness beneath the adjoining oceans, the northern block of Southern Granulite Terrane and the Dharwar Craton. The thin lithosphere below the southern block of Southern Granulite Terrane including Sri Lanka is, however, atypical considering its age. Our results suggest that the southern Indian shield as a whole cannot be supported isostatically only by thickened crust; a thin and hot lithosphere beneath the southern block of Southern Granulite Terrane including Sri Lanka is required to explain the high topography, gravity, geoid and crustal temperatures. The widespread thermal perturbation during Pan-African (550 Ma) metamorphism and the breakup of Gondwana during late Cretaceous are proposed as twin cause mechanism for the stretching and/or convective removal of the lower part of lithospheric mantle and its replacement by hotter and lighter asthenosphere in the southern block of Southern Granulite Terrane including Sri Lanka

  8. The continental lithosphere

    DEFF Research Database (Denmark)

    Artemieva, Irina

    2009-01-01

    The goal of the present study is to extract non-thermal signal from seismic tomography models in order to distinguish compositional variations in the continental lithosphere and to examine if geochemical and petrologic constraints on global-scale compositional variations in the mantle...... are consistent with modern geophysical data. In the lithospheric mantle of the continents, seismic velocity variations of a non-thermal origin (calculated from global Vs seismic tomography data [Grand S.P., 2002. Mantle shear-wave tomography and the fate of subducted slabs. Philosophical Transactions...... and evolution of Precambrian lithosphere: A global study. Journal of Geophysical Research 106, 16387–16414.] show strong correlation with tectono-thermal ages and with regional variations in lithospheric thickness constrained by surface heat flow data and seismic velocities. In agreement with xenolith data...

  9. Lithospheric low-velocity zones associated with a magmatic segment of the Tanzanian Rift, East Africa

    Science.gov (United States)

    Plasman, M.; Tiberi, C.; Ebinger, C.; Gautier, S.; Albaric, J.; Peyrat, S.; Déverchère, J.; Le Gall, B.; Tarits, P.; Roecker, S.; Wambura, F.; Muzuka, A.; Mulibo, G.; Mtelela, K.; Msabi, M.; Kianji, G.; Hautot, S.; Perrot, J.; Gama, R.

    2017-07-01

    Rifting in a cratonic lithosphere is strongly controlled by several interacting processes including crust/mantle rheology, magmatism, inherited structure and stress regime. In order to better understand how these physical parameters interact, a 2 yr long seismological experiment has been carried out in the North Tanzanian Divergence (NTD), at the southern tip of the eastern magmatic branch of the East African rift, where the southward-propagating continental rift is at its earliest stage. We analyse teleseismic data from 38 broad-band stations ca. 25 km spaced and present here results from their receiver function (RF) analysis. The crustal thickness and Vp/Vs ratio are retrieved over a ca. 200 × 200 km2 area encompassing the South Kenya magmatic rift, the NTD and the Ngorongoro-Kilimanjaro transverse volcanic chain. Cratonic nature of the lithosphere is clearly evinced through thick (up to ca. 40 km) homogeneous crust beneath the rift shoulders. Where rifting is present, Moho rises up to 27 km depth and the crust is strongly layered with clear velocity contrasts in the RF signal. The Vp/Vs ratio reaches its highest values (ca. 1.9) beneath volcanic edifices location and thinner crust, advocating for melting within the crust. We also clearly identify two major low-velocity zones (LVZs) within the NTD, one in the lower crust and the second in the upper part of the mantle. The first one starts at 15-18 km depth and correlates well with recent tomographic models. This LVZ does not always coexist with high Vp/Vs ratio, pleading for a supplementary source of velocity decrease, such as temperature or composition. At a greater depth of ca. 60 km, a mid-lithospheric discontinuity roughly mimics the step-like and symmetrically outward-dipping geometry of the Moho but with a more slanting direction (NE-SW) compared to the NS rift. By comparison with synthetic RF, we estimate the associated velocity reduction to be 8-9 per cent. We relate this interface to melt ponding

  10. Anomalous South Pacific lithosphere dynamics derived from new total sediment thickness estimates off the West Antarctic margin

    Science.gov (United States)

    Wobbe, Florian; Lindeque, Ansa; Gohl, Karsten

    2014-12-01

    Paleotopographic models of the West Antarctic margin, which are essential for robust simulations of paleoclimate scenarios, lack information on sediment thickness and geodynamic conditions, resulting in large uncertainties. A new total sediment thickness grid spanning the Ross Sea-Amundsen Sea-Bellingshausen Sea basins is presented and is based on all the available seismic reflection, borehole, and gravity modeling data offshore West Antarctica. This grid was combined with NGDC's global 5 arc minute grid of ocean sediment thickness (Whittaker et al., 2013) and extends the NGDC grid further to the south. Sediment thickness along the West Antarctic margin tends to be 3-4 km larger than previously assumed. The sediment volume in the Bellingshausen, Amundsen, and Ross Sea basins amounts to 3.61, 3.58, and 2.78 million km3, respectively. The residual basement topography of the South Pacific has been revised and the new data show an asymmetric trend over the Pacific-Antarctic Ridge. Values are anomalously high south of the spreading ridge and in the Ross Sea area, where the topography seems to be affected by persistent mantle processes. In contrast, the basement topography offshore Marie Byrd Land cannot be attributed to dynamic topography, but rather to crustal thickening due to intraplate volcanism. Present-day dynamic topography models disagree with the presented revised basement topography of the South Pacific, rendering paleotopographic reconstructions with such a limited dataset still fairly uncertain.

  11. Utilization of high resolution satellite geoid data for estimation of lithospheric thickness in the Bay of Bengal

    Digital Repository Service at National Institute of Oceanography (India)

    Majumdar, T.J.; Bhattacharyya, R.; Chatterjee, S.; Krishna, K.S.

    been estimated using residual geoid data. In a similar fashion, the age offset from the age contours have been observed. (Figs. 5 a and b). Geoid models for ocean ridges and fracture zones If h(t) is the blope is the geoid as a function of seafloor... age, resulting from changes in seafloor depth and plate thickness due to cooling, then the quantity (∆h/age offset) estimated above is simply the derivative of h(t) with respect to age, where t is the mean age between two sides of a fracture8. From...

  12. Electrical structure of the lithosphere across the Western Paraná suture zone: the role of a Neoproterozoic-Cambrian subduction in generating the Paraná Magmatic Province

    Science.gov (United States)

    Dragone, G. N.; Bologna, M.; Gimenez, M. E.; Alvarez, O.; Lince Klinger, F. G.; Correa-Otto, S.; Ussami, N.

    2017-12-01

    The Paraná Magmatic Province (PMP) together with the Etendeka Province (EP) in Africa is one of the Earth's largest igneous provinces originated prior to the Western Gondwanaland break-up and the inception of the South Atlantic Ocean in the Lower Cretaceous. Geochemical data of PMP-EP basalts collected since late 1980's indicate the origin of PMP-EP by melting of a heterogeneous and enriched subcontinental lithospheric mantle with fast rate of eruption (borders of the PMP, the Western Paraná suture zone (WPS in Fig. 1). We discuss the electrical properties of the lithosphere along three MT profiles across the WPS. MT-A profile (Padilha et al., 2015, JGR) extends from Rio Apa craton towards the center of PMP (high-TiO2 basalts). Profile MT-B extends from Tebicuary craton towards the center of PMP (low-TiO2) and profile MT-C extends from Rio de la Plata craton towards the southern PMP (low- and high-TiO2). All profiles show a resistive ( 104 ohm m) and thick (> 150 km) lithosphere in the cratonic areas whereas the electrical lithosphere is thinner (<100 km) with alternating high and low resistivities within PMP. Vertically elongated and high electrical conductivity anomalies ( 10 ohm m) centered at 40 km depth occur along the -30 mGal contour line in the three profiles, and are interpreted as the location of the suture and former subduction zone. We will discuss the correlation between geochemical and petrological characteristics of basalts and the electrical properties of the lithospheric mantle underneath.

  13. Lithospheric expression of geological units in central and eastern North America from full waveform tomography

    Science.gov (United States)

    Yuan, Huaiyu; French, Scott; Cupillard, Paul; Romanowicz, Barbara

    2014-09-01

    The EarthScope TA deployment has provided dense array coverage throughout the continental US and with it, the opportunity for high resolution 3D seismic velocity imaging of both lithosphere and asthenosphere in the continent. Building upon our previous long-period waveform tomographic modeling in North America, we present a higher resolution 3D isotropic and radially anisotropic shear wave velocity model of the North American lithospheric mantle, constructed tomographically using the spectral element method for wavefield computations and waveform data down to 40 s period. The new model exhibits pronounced spatial correlation between lateral variations in seismic velocity and anisotropy and major tectonic units as defined from surface geology. In the center of the continent, the North American craton exhibits uniformly thick lithosphere down to 200-250 km, while major tectonic sutures of Proterozoic age visible in the surface geology extend down to 100-150 km as relatively narrow zones of distinct radial anisotropy, with Vsv >Vsh. Notably, the upper mantle low velocity zone is present everywhere under the craton between 200 and 300 km depth. East of the continental rift margin, the lithosphere is broken up into a series of large, somewhat thinner (150 km) high velocity blocks, which extend laterally 200-300 km offshore into the Atlantic Ocean. Between the craton and these deep-rooted blocks, we find a prominent narrow band of low velocities that roughly follows the southern and eastern Laurentia rift margin and extends into New England. We suggest that the lithosphere along this band of low velocities may be thinned due to the combined effects of repeated rifting processes and northward extension of the hotspot related Bermuda low-velocity channel across the New England region. We propose that the deep rooted high velocity blocks east of the Laurentia margin represent the Proterozoic Gondwanian terranes of pan-African affinity, which were captured during the Rodinia

  14. Origins of cratonic mantle discontinuities: A view from petrology, geochemistry and thermodynamic models

    Science.gov (United States)

    Aulbach, Sonja; Massuyeau, Malcolm; Gaillard, Fabrice

    2017-01-01

    Geophysically detectible mid-lithospheric discontinuities (MLD) and lithosphere-asthenosphere boundaries (LAB) beneath cratons have received much attention over recent years, but a consensus on their origin has not yet emerged. Cratonic lithosphere composition and origin is peculiar due to its ultra-depletion during plume or accretionary tectonics, cool present-day geothermal gradients, compositional and rheological stratification and multiple metasomatic overprints. Bearing this in mind, we integrate current knowledge on the physical properties, chemical composition, mineralogy and fabric of cratonic mantle with experimental and thermodynamic constraints on the formation and migration of melts, both below and within cratonic lithosphere, in order to find petrologically viable explanations for cratonic mantle discontinuities. LABs characterised by strong seismic velocity gradients and increased conductivity require the presence of melts, which can form beneath intact cratonic roots reaching to 200-250 km depth only in exceptionally warm and/or volatile-rich mantle, thus explaining the paucity of seismical LAB observations beneath cratons. When present, pervasive interaction of these - typically carbonated - melts with the deep lithosphere leads to densification and thermochemical erosion, which generates topography at the LAB and results in intermittent seismic LAB signals or conflicting seismic, petrologic and thermal LAB depths. In rare cases (e.g. Tanzanian craton), the tops of live melt percolation fronts may appear as MLDs and, after complete lithosphere rejuvenation, may be sites of future, shallower LABs (e.g. North China craton). Since intact cratons are presently tectonomagmatically quiescent, and since MLDs produce both positive and negative velocity gradients, in some cases with anisotropy, most MLDs may be best explained by accumulations (metasomes) of seismically slow minerals (pyroxenes, phlogopite, amphibole, carbonates) deposited during past

  15. U-Pb thermochronology of the lower crust: producing a long-term record of craton thermal evolution

    Science.gov (United States)

    Blackburn, T.; Bowring, S. A.; Mahan, K. H.; Perron, T.; Schoene, B.; Dudas, F. O.

    2010-12-01

    within the lithosphere including exhumation, conduction, decay of heat producing elements and thickness of crustal layers/lithospheric mantle. The thermal histories produced from this numerical model can in turn be used to calculate model U-Pb thermochronometric data using a numerical solution to the diffusion/production equation. Integration of thermal and volume diffusion models for the U-Pb system suggests that the extreme slow cooling recorded by U-Pb thermochronology is consistent with low integrated exhumation rates (<0.005 km/Ma). This exhumation rate is integrated over time-scales of hundreds of million to a billion years and does not preclude the possibility for rapid or short-wave length uplift/subsidence. This long-term record of continental lithosphere stability and apparent neutral buoyancy of the craton within a cooling mantle may be further used to refine our estimates of secular cooling within the mantle.

  16. Plume-induced dynamic instabilities near cratonic blocks: Implications for P-T-t paths and metallogeny

    NARCIS (Netherlands)

    Guillou-Frottier, L.; Burov, E.; Cloetingh, S.; Le Goff, E.; Deschamps, Y.; Huet, B.; Bouchot, V.

    2012-01-01

    Plume head - lithosphere interactions around cratonic blocks result in thermo-mechanical disturbances that lead to heating and burial phases of crustal rocks. We present results from numerical models of plume head - cratonic blocks interactions where a free upper surface condition and realistic

  17. Geophysical constraints on the mantle structure of the Canadian Cordillera and North America Craton

    Science.gov (United States)

    Yu, T. C.; Currie, C. A.; Unsworth, M. J.

    2017-12-01

    In western Canada, geophysical data indicate that there is a pronounced contrast in mantle structure between the Canadian Cordillera (CC) and North America craton (NAC). The CC is characterized by lower mantle seismic velocity, higher surface heat flow, lower mantle electrical resistivity and lower effective elastic thickness. These observations are consistent with two distinct thermal regimes: the CC has hot and thin lithosphere, while the NAC lithosphere is cool and thick. The boundary between the CC and NAC coincides with the south-north trending Rocky Mountain Trench - Tintina Fault system. Earlier studies have hypothesized that the thin CC lithosphere is maintained by small-scale convection of hydrated mantle, whereas the NAC lithosphere is dry and resistant to thinning. Here, we test this hypothesis through a detailed examination of two independent data sets: (1) seismic shear-wave (Vs) tomography models and (2) magnetotelluric (MT) measurements of mantle electrical resistivity. We analyze tomography model NA07 at 50-250 km depth and create a mapping of Vs to temperature based on mantle composition (via Perple_X) and a correction for anelasticity. For the CC, the calculated temperature is relatively insensitive to mantle composition but strongly depends on the water content and anelastic correction. With a laboratory-based correction, the estimated temperature is 1150 °C at 100 km depth for wet mantle, compared to 1310 °C for dry mantle; no melt is predicted in either case. An empirical anelastic correction predicts a 115 °C hotter mantle and likely some melt. In contrast, composition is the main control on the calculated temperature for the NAC, especially at depths electrical resistivity is sensitive to mantle temperature and hydration.

  18. Probing the edge of the West African Craton: A first seismic glimpse from Niger

    Science.gov (United States)

    Di Leo, Jeanette F.; Wookey, James; Kendall, J.-Michael; Selby, Neil D.

    2015-03-01

    Constraints on crustal and mantle structure of the Eastern part of the West African Craton have to date been scarce. Here we present results of P receiver function and SK(K)S wave splitting analyses of data recorded at International Monitoring System array TORD in SW Niger. Despite lacking in lateral coverage, our measurements sharply constrain crustal thickness (˜41 km), VP/VS ratio (1.69 ± 0.03), mantle transition zone (MTZ) thickness (˜247 km), and a midlithospheric discontinuity at ˜67 km depth. Splitting delay times are low with an average of 0.63 ± 0.01 s. Fast directions follow the regional surface geological trend with an average of 57 ± 1°. We suggest that splitting is due to fossil anisotropic fabrics in the crust and lithosphere, incurred during the Paleoproterozoic Eburnean Orogeny, with possible contributions from the later Pan-African Orogeny and present-day mantle flow. The MTZ appears to be unperturbed, despite the proximity of the sampled region to the deep cratonic root.

  19. The 3-dimensional construction of the Rae craton, central Canada

    Science.gov (United States)

    Snyder, David B.; Craven, James A.; Pilkington, Mark; Hillier, Michael J.

    2015-10-01

    Reconstruction of the 3-dimensional tectonic assembly of early continents, first as Archean cratons and then Proterozoic shields, remains poorly understood. In this paper, all readily available geophysical and geochemical data are assembled in a 3-D model with the most accurate bedrock geology in order to understand better the geometry of major structures within the Rae craton of central Canada. Analysis of geophysical observations of gravity and seismic wave speed variations revealed several lithospheric-scale discontinuities in physical properties. Where these discontinuities project upward to correlate with mapped upper crustal geological structures, the discontinuities can be interpreted as shear zones. Radiometric dating of xenoliths provides estimates of rock types and ages at depth beneath sparse kimberlite occurrences. These ages can also be correlated to surface rocks. The 3.6-2.6 Ga Rae craton comprises at least three smaller continental terranes, which "cratonized" during a granitic bloom. Cratonization probably represents final differentiation of early crust into a relatively homogeneous, uniformly thin (35-42 km), tonalite-trondhjemite-granodiorite crust with pyroxenite layers near the Moho. The peak thermotectonic event at 1.86-1.7 Ga was associated with the Hudsonian orogeny that assembled several cratons and lesser continental blocks into the Canadian Shield using a number of southeast-dipping megathrusts. This orogeny metasomatized, mineralized, and recrystallized mantle and lower crustal rocks, apparently making them more conductive by introducing or concentrating sulfides or graphite. Little evidence exists of thin slabs similar to modern oceanic lithosphere in this Precambrian construction history whereas underthrusting and wedging of continental lithosphere is inferred from multiple dipping discontinuities.

  20. Origin and diamond prospectivity of Mesoproterozoic kimberlites from the Narayanpet field, Eastern Dharwar Craton, southern India

    DEFF Research Database (Denmark)

    Chalapathi Rao, N.V.; Paton, Chad; Lehmann, B.

    2012-01-01

    to both archetypal kimberlites and to orangeites, and it is not straight forward to apply conventional mineral-genetic schemes in the nomenclature of the NKF pipes. Low fO of the NKF magma (ΔNNO (nickel-nickel oxide)=-1.9 to -3.2), indistinguishable from that of diamondiferous kimberlites world-wide...... is a characteristic feature of the NKF. This has been attributed alternately to the derivation of NKF magmas from a shallower depth, or to variability in thickness of the Sub-Continental Lithospheric Mantle (SCLM) beneath the Eastern Dharwar Craton. Recently, exploration by De Beers resulted in the discovery...... of a number of new kimberlite occurrences from the NKF, with some of their geochemical features and radiogenic isotope systematics subsequently becoming available. In this paper, we present detailed petrography, groundmass mineral composition and new bulk-rock geochemistry data for a number of NKF rocks...

  1. The Role of Water in the Stability of Cratonic Keels

    Science.gov (United States)

    Peslier, Anne H.; Woodland, Alan B.; Bell, David R.; Lazarov, Marina

    2011-01-01

    Cratons are typically underlain by large, deep, and old lithospheric keels (to greater than 200 km depth, greater than 2.5 Ga old) projecting into the asthenosphere (e.g., Jordan, 1978; Richardson et al., 1984). This has mystified Earth scientists as the dynamic and relatively hot asthenosphere should have eroded away these keels over time (e.g., Sleep, 2003; O'Neill et al., 2008; Karato, 2010). Three key factors have been invoked to explain cratonic root survival: 1) Low density makes the cratonic mantle buoyant (e.g., Poudjom Djomani et al., 2001). 2) Low temperatures (e.g., Pollack, 1986; Boyd, 1987), and 3) low water contents (e.g., Pollack, 1986), would make cratonic roots mechanically strong. Here we address the mechanism of the longevity of continental mantle lithosphere by focusing on the water parameter. Although nominally anhydrous , olivine, pyroxene and garnet can accommodate trace amounts of water in the form of H bonded to structural O in mineral defects (e.g., Bell and Rossman, 1992). Olivine softens by orders of magnitude if water (1-1000 ppm H2O) is added to its structure (e.g., Mackwell et al., 1985). Our recent work has placed constraints on the distribution of water measured in peridotite minerals in the cratonic root beneath the Kaapvaal in southern Africa (Peslier et al., 2010). At P greater than 5 GPa, the water contents of pyroxene remain relatively constant while those of olivine systematically decrease from 50 to less than 10 ppm H2O at 6.4 GPa. We hypothesized that at P greater than 6.4 GPa, i.e. at the bottom of the cratonic lithosphere, olivines are essentially dry (greater than 10 ppm H2O). As olivine likely controls the rheology of the mantle, we calculated that the dry olivines could be responsible for a contrast in viscosity between cratonic lithosphere and surrounding asthenosphere large enough to explain the resistance of cratonic root to asthenospheric delamination.

  2. New insight on the paleoproterozoic evolution of the São Francisco Craton: Reinterpretation of the geology, the suture zones and the thicknesses of the crustal blocks using geophysical and geological data

    Science.gov (United States)

    Sampaio, Edson E. S.; Barbosa, Johildo S. F.; Correa-Gomes, Luiz C.

    2017-07-01

    The Archean-Paleoproterozoic Jequié (JB) and Itabuna-Salvador-Curaçá (ISCB) blocks and their tectonic transition zone in the Valença region, Bahia, Brazil are potentially important for ore deposits, but the geological knowledge of the area is still meager. The paucity of geological information restricts the knowledge of the position and of the field characteristics of the tectonic suture zone between these two crustal segments JB and ISCB. Therefore, interpretation of geophysical data is necessary to supplement the regional structural and petrological knowledge of the area as well as to assist mining exploration programs. The analysis of the airborne radiometric and magnetic data of the region has established, respectively, five radiometric domains and five magnetic zones. Modeling of a gravity profile has defined the major density contrasts of the deep structures. The integrated interpretation of the geophysical data fitted to the known geological information substantially improved the suture zone (lower plate JB versus upper plate ISCB) delimitation, the geological map of the area and allowed to estimate the thicknesses of these two blocks, and raised key questions about the São Francisco Craton tectonic evolution.

  3. The Thermal Structure and Strength of Cratons and their Margins

    Science.gov (United States)

    Jaupart, C. P.; Mareschal, J. C.

    2015-12-01

    The large cratons of today are made of younger terranes that wrap around older cores. Deformation due to accretion did not proceed in homogeneous fashion and was concentrated in the younger belts. This is illustrated clearly in the Archean Superior Province, Canada. In this area, one observes little imbrication of accreted crust and craton core, in contrast to the laterally extensive thrusting that has affected the younger terranes to the South. The boundary between the craton core and accreted belts is a nearly vertical interface delineated by steeply dipping electrical and seismic anomalies extending to the base of the lithosphere. These steeply dipping structures have been interpreted as relicts of the subduction that drove accretion. By contrast, the sub-crustal subduction remnant that is imaged beneath younger terranes to the south shows up as a moderately dipping (≈30°) structure. These observations suggest a stiff craton surrounded by weaker belts. This strength contrast may have affected later events, such as the Keweenawan rifing, which propagated northward through the accreted terranes but stopped short of impinging the craton core. In the Superior Province, crustal heat production is much higher in the accreted terranes than in the craton core, implying higher temperatures and lower mechanical strength. Such a remarkable dichotomy also exists in South Africa, where the Limpopo and Namaqua belts are characterized by higher heat flux and crustal heat production than the adjacent Archean Kaapvaal and Zimbabwe cratons. The generality of this cannot be assessed on the basis of heat flow and heat production data which are scarce in most other cratons. These cratons, however, are characterized by post-orogenic high temperature metamorphism which is best explained by high crustal heat production. This is true, for example, for the Jimperding metamorphic belt at the edge of the Yilgarn craton, Western Australia. Thus, cratons appear to be surrounded, and

  4. Lithospheric Expressions of the Precambrian Shield, Mesozoic Rifting, and Cenozoic Subduction and Mountain Building in Venezuela

    Science.gov (United States)

    Levander, A.; Masy, J.; Niu, F.

    2013-05-01

    The Caribbean (CAR)-South American (SA) plate boundary in Venezuela is a broad zone of faulting and diffuse deformation. GPS measurements show the CAR moving approximately 2 cm/yr relative to SA, parallel to the strike slip fault system in the east, with more oblique convergence in the west (Weber et al., 2001) causing the southern edge of the Caribbean to subduct beneath northwestern South America. The west is further complicated by the motion of the triangular Maracaibo block, which is escaping northeastward relative to SA along the Bocono and Santa Marta Faults. In central and eastern Venezuela, plate motion is accommodated by transpression and transtension along the right lateral San Sebastian- El Pilar strike-slip fault system. The strike-slip system marks the northern edge of coastal thrust belts and their associated foreland basins. The Archean-Proterozoic Guayana Shield, part of the Amazonian Craton, underlies southeastern and south-central Venezuela. We used the 87 station Venezuela-U.S. BOLIVAR array (Levander et al., 2006) to investigate lithospheric structure in northern South America. We combined finite-frequency Rayleigh wave tomography with Ps and Sp receiver functions to determine lithosphere-asthenosphere boundary (LAB) depth. We measured Rayleigh phase velocities from 45 earthquakes in the period band 20-100s. The phase velocities were inverted for 1D shear velocity structure on a 0.5 by 0.5 degree grid. Crustal thickness for the starting model was determined from active seismic experiments and receiver function analysis. The resulting 3D shear velocity model was then used to determine the depth of the LAB, and to CCP stack Ps and Sp receiver functions from ~45 earthquakes. The receiver functions were calculated in several frequency bands using iterative deconvolution and inverse filtering. Lithospheric thickness varies by more a factor of 2.5 across Venezuela. We can divide the lithosphere into several distinct provinces, with LAB depth

  5. What is a craton? How many are there? How do they relate? And how did they form?

    Science.gov (United States)

    Bleeker, W.; Davis, B. W.

    2004-05-01

    What is a craton? A craton is a large, coherent domain of Earth's continental crust that has attained and maintained long-term stability, having undergone little internal deformation, except perhaps near its margins due to interaction with neighbouring terranes. Stable continental crust is an end product of intense magmatic, tectonic, and metamorphic reworking; hence, cratons consist of polydeformed and metamorphosed crystalline and metamorphic rocks (e.g., typically "granite-greenstone terrains" in the most ancient cratons). Reworked crust only becomes a craton once the cumulative tectonic, magmatic, and metamorphic reprocessing has self-organized the crust and underlying lithosphere into a stable density, compositional, and thermal profile. Major late-stage "granite bloom" events play a critical role in attaining such stable lithospheric profiles. Once above average stability has been reached, deformation will be concentrated in adjacent domains with weaker strength profiles. Significant rifting events, assisted by mantle plume activity and mafic dyke swarms, are then needed to break up cratonic lithosphere. Where cratons are exposed, they form "shields" dominated by crystalline and metamorphic rocks; where younger, weakly deformed cover overlies cratonic basement, these areas are referred to as "platforms". Shields and platforms are physiographic terms rather than tectonic entities. Another concept, related but not identical to cratons is that of "structural provinces" and the two are commonly confused. Perhaps there is a slight bias for Archean cratons with buoyant mantle keels to form relatively high-standing areas, thus forming shields. However, large parts of Archean cratons are buried underneath platformal cover. There is no strict age connotation to the term "craton", and implied age depends on context. In a context of mantle keels, diamonds and kimberlites, there often is an implicit tendency to equate cratons with stable crust of Archean age. Elsewhere

  6. Intraplate Earthquakes and Deformation within the East Antarctic Craton

    Science.gov (United States)

    Lough, A. C.; Wiens, D.; Nyblade, A.

    2017-12-01

    The apparent lack of tectonic seismicity within Antarctica has long been discussed. Explanations have ranged from a lack of intraplate stress due to the surrounding spreading ridges and low absolute plate velocity (Sykes, 1978), to the weight of ice sheets increasing the vertical normal stress (Johnston, 1987). The 26 station GAMSEIS/AGAP array deployed in East Antarctica from late 2008 to early 2010 provides the first opportunity to study the intraplate seismicity of the Antarctic interior using regional data. Here we report 27 intraplate tectonic earthquakes that occurred during 2009. Depth determination together with their corresponding uncertainty estimates, show that most events originate in the shallow to middle crust, indicating a tectonic and not a cryoseismic origin. The earthquakes are primarily located beneath linear alignments of basins adjacent to the Gamburtsev Subglacial Mountains (GSM) that have been denoted as the East Antarctic rift system (Ferraccioli et al, 2011). The geophysical properties of the `rift' system contrast sharply with those of the GSM and Vostok Subglacial Highlands on either side. Crustal thickness, seismic velocity, and gravity anomalies all indicate large lateral variation in lithospheric properties. We propose the events outline an ancient continental rift, a terrain boundary feature, or a combination of the two where rifting exploited pre-existing weakness. It is natural to draw parallels between East Antarctica and the St. Lawrence depression where rifting and a collisional suture focus intraplate earthquakes within a craton (Schulte and Mooney, 2005). We quantify the East Antarctic seismicity by developing a frequency-magnitude relation, constraining the lower magnitudes with the 2009 results and the larger magnitudes with 1982-2012 teleseismic seismicity. East Antarctica and the Canadian Shield show statistically indistinguishable b-values (near 1) and seismicity rates as expressed as the number of events with mb > 4 per

  7. The continental lithosphere: a geochemical perspective

    International Nuclear Information System (INIS)

    Hawkesworth, C.J.; Person, G.; Turner, S.P.; Calsteren, P. Van; Gallagher, K.

    1993-01-01

    The lithosphere is the cool strong outler layer of the Earth that is effectively a boundary layer to the convecting interior. The evidence from mantle xenoliths and continental basalts is that the lower continental crust and uppermost mantle are different beneath Archaen and proterozoic areas. Mantle xenoliths from Archaen terrains, principally the Kaapvaal craton in southern Africa, are significantly depleted in Fe and other major elements which are concentrated in basalts. Nd and Os isotope data on inclusions in diamonds and peridoties respectively, indicate that such mantle is as old as the overlying Archaen crust. Since it appears to have been coupled to the overlying crust, and to have been isolated from the homogenising effects of convection for long periods of time, it is inferred to be within the continental lithosphere. The mantle lithosphere beneath Proterozoic and younger areas is less depleted in major elements, and so it is more fertile, less buoyant, and therefore thinner, than the Archaen mantle lithosphere. (author). 136 refs, 14 figs

  8. Rifting an Archaean Craton: Insights from Seismic Anisotropy Patterns in E. Africa

    Science.gov (United States)

    Ebinger, C. J.; Tiberi, C.; Currie, C. A.; van Wijk, J.; Albaric, J.

    2016-12-01

    Few places worldwide offer opportunities to study active deformation of deeply-keeled cratonic lithosphere. The magma-rich Eastern rift transects the eastern edge of the Archaean Tanzania craton in northeastern Tanzania, which has been affected by a large-scale mantle upwelling. Abundant xenolith locales offer constraints on mantle age, composition, and physical properties. Our aim is to evaluate models for magmatic fluid-alteration (metasomatism) and deformation of mantle lithosphere along the edge of cratons by considering spatial variations in the direction and magnitude of seismic anisotropy, which is strongly influenced by mantle flow patterns along lithosphere-asthenosphere topography, fluid-filled cracks (e.g., dikes), and pre-existing mantle lithosphere strain fabrics. Waveforms of teleseismic earthquakes (SKS, SKKS) recorded on the 39-station CRAFTI-CoLiBREA broadband array in southern Kenya and northern Tanzania are used to determine the azimuth and amount of shear-wave splitting accrued as seismic waves pass through the uppermost mantle and lithosphere at the craton edge. Lower crustal earthquakes enable evaluation of seismic anisotropy throughout the crust along the rift flanks and beneath the heavily intruded Magadi and Natron basins, and the weakly intruded Manyara basin. Our results and those of earlier studies show a consistent N50E splitting direction within the craton, with delay times of ca. 1.5 s, and similar direction east of the rift in thinner Pan-African lithosphere. Stations within the rift zone are rotated to a N15-35E splitting, with the largest delay times of 2.5 s at the margin of the heavily intruded Magadi basin. The short length scale of variations and rift-parallel splitting directions are similar to patterns in the Main Ethiopian rift attributed to melt-filled cracks or oriented pockets rising from the base of the lithosphere. The widespread evidence for mantle metasomatism and magma intrusion to mid-crustal levels suggests that

  9. Preliminary three-dimensional model of mantle convection with deformable, mobile continental lithosphere

    Science.gov (United States)

    Yoshida, Masaki

    2010-06-01

    Characteristic tectonic structures such as young orogenic belts and suture zones in a continent are expected to be mechanically weaker than the stable part of the continental lithosphere with the cratonic root (or cratonic lithosphere) and yield lateral viscosity variations in the continental lithosphere. In the present-day Earth's lithosphere, the pre-existing, mechanically weak zones emerge as a diffuse plate boundary. However, the dynamic role of a weak (low-viscosity) continental margin (WCM) in the stability of continental lithosphere has not been understood in terms of geophysics. Here, a new numerical simulation model of mantle convection with a compositionally and rheologically heterogeneous, deformable, mobile continental lithosphere is presented for the first time by using three-dimensional regional spherical-shell geometry. A compositionally buoyant and highly viscous continental assemblage with pre-existing WCMs, analogous to the past supercontinent, is modeled and imposed on well-developed mantle convection whose vigor of convection, internal heating rate, and rheological parameters are appropriate for the Earth's mantle. The visco-plastic oceanic lithosphere and the associated subduction of oceanic plates are incorporated. The time integration of the advection of continental materials with zero chemical diffusion is performed by a tracer particle method. The time evolution of mantle convection after setting the model supercontinent is followed over 800 Myr. Earth-like continental drift is successfully reproduced, and the characteristic thermal interaction between the mantle and the continent/supercontinent is observed in my new numerical model. Results reveal that the WCM protects the cratonic lithosphere from being stretched by the convecting mantle and may play a significant role in the stability of the cratonic lithosphere during the geological timescale because it acts as a buffer that prevents the cratonic lithosphere from undergoing global

  10. Crustal response to lithosphere evolution

    DEFF Research Database (Denmark)

    Artemieva, Irina; Thybo, Hans; Cherepanova, Yulia

    2012-01-01

    We present a new model for the structure of the crust in an area which stretches from the North Atlantic region in the west to the Verkhoyansk Ridge in the east and encompasses Greenland, Iceland, most of Europe, West Siberian basin, and the Siberian cratons. The model is based on critically asse......, thicknesses of different crustal layers, and Pn seismic velocities....... assessed results from various seismic studies, including reflection and refraction profiles and receiver function studies. The region includes a nearly continuous age record for crustal evolution over ca. 3.6-3.8 billion years. We present an analysis of the crustal structure heterogeneity in relation...

  11. COMPOSITIONAL AND THERMAL DIFFERENCES BETWEEN LITHOSPHERIC AND ASTHENOSPHERIC MANTLE AND THEIR INFLUENCE ON CONTINENTAL DELAMINATION

    Directory of Open Access Journals (Sweden)

    A. I. Kiselev

    2015-01-01

    Full Text Available The lower part of lithosphere in collisional orogens may delaminate due to density inversion between the asthenosphere and the cold thickened lithospheric mantle. Generally, standard delamination models have neglected density changes within the crust and the lithospheric mantle, which occur due to phase transitions and compositional variations upon changes of P-T parameters. Our attention is focused on effects of phase and density changes that may be very important and even dominant when compared with the effect of a simple change of the thermal mantle structure. The paper presents the results of numerical modeling for eclogitization of basalts of the lower crust as well as phase composition changes and density of underlying peridotite resulted from tectonic thickening of the lithosphere and its foundering into the asthenosphere. As the thickness of the lower crust increases, the mafic granulite (basalt passes into eclogite, and density inversion occurs at the accepted crust-mantle boundary (P=20 kbar because the newly formed eclogite is heavier than the underlying peridotite by 6 % (abyssal peridotite, according to [Boyd, 1989]. The density difference is a potential energy for delamination of the eclogitic portion of the crust. According to the model, P=70 kbar and T=1300 °C correspond to conditions at the lower boundary of the lithosphere. Assuming the temperature adiabatic distribution within the asthenosphere, its value at the given parameters ranges from 1350 °C to 1400 °C. Density inversion at dry conditions occurs with the identical lithospheric and asthenospheric compositions at the expense of the temperature difference at 100 °C with the density difference of only 0.0022 %. Differences of two other asthenospheric compositions (primitive mantle, and lherzolite KH as compared to the lithosphere (abyssal peridotite are not compensated for by a higher temperature. The asthenospheric density is higher than that of the lithospheric base

  12. Mesozoic mafic dikes from the Shandong Peninsula, North China Craton: Petrogenesis and tectonic implications

    International Nuclear Information System (INIS)

    Liu Shen; Hu Ruizhong; Zhao Junhong; Feng Caixia; Zou, Haibo

    2006-01-01

    Mesozoic mafic dikes are widely distributed in Luxi (Mengyin and Zichuan) and Jiaodong regions of the Shandong Peninsula, China, providing an opportunity of investigating the nature of the lost lithospheric mantle beneath the North China Craton (NCC). The mafic dikes are characterized by strong depletion in high field strength elements (HFSE), enrichment in light rare earth elements (LREE), highly variable Th/U ratios, high initial ( 87 Sr/ 86 Sr) i (0.7050-0.7099) and negative ε Nd (T) (-6.0 to -17.6). They were derived from melting of metasomatized portions of the subcontinental lithospheric mantle, followed by fractionation of clinopyroxenes. The similarity in Nd isotopic compositions between the Mengyin gabbro dikes and the Paleozoic peridotite xenoliths suggests that ancient lithospheric mantle was still retained at 120 Ma below Mengyin, although the ancient lithospheric mantle in many other places beneath NCC had been severely modified. There might be multiple enrichment events in the lithospheric mantle. An early-stage (before or during Paleozoic) rutile-rich metasomatism affected the lithospheric mantle below Mengyin, Jiaodong and Zichuan. Since then, the lithospheric mantle beneath Mengyin was isolated. A late-stage metasomatism by silicate melts modified the lithospheric mantle beneath Jiaodong and Zichuan but not Mengyin. The removal of the enriched lithospheric mantle and the generation of the mafic dikes may be mainly related to the convective overturn accompanying Jurassic-Cretaceous subduction of the paleo-Pacific plate. (author)

  13. Sub-Moho Reflectors, Mantle Faults and Lithospheric Rheology

    Science.gov (United States)

    Brown, L. D.

    2013-12-01

    One of the most unexpected and dramatic observations from the early years of deep reflection profiling of the continents using multichannel CMP techniques was the existing of prominent reflections from the upper mantle. The first of these, the Flannan thrust/fault/feature, was traced by marine profiling of the continental margin offshore Britain by the BIRPS program, which soon found them to be but one of several clear sub-crustal discontinuities in that area. Subsequently, similar mantle reflectors have been observed in many areas around the world, most commonly beneath Precambrian cratonic areas. Many, but not all, of these mantle reflections appear to arise from near the overlying Moho or within the lower crust before dipping well into the mantle. Others occur as subhorizontal events at various depths with the mantle, with one suite seeming to cluster at a depth of about 75 km. The dipping events have been variously interpreted as mantle roots of crustal normal faults or the deep extension of crustal thrust faults. The most common interpretation, however, is that these dipping events are the relicts of ancient subduction zones, the stumps of now detached Benioff zones long since reclaimed by the deeper mantle. In addition to the BIRPS reflectors, the best known examples include those beneath Fennoscandia in northern Europe, the Abitibi-Grenville of eastern Canada, and the Slave Province of northwestern Canada (e.g. on the SNORCLE profile). The most recently reported example is from beneath the Sichuan Basin of central China. The preservation of these coherent, and relatively delicate appearing, features beneath older continental crust and presumably within equally old (of not older) mantle lithosphere, has profound implications for the history and rheology of the lithosphere in these areas. If they represent, as widely believe, some form of faulting with the lithosphere, they provide corollary constraints on the nature of faulting in both the lower crust and

  14. The lithosphere-asthenosphere: Italy and surroundings

    International Nuclear Information System (INIS)

    Panza, G.F.; Aoudia, A.; Pontevivo, A.; Chimera, G.; Raykova, R.

    2003-02-01

    The velocity-depth distribution of the lithosphere-asthenosphere in the Italian region and surroundings is imaged, with a lateral resolution of about 100 km, by surface wave velocity tomography and non-linear inversion. Maps of the Moho depth, of the thickness of the lithosphere and of the shear-wave velocities, down to depths of 200 km and more, are constructed. A mantle wedge, identified in the uppermost mantle along the Apennines and the Calabrian Arc, underlies the principal recent volcanoes, and partial melting can be relevant in this part of the uppermost mantle. In Calabria a lithospheric doubling is seen, in connection with the subduction of the Ionian lithosphere. The asthenosphere is shallow in the Southern Tyrrhenian Sea. High velocity bodies, cutting the asthenosphere, outline the Adria-lonian subduction in the Tyrrhenian Sea and the deep-reaching lithospheric root in the Western Alps. Less deep lithospheric roots are seen in the Central Apennines. The lithosphere-asthenosphere properties delineate a differentiation between the northern and the southern sectors of the Adriatic Sea, likely attesting the fragmentation of Adria. (author)

  15. The lithosphere-asthenosphere Italy and surroundings

    CERN Document Server

    Panza, G F; Chimera, G; Pontevivo, A; Raykova, R

    2003-01-01

    The velocity-depth distribution of the lithosphere-asthenosphere in the Italian region and surroundings is imaged, with a lateral resolution of about 100 km, by surface wave velocity tomography and non-linear inversion. Maps of the Moho depth, of the thickness of the lithosphere and of the shear-wave velocities, down to depths of 200 km and more, are constructed. A mantle wedge, identified in the uppermost mantle along the Apennines and the Calabrian Arc, underlies the principal recent volcanoes, and partial melting can be relevant in this part of the uppermost mantle. In Calabria a lithospheric doubling is seen, in connection with the subduction of the Ionian lithosphere. The asthenosphere is shallow in the Southern Tyrrhenian Sea. High velocity bodies, cutting the asthenosphere, outline the Adria-lonian subduction in the Tyrrhenian Sea and the deep-reaching lithospheric root in the Western Alps. Less deep lithospheric roots are seen in the Central Apennines. The lithosphere-asthenosphere properties delineat...

  16. Lithosphere formation in the central Slave Craton (Canada): plume subcretion or lithosphere accretion?

    Science.gov (United States)

    Aulbach, Sonja; Griffin, William L.; Pearson, Norman J.; O'Reilly, Suzanne Y.; Doyle, Buddy J.

    2007-08-01

    Pod drives are modern outboard ship propulsion systems with a motor encapsulated in a watertight pod, whose shaft is connected directly to one or two propellers. The whole unit hangs from the stern of the ship and rotates azimuthally, thus providing thrust and steering without the need of a rudder. Force/momentum and phase-resolved laser Doppler anemometry (LDA) measurements were performed for in line co-rotating and contra-rotating propellers pod drive models. The measurements permitted to characterize these ship propulsion systems in terms of their hydrodynamic characteristics. The torque delivered to the propellers and the thrust of the system were measured for different operation conditions of the propellers. These measurements lead to the hydrodynamic optimization of the ship propulsion system. The parameters under focus revealed the influence of distance between propeller planes, propeller frequency of rotation ratio and type of propellers (co- or contra-rotating) on the overall efficiency of the system. Two of the ship propulsion systems under consideration were chosen, based on their hydrodynamic characteristics, for a detailed study of the swirling wake flow by means of laser Doppler anemometry. A two-component laser Doppler system was employed for the velocity measurements. A light barrier mounted on the axle of the rear propeller motor supplied a TTL signal to mark the beginning of each period, thus providing angle information for the LDA measurements. Measurements were conducted for four axial positions in the slipstream of the pod drive models. The results show that the wake of contra-rotating propeller is more homogeneous than when they co-rotate. In agreement with the results of the force/momentum measurements and with hypotheses put forward in the literature (see e.g. Poehls in Entwurfsgrundlagen für Schraubenpropeller, 1984; Schneekluth in Hydromechanik zum Schiffsentwurf, 1988; Breslin and Andersen in Hydrodynamics of ship propellers, 1996; Schneekluth and Bertram in Ship design for efficiency and economy, 1998), the co-rotating propellers model showed a much stronger swirl in the wake of the propulsor. The anisotropy of turbulence was analyzed using the anisotropy tensor introduced by Lumley and Newman (J Fluid Mech 82(1):161 178, 1977). The invariants of the anisotropy tensor of the wake flow were computed and were plotted in the Lumley Newman-diagram. These measurements revealed that the anisotropy tensor in the wake of ship propellers is located near to the borders of the invariant map, showing a large degree of anisotropy. They will be presented and will be discussed with respect to applications of turbulence models to predict swirling flows.

  17. Infrared spectral and carbon isotopic characteristics of micro- and macro-diamonds from the Panda kimberlite (Central Slave Craton, Canada)

    Science.gov (United States)

    Melton, G. L.; Stachel, T.; Stern, R. A.; Carlson, J.; Harris, J. W.

    2013-09-01

    One hundred and twenty-one micro-diamonds (Panda kimberlite (Ekati mine, Central Slave Craton, Canada) were analyzed for nitrogen content, nitrogen aggregation state (%B) and platelet and hydrogen peak areas (cm- 2). Micro-diamond nitrogen concentrations range from 2‰, but mostly vary by bearing metasomatic fluid/melt that isotopically evolves as it percolates upward through the lithosphere.

  18. Proterozoic orogenic belts and rifting of Indian cratons: Geophysical constraints

    Directory of Open Access Journals (Sweden)

    D.C. Mishra

    2014-01-01

    Full Text Available The Aravalli–Delhi and Satpura Mobile Belts (ADMB and SMB and the Eastern Ghat Mobile Belt (EGMB in India form major Proterozoic mobile belts with adjoining cratons and contemporary basins. The most convincing features of the ADMB and the SMB have been the crustal layers dipping from both sides in opposite directions, crustal thickening (∼45 km and high density and high conductivity rocks in upper/lower crust associated with faults/thrusts. These observations indicate convergence while domal type reflectors in the lower crust suggest an extensional rifting phase. In case of the SMB, even the remnant of the subducting slab characterized by high conductive and low density slab in lithospheric mantle up to ∼120 km across the Purna–Godavari river faults has been traced which may be caused by fluids due to metamorphism. Subduction related intrusives of the SMB south of it and the ADMB west of it suggest N–S and E–W directed convergence and subduction during Meso–Neoproterozoic convergence. The simultaneous E–W convergence between the Bundelkhand craton and Marwar craton (Western Rajasthan across the ADMB and the N–S convergence between the Bundelkhand craton and the Bhandara and Dharwar cratons across the SMB suggest that the forces of convergence might have been in a NE–SW direction with E–W and N–S components in the two cases, respectively. This explains the arcuate shaped collision zone of the ADMB and the SMB which are connected in their western part. The Eastern Ghat Mobile Belt (EGMB also shows signatures of E–W directed Meso–Neoproterozoic convergence with East Antarctica similar to ADMB in north India. Foreland basins such as Vindhyan (ADMB–SMB, and Kurnool (EGMB Supergroups of rocks were formed during this convergence. Older rocks such as Aravalli (ADMB, Mahakoshal–Bijawar (SMB, and Cuddapah (EGMB Supergroups of rocks with several basic/ultrabasic intrusives along these mobile belts, plausibly formed during

  19. Upper mantle beneath foothills of the western Himalaya: subducted lithospheric slab or a keel of the Indian shield?

    Science.gov (United States)

    Vinnik, L.; Singh, A.; Kiselev, S.; Kumar, M. Ravi

    2007-12-01

    The fate of the mantle lithosphere of the Indian Plate in the India-Eurasia collision zone is not well understood. Tomographic studies reveal high P velocity in the uppermost mantle to the south of the western Himalaya, and these high velocities are sometimes interpreted as an image of subducting Indian lithosphere. We suggest that these high velocities are unrelated to the ongoing subduction but correspond to a near-horizontal mantle keel of the Indian shield. In the south of the Indian shield upper-mantle velocities are anomalously low, and relatively high velocities may signify a recovery of the normal shield structure in the north. Our analysis is based on the recordings of seismograph station NIL in the foothills of the western Himalaya. The T component of the P receiver functions is weak relative to the Q component, which is indicative of a subhorizontally layered structure. Joint inversion of the P and S receiver functions favours high uppermost mantle velocities, typical of the lithosphere of Archean cratons. The arrival of the Ps converted phase from 410 km discontinuity at NIL is 2.2 s earlier than in IASP91 global model. This can be an effect of remnants of Tethys subduction in the mantle transition zone and of high velocities in the keel of the Indian shield. Joint inversion of SKS particle motions and P receiver functions reveals a change in the fast direction of seismic azimuthal anisotropy from 60° at 80-160 km depths to 150° at 160-220 km. The fast direction in the lower layer is parallel to the trend of the Himalaya. The change of deformation regimes at a depth of 160 km suggests that this is the base of the lithosphere of the Indian shield. A similar boundary was found with similar techniques in central Europe and the Tien Shan region, but the base of the lithosphere in these regions is relatively shallow, in agreement with the higher upper-mantle temperatures. The ongoing continental collision is expressed in crustal structure: the crust

  20. Numerical simulations of the mantle lithosphere delamination

    Science.gov (United States)

    Morency, C.; Doin, M.-P.

    2004-03-01

    Sudden uplift, extension, and increased igneous activity are often explained by rapid mechanical thinning of the lithospheric mantle. Two main thinning mechanisms have been proposed, convective removal of a thickened lithospheric root and delamination of the mantle lithosphere along the Moho. In the latter case, the whole mantle lithosphere peels away from the crust by the propagation of a localized shear zone and sinks into the mantle. To study this mechanism, we perform two-dimensional (2-D) numerical simulations of convection using a viscoplastic rheology with an effective viscosity depending strongly on temperature, depth, composition (crust/mantle), and stress. The simulations develop in four steps. (1) We first obtain "classical" sublithospheric convection for a long time period (˜300 Myr), yielding a slightly heterogeneous lithospheric temperature structure. (2) At some time, in some simulations, a strong thinning of the mantle occurs progressively in a small area (˜100 km wide). This process puts the asthenosphere in direct contact with the lower crust. (3) Large pieces of mantle lithosphere then quickly sink into the mantle by the horizontal propagation of a detachment level away from the "asthenospheric conduit" or by progressive erosion on the flanks of the delaminated area. (4) Delamination pauses or stops when the lithospheric mantle part detaches or when small-scale convection on the flanks of the delaminated area is counterbalanced by heat diffusion. We determine the parameters (crustal thicknesses, activation energies, and friction coefficients) leading to delamination initiation (step 2). We find that delamination initiates where the Moho temperature is the highest, as soon as the crust and mantle viscosities are sufficiently low. Delamination should occur on Earth when the Moho temperature exceeds ˜800°C. This condition can be reached by thermal relaxation in a thickened crust in orogenic setting or by corner flow lithospheric erosion in the

  1. A numerical model of mantle convection with deformable, mobile continental lithosphere within three-dimensional spherical geometry

    Science.gov (United States)

    Yoshida, M.

    2010-12-01

    A new numerical simulation model of mantle convection with a compositionally and rheologically heterogeneous, deformable, mobile continental lithosphere is presented for the first time by using three-dimensional regional spherical-shell geometry (Yoshida, 2010, Earth Planet. Sci. Lett.). The numerical results revealed that one of major factor that realizes the supercontinental breakup and subsequent continental drift is a pre-existing, weak (low-viscosity) continental margin (WCM) in the supercontinent. Characteristic tectonic structures such as young orogenic belts and suture zones in a continent are expected to be mechanically weaker than the stable part of the continental lithosphere with the cratonic root (or cratonic lithosphere) and yield lateral viscosity variations in the continental lithosphere. In the present-day Earth's lithosphere, the pre-existing, mechanically weak zones emerge as a diffuse plate boundary. However, the dynamic role of the WCM in the stability of continental lithosphere has not been understood in terms of geophysics. In my numerical model, a compositionally buoyant and highly viscous continental assemblage with pre-existing WCMs, analogous to the past supercontinent, is modeled and imposed on well-developed mantle convection whose vigor of convection, internal heating rate, and rheological parameters are appropriate for the Earth's mantle. The visco-plastic oceanic lithosphere and the associated subduction of oceanic plates are incorporated. The time integration of the advection of continental materials with zero chemical diffusion is performed by a tracer particle method. The time evolution of mantle convection after setting the model supercontinent is followed over 800 Myr. Earth-like continental drift is successfully reproduced, and the characteristic thermal interaction between the mantle and the continent/supercontinent is observed in my new numerical model. Results reveal that the WCM protects the cratonic lithosphere from being

  2. Impact of lithospheric rheology on surface topography

    Science.gov (United States)

    Liao, K.; Becker, T. W.

    2017-12-01

    The expression of mantle flow such as due to a buoyant plume as surface topography is a classical problem, yet the role of rheological complexities could benefit from further exploration. Here, we investigate the topographic expressions of mantle flow by means of numerical and analytical approaches. In numerical modeling, both conventional, free-slip and more realistic, stress-free boundary conditions are applied. For purely viscous rheology, a high viscosity lithosphere will lead to slight overestimates of topography for certain settings, which can be understood by effectively modified boundary conditions. Under stress-free conditions, numerical and analytical results show that the magnitude of dynamic topography decreases with increasing lithosphere thickness (L) and viscosity (ηL), as L-1 and ηL-3. The wavelength of dynamic topography increases linearly with L and (ηL/ ηM) 1/3. We also explore the time-dependent interactions of a rising plume with the lithosphere. For a layered lithosphere with a decoupling weak lower crust embedded between stronger upper crust and lithospheric mantle, dynamic topography increases with a thinner and weaker lower crust. The dynamic topography saturates when the decoupling viscosity is 3-4 orders lower than the viscosity of upper crust and lithospheric mantle. We further explore the role of visco-elastic and visco-elasto-plastic rheologies.

  3. Permeability Barrier Generation in the Martian Lithosphere

    Science.gov (United States)

    Schools, Joe; Montési, Laurent

    2015-11-01

    Permeability barriers develop when a magma produced in the interior of a planet rises into the cooler lithosphere and crystallizes more rapidly than the lithosphere can deform (Sparks and Parmentier, 1991). Crystallization products may then clog the porous network in which melt is propagating, reducing the permeability to almost zero, i.e., forming a permeability barrier. Subsequent melts cannot cross the barrier. Permeability barriers have been useful to explain variations in crustal thickness at mid-ocean ridges on Earth (Magde et al., 1997; Hebert and Montési, 2011; Montési et al., 2011). We explore here under what conditions permeability barriers may form on Mars.We use the MELTS thermodynamic calculator (Ghiorso and Sack, 1995; Ghiorso et al., 2002; Asimow et al., 2004) in conjunction with estimated Martian mantle compositions (Morgan and Anders, 1979; Wänke and Dreibus, 1994; Lodders and Fegley, 1997; Sanloup et al., 1999; Taylor 2013) to model the formation of permeability barriers in the lithosphere of Mars. In order to represent potential past and present conditions of Mars, we vary the lithospheric thickness, mantle potential temperature (heat flux), oxygen fugacity, and water content.Our results show that permeability layers can develop in the thermal boundary layer of the simulated Martian lithosphere if the mantle potential temperature is higher than ~1500°C. The various Martian mantle compositions yield barriers in the same locations, under matching variable conditions. There is no significant difference in barrier location over the range of accepted Martian oxygen fugacity values. Water content is the most significant influence on barrier development as it reduces the temperature of crystallization, allowing melt to rise further into the lithosphere. Our lower temperature and thicker lithosphere model runs, which are likely the most similar to modern Mars, show no permeability barrier generation. Losing the possibility of having a permeability

  4. Lithospheric discontinuities beneath the U.S. Midcontinent - signatures of Proterozoic terrane accretion and failed rifting

    Science.gov (United States)

    Chen, Chen; Gilbert, Hersh; Fischer, Karen M.; Andronicos, Christopher L.; Pavlis, Gary L.; Hamburger, Michael W.; Marshak, Stephen; Larson, Timothy; Yang, Xiaotao

    2018-01-01

    Seismic discontinuities between the Moho and the inferred lithosphere-asthenosphere boundary (LAB) are known as mid-lithospheric discontinuities (MLDs) and have been ascribed to a variety of phenomena that are critical to understanding lithospheric growth and evolution. In this study, we used S-to-P converted waves recorded by the USArray Transportable Array and the OIINK (Ozarks-Illinois-Indiana-Kentucky) Flexible Array to investigate lithospheric structure beneath the central U.S. This region, a portion of North America's cratonic platform, provides an opportunity to explore how terrane accretion, cratonization, and subsequent rifting may have influenced lithospheric structure. The 3D common conversion point (CCP) volume produced by stacking back-projected Sp receiver functions reveals a general absence of negative converted phases at the depths of the LAB across much of the central U.S. This observation suggests a gradual velocity decrease between the lithosphere and asthenosphere. Within the lithosphere, the CCP stacks display negative arrivals at depths between 65 km and 125 km. We interpret these as MLDs resulting from the top of a layer of crystallized melts (sill-like igneous intrusions) or otherwise chemically modified lithosphere that is enriched in water and/or hydrous minerals. Chemical modification in this manner would cause a weak layer in the lithosphere that marks the MLDs. The depth and amplitude of negative MLD phases vary significantly both within and between the physiographic provinces of the midcontinent. Double, or overlapping, MLDs can be seen along Precambrian terrane boundaries and appear to result from stacked or imbricated lithospheric blocks. A prominent negative Sp phase can be clearly identified at 80 km depth within the Reelfoot Rift. This arrival aligns with the top of a zone of low shear-wave velocities, which suggests that it marks an unusually shallow seismic LAB for the midcontinent. This boundary would correspond to the top of a

  5. Cratonic roots and lower crustal seismicity: Investigating the role of deep intrusion in the Western rift, Africa

    Science.gov (United States)

    Drooff, C.; Ebinger, C. J.; Lavayssiere, A.; Keir, D.; Oliva, S. J.; Tepp, G.; Gallacher, R. J.

    2017-12-01

    Improved seismic imaging beneath the African continent reveals lateral variations in lithospheric thickness, and crustal structure, complementing a growing crust and mantle xenolith data base. Border fault systems in the active cratonic rifts of East Africa are characterized by lower crustal seismicity, both in magmatic sectors and weakly magmatic sectors, providing constraints on crustal rheology and, in some areas, magmatic fluid migration. We report new seismicity data from magmatic and weakly magmatic sectors of the East African rift zone, and place the work in the context of independent geophysical and geochemical studies to models for strain localization during early rifting stages. Specifically, multidisciplinary studies in the Magadi Natron rift sectors reveal volumetrically large magmatic CO2 degassing along border faults with seismicity along projections of surface dips to the lower crust. The magmatic CO2 degassing and high Vp/Vs ratios and reflectivity of the lower crust implies that the border fault serves a conduit between the lower crustal underplating and the atmospheric. Crustal xenoliths in the Eastern rift sector indicate a granulitic lower crust, which is relatively weak in the presence of fluids, arguing against a strong lower crust. Within magmatic sectors, seismic, structural, and geochemistry results indicate that frequent lower crustal earthquakes are promoted by elevated pore pressures from volatile degassing along border faults, and hydraulic fracture around the margins of magma bodies. Within some weakly magmatic sectors, lower crustal earthquakes also occur along projections of border faults to the lower crust (>30 km), and they are prevalent in areas with high Vp/Vs in the lower crust. Within the southern Tanganyika rift, focal mechanisms are predominantly normal with steep nodal planes. Our comparative studies suggest that pervasive metasomatism above a mantle plume, and melt extraction in thin zones between cratonic roots, lead to

  6. Strong crustal seismic anisotropy in the Kalahari Craton based on Receiver Functions

    DEFF Research Database (Denmark)

    Thybo, Hans; Soliman, Mohammad Youssof Ahmad; Artemieva, Irina

    2015-01-01

    Earlier seismic studies of the Kalahari Craton in southern Africa infer deformation of upper mantle by flow with fast direction of seismic anisotropy being parallel to present plate motion, and/or report anisotropy frozen into the lithospheric mantle. We present evidence for very strong seismic...... is uniform within tectonic units and parallel to orogenic strike in the Limpopo and Cape fold belts. It is further parallel to the strike of major dyke swarms which indicates that a large part of the observed anisotropy is controlled by lithosphere fabrics and macroscopic effects. The directions of the fast...... that the crust and lithospheric mantle may have been coupled since cratonisation. If so, the apparent match between mantle anisotropy and the present plate motion is coincidental....

  7. New Constraints on Upper Mantle Structure Underlying the Diamondiferous Central Slave Craton, Canada, from Teleseismic Body Wave Tomography

    Science.gov (United States)

    Esteve, C.; Schaeffer, A. J.; Audet, P.

    2017-12-01

    Over the past number of decades, the Slave Craton (Canada) has been extensively studied for its diamondiferous kimberlites. Not only are diamonds a valuable resource, but their kimberlitic host rocks provide an otherwise unique direct source of information on the deep upper mantle (and potentially transition zone). Many of the Canadian Diamond mines are located within the Slave Craton. As a result of the propensity for diamondiferous kimberlites, it is imperative to probe the deep mantle structure beneath the Slave Craton. This work is further motivated by the increase in high-quality broadband seismic data across the Northern Canadian Cordillera over the past decade. To this end we have generated a P and S body wave tomography model of the Slave Craton and its surroundings. Furthermore, tomographic inversion techniques are growing ever more capable of producing high resolution Earth models which capture detailed structure and dynamics across a range of scale lengths. Here, we present preliminary results on the structure of the upper mantle underlying the Slave Craton. These results are generated using data from eight different seismic networks such as the Canadian National Seismic Network (CNSN), Yukon Northwest Seismic Network (YNSN), older Portable Observatories for Lithospheric Analysis and Reseach Investigating Seismicity (POLARIS), Regional Alberta Observatory for Earthquake Studies Network (RV), USArray Transportable Array (TA), older Canadian Northwest Experiment (CANOE), Batholith Broadband (XY) and the Yukon Observatory (YO). This regional model brings new insights about the upper mantle structure beneath the Slave Craton, Canada.

  8. Antarctic Lithosphere Studies: Progress, Problems and Promise

    Science.gov (United States)

    Dalziel, I. W. D.; Wilson, T. J.

    2017-12-01

    In the sixty years since the International Geophysical Year, studies of the Antarctic lithosphere have progressed from basic geological observations and sparse geophysical measurements to continental-scale datasets of radiometric dates, ice thickness, bedrock topography and characteristics, seismic imaging and potential fields. These have been augmented by data from increasingly dense broadband seismic and geodetic networks. The Antarctic lithosphere is known to have been an integral part, indeed a "keystone" of the Pangea ( 250-185Ma) and Gondwanaland ( 540-180 Ma) supercontinents. It is widely believed to have been part of hypothetical earlier supercontinents Rodinia ( 1.0-0.75 Ga) and Columbia (Nuna) ( 2.0-1.5 Ga). Despite the paucity of exposure in East Antarctica, the new potential field datasets have emboldened workers to extrapolate Precambrian geological provinces and structures from neighboring continents into Antarctica. Hence models of the configuration of Columbia and its evolution into Rodinia and Gondwana have been proposed, and rift-flank uplift superimposed on a Proterozoic orogenic root has been hypothesized to explain the Gamburtsev Subglacial Mountains. Mesozoic-Cenozoic rifting has imparted a strong imprint on the West Antarctic lithosphere. Seismic tomographic evidence reveals lateral variation in lithospheric thickness, with the thinnest zones within the West Antarctic rift system and underlying the Amundsen Sea Embayment. Upper mantle low velocity zones are extensive, with a deeper mantle velocity anomaly underlying Marie Byrd Land marking a possible mantle plume. Misfits between crustal motions measured by GPS and GIA model predictions can, in part, be linked with the changes in lithosphere thickness and mantle rheology. Unusually high uplift rates measured by GPS in the Amundsen region can be interpreted as the response of regions with thin lithosphere and weak mantle to late Holocene ice mass loss. Horizontal displacements across the TAM

  9. Large-scale variation in lithospheric structure along and across the Kenya rift

    Science.gov (United States)

    Prodehl, C.; Mechie, J.; Kaminski, W.; Fuchs, K.; Grosse, C.; Hoffmann, H.; Stangl, R.; Stellrecht, R.; Khan, M.A.; Maguire, Peter K.H.; Kirk, W.; Keller, Gordon R.; Githui, A.; Baker, M.; Mooney, W.; Criley, E.; Luetgert, J.; Jacob, B.; Thybo, H.; Demartin, M.; Scarascia, S.; Hirn, A.; Bowman, J.R.; Nyambok, I.; Gaciri, S.; Patel, J.; Dindi, E.; Griffiths, D.H.; King, R.F.; Mussett, A.E.; Braile, L.W.; Thompson, G.; Olsen, K.; Harder, S.; Vees, R.; Gajewski, D.; Schulte, A.; Obel, J.; Mwango, F.; Mukinya, J.; Riaroh, D.

