On the relative motions of long-lived Pacific mantle plumes.

Science.gov (United States)

Konrad, Kevin; Koppers, Anthony A P; Steinberger, Bernhard; Finlayson, Valerie A; Konter, Jasper G; Jackson, Matthew G

2018-02-27

Mantle plumes upwelling beneath moving tectonic plates generate age-progressive chains of volcanos (hotspot chains) used to reconstruct plate motion. However, these hotspots appear to move relative to each other, implying that plumes are not laterally fixed. The lack of age constraints on long-lived, coeval hotspot chains hinders attempts to reconstruct plate motion and quantify relative plume motions. Here we provide 40 Ar/ 39 Ar ages for a newly identified long-lived mantle plume, which formed the Rurutu hotspot chain. By comparing the inter-hotspot distances between three Pacific hotspots, we show that Hawaii is unique in its strong, rapid southward motion from 60 to 50 Myrs ago, consistent with paleomagnetic observations. Conversely, the Rurutu and Louisville chains show little motion. Current geodynamic plume motion models can reproduce the first-order motions for these plumes, but only when each plume is rooted in the lowermost mantle.

Multiple-frequency tomography of the upper mantle beneath the African/Iberian collision zone

Science.gov (United States)

Bonnin, Mickaël; Nolet, Guust; Villaseñor, Antonio; Gallart, Josep; Thomas, Christine

2014-09-01

During the Cenozoic, the geodynamics of the western Mediterranean domain has been characterized by a complex history of subduction of Mesozoic oceanic lithosphere. The final stage of these processes is proposed to have led to the development of the Calabria and Gibraltar arcs, whose formation is still under debate. In this study, we take advantage of the dense broad-band station networks now available in the Alborán Sea region, to develop a high-resolution 3-D tomographic P velocity model of the upper mantle beneath the African/Iberian collision zone that will better constraint the past dynamics of this zone. The model is based on 13200 teleseismic arrival times recorded between 2008 and 2012 at 279 stations for which cross-correlation delays are measured with a new technique in different frequency bands centred between 0.03 and 1.0 Hz, and for the first time interpreted using multiple frequency tomography. Our model shows, beneath the Alborán Sea, a strong (4 per cent) fast vertically dipping anomaly observed to at least 650 km depth. The arched shape of this anomaly, and its extent at depth, are coherent with a lithospheric slab, thus favouring the hypothesis of a westward consumption of the Ligurian ocean slab by roll-back during Cenozoic. In addition to this fast anomaly in the deep upper mantle, high intensity slow anomalies are widespread in the lithosphere and asthenosphere beneath Morocco and southern Spain. These anomalies are correlated at the surface with the position of the Rif and Atlas orogens and with Cenozoic volcanic fields. We thus confirm the presence, beneath Morocco, of an anomalous (hot?) upper mantle, but without clear indication for a lateral spreading of the Canary plume to the east.

Transdimensional Bayesian tomography of the lowermost mantle from shear waves

Science.gov (United States)

Richardson, C.; Mousavi, S. S.; Tkalcic, H.; Masters, G.

2017-12-01

The lowermost layer of the mantle, known as D'', is a complex region that contains significant heterogeneities on different spatial scales and a wide range of physical and chemical features such as partial melting, seismic anisotropy, and variations in thermal and chemical composition. The most powerful tools we have to probe this region are seismic waves and corresponding imaging techniques such as tomography. Recently, we developed compressional velocity tomograms of D'' using a transdimensional Bayesian inversion, where the model parameterization is not explicit and regularization is not required. This has produced a far more nuanced P-wave velocity model of D'' than that from traditional S-wave tomography. We also note that P-wave models of D'' vary much more significantly among various research groups than the corresponding S-wave models. This study therefore seeks to develop a new S-wave velocity model of D'' underneath Australia by using predominantly ScS-S differential travel times measured through waveform correlation and Bayesian transdimensional inversion to further understand and characterize heterogeneities in D''. We used events at epicentral distances between 45 and 75 degrees from stations in Australia at depths of over 200 km and with magnitudes between 6.0 and 6.7. Because of globally incomplete coverage of station and earthquake locations, a major limitation of deep earth tomography has been the explicit parameterization of the region of interest. Explicit parameterization has been foundational in most studies, but faces inherent problems of either over-smoothing the data, or allowing for too much noise. To avoid this, we use spherical Voronoi polygons, which allow for a high level of flexibility as the polygons can grow, shrink, or be altogether deleted throughout a sequence of iterations. Our technique also yields highly desired model parameter uncertainties. While there is little doubt that D'' is heterogeneous, there is still much that is

Determination of Mantle Discontinuity Depths beneath the South Pacific Superswell As Inferred Using Data From Broadband OBS Array

Science.gov (United States)

Suetsugu, D.; Shiobara, H.; Sugioka, H.; Kanazawa, T.; Fukao, Y.

2005-12-01

We determined depths of the mantle discontinuities (the 410-km and 660-km discontinuities) beneath the South Pacific Superswell using waveform data from broadband ocean bottom seismograph (BBOBS) array to image presumed mantle plumes and their temperature anomalies. Seismic structure beneath this region had not previously been well explored in spite of its significance for mantle dynamics. The region is characterized by a topographic high of more than 680 m (Adam and Bonneville, 2005), a concentration of hotspot chains (e.g., Society, Cook-Austral, Marquesas, and Pitcairn) whose volcanic rocks have isotopic characteristics suggesting deep mantle origin, and a broad low velocity anomaly in the lower mantle revealed by seismic tomography. These observations suggest the presence of a whole-mantle scale upwelling beneath the region, which is called a 'superplume' (McNutt, 1998). However, the seismic structure has been only poorly resolved so far and the maximum depth of anomalous material beneath the hotspots has not yet been determined, mainly due to the sparseness of seismic stations in the region. To improve the seismic coverage, we deployed an array of 10 BBOBS over the French Polynesia area from 2003 to 2005. The BBOBS has been developed by Earthquake Research Institute of University of Tokyo and are equipped with the broadband CMG-3T/EBB sensor. The observation was conducted as a Japan-France cooperative project (Suetsugu et al., 2005, submitted to EOS). We computed receiver functions from the BBOBS data to detect Ps waves from the mantle discontinuities. The Velocity Spectrum Stacking method (Gurrola et al., 1994) were employed to enhance the Ps waves for determination of the discontinuity depths, in which receiver functions were stacked in a depth-velocity space. The Ps-waves from the mantle discontinuities were successfully detected at the most of the BBOBS stations, from which the discontinuity depths were determined with the Iasp91 velocity model. The 410-km

Upper mantle seismic velocity anomaly beneath southern Taiwan as revealed by teleseismic relative arrival times

Science.gov (United States)

Chen, Po-Fei; Huang, Bor-Shouh; Chiao, Ling-Yun

2011-01-01

Probing the lateral heterogeneity of the upper mantle seismic velocity structure beneath southern and central Taiwan is critical to understanding the local tectonics and orogeny. A linear broadband array that transects southern Taiwan, together with carefully selected teleseismic sources with the right azimuth provides useful constraints. They are capable of differentiating the lateral heterogeneity along the profile with systematic coverage of ray paths. We implement a scheme based on the genetic algorithm to simultaneously determine the relative delayed times of the teleseismic first arrivals of array data. The resulting patterns of the delayed times systematically vary as a function of the incident angle. Ray tracing attributes the observed variations to a high velocity anomaly dipping east in the mantle beneath the southeast of Taiwan. Combining the ray tracing analysis and a pseudo-spectral method to solve the 2-D wave propagations, we determine the extent of the anomaly that best fits the observations via the forward grid search. The east-dipping fast anomaly in the upper mantle beneath the southeast of Taiwan agrees with the results from several previous studies and indicates that the nature of the local ongoing arc-continent collision is likely characterized by the thin-skinned style.

The upper-mantle transition zone beneath the Chile-Argentina flat subduction zone

Science.gov (United States)

Bagdo, Paula; Bonatto, Luciana; Badi, Gabriela; Piromallo, Claudia

2016-04-01

The main objective of the present work is the study of the upper mantle structure of the western margin of South America (between 26°S and 36°S) within an area known as the Chile-Argentina flat subduction zone. For this purpose, we use teleseismic records from temporary broad band seismic stations that resulted from different seismic experiments carried out in South America. This area is characterized by on-going orogenic processes and complex subduction history that have profoundly affected the underlying mantle structure. The detection and characterization of the upper mantle seismic discontinuities are useful to understand subduction processes and the dynamics of mantle convection; this is due to the fact that they mark changes in mantle composition or phase changes in mantle minerals that respond differently to the disturbances caused by mantle convection. The discontinuities at a depth of 410 km and 660 km, generally associated to phase changes in olivine, vary in width and depth as a result of compositional and temperature anomalies. As a consequence, these discontinuities are an essential tool to study the thermal and compositional structure of the mantle. Here, we analyze the upper-mantle transition zone discontinuities at a depth of 410 km and 660 km as seen from Pds seismic phases beneath the Argentina-Chile flat subduction.

Constraining P-wave velocity variations in the upper mantle beneath Southeast Asia

NARCIS (Netherlands)

Li, Chang; Hilst, R.D. van der; Toksöz, M. Nafi

2006-01-01

We have produced a P-wave model of the upper mantle beneath Southeast (SE) Asia from reprocessed short period International Seismological Centre (ISC) P and pP data, short period P data of the Annual Bulletin of Chinese Earthquakes (ABCE), and long period PP-P data.We used 3D sensitivity kernels

Interaction of the Cyprus/Tethys Slab With the Mantle Transition Zone Beneath Anatolia

Science.gov (United States)

Thompson, D. A.; Rost, S.; Taylor, G.; Cornwell, D. G.

2017-12-01

The geodynamics of the eastern Mediterranean are dominated by northward motion of the Arabian/African continents and subduction of the oldest oceanic crust on the planet along the Aegean and Cyprean trenches. These slabs have previously been imaged using seismic tomography on a continental scale, but detailed information regarding their descent from upper to lower mantle and how they interact with the mantle transition zone have been severely lacking. The Dense Array for North Anatolia (DANA) was a 73 station passive seismic deployment active between 2012-2013 with the primary aim of imaging shallow structure beneath the North Anatolian Fault. However, we exploit the exceptional dataset recorded by DANA to characterise a region where the Cyprus Slab impinges upon the mantle transition zone beneath northern Turkey, providing arguably the most detailed view of a slab as it transits from the upper to lower mantle. We map varying depths and amplitudes of the transition zone seismic discontinuities (`410', `520' and `660') in 3D using over 1500 high quality receiver functions over an area of approximately 200km x 300km. The `410' is observed close to its predicted depth, but the `660' is depressed to >670 km across the entirety of the study region. This is consistent with an accumulation of cold subducted material at the base of the upper mantle, and the presence of a `520' discontinuity in the vicinity of the slab surface also suggests that the slab is present deep within the transition zone. Anomalous low velocity layers above and within the transition zone are constrained and may indicate hydration and ongoing mass/fluid flux between upper and lower mantle in the presence of subduction. The results of the study have implications not only for the regional geodynamics of Anatolia, but also for slab dynamics globally.

Deformation in D″ Beneath North America From Anisotropy

Science.gov (United States)

Nowacki, A. J.; Wookey, J.; Kendall, J. M.

2009-12-01

The lowermost few hundred kilometres of the Earth's mantle—known as D″—form the boundary between it and the core below, control the Earth's convective system, and are the site of probable large thermochemical heterogeneity. Seismic observations of D″ show a strong heterogeneity in seismic wave velocity and significant seismic anisotropy (the variation of wave speed with direction) are present in many parts of the region. On the basis of continuous regions of fast shear velocity (VS) anomalies in global models, it is also proposed as the resting place of subducted slabs, notably the Farallon beneath North America. A phase change of MgSiO3-perovskite (pv) to a post-perovskite (ppv) structure at near-core-mantle boundary (CMB) conditions is a compelling mechanism to explain the seismic features of D″. An outstanding question is how this and other mineral phases may deform to produce anisotropy, with different mechanisms possible. With knowledge either of mantle flow or which slip system is responsible for causing deformation, we can potentially determine the other with observations of the resulting seismic anisotropy. We investigate the dynamics at the CMB beneath North America using differential shear wave splitting in S and ScS phases from earthquakes of magnitude MW>5.5 in South and Central America, Hawaii the Mid-Atlantic Ridge and East Pacific Rise. They are detected on ~500 stations in North America, giving ~700 measurements of anisotropy in D″. We achieve this by correcting for anisotropy in the upper mantle (UM) beneath both the source and receiver. The measurements cover three regions beneath western USA, the Yucatan peninsula and Florida. In each case, two different, crossing ray paths are used, so that the style of anisotropy can be constrained—a single azimuth cannot distinguish differing cases. Our results showing ~1% anisotropy dependent on azimuth are not consistent with transverse isotropy with a vertical symmetry axis (VTI) anywhere. The

Large-scale global convection in the mantle beneath Australia from 55 Ma to now

International Nuclear Information System (INIS)

Zhang, M.

1999-01-01

Full text: The global-scale mantle convection cells in the asthenosphere are not geochemically homogeneous. The heterogeneity is most prominently reflected in the isotopic compositions (Pb-Sr-Nd) of the mid-ocean ridge basalts (MORB) that are direct partial melts from the underlying asthenosphere. Of particular relevance to Australia's geodynamic evolution from about 100 million years, are the distinctive geochemical signatures of the asthenosphere beneath the Pacific Ocean (Pacific MORB) and Indian Ocean (Indian MORB). Therefore, delineation of the boundary between the two distinct mantle reservoirs and any change in that boundary with time provide information about the patterns of global-scale asthenospheric mantle convection. This information has also allowed us to track large-scale mantle chemical reservoirs such as the distinctive Gondwana lithospheric mantle, and hence better understand the geodynamic evolution of the Australian continent from the time of Gondwana dispersal. Pb-Sr-Nd isotope data for Cenozoic basalts in eastern Australia (Zhang et al, 1999) indicate that Pacific-MORB type isotopic signatures characterise the lava-field basalts (55-14 Ma) in southeastern Australia, whereas Indian-MORB type isotopic signatures characterise younger basalts (6-0 Ma) from northeastern Australia. This discovery helps to constrain the changing locus of the major asthenospheric mantle convection cells represented by the Pacific and Indian MORB sources during and following the breakup of the eastern part of Gondwana, and locates, for the first time, the boundary of these convection cells beneath the Australian continent. This extends previous work in the SW Pacific back-arc basins (eg Hickey-Vargas et al., 1995) and the Southern Ocean (Lanyon et al., 1995) that indicates that the 1- and P-MORB mantle convection cells have been moving in opposite directions since the early Tertiary. These new data also indicate that the Indian-MORB source is a long-term asthenospheric

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.

Eutectic melting temperature of the lowermost Earth's mantle

Science.gov (United States)

Andrault, D.; Lo Nigro, G.; Bolfan-Casanova, N.; Bouhifd, M.; Garbarino, G.; Mezouar, M.

2009-12-01

Partial melting of the Earth's deep mantle probably occurred at different stages of its formation as a consequence of meteoritic impacts and seismology suggests that it even continues today at the core-mantle boundary. Melts are important because they dominate the chemical evolution of the different Earth's reservoirs and more generally the dynamics of the whole planet. Unfortunately, the most critical parameter, that is the temperature profile inside the deep Earth, remains poorly constrained accross the planet history. Experimental investigations of the melting properties of materials representative of the deep Earth at relevant P-T conditions can provide anchor points to refine past and present temperature profiles and consequently determine the degree of melting at the different geological periods. Previous works report melting relations in the uppermost lower mantle region, using the multi-anvil press [1,2]. On the other hand, the pyrolite solidus was determined up to 65 GPa using optical observations in the laser-heated diamond anvil cell (LH-DAC) [3]. Finally, the melting temperature of (Mg,Fe)2SiO4 olivine is documented at core-mantle boundary (CMB) conditions by shock wave experiments [4]. Solely based on these reports, experimental data remain too sparse to draw a definite melting curve for the lower mantle in the relevant 25-135 GPa pressure range. We reinvestigated melting properties of lower mantle materials by means of in-situ angle dispersive X-ray diffraction measurements in the LH-DAC at the ESRF [5]. Experiments were performed in an extended P-T range for two starting materials: forsterite and a glass with chondrite composition. In both cases, the aim was to determine the onset of melting, and thus the eutectic melting temperatures as a function of pressure. Melting was evidenced from drastic changes of diffraction peak shape on the image plate, major changes in diffraction intensities in the integrated pattern, disappearance of diffraction rings

The structure of the crust and uppermost mantle beneath Madagascar

Science.gov (United States)

Andriampenomanana, Fenitra; Nyblade, Andrew A.; Wysession, Michael E.; Durrheim, Raymond J.; Tilmann, Frederik; Julià, Jordi; Pratt, Martin J.; Rambolamanana, Gérard; Aleqabi, Ghassan; Shore, Patrick J.; Rakotondraibe, Tsiriandrimanana

2017-09-01

The lithosphere of Madagascar was initially amalgamated during the Pan-African events in the Neoproterozoic. It has subsequently been reshaped by extensional processes associated with the separation from Africa and India in the Jurassic and Cretaceous, respectively, and been subjected to several magmatic events in the late Cretaceous and the Cenozoic. In this study, the crust and uppermost mantle have been investigated to gain insights into the present-day structure and tectonic evolution of Madagascar. We analysed receiver functions, computed from data recorded on 37 broad-band seismic stations, using the H-κ stacking method and a joint inversion with Rayleigh-wave phase-velocity measurements. The thickness of the Malagasy crust ranges between 18 and 46 km. It is generally thick beneath the spine of mountains in the centre part (up to 46 km thick) and decreases in thickness towards the edges of the island. The shallowest Moho is found beneath the western sedimentary basins (18 km thick), which formed during both the Permo-Triassic Karro rifting in Gondwana and the Jurassic rifting of Madagascar from eastern Africa. The crust below the sedimentary basin thickens towards the north and east, reflecting the progressive development of the basins. In contrast, in the east there was no major rifting episode. Instead, the slight thinning of the crust along the east coast (31-36 km thick) may have been caused by crustal uplift and erosion when Madagascar moved over the Marion hotspot and India broke away from it. The parameters describing the crustal structure of Archean and Proterozoic terranes, including average thickness (40 km versus 35 km), Poisson's ratio (0.25 versus 0.26), average shear-wave velocity (both 3.7 km s-1), and thickness of mafic lower crust (7 km versus 4 km), show weak evidence of secular variation. The uppermost mantle beneath Madagascar is generally characterized by shear-wave velocities typical of stable lithosphere (∼4.5 km s-1). However

New constraints on the textural and geochemical evolution of the upper mantle beneath the Styrian basin

Science.gov (United States)

Aradi, Laszlo; Hidas, Károly; Zanetti, Alberto; János Kovács, István; Patkó, Levente; Szabó, Csaba

2016-04-01

Plio-Pleistocene alkali basaltic volcanism sampled sporadically the upper mantle beneath the Carpathian-Pannonian Region (CPR, e.g. [1]). Lavas and pyroclasts often contain mantle derived xenoliths, and the majority of them have been extensively studied [1], except the westernmost Styrian Basin Volcanic Field (SBVF, Eastern Austria and Slovenia). In the SBVF only a few volcanic centers have been studied in details (e.g. Kapfenstein & Tobaj). Based on these studies, the upper mantle beneath the SBVF is consists of dominantly high temperature, texturally and geochemically homogeneous protogranular spinel lherzolite. New major and trace element data from rock-forming minerals of ultramafic xenoliths, coupled with texture and deformation analysis from 12 volcanic outcrops across the SBVF, suggest that the lithospheric roots of the region are more heterogeneous than described previously. The studied xenoliths are predominantly lherzolite, amphibole is a common phase that replaces pyroxenes and spinels and proves modal metasomatism. Phlogopite coupled with apatite is also present in amphibole-rich samples. The texture of the xenoliths is usually coarse-grained and annealed with low abundance of subgrain boundaries in both olivine and pyroxenes. Olivine crystal preferred orientation (CPO) varies between the three most abundant one: [010]-fiber, orthogonal and [100]-fiber symmetry [2]. The CPO of pyroxenes is usually coherent with coeval deformation with olivine, however the CPO of amphibole is suggesting postkinematic epitaxial overgrowth on the precursor pyroxenes. According to equilibrium temperatures, the studied xenolith suite samples a broader temperature range (850-1100 °C) than the literature data, corresponding to mantle depths between 30 and 60 km, which indicates that the xenolith suite only represents the shallower part of the recent 100 km thick lithospheric mantle beneath the SBVF. The equilibrium temperatures show correlation with the varying CPO symmetries

Mapping the mantle transition zone beneath the central Mid-Atlantic Ridge using Ps receiver functions.

Science.gov (United States)

Agius, M. R.; Rychert, C.; Harmon, N.; Kendall, J. M.

2017-12-01

Determining the mechanisms taking place beneath ridges is important in order to understand how tectonic plates form and interact. Of particular interest is establishing the depth at which these processes originate. Anomalies such as higher temperature within the mantle transition zone may be inferred seismically if present. However, most ridges are found in remote locations beneath the oceans restricting seismologists to use far away land-based seismometers, which in turn limits the imaging resolution. In 2016, 39 broadband ocean-bottom seismometers were deployed across the Mid-Atlantic Ridge, along the Romanche and Chain fracture zones as part of the PI-LAB research project (Passive Imaging of the Lithosphere and Asthenosphere Boundary). The one-year long seismic data is now retrieved and analysed to image the mantle transition zone beneath the ridge. We determine P-to-s (Ps) receiver functions to illuminate the 410- and 660-km depth mantle discontinuities using the extended multitaper deconvolution. The data from ocean-bottom seismometers have tilt and compliance noise corrections and is filtered between 0.05-0.2 Hz to enhance the signal. 51 teleseismic earthquakes generated hundreds of good quality waveforms, which are then migrated to depth in 3-D. The topography at the d410 deepens towards the west of the Romanche and Chain fracture zone by 15 km, whereas the topography of d660 shallows beneath the ridge between the two zones. Transition zone thickness thins from 5 to 20 km. Thermal anomalies determined from temperature relationships with transition zone thickness and depth variations of the d410 and d660 suggests hotter temperatures of about 200 K. Overall, the result suggests mid-ocean ridges may have associated thermal signatures as deep as the transition zone.

Constraining spatial variations in P-wave velocity in the upper mantle beneath SE Asia

NARCIS (Netherlands)

Li, C.; Hilst, R.D. van der; Toksoz, N.M.

2006-01-01

We have produced a P-wave model of the upper mantle beneath Southeast (SE) Asia from reprocessed short period International Seismological Centre (ISC) P and pP data, short period P data of the Annual Bulletin of Chinese Earthquakes (ABCE), and long period PP-P data.We used 3D sensitivity kernels

Ps mantle transition zone imaging beneath the Colorado Rocky Mountains: Evidence for an upwelling hydrous mantle

Science.gov (United States)

Zhang, Zhu; Dueker, Kenneth G.; Huang, Hsin-Hua

2018-06-01

We analyze teleseismic P-to-S conversions for high-resolution imaging of the mantle transition zone beneath the Colorado Rocky Mountains using data from a dense PASSCAL seismic broadband deployment. A total of 6,021 P-to-S converted receiver functions are constructed using a multi-channel minimum-phase deconvolution method and migrated using the common converted point technique with the 3-D teleseismic P- and S-wave tomography models of Schmandt and Humphreys (2010). The image finds that the average depths of the 410-km discontinuity (the 410) and 660-km discontinuity (the 660) at 408 ± 1.9 km and 649 ± 1.6 km respectively. The peak-to-peak topography of both discontinuities is 33 km and 27 km respectively. Additionally, prominent negative polarity phases are imaged both above and below the 410. To quantify the mean properties of the low-velocity layers about 410 km, we utilize double gradient layer models parameterization to fit the mean receiver function waveform. This waveform fitting is accomplished as a grid-search using anelastic synthetic seismograms. The best-fitting model reveals that the olivine-wadsleyite phase transformation width is 21 km, which is significantly larger than anhydrous mineral physics prediction (4-10 km) (Smyth and Frost, 2002). The findings of a wide olivine-wadsleyite phase transformation and the negative polarity phases above and below the 410, suggest that the mantle, at least in the 350-450 km depth range, is significantly hydrated. Furthermore, a conspicuous negative polarity phase below the 660 is imaged in high velocity region, we speculate the low velocity layer is due to dehydration flux melting in an area of convective downwelling. Our interpretation of these results, in tandem with the tomographic image of a Farallon slab segment at 800 km beneath the region (Schmandt and Humphreys, 2010), is that hydrous and upwelling mantle contributes to the high-standing Colorado Rocky Mountains.

Local Upper Mantle Upwelling beneath New England: Evidence from Seismic Anisotropy.

Science.gov (United States)

Levin, V. L.; Long, M. D.; Lopez, I.; Li, Y.; Skryzalin, P. A.

2017-12-01

The upper mantle beneath eastern North America contains regions where seismic wave speed is significantly reduced. As they cut across the trend of the Appalachian terranes, these anomalies likely post-date the Paleozoic assembly of Pangea. Most prominent of them, the North Appalachian Anomaly (NAA), has been alternatively explained by the localized disruption of lithospheric fabric, the passage of the Great Meteor Hot Spot, and the current local upwelling of the asthenosphere. Comprehensive mapping of shear wave splitting identified a local perturbation of an otherwise uniform regional pattern, with no apparent splitting occurring at a site within the NAA. To evaluate the reality of this apparent localized disruption in the anisotropic fabric of the upper mantle beneath northeastern North America we used observations of shear wave splitting from a set of long-running observatories not included in previous studies. Three methods of evaluating shear wave splitting (rotation-correlation, minimization of the transverse component, and the splitting intensity) yield complementary results. We show that splitting of core-refracted shear waves within the outline of the NAA is significantly weaker than towards its edges and beyond them (Figure 1). Average fast orientations are close to the absolute plate motion in the hot-spot reference frame, thus we can attribute a large fraction of this signal to the coherently sheared sub-lithospheric upper mantle. A decrease in average delay we observe, from 1 s outside the NAA to under 0.2 s within it, translates into a reduction of the vertical extent of the sheared layer from 130 km to 16 km (assuming 4% anisotropy), or alternatively into a weakening of the azimuthal anisotropy from 5% to 0.6% (assuming a 100 km thick layer). The splitting reduction within the NAA is consistent with a localized change in anisotropic fabric that would be expected in case of geologically recent sub-vertical flow overprinting the broadly uniform upper

Broad plumes rooted at the base of the Earth's mantle beneath major hotspots.

Science.gov (United States)

French, Scott W; Romanowicz, Barbara

2015-09-03

Plumes of hot upwelling rock rooted in the deep mantle have been proposed as a possible origin of hotspot volcanoes, but this idea is the subject of vigorous debate. On the basis of geodynamic computations, plumes of purely thermal origin should comprise thin tails, only several hundred kilometres wide, and be difficult to detect using standard seismic tomography techniques. Here we describe the use of a whole-mantle seismic imaging technique--combining accurate wavefield computations with information contained in whole seismic waveforms--that reveals the presence of broad (not thin), quasi-vertical conduits beneath many prominent hotspots. These conduits extend from the core-mantle boundary to about 1,000 kilometres below Earth's surface, where some are deflected horizontally, as though entrained into more vigorous upper-mantle circulation. At the base of the mantle, these conduits are rooted in patches of greatly reduced shear velocity that, in the case of Hawaii, Iceland and Samoa, correspond to the locations of known large ultralow-velocity zones. This correspondence clearly establishes a continuous connection between such zones and mantle plumes. We also show that the imaged conduits are robustly broader than classical thermal plume tails, suggesting that they are long-lived, and may have a thermochemical origin. Their vertical orientation suggests very sluggish background circulation below depths of 1,000 kilometres. Our results should provide constraints on studies of viscosity layering of Earth's mantle and guide further research into thermochemical convection.

Mantle transition zone beneath northeast China from P-receiver function

Science.gov (United States)

Zhang, R.; Wu, Q.

2015-12-01

We used receiver functions to examine lateral topographical variations on the 410- and 660-km beneath northeast China and particularly the Kuril-Japan arc junctions. Compared to other receiver functions studies, our analysis was based on greater station coverage of higher density by combining all recent seismic arrays so far deployed in northeast China. Our image shows that the 410-km is featured by a ~10-20 km uplift extending in the NNE direction beneath some areas of the Quaternary basaltic rocks distributed at Abaga and at Wudalianchi. The Clapeyron slope of the olivine phase transiton at 410-km suggests that the uplift is compatible with a negative thermal anomaly. We also confirm a significant depression of the 660 from the Changbai volcanism in the north to Korea in the south along the NW-SE direction. The depression is also accompanied by an uplift of the 660 to the west. The shallow 660-km discontinuity is also particularly detected beneath the Kuril-Japan arc junctions, while it was not detected before. The thermal anomaly at 410 km depth is most likely a remnant of a detached mantle lithosphere that recently sank to depth, thus providing robust evidence for the source and evolution of these basalts. The depression of the 660-km discontinuity may support that the subducting Pacific slab bends sharply and becomes stagnant when it meets strong resistance at a depth of about 670 km. After accumulation to a great extent the stagnant slab finally penetrates into the lower mantle. Combined with the previous triplicated studies, the shallow 660-km may suggest that descending Pacific slab at its leading and junction edges might be accommodated by a tearing near a depth of 660 km. Acknowledgements. Two liner seismic arrays were deployed by the Institute of Geophysics, China Earthquake Administration. The data of the permanent stations were provided by the Data Management Centre of China, National Seismic Network at the Institute of Geophysics, China Earthquake

Complex Anisotropic Structure of the Mantle Wedge Beneath Kamchatka Volcanoes

Science.gov (United States)

Levin, V.; Park, J.; Gordeev, E.; Droznin, D.