    1991-01-01

    The Kenya rift is one of the classic examples of a continental rift zone: models for its evolution range from extension of the lithosphere by pure shear1, through extension by simple shear2, to diapiric upwelling of an asthenolith3. Following a pilot study in 19854, the present work involved the shooting of three seismic refraction and wide-angle reflection profiles along the axis, across the margins, and on the northeastern flank of the rift (Fig. 1). These lines were intended to reconcile the different crustal thickness estimates for the northern and southern parts of the rift4-6 and to reveal the structure across the rift, including that beneath the flanks. The data, presented here, reveal significant lateral variations in structure both along and across the rift. The crust thins along the rift axis from 35 km in the south to 20 km in the north; there are abrupt changes in Mono depth and uppermost-mantle seismic velocity across the rift margins, and crustal thickening across the boundary between the Archaean craton and PanAfrican orogenic belt immediately west of the rift. These results suggest that thickened crust may have controlled the rift's location, that there is a decrease in extension from north to south, and that the upper mantle immediately beneath the rift may contain reservoirs of magma generated at greater depth.

  10. The geological record of base metal sulfides in the cratonic mantle: A microscale 187Os/188Os study of peridotite xenoliths from Somerset Island, Rae Craton (Canada)

    Science.gov (United States)

    Bragagni, A.; Luguet, A.; Fonseca, R. O. C.; Pearson, D. G.; Lorand, J.-P.; Nowell, G. M.; Kjarsgaard, B. A.

    2017-11-01

    We report detailed petrographic investigations along with 187Os/188Os data in Base Metal Sulfide (BMS) on four cratonic mantle xenoliths from Somerset Island (Rae Craton, Canada). The results shed light on the processes affecting the Re-Os systematics and provide time constraints on the formation and evolution of the cratonic lithospheric mantle beneath the Rae craton. When devoid of alteration, BMS grains mainly consist of pentlandite + pyrrhotite ± chalcopyrite. The relatively high BMS modal abundance of the four investigated xenoliths cannot be reconciled with the residual nature of these peridotites, but requires addition of metasomatic BMS. This is especially evident in the two peridotites with the highest bulk Pd/Ir and Pd/Pt. Metasomatic BMS likely formed during melt/fluid percolation in the Sub Continental Lithospheric Mantle (SCLM) as well as during infiltration of the host kimberlite magma, when djerfisherite crystallized around older Fe-Ni-sulfides. On the whole-rock scale, kimberlite metasomatism is visible in a subset of bulk xenoliths, which defines a Re-Os errorchron that dates the host magma emplacement. The 187Os/188Os measured in the twenty analysed BMS grains vary from 0.1084 to >0.17 and it shows no systematic variation depending on the sulfide mineralogical assemblage. The largest range in 187Os/188Os is observed in BMS grains from the two xenoliths with the highest Pd/Ir, Pd/Pt, and sulfide modal abundance. The whole-rock TRD ages of these two samples underestimate the melting age obtained from BMS, demonstrating that bulk Re-Os model ages from peridotites with clear evidence of metasomatism should be treated with caution. The TRD ages determined in BMS grains are clustered around 2.8-2.7, ∼2.2 and ∼1.9 Ga. The 2.8-2.7 Ga TRD ages document the main SCLM building event in the Rae craton, which is likely related to the formation of the local greenstone belts in a continental rift setting. The Paleoproterozoic TRD ages can be explained by

  11. Characterising East Antarctic Lithosphere and its Rift Systems using Gravity Inversion

    Science.gov (United States)

    Vaughan, Alan P. M.; Kusznir, Nick J.; Ferraccioli, Fausto; Leat, Phil T.; Jordan, Tom A. R. M.; Purucker, Michael E.; Golynsky, A. V. Sasha; Rogozhina, Irina

    2013-04-01

    Since the International Geophysical Year (1957), a view has prevailed that East Antarctica has a relatively homogeneous lithospheric structure, consisting of a craton-like mosaic of Precambrian terranes, stable since the Pan-African orogeny ~500 million years ago (e.g. Ferracioli et al. 2011). Recent recognition of a continental-scale rift system cutting the East Antarctic interior has crystallised an alternative view of much more recent geological activity with important implications. The newly defined East Antarctic Rift System (EARS) (Ferraccioli et al. 2011) appears to extend from at least the South Pole to the continental margin at the Lambert Rift, a distance of 2500 km. This is comparable in scale to the well-studied East African rift system. New analysis of RadarSat data by Golynsky & Golynsky (2009) indicates that further rift zones may form widely distributed extension zones within the continent. A pilot study (Vaughan et al. 2012), using a newly developed gravity inversion technique (Chappell & Kusznir 2008) with existing public domain satellite data, shows distinct crustal thickness provinces with overall high average thickness separated by thinner, possibly rifted, crust. Understanding the nature of crustal thickness in East Antarctica is critical because: 1) this is poorly known along the ocean-continent transition, but is necessary to improve the plate reconstruction fit between Antarctica, Australia and India in Gondwana, which will also better define how and when these continents separated; 2) lateral variation in crustal thickness can be used to test supercontinent reconstructions and assess the effects of crystalline basement architecture and mechanical properties on rifting; 3) rift zone trajectories through East Antarctica will define the geometry of zones of crustal and lithospheric thinning at plate-scale; 4) it is not clear why or when the crust of East Antarctica became so thick and elevated, but knowing this can be used to test models of

  12. Lithospheric Structure, Crustal Kinematics, and Earthquakes in North China: An Integrated Study

    Science.gov (United States)

    Liu, M.; Yang, Y.; Sandvol, E.; Chen, Y.; Wang, L.; Zhou, S.; Shen, Z.; Wang, Q.

    2007-12-01

    The North China block (NCB) is geologically part of the Archaean Sino-Korean craton. But unusual for a craton, it was thermally rejuvenated since late Mesozoic, and experienced widespread extension and volcanism through much of the Cenozoic. Today, the NCB is characterized by strong internal deformation and seismicity, including the 1976 Tangshan earthquake that killed ~250,000 people. We have started a multidisciplinary study to image the lithospheric and upper mantle structure using seismological methods, to delineate crustal kinematics and deformation via studies of neotectonics and space geodesy, and to investigate the driving forces, the stress states and evolution, and seismicity using geodynamic modeling. Both seismic imaging and GPS results indicate that the Ordos plateau, which is the western part of the NCB and a relic of the Sino-Korean craton, has been encroached around its southern margins by mantle flow and thus is experiencing active cratonic destruction. Some of the mantle flow may be driven by the Indo-Asian collision, although the cause of the broad mantle upwelling responsible for the Mesozoic thinning of the NCB lithosphere remains uncertain. At present, crustal deformation in the NCB is largely driven by gravitational spreading of the expanding Tibetan Plateau. Internal deformation within the NCB is further facilitated by the particular tectonic boundary conditions around the NCB, and the large lateral contrasts of lithospheric strength and rheology. Based on the crustal kinematics and lithospheric structure, we have developed a preliminary geodynamic model for stress states and strain energy in the crust of the NCB. The predicted long-term strain energy distribution is comparable with the spatial pattern of seismic energy release in the past 2000 years. We are exploring the cause of the spatiotemporal occurrence of large earthquakes in the NCB, especially the apparent migration of seismicity from the Weihe-Shanxi grabens around the Ordos to

  13. Lithospheric structure and deformation of the North American continent

    OpenAIRE

    Magdala Tesauro; Mikhail Kaban; S. Cloetingh; W. D. Mooney

    2013-01-01

    We estimate the integrated strength and elastic thickness (Te) of the North American lithosphere based on thermal, density and structural (seismic) models of the crust and upper mantle. The temperature distribution in the lithosphere is estimated considering for the first time the effect of composition as a result of the integrative approach based on a joint analysis of seismic and gravity data. We do this via an iterative adjustment of the model. The upper mantle temperatures are initially e...

  14. Estimating lithospheric properties at Atla Regio, Venus

    Science.gov (United States)

    Phillips, Roger J.

    1994-01-01

    Magellan spehrical harmonic gravity and topography models are used to estimate lithospheric properties at Alta Regio, Venus, a proposed hotspot with dynamic support from mantle plume(s). Global spherical harmonic and local representations of the gravity field share common properties in the Atla region interms of their spectral behavior over a wavelength band from approximately 2100 to approximately 700 km. The estimated free-air admittance spectrum displays a rather featureless long-wavelength portion followed by a sharp rise at wavelengths shorter than about 1000 km. This sharp rise requires significant flexural support of short-wavelength structures. The Bouguer coherence also displays a sharp drop in this wavelength band, indicating a finite flexural rigidity of the lithosphere. A simple model for lithospheric loading from above and below is introduced (D. W. Forsyth, 1985) with four parameters: f, the ratio of bottom loading to top loading; z(sub m), crustal thickness; z(sub l) depth to bottom loading source; and T(sub e) elastic lithosphere thickness. A dual-mode compensation model is introduced in which the shorter wavelengths (lambda approximately less than 1000 km) might be explained best by a predominance of top loading by the large shield volcanoes Maat Mons, Ozza Mons, and Sapas Mons, and the longer wavelengths (lambda approximately greater than 1500 km) might be explained best by a deep depth of compensation, possibly representing bottom loading by a dynamic source. A Monte Carlo inversion technique is introduced to thoroughly search out the four-space of the model parameters and to examine parameter correlation in the solutions. Venus either is a considerabe deficient in heat sources relative to Earth, or the thermal lithosphere is overthickened in response to an earlier episode of significant heat loss from the planet.

  15. Lithospheric flexure beneath the Freyja Montes Foredeep, Venus: Constraints on lithospheric thermal gradient and heat flow

    International Nuclear Information System (INIS)

    Solomon, S.C.; Head, J.W.

    1990-01-01

    Analysis of Venera 15 and 16 radar images and topographic data from the Freyja Montes region on Venus suggest that this mountain belt formed as a result of a sequence of underthrusts of the lithosphere of the North Polar Plains beneath the highlands of Ishtar Terra. The Freyja Montes deformation zone consists, south to north, of a linear orogenic belt, an adjacent plateau, a steep scarp separating the plateau from the North Polar Plains, a linear depression at the base of the scarp, and an outer rise. The topographic profile of the depression and outer rise are remarkably similar to that of a foreland deep and rise formed by the flexure of the underthrusting plate beneath a terrestrial mountain range. The authors test the lithospheric flexure hypothesis and they estimate the effective thickness T e of the elastic lithosphere of the underthrusting portion of the North Polar Plains by fitting individual topographic profiles to deflection curves for a broken elastic plate. The theoretical curves fit the observed topographic profiles to within measurement error for values of flexural rigidity D in the range (0.8-3) x 10 22 N m, equivalent to T e in the range 11-18 km. Under the assumption that the base of the mechanical lithosphere is limited by the creep strength of olivine, the mean lithospheric thermal gradient is 14-23 K/km. That the inferred thermal gradient is similar to the value expected for the global mean gradient on the basis of scaling from Earth provides support for the hypothesis that simple conduction dominates lithospheric heat transport on Venus relative to lithospheric recycling and volcanism

  16. The Lithosphere in Italy: Structure and Seismicity

    International Nuclear Information System (INIS)

    Brandmayr, Enrico; Blagoeva Raykova, Reneta; Zuri, Marco; Romanelli, Fabio; Doglioni, Carlo; Panza, Giuliano Francesco

    2010-07-01

    We propose a structural model for the lithosphere-asthenosphere system for the Italic region by means of the S-wave velocity (V S ) distribution with depth. To obtain the velocity structure the following methods are used in the sequence: frequency-time analysis (FTAN); 2D tomography (plotted on a grid 1 o x 1 o ); non-linear inversion; smoothing optimization method. The 3D V S structure (and its uncertainties) of the study region is assembled as a juxtaposition of the selected representative cellular models. The distribution of seismicity and heat flow is used as an independent constraint for the definition of the crustal and lithospheric thickness. The moment tensor inversion of recent damaging earthquakes which occurred in the Italic region is performed through a powerful non-linear technique and it is related to the different rheologic-mechanic properties of the crust and uppermost mantle. The obtained picture of the lithosphere-asthenosphere system for the Italic region confirms a mantle extremely vertically stratified and laterally strongly heterogeneous. The lateral variability in the mantle is interpreted in terms of subduction zones, slab dehydration, inherited mantle chemical anisotropies, asthenospheric upwellings, and so on. The western Alps and the Dinarides have slabs with low dip, whereas the Apennines show a steeper subduction. No evidence for any type of mantle plume is observed. The asymmetric expansion of the Tyrrhenian Sea, which may be interpreted as related to a relative eastward mantle flow with respect to the overlying lithosphere, is confirmed. (author)

  17. The lithosphere-asthenosphere boundary observed with USArray receiver functions

    Directory of Open Access Journals (Sweden)

    P. Kumar

    2012-05-01

    Full Text Available The dense deployment of seismic stations so far in the western half of the United States within the USArray project provides the opportunity to study in greater detail the structure of the lithosphere-asthenosphere system. We use the S receiver function technique for this purpose, which has higher resolution than surface wave tomography, is sensitive to seismic discontinuities, and is free from multiples, unlike P receiver functions. Only two major discontinuities are observed in the entire area down to about 300 km depth. These are the crust-mantle boundary (Moho and a negative boundary, which we correlate with the lithosphere-asthenosphere boundary (LAB, since a low velocity zone is the classical definition of the seismic observation of the asthenosphere by Gutenberg (1926. Our S receiver function LAB is at a depth of 70–80 km in large parts of westernmost North America. East of the Rocky Mountains, its depth is generally between 90 and 110 km. Regions with LAB depths down to about 140 km occur in a stretch from northern Texas, over the Colorado Plateau to the Columbia basalts. These observations agree well with tomography results in the westernmost USA and on the east coast. However, in the central cratonic part of the USA, the tomography LAB is near 200 km depth. At this depth no discontinuity is seen in the S receiver functions. The negative signal near 100 km depth in the central part of the USA is interpreted by Yuan and Romanowicz (2010 and Lekic and Romanowicz (2011 as a recently discovered mid-lithospheric discontinuity (MLD. A solution for the discrepancy between receiver function imaging and surface wave tomography is not yet obvious and requires more high resolution studies at other cratons before a general solution may be found. Our results agree well with petrophysical models of increased water content in the asthenosphere, which predict a sharp and shallow LAB also in continents (Mierdel et al., 2007.

  18. Mercury's Lithospheric Magnetization

    Science.gov (United States)

    Johnson, C.; Phillips, R. J.; Philpott, L. C.; Al Asad, M.; Plattner, A.; Mast, S.; Kinczyk, M. J.; Prockter, L. M.

    2017-12-01

    Magnetic field data obtained by the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft have been used to demonstrate the presence of lithospheric magnetization on Mercury. Larger amplitude fields resulting from the core dynamo and the strongly time-varying magnetospheric current systems are first estimated and subtracted from the magnetic field data to isolate lithospheric signals with wavelengths less than 500 km. These signals (hereafter referred to as data) are only observed at spacecraft altitudes less than 120 km, and are typically a few to 10 nT in amplitude. We present and compare equivalent source dipole magnetization models for latitudes 35°N to 75°N obtained from two distinct approaches to constrain the distribution and origin of lithospheric magnetization. First, models that fit either the data or the surface field predicted from a regional spherical harmonic representation of the data (see Plattner & Johnson abstract) and that minimize the root mean square (RMS) value of the magnetization are derived. Second, models in which the spatial distribution of magnetization required to fit the data is minimized are derived using the approach of Parker (1991). As seen previously, the largest amplitudes of lithospheric magnetization are concentrated around the Caloris basin. With this exception, across the northern hemisphere there are no overall correlations of magnetization with surface geology, although higher magnetizations are found in regions with darker surfaces. Similarly, there is no systematic correlation of magnetization signatures with crater materials, although there are specific instances of craters with interiors or ejecta that have magnetizations distinct from the surrounding region. For the latter case, we observe no correlation of the occurrence of these signatures with crater degradation state (a proxy for age). At the lowest spacecraft altitudes (source depths less than O(10 km) are unlikely in most regions

  19. Imaging the lithosphere and underlying mantle of the South Atlantic, South America and Africa using waveform tomography with massive datasets

    Science.gov (United States)

    Celli, N. L.; Lebedev, S.; Schaeffer, A. J.; Ravenna, M.; Gaina, C.

    2017-12-01

    Recent growth in global seismic station coverage has created dense data sampling of the previously poorly constrained lithosphere and underlying mantle beneath the South Atlantic, South America and Africa. The new data enable us to image the vast region at a new level of detail and address important open questions regarding its lithospheric architecture and mantle dynamics. In order to fully exploit the data sampling, we use an efficient, multimode waveform tomography scheme that enables the extraction of structural information from millions of seismograms and use the inherent data redundancy to minimize effects of errors in the data. Our tomographic model is constrained by waveform fits of over 1.2 million vertical-component seismograms, computed using the Automated Multimode Inversion of surface, S- and multiple S-waves. Each successful seismogram fit provides a set of linear equations describing 1D average velocity perturbations within approximate sensitivity volumes, with respect to a 3D reference model. We then combine all equations into a large linear system and invert jointly for a model of S- and P-wave speeds and azimuthal anisotropy within the lithosphere and underlying mantle. We are now able to image the detailed structure of various African shields. For example, in West Africa, two clearly separate high-velocity units underlay the Reguibat and Man-Léo Shields; in the Congo area, a single high-velocity body, formed by three main units correspond to the Gabon-Cameroon, Bomu-Kibali and Kasai Shields. Strong low-velocity anomalies underlay the Afar Hotspot and the East African Rift; pronounced low velocities are also seen beneath parts of the Sahara Desert. We discuss the shape of the deep Afar anomaly and its possible relationships with the Saharan volcanism and the neighboring Tanzania Craton. In the South Atlantic, we retrieve fine-scale velocity structure along the Mid-Atlantic Ridge (MAR), indicative of hotspot-ridge interactions. Major hotspots show

  20. Gravity and magnetic modelling in the Vrancea Zone, south-eastern Carpathians: Redefinition of the edge of the East European Craton beneath the south-eastern Carpathians

    Science.gov (United States)

    Bocin, A.; Stephenson, R.; Matenco, L.; Mocanu, V.

    2013-11-01

    A 2D gravity and magnetic data model has been constructed along a 71 km densely observed profile, called DACIA PLAN GRAV MAN's. The profile crosses part of the nappe pile of the south-eastern Carpathians and includes the seismically active Vrancea Zone and was acquired with the objective to illuminate the basement structure and affinity in this area. The modelling approach was to create an initial model from well constrained geological information, integrate it with previous seismic ray tracing and tomographic models and then alter it outside the a priori constraints in order to reach the best fit between observed and calculated potential field anomalies. The results support a realignment of the position of the TTZ (Tornquist-Teisseyre Zone), the profound tectonic boundary within Europe that separates Precambrian cratonic lithosphere of the East European Craton (EEC) from younger accreted lithosphere of Phanerozoic mobile belts to its west. The TTZ is shown to lie further to the south-west than was previously inferred within Romania, where it is largely obscured by the Carpathian nappes. The crust of the EEC beneath the south-eastern Carpathians is inferred to terminate along a major crustal structure lying just west of the Vrancea seismogenic zone. The intermediate depth seismicity of the Vrancea Zone therefore lies within the EEC lithosphere, generally supporting previously proposed models invoking delamination of cratonic lithosphere as the responsible mechanism.

  1. Insights on the lithospheric structure of the Zagros mountain belt from seismological data analysis

    Science.gov (United States)

    Paul, A.; Kaviani, A.; Vergne, J.; Hatzfeld, D.; Mokhtari, M.

    2003-04-01

    As part of a French-Iranian collaboration, we installed a temporary seismological network across the Zagros for 4.5 months in 2000-2001 to investigate the lithospheric structure of the mountain belt. The network included 65 stations located along a 600-km long line (average spacing of ˜10 km) from the coast of the Persian Gulf to the stable block of Central Iran. A migrated depth cross-section computed from radial receiver functions displays clear P-to-S conversions at the Moho beneath most of the profile. The average Moho depth is 45 to 50 km beneath the folded belt. It deepens rather abruptly beneath the suture zone of the MZT (Main Zagros Thrust) and the Sanandaj-Sirjan (SS) metamorphic zone. The maximum crustal thickness of ˜65 km is reached 50 km NE of the surface trace of the MZT. The region of over-thickened crust is shifted to the NE with respect to the areas of highest elevations and the strongest negative Bouguer anomaly. To the NE, the crust of the block of Central Iran is 40-km thick on average. Two patches of Ps converted energy can be seen below the Moho in the northern half of the transect that cannot be attributed to multiple reflections. Teleseismic P residual travel time curves display lateral variations as large as 1.5 s with both long (faster arrivals in the SW than in the NE) and short-scale variations (in the MZT region). They were inverted for variations of P wave velocity with the ACH technique. The crustal layer exhibits rather strong lateral variations of Vp with lower velocities under the MZT and the Urumieh-Dokhtar magmatic assemblage, and faster velocities under the SS zone. In the mantle, a clear difference appears between the faster P wave velocities of the Arabian craton and the relatively lower velocities of the mantle of Central Iran.

  2. Developing regionalized models of lithospheric thickness and velocity structure across Eurasia and the Middle East from jointly inverting P-wave and S-wave receiver functions with Rayleigh wave group and phase velocities

    Energy Technology Data Exchange (ETDEWEB)

    Julia, J; Nyblade, A; Hansen, S; Rodgers, A; Matzel, E

    2009-07-06

    In this project, we are developing models of lithospheric structure for a wide variety of tectonic regions throughout Eurasia and the Middle East by regionalizing 1D velocity models obtained by jointly inverting P-wave and S-wave receiver functions with Rayleigh wave group and phase velocities. We expect the regionalized velocity models will improve our ability to predict travel-times for local and regional phases, such as Pg, Pn, Sn and Lg, as well as travel-times for body-waves at upper mantle triplication distances in both seismic and aseismic regions of Eurasia and the Middle East. We anticipate the models will help inform and strengthen ongoing and future efforts within the NNSA labs to develop 3D velocity models for Eurasia and the Middle East, and will assist in obtaining model-based predictions where no empirical data are available and for improving locations from sparse networks using kriging. The codes needed to conduct the joint inversion of P-wave receiver functions (PRFs), S-wave receiver functions (SRFs), and dispersion velocities have already been assembled as part of ongoing research on lithospheric structure in Africa. The methodology has been tested with synthetic 'data' and case studies have been investigated with data collected at an open broadband stations in South Africa. PRFs constrain the size and S-P travel-time of seismic discontinuities in the crust and uppermost mantle, SRFs constrain the size and P-S travel-time of the lithosphere-asthenosphere boundary, and dispersion velocities constrain average S-wave velocity within frequency-dependent depth-ranges. Preliminary results show that the combination yields integrated 1D velocity models local to the recording station, where the discontinuities constrained by the receiver functions are superimposed to a background velocity model constrained by the dispersion velocities. In our first year of this project we will (i) generate 1D velocity models for open broadband seismic stations

  3. An integrated geological and geophysical study of the Parnaíba cratonic basin, North-East Brazil

    Science.gov (United States)

    Tozer, B.; Watts, A. B.; Daly, M.

    2015-12-01

    Cratonic basins are characterized by their sub-circular shape, long-lived (>100 Myr) subsidence, shallow marine/terrestrial sediments that young towards the center of the basin and exhibit little internal deformation, and thick seismic lithosphere. Despite the recognition of >30 world-wide, the paucity of geological and geophysical data over these basins means their origin remains enigmatic. In order to address this problem, we have used a recently acquired 1400 km long seismic reflection profile recorded to 20 s TWTT, field observations and well logs, gravity and magnetic data acquired at 1 km intervals, and five wide-angle refection/refraction receiver gathers recorded at offsets up to 100 km, to constrain the origin of the Parnaíba basin, North-East Brazil. We find a depth to pre-Paleozoic basement and Moho of ~ 3.5 and ~ 40 - 42 km respectively beneath the basin center. A prominent mid-crustal reflection (MCR) can be tracked laterally for ~ 300 km at depths between 17 - 25 km and a low-fold wide-angle receiver gather stack shows that the crust below the MCR is characterized by a ~ 4 s TWTT package of anastomosing reflections. Gravity modelling suggests that the MCR represents the upper surface of a high density (+0.14 kg m-3) lower crustal body, which is probably of magmatic origin. Backstripping of biostratigraphic data from wells in the center of the basin show an exponentially decreasing subsidence. We show that although cooling of a thick (180 km) lithosphere following prolonged rifting (~ 65 Myr) can provide a good fit to the tectonic subsidence curves, process-oriented gravity and flexure modelling suggest that other processes must be important, as rifting does not account for the observed gravity anomaly and predicts too thin a crust (~ 34 km). The thicker than expected crust suggests warping due, for example, to far-field stresses or basal tractions. Another possibility, which is compatible with existing geophysical data, is a dense magmatic intrusion

  4. Present-day geothermal characteristics of the Ordos Basin, western North China Craton: new findings from deep borehole steady-state temperature measurements

    Science.gov (United States)

    Gao, Peng; Qiu, Qianfeng; Jiang, Guangzheng; Zhang, Chao; Hu, Shengbiao; Lei, Yuhong; Wang, Xiangzeng

    2018-07-01

    Heat flow and associated thermal regimes are related to the tectonic evolution and geophysical properties of the lithosphere. The Ordos Basin is located in a tectonic transitional zone: areas to the east of the basin are characterized as tectonically active, while regions to the west of the basin are characterized as tectonically stable. It is of general interest to learn the geothermal characteristics of the basin in such tectonic conditions. To clarify the spatial variability of the present-day geothermal field across the basin and its implications, we report 13 terrestrial heat flow points based on the first systematic steady-state deep borehole temperature measurements in the basin. The new data together with existing data show that the geothermal gradients in the basin range from 12.6 to 42.3 °C km-1 with a mean of 27.7 ± 5.3 °C km-1; the terrestrial heat flow values range from 43.3 to 88.7 mW m-2 with a mean of 64.7 ± 8.9 mW m-2. Such values are higher than those of typical cratonic basins and lower than those of tectonically active areas. By using all these data in the basin and adjacent areas, we plot geothermal gradient and heat flow distribution maps. The maps reveal that the basin is cooling westwards and northwards. The distribution pattern of the geothermal field is consistent with the lithospheric thickness variation in the basin. This similarity suggests that the geothermal spatial variability of the Ordos Basin is mainly influenced by heat from the deep mantle. In the southeastern basin, we locate a positive geothermal anomaly caused by the convergence of heat flow in basement highs and the high radiogenic heat production. In addition, the high heat flow in the eastern basin is related to the intense uplift during the Cenozoic Era.

  5. Present-day geothermal characteristics of the Ordos Basin, western North China Craton: new findings from deep borehole steady-state temperature measurements

    Science.gov (United States)

    Gao, Peng; Qiu, Qianfeng; Jiang, Guangzheng; Zhang, Chao; Hu, Shengbiao; Lei, Yuhong; Wang, Xiangzeng

    2018-03-01

    Heat flow and associated thermal regimes are related to the tectonic evolution and geophysical properties of the lithosphere. The Ordos Basin is located in a tectonic transitional zone: areas to the east of the basin are characterized as tectonically active, while regions to the west of the basin are characterized as tectonically stable. It is of general interest to learn the geothermal characteristics of the basin in such tectonic conditions. To clarify the spatial variability of the present-day geothermal field across the basin and its implications, we report 13 terrestrial heat flow points based on the first systematic steady-state deep borehole temperature measurements in the basin. The new data together with existing data show that the geothermal gradients in the basin range from 12.6 to 42.3° C km-1 with a mean of 27.7 ± 5.3° C km-1; the terrestrial heat flow values range from 43.3 to 88.7 mW/m2 with a mean of 64.7 ± 8.9 mW/m2. Such values are higher than those of typical cratonic basins and lower than those of tectonically active areas. By using all these data in the basin and adjacent areas, we plot geothermal gradient and heat flow distribution maps. The maps reveal that the basin is cooling westward and northward. The distribution pattern of the geothermal field is consistent with the lithospheric thickness variation in the basin. This similarity suggests that the geothermal spatial variability of the Ordos Basin is mainly influenced by heat from the deep mantle. In the southeastern basin, we locate a positive geothermal anomaly caused by the convergence of heat flow in basement highs and the high radiogenic heat production. In addition, the high heat flow in the eastern basin is related to the intense uplift during the Cenozoic Era.

  6. Thermal classification of lithospheric discontinuities beneath USArray

    Science.gov (United States)

    Hansen, Steven M.; Dueker, Ken; Schmandt, Brandon

    2015-12-01

    Broadband seismic data from the United States were processed into Ps and Sp receiver function image volumes for the purpose of constraining negative velocity gradients (NVG) at depths between the Moho and 200 km. Moho depth picks from the two independent datasets are in good agreement, however, large discrepancies in NVG picks occur and are attributed to free-surface multiples which obscure deep NVG arrivals in the Ps data. From the Sp data, shallow NVG are found west of the Rockies and in the central US while deep and sporadic NVG are observed beneath the Great Plains and northern Rockies. To aid the interpretation of the observed NVG arrivals, the mantle thermal field is estimated by mapping surface wave tomography velocities to temperature assuming an anelastic olivine model. The distribution of temperature versus NVG depth is bi-modal and displays two distinct thermal populations that are interpreted to represent both the lithosphere-asthenosphere boundary (LAB) and mid-lithosphere discontinuities (MLD). LAB arrivals occur in the western US at 60-85 km and 1200-1400 °C depth suggesting that they manifest partial melt near the base of the thermal plate. MLD arrivals primarily occur at 70-110 km depth and 700-900 °C and we hypothesize that these arrivals are caused by a low-velocity metasomatic layer containing phlogopite resulting from magma crystallization products that accumulate within long-lived thick lithosphere.

  7. Building and Modification of the Continental Lithosphere: the History of the Contiguous U.S. as told by MLDs and LABs

    Science.gov (United States)

    Hopper, E.; Fischer, K. M.

    2016-12-01

    The lithosphere preserves a record of past and present tectonic processes in its internal structures and its boundary with the underlying asthenosphere. We use common conversion point stacked Sp converted waves recorded by EarthScope's Transportable Array, as well as other available permanent and temporary broadband stations, to image such structures in the lithospheric mantle of the contiguous U.S. In the tectonically youngest western U.S., a shallow, sharp velocity gradient at the base of the lithosphere suggests a boundary defined by ponded melt. The lithosphere thickens with age of volcanism, implying the lithosphere is a melt-mitigated, conductively cooling thermal boundary layer. Beneath older, colder lithosphere where melt fractions are likely much lower, the velocity gradient at the base of such a layer should be a more diffuse, primarily thermal boundary. This is consistent with observations in the eastern U.S. where the lithosphere-asthenosphere boundary (LAB) is locally sharp and shallower only in areas of inferred enhanced upwelling - such as ancient hot spot tracks and areas of inferred delamination. In the cratonic interior, the LAB is even more gradual in depth, and is transparent to Sp waves with dominant periods of 10 s. Although seismic imaging only provides a snapshot of the lithosphere as it is today, preserved internal structures extend the utility of this imaging back into deep geological time. Ancient accretion within the cratonic lithospheric mantle is preserved as dipping structures associated with relict subducted slabs from Paleoproterozoic continental accretion, suggesting that lateral accretion was integral to the cratonic mantle root formation process. Metasomatism, melt migration and ponding below a carbonated peridotite solidus explain a sub-horizontal mid-lithospheric discontinuity (MLD) commonly observed at 70-100 km depth. This type of MLD is strongest in Mesoproterozoic and older lithosphere, suggesting that it formed more

  8. Gravity Maps of Antarctic Lithospheric Structure from Remote-Sensing and Seismic Data

    Science.gov (United States)

    Tenzer, Robert; Chen, Wenjin; Baranov, Alexey; Bagherbandi, Mohammad

    2018-02-01

    Remote-sensing data from altimetry and gravity satellite missions combined with seismic information have been used to investigate the Earth's interior, particularly focusing on the lithospheric structure. In this study, we use the subglacial bedrock relief BEDMAP2, the global gravitational model GOCO05S, and the ETOPO1 topographic/bathymetric data, together with a newly developed (continental-scale) seismic crustal model for Antarctica to compile the free-air, Bouguer, and mantle gravity maps over this continent and surrounding oceanic areas. We then use these gravity maps to interpret the Antarctic crustal and uppermost mantle structure. We demonstrate that most of the gravity features seen in gravity maps could be explained by known lithospheric structures. The Bouguer gravity map reveals a contrast between the oceanic and continental crust which marks the extension of the Antarctic continental margins. The isostatic signature in this gravity map confirms deep and compact orogenic roots under the Gamburtsev Subglacial Mountains and more complex orogenic structures under Dronning Maud Land in East Antarctica. Whereas the Bouguer gravity map exhibits features which are closely spatially correlated with the crustal thickness, the mantle gravity map reveals mainly the gravitational signature of the uppermost mantle, which is superposed over a weaker (long-wavelength) signature of density heterogeneities distributed deeper in the mantle. In contrast to a relatively complex and segmented uppermost mantle structure of West Antarctica, the mantle gravity map confirmed a more uniform structure of the East Antarctic Craton. The most pronounced features in this gravity map are divergent tectonic margins along mid-oceanic ridges and continental rifts. Gravity lows at these locations indicate that a broad region of the West Antarctic Rift System continuously extends between the Atlantic-Indian and Pacific-Antarctic mid-oceanic ridges and it is possibly formed by two major

  9. Mg-Fe Isotope Systems of Mantle Xenoliths: Constrains on the Evolution of Siberian Craton

    Science.gov (United States)

    An, Y.; Kiseeva, E. S.; Sobolev, N. V.; Zhang, Z.

    2017-12-01

    Mantle xenoliths bring to the surface a variety of lithologies (dunites, lherzolites, harzburgites, wehrlites, eclogites, pyroxenites, and websterites) and represent snapshots of the geochemical processes that occur deep within the Earth. Recent improvements in the precision of the MC-ICP-MS measurements have allowed us to expand the amount of data on Mg and Fe isotopes for mantle-derived samples. For instance, to constrain the isotopic composition of the Earth based on the study of spinel and garnet peridotites (An et al., 2017; Teng et al., 2010), to trace the origin and to investigate the isotopic fractionation mechanism during metamorphic process using cratonic or orogenic eclogites (Li et al., 2011; Wang et al., 2012) and to reveal the metasomatism-induced mantle heterogeneity by pyroxenites (Hu et al., 2016). Numerous multi-stage modification events and mantle layering are detected in the subcontinental lithospheric mantle under the Siberian craton (Ashchepkov et al., 2008a; Sobolev et al., 1975, etc). Combined analyses of Mg and Fe isotopic systems could provide new constraints on the formation and evolution of the ancient cratonic mantle. In order to better constrain the magnitude and mechanism of inter-mineral Mg and Fe isotopic fractionations at high temperatures, systematic studies of mantle xenoliths are needed. For example, theoretical calculations and natural samples measurements have shown that large equilibrium Mg isotope fractionations controlled by the difference in coordination number of Mg among minerals could exist (Huang et al., 2013; Li et al., 2011). Thus, the Mg isotope geothermometer could help us trace the evolution history of ancient cratons. In this study we present Mg and Fe isotopic data for whole rocks and separated minerals (clinopyroxene (cpx) and garnet (grt)) from different types of mantle xenoliths (garnet pyroxenites, eclogites, grospydites and garnet peridotites) from a number of kimberlite pipes in Siberian craton (Udachnaya

  10. Structure of the mantle lithosphere in continental collision zones of Europe, North America and China from S-receiver functions

    Science.gov (United States)

    Kind, R.; Shen, X.

    2017-12-01

    Seismic tomography and receiver functions are the most common methods to study the structure of the mantle lithosphere. We use S-receiver functions to study continent-continent collision zones in Europe, North America and China. In order to avoid possible numerical problems caused by filtering effects (side lobes) we process the data practically without filtering (also excluding deconvolution). Side lobes are still a fundamental question to check the reality of the Mid-Lithospheric Discontinuity (MLD). We use openly available data of mostly permanent seismic broadband stations from the European portal EIDA, from IRIS and from the Chinese Seismic Network. We obtained several ten thousands of useful records in each region by visual and fully automatic processing. We observed the MLD in all cratonic regions near 100 km depth and the Lithosphere-Asthenosphere Boundary (LAB) partly in cratonic regions near 200 km depth. The observation of the cratonic LAB with converted waves requires a relatively sharp discontinuity which excludes temperature as only cause of the LAB. In younger tectonic active regions we observed the LAB near 100 km depth. TheLAB and MLD are in collision zones significantly structured. In central Europe we observed the deep cratonic LAB reaching far to the west of the Tornquist-Teisseyre Zone below Phanerozoic cover. Below the northern edge of the Bohemian Massif seems to be a tear in the LAB leading to a jump in its depth of about 100 km. In North America we see north of Yellowstone a smooth deepening of the western LAB from about 100 km depth to 200 km depth at the Mid-Continental Rift System. Similarly to the LAB jump below the Bohemian Massif in Europe, we see below the Sevier Thrust Belt also a jump of about 100 km in the LAB depth. In China we see the cratonic LAB deepening to the south-west far below eastern Tibet. Below the craton in north-east China is only the shallow LAB/MLD visible. These observations in three continents show that the

  11. Updated Reference Model for Heat Generation in the Lithosphere

    Science.gov (United States)

    Wipperfurth, S. A.; Sramek, O.; Roskovec, B.; Mantovani, F.; McDonough, W. F.

    2017-12-01

    Models integrating geophysics and geochemistry allow for characterization of the Earth's heat budget and geochemical evolution. Global lithospheric geophysical models are now constrained by surface and body wave data and are classified into several unique tectonic types. Global lithospheric geochemical models have evolved from petrological characterization of layers to a combination of petrologic and seismic constraints. Because of these advances regarding our knowledge of the lithosphere, it is necessary to create an updated chemical and physical reference model. We are developing a global lithospheric reference model based on LITHO1.0 (segmented into 1°lon x 1°lat x 9-layers) and seismological-geochemical relationships. Uncertainty assignments and correlations are assessed for its physical attributes, including layer thickness, Vp and Vs, and density. This approach yields uncertainties for the masses of the crust and lithospheric mantle. Heat producing element abundances (HPE: U, Th, and K) are ascribed to each volume element. These chemical attributes are based upon the composition of subducting sediment (sediment layers), composition of surface rocks (upper crust), a combination of petrologic and seismic correlations (middle and lower crust), and a compilation of xenolith data (lithospheric mantle). The HPE abundances are correlated within each voxel, but not vertically between layers. Efforts to provide correlation of abundances horizontally between each voxel are discussed. These models are used further to critically evaluate the bulk lithosphere heat production in the continents and the oceans. Cross-checks between our model and results from: 1) heat flux (Artemieva, 2006; Davies, 2013; Cammarano and Guerri, 2017), 2) gravity (Reguzzoni and Sampietro, 2015), and 3) geochemical and petrological models (Rudnick and Gao, 2014; Hacker et al. 2015) are performed.

  12. Early Cretaceous Ductile Deformation of Marbles from the Western Hills of Beijing, North China Craton

    Science.gov (United States)

    Feng, H.; Liu, J.

    2017-12-01

    During the Early Cretaceous tectonic lithosphere extension, the pre-mesozoic rocks from the Western Hills in the central part of the North China Craton suffered from weak metamorphism but intense shear deformation. The prominent features of the deformation structures are the coexisting layer-parallel shear zones and intrafolia folds, and the along-strike thickness variations of the marble layers from the highly sheared Mesoproterozoic Jing'eryu Formation. Platy marbles are well-developed in the thinner layers, while intrafolia folds are often observed in the thicker layers. Most folds are tight recumbent folds and their axial planes are parallel to the foliations and layerings of the marbles. The folds are A-type folds with hinges being always paralleling to the stretching lineations consistently oriented at 130°-310° directions throughout the entire area. SPO and microstructural analyses of the sheared marbles suggest that the thicker layers suffered from deformations homogeneously, while strain localization can be distinguished in the thinner layers. Calcite twin morphology and CPO analysis indicate that the deformation of marbles from both thinner and thicker layers happened at temperatures of 300 to 500°C. The above analysis suggests that marbles in the thicker layers experienced a progressive sequence of thermodynamic events: 1) regional metamorphism, 2) early ductile deformation dominated by relatively higher temperature conditions, during which all the mineral particles elongated and oriented limitedly and the calcite grains are deformed mainly by mechanical twinning, and 3) late superimposition of relatively lower temperature deformation and recrystallization, which superposed the early deformation, and made the calcites finely granulated, elongated and oriented by dynamical recrystallization along with other grains. Marbles from the thinner layers, however, experienced a similar, but different sequence of thermo-dynamic events, i.e. regional

  13. Seismic structure of the lithosphere beneath NW Namibia: Impact of the Tristan da Cunha mantle plume

    Science.gov (United States)

    Yuan, Xiaohui; Heit, Benjamin; Brune, Sascha; Steinberger, Bernhard; Geissler, Wolfram H.; Jokat, Wilfried; Weber, Michael

    2017-01-01

    Northwestern Namibia, at the landfall of the Walvis Ridge, was affected by the Tristan da Cunha mantle plume during continental rupture between Africa and South America, as evidenced by the presence of the Etendeka continental flood basalts. Here we use data from a passive-source seismological network to investigate the upper mantle structure and to elucidate the Cretaceous mantle plume-lithosphere interaction. Receiver functions reveal an interface associated with a negative velocity contrast within the lithosphere at an average depth of 80 km. We interpret this interface as the relic of the lithosphere-asthenosphere boundary (LAB) formed during the Mesozoic by interaction of the Tristan da Cunha plume head with the pre-existing lithosphere. The velocity contrast might be explained by stagnated and "frozen" melts beneath an intensively depleted and dehydrated peridotitic mantle. The present-day LAB is poorly visible with converted waves, indicating a gradual impedance contrast. Beneath much of the study area, converted phases of the 410 and 660 km mantle transition zone discontinuities arrive 1.5 s earlier than in the landward plume-unaffected continental interior, suggesting high velocities in the upper mantle caused by a thick lithosphere. This indicates that after lithospheric thinning during continental breakup, the lithosphere has increased in thickness during the last 132 Myr. Thermal cooling of the continental lithosphere alone cannot produce the lithospheric thickness required here. We propose that the remnant plume material, which has a higher seismic velocity than the ambient mantle due to melt depletion and dehydration, significantly contributed to the thickening of the mantle lithosphere.

  14. Origin and Distribution of Water Contents in Continental and Oceanic Lithospheric Mantle

    Science.gov (United States)

    Peslier, Anne H.

    2013-01-01

    The water content distribution of the upper mantle will be reviewed as based on the peridotite record. The amount of water in cratonic xenoliths appears controlled by metasomatism while that of the oceanic mantle retains in part the signature of melting events. In both cases, the water distribution is heterogeneous both with depth and laterally, depending on localized water re-enrichments next to melt/fluid channels. The consequence of the water distribution on the rheology of the upper mantle and the location of the lithosphere-asthenosphere boundary will also be discussed.

  15. Constraints on the Chemistry and Abundance of Hydrous Phases in Sub Continental Lithospheric Mantle: Implications for Mid-Lithospheric Discontinuities

    Science.gov (United States)

    Saha, S.; Dasgupta, R.; Fischer, K. M.; Mookherjee, M.

    2017-12-01

    The origins of a 2-10% reduction in seismic shear wave velocity (Vs) at depths of 60-160 km in sub continental lithospheric mantle (SCLM) regions, identified as the Mid Lithospheric Discontinuity (MLD) [e.g., 1] are highly debated [e.g., 2, 3]. One of the proposed explanations for MLDs is the presence of hydrous minerals such as amphibole and phlogopite at these depths [e.g., 2, 4, 5]. Although the stability and compositions of these phases in peridotite + H2O ± CO2 have been widely explored [e.g., 6], their composition and abundance as a function of permissible SCLM chemistry remain poorly understood. We have compiled phase equilibria experiments conducted over a range of pressure (0.5-8 GPa), temperature (680-1300 °C), major element peridotite compositions, and volatiles (H2O: 0.05-13.79 wt.% and CO2: 0.25-5.3 wt.%). The goal was to constrain how compositional parameters such as CaO and alkali/H2O affect the chemistry and abundance of amphibole and phlogopite. We observe that the abundance of amphibole increases with CaO content and decreasing alkali/H2O. The abundance of phlogopite varies directly with K2O content. Unlike phlogopite compositions that remain consistent, amphibole compositions show variability (pargastitic to K-richterite) depending on bulk CaO and Na2O. Mineral modes, obtained by mass balance on a melt/fluid free basis, were used to calculate aggregate shear wave velocity, Vs for the respective assemblages [e.g., 7] and compared with absolute values observed at MLD depths [e.g., 8]. Vs shows a strong inverse correlation with phlogopite and amphibole modes (particularly where phlogopite is absent). For the Mg# range of cratonic xenoliths, 5-10% phlogopite at MLD depths can match the observed Vs values, while CaO contents in cratonic xenoliths limit the amphibole abundance to 10%, which is lower than previous estimates based on heat flow calculations [e.g., 4]. The modes of hydrous and other phases and corresponding Vs values could be used to

  16. Contrast of lithospheric dynamics across the southern and eastern margins of the Tibetan Plateau: a numerical study

    Science.gov (United States)

    Sun, Yujun; Fan, Taoyuan; Wu, Zhonghai

    2018-05-01

    Both of the southern and eastern margins of the Tibetan Plateau are bounded by the cratonic blocks (Indian plate and Sichuan basin). However, there are many differences in tectonic deformation, lithospheric structure and surface heat flow between these two margins. What dynamics cause these differences? With the constraints of the lithospheric structure and surface heat flow across the southern and eastern margins of Tibetan Plateau, we constructed 2-D thermal-mechanical finite-element models to investigate the dynamics across these two margins. The results show that the delamination of mantle lithosphere beneath the Lhasa terrane in Oligocene and the rheological contrast between the Indian and Tibetan crust are the two main factors that control the subduction of the Indian plate. The dynamics across the eastern margin of the Tibetan Plateau are different from the southern margin. During the lateral expansion of the Tibetan Plateau, pure shear thickening is the main deformation characteristic for the Songpan-Ganzi lithosphere. This thickening results in the reduction of geothermal gradient and surface heat flow. From this study, it can be seen that the delamination of the mantle lithosphere and the rheological contrast between the Tibetan Plateau and its bounding blocks are the two main factors that control the lithospheric deformation and surface heat flow.

  17. Cyclic Cratonic Carbonates and Phanerozoic Calcite Seas.

    Science.gov (United States)

    Wilkinson, Bruce H.

    1982-01-01

    Discusses causes of cyclicity in cratonic carbonate sequences and evidence for and potential significance of postulated primary calcite sediment components in past Paleozoic seas, outlining problems, focusing on models explaining existing data, and identifying background. Future sedimentary geologists will need to address these and related areas…

  18. Western cratonic domain in Uruguay: geochronology

    International Nuclear Information System (INIS)

    Preciozzi, F.; Peel, F.; Muzio, R.; Ledesma, J.; Guerequiz, R.

    2009-01-01

    In this article has been studied the Western cratonic in Uruguay are divided into three major units: Piedra Alta Terrane, Valentines Block and Pavas Terrane. Piedra Alta Terrane has of evidence of Neo proterozoic orogenesis . Sarandi del Yi -Arroyo Solis Grande shear zone separate, it from Valentine block . Valentine Block separate it from Pavas terrane by Cueva del Tigre shear zone

  19. Mesozoic(?) lithosphere-scale buckling of the East European Craton in southern Ukraine

    DEFF Research Database (Denmark)

    Starostenko, V.; Janik, T.; Lysynchuk, D.

    2013-01-01

    as yet revealed elsewhere by either deep reflection or refraction seismics. The presence of several detachment horizons in the folded crust calculated in the velocity model, is compatible with the existence of fold systems with various dominant wavelengths at different crustal levels. Such a situation...

  20. Rift propagation at craton margin.: Distribution of faulting and volcanism in the North Tanzanian Divergence (East Africa) during Neogene times

    Science.gov (United States)

    Le Gall, B.; Nonnotte, P.; Rolet, J.; Benoit, M.; Guillou, H.; Mousseau-Nonnotte, M.; Albaric, J.; Deverchère, J.

    2008-02-01

    A revised kinematic model is proposed for the Neogene tectono-magmatic development of the North Tanzanian Divergence where the axial valley in S Kenya splits southwards into a wide diverging pattern of block faulting in association with the disappearance of volcanism. Propagation of rifting along the S Kenya proto-rift during the last 8 Ma is first assumed to have operated by linkage of discrete magmatic cells as far S as the Ngorongoro-Kilimanjaro transverse volcanic belt that follows the margin of cratonic blocks in N Tanzania. Strain is believed to have nucleated throughout the thermally-weakened lithosphere in the transverse volcanic belt that might have later linked the S Kenya and N Tanzania rift segments with marked structural changes along-strike. The North Tanzanian Divergence is now regarded as a two-armed rift pattern involving: (1) a wide domain of tilted fault blocks to the W (Mbulu) that encompasses the Eyasi and Manyara fault systems, in direct continuation with the Natron northern trough. The reactivation of basement fabrics in the cold and intact Precambrian lithosphere in the Mbulu domain resulted in an oblique rift pattern that contrasts with the orthogonal extension that prevailed in the Magadi-Natron trough above a more attenuated lithosphere. (2) To the E, the Pangani horst-like range is thought to be a younger (< 1 Ma) structure that formed in response to the relocation of extension S of the Kilimanjaro magmatic center. A significant contrast in the mechanical behaviour of the stretched lithosphere in the North Tanzanian diverging rift is assumed to have occurred on both sides of the Masai cratonic block with a mid-crustal decoupling level to the W where asymmetrical fault-basin patterns are dominant (Magadi-Natron and Mbulu), whereas a component of dynamical uplift is suspected to have caused the topographic elevation of the Pangani range in relation with possible far-travelled mantle melts produced at depth further N.

  1. Lithospheric controls on magma composition along Earth's longest continental hotspot track.

    Science.gov (United States)

    Davies, D R; Rawlinson, N; Iaffaldano, G; Campbell, I H

    2015-09-24

    Hotspots are anomalous regions of volcanism at Earth's surface that show no obvious association with tectonic plate boundaries. Classic examples include the Hawaiian-Emperor chain and the Yellowstone-Snake River Plain province. The majority are believed to form as Earth's tectonic plates move over long-lived mantle plumes: buoyant upwellings that bring hot material from Earth's deep mantle to its surface. It has long been recognized that lithospheric thickness limits the rise height of plumes and, thereby, their minimum melting pressure. It should, therefore, have a controlling influence on the geochemistry of plume-related magmas, although unambiguous evidence of this has, so far, been lacking. Here we integrate observational constraints from surface geology, geochronology, plate-motion reconstructions, geochemistry and seismology to ascertain plume melting depths beneath Earth's longest continental hotspot track, a 2,000-kilometre-long track in eastern Australia that displays a record of volcanic activity between 33 and 9 million years ago, which we call the Cosgrove track. Our analyses highlight a strong correlation between lithospheric thickness and magma composition along this track, with: (1) standard basaltic compositions in regions where lithospheric thickness is less than 110 kilometres; (2) volcanic gaps in regions where lithospheric thickness exceeds 150 kilometres; and (3) low-volume, leucitite-bearing volcanism in regions of intermediate lithospheric thickness. Trace-element concentrations from samples along this track support the notion that these compositional variations result from different degrees of partial melting, which is controlled by the thickness of overlying lithosphere. Our results place the first observational constraints on the sub-continental melting depth of mantle plumes and provide direct evidence that lithospheric thickness has a dominant influence on the volume and chemical composition of plume-derived magmas.

  2. Extensive crustal melting during craton destruction: Evidence from the Mesozoic magmatic suite of Junan, eastern North China Craton

    Science.gov (United States)

    Yang, Fan; Santosh, M.; Tang, Li

    2018-05-01

    The cratonic destruction associated with the Pacific plate subduction beneath the eastern North China Craton (NCC) shows a close relationship with the widespread magmatism during the Late Mesozoic. Here we investigate a suite of intrusive and extrusive magmatic rocks from the Junan region of the eastern NCC in order to evaluate the role of extensive crustal melting related to decratonization. We present petrological, geochemical, zircon U-Pb geochronological and Lu-Hf isotopic data to evaluate the petrogenesis, timing and tectonic significance of the Early Cretaceous magmatism. Zircon grains in the basalt from the extrusive suite of Junan show multiple populations with Neoproterozoic and Early Paleozoic xenocrystic grains ranging in age from 764 Ma to 495 Ma as well as Jurassic grains with an age range of 189-165 Ma. The dominant population of magmatic zircon grains in the syenite defines three major age peaks of 772 Ma, 132 Ma and 126 Ma. Zircons in the granitoids including alkali syenite, monzonite and granodiorite yield a tightly restricted age range of 124-130 Ma representing their emplacement ages. The Neoproterozoic (841-547 Ma) zircon grains from the basalt and the syenite possess εHf(t) values of -22.9 to -8.4 and from -18.8 to -17.3, respectively. The Early Paleozoic (523-494 Ma) zircons from the basalt and the syenite also show markedly negative εHf(t) values of -22.7 to -18.0. The dominant population of Early Cretaceous (134-121 Ma) zircon grains presented in all the samples also displays negative εHf(t) values range from -31.7 to -21.1, with TDM of 1653-2017 Ma and TDMC in the range of 2193-3187 Ma. Accordingly, the Lu-Hf data suggest that the parent magma was sourced through melting of Mesoarchean to Paleoproterozoic basement rocks. Geochemical data on the Junan magmatic suite display features similar to those associated with the arc magmatic rocks involving subduction-related components, with interaction of fluids and melts in the suprasubduction

  3. Lithosphere erosion atop mantle plumes

    Science.gov (United States)

    Agrusta, R.; Arcay, D.; Tommasi, A.

    2012-12-01

    Mantle plumes are traditionally proposed to play an important role in lithosphere erosion. Seismic images beneath Hawaii and Cape Verde show a lithosphere-asthenosphere-boundary (LAB) up to 50 km shallower than the surroundings. However, numerical models show that unless the plate is stationary the thermo-mechanical erosion of the lithosphere does not exceed 30 km. We use 2D petrological-thermo-mechanical numerical models based on a finite-difference method on a staggered grid and marker in cell method to study the role of partial melting on the plume-lithosphere interaction. A homogeneous peridotite composition with a Newtonian temperature- and pressure-dependent viscosity is used to simulate both the plate and the convective mantle. A constant velocity, ranging from 5 to 12.5 cm/yr, is imposed at the top of the plate. Plumes are created by imposing a thermal anomaly of 150 to 350 K on a 50 km wide domain at the base of the model (700 km depth); the plate right above the thermal anomaly is 40 Myr old. Partial melting is modeled using batch-melting solidus and liquidus in anhydrous conditions. We model the progressive depletion of peridotite and its effect on partial melting by assuming that the melting degree only strictly increases through time. Melt is accumulated until a porosity threshold is reached and the melt in excess is then extracted. The rheology of the partially molten peridotite is determined using viscous constitutive relationship based on a contiguity model, which enables to take into account the effects of grain-scale melt distribution. Above a threshold of 1%, melt is instantaneously extracted. The density varies as a function of partial melting degree and extraction. Besides, we analyze the kinematics of the plume as it impacts a moving plate, the dynamics of time-dependent small-scale convection (SSC) instabilities developing in the low-viscosity layer formed by spreading of hot plume material at the lithosphere base, and the resulting thermal

  4. Earthquake rupture below the brittle-ductile transition in continental lithospheric mantle.

    Science.gov (United States)

    Prieto, Germán A; Froment, Bérénice; Yu, Chunquan; Poli, Piero; Abercrombie, Rachel

    2017-03-01

    Earthquakes deep in the continental lithosphere are rare and hard to interpret in our current understanding of temperature control on brittle failure. The recent lithospheric mantle earthquake with a moment magnitude of 4.8 at a depth of ~75 km in the Wyoming Craton was exceptionally well recorded and thus enabled us to probe the cause of these unusual earthquakes. On the basis of complete earthquake energy balance estimates using broadband waveforms and temperature estimates using surface heat flow and shear wave velocities, we argue that this earthquake occurred in response to ductile deformation at temperatures above 750°C. The high stress drop, low rupture velocity, and low radiation efficiency are all consistent with a dissipative mechanism. Our results imply that earthquake nucleation in the lithospheric mantle is not exclusively limited to the brittle regime; weakening mechanisms in the ductile regime can allow earthquakes to initiate and propagate. This finding has significant implications for understanding deep earthquake rupture mechanics and rheology of the continental lithosphere.

  5. Moho Depth Variations in the Northeastern North China Craton Revealed by Receiver Function Imaging

    Science.gov (United States)

    Zhang, P.; Chen, L.; Yao, H.; Fang, L.

    2016-12-01

    The North China Craton (NCC), one of the oldest cratons in the world, has attracted wide attention in Earth Science for decades because of the unusual Mesozoic destruction of its cratonic lithosphere. Understanding the deep processes and mechanism of this craton destruction demands detailed knowledge about the deep structure of the region. In this study, we used two-year teleseismic receiver function data from the North China Seismic Array consisting of 200 broadband stations deployed in the northeastern NCC to image the Moho undulation of the region. A 2-D wave equation-based poststack depth migration method was employed to construct the structural images along 19 profiles, and a pseudo 3D crustal velocity model of the region based on previous ambient noise tomography and receiver function study was adopted in the migration. We considered both the Ps and PpPs phases, but in some cases we also conducted PpSs+PsPs migration using different back azimuth ranges of the data, and calculated the travel times of all the considered phases to constrain the Moho depths. By combining the structure images along the 19 profiles, we got a high-resolution Moho depth map beneath the northeastern NCC. Our results broadly consist with the results of previous active source studies [http://www.craton.cn/data], and show a good correlation of the Moho depths with geological and tectonic features. Generally, the Moho depths are distinctly different on the opposite sides of the North-South Gravity Lineament. The Moho in the west are deeper than 40 km and shows a rapid uplift from 40 km to 30 km beneath the Taihang Mountain Range in the middle. To the east in the Bohai Bay Basin, the Moho further shallows to 30-26 km depth and undulates by 3 km, coinciding well with the depressions and uplifts inside the basin. The Moho depth beneath the Yin-Yan Mountains in the north gradually decreases from 42 km in the west to 25 km in the east, varying much smoother than that to the south.

  6. Low Seismic Attenuation in Southern New England Lithosphere Implies Little Heating by the Upwelling Asthenosphere

    Science.gov (United States)

    Lamoureux, J. M.; Menke, W. H.

    2017-12-01

    The Northern Appalachian Anomaly (NAA) is a patch of the asthenosphere in southern New England that is unusually hot given its passive margin setting. Previous research has detected large seismic wave delays that imply a temperature of 770 deg C higher than the mantle below the adjacent craton at the same depth. A key outstanding issue is whether the NAA interacts with the lithosphere above it (e.g. by heating it up). We study this issue using Po and So waves from two magnitude >5.5 earthquakes near the Puerto Rico Trench. These waves, propagating in the cold oceanic lithosphere at near Moho speeds, deliver high frequency energy to the shallow continental lithosphere. We hypothesized that: (1) once within the continental lithosphere, Po and So experience attenuation with distance that can be quantified by a quality factor Q, and that (2) any heating of the lithosphere above the NAA would lead to a higher Q than in regions further north or south along the continental margin. Corresponding Po and So velocities would also be lower. The decay rates of Po and So are estimated using least-squares applied to RMS coda amplitudes measured from digital seismograms from stations in northeastern North America, corrected for instrument response. A roughly log-linear decrease in amplitude is observed, corresponding to P and S wave quality factors in the range of 394-1500 and 727-6847, respectively. Measurements are made for four margin-perpendicular geographical bands, with one band overlapping the NAA. We detect no effect on these amplitudes by the NAA; 95% confidence bounds overlap in every case; Furthermore, all quality factors are much higher than the 100 predicted by lab experiments for near-solidus mantle rocks. These results suggest that the NAA is not causing significant heating of the lithosphere above it. The shear velocities, however, are about 10% slower above the NAA - an effect that may be fossil, reflecting processes that occurred millions of years ago.

  7. Non-uniform splitting of a single mantle plume by double cratonic roots : Insight into the origin of the central and southern East African Rift System

    NARCIS (Netherlands)

    Koptev, Alexander; Cloetingh, Sierd; Gerya, Taras; Calais, Eric; Leroy, Sylvie

    Using numerical thermo-mechanical experiments we analyse the role of an active mantle plume and pre-existing lithospheric thickness differences in the structural development of the central and southern East African Rift system. The plume-lithosphere interaction model setup captures the essential

  8. Neoproterozoic stratigraphic framework of the Tarim Craton in NW China: Implications for rift evolution

    Science.gov (United States)

    Wu, Lin; Guan, Shuwei; Zhang, Shuichang; Yang, Haijun; Jin, Jiuqiang; Zhang, Xiaodan; Zhang, Chunyu

    2018-06-01

    The Tarim Craton is overlain by thick Neoproterozoic sedimentary successions in rift tectonic setting. This study examines the latest outcrop, seismic, and drilling core data with the objective of investigating the regional stratigraphy to deeply recognize the evolution of rifting in the craton. Cryogenian to Lower Ediacaran successions are mainly composed of clastic rocks with thicknesses of 2000-3000 m, and the Upper Ediacaran successions are composed of carbonate rocks with thicknesses of 500-800 m. The rift basins and stratigraphic zones are divided into northern and southern parts by a central paleo-uplift. The northern rift basin extends through the northern Tarim Craton in an E-W direction with two depocenters (Aksu and Kuruktag). The southern rift basin is oriented NE-SW. There are three or four phases of tillites in the northern zone, while there are two in the southern zone. Given the north-south difference of the stratigraphic framework, the northern rift basin initiated at ca. 740 Ma and the southern rift basin initiated at ca. 780 Ma. During the Cryogenian and Ediacaran, the northern and southern rift basins were separated by the central paleo-uplift, finally connecting with each other in the early Cambrian. Tectonic deformation in the Late Ediacaran led to the formation of a parallel unconformity in the rift basins and an angular unconformity in the central paleo-uplift. The Neoproterozoic rift basins continued to affect the distribution of Lower Cambrian hydrocarbon source rocks. The north-south distribution and evolution of the rift basins in the Tarim Craton have implications for reconstructions of the Rodinia supercontinent.

  9. The helium flux from the continents and ubiquity of low-3He/4He recycled crust and lithosphere

    Science.gov (United States)

    Day, James M. D.; Barry, Peter H.; Hilton, David R.; Burgess, Ray; Pearson, D. Graham; Taylor, Lawrence A.

    2015-03-01

    New helium isotope and trace-element abundance data are reported for pyroxenites and eclogites from South Africa, Siberia, and the Beni Bousera Massif, Morocco that are widely interpreted to form from recycled oceanic crustal protoliths. The first He isotope data are also presented for Archaean peridotites from the Kaapvaal (South Africa), Slave (Canada), and Siberian cratons, along with recently emplaced off-craton peridotite xenoliths from Kilbourne Hole, San Carlos (USA) and Vitim (Siberia), to complement existing 3He/4He values obtained for continental and oceanic peridotites. Helium isotope compositions of peridotite xenoliths vary from 7.3 to 9.6 RA in recently (volcanics that contain a contribution from asthenospheric sources. Using the new He isotope data for cratonic peridotites and assuming that significant portions (>50%) of the Archaean and Proterozoic continental lithospheric mantle are stable and unaffected by melt or fluid infiltration on geological timescales (>0.1 Ga), and that U and Th contents vary between cratonic lithosphere and non-cratonic lithosphere, calculations yield a 3He flux of 0.25-2.2 atoms/s/cm2 for the continental lithospheric mantle. These estimates differ by a factor of ten from non-cratonic lithospheric mantle and are closer to the observed 3He flux from the continents (<1 atoms/s/cm2). Pyroxenites and eclogites from the continental regions are all characterized by 3He/4He (0.03-5.6 RA) less than the depleted upper mantle, and relatively high U and Th contents. Together with oceanic and continental lithospheric peridotites, these materials represent reservoirs with low time-integrated 3He/(U + Th) in the mantle. Pyroxenites and eclogites are also characterized by higher Fe/Mg, more radiogenic Os-Pb isotope compositions, and more variable δ18O values (∼3‰ to 7‰), compared with peridotitic mantle. These xenoliths are widely interpreted to be the metamorphic/metasomatic equivalents of recycled oceanic crustal protoliths. The

  10. (142)Nd evidence for an enriched Hadean reservoir in cratonic roots.

    Science.gov (United States)

    Upadhyay, Dewashish; Scherer, Erik E; Mezger, Klaus

    2009-06-25

    The isotope (146)Sm undergoes alpha-decay to (142)Nd, with a half-life of 103 million years. Measurable variations in the (142)Nd/(144)Nd values of rocks resulting from Sm-Nd fractionation could therefore only have been produced within about 400 million years of the Solar System's formation (that is, when (146)Sm was extant). The (142)Nd/(144)Nd compositions of terrestrial rocks are accordingly a sensitive monitor of the main silicate differentiation events that took place in the early Earth. High (142)Nd/(144)Nd values measured in some Archaean rocks from Greenland hint at the existence of an early incompatible-element-depleted mantle. Here we present measurements of low (142)Nd/(144)Nd values in 1.48-gigayear-(Gyr)-old lithospheric mantle-derived alkaline rocks from the Khariar nepheline syenite complex in southeastern India. These data suggest that a reservoir that was relatively enriched in incompatible elements formed at least 4.2 Gyr ago and traces of its isotopic signature persisted within the lithospheric root of the Bastar craton until at least 1.48 Gyr ago. These low (142)Nd/(144)Nd compositions may represent a diluted signature of a Hadean (4 to 4.57 Gyr ago) enriched reservoir that is characterized by even lower values. That no evidence of the early depleted mantle has been observed in rocks younger than 3.6 Gyr (refs 3, 4, 7) implies that such domains had effectively mixed back into the convecting mantle by then. In contrast, some early enriched components apparently escaped this fate. Thus, the mantle sampled by magmatism since 3.6 Gyr ago may be biased towards a depleted composition that would be balanced by relatively more enriched reservoirs that are 'hidden' in Hadean crust, the D'' layer of the lowermost mantle or, as we propose here, also within the roots of old cratons.

  11. On the Implications of A Priori Constraints in Transdimensional Bayesian Inversion for Continental Lithospheric Layering

    Science.gov (United States)

    Roy, C.; Romanowicz, B. A.

    2017-12-01

    Monte Carlo methods are powerful approaches to solve nonlinear problems and are becoming very popular in Earth sciences. One reason being that, at first glance, no constraints or explicit regularization of model parameters are required. At second glance, one might realize that regularization is done through a prior. The choice of this prior, however, is subjective, and with its choice, unintended or undesired extra information can be injected into the problem. The principal criticism of Bayesian methods is that the prior can be "tuned" in order to get the expected solution. Consequently, detractors of the Bayesian method could easily argue that the solution is influenced by the form of the prior distribution, which choice is subjective. Hence, models obtained with Monte Carlo methods are still highly debated. Here we investigate the influence of a priori constraints (i.e., fixed crustal discontinuities) on the posterior probability distributions of estimated parameters, that is, vertical polarized shear velocity VSV and radial anisotropy ξ, in a transdimensional Bayesian inversion for continental lithospheric structure. We follow upon the work of Calò et al. (2016), who jointly inverted converted phases (P to S) without deconvolution and surface wave dispersion data, to obtain 1-D radial anisotropic shear wave velocity profiles in the North American craton. We aim at verifying whether the strong lithospheric layering found in the stable part of the craton is robust with respect to artifacts that might be caused by the methodology used. We test the hypothesis that the observed midlithospheric discontinuities result from (1) fixed crustal discontinuities in the reference model and (2) a fixed Vp/Vs ratio. The synthetic tests on two Earth models show that a fixed Vp/Vs ratio does not introduce artificial layering, even if the assumed value is slightly wrong. This is an important finding for real data inversion where the true value is not always available or accurate

  12. The crustal thickness of Australia

    Science.gov (United States)

    Clitheroe, G.; Gudmundsson, O.; Kennett, B.L.N.

    2000-01-01

    We investigate the crustal structure of the Australian continent using the temporary broadband stations of the Skippy and Kimba projects and permanent broadband stations. We isolate near-receiver information, in the form of crustal P-to-S conversions, using the receiver function technique. Stacked receiver functions are inverted for S velocity structure using a Genetic Algorithm approach to Receiver Function Inversion (GARFI). From the resulting velocity models we are able to determine the Moho depth and to classify the width of the crust-mantle transition for 65 broadband stations. Using these results and 51 independent estimates of crustal thickness from refraction and reflection profiles, we present a new, improved, map of Moho depth for the Australian continent. The thinnest crust (25 km) occurs in the Archean Yilgarn Craton in Western Australia; the thickest crust (61 km) occurs in Proterozoic central Australia. The average crustal thickness is 38.8 km (standard deviation 6.2 km). Interpolation error estimates are made using kriging and fall into the range 2.5-7.0 km. We find generally good agreement between the depth to the seismologically defined Moho and xenolith-derived estimates of crustal thickness beneath northeastern Australia. However, beneath the Lachlan Fold Belt the estimates are not in agreement, and it is possible that the two techniques are mapping differing parts of a broad Moho transition zone. The Archean cratons of Western Australia appear to have remained largely stable since cratonization, reflected in only slight variation of Moho depth. The largely Proterozoic center of Australia shows relatively thicker crust overall as well as major Moho offsets. We see evidence of the margin of the contact between the Precambrian craton and the Tasman Orogen, referred to as the Tasman Line. Copyright 2000 by the American Geophysical Union.

  13. Multitaper spectral method to estimate the elastic thickness of South China: Implications for intracontinental deformation

    Directory of Open Access Journals (Sweden)

    Yangfan Deng

    2014-03-01

    Full Text Available The effective elastic thickness (Te represents the thickness of the elastic layer or the flexural rigidity of the lithosphere, the equivalent of which can be calculated from the spectral analysis of gravity and topographic data. Studies of Te have profound influence on intracontinental deformation, and coupling of the tectonic blocks. In this paper, we use the multitaper spectral estimation method to calculate the coherence between Bouguer gravity and topography data, and to obtain the Te map of South China. Through the process of correction, we discuss the relationships of Te versus heat flow, and Te versus seismicity. The results show that Te distribution of South China is affected by three factors: the original age, which controls the basic feature; the Mesozoic evolution, which affects the Te distribution; and the neotectonic movement, which shaped the final distribution. The crust age has a positive correlation with the first-order Te distribution; thus the Yangtze Craton has a relatively higher Te (about 50 km whereas the Te in Cathaysia block is only 10–20 km. By analysis and comparison among the tectonic models of South China, the Te distribution can be well explained using the flat-subduction model. As is typical with neotectonics, the region with a higher heat flow is related with a lower Te. The seismicity does not have a clear relationship with Te, but the strong seismicity could cause a low Te. Seismogenic layer (Ts has a similar trend as Te in the craton, whereas in other areas the relationship is complex.

  14. Did clockwise rotation of Antarctica cause the break-up of Gondwanaland? An investigation in the 'deep-keeled cratons' frame for global dynamics

    Science.gov (United States)

    Osmaston, M. F.

    2012-04-01

    considered a major agent in plate motion dynamics for the period during which East Antarctica, or any other sufficiently deep-keeled craton previously, was located at one of the Earth's poles. [1] Osmaston M. F. (2006) Global tectonic actions emanating from Arctic opening in the circumstances of a two-layer mantle and a thick-plate paradigm involving deep cratonic tectospheres: the Eurekan (Eocene) compressive motion of Greenland and other examples. In Proc. ICAM IV, 2003 (ed. R. Scott & D. Thurston). OCS Study MMS 2006-003, p.105-124: Also at: http://www.mms.gov/alaska/icam. [2] Osmaston M. (2005) Interrelationships between large-scale plate motions as indicators of mantle structure: new constraints on mantle modelling and compositional layout. In 3rd Workshop on "Earth's mantle composition, structure and phase transitions" St Malo, France. http://deep.earth.free.fr/participants.php. [3] Osmaston M. F. (2007) Cratonic keels and a two-layer mantle tested: mantle expulsion during Arabia-Russia closure linked to westward enlargement of the Black Sea, formation of the Western Alps and subduction of the Tyrrhenian (not the Ionian) Sea. XXIV IUGG, Session JSS 011, Abstr #2105 http://www.iugg2007perugia.it/webbook/. [4] Osmaston M. F. (2009) Deep cratonic keels and a 2-layer mantle? Tectonic basis for some far-reaching new insights on the dynamical properties of the Earth's mantle: example motions from Mediterranean, Atlantic-Arctic and India. Geophys. Res. Abstr. 11, EGU2009-6359 (Solicited). [5] Karato S. (1986) Does partial melting reduce the creep strength of the upper mantle? Nature 319, 309-310. [6] Hirth G. & Kohlstedt D. L. (1996) Water in the oceanic upper mantle: implication for rheology, melt extraction, and the evolution of the lithosphere. EPSL 144, 93-108. [7] Osmaston M. F. (2010) On the actual variety of plate dynamical mechanisms and how mantle evolution affected them through time, from core formation to the Indian collision. Geophys. Res. Abstr. 12, EGU2010

  15. Seismological Constraints on Lithospheric Evolution in the Appalachian Orogen

    Science.gov (United States)

    Fischer, K. M.; Hopper, E.; Hawman, R. B.; Wagner, L. S.

    2017-12-01

    Crust and mantle structures beneath the Appalachian orogen, recently resolved by seismic data from the EarthScope SESAME Flexible Array and Transportable Array, provide new constraints on the scale and style of the Appalachian collision and subsequent lithospheric evolution. In the southern Appalachians, imaging with Sp and Ps phases reveals the final (Alleghanian) suture between the crusts of Laurentia and the Gondwanan Suwannee terrane as a low angle (Kellogg, 2017) isostatic arguments indicate crustal thicknesses were 15-25 km larger at the end of the orogeny, indicating a thick crustal root across the region. The present-day residual crustal root beneath the Blue Ridge mountains is estimated to have a density contrast with the mantle of only 104±20 kg/m3. This value is comparable to other old orogens but lower than values typical of young or active orogens, indicating a loss of lower crustal buoyancy over time. At mantle depths, the negative shear velocity gradient that marks the transition from lithosphere to asthenosphere, as illuminated by Sp phases, varies across the Appalachian orogen. This boundary is shallow beneath the northeastern U.S. and in the zone of Eocene volcanism in Virginia, where low velocity anomalies occur in the upper mantle. These correlations suggest recent active lithosphere-asthenosphere interaction.

  16. Neoarchean granite-greenstone belts and related ore mineralization in the North China Craton: An overview

    Directory of Open Access Journals (Sweden)

    Li Tang

    2018-05-01

    process of oceanic slab subduction and mantle plume. The Neoarchean cratonization of the NCC appears to have involved two stages of tectonic process along the 2.75–2.6 Ga GGB and ∼2.5 Ga GGBs, the former involve plume–arc interaction process, and the latter involving oceanic lithospheric subduction, with or without arc-plume interaction. Keywords: North China Craton, Granite greenstone belt, Neoarchean, Plate and plume tectonics, Metallogeny

  17. Metasomatised ancient lithospheric mantle beneath the young Zealandia microcontinent and its role in HIMU-like intraplate magmatism

    DEFF Research Database (Denmark)

    Scott, James M; Waight, Tod Earle; van der Meer, Quinten

    2014-01-01

    There has been long debate on the asthenospheric versus lithospheric source for numerous intraplate basalts with ocean island basalt (OIB) and high time-integrated U/Pb (HIMU)-like source signatures that have erupted through the Zealandia continental crust. Analysis of 157 spinel facies peridotitic...... is highly heterogeneous. It is composed of a refractory craton-like domain (West Otago) adjacent to several moderately fertile domains (East Otago, North Otago, Auckland Islands). Each domain has an early history decoupled from the overlying Carboniferous and younger continental crust, and each domain has...

  18. Magnetic anomalies across Bastar craton and Pranhita–Godavari ...

    Indian Academy of Sciences (India)

    Such intrusions can be explained considering the collision of the Bastar and Dharwar cratons by the ... that there was no imprint of magnetization of a later date, it is concluded that the Indian plate was located in the .... swarms, that occur in this craton. Thus the .... b, c and d, needed to explain the anomalies along with the ...

  19. Lithospheric strucutre and relationship to seismicity beneath the Southeastern US using reciever functions

    Science.gov (United States)

    Cunningham, E.; Lekic, V.

    2017-12-01

    Despite being on a passive margin for millions of years, the Southeastern United States (SEUS) contains numerous seismogenic zones with the ability to produce damaging earthquakes. However, mechanisms controlling these intraplate earthquakes are poorly understood. Recently, Biryol et al. 2016 use P-wave tomography suggest that upper mantle structures beneath the SEUS correlate with areas of seismicity and seismic quiescence. Specifically, thick and fast velocity lithosphere beneath North Carolina is stable and indicative of areas of low seismicity. In contrast, thin and slow velocity lithosphere is weak, and the transition between the strong and weak lithosphere may be correlated with seismogenic zones found in the SEUS. (eg. Eastern Tennessee seismic zone and the Central Virginia seismic zone) Therefore, I systematically map the heterogeneity of the mantle lithosphere using converted seismic waves and quantify the spatial correlation between seismicity and lithospheric structure. The extensive network of seismometers that makes up the Earthscope USArray combined with the numerous seismic deployments in the Southeastern United States allows for unprecedented opportunity to map changes in lithospheric structure across seismogenic zones and seismic quiescent regions. To do so, I will use both P-to-s and S-to-p receiver functions (RFS). Since RFs are sensitive to seismic wavespeeds and density discontinuities with depth, they particularly useful for studying lithospheric structure. Ps receiver functions contain high frequency information allowing for high resolution, but can become contaminated by large sediment signals; therefore, I removed sediment multiples and correct for time delays of later phases using the method of Yu et. al 2015 which will allow us to see later arriving phases associated with lithospheric discontinuities. S-to-p receiver functions are not contaminated by shallow layers, making them ideal to study deep lithospheric structures but they can

  20. Formation and Evolution of the Continental Lithospheric Mantle: Perspectives From Radiogenic Isotopes of Silicate and Sulfide Inclusions in Macrodiamonds

    Science.gov (United States)

    Shirey, S. B.; Richardson, S. H.

    2007-12-01

    Silicate and sulfide inclusions that occur in diamonds comprise the oldest (>3 Ga), deepest (>140 km) samples of mantle-derived minerals available for study. Their relevance to the evolution of the continental lithosphere is clear because terrestrial macrodiamonds are confined to regions of the Earth with continental lithospheric mantle keels. The goals of analytical work on inclusions in diamond are to obtain paragenesis constraints, radiogenic ages, and initial isotopic compositions. The purpose is to place diamond formation episodes into the broader framework of the geological processes that create and modify the continental lithosphere and to relate the source of the C and N in diamond-forming fluids to understanding the Earth's C and N cycles in the Archean. Although sulfide and silicate inclusions rarely occur in the same diamond, they both can be grouped according to their geochemical similarity with the chief rock types that comprise the mantle keel: peridotite and eclogite. Silicate inclusions are classified as harzburgitic (depleted; olivine > Fo91, garnet Cr2O3 > 3 wt% and CaO from 0 to 5 wt%), lherzolitic (fertile), or eclogitic (basaltic; garnet Cr2O3 14 wt%; Os > 2 ppm) versus eclogitic (Ni bearing kimberlites, and the generosity of mining companies because of the extreme rarity of inclusions in suites of mostly gem-quality diamonds. Most isotopic work has been on the Kaapvaal-Zimbabwe craton with lesser work on the Slave, Siberian, and Australian cratons. Sm-Nd ages on silicate suites and Re-Os ages on sulfide suites confirm diamond formation from the Mesoarchean though the Neoproterozoic. Most important are the systematics across cratons in the context of crustal geology that lead to generalities about craton evolution. Inclusion suites date mantle keels as Mesoarchean and clearly point to subduction as the major process to form the earliest continental nuclei and to amalgamate the cratons in their present form. This is evident from the elevated

  1. APW path traced for the Guiana Shield (2070-1960 Ma) and Paleogeographic Implications: Paleomagnetic data from the 1.98-1.96 Ga Surumu Group (Northern Amazonian Craton)

    Science.gov (United States)

    Bispo-Santos, F.; Dagrella Filho, M. S.; Reis, N. J.; Trindade, R. I.

    2013-05-01

    Definition of continental paleogeography for times prior to formation of Columbia Supercontinent (1900-1850 Ma) is very complex, since amalgamation of some continental blocks of Earth was still in progress, as in the case of Laurentia, Baltica and Amazonian Craton. So, paleogeographic models proposed for this time are still very speculative and/or subjective. The use of the paleomagnetic technique tracing apparent polar wander (APW) paths for the various cratonic blocks can contribute to understand the continental amalgamation and breakup, especially for times where all created oceanic lithosphere was fully consumed. In this study, we present the paleomagnetic data obtained for samples collected from 39 sites from the well-dated 1980-1960 Ma (U-Pb) volcanic rocks belonging to the Surumu Group, cropping out in the northern Roraima State (Guiana Shield, Amazonian Craton). AF and thermal treatment revealed northwestern directions with moderate downward inclinations on samples from 20 out of the 39 analyzed sites. Site mean directions cluster around the mean, Dm = 298.6°; Im = 39.4° (N = 20; α95 = 10.1°), which yielded a key paleomagnetic pole (SG) for the Guiana Shield, located at 234.8°E, 27.4°N (A95 = 9.8°). Magnetic mineralogy experiments show that the magnetization of these rocks, probably of primary origin, is carried by magnetite and/or hematite. The SG pole contributes to a better fit of the APW path traced for Guiana Shield during the Paleoproterozoic (2070-1960 Ma). Comparison with the APW path traced for the West-Africa Craton for the same time interval suggests that these cratonic blocks were linked at 2000-1960 Ma ago, forming a paleogeography in which the Guri (Guiana Shield) and Sassandra (West-Africa Craton) shear zones were aligned as suggested in previous geologic models. KEYWORDS: Paleoproterozoic, Paleomagnetism, APWP, Amazonian Craton, Surumu Group.

  2. Interaction between mantle and crustal detachments: a non-linear system controlling lithospheric extension

    Science.gov (United States)

    Rosenbaum, G.; Regenauer-Lieb, K.; Weinberg, R. F.

    2009-12-01

    We use numerical modelling to investigate the development of crustal and mantle detachment faults during lithospheric extension. Our models simulate a wide range of rift systems with varying values of crustal thickness and heat flow, showing how strain localization in the mantle interacts with localization in the upper crust and controls the evolution of extensional systems. Model results reveal a richness of structures and deformation styles, which grow in response to a self-organized mechanism that minimizes the internal stored energy of the system by localizing deformation at different levels of the lithosphere. Crustal detachment faults are well developed during extension of overthickened (60 km) continental crust, even when the initial heat flow is relatively low (50 mW/m2). In contrast, localized mantle deformation is most pronounced when the extended lithosphere has a normal crustal thickness (30-40 km) and an intermediate (60-70 mW/m2) heat flow. Results show a non-linear response to subtle changes in crustal thickness or heat flow, characterized by abrupt and sometime unexpected switches in extension modes (e.g. from diffuse rifting to effective lithospheric-scale rupturing) or from mantle- to crust-dominated strain localization. We interpret this non-linearity to result from the interference of doming wavelengths. Disharmony of crust and mantle doming wavelengths results in efficient communication between shear zones at different lithospheric levels, leading to rupturing of the whole lithosphere. In contrast, harmonious crust and mantle doming inhibits interaction of shear zones across the lithosphere and results in a prolonged rifting history prior to continental breakup.

  3. Magma explains low estimates of lithospheric strength based on flexure of ocean island loads

    Science.gov (United States)

    Buck, W. Roger; Lavier, Luc L.; Choi, Eunseo

    2015-04-01

    One of the best ways to constrain the strength of the Earth's lithosphere is to measure the deformation caused by large, well-defined loads. The largest, simple vertical load is that of the Hawaiian volcanic island chain. An impressively detailed recent analysis of the 3D response to that load by Zhong and Watts (2013) considers the depth range of seismicity below Hawaii and the seismically determined geometry of lithospheric deflection. These authors find that the friction coefficient for the lithosphere must be in the normal range measured for rocks, but conclude that the ductile flow strength has to be far weaker than laboratory measurements suggest. Specifically, Zhong and Watts (2013) find that stress differences in the mantle lithosphere below the island chain are less than about 200 MPa. Standard rheologic models suggest that for the ~50 km thick lithosphere inferred to exist below Hawaii yielding will occur at stress differences of about 1 GPa. Here we suggest that magmatic accommodation of flexural extension may explain Hawaiian lithospheric deflection even with standard mantle flow laws. Flexural stresses are extensional in the deeper part of the lithosphere below a linear island load (i.e. horizontal stresses orthogonal to the line load are lower than vertical stresses). Magma can accommodate lithospheric extension at smaller stress differences than brittle and ductile rock yielding. Dikes opening parallel to an island chain would allow easier downflexing than a continuous plate, but wound not produce a freely broken plate. The extensional stress needed to open dikes at depth depends on the density contrast between magma and lithosphere, assuming magma has an open pathway to the surface. For a uniform lithospheric density ρL and magma density ρM the stress difference to allow dikes to accommodate extension is: Δσxx (z) = g z (ρM - gρL), where g is the acceleration of gravity and z is depth below the surface. For reasonable density values (i.e.

  4. Lithosphere, crust and basement ridges across Ganga and Indus basins and seismicity along the Himalayan front, India and Western Fold Belt, Pakistan

    Science.gov (United States)

    Ravi Kumar, M.; Mishra, D. C.; Singh, B.

    2013-10-01

    Spectral analysis of the digital data of the Bouguer anomaly of North India including Ganga basin suggest a four layer model with approximate depths of 140, 38, 16 and 7 km. They apparently represent lithosphere-asthenosphere boundary (LAB), Moho, lower crust, and maximum depth to the basement in foredeeps, respectively. The Airy's root model of Moho from the topographic data and modeling of Bouguer anomaly constrained from the available seismic information suggest changes in the lithospheric and crustal thicknesses from ˜126-134 and ˜32-35 km under the Central Ganga basin to ˜132 and ˜38 km towards the south and 163 and ˜40 km towards the north, respectively. It has clearly brought out the lithospheric flexure and related crustal bulge under the Ganga basin due to the Himalaya. Airy's root model and modeling along a profile (SE-NW) across the Indus basin and the Western Fold Belt (WFB), (Sibi Syntaxis, Pakistan) also suggest similar crustal bulge related to lithospheric flexure due to the WFB with crustal thickness of 33 km in the central part and 38 and 56 km towards the SE and the NW, respectively. It has also shown the high density lower crust and Bela ophiolite along the Chamman fault. The two flexures interact along the Western Syntaxis and Hazara seismic zone where several large/great earthquakes including 2005 Kashmir earthquake was reported. The residual Bouguer anomaly maps of the Indus and the Ganga basins have delineated several basement ridges whose interaction with the Himalaya and the WFB, respectively have caused seismic activity including some large/great earthquakes. Some significant ridges across the Indus basin are (i) Delhi-Lahore-Sargodha, (ii) Jaisalmer-Sibi Syntaxis which is highly seismogenic. and (iii) Kachchh-Karachi arc-Kirthar thrust leading to Sibi Syntaxis. Most of the basement ridges of the Ganga basin are oriented NE-SW that are as follows (i) Jaisalmer-Ganganagar and Jodhpur-Chandigarh ridges across the Ganga basin intersect

  5. Exploring Moho sharpness in Northeastern North China Craton with frequency-dependence analysis of Ps receiver function

    Science.gov (United States)

    Zhang, P.; Yao, H.; Chen, L.; WANG, X.; Fang, L.

    2017-12-01

    The North China Craton (NCC), one of the oldest cratons in the world, has attracted wide attention in Earth Science for decades because of the unusual Mesozoic destruction of its cratonic lithosphere. Understanding the deep processes and mechanism of this craton destruction demands detailed knowledge about the deep structure of this region. In this study, we calculate P-wave receiver functions (RFs) with two-year teleseismic records from the North China Seismic Array ( 200 stations) deployed in the northeastern NCC. We observe both diffused and concentered PpPs signals from the Moho in RF waveforms, which indicates heterogeneous Moho sharpness variations in the study region. Synthetic Ps phases generated from broad positive velocity gradients at the depth of the Moho (referred as Pms) show a clear frequency dependence nature, which in turn is required to constrain the sharpness of the velocity gradient. Practically, characterizing such a frequency dependence feature in real data is challenging, because of low signal-to-noise ratio, contaminations by multiples generated from shallow structure, distorted signal stacking especially in double-peak Pms signals, etc. We attempt to address these issues by, firstly, utilizing a high-resolution Moho depth model of this region to predict theoretical delay times of Pms that facilitate more accurate Pms identifications. The Moho depth model is derived by wave-equation based poststack depth migration on both Ps phase and surface-reflected multiples in RFs in our previous study (Zhang et al., submitted to JGR). Second, we select data from a major back azimuth range of 100° - 220° that includes 70% teleseismic events due to the uneven data coverage and to avoid azimuthal influence as well. Finally, we apply an adaptive cross-correlation stacking of Pms signals in RFs for each station within different frequency bands. High-quality Pms signals at different frequencies will be selected after careful visual inspection and adaptive

  6. Seismic crustal structure of the North China Craton and surrounding area: Synthesis and analysis

    Science.gov (United States)

    Xia, B.; Thybo, H.; Artemieva, I. M.

    2017-07-01

    We present a new digital model (NCcrust) of the seismic crustal structure of the Neoarchean North China Craton (NCC) and its surrounding Paleozoic-Mesozoic orogenic belts (30°-45°N, 100°-130°E). All available seismic profiles, complemented by receiver function interpretations of crustal thickness, are used to constrain a new comprehensive crustal model NCcrust. The model, presented on a 0.25° × 0.25°grid, includes the Moho depth and the internal structure (thickness and velocity) of the crust specified for four layers (the sedimentary cover, upper, middle, and lower crust) and the Pn velocity in the uppermost mantle. The crust is thin (30-32 km) in the east, while the Moho depth in the western part of the NCC is 38-44 km. The Moho depth of the Sulu-Dabie-Qinling-Qilian orogenic belt ranges from 31 km to 51 km, with a general westward increase in crustal thickness. The sedimentary cover is 2-5 km thick in most of the region, and typical thicknesses of the upper crust, middle crust, and lower crust are 16-24 km, 6-24 km, and 0-6 km, respectively. We document a general trend of westward increase in the thickness of all crustal layers of the crystalline basement and as a consequence, the depth of the Moho. There is no systematic regional pattern in the average crustal Vp velocity and the Pn velocity. We examine correlation between the Moho depth and topography for seven tectonic provinces in the North China Craton and speculate on mechanisms of isostatic compensation.

  7. Western cratonic domains in Uruguay: geochronology

    International Nuclear Information System (INIS)

    Preciozzi, F.; Peel, E.; Muzio, E.; Ledesma, R.; Guerequiz, R.

    2001-01-01

    The western cratonic domains in Uruguay are divided into three major units: Piedra Alta Terrane, Valentines Block and Pavas Block. Piedra Alta Terrane lacks of evidence of Neoproterozoic orogenesis (deformation, metamorphism or magmatism). Sarandí del Yi - Arroyo Solís Grande shear zone, separates it from Valentines Block. Valentines Block is separated from Pavas Block by Cueva del Tigre shear zone. Magmatic rocks with different ages, compositions and emplacements occur all over the Piedra Alta Terrane distributed in three metamorphic belts (Arroyo Grande, San José and Montevideo) as well as in the Central Gneissic-Migmatitic Complex (Figure 1). Samples from the Gneissic-Migmatitic complex, late tectonic granitoids and basic rocks associated to the metamorphic belts were analyzed using Rb/Sr, U/Pb, K/Ar and Sm/Nd methodologies. The age ranges obtained for granitoids

  8. The lithosphere-asthenosphere system in the Calabrian Arc and surrounding seas

    Energy Technology Data Exchange (ETDEWEB)

    Panza, G F [Department of Earth Sciences, University of Trieste, Trieste (Italy); [Abdus Salam International Centre for Theoretical Physics, SAND Group, Trieste (Italy)]. E-mail: panza@dst.univ.trieste.it; Pontevivo, A [Department of Earth Sciences, University of Trieste, Trieste (Italy)

    2002-10-01

    Through the non-linear inversion of Surface-Wave Tomography data, using as a priori constraints seismic data from literature, it has been possible to define a fairly detailed structural model of the lithosphere-asthenosphere system (thickness, S-wave and P-wave velocities of the crust and of the upper mantle layers) in the Calabrian Arc region (Southern Tyrrhenian Sea, Calabria and the Northern-Western part of the Ionian Sea). The main features identified by our study are: (1) a very shallow (less then 10 km deep) crust-mantle transition in the Southern Tyrrhenian Sea and very low S-wave velocities just below a very thin lid in correspondence of the submarine volcanic bodies in the study area; (2) a shallow and very low S-wave velocity layer in the mantle in the areas of Aeolian islands, of Vesuvius, Ischia and Phlegraean Fields, representing their shallow-mantle magma source; (3) a thickened continental crust and lithospheric doubling in Calabria; (4) a crust about 25 km thick and a mantle velocity profile versus depth consistent with the presence of a continental rifled, now thermally relaxed, lithosphere in the investigated part of the Ionian Sea; (5) the subduction of the Ionian lithosphere towards NW below the Tyrrhenian Basin; (6) the subduction of the Adriatic lithosphere underneath the Vesuvius and Phlegraean Fields. (author)

  9. Preservation of an Archaean whole rock Re-Os isochron for the Venetia lithospheric mantle: Evidence for rapid crustal recycling and lithosphere stabilisation at 3.3 Ga

    Science.gov (United States)

    van der Meer, Quinten H. A.; Klaver, Martijn; Reisberg, Laurie; Riches, Amy J. V.; Davies, Gareth R.

    2017-11-01

    Re-Os and platinum group element analyses are reported for peridotite xenoliths from the 533 Ma Venetia kimberlite cluster situated in the Limpopo Mobile Belt, the Neoarchaean collision zone between the Kaapvaal and Zimbabwe Cratons. The Venetian xenoliths provide a rare opportunity to examine the state of the cratonic lithosphere prior to major regional metasomatic disturbance of Re-Os systematics throughout the Phanerozoic. The 32 studied xenoliths record Si-enrichment that is characteristic of the Kaapvaal lithospheric mantle and can be subdivided into five groups based on Re-Os analyses. The most pristine group I samples (n = 13) display an approximately isochronous relationship and fall on a 3.28 ± 0.17 Ga (95 % conf. int.) reference line that is based on their mean TMA age. This age overlaps with the formation age of the Limpopo crust at 3.35-3.28 Ga. The group I samples derive from ∼50 to ∼170 km depth, suggesting coeval melt depletion of the majority of the Venetia lithospheric mantle column. Group II and III samples have elevated Re/Os due to Re addition during kimberlite magmatism. Group II has otherwise undergone a similar evolution as the group I samples with overlapping 187Os/188Os at eruption age: 187Os/188OsEA, while group III samples have low Os concentrations, unradiogenic 187Os/188OsEA and were effectively Re-free prior to kimberlite magmatism. The other sample groups (IV and V) have disturbed Re-Os systematics and provide no reliable age information. A strong positive correlation is recorded between Os and Re concentrations for group I samples, which is extended to groups II and III after correction for kimberlite addition. This positive correlation precludes a single stage melt depletion history and indicates coupled remobilisation of Re and Os. The combination of Re-Os mobility, preservation of the isochronous relationship, correlation of 187Os/188Os with degree of melt depletion and lack of radiogenic Os addition puts tight constraints on

  10. Geological evolution of the Antongil Craton, NE Madagascar

    Science.gov (United States)

    Schofield, D.I.; Thomas, Ronald J.; Goodenough, K.M.; De Waele, B.; Pitfield, P.E.J.; Key, R.M.; Bauer, W.; Walsh, G.J.; Lidke, D.J.; Ralison, A.V.; Rabarimanana, M.; Rafahatelo, J.-M.; Randriamananjara, T.

    2010-01-01

    The Antongil Craton, along with the Masora and Antananarivo cratons, make up the fundamental Archaean building blocks of the island of Madagascar. They were juxtaposed during the late-Neoproterozoic to early Palaeozoic assembly of Gondwana. In this paper we give a synthesis of the geology of the Antongil Craton and present previously published and new geochemical and U-Pb zircon analyses to provide an event history for its evolution.The oldest rocks in the Antongil Craton form a nucleus of tonalitic gneiss, characteristic of Palaeo-Mesoarchaean cratons globally, including phases dated between 3320 ?? 14. Ma to 3231 ?? 6. Ma and 3187 ?? 2. Ma to 3154 ?? 5. Ma. A series of mafic dykes was intruded into the Mesoarchaean tonalites and a sedimentary succession was deposited on the craton prior to pervasive deformation and migmatisation of the region. The age of deposition of the metasediments has been constrained from a volcanic horizon to around 3178 ?? 2. Ma and subject to migmatisation at around 2597 ?? 49. Ma. A subsequent magmatic episode generated voluminous, weakly foliated granitic rocks, that also included additions from both reworked older crustal material and younger source components. An earlier granodiorite-dominated assemblage, dated between 2570 ?? 18. Ma and 2542 ?? 5. Ma, is largely exposed in xenoliths and more continuously in the northern part of the craton, while a later monzogranite-dominated phase, dated between 2531 ?? 13. Ma and 2513 ?? 0.4. Ma is more widely developed. Together these record the stabilisation of the craton, attested to by the intrusion of a younger dyke swarm, the age of which is constrained by a sample of metagabbro dated at 2147 ?? 6. Ma, providing the first evidence for Palaeoproterozoic rocks from the Antongil Craton.The youngest events recorded in the isotopic record of the Antongil Craton are reflected in metamorphism, neocrystallisation and Pb-loss at 792 ?? 130. Ma to 763 ?? 13. Ma and 553 ?? 68. Ma. These events are

  11. Interaction between mantle and crustal detachments: A nonlinear system controlling lithospheric extension

    Science.gov (United States)

    Rosenbaum, Gideon; Regenauer-Lieb, Klaus; Weinberg, Roberto F.

    2010-11-01

    We use numerical modeling to investigate the development of crustal and mantle detachments during lithospheric extension. Our models simulate a wide range of extensional systems with varying values of crustal thickness and heat flow, showing how strain localization in the mantle interacts with localization in the upper crust and controls the evolution of extensional systems. Model results reveal a richness of structures and deformation styles as a response to a self-organized mechanism that minimizes the internal stored energy of the system by localizing deformation. Crustal detachments, here referred as low-angle normal decoupling horizons, are well developed during extension of overthickened (60 km) continental crust, even when the initial heat flow is relatively low (50 mW m-2). In contrast, localized mantle deformation is most pronounced when the extended lithosphere has a normal crustal thickness (30-40 km) and an intermediate heat flow (60-70 mW m-2). Results show a nonlinear response to subtle changes in crustal thickness or heat flow, characterized by abrupt and sometimes unexpected switches in extension modes (e.g., from diffuse extensional deformation to effective lithospheric-scale rupturing) or from mantle- to crust-dominated strain localization. We interpret this nonlinearity to result from the interference of doming wavelengths in the presence of multiple necking instabilities. Disharmonic crust and mantle doming wavelengths results in efficient communication between shear zones at different lithospheric levels, leading to rupturing of the whole lithosphere. In contrast, harmonic crust and mantle doming inhibits interaction of shear zones across the lithosphere and results in a prolonged history of extension prior to continental breakup.

  12. Inclusions in diamonds constrain thermo-chemical conditions during Mesozoic metasomatism of the Kaapvaal cratonic mantle

    Science.gov (United States)

    Weiss, Yaakov; Navon, Oded; Goldstein, Steven L.; Harris, Jeff W.

    2018-06-01

    ) differs as well. The fO2 calculated for the saline HDF compositions (Δlog ⁡ fO 2 (FMQ) = - 2.47 to -1.34) are higher by about a log unit compared with that recorded by xenoliths at 4-7 GPa. We conclude that enriched saline HDFs mediated the metasomatism that preceded Group I kimberlite eruptions in the southwestern Kaapvaal craton, and that their 'cold and oxidized' nature reflects their derivation from a deep subducting slab. This event had little impact on the temperature and redox state of the Kaapvaal lithosphere as a reservoir, however, it likely affected its properties along limited metasomatized veins and their wall rock. To reconcile the temperature and oxygen fugacity discrepancy between inclusions in diamonds and xenoliths, we argue that xenoliths did not equilibrate during the last saline metasomatic event or kimberlite eruption. Thus the P-T- fO2 gradients they record express pre-existing lithospheric conditions that were likely established during the last major thermal event in the Kaapvaal craton (i.e. the Karoo magmatism at ca. 180 Ma).

  13. Constraining the dynamic response of subcontinental lithospheric mantle to rifting using Re-Os model ages in the Western Ross Sea, Antarctica

    Science.gov (United States)

    Doherty, C.; Class, C.; Goldstein, S. L.; Shirey, S. B.; Martin, A. P.; Cooper, A. F.; Berg, J. H.; Gamble, J. A.

    2012-12-01

    In order to understand the dynamic response of the subcontinental lithospheric mantle (SCLM) to rifting, it is important to be able to distinguish the geochemical signatures of SCLM vs. asthenosphere. Recent work demonstrates that unradiogenic Os isotope ratios can indicate old depletion events in the convecting upper mantle (e.g. Rudnick & Walker, 2009), and allow us to make these distinctions. Thus, if SCLM can be traced across a rifted margin, its fate during rifting can be established. The Western Ross Sea provides favorable conditions to test the dynamic response of SCLM to rifting. Re-Os measurements from 8 locations extending from the rift shoulder to 200 km into the rift basin reveal 187Os/188Os ranging from 0.1056 at Foster Crater on the shoulder, to 0.1265 on Ross Island within the rift. While individual sample model ages vary widely throughout the margin, 'aluminochron' ages (Reisberg & Lorand, 1995) reveal a narrower range of lithospheric stabilization ages. Franklin Island and Sulfur Cones show a range of Re-depletion ages (603-1522 Ma and 436-1497 Ma) but aluminochrons yield Paleoproterozoic stabilization ages of 1680 Ma and 1789 Ma, respectively. These ages coincide with U-Pb zircon ages from Transantarctic Mountain (TAM) crustal rocks, in support of SCLM stabilization at the time of crust formation along the central TAM. The Paleoproterozoic stabilization age recorded at Franklin Island is especially significant, since it lies 200km off of the rift shoulder. The similar ages beneath the rift shoulder and within the rift suggests stretched SCLM reaches into the rift and thus precludes replacement by asthenospheric mantle. The persistence of thinned Paleoproterozoic SCLM into the rifted zone in WARS suggests that it represents a 'type I' margin of Huismans and Beaumont (2011), which is characterized by crustal breakup before loss of lithospheric mantle. The Archean Re-depletion age of 3.2 Ga observed on the rift shoulder suggests that cratonic

  14. Continents as lithological icebergs: The importance of buoyant lithospheric roots

    Science.gov (United States)

    Abbott, D.H.; Drury, R.; Mooney, W.D.

    1997-01-01

    An understanding of the formation of new continental crust provides an important guide to locating the oldest terrestrial rocks and minerals. We evaluated the crustal thicknesses of the thinnest stable continental crust and of an unsubductable oceanic plateau and used the resulting data to estimate the amount of mantle melting which produces permanent continental crust. The lithospheric mantle is sufficiently depleted to produce permanent buoyancy (i.e., the crust is unsubductable) at crustal thicknesses greater than 25-27 km. These unsubductable oceanic plateaus and hotspot island chains are important sources of new continental crust. The newest continental crust (e.g., the Ontong Java plateau) has a basaltic composition, not a granitic one. The observed structure and geochemistry of continents are the result of convergent margin magmatism and metamorphism which modify the nascent basaltic crust into a lowermost basaltic layer overlain by a more silicic upper crust. The definition of a continent should imply only that the lithosphere is unsubductable over ??? 0.25 Ga time periods. Therefore, the search for the oldest crustal rocks should include rocks from lower to mid-crustal levels.

  15. Lithospheric deformation inferred from electrical anisotropy of magnetotelluric data

    Science.gov (United States)

    Yin, Y.; Wei, W.; Jin, S.; Ye, G.; Unsworth, M. J.; Zhang, L.

    2013-12-01

    In our research, a comprehensive procedure of analyzing and modeling electrical anisotropy for MT data is suggested, based on the field examples of the Great Slave Lake shear zone (GSLsz) in western Canada, the North China Craton (NCC) and the Altyn Tagh fault in northern Tibet. Diverse dimensionality tools are used to distinguish heterogeneity and anisotropy from MT data. In addition to the phase splits and phase tensor polarizations, a combination of the phase tensor and induction arrows is applied to judge anisotropy. The skin depths of specific period band are considered to determine whether these features result from anisotropy or heterogeneity. Specific resistivity structures in the 2-D isotropic inversion models can indicate electrical anisotropy as well, like the dike-like media or a series of conductive ';blobs' can be observed in the 2-D isotropic inversion models of the GSLsz and NCC data. Anisotropic inversions can be undertaken using an improved inversion code based on isotropic code but incorporating a trade-off parameter for electrical anisotropy named anisotropic tau. A series of anisotropic tau have been applied to test its effect and to get a best trade-off between anisotropy and heterogeneity. Then, 2-D and 3-D forward modeling works are undertaken to test the robustness of the major anisotropic features. The anisotropic structures inferred from the inversion models are replaced by various alternating isotropic or anisotropic structures to see if they are required. The fitting of the response curves compared with the field data and corresponding r.m.s misfits can help us choose the best model that can generally illustrate the underground structure. Finally, the analysis and modeling result of the MT data from North China Craton is taken as an example to demonstrate how the electrical anisotropy can be linked with the lithospheric deformation. According to the reliable models we got, there may be an anisotropic layer at the mid-lower crustal to

  16. Linking plate reconstructions with deforming lithosphere to geodynamic models

    Science.gov (United States)

    Müller, R. D.; Gurnis, M.; Flament, N.; Seton, M.; Spasojevic, S.; Williams, S.; Zahirovic, S.

    2011-12-01

    While global computational models are rapidly advancing in terms of their capabilities, there is an increasing need for assimilating observations into these models and/or ground-truthing model outputs. The open-source and platform independent GPlates software fills this gap. It was originally conceived as a tool to interactively visualize and manipulate classical rigid plate reconstructions and represent them as time-dependent topological networks of editable plate boundaries. The user can export time-dependent plate velocity meshes that can be used either to define initial surface boundary conditions for geodynamic models or alternatively impose plate motions throughout a geodynamic model run. However, tectonic plates are not rigid, and neglecting plate deformation, especially that of the edges of overriding plates, can result in significant misplacing of plate boundaries through time. A new, substantially re-engineered version of GPlates is now being developed that allows an embedding of deforming plates into topological plate boundary networks. We use geophysical and geological data to define the limit between rigid and deforming areas, and the deformation history of non-rigid blocks. The velocity field predicted by these reconstructions can then be used as a time-dependent surface boundary condition in regional or global 3-D geodynamic models, or alternatively as an initial boundary condition for a particular plate configuration at a given time. For time-dependent models with imposed plate motions (e.g. using CitcomS) we incorporate the continental lithosphere by embedding compositionally distinct crust and continental lithosphere within the thermal lithosphere. We define three isostatic columns of different thickness and buoyancy based on the tectonothermal age of the continents: Archean, Proterozoic and Phanerozoic. In the fourth isostatic column, the oceans, the thickness of the thermal lithosphere is assimilated using a half-space cooling model. We also

  17. Crustal thickness controlled by plate tectonics

    DEFF Research Database (Denmark)

    Artemieva, Irina M.; Meissner, Rolf

    2012-01-01

    /gabbro–eclogite phase transition in crustal evolution and the links between lithosphere recycling, mafic magmatism, and crustal underplating. We advocate that plate tectonics processes, togetherwith basalt/gabbro–eclogite transition, limit crustal thickness worldwide by providing effective mechanisms of crustal...

  18. Estimation of Water Within the Lithospheric Mantle of Central Tibet from Petrological-Geophysical Investigations

    Science.gov (United States)

    Vozar, J.; Fullea, J.; Jones, A. G.

    2013-12-01

    Investigations of the lithosphere and sub-lithospheric upper mantle by integrated petrological-geophysical modeling of magnetotelluric (MT) and seismic surface-wave data, which are differently sensitive to temperature and composition, allows us to reduce the uncertainties associated with modeling these two data sets independently, as commonly undertaken. We use selected INDEPTH MT data, which have appropriate dimensionality and large penetration depths, across central Tibet for 1D modeling. Our deep resistivity models from the data can be classified into two different and distinct groups: (i) the Lhasa Terrane and (ii) the Qiangtang Terrane. For the Lhasa Terrane group, the models show the existence of upper mantle conductive layer localized at depths of 200 km, whereas for the Qiangtang Terrane, this conductive layer is shallower at depths of 120 km. We perform the integrated geophysical-petrological modeling of the MT and surface-wave data using the software package LitMod. The program facilitates definition of realistic temperature and pressure distributions within the upper mantle for given thermal structure and oxide chemistry in the CFMAS system. This allows us to define a bulk geoelectric and seismic model of the upper mantle based on laboratory and xenolith data for the most relevant mantle minerals, and to compute synthetic geophysical observables. Our results suggest an 80-120 km-thick, dry lithosphere in the central part of the Qiangtang Terrane. In contrast, in the central Lhasa Terrane the predicted MT responses are too resistive for a dry lithosphere regardless its thickness; according to seismic and topography data the expected lithospheric thickness is about 200 km. The presence of small amounts of water significantly decreases the electrical resistivity of mantle rocks and is required to fit the MT responses. We test the hypothesis of small amounts of water (ppm scale) in the nominally anhydrous minerals of the lithospheric mantle. Such a small

  19. Numerical modeling of convective erosion and peridotite-melt interaction in big mantle wedge: Implications for the destruction of the North China Craton

    Science.gov (United States)

    He, Lijuan

    2014-04-01

    The deep subduction of the Pacific Plate underneath East Asia is thought to have played a key role in the destruction of the North China Craton (NCC). To test this hypothesis, this paper presents a new 2-D model that includes an initial stable equilibrated craton, the formation of a big mantle wedge (BMW), and erosion by vigorous mantle convection. The model shows that subduction alone cannot thin the cold solid craton, but it can form a low-viscosity BMW. The amount of convective erosion is directly proportional to viscosity within the BMW (η0bmw), and the rheological boundary layer thins linearly with decreasing log10(η0bmw), thereby contributing to an increase in heat flow at the lithospheric base. This model also differs from previous modeling in that the increase in heat flow decays linearly with t1/2, meaning that the overall thinning closely follows a natural log relationship over time. Nevertheless, convection alone can only cause a limited thinning due to a minor increase in basal heat flow. The lowering of melting temperature by peridotite-melt interaction can accelerate thinning during the early stages of this convection. The two combined actions can thin the craton significantly over tens of Myr. This modeling, combined with magmatism and heat flow data, indicates that the NCC evolution has involved four distinct stages: modification in the Jurassic by Pacific Plate subduction and BMW formation, destruction during the Early Cretaceous under combined convective erosion and peridotite-melt interaction, extension in the Late Cretaceous, and cooling since the late Cenozoic.

  20. Using natural laboratories and modeling to decipher lithospheric rheology

    Science.gov (United States)

    Sobolev, Stephan

    2013-04-01

    Rheology is obviously important for geodynamic modeling but at the same time rheological parameters appear to be least constrained. Laboratory experiments give rather large ranges of rheological parameters and their scaling to nature is not entirely clear. Therefore finding rheological proxies in nature is very important. One way to do that is finding appropriate values of rheological parameter by fitting models to the lithospheric structure in the highly deformed regions where lithospheric structure and geologic evolution is well constrained. Here I will present two examples of such studies at plate boundaries. One case is the Dead Sea Transform (DST) that comprises a boundary between African and Arabian plates. During the last 15- 20 Myr more than 100 km of left lateral transform displacement has been accumulated on the DST and about 10 km thick Dead Sea Basin (DSB) was formed in the central part of the DST. Lithospheric structure and geological evolution of DST and DSB is rather well constrained by a number of interdisciplinary projects including DESERT and DESIRE projects leaded by the GFZ Potsdam. Detailed observations reveal apparently contradictory picture. From one hand widespread igneous activity, especially in the last 5 Myr, thin (60-80 km) lithosphere constrained from seismic data and absence of seismicity below the Moho, seem to be quite natural for this tectonically active plate boundary. However, surface heat flow of less than 50-60mW/m2 and deep seismicity in the lower crust ( deeper than 20 km) reported for this region are apparently inconsistent with the tectonic settings specific for an active continental plate boundary and with the crustal structure of the DSB. To address these inconsistencies which comprise what I call the "DST heat-flow paradox", a 3D numerical thermo-mechanical model was developed operating with non-linear elasto-visco-plastic rheology of the lithosphere. Results of the numerical experiments show that the entire set of

  1. Reworking of Archean mantle in the NE Siberian craton by carbonatite and silicate melt metasomatism: Evidence from a carbonate-bearing, dunite-to-websterite xenolith suite from the Obnazhennaya kimberlite

    Science.gov (United States)

    Ionov, Dmitri A.; Doucet, Luc S.; Xu, Yigang; Golovin, Alexander V.; Oleinikov, Oleg B.

    2018-03-01

    The Obnazhennaya kimberlite in the NE Siberian craton hosts a most unusual cratonic xenolith suite, with common rocks rich in pyroxenes and garnet, and no sheared peridotites. We report petrographic and chemical data for whole rocks (WR) and minerals of 20 spinel and garnet peridotites from Obnazhennaya with Re-depletion Os isotope ages of 1.8-2.9 Ga (Ionov et al., 2015a) as well as 2 pyroxenites. The garnet-bearing rocks equilibrated at 1.6-2.8 GPa and 710-1050 °C. Some xenoliths contain vermicular spinel-pyroxene aggregates with REE patterns in clinopyroxene mimicking those of garnet. The peridotites show significant scatter of Mg# (0.888-0.924), Cr2O3 (0.2-1.4 wt.%) and high NiO (0.3-0.4 wt.%). None are pristine melting residues. Low-CaO-Al2O3 (≤0.9 wt.%) dunites and harzburgites are melt-channel materials. Peridotites with low to moderate Al2O3 (0.4-1.8 wt.%) usually have CaO > Al2O3, and some have pockets of calcite texturally equilibrated with olivine and garnet. Such carbonates, exceptional in mantle xenoliths and reported here for the first time for the Siberian mantle, provide direct evidence for modal makeover and Ca and LREE enrichments by ephemeral carbonate-rich melts. Peridotites rich in CaO and Al2O3 (2.7-8.0 wt.%) formed by reaction with silicate melts. We infer that the mantle lithosphere beneath Obnazhennaya, initially formed in the Mesoarchean, has been profoundly modified. Pervasive inter-granular percolation of highly mobile and reactive carbonate-rich liquids may have reduced the strength of the mantle lithosphere leading the way for reworking by silicate melts. The latest events before the kimberlite eruption were the formation of the carbonate-phlogopite pockets, fine-grained pyroxenite veins and spinel-pyroxene symplectites. The reworked lithospheric sections are preserved at Obnazhennaya, but similar processes could erode lithospheric roots in the SE Siberian craton (Tok) and the North China craton, where ancient melting residues and

  2. Intracratonic asthenosphere upwelling and lithosphere rejuvenation beneath the Hoggar swell (Algeria): Evidence from HIMU metasomatised lherzolite mantle xenoliths

    Science.gov (United States)

    Beccaluva, L.; Azzouni-Sekkal, A.; Benhallou, A.; Bianchini, G.; Ellam, R. M.; Marzola, M.; Siena, F.; Stuart, F. M.

    2007-08-01

    The mantle xenoliths included in Quaternary alkaline volcanics from the Manzaz-district (Central Hoggar) are proto-granular, anhydrous spinel lherzolites. Major and trace element analyses on bulk rocks and constituent mineral phases show that the primary compositions are widely overprinted by metasomatic processes. Trace element modelling of the metasomatised clinopyroxenes allows the inference that the metasomatic agents that enriched the lithospheric mantle were highly alkaline carbonate-rich melts such as nephelinites/melilitites (or as extreme silico-carbonatites). These metasomatic agents were characterized by a clear HIMU Sr-Nd-Pb isotopic signature, whereas there is no evidence of EM1 components recorded by the Hoggar Oligocene tholeiitic basalts. This can be interpreted as being due to replacement of the older cratonic lithospheric mantle, from which tholeiites generated, by asthenospheric upwelling dominated by the presence of an HIMU signature. Accordingly, this rejuvenated lithosphere (accreted asthenosphere without any EM influence), may represent an appropriate mantle section from which deep alkaline basic melts could have been generated and shallower mantle xenoliths sampled, respectively. The available data on lherzolite xenoliths and alkaline lavas (including He isotopes, Ra Pan-African basement. This can be considered a far-field foreland reaction of the Africa-Europe collisional system since the Eocene.

  3. Geodynamic inversion to constrain the non-linear rheology of the lithosphere

    Science.gov (United States)

    Baumann, T. S.; Kaus, Boris J. P.

    2015-08-01

    One of the main methods to determine the strength of the lithosphere is by estimating it's effective elastic thickness. This method assumes that the lithosphere is a thin elastic plate that floats on the mantle and uses both topography and gravity anomalies to estimate the plate thickness. Whereas this seems to work well for oceanic plates, it has given controversial results in continental collision zones. For most of these locations, additional geophysical data sets such as receiver functions and seismic tomography exist that constrain the geometry of the lithosphere and often show that it is rather complex. Yet, lithospheric geometry by itself is insufficient to understand the dynamics of the lithosphere as this also requires knowledge of the rheology of the lithosphere. Laboratory experiments suggest that rocks deform in a viscous manner if temperatures are high and stresses low, or in a plastic/brittle manner if the yield stress is exceeded. Yet, the experimental results show significant variability between various rock types and there are large uncertainties in extrapolating laboratory values to nature, which leaves room for speculation. An independent method is thus required to better understand the rheology and dynamics of the lithosphere in collision zones. The goal of this paper is to discuss such an approach. Our method relies on performing numerical thermomechanical forward models of the present-day lithosphere with an initial geometry that is constructed from geophysical data sets. We employ experimentally determined creep-laws for the various parts of the lithosphere, but assume that the parameters of these creep-laws as well as the temperature structure of the lithosphere are uncertain. This is used as a priori information to formulate a Bayesian inverse problem that employs topography, gravity, horizontal and vertical surface velocities to invert for the unknown material parameters and temperature structure. In order to test the general methodology

  4. The 1000Ma reassembly of Dharwar and Bastar cratons – Evidence ...

    Indian Academy of Sciences (India)

    8

    2. General Geology. The geology of the study area comprises of the rocks of the Dharwar craton, the ...... Craton-mobile belt relations in in Southern Granulite Terrain. Geol. Soc. .... chemistry: a case study from the Eastern Ghats Belt. India.

  5. East-China Geochemistry Database (ECGD):A New Networking Database for North China Craton

    Science.gov (United States)

    Wang, X.; Ma, W.

    2010-12-01

    generate classification thematic maps using query results, according different parameters. 3.Data analysis on-line. Here we designed lots of geochemical online analysis tools, including geochemical diagrams, CIPW computing, and so on, which allows researchers to analyze query data without download query results. Operation of all these analysis tools is very easy; users just do it by click mouse one or two time. In summary, ECGD provide a geochemical platform for researchers, whom to know where various data are, to view various data in a synthetic and dynamic way, and analyze interested data online. REFERENCES [1] S. Gao, R.L. Rudnick, and W.L. Xu, “Recycling deep cratonic lithosphere and generation of intraplate magmatism in the North China Craton,” Earth and Planetary Science Letters,270,41-53,2008. [2] K.A. Lehnert, U. Harms, and E. Ito, “Promises, Achievements, and Challenges of Networking Global Geoinformatics Resources - Experiences of GeosciNET and EarthChem,” Geophysical Research Abstracts, Vol.10, EGU2008-A-05242,2008.

  6. Global thermal models of the lithosphere

    Science.gov (United States)

    Cammarano, F.; Guerri, M.

    2017-12-01

    Unraveling the thermal structure of the outermost shell of our planet is key for understanding its evolution. We obtain temperatures from interpretation of global shear-velocity (VS) models. Long-wavelength thermal structure is well determined by seismic models and only slightly affected by compositional effects and uncertainties in mineral-physics properties. Absolute temperatures and gradients with depth, however, are not well constrained. Adding constraints from petrology, heat-flow observations and thermal evolution of oceanic lithosphere help to better estimate absolute temperatures in the top part of the lithosphere. We produce global thermal models of the lithosphere at different spatial resolution, up to spherical-harmonics degree 24, and provide estimated standard deviations. We provide purely seismic thermal (TS) model and hybrid models where temperatures are corrected with steady-state conductive geotherms on continents and cooling model temperatures on oceanic regions. All relevant physical properties, with the exception of thermal conductivity, are based on a self-consistent thermodynamical modelling approach. Our global thermal models also include density and compressional-wave velocities (VP) as obtained either assuming no lateral variations in composition or a simple reference 3-D compositional structure, which takes into account a chemically depleted continental lithosphere. We found that seismically-derived temperatures in continental lithosphere fit well, overall, with continental geotherms, but a large variation in radiogenic heat is required to reconcile them with heat flow (long wavelength) observations. Oceanic shallow lithosphere below mid-oceanic ridges and young oceans is colder than expected, confirming the possible presence of a dehydration boundary around 80 km depth already suggested in previous studies. The global thermal models should serve as the basis to move at a smaller spatial scale, where additional thermo-chemical variations

  7. Crustal-scale pop-up structure in cratonic lithosphere: DOBREdeep seismic reflection study of the Donbas fold belt, Ukraine.

    NARCIS (Netherlands)

    Maystrenko, Yu.; Stovba, S.; Stephenson, R.A.; Bayer, U.; Menyoli, E.; Gajewski, D.; Huebscher, Ch.; Rabbel, W.; Saintot, A.N.; Starostenko, V.I.; Thybo, H.; Tolkunov, A.P.

    2003-01-01

    The DOBRE project investigated the interplay of geologic and geodynamic processes that controlled the evolution of the Donbas fold belt, Ukraine, as an example of an inverted intracratonic rift basin. A deep seismic reflection profile provides an excellent image of the structure of the Donbas fold

  8. 3D Crustal Velocity Structure Model of the Middle-eastern North China Craton

    Science.gov (United States)

    Duan, Y.; Wang, F.; Lin, J.; Wei, Y.

    2017-12-01

    Lithosphere thinning and destruction in the middle-eastern North China Craton (NCC), a region susceptible to strong earthquakes, is one of the research hotspots in solid earth science. Up to 42 wide-angle reflection/refraction deep seismic sounding (DSS) profiles have been completed in the middle-eastern NCC, we collect all the 2D profiling results and perform gridding of the velocity and interface depth data, and build a 3D crustal velocity structure model for the middle-eastern NCC, named HBCrust1.0, using the Kriging interpolation method. In this model, four layers are divided by three interfaces: G is the interface between the sedimentary cover and crystalline crust, with velocities of 5.0-5.5 km/s above and 5.8-6.0 km/s below. C is the interface of the upper and lower crust, with velocity jump from 6.2-6.4 km/s to 6.5-6.6 km/s. M is the interface between the crust and upper mantle, with velocity 6.7-7.0 km/s at the crust bottom and 7.9-8.0 km/s on mantle top. Our results show that the first arrival time calculated from HBCust1.0 fit well with the observation. It also demonstrates that the upper crust is the main seismogenic layer, and the brittle-ductile transition occurs at depths near interface C. The depth of interface Moho varies beneath the source area of the Tangshan earth-quake, and a low-velocity structure is found to extend from the source area to the lower crust. Based on these observations, it can be inferred that stress accumulation responsible for the Tangshan earthquake may have been closely related to the migration and deformation of the mantle materials. Comparisons of the average velocities of the whole crust, the upper and the lower crust show that the average velocity of the lower crust under the central part of the North China Basin (NCB) in the east of the craton is obviously higher than the regional average, this high-velocity probably results from longterm underplating of the mantle magma. This research is founded by the Natural Science

  9. A historical overview of Moroccan magmatic events along northwest edge of the West African Craton

    Science.gov (United States)

    Ikenne, Moha; Souhassou, Mustapha; Arai, Shoji; Soulaimani, Abderrahmane

    2017-03-01

    Located along the northwestern edge of the West African Craton, Morocco exhibits a wide variety of magmatic events from Archean to Quaternary. The oldest magmatic rocks belong to the Archean Reguibat Shield outcrops in the Moroccan Sahara. Paleoproterozoic magmatism, known as the Anti-Atlas granitoids, is related to the Eburnean orogeny and initial cratonization of the WAC. Mesoproterozoic magmatism is represented by a small number of mafic dykes known henceforth as the Taghdout mafic volcanism. Massive Neoproterozoic magmatic activity, related to the Pan-African cycle, consists of rift-related Tonian magmatism associated with the Rodinia breakup, an Early Cryogenian convergent margin event (760-700 Ma), syn-collisional Bou-Azzer magmatism (680-640 Ma), followed by widespread Ediacaran magmatism (620-555 Ma). Each magmatic episode corresponded to a different geodynamic environment and produced different types of magma. Phanerozoic magmatism began with Early Cambrian basaltic (rift?) volcanism, which persisted during the Middle Cambrian, and into the Early Ordovician. This was succeeded by massive Late Devonian and Carboniferous, pre-Variscan tholeiitic and calc-alkaline (Central Morocco) volcanic flows in basins of the Moroccan Meseta. North of the Atlas Paleozoic Transform Zone, the Late Carboniferous Variscan event was accompanied by the emplacement of 330-300 Ma calc-alkaline granitoids in upper crustal shear zones. Post-Variscan alkaline magmatism was associated with the opening of the Permian basins. Mesozoic magmatism began with the huge volumes of magma emplaced around 200 Ma in the Central Atlantic Magmatic Province (CAMP) which was associated with the fragmentation of Pangea and the subsequent rifting of Central Atlantic. CAMP volcanism occurs in all structural domains of Morocco, from the Anti-Atlas to the External Rif domain with a peak activity around 199 Ma. A second Mesozoic magmatic event is represented by mafic lava flows and gabbroic intrusions in

  10. Thermodynamic, geophysical and rheological modeling of the lithosphere underneath the North Atlantic Porcupine Basin (Ireland).

    Science.gov (United States)

    Botter, C. D.; Prada, M.; Fullea, J.

    2017-12-01

    The Porcupine is a North-South oriented basin located southwest of Ireland, along the North Atlantic continental margin, formed by several rifting episodes during Late Carboniferous to Early Cretaceous. The sedimentary cover is underlined by a very thin continental crust in the center of the basin (10 in the South. In spite of the abundant literature, most of the oil and gas exploration in the Porcupine Basin has been targeting its northern part and is mostly restricted to relatively shallow depths, giving a restrained overview of the basin structure. Therefore, studying the thermodynamic and composition of the deep and broader structures is needed to understand the processes linked to the formation and the symmetry signature of the basin. Here, we model the present-day thermal and compositional structure of the continental crust and lithospheric mantle underneath the Porcupine basin using gravity, seismic, heat flow and elevation data. We use an integrated geophysical-petrological framework where most relevant rock properties (density, seismic velocities) are determined as a function of temperature, pressure and composition. Our modelling approach solves simultaneously the heat transfer, thermodynamic, geopotential, seismic and isostasy equations, and fit the results to all available geophysical and petrological observables (LitMod software). In this work we have implemented a module to compute self-consistently a laterally variable lithospheric elastic thickness based on mineral physics rheological laws (yield strength envelopes over the 3D volume). An appropriate understanding of local and flexural isostatic behavior of the basin is essential to unravel its tectonic history (i.e. stretching factors, subsidence etc.). Our Porcupine basin 3D model is defined by four lithological layers, representing properties from post- and syn-rift sequences to the lithospheric mantle. The computed yield strength envelopes are representative of hyperextended lithosphere and

  11. Elysium region, mars: Tests of lithospheric loading models for the formation of tectonic features

    International Nuclear Information System (INIS)

    Hall, J.L.; Solomon, S.C.; Head, J.W.