2002-12-01

the slab. To explain the vertical stratification of anisotropy implied from receiver functions, and the strong lateral dependence of shear-wave splitting observations, we cannot rely on simple models of mantle wedge behaviour e.g., olivine-crystal alignment through subduction-driven corner flow. Diverse mechanisms can contribute to the observed pattern of anisotropic properties, with volatiles likely being a key influence. For instance, we find evidence in favor of a slow-symmetry-axis anisotropy within the uppermost 10-20 km of the mantle wedge, implying either excessive hydration of the mantle or else a presence of systematically aligned volatile-filled cracks or lenses. Also, shear-wave splitting is weak beneath the Avachinsky-Koryaksky volcanic center, suggesting either vertical flow or the influence of volatiles and/or thermally-enhanced diffusion creep.

Heterogeneous distribution of water in the mantle transition zone beneath United States inferred from seismic observations

Science.gov (United States)

Wang, Y.; Pavlis, G. L.; Li, M.

2017-12-01

The amount of water in the Earth's deep mantle is critical for the evolution of the solid Earth and the atmosphere. Mineral physics studies have revealed that Wadsleyite and Ringwoodite in the mantle transition zone could store several times the volume of water in the ocean. However, the water content and its distribution in the transition zone remain enigmatic due to lack of direct observations. Here we use seismic data from the full deployment of the Earthscope Transportable Array to produce 3D image of P to S scattering of the mantle transition zone beneath the United States. We compute the image volume from 141,080 pairs of high quality receiver functions defined by the Earthscope Automated Receiver Survey, reprocessed by the generalized iterative deconvolution method and imaged by the plane wave migration method. We find that the transition zone is filled with previously unrecognized small-scale heterogeneities that produce pervasive, negative polarity P to S conversions. Seismic synthetic modeling using a point source simulation method suggests two possible structures for these objects: 1) a set of randomly distributed blobs of slight difference in size, and 2) near vertical diapir structures from small scale convections. Combining with geodynamic simulations, we interpret the observation as compositional heterogeneity from small-scale, low-velocity bodies that are water enriched. Our results indicate there is a heterogeneous distribution of water through the entire mantle transition zone beneath the contiguous United States.

Comparative Study on the Electrical Properties of the Oceanic Mantle Beneath the Northwest Pacific Ocean

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Toh, H.

2013-12-01

We have been conducting long-term seafloor electromagnetic (EM) observations at two sites in the northwest Pacific since 2001. The older site was established at the deep seafloor (~5600m) on the northwest Pacific basin (Site NWP), while the new one was installed on the west Philippine basin (Site WPB) in 2006 at the slightly deeper (~5700m) seafloor. The ages of the oceanic basins at those sites are approximately 129 Ma for Site NWP (Shipboard Scientific Party of ODP Leg 191, 2000) and 49 Ma for Site WPB (Salisbury et al., 2006), respectively. The EM instruments deployed at those sites are seafloor EM stations (SFEMS; Toh et al., 2004 and 2006) and capable of measuring vector EM fields at the seafloor for as long as one year or more with other physical quantities such as the instruments' attitude, orientation and temperature. One of the objectives of the seafloor long-term EM observations by SFEMSs is to make a comparative study of the oceanic mantle with and without influence of the so-called 'stagnant slabs' in terms of their electrical conductivity. It is anticipated that the mantle transition zone under the influence of the stagnant slab has a higher electrical conductivity because the transition zone there could be wetter than that in the absence of the stagnant slab. In this context, the mantle transition zone beneath Site WPB can be said to have influence by the stagnant slab, while that beneath Site NWP does not. It, therefore, is basically possible to estimate how much water is present in each transition zone by comparison of the electrical conductivity profiles of the two. The one-dimensional electrical profile beneath Site NWP has been derived so far using the magnetotelluric (MT) and geomagnetic depth sounding (GDS) methods with significant jumps in the electrical property at 410 and 660km discontinuities. The jumps are approximately factors of 10 and 2, respectively (Ichiki et al., 2009). Here we show a profile beneath Site WPB using both MT and GDS

Receiver Function Imaging of Mantle Transition Zone Discontinuities Beneath Alaska

Science.gov (United States)

Dahm, Haider Hassan Faraj

Subduction of tectonic plates is one of the most important tectonic processes, yet many aspects of subduction zone geodynamics remain unsolved and poorly understood, such as the depth extent of the subducted slab and its geometry. The Alaska subduction zone, which is associated with the subduction of the Pacific Plate beneath the North America plate, has a complex tectonic setting and carries a series of subduction episodes, and represents an excellent target to study such plate tectonic processes. Previous seismological studies in Alaska have proposed different depth estimations and geometry for the subducted slab. The Mantle transition zone discontinuities of the 410km and the 660 km provide independent constraints on the depth extent of the subducted slabs. We conducted a receiver function study to map the topography of the 410 km and the 660 km discontinuities beneath Alaska and its adjacent areas by taking advantage of the teleseismic data from the new USArray deployment in Alaska and northwestern Canada. Stacking over 75,000 high-quality radial receiver functions recorded in Alaska with more than 40 years of recording period, the topographies of the 410 km and 660 km are mapped. The depths of both d410 and d660 show systematic spatial variations, the mean depth of d410 and d660 are within 6 km and 6 km from the global average, respectively. The mean MTZ thickness of the entire study area is within -2 km from the global average of 250 km, suggesting normal MTZ conditions on average. Central and south-central Alaska are characterized by a larger than normal MTZ thickness, suggesting that the subducting Pacific slab is thermally interacted with the MTZ. This study shows that lateral upper mantle velocity variations contribute the bulk of the observed apparent undulations of the MTZ discontinuities.

Seismic scatterers in the mid-lower mantle beneath Tonga-Fiji

Science.gov (United States)

Kaneshima, Satoshi

2018-01-01

We analyze deep and intermediate-depth earthquakes at the Tonga-Fiji region in order to reveal the distribution of scattering objects in the mid-lower mantle. By array processing waveform data recorded at regional seismograph stations in the US, Alaska, and Japan, we investigate S-to-P scattering waves in the P coda, which arise from kilometer-scale chemically distinct objects in the mid-lower mantle beneath Tonga-Fiji. With ten scatterers previously reported by the author included, twenty-three mid-lower mantle scatterers have been detected below 900 km depth, while scatterers deeper than 1900 km have not been identified. Strong mid-lower mantle S-to-P scattering most frequently occurs at the scatterers located within a depth range between 1400 km and 1600 km. The number of scatterers decreases below 1600 km depth, and the deeper objects tend to be weaker. The scatterer distribution may reflect diminishing elastic anomalies of basaltic rocks with depth relative to the surrounding mantle rocks, which mineral physics has predicted to occur. The predominant occurrence of strong S-to-P scattering waves within a narrow depth range may reflect significant reduction of rigidity due to the ferro-elastic transformation of stishovite in basaltic rocks. Very large signals associated with mid-mantle scatterers are observed only for a small portion of the entire earthquake-array pairs. Such infrequent observations of large scattering signals, combined with quite large event-to-event differences in the scattering intensity for each scatterer, suggest both that the strong arrivals approximately represent ray theoretical S-to-P converted waves at objects with a plane geometry. The plane portions of the strong scatterers may often dip steeply, with the size exceeding 100 km. For a few strong scatterers, the range of receivers showing clear scattered waves varies substantially from earthquake-array pair to pair. Some of the scatterers are also observed at different arrays that have

Upper mantle velocity structure beneath Italy from direct and secondary P-wave teleseismic tomography

Directory of Open Access Journals (Sweden)

P. De Gori

1997-06-01

Full Text Available High-quality teleseismic data digitally recorded by the National Seismic Network during 1988-1995 have been analysed to tomographically reconstruct the aspherical velocity structure of the upper mantle beneath the Italian region. To improve the quality and the reliability of the tomographic images, both direct (P, PKPdf and secondary (pP,sP,PcP,PP,PKPbc,PKPab travel-time data were used in the inversion. Over 7000 relative residuals were computed with respect to the IASP91 Earth velocity model and inverted using a modified version of the ACH technique. Incorporation of data of secondary phases resulted in a significant improvement of the sampling of the target volume and of the spatial resolution of the heterogeneous zones. The tomographic images show that most of the lateral variations in the velocity field are confined in the first ~250 km of depth. Strong low velocity anomalies are found beneath the Po plain, Tuscany and Eastern Sicily in the depth range between 35 and 85 km. High velocity anomalies dominate the upper mantle beneath the Central-Western Alps, Northern-Central Apennines and Southern Tyrrhenian sea at lithospheric depths between 85 and 150 km. At greater depth, positive anomalies are still observed below the northernmost part of the Apenninic chain and Southern Tyrrhenian sea. Deeper anomalies present in the 3D velocity model computed by inverting only the first arrivals dataset, generally appear less pronounced in the new tomographic reconstructions. We interpret this as the result of the ray sampling improvement on the reduction of the vertical smearing effects.

S-P wave travel time residuals and lateral inhomogeneity in the mantle beneath Tibet and the Himalaya

Science.gov (United States)

Molnar, P.; Chen, W.-P.

1984-01-01

S-P wave travel time residuals were measured in earthquakes in Tibet and the Himalaya in order to study lateral inhomogeneities in the earth's mantle. Average S-P residuals, measured with respect to Jeffrey-Bullen (J-B) tables for 11 earthquakes in the Himalaya are less than +1 second. Average J-B S-P from 10 of 11 earthquakes in Tibet, however, are greater than +1 second even when corrected for local crustal thickness. The largest values, ranging between 2.5 and 4.9 seconds are for five events in central and northern Tibet, and they imply that the average velocities in the crust and upper mantle in this part of Tibet are 4 to 10 percent lower than those beneath the Himalaya. On the basis of the data, it is concluded that it is unlikely that a shield structure lies beneath north central Tibet unless the S-P residuals are due to structural variations occurring deeper than 250 km.

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.

Scattering beneath Western Pacific subduction zones: evidence for oceanic crust in the mid-mantle

Science.gov (United States)

Bentham, H. L. M.; Rost, S.

2014-06-01

Small-scale heterogeneities in the mantle can give important insight into the dynamics and composition of the Earth's interior. Here, we analyse seismic energy found as precursors to PP, which is scattered off small-scale heterogeneities related to subduction zones in the upper and mid-mantle. We use data from shallow earthquakes (less than 100 km depth) in the epicentral distance range of 90°-110° and use array methods to study a 100 s window prior to the PP arrival. Our analysis focuses on energy arriving off the great circle path between source and receiver. We select coherent arrivals automatically, based on a semblance weighted beampower spectrum, maximizing the selection of weak amplitude arrivals. Assuming single P-to-P scattering and using the directivity information from array processing, we locate the scattering origin by ray tracing through a 1-D velocity model. Using data from the small-aperture Eielson Array (ILAR) in Alaska, we are able to image structure related to heterogeneities in western Pacific subduction zones. We find evidence for ˜300 small-scale heterogeneities in the region around the present-day Japan, Izu-Bonin, Mariana and West Philippine subduction zones. Most of the detected heterogeneities are located in the crust and upper mantle, but 6 per cent of scatterers are located deeper than 600 km. Scatterers in the transition zone correlate well with edges of fast features in tomographic images and subducted slab contours derived from slab seismicity. We locate deeper scatterers beneath the Izu-Bonin/Mariana subduction zones, which outline a steeply dipping pseudo-planar feature to 1480 km depth, and beneath the ancient (84-144 Ma) Indonesian subduction trench down to 1880 km depth. We image the remnants of subducted crustal material, likely the underside reflection of the subducted Moho. The presence of deep scatterers related to past and present subduction provides evidence that the subducted crust does descend into the lower mantle at

Understanding seismic heterogeneities in the lower mantle beneath the Americas from seismic tomography and plate tectonic history

NARCIS (Netherlands)

Ren, Y.; Stutzmann, E.; Hilst, R.D. van der; Besse, J.

2007-01-01

We combine results from seismic tomography and plate motion history to investigate slabs of subducted lithosphere in the lower mantle beneath the Americas. Using broadband waveform cross correlation, we measured 37,000 differential P and S traveltimes, 2000 PcP-P and ScS-S times along a wide

Shear velocity structure of the laterally heterogeneous crust and uppermost mantle beneath the Indian region

Science.gov (United States)

Mohan, G.; Rai, S. S.; Panza, G. F.

1997-08-01

The shear velocity structure of the Indian lithosphere is mapped by inverting regionalized Rayleigh wave group velocities in time periods of 15-60 s. The regionalized maps are used to subdivide the Indian plate into several geologic units and determine the variation of velocity with depth in each unit. The Hedgehog Monte Carlo technique is used to obtain the shear wave velocity structure for each geologic unit, revealing distinct velocity variations in the lower crust and uppermost mantle. The Indian shield has a high-velocity (4.4-4.6 km/s) upper mantle which, however, is slower than other shields in the world. The central Indian platform comprised of Proterozoic basins and cratons is marked by a distinct low-velocity (4.0-4.2 km/s) upper mantle. Lower crustal velocities in the Indian lithosphere generally range between 3.8 and 4.0 km/s with the oceanic segments and the sedimentary basins marked by marginally higher and lower velocities, respectively. A remarkable contrast is observed in upper mantle velocities between the northern and eastern convergence fronts of the Indian plate. The South Bruma region along the eastern subduction front of the Indian oceanic lithosphere shows significant velocity enhancement in the lower crust and upper mantle. High velocities (≈4.8 km/s) are also observed in the upper mantle beneath the Ninetyeast ridge in the northeastern Indian Ocean.

Seismic anisotropy in the upper mantle beneath the MAGIC array, mid-Atlantic Appalachians: Constraints from SKS splitting and quasi-Love wave propagation

Science.gov (United States)

Aragon, J. C.; Long, M. D.; Benoit, M. H.; Servali, A.