    1986-01-01

    The second largest volcanic province on Mars lies in the Elysium region. Like the larger Tharsis province, Elysium is marked by a topographic rise and a broad free air gravity anomaly and also exhibits a complex assortment of tectonic and volcanic features. We test the hypothesis that the tectonic features in the Elysium region are the product of stresses produced by loading of the Martian lithosphere. We consider loading at three different scales: local loading by individual volcanoes, regional loading of the lithosphere from above or below, and quasi-global loading by Tharsis. A comparison of flexural stresses with lithospheric strength and with the inferred maximum depth of faulting confirms that concentric graben around Elysium Mons can be explained as resulting from local flexure of an elastic lithosphere about 50 km thick in response to the volcano load. Volcanic loading on a regional scale, however, leads to predicted stresses inconsistent with all observed tectonic features, suggesting that loading by widespread emplacement of thick plains deposits was not an important factor in the tectonic evolution of the Elysium region. A number of linear extensional features oriented generally NW-SE may have been the result of flexural uplift of the lithosphere on the scale of the Elysium rise. The global stress field associated with the support of the Tharsis rise appears to have influenced the development of many of the tectonic features in the Elysium region, including Cerberus Rupes and the systems of ridges in eastern and western Elysium. The comparisons of stress models for Elysium with the preserved tectonic features support a succession of stress fields operating at different times in the region

  12. The peculiar case of Marosticano xenoliths: a cratonic mantle fragment affected by carbonatite metasomatism in the Veneto Volcanic Province (Northern Italy)

    Science.gov (United States)

    Brombin, Valentina; Bonadiman, Costanza; Coltorti, Massimo; Florencia Fahnestock, M.; Bryce, Julia G.; Marzoli, Andrea

    2017-04-01

    The Tertiary Magmatic Province of Veneto, known as Veneto Volcanic Province (VVP), in the Northern Italy, represents one of the most important volcanic provinces of the Adria Plate. It is composed by five volcanic districts: Val d'Adige, Marosticano, Mts. Lessini, Berici Hills and Euganean Hills. Most of the volcanic products are relatively undifferentiated lavas, from nephelinites to tholeiites in composition. Commonly VVP nephelinites and basanites carry mantle xenoliths. This study presents a petrological characterization of the new xenolith occurrence of Marosticano and comparison with previously studied VVP xenolith populations (i.e. from the Lessinean and Val d'Adige areas), which represent off-craton lithospheric mantle fragment affected by Na-alkaline silicate metasomatism (Siena & Coltorti 1989; Beccaluva et al., 2001; Gasperini et al., 2006). Marosticano (MA) peridotites are anhydrous spinel-bearing lherzolites and harzburgites, which are geochemically well distinguishible from the other VVP mantle xenoliths. Primary minerals record the "most restitic" composition of the VVP sampled mantle, even calling the geochemical features of a sub-cratonic mantle. Olivines in both lherzolites and harzburgites show high Ni contents compared with the Fo values (Ni→ lherzolite: 2600-3620 ppm; harzburgite: 2600-3540 ppm; Fo → lh: 91-92; hz: 90-93) that follow the trend of olivine from a cratonic area (Kelemen, 1998). Orthopyroxenes have mg# values with 1:1 ratio with coexisting olivines and Al2O3 contents always 0.5 wt%) contents are also the chemical characteristics of the clinopyroxenes. On the whole both MA pyroxenes show major element contents that recall the characteristics of those from cratonic (sp-bearing) peridotites (e.g. from Greenland, South Africa and Tanzania; Downes et al., 2004). In addition, the relationship between the high Fo content of olivine and the high chromium contents (cr#=(Cr/(Cr+Al)X100); lh: 30-53; hz: 38-67) in coexisting spinel, out of

  13. The lithospheric mantle below southern West Greenland

    DEFF Research Database (Denmark)

    Sand, Karina Krarup; Waight, Tod Earle; Pearson, D. Graham

    2009-01-01

    Geothermobarometry of primarily garnet lherzolitic xenoliths from several localities in southern West Greenland is applied to address the diamond potential, pressure and temperature distribution and the stratigraphy of the subcontinental lithospheric mantle ~600 Ma ago. The samples are from kimbe...... into the reworked Archean North of the Naqssugtoqidian deformation front....

  14. Do cratons preserve evidence of stagnant lid tectonics?

    Directory of Open Access Journals (Sweden)

    Derek Wyman

    2018-01-01

    Full Text Available Evidence for episodic crustal growth extending back to the Hadean has recently prompted a number of numerically based geodynamic models that incorporate cyclic changes from stagnant lid to mobile lid tectonics. A large part of the geologic record is missing for the times at which several of these cycles are inferred to have taken place. The cratons, however, are likely to retain important clues relating to similar cycles developed in the Mesoarchean and Neoarchean. Widespread acceptance of a form of plate tectonics by ∼3.2 Ga is not at odds with the sporadic occurrence of stagnant lid tectonics after this time. The concept of scale as applied to cratons, mantle plumes and Neoarchean volcanic arcs are likely to provide important constraints on future models of Earth's geodynamic evolution. The Superior Province will provide some of the most concrete evidence in this regard given that its constituent blocks may have been locked into a stagnant lid relatively soon after their formation and then assembled in the next global plate tectonic interval. Perceived complexities associated with inferred mantle plume – volcanic arc associations in the Superior Province and other cratons may be related to an over estimation of plume size. A possible stagnant lid episode between ∼2.9 Ga and ∼2.8 Ga is identified by previously unexplained lapses in volcanism on cratons, including the Kaapvaal, Yilgarn and Superior Province cratons. If real, then mantle dynamics associated with this episode likely eliminated any contemporaneous mantle plume incubation sites, which has important implications for widespread plumes developed at ∼2.7 Ga and favours a shallow mantle source in the transition zone. The Superior Province provides a uniquely preserved local proxy for this global event and could serve as the basis for detailed numerical models in the future.

  15. Lithospheric controls on crustal reactivation and intraplate mountain building in the Gobi Corridor, Central Asia

    Science.gov (United States)

    Cunningham, D.

    2017-12-01

    This talk will review the Permian-Recent tectonic history of the Gobi Corridor region which includes the actively deforming Gobi Altai-Altai, Eastern Tien Shan, Beishan and North Tibetan foreland. Since terrane amalgamation in the Permian, Gobi Corridor crust has been repeatedly reactivated by Triassic-Jurassic contraction/transpression, Late Cretaceous extension and Late Cenozoic transpression. The tectonic history of the region suggests the following basic principle for intraplate continental regions: non-cratonized continental interior terrane collages are susceptible to repeated intraplate reactivation events, driven by either post-orogenic collapse and/or compressional stresses derived from distant plate boundary convergence. Thus, important related questions are: 1) what lithospheric pre-conditions favor intraplate crustal reactivation in the Gobi Corridor (simple answer: crustal thinning, thermal weakening, strong buttressing cratons), 2) what are the controls on the kinematics of deformation and style of mountain building in the Gobi-Altai-Altai, Beishan and North Tibetan margin (simple answer: many factors, but especially angular relationship between SHmax and `crustal grain'), 3) how does knowledge of the array of Quaternary faults and the historical earthquake record influence our understanding of modern earthquake hazards in continental intraplate regions (answer: extrapolation of derived fault slip rates and recurrence interval determinations are problematic), 4) what important lessons can we learn from the Mesozoic-Cenozoic tectonic history of Central Asia that is applicable to the tectonic evolution of all intraplate continental regions (simple answer: ancient intraplate deformation events may be subtly expressed in the rock record and only revealed by low-temperature thermochronometers, preserved orogen-derived sedimentary sequences, fault zone evidence for younger brittle reactivation, and recognition of a younger class of cross-cutting tectonic

  16. Fluid-induced transition from banded kyanite- to bimineralic eclogite and implications for the evolution of cratons

    Science.gov (United States)

    Sommer, H.; Jacob, D. E.; Stern, R. A.; Petts, D.; Mattey, D. P.; Pearson, D. G.

    2017-06-01

    lithosphere. As kyanite contains around 100 ppm of H2O it is suggested that the kyanite-out reaction, once initiated by heating and restricted metasomatic influx, was promoted by the release of water contained in the kyanite. The steep (high-P low-T) prograde P-T path defining rapid compression at low heating rates is atypical for subduction transport of eclogites into the lithospheric mantle. Such a trajectory is best explained in a model where strong lateral compression forces eclogites downward to higher pressures, supporting models of cratonic lithosphere formation by lateral collision and compression.

  17. Influence of the lithosphere-asthenosphere boundary on the stress field northwest of the Alps

    Science.gov (United States)

    Maury, J.; Cornet, F. H.; Cara, M.

    2014-11-01

    In 1356, a magnitude 6-7 earthquake occurred near Basel, in Switzerland. But recent compilations of GPS measurements reveal that measured horizontal deformation rates in northwestern continental Europe are smaller than error bars on the measurements, proving present tectonic activity, if any, is very small in this area. We propose to reconcile these apparently antinomic observations with a mechanical model of the lithosphere that takes into account the geometry of the lithosphere-asthenosphere boundary, assuming that the only loading mechanism is gravity. The lithosphere is considered to be an elastoplastic material satisfying a Von Mises plasticity criterion. The model, which is 400 km long, 360 km wide and 230 km thick, is centred near Belfort in eastern France, with its width oriented parallel to the N145°E direction. It also takes into account the real topography of both the ground surface and that of the Moho discontinuity. Not only does the model reproduce observed principal stress directions orientations, it also identifies a plastic zone that fits roughly the most seismically active domain of the region. Interestingly, a somewhat similar stress map may be produced by considering an elastic lithosphere and an ad-hoc horizontal `tectonic' stress field. However, for the latter model, examination of the plasticity criterion suggests that plastic deformation should have taken place. It is concluded that the present-day stress field in this region is likely controlled by gravity and rheology, rather than by active Alpine tectonics.

  18. Petrogenesis of the middle Jurassic appinite and coeval granitoids in the Eastern Hebei area of North China Craton

    Science.gov (United States)

    Fan, Wenbo; Jiang, Neng; Xu, Xiyang; Hu, Jun; Zong, Keqing

    2017-05-01

    An integrated study of zircon U-Pb ages and Hf-O isotopic compositions, whole rock elemental and Sr-Nd isotope geochemistry was conducted on three lithologically diverse middle Jurassic plutons from the Eastern Hebei area of the North China Craton (NCC), in order to reveal both their petrogenesis and possible tectonic affinity. The three plutons have consistent magmatic zircon U-Pb ages from 167 ± 1 Ma to 173 ± 1 Ma. The Nianziyu pluton has typical characteristics of appinite with low SiO2 (43.7-52.6%), high Ca, Mg, Fe and H2O contents. It possesses subduction-related trace element patterns, enriched Nd-Hf isotopic signatures as well as elevated zircon δ18O values (6.2-7.2‰), arguing for an enriched mantle source metasomatized by fluids related to subduction. The Shuihutong monzogranites have high silica (SiO2 = 75.4-75.9%) and alkali contents, low Ca contents and striking negative Ba, Sr and Eu anomalies. Samples from the pluton have more evolved Nd-Hf isotopic values and are considered to be most likely derived from anatexis of ancient lower continental crust. Hybridization between mantle- and ancient lower crust-derived magmas is proposed for the mafic microgranular enclave-bearing Baijiadian granitoids, which are characterized by variable εNd (t) and εHf(t) values. Integrated with the regional geologic history, we suggest that the formation of the three middle Jurassic plutons were related to the subduction of the Paleo-Pacific ocean plate beneath the NCC. Their petrogenetic differences reflect complex magmatic processes in subduction settings involving melting of multiple sources, possible partly facilitated by fluid metasomatism and water-rich magma injection, accompanied with various degrees of magma mixing. The appearance of middle Jurassic appinitic rocks leads us to propose that the NCC destruction and lithosphere thinning were facilitated and controlled by the weakening of the lithospheric mantle after hydration because of the subduction of the

  19. Diamond ages from Victor (Superior Craton): Intra-mantle cycling of volatiles (C, N, S) during supercontinent reorganisation

    Science.gov (United States)

    Aulbach, S.; Creaser, Robert A.; Stachel, Thomas; Heaman, Larry M.; Chinn, Ingrid L.; Kong, Julie

    2018-05-01

    The central Superior Craton hosts both the diamondiferous 1.1 Ga Kyle Lake and Jurassic Attawapiskat kimberlites. A major thermal event related to the Midcontinent Rift at ca. 1.1 Ga induced an elevated geothermal gradient that largely destroyed an older generation of diamonds, raising the question of when, and how, the diamond inventory beneath Attawapiskat was formed. We determined Re-Os isotope systematics of sulphides included in diamonds from Victor by isotope dilution negative thermal ionisation mass spectrometry in order to obtain insights into the age and nature of the diamond source in the context of regional tectonothermal evolution. Regression of the peridotitic inclusion data (n = 14 of 16) yields a 718 ± 49 Ma age, with an initial 187Os/188Os ratio of 0.1177 ± 0.0016, i.e. depleted at the time of formation (γOs -3.7 ± 1.3). Consequently, Re depletion model ages calculated for these samples are systematically overestimated. Given that reported 187Os/188Os in olivine from Attawapiskat xenoliths varies strongly (0.1012-0.1821), the low and nearly identical initial Os of sulphide inclusions combined with their high 187Re/188Os (median 0.34) suggest metasomatic formation from a mixed source. This was likely facilitated by percolation of amounts of melt sufficient to homogenise Os, (re)crystallise sulphide and (co)precipitate diamond; that is, the sulphide inclusions and their diamond host are synchronous if not syngenetic. The ∼720 Ma age corresponds to rifting beyond the northern craton margin during Rodinia break-up. This suggests mobilisation of volatiles (C, N, S) and Os due to attendant mantle stretching and metasomatism by initially oxidising and S-undersaturated melts, which ultimately produced lherzolitic diamonds with high N contents compared to older Kyle Lake diamonds. Thus, some rift-influenced settings are prospective with respect to diamond formation. They are also important sites of hidden, intra-lithospheric volatile redistribution

  20. Western cratonic domains in Uruguay, geochronology

    International Nuclear Information System (INIS)

    Preciozzi, F; Pell, E; Muzio, R; Ledesma, J.J; Guerequiz, R

    2001-01-01

    The western cratonic domains in Uruguay are divided into three major units: Piedra Alta Terrane, Valentines Block and Pavas Terrane. Piedra Alta Terrane lacks of evidence of Neo proterozoic oro geneses (deformation, metamorphism or magmatism). Sarandi del Yi - Arroyo Solis Grande shear zone, separates it from Valentines Block. Valentines Block is separated from Pavas Terrane by Cueva del Tigre shear zone. Magmatic rocks with different ages, compositions and emplacements occur all over the Piedra Alta Terrane distributed in three metamorphic belts (Arroyo Grande, San Jose and Montevideo) as well as in the Central Gneissic-Migmatitic Complex. Samples from the gneissic-migmatitic complex, late tectonic granitoids and basic rocks associated to the metamorphic belts were analyzed using Rb/Sr, U/Pb, K/Ar and Sm/Nd methodologies. The age ranges obtained for granitoids and gneissicmigmatitic samples using Rb/Sr whole rock (WR) systematics are 1.7 to 2.5 Ga, showing two intervals: 1.9 to 2.05 Ga (intrusion of late granites) and 2.1 to 2.2 Ga (deformation and metamorphism). K/Ar cooling ages present several ranges: 1.3 to 1.35 Ga (probable local heating of the crust), 1.7 to 1.8 Ga (microgabbro magmatism, data confirmed by the Ar/Ar method) and ages between 2.0 to 2.2 Ga. Rb/Sr (WR) data yielded an isochronic age of 2094 ± 28.3 Ma, Ro = 0.70174 ± 0.00009, MSWD 19.74, interpreted as the time of the metamorphic event recognized for all the Piedra Alta Terrane. TDM Sm/Nd model ages presented a range from 2065 Ma to 2450 Ma. U/Pb systematics yield ages in migmatitic and gneissic rocks from 2.16 Ga to 2.21 Ga, showing metamorphism and deformation phenomena. While the ages of granitoids associated to the San Jose Belt are between 2.06 Ga and 2.08 Ga (Isla Mala) to 2.1 Ga (Granito de Cufre). None of them show metamorphic phenomena. Valentines Block (Preciozzi et al., 1979) comprises granulitic gneisses, granitoids and mafic rocks of different compositions. The geologic complexity

  1. Timing and implications for the late Mesozoic geodynamic settings of eastern North China Craton: Evidences from K-Ar dating age and sedimentary-structural characteristics records of Lingshan Island, Shandong Province

    Science.gov (United States)

    Li, Jie; Jin, Aiwen; Hou, Guiting

    2017-12-01

    The Lingshan Island in Shandong Province in the eastern North China Craton, well known for the Late Mesozoic multi-scale slide-slump structures is related to paleo-earthquake. Terrigenous clastic rocks, volcanic clastic rocks and volcanic lavas are extensively exposed in the Lingshan Island and its adjacent regions of the Shandong Province, which led to fierce debates on their ages, sedimentary characteristics and tectono-sedimentary evolution. In this contribution, we present the characteristics of the Late Mesozoic stratigraphy in the Lingshan Island. Whole-rock K-Ar dating of dyke at Beilaishi and rhyolites at Laohuzui of the Lingshan Island yielded ages of 159 Ma and 106-92 Ma which coincides with the Laiyang Period rifting and the Qingshan Period rifting in the Jiaolai Basin, respectively. On the basis of the analysis to the Late Mesozoic sedimentary environment of `flysch' and `molasse'-like formations as well as tectonic stress fields reconstruction, four episodes of the tectono-sedimentary evolution were established in the Lingshan Island and its adjacent regions in the eastern North China Craton. They consist of two episodes of extensional events for the syn-rift, and two episodes of compression events for the inversion of the post-rift. The entire episodes can be summarized as follows: (1) the first syn-rift NW-SE extension in Laiyang Period can be identified by the `flysch' formation (Unit 1) and by emplacement of the NE-trending dyke in the Laiyang Group. This syn-rift episode can be related to the NW-SE post-orogenic extension resulted from the gravity collapse of the thickened lithosphere along the Sulu Orogen. (2) The first post-rift NW-SE inversion, which was caused by the NW-directed subduction of Izanaqi Plate, can be well documented by the `X' type conjugate joints as well as slide slump folds in Unit 1. (3) The second syn-rift NW-SE extension in Qingshan Period is characterized by rhyolite rocks (Unit 2). This syn-rift episode can be considered

  2. Finite-frequency Rayleigh wave tomography of the western Mediterranean: Mapping its lithospheric structure

    Science.gov (United States)

    Palomeras, I.; Thurner, S.; Levander, A.; Liu, K.; Villasenor, A.; Carbonell, R.; Harnafi, M.

    2014-01-01

    We present a 3-D shear wave velocity model for the crust and upper mantle of the western Mediterranean from Rayleigh wave tomography. We analyzed the fundamental mode in the 20-167 s period band (6.0-50.0 mHz) from earthquakes recorded by a number of temporary and permanent seismograph arrays. Using the two-plane wave method, we obtained phase velocity dispersion curves that were inverted for an isotropic Vs model that extends from the southern Iberian Massif, across the Gibraltar Arc and the Atlas mountains to the Saharan Craton. The area of the western Mediterranean that we have studied has been the site of complex subduction, slab rollback, and simultaneous compression and extension during African-European convergence since the Oligocene. The shear velocity model shows high velocities beneath the Rif from 65 km depth and beneath the Granada Basin from ˜70 km depth that extend beneath the Alboran Domain to more than 250 km depth, which we interpret as a near-vertical slab dangling from beneath the western Alboran Sea. The slab appears to be attached to the crust beneath the Rif and possibly beneath the Granada Basin and Sierra Nevada where low shear velocities (3.8 km/s) are mapped to >55 km depth. The attached slab is pulling down the Gibraltar Arc crust, thickening it, and removing the continental margin lithospheric mantle beneath both Iberia and Morocco as it descends into the deeper mantle. Thin lithosphere is indicated by very low upper mantle velocities beneath the Alboran Sea, above and east of the dangling slab and beneath the Cenozoic volcanics.

  3. Cover sequences at the northern margin of the Antongil Craton, NE Madagascar

    Science.gov (United States)

    Bauer, W.; Walsh, G.J.; De Waele, B.; Thomas, Ronald J.; Horstwood, M.S.A.; Bracciali, L.; Schofield, D.I.; Wollenberg, U.; Lidke, D.J.; Rasaona, I.T.; Rabarimanana, M.H.

    2011-01-01

    The island of Madagascar is a collage of Precambrian, generally high-grade metamorphic basement domains, that are locally overlain by unmetamorphosed sedimentary rocks and poorly understood low-grade metasediments. In the Antalaha area of NE Madagascar, two distinct cover sequences rest on high-grade metamorphic and igneous basement rocks of the Archaean Antongil craton and the Neoproterozoic Bemarivo belt. The older of these two cover sequences, the Andrarona Group, consists of low-grade metasedimentary rocks. The younger sequence, the newly defined Ampohafana Formation, consists of unmetamorphosed sedimentary rocks. The Andrarona Group rests on Neoarchaean granites and monzogranites of the Antongil craton and consists of a basal metagreywacke, thick quartzites and an upper sequence of sericite-chlorite meta-mudstones, meta-sandstones and a volcaniclastic meta-sandstone. The depositional age of the volcaniclastic meta-sandstone is constrained in age by U–Pb laser-ablation ICP-MS analyses of euhedral zircons to 1875 ± 8 Ma (2σ). Detrital zircons of Archaean and Palaeoproterozoic age represent an input from the Antongil craton and a newly defined Palaeoproterozoic igneous unit, the Masindray tonalite, which underlies the Andrarona Group, and yielded a U–Pb zircon age of 2355 ± 11 Ma (2σ), thus constraining the maximum age of deposition of the basal part of the Andrarona Group. The Andrarona Group shows a low-grade metamorphic overprint in the area near Antalaha; illite crystallinity values scatter around 0.17°Δ2Θ CuKα, which is within the epizone. The Ampohafana Formation consists of undeformed, polymict conglomerate, cross-bedded sandstone, and red mudstone. An illite crystallinity value of >0.25°Δ2Θ CuKα obtained from the rocks is typical of the diagenetic zone. Occurrences of rhyodacite pebbles in the Ampohafana Formation and the intrusion of a basaltic dyke suggest a deposition in a WSW-ENE-trending graben system during the opening of the Indian

  4. Lithospheric scale conductivity anomalies at the Proterozoic-Phanerozoic transition of Australia- Insights from AusLAMP MT data

    Science.gov (United States)

    Robertson, K. E.; Thiel, S.; Heinson, G. S.

    2017-12-01

    The intraplate deformation of the north-south trending Neoproterozoic Ikara-Flinders Ranges in South Australia, Australia, draws interest due to its high heat flow, elevated seismicity and the presence of diamondiferous kimberlites and mineral deposits. To the west lies the highly prospective Archean-Paleoproterozoic Gawler Craton, boasting the world's largest IOCG-U deposit, Olympic Dam. The Paleo-Mesoproterozoic Curnamona Province lies to the east, thought to have once been connected to the Gawler Craton and host to the world-class Broken Hill Ag-Pb-Zn deposit. A total of 162 long-period (10 s - 10,000 s) magnetotelluric (MT) stations from the Australia-wide AusLAMP (Australian Lithospheric Architecture Magnetotelluric Project) dataset were used to image the electrical resistivity beneath the Ikara-Flinders Ranges and adjacent Curnamona Province. The most recent acquisition extends this survey region northward to an area predominantly covered with Paleo-Mesozoic sedimentary basins including the most significant on-shore oil and gas region in Australia, the Cooper Basin. The resultant model from 3D inversions using ModEM software shows a relatively resistive Ikara-Flinders Ranges, with two parallel arcuate conductors (the WNAC and ENAC) at 20 to 80 km depth in the Nackara Arc. These conductors correlate well with locations of diamondiferous kimberlites which suggests that the conductors may have derived from the ascent of carbon-rich kimberlite-hosting magma and volatiles up large lithospheric scale structures. The conductors appear to have no correlation with regions of intraplate seismicity within the Ikara-Flinders Ranges which may mean that enhanced pore fluid pressure is not the main cause for the seismicity as was recently proposed. A large conductor covering most of the Curnamona Province (the CC) extends over depths of 10-40 km. The Curnamona Province's most recent tectonothermal activity is from Delamerian reworking during the Cambrian at its margins but

  5. The lithosphere-asthenosphere boundary beneath the Korean Peninsula from S receiver functions

    Science.gov (United States)

    Lee, S. H.; Rhie, J.

    2017-12-01

    The shallow lithosphere in the Eastern Asia at the east of the North-South Gravity Lineament is well published. The reactivation of the upper asthenosphere induced by the subducting plates is regarded as a dominant source of the lithosphere thinning. Additionally, assemblage of various tectonic blocks resulted in complex variation of the lithosphere thickness in the Eastern Asia. Because, the Korean Peninsula located at the margin of the Erasian Plate in close vicinity to the trench of subducting oceanic plate, significant reactivation of the upper asthenosphere is expected. For the study of the tectonic history surrounding the Korean Peninsula, we determined the lithosphere-asthenosphere boundary (LAB) beneath the Korean Peninsula using common conversion point stacking method with S receiver functions. The depth of the LAB beneath the Korean Peninsula ranges from 60 km to 100 km and confirmed to be shallower than that expected for Cambrian blocks as previous global studies. The depth of the LAB is getting shallower to the south, 95 km at the north and 60 km at the south. And rapid change of the LAB depth is observed between 36°N and 37°N. The depth change of the LAB getting shallower to the south implies that the source of the lithosphere thinning is a hot mantle upwelling induced by the northward subduction of the oceanic plates since Mesozoic. Unfortunately, existing tectonic models can hardly explain the different LAB depth in the north and in the south as well as the rapid change of the LAB depth.

  6. Geochemistry of organic-rich river waters in Amazonia: Insights on weathering processes of intertropical cratonic terrain

    International Nuclear Information System (INIS)

    Horbe, Adriana Maria Coimbra; Lages, Anderson da Silva; Moquet, Jean-Sébastien; Santos, Roberto Ventura; Seyler, Patrick

    2016-01-01

    In this study, eight organic-rich rivers that flow through the Brazilian craton in the southwestern Amazon rainforest are investigated. This investigation is the first of its type in this area and focuses on the effects of lithology, long-term weathering, thick soils, forest cover and hydrological period on the dissolved load compositions in rivers draining cratonic terrain. The major dissolved ion concentrations, alkalinity (TAlk), SiO 2 , trace element concentrations, and Sr isotope contents in the water were determined between April 2009 and January 2010. In addition, the isotopic values of oxygen and hydrogen were determined between 2011 and 2013. Overall, the river water is highly dilute and dominated by the major dissolved elements TAlk, SiO 2 and K + and the major dissolved trace elements Al, Fe, Ba, Mn, P, Zn and Sr, which exhibit large temporal and spatial variability and are closely correlated with the silicatic bedrock and hydrology. Additionally, rainwater and recycled water vapor and the size of the basin contribute to the geochemistry of the waters. The total weathering flux estimated from our results is 2–4 t km −2 .yr −1 , which is one of the lowest fluxes in the world. The CO 2 consumption rate is approximately 21–61 10 3  mol km −2  yr −1 , which is higher than expected given the stability of the felsic to basic igneous and metamorphic to siliciclastic basement rocks and the thick tropical soil cover. Thus, weathering of the cratonic terrain under intertropical humid conditions is still an important consumer of CO 2 . - Highlights: • Were studied rivers flowing the Brazilian craton covered by lateritic soils. • The river waters are highly diluted and dominated by TAlk, SiO 2 and K + . • There is spatial and temporal variability in the chemical composition. • The rain amount and recycled water vapor affect the O and D isotopes. • Geology, weathering, discharge and seasonality highlight a singular composition.

  7. Fossil plume head beneath the Arabian lithosphere?

    Science.gov (United States)

    Stein, Mordechai; Hofmann, Albrecht W.

    1992-12-01

    Phanerozoic alkali basalts from Israel, which have erupted over the past 200 Ma, have isotopic compositions similar to PREMA ("prevalent mantle") with narrow ranges of initial ɛ Nd(T) = +3.9-+5.9; 87Sr/ 86Sr(T)= 0.70292-0.70334; 206Pb/ 204Pb(T)= 18.88-19.99; 207Pb/ 204Pb(T)= 15.58-15.70; and 208Pb/ 204Pb(T)= 38.42-39.57. Their Nb/U(43 ± 9) and Ce/Pb(26 ± 6) ratios are identical to those of normal oceanic basalts, demonstrating that the basalts are essentially free of crustal contamination. Overall, the basalts are chemically and isotopically indistinguishable from many ordinary plume basalts, but no plume track can be identified. We propose that these and other, similar, magmas from the Arabian plate originated from a "fossilized" head of a mantle plume, which was unable to penetrate the continental lithosphere and was therefore trapped and stored beneath it. The plume head was emplaced some time between the late Proterozoic crust formation and the initiation of the Phanerozoic magmatic cycles. Basalts from rift environments in other continental localities show similar geochemistry to that of the Arabian basalts and their sources may also represent fossil plume heads trapped below the continents. We suggest that plume heads are, in general, characterized by the PREMA isotopic mantle signature, because the original plume sources (which may have HIMU or EM-type composition) have been diluted by overlying mantle material, which has been entrained by the plume heads during ascent. On the Arabian plate, rifting and thinning of the lithosphere caused partial melting of the stored plume, which led to periodic volcanism. In the late Cenozoic, the lithosphere broke up and the Red Sea opened. N-MORB tholeiites are now erupting in the central trough of the Red Sea, where the lithosphere has moved apart and the fossil plume has been exhausted, whereas E-MORBs are erupting in the northern and southern troughs, still tapping the plume reservoir. Fossil plumes, which are

  8. Satellite Tidal Magnetic Signals Constrain Oceanic Lithosphere-Asthenosphere Boundary Earth Tomography with Tidal Magnetic Signals

    Science.gov (United States)

    Grayver, Alexander V.; Schnepf, Neesha R.; Kuvshinov, Alexey V.; Sabaka, Terence J.; Chandrasekharan, Manoj; Olsen, Niles

    2016-01-01

    The tidal flow of electrically conductive oceans through the geomagnetic field results in the generation of secondary magnetic signals, which provide information on the subsurface structure. Data from the new generation of satellites were shown to contain magnetic signals due to tidal flow; however, there are no reports that these signals have been used to infer subsurface structure. Here we use satellite-detected tidal magnetic fields to image the global electrical structure of the oceanic lithosphere and upper mantle down to a depth of about 250 km. The model derived from more than 12 years of satellite data reveals an Approximately 72 km thick upper resistive layer followed by a sharp increase in electrical conductivity likely associated with the lithosphere-asthenosphere boundary, which separates colder rigid oceanic plates from the ductile and hotter asthenosphere.

  9. Gravity signals from the lithosphere in the Central European Basin System

    Science.gov (United States)

    Yegorova, T.; Bayer, U.; Thybo, H.; Maystrenko, Y.; Scheck-Wenderoth, M.; Lyngsie, S. B.

    2007-01-01

    We study the gravity signals from different depth levels in the lithosphere of the Central European Basin System (CEBS). The major elements of the CEBS are the Northern and Southern Permian Basins which include the Norwegian-Danish Basin (NDB), the North-German Basin (NGB) and the Polish Trough (PT). An up to 10 km thick sedimentary cover of Mesozoic-Cenozoic sediments, hides the gravity signal from below the basin and masks the heterogeneous structure of the consolidated crust, which is assumed to be composed of domains that were accreted during the Paleozoic amalgamation of Europe. We performed a three-dimensional (3D) gravity backstripping to investigate the structure of the lithosphere below the CEBS. Residual anomalies are derived by removing the effect of sediments down to the base of Permian from the observed field. In order to correct for the influence of large salt structures, lateral density variations are incorporated. These sediment-free anomalies are interpreted to reflect Moho relief and density heterogeneities in the crystalline crust and uppermost mantle. The gravity effect of the Moho relief compensates to a large extent the effect of the sediments in the CEBS and in the North Sea. Removal of the effects of large-scale crustal inhomogeneities shows a clear expression of the Variscan arc system at the southern part of the study area and the old crust of Baltica further north-east. The remaining residual anomalies (after stripping off the effects of sediments, Moho topography and large-scale crustal heterogeneities) reveal long wavelength anomalies, which are caused mainly by density variations in the upper mantle, though gravity influence from the lower crust cannot be ruled out. They indicate that the three main subbasins of the CEBS originated on different lithospheric domains. The PT originated on a thick, strong and dense lithosphere of the Baltica type. The NDB was formed on a weakened Baltica low-density lithosphere formed during the Sveco

  10. On the origin of cratonic `high-mu' isotopic signatures

    Science.gov (United States)

    Reimink, J. R.; Carlson, R.; Shirey, S. B.; Pearson, D. G.; Kamber, B. S.

    2017-12-01

    Some Archean cratons (i.e. Slave, Wyoming) contain Neoarchean granitoids with initial Pb isotopic compositions indicative of derivation from sources characterized by high time-integrated U/Pb ratios (high-mu [1]). Single-stage high-m precursor source reservoir separation from the depleted mantle occurred no later than 3.9 Ga [2]. However, multi-stage separation could have occurred in the Hadean, suggesting that recycling or reworking of Eoarchean/Hadean crust played a significant role in the generation of Neoarchean granitic crust in many cratons. The Sm-Nd system is similar to the U-Pb system in that it has a short-lived parent-daughter pair (146Sm-142Nd) that is sensitive to very early differentiation events, as well as a long-lived parent-daughter pair (147Sm-143Nd) that is sensitive to differentiation throughout all of Earth history. The 103 Ma half-life of 146Sm makes it sensitive only to Sm/Nd fractionation that occurred in the Hadean, providing a useful tracker for very early differentiation events. Indeed, evidence for Neoarchean remelting of ancient crust in another craton has come from analyses of the paired Sm-Nd isotope systems from the Hudson Bay terrane of the northeastern Superior Province. These results indicate that the source of 2.7 Ga Hudson Bay terrane granitoids was Hadean mafic crust, and not Eoarchean felsic crust [3]. Here, we present new data from Neoarchean granites located in the Slave and Wyoming cratons, along with modeling of the dual paired-isotope systems of U-Pb and Sm-Nd to achieve a tighter constraint on the composition of the precursors and the timing of their melting. Combining our newly collected 142Nd data with the high-m signature of these Neoarchean rocks, we evaluate precursor source separation ages along with the source Sm/Nd and U/Pb compositions. In the simplest end-member scenarios, use of the 142Nd system allows us to test whether the cratonic high-mu signature was created by melting of Hadean mafic crust or Eoarchean

  11. Seismicity and lithospheric structure of Central Mozambique: implications for the southward propagation of the East African Rift System

    Science.gov (United States)

    Fonseca, J. F. B. D.; Domingues, A.

    2017-12-01

    South of latitude 5ºS, there is scarce support for a single preferred location of continental rifting in SE Africa. Building on the complexity already displayed further north around the Victoria microplate, the structures associated with rifting activity are now distributed over three branches: one directed towards the SW through Zambia and into the Okawango rift in Botswana; one running offshore along the Mozambique Channel; and a central rift system through lake Malawi and Central Mozambique. Our investigation focuses on this central branch, whose tectonic relevance was highlighted by the M7 Machaze earthquake in 2006. Through the temporary deployment of 30 broadband stations in central Mozambique we were able to document that the Shire and Urema grabens linking the Malawi rift to the Machaze epicentral area are seismically active, correlating with a 300 km long narrow band of seismicity reaching the lower crust. No significant seismicity was recorded along the Mazenga graben, south of the Machaze epicentral area. A tomographic model derived from ambient noise analysis showed a strong correlation between the seismicity and a sharp NNE-SSW boundary between the fast crust of the Zimbabwe and Kaapvaal cratons and slower crust underneath the Mozambique Coastal Plains. The seismicity shuts down were this trend rotates to a more N-S direction as the Lebombo monocline is approached. 20th Century seismicity of SE Africa shows a clear cluster in time, with five M>6 earthquakes concentrated in the 1950's, distributed along the edges of the Zimbabwe craton and spanning distances of 600 km. Spatial correlation with such range is hard to reconcile with stress transmission in the crust and may point to the interaction of the cratonic root with asthenospheric flow. Under this light, the M6.5 Central Botswana earthquake of April 2017 and the M7 Machaze earthquake of 2006, both located in the vicinity of the borders of the Kaapvaal craton, may bear a similar correlation. The

  12. Asthenosphere versus lithosphere as possible sources for basaltic magmas erupted during formation of the Red Sea

    International Nuclear Information System (INIS)

    Altherr, R.; Henjes-Kunst, F.; Baumann, A.

    1990-01-01

    Representative basalts from the axial trough of the Red Sea and from volcanic fields of the Arabian Peninsula ranging in composition from N-type MORB to basanite and in age from Early Miocene to Recent show a limited variation in their isotopic compositions: 87 Sr/ 86 Sr = 0.70240-0.70361, 206 Pb/ 204 Pb = 18.040-19.634, 207 Pb/ 204 Pb = 15.496-15.666, 208 Pb/ 204 Pb = 37.808-39.710, 143 Nd/ 144 Nd = 0.513194-0.512670. There is a poorly constrained correlation between chemical composition and isotope ratios: with increasing alkalinity, Sr and Pb isotope ratios increase and the Nd isotope ratio tends to decrease. In Pb isotope variation diagrams most of the basalts plot significantly above the NHRLs, irrespective of tectonic setting, i.e. thickness of underlying crust and/or lithosphere. MORBs from the axial trough of the Red Sea have higher Pb isotope ratios for a given 87 Sr/ 86 Sr than MORBs from the Indian Ocean ridges, including the Carlsberg Ridge. It is therefore suggested that both spreading ridges tap different convective systems in the asthenosphere. The tectonic setting of the basalts is reflected in their Nd-Sr isotope characteristics. Basalts from areas where the continental lithosphere is drastically thinned or absent (i.e. Red Sea axial trough and coastal plain, Afar) plot along a reference line defined by N-type MORB and Tristan da Cunha. Basalts erupted in areas with Pan-African crust of normal thickness and moderately thinned lithospheric mantle (i.e. rift shoulder) are characterized by relative low 143 Nd/ 144 Nd ratios and plot below the reference line towards an EM I component which is also found in the subcontinental lithospheric mantle. These differences in the Nd-Sr isotopic compositions of the basalts are independent of bulk-rock chemistry and are therefore controlled by tectonic setting alone. (orig./WL)

  13. Saghro Group in the Ougnat Massif (Morocco), an evidence for a continuous Cadomian basin along the northern West African Craton

    Science.gov (United States)

    Michard, André; Soulaimani, Abderrahmane; Ouanaimi, Hassan; Raddi, Youssef; Aït Brahim, Lahsen; Rjimati, Ech-Cherki; Baidder, Lahssen; Saddiqi, Omar

    2017-03-01

    The Saghro Group (SG) is a folded, low-grade volcano-sedimentary series up to 8 km thick that crops out within and to the north of the Pan-African suture zone in the central and eastern Anti-Atlas. Here we describe the SG of the Ougnat inliers that are exposed in the easternmost Anti-Atlas beneath the unconformable, Late Ediacaran Ouarzazate Group (OZG) volcanic rocks. The Ougnat SG mostly consists of volcaniclastic greywackes accumulated in a peritidal-to-shallow basin. The basin infilling was deformed by NNE-trending, mostly upright folds with axial-planar slaty cleavage and low-grade metamorphism. The deformed SG rocks were intruded by the ∼550 Ma Mellab hypovolcanic granodiorite. The latter also crosscuts the lowest OZG rocks that are dated to 574-571 Ma in the western Saghro region. The SG rocks that form the Siroua and Saghro inliers have an oldest age of 620-610 Ma and were folded at ∼610-580 Ma at the onset of the Cadomian orogenic events. We show that the SG rocks are similar to the ;Série verte; (SV) rocks that are exposed in the Ougarta and western Hoggar east of the Pan-African suture. We infer that the SG and SV rocks accumulated in a same, continuous basin that was bounding the West African Craton to the north and the east. This strongly subsiding basin formed close to a volcanic arc and was folded during the last Pan-African synmetamorphic events. Fold orientation and age of folding differ however along the edge of the West African Craton. The orogenic greywackes that form the remnants of the SG-SV basin thus constitute a precious record of the diachronic Cadomian event s.l. along the West African Craton northern margin.

  14. Petrology of Serpentinites and Rodingites in the Oceanic Lithosphere

    OpenAIRE

    Klein, Frieder

    2009-01-01

    Serpentinization, steatitization, and rodingitization are consequences of seawater reaction with lithospheric mantle. These processes take place coevally within the oceanic lithosphere and are related to circulation pathways, lithologic makeup of rocks along the flow path, fluid flux, and temperature. While the boundary conditions are set by the history of magmatic and tectonic accretion of the lithosphere, fluid-rock equilibria determine what reactions take place and where in the system. Pet...

  15. Between carbonatite and lamproite—Diamondiferous Torngat ultramafic lamprophyres formed by carbonate-fluxed melting of cratonic MARID-type metasomes

    Science.gov (United States)

    Tappe, Sebastian; Foley, Stephen F.; Kjarsgaard, Bruce A.; Romer, Rolf L.; Heaman, Larry M.; Stracke, Andreas; Jenner, George A.

    2008-07-01

    New U-Pb perovskite ages reveal that diamondiferous ultramafic lamprophyre magmas erupted through the Archean crust of northern Labrador and Quebec (eastern Canada) between ca. 610 and 565 Ma, a period of strong rifting activity throughout contiguous Laurentia and Baltica. The observed Torngat carbonate-rich aillikite/carbonatite and carbonate-poor mela-aillikite dyke varieties show a large spread in Sr-Nd-Hf-Pb isotope ratios with pronounced correlations between isotope systems. An isotopically depleted component is identified solely within aillikites ( 87Sr/ 86Sr i = 0.70323-0.70377; ɛNd i = +1.2-+1.8; ɛHf i = +1.4-+3.5; 206Pb/ 204Pb i = 18.2-18.5), whereas some aillikites and all mela-aillikites range to more enriched isotope signatures ( 87Sr/ 86Sr i = 0.70388-0.70523; ɛNd i = -0.5 to -3.9; ɛHf i = -0.6 to -6.0; 206Pb/ 204Pb i = 17.8-18.2). These contrasting isotopic characteristics of aillikites/carbonatites and mela-aillikites, along with subtle differences in their modal carbonate, SiO 2, Al 2O 3, Na 2O, Cs-Rb, and Zr-Hf contents, are consistent with two distinctive metasomatic assemblages of different age in the mantle magma source region. Integration of petrologic, geochemical, and isotopic information leads us to propose that the isotopically enriched component originated from a reduced phlogopite-richterite-Ti-oxide dominated source assemblage that is reminiscent of MARID suite xenoliths. In contrast, the isotopically depleted component was derived from a more oxidized phlogopite-carbonate dominated source assemblage. We argue that low-degree CO 2-rich potassic silicate melts from the convective upper mantle were preferentially channelled into an older, pre-existing MARID-type vein network at the base of the North Atlantic craton lithosphere, where they froze to form new phlogopite-carbonate dominated veins. Continued stretching and thinning of the cratonic lithosphere during the Late Neoproterozoic remobilized the carbonate-rich vein material and

  16. Thick Toenails

    Science.gov (United States)

    ... in individuals with nail fungus (onychomycosis), psoriasis and hypothyroidism. Those who have problems with the thickness of their toenails should consult a foot and ankle surgeon for proper diagnosis and treatment. Find an ACFAS Physician Search Search Tools Find ...

  17. Constraints on the Lithospheric Strength at Volcanic Rifted Margins from the Geometry of Seaward Dipping Reflectors Using Analytic and Numerical Models

    Science.gov (United States)

    Tian, X.; Buck, W. R.

    2017-12-01

    Seaward dipping reflectors (SDRs) are found at many rifted margins. Drilling indicates SDRs are interbedded layers of basalts and sediments. Multi-channel seismic reflection data show SDRs with various width (2 100 km), thickness (1 15 km) and dip angles (0 30). Recent studies use analytic thin plate models (AtPM) to describe plate deflections under volcanic loads. They reproduce a wide range of SDRs structures without detachment faulting. These models assume that the solidified dikes provide downward loads at the rifting center. Meanwhile, erupted lava flows and sediments fill in the flexural depression and further load the lithosphere. Because the strength of the lithosphere controls the amount and wavelength of bending, the geometries of SDRs provide a window into the strength of the lithosphere during continental rifting. We attempt to provide a quantitative mapping between the SDR geometry and the lithospheric strength and thickness during rifting. To do this, we first derive analytic solutions to two observables that are functions of effective elastic thickness (Te). One observable (Xf) is the horizontal distance for SDRs to evolve from flat layers to the maximum bent layers. Another observable is the ratio between the thickness and the tangent of the maximum slope of SDRs at Xf. We then extend the AtPM to numerical thin plate models (NtPM) with spatially restricted lava flows. AtPM and NtPM show a stable and small relative difference in terms of the two observables with different values of Te. This provides a mapping of Te between NtPM and AtPM models. We also employ a fully two-dimensional thermal-mechanical treatment with elasto-visco-plastic rheology to simulate SDRs formation. These models show that brittle yielding due to bending can reduce the Te of the lithosphere by as much as 50% of the actual brittle lithospheric thickness. Quantification of effects of plastic deformation on bending allow us to use Te to link SDRs geometries to brittle lithospheric

  18. Global equivalent magnetization of the oceanic lithosphere

    Science.gov (United States)

    Dyment, J.; Choi, Y.; Hamoudi, M.; Lesur, V.; Thebault, E.

    2015-11-01

    As a by-product of the construction of a new World Digital Magnetic Anomaly Map over oceanic areas, we use an original approach based on the global forward modeling of seafloor spreading magnetic anomalies and their comparison to the available marine magnetic data to derive the first map of the equivalent magnetization over the World's ocean. This map reveals consistent patterns related to the age of the oceanic lithosphere, the spreading rate at which it was formed, and the presence of mantle thermal anomalies which affects seafloor spreading and the resulting lithosphere. As for the age, the equivalent magnetization decreases significantly during the first 10-15 Myr after its formation, probably due to the alteration of crustal magnetic minerals under pervasive hydrothermal alteration, then increases regularly between 20 and 70 Ma, reflecting variations in the field strength or source effects such as the acquisition of a secondary magnetization. As for the spreading rate, the equivalent magnetization is twice as strong in areas formed at fast rate than in those formed at slow rate, with a threshold at ∼40 km/Myr, in agreement with an independent global analysis of the amplitude of Anomaly 25. This result, combined with those from the study of the anomalous skewness of marine magnetic anomalies, allows building a unified model for the magnetic structure of normal oceanic lithosphere as a function of spreading rate. Finally, specific areas affected by thermal mantle anomalies at the time of their formation exhibit peculiar equivalent magnetization signatures, such as the cold Australian-Antarctic Discordance, marked by a lower magnetization, and several hotspots, marked by a high magnetization.

  19. The extending lithosphere (Arthur Holmes Medal Lecture)

    Science.gov (United States)

    Brun, Jean-Pierre

    2017-04-01

    Extension of the lithosphere gives birth to a wide range of structures, with characteristic widths between 10 and 1000 km, which includes continental rifts, passive margins, oceanic rifts, core complexes, or back-arc basins. Because the rheology of rocks strongly depends on temperature, this variety of extensional structures falls in two broad categories of extending lithospheres according to the initial Moho temperature TM. "Cold extending systems", with TM 750°C and crustal-dominated strength, lead, depending on strain rate, to either wide rifts or metamorphic core complexes. A much less quoted product of extension is the exhumation of high-pressure (HP ) metamorphic rocks occurring in domains of back-arc extension driven by slab rollback (e.g. Aegean; Appennines-Calabrian) or when the subduction upper plate undergoes extension for plate kinematics reasons (e.g. Norwegian Caledonides; Papua New Guinea). In these tectonic environments, well-documented pressure-temperature-time (P - T - t) paths of HP rocks show a two-stage retrogression path whose the first part corresponds to an isothermal large pressure drop ΔP proportional to the maximum pressure Pmax recorded by the rocks. This linear relation between ΔP and Pmax, which likely results from a stress switch between compression and extension at the onset of exhumation, is in fact observed in all HP metamorphism provinces worldwide, suggesting that the exhumation of HP rocks in extension is a general process rather than an uncommon case. In summary, the modes and products of extension are so diverse that, taken all together, they constitute a very versatile natural laboratory to decipher the rheological complexities of the continental lithosphere and their mechanical implications.

  20. Juvenile crust formation in the Zimbabwe Craton deduced from the O-Hf isotopic record of 3.8-3.1 Ga detrital zircons

    Science.gov (United States)

    Bolhar, Robert; Hofmann, Axel; Kemp, Anthony I. S.; Whitehouse, Martin J.; Wind, Sandra; Kamber, Balz S.

    2017-10-01

    Hafnium and oxygen isotopic compositions measured in-situ on U-Pb dated zircon from Archaean sedimentary successions belonging to the 2.9-2.8 Ga Belingwean/Bulawayan groups and previously undated Sebakwian Group are used to characterize the crustal evolution of the Zimbabwe Craton prior to 3.0 Ga. Microstructural and compositional criteria were used to minimize effects arising from Pb loss due to metamorphic overprinting and interaction with low-temperature fluids. 207Pb/206Pb age spectra (concordance >90%) reveal prominent peaks at 3.8, 3.6, 3.5, and 3.35 Ga, corresponding to documented geological events, both globally and within the Zimbabwe Craton. Zircon δ18O values from +4 to +10‰ point to both derivation from magmas in equilibrium with mantle oxygen and the incorporation of material that had previously interacted with water in near-surface environments. In εHf-time space, 3.8-3.6 Ga grains define an array consistent with reworking of a mafic reservoir (176Lu/177Hf ∼0.015) that separated from chondritic mantle at ∼3.9 Ga. Crustal domains formed after 3.6 Ga depict a more complex evolution, involving contribution from chondritic mantle sources and, to a lesser extent, reworking of pre-existing crust. Protracted remelting was not accompanied by significant mantle depletion prior to 3.35 Ga. This implies that early crust production in the Zimbabwe Craton did not cause complementary enriched and depleted reservoirs that were tapped by later magmas, possibly because the volume of crust extracted and stabilised was too small to influence (asthenospheric) mantle isotopic evolution. Growth of continental crust through pulsed emplacement of juvenile (chondritic mantle-derived) melts, into and onto the existing cratonic nucleus, however, involved formation of complementary depleted subcontinental lithospheric mantle since the early Archaean, indicative of strongly coupled evolutionary histories of both reservoirs, with limited evidence for recycling and lateral

  1. Inclusions of crichtonite-group minerals in Cr-pyropes from the Internatsionalnaya kimberlite pipe, Siberian Craton: Crystal chemistry, parageneses and relationships to mantle metasomatism

    Science.gov (United States)

    Rezvukhin, Dmitriy I.; Malkovets, Vladimir G.; Sharygin, Igor S.; Tretiakova, Irina G.; Griffin, William L.; O'Reilly, Suzanne Y.

    2018-05-01

    Cr-pyrope xenocrysts and associated inclusions of crichtonite-group minerals from the Internatsionalnaya kimberlite pipe were studied to provide new insights into processes in the lithospheric mantle beneath the Mirny kimberlite field, Siberian craton. Pyropes are predominantly of lherzolitic paragenesis (Cr2O3 2-6 wt%) and have trace-element spectra typical for garnets from fertile mantle (gradual increase in chondrite-normalized values from LREE to MREE-HREE). Crichtonite-group minerals commonly occur as monomineralic elongated inclusions, mostly in association with rutile, Mg-ilmenite and Cr-spinel within individual grains of pyrope. Sample INT-266 hosts intergrowth of crichtonite-group mineral and Cl-apatite, while sample INT-324 contains polymineralic apatite- and dolomite-bearing assemblages. Crichtonite-group minerals are Al-rich (1.1-4.5 wt% Al2O3), moderately Zr-enriched (1.3-4.3 wt% ZrO2), and are Ca-, Sr-, and occasionally Ba-dominant in terms of A-site occupancy; they also contain significant amounts of Na and LREE. T-estimates and chemical composition of Cr-pyropes imply that samples represent relatively low-T peridotite assemblages with ambient T ranging from 720 to 820°С. Projected onto the 35 mW/m2 cratonic paleogeotherm for the Mirny kimberlite field (Griffin et al., 1999b. Tectonophysics 310, 1-35), temperature estimates yield a P range of 34-42 kbar ( 110-130 km), which corresponds to a mantle domain in the uppermost part of the diamond stability field. The presence of crichtonite-group minerals in Cr-pyropes has petrological and geochemical implications as evidence for metasomatic enrichment of some incompatible elements in the lithospheric mantle beneath the Mirny kimberlite field. The genesis of Cr-pyropes with inclusions of crichtonite-group minerals is attributed to the percolation of Ca-Sr-Na-LREE-Zr-bearing carbonate-silicate metasomatic agents through Mg- and Cr-rich depleted peridotite protoliths. The findings of several potentially

  2. High-temperature peridotites - lithospheric or asthenospheric?

    International Nuclear Information System (INIS)

    Hops, J.J.; Gurney, J.J.

    1990-01-01

    High-temperature peridotites by definition yield equilibration temperatures greater than 1100 degrees C. On the basis of temperature and pressure calculations, these high-temperature peridotites are amongst the deepest samples entrained by kimberlites on route to the surface. Conflicting models proposing either a lithospheric or asthenospheric origin for the high-temperature peridotites have been suggested. A detailed study of these xenoliths from a single locality, the Jagersfontein kimberlite in the Orange Free State, has been completed as a means of resolving this controversy. 10 refs., 2 figs

  3. Craton-derived alluvium as a major sediment source in the Himalayan Foreland Basin of India

    DEFF Research Database (Denmark)

    Sinha, R.; Kettanah, Y.; Gibling, M.R.

    2009-01-01

    of the Bundelkhand Complex. Along the Yamuna Valley the red alluvium is overlain by gray alluvium dated at 82–35 ka ago, which also yields a cratonic signature, with large amounts of smectite derived from the Deccan Traps. Cratonic contributions are evident in alluvium as young as 9 ka ago in a section 25 km north...

  4. The Vendian-Early Palaeozoic sedimentary basins of the East European Craton

    NARCIS (Netherlands)

    Sliaupa, S.; Fokin, P.A.; Lazauskiene, J.; Stephenson, R.A.

    2006-01-01

    Vendian-Early Palaeozoic sedimentation on the East European Craton (EEC) was confined to the cratonic margins with limited intracratonic subsidence. Generally, there are two geodynamic stages involved: in stage 1, basins formed in response to continental break-up processes; in stage 2, basins formed

  5. Palaeomagnetism of the Palaeoproterozoic Boonadgin Dyke Suite, Yilgarn Craton: Possible connection with India

    Science.gov (United States)

    Liu, Y.; Li, Z. X.; Pisarevsky, S.; Kirscher, U.; Mitchell, R.; Stark, J. C.

    2017-12-01

    A palaeomagnetic study was carried out on the newly identified 1.9 Ga Boonadgin dyke swarm in the Yilgarn Craton, Western Australia. Ten dykes revealed a high-temperature characteristic remanent magnetisation (ChRM) with dual polarities, directing either SW shallow downward (4 sites) or NE shallow upward (6 sites). Our results reveal that the Yilgarn Craton was at an equatorial palaeolatitude at 1.9 Ga. Meanwhile, a paleopole from the ca. 1.9 Ga Dharwar dykes of South India, supported by a positive baked-contact test, puts India at a similar paleolatitude. The Boonadgin dyke swarm can be interpreted to represent an arm of a radiating dyke swarm that shared the same plume centre with coeval mafic dykes in the Dharwar and Bastar cratons of southern India. We therefore propose that the Western Australia Craton (WAC, consisting of the the Yilgarn and Pilbara cratons) and South India were connected at ca. 1.89 Ga.

  6. Insights into the lithospheric architecture of Iberia and Morocco from teleseismic body-wave attenuation

    Science.gov (United States)

    Bezada, Maximiliano J.

    2017-11-01

    The long and often complicated tectonic history of continental lithosphere results in lateral strength heterogeneities which in turn affect the style and localization of deformation. In this study, we produce a model for the attenuation structure of Iberia and northern Morocco using a waveform-matching approach on P-wave data from teleseismic deep-focus earthquakes. We find that attenuation is correlated with zones of intraplate deformation and seismicity, but do not find a consistent relationship between attenuation and recent volcanism. The main features of our model are low to moderate Δt* in the undeformed Tertiary basins of Spain and high Δt* in areas deformed by the Alpine orogeny. Additionally, low Δt* is found in areas where the Alboran slab is thought to be attached to the Iberian and African lithosphere, and high Δt* where it has detached. These features are robust with respect to inversion parameters, and are consistent with independent data. Very mild backazimuthal dependence of the measurements and comparison with previous results suggest that the source of the attenuation is sub-crustal. In line with other recent studies, the range of Δt* we observe is much larger than can be expected from lithospheric thickness or temperature variations.

  7. Implications for anomalous mantle pressure and dynamic topography from lithospheric stress patterns in the North Atlantic Realm

    Science.gov (United States)

    Schiffer, Christian; Nielsen, Søren Bom

    2016-08-01

    With convergent plate boundaries at some distance, the sources of the lithospheric stress field of the North Atlantic Realm are mainly mantle tractions at the base of the lithosphere, lithospheric density structure and topography. Given this, we estimate horizontal deviatoric stresses using a well-established thin sheet model in a global finite element representation. We adjust the lithospheric thickness and the sub-lithospheric pressure iteratively, comparing modelled in plane stress with the observations of the World Stress Map. We find that an anomalous mantle pressure associated with the Iceland and Azores melt anomalies, as well as topography are able to explain the general pattern of the principle horizontal stress directions. The Iceland melt anomaly overprints the classic ridge push perpendicular to the Mid Atlantic ridge and affects the conjugate passive margins in East Greenland more than in western Scandinavia. The dynamic support of topography shows a distinct maximum of c. 1000 m in Iceland and amounts <150 m along the coast of south-western Norway and 250-350 m along the coast of East Greenland. Considering that large areas of the North Atlantic Realm have been estimated to be sub-aerial during the time of break-up, two components of dynamic topography seem to have affected the area: a short-lived, which affected a wider area along the rift system and quickly dissipated after break-up, and a more durable in the close vicinity of Iceland. This is consistent with the appearance of a buoyancy anomaly at the base of the North Atlantic lithosphere at or slightly before continental breakup, relatively fast dissipation of the fringes of this, and continued melt generation below Iceland.

  8. Piecewise delamination of Moroccan lithosphere from beneath the Atlas Mountains

    Science.gov (United States)

    Bezada, M. J.; Humphreys, E. D.; Davila, J. M.; Carbonell, R.; Harnafi, M.; Palomeras, I.; Levander, A.

    2014-04-01

    The elevation of the intracontinental Atlas Mountains of Morocco and surrounding regions requires a mantle component of buoyancy, and there is consensus that this buoyancy results from an abnormally thin lithosphere. Lithospheric delamination under the Atlas Mountains and thermal erosion caused by upwelling mantle have each been suggested as thinning mechanisms. We use seismic tomography to image the upper mantle of Morocco. Our imaging resolves the location and shape of lithospheric cavities and of delaminated lithosphere ˜400 km beneath the Middle Atlas. We propose discontinuous delamination of an intrinsically unstable Atlas lithosphere, enabled by the presence of anomalously hot mantle, as a mechanism for producing the imaged structures. The Atlas lithosphere was made unstable by a combination of tectonic shortening and eclogite loading during Mesozoic rifting and Cenozoic magmatism. The presence of hot mantle sourced from regional upwellings in northern Africa or the Canary Islands enhanced the instability of this lithosphere. Flow around the retreating Alboran slab focused upwelling mantle under the Middle Atlas, which we infer to be the site of the most recent delamination. The Atlas Mountains of Morocco stand as an example of large-scale lithospheric loss in a mildly contractional orogen.

  9. Low crustal velocities and mantle lithospheric variations in southern Tibet from regional Pnl waveforms

    Science.gov (United States)

    Rodgers, Arthur J.; Schwartz, Susan Y.

    We report low average crustal P-wave velocities (5.9-6.1 km/s, Poisson's ratio 0.23-0.27, thickness 68-76 km) in southern Tibet from modelling regional Pnl waveforms recorded by the 1991-1992 Tibetan Plateau Experiment. We also find that the mantle lithosphere beneath the Indus-Tsangpo Suture and the Lhasa Terrane is shield-like (Pn velocity 8.20-8.25 km/s, lid thickness 80-140 km, positive velocity gradient 0.0015-0.0025 s-1). Analysis of relative Pn travel time residuals requires a decrease in the mantle velocities beneath the northern Lhasa Terrane, the Banggong-Nujiang Suture and the southern Qiangtang Terrane. Tectonic and petrologic considerations suggest that low bulk crustal velocities could result from a thick (50-60 km) felsic upper crust with vertically limited and laterally pervasive partial melt. These results are consistent with underthrusting of Indian Shield lithosphere beneath the Tibetan Plateau to at least the central Lhasa Terrane.

  10. The westward drift of the lithosphere: A tidal ratchet?

    Directory of Open Access Journals (Sweden)

    A. Carcaterra

    2018-03-01

    Full Text Available Is the westerly rotation of the lithosphere an ephemeral accidental recent phenomenon or is it a stable process of Earth's geodynamics? The reason why the tidal drag has been questioned as the mechanism determining the lithospheric shift relative to the underlying mantle is the apparent too high viscosity of the asthenosphere. However, plate boundaries asymmetries are a robust indication of the ‘westerly’ decoupling of the entire Earth's outer lithospheric shell and new studies support lower viscosities in the low-velocity layer (LVZ atop the asthenosphere. Since the solid Earth tide oscillation is longer in one side relative to the other due to the contemporaneous Moon's revolution, we demonstrate that a non-linear rheological behavior is expected in the lithosphere mantle interplay. This may provide a sort of ratchet favoring lowering of the LVZ viscosity under shear, allowing decoupling in the LVZ and triggering the westerly motion of the lithosphere relative to the mantle.

  11. Mars: Lithospheric Flexure of the Tharsis Montes Volcanoes and the Evolutionary Relationship to Their Tectonic History

    Science.gov (United States)

    Chute, H.; Dombard, A. J.; Byrne, P. K.

    2017-12-01

    Lithospheric flexure associated with Arsia, Pavonis, and Ascraeus Montes has been previously studied to constrain the timeline and breadth of endogenic surface features surrounding these volcanoes. Here, we simulate the radial extent of two specific load-related features: annular graben and flank terraces. Detailed mapping of Ascraeus Mons (the youngest of the three volcanoes) showed a phase of compression of the edifice, forming the terraces and an annulus of graben immediately off the flanks, followed by a period of extension that formed additional graben superposed on the terraces on the lower flanks of the edifice. This transition from compression to extension on the lower flanks has been difficult to reconcile in mechanical models. We explore, with finite-element simulations, the effects of a thermal anomaly associated with an intrusive crustal underplate, which results in locally thinning the lithosphere (in contrast to past efforts that assumed a constant-thickness lithosphere). We find that it is primarily the horizontal extent of this thermal anomaly that governs how the lithosphere flexes under a volcano, as well as the transition from flank compression to a tight annulus of extensional stresses. Specifically, we propose that the structures on Ascraeus may be consistent with an early stage of volcanic growth accompanied by an underplate about the same width as the edifice that narrowed as volcanism waned, resulting in an inward migration of the extensional horizontal stresses from the surrounding plains onto the lower flanks. By linking the surface strains on the volcano with the volcano-tectonic evolution predicted by our flexure model, we can further constrain a more accurate timeline for the tectonic history of Ascraeus Mons. More broadly, because these tectonic structures are commonly observed, our results provide a general evolutionary model for large shield volcanoes on Mars.

  12. Long wavelength magnetic anomalies over continental rifts in cratonic region

    Science.gov (United States)

    Friedman, S. A.; Persaud, P.; Ferre, E. C.; Martín-Hernández, F.; Feinberg, J. M.