2016-12-01

North America's eastern passive continental margin has been modified by several cycles of supercontinent assembly. Its complex surface geology and distinct topography provide evidence of these events, while also raising questions about the extent of deformation in the continental crust, lithosphere, and mantle during past episodes of rifting and mountain building. The Mid-Atlantic Geophysical Integrative Collaboration (MAGIC) is an EarthScope and GeoPRISMS-funded project that involves a collaborative effort among seismologists, geodynamicists, and geomorphologists. One component of the project is a broadband seismic array consisting of 28 instruments in a linear path from coastal Virginia to western Ohio, which operated between October 2013 and October 2016. A key science question addressed by the MAGIC project is the geometry of past lithospheric deformation and present-day mantle flow beneath the Appalachians, which can be probed using observations of seismic anisotropy Here we present observations of SKS splitting and quasi-Love wave arrivals from stations of the MAGIC array, which together constrain seismic anisotropy in the upper mantle. SKS splitting along the array reveals distinct regions of upper mantle anisotropy, with stations in and to the west of the range exhibiting fast directions parallel to the strike of the mountains. In contrast, weak splitting and null SKS arrivals dominate eastern stations in the coastal plain. Documented Love-to-Rayleigh wave scattering for surface waves originating the magnitude 8.3 Illapel, Chile earthquakes in September 2015 provides complementary constraints on anisotropy. These quasi-Love wave arrivals suggest a pronounced change in upper mantle anisotropy at the eastern edge of present-day Appalachian topography. Together, these observations increase our understanding of the extent of lithospheric deformation beneath North America associated with Appalachian orogenesis, as well as the pattern of present-day mantle flow

Topography of upper mantle seismic discontinuities beneath the North Atlantic: the Azores, Canary and Cape Verde plumes

Science.gov (United States)

Saki, Morvarid; Thomas, Christine; Nippress, Stuart E. J.; Lessing, Stephan

2015-04-01

We are mapping the topography of upper mantle seismic discontinuities beneath the North Atlantic and surrounding regions by using precursor arrivals to PP and SS seismic waves that reflect off the seismic discontinuities. Many source-receiver combinations have been used in order to collect a large dataset of reflection points beneath our investigating area. We analyzed over 1700 seismograms from MW>5.8 events using array seismic methods to enhance the signal to noise ratio. The measured time lag between PP (SS) arrivals and their corresponding precursors on robust stacks are used to measure the depth of the transition zone boundaries. The reflectors' depths show a correlation between the location of hotspots and a significantly depressed 410 km discontinuity indicating a temperature increase of 200-300 K compared to the surrounding mantle. For the 660 km discontinuity three distinct behaviours are visible: i) normal depths beneath Greenland and at a distance of a few hundred kilometres away from the hotspots and ii) shallower 660 km discontinuity compared with the global average value near hotspots closer to the Mid-Atlantic Ridge and iii) very few observations of a 660 km discontinuity at the hotspot locations. We interpret our observations as a large upwelling beneath the southern parts of our study region, possibly due to the South Atlantic convection cell. The thermal anomaly may be blocked by endothermic phase transformation and likely does not extend through the top of the transition zone as whole except for those branches which appear as the Azores, Canaries and Cape Verde hotspots at the surface.

Seismological evidence for a sub-volcanic arc mantle wedge beneath the Denali volcanic gap, Alaska

Science.gov (United States)

McNamara, D.E.; Pasyanos, M.E.

2002-01-01

Arc volcanism in Alaska is strongly correlated with the 100 km depth contour of the western Aluetian Wadati-Benioff zone. Above the eastern portion of the Wadati-Benioff zone however, there is a distinct lack of volcanism (the Denali volcanic gap). We observe high Poisson's ratio values (0.29-0.33) over the entire length of the Alaskan subduction zone mantle wedge based on regional variations of Pn and Sn velocities. High Poisson's ratios at this depth (40-70 km), adjacent to the subducting slab, are attributed to melting of mantle-wedge peridotites, caused by fluids liberated from the subducting oceanic crust and sediments. Observations of high values of Poisson's ratio, beneath the Denali volcanic gap suggest that the mantle wedge contains melted material that is unable to reach the surface. We suggest that its inability to migrate through the overlying crust is due to increased compression in the crust at the northern apex of the curved Denali fault.

Ultra-low velocity zone heterogeneities at the core-mantle boundary from diffracted PKKPab waves

Science.gov (United States)

Ma, Xiaolong; Sun, Xinlei

2017-08-01

Diffracted waves around Earth's core could provide important information of the lowermost mantle that other seismic waves may not. We examined PKKPab diffraction waves from 52 earthquakes occurring at the western Pacific region and recorded by USArray to probe the velocity structure along the core-mantle boundary (CMB). These diffracted waves emerge at distances up to 10° past the theoretical cutoff epicentral distance and show comparable amplitudes. We measured the ray parameters of PKKPab diffraction waves by Radon transform analysis that is suitable for large-aperture arrays. These ray parameters show a wide range of values from 4.250 to 4.840 s/deg, suggesting strong lateral heterogeneities in sampling regions at the base of the mantle. We further estimated the P-wave velocity variations by converting these ray parameters and found the CMB regions beneath the northwestern edge of African Anomaly (Ritsma et al. in Science 286:1925-1928, 1999) and southern Sumatra Islands exhibit velocity reductions up to 8.5% relative to PREM. We suggest that these low velocity regions are Ultra-low velocity zones, which may be related to partial melt or iron-enriched solids.[Figure not available: see fulltext.

Anisotropic structure of the mantle wedge beneath the Ryukyu arc from teleseismic receiver function analysis

Science.gov (United States)

McCormack, K. A.; Wirth, E. A.; Long, M. D.

2011-12-01

The recycling of oceanic plates back into the mantle through subduction is an important process taking place within our planet. However, many fundamental aspects of subduction systems, such as the dynamics of mantle flow, have yet to be completely understood. Subducting slabs transport water down into the mantle, but how and where that water is released, as well as how it affects mantle flow, is still an open question. In this study, we focus on the Ryukyu subduction zone in southwestern Japan and use anisotropic receiver function analysis to characterize the structure of the mantle wedge. We compute radial and transverse P-to-S receiver functions for eight stations of the broadband F-net array using a multitaper receiver function estimator. We observe coherent P-to-SV converted energy in the radial receiver functions at ~6 sec for most of the stations analyzed consistent with conversions originating at the top of the slab. We also observe conversions on the transverse receiver functions that are consistent with the presence of multiple anisotropic and/or dipping layers. The character of the transverse receiver functions varies significantly along strike, with the northernmost three stations exhibiting markedly different behavior than stations located in the center of the Ryukyu arc. We compute synthetic receiver functions using a forward modeling scheme that can handle dipping interfaces and anisotropic layers to create models for the depths, thicknesses, and strengths of anisotropic layers in the mantle wedge beneath Ryukyu.

Simulating the Seismic Signal of Phase Transitions in the Deepest Mantle (Invited)

Science.gov (United States)

Walker, A.; Dobson, D. P.; Nowacki, A.; Wookey, J. M.; Forte, A. M.; Kendall, J. M.

2013-12-01

The discovery of the perovskite to post-perovskite phase transition in (Mg,Fe)SiO3 explains many of the seismic observations of the lowermost mantle including the presence of multiple seismic discontinuities and significant seismic anisotropy. However, the explanations of many detailed features remain elusive. The recent discovery of a topotactic relationship between the orientation of perovskite and post-perovskite crystals in a partially transformed analogue opens the possibility of texture inheritance through the phase transition [1]. This must be captured in simulations designed to explain the anisotropy of the lowermost mantle, especially those which link mantle dynamics with seismic observations. We have extended our previous work linking models of flow in the lowermost mantle with simulations of texture development and predictions of seismic anisotropy [2] to account for the topotaxy between perovskite and post-perovskite. In particular, we compare four cases: (1) As in [2], anisotropy is only generated in post-perovskite by dislocation mediated deformation dominated by one of a number of slip systems, phase transitions destroy texture and ferropericlase and perovskite dominated rocks are isotropic. (2) Although phase transitions destroy texture, ferropericlase and/or perovskite deform by dislocation motion permitting the generation of seismic anisotropy in warmer regions of the mantle where post-perovskite is unstable. We account for the possibility of the inversion of slip-system activities in ferropericlase at high pressure as suggested by models of dislocation motion based on atomic scale simulations [3]. (3) Allow texture development by dislocation motion in perovskite and post-perovskite and texture inheritance through phase transitions by the mechanism described in [1]. However, we assume that the bulk of the lower mantle deforms by a mechanism that does not lead to the development of texture and so begin the simulation from a random distribution of

Finite-Frequency Seismic Tomography of Body Waves and Surface Waves from Ambient Seismic Noise: Crustal and Mantle Structure Beneath Eastern Eurasia

National Research Council Canada - National Science Library

Ren, Yong; Zhang, Wei; Yang, Ting; Shen, Yang; Yang, Xiaoping

2008-01-01

To improve seismic calibration for nuclear explosion monitoring, we use 3D sensitivity kernels of finite-frequency body and surface waves to develop models of the crustal and mantle structures beneath eastern Eurasia...

Mantle Convection beneath the Aegir Ridge, a Shadow in the Iceland Hotspot

Science.gov (United States)

Howell, S. M.; Ito, G.; Breivik, A. J.; Hanan, B. B.; Mjelde, R.; Sayit, K.; Vogt, P. R.

2012-12-01

The Iceland Hotspot has produced extensive volcanism spanning much of the ocean basin between Greenland and Norway, forming one of the world's largest igneous provinces. However, an apparent igneous "shadow" in hotspot activity is located at the fossil Aegir Ridge, which formed anomalously thin crust, despite this ridge being near the Iceland hotspot when it was active. The Aegir Ridge accommodated seafloor spreading northeast of present-day Iceland from the time of continental breakup at ~55 Ma until ~25 Ma, at which point spreading shifted west to the Kolbeinsey Ridge. To address the cause of the anomalously thin crust produced by the Aegir Ridge, we use three-dimensional numerical models to simulate the interaction between a mantle plume beneath the Iceland hotspot, rifting continental lithosphere, and the time-evolving North Atlantic ridge system. Two end-member hypotheses were investigated: (1) Material emanating from the Iceland mantle plume was blocked from reaching the Aegir Ridge by the thick lithosphere of the Jan Mayen Microcontinent as the Kolbeinsey Ridge began rifting it from Greenland at ~30 Ma, just east of the plume center; (2) Plume material was not blocked and did reach the Aegir Ridge, but had already experienced partial melting closer to the hotspot. This material was then unable to produce melt volumes at the Aegir Ridge comparable to those of pristine mantle. To test these hypotheses, we vary the volume flux and viscosity of the plume, and identify which conditions do and do not lead to the Aegir Ridge forming anomalously thin crust. Results show that the combination of plume material being drawn into the lithospheric channels beneath the Reykjanes Ridge and Kolbeinsey Ridge after their respective openings, and the impedance of plume flow by the Jan Mayen Microcontinent (hypothesis 1), can deprive the Aegir Ridge of plume influence. This leads to low crustal thicknesses that are comparable to those observed. We have yet to produce a model

Upper Mantle Shear Wave Structure Beneath North America From Multi-mode Surface Wave Tomography

Science.gov (United States)

Yoshizawa, K.; Ekström, G.

2008-12-01

The upper mantle structure beneath the North American continent has been investigated from measurements of multi-mode phase speeds of Love and Rayleigh waves. To estimate fundamental-mode and higher-mode phase speeds of surface waves from a single seismogram at regional distances, we have employed a method of nonlinear waveform fitting based on a direct model-parameter search using the neighbourhood algorithm (Yoshizawa & Kennett, 2002). The method of the waveform analysis has been fully automated by employing empirical quantitative measures for evaluating the accuracy/reliability of estimated multi-mode phase dispersion curves, and thus it is helpful in processing the dramatically increasing numbers of seismic data from the latest regional networks such as USArray. As a first step toward modeling the regional anisotropic shear-wave velocity structure of the North American upper mantle with extended vertical resolution, we have applied the method to long-period three-component records of seismic stations in North America, which mostly comprise the GSN and US regional networks as well as the permanent and transportable USArray stations distributed by the IRIS DMC. Preliminary multi-mode phase-speed models show large-scale patterns of isotropic heterogeneity, such as a strong velocity contrast between the western and central/eastern United States, which are consistent with the recent global and regional models (e.g., Marone, et al. 2007; Nettles & Dziewonski, 2008). We will also discuss radial anisotropy of shear wave speed beneath North America from multi-mode dispersion measurements of Love and Rayleigh waves.

Importance of initial buoyancy field on evolution of mantle thermal structure: Implications of surface boundary conditions

Directory of Open Access Journals (Sweden)

Petar Glišović

2015-01-01

Full Text Available Although there has been significant progress in the seismic imaging of mantle heterogeneity, the outstanding issue that remains to be resolved is the unknown distribution of mantle temperature anomalies in the distant geological past that give rise to the present-day anomalies inferred by global tomography models. To address this question, we present 3-D convection models in compressible and self-gravitating mantle initialised by different hypothetical temperature patterns. A notable feature of our forward convection modelling is the use of self-consistent coupling of the motion of surface tectonic plates to the underlying mantle flow, without imposing prescribed surface velocities (i.e., plate-like boundary condition. As an approximation for the surface mechanical conditions before plate tectonics began to operate we employ the no-slip (rigid boundary condition. A rigid boundary condition demonstrates that the initial thermally-dominated structure is preserved, and its geographical location is fixed during the evolution of mantle flow. Considering the impact of different assumed surface boundary conditions (rigid and plate-like on the evolution of thermal heterogeneity in the mantle we suggest that the intrinsic buoyancy of seven superplumes is most-likely resolved in the tomographic images of present-day mantle thermal structure. Our convection simulations with a plate-like boundary condition reveal that the evolution of an initial cold anomaly beneath the Java-Indonesian trench system yields a long-term, stable pattern of thermal heterogeneity in the lowermost mantle that resembles the present-day Large Low Shear Velocity Provinces (LLSVPs, especially below the Pacific. The evolution of subduction zones may be, however, influenced by the mantle-wide flow driven by deeply-rooted and long-lived superplumes since Archean times. These convection models also detect the intrinsic buoyancy of the Perm Anomaly that has been identified as a unique

Noble gas composition of subcontinental lithospheric mantle: An extensively degassed reservoir beneath Southern Patagonia

Science.gov (United States)

Jalowitzki, Tiago; Sumino, Hirochika; Conceição, Rommulo V.; Orihashi, Yuji; Nagao, Keisuke; Bertotto, Gustavo W.; Balbinot, Eduardo; Schilling, Manuel E.; Gervasoni, Fernanda

2016-09-01

Patagonia, in the Southern Andes, is one of the few locations where interactions between the oceanic and continental lithosphere can be studied due to subduction of an active spreading ridge beneath the continent. In order to characterize the noble gas composition of Patagonian subcontinental lithospheric mantle (SCLM), we present the first noble gas data alongside new lithophile (Sr-Nd-Pb) isotopic data for mantle xenoliths from Pali-Aike Volcanic Field and Gobernador Gregores, Southern Patagonia. Based on noble gas isotopic compositions, Pali-Aike mantle xenoliths represent intrinsic SCLM with higher (U + Th + K)/(3He, 22Ne, 36Ar) ratios than the mid-ocean ridge basalt (MORB) source. This reservoir shows slightly radiogenic helium (3He/4He = 6.84-6.90 RA), coupled with a strongly nucleogenic neon signature (mantle source 21Ne/22Ne = 0.085-0.094). The 40Ar/36Ar ratios vary from a near-atmospheric ratio of 510 up to 17700, with mantle source 40Ar/36Ar between 31100-6800+9400 and 54000-9600+14200. In addition, the 3He/22Ne ratios for the local SCLM endmember, at 12.03 ± 0.15 to 13.66 ± 0.37, are higher than depleted MORBs, at 3He/22Ne = 8.31-9.75. Although asthenospheric mantle upwelling through the Patagonian slab window would result in a MORB-like metasomatism after collision of the South Chile Ridge with the Chile trench ca. 14 Ma, this mantle reservoir could have remained unhomogenized after rapid passage and northward migration of the Chile Triple Junction. The mantle endmember xenon isotopic ratios of Pali-Aike mantle xenoliths, which is first defined for any SCLM-derived samples, show values indistinguishable from the MORB source (129Xe/132Xe =1.0833-0.0053+0.0216 and 136Xe/132Xe =0.3761-0.0034+0.0246). The noble gas component observed in Gobernador Gregores mantle xenoliths is characterized by isotopic compositions in the MORB range in terms of helium (3He/4He = 7.17-7.37 RA), but with slightly nucleogenic neon (mantle source 21Ne/22Ne = 0.065-0.079). We

Seismic imaging of the upper mantle beneath the northern Central Andean Plateau: Implications for surface topography

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Ward, K. M.; Zandt, G.; Beck, S. L.; Wagner, L. S.