    2017-12-01

    New collections of unaltered mantle xenoliths shed light on potential upper mantle contributions to long wavelength magnetic anomalies (LWMA) in continental rifts in cratonic / shield areas. The new material originates from the East African Rift (Tanzania), the Rio Grande Rift (U.S.A.), the Rhine Rift (Germany), and the West Antarctic Rift (Antarctica). The xenoliths sample the uppermost ( 0.2 or Fe geotherms (>60ºC/km) that are characteristic of rifted regions preclude any contribution to LWMA at depths >10 km. Hence, only upper basalts and hypovolcanic mafic sills would constitute potential magnetic sources. In contrast, the margins of these rifted regions consist of refractory cratonic domains, often characterized by oxidized sublithospheric mantle that host significant concentrations of primary magnetite. The higher NRMs of these peridotites (up to 15 A/m, Qn > 2.5) combined with much lower geotherms (as low as 15ºC/km) allows for a 5 to 10 km layer of uppermost mantle to potentially contribute to LWMA. Assuming that Qn values in rift margins are also gradient across the rift would primarily reflect thermal equilibration over time.

  13. Probing the Cypriot Lithosphere: Insights from Broadband Seismology

    Science.gov (United States)

    Ogden, C. S.; Bastow, I. D.; Pilidou, S.; Dimitriadis, I.; Iosif, P.; Constantinou, C.; Kounoudis, R.

    2017-12-01

    Cyprus, an island in the eastern Mediterranean Sea, is an ideal study locale for understanding both the final stages of subduction, and the internal structure of so-called `ophiolites' - rare, on-land exposures of oceanic crust. The Troodos ophiolite offers an excellent opportunity to interrogate a complete ophiolite sequence from mantle rocks to pillow lavas. However, determining its internal architecture, and that of the subducting African plate deep below it, cannot be easily achieved using traditional field geology. To address this issue, we have built a new network of five broadband seismograph stations across the island. These, along with existing permanent stations, record both local and teleseismic earthquakes that we are now using to image Cyprus' crust and mantle seismic structure. Receiver functions are time series, computed from three-component seismograms, which contain information about lithospheric seismic discontinuities. When a P-wave strikes a velocity discontinuity such as the Moho, energy is converted to S-waves (direct Ps phase). The widely-used H-K Stacking technique utilises this arrival, and subsequent crustal reverberations (PpPs and PsPs+PpSs), to calculate crustal thickness (H) and bulk-crustal Vp/Vs ratio (K). Central to the method is the assumption that the Moho produces the largest amplitude conversions, after the direct P-arrival, which is valid where the Moho is sharp. Where the Moho is gradational or upper crustal discontinuities are present, the Moho signals are weakened and masked by shallow crustal conversions, potentially rendering the H-K stacking method unreliable. Using a combination of synthetic and observed seismograms, we explore Cyprus' crustal structure and, specifically, the reliability of the H-K method in constraining it. Data quality is excellent across the island, but the receiver function Ps phase amplitude is low, and crustal reverberations are almost non-existent. Therefore, a simple, abrupt wavespeed jump at the

  14. Genesis of the Hengling magmatic belt in the North China Craton: Implications for Paleoproterozoic tectonics

    Science.gov (United States)

    Peng, Peng; Guo, Jinghui; Zhai, Mingguo; Windley, Brian F.; Li, Tiesheng; Liu, Fu

    2012-09-01

    The 2200-1880 Ma igneous rocks in the central and eastern parts of the North China Craton (NCC) constitute a new Hengling magmatic belt (HMB), which includes the ~ 2147 Ma Hengling mafic sill/dyke swarm, the ~ 2060 Ma Yixingzhai mafic dyke swarm, and the ~ 1973 Ma Xiwangshan mafic dyke swarm. The three swarms are contiguous and have experienced variable degrees of metamorphism from greenschist to low amphibolite facies (Hengling), medium granulite facies (Yixingzhai), and medium/high-pressure granulite facies (Xiwangshan). They are all tholeiitic in composition typically with 47-52 wt.% SiO2 and 4-10 wt.% MgO, and all show light rare earth element enrichments and Nb- and Ta-depletion. Their Nd TDM ages are in the range of 2.5-3.0 Ga. Specifically, the Hengling and Yixingzhai dykes/sills are depleted in Th, U, Zr, Hf and Ti, whereas the Xiwangshan dykes are enriched in U and weakly depleted in other elements. Variable Sr-anomalies indicate significant feldspar accumulation (positive anomalies) or fractionation. The ɛNd(t) values of the three swarms are: - 3.2-+3.0 (Hengling), - 1.7-+ 1.8 (Yixingzhai) and - 1.4-+ 1.0 (Xiwangshan). These mafic representatives of the HMB originated from the > 2.5 Ga sub-continental lithospheric mantle of the NCC, and with A-type granites and other igneous associations in this belt they likely evolved in an intra-continental rift. The progressive changing compositions of the three swarms are interpreted in terms of their source regions at different depths, i.e., shallower and shallower through time. And the decrease in scale and size of the intrusions and their magma volumes indicate the progressive weakening of magmatism in this rift. The rocks in this belt are different chronologically, petrologically and chemically from those in the Xuwujia magmatic belt (XMB). We propose that the two magmatic belts represent two different magmatic systems in different blocks of the NCC, i.e., an eastern block (with the HMB) and a western block

  15. Lithosphere and upper-mantle structure of the southern Baltic Sea estimated from modelling relative sea-level data with glacial isostatic adjustment

    Science.gov (United States)

    Steffen, H.; Kaufmann, G.; Lampe, R.

    2014-06-01

    During the last glacial maximum, a large ice sheet covered Scandinavia, which depressed the earth's surface by several 100 m. In northern central Europe, mass redistribution in the upper mantle led to the development of a peripheral bulge. It has been subsiding since the begin of deglaciation due to the viscoelastic behaviour of the mantle. We analyse relative sea-level (RSL) data of southern Sweden, Denmark, Germany, Poland and Lithuania to determine the lithospheric thickness and radial mantle viscosity structure for distinct regional RSL subsets. We load a 1-D Maxwell-viscoelastic earth model with a global ice-load history model of the last glaciation. We test two commonly used ice histories, RSES from the Australian National University and ICE-5G from the University of Toronto. Our results indicate that the lithospheric thickness varies, depending on the ice model used, between 60 and 160 km. The lowest values are found in the Oslo Graben area and the western German Baltic Sea coast. In between, thickness increases by at least 30 km tracing the Ringkøbing-Fyn High. In Poland and Lithuania, lithospheric thickness reaches up to 160 km. However, the latter values are not well constrained as the confidence regions are large. Upper-mantle viscosity is found to bracket [2-7] × 1020 Pa s when using ICE-5G. Employing RSES much higher values of 2 × 1021 Pa s are obtained for the southern Baltic Sea. Further investigations should evaluate whether this ice-model version and/or the RSL data need revision. We confirm that the lower-mantle viscosity in Fennoscandia can only be poorly resolved. The lithospheric structure inferred from RSES partly supports structural features of regional and global lithosphere models based on thermal or seismological data. While there is agreement in eastern Europe and southwest Sweden, the structure in an area from south of Norway to northern Germany shows large discrepancies for two of the tested lithosphere models. The lithospheric

  16. On the nature and origin of garnet in highly-refractory Archean lithosphere: implications for continent stabilisation

    Science.gov (United States)

    Gibson, Sally

    2014-05-01

    The nature and timescales of garnet formation in the Earth's subcontinental lithospheric mantle (SCLM) are important to our understanding of how this rigid outer shell has evolved and stabilised since the Archean. Nevertheless, the widespread occurrence of pyrope garnet in the sub-cratonic mantle remains one of the 'holy grails' of mantle petrology. The paradox is that garnet often occurs in mantle lithologies (dunites and harzburgites) which represent residues of major melting events (up to 40 %) whereas experimental studies on fertile peridotite suggest this phase should be exhausted by years. The garnets display systematic trends from ultra-depleted to enriched compositions that have not been recognised in peridotite suites from elsewhere (Gibson et al., 2013). Certain harzburgite members of the xenolith suite contain the first reported occurrence of pyrope garnets with rare-earth element (REE) patterns similar to hypothetical garnets proposed by Stachel et al. (2004) to have formed in the Earth's SCLM during the Archean, prior to metasomatism. These rare ultra-depleted low-Cr garnets occur in low temperature (~1050 oC) xenoliths derived from depths of ~120 km and coexist in chemical and textural equilibrium with highly-refractory olivine (Fo95.4) and orthopyroxene (Mg#=96.4). These phases are all more magnesian than generally encountered in global samples of depleted mantle, i.e. harzburgites and diamond inclusion suites. The Tanzanian ultra-depleted garnets form interconnecting networks ('necklaces') around grains of orthopyroxene, which is of key importance to their origin. This close spatial relationship of garnet and orthopyroxene together with the major, trace and REE contents of the ultra-depleted garnets, are consistent with an origin by isochemical exsolution. The significance of ultra-depleted low-Cr garnets has not previously been recognised in global suites of mantle xenoliths or diamond inclusions: they appear to have been overlooked, primarily

  17. Platinum-group element contents of Karelian kimberlites: Implications for the PGE budget of the sub-continental lithospheric mantle

    Science.gov (United States)

    Maier, W. D.; O'Brien, H.; Peltonen, P.; Barnes, Sarah-Jane

    2017-11-01

    We present high-precision isotope dilution data for Os, Ir, Ru, Pt, Pd and Re in Group I and Group II kimberlites from the Karelian craton, as well as 2 samples of the Premier Group I kimberlite pipe from the Kaapvaal craton. The samples have, on average, 1.38 ppb Pt and 1.33 ppb Pd, with Pt/Pd around unity. These PGE levels are markedly lower, by as much as 80%, than those reported previously for kimberlites from South Africa, Brazil and India, but overlap with PGE results reported recently from Canadian kimberlites. Primitive-mantle-normalised chalcophile element patterns are relatively flat from Os to Pt, but Cu, Ni and, somewhat less so, Au are enriched relative to the PGE (e.g., Cu/Pd > 25.000). Pd/Ir ratios are 3,6 on average, lower than in most other mantle melts. The PGE systematics can be largely explained by two components, (i) harzburgite/lherzolite detritus of the SCLM with relatively high IPGE (Os-Ir-Ru)/PPGE (Rh-Pt-Pd) ratios, and (ii) a melt component that has high PPGE/IPGE ratios. By using the concentrations of iridium in the kimberlites as a proxy for the proportion of mantle detritus in the magma, we estimate that the analysed kimberlites contain 3-27% entrained and partially dissolved detritus from the sub-continental lithospheric mantle, consistent with previous estimates of kimberlites elsewhere (Tappe S. et al., 2016, Chem. Geol. 10.1016/j.chemgeo.2016.08.019).

  18. Lithospheric Structure of Antarctica and Implications for Geological and Cryospheric Evolution

    Science.gov (United States)

    Wiens, Douglas; Heeszel, David; Sun, Xinlei; Lloyd, Andrew; Nyblade, Andrew; Anandakrishnan, Sridhar; Aster, Richard; Chaput, Julien; Huerta, Audrey; Hansen, Samantha; Wilson, Terry

    2013-04-01

    Recent broadband seismic deployments, including the AGAP/GAMSEIS array of 24 broadband seismographs over the Gamburtsev Subglacial Mountains (GSM) in East Antarctica and the POLENET/ANET deployment of 33 seismographs across much of West Antarctica, reveal the detailed crust and upper mantle structure of Antarctica for the first time. The seismographs operate year-around even in the coldest parts of Antarctica, due to novel insulated boxes, power systems, and modified instrumentation developed in collaboration with the IRIS PASSCAL Instrument Center. We analyze the data using several different techniques to develop high-resolution models of Antarctic seismic structure. We use Rayleigh wave phase velocities at periods of 20-180 s determined using a modified two-plane wave decomposition of teleseismic Rayleigh waves to invert for the three dimensional shear velocity structure. In addition, Rayleigh wave group and phase velocities obtained by ambient seismic noise correlation methods provide constraints at shorter periods and shallower depths. Receiver functions provide precise estimates of crustal structure beneath the stations, and P and S wave tomography provides models of upper mantle structure down to ~ 500 km depth along transects of greater seismic station density. The new seismic results show that the high elevations of the GSM are supported by thick crust (~ 55 km), and are underlain by thick Precambrian continental lithosphere that initially formed during Archean to mid-Proterozoic times. The absence of lithospheric thermal anomalies suggests that the mountains were formed by a compressional orogeny during the Paleozoic, thus providing a locus for ice sheet nucleation throughout a long period of geological time. Within West Antarctica, the crust and lithosphere are extremely thin near the Transantarctic Mountain Front and topographic lows such as the Bentley Trench and Byrd Basin, which represent currently inactive Cenozoic rift systems. Slow seismic

  19. LITHOSPHERIC STRUCTURE OF THE CARPATHIAN-PANNONIAN REGION BASED ON THE GRAVITY MODELING BY INTEGRATING THE CELEBRATION2000 SEISMIC EXPERIMENT AND NEW GEOPHYSICAL RESULTS

    Science.gov (United States)

    Bielik, M.; Alasonati Tašárová, Z.; Zeyen, H. J.; Afonso, J.; Goetze, H.; Dérerová, J.

    2009-12-01

    Two different methods for the 3-D interpretation of the gravity field have been applied to the study of the structure and tectonics of the Carpathian-Pannonian lithosphere. The first (second) method provided a set of the different stripped gravity maps (the new lithosphere thickness map). The contribution presents the interpretation of the gravity field, which takes into account the CELEBRATION2000 seismic as well as new geophysical results. The sediment stripped gravity map is characterized by gravity minima in the Eastern Alps and Western Carpathians, and gravity maxima in the Pannonian Back-arc Basin system and the European platform. The gravity low in the Eastern Alps is produced by the thick crust (more than 45 km). The Western Carpathian gravity minimum is a result of the interference of two main gravitational effects. The first one comes from the low-density sediments of the Outer Western Carpathians and Carpathian Foredeep. The second one is due to the thick low-density upper and middle crust, reaching up to 25 km. In the Pannonian Back-arc Basin system can be observed the regional gravity high which is a result of the gravity effect of the anomalously shallow Moho. The most dominant feature of the complete 3-D stripped gravity map (crustal gravity effect map) is the abrupt change of the gravity field along the Klippen Belt zone. While the European platform is characterized by positive anomalies, the Western Carpathian orogen and the Pannonian Back-arc Basin system by relatively long-wavelength gravity low (several hundred kilometers). The lowest values are associated with the thick low-density upper and middle crust of the Inner Western Carpathians. That is why we suggest that the European Platform consists of the significantly denser crust with respect to the less dense crust of the microplates ALCAPA and Tisza-Dacia. The contrast in the gravity fields over the European platform and microplates ALCAPA and Tisza-Dacia reflect also their different crustal

  20. Elasticity of Pargasite Amphibole: A Hydrous Phase at Mid Lithospheric Discontinuity

    Science.gov (United States)

    Peng, Y.; Mookherjee, M.

    2017-12-01

    Mid Lithospheric Discontinuity (MLD) is characterized by a low shear wave velocity ( 3 to 10 %). In cratons, the depth of MLD varies between 80 and 100 km. The reduction of the shear wave velocity at MLD is similar to what is observed in the lithosphere-asthenosphere boundary (LAB). Such low velocity at MLD could be caused by partial melting, temperature induced grain boundary sliding, changes in the elastic anisotropy, and/or metasomatism which may lead to the formation of hydrous phases including mica and amphibole. Thus, it is clear that in order to assess the role of metasomatism at MLD, we need better constraints on the elasticity of hydrous phases. However, such elasticity data are scarce. In this study, we explore elasticity of pargasite amphibole [NaCa2(Mg4Al)(Si6Al2)O22(OH)2] using density functional theory (DFT) with local density approximation (LDA) and generalized gradient approximation (GGA). We find that the pressure-volume results can be adequately described by a finite strain equation with the bulk modulus, K0 being 102 and 85 GPa for LDA and GGA respectively. We also determined the full elastic constant tensor (Cij) using the finite difference method. The bulk modulus, K0 determined from the full elastic constant tensor is 104 GPa for LDA and 87 GPa for GGA. The shear modulus, G0 determined from the full elastic constant tensor is 64 GPa for LDA and 58 GPa for GGA. The bulk and shear moduli predicted with LDA are 5 and 1 % stiffer than the recent results [1]. In contrast, the bulk and shear moduli predicted with GGA are 12 and 10 % softer compared to the recent results [1]. The full elastic constant tensor for pargasite shows significant anisotropy. For instance, LDA predicts compressional (AVP) and shear (AVS) wave anisotropy of 22 and 20 % respectively. At higher pressure, elastic moduli stiffen. However, temperature is likely to have an opposite effect on the elasticity and this remains largely unknown for pargasite. Compared to the major mantle

  1. Thermal regime of the lithosphere and prediction of seismic hazard in the Caspian region

    International Nuclear Information System (INIS)

    Levin, L.E.; Solodilov, L.N.; Kondorskaya, N.V.; Gasanov, A.G; Panahi, B.M.

    2002-01-01

    Full text : Prediction of seicmicity is one of elements of ecology hazard warning. In this collective research, it is elaborated in three directions : quantitative estimate of regional faults by level of seismic activity; ascertainment of space position of earthquake risk zones, determination of high seismic potential sites for the period of the next 3-5 years. During elaboration of prediction, it takes into account that peculiar feature all over the is determined by relationship of about 90 percent of earthquake hypocenters and released energy of seismic waves with elactic-brittle ayer of the lithosphere. Concetration of earthquakes epicenters is established predominantly in zones of complex structure of elastic-brittle layer where gradient of it thickness is 20-30 km. Directions of hypocenters migration in the plastic-viscous layer reveal a space position of seismic dangerous zones. All this provides a necessity for generalization of data on location of earthquakes epicenters; determination of their magnitudes, space position of regional faults and heat flow with calculation of thermal regime being made for clarification of the lithosphere and elastic-brittle thickness variations separately. General analysis includes a calculation of released seismic wave energy and determination of peculiar features of its distribution in the entire region and also studies of hypocenters migration in the plastic-viscous layer of the litosphere in time.

  2. DESTRUCTION OF THE LITHOSPHERE: FAULTBLOCK DIVISIBILITY AND ITS TECTONOPHYSICAL REGULARITIES

    Directory of Open Access Journals (Sweden)

    Semen I. Sherman

    2012-01-01

    Full Text Available A new concept is proposed concerning the origin and inception of ‘initial’ faults and formation of large blocks as a result of cooling of the Archaean lithosphere, during which Benard cells had formed (Fig. 5. At locations where cooling convection currents went down, partial crystallization took place, stresses were localized, and initial fault occurred there. The systems of such fault developed mainly in two directions and gradually formed an initial block pattern of the lithosphere. This pattern is now represented by the largest Archaean faults acting as boundaries of the lithospheric plates and large intraplate blocks (Fig. 6. This group of faults represents the first scaletime level of destruction of the lithosphere. Large blocks of the first (and may be the second order, which are located on the viscous foundation, interacted with each other under the influence of the sublithospheric movements or endogenous sources and thus facilitated the occurrence of high stresses inside the blocks. When the limits of strength characteristics of the block medium were exceeded, the intrablock stresses were released and caused formation of fractures/faults and blocks of various ranks (Fig. 14. This large group, including faultblock structures of various ranks and ages, comprises the second level of the scaletime destruction of the lithosphere.The intense evolution of ensembles of faults and blocks of the second scaletime level is facilitated by shortterm activation of faultblock structures of the lithosphere under the influence of strain waves. Periods of intensive shortterm activation are reliably detected by seismic monitoring over the past fifty years. Investigations of periodical processes specified in the geological records over the post-Proterozoic periods [Khain, Khalilov, 2009] suggest that in so far uninvestigated historical and more ancient times, the top of the lithosphere was subject to wave processes that

  3. Evidence for multiphase folding of the central Indian Ocean lithosphere

    Digital Repository Service at National Institute of Oceanography (India)

    Krishna, K.S.; Bull, J.M.; Scrutton, R.A.

    Long-wavelength (100-300 km) folding in the central Indian Ocean associated with the diffuse plate boundary separating the Indian, Australian, and Capricorn plates is Earth's most convincing example of organized large-scale lithospheric deformation...

  4. Lithospheric Strength Beneath the Zagros Mountains of Southwestern Iran

    Science.gov (United States)

    Adams, A. N.; Nyblade, A.; Brazier, R.; Rodgers, A.; Al-Amri, A.

    2006-05-01

    The Zagros Mountain Belt of southwestern Iran is among the most seismically active mountain belts in the world. Early seismic studies of this area found that the lithosphere underlying the Zagros Mountains follows the "jelly sandwich" model, having a strong upper crust and a strong lithospheric mantle, separated by a weak lower crust. More recent studies, which analyzed earthquakes originating within the Zagros Mountains that were recorded at teleseismic distances, however, found that these earthquakes occurred only within the upper crust, thus indicating that the strength of the Zagros Mountains' lithosphere lies only within the upper crust, in accordance with the "creme brulee" lithospheric model. Preliminary analysis of regionally recorded earthquakes that originated within the Zagros Mountains is presented here. Using earthquakes recorded at regional distances will allow the analysis of a larger dataset than has been used in previous studies. Preliminary results show earthquakes occurring throughout the crust and possibly extending into the upper mantle.

  5. Petrologically-constrained thermo-chemical modelling of cratonic upper mantle consistent with elevation, geoid, surface heat flow, seismic surface waves and MT data

    Science.gov (United States)

    Jones, A. G.; Afonso, J. C.

    2015-12-01

    The Earth comprises a single physio-chemical system that we interrogate from its surface and/or from space making observations related to various physical and chemical parameters. A change in one of those parameters affects many of the others; for example a change in velocity is almost always indicative of a concomitant change in density, which results in changes to elevation, gravity and geoid observations. Similarly, a change in oxide chemistry affects almost all physical parameters to a greater or lesser extent. We have now developed sophisticated tools to model/invert data in our individual disciplines to such an extent that we are obtaining high resolution, robust models from our datasets. However, in the vast majority of cases the different datasets are modelled/inverted independently of each other, and often even without considering other data in a qualitative sense. The LitMod framework of Afonso and colleagues presents integrated inversion of geoscientific data to yield thermo-chemical models that are petrologically consistent and constrained. Input data can comprise any combination of elevation, geoid, surface heat flow, seismic surface wave (Rayleigh and Love) data and receiver function data, and MT data. The basis of LitMod is characterization of the upper mantle in terms of five oxides in the CFMAS system and a thermal structure that is conductive to the LAB and convective along the adiabat below the LAB to the 410 km discontinuity. Candidate solutions are chosen from prior distributions of the oxides. For the crust, candidate solutions are chosen from distributions of crustal layering, velocity and density parameters. Those candidate solutions that fit the data within prescribed error limits are kept, and are used to establish broad posterior distributions from which new candidate solutions are chosen. Examples will be shown of application of this approach fitting data from the Kaapvaal Craton in South Africa and the Rae Craton in northern Canada. I

  6. Late Cretaceous to Present evolution of the NW Africa peri-cratonic in the Africa-Eurasia plate convergence context

    Science.gov (United States)

    Ghorbal, B.; Bertotti, G.; Andriessen, P. A. M.

    2009-04-01

    Africa-Eurasia plate convergence is the main mechanism to explain topographic evolution and patterns of Tertiary vertical motions recorded around the entire Mediterranean and even further east. However, most of the studies are concentrated on the Eurasian side of the Mediterranean Realm. Along the NW Africa pericratonic zone (western Mediterranean side) extending longitudinally from the Anti-Atlas to the Rif Mountains, the highest topography is observed in the High Atlas intracontinental belt and in the Pan-African Anti-Atlas belt, and not in the youngest belt, the Rif. The combination of AFT and (U-Th)/He low-thermal dating, performed on pre-Cenozoic basement rocks along the Moroccan pericratonic transect (500km) yield ages ranging respectively between 90-9Ma and 65-7Ma, documenting vertical motions of subsidence and exhumation in between Late Cretaceous and Present. Time-Temperature models show that vertical movements are spatially zoned through Morocco, with the highest amplitude of vertical movements in the High Atlas (>4-5km) and more modest amounts in the Anti-Atlas and the Western Meseta (African peri-cratonic zone including the Western Meseta and the Anti-Atlas in addition to the Atlas and the Rif systems experienced Tertiary deformation. Two stages of folding are distinguished on the basis of low-thermal dating results along the pericratonic transect. The first is a lithospherical folding of ~500km in the Late Cretaceous (confirming that this process is a primary response to recently induced compressional stress fields) and the second is a crustal folding of ~100-150km wavelength in the Late Eocene that occurred independently to the mantle, requiring therefore the existence of a decoupling in between the base of the crust and the high mantle.

  7. Le craton ouest-africain et le bouclier guyanais: un seul craton au Protérozoïque inférieur?

    Science.gov (United States)

    Caen-Vachette, Michelle

    Geochronological and paleomagnetism data for southern West African craton and Guyana shield in South America, are concordant and suggest the existence of a large unit grouping them during Archean and Lower Proterozoic times. The paleomagnetism data allow to put on a single line, the Zednes (Mauritania), Sassandra (Ivory Coast) and Guri (Venezuela) fault zones, the mylonites of which were dated 1670 Ma. This age reflects the end of the eburnean-transamazonian shearing tectonic, which affected the large West Africa-Guyana unit. This line separates the western Archean domain from the eastern lower Proterozoic one; thence it is possible to correlate the Sasca (Ivory Coast) and Pastora (Venezuela) areas. Archean relics have been found in mobile pan-african-bresiliano zones which surround the Precambrian cratons; this fact suggests the existence of still more extended Archean craton than defined above.

  8. Lithospheric Shear Velocity Structure of South Island, New Zealand from Rayleigh Wave Tomography of Amphibious Array Data

    Science.gov (United States)

    Ball, J. S.; Sheehan, A. F.; Stachnik, J. C.; Lin, F. C.; Collins, J. A.

    2015-12-01

    We present the first 3D shear velocity model extending well offshore of New Zealand's South Island, imaging the lithosphere beneath Campbell and Challenger plateaus. Our model is constructed via linearized inversion of both teleseismic (18 -70 s period) and ambient noise-based (8 - 25 s period) Rayleigh wave dispersion measurements. We augment an array of 29 ocean-bottom instruments deployed off the South Island's east and west coasts in 2009-2010 with 28 New Zealand land-based seismometers. The ocean-bottom seismometers and 4 of the land seismometers were part of the Marine Observations of Anisotropy Near Aotearoa (MOANA) experiment, and the remaining land seismometers are from New Zealand's permanent GeoNet array. Major features of our shear wave velocity (Vs) model include a low-velocity (Vs<4.3km/s) body extending to at least 75km depth beneath the Banks and Otago peninsulas, a high-velocity (Vs~4.7km/s) upper mantle anomaly underlying the Southern Alps to a depth of 100km, and discontinuous lithospheric velocity structure between eastern and western Challenger Plateau. Using the 4.5km/s contour as a proxy for the lithosphere-asthenosphere boundary, our model suggests that the lithospheric thickness of Challenger Plateau is substantially greater than that of Campbell Plateau. The high-velocity anomaly we resolve beneath the central South Island exhibits strong spatial correlation with subcrustal earthquake hypocenters along the Alpine Fault (Boese et al., 2013). The ~400km-long low velocity zone we image beneath eastern South Island underlies Cenozoic volcanics and mantle-derived helium observations (Hoke et al., 2000) on the surface. The NE-trending low-velocity zone dividing Challenger Plateau in our model underlies a prominent magnetic discontinuity (Sutherland et al., 1999). The latter feature has been interpreted to represent a pre-Cretaceous crustal boundary, which our results suggest may involve the entire mantle lithosphere.

  9. Imaging rifting at the lithospheric scale in the northern East African Rift using S-to-P receiver functions

    Science.gov (United States)

    Lavayssiere, A.; Rychert, C.; Harmon, N.; Keir, D.; Hammond, J. O. S.; Kendall, J. M.; Leroy, S. D.; Doubre, C.

    2017-12-01

    The lithosphere is modified during rifting by a combination of mechanical stretching, heating and potentially partial melt. We image the crust and upper mantle discontinuity structure beneath the northern East African Rift System (EARS), a unique tectonically active continental rift exposing along strike the transition from continental rifting in the Main Ethiopian rift (MER) to incipient seafloor spreading in Afar and the Red Sea. S-to-P receiver functions from 182 stations across the northern EARS were generated from 3688 high quality waveforms using a multitaper technique and then migrated to depth using a regional velocity model. Waveform modelling of data stacked in large conversion point bins confirms the depth and strength of imaged discontinuities. We image the Moho at 29.6±4.7 km depth beneath the Ethiopian plateaux with a variability in depth that is possibly due to lower crustal intrusions. The crust is 27.3±3.9 km thick in the MER and thinner in northern Afar, 17.5±0.7 km. The model requires a 3±1.2% reduction in shear velocity with increasing depth at 68.5±1.5 km beneath the Ethiopian plateaux, consistent with the lithosphere-asthenosphere boundary (LAB). We do not resolve a LAB beneath Afar and the MER. This is likely associated with partial melt near the base of the lithosphere, reducing the velocity contrast between the melt-intruded lithosphere and the partially molten asthenosphere. We identify a 4.5±0.7% increase in velocity with depth at 91±3 km beneath the MER. This change in velocity is consistent with the onset of melting found by previous receiver functions and petrology studies. Our results provide independent constraints on the depth of melt production in the asthenosphere and suggest melt percolation through the base of the lithosphere beneath the northernmost East African rift.

  10. Flexural modeling of the elastic lithosphere at an ocean trench: A parameter sensitivity analysis using analytical solutions

    Science.gov (United States)

    Contreras-Reyes, Eduardo; Garay, Jeremías

    2018-01-01

    The outer rise is a topographic bulge seaward of the trench at a subduction zone that is caused by bending and flexure of the oceanic lithosphere as subduction commences. The classic model of the flexure of oceanic lithosphere w (x) is a hydrostatic restoring force acting upon an elastic plate at the trench axis. The governing parameters are elastic thickness Te, shear force V0, and bending moment M0. V0 and M0 are unknown variables that are typically replaced by other quantities such as the height of the fore-bulge, wb, and the half-width of the fore-bulge, (xb - xo). However, this method is difficult to implement with the presence of excessive topographic noise around the bulge of the outer rise. Here, we present an alternative method to the classic model, in which lithospheric flexure w (x) is a function of the flexure at the trench axis w0, the initial dip angle of subduction β0, and the elastic thickness Te. In this investigation, we apply a sensitivity analysis to both methods in order to determine the impact of the differing parameters on the solution, w (x). The parametric sensitivity analysis suggests that stable solutions for the alternative approach requires relatively low β0 values (rise bulge. The alternative method is a more suitable approach, assuming that accurate geometric information at the trench axis (i.e., w0 and β0) is available.

  11. The tectonic evolution of Southern part of the Sao Francisco Craton

    International Nuclear Information System (INIS)

    Teixeira, W.

    1986-01-01

    The potentiality of geochronology when it is applied to the geologic context of craton basement of archean areas is shown. Samples from southern part of the Sao Francisco Craton, in Brazil, were collected for petrographic analysis and geochronological data interpretation. The set of radiometric determinations was obtained by K-Ar, Rb-Sr, Pb-Pb and U-Pb methods. (M.C.K.) [pt

  12. Continental lithosphere of the Arabian Plate: A geologic, petrologic, and geophysical synthesis

    Science.gov (United States)

    Stern, Robert J.; Johnson, Peter

    2010-07-01

    The Arabian Plate originated ˜ 25 Ma ago by rifting of NE Africa to form the Gulf of Aden and Red Sea. It is one of the smaller and younger of the Earth's lithospheric plates. The upper part of its crust consists of crystalline Precambrian basement, Phanerozoic sedimentary cover as much as 10 km thick, and Cenozoic flood basalt (harrat). The distribution of these rocks and variations in elevation across the Plate cause a pronounced geologic and topographic asymmetry, with extensive basement exposures (the Arabian Shield) and elevations of as much as 3000 m in the west, and a Phanerozoic succession (Arabian Platform) that thickens, and a surface that descends to sea level, eastward between the Shield and the northeastern margin of the Plate. This tilt in the Plate is partly the result of marginal uplift during rifting in the south and west, and loading during collision with, and subduction beneath, the Eurasian Plate in the northeast. But a variety of evidence suggests that the asymmetry also reflects a fundamental crustal and mantle heterogeneity in the Plate that dates from Neoproterozoic time when the crust formed. The bulk of the Plate's upper crystalline crust is Neoproterozoic in age (1000-540 Ma) reflecting, in the west, a 300-million year process of continental crustal growth between ˜ 850 and 550 Ma represented by amalgamated juvenile magmatic arcs, post-amalgamation sedimentary and volcanic basins, and granitoid intrusions that make up as much as 50% of the Shield's surface. Locally, Archean and Paleoproterozoic rocks are structurally intercalated with the juvenile Neoproterozoic rocks in the southern and eastern parts of the Shield. The geologic dataset for the age, composition, and origin of the upper crust of the Plate in the east is smaller than the database for the Shield, and conclusions made about the crust in the east are correspondingly less definitive. In the absence of exposures, furthermore, nothing is known by direct observation about the

  13. Structure of the lithosphere-asthenosphere and volcanism in the Tyrrhenian Sea and surroundings

    International Nuclear Information System (INIS)

    Panza, G.F.; Aoudia, A.; Pontevivo, A.; Sarao, A.; Peccerillo, A.

    2003-01-01

    The Italian peninsula and the Tyrrhenian Sea are some of the geologically most complex regions on Earth. Such a complexity is expressed by large lateral and vertical variations of the physical properties as inferred from the lithosphere-asthenosphere structure and by the wide varieties of Polio-Quaternary magmatic rocks ranging from teacloth to calcalkaline to sodium- and potassium-alkaline and ultra- alkaline compositions. The integration of geophysical, petrological and geochemical data allows us to recognise various sectors in the Tyrrhenian Sea and surrounding areas and compare different volcanic complexes in order to better constrain the regional geodynamics. A thin crust overlying a soft mantle (10% of partial melting) is typical of the back arc volcanism of the central Tyrrhenian Sea (Magnaghi, Vavilov and Marsili) where tholeiitic rocks dominate. Similar lithosphere-asthenosphere structure is observed for Ustica, Vulture and Etna volcanoes where the geochemical signatures could be related to the contamination of the side intraplate mantle by material coming from the either ancient or active roll-back. The lithosphere-asthenosphere structure and geochemical-isotopic composition do not change significantly when we move to the Stromboli-Campanian volcanoes, where we identify a well developed low-velocity layer, about 10 km thick, below a thin lid, overlain by a thin continental crust. The geochemical signature of the nearby Ischia volcano is characteristic of the Campanian sector and the relative lithosphere-asthenosphere structure may likely represent a transition to the back arc volcanism sector acting in the central Tyrrhenian. The difference in terms of structure beneath Stromboli and the nearby Vulcano and Lipari is confirmed by different geochemical signatures. The affinity between Vulcano, Lipari and Etna could be explained by their common position along the Tindari-Letoianni-Malta fault zone. A low velocity mantle wedge, just below the Moho, is present

  14. Seismic Constraints on the Lithosphere-Asthenosphere Boundary Beneath the Izu-Bonin Area: Implications for the Oceanic Lithospheric Thinning

    Science.gov (United States)

    Cui, Qinghui; Wei, Rongqiang; Zhou, Yuanze; Gao, Yajian; Li, Wenlan

    2018-01-01

    The lithosphere-asthenosphere boundary (LAB) is the seismic discontinuity with negative velocity contrasts in the upper mantle. Seismic detections on the LAB are of great significance in understanding the plate tectonics, mantle convection and lithospheric evolution. In this paper, we study the LAB in the Izu-Bonin subduction zone using four deep earthquakes recorded by the permanent and temporary seismic networks of the USArray. The LAB is clearly revealed with sP precursors (sdP) through the linear slant stacking. As illustrated by reflected points of the identified sdP phases, the depth of LAB beneath the Izu-Bonin Arc (IBA) is about 65 km with a range of 60-68 km. The identified sdP phases with opposite polarities relative to sP phases have the average relative amplitude of 0.21, which means a 3.7% velocity drop and implies partial melting in the asthenosphere. On the basis of the crustal age data, the lithosphere beneath the IBA is located at the 1100 °C isotherm calculated with the GDH1 model. Compared to tectonically stable areas, such as the West Philippine Basin (WPB) and Parece Vela Basin (PVB) in the Philippine Sea, the lithosphere beneath the Izu-Bonin area shows the obvious lithospheric thinning. According to the geodynamic and petrological studies, the oceanic lithospheric thinning phenomenon can be attributed to the strong erosion of the small-scale convection in the mantle wedge enriched in volatiles and melts.

  15. A Review of Recent Developments in the Study of Regional Lithospheric Electrical Structure of the Asian Continent

    Science.gov (United States)

    Zhang, Letian

    2017-09-01

    The Asian continent was formed through the amalgamation of several major continental blocks that were formerly separated by the Paleo-Asian and Tethyan Oceans. During this process, the Asian continent underwent a long period of continental crustal growth and tectonic deformation, making it the largest and youngest continent on Earth. This paper presents a review of the application of geophysical electromagnetic methods, mainly the magnetotelluric (MT) method, in recent investigations of the diverse tectonic features across the Asian continent. The case studies cover the major continental blocks of Asia, the Central Asian orogenic system, the Tethyan orogenic system, as well as the western Pacific subduction system. In summary, most of the major continental blocks of Asia exhibit a three-layer structure with a resistive upper crust and upper mantle and a relatively conductive mid-lower crust. Large-scale conductors in the upper mantle were interpreted as an indication of lithospheric modification at the craton margins. The electrical structure of the Central Asian orogenic system is generally more resistive than the bordering continental blocks, whereas the Tethyan orogenic system displays more conductive, with pervasive conductors in the lower crust and upper mantle. The western Pacific subduction system shows increasing complexity in its electrical structure from its northern extent to its southern extent. In general, the following areas of the Asian continent have increasingly conductive lithospheric electrical structures, which correspond to a transition from the most stable areas to the most active tectonic areas of Asia: the major continental blocks, the accretionary Central Asian orogenic system, the collisional Tethyan orogenic system, and the western Pacific subduction system. As a key part of this review, a three-dimensional (3-D) model of the lithospheric electrical structure of a large portion of the Tibetan Plateau is presented and discussed in detail

  16. S-Wave's Velocities of the Lithosphere-Asthenosphere System in the Caribbean Region

    International Nuclear Information System (INIS)

    Gonzalez, O'Leary; Alvarez, Jose Leonardo; Moreno, Bladimir; Panza, Giuliano F.

    2010-06-01

    An overview of the S-wave velocity (Vs) structural model of the Caribbean is presented with a resolution of 2 o x2 o . As a result of the frequency time analysis (FTAN) of more than 400 trajectories epicenter-stations in this region, new tomographic maps of Rayleigh waves group velocity dispersion at periods ranging from 10 s to 40 s have been determined. For each 2 o x2 o cell, group velocity dispersion curves were determined and extended to 150 s adding data from a larger scale tomographic study (Vdovin et al., 1999). Using, as independent a priori information, the available geological and geophysical data of the region, each dispersion curve has been mapped, by non-linear inversion, into a set of Vs vs. depth models in the depth range from 0 km to 300 km. Due to the non-uniqueness of the solutions for each cell a Local Smoothness Optimization (LSO) has been applied to the whole region to identify a tridimensional model of Vs vs. depth in cells of 2 o x2 o , thus satisfying the Occam razor concept. Through these models some main features of the lithosphere and asthenosphere are evidenced, such as: the west directed subduction zone of the eastern Caribbean region with a clear mantle wedge between the Caribbean lithosphere and the subducted slab; the complex and asymmetric behavior of the crustal and lithospheric thickness in the Cayman ridge; the diffused presence of oceanic crust in the region; the presence of continental type crust in the South America, Central America and North America plates, as well as the bottom of the upper asthenosphere that gets shallower going from west to east. (author)

  17. Heat flow, heat transfer and lithosphere rheology in geothermal areas: Features and examples

    Science.gov (United States)

    Ranalli, G.; Rybach, L.

    2005-10-01

    Surface heat flow measurements over active geothermal systems indicate strongly positive thermal anomalies. Whereas in "normal" geothermal settings, the surface heat flow is usually below 100-120 mW m - 2 , in active geothermal areas heat flow values as high as several watts per meter squared can be found. Systematic interpretation of heat flow patterns sheds light on heat transfer mechanisms at depth on different lateral, depth and time scales. Borehole temperature profiles in active geothermal areas show various signs of subsurface fluid movement, depending on position in the active system. The heat transfer regime is dominated by heat advection (mainly free convection). The onset of free convection depends on various factors, such as permeability, temperature gradient and fluid properties. The features of heat transfer are different for single or two-phase flow. Characteristic heat flow and heat transfer features in active geothermal systems are demonstrated by examples from Iceland, Italy, New Zealand and the USA. Two main factors affect the rheology of the lithosphere in active geothermal areas: steep temperature gradients and high pore fluid pressures. Combined with lithology and structure, these factors result in a rheological zonation with important consequences both for geodynamic processes and for the exploitation of geothermal energy. As a consequence of anomalously high temperature, the mechanical lithosphere is thin and its total strength can be reduced by almost one order of magnitude with respect to the average strength of continental lithosphere of comparable age and thickness. The top of the brittle/ductile transition is located within the upper crust at depths less than 10 km, acts as the root zone of listric normal faults in extensional environments and, at least in some cases, is visible on seismic reflection lines. These structural and rheological features are well illustrated in the Larderello geothermal field in Tuscany.

  18. Lithospheric electrical structure of the middle Lhasa terrane in the south Tibetan plateau

    Science.gov (United States)

    Liang, Hongda; Jin, Sheng; Wei, Wenbo; Gao, Rui; Ye, Gaofeng; Zhang, Letian; Yin, Yaotian; Lu, Zhanwu

    2018-04-01

    The Lhasa terrane in southern Tibetan plateau is a huge tectono-magmatic belt and an important metallogenic belt. Its formation evolution process and mineralization are affected by the subduction of oceanic plate and subsequent continental collision. However, the evolution of Lhasa terrane has been a subject of much debate for a long time. The Lithospheric structure records the deep processes of the subduction of oceanic plate and continental collision. The magnetotelluric (MT) method can probe the sub-surface electrical conductivity, newly dense broadband and long period magnetotelluric data were collected along a south-north trending profile that across the Lhasa terrane at 88°-89°E. Dimensionality analyses demonstrated that the MT data can be interpreted using two-dimensional approaches, and the regional strike direction was determined as N110°E.Based on data analysis results, a two-dimensional (2-D) resistivity model of crust and upper mantle was derived from inversion of the transverse electric mode, transverse magnetic mode and vertical magnetic field data. Inversion model shows a large north-dipping resistor that extended from the upper crust to upper mantle beneath the Himalaya and the south of Lhasa Terrane, which may represent the subducting Indian continental lithosphere. The 31°N may be an important boundary in the Lhasa Terrane, the south performs a prominent high-conductivity anomaly from the lower crust to upper mantle which indicates the existence of asthenosphere upwelling, while the north performs a higher resistivity and may have a reworking ancient basement. The formation of the ore deposits in the study area may be related to the upwelling of the mantle material triggered by slab tearing and/or breaking off of the Indian lithosphere, and the mantle material input also contributed the total thickness of the present-day Tibetan crust. The results provide helpful constrains to understand the mechanism of the continent-continent collision and

  19. The stretching amplitude and thermal regime of the lithosphere in the nonvolcanic passive margin of Antarctica in the Mawson Sea region

    Science.gov (United States)

    Galushkin, Yu. I.; Leitchenkov, G. L.; Guseva, Yu. B.; Dubinin, E. P.

    2018-01-01

    The burial history and thermal evolution of the lithosphere within the passive nonvolcanic Antarctic margin in the region of the Mawson Sea are numerically reconstructed for the margin areas along the seismic profile 5909 with the use of the GALO basin modeling system. The amplitudes of the lithosphere stretching at the different stages of continental rifting which took place from 160 to 90 Ma ago are calculated from the geophysical estimates of the thickness of the consolidated crust and the tectonic analysis of the variations in the thickness of the sedimentary cover and sea depths during the evolution of the basin. It is hypothesized that the formation of the recent sedimentary section sequence in the studied region of the Antarctic margin began 140 Ma ago on a basement that was thinned by a factor of 1.6 to 4.5 during the first episode of margin stretching (160-140 Ma) under a fairly high heat flux. The reconstruction of the thermal regime of the lithosphere has shown that the mantle rocks could occur within the temperature interval of serpentinization and simultaneously within the time interval of lithospheric stretching (-160 serpentinization could take place in these areas as in the other margin segments at the stage of presedimentation ultra slow basement stretching.

  20. Post-processing scheme for modelling the lithospheric magnetic field

    Directory of Open Access Journals (Sweden)

    V. Lesur

    2013-03-01

    Full Text Available We investigated how the noise in satellite magnetic data affects magnetic lithospheric field models derived from these data in the special case where this noise is correlated along satellite orbit tracks. For this we describe the satellite data noise as a perturbation magnetic field scaled independently for each orbit, where the scaling factor is a random variable, normally distributed with zero mean. Under this assumption, we have been able to derive a model for errors in lithospheric models generated by the correlated satellite data noise. Unless the perturbation field is known, estimating the noise in the lithospheric field model is a non-linear inverse problem. We therefore proposed an iterative post-processing technique to estimate both the lithospheric field model and its associated noise model. The technique has been successfully applied to derive a lithospheric field model from CHAMP satellite data up to spherical harmonic degree 120. The model is in agreement with other existing models. The technique can, in principle, be extended to all sorts of potential field data with "along-track" correlated errors.

  1. Melt evolution beneath thick lithosphere: A magmatic inclusions study of La Palma, Canary Islands

    NARCIS (Netherlands)

    Nikogosian, I.; Elliott, T.R.; Touret, J.L.R.

    2002-01-01

    Volcanism in the Canary Islands is notable for its highly alkalic character even in the shield building lavas of the currently most active island, La Palma. In order to understand better the processes responsible for this alkalic end of the compositional spectrum of ocean island basalts (OIB), we

  2. Topographic asymmetry of the South Atlantic from global models of mantle flow and lithospheric stretching

    Science.gov (United States)

    Flament, Nicolas; Gurnis, Michael; Williams, Simon; Seton, Maria; Skogseid, Jakob; Heine, Christian; Müller, Dietmar

    2014-05-01

    The relief of the South Atlantic is characterized by elevated passive continental margins along southern Africa and eastern Brazil, and by the bathymetric asymmetry of the southern oceanic basin where the western flank is much deeper than the eastern flank. We investigate the origin of these topographic features in the present and over time since the Jurassic with a model of global mantle flow and lithospheric deformation. The model progressively assimilates plate kinematics, plate boundaries and lithospheric age derived from global tectonic reconstructions with deforming plates, and predicts the evolution of mantle temperature, continental crustal thickness, long-wavelength dynamic topography, and isostatic topography. Mantle viscosity and the kinematics of the opening of the South Atlantic are adjustable parameters in multiple model cases. Model predictions are compared to observables both for the present-day and in the past. Present-day predictions are compared to topography, mantle tomography, and an estimate of residual topography. Predictions for the past are compared to tectonic subsidence from backstripped borehole data along the South American passive margin, and to dynamic uplift as constrained by thermochronology in southern Africa. Comparison between model predictions and observations suggests that the first-order features of the topography of the South Atlantic are due to long-wavelength dynamic topography, rather than to asthenospheric processes. We find the uplift of southern Africa to be best reproduced with a lower mantle that is at least 40 times more viscous than the upper mantle.

  3. Nature of the basement of the East Anatolian plateau: Implications for the lithospheric foundering processes

    Science.gov (United States)

    Topuz, G.; Candan, O.; Zack, T.; Yılmaz, A.

    2017-12-01

    The East Anatolian Plateau (Turkey) is characterized by (1) an extensive volcanic-sedimentary cover of Neogene to Quaternary age, (2) crustal thicknesses of 42-50 km, and (3) an extremely thinned lithospheric mantle. Its basement beneath the young cover is thought to consist of oceanic accretionary complexes of Late Cretaceous to Oligocene age. The attenuated state of the lithospheric mantle and the causes of the young volcanism are accounted for by slab steepening and subsequent break-off. We present field geological, petrological and geochronological data on three basement inliers (Taşlıçay, Akdağ and Ilıca) in the region. These areas are made up of amphibolite- to granulite-facies rocks, comprising marble, amphibolite, metapelite, quartzite and metagranite. The granulite-facies domain is equilibrated at 0.7 GPa and 800 ˚C at 83 ± 2 Ma (2σ). The metamorphic rocks are intruded by subduction-related coeval gabbroic, quartz monzonitic to tonalitic rocks. Both the metamorphic rocks and the intrusions are tectonically overlain by ophiolitic rocks. All these crystalline rocks are unconformably overlain by lower Maastrichtien clastic rocks and reefal limestone, suggesting that the exhumation at the earth's surface and juxtaposition with ophiolitic rocks occurred by early Maastrichtien. U-Pb dating on igneous zircon from metagranite yielded a protolith age of 445 ± 10 Ma (2σ). The detrital zircons from a metaquartzite point to Neoproterozoic to Early Paleozoic provenance. All these data favor a more or less continuous continental substrate to the allochthonous ophiolitic rocks beneath the young volcanic-sedimentary cover. The metamorphism and coeval magmatism can be regarded as the middle- to lower-crustal root of the Late Cretaceous magmatic arc that developed due to northward subduction along the Bitlis-Zagros suture. The presence of a continental basement beneath the young cover requires that the loss of the lithospheric mantle from beneath the East

  4. Earth's oldest stable crust in the Pilbara Craton formed by cyclic gravitational overturns

    Science.gov (United States)

    Wiemer, Daniel; Schrank, Christoph E.; Murphy, David T.; Wenham, Lana; Allen, Charlotte M.

    2018-05-01

    During the early Archaean, the Earth was too hot to sustain rigid lithospheric plates subject to Wilson Cycle-style plate tectonics. Yet by that time, up to 50% of the present-day continental crust was generated. Preserved continental fragments from the early Archaean have distinct granite-dome/greenstone-keel crust that is interpreted to be the result of a gravitationally unstable stratification of felsic proto-crust overlain by denser mafic volcanic rocks, subject to reorganization by Rayleigh-Taylor flow. Here we provide age constraints on the duration of gravitational overturn in the East Pilbara Terrane. Our U-Pb ages indicate the emplacement of 3,600-3,460-million-year-old granitoid rocks, and their uplift during an overturn event ceasing about 3,413 million years ago. Exhumation and erosion of this felsic proto-crust accompanied crustal reorganization. Petrology and thermodynamic modelling suggest that the early felsic magmas were derived from the base of thick ( 43 km) basaltic proto-crust. Combining our data with regional geochronological studies unveils characteristic growth cycles on the order of 100 million years. We propose that maturation of the early crust over three of these cycles was required before a stable, differentiated continent emerged with sufficient rigidity for plate-like behaviour.

  5. Constraints on Composition, Structure and Evolution of the Lithosphere

    Science.gov (United States)

    Bianchini, Gianluca; Bonadiman, Costanza; Aulbach, Sonja; Schutt, Derek

    2015-05-01

    The idea for this special issue was triggered at the Goldschmidt Conference held in Florence (August 25-30, 2013), where we convened a session titled "Integrated Geophysical-Geochemical Constraints on Composition and Structure of the Lithosphere". The invitation to contribute was extended not only to the session participants but also to a wider spectrum of colleagues working on related topics. Consequently, a diverse group of Earth scientists encompassing geophysicists, geodynamicists, geochemists and petrologists contributed to this Volume, providing a comprehensive overview on the nature and evolution of lithospheric mantle by combining studies that exploit different types of data and interpretative approaches. The integration of geochemical and geodynamic datasets and their interpretation represents the state of the art in our knowledge of the lithosphere and beyond, and could serve as a blueprint for future strategies in concept and methodology to advance our knowledge of this and other terrestrial reservoirs.

  6. Extensional and compressional instabilities in icy satellite lithospheres

    International Nuclear Information System (INIS)

    Herrick, D.L.; Stevenson, D.J.

    1990-01-01

    The plausibility of invoking a lithospheric instability mechanism to account for the grooved terrains on Ganymede, Encedalus, and Miranda is presently evaluated in light of the combination of a simple mechanical model of planetary lithospheres and asthenospheres with recent experimental data for the brittle and ductile deformation of ice. For Ganymede, high surface gravity and warm temperatures render the achievement of an instability sufficiently great for the observed topographic relief virtually impossible; an instability of sufficient strength, however, may be able to develop on such smaller, colder bodies as Encedalus and Miranda. 15 refs

  7. How does continental lithosphere break-apart? A 3D seismic view on the transition from magma-poor rifted margin to magmatic oceanic lithosphere

    Science.gov (United States)

    Emmanuel, M.; Lescanne, M.; Picazo, S.; Tomasi, S.

    2017-12-01

    In the last decade, high-quality seismic data and drilling results drastically challenged our ideas about how continents break apart. New models address their observed variability and are presently redefining basics of rifting as well as exploration potential along deepwater rifted margins. Seafloor spreading is even more constrained by decades of scientific exploration along Mid Oceanic Ridges. By contrast, the transition between rifting and drifting remains a debated subject. This lithospheric breakup "event" is geologically recorded along Ocean-Continent Transitions (OCT) at the most distal part of margins before indubitable oceanic crust. Often lying along ultra-deepwater margin domains and buried beneath a thick sedimentary pile, high-quality images of these domains are rare but mandatory to get strong insights on the processes responsible for lithospheric break up and what are the consequences for the overlying basins. We intend to answer these questions by studying a world-class 3D seismic survey in a segment of a rifted margin exposed in the Atlantic. Through these data, we can show in details the OCT architecture between a magma-poor hyper-extended margin (with exhumed mantle) and a classical layered oceanic crust. It is characterized by 1- the development of out-of-sequence detachment systems with a landward-dipping geometry and 2- the increasing magmatic additions oceanwards (intrusives and extrusives). Geometry of these faults suggests that they may be decoupled at a mantle brittle-ductile interface what may be an indicator on thermicity. Furthermore, magmatism increases as deformation migrates to the future first indubitable oceanic crust what controls a progressive magmatic crustal thickening below, above and across a tapering rest of margin. As the magmatic budget increases oceanwards, full-rate divergence is less and less accommodated by faulting. Magmatic-sedimentary architectures of OCT is therefore changing from supra-detachment to magmatic

  8. Comparison of Oceanic and Continental Lithosphere, Asthenosphere, and the LAB Through Shear Velocity Inversion of Rayleigh Wave Data from the ALBACORE Amphibious Array in Southern California

    Science.gov (United States)

    Amodeo, K.; Rathnayaka, S.; Weeraratne, D. S.; Kohler, M. D.

    2016-12-01

    Continental and oceanic lithosphere, which form in different tectonic environments, are studied in a single amphibious seismic array across the Southern California continental margin. This provides a unique opportunity to directly compare oceanic and continental lithosphere, asthenosphere, and the LAB (Lithosphere-Asthenosphere Boundary) in a single data set. The complex history of the region, including spreading center subduction, block rotation, and Borderland extension, allows us to study limits in the rigidity and strength of the lithosphere. We study Rayleigh wave phase velocities obtained from the ALBACORE (Asthenospheric and Lithospheric Broadband Architecture from the California Offshore Region Experiment) offshore seismic array project and invert for shear wave velocity structure as a function of depth. We divide the study area into several regions: continent, inner Borderland, outer Borderland, and oceanic seafloor categorized by age. A unique starting Vs model is used for each case including layer thicknesses, densities, and P and S velocities which predicts Rayleigh phase velocities and are compared to observed phase velocities in each region. We solve for shear wave velocities with the best fit between observed and predicted phase velocity data in a least square sense. Preliminary results indicate that lithospheric velocities in the oceanic mantle are higher than the continental region by at least 2%. The LAB is observed at 50 ± 20 km beneath 15-35 Ma oceanic seafloor. Asthenospheric low velocities reach a minimum of 4.2 km/s in all regions, but have a steeper positive velocity gradient at the base of the oceanic asthenosphere compared to the continent. Seismic tomography images in two and three dimensions will be presented from each study region.

  9. Surface Curvature in Island Groups and Discontinuous Cratonic Structures

    Science.gov (United States)

    McDowell, M. S.

    2002-05-01

    The Canadian Archipelago includes eight major islands and a host of smaller ones. They are separated by water bodies, of varying widths attributable to glacial activity and ocean currents. Land form varies from relatively rugged mountains (~2000 m) in eastern, glacial, islands, to low lying western, similar to the continental topography adjacent. The Arctic region is thought to have been low average elevation before the Pleistocene. To a picture puzzler, it all looks like it fit together. Experimentally cutting apart the islands from large scale maps shows that the rough edges match fairly well. However, when those independent pieces are sutured together, without restraint, as in free air, the fit is far better. Far more importantly, they consistently form a noticeably concave surface. This tendency is not at all apparent in flat surface or computer screen manipulation; the pieces need to be "hand joined" or on a molded surface to allow the assembly to freely form as it will. Fitting together the coastlines above 60 \\deg north, from 120 \\deg west to 45 \\deg east, and comparing the resulting contracted area to the original, obtains an 8 percent area reduction. The curvature "humps" a trial planar section of 15 cms by 1.6 cm, a substantial difference in the radius of curvature. If you rashly suggest applying that formula globally, the resulting sphere would have a surface area of 4.7 x108,(down from 5 x108), and therefore radius of 6117 km, down from 6400, which is a rather preposterous conclusion. As nobody would believe it, I tested the idea elsewhere. The Huronian succession of six named cratons is adjacent on the south. I cut this map apart, too, and fit it together, once again getting a curvature, this time more pronounced. I am trying it with the Indonesian Archipelago, although this area has volcanic complications, and with Precambrian Basins in western Australia and Nimibia, Africa. Indications are - an essentially similar pattern of fit, but non uniform

  10. Data science implications in diamond formation and craton evolution

    Science.gov (United States)

    Pan, F.; Huang, F.; Fox, P. A.

    2017-12-01

    Diamonds are so-called "messengers" from the deep Earth. Fluid and mineral inclusions in diamonds could reflect the compositions of fluids/melts and wall-rocks in which diamond formed. Recently many diamond samples are examined to study the water content in the mantle transition zone1, the mechanism of diamond formation2 and the mantle evolution history3. However, most of the studies can only explain local activities. Therefore, an overall project of data grouping, comparison and correlation is needed, but limited progress has been made due to a lack of benchmark datasets on diamond formation and effective computing algorithms. In this study, we start by proposing the very first complete and easily-accessible dataset on mineral and fluid inclusions in diamonds. We rescue, collect and organize the data available from papers, journals and other publications resources ([2-4] and more), and then apply several state-of-the-art machine learning methods to tackle this earth science problem by clustering diamond formation process into distinct groups primarily based on the compositions, the formation temperature and pressure, the age and so on. Our ongoing work includes further data exploration and training existing models. Our preliminary results show that diamonds formed from older cratons usually have higher formation temperature. Also peridotitic diamonds take a much larger population than the ecologitic ones. More details are being discovered when we finish constructing the database and training our model. We expect the result to demonstrate the advantages of using machine learning and data science in earth science research problems. Our methodology for knowledge discovery are very general and can be broadly applied to other earth science research problems under the same framework.[1] Pearson et al, Nature (2014); [2] Tomlinson et al, EPSL (2006); [3] Weiss et al, Nature (2016); [4] Stachel and Harris, Ore Geology Reviews (2008); Weiss et al, EPSL (2013)

  11. Deformation of the Pannonian lithosphere and related tectonic topography: a depth-to-surface analysis

    NARCIS (Netherlands)

    Dombrádi, E.

    2012-01-01

    Fingerprints of deep-seated, lithospheric deformation are often recognised on the surface, contributing to topographic evolution, drainage organisation and mass transport. Interactions between deep and surface processes were investigated in the Carpathian-Pannonian region. The lithosphere beneath

  12. Heat Flow, Regional Geophysics and Lithosphere Structure In The Czech Republic

    Science.gov (United States)

    Safanda, J.; Cermak, V.; Kresl, M.; Dedecek, P.

    Paper summarises and critically revises heat flow data that have been collected in the Czech Republic to date. The regional heat flow density map was prepared in view of all existing heat flow data completed with the similar in the surrounding countries and taking into consideration also temperature measurements in deep boreholes. Crustal temperature profiles were calculated by using the available geological information, results of deep seismic sounding and the laboratory data on radiogenic heat produc- tion and thermal conductivity. Special attention was paid to numerous temperature logs in two sedimentary basins, namely in the Cheb and Ostrava-Karvina coal basins, for which detailed heat flow patterns were proposed. Relationships between heat flow distribution and the crustal/lithosphere evolution, between heat flow and the heat pro- duction of the crustal rocks, heat flow and crustal thickness and the steady-state vs. transient heat transport are discussed.

  13. Sensitivity analysis of crustal correction for calculation of lithospheric mantle density from gravity data

    DEFF Research Database (Denmark)

    Herceg, Matija; Artemieva, Irina; Thybo, Hans

    2016-01-01

    for the crust and (ii) uncertainties in the seismic crustal structure (thickness and average VP velocities of individual crustal layers, including the sedimentary cover). We examine the propagation of these uncertainties into determinations of lithospheric mantle density and analyse both sources of possible......We investigate how uncertainties in seismic and density structure of the crust propagate to uncertainties in mantle density structure. The analysis is based on interpretation of residual upper-mantle gravity anomalies which are calculated by subtracting (stripping) the gravitational effect...... mantle, knowledge on uncertainties associated with incomplete information on crustal structure is of utmost importance for progress in gravity modelling. Uncertainties in the residual upper-mantle gravity anomalies result chiefly from uncertainties in (i) seismic VP velocity-density conversion...

  14. The geotectonic evolution of southern part of Sao Francisco Craton, based in geochronologic interpretation

    International Nuclear Information System (INIS)

    Teixeira, W.

    1985-01-01

    Interpretation of available radiometric data from poly metamorphic terranes of southern part of the Sao Francisco Craton demonstrates the importance of geochronology as a tool in the study of ancient crustal evolution. In addition, radiometric study of basic intrusive magmatism helps define the most important epochs of crustal rifting during the Proterozoic. The definition of the southern border of the cratonic area based on distinctive age patterns of the geochronological provinces is also discussed. Finally, the geochronologic evolution of the Bambui platform cover is presented. Approximately 250 radiometric age determinations (Rb-Sr, K-Ar and Pb-Pb methods) were interpreted principally through the use of iso chronic diagrams. The geologic history tectonomagnetic events identified in this study is compared to the crustal evolution of similar segments of the Sao Francisco Craton and elsewhere. (author)

  15. Compositional trends among Kaapvaal Craton garnet peridotite xenoliths and their effects on seismic velocity and density

    DEFF Research Database (Denmark)

    Schutt, Derek; Lesher, Charles

    2010-01-01

    garnet and clinopyroxene enrichment. Using the parameterization of Schutt and Lesher (2006) we show that at cratonic mantle temperatures and pressures, orthopyroxene enrichment results in little change in bulk density (ρbulk) and shear-wave velocity (VS), but decreases compressional wave velocities (VP......We examine the modes and compositions of garnet-bearing peridotite xenoliths from the Kaapvaal Craton to quantify factors governing density and seismic velocity variations within metasomatically altered cratonic mantle. Three distinct compositional trends are resolved by principal component...... analysis. The first reflects differences in residue composition resulting from partial melting. The second is associated with orthopyroxene (opx) enrichment, possibly due to silica addition by subduction zone fluids in the source region of the xenoliths. The third principal component reflects garnet...

  16. Seismic velocity model of the crust and upper mantle along profile PANCAKE across the Carpathians between the Pannonian Basin and the East European Craton

    DEFF Research Database (Denmark)

    Starostenko, V.; Janik, T.; Kolomiyets, K.

    2013-01-01

    the Transcarpathian Depression and the Carpathian Foredeep; and the south-western part of the EEC, including the Trans European Suture Zone (TESZ). Seismic data support a robust model of the Vp velocity structure of the crust. In the PB, the 22-23km thick crust consists of a 2-5km thick sedimentary layer (Vp=2......Results are presented of a seismic wide-angle reflection/refraction survey along a profile between the Pannonian Basin (PB) and the East European Craton (EEC) called PANCAKE. The P- and S-wave velocity model derived can be divided into three sectors: the PB; the Carpathians, including.......4-3.7km/s), 17-20km thick upper crystalline crust (5.9-6.3km/s) and an up to 3km thick lower crustal layer (Vp=6.4km/s). In the central part of the Carpathians, a 10-24km thick uppermost part of the crust with Vp≤6.0km/s may correspond to sedimentary rocks of different ages; several high velocity bodies...

  17. Lithosphere structure in Madagascar as revealed from receiver functions and surface waves analysis.

    Science.gov (United States)

    Rindraharisaona, E. J.; Tilmann, F. J.; Yuan, X.; Dreiling, J.; Priestley, K. F.; Barruol, G.; Wysession, M. E.

    2017-12-01

    The geological history of Madagascar makes it an ideal place to study the lithospheric structure and its evolution. It comprises Archean to Proterozoic units on the central eastern part, which is surrounded by a Triassic to Jurassic basin formation in the west and Cretaceous volcanics along the coasts. Quaternary volcanic rocks have been embedded in crystalline and sedimentary rocks. The aim of the present work is to characterize the crustal structure and determine the imprint of the dominant geodynamic events that have affected Madagascar: the Pan-African orogeny, the breakup of Gondwanaland and Neogene tectonic activity. From 2011 to 2014 different temporary seismic arrays were deployed in Madagascar. We based the current study mostly on SELASOMA project, which is composed of 50 seismic stations that were installed traversing southern Madagascar from the west to the east, sampling the different geological units. To measured seismic dispersion curves, one a wide period ranges using ambient noise, Rayleigh and Love surface waves. To compute the average crustal Vp/Vs ratio internal crustal structure and discontinuities in the mantle, we use both P- and S-waves receiver functions. To better resolve of the crustal structure, we jointly inverted P-wave receiver functions and Rayleigh wave group velocity.The crustal extension during the Carboniferous to Cenozoic has thinned the igneous crust down to 15 km in the western Morondava basin by removing much of the lower crust, while the thickness of the upper crust is nearly identical in the sedimentary basin and under Proterozoic and Archaean rocks of the eastern two thirds of Southern Madagascar. In general, the Archean crust is thicker than the Proterozoic, because mafic component is missing in the Proterozoic domain while it forms the bottom of the Archean crust. The lithosphere thickness in the southern part of Madagascar is estimated to be between 90 and 125 km.

  18. Geochemical Characteristics of Cenozoic Jining Basalts of the Western North China Craton: Evidence for the Role of the Lower Crust, Lithosphere, and Asthenosphere in Petrogenesis

    Directory of Open Access Journals (Sweden)

    Kung-Suan Ho

    2011-01-01

    Full Text Available The Jining volcanic field located in the southern margin of the Mongolian plateau and the western North China Block consists of four rock types: quartz tholeiite, olivine tholeiite, alkali olivine basalt and basanite. These rocks have a wide range of K-Ar ages from ~36 to < 0.2 Ma. The early volcanism was voluminous and dominated by flood-type fissure eruptions of tholeiites, whereas the later phase was represented by sparse eruptions of basanitic lavas. Thirty-six samples analyzed in this study show a wide range in SiO2 contents from 44% ~ 54%. They all are sodium-rich and high-Ti basalts that, however, show marked isotopic variations between two end-members: (1 tholeiites that have higher 87Sr/86Sr of 0.7048 ~ 0.7052, and lower £`Nd of -0.8 to -2.4 and Pb isotope ratios (206Pb/204Pb of 16.9 ~ 17.2, 207Pb/204Pb of 15.3 ~ 15.4 and 208Pb/204Pb of 37.1 ~ 37.7; and (2 basanites that have lower 87Sr/86Sr of 0.7035 ~ 0.7044, and higher £`Nd of +1.3 to +4.9 and Pb isotope ratios (206Pb/204Pb of 17.7 ~ 18.0, 207Pb/204Pb of 15.4 ~ 15.5 and 208Pb/204Pb of 37.8 ~ 38.2. Alkali olivine basalt that occurs as a subordinate rock type is geochemically similar to the basanites, but isotopically similar to the tholeiites, characterized by the highest 87Sr/86Sr ratio among the three basaltic suites, coupled with a low Nb/U value (~33.

  19. Lithospheric Layering beneath the Contiguous United States Constrained by S-to-P Receiver Functions

    Science.gov (United States)

    Liu, L.; Liu, K. H.; Kong, F.; Gao, S. S.

    2017-12-01

    The greatly-improved spatial coverage of broadband seismic stations as a result of the deployment of the EarthScope Transportable Array (TA) stations and the diversity of tectonic environments in the contiguous United States provide a unique opportunity to investigate the depth variation and nature of intra-lithospheric interfaces in different tectonic regimes. A total of 284,121 high-quality S-to-P receiver functions (SRFs) are obtained from 3,809 broadband seismic stations in the TA and other permanent and temporary deployments in the contiguous United States. The SRFs are computed using frequency domain deconvolution, and are stacked in consecutive circles with a radius of 2°. They are converted to depth series after move-out corrections using the IASP91 Earth model. Similar to previous SRF studies, a robust negative arrival, representing a sharp discontinuity of velocity reduction with depth, is visible in virtually all the stacked traces in the depth range of 30-110 km. Beneath the western US, the depth of this discontinuity is 69±17 km, and beneath the eastern US, it ranges from 75 to 90 km, both of which are comparable to the depth of the tomographically-determined lithosphere-asthenosphere boundary (LAB). In contrast, the depth of the discontinuity beneath the central US is 83±10 km which is significantly smaller than the 250 km LAB depth determined by seismic surface wave tomography. Based on previous seismic tomography, shear-wave splitting and mantle xenolith studies, we interpret this discontinuity as the top of a frozen-in layer of volatile-rich melt beneath the central US. The observations and the discrepancy between the SRF and seismic tomography results for the central US as well as the amplitude of the corresponding arrival on the SRFs may be explained by spatial variations of the thickness of the transitional layer between the "pure" lithosphere and the "pure" asthenosphere. Under this hypothesis, the consistency between the results from the

  20. Geochronology of sedimentary and metasedimentary Precambrian rocks of the West African craton

    International Nuclear Information System (INIS)

    Clauer, N.; Jeannette, D.; Trompette, R.

    1982-01-01

    This contribution summarizes current knowledge of the geochronology of the Upper Proterozoic sedimentary rocks covering the West African craton. This was done by using direct dating methods. Correlations between the northern edge of the Tindouf basin and the northern and southern part of the Taoudeni basin, as well as the Volta basin, are proposed. Tectonic, volcanic and thermal activities in connection with the Pan-African orogeny are recorded only around the craton. They induced either sedimentation lacks in Morocco or sedimentation excesses in Hoggar. Unsolved problems such as the precise stratigraphic position of the uppermost Proterozoic tillitic episode and the correlation within the Moroccan Anti-Atlas are also raised. (Auth.)

  1. Crustal structure of the Siberian craton and the West Siberian basin

    DEFF Research Database (Denmark)

    Cherepanova, Yulia; Artemieva, Irina; Thybo, Hans

    2013-01-01

    We present a digital model SibCrust of the crustal structure of the Siberian craton (SC) and the West Siberian basin (WSB), based on all seismic profiles published since 1960 and sampled with a nominal interval of 50. km. Data quality is assessed and quantitatively assigned to each profile based...... and ~. 6.2-6.6. km/s in parts of the WSB and SC. Exceptionally high basement Vp velocities (6.8-7.0. km/s) at the northern border between the SC and the WSB indicate the presence of magmatic intrusions and are proposed to mark the source zone of the Siberian LIP. The cratonic crust generally consists...

  2. Extension of thickened and hot lithospheres: Inferences from laboratory modeling

    NARCIS (Netherlands)

    Tirel, C.; Brun, J.P.; Sokoutis, D.

    2006-01-01

    The extension of a previously thickened lithosphere is studied through a series of analogue experiments. The models deformed in free and boundary-controlled gravity spreading conditions that simulate the development of wide rift-type and core complex-type structures. In models, the development of

  3. European Lithospheric Mantle; geochemical, petrological and geophysical processes

    Science.gov (United States)

    Ntaflos, Th.; Puziewicz, J.; Downes, H.; Matusiak-Małek, M.

    2017-04-01

    The second European Mantle Workshop occurred at the end of August 2015, in Wroclaw, Poland, attended by leading scientists in the study the lithospheric mantle from around the world. It built upon the results of the first European Mantle Workshop (held in 2007, in Ferrara, Italy) published in the Geological Society of London Special Publication 293 (Coltorti & Gregoire, 2008).

  4. Lithospheric strength variations in Mainland China : Tectonic implications

    NARCIS (Netherlands)

    Deng, Yangfan; Tesauro, M.

    2016-01-01

    We present a new thermal and strength model for the lithosphere of Mainland China. To this purpose, we integrate a thermal model for the crust, using a 3-D steady state heat conduction equation, with estimates for the upper mantle thermal structure, obtained by inverting a S wave tomography model.

  5. Numerical modeling of continental lithospheric weak zone over plume

    Science.gov (United States)

    Perepechko, Y. V.; Sorokin, K. E.

    2011-12-01

    The work is devoted to the development of magmatic systems in the continental lithosphere over diffluent mantle plumes. The areas of tension originating over them are accompanied by appearance of fault zones, and the formation of permeable channels, which are distributed magmatic melts. The numerical simulation of the dynamics of deformation fields in the lithosphere due to convection currents in the upper mantle, and the formation of weakened zones that extend up to the upper crust and create the necessary conditions for the formation of intermediate magma chambers has been carried out. Thermodynamically consistent non-isothermal model simulates the processes of heat and mass transfer of a wide class of magmatic systems, as well as the process of strain localization in the lithosphere and their influence on the formation of high permeability zones in the lower crust. The substance of the lithosphere is a rheologic heterophase medium, which is described by a two-velocity hydrodynamics. This makes it possible to take into account the process of penetration of the melt from the asthenosphere into the weakened zone. The energy dissipation occurs mainly due to interfacial friction and inelastic relaxation of shear stresses. The results of calculation reveal a nonlinear process of the formation of porous channels and demonstrate the diversity of emerging dissipative structures which are determined by properties of both heterogeneous lithosphere and overlying crust. Mutual effect of a permeable channel and the corresponding filtration process of the melt on the mantle convection and the dynamics of the asthenosphere have been studied. The formation of dissipative structures in heterogeneous lithosphere above mantle plumes occurs in accordance with the following scenario: initially, the elastic behavior of heterophase lithosphere leads to the formation of the narrow weakened zone, though sufficiently extensive, with higher porosity. Further, the increase in the width of

  6. Lithospheric origin for Neogene-Quaternary Middle Atlas lavas (Morocco): Clues from trace elements and Sr-Nd-Pb-Hf isotopes

    Science.gov (United States)

    Bosch, Delphine; Maury, René C.; El Azzouzi, M'hammed; Bollinger, Claire; Bellon, Hervé; Verdoux, Patrick

    2014-09-01

    This study presents new geochemical data on 26 mafic lavas from the Middle Atlas and Central Morocco volcanic provinces, including Miocene nephelinites and Pliocene-Quaternary (3.9-0.6 Ma) nephelinites, basanites, alkali and subalkaline basalts. Most of them represent near-primary magmas, although some alkali basalts were derived from the minor fractionation of olivine and diopside phenocrysts. These evolved samples and the subalkaline basalt display higher 207Pb/204Pb and Zr/Nb ratios and lower εNd consistent with their contamination by lower crustal granulites during an open fractionation process. The progressive enrichment in incompatible elements observed from alkali basalts to nephelinites suggests their derivation from decreasing partial melting degrees of an enriched mantle source located at the garnet-spinel transition zone. The strong negative spikes observed for K in multielement patterns indicate that this source contained a residual pargasitic amphibole. We propose that partial melting occurred at around 2 GPa, i.e. near the lithosphere-asthenosphere boundary beneath the Middle Atlas (60-80 km). The trace element and isotopic Sr-Nd-Pb-Hf signature of the uncontaminated lavas displays a geochemical flavour intermediate between those of high μ (HIMU), “C”, and enriched mantle components. It is very similar to that of abundant metasomatic amphibole- and clinopyroxene-rich lithospheric peridotites and pyroxenites carried by Middle Atlas lavas, which likely represent an analog of the source of these lavas. It is therefore not necessary to postulate the contribution of a “fresh” asthenospheric mantle to their genesis. We propose that they resulted from the partial melting of the base of a veined lithospheric mantle metasomatised during the late Cretaceous by alkaline melts from the Central Atlantic plume, the ancestor of the Canary plume. Melting was probably triggered by the flux of a hot mantle within a regional SW-NE sub-lithospheric channel, in

  7. Petrogenesis of postcollisional magmatism at Scheelite Dome, Yukon, Canada: Evidence for a lithospheric mantle source for magmas associated with intrusion-related gold systems

    Science.gov (United States)

    Mair, John L.; Farmer, G. Lang; Groves, David I.; Hart, Craig J.R.; Goldfarb, Richard J.

    2011-01-01

    are attributes of the ancient North American cratonic margin that appear to be essential prerequisites to this style of postcollisional magmatism and associated gold-rich fluid exsolution. This type of magmatic hydrothermal activity occurs in a very specific tectonic setting that typically sets intrusion-related gold deposits apart from orogenic gold deposits, which are synorogenic in timing and have no consistent direct relationship to such diverse and contemporaneous lithospheric mantle-derived magmas, although they too are commonly sited adjacent to lithospheric boundaries.

  8. Lithospheric-scale centrifuge models of pull-apart basins

    Science.gov (United States)

    Corti, Giacomo; Dooley, Tim P.

    2015-11-01

    We present here the results of the first lithospheric-scale centrifuge models of pull-apart basins. The experiments simulate relative displacement of two lithospheric blocks along two offset master faults, with the presence of a weak zone in the offset area localising deformation during strike-slip displacement. Reproducing the entire lithosphere-asthenosphere system provides boundary conditions that are more realistic than the horizontal detachment in traditional 1 g experiments and thus provide a better approximation of the dynamic evolution of natural pull-apart basins. Model results show that local extension in the pull-apart basins is accommodated through development of oblique-slip faulting at the basin margins and cross-basin faults obliquely cutting the rift depression. As observed in previous modelling studies, our centrifuge experiments suggest that the angle of offset between the master fault segments is one of the most important parameters controlling the architecture of pull-apart basins: the basins are lozenge shaped in the case of underlapping master faults, lazy-Z shaped in case of neutral offset and rhomboidal shaped for overlapping master faults. Model cross sections show significant along-strike variations in basin morphology, with transition from narrow V- and U-shaped grabens to a more symmetric, boxlike geometry passing from the basin terminations to the basin centre; a flip in the dominance of the sidewall faults from one end of the basin to the other is observed in all models. These geometries are also typical of 1 g models and characterise several pull-apart basins worldwide. Our models show that the complex faulting in the upper brittle layer corresponds at depth to strong thinning of the ductile layer in the weak zone; a rise of the base of the lithosphere occurs beneath the basin, and maximum lithospheric thinning roughly corresponds to the areas of maximum surface subsidence (i.e., the basin depocentre).

  9. Space geodesy validation of the global lithospheric flow

    Science.gov (United States)

    Crespi, M.; Cuffaro, M.; Doglioni, C.; Giannone, F.; Riguzzi, F.

    2007-02-01

    Space geodesy data are used to verify whether plates move chaotically or rather follow a sort of tectonic mainstream. While independent lines of geological evidence support the existence of a global ordered flow of plate motions that is westerly polarized, the Terrestrial Reference Frame (TRF) presents limitations in describing absolute plate motions relative to the mantle. For these reasons we jointly estimated a new plate motions model and three different solutions of net lithospheric rotation. Considering the six major plate boundaries and variable source depths of the main Pacific hotspots, we adapted the TRF plate kinematics by global space geodesy to absolute plate motions models with respect to the mantle. All three reconstructions confirm (i) the tectonic mainstream and (ii) the net rotation of the lithosphere. We still do not know the precise trend of this tectonic flow and the velocity of the differential rotation. However, our results show that assuming faster Pacific motions, as the asthenospheric source of the hotspots would allow, the best lithospheric net rotation estimate is 13.4 +/- 0.7 cm yr-1. This superfast solution seems in contradiction with present knowledge on the lithosphere decoupling, but it matches remarkably better with the geological constraints than those retrieved with slower Pacific motion and net rotation estimates. Assuming faster Pacific motion, it is shown that all plates move orderly `westward' along the tectonic mainstream at different velocities and the equator of the lithospheric net rotation lies inside the corresponding tectonic mainstream latitude band (~ +/-7°), defined by the 1σ confidence intervals.

  10. Plume-stagnant slab-lithosphere interactions: Origin of the late Cenozoic intra-plate basalts on the East Eurasia margin

    Science.gov (United States)

    Kimura, Jun-Ichi; Sakuyama, Tetsuya; Miyazaki, Takashi; Vaglarov, Bogdan S.; Fukao, Yoshio; Stern, Robert J.

    2018-02-01

    Intra-plate basalts of 35-0 Ma in East Eurasia formed in a broad backarc region above the stagnant Pacific Plate slab in the mantle transition zone. These basalts show regional-scale variations in Nd-Hf isotopes. The basalts with the most radiogenic Nd-Hf center on the Shandong Peninsula with intermediate Nd-Hf at Hainan and Datong. The least radiogenic basalts occur in the perimeters underlain by the thick continental lithosphere. Shandong basalts possess isotopic signatures of the young igneous oceanic crust of the subducted Pacific Plate. Hainan and Datong basalts have isotopic signatures of recycled subduction materials with billions of years of storage in the mantle. The perimeter basalts have isotopic signatures similar to pyroxenite xenoliths from the subcontinental lithospheric mantle beneath East Eurasia. Hainan basalts exhibit the highest mantle potential temperature (Tp), while the Shandong basalts have the lowest Tp. We infer that a deep high-Tp plume interacted with the subducted Pacific Plate slab in the mantle transition zone to form a local low-Tp plume by entraining colder igneous oceanic lithosphere. We infer that the subducted Izanagi Plate slab, once a part of the Pacific Plate mosaic, broke off from the Pacific Plate slab at 35 Ma to sink into the lower mantle. The sinking Izanagi slab triggered the plume that interacted with the stagnant Pacific slab and caused subcontinental lithospheric melting. This coincided with formation of the western Pacific backarc marginal basins due to Pacific Plate slab rollback and stagnation.

  11. Geology of the Terre Adélie Craton (135 – 146˚ E)

    Science.gov (United States)

    Ménot, R.P.; Duclaux, G.; Peucat, J.J.; Rolland, Y.; Guillot, S.; Fanning, M.; Bascou, J.; Gapais, D.; Pêcher, A.

    2007-01-01

    More than 15 years of field and laboratory investigations on samples from Terre Adélie to the western part of George Vth Land (135 to 146°E) during the GEOLETA program allow a reassessment of the Terre Adélie Craton (TAC) geology. The TAC represents the largest exposed fragment of the East Antarctic Shield preserved from both Grenville and Ross tectono-metamorphic events. Therefore it corresponds to a well-preserved continental segment that developed from the Neoarchean to the Paleoproterozoic. Together with the Gawler Craton in South Australia, the TAC is considered as part of the Mawson continent, i.e. a striking piece of the Rodinia Supercontinent. However, this craton represents one of the less studied parts of the East Antarctic Shield. The three maps presented here clearly point out the extent of two distinct domains within the Terre Adélie Craton and suggest that the TAC was built up through a polyphased evolution during the Neoarchean-Siderian (c.a. 2.5Ga) and the Statherian (c.a. 1.7Ga) periods. These data support a complete re-assessment of the TAC geology and represent a valuable base for the understanding of global geodynamics changes during Paleoproterozoic times.

  12. Compositional trends among Kaapvaal Craton garnet peridotite xenoliths and their effects on seismic velocity and density

    DEFF Research Database (Denmark)

    Schutt, Derek; Lesher, Charles

    2010-01-01

    and clinopyroxene enrichment possibly as a consequence of melt infiltration. More than half of the mineral mode variance among Kaapvaal Craton xenoliths can be accounted for by opx enrichment. Melt depletion effects can account for as much as 30% of the variance, while less than 20% of the variance is associated...

  13. The Amazonian Craton and its influence on past fluvial systems (Mesozoic-Cenozoic, Amazonia)

    NARCIS (Netherlands)

    Hoorn, C.; Roddaz, M.; Dino, R.; Soares, E.; Uba, C.; Ochoa-Lozano, D.; Mapes, R.; Hoorn, C.; Wesselingh, F.P.

    2010-01-01

    The Amazonian Craton is an old geological feature of Archaean/Proterozoic age that has determined the character of fluvial systems in Amazonia throughout most of its past. This situation radically changed during the Cenozoic, when uplift of the Andes reshaped the relief and drainage patterns of

  14. An isotopic perspective on growth and differentiation of Proterozoic orogenic crust: From subduction magmatism to cratonization

    Science.gov (United States)

    Johnson, Simon P.; Korhonen, Fawna J.; Kirkland, Christopher L.; Cliff, John B.; Belousova, Elena A.; Sheppard, Stephen

    2017-01-01

    The in situ chemical differentiation of continental crust ultimately leads to the long-term stability of the continents. This process, more commonly known as 'cratonization', is driven by deep crustal melting with the transfer of those melts to shallower regions resulting in a strongly chemically stratified crust, with a refractory, dehydrated lower portion overlain by a complementary enriched upper portion. Since the lower to mid portions of continental crust are rarely exposed, investigation of the cratonization process must be through indirect methods. In this study we use in situ Hf and O isotope compositions of both magmatic and inherited zircons from several felsic magmatic suites in the Capricorn Orogen of Western Australia to highlight the differentiation history (i.e. cratonization) of this portion of late Archean to Proterozoic orogenic crust. The Capricorn Orogen shows a distinct tectonomagmatic history that evolves from an active continental margin through to intracratonic reworking, ultimately leading to thermally stable crust that responds similarly to the bounding Archean Pilbara and Yilgarn Cratons. The majority of magmatic zircons from the main magmatic cycles have Hf isotopic compositions that are generally more evolved than CHUR, forming vertical arrays that extend to moderately radiogenic compositions. Complimentary O isotope data, also show a significant variation in composition. However, combined, these data define not only the source components from which the magmas were derived, but also a range of physio-chemical processes that operated during magma transport and emplacement. These data also identify a previously unknown crustal reservoir in the Capricorn Orogen.

  15. Provenance of zircon of the lowermost sedimentary cover, Estonia, East-European Craton

    Directory of Open Access Journals (Sweden)

    Konsa, M.

    1999-12-01

    Full Text Available Bulk and accessory mineral composition of fresh and weathered crystalline rocks, and sedimentary deposits overlying the crystalline-sedimentary unconformity have been examined in core samples from 28 drill holes in Estonia. Before the Late Vendian to Early Cambrian regional subsidence and sedimentation, the region represented a flat plateau within the Svecofennian Domain. Palaeo-and Mesoproterozoic crystalline rocks, regardless their different initial mineral composition, subcrop under the Upper Vendian/Lower Cambrian sedimentary cover as usually intensely weathered rocks (saprolites composed of residual quartz, altered micas and prevailing clay minerals mainly of the kaolinite group. Thus, the bulk mineral composition of any basement crystalline rocks imparts no specific inherited rock-forming minerals into the covering sedimentary rocks. From the variety of accessory and opaque minerals of crystalline rocks, only zircon populations survived in saprolites. Crystalline rocks of different origin yield different zircons. Relationships between the zircon typology of the basement rocks having specific areas of distribution and the sedimentary rocks immediately overlying those crystalline rocks were the main subject of this study. The result is that siliciclastic sedimentary rocks covering weathered crystalline rocks only in places inherited zircons with typological features characteristic of specific basement areas. In northeastern Estonia, local lenses of the Oru Member (the earliest Upper Vendian sedimentary rocks in Estonia resembling the debris of weathered crystalline rocks yield accessory zircon which in a 1-2 m thick layer above the basement surface is similar to the zircons of the underlying weathering mantle of certain crystalline rocks. In the next unit, the Moldova Member, up to 43 m above the basement surface, a mixture of zircons resembling those of various local basement rocks has been found. Further upwards, in the Vendian and Lower

  16. Seismic Refraction & Wide-angle Reflection Experiment on the Northern Margin of North China Craton -Data Acquisition and Preliminary Processing Result

    Science.gov (United States)

    Li, W.; Gao, R.; Keller, G. R.; Hou, H.; Li, Q.; Cox, C. M.; Chang, J. C.; Zhang, J.; Guan, Y.

    2010-12-01

    The evolution history of Central Asian Orogen Belt (CAOB) is still the main tectonic problems in northeastern Asia. The Siberia Craton (NC), North China Craton (NCC) and several blocks collided, and the resulting tectonic collage formed as the Paleo-Asian Ocean disappeared. Concerning the northern margin of North China Craton, many different geological questions remain unanswered, such as: the intracontinental orogenic process in the Yanshan orogen and the nature and location of the suture between the southern NC and the northern NCC. In Dec 2009, a 400 km long seismic refraction and wide-angle reflection profile was completed jointly by Institute of Geology, CAGS and University of Oklahoma. The survey line extended from the west end of the Yanshan orogen, across a granitoid belt to the Solonker suture zone. The recording of seismic waves from 8 explosions (500~1500 kg each) was conducted in four deployments of 300 Reftek125 (Texan) seismic recorders, with an average spacing of 1 km. For the calculations, we used the Rayinvr, Vmed and Zplot programs for ray tracing, model modification and phase picking. The initial result show that: 1)the depth of low velocity sediment cover ranges from 0.6 to 2.7 km (velocity: 2.8~5.6 km/s); 2)the depth of basement is 5.6~10 km (the depth of basement under the granitoid belt deepens to 10 km and velocity increases to 6.2 km/s); 3)the upper crust extends to a depth of 15.5~21 km and has the P-wave velocities between 5.6 and 6.4 km/s; 4)the thickness of the lower crust ranges from 22~28 km(velocity: 6.4~6.9 km/s); and 5)the depth of Moho varies from 39.5 km under the granitoid belt to 49 km under the Yanshan orogen. Based on these results, we can preliminarily deduce that: 1) the concave depression of the Moho observed represents the root of the Yanshan orogen, and it may prove that the orogen is dominated by thick-skinned tectonics; 2) the shape of velocity variations under the granitoid belt is suggestive of a magma conduit. It

  17. Recognized Multiple Rifts of the Neoproterozoic in the Initiation of the Tarim Craton (NW China) and Their Tectonic Implications

    Science.gov (United States)

    He, B.; Jiao, C.; Huang, T.; Zhou, X.; Cai, Z.; Cao, Z.; Jiang, Z.; Cui, J.; Yu, Z.; Chen, W.

    2017-12-01

    The Tarim Basin is the largest, oil-bearing and superimposed basin in the northwest of China. The development and tectonic property of the initial Tarim basin have been acutely disputed and remain enigmatic. Urgently need to reveal the origin and formation dynamics of the Tarim Carton and evaluate the potential of the deep energy resources. However, covered by vast desert and huge-thickness sedimentary strata, suffered by multiple tectonic movements, seismic data with low signal- to- noise ratio in the deep are the critical difficulties. We analyse 4 field outcrops, 18 wells, 27 reprocessed seismic reflection profiles with high SNR across the basin and many ancillary ones and aeromagnetic data. We find about 20 normal fault-controlled rift depressions of the Cryogenian and Ediacaran scattered in the Tarim basin, which developed on the Precambrian metamorphic and crystalline basements and covered by the epeiric sea and basin facies sediments of the Lower Cambrian. The structural styles of the rifts are mainly half grabens, symmetrical troughs and horst-grabens. The regional differences exist obviously in spatial and temporal. The WNW-ESE-trending faults occur in the central part and northern of the basin and the NE, and the NEE-trending faults occur in the southern parts, which response with the anomaly of aeromagnetic. Some main faults of the Ediacaran inherited from the Cryogenian and some occurred newly, the more rifting depressions occurred during the Ediacaran. The extensional NNW-SSE-oriented and NNE-SSW-oriented paleostress field occurred simultaneously during rifting, and accompanied with the clockwise shearing. According to the activities of syn-sedimentary faults, magmatic events and sediments, the tectonic properties of the rifts are different depending on their locations in the Tarim craton. The rifting phases mainly occurred from 780 Ma to 615 Ma. The formation of rifts were associated with the opening of the South Tianshan Ocean and the South Altun

  18. Spectral characteristics of banded iron formations in Singhbhum craton, eastern India: Implications for hematite deposits on Mars

    Directory of Open Access Journals (Sweden)

    Mahima Singh

    2016-11-01

    Full Text Available Banded iron formations (BIFs are major rock units having hematite layers intermittent with silica rich layers and formed by sedimentary processes during late Archean to mid Proterozoic time. In terrestrial environment, hematite deposits are mainly found associated with banded iron formations. The BIFs in Lake Superior (Canada and Carajas (Brazil have been studied by planetary scientists to trace the evolution of hematite deposits on Mars. Hematite deposits are extensively identified in Meridiani region on Mars. Many hypotheses have been proposed to decipher the mechanism for the formation of these deposits. On the basis of geomorphological and mineralogical studies, aqueous environment of deposition is found to be the most supportive mechanism for its secondary iron rich deposits. In the present study, we examined the spectral characteristics of banded iron formations of Joda and Daitari located in Singhbhum craton in eastern India to check its potentiality as an analog to the aqueous/marine environment on Mars. The prominent banding feature of banded iron formations is in the range of few millimeters to few centimeters in thickness. Fe rich bands are darker (gray in color compared to the light reddish jaspilitic chert bands. Thin quartz veins (<4 mm are occasionally observed in the hand-specimens of banded iron formations. Spectral investigations have been conducted in VIS/NIR region of electromagnetic spectrum in the laboratory conditions. Optimum absorption bands identified include 0.65, 0.86, 1.4 and 1.9 μm, in which 0.56 and 0.86 μm absorption bands are due to ferric iron and 1.4 and 1.9 μm bands are due to OH/H2O. To validate the mineralogical results obtained from VIS/NIR spectral radiometry, laser Raman and Fourier transform infrared spectroscopic techniques were utilized and the results were found to be similar. Goethite-hematite association in banded iron formation in Singhbhum craton suggests dehydration activity, which has

  19. Geochemistry and geochronology of the Archean and palaeo-Proterozoic formations of southern Cameroon (Ntem group, Congo craton)

    International Nuclear Information System (INIS)

    Rchameni, R.

    1997-01-01

    The aim of this work is to understand the crustal evolution of the NW margin of the Congo craton using structural, petrography, isotopic, geochemical and geochronological studies of the Archean and palaeo-Proterozoic formations of the Ntem group of southern Cameroon. The synthesis of these studies allows to propose a diapir-type gravity model linked with the genesis of granitoids to explain the geodynamical evolution of this part of the craton during the Archean. A convergence model with the collision of the Congo and Sao-Francisco cratons and with crust thickening followed by a relaxation phase is proposed for the palaeo-Proterozoic. (J.S.)

  20. Spatial variations of current tectonic stress field and its relationship to the structure and rheology of lithosphere around the Bohai Sea, North China

    Science.gov (United States)

    Li, Xianrui; Wang, Jie; Zeng, Zuoxun; Dai, Qingqin

    2017-05-01

    The tectonic stress field in the middle-upper crust is closely related to the structure and rheology of the lithosphere. To determine the stress field in the deep crust, we inversed the focal mechanism solutions (FMSs) of 62 earthquakes that occurred between 2009 and 2015 in the Bohai Sea and its surrounding areas using broadband seismic waveforms collected from 140 stations. We then derived the tectonic stress field using the software SATSI (Spatial And Temporal Stress Inversion) based on the damped linear inversion method. The inversion results show that both the maximum (σ1) and minimum (σ3) principle stress axes throughout the entire region are nearly horizontal except in the Tangshan and Haicheng areas, suggesting that the study area is predominantly under a strike-slip faulting stress regime. The σ1 and σ3 axes are found to be oriented in the NEE-SWW or nearly E-W and NNW-SSE or nearly S-N directions, respectively. These results indicate that the stress field in the North China Craton is controlled by the combined effects of the Pacific Plate westward subduction and the India-Eurasia Plate collision. However, localized normal faulting stress regimes (where the vertical stress σv ≈ σ1) are observed in the Tangshan and Haicheng areas, where low viscosity bodies (LVBs) were identified using geophysical data. Based on the analysis of focal mechanism solutions, active faults and lithosphere rheology characteristics in the Tangshan and Haicheng areas, we speculate that the anomalous stress regime is caused by the local extension resulting from the movement of strike-slip faults under the action of the regional stress field. The existence of LVB may indicate weakness in the crust that favors the accumulation of tectonic stress and triggers large earthquakes.

  1. Rayleigh and S wave tomography constraints on subduction termination and lithospheric foundering in central California

    Science.gov (United States)

    Jiang, Chengxin; Schmandt, Brandon; Hansen, Steven M.; Dougherty, Sara L.; Clayton, Robert W.; Farrell, Jamie; Lin, Fan-Chi

    2018-01-01

    The crust and upper mantle structure of central California have been modified by subduction termination, growth of the San Andreas plate boundary fault system, and small-scale upper mantle convection since the early Miocene. Here we investigate the contributions of these processes to the creation of the Isabella Anomaly, which is a high seismic velocity volume in the upper mantle. There are two types of hypotheses for its origin. One is that it is the foundered mafic lower crust and mantle lithosphere of the southern Sierra Nevada batholith. The alternative suggests that it is a fossil slab connected to the Monterey microplate. A dense broadband seismic transect was deployed from the coast to the western Sierra Nevada to fill in the least sampled areas above the Isabella Anomaly, and regional-scale Rayleigh and S wave tomography are used to evaluate the two hypotheses. New shear velocity (Vs) tomography images a high-velocity anomaly beneath coastal California that is sub-horizontal at depths of ∼40–80 km. East of the San Andreas Fault a continuous extension of the high-velocity anomaly dips east and is located beneath the Sierra Nevada at ∼150–200 km depth. The western position of the Isabella Anomaly in the uppermost mantle is inconsistent with earlier interpretations that the Isabella Anomaly is connected to actively foundering foothills lower crust. Based on the new Vs images, we interpret that the Isabella Anomaly is not the dense destabilized root of the Sierra Nevada, but rather a remnant of Miocene subduction termination that is translating north beneath the central San Andreas Fault. Our results support the occurrence of localized lithospheric foundering beneath the high elevation eastern Sierra Nevada, where we find a lower crustal low Vs layer consistent with a small amount of partial melt. The high elevations relative to crust thickness and lower crustal low Vs zone are consistent with geological inferences that lithospheric foundering drove

  2. Rayleigh and S wave tomography constraints on subduction termination and lithospheric foundering in central California

    Science.gov (United States)

    Jiang, Chengxin; Schmandt, Brandon; Hansen, Steven M.; Dougherty, Sara L.; Clayton, Robert W.; Farrell, Jamie; Lin, Fan-Chi

    2018-04-01

    The crust and upper mantle structure of central California have been modified by subduction termination, growth of the San Andreas plate boundary fault system, and small-scale upper mantle convection since the early Miocene. Here we investigate the contributions of these processes to the creation of the Isabella Anomaly, which is a high seismic velocity volume in the upper mantle. There are two types of hypotheses for its origin. One is that it is the foundered mafic lower crust and mantle lithosphere of the southern Sierra Nevada batholith. The alternative suggests that it is a fossil slab connected to the Monterey microplate. A dense broadband seismic transect was deployed from the coast to the western Sierra Nevada to fill in the least sampled areas above the Isabella Anomaly, and regional-scale Rayleigh and S wave tomography are used to evaluate the two hypotheses. New shear velocity (Vs) tomography images a high-velocity anomaly beneath coastal California that is sub-horizontal at depths of ∼40-80 km. East of the San Andreas Fault a continuous extension of the high-velocity anomaly dips east and is located beneath the Sierra Nevada at ∼150-200 km depth. The western position of the Isabella Anomaly in the uppermost mantle is inconsistent with earlier interpretations that the Isabella Anomaly is connected to actively foundering foothills lower crust. Based on the new Vs images, we interpret that the Isabella Anomaly is not the dense destabilized root of the Sierra Nevada, but rather a remnant of Miocene subduction termination that is translating north beneath the central San Andreas Fault. Our results support the occurrence of localized lithospheric foundering beneath the high elevation eastern Sierra Nevada, where we find a lower crustal low Vs layer consistent with a small amount of partial melt. The high elevations relative to crust thickness and lower crustal low Vs zone are consistent with geological inferences that lithospheric foundering drove uplift

  3. Preferential mantle lithospheric extension under the South China margin

    International Nuclear Information System (INIS)

    Clift, P.; Jian Lin

    2001-01-01

    Continental rifting in the South China Sea culminated in seafloor spreading at ∼ 30Ma (Late Oligocene). The basin and associated margins form a classic example of break-up in a relatively juvenile arc crust environment. In this study, we documented the timing, distribution and amount of extension in the crust and mantle lithosphere on the South China Margin during this process. Applying a one-dimensional backstripping modeling technique to drilling data from the Pearl River Mouth Basin (PRMB) and Beibu Gulf Basin, we calculated subsidence rates of the wells and examined the timing and amount of extension. Our results show that extension of the crust exceeded that in the mantle lithosphere under the South China Shelf, but that the two varied in phase, suggesting depth-dependent extension rather than a lithospheric-scale detachment. Estimates of total crustal extension derived in this way are similar to those measured by seismic refraction, indicating that isostatic compensation is close to being local. Extension in the Beibu Gulf appears to be more uniform with depth, a difference that we attribute to the different style of strain accommodation during continental break-up compared to intra-continental rifting. Extension in PRMB and South China slope continues for ∼ 5m.y. after the onset of seafloor spreading due to the weakness of the continental lithosphere. The timing of major extension is broadly mid-late Eocene to late Oligocene (∼ 45-25Ma), but is impossible to correlate in detail with poorly dated strike-slip deformation in the Red River Fault Zone. (author)

  4. A Swarm lithospheric magnetic field model to SH degree 80

    OpenAIRE

    Thébault, Erwan; Vigneron, Pierre; Langlais, Benoit; Hulot, Gauthier

    2016-01-01

    International audience; The Swarm constellation of satellites was launched in November 2013 and since then has delivered high-quality scalar and vector magnetic field measurements. A consortium of several research institutions was selected by the European Space Agency to provide a number of scientific products to be made available to the scientific community on a regular basis. In this study, we present the dedicated lithospheric field inversion model. It uses carefully selected magnetic fiel...

  5. Imaging pockets and conduits of low velocity material beneath the lithosphere of the Atlas Mountains of Morocco: links to volcanism and orogenesis

    Science.gov (United States)

    Miller, M. S.; Sun, D.; O'Driscoll, L.; Holt, A.; Butcher, A.; Becker, T. W.; Diaz Cusi, J.; Thomas, C.

    2014-12-01

    The Atlas Mountains of Morocco have unusually high topography, with no apparent deep crustal root, and regions of localized Cenozoic alkaline volcanism. Previous seismic imaging and geophysical studies have implied a hot mantle upwelling as the source of the volcanism and high elevation, but the existence and physical properties of such an upwelling are debated. Recent temporary deployments of over 100 broadband seismometers that extended across Morocco as part of the PICASSO, Morocco-Münster, and IberArray experiments along with select permanent stations have provided a dataset to image the detailed mantle and lithospheric structure beneath the Atlas. We present results from S receiver functions (SRF), shear wave splitting, waveform modeling, and geodynamic models that help constrain the tectonic evolution of the Atlas and the localized alkaline volcanism. The receiver functions show that the lithosphere is thin (~65 km) beneath the Atlas, but thickens (~105 km) over a very short length scale at the flanks of the mountains and near the Quaternary volcanoes. These changes in lithospheric thickness also correspond to dramatic decreases in delay times inferred from S and SKS splitting observations. SRFs also indicate a broad, low seismic velocity anomaly (~150 km) below the shallow lithosphere that extends along much of the Atlas and beneath the Anti-Atlas and correlates with the location of Pliocene-Quaternary magmatism. Waveform analysis from the linear array across the Middle and High Atlas constrains the position, shape, and physical characteristics of a localized, low velocity conduit that extends up from the uppermost mantle (~200 km). The shape, position and temperature of the imaged low velocity anomaly, offsets in the lithosphere-asthenosphere boundary, and correlation with mantle flow inferred from shear wave splitting suggest that the unusually high topography of the Atlas Mountains is due to active mantle support.

  6. The lithospheric structure of the Western Carpathian-Pannonian Basin region based on the CELEBRATION 2000 seismic experiment and gravity modelling

    Science.gov (United States)

    Tašárová, Alasonati; Afonso, J. C.; Bielik, M.; Götze, H.-J.; Hók, J.

    2009-10-01

    The lithospheric structure of the Western Carpathian-Pannonian Basin region was studied using 3-D modelling of the Bouguer gravity anomaly constrained by seismic models and other geophysical data. The thermal structure and density distribution in the shallow upper mantle were also estimated using a combination of petrological, geophysical, and mineral physics information (LitMod). This approach is necessary if the more complicated structure of the Pannonian Basin is to be better constrained. As a result, we have constructed the first 3-D gravity model of the region that combines various geophysical datasets and is consistent with petrological data. The model provides improved estimates of both the density distribution within the lithosphere and the depth to major density discontinuities. We present new maps of the thickness of major sedimentary basins and of the depth to the Moho and the lithosphere-asthenosphere boundary. In our best-fitting model, the Pannonian Basin is characterised by extremely thin crust and lithospheric mantle, both of which have low density. A low-density uppermost asthenospheric mantle layer is also included at depths of 60-100 km. The Western Carpathians have only a thin crustal root and moderate densities. In contrast, the European Platform and Eastern Alps are characterised by lithosphere that is considerably thicker and denser. This inference is also supported by stripped gravity anomalies from which sediment, Moho and asthenospheric gravity contributions have been removed. These residual anomalies are characteristically low in the Western Carpathian-Pannonian Basin region, which suggests that both the ALCAPA and Tisza-Dacia microplates are 'exotic terranes' that are markedly different to the European Platform.

  7. New large-scale lithospheric model of the Western Carpathian-Pannonian Basin region based on the 3-D gravity modelling.

    Science.gov (United States)

    Alasonati Tasarova, Zuzana; Bielik, Miroslav; Götze, Hans-Jürgen; Afonso, Jaun Carlos; Fullea, Javier

    2010-05-01

    A 3-D forward modelling of the Bouguer gravity field was performed for the Western Carpathian-Pannonian Basin region. The gravity model extends to depth of 220 km and includes also the surrounding units (the Eastern Alps, Bohemian Massif, Trans-European Suture Zone and East European Craton). It is constrained by seismic models, mainly from the CELEBRATION 2000 seismic experiment, and other geophysical data. Additionally, the density distribution and thermal structure in the shallow upper mantle were estimated using a combination of petrological, geophysical, and mineral physics information (LitMod). This approach is necessary in order to better constrain the more complicated structure of the Pannonian Basin. As a result, we present the first 3-D gravity model of the region that combines various geophysical datasets and is consistent with petrological data. Realistic density values within the uppermost mantle provide a better control on the regional gravity signal. In turn, this generates a model with refined and enhanced crustal structure. This means that deeper parts of the model are better accounted for, which helps to better constrain the nature of shallower crustal layers. Although not commonly applied in potential field modelling, we find that this approach is advantageous when modelling large areas with insufficient near-surface constraints. Also, a density distribution within the crust and uppermost mantle that is consistent with petrological data allows better estimates of the depth to the Moho (where it is not constrained by seismic data) and to the lithosphere-asthenosphere boundary. Hence, our model provides improved estimates of both the density distribution within the crust and uppermost mantle and the depth to major density discontinuities (sediments, Moho, lithosphere-asthenosphere boundary). The results of the modelling reveal a markedly different nature of the Western Carpathian-Pannonian region (ALACAPA and Tisza-Dacia microplates) from the

  8. Crust-Lithosphere-Asthenosphere Dynamics in Mantle Plume Provinces with Emphasis on the Galapagos =

    Science.gov (United States)

    Orellana, Felipe

    Hotspot tracks, which most geoscientists attribute to the effects of mantle plumes on the overlying lithospheric plates, are characterized by distinct bathymetry, gravity signatures, structural geology, volcanology, petrology, and geochemistry; motivating us to try to understand the dynamics behind the space-time-histories of these systems. Making use of classical geodynamic paradigms, such as highly-viscous fluids (Stokes flow, for the mantle and/or lithosphere), elastic plate behavior, and heat flow, we develop conceptual frameworks to explain a number of distinct hotspot track features, and present quantitative models aimed at elucidating their origins. There is much diversity among the population of mantle plume hotspot tracks on the Earth's oceanic crust. For example, there are marked differences in the style of their bathymetry, as well as in their gravimetric signals, and also in the isotopic signature of extruded lavas. At the same time, important underlying differences are given by the age of the lithospheric plates under which the mantle plumes are impinging, lithospheric elastic thickness, the heat (or buoyancy) flux of individual mantle plumes, their melt production, crustal thickening, the proximity of spreading centers, etc. In the first chapter of this dissertation, making use of scaling theory, we show that for most oceanic hotspot tracks, the character of bathymetric expression (primarily rough vs. smooth topography) can be explained by three independent primary underlying factors - plate thickness, or equivalently plate age; plate speed; and plume buoyancy flux - combined into a single parameter, R, the ratio of plume heat flux to the effective thermal capacity of the moving plate overlying the plume. The Galapagos archipelago (off the west coast of equatorial South America), part of a >20 Ma old hotspot track formed by the underlying Galapagos mantle plume, currently exhibits a broad geographic distribution of volcanic centers of surprisingly

  9. SPECIFIC FEATURES OF DEFORMATION OF THE CONTINENTAL AND OCEANIC LITHOSPHERE AS A RESULT OF THE EARTH CORE NORTHERN DRIFT

    Directory of Open Access Journals (Sweden)

    Mikhail A. Goncharov

    2012-01-01

    drifting (Figures 26, and 27.By comparing the equations that describe the model of the northern drift of the lithosphere and the model of the core drift towards the North Pole, it is possible to establish a quantitative ‘bridge’ between the structures of meridional compression of the lithosphere and the core drifting structures.Conclusions based on the model of the northern drift of the lithosphere conform to many independent data and concepts, such as disturbance of the isostatic equilibrium of the Antarctica lithosphere and its high standing; the anomalously wide shelf of the Arctic ocean (Figure 28а and the increased thickness of the sediment cover, that is rich in hydrocarbons, in combination with the ultralow velocity of spreading in Gakkel Ridge; the approximately equal areas of Antarctica and the Arctic ocean as antipodes (Figure 28б; elongation (according to GPS data of the parallels in the Southern hemisphere, and their shortening in the Northern hemisphere (Figure 26; radial (relative to the South Pole rifts and other lineaments in Antarctica (Figures 29, and 30; the sub-concentric (relative to the same pole system of spreading around Antarctica, which develops northward into the submeridional system including three ‘trunks’ at a distance of about 90° (Figure 31.Due to the higher velocity of the northern drift of the lithosphere within the band with the middle meridian 100° E – 80° W, wherein the main mass of the continental lithosphere is concentrated and whose two ‘poles’ are marked by the axes of the African and Pacific superplumes (Figures 3, 4, 5, and 32, the following specific features have developed: maximum elongation of the Antarctic continent in the Southern (‘stretched’ hemisphere (Figure 28 б; maximum shortening of the Arctic ocean in the Northern (‘compressed’ hemisphere (Figure 28а; maximum spreading velocity in the SouthEastern Indian Ridge (Figure 33; maximum northern component of the horizontal displacements velocity

  10. Rock mechanics observations pertinent to the rheology of the continental lithosphere and the localization of strain along shear zones

    Science.gov (United States)

    Kirby, S.H.

    1985-01-01

    Emphasized in this paper are the deformation processes and rheologies of rocks at high temperatures and high effective pressures, conditions that are presumably appropriate to the lower crust and upper mantle in continental collision zones. Much recent progress has been made in understanding the flexure of the oceanic lithosphere using rock-mechanics-based yield criteria for the inelastic deformations at the top and base. At mid-plate depths, stresses are likely to be supported elastically because bending strains and elastic stresses are low. The collisional tectonic regime, however, is far more complex because very large permanent strains are sustained at mid-plate depths and this requires us to include the broad transition between brittle and ductile flow. Moreover, important changes in the ductile flow mechanisms occur at the intermediate temperatures found at mid-plate depths. Two specific contributions of laboratory rock rheology research are considered in this paper. First, the high-temperature steady-state flow mechanisms and rheology of mafic and ultramafic rocks are reviewed with special emphasis on olivine and crystalline rocks. Rock strength decreases very markedly with increases in temperature and it is the onset of flow by high temperature ductile mechanisms that defines the base of the lithosphere. The thickness of the continental lithosphere can therefore be defined by the depth to a particular isotherm Tc above which (at geologic strain rates) the high-temperature ductile strength falls below some arbitrary strength isobar (e.g., 100 MPa). For olivine Tc is about 700??-800??C but for other crustal silicates, Tc may be as low as 400??-600??C, suggesting that substantial decoupling may take place within thick continental crust and that strength may increase with depth at the Moho, as suggested by a number of workers on independent grounds. Put another way, the Moho is a rheological discontinuity. A second class of laboratory observations pertains to

  11. The Cretaceous Duimiangou adakite-like intrusion from the Chifeng region, northern North China Craton: Crustal contamination of basaltic magma in an intracontinental extensional environment

    Science.gov (United States)

    Fu, Lebing; Wei, Junhao; Kusky, Timothy M.; Chen, Huayong; Tan, Jun; Li, Yanjun; Shi, Wenjie; Chen, Chong; Zhao, Shaoqing

    2012-03-01

    Zircon U-Pb ages, major and trace element and Sr, Nd and Pb isotope compositions of the Duimiangou (DMG) quartz monzonite from the Chifeng region on the northern North China Craton (NCC) were studied to investigate its derivation, evolution and geodynamic significance. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) zircon U-Pb dating yields an emplacement age of 128 ± 1 Ma for this intrusion, with numerous Mesozoic inherited zircons clustering at 219 ± 12 Ma and 161 ± 3 Ma, along with some ancient zircons with ages of 2.5 Ga, 1.77 Ga and 324 Ma. Bulk-rock analyses show that this intrusion is characterized by variable SiO2 (63.4-69.4 wt.%), Al2O3 (14.5-16.3 wt.%), Na2O + K2O (8.01-8.95 wt.%), and Mg# (41.3-48.0). They are enriched in large ion lithophile elements and light rare earth elements without significant Eu anomalies (mostly between 0.89-1.10), and depleted in heavy rare earth elements and high field strength elements, with high Sr/Y (63.7-101.7) and (La/Yb)N (20.5-31.0) ratios. The DMG intrusion formed in an intracontinental extensional setting contemporaneous with the formation of pull-apart basins, metamorphic core complexes and intense magmatism, rather than in a convergent margin. It has homogeneous Sr ((87Sr/86Sr)i = 0.7059-0.7066), Nd (εNd(t) = - 6.2 to - 7.2) and Pb ((206Pb/204Pb)i = 17.289-17.375, (207Pb/204Pb)i = 15.359-15.463, (208Pb/204Pb)i = 37.130-37.472) isotope compositions. Sr-Nd isotope modeling results, plus relatively young Nd model ages (1522-1618 Ma) and the presence of relict zircons, suggest that this intrusion could have originated from crustal contamination of newly formed basaltic melts derived from asthenospheric mantle, accompanied by fractional crystallization of K-feldspar, biotite, apatite, Fe-Ti oxides and minor hornblende and plagioclase. Thus, the DMG adakite-like intrusion may record the magmatic event associated with underplating of asthenospheric magma in an intracontinental extensional

  12. Os-Hf-Sr-Nd isotope and PGE systematics of spinel peridotite xenoliths from Tok, SE Siberian craton: Effects of pervasive metasomatism in shallow refractory mantle

    Science.gov (United States)

    Ionov, Dmitri A.; Shirey, Steven B.; Weis, Dominique; Brügmann, Gerhard

    2006-01-01

    Os-Hf-Sr-Nd isotopes and PGE were determined in peridotite xenoliths carried to the surface by Quaternary alkali basaltic magmas in the Tokinsky Stanovik Range on the Aldan shield. These data constrain the timing and nature of partial melting and metasomatism in the lithospheric mantle beneath SE Siberian craton. The xenoliths range from the rare fertile spinel lherzolites to the more abundant, strongly metasomatised olivine-rich (70-84%) rocks. Hf-Sr-Nd isotope compositions of the xenoliths are mainly within the fields of oceanic basalts. Most metasomatised xenoliths have lower 143Nd / 144Nd and 176Hf / 177Hf and higher 87Sr / 86Sr than the host basalts indicating that the metasomatism is older and has distinct sources. A few xenoliths have elevated 176Hf / 177Hf (up to 0.2838) and plot above the Hf-Nd mantle array defined by oceanic basalts. 187Os / 188Os in the poorly metasomatised, fertile to moderately refractory (Al2O3 ≥ 1.6%) Tok peridotites range from 0.1156 to 0.1282, with oldest rhenium depletion ages being about 2 Ga. The 187Os / 188Os in these rocks show good correlations with partial melting indices (e.g. Al2O3, modal cpx); the intercept of the Al-187Os / 188Os correlation with lowest Al2O3 estimates for melting residues (∼0.3-0.5%) has a 187Os / 188Os of ∼0.109 suggesting that these peridotites may have experienced melt extraction as early as 2.8 Gy ago. 187Os / 188Os in the strongly metasomatised, olivine-rich xenoliths (0.6-1.3% Al2O3) ranges from 0.1164 to 0.1275 and shows no apparent links to modal or chemical compositions. Convex-upward REE patterns and high abundances of heavy to middle REE in these refractory rocks indicate equilibration with evolved silicate melts at high melt / rock ratios, which may have also variably elevated their 187Os / 188Os. This inference is supported by enrichments in Pd and Pt on chondrite-normalised PGE abundance patterns in some of the rocks. The melt extraction ages for the Tok suite of 2.0 to 2.8 Ga are

  13. Metamorphism and Shear Localization in the Oceanic and Continental Lithosphere: A Local or Lithospheric-Scale Effect?

    Science.gov (United States)

    Montesi, L.

    2017-12-01

    Ductile rheologies are characterized by strain rate hardening, which favors deformation zones that are as wide as possible, thus minimizing strain rate and stress. By contrast, plate tectonics and the observation of ductile shear zones in the exposed middle to lower crust show that deformation is often localized, that is, strain (and likely strain rate) is locally very high. This behavior is most easily explained if the material in the shear zone is intrinsically weaker than the reference material forming the wall rocks. Many origins for that weakness have been proposed. They include higher temperature (shear heating), reduced grain size, and fabric. The latter two were shown to be the most effective in the middle crust and upper mantle (given observational limits restricting heating to 50K or less) but they were not very important in the lower crust. They are not sufficient to explain the generation of narrow plate boundaries in the oceans. We evaluate here the importance of metamorphism, especially related to hydration, in weakening the lithosphere. Serpentine is a major player in the dynamics of the oceanic lithosphere. Although its ductile behavior is poorly constrained, serpentine is likely to behave in a brittle or quasi-plastic manner with a reduced coefficient of friction, replacing stronger peridotite. Serpentinization sufficiently weakens the oceanic lithosphere to explain the generation of diffuse plate boundaries and, combined with grain size reduction, the development of narrow plate boundaries. Lower crust outcrops, especially in the Bergen Arc (Norway), display eclogite shear zones hosted in metastable granulites. The introduction of water triggered locally a metamorphic reaction that reduces rock strength and resulted in a ductile shear zone. The presence of these shear zones has been used to explain the weakness of the lower crust perceived from geodesy and seismic activity. We evaluate here how much strain rate may increase as a result of

  14. Geothermal structure of the eastern Black Sea basin and the eastern Pontides orogenic belt: Implications for subduction polarity of Tethys oceanic lithosphere

    Directory of Open Access Journals (Sweden)

    Nafiz Maden

    2013-07-01

    Full Text Available The numerical results of thermal modeling studies indicate that the lithosphere is cold and strong beneath the Black Sea basin. The thermal lithospheric thickness increases southward from the eastern Pontides orogenic belt (49.4 km to Black Sea basin (152.2 km. The Moho temperature increases from 367 °C in the trench to 978 °C in the arc region. The heat flow values for the Moho surface change between 16.4 mW m−2 in the Black Sea basin and 56.9 mW m−2 in the eastern Pontides orogenic belt. Along the southern Black Sea coast, the trench region has a relatively low geothermal potential with respect to the arc and back-arc region. The numerical studies support the existence of southward subduction beneath the Pontides during the late Mesozoic–Cenozoic.

  15. Structures, microfabrics, fractal analysis and temperature-pressure estimation of the Mesozoic Xingcheng-Taili ductile shear zone in the North China craton

    Science.gov (United States)

    Liang, Chenyue; Neubauer, Franz; Liu, Yongjiang; Jin, Wei; Zeng, Zuoxun; Bernroider, Manfred; Li, Weimin; Wen, Quanbo; Han, Guoqing; Zhao, Yingli

    2014-05-01

    The ductile shear zone in Xingcheng-Taili area (western Liaoning Province in China) is tectonically located in the eastern section of the northern margin of the North China craton, and dominantly comprises deformed granitic rocks of Neoarchean and Triassic to Late Jurassic age, which were affected by shearing within middle- to low-grade metamorphic conditions. Because a high-temperature metamorphic overprint is lacking, microstructures attesting to low-temperature ductile deformation are well preserved. However, the rocks and its structures have not been previously analyzed in detail except by U-Pb zircon dating and some geochemistry. Here, we describe the deformation characteristics and tectonic evolution of the Xingcheng-Taili ductile shear zone, in order to understand the mode of lithosphericscale reactivation, extension and thinning of the North China craton. The ductile deformation history comprises four successive deformation phases: (1) In the Neoarchean granitic rocks, a steep gneissosity and banded structures trend nearly E-W (D1). (2) A NE-striking sinistral structure of Upper Triassic rocks may indicate a deformation event (D2) in Late Triassic times, which ductile deformation structures superimposed on Neoarchean granitic rocks. (3) A gneissose structure with S-C fabrics as well as an ENE-trending sinistral strike-slip characteristic (D3) developed in Upper Jurassic biotite adamellite and show the deformation characteristics of a shallow crustal level and generated mylonitic fabrics superimposed on previous structures. (4) Late granitic dykes show different deformational behavior, and shortening with D4 folds. The attitude of the foliation S and mineral stretching lineation of three main types of rocks shows remarkable differences in orientation. The shapes of recrystallized quartz grains from three main types of granitic rocks with their jagged and indented boundaries were natural records of deformation conditions (D1to D3). Crystal preferred

  16. The 3.26-3.24 Ga Barberton asteroid impact cluster: Tests of tectonic and magmatic consequences, Pilbara Craton, Western Australia

    Science.gov (United States)

    Glikson, Andrew; Vickers, John

    2006-01-01

    The location in the Barberton Greenstone Belt (Kaapvaal Craton) of ∼3.26-3.24 Ga asteroid impact ejecta units at, and immediately above, a sharp break between a > 12 km-thick mafic-ultramafic volcanic crust (Onverwacht Group ∼3.55-3.26 Ga, including the ∼3.298 > 3.258 Ga Mendon Formation) and a turbidite-felsic volcanic rift-facies association (Fig Tree Group ∼3.258-3.225 Ga), potentially represents the first documented example of cause-effect relations between extraterrestrial bombardment and major tectonic and igneous events [D.R. Lowe, G.R. Byerly, F. Asaro, F.T. Kyte, Geological and geochemical record of 3400 Ma old terrestrial meteorite impacts, Science 245 (1989) 959-962; D.R. Lowe, G.R. Byerly, F.T. Kyte, A. Shukolyukov, F. Asaro, A. Krull, Spherule beds 3.47-3.34 Ga-old in the Barberton greenstone belt, South Africa: a record of large meteorite impacts and their influence on early crustal and biological evolution, Astrobiology 3 (2003) 7-48; A.Y. Glikson, The astronomical connection of terrestrial evolution: crustal effects of post-3.8 Ga mega-impact clusters and evidence for major 3.2 ± 0.1 Ga bombardment of the Earth-Moon system, J. Geodyn. 32 (2001) 205-229]. Here we correlate this boundary with a contemporaneous break and peak magmatic and faulting events in the Pilbara Craton, represented by the truncation of a 3.255-3.235 Ga-old volcanic sequence (Sulphur Springs Group-SSG) by a turbidite-banded iron formation-felsic volcanic association (Pincunah Hill Formation, basal Gorge Creek Group). These events are accompanied by ∼3.252-3.235 Ga granitoids (Cleland plutonic suite). The top of the komatiite-tholeiite-rhyolite sequence of the SSG is associated with a marker chert defined at 3.238 ± 3-3.235 ± 3 Ga, abruptly overlain by an olistostrome consisting of mega-clasts of felsic volcanics, chert and siltstone up to 250 × 150 m-large, intercalated with siliciclastic sedimentary rocks and felsic volcanics (Pincunah Hill Formation-basal Gorge

  17. Long-lived melting of ancient lower crust of the North China Craton in response to paleo-Pacific plate subduction, recorded by adakitic rhyolite

    Science.gov (United States)

    Wang, Chao; Song, Shuguang; Niu, Yaoling; Allen, Mark B.; Su, Li; Wei, Chunjing; Zhang, Guibin; Fu, Bin

    2017-11-01

    Magmatism in eastern China in response to paleo-Pacific plate subduction during the Mesozoic was complex, and it is unclear how and when exactly the magmas formed via thinning and partial destruction of the continental lithosphere. To better understand this magmatism, we report the results of a geochronological and geochemical study of Early Cretaceous adakitic rhyolite (erupted at 125.4 ± 2.2 Ma) in the Xintaimen area within the eastern North China Craton (NCC). In situ zircon U-Pb dating shows that this adakitic rhyolite records a long ( 70 Myrs) and complicated period of magmatism with concordant 206Pb/238U ages from 193 Ma to 117 Ma. The enriched bulk rock Sr-Nd isotopic compositions of the Xintaimen adakitic rhyolite, as well as the enriched zircon Hf and O isotopic compositions, indicate that the magmas parental to the adakitic rhyolite were derived from partial melting of the Paleoproterozoic mafic lower crust, heated by mafic melts derived from the mantle during the paleo-Pacific plate subduction. A minor older basement component is indicated by the presence of captured Neoarchean to Early Paleoproterozoic zircons. The Mesozoic zircons have restricted Hf and O isotopic compositions irrespective of their ages, suggesting that they formed from similar sources at similar melting conditions. The Xintaimen adakitic rhyolite offers an independent line of evidence that the ancient lower crust of eastern China underwent a long period ( 70 Myrs) of destruction, melting or remelting, from 193 to 120 Ma, related to the subduction of the paleo-Pacific plate beneath eastern China.

  18. A subduction wedge origin for Paleoarchean peridotitic diamonds and harzburgites from the Panda kimberlite, Slave craton: evidence from Re-Os isotope systematics

    Science.gov (United States)

    Westerlund, K. J.; Shirey, S. B.; Richardson, S. H.; Carlson, R. W.; Gurney, J. J.; Harris, J. W.

    2006-09-01

    An extensive study of peridotitic sulfide inclusion bearing diamonds and their prospective harzburgitic host rocks from the 53 Ma Panda kimberlite pipe, Ekati Mine, NWT Canada, has been undertaken with the Re-Os system to establish their age and petrogenesis. Diamonds with peridotitic sulfide inclusions have poorly aggregated nitrogen (bearing diamonds and indicates residence in the cooler portion of the Slave craton lithospheric mantle. For most of the sulfide inclusions, relatively low Re contents (average 0.457 ppm) and high Os contents (average 339 ppm) lead to extremely low 187Re/188Os, typically << 0.05. An age of 3.52 ± 0.17 Ga (MSWD = 0.46) and a precise initial 187Os/188Os of 0.1093 ± 0.0001 are given by a single regression of 11 inclusions from five diamonds that individually provide coincident internal isochrons. This initial Os isotopic composition is 6% enriched in 187Os over 3.5 Ga chondritic or primitive mantle. Sulfide inclusions with less radiogenic initial Os isotopic compositions reflect isotopic heterogeneity in diamond forming fluids. The harzburgites have even lower initial 187Os/188Os than the sulfide inclusions, some approaching the isotopic composition of 3.5 Ga chondritic mantle. In several cases isotopically distinct sulfides occur in different growth zones of the same diamond. This supports a model where C-O-H-S fluids carrying a radiogenic Os signature were introduced into depleted harzburgite and produced diamonds containing sulfides conforming to the 3.5 Ga isochron. Reaction of this fluid with harzburgite led to diamonds with less radiogenic inclusions while elevating the Os isotope ratios of some harzburgites. Subduction is a viable way of introducing such fluids. This implies a role for subduction in creating early continental nuclei at 3.5 Ga and generating peridotitic diamonds.