2015-12-01

Extending over 1,800 km along the active South American Cordilleran margin, the Central Andean Plateau (CAP) as defined by the 3 km elevation contour is second only to the Tibetan Plateau in geographic extent. The uplift history of the 4 km high Plateau remains uncertain with paleoelevation studies along the CAP suggesting a complex, non-uniform uplift history. As part of the Central Andean Uplift and the Geodynamics of High Topography (CAUGHT) project, we use surface waves measured from ambient noise and two-plane wave tomography to image the S-wave velocity structure of the crust and upper mantle to investigate the upper mantle component of plateau uplift. We observe three main features in our S-wave velocity model including (1), a high velocity slab (2), a low velocity anomaly above the slab where the slab changes dip from near horizontal to a normal dip, and (3), a high-velocity feature in the mantle above the slab that extends along the length of the Altiplano from the base of the Moho to a depth of ~120 km with the highest velocities observed under Lake Titicaca. A strong spatial correlation exists between the lateral extent of this high-velocity feature beneath the Altiplano and the lower elevations of the Altiplano basin suggesting a potential relationship. Non-uniqueness in our seismic models preclude uniquely constraining this feature as an uppermost mantle feature bellow the Moho or as a connected eastward dipping feature extending up to 300 km in the mantle as seen in deeper mantle tomography studies. Determining if the high velocity feature represents a small lithospheric root or a delaminating lithospheric root extending ~300 km into the mantle requires more integration of observations, but either interpretation shows a strong geodynamic connection with the uppermost mantle and the current topography of the northern CAP.

Upper mantle seismic structure beneath southwest Africa from finite-frequency P- and S-wave tomography

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Youssof, Mohammad; Yuan, Xiaohui; Tilmann, Frederik; Heit, Benjamin; Weber, Michael; Jokat, Wilfried; Geissler, Wolfram; Laske, Gabi; Eken, Tuna; Lushetile, Bufelo

2015-04-01

We present a 3D high-resolution seismic model of the southwestern Africa region from teleseismic tomographic inversion of the P- and S- wave data recorded by the amphibious WALPASS network. We used 40 temporary stations in southwestern Africa with records for a period of 2 years (the OBS operated for 1 year), between November 2010 and November 2012. The array covers a surface area of approximately 600 by 1200 km and is located at the intersection of the Walvis Ridge, the continental margin of northern Namibia, and extends into the Congo craton. Major questions that need to be understood are related to the impact of asthenosphere-lithosphere interaction, (plume-related features), on the continental areas and the evolution of the continent-ocean transition that followed the break-up of Gondwana. This process is supposed to leave its imprint as distinct seismic signature in the upper mantle. Utilizing 3D sensitivity kernels, we invert traveltime residuals to image velocity perturbations in the upper mantle down to 1000 km depth. To test the robustness of our tomographic image we employed various resolution tests which allow us to evaluate the extent of smearing effects and help defining the optimum inversion parameters (i.e., damping and smoothness) used during the regularization of inversion process. Resolution assessment procedure includes also a detailed investigation of the effect of the crustal corrections on the final images, which strongly influenced the resolution for the mantle structures. We present detailed tomographic images of the oceanic and continental lithosphere beneath the study area. The fast lithospheric keel of the Congo Craton reaches a depth of ~250 km. Relatively low velocity perturbations have been imaged within the orogenic Damara Belt down to a depth of ~150 km, probably related to surficial suture zones and the presence of fertile material. A shallower depth extent of the lithospheric plate of ~100 km was observed beneath the ocean

Iron-carbonate interaction at Earth's core-mantle boundary

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Dorfman, S. M.; Badro, J.; Nabiei, F.; Prakapenka, V.; Gillet, P.

2015-12-01

Carbon storage and flux in the deep Earth are moderated by oxygen fugacity and interactions with iron-bearing phases. The amount of carbon stored in Earth's mantle versus the core depends on carbon-iron chemistry at the core-mantle boundary. Oxidized carbonates subducted from Earth's surface to the lowermost mantle may encounter reduced Fe0 metal from disproportionation of Fe2+ in lower mantle silicates or mixing with the core. To understand the fate of carbonates in the lowermost mantle, we have performed experiments on sandwiches of single-crystal (Ca0.6Mg0.4)CO3 dolomite and Fe foil in the laser-heated diamond anvil cell at lower mantle conditions of 49-110 GPa and 1800-2500 K. Syntheses were conducted with in situ synchrotron X-ray diffraction to identify phase assemblages. After quench to ambient conditions, samples were sectioned with a focused Ga+ ion beam for composition analysis with transmission electron microscopy. At the centers of the heated spots, iron melted and reacted completely with the carbonate to form magnesiowüstite, iron carbide, diamond, magnesium-rich carbonate and calcium carbonate. In samples heated at 49 and 64 GPa, the two carbonates exhibit a eutectoid texture. In the sample heated at 110 GPa, the carbonates form rounded ~150-nm-diameter grains with a higher modal proportion of interspersed diamonds. The presence of reduced iron in the deep lower mantle and core-mantle boundary region will promote the formation of diamonds in carbonate-bearing subducted slabs. The complete reaction of metallic iron to oxides and carbides in the presence of mantle carbonate supports the formation of these phases at the Earth's core-mantle boundary and in ultra-low velocity zones.

New Pn and Sn tomographic images of the uppermost mantle beneath the Mediterranean region

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Gil, A.; Díaz, J.; Gallart, J.

2012-04-01

We present here new images of the seismic velocity and anisotropy variations in the uppermost mantle beneath the Mediterranean region, compiled from inversion of Pn and Sn phases. The method of Hearn (1996) has been applied to Pn and Sn lectures from the catalogs of the International Seismological Center and the Spanish Instituto Geografico Nacional. A total of 1,172,293 Pn arrivals coming from 16,527 earthquakes recorded at 1,657 stations with epicentral distances between 220 km and 1400 km have been retained (331,567 arrivals from 15,487events at 961 stations for Sn). Our results, grossly consistent with available 3D tomography images, show significant features well correlated with surface geology. The Pn velocities are high (>8.2 km/s) beneath major sedimentary basins (western Alboran Sea, Valencia Trough, Adriatic Sea, Aquitaine, Guadalquivir, Rharb, Aquitaine and Po basins), and low (Islands, probably related to a thermal anomaly associated to the westward displacement of the Alboran block along the Emile Baudot escarpment 16 Ma ago. The Pn anisotropic image shows consistent orientations sub-parallel to major orogenic structures, such as Betics, Apennines, Calabrian Arc and Alps. The station delays beneath Betic and Rif ranges are strongly negative, suggesting the presence of crustal thickening all along the Gibraltar Arc. However, only the Betics have a very strong low-velocity anomaly and a pronounced anisotropy pattern. The Sn tomographic image correlates well with the Pn image, even if some relevant differences can be observed beneath particular regions.

Alakit and Daldyn kimberlite fields, Siberia, Russia: Two types of mantle sub-terranes beneath central Yakutia?

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I.V. Ashchepkov

2017-07-01

Full Text Available Mineral data from Yakutian kimberlites allow reconstruction of the history of lithospheric mantle. Differences occur in compositions of mantle pyropes and clinopyroxenes from large kimberlite pipes in the Alakit and Daldyn fields. In the Alakit field, Cr-diopsides are alkaline, and Stykanskaya and some other pipes contain more sub-calcic pyropes and dunitic-type diamond inclusions, while in the Daldyn field harzburgitic pyropes are frequent. The eclogitic diamond inclusions in the Alakit field are sharply divided in types and conditions, while in the Daldyn field they show varying compositions and often continuous Pressure–Temperature (P–T ranges with increasing Fe# with decreasing pressures. In Alakit, Cr-pargasites to richterites were found in all pipes, while in Daldyn, pargasites are rare Dalnyaya and Zarnitsa pipes. Cr-diopsides from the Alakit region show higher levels of light Rare Earth Elements (LREE and stronger REE-slopes, and enrichment in light Rare Earth Elements (LREE, sometimes Th-U, and small troughs in Nb-Ta-Zr. In the Daldyn field, the High Field Strength Elements HFSE troughs are more common in clinopyroxenes with low REE abundances, while those from sheared and refertilized peridotites have smooth patterns. Garnets from Alakit show HREE minima, but those from Daldyn often have a trough at Y and high U and Pb. PTXfO2 diagrams from both regions show similarities, suggesting similar layering and structures. The degree of metasomatism is often higher for pipes which show dispersion in P–Fe# trends for garnets. In the mantle beneath Udachnaya and Aykhal, pipes show 6–7 linear arrays of P–Fe# in the lower part of the mantle section at 7.5–3.0 GPa, probably reflecting primary subduction horizons. Beneath the Sytykanskaya pipe, there are several horizons with opposite inclinations which reflect metasomatic processes. The high dispersion of the P–Fe# trend indicating widespread metasomatism is associated with decreased

Upper Mantle Structure beneath Afar: inferences from surface waves.

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Sicilia, D.; Montagner, J.; Debayle, E.; Lepine, J.; Leveque, J.; Cara, M.; Ataley, A.; Sholan, J.

2001-12-01

The Afar hotspot is related to one of the most important plume from a geodynamic point of view. It has been advocated to be the surface expression of the South-West African Superswell. Below the lithosphere, the Afar plume might feed other hotspots in central Africa (Hadiouche et al., 1989; Ebinger & Sleep, 1998). The processes of interaction between crust, lithosphere and plume are not well understood. In order to gain insight into the scientific issue, we have performed a surface-wave tomography covering the Horn of Africa. A data set of 1404 paths for Rayleigh waves and 473 paths for Love waves was selected in the period range 45-200s. They were collected from the permanent IRIS and GEOSCOPE networks and from the PASSCAL experiment, in Tanzania and Saudi Arabia. Other data come from the broadband stations deployed in Ethiopia and Yemen in the framework of the French INSU program ``Horn of Africa''. The results presented here come from a path average phase velocities obtained with a method based on a least-squares minimization (Beucler et al., 2000). The local phase velocity distribution and the azimuthal anisotropy were simultaneously retrieved by using the tomographic technique of Montagner (1986). A correction of the data is applied according to the crustal structure of the 3SMAC model (Nataf & Ricard, 1996). We find low velocities down to 200 km depth beneath the Red Sea, the Gulf of Aden, Afars, the Ethiopian Plateau and southern Arabia. High velocities are present in the eastern Arabia and the Tanzania Craton. The anisotropy beneath Afar seems to be complex, but enables to map the flow pattern at the interface lithosphere-asthenosphere. The results presented here are complementary to those obtained by Debayle et al. (2001) at upper-mantle transition zone depths using waveform inversion of higher Rayle igh modes.

Mantle upwelling beneath Madagascar: evidence from receiver function analysis and shear wave splitting

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Paul, Jonathan D.; Eakin, Caroline M.

2017-07-01

Crustal receiver functions have been calculated from 128 events for two three-component broadband seismomenters located on the south coast (FOMA) and in the central High Plateaux (ABPO) of Madagascar. For each station, crustal thickness and V p / V s ratio were estimated from H- κ plots. Self-consistent receiver functions from a smaller back-azimuthal range were then selected, stacked and inverted to determine shear wave velocity structure as a function of depth. These results were corroborated by guided forward modeling and by Monte Carlo error analysis. The crust is found to be thinner (39 ± 0.7 km) beneath the highland center of Madagascar compared to the coast (44 ± 1.6 km), which is the opposite of what would be expected for crustal isostasy, suggesting that present-day long wavelength topography is maintained, at least in part, dynamically. This inference of dynamic support is corroborated by shear wave splitting analyses at the same stations, which produce an overwhelming majority of null results (>96 %), as expected for vertical mantle flow or asthenospheric upwelling beneath the island. These findings suggest a sub-plate origin for dynamic support.

A study of upper mantle discontinuities beneath the Korean Peninsula using teleseismic receiver functions

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Lee, S.; Park, Y.; Kim, K.; Rhie, J.

2010-12-01

The study on the topography of the upper mantle discontinuities helps us to understand the complex interactions between the subducting slabs and upper mantle discontinuities. To investigate the depth variation of the upper mantle discontinuities beneath the Korean Peninsula and surrounding regions, we applied the common conversion point stacking of the P-to-s receiver functions. The broadband seismic networks in South Korea and Japan were used to produce the high-resolution receiver function images of the region. The 410- and 660-km discontinuities (hereafter referred to as the 410 and the 660) are clearly imaged and their depth variations show interesting features, especially for the 660. In this region, the subducting Pacific slab bends to flatten over the 660 and several tomographic images indicate that the stagnant slab is extending to the west under China. If the depth of the 660 is affected by the temperature, the broad depression of the 660 is expected and several SS precursor studies support this idea. However, our observation shows that the 660 is locally depressed and its pattern is spatially changing. While the depressed 660 due to the Pacific slab is clearly imaged at lower latitudes (depressed 660 to the north. It indicates that the effect of the Pacific slab on the depth variation of the 660 is changing significantly in our study area.

A geochemical study of lithospheric mantle beneath Northern Victoria Land (Antarctica): main evidences from volatile content in ultramafic xenoliths

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Correale, Alessandra; Pelorosso, Beatrice; Rizzo, Andrea Luca; Coltorti, Massimo; Italiano, Francesco; Bonadiman, Costanza

2017-04-01

A geochemical study of ultramafic xenoliths from Northern Victoria Land (Green Point, GP and Handler Ridge, HR), is carried out in order to investigate the features of the lithosphere mantle beneath the Western Antarctic Ridge System (WARS). The majority of samples is spinel anhydrous lherzolite with rare presence of secondary phases (secondary cpx and glass). Geothermobarometric calculations, based on the Fe/Mg distribution among the peridotite minerals reveal that Sub Continental Lithospheric Mantle (SCLM) beneath Handler Ridge records temperatures and redox conditions higher then Greene Point (P fixed at 15 Kbar). Moreover, geochemical models evidence that, GP mantle domain represents a residuum after ˜7 to 21 % of partial melting in the spinel stability field, which was variably affected by interaction with infiltrating melts, acting in different times, from at least Jurassic to Cenozoic (Pelorosso et al., 2016). Fluid inclusions (FI) entrapped in olivine and pyroxene crystals were investigated for elemental and isotopic contents of both, noble gases (He, Ne, Ar) and CO2. He, Ar and Ne concentrations range from 1.52×10-14 to 1.07×10-12, from 4.09×10-13 to 3.47×10-11and from 2.84×10-16 to 7.57×10-14 mol/g, respectively, while the CO2amounts are between 7.08×10-10 and 8.12×10-7 mol/g. The 3He/4He varies between 5.95 and 20.18 Ra (where Ra is the 3He/4He ratio of air), being the lowest and the highest values measured in the He-poorer samples. Post-eruptive input of cosmogenic 3He and radiogenic 4He seems to influence mainly the samples associated to a lower He concentrations, increasing and decreasing respectively their primordial 3He/4He values, that for all the other samples range between 6.76 and 7.45 Ra. This range reasonably reflects the isotope signature of mantle beneath the investigated areas. The 4He/40Ar* ratio corrected for atmospheric-derived contamination ranges between 0.004 and 0.39. The lowest 4He/40Ar* values (4He/40Ar*correspondence of

Tomography of core-mantle boundary and lowermost mantle coupled by geodynamics: joint models of shear and compressional velocity

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Gaia Soldati

2015-03-01

Full Text Available We conduct joint tomographic inversions of P and S travel time observations to obtain models of delta v_P  and delta v_S in the entire mantle. We adopt a recently published method which takes into account the geodynamic coupling between mantle heterogeneity and core-mantle boundary (CMB topography by viscous flow, where sensitivity of the seismic travel times to the CMB is accounted for implicitly in the inversion (i.e. the CMB topography is not explicitly inverted for. The seismic maps of the Earth's mantle and CMB topography that we derive can explain the inverted seismic data while being physically consistent with each other. The approach involved scaling P-wave velocity (more sensitive to the CMB to density anomalies, in the assumption that mantle heterogeneity has a purely thermal origin, so that velocity and density heterogeneity are proportional to one another. On the other hand, it has sometimes been suggested that S-wave velocity might be more directly sensitive to temperature, while P heterogeneity is more strongly influenced by chemical composition. In the present study, we use only S-, and not P-velocity, to estimate density heterogeneity through linear scaling, and hence the sensitivity of core-reflected P phases to mantle structure. Regardless of whether density is more closely related to P- or S-velocity, we think it is worthwhile to explore both scaling approaches in our efforts to explain seismic data. The similarity of the results presented in this study to those obtained by scaling P-velocity to density suggests that compositional anomaly has a limited impact on viscous flow in the deep mantle.

Lateral variation in upper mantle temperature and composition beneath mid-ocean ridges inferred from shear-wave propagation, geoid, and bathymetry. Ph.D. Thesis

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Sheehan, Anne Francis

1991-01-01

Resolution of both the extent and mechanism of lateral heterogeneity in the upper mantle constraints the nature and scales of mantle convection. Oceanic regions are of particular interest as they are likely to provide the closest glimpse at the patterns of temperature anomalies and convective flow in the upper mantle because of their young age and simple crustal structure relative to continental regions. Lateral variations were determined in the seismic velocity and attenuation structure of the lithosphere and astenosphere beneath the oceans, and these seismological observations were combined with the data and theory of geoid and bathymetry anomalies in order to test and improve current models for seafloor spreading and mantle convection. Variations were determined in mantle properties on a scale of about 1000 km, comparable to the thickness of the upper mantle. Seismic velocity, geoid, and bathymetry anomalies are all sensitive to variations in upper mantle density, and inversions were formulated to combine quantitatively these different data and to search for a common origin. Variations in mantle density can be either of thermal or compositional origin and are related to mantle convection or differentiation.

Silicate melt metasomatism in the lithospheric mantle beneath SW Poland

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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

Seismic and thermal structure of the crust and uppermost mantle beneath Antarctica from inversion of multiple seismic datasets

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Wiens, D.; Shen, W.; Anandakrishnan, S.; Aster, R. C.; Gerstoft, P.; Bromirski, P. D.; Dalziel, I.; Hansen, S. E.; Heeszel, D.; Huerta, A. D.; Nyblade, A.; Stephen, R. A.; Wilson, T. J.; Winberry, J. P.; Stern, T. A.

2017-12-01

Since the last decade of the 20th century, over 200 broadband seismic stations have been deployed across Antarctica (e.g., temporary networks such as TAMSEIS, AGAP/GAMSEIS, POLENET/ANET, TAMNNET and RIS/DRIS by U.S. geoscientists as well as stations deployed by Japan, Britain, China, Norway, and other countries). In this presentation, we discuss our recent efforts to build reference crustal and uppermost mantle shear velocity (Vs) and thermal models for continental Antarctica based on those seismic arrays. By combing the high resolution Rayleigh wave dispersion maps derived from both ambient noise and teleseismic earthquakes, together with P receiver function waveforms, we develop a 3-D Vs model for the crust and uppermost mantle beneath Central and West Antarctica to a depth of 200 km. Additionally, using this 3-D seismic model to constrain the crustal structure, we re-invert for the upper mantle thermal structure using the surface wave data within a thermodynamic framework and construct a 3-D thermal model for the Antarctic lithosphere. The final product, a high resolution thermal model together with associated uncertainty estimates from the Monte Carlo inversion, allows us to derive lithospheric thickness and surface heat flux maps for much of the continent. West Antarctica shows a much thinner lithosphere ( 50-90 km) than East Antarctica ( 130-230 km), with a sharp transition along the Transantarctic Mountains (TAM). A variety of geological features, including a slower/hotter but highly heterogeneous West Antarctica and a much faster/colder East Antarctic craton, are present in the 3-D seismic/thermal models. Notably, slow seismic velocities observed in the uppermost mantle beneath the southern TAM are interpreted as a signature of lithospheric foundering and replacement with hot asthenosphere. The high resolution image of these features from the 3-D models helps further investigation of the dynamic state of Antarctica's lithosphere and underlying asthenosphere

2-dimensional triplicated waveform modeling of the mantle transition zone beneath Northeast Asia

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Lai, Y.; Chen, L.; Wang, T.

2017-12-01

The Mantle Transition Zone (MTZ) of Northeast Asia has long been investigated by geoscientists for its critical importance where the subducted Pacific slab is stagnant above the 660km discontinuity, accompanied by complicated mantle processes. Taking advantages of the frequent occurrent deep earthquakes in subduction zone and dense seismic arrays in Northeast China, we successfully constructed the fine-scale P and SH velocity structure of a narrow azimuthal fan area based on 2-Dimensional (2D) triplicated waveform modeling for three deep close earthquakes, in which the triplicated waveforms are very sensitive to MTZ velocity structure in general, particularly the morphology of the stagnant slab in Northeast Asia. In our 2D triplication study, for the first time, we show a quite consistent feature of a high velocity layer for both Vp and Vs with the thickness of 140km and the length of 1200km just atop the 660km discontinuity, the western edge of the stagnant slab intersect with the North-South Gravity Lineament in China and has the subducting age of 30 Ma. Compared with a quite normal Vp, the Shear wave velocity reduction of -0.5% in the slab and -2.5% in the upper MTZ is required to reconcile the SH waves featured by the broad BOD. The high Vp/Vs ratio beneath Northeast Asia may imply a water-rich MTZ with the H2O content of 0.1-0.3 wt%. Particularly, a low velocity anomaly of about 150km wide was detected in the overall high-velocity stagnant slab by both P and SH triplicated waveform modeling, with the velocity anomaly value of -1% and -3%, respectively. The gap/window in the stagnant slab may provide a passage for hot deeper mantle materials to penetrate through the thick slab and feed the surface Changbaishan volcano. We also speculate that the existence of such a gap can be the manifestation of the original heterogeneity in the subducted slab and will further exacerbatethe impending gravitational instability and speed up mantle avalanche.

Sub-crustal seismic activity beneath Klyuchevskoy Volcano

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Carr, M. J.; Droznina, S.; Levin, V. L.; Senyukov, S.

2013-12-01

Seismic activity is extremely vigorous beneath the Klyuchevskoy Volcanic Group (KVG). The unique aspect is the distribution in depth. In addition to upper-crustal seismicity, earthquakes take place at depths in excess of 20 km. Similar observations are known in other volcanic regions, however the KVG is unique in both the number of earthquakes and that they occur continuously. Most other instances of deep seismicity beneath volcanoes appear to be episodic or transient. Digital recording of seismic signals started at the KVG in early 2000s.The dense local network reliably locates earthquakes as small as ML~1. We selected records of 20 earthquakes located at depths over 20 km. Selection was based on the quality of the routine locations and the visual clarity of the records. Arrivals of P and S waves were re-picked, and hypocentral parameters re-established. Newl locations fell within the ranges outlined by historical seismicity, confirming the existence of two distinct seismically active regions. A shallower zone is at ~20 km depth, and all hypocenters are to the northeast of KVG, in a region between KVG and Shiveluch volcano. A deeper zone is at ~30 km, and all hypocenters cluster directly beneath the edifice of the Kyuchevskoy volcano. Examination of individual records shows that earthquakes in both zones are tectonic, with well-defined P and S waves - another distinction of the deep seismicity beneath KVG. While the upper seismic zone is unquestionably within the crust, the provenance of the deeper earthquakes is enigmatic. The crustal structure beneath KVG is highly complex, with no agreed-upon definition of the crust-mantle boundary. Rather, a range of values, from under 30 to over 40 km, exists in the literature. Similarly, a range of velocity structures has been reported. Teleseismic receiver functions (RFs) provide a way to position the earthquakes with respect to the crust-mantle boundary. We compare the differential travel times of S and P waves from deep

Petrological Constraints on Melt Generation Beneath the Asal Rift (Djibouti)

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Pinzuti, P.; Humler, E.; Manighetti, I.; Gaudemer, Y.; Bézos, A.

2010-12-01

The temporal evolution of the mantle melting processes in the Asal Rift is evaluated from the chemical composition of 95 lava flows sampled along 10 km of the rift axis and 8 km off-axis (that is for the last 650 ky). The major element composition and the trace element ratios of aphyric basalts across the Asal Rift show a symmetric pattern relative to the rift axis and preserved a clear signal of mantle melting depth variations. FeO, Fe8.0, Sm/YbN and Zr/Y increase, whereas SiO2 and Lu/HfN decrease from the rift axis to the rift shoulders. These variations are qualitatively consistent with a shallower melting beneath the rift axis than off-axis and the data show that the melting regime is inconsistent with a passive upwelling model. In order to quantify the depth range and extent of melting, we invert Na8.0 and Fe8.0 contents of basalts based on a pure active upwelling model. Beneath the rift axis, melting paths are shallow, from 60 to 30 km. These melting paths are consistent with adiabatic melting in normal-temperature asthenosphere, beneath an extensively thinned mantle lithosphere. In contrast, melting on the rift shoulders occurred beneath a thick mantle lithosphere and required mantle solidus temperature 180°C hotter than normal (melting paths from 110 to 75 km). The calculated rate of lithospheric thinning is high (6.0 cm yr-1) and could explain the survival of a metastable garnet within the mantle at depth shallower than 90 km beneath the modern Asal Rift.

Depth variations of P-wave azimuthal anisotropy beneath East Asia

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Wei, W.; Zhao, D.; Xu, J.

2017-12-01

We present a new P-wave anisotropic tomographic model beneath East Asia by inverting a total of 1,488,531 P wave arrival-time data recorded by the regional seismic networks in East Asia and temporary seismic arrays deployed on the Tibetan Plateau. Our results provide important new insights into the subducting Indian, Pacific and Philippine Sea plates and mantle dynamics in East Asia. Our tomographic images show that the northern limit of the subducting Indian plate has reached the Jinsha River suture in eastern Tibet. A striking variation of P-wave azimuthal anisotropy is revealed in the Indian lithosphere: the fast velocity direction (FVD) is NE-SW beneath the Indian continent, whereas the FVD is arc parallel beneath the Himalaya and Tibetan Plateau, which may reflect re-orientation of minerals due to lithospheric extension, in response to the India-Eurasia collision. The FVD in the subducting Philippine Sea plate beneath the Ryukyu arc is NE-SW(trench parallel), which is consistent with the spreading direction of the West Philippine Basin during its initial opening stage, suggesting that it may reflect the fossil anisotropy. A circular pattern of FVDs is revealed around the Philippine Sea slab beneath SE China. We suggest that it reflects asthenospheric strain caused by toroidal mantle flow around the edge of the subducting slab. We find a striking variation of the FVD with depth in the subducting Pacific slab beneath the Northeast Japan arc. It may be caused by slab dehydration that changed elastic properties of the slab with depth. The FVD in the mantle wedge beneath the Northeast Japan and Ryukyu arcs is trench normal, which reflects subduction-induced convection. Beneath the Kuril and Izu-Bonin arcs where oblique subduction occurs, the FVD in the mantle wedge is nearly normal to the moving direction of the downgoing Pacific plate, suggesting that the oblique subduction together with the complex slab morphology have disturbed the mantle flow.

The African upper mantle and its relationship to tectonics and surface geology

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Priestley, Keith; McKenzie, Dan; Debayle, Eric; Pilidou, Sylvana

2008-12-01

This paper focuses on the upper-mantle velocity structure of the African continent and its relationship to the surface geology. The distribution of seismographs and earthquakes providing seismograms for this study results in good fundamental and higher mode path coverage by a large number of relatively short propagation paths, allowing us to image the SV-wave speed structure, with a horizontal resolution of several hundred kilometres and a vertical resolution of ~50 km, to a depth of about 400 km. The difference in mantle structure between the Archean and Pan-African terranes is apparent in our African upper-mantle shear wave model. High-velocity (4-7 per cent) roots exist beneath the cratons. Below the West African, Congo and Tanzanian Cratons, these extend to 225-250 km depth, but beneath the Kalahari Craton, the high wave speed root extends to only ~170 km. With the exception of the Damara Belt that separates the Congo and Kalahari Cratons, any high-speed upper-mantle lid below the Pan-African terranes is too thin to be resolved by our long-period surface wave technique. The Damara Belt is underlain by higher wave speeds, similar to those observed beneath the Kalahari Craton. Extremely low SV-wave speeds occur to the bottom of our model beneath the Afar region. The temperature of the African upper mantle is determined from the SV-wave speed model. Large temperature variations occur at 125 km depth with low temperatures beneath west Africa and all of southern Africa and warm mantle beneath the Pan-African terrane of northern Africa. At 175 km depth, cool upper mantle occurs below the West African, Congo, Tanzanian and Kalahari Cratons and anomalously warm mantle occurs below a zone in northcentral Africa and beneath the region surrounding the Red Sea. All of the African volcanic centres are located above regions of warm upper mantle. The temperature profiles were fit to a geotherm to determine the thickness of the African lithosphere. Thick lithosphere exists

Keweenaw hot spot: Geophysical evidence for a 1.1 Ga mantle plume beneath the Midcontinent Rift System

Science.gov (United States)

Hutchinson, D.R.; White, R.S.; Cannon, W.F.; Schulz, K.J.