  19. Trace element distribution in peridotite xenoliths from Tok, SE Siberian craton: A record of pervasive, multi-stage metasomatism in shallow refractory mantle

    Science.gov (United States)

    Ionov, Dmitri A.; Chazot, Gilles; Chauvel, Catherine; Merlet, Claude; Bodinier, Jean-Louis

    2006-03-01

    Spinel peridotite xenoliths in alkali basalts at Tok, SE Siberian craton range from fertile lherzolites to harzburgites and wehrlites; olivine-rich (70-84%) rocks are dominant. REE patterns in the lherzolites range from nearly flat for fertile rocks (14-17% cpx) to LREE-enriched; the enrichments are positively correlated with modal olivine, consistent with high-permeability of olivine-rich rocks during melt percolation. Clinopyroxene in olivine-rich Tok peridotites typically has convex-upward trace element patterns (La/Nd PM 1); the LREE-enrichments are positively correlated with phosphorus abundances and are mainly hosted by accessory phosphates and P-rich cryptocrystalline materials. In addition to apatite, some Tok xenoliths contain whitlockite (an anhydrous, halogen-poor and Na-Mg-rich phosphate), which is common in meteorites and lunar rocks, but has not been reported from any terrestrial mantle samples. Some olivine-rich peridotites have generations of clinopyroxene with distinct abundances of Na, LREE, Sr and Zr. The mineralogical and trace element data indicate that the lithospheric mantle section represented by the xenoliths experienced a large-scale metasomatic event produced by upward migration of mafic silicate melts followed by percolation of low- T, alkali-rich melts and fluids. Chromatographic fractionation and fractional crystallisation of the melts close to the percolation front produced strong LREE-enrichments, which are most common in the uppermost mantle and are related to carbonate- and P 2O 5-rich derivatives of the initial melt. Reversal and gradual retreat of the percolation front during thermal relaxation to ambient geotherm ("retrograde" metasomatism) caused local migration and entrapment of small-volume residual fluids and precipitation of volatile-rich accessory minerals. A distinct metasomatic episode, which mainly produced "anhydrous" late-stage interstitial materials was concomitant with the alkali basaltic magmatism, which brought

  20. Paleomagnetic study of 1765 Ma dyke swarm from the Singhbhum Craton: Implications to the paleogeography of India

    Science.gov (United States)

    Shankar, Ravi; Srinivasa Sarma, D.; Ramesh Babu, N.; Parashuramulu, V.

    2018-05-01

    We report the first key paleopole as a result of paleomagnetic study on a precisely dated 1765.3 ± 1.0 Ma WNW-ESE trending dyke swarm from Singhbhum Craton. This pole has been used in this study to propose the paleogeographic reconstruction of India with Baltica Craton and North China Craton. Incremental alternating field (AF) and thermal demagnetization, isolated high coercivity components with north to north-westerly declination and shallow negative inclination from 9 sampling sites which are representing different individual dykes. The primary origin of the ChRM is supported by the positive baked contact test. The WNW-ESE trending dykes yield a mean paleomagnetic direction with a declination = 329.2° and an inclination = -22.8° (k = 31.6; α95 = 9.3°). The positive bake contact test proves the primary nature of remanence. The pole position of Singhbhum Craton at 1765 Ma is 45°N, 311°E (dp = 5.2 and dm = 9.9). Paleogeographic reconstruction at ca. 1770 Ma, supported by geological, tectonic and metallogenic evidences indicate that the Baltica Craton and India linkage can be stable for at least ∼370 Ma (∼1770-1400 Ma). There is also reasonable evidence in support of India-North China Craton spatial proximity at ∼1770 Ma.

  1. Mantle weakening and strain localization: Implications for the long-term strength of the continental lithosphere

    OpenAIRE

    Précigout , Jacques; Gueydan , Frédéric

    2009-01-01

    International audience; Mechanics of the continental lithosphere require the presence of a high-strength uppermost mantle that defines the "jelly sandwich" model for lithosphere strength layering. However, in deforming regions, growing numbers of geological and geophysical data predict a sub-Moho mantle strength lower than the crustal strength, or a "crème brûlée" model. To reconcile these two opposite views of lithosphere strength layering, we account for a new olivine rheology, which could ...

  2. Impact of the lithosphere on dynamic topography: Insights from analogue modeling

    OpenAIRE

    Sembroni, Andrea; Kiraly, Agnes; Faccenna, Claudio; Funiciello, Francesca; Becker, Thorsten W.; Goblig, Jan; Fernandez, Manel

    2017-01-01

    Density anomalies beneath the lithosphere are expected to generate dynamic topography at the Earth's surface due to the induced mantle flow stresses which scale linearly with density anomalies, while the viscosity of the upper mantle is expected to control uplift rates. However, limited attention has been given to the role of the lithosphere. Here we present results from analogue modeling of the interactions between a density anomaly rising in the mantle and the lithosphere in a Newtonian sys...

  3. Lithospheric structure of the westernmost Mediterranean inferred from finite frequency Rayleigh wave tomography S-velocity model.

    Science.gov (United States)

    Palomeras, Imma; Villasenor, Antonio; Thurner, Sally; Levander, Alan; Gallart, Josep; Harnafi, Mimoun

    2016-04-01

    The Iberian Peninsula and Morocco, separated by the Alboran Sea and the Algerian Basin, constitute the westernmost Mediterranean. From north to south this region consists of the Pyrenees, the result of interaction between the Iberian and Eurasian plates; the Iberian Massif, a region that has been undeformed since the end of the Paleozoic; the Central System and Iberian Chain, regions with intracontinental Oligocene-Miocene deformation; the Gibraltar Arc (Betics, Rif and Alboran terranes) and the Atlas Mountains, resulting from post-Oligocene subduction roll-back and Eurasian-Nubian plate convergence. In this study we analyze data from recent broad-band array deployments and permanent stations on the Iberian Peninsula and in Morocco (Spanish IberArray and Siberia arrays, the US PICASSO array, the University of Munster array, and the Spanish, Portuguese, and Moroccan National Networks) to characterize its lithospheric structure. The combined array of 350 stations has an average interstation spacing of ~60 km, comparable to USArray. We have calculated the Rayleigh waves phase velocities from ambient noise for short periods (4 s to 40 s) and teleseismic events for longer periods (20 s to 167 s). We inverted the phase velocities to obtain a shear velocity model for the lithosphere to ~200 km depth. The model shows differences in the crust for the different areas, where the highest shear velocities are mapped in the Iberian Massif crust. The crustal thickness is highly variable ranging from ~25 km beneath the eastern Betics to ~55km beneath the Gibraltar Strait, Internal Betics and Internal Rif. Beneath this region a unique arc shaped anomaly with high upper mantle velocities (>4.6 km/s) at shallow depths (volcanic fields in Iberia and Morocco, indicative of high temperatures at relatively shallow depths, and suggesting that the lithosphere has been removed beneath these areas

  4. Some problems of geologic relations between the Amazon craton and east margins fold belts

    International Nuclear Information System (INIS)

    Almeida, F.F.M. de

    1986-01-01

    This paper deals with some geologic problems related to the limits between the Amazon craton and the fold belts developed at its margins during the Precambrian. These limits are diversified but clearly recognized. To the north, the Araguaia-Tocantins fold belt, of presumed Middle Proterozoic age, is separated from the cratonic block by a deep marginal fracture zone permeated by mafic and ultramafic rocks. The geologic, magmatic and aeromagnetic characteristics of this zone point out the presence of deep faults, supposed to be of Middle Proterozoic age. The southern Paraguay fold belt constitutes and accurated zone of linear structures supposed to be of Late Proterozoic development. Despite the great increase of knowledge during the last ten years many tectonic, stratigraphic and geochronologic problems remain unsolved. The aim of this paper is to point out some of these problems and suggest specific studies to solve them. (author)

  5. Gravity Data Interpretation in the Northern Edge of the Congo Craton, South-Cameroon

    Directory of Open Access Journals (Sweden)

    James Derek Fairhead

    2010-01-01

    Full Text Available Gravity data in the southern Cameroon are interpreted to better understand the organization of underlying structuresthroughout the northern edge of the Congo craton. The Bouguer anomaly maps of the region are characterized by an elongated trending trending negative gravity anomaly which correspond to a collapsed structure associated with a granitic intrusion beneath the cente center of the region r of the region of the region and limited by fault systems. �e applied 3�D gravity modelling and inversion in order to obtain the 3D density structure of the area. Our result demonstrated that observed gravity anomalies in the region are associated to tectonic structures in the subsurface. The resulting model agrees with the hypothesis of the existence of a major continental collision zone between the Congo Craton and the Pan�African belt. The presence of deep granulites structures in the northern part of the region expresses a continental collision.

  6. PALEOARCHEAN MAFIC ROCKS OF THE SOUTHWESTERN SIBERIAN CRATON: PRELIMINARY GEOCHRONOLOGY AND GEOCHEMICAL CHARACTERIZATION

    Directory of Open Access Journals (Sweden)

    A. V. Ivanov

    2017-01-01

    Full Text Available The Siberian craton consists of Archean blocks, which were welded up into the same large unit by ca 1.9 Ga [Gladkochub et al., 2006; Rojas-Agramonte et al., 2011]. The history of the constituent Archean blocks is mosaic because of limited number of outcrops, insufficient sampling coverage because of their location in remote regions and deep forest and difficulties with analytical studies of ancient rocks, which commonly underwent metamorphic modifications and secondary alterations. In this short note, we report data on discovery of unusual for Archean mafic rocks of ultimate fresh appearance. These rocks were discovered within southwestern Siberian craton in a region near a boundary between Kitoy granulites of the Sharyzhalgai highgrade metamorphic complex and Onot green-schist belt (Fig. 1. Here we present preliminary data on geochronology of these rocks and provide their geochemical characterization.

  7. Towards an improved determination of Earth’s lithospheric field from satellite observations

    DEFF Research Database (Denmark)

    Kotsiaros, Stavros; Olsen, Nils; Finlay, Chris

    Perhaps one of the biggest difficulties in modelling the Earth’s lithospheric magnetic field is the separation of contributions from sources of internal and external origin. In particular, the determination of smaller-scale lithospheric magnetic field features is problematic because the lithosphe......Perhaps one of the biggest difficulties in modelling the Earth’s lithospheric magnetic field is the separation of contributions from sources of internal and external origin. In particular, the determination of smaller-scale lithospheric magnetic field features is problematic because...

  8. Signatures of Lithospheric Flexure and Elevated Heat Flow in Stereo Topography at Coronae on Venus

    Science.gov (United States)

    O'Rourke, Joseph G.; Smrekar, Suzanne E.

    2018-02-01

    Signatures of lithospheric flexure were previously identified at a dozen or more large coronae on Venus. Thin plate models fit to topographic profiles return elastic parameters, allowing derivation of mechanical thickness and surface heat flows given an assumed yield strength envelope. However, the low resolution of altimetry data from the NASA Magellan mission has hindered studying the vast majority of coronae, particularly those less than a few hundred kilometers in diameter. Here we search for flexural signatures around 99 coronae over ˜20% of the surface in Magellan altimetry data and stereo-derived topography that was recently assembled from synthetic aperture radar images. We derive elastic thicknesses of ˜2 to 30 km (mostly ˜5 to 15 km) with Cartesian and axisymmetric models at 19 coronae. We discuss the implications of low values that were also noted in earlier gravity studies. Most mechanical thicknesses are estimated as 24 K km-1. Implied surface heat flows >95 mW m-2—twice the global average in many thermal evolution models—imply that coronae are major contributors to the total heat budget or Venus is cooling faster than expected. Binomial statistics show that "Type 2" coronae with incomplete fracture annuli are significantly less likely to host flexural signatures than "Type 1" coronae with largely complete annuli. Stress calculations predict extensional faulting where nearly all profiles intersect concentric fractures. We failed to identify systematic variations in flexural parameters based on type, geologic setting, or morphologic class. Obtaining quality, high-resolution topography from a planetwide survey is vital to verifying our conclusions.

  9. Lithospheric Structure of the Zagros and Alborz Mountain Belts (Iran) from Seismic Imaging

    Science.gov (United States)

    Paul, A.; Hatzfeld, D.; Kaviani, A.; Tatar, M.

    2008-12-01

    We present a synthesis of the results of two dense temporary passive seismic experiments installed for a few months across Central Zagros for the first one, and from North-western Zagros to Alborz for the second one. On both transects, the receiver function analysis shows that the crust has an average thickness of ~ 43 km beneath the Zagros fold-and-thrust belt and the Iranian plateau. The crust is thicker in the back side of the Main Zagros Reverse Fault (MZRF), with a larger maximum Moho depth in Central Zagros (69 ± 2 km) than in North-western Zagros (56 ± 2 km). To reconcile Bouguer anomaly data and Moho depth profile of Central Zagros, we proposed that the thickening is related to overthrusting of the Arabian margin by Central Iran on the MZRF considered as a major thrust fault rooted at Moho depth. The better-quality receiver functions of NW Zagros display clear conversions on a low-velocity channel which cross-cuts the whole crust from the surface trace of the MZRF to the Moho on 250-km length. Waveform modeling shows that the crustal LVZ is ~ 10-km thick with a S-wave velocity 8-30 % smaller than the average crustal velocity. We interpret the low-velocity channel as the trace of the thrust fault and the suture between the Arabian and the Iranian lithospheres. We favour the hypothesis of the LVZ being due to sediments of the Arabian margin dragged to depth during the subduction of the Neotethyan Ocean. At upper mantle depth, we find shield-like shear-wave velocities in the Arabian upper-mantle, and lower velocities in the Iranian shallow mantle (50-150 km) which are likely due to higher temperature. The lack of a high-velocity anomaly in the mantle northeast of the MZRF suture suggests that the Neotethian oceanic lithosphere is now detached from the Arabian margin. The crust of the Alborz mountain range is not thickened in relation with its high elevations, but its upper mantle has low P-wave velocities.

  10. Patterns of deformation and volcanic flows associated with lithospheric loading by large volcanoes on Venus

    Science.gov (United States)

    Mcgovern, Patrick J.; Solomon, Sean C.

    1993-01-01

    Magellan radar imaging and topography data are now available for a number of volcanoes on Venus greater than 100 km in radius. These data can be examined to reveal evidence of the flexural response of the lithosphere to the volcanic load. On Venus, erosion and sediment deposition are negligible, so tectonic evidence of deformation around large volcanoes should be evident except where buried by very young flows. Radar images reveal that most tectonic features and flow units on the flanks of these volcanoes have predominantly radial orientations. However, both Tepev Mons in Bell Regio and Sapas Mons in Atla Regio exhibit circumferential graben on their flanks. In addition, images reveal several flow units with an annular character around the north and west flanks of Tepev Mons. This pattern most likely results from ponding of flows in an annular flexural moat. Maat Mons in Atla Regio and Sif Mons in Eistla Regio are examples of volcanoes that lack circumferential graben and annular flows; discernible flow units and fractures on these constructs appear to be predominantly radial. Altimetry data can also provide evidence of flexural response. Tepev Mons is partially encircled by depressions that may be sections of a flexural moat that has not been completely filled. The locations of these depressions generally coincide with the annular flows described above. There is weaker evidence for such depressions around Maat Mons as well. The lack of circumferential tectonic features around most volcanoes on Venus might be explained by gradual moat filling and coverage by radial flows. The depressions around Tepev (and possible Maat) may indicate that this process is currently continuing. We use analytic models of plate flexure in an axisymmetric geometry to constrain the elastic plate thickness supporting Tepev Mons. If we consider the outer radius of the ponded flows to be the edge of a moat, we find that models with elastic plate thickness of 10-20 km fit best. Finite element

  11. Introduction to nickel sulfide orebodies and komatiites of the Black Swan area, Yilgarn Craton, Western Australia

    Science.gov (United States)

    Barnes, S. J.

    2004-11-01

    The Black Swan district, 70 km north east of Kalgoorlie in the Archaean Yilgarn Craton of Western Australia, hosts massive and disseminated nickel sulfide orebodies associated with komatiites. The host rocks and ores preserve some remarkable primary igneous features, which provide important clues as to the origin of komatiite-hosted nickel ores. The series of papers that follow report an extremely detailed study of the petrology, volcanology and geochemistry of these unusually well-preserved orebodies and their host rocks.

  12. THE SOUTHERN FRAGMENT OF THE SIBERIAN CRATON: “LANDSCAPE” HISTORY OVER TWO BILLION YEARS

    Directory of Open Access Journals (Sweden)

    Arkady M. Stanevich

    2010-01-01

    Full Text Available In the state-of-the-art geology, concepts of evolution of interrelated geodynamic and biotic events throughout the history of the Earth have been developed (Fig. 1. Research results on sediments, bio-stratigraphy and geodynamics of the southern fragment of the Siberian craton (SSC, Fig. 2 provide for more or less reliable assessments of the status and evolution of ancient landscapes and biotas from the Lower Proterozoic to the Cenozoic.In the Lower Proterozoic, the geodynamic regime of the Urik-Iyskiy graben was similar to those of the westernpacific island-arc systems, which resulted in the orogen formation and established post-orogen granitoids of 1.86 bln years of age. At the beginning of the Early Riphean, volcano-sedimentary masses were accumulated in continental basins (Fig. 2, 3A. Collision orogenesis also resulted in the occurrence of the terrigeno-volcanogenic complex of the Akitkanskaya suite in the Western Pribaikalie and the transecting Irelskiy granitoids, aged 1.86 bln years, at the edge of the craton. Later on, most probably before the Riphean, peneplanation took place, and a shallow peripheral sea was formed with highly-mature sediments of the Purpolskaya suite. Different environments are reconstructed in the KodarUdokan zone. Sediments of the Udokanskaya suite, varying in thicknesses from 11 to 14 km, suggest a complicated evolution of sedimentation in the peripheral marine basin. Dozens of radiochronological datings of granitoids of the Chuiskiy and Kodarskiy complex which transect the Udokanskaya suite are within the range from 1.7 to 2.0 bln years. From the deposit composition and texture, it can be suggested that the middle, Chineiskaya sub-suite was formed under island-arc conditions; and glacial phenomena occurred in the late Udokan time.Further geological history of the SSC can be described only within the period after the Late Riphean sedimentations (see Fig. 3Б, В. The SSC evolution in the Neo-Proterozoic began with

  13. An isotopic perspective on growth and differentiation of Proterozoic orogenic crust: From subduction magmatism to cratonization

    Energy Technology Data Exchange (ETDEWEB)

    Johnson, Simon P.; Korhonen, Fawna J.; Kirkland, Christopher L.; Cliff, John B.; Belousova, Elena A.; Sheppard, Stephen

    2017-01-01

    The in situ chemical differentiation of continental crust ultimately leads to the long-term stability of the continents. This process, more commonly known as ‘cratonization’, is driven by deep crustal melting with the transfer of those melts to shallower regions resulting in a strongly chemically stratified crust, with a refractory, dehydrated lower portion overlain by a complementary enriched upper portion. Since the lower to mid portions of continental crust are rarely exposed, investigation of the cratonization process must be through indirect methods. In this study we use in situ Hf and O isotope compositions of both magmatic and inherited zircons from several felsic magmatic suites in the Capricorn Orogen of Western Australia to highlight the differentiation history (i.e. cratonization) of this portion of late Archean to Proterozoic orogenic crust. The Capricorn Orogen shows a distinct tectonomagmatic history that evolves from an active continental margin through to intracratonic reworking, ultimately leading to thermally stable crust that responds similarly to the bounding Archean Pilbara and Yilgarn Cratons.

  14. Anomalous variations of lithosphere magnetic field before several earthquakes

    Science.gov (United States)

    Ni, Z.; Chen, B.

    2015-12-01

    Based on the geomagnetic vector data measured each year since 2011 at more than 500 sites with a mean spatial interval of ~70km.we observed anomalous variations of lithospheric magnetic field before and after over 15 earthquakes having magnitude > 5. We find that the field in near proximity (about 50km) to the epicenter of large earthquakes shows high spatial and temporal gradients before the earthquake. Due to the low frequency of repeat measurements it is unclear when these variations occurred and how do them evolve. We point out anomalous magnetic filed using some circles with radius of 50km usually in June of each year, and then we would check whether quake will locat in our circles during one year after that time (June to next June). Now we caught 10 earthquakes of 15 main shocks having magnitude > 5, most of them located at less than10km away from our circles and some of them were in our circles. Most results show that the variations of lithosphere magnetic filed at the epicenter are different with surrending backgroud usually. When we figure out horizontal variations (vector) of lithosphere magnetic field and epicenter during one year after each June, we found half of them show that the earthquakes will locat at "the inlands in a flowing river", that means earthquakes may occur at "quiet"regions while the backgroud show character as"flow" as liquid. When we compared with GPS results, it appears that these variations of lithospere magnetic field may also correlate with displacement of earth's surface. However we do not compared with GPS results for each earthquake, we are not clear whether these anomalous variations of lithospere magnetic field may also correlate with anomalous displacement of earth's surface. Future work will include developing an automated method for identifying this type of anomalous field behavior and trying to short repeat measurement period to 6 month to try to find when these variations occur.

  15. Dynamics of Lithospheric Extension and Residual Topography in Southern Tibet

    Science.gov (United States)

    Chen, B.; Shahnas, M. H.; Pysklywec, R.; Sengul Uluocak, E.

    2017-12-01

    Although the north-south (N-S) convergence between India and Eurasia is ongoing, a number of north-south trending rifts (e.g., Tangra Yum Co Rift, Yadong-Gulu Rift and Cona Rift) and normal faulting are observed at the surface of southern Tibet, suggesting an east-west (E-W) extension tectonic regime. The earthquake focal mechanisms also show that deformation of southern Tibet is dominated by E-W extension across these N-S trending rifts. Because the structure of the lithosphere and underlying mantle is poorly understood, the origin of the east-west extension of southern Tibet is still under debate. Gravitational collapse, oblique convergence, and mantle upwelling are among possible responsible mechanisms. We employ a 3D-spherical control volume model of the present-day mantle flow to understand the relationship between topographic features (e.g., rifts and the west-east extension), intermediate-depth earthquakes, and tectonic stresses induced by mantle flow beneath the region. The thermal structure of the mantle and crust is obtained from P and S-wave seismic inversions and heat flow data. Power-law creep with viscous-plastic rheology, describing the behavior of the lithosphere and mantle material is employed. We determine the models which can best reconcile the observed features of southern Tibet including surface heat flow, residual topography with uplift and subsidence, reported GPS rates of the vertical movements, and the earthquake events. The 3D geodynamic modeling of the contemporary mantle flow-lithospheric response quantifies the relative importance of the various proposed mechanism responsible for the E-W extension and deep earthquakes in southern Tibet. The results also have further implications for the magmatic activities and crustal rheology of the region.

  16. Generation of continental rifts, basins, and swells by lithosphere instabilities

    Science.gov (United States)

    Fourel, Loïc.; Milelli, Laura; Jaupart, Claude; Limare, Angela

    2013-06-01

    Continents may be affected simultaneously by rifting, uplift, volcanic activity, and basin formation in several different locations, suggesting a common driving mechanism that is intrinsic to continents. We describe a new type of convective instability at the base of the lithosphere that leads to a remarkable spatial pattern at the scale of an entire continent. We carried out fluid mechanics laboratory experiments on buoyant blocks of finite size that became unstable due to cooling from above. Dynamical behavior depends on three dimensionless numbers, a Rayleigh number for the unstable block, a buoyancy number that scales the intrinsic density contrast to the thermal one, and the aspect ratio of the block. Within the block, instability develops in two different ways in an outer annulus and in an interior region. In the outer annulus, upwellings and downwellings take the form of periodically spaced radial spokes. The interior region hosts the more familiar convective pattern of polygonal cells. In geological conditions, such instabilities should manifest themselves as linear rifts striking at a right angle to the continent-ocean boundary and an array of domal uplifts, volcanic swells, and basins in the continental interior. Simple scaling laws for the dimensions and spacings of the convective structures are derived. For the subcontinental lithospheric mantle, these dimensions take values in the 500-1000 km range, close to geological examples. The large intrinsic buoyancy of Archean lithospheric roots prevents this type of instability, which explains why the widespread volcanic activity that currently affects Western Africa is confined to post-Archean domains.

  17. Lithosphere Response to Intracratonic Rifting: Examples from Europe and Siberia

    DEFF Research Database (Denmark)

    Artemieva, I. M.; Thybo, H.; Herceg, M.

    2012-01-01

    is based on critically assessed results from various seismic studies, including reflection and refraction profiles and receiver function studies. We also use global shear-wave tomography models, gravity constraints based on GOCE data, and thermal models for the lithosphere to speculate on thermo...... of basaltic magmas and consequently in a change in mantle density and seismic velocities. Although kimberlite magmatism is commonly not considered as a rifting events, its deep causes may be similar to the mantle-driven rifting and, as a consequence, modification of mantle density and velocity structure may...... in it seismic wave velocity and density structure....

  18. Proterozoic biotite Rb-Sr dates in the northwestern part of the Yilgarn Craton, Western Australia

    International Nuclear Information System (INIS)

    Libby, W.G.; De Laeter, J.R.; Armstrong, R.A.

    1999-01-01

    Rb-Sr dating of biotite in the northwestern corner of the Yilgarn Craton identified four areas with distinctive age ranges. Biotite in the northwestern area, which includes the Narryer Terrane and part of the Murchison Terrane, yields reset Rb-Sr dates of ca 1650 Ma. In the western area, along the margin of the craton, biotite has been reset to 629 Ma. Eastward of these areas, mainly in the Murchison Terrane, the modal biotite date is near 2450 Ma, though because of a skewed distribution the mean date is closer to 2300 Ma. Dates in a transition zone between the western and eastern areas range broadly between 2000 and 1000 Ma, averaging about 1775 Ma. The western area and the transition zone are continuous with analogous areas south of the limits of the present study. The 1650 Ma dates in the northwestern area are probably related to plutonic and tectonic activity of similar age in the Gascoyne Province to the north. They may represent cooling after thermal resetting during tectonic loading by southward thrust-stacking of slices of Narryer Terrane and allochthonous Palaeoproterozoic volcanic arc and back arc rocks during the Capricorn Orogeny. This episode of crustal shortening resulted from the collision of the Yilgarn and Pilbara Cratons to form the West Australian Craton. The dates reflect cooling associated with subsequent erosion-induced rebound. The 2450 Ma biotite dates of the eastern area are similar to biotite dates found over most of the Yilgarn Craton and represent a background upon which the later dates have been superimposed. The origin of dates in the western area is unknown but may be related to an associated dolerite dyke swarm or to possible thrusting from the west. There is some evidence of minor later intrusion of felsic hypabyssal rock between 2000 and 2200 Ma and localised shearing in the Narryer area at about 1350 to 1400 Ma. One small area near Yalgoo with biotite Rb-Sr dates near 2200 Ma may be co genetic with the Muggamurra Swarm of dolerite

  19. Evolving lithospheric flexure and paleotopography of the Pyrenean Orogen from 3D flexural modeling and basin analysis

    Science.gov (United States)

    Curry, M. E.; van der Beek, P.; Huismans, R. S.; Muñoz, J. A.

    2017-12-01

    The Pyrenees are an asymmetric, doubly-vergent orogen with retro- and pro- foreland basins that preserve a record of deformation since the Mesozoic. The extensive research and exploration efforts on the mountain belt and flanking foreland basins provide an exceptional dataset for investigating geodynamics and surface processes over large spatial and temporal scales in western Europe. We present the results of a numerical modeling study investigating the spatio-temporal variation in lithospheric flexure in response to the developing orogen. We employ a finite element method to model the 3D flexural deformation of the lithosphere beneath the Pyrenean orogen since the onset of convergence in the late Cretaceous. Using subsurface, geophysical, and structural data, we describe the evolving geometry of both the French Aquitaine and Spanish Ebro foreland basins at the present (post-orogenic), the mid-Eocene (peak orogenic), the Paleocene (early orogenic), and the end of the Cretaceous (pre- to early orogenic). The flexural modeling provides insight into how both the rigidity of the lithosphere and the paleotopographic load have varied over the course of orogenesis to shape the basin geometry. We find that the overriding European plate has higher rigidity than the subducting Iberian plate, with modern Effective Elastic Thickness (EET) values of 20 ± 2 and 12 ± 2 km, respectively. Modeling indicates that the modern rigidity of both plates decreases westward towards the Bay of Biscay. The lithospheric rigidity has increased by 50% since the Mesozoic with early Cenozoic EET values of 13 ± 2 and 8 ± 1 km for the European and Iberian plates, respectively. The topographic load began increasing with convergence in the late Cretaceous, reaching modern levels in the central and eastern Pyrenees by the Eocene. In contrast, the topographic load in the western Pyrenees was 70% of the modern value in the Eocene, and experienced topographic growth through the Oligo-Miocene. The

  20. On the Elastic Strength (and Its Anisotropy) of the North American Continental Lithosphere (in a Global Perspective)

    Science.gov (United States)

    Wang, D. V.; Simons, F. J.

    2010-12-01

    While we are have become aware of the difficulties and pitfalls of estimating the effective elastic strength of the lithosphere via the spectral analysis and inversion of the coherence between gravity anomalies and topography - and will provide a brief summary on what we have discovered in this regard - we have made one final attempt at characterizing the elastic lithosphere via the coherence of Bouguer anomalies with topography. We have conducted a global study, on each of the worlds' continents, and will use this global scope to guide our presentation of the results for the North American region. Two key innovations distinguish our approach. The first is that we are investigating a possible anisotropy in the coherence or isostatic response, based on advanced spectral analysis methods and sound statistical judgment. For North America, the results are not unambiguous: lithospheric elastic anisotropy may be weakly expressed but is certainly hard to measure with confidence. The second innovation is that we have definitely thrown off the yoke of needing to analyze rectangular regions when Fourier-based methods are involved. For this we developed a Cartesian technique of spatiospectral localization in the sense of Slepian, with which regions of arbitrary geometry can be handled, and directionally sensitive (or agnostic) analyses carried out. This in contrast to any other Fourier based method. By "final attempt" we mean that while we are now in the position to analyze gravity/topography with the theoretically "best", geologically sensitive, method of spectral analysis, the quality of the results remains strongly influenced by the marriage (of convenience and of popular choice) to coherence or admittance, as we will show. Thus, after presenting what we have learned about lithospheric-thickness estimation in general, and the results for the North American continent in particular, we will conclude our presentation with a series of caveats as to the general applicablity of

  1. ∼2.5 Ga late cratonisation events in Dharwar craton: insights from the gold mineralisation ages

    International Nuclear Information System (INIS)

    Srinivasa Sarma, D.; Ram Mohan, M.; McNaughton, Neal

    2013-01-01

    The history of volcanism, granitic magmatism, and gold mineralization is defined by U-Pb geochronology of magmatic zircons and hydrothermal monazite and xenotime respectively. The felsic volcanic host rocks from Hutti greenstone belt have a U-Pb zircon age of 258 ±7 Ma, about 40 m.y. older than the age of gold mineralization at 2547±10 Ma determined from hydrothermal monazite in the Hutti gold deposit. The syntectonic Kavital granitoid in the Hutti greenstone belt has a U-Pb zircon age of 2545±7 Ma, which overlaps with the timing of gold deposition and is consistent with structural interpretations. Zircon U-Pb ages for a felsic volcanic rock (2,588±10 Ma) and an intrusive granite (e''2,555±6 Ma) in the Gadag greenstone belt in the Western Dharwar Craton. In situ U-Pb dating of monazite and xenotime in gold reefs of the Gadag (2,522±6 Ma) and Ajjanahalli (2,520±9 Ma) gold deposits reveal a previously undated episode of gold mineralization at 2.52 Ga, substantially younger than the 2.55 Ga Hutti deposit in the eastern Dharwar Craton. The Hutti, Gadag and Ajjanahalli gold geochronology suggests that gold mineralization occurred throughout the Dharwar craton some 80 to 120 m.y. later than the major peak of Late Archean world-class orogenic gold mineralization in most other Archean cratons. Although gold mineralization across the craton postdates most of the magmatic activity and metamorphism at upper crustal levels, widespread thermal reworking of the lower middle crust, involving partial melting, metamorphism, and lower crustal granitoid intrusion, occurred concurrently with gold mineralization. It is likely that the large-scale hydrothermal fluid flow that produced widespread gold deposition was also part of this tectono-thermal event during the final stages of cratonization of the Dharwar Craton in southern India. (author)

  2. Colorado Plateau magmatism and uplift by warming of heterogeneous lithosphere.

    Science.gov (United States)

    Roy, Mousumi; Jordan, Thomas H; Pederson, Joel

    2009-06-18

    The forces that drove rock uplift of the low-relief, high-elevation, tectonically stable Colorado Plateau are the subject of long-standing debate. While the adjacent Basin and Range province and Rio Grande rift province underwent Cenozoic shortening followed by extension, the plateau experienced approximately 2 km of rock uplift without significant internal deformation. Here we propose that warming of the thicker, more iron-depleted Colorado Plateau lithosphere over 35-40 Myr following mid-Cenozoic removal of the Farallon plate from beneath North America is the primary mechanism driving rock uplift. In our model, conductive re-equilibration not only explains the rock uplift of the plateau, but also provides a robust geodynamic interpretation of observed contrasts between the Colorado Plateau margins and the plateau interior. In particular, the model matches the encroachment of Cenozoic magmatism from the margins towards the plateau interior at rates of 3-6 km Myr(-1) and is consistent with lower seismic velocities and more negative Bouguer gravity at the margins than in the plateau interior. We suggest that warming of heterogeneous lithosphere is a powerful mechanism for driving epeirogenic rock uplift of the Colorado Plateau and may be of general importance in plate-interior settings.

  3. S-wave velocities of the lithosphere-asthenosphere system in the Lesser Antilles from the joint inversion of surface wave dispersion and receiver function analysis

    Science.gov (United States)

    González, O'Leary; Clouard, Valerie; Tait, Stephen; Panza, Giuliano F.

    2018-06-01

    We present an overview of S-wave velocities (Vs) within the crust and upper mantle of the Lesser Antilles as determined with 19 seismic broadband stations. Receiver functions (RF) have been computed from teleseismic recordings of earthquakes, and Rayleigh wave group velocity dispersion relations have been taken from earlier surface wave tomographic studies in the Caribbean area. Local smoothness optimization (LSO) procedure has been applied, combined with an H-K stacking method, the spatial distribution of hypocenters of local earthquakes and of the energy they released, in order to identify an optimum 1D model of Vs below each station. Several features of the Caribbean plate and its interaction with the Atlantic subducting slab are visible in the resulting models: (a) relatively thick oceanic crust below these stations ranges from 21 km to 33 km, being slight thinner in the middle of the island arc; (b) crustal low velocity zones are present below stations SABA, SEUS, SKI, SMRT, CBE, DSD, GCMP and TDBA; (c) lithospheric thickness range from 40 km to 105 km but lithosphere-asthenosphere boundary was not straightforward to correlate between stations; (d) the aseismic mantle wedge between the Caribbean seismic lithosphere and the subducted slab varies in thickness as well as Vs values which are, in general, lower below the West of Martinique than below the West of Guadeloupe; (e) the depth of the subducted slab beneath the volcanic arc, appears to be greater to the North, and relatively shallower below some stations (e.g. DLPL, SAM, BIM and FDF) than was estimated in previous studies based on the depth-distribution of seismicity; f) the WBZ is >10-15 km deeper than the top of the slab below the Central Lesser Antilles (Martinique and Dominica) where the presence of partial melt in the mantle wedge seems also to be more evident.

  4. Strain localization at the margins of strong lithospheric domains: insights from analogue models

    NARCIS (Netherlands)

    Calignano, Elisa; Sokoutis, Dimitrios; Willingshofer, Ernst; Gueydan, Frederic; Cloetingh, Sierd

    The lateral variation of the mechanical properties of continental lithosphere is an important factor controlling the localization of deformation and thus the deformation history and geometry of intra-plate mountain belts. A series of three-layer lithospheric-scale analog models, with a strong domain

  5. Implications of a visco-elastic model of the lithosphere for calculating yield strength envelopes

    NARCIS (Netherlands)

    Ershov, A.V.; Stephenson, R.A.

    2006-01-01

    The dominant deformation mechanism in the ductile part of the lithosphere is creep. From a mechanical point of view, creep can be modelled as a viscous phenomenon. On the other hand, yield-strength envelopes (YSEs), commonly used to describe lithosphere rheology, are constructed supposing creep to

  6. Lithosphere erosion and continental breakup : Interaction of extension, plume upwelling and melting

    NARCIS (Netherlands)

    Lavecchia, Alessio; Thieulot, Cedric; Beekman, Fred; Cloetingh, Sierd; Clark, Stuart

    2017-01-01

    We present the results of thermo-mechanical modelling of extension and breakup of a heterogeneous continental lithosphere, subjected to plume impingement in presence of intraplate stress field. We incorporate partial melting of the extending lithosphere, underlying upper mantle and plume, caused by

  7. Robust high resolution models of the continental lithosphere: Methodology and application to Asia

    NARCIS (Netherlands)

    Stolk, W.|info:eu-repo/dai/nl/323259170

    2013-01-01

    Asia is a key natural laboratory for the study of active intra-continental deformation in far-field response to the ongoing collision ofIndiaandEurasia. The resulting tectonic processes strongly depend on the thermo-mechanical structure of the lithosphere. This lithosphere can be separated into

  8. Lead Thickness Measurements

    International Nuclear Information System (INIS)

    Rucinski, R.

    1998-01-01

    The preshower lead thickness applied to the outside of D-Zero's superconducting solenoid vacuum shell was measured at the time of application. This engineering documents those thickness measurements. The lead was ordered in sheets 0.09375-inch and 0.0625-inch thick. The tolerance on thickness was specified to be +/- 0.003-inch. The sheets all were within that thickness tolerance. The nomenclature for each sheet was designated 1T, 1B, 2T, 2B where the numeral designates it's location in the wrap and 'T' or 'B' is short for 'top' or 'bottom' half of the solenoid. Micrometer measurements were taken at six locations around the perimeter of each sheet. The width,length, and weight of each piece was then measured. Using an assumed pure lead density of 0.40974 lb/in 3 , an average sheet thickness was calculated and compared to the perimeter thickness measurements. In every case, the calculated average thickness was a few mils thinner than the perimeter measurements. The ratio was constant, 0.98. This discrepancy is likely due to the assumed pure lead density. It is not felt that the perimeter is thicker than the center regions. The data suggests that the physical thickness of the sheets is uniform to +/- 0.0015-inch.

  9. In situ rheology of the oceanic lithosphere along the Hawaiian ridge

    Science.gov (United States)

    Pleus, A.; Ito, G.; Wessel, P.; Frazer, L. N.

    2017-12-01

    Much of our quantitative understanding of lithospheric rheology is based on rock deformation experiments carried out in the laboratory. The accuracy of the relationships between stress and lithosphere deformation, however, are subject to large extrapolations, given that laboratory strain rates (10-7 s-1) are much greater than geologic rates (10-15 to 10-12 s-1). In situ deformation experiments provide independent constraints and are therefore needed to improve our understanding of natural rheology. Zhong and Watts [2013] presented such a study around the main Hawaiian Islands and concluded that the lithosphere flexure requires a much weaker rheology than predicted by laboratory experiments. We build upon this study by investigating flexure around the older volcanoes of the Hawaiian ridge. The ridge is composed of a diversity of volcano sizes that loaded seafloor of nearly constant age (85+/-8 Ma); this fortunate situation allows for an analysis of flexural responses to large variations in applied loads at nearly constant age-dependent lithosphere thermal structure. Our dataset includes new marine gravity and multi-beam bathymetry data collected onboard the Schmidt Ocean Institute's R/V Falkor. These data, along with forward models of lithospheric flexure, are used to obtain a joint posterior probability density function for model parameters that control the lithosphere's flexural response to a given load. These parameters include the frictional coefficient constraining brittle failure in the shallow lithosphere, the activation energy for the low-temperature plasticity regime, and the geothermal gradient of the Hawaiian lithosphere. The resulting in situ rheological parameters may be used to verify or update those derived in the lab. Attaining accurate lithospheric rheological properties is important to our knowledge, not only of the evolution of the Hawaiian lithosphere, but also of other solid-earth geophysical problems, such as oceanic earthquakes, subduction

  10. Recent progress in modelling 3D lithospheric deformation

    Science.gov (United States)

    Kaus, B. J. P.; Popov, A.; May, D. A.

    2012-04-01

    Modelling 3D lithospheric deformation remains a challenging task, predominantly because the variations in rock types, as well as nonlinearities due to for example plastic deformation result in sharp and very large jumps in effective viscosity contrast. As a result, there are only a limited number of 3D codes available, most of which are using direct solvers which are computationally and memory-wise very demanding. As a result, the resolutions for typical model runs are quite modest, despite the use of hundreds of processors (and using much larger computers is unlikely to bring much improvement in this situation). For this reason we recently developed a new 3D deformation code,called LaMEM: Lithosphere and Mantle Evolution Model. LaMEM is written on top of PETSc, and as a result it runs on massive parallel machines and we have a large number of iterative solvers available (including geometric and algebraic multigrid methods). As it remains unclear which solver combinations work best under which conditions, we have implemented most currently suggested methods (such as schur complement reduction or Fully coupled iterations). In addition, we can use either a finite element discretization (with Q1P0, stabilized Q1Q1 or Q2P-1 elements) or a staggered finite difference discretization for the same input geometry, which is based on a marker and cell technique). This gives us he flexibility to test various solver methodologies on the same model setup, in terms of accuracy, speed, memory usage etc. Here, we will report on some features of LaMEM, on recent code additions, as well as on some lessons we learned which are important for modelling 3D lithospheric deformation. Specifically we will discuss: 1) How we combine a particle-and-cell method to make it work with both a finite difference and a (lagrangian, eulerian or ALE) finite element formulation, with only minor code modifications code 2) How finite difference and finite element discretizations compare in terms of

  11. Implications for anomalous mantle pressure and dynamic topography from lithospheric stress patterns in the North Atlantic Realm

    DEFF Research Database (Denmark)

    Schiffer, Christian; Nielsen, Søren Bom

    2016-01-01

    With convergent plate boundaries at some distance, the sources of the lithospheric stress field of the North Atlantic Realm are mainly mantle tractions at the base of the lithosphere, lithospheric density structure and topography. Given this, we estimate horizontal deviatoric stresses using a wel...

  12. INVESTIGATION OF LITHOSPHERIC STRUCTURE IN MONGOLIA: INSIGHTS FROM INSAR OBSERVATIONS AND MODELLING

    Directory of Open Access Journals (Sweden)

    Z. Jing

    2017-09-01

    Full Text Available The western Mongolia is a seismically active intracontinental region, with ongoing tectonic deformation and widespread seismicity related to the far-field effects of India-Eurasia collision. During the 20th century, four earthquakes with the magnitude larger than 8 occurred in the western Mongolia and its surrounding regions, providing a unique opportunity to study the geodynamics of intracontinental tectonic deformations. The 1957 magnitude 8.3 Gobi-Altai earthquake is one of the largest seismic events. The deformation pattern of rupture zone associated with this earthquake is complex, involving left-lateral strike-slip and reverse dip-slip faulting on several distinct geological structures in a 264 × 40 km wide zone. To understand the relationship between the observed postseismic surface deformation and the rheological structure of the upper lithosphere, Interferometric Synthetic Aperture Radar (InSAR data are used to study the 1957 earthquake. Then we developed a postseismic model in a spherical, radially layered elastic-viscoelastic Earth based on InSAR results, and further analysed the dominant contribution to the surface deformation. This work is important for understanding not only the regional tectonics, but also the structure and dynamics of the lithosphere. SAR data were acquired from the ERS1/2 and Envisat from 1996 to 2010. Using the Repeat Orbit Interferometry Package (ROI_PAC, 124 postseismic interferograms are produced on four adjacent tracks. By stacking these interferograms, the maximum InSAR line-of-sight deformation rate along the Gobi-Altai fault zone is obtained. The main results are as follows: (1 The maximum InSAR line-of-sight deformation velocity along this large fault zone is about 6 mm/yr; (2 The modelled surface deformation suggests that the viscoelastic relaxation is the most reasonable mechanism to explain the observed surface motion; (3 The optimal model cover the Gobi-Altai seismogenic thickness is 10

  13. Education and "Thick" Epistemology

    Science.gov (United States)

    Kotzee, Ben

    2011-01-01

    In this essay Ben Kotzee addresses the implications of Bernard Williams's distinction between "thick" and "thin" concepts in ethics for epistemology and for education. Kotzee holds that, as in the case of ethics, one may distinguish between "thick" and "thin" concepts of epistemology and, further, that this distinction points to the importance of…

  14. Thick film hydrogen sensor

    Science.gov (United States)

    Hoffheins, Barbara S.; Lauf, Robert J.

    1995-01-01

    A thick film hydrogen sensor element includes an essentially inert, electrically-insulating substrate having deposited thereon a thick film metallization forming at least two resistors. The metallization is a sintered composition of Pd and a sinterable binder such as glass frit. An essentially inert, electrically insulating, hydrogen impermeable passivation layer covers at least one of the resistors.

  15. Dynamics of the Pacific Northwest Lithosphere and Asthenosphere

    Science.gov (United States)

    Humphreys, E.

    2013-12-01

    Seismic imaging resolves a complex structure beneath the Pacific Northwest (PNW) that is interpreted as: an high-velocity piece of accreted (~50 Ma) Farallon lithosphere that deepens from being exposed (at coast, where it is called Siletzia) to lower crust in SE Washington and then descending vertically to ~600 km as a 'curtain' beneath central Idaho; a stubby Juan de Fuca slab (to directed tractions on the Cascadia mega-thrust average ~4 TN per meter of along-strike fault length, or probably a shear stress of ~40 MPa over much of the locked mega-thrust (i.e., much more shear stress than the typical earthquake stress drop of 1-10 MPa). Normal to the coast, southern Cascadia is relatively tensional (where margin-normal compression is less than typical ridge push by ~4 TN/m of along-strike fault length) whereas northern Cascadia is compressional. This indicates that the southern Cascadia mega-thrust is more weakly coupled than the northern mega-thrust. Southern Cascadia slab rollback and extension of the Cascade graben and Basin-and-Range are enabled by the weak coupling, in conjunction with high gravitational potential energy of the southern Oregon arc and back-arc. Juan de Fuca-Gorda lithosphere experiences the same stress on its eastern margin as North America does on the PNW Cascadia margin (by stress continuity), although current models of the individual plates do not show this continuity. Gorda plate is strongly compressed across the Mendocino transform by the north-moving Pacific Plate. Development of the NW-trending Blanco transform has created a fault that avoids this strong compression.

  16. Power law olivine crystal size distributions in lithospheric mantle xenoliths

    Science.gov (United States)

    Armienti, P.; Tarquini, S.

    2002-12-01

    Olivine crystal size distributions (CSDs) have been measured in three suites of spinel- and garnet-bearing harzburgites and lherzolites found as xenoliths in alkaline basalts from Canary Islands, Africa; Victoria Land, Antarctica; and Pali Aike, South America. The xenoliths derive from lithospheric mantle, from depths ranging from 80 to 20 km. Their textures vary from coarse to porphyroclastic and mosaic-porphyroclastic up to cataclastic. Data have been collected by processing digital images acquired optically from standard petrographic thin sections. The acquisition method is based on a high-resolution colour scanner that allows image capturing of a whole thin section. Image processing was performed using the VISILOG 5.2 package, resolving crystals larger than about 150 μm and applying stereological corrections based on the Schwartz-Saltykov algorithm. Taking account of truncation effects due to resolution limits and thin section size, all samples show scale invariance of crystal size distributions over almost three orders of magnitude (0.2-25 mm). Power law relations show fractal dimensions varying between 2.4 and 3.8, a range of values observed for distributions of fragment sizes in a variety of other geological contexts. A fragmentation model can reproduce the fractal dimensions around 2.6, which correspond to well-equilibrated granoblastic textures. Fractal dimensions >3 are typical of porphyroclastic and cataclastic samples. Slight bends in some linear arrays suggest selective tectonic crushing of crystals with size larger than 1 mm. The scale invariance shown by lithospheric mantle xenoliths in a variety of tectonic settings forms distant geographic regions, which indicate that this is a common characteristic of the upper mantle and should be taken into account in rheological models and evaluation of metasomatic models.

  17. Bouguer images of the North American craton and its structural evolution

    Science.gov (United States)

    Arvidson, R. E.; Bowring, S.; Eddy, M.; Guinness, E.; Leff, C.; Bindschadler, D.

    1984-01-01

    Digital image processing techniques have been used to generate Bouguer images of the North American craton that diplay more of the granularity inherent in the data as compared with existing contour maps. A dominant NW-SE linear trend of highs and lows can be seen extending from South Dakota, through Nebraska, and into Missouri. The structural trend cuts across the major Precambrian boundary in Missouri, separating younger granites and rhyolites from older sheared granites and gneisses. This trend is probably related to features created during an early and perhaps initial episode of crustal assembly by collisional processes. The younger granitic materials are probably a thin cover over an older crust.

  18. Sulfur and lead isotope characteristics of the Pontes e Lacerda gold deposits, SW Amazonian Craton Brazil

    International Nuclear Information System (INIS)

    Geraldes, M.C.; Tassinari, C.C.G.; Babinski; M; Iyer, S

    2001-01-01

    This work deals with the characterization of the S and Pb isotope signatures in sulfides from the Pontes e Lacerda mesothermal gold deposits located in the SW sector of Amazonian craton. Stable and radiogenic isotopes have played an important role in the study of ore deposited and hydrothermal processes and they are most useful when can be used together. The purpose of this study is to constrain the sources and the mechanisms of gold deposition in Pontes e Lacerda region which may be a helpful contribution to an exploratory model in the area (au)

  19. Gravimetric study on the western edge of the Rio de La Plata craton

    International Nuclear Information System (INIS)

    Rame, G; Miro, R

    2010-01-01

    This work is about the gravimetric study on the western edge of the Rio de la Plata craton which belongs to the Gondwana fragment in the south of Brazil, Uruguay and central eastern of Argentina. The work consisted of a survey of 332 gravimetric and topographic stations extended from the western edge of the Sierra Chica de Cordoba up to 200 km east on the pampas. The gravity values observed (gobs) were obtained using a LaCoste §Rom berg gravimeter G-961 and 200T Sodin both with 0.01 mGal, referred to IGSN71 (International Gravity Standardization Net 1971) network

  20. Accretionary Tectonics of Rock Complexes in the Western Margin of the Siberian Craton

    Science.gov (United States)

    Likhanov, I. I.; Nozhkin, A. D.; Savko, K. A.

    2018-01-01

    The geological, geochemical, and isotope-geochronological evidence of the events at the final stage of the Neoproterozoic history of the Yenisei Range is considered (beginning from the formation of fragments of the oceanic crust in the region and their accretion to the Siberian Craton until the postaccretionary stage of crustal tension and onset of the Caledonian orogeny). Based on an analysis of new data on the petrogeochemical composition, age, and geodynamic nature of the formation of contrasting rocks in the composition of tectonic mélange of the Near-Yenisei (Prieniseiskaya) regional shear zone, we have found the chronological sequence of events that marks the early stages of the Paleoasian Ocean evolution in the zone of its junction with the Siberian Craton. These events are documented by the continental marginal, ophiolitic, and island-arc geological complexes, each of which has different geochemical features. The most ancient structures are represented by fragments of oceanic crust and island arcs from the Isakovka terrane (700-620 Ma). The age of glaucophane-schist metamorphic units that formed in the paleosubduction zone corresponds to the time interval of 640-620 Ma. The formation of high-pressure tectonites in the suture zone, about 600 Ma in age, marks the finishing stage of accretion of the Isakovka block to the western margin of the Siberian Craton. The final events in the early history of the Asian Paleoocean were related to the formation of Late Vendian riftogenic amygdaloidal basalts (572 ± 6.5 Ma) and intrusion of postcollisional leucogranites of the Osinovka massif (550-540 Ma), which intruded earlier fragments of the oceanic crust in the Isakovka terrane. These data allow us to refine the Late Precambrian stratigraphic scheme in the northwestern Trans-Angarian part of the Yenisei Range and the evolutionary features of the Sayan-Yenisei accretionary belt. The revealed Late Neoproterozoic landmarks of the evolution of the Isakovka terrane are

  1. Updating the Geologic Barcodes for South China: Discovery of Late Archean Banded Iron Formations in the Yangtze Craton.

    Science.gov (United States)

    Ye, Hui; Wu, Chang-Zhi; Yang, Tao; Santosh, M; Yao, Xi-Zhu; Gao, Bing-Fei; Wang, Xiao-Lei; Li, Weiqiang

    2017-11-08

    Banded iron formations (BIFs) in Archean cratons provide important "geologic barcodes" for the global correlation of Precambrian sedimentary records. Here we report the first finding of late Archean BIFs from the Yangtze Craton, one of largest Precambrian blocks in East Asia with an evolutionary history of over 3.3 Ga. The Yingshan iron deposit at the northeastern margin of the Yangtze Craton, displays typical features of BIF, including: (i) alternating Si-rich and Fe-rich bands at sub-mm to meter scales; (ii) high SiO 2  + Fe 2 O 3total contents (average 90.6 wt.%) and Fe/Ti ratios (average 489); (iii) relative enrichment of heavy rare earth elements and positive Eu anomalies (average 1.42); (iv) and sedimentary Fe isotope compositions (δ 56 Fe IRMM-014 as low as -0.36‰). The depositional age of the BIF is constrained at ~2464 ± 24 Ma based on U-Pb dating of zircon grains from a migmatite sample of a volcanic protolith that conformably overlied the Yingshan BIF. The BIF was intruded by Neoproterozoic (805.9 ± 4.7 Ma) granitoids that are unique in the Yangtze Craton but absent in the North China Craton to the north. The discovery of the Yingshan BIF provides new constraints for the tectonic evolution of the Yangtze Craton and has important implications in the reconstruction of Pre-Nuna/Columbia supercontinent configurations.

  2. 207Pb-206Pb zircon ages of eastern and western Dharwar craton, southern India : Evidence for contemporaneous Archaean crust

    Science.gov (United States)

    Maibam, B.; Goswami, J. N.; Srinivasan, R.

    2009-04-01

    Dharwar craton is one of the major Archaean crustal blocks in the Indian subcontinent. The craton is comprised of two blocks, western and eastern. The western domain is underlain by orthogneisses and granodiorites (ca. 2.9-3.3 Ga) collectively termed as Peninsular Gneiss [e.g., 1] interspersed with older tracts of metasedimentary and metamorphosed igneous suites (Sargur Group and Dharwar Group; [2]). The eastern part of the craton is dominated by Late Archaean (2.50-2.75 Ga) granitoids and their gneissic equivalents. They are interspersed with schist belts (also of Sargur Group and Dharwar Group), which are lithologically similar to the Dharwar Supergroup in the western block, but are in different metamorphic dress. Here we report 207Pb-206Pb age of zircons separated from the metasedimentary and gneissic samples from the two blocks to constrain the evolution of the Dharwar craton during the early Archaean. Detrital zircons of the metasedimentary rocks from both the blocks show a wide range of overlapping ages between ~2.9 to >3.5 Ga. Zircon ages of the orthogneisses from the two blocks showed that most of the analysed grains of the eastern Dharwar block are found to be of the age as old as the western Dharwar gneisses. Imprints of younger events could be discerned from the presence of overgrowths in zircons from the studied samples throughout the craton. Our data suggest that crust forming cycles in the two blocks of the Dharwar craton occurred contemporaneously during the Archaean. References [1] Beckinsale, R.D., Drury, S.A., Holt, R.W. (1980) Nature 283, 469-470. [2] Swami Nath J., Ramakrishnan M., Viswanatha M.N. (1976) Rec. Geol. Surv. Ind., 107, 149-175.

  3. Thinning of heterogeneous lithosphere: insights from field observations and numerical modelling

    Science.gov (United States)

    Petri, B.; Duretz, T.; Mohn, G.; Schmalholz, S. M.

    2017-12-01

    The nature and mechanisms of formation of extremely thinned continental crust (N Italy) and in the Southern Alps (N Italy) were selected for their exceptional level of preservation of rift-related structures. This situation enables us to characterize (1) the pre-rift architecture of the continental lithosphere, (2) the localization of rift-related deformation in distinct portion of the lithosphere and (3) the interaction between initial heterogeneities of the lithosphere and rift-related structures. In a second stage, these observations are integrated in high-resolution, two-dimensional thermo-mechanical models taking into account various patterns of initial mechanical heterogeneities. Our results show the importance of initial pre-rift architecture of the continental lithosphere during rifting. Key roles are given to high-angle and low-angle normal faults, anastomosing shear-zones and decoupling horizons. We propose that during the first stages of thinning, deformation is strongly controlled by the complex pre-rift architecture of the lithosphere, localized along major structures responsible for the lateral extrusion of mid to lower crustal levels. This extrusion juxtaposes mechanically stronger levels in the hyper-thinned continental crust, being exhumed by subsequent low-angle normal faults. Altogether, these results highlight the critical role of the extraction of mechanically strong layers of the lithosphere during the extreme thinning of the continental lithosphere and allows to propose a new model for the formation of continental passive margins.

  4. Modelling the possible interaction between edge-driven convection and the Canary Islands mantle plume

    Science.gov (United States)

    Negredo, A. M.; Rodríguez-González, J.; Fullea, J.; Van Hunen, J.

    2017-12-01

    The close location between many hotspots and the edges of cratonic lithosphere has led to the hypothesis that these hotspots could be explained by small-scale mantle convection at the edge of cratons (Edge Driven Convection, EDC). The Canary Volcanic Province hotspot represents a paradigmatic example of this situation due to its close location to the NW edge of the African Craton. Geochemical evidence, prominent low seismic velocity anomalies in the upper and lower mantle, and the rough NE-SW age-progression of volcanic centers consistently point out to a deep-seated mantle plume as the origin of the Canary Volcanic Province. It has been hypothesized that the plume material could be affected by upper mantle convection caused by the thermal contrast between thin oceanic lithosphere and thick (cold) African craton. Deflection of upwelling blobs due to convection currents would be responsible for the broader and more irregular pattern of volcanism in the Canary Province compared to the Madeira Province. In this study we design a model setup inspired on this scenario to investigate the consequences of possible interaction between ascending mantle plumes and EDC. The Finite Element code ASPECT is used to solve convection in a 2D box. The compositional field and melt fraction distribution are also computed. Free slip along all boundaries and constant temperature at top and bottom boundaries are assumed. The initial temperature distribution assumes a small long-wavelength perturbation. The viscosity structure is based on a thick cratonic lithosphere progressively varying to a thin, or initially inexistent, oceanic lithosphere. The effects of assuming different rheologies, as well as steep or gradual changes in lithospheric thickness are tested. Modelling results show that a very thin oceanic lithosphere (models assuming temperature-dependent viscosity and large viscosity variations evolve to large-scale (upper mantle) convection cells, with upwelling of hot material being

  5. P-T Equilibrium Conditions of Xenoliths from the Udachnaya Kimberlite Pipe: Thermal Perturbations in the Lithospheric Mantle

    Science.gov (United States)

    Tychkov, Nikolay; Agashev, Alexey; Malygina, Elena; Pokhilenko, Nikolay

    2014-05-01

    Integrated study of 250 peridotite xenoliths from Udachnaya -East pipe show difference in mineral paragenesises and textural-structural peculiarities in the different level of cratonic lithosphere mantle (CLM). The compositions of minerals were determined using EPMA. Thermobarometric parameters (Brey, Kohller, 1990) were determined for all rocks occupying different fields on geothermal curve. The deepest layer (the pressure interval of 5.0-7.0 GPa) contains mostly pophyroclastic lherzolites. Anyway, some rocks of this layer have an idiomorphic texture being also enriched in incompatible components. Higher in the CLM sequence, the interval (4.2-6.3 GPa) is composed of the most depleted rocks: megacristalline ultradepleted harzburgite-dunites and depleted granular harzburgite-dunites, as well as lherzolites in a subordinate amount. They correspond strate to 35 mW/m2 and partly overlap the deeper layer in dapth. It is likely that rocks of this layer are in equilibrium and were not subject to significant secondary changes due to kimberlite magma intrusion. Thus, this interval of the CLM sequence reflects the true (relic) geotherm for the area of the Udachnaya kimberlite pipe. Moreover, it is obvious that this interval was a major supplier of diamonds into kimberlites of the Udachnaya pipe. The interval of 4.2-2.0 GPa in the CLM sequence is also composed of coarse depleted lherzolites and harzburgites. Rocks of this interval are slightly more enriched than those of the underlying interval. This is confirmed by the distinct predominance of lherzolites over harzburgite-dunites. The heat flow in this layer varies in the range of 38-45 mW/m2 and shows a general tendency to increase with decreasing depth. According to occurrence of nonequilibrium mineral assemblages and increased heat flow relative to the major heat flow of 35 mW/m2, this interval is similar to the deepest interval of secondary enriched rocks. Interval of less than 2.0 GPa composed of spinel lherzolites and

  6. Effective elastic thickness along the conjugate passive margins of India, Madagascar and Antarctica: A re-evaluation using the Hermite multitaper Bouguer coherence application

    Science.gov (United States)

    Ratheesh-Kumar, R. T.; Xiao, Wenjiao

    2018-05-01

    Gondwana correlation studies had rationally positioned the western continental margin of India (WCMI) against the eastern continental margin of Madagascar (ECMM), and the eastern continental margin of India (ECMI) against the eastern Antarctica continental margin (EACM). This contribution computes the effective elastic thickness (Te) of the lithospheres of these once-conjugated continental margins using the multitaper Bouguer coherence method. The results reveal significantly low strength values (Te ∼ 2 km) in the central segment of the WCMI that correlate with consistently low Te values (2-3 km) obtained throughout the entire marginal length of the ECMM. This result is consistent with the previous Te estimates of these margins, and confirms the idea that the low-Te segments in the central part of the WCMI and along the ECMM represents paleo-rift inception points of the lithospheric margins that was thermally and mechanically weakened by the combined action of the Marion hotspot and lithospheric extension during the rifting. The uniformly low-Te value (∼2 km) along the EACM indicates a mechanically weak lithospheric margin, probably due to considerable stretching of the lithosphere, considering the fact that this margin remained almost stationary throughout its rift history. In contrast, the ECMI has comparatively high-Te variations (5-11 km) that lack any correlation with the regional tectonic setting. Using gravity forward and inversion applications, we find a leading order of influence of sediment load on the flexural properties of this marginal lithosphere. The study concludes that the thick pile of the Bengal Fan sediments in the ECMI masks and has erased the signal of the original load-induced topography, and its gravity effect has biased the long-wavelength part of the observed gravity signal. The hence uncorrelated flat topography and deep lithospheric flexure together contribute a bias in the flexure modeling, which likely accounts a relatively high Te

  7. Ocean Sediment Thickness Contours

    Data.gov (United States)

    National Oceanic and Atmospheric Administration, Department of Commerce — Ocean sediment thickness contours in 200 meter intervals for water depths ranging from 0 - 18,000 meters. These contours were derived from a global sediment...

  8. Archaean ultra-depleted komatiites formed by hydrous melting of cratonic mantle.

    Science.gov (United States)

    Wilson, A H; Shirey, S B; Carlson, R W

    2003-06-19

    Komatiites are ultramafic volcanic rocks containing more than 18 per cent MgO (ref. 1) that erupted mainly in the Archaean era (more than 2.5 gigayears ago). Although such compositions occur in later periods of Earth history (for example, the Cretaceous komatiites of Gorgona Island), the more recent examples tend to have lower MgO content than their Archaean equivalents. Komatiites are also characterized by their low incompatible-element content, which is most consistent with their generation by high degrees of partial melting (30-50 per cent). Current models for komatiite genesis include the melting of rock at great depth in plumes of hot, diapirically rising mantle or the melting of relatively shallow mantle rocks at less extreme, but still high, temperatures caused by fluxing with water. Here we report a suite of ultramafic lava flows from the Commondale greenstone belt, in the southern part of the Kaapvaal Craton, which represents a previously unrecognized type of komatiite with exceptionally high forsterite content of its igneous olivines, low TiO(2)/Al(2)O(3) ratio, high silica content, extreme depletion in rare-earth elements and low Re/Os ratio. We suggest a model for their formation in which a garnet-enriched residue left by earlier cratonic volcanism was melted by hydration from a subducting slab.