1990-01-01

The Proterozoic Midcontinent Rift System of North America is remarkably similar to Phanerozoic rifted continental margins and flood basalt provinces. Like the younger analogues, the volcanism within this older rift can be explained by decompression melting and rapid extrusion of igneous material during lithospheric extension above a broad, asthenospheric, thermal anomaly which we call the Keweenaw hot spot. Great Lakes International Multidisciplinary Program on Crustal Evolution seismic reflection profiles constrain end-member models of melt thickness and stretching factors, which yield an inferred mantle potential temperature of 1500°–1570°C during rifting. Combined gravity modeling and subsidence calculations are consistent with stretching factors that reached 3 or 4 before rifting ceased, and much of the lower crust beneath the rift consists of relatively high density intruded or underplated synrift igneous material. The isotopic signature of Keweenawan volcanic rocks, presented in a companion paper by Nicholson and Shirey (this issue), is consistent with our model of passive rifting above an asthenospheric mantle plume.

Dynamical links between small- and large-scale mantle heterogeneity: Seismological evidence

Science.gov (United States)

Frost, Daniel A.; Garnero, Edward J.; Rost, Sebastian

2018-01-01

We identify PKP • PKP scattered waves (also known as P‧ •P‧) from earthquakes recorded at small-aperture seismic arrays at distances less than 65°. P‧ •P‧ energy travels as a PKP wave through the core, up into the mantle, then scatters back down through the core to the receiver as a second PKP. P‧ •P‧ waves are unique in that they allow scattering heterogeneities throughout the mantle to be imaged. We use array-processing methods to amplify low amplitude, coherent scattered energy signals and resolve their incoming direction. We deterministically map scattering heterogeneity locations from the core-mantle boundary to the surface. We use an extensive dataset with sensitivity to a large volume of the mantle and a location method allowing us to resolve and map more heterogeneities than have previously been possible, representing a significant increase in our understanding of small-scale structure within the mantle. Our results demonstrate that the distribution of scattering heterogeneities varies both radially and laterally. Scattering is most abundant in the uppermost and lowermost mantle, and a minimum in the mid-mantle, resembling the radial distribution of tomographically derived whole-mantle velocity heterogeneity. We investigate the spatial correlation of scattering heterogeneities with large-scale tomographic velocities, lateral velocity gradients, the locations of deep-seated hotspots and subducted slabs. In the lowermost 1500 km of the mantle, small-scale heterogeneities correlate with regions of low seismic velocity, high lateral seismic gradient, and proximity to hotspots. In the upper 1000 km of the mantle there is no significant correlation between scattering heterogeneity location and subducted slabs. Between 600 and 900 km depth, scattering heterogeneities are more common in the regions most remote from slabs, and close to hotspots. Scattering heterogeneities show an affinity for regions close to slabs within the upper 200 km of the

Flow in the Deep Mantle from Seisimc Anisotropy: Progress and Prospects

Science.gov (United States)

Long, M. D.

2017-12-01

Observations of seismic anisotropy, or the directional dependence of seismic wavespeeds, provide one some of the most direct constraints on the pattern of flow in the Earth's mantle. In particular, as our understanding of crystallographic preferred orientation (CPO) of olivine aggregates under a range of deformation conditions has improved, our ability to exploit observations of upper mantle anisotropy has led to fundamental discoveries about the patterns of flow in the upper mantle and the drivers of that flow. It has been a challenge, however, to develop a similar framework for understanding flow in the deep mantle (transition zone, uppermost lower mantle, and lowermost mantle), even though there is convincing observational evidence for seismic anisotropy at these depths. Recent progress on the observational front has allowed for an increasingly detailed view of mid-mantle anisotropy (transition zone and uppermost lower mantle), particularly in subduction systems, which may eventually lead to a better understanding of mid-mantle deformation and the dynamics of slab interaction with the surrounding mid-mantle. New approaches to the observation and modeling of lowermost mantle anisotropy, in combination with constraints from mineral physics, are progressing towards interpretive frameworks that allow for the discrimination of different mantle flow geometries in different regions of D". In particular, observational strategies that involve the use of multiple types of body wave phases sampled over a range of propagation azimuths enable detailed forward modeling approaches that can discriminate between different mechanisms for D" anisotropy (e.g., CPO of post-perovskite, bridgmanite, or ferropericlase, or shape preferred orientation of partial melt) and identify plausible anisotropic orientations. We have recently begun to move towards a full waveform modeling approach in this work, which allows for a more accurate simulation for seismic wave propagation. Ongoing

Regional variations in upper mantle compressional velocities beneath southern California 1. Post-shock temperatures: Their experimental determination, calculation, and implications, 2.. Ph.D. Thesis

Science.gov (United States)

Raikes, S. A.

1978-01-01

The compressional velocity within the upper mantle beneath Southern California is investigated through observations of the dependence of teleseismic P-delays at all stations of the array on the distance and azimuth to the event. The variation of residuals with azimuth was found to be as large as 1.3 sec at a single station; the delays were stable as a function of time, and no evidence was found for temporal velocity variations related to seismic activity in the area. These delays were used in the construction of models for the upper mantle P-velocity structure to depths of 150 km, both by ray tracing and inversion techniques. The models exhibit considerable lateral heterogeneity including a region of low velocity beneath the Imperial Valley, and regions of increased velocity beneath the Sierra Nevada and much of the Transverse Ranges. The development is described of a technique for the experimental determination of post-shock temperatures, and its application to several metals and silicates shocked to pressures in the range 5 to 30 GPa. The technique utilizes an infra-red radiation detector to determine the brightness temperature of the free surface of the sample after the shock wave has passed through it.

Seismic Discontinuities within the Crust and Mantle Beneath Indonesia as Inferred from P Receiver Functions

Science.gov (United States)

Woelbern, I.; Rumpker, G.

2015-12-01

Indonesia is situated at the southern margin of SE Asia, which comprises an assemblage of Gondwana-derived continental terranes, suture zones and volcanic arcs. The formation of SE Asia is believed to have started in Early Devonian. Its complex history involves the opening and closure of three distinct Tethys oceans, each accompanied by the rifting of continental fragments. We apply the receiver function technique to data of the temporary MERAMEX network operated in Central Java from May to October 2004 by the GeoForschungsZentrum Potsdam. The network consisted of 112 mobile stations with a spacing of about 10 km covering the full width of the island between the southern and northern coast lines. The tectonic history is reflected in a complex crustal structure of Central Java exhibiting strong topography of the Moho discontinuity related to different tectonic units. A discontinuity of negative impedance contrast is observed throughout the mid-crust interpreted as the top of a low-velocity layer which shows no depth correlation with the Moho interface. Converted phases generated at greater depth beneath Indonesia indicate the existence of multiple seismic discontinuities within the upper mantle and even below. The strongest signal originates from the base of the mantle transition zone, i.e. the 660 km discontinuity. The phase related to the 410 km discontinuity is less pronounced, but clearly identifiable as well. The derived thickness of the mantle-transition zone is in good agreement with the IASP91 velocity model. Additional phases are observed at roughly 33 s and 90 s relative to the P onset, corresponding to about 300 km and 920 km, respectively. A signal of reversed polarity indicates the top of a low velocity layer at about 370 km depth overlying the mantle transition zone.

Rheological properties of the lower crust and upper mantle beneath Baja California: a microstructural study of xenoliths from San Quintin

Science.gov (United States)

Van der Werf, Thomas F.; Chatzaras, Vasileios; Tikoff, Basil; Drury, Martyn R.

2016-04-01

Baja California is an active transtensional rift zone, which links the San Andreas Fault with the East Pacific Rise. The erupted basalts of the Holocene San Quintin volcanic field contain xenoliths, which sample the lower crust and upper mantle beneath Baja California. The aim of this research is to gain insight in the rheology of the lower crust and the upper mantle by investigating the xenolith microstructure. Microstructural observations have been used to determine the dominant deformation mechanisms. Differential stresses were estimated from recrystallized grain size piezometry of plagioclase and clinopyroxene for the lower crust and olivine for the upper mantle. The degree of deformation can be inferred from macroscopic foliations and the deformation microstructures. Preliminary results show that both the lower crust and the upper mantle have been affected by multiple stages of deformation and recrystallization. In addition the dominant deformation mechanism in both the lower crust and the upper mantle is dislocation creep based on the existence of strong crystallographic preferred orientations. The differential stress estimates for the lower crust are 10-29 MPa using plagioclase piezometry and 12-35 MPa using clinopyroxene piezometry. For the upper mantle, differential stress estimates are 10-20 MPa. These results indicate that the strength of the lower crust and the upper mantle are very similar. Our data do not fit with the general models of lithospheric strength and may have important implications for the rheological structure of the lithosphere in transtensional plate margins and for geodynamic models of the region.

Crustal and mantle velocity models of southern Tibet from finite frequency tomography

Science.gov (United States)

Liang, Xiaofeng; Shen, Yang; Chen, Yongshun John; Ren, Yong

2011-02-01

Using traveltimes of teleseismic body waves recorded by several temporary local seismic arrays, we carried out finite-frequency tomographic inversions to image the three-dimensional velocity structure beneath southern Tibet to examine the roles of the upper mantle in the formation of the Tibetan Plateau. The results reveal a region of relatively high P and S wave velocity anomalies extending from the uppermost mantle to at least 200 km depth beneath the Higher Himalaya. We interpret this high-velocity anomaly as the underthrusting Indian mantle lithosphere. There is a strong low P and S wave velocity anomaly that extends from the lower crust to at least 200 km depth beneath the Yadong-Gulu rift, suggesting that rifting in southern Tibet is probably a process that involves the entire lithosphere. Intermediate-depth earthquakes in southern Tibet are located at the top of an anomalous feature in the mantle with a low Vp, a high Vs, and a low Vp/Vs ratio. One possible explanation for this unusual velocity anomaly is the ongoing granulite-eclogite transformation. Together with the compressional stress from the collision, eclogitization and the associated negative buoyancy force offer a plausible mechanism that causes the subduction of the Indian mantle lithosphere beneath the Higher Himalaya. Our tomographic model and the observation of north-dipping lineations in the upper mantle suggest that the Indian mantle lithosphere has been broken laterally in the direction perpendicular to the convergence beneath the north-south trending rifts and subducted in a progressive, piecewise and subparallel fashion with the current one beneath the Higher Himalaya.

Anisotropy of the upper mantle beneath the equatorial part of the Mid-Atlantic Ridge

Science.gov (United States)

Kendall, J. M.; Rychert, C.; Harmon, N.; Tharimena, S.; Agius, M. R.

2017-12-01

It has been long-known that the mantle beneath ocean spreading centres is anisotropic, holding the signature of the formation of new oceanic lithosphere and its coupling with the underlying convecting asthenosphere. Numerical studies have suggested that there should be significant differences between the anisotropy at slow versus fast spreading centres, but there is little observational evidence to calibrate these simulations, especially at slow spreading centres. Near the ridge axis, the anisotropic effects of melt versus the lattice preferred orientation of minerals is not well understood. Finally, the mantle flow near ridge-transform interactions is also poorly understood. Here we present observations of SKS splitting in a region of the Mid-Atlantic Ridge near the equator and offset by the Romanche and Chain Fracture Zones. An array of 37 ocean-bottom seismometers were deployed for a year in depths of up to nearly 6000m, with the aim of studying the nature of the lithosphere-asthenosphere boundary as it forms (the PiLAB - Passive Imaging of the lithosphere-asthenosphere boundary - experiment). Stations were deployed on crust that varies from newly formed to 80 My old. We analyse 40 teleseismic events of magnitude greater than 5.8 and with epicentral distances between 88 and 130 degrees. The ocean-bottom is a noisy environment and a range of filters are used to isolate the SKS, SKKS, and related signals. Furthermore, stacking splitting error envelopes is used to improve confidence in the splitting parameters. Many of the splitting measurements show an orientation parallel to the direction of plate spreading, as expected, but variability in the orientation of the anisotropy increases towards the ridge axis. The magnitude of the anisotropy is also quite variable and suggests larger delay times near the ridge axis. Off-axis anisotropy is interpreted in terms of deformation of peridotite due to mantle flow. Near the ridge axis, the effect of ridge-parallel melt

Carbonate stability in the reduced lower mantle

Science.gov (United States)

Dorfman, Susannah M.; Badro, James; Nabiei, Farhang; Prakapenka, Vitali B.; Cantoni, Marco; Gillet, Philippe

2018-05-01

Carbonate minerals are important hosts of carbon in the crust and mantle with a key role in the transport and storage of carbon in Earth's deep interior over the history of the planet. Whether subducted carbonates efficiently melt and break down due to interactions with reduced phases or are preserved to great depths and ultimately reach the core-mantle boundary remains controversial. In this study, experiments in the laser-heated diamond anvil cell (LHDAC) on layered samples of dolomite (Mg, Ca)CO3 and iron at pressure and temperature conditions reaching those of the deep lower mantle show that carbon-iron redox interactions destabilize the MgCO3 component, producing a mixture of diamond, Fe7C3, and (Mg, Fe)O. However, CaCO3 is preserved, supporting its relative stability in carbonate-rich lithologies under reducing lower mantle conditions. These results constrain the thermodynamic stability of redox-driven breakdown of carbonates and demonstrate progress towards multiphase mantle petrology in the LHDAC at conditions of the lowermost mantle.

Lateral Variations of the Mantle Transition Zone Structure beneath the Southeastern Tibetan Plateau Revealed by P-wave Receiver Functions

Science.gov (United States)

Bai, Y.; Ai, Y.; Jiang, M.; He, Y.; Chen, Q.