  9. Correlations between the North China Craton and the Indian Shield: Constraints from regional metallogeny

    Directory of Open Access Journals (Sweden)

    Caifeng Li

    2015-11-01

    Full Text Available The correlation between the North China Craton (NCC and the Indian Shield (IND has been a hot topic in recent years. On the basis of ore deposit databases, the NCC and IND have shown broad similarity in metallogenesis from the middle Archaean to the Mesoproterozoic. The two blocks both have three major metallogenic systems: (1 the Archaean BIF metallogenic system; (2 the Paleoproterozoic Cu-Pb-Zn metallogenic system; and (3 the Mesoproterozoic Fe-Pb-Zn system. In the north margin of the NCC and the west margin of the IND, the Archaean BIF-Au-Cu-Pb-Zn deposits had the same petrogenesis and host rocks, the Paleoproterozoic Cu-Pb-Zn deposits were controlled by active belts, and the Mesoproterozoic Fe-Pb-Zn deposits were mainly related to multi-stage rifting. Matching regional mineralization patterns and geological features has established the continental assembly referred to as “NCWI”, an acronym for the north margin of the NCC (NC and the west margin of the IND (WI during the middle Archaean to the Mesoproterozoic. In this assembly, the available geological and metallogenic data from the Eastern Block and active belts of NC fit those from the Dharwar craton and the Aravalli–Delhi–Vindhyan belt of WI, respectively. Moreover, the depositional model and environment of Paleoproterozoic metasedimentary manganese deposits in NCWI implied that the assembly may be located at low latitudes, where the conditions were favorable for dissolving ice and precipitating manganese deposits.

  10. Early tectonic history of the Marymia Inlier and correlation with the Archaean Yilgarn Craton, Western Australia

    International Nuclear Information System (INIS)

    Bagas, L.

    1999-01-01

    The Archaean granite-greenstone rocks of the Marymia Inlier outcrop within Proterozoic rocks forming the Capricorn Orogen. Five major deformation events are recognised in the rocks of the Plutonic Well and Baumgarten greenstone belts. The first two events were Late Archaean and synchronous with major epithermal gold mineralisation in the belts. Palaeoproterozoic extensional faulting was probably related to the early stages of the Capricorn Orogeny. The fourth event records a compressional phase of the Capricorn Orogeny associated with greenschist-facies metamorphism, whereas the last major event involved wrench faulting associated with minor folding. The Archaean tectonic history, rock types and timing of mineralisation strongly suggest that the Marymia Inlier is part of the Yilgarn Craton, and that each of the provinces in the craton experienced the same geological history since 2.72 Ga. The inlier is now interpreted to include two components, one is the eastern or northern extension of either the Narryer Terrane. Murchison Province or Southern Cross Province, and the other is the northwestern extension of the Eastern Goldfields Province. The Jenkin Fault, which was active in Proterozoic times, separates these two components. Copyright (1999) Blackwell Science Pty Ltd

  11. Time constraint based on zircon dating for the Jacareacanga group (Tapajos province, Amazon craton, Brazil)

    International Nuclear Information System (INIS)

    Almeida, M.E.; Ferreira, A.L; Macambira, M.J.B.; Sachett, C.R

    2001-01-01

    During long time the Jacareacanga meta-volcano sedimentary sequence have been interpreted as Archean greenstone belt terrain. However, recent data are indicating younger U-Pb ages about 2.1 Ga. In the Tapajos Province (Amazon Craton), the Cuiu-Cuiu Complex (2.00-2.03 Ga), Creporizao granitoids (1.99-1.96 Ga) and Jacareacanga Group are the oldest rocks. The Jacareancaga Group is composed by quartzmica schists, quartzites, ferruginous quartzite, metachert, and minor talc-tremolite-chlorite schist, actinolite-epidote schist, hornfels, metargilites and metawackes metamorphosed in low to medium-grade conditions. The aim of the present paper is to estabilish the maximum age of Jacareacanga sedimentation and identify probable sources in Espirito Santo region (Espirito Santo muscovite-biotite schist). In this research, similar and new results are obtained by zircon evaporation methodology. This research shows geochronological data about the Espirito Santo muscovite-biotite schist related to Jacareacanga Group (Ferreira et al., 2000) in Tapajos Province (Amazon Craton). The area is located near Amazonas and Para States boundary (Northern Brazil) and the sample was obtained at Espirito Santo (garimpo) small-scale gold mine (06 o 00min.48seg.S,58 o 08min.17seg.W) (au)

  12. A mantle origin for Paleoarchean peridotitic diamonds from the Panda kimberlite, Slave Craton: Evidence from 13C-, 15N- and 33,34S-stable isotope systematics

    Science.gov (United States)

    Cartigny, Pierre; Farquhar, James; Thomassot, Emilie; Harris, Jeffrey W.; Wing, Bozwell; Masterson, Andy; McKeegan, Kevin; Stachel, Thomas

    2009-11-01

    In order to address diamond formation and origin in the lithospheric mantle underlying the Central Slave Craton, we report N- and C-stable isotopic compositions and N-contents and aggregation states for 85 diamonds of known paragenesis (73 peridotitic, 8 eclogitic and 4 from lower mantle) from the Panda kimberlite (Ekati Mine, Lac de Gras Area, Canada). For 12 peridotitic and two eclogitic sulfide inclusion-bearing diamonds from this sample set, we also report multiple-sulfur isotope ratios. The 73 peridotitic diamonds have a mean δ13C-value of - 5.2‰ and range from - 6.9 to - 3.0‰, with one extreme value at - 14.1‰. The associated δ15N-values range from - 17.0 to + 8.5‰ with a mean value of - 4.0‰. N-contents range from 0 to 1280 ppm. The 8 eclogitic diamonds have δ13C-values ranging from - 11.2 to - 4.4‰ with one extreme value at - 19.4‰. Their δ15N ranges from - 2.1 to + 7.9‰ and N-contents fall between 0 and 3452 ppm. Four diamonds with an inferred lower mantle origin are all Type II (i.e. nitrogen-free) and have a narrow range of δ13C values, between - 4.5 and - 3.5‰. The δ34S of the 14 analyzed peridotitic and eclogitic sulfide inclusions ranges from - 3.5 to +5.7‰. None of them provide evidence for anomalous δ33S-values; observed variations in δ33S are from +0.19 to - 0.33‰, i.e. within the 2 sigma uncertainties of mantle sulfur ( δ33S = 0‰). At Panda, the N contents and the δ13C of sulfide-bearing peridotitic diamonds show narrower ranges than silicate-bearing peridotitic diamonds. This evidence supports the earlier suggestion established from eclogitic diamonds from the Kaapvaal that sulfide-(±silicate) bearing diamonds sample a more restricted portion of sublithospheric mantle than silicate-(no sulfide) bearing diamonds. Our findings at Panda suggest that sulfide-bearing diamonds should be considered as a specific diamond population on a global-scale. Based on our study of δ34S, Δ 33S, δ15N and δ13C, we find no

  13. Anomalous Structure of Oceanic Lithosphere in the North Atlantic and Arctic Oceans: A Preliminary Analysis Based on Bathymetry, Gravity and Crustal Structure

    Science.gov (United States)

    Barantsrva, O.

    2014-12-01

    We present a preliminary analysis of the crustal and upper mantle structure for off-shore regions in the North Atlantic and Arctic oceans. These regions have anomalous oceanic lithosphere: the upper mantle of the North Atlantic ocean is affected by the Iceland plume, while the Arctic ocean has some of the slowest spreading rates. Our specific goal is to constrain the density structure of the upper mantle in order to understand the links between the deep lithosphere dynamics, ocean spreading, ocean floor bathymetry, heat flow and structure of the oceanic lithosphere in the regions where classical models of evolution of the oceanic lithosphere may not be valid. The major focus is on the oceanic lithosphere, but the Arctic shelves with a sufficient data coverage are also included into the analysis. Out major interest is the density structure of the upper mantle, and the analysis is based on the interpretation of GOCE satellite gravity data. To separate gravity anomalies caused by subcrustal anomalous masses, the gravitational effect of water, crust and the deep mantle is removed from the observed gravity field. For bathymetry we use the global NOAA database ETOPO1. The crustal correction to gravity is based on two crustal models: (1) global model CRUST1.0 (Laske, 2013) and, for a comparison, (2) a regional seismic model EUNAseis (Artemieva and Thybo, 2013). The crustal density structure required for the crustal correction is constrained from Vp data. Previous studies have shown that a large range of density values corresponds to any Vp value. To overcome this problem and to reduce uncertainty associated with the velocity-density conversion, we account for regional tectonic variations in the Northern Atlantics as constrained by numerous published seismic profiles and potential-field models across the Norwegian off-shore crust (e.g. Breivik et al., 2005, 2007), and apply different Vp-density conversions for different parts of the region. We present preliminary results

  14. Melt Origin Across a Rifted Continental Margin: A Case for Subduction-related Metasomatic Agents in the Lithospheric Source of Alkaline Basalt, Northwest Ross Sea, Antarctica

    Science.gov (United States)

    Panter, K. S.; Castillo, P.; Krans, S. R.; Deering, C. D.; McIntosh, W. C.; Valley, J. W.; Kitajima, K.; Kyle, P. R.; Hart, S. R.; Blusztajn, J.

    2017-12-01

    Alkaline magmatism within the West Antarctic rift system in the NW Ross Sea (NWRS) includes a chain of shield volcanoes extending 260 km along the coast, numerous seamounts located on the continental shelf and hundreds more within the oceanic Adare Basin. Dating and geochemistry confirm that the seamounts are Pliocene‒Pleistocene in age and petrogenetically akin to the mostly Miocene volcanism on the continent as well as to a much broader region of alkaline volcanism that altogether encompasses areas of West Antarctica, Zealandia and Australia. All of these regions were contiguous prior to Gondwana breakup at 100 Ma, suggesting that the magmatism is interrelated. Mafic alkaline magmas (> 6 wt.% MgO) erupted across the transition from continent to ocean in the NWRS show a remarkable systematic increase in Si-undersaturation, P2O5, Sr, Zr, Nb and light rare earth element (LREE) concentrations, LREE/HREE and Nb/Y ratios. Radiogenic isotopes also vary with Nd and Pb ratios increasing and Sr ratios decreasing ocean-ward. The variations are not explained by crustal contamination or by changes in degree of mantle partial melting but are likely a function of the thickness and age of mantle lithosphere. The isotopic signature of the most Si-undersaturated and incompatible element enriched basalts best represent the composition of the sub-lithospheric source with low 87Sr/86Sr (≤ 0.7030) and δ18Oolivine (≤ 5.0 ‰), high 143Nd/144Nd ( 0.5130) and 206Pb/204Pb (≥ 20) ratios. The isotopic `endmember' is derived from recycled material and was transferred to the lithospheric mantle by small degree melts to form amphibole-rich metasomes. Later melting of the metasomes produced silica-undersaturated liquids that reacted with the surrounding peridotite. This reaction occurred to a greater extent as the melt traversed through thicker and older lithosphere continent-ward. Ancient or more recent ( 550‒100 Ma) subduction along the margin of Gondwana supplied the recycled

  15. Silicate melt metasomatism in the lithospheric mantle beneath SW Poland

    Science.gov (United States)

    Puziewicz, Jacek; Matusiak-Małek, Magdalena; Ntaflos, Theodoros; Grégoire, Michel; Kukuła, Anna

    2014-05-01

    The xenoliths of peridotites representing the subcontinental lithospheric mantle (SCLM) beneath SW Poland and adjacent parts of Germany occur in the Cenozoic alkaline volcanic rocks. Our study is based on detailed characterization of xenoliths occurring in 7 locations (Steinberg in Upper Lusatia, Księginki, Pilchowice, Krzeniów, Wilcza Góra, Winna Góra and Lutynia in Lower Silesia). One of the two major lithologies occurring in the xenoliths, which we call the "B" lithology, comprises peridotites (typically harzburgites) with olivine containing from 90.5 to 84.0 mole % of forsterite. The harzburgites contain no clinopyroxene or are poor in that mineral (eg. in Krzeniów the group "B" harzburgites contain pfu in ortho-, and pfu in clinopyroxene). The exception are xenoliths from Księginki, which contain pyroxenes characterised by negative correlation between mg# and Al. The REE patterns of both ortho- and clinopyroxene in the group "B" peridotites suggest equilibration with silicate melt. The rocks of "B" lithology were formed due to alkaline silicate melt percolation in the depleted peridotitic protolith. The basaltic melts formed at high pressure are usually undersaturated in both ortho- and clinopyroxene at lower pressures (Kelemen et al. 1992). Because of cooling and dissolution of ortho- and clinopyroxene the melts change their composition and become saturated in one or both of those phases. Experimental results (e.g. Tursack & Liang 2012 and references therein) show that the same refers to alkaline basaltic silicate melts and that its reactive percolation in the peridotitic host leads to decrease of Mg/(Mg+Fe) ratios of olivine and pyroxenes. Thus, the variation of relative volumes of olivine and orthopyroxene as well as the decrease of mg# of rock-forming silicates is well explained by reactive melt percolation in the peridotitic protolith consisting of high mg# olivine and pyroxenes (in the area studied by us that protolith was characterised by olivine

  16. High-Resolution Gravity Field Modeling for Mercury to Estimate Crust and Lithospheric Properties

    Science.gov (United States)

    Goossens, S.; Mazarico, E.; Genova, A.; James, P. B.

    2018-05-01

    We estimate a gravity field model for Mercury using line-of-sight data to improve the gravity field model at short wavelengths. This can be used to infer crustal density and infer the support mechanism of the lithosphere.

  17. A seismic tomography study of lithospheric structure under the Norwegian Caledonides

    DEFF Research Database (Denmark)

    Hejrani, Babak; Jacobsen, B. H.; Balling, N.

    2012-01-01

    A deep lithospheric transition between southern Norway and southern Sweden has been revealed in papers by Medhus et al. (2009,) and Medhus (2010). This lithospheric transition is crossing various tectonic units including the Caledonides.. We address the question of whether this transition continu...... (Hejrani et al., 2011) (optimizes 2D ray coverage under a crooked profile) is used to resolve the details of the transition boundaries in lithosphere structure across the mountains and its relation to the geological surface settings....... in this area. These results are compared the upper mantle structure obtained by Medhus (2010) and Hejrani et al. (2011) for Caledonian and shield units to the south in southern Norway and Sweden, where the lithospheric transition follows the eastern margin of the Oslo Graben. Crooked line seismic tomography...

  18. A geochemical and Pb, Sr isotopic study of the evolution of granite-gneisses from the Bastar craton, Central India

    International Nuclear Information System (INIS)

    Sarkar, G.; Paul, D.K.; Misra, V.P.; de Laeter, J.R.; Mc Naughton, N.J.

    1990-01-01

    Preliminary Pb-Pb and Rb-Sr geochronology of granitic and gneissic rocks from the Sukma area of the Bastar craton, Central India, provides important constraints on crustal evolution. Much of the craton is made up of felsic orthogneisses and younger granitic intrusives, compositionally ranging from tonalite to granite. Pb-Pb isotopic data suggest the presence of ca. 3.0 Ga old gneisses. Younger granitic intrusives have been dated at ca. 2.6 Ga which represents a widespread resetting and/or emplacement event. Comparison of the Pb-Pb and Rb-Sr whole rock ages suggests that the latter were more perturbed after the gneiss-forming or emplacement events. All rock suites show significant geological scatter of isotopic data probably because of sampling on a regional scale, and reflect multi-stage isotopic evolution in a complex terrain. The present isotopic data indicate the presence of Archaean rock in the Bastar craton and suggest temporal similarity with the oldest crustal rocks in the Singhbhum and Dharwar cratons. (author). 18 refs., 4 tabs., 8 figs

  19. Paleomagnetic Results of the 925 Ma Mafic Dykes From the North China Craton: Implications for the Neoproterozoic Paleogeography of Rodinia

    Science.gov (United States)

    Zhao, X.; Peng, P.

    2017-12-01

    Precambrian mafic dyke swarms are useful geologic records for Neoproterozoic paleogeographic reconstruction. We present a paleomagnetic study of the 925 Ma Dashigou dyke swarm from 3 widely separated locations in the central and northern parts of the North China Craton, which are previously unsampled regions. Stepwise thermal and alternating field demagnetizations were successful in isolating two magnetic components. The lower unblocking temperature component represents the recent Earth magnetic field. The higher unblocking temperature component is the characteristic remanent magnetization and yields positive baked contact test. Results from detailed rock magnetic measurements corroborate the demagnetization behavior and show that titanomagnetites are the main magnetic carrier in these rocks. There was no regional event that has reset the remanent magnetization of all the dyke sites, as indicated by the magnetization directions of both overlying and underlying strata. The similarity of the virtual paleomagnetic poles for the 3 sampled regions also argues that the characteristic remanent magnetizations are primary magnetization when the dykes were emplaced. The paleomagnetic poles from the Dashigou dyke swarm of the North China Craton are not similar to those of the identical aged Bahia dykes from the São Francisco Craton, Brazil, indicating that these mafic dykes may be not parts of a common regional magmatic event that affected North China Craton and NE Brazil at about 925 Ma.

  20. NEW DATA ON AGE AND NATURE OF CARBONIZATION WITHIN SOUTHERN FLANK OF THE BAIKAL LEDGE OF THE SIBERIAN CRATON BASEMENT

    Directory of Open Access Journals (Sweden)

    Yu. V. Danilova

    2017-01-01

    Full Text Available The Baikal ledge rock formations in the Siberian craton structure are included in the Akitkan mobile belt which is considered as the Late Paleoproterozoic independent island arc system moved up to the ancient basement during the terrains amalgamation 1.91–2.00 Ga ago (Fig. 1 [Rosen, 2003; Gladkochub et al., 2009; Didenko et al., 2013].

  1. Short-Wavelength Infrared (SWIR) spectroscopy of low-grade metamorphic volcanic rocks of the Pilbara Craton

    NARCIS (Netherlands)

    Abweny, Mohammad S.; van Ruitenbeek, Frank J A; de Smeth, Boudewijn; Woldai, Tsehaie; van der Meer, Freek D.; Cudahy, Thomas; Zegers, Tanja; Blom, Jan Kees; Thuss, Barbara

    This paper shows the results of Short-Wavelength Infrared (SWIR) spectroscopy investigations of volcanic rocks sampled from low-grade metamorphic greenstone belts of the Archean Pilbara Craton in Western Australia. From the reflectance spectra a range of spectrally active minerals were identified,

  2. The extent of the Cratonic keel underneath the Southern African region: A 3D image using Finite-Frequency Tomograph

    DEFF Research Database (Denmark)

    Soliman, Mohammad Youssof Ahmad; Bezada, Max; Thybo, Hans

    2010-01-01

    We have re-examined the P body wave data from the South Africa Seismic Experiment (Carlson et al, EOS 77, 1996) across the Kaapvaal and Zimbabwe cratons and the Bushveld complex. Using finite-frequency kernels, we inverted the P-wave delay times to obtain 3-D images of compressional velocity...

  3. Deformation of the Pannonian lithosphere and related tectonic topography: a depth-to-surface analysis

    OpenAIRE

    Dombrádi, E.

    2012-01-01

    Fingerprints of deep-seated, lithospheric deformation are often recognised on the surface, contributing to topographic evolution, drainage organisation and mass transport. Interactions between deep and surface processes were investigated in the Carpathian-Pannonian region. The lithosphere beneath the Pannonian basin has formerly been extended, significantly stretched and heated up and thus became extremely weak from a rheological point of view. From Pliocene times onward the ‘crème brulee’ ty...

  4. Amount of Asian lithospheric mantle subducted during the India/Asia collision

    OpenAIRE

    Replumaz, A.; Guillot, S.; Villaseñor, Antonio; Negredo, A. M.

    2013-01-01

    Body wave seismic tomography is a successful technique for mapping lithospheric material sinking into the mantle. Focusing on the India/Asia collision zone, we postulate the existence of several Asian continental slabs, based on seismic global tomography. We observe a lower mantle positive anomaly between 1100 and 900 km depths, that we interpret as the signature of a past subduction process of Asian lithosphere, based on the anomaly position relative to positive anomalies related to Indian c...

  5. Analysis of Lithospheric Stresses Using Satellite Gravimetry: Hypotheses and Applications to North Atlantic

    Science.gov (United States)

    Minakov, A.; Medvedev, S.

    2017-12-01

    Analysis of lithospheric stresses is necessary to gain understanding of the forces that drive plate tectonics and intraplate deformations and the structure and strength of the lithosphere. A major source of lithospheric stresses is believed to be in variations of surface topography and lithospheric density. The traditional approach to stress estimation is based on direct calculations of the Gravitational Potential Energy (GPE), the depth integrated density moment of the lithosphere column. GPE is highly sensitive to density structure which, however, is often poorly constrained. Density structure of the lithosphere may be refined using methods of gravity modeling. However, the resulted density models suffer from non-uniqueness of the inverse problem. An alternative approach is to directly estimate lithospheric stresses (depth integrated) from satellite gravimetry data. Satellite gravity gradient measurements by the ESA GOCE mission ensures a wealth of data for mapping lithospheric stresses if a link between data and stresses or GPE can be established theoretically. The non-uniqueness of interpretation of sources of the gravity signal holds in this case as well. Therefore, the data analysis was tested for the North Atlantic region where reliable additional constraints are supplied by both controlled-source and earthquake seismology. The study involves comparison of three methods of stress modeling: (1) the traditional modeling approach using a thin sheet approximation; (2) the filtered geoid approach; and (3) the direct utilization of the gravity gradient tensor. Whereas the first two approaches (1)-(2) calculate GPE and utilize a computationally expensive finite element mechanical modeling to calculate stresses, the approach (3) uses a much simpler numerical treatment but requires simplifying assumptions that yet to be tested. The modeled orientation of principal stresses and stress magnitudes by each of the three methods are compared with the World Stress Map.

  6. Intermediate-Depth Subduction Earthquakes Recorded by Pseudotachylyte in Dry Eclogite-Facies Oceanic Lithosphere from the Alps

    Science.gov (United States)

    Scambelluri, M.; Pennacchioni, G.; Gilio, M.; Bestmann, M.

    2016-12-01

    While geophysical studies and laboratory experiments provide much information on subduction earthquakes, field studies identifying the rock types for earthquake development and the deep seismogenic environments are still scarce. To date, fluid overpressure and volume decrease during hydrous mineral breakdown the widely favoured trigger of subduction earthquakes in serpentinized lithospheric mantle and hydrated low-velocity layers atop slabs. Here we document up to 40 cm-thick pseudotachylyte (PST) in Alpine oceanic gabbro and peridotite (2-2.5 GPa-550-620°C), the analogue of a modern cold subducting oceanic lithosphere. These rocks mostly remained unaltered dry systems; only very minor domains (<1%) record partial hydration and static eclogitic metamorphism. Meta-peridotite shows high-pressure olivine + antigorite (garnet + zoisite + chlorite after mantle plagioclase); meta-gabbro develops omphacite + zoisite + talc + chloritoid + garnet. Abundant syn-eclogitic pseudotachylyte cut the dry gabbro-peridotite and the eclogitized domains. In meta-peridotite, PST shows olivine, orthopyroxene, spinel microliths and clasts of high-pressure olivine + antigorite and garnet + zoisite + chlorite aggregates. In metagabbro, microfaults in damage zones near PST cut brecciated igneous pyroxene cemented by omphacite. In unaltered gabbro, glassy PST contains micron-scale garnet replacing plagioclase microliths during, or soon after, PST cooling. In the host rock, garnet coronas between igneous olivine and plagioclase only occur near PST and between closely spaced PST veins. Absence of garnet away from PST indicates that garnet growth was triggered by mineral seeds and by heat released by PST. The above evidence shows that pseudotachylyte formed at eclogite-facies conditions. In such setting, strong, dry, metastable gabbro-peridotite concentrate stress to generate large intermediate depth subduction earthquakes without much involvement of free fluid.

  7. Magmatism during the accretion of the late Archaean Dharwar Craton (South India): sanukitoids and related rocks in their geological context.

    Science.gov (United States)

    Moyen, J.-F.; Martin, H.; Jayananda, M.; Peucat, J.-J.

    2003-04-01

    The South Indian Dharwar Craton assembled during the late-Archaean (ca. 2.5 Ga). This event was associated with intense granite genesis and emplacement. Based on petrography and geochemistry, 4 main types of late Archaean granitoids were distinguished: (1) Anatectic granites (and diatexites), formed by partial melting of TTG gneisses; (2) Classical TTGs; (3) Sanukitoids, generated by interaction between slab melts (TTG) and mantle peridotite; (4) The high HFSE Closepet granite, interpreted as derived from partial melting of a mantle metasomatized by slab melts (TTG). While the 3 later groups all are interpreted as resulting from slab melt/mantle wedge interactions, their differences are related to decreasing felsic melt/peridotite ratios during the ascent “slab melts” in the mantle wedge above an active subduction zone. Field data together with geochronology and isotope geochemistry allow to subdivide the Dharwar craton into three main domains: (1) The Western Dharwar Craton (WDC) is an old (3.3 2.9 Ga ), stable continental block with limited amounts of 2.5 Ga old anatectic granites. (2) The Eastern Dharwar Craton (EDC) is subdivided into two parts: (2a) West of Kolar Schist Belt, a region of 3.0-2.7 Ga old basement intruded by 2.5 Ga old anatectic granites; (2b) East of Kolar, an area featuring mainly 2.5 Ga old diatexites and granites, derived of partial melting of a newly accreted TTG crust. Anatectic granites are ubiquitous, and late in the cratonic evolution; they witnessed generalized melting of a juvenile crust. In contrast, deep-originated granites emplaced before this melting and are restricted to the boundaries between the blocks. This structure of distinct terranes separated by narrow bands operating as channels for deep-originated magmas provides independent evidences for a two-stage evolution: an arc accretion context for the TTG, sanukitoids and related rocks, immediately followed by high temperature reworking of the newly accreted craton

  8. Is Nubia plate rigid? A geodetic study of the relative motion of different cratonic areas within Africa.

    Science.gov (United States)

    Njoroge, M. W.; Malservisi, R.; Hugentobler, U.; Mokhtari, M.; Voytenko, D.

    2014-12-01

    Plate rigidity is one of the main paradigms of plate tectonics and a fundamental assumption in the definition of a global reference frame as ITRF. Although still far for optimal, the increased GPS instrumentation of the African region can allow us to understand how rigid one of the major plate can be. The presence of diffused band of seismicity, the Cameroon volcanic line, Pan African Kalahari orogenic belt and East Africa Rift suggest the possibility of relative motion among the different regions within the Nubia. The study focuses on the rigidity of Nubia plate. We divide the plate into three regions: Western (West Africa craton plus Nigeria), Central (approximately the region of the Congo craton) and Southern (Kalahari craton plus South Africa) and we utilize Euler Vector formulation to study internal rigidity and eventual relative motion. Developing five different reference frames with different combinations of the 3 regions, we try to understand the presence of the relative motion between the 3 cratons thus the stability of the Nubia plate as a whole. All available GPS stations from the regions are used separately or combined in creation of the reference frames. We utilize continuous stations with at least 2.5 years of data between 1994 and 2014. Given the small relative velocity, it is important to eliminate eventual biases in the analysis and to have a good estimation in the uncertainties of the observed velocities. For this reason we perform our analysis using both Bernese and Gipsy-oasis codes to generate time series for each station. Velocities and relative uncertainties are analyzed using the Allan variance of rate technique, taking in account for colored noise. An analysis of the color of the noise as function of latitude and climatic region is also performed to each time series. Preliminary results indicate a slight counter clockwise motion of West Africa craton with respect to South Africa Kalahari, and South Africa Kalahari-Congo Cratons. In addition

  9. Elastic Thickness Estimates for Coronae Associated with Chasmata on Venus

    Science.gov (United States)

    Hoogenboom, T.; Martin, P.; Housean, G. A.

    2005-01-01

    Coronae are large-scale circular tectonic features surrounded by annular ridges. They are generally considered unique to Venus and may offer insights into the differences in lithospheric structure or mantle convective pattern between Venus and Earth. 68% of all coronae are associated with chasmata or fracture belts. The remaining 32% are located at volcanic rises or in the plains. Chasmata are linear to arcuate troughs, with trough parallel fractures and faults which extend for 1000 s of kilometers. Estimates of the elastic thickness of the lithosphere (T(sub e)) have been calculated in a number of gravity/topography studies of Venus and for coronae specifically. None of these studies, however, have explored the dependence of T(sub e) on the tectonic history of the region, as implied from the interpretation of relative timing relationships between coronae and surrounding features. We examine the relationship between the local T(sub e) and the relative ages of coronae and chasmata with the aim of further constraining the origin and evolution of coronae and chasmata systems.

  10. Monitoring production target thickness

    International Nuclear Information System (INIS)

    Oothoudt, M.A.

    1993-01-01

    Pion and muon production targets at the Clinton P. Anderson Meson Physics Facility consist of rotating graphite wheels. The previous target thickness monitoring Procedure scanned the target across a reduced intensity beam to determine beam center. The fractional loss in current across the centered target gave a measure of target thickness. This procedure, however, required interruption of beam delivery to experiments and frequently indicated a different fractional loss than at normal beam currents. The new monitoring Procedure compares integrated ups and downs toroid current monitor readings. The current monitors are read once per minute and the integral of readings are logged once per eight-hour shift. Changes in the upstream to downstream fractional difference provide a nonintrusive continuous measurement of target thickness under nominal operational conditions. Target scans are now done only when new targets are installed or when unexplained changes in the current monitor data are observed

  11. Migration of plutonium and americium in the lithosphere

    International Nuclear Information System (INIS)

    Fried, S.; Friedman, A.M.; Hines, J.J.; Atcher, R.W.; Quarterman, L.A.; Volesky, A.

    1976-01-01

    When radionuclides are stored as wastes either in permanent repositories or in waste storage areas, the possibility of escape into the environment must be considered. Surface contamination and the transport and migration of radionuclides into the lithosphere through the agency of water are discussed. Water in the form of rain will inevitably wash contaminants into soils and thence into conducting rocks. The migration of radionuclides must follow widely varying paths. In porous rocks, water percolates easily under a slight pressure gradient and rapid movement of large volumes of water can result with concommitant transport of large amounts of contaminating materials. In relatively non-porous rocks such as Niagara limestones the transport meets much more resistance and the volumes of water conducted are correspondingly reduced. In such situations much of the migration of water and its solutes may be through cracks and fissures in the rock. Certain strata of rock or rock products may be almost impervious to flow of water and by this token may be considered to be an especially suitable container for long term safe storage of nuclear wastes, particularly if these strata are quiescent. A series of investigations was undertaken to examine the properties of rocks in acting as a retarding agent in the migration of radionuclides. The rocks that are discussed are Niagara limestone (chosen for its density and fine porosity), basalt from the National Reactor Test site, and Los Alamos tuff

  12. The Lithospheric Structure Beneath Canary Islands from Receiver Function Analysis

    Science.gov (United States)

    Martinez-Arevalo, C.; Mancilla, F.; Helffrich, G. R.; Garcia, A.

    2009-12-01

    The Canary Archipelago is located a few hundred kilometers off the western Moroccan coast, extending 450 km west-to-east. It is composed of seven main islands. All but one have been active in the last million years. The origin of the Canary Islands is not well established and local and regional geology features cannot be completely explained by the current models. The main aim of this study is to provide new data that help us to understand and constrain the archipelago's origin and tectonic evolution. The crustal structure under each station is obtained applying P-receiver function technique to the teleseismic P arrivals recorded by the broadband seismic network installed at the Canary Island by the Instituto Geográfico Nacional (IGN) and two temporary stations (MIDSEA and IRIS). We computed receiver functions using the Extended-Time Multitaper Frequency Domain Cross-Correlation Receiver Function (ET-MTRF) method. The results show that the crust is thicker, around 22 km, in the eastern islands (Fuerteventura and Lanzarote) than in the western ones (El Hierro, La Palma, Tenerife), around 17 km, with the exception of La Gomera island. This island, located in the west, exhibits similar crustal structure to Fuerteventura and Lanzarote. A discontinuity at 70-80 km, possibly the LAB (Lithosphere Asthenosphere Boundary) is clearly observed in all the stations. It appears that Moho depths do not track the LAB discontinuity.

  13. Risk and Geodynamically active areas of Carpathian lithosphere

    Directory of Open Access Journals (Sweden)

    Lubomil Pospíšil

    2007-01-01

    Full Text Available This paper illustrates an application of multidisciplinary data analysis to the Carpathian–Pannonian region and presents a verification of a Complex model of the Carpathian - Pannonian lithosphere by recent data sets and geophysical data analyses and its utilization for the determination of risk and active geodynamic and tectonic zones of Ist order . This model can be used for the analysing any Carpathian area from the point of view of the seismic risk, hazards and geodynamic activity, which is important to know for the building of a repository for the radioactive wasted material. Besides the traditionally used geological (sedimentological and volcanological data and geomorphological data (Remote Sensing, an emphasis was laid on geodetic, grav/mag data, seismic, seismological and other geophysical data (magnetotelluric, heat flow, paleomagnetic etc.. All available geonomic (geologic, geodetic, geophysical, geomorphological data were verified and unified on the basis of the same scale and in the Western Carpathians on the Remote Sensing data. The paper concentrates on two problematic areas – the so call “rebounding area” in the Eastern Carpathians and the Raba – Muran - Malcov tectonic systems.

  14. Detachments of the subducted Indian continental lithosphere based on 3D finite-frequency tomographic images

    Science.gov (United States)

    Liang, X.; Tian, X.; Wang, M.

    2017-12-01

    Indian plate collided with Eurasian plate at 60 Ma and there are about 3000 km crustal shortening since the continental-continental collision. At least one third of the total amount of crustal shortening between Indian and Eurasian plates could not be accounted by thickened Tibetan crust and surface erosion. It will need a combination of possible transfer of lower crust to the mantle by eclogitization and lateral extrusion. Based on the lithosphere-asthenosphere boundary images beneath the Tibetan plateau, there is also at least the same amount deficit for lithospheric mantle subducted into upper/lower mantle or lateral extrusion with the crust. We have to recover a detailed Indian continental lithosphere image beneath the plateau in order to explain this deficit of mass budget. Combining the new teleseismic body waves recorded by SANDWICH passive seismic array with waveforms from several previous temporary seismic arrays, we carried out finite-frequency tomographic inversions to image three-dimensional velocity structures beneath southern and central Tibetan plateau to examine the possible image of subducted Indian lithosphere in the Tibetan upper mantle. We have recovered a continuous high velocity body in upper mantle and piece-wised high velocity anomalies in the mantle transition zone. Based on their geometry and relative locations, we interpreted these high velocity anomalies as the subducted and detached Indian lithosphere at different episodes of the plateau evolution. Detachments of the subducted Indian lithosphere should have a crucial impact on the volcanism activities and uplift history of the plateau.

  15. Three-dimensional lithospheric density distribution of China and surrounding regions

    Directory of Open Access Journals (Sweden)

    Chuantao Li

    2014-01-01

    Full Text Available In this paper, we analyze lithospheric density distribution of China and surrounding regions on the basis of 30′ × 30′ gravity data and 1° × 1° P-wave velocity data. Firstly, we used the empirical equation between the density and the P-wave velocity difference as the base of the initial model of the Asian lithospheric density. Secondly, we calculated the gravity anomaly, caused by the Moho discontinuity and the sedimentary layer discontinuity, by the Parker formula. Thirdly, the gravity anomaly of the spherical harmonics with 2–40 order for the anomalous body below the lithosphere is calculated based on the model of EGM96. Finally, by using Algebra Reconstruction Techniques (ART, the inversion of 30′ × 30′ residual lithospheric Bouguer gravity anomaly caused by the lithosphere yields a rather detailed structural model. The results show that the lithospheric density distribution of China and surrounding regions has a certain connection with the tectonic structure. The density is relatively high in the Philippine Sea plate, Japan Sea, the Indian plate, the Kazakhstan shield and the Western Siberia plain, whereas the Tibetan Plateau has low-density characteristics. The minimum value of density lies in the north of Philippines, in the Taiwan province and in the Ryukyu island arc.

  16. Peeling back the lithosphere: Controlling parameters, surface expressions and the future directions in delamination modeling

    Science.gov (United States)

    Göğüş, Oğuz H.; Ueda, Kosuke

    2018-06-01

    Geodynamical models investigate the rheological and physical properties of the lithosphere that peels back (delaminates) from the upper-middle crust. Meanwhile, model predictions are used to relate to a set of observations in the geological context to the test the validity of delamination. Here, we review numerical and analogue models of delamination from these perspectives and provide a number of first-order topics which future modeling studies may address. Models suggest that the presence of the weak lower crust that resides between the strong mantle lithosphere (at least 100 times more viscous/stronger) and the strong upper crust is necessary to develop delamination. Lower crustal weakening may be induced by melt infiltration, shear heating or it naturally occurs through the jelly sandwich type strength profile of the continental lithosphere. The negative buoyancy of the lithosphere required to facilitate the delamination is induced by the pre-existing ocean subduction and/or the lower crustal eclogitization. Surface expression of the peeling back lithosphere has a distinct transient and migratory imprint on the crust, resulting in rapid surface uplift/subsidence, magmatism, heating and shortening/extension. New generation of geodynamical experiments can explain how different types of melting (e.g hydrated, dry melting) occurs with delamination. Reformation of the lithosphere after removal, three dimensional aspects, and the termination of the process are key investigation areas for future research. The robust model predictions, as with other geodynamic modeling studies should be reconciled with observations.

  17. The rheological structure of the lithosphere in the Eastern Marmara region, Turkey

    Science.gov (United States)

    Oruç, Bülent; Sönmez, Tuba

    2017-05-01

    The aim of this work is to propose the geometries of the crustal-lithospheric mantle boundary (Moho) and lithosphere-asthenosphere boundary (LAB) and the 1D thermal structure of the lithosphere, in order to establish a rheological model of the Eastern Marmara region. The average depths of Moho and LAB are respectively 35 km and 51 km from radially averaged amplitude spectra of EGM08 Bouguer anomalies. The geometries of Moho and LAB interfaces are estimated from the Parker-Oldenburg gravity inversion algorithm. Our results show the Moho depth varies from 31 km at the northern part of North Anatolian Fault Zone (NAFZ) to 39 km below the mountain belt in the southern part of the NAFZ. The depth to the LAB beneath the same parts of the region ranges from 45 km to 55 km. Having lithospheric strength and thermal boundary layer structure, we analyzed the conditions of development of lithosphere thinning. A two-dimensional strength profile has been estimated for rheology model of the study area. Thus we suggest that the rheological structure consists of a strong upper crust, a weak lower crust, and a partly molten upper lithospheric mantle.

  18. An Integrated Geochronological, Petrological, Geochemical and Paleomagnetic Study of Paleoproterozoic and Mesoproterozoic Mafic Dyke Swarms in the Nain Craton, Labrador

    Science.gov (United States)

    Sahin, Tugce

    The Nain craton comprises the western, Labrador segment of the larger North Atlantic craton (NAC) which exposes Early through Late Archean gneisses. The NAC is bounded on all sides by Paleoproterozoic collisional orogens that involved either considerable structural reworking (Torngat-Nagssugtoqidian-Lewisian) or the accretion of juvenile arc magmas (Ketilidian-Makkovik). The NAC remains poorly understood compared to other Archean crustal blocks now dispersed globally. Compounding this problem is a lack of reliable paleomagnetic poles for NAC units that predate its assembly into the supercontinent Laurentia by ca. 1800 Ma, which could be used to test neighboring relationships with other cratonic fragments. In order to understand the history of the NAC as part of a possible, larger supercontinent, the record of mafic dyke swarms affecting the craton, particularly those that postdate the Late Archean terrane assembly, were examined in this study. Diabase or gabbroic dyke swarms are invaluable in such studies because their geometries offer possible locus points, they often have a punctuated emplacement and precisely datable crystallization histories, and they have cooling histories and oxide mineralogy amenable to recovering robust paleopoles. Coastal Labrador exposes a number of mafic dykes, some of which are demonstrably Paleoproterozoic (e.g. 2235 Ma Kikkertavak dykes; 2121 Ma Tikkigatsiagak dykes) or Mesoproterozoic (e.g. 1280-1270 Ma Nain and Harp dykes) in age (U-Pb; baddeleyite or zircon). The southern half of the Nain craton (Hopedale block) in particular preserves a rich array of mafic dykes. Dyke cross-cutting relationships are numerous and relatively well exposed, permitting multiple opportunities for paleomagnetic field tests (e.g. baked contact). The results presented here allow understanding of the tectonic evolution of the NAC with implications for strengthened Labrador-Greenland correlations, and testing possible Paleoproterozoic supercontinent

  19. Coating thickness measurement

    International Nuclear Information System (INIS)

    1976-12-01

    The standard specifies measurements of the coating thickness, which make use of beta backscattering and/or x-ray fluorescence. For commonly used combinations of coating material and base material the appropriate measuring ranges and radionuclides to be used are given for continuous as well as for discontinuous measurements

  20. Using gravimetry to probe small to large scale lithospheric structure at Fogo Island (Cape Verde)

    Science.gov (United States)

    Fernandes, R.; Bos, M. S.; Dumont, S.; Oliveira, C. S.; Miranda, E. H.; Almeida, P. G.

    2017-12-01

    The Fogo volcano is located on one of the Cape Verde islands in the Atlantic Ocean. Its last eruption was from November 2014 to February 2015 and destroyed two villages. To better understand its volcanic plumbing system, but also its eruptive dynamics that would contribute to hazard assessment and risk mitigation, the project "FIRE - Fogo Island volcano: multi-disciplinary Research on 2014/15 Eruption" was rapidly conceived in collaboration with local institutions. Despite a regular eruptive activity in the last centuries ( 20 years), no magmatic chamber has been evidenced yet and this is what we are investigating using gravimetry in the FIRE project. Approximately 70 relative new gravity observations were made by the C4G mission during the 2014 eruption, using a SCINTREX CG­3M gravimeter. The spacing between observation points was around 5 km for most of the island and around 2 km in Chã das Caldeiras, close to the 2014 eruption vent. In January 2017, 48 additional observations were made which densified the post-eruption observations in Chã das Caldeiras. The exact location of each observation point was determined with an accuracy of <10 cm from co­located GNSS receivers. The gravity residuals are fitted to the predicted gravity effect from modelled magma chambers with various diameters, depths and density contrasts in order to investigate if such a chamber can explain the measurements. A digital terrain model of the island, refined with a high-resolution terrain model of 10m resolution in the Chã das Caldeiras produced as part of the project, will be used to remove the gravitational effect of the topography. In addition, the new gravity observations can be used to improve global geopotential models such as EGM2008/EIGEN6C4 over the island and to compute a new elastic thickness of the crust underneath the island. Pim (2006) estimated that the elastic thickness Te is around 30 km in this region which is normal for the age of lithosphere, suggesting that it has

  1. EARLY STAGE OF THE CENTRAL ASIAN OROGENIC BELT BUILDING: EVIDENCES FROM THE SOUTHERN SIBERIAN CRATON

    Directory of Open Access Journals (Sweden)

    D. P. Gladkochub

    2017-01-01

    Full Text Available The origin of the Central-Asian Orogenic Belt (CAOB, especially of its northern segment nearby the southern margin of the Siberian craton (SC is directly related to development and closure of the Paleo-Asian Ocean (PAO. Signatures of early stages of the PAO evolution are recorded in the Late Precambrian sedimentary successions of the Sayan-Baikal-Patom Belt (SBPB on the southern edge of SC. These successions are spread over 2000 km and can be traced along this edge from north-west (Sayan area to south-east (Baikal area and further to north-east (Patom area. Here we present the synthesis of all available and reliable LA-ICP-MS U-Pb geochronological studies of detrital zircons from these sedimentary successions.

  2. Paleointensity determination on Neoarchaean dikes within the Vodlozerskii terrane of the Karelian craton

    Science.gov (United States)

    Shcherbakova, V. V.; Lubnina, N. V.; Shcherbakov, V. P.; Zhidkov, G. V.; Tsel'movich, V. A.

    2017-09-01

    The results of paleomagnetic studies and paleointensity determinations from two Neoarchaean Shala dikes with an age of 2504 Ma, located within the Vodlozerskii terrane of the Karelian craton, are presented. The characteristic components of primary magnetization with shallow inclinations I = -5.7 and 1.9 are revealed; the reliability of the determinations is supported by two contact tests. High paleointensity values are obtained by the Thellier-Coe and Wilson techniques. The calculated values of the virtual dipole moment (11.5 and 13.8) × 1022 A m2 are noticeably higher than the present value of 7.8 × 1022 A m2. Our results, in combination with the previous data presented in the world database, support the hypothesized existence of a period of high paleointensity in the Late Archaean-Early Proterozoic.

  3. The West-African craton margin in eastern Senegal: a seismological study

    International Nuclear Information System (INIS)

    Dorbath, Catherine; Dorbath, Louis; Gaulon, Roland; Le Page, Alain

    1983-01-01

    A vertical short period seismological array was operated for six months in earstern Senegal. Large P wave travel-time anomalies are in fairly good relation with the gravity and geological features. Two-dimensional inversion of the data shows the existence of a major vertical discontinuity extending from the surface to 150-200 km depth. The other heterogeneities are mainly located in the crust and related to specific segments of the regional geology: craton, Mauritanides and Senegalo-Mauritanian basin. The main discontinuity dipping to the east is interpreted as the trace of an old subduction slab. We propose the following geodynamical process to explain the formation of the Mauritanides orogenic belt: continental collision after opening of a back-arc marginal basin in late Precambrian and its closure until Devonian

  4. The 3.1 Ga Nuggihalli chromite deposits, Western Dhawar craton (India)

    DEFF Research Database (Denmark)

    Mukherjee, Ria; Mondal, Sisir K.; Frei, Robert

    2012-01-01

    The Nuggihalli greenstone belt is part of the older greenstone belts (3.4 - 3.0 Ga) in the Western Dharwar Craton, southern India. This greenstone sequence consists of conformable metavolcanic and metasedimentary supracrustal rock assemblages that belong to the Sargur Group. Sill-like ultramafic......-mafic plutonic bodies are present within these supracrustal rocks (schist rocks) which are in turn enclosed by tonalite-trondhjemite-granodiorite gneiss (TTG). The sill-like ultramafic-mafic rocks are cumulates derived from a high-Mg parental magma that are represented by chromitite-hosted serpentinite...... and tremolite-chlorite-actinolite- schist (altered peridotite), anorthosite, pyroxenite, and gabbro hosting magnetite bands. The first whole-rock Sm-Nd data for the peridotite anorthosite- pyroxenite-gabbro unit has been obtained yielding an age of 3125 ± 120 Ma (MSWD = 1.3) which is similar to reported ages...

  5. Lithospheric-scale analogue modelling of collision zones with a pre-existing weak zone, in "Deformation Mechanisms, Rheology and Tectonics: from Minerals to the Lithosphere"

    NARCIS (Netherlands)

    Willingshofer, E.; Sokoutis, D.; Burg, J.P.

    2005-01-01

    Lithospheric-scale analogue experiments have been conducted to investigate the influence of strength heterogeneities on the distribution and mode of crustal-scale deformation, on the resulting geometry of the deformed area, and on its topographic expression. Strength heterogeneities were

  6. U-Pb baddeleyite ages and geochemistry of dolerite dykes in the Bas Drâa Inlier of the Anti-Atlas of Morocco: Newly identified 1380 Ma event in the West African Craton

    Science.gov (United States)

    El Bahat, Abdelhakim; Ikenne, Moha; Söderlund, Ulf; Cousens, Brian; Youbi, Nasrrddine; Ernst, Richard; Soulaimani, Abderrahmane; El Janati, M'hamed; Hafid, Ahmid

    2013-08-01

    In the Bas-Drâa Inlier (Anti-Atlas, Morocco), the Paleoproterozoic basement which is cut by the Ediacaran Taourgha granite is also crosscut by numerous dykes of a variety of trends, mostly of uncertain age. Two doleritic dykes are dated by the ID-TIMS U-Pb method on baddeleyite and yield emplacement ages of 1381 ± 8 Ma (MSWD = 0.84) and 1384 ± 6 Ma (MSWD = 1.4) determined for a N135°E and a N40°E trending dyke, respectively. These dates represent the first geochronological evidence of a Mesoproterozoic magmatic event in the Anti-Atlas. This magmatic event falls in the previously considered ca 1.7-1.0 Ga (Mesoproterozoic) gap in geological activity in the Anti-Atlas. The poorly dated Taghdout and Taarotihate sequences could represent remnants of the ca. 1380 Ma magmatism and rift-related sedimentation. The Mesoproterozoic sedimentary succession of the Atar Group in the Taoudeni basin (Mauritania) could also represent a good candidate for rift-related sedimentation but it postdates the 1380 Ma magmatic event by 270 Ma. The dated 1380 Ma dykes are transitional to mildly alkaline basalts, not unlike some Hawaiian lavas. However, these dykes have a distinct negative Nb anomaly (a common features in many Large Igneous Provinces, LIPs), and this requires interaction with the lithosphere. This interaction may have occurred at the level of the lithospheric mantle or the crust. These newly dated 1380 Ma dykes may converge to the north, speculatively suggesting a magmatic center (associated with a 1380 Ma mantle plume?) along the northern margin, and possibly linked to rifting and possible breakup on that margin, and also to a regional uplift that largely removed the evidence of a 1380 Ma cover sequence. Contemporaneous 1380-1390 Ma magmatism is reported elsewhere on other crustal blocks, and that in northeastern Laurentia (northern Greenland), northern Siberia (Anabar shield), and Baltica (southern Urals) can be reconstructed with that of the Bas Drâa Inlier (Anti

  7. Double subduction of continental lithosphere, a key to form wide plateau

    Science.gov (United States)

    Replumaz, Anne; Funiciello, Francesca; Reitano, Riccardo; Faccenna, Claudio; Balon, Marie

    2016-04-01

    The mechanisms involved in the creation of the high and wide topography, like the Tibetan Plateau, are still controversial. In particular, the behaviour of the indian and asian lower continental lithosphere during the collision is a matter of debate, either thickening, densifying and delaminating, or keeping its rigidity and subducting. But since several decades seismicity, seismic profiles and global tomography highlight the lithospheric structure of the Tibetan Plateau, and make the hypotheses sustaining the models more precise. In particular, in the western syntaxis, it is now clear that the indian lithosphere subducts northward beneath the Hindu Kush down to the transition zone, while the asian one subducts southward beneath Pamir (e.g. Negredo et al., 2007; Kufner et al., 2015). Such double subduction of continental lithospheres with opposite vergence has also been inferred in the early collision time. Cenozoic volcanic rocks between 50 and 30 Ma in the Qiangtang block have been interpreted as related to an asian subduction beneath Qiangtang at that time (De Celles et al., 2011; Guillot and Replumaz, 2013). We present here analogue experiments silicone/honey to explore the subduction of continental lithosphere, using a piston as analogue of far field forces. We explore the parameters that control the subductions dynamics of the 2 continental lithospheres and the thickening of the plates at the surface, and compare with the Tibetan Plateau evolution. We show that a continental lithosphere is able to subduct in a collision context, even lighter than the mantle, if the plate is rigid enough. In that case the horizontal force due to the collision context, modelled by the piston push transmitted by the indenter, is the driving force, not the slab pull which is negative. It is not a subduction driving by the weight of the slab, but a subduction induced by the collision, that we could call "collisional subduction".

  8. Effects of upper mantle heterogeneities on the lithospheric stress field and dynamic topography

    Science.gov (United States)

    Osei Tutu, Anthony; Steinberger, Bernhard; Sobolev, Stephan V.; Rogozhina, Irina; Popov, Anton A.

    2018-05-01

    The orientation and tectonic regime of the observed crustal/lithospheric stress field contribute to our knowledge of different deformation processes occurring within the Earth's crust and lithosphere. In this study, we analyze the influence of the thermal and density structure of the upper mantle on the lithospheric stress field and topography. We use a 3-D lithosphere-asthenosphere numerical model with power-law rheology, coupled to a spectral mantle flow code at 300 km depth. Our results are validated against the World Stress Map 2016 (WSM2016) and the observation-based residual topography. We derive the upper mantle thermal structure from either a heat flow model combined with a seafloor age model (TM1) or a global S-wave velocity model (TM2). We show that lateral density heterogeneities in the upper 300 km have a limited influence on the modeled horizontal stress field as opposed to the resulting dynamic topography that appears more sensitive to such heterogeneities. The modeled stress field directions, using only the mantle heterogeneities below 300 km, are not perturbed much when the effects of lithosphere and crust above 300 km are added. In contrast, modeled stress magnitudes and dynamic topography are to a greater extent controlled by the upper mantle density structure. After correction for the chemical depletion of continents, the TM2 model leads to a much better fit with the observed residual topography giving a good correlation of 0.51 in continents, but this correction leads to no significant improvement of the fit between the WSM2016 and the resulting lithosphere stresses. In continental regions with abundant heat flow data, TM1 results in relatively small angular misfits. For example, in western Europe the misfit between the modeled and observation-based stress is 18.3°. Our findings emphasize that the relative contributions coming from shallow and deep mantle dynamic forces are quite different for the lithospheric stress field and dynamic

  9. Coating thickness measuring device

    International Nuclear Information System (INIS)

    Joffe, B.B.; Sawyer, B.E.; Spongr, J.J.

    1984-01-01

    A device especially adapted for measuring the thickness of coatings on small, complexly-shaped parts, such as, for example, electronic connectors, electronic contacts, or the like. The device includes a source of beta radiation and a radiation detector whereby backscatter of the radiation from the coated part can be detected and the thickness of the coating ascertained. The radiation source and detector are positioned in overlying relationship to the coated part and a microscope is provided to accurately position the device with respect to the part. Means are provided to control the rate of descent of the radiation source and radiation detector from its suspended position to its operating position and the resulting impact it makes with the coated part to thereby promote uniformity of readings from operator to operator, and also to avoid excessive impact with the part, thereby improving accuracy of measurement and eliminating damage to the parts

  10. Anatomy of the dead sea transform from lithospheric to microscopic scale

    Science.gov (United States)

    Weber, M.; Abu-Ayyash, K.; Abueladas, A.; Agnon, A.; Alasonati-Tasarova, Z.; Al-Zubi, H.; Babeyko, A.; Bartov, Y.; Bauer, K.; Becken, M.; Bedrosian, P.A.; Ben-Avraham, Z.; Bock, G.; Bohnhoff, M.; Bribach, J.; Dulski, P.; Ebbing, J.; El-Kelani, R.; Forster, A.; Forster, H.-J.; Frieslander, U.; Garfunkel, Z.; Goetze, H.J.; Haak, V.; Haberland, C.; Hassouneh, M.; Helwig, S.; Hofstetter, A.; Hoffmann-Rotrie, A.; Jackel, K.H.; Janssen, C.; Jaser, D.; Kesten, D.; Khatib, M.; Kind, R.; Koch, O.; Koulakov, I.; Laske, Gabi; Maercklin, N.; Masarweh, R.; Masri, A.; Matar, A.; Mechie, J.; Meqbel, N.; Plessen, B.; Moller, P.; Mohsen, A.; Oberhansli, R.; Oreshin, S.; Petrunin, A.; Qabbani, I.; Rabba, I.; Ritter, O.; Romer, R.L.; Rumpker, G.; Rybakov, M.; Ryberg, T.; Saul, J.; Scherbaum, F.; Schmidt, S.; Schulze, A.; Sobolev, S.V.; Stiller, M.; Stromeyer, D.; Tarawneh, K.; Trela, C.; Weckmann, U.; Wetzel, U.; Wylegalla, K.

    2009-01-01

    Fault zones are the locations where motion of tectonic plates, often associated with earthquakes, is accommodated. Despite a rapid increase in the understanding of faults in the last decades, our knowledge of their geometry, petrophysical properties, and controlling processes remains incomplete. The central questions addressed here in our study of the Dead Sea Transform (DST) in the Middle East are as follows: (1) What are the structure and kinematics of a large fault zone? (2) What controls its structure and kinematics? (3) How does the DST compare to other plate boundary fault zones? The DST has accommodated a total of 105 km of leftlateral transform motion between the African and Arabian plates since early Miocene (???20 Ma). The DST segment between the Dead Sea and the Red Sea, called the Arava/ Araba Fault (AF), is studied here using a multidisciplinary and multiscale approach from the ??m to the plate tectonic scale. We observe that under the DST a narrow, subvertical zone cuts through crust and lithosphere. First, from west to east the crustal thickness increases smoothly from 26 to 39 km, and a subhorizontal lower crustal reflector is detected east of the AF. Second, several faults exist in the upper crust in a 40 km wide zone centered on the AF, but none have kilometer-size zones of decreased seismic velocities or zones of high electrical conductivities in the upper crust expected for large damage zones. Third, the AF is the main branch of the DST system, even though it has accommodated only a part (up to 60 km) of the overall 105 km of sinistral plate motion. Fourth, the AF acts as a barrier to fluids to a depth of 4 km, and the lithology changes abruptly across it. Fifth, in the top few hundred meters of the AF a locally transpressional regime is observed in a 100-300 m wide zone of deformed and displaced material, bordered by subparallel faults forming a positive flower structure. Other segments of the AF have a transtensional character with small pull

  11. Thick melanoma in Tuscany.

    Science.gov (United States)

    Chiarugi, Alessandra; Nardini, Paolo; Borgognoni, Lorenzo; Brandani, Paola; Gerlini, Gianni; Rubegni, Pietro; Lamberti, Arianna; Salvini, Camilla; Lo Scocco, Giovanni; Cecchi, Roberto; Sirna, Riccardo; Lorenzi, Stefano; Gattai, Riccardo; Battistini, Silvio; Crocetti, Emanuele

    2017-03-14

    The epidemiologic trends of cutaneous melanoma are similar in several countries with a Western-type life style, where there is a progressive increasing incidence and a low but not decreasing mor- tality, or somewhere an increase too, especially in the older age groups. Also in Tuscany there is a steady rise in incidence with prevalence of in situ and invasive thin melanomas, with also an increase of thick melanomas. It is necessary to reduce the frequency of thick melanomas to reduce specific mortality. The objective of the current survey has been to compare, in the Tuscany population, by a case- case study, thin and thick melanoma cases, trying to find out those personal and tumour characteristics which may help to customize preventive interventions. RESULTS The results confirmed the age and the lower edu- cation level are associated with a later detection. The habit to perform skin self-examination is resulted protec- tive forward thick melanoma and also the diagnosis by a doctor. The elements emerging from the survey allow to hypothesize a group of subjects resulting at higher risk for a late diagnosis, aged over 50 and carrier of a fewer constitutional and environmental risk factors: few total and few atypical nevi, and lower sun exposure and burning. It is assumable that a part of people did not be reached from messages of prevention because does not recognize oneself in the categories of people at risk for skin cancers described in educational cam- paigns. If we want to obtain better results on diagnosis of skin melanoma we have to think a new strategy. At least to think over the educational messages discriminating people more at risk of incidence of melanoma from people more at risk to die from melanoma, and to renewed active involvement of the Gen- eral Practitioners .

  12. Thick brane solutions

    International Nuclear Information System (INIS)

    Dzhunushaliev, Vladimir; Minamitsuji, Masato; Folomeev, Vladimir

    2010-01-01

    This paper gives a comprehensive review on thick brane solutions and related topics. Such models have attracted much attention from many aspects since the birth of the brane world scenario. In many works, it has been usually assumed that a brane is an infinitely thin object; however, in more general situations, one can no longer assume this. It is also widely considered that more fundamental theories such as string theory would have a minimal length scale. Many multidimensional field theories coupled to gravitation have exact solutions of gravitating topological defects, which can represent our brane world. The inclusion of brane thickness can realize a variety of possible brane world models. Given our understanding, the known solutions can be classified into topologically non-trivial solutions and trivial ones. The former class contains solutions of a single scalar (domain walls), multi-scalar, gauge-Higgs (vortices), Weyl gravity and so on. As an example of the latter class, we consider solutions of two interacting scalar fields. Approaches to obtain cosmological equations in the thick brane world are reviewed. Solutions with spatially extended branes (S-branes) and those with an extra time-like direction are also discussed.

  13. Lithospheric structure of the Arabian Shield and Platform from complete regional waveform modelling and surface wave group velocities

    Science.gov (United States)

    Rodgers, Arthur J.; Walter, William R.; Mellors, Robert J.; Al-Amri, Abdullah M. S.; Zhang, Yu-Shen

    1999-09-01

    Regional seismic waveforms reveal significant differences in the structure of the Arabian Shield and the Arabian Platform. We estimate lithospheric velocity structure by modelling regional waveforms recorded by the 1995-1997 Saudi Arabian Temporary Broadband Deployment using a grid search scheme. We employ a new method whereby we narrow the waveform modelling grid search by first fitting the fundamental mode Love and Rayleigh wave group velocities. The group velocities constrain the average crustal thickness and velocities as well as the crustal velocity gradients. Because the group velocity fitting is computationally much faster than the synthetic seismogram calculation this method allows us to determine good average starting models quickly. Waveform fits of the Pn and Sn body wave arrivals constrain the mantle velocities. The resulting lithospheric structures indicate that the Arabian Platform has an average crustal thickness of 40 km, with relatively low crustal velocities (average crustal P- and S-wave velocities of 6.07 and 3.50 km s^-1 , respectively) without a strong velocity gradient. The Moho is shallower (36 km) and crustal velocities are 6 per cent higher (with a velocity increase with depth) for the Arabian Shield. Fast crustal velocities of the Arabian Shield result from a predominantly mafic composition in the lower crust. Lower velocities in the Arabian Platform crust indicate a bulk felsic composition, consistent with orogenesis of this former active margin. P- and S-wave velocities immediately below the Moho are slower in the Arabian Shield than in the Arabian Platform (7.9 and 4.30 km s^-1 , and 8.10 and 4.55 km s^-1 , respectively). This indicates that the Poisson's ratios for the uppermost mantle of the Arabian Shield and Platform are 0.29 and 0.27, respectively. The lower mantle velocities and higher Poisson's ratio beneath the Arabian Shield probably arise from a partially molten mantle associated with Red Sea spreading and continental

  14. Thermal modeling and geomorphology of the south border of the Sao Francisco Craton: thermochronology by fission tracks in apatites;Modelagem termica e geomorfologia da borda sul do Craton do Sao Francisco: termocronologia por tracos de fissao em apatita

    Energy Technology Data Exchange (ETDEWEB)

    Hackspacher, Peter Christian [UNESP, Rio Claro, SP (Brazil). Inst. de Geociencias e Ciencias Exatas; Godoy, Daniel Francoso de; Franco, Ana Olivia Barufi [UNESP, Rio Claro, SP (Brazil). Pos-Graduacao em Geologia Regional; Ribeiro, Luiz Felipe Brandini [NUCLEARGEO, Rio Claro, SP (Brazil); Hadler Neto, Julio Cesar [Universidade Estadual de Campinas (IFGW/UNICAMP), SP (Brazil). Inst. de Fisica Gleb Wataghin

    2007-12-15

    Recent developments in Fission Track thermochronology associated to mesozoic-cenozoic erosion and tectonic presented trough thematic maps (isotemperature), permit to model the landscape evolution in the southern border of the Sao Francisco craton, southeastern Brazil. Paleotemperature, obtained by fission track analysis in apatite, is closely related to geomorphologic interpretations. The area suffered a complex imprint of endogenous and exogenous processes resulting diversified and differentiated relieves. The landscape is strongly controlled by exhumation between Jurassic and Lower Cretaceous, uplift with tectonic denudation related to crustal heating at the Upper Cretaceous and reactivation of faults until the Miocene. This scenario is a result of reactivations of different brittle structures that accommodate the deformation in the southern border of the Sao Francisco craton. The landscape reflects denudations of up to 3 km with preserved remains of erosive surfaces in the topographical tops and chronocorrelates deposits in the basins of the region. (author)

  15. Life in the lithosphere, kinetics and the prospects for life elsewhere.

    Science.gov (United States)

    Cockell, Charles S

    2011-02-13

    The global contiguity of life on the Earth today is a result of the high flux of carbon and oxygen from oxygenic photosynthesis over the planetary surface and its use in aerobic respiration. Life's ability to directly use redox couples from components of the planetary lithosphere in a pre-oxygenic photosynthetic world can be investigated by studying the distribution of organisms that use energy sources normally bound within rocks, such as iron. Microbiological data from Iceland and the deep oceans show the kinetic limitations of living directly off igneous rocks in the lithosphere. Using energy directly extracted from rocks the lithosphere will support about six orders of magnitude less productivity than the present-day Earth, and it would be highly localized. Paradoxically, the biologically extreme