2017-12-01

The deep structure of the southeastern Tibetan plateau is of great scientific importance to a better understanding of the India-Eurasia collision as well as the evolution of the magnificent Tibetan plateau. In this study, we collected 566 permanent and temporary seismic stations deployed in SE Tibet, with a total of 77853 high quality P-wave receiver functions been extracted by maximum entropy deconvolution method. On the basis of the Common Conversion Point (CCP) stacking technique, we mapped the topography of the 410km and 660km discontinuities (hereinafter called the `410' and the `660'), and further investigated the lateral variation of the mantle transition zone (MTZ) thickness beneath this region. The background velocity model deduced from H-κ stacking results and a previous body-wave tomographic research was applied for the correction of the crustal and upper mantle heterogeneities beneath SE Tibet for CCP stacking. Our results reveal two significantly thickened MTZ anomalies aligned nearly in the south-north direction. The magnitude of both anomalies are 30km above the global average of 250km. The southern anomaly located beneath the Dianzhong sub-block and the Indo-China block is characterized by a slightly deeper `410' and a greater-than-normal `660', while the northern anomaly beneath western Sichuan has an uplifted `410' and a depressed `660'. Combining with previous studies in the adjacent region, we suggest that slab break-off may occurred during the eastward subduction of the Burma plate, with the lower part of the cold slab penetrated into the MTZ and stagnated at the bottom of the `660' which may cause the southern anomaly in our receiver function images. The origin of the Tengchong volcano is probably connected to the upwelling of the asthenospheric material caused by the slab break-off or to the ascending of the hot and wet material triggered by the dehydration of stagnant slab in the MTZ. The anomaly in the north, on the other hand, might be

Amphibious Magnetotelluric Investigation of the Aleutian Arc: Mantle Melt Generation and Migration beneath Okmok Caldera

Science.gov (United States)

Zelenak, G.; Key, K.; Bennington, N. L.; Bedrosian, P.

2015-12-01

Understanding the factors controlling the release of volatiles from the downgoing slab, the subsequent generation of melt in the overlying mantle wedge, the migration of melt to the crust, and its evolution and emplacement within the crust are important for advancing our understanding of arc magmatism and crustal genesis. Because melt and aqueous fluids are a few orders of magnitude more electrically conductive than unmelted peridotite, the conductivity-mapping magnetotelluric (MT) method is well-suited to imaging fluids and melt beneath arc volcanoes. Here we present conductivity results from an amphibious MT profile crossing Okmok volcano in the central Aleutian arc. The Aleutian arc is one of the most volcanically active regions in North America, making it an ideal location for studying arc magnetism. Okmok volcano, located on the northeastern portion of Umnak Island, is among the most active volcanoes in the Aleutian chain. In addition to two caldera-forming events in the Holocene, numerous eruptions in the past century indicate a robust magmatic supply. Previous coarse resolution seismic studies have inferred a crustal magma reservoir. In order to investigate the role fluids play in melting the mantle wedge, how melts ascend through the corner flow regime of the mantle wedge, how melt migrates and is stored within the upper mantle and crust, and how this impacts explosive caldera forming eruptions, we carried out an amphibious geophysical survey across the arc in June-July 2015. Twenty-nine onshore MT stations and 10 offshore stations were collected in a 3D array covering Okmok, and 43 additional offshore MT stations completed a 300 km amphibious profile starting at the trench, crossing the forearc, arc and backarc. Thirteen onshore passive seismic stations were also installed and will remain in place for one year to supplement the twelve permanent stations on the island. Data collected by this project will be used to map seismic velocity and electrical

S-wave attenuation structure beneath the northern Izu-Bonin arc

Science.gov (United States)

Takahashi, Tsutomu; Obana, Koichiro; Kodaira, Shuichi

2016-04-01

To understand temperature structure or magma distribution in the crust and uppermost mantle, it is essential to know their attenuation structure. This study estimated the 3-D S-wave attenuation structure in the crust and uppermost mantle at the northern Izu-Bonin arc, taking into account the apparent attenuation due to multiple forward scattering. In the uppermost mantle, two areas of high seismic attenuation (high Q -1) imaged beneath the volcanic front were mostly colocated with low-velocity anomalies. This coincidence suggests that these high- Q -1 areas in low-velocity zones are the most likely candidates for high-temperature regions beneath volcanoes. The distribution of random inhomogeneities indicated the presence of three anomalies beneath the volcanic front: Two were in high- Q -1 areas but the third was in a moderate- Q -1 area, indicating a low correlation between random inhomogeneities and Q -1. All three anomalies of random inhomogeneities were rich in short-wavelength spectra. The most probable interpretation of such spectra is the presence of volcanic rock, which would be related to accumulated magma intrusion during episodes of volcanic activity. Therefore, the different distributions of Q -1 and random inhomogeneities imply that the positions of hot regions in the uppermost mantle beneath this arc have changed temporally; therefore, they may provide important constraints on the evolutionary processes of arc crust and volcanoes.

Elastic and Anelastic Structure Beneath Eurasia

National Research Council Canada - National Science Library

Ekstrom, Goran

1997-01-01

The primary objective of this work has been to map the variations of elastic mantle properties beneath Eurasia over horizontal length scales of approximately 1000-1500 kilometers and vertial length...

Upper mantle compositional variations and discontinuity topography imaged beneath Australia from Bayesian inversion of surface-wave phase velocities and thermochemical modeling

DEFF Research Database (Denmark)

Khan, A.; Zunino, Andrea; Deschamps, F.

2013-01-01

Here we discuss the nature of velocity heterogeneities seen in seismic tomography images of Earth's mantle whose origins and relation to thermochemical variations are yet to be understood. We illustrate this by inverting fundamental-mode and higher-order surface-wave phase velocities for radial....../Fe and Mg/Si values relative to surrounding mantle. Correlated herewith are thermal variations that closely follow surface tectonics. We also observe a strong contribution to lateral variations in structure and topography across the “410 km” seismic discontinuity from thermochemically induced phase......-wave tomography models with other regional models is encouraging. Radial anisotropy is strongest at 150/200 km depth beneath oceanic/continental areas, respectively, and appears weak and homogeneous below. Finally, geoid anomalies are computed for a subset of sampled model and compared to observations....

Composition of uppermost mantle beneath the Northern Fennoscandia - numerical modeling and petrological interpretation

Science.gov (United States)

Virshylo, Ivan; Kozlovskaya, Elena; Prodaivoda, George; Silvennoinen, Hanna

2013-04-01

Studying of the uppermost mantle beneath the northern Fennoscandia is based on the data of the POLENET/LAPNET passive seismic array. Firstly, arrivals of P-waves of teleseismic events were inverted into P-wave velocity model using non-linear tomography (Silvennoinen et al., in preparation). The second stage was numerical petrological interpretation of referred above velocity model. This study presents estimation of mineralogical composition of the uppermost mantle as a result of numerical modeling. There are many studies concerning calculation of seismic velocities for polymineral media under high pressure and temperature conditions (Afonso, Fernàndez, Ranalli, Griffin, & Connolly, 2008; Fullea et al., 2009; Hacker, 2004; Xu, Lithgow-Bertelloni, Stixrude, & Ritsema, 2008). The elastic properties under high pressure and temperature (PT) conditions were modelled using the expanded Hook's law - Duhamel-Neumann equation, which allows computation of thermoelastic strains. Furthermore, we used a matrix model with multi-component inclusions that has no any restrictions on shape, orientation or concentration of inclusions. Stochastic method of conditional moment with computation scheme of Mori-Tanaka (Prodaivoda, Khoroshun, Nazarenko, & Vyzhva, 2000) is applied instead of traditional Voigt-Reuss-Hill and Hashin-Shtrikman equations. We developed software for both forward and inverse problem calculation. Inverse algorithm uses methods of global non-linear optimization. We prefer a "model-based" approach for ill-posed problem, which means that the problem is solved using geological and geophysical constraints for each parameter of a priori and final models. Additionally, we are checking at least several different hypothesis explaining how it is possible to get the solution with good fit to the observed data. If the a priori model is close to the real medium, the nearest solution would be found by the inversion. Otherwise, the global optimization is searching inside the

Contrasting isotopic mantle sources for proterozoic lamproites and kimberlites from the Cuddapah basin and eastern Dharwar craton: implication for proterozoic mantle heterogeneity beneath southern India

International Nuclear Information System (INIS)

Chalapathi Rao, N.V.; Gibson, S.A.; Pyle, D.M.; Dickin, A.P.

1998-01-01

Kimberlites intruding the Precambrian basement towards the western margin of the Cuddapah basin near Anantapur (1090 Ma) and Mahbubnagar (1360 Ma) in Andhra Pradesh have initial 87 Sr/ 86 Sr between 0.70205 to 0.70734 and σNd between +0.5 to +4.68. Mesoproterozoic lamproites (1380 Ma) from the Cuddapah basin (Chelima and Zangamarajupalle) and its NE margin (Ramannapeta) have initial 87 Sr/ 86 Sr between 0.70520 and 0.7390 and εNd from -6.43 to -8.29. Combined Sr- and Nd- isotopic ratios suggest that lamproites were derived from enriched sources which have time-averaged higher Rb/Sr and lower Sm/Nd ratios than the Bulk Earth whereas kimberlites were derived from depleted source with lower Rb/Sr and higher Sm/Nd ratios. Calculated T DM model ages suggest that the lamproite source enrichment (∼2 Ga) preceded that of kimberlites (∼1.37 Ga). Our work demonstrates the existence of isotopically contrasting upper mantle sources for southern Indian kimberlites and lamproites and provides evidence for a lateral, isotopically heterogeneous mantle beneath the Cuddapah basin and eastern Dharwar craton. The significance of our results in the context of diamond exploration is also highlighted. (author)

Slab-derived components in the subcontinental lithospheric mantle beneath Chilean Patagonia: Geochemistry and Sr-Nd-Pb isotopes of mantle xenoliths and host basalt

Science.gov (United States)

Jalowitzki, Tiago; Gervasoni, Fernanda; Conceição, Rommulo V.; Orihashi, Yuji; Bertotto, Gustavo W.; Sumino, Hirochika; Schilling, Manuel E.; Nagao, Keisuke; Morata, Diego; Sylvester, Paul

2017-11-01

modified oceanic crust throughout the initial stages of the Farallón-Aluk ridge collision during Paleocene to Eocene time. However, based on the tectonic evolution of southern South America, we cannot exclude the influence of long-lived subduction events beneath south Patagonia. Although we believe that the studied samples were brought to the surface in this geodynamic context, there is no evidence that ocean island basalt (OIB)-like melts related to the Farallón-Aluk asthenospheric slab window affected the peridotite composition. The host alkaline basalt is a single unit with a HIMU-like OIB signature characterized by marked positive Nb-Ta anomalies coupled with negative anomalies in highly incompatible and fluid-mobile elements (Rb, K, Pb, and Sr). The compositional similarity between the HIMU-like OIB mantle source and the host basalt is also evident from trace element ratios [(Ba-Th-K-La-Zr)/Nb] as well as by the low 87Sr/86Sri (0.703039-0.703058) and relatively high 143Nd/144Ndi (0.512880-0.512874) and 206Pb/204Pb (19.333-19.389) isotopic ratios. The low 206Pb/204Pb ratios compared to end-member HIMU lavas (e.g., Sta. Helena and the Cook-Austral Islands) suggest that this region was modified by processes associated with a prolonged period of subduction related to the Andean orogenesis and the recycling of several oceanic plates beneath the continent, following the Mesozoic breakup of Gondwana or an even older subduction-related event with young recycling ages (< 2 Ga).

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

The subcontinental mantle beneath southern New Zealand, characterised by helium isotopes in intraplate basalts and gas-rich springs

Science.gov (United States)

Hoke, L.; Poreda, R.; Reay, A.; Weaver, S. D.

2000-07-01

relationship with either age or proximity to the Cenozoic intraplate volcanic centres or with major faults. In general, areas characterised by mantle 3He emission are interpreted to define those regions beneath which mantle melting and basalt magma addition to the crust are recent. The strongest mantle 3He anomaly (equivalent to >80% mantle helium component) is centred over southern Dunedin, measured in magmatic CO 2-rich mineral water springs issuing from crystalline basement rocks which outcrop at the southern extent of Miocene intraplate basaltic volcanism which ceased 9 Ma ago. This mantle helium anomaly overlaps with an area characterised by elevated surface high heat flow, compatible with a long-lived mantle melt/heat input into the crust. In comparison Banks Peninsula, another Miocene intraplate basaltic centre, is characterised by relatively low surface heat flow and a small mantle helium contribution measured in a nitrogen-rich spring. Here the thermal transient induced by the magmatic event has either dissipated or has not reached the surface. In the former case one might be dealing with storage and mixing of magmatic and crustal gases at shallow crustal levels and in the latter with active to recent mantle-melt degassing at depth. Along the most actively deforming part of the plate boundary zone, the transpressional Alpine Fault and Marlborough fault systems, mantle helium is present in gas-rich springs in all those areas underlain by actively subducting oceanic crust (the Australian plate in the south and Pacific plate in the north), whereas the central part of the Alpine transpressional fault is characterised by pure crustal radiogenic helium. Areas where the mantle helium component is negligible are restricted to the centre part of the South Island, extending along its length from Southland to northern Canterbury and Murchison. These areas are interpreted to delineate the extent of thicker and colder lithosphere compared to all other areas where mantle helium

Formation of mantle "lone plumes" in the global downwelling zone - A multiscale modelling of subduction-controlled plume generation beneath the South China Sea

Science.gov (United States)

Zhang, Nan; Li, Zheng-Xiang

2018-01-01

It has been established that almost all known mantle plumes since the Mesozoic formed above the two lower mantle large low shear velocity provinces (LLSVPs). The Hainan plume is one of the rare exceptions in that instead of rising above the LLSVPs, it is located within the broad global mantle downwelling zone, therefore classified as a "lone plume". Here, we use the Hainan plume example to investigate the feasibility of such lone plumes being generated by subducting slabs in the mantle downwelling zone using 3D geodynamic modelling. Our geodynamic model has a high-resolution regional domain embedded in a relatively low resolution global domain, which is set up in an adaptive-mesh-refined, 3D mantle convection code ASPECT (Advanced Solver for Problems in Earth's ConvecTion). We use a recently published plate motion model to define the top mechanical boundary condition. Our modelling results suggest that cold slabs under the present-day Eurasia, formed from the Mesozoic subduction and closure of the Tethys oceans, have prevented deep mantle hot materials from moving to the South China Sea from regions north or west of the South China Sea. From the east side, the Western Pacific subduction systems started to promote the formation of a lower-mantle thermal-chemical pile in the vicinity of the future South China Sea region since 70 Ma ago. As the top of this lower-mantle thermal-chemical pile rises, it first moved to the west, and finally rested beneath the South China Sea. The presence of a thermochemical layer (possible the D″ layer) in the model helps stabilizing the plume root. Our modelling is the first implementation of multi-scale mesh in the regional model. It has been proved to be an effective way of modelling regional dynamics within a global plate motion and mantle dynamics background.

Magnitude corrections for attenuation in the upper mantle

International Nuclear Information System (INIS)

Anon.

1978-01-01

Since 1969, a consistent discrepancy in seismic magnitudes of nuclear detonations at NTS compared with magnitudes of detonations elsewhere in the world has been observed. This discrepancy can be explained in terms of a relatively high seismic attenuation for compressional waves in the upper mantle beneath the NTS and in certain other locations. A correction has been developed for this attenuation based on a relationship between the velocity of compressional waves at the top of the earth's mantle (just beneath the Mohorovicic discontinuity) and the seismic attenuation further down in the upper mantle. Our new definition of body-wave magnitude includes corrections for attenuation in the upper mantle at both ends of the teleseismic body-wave path. These corrections bring the NTS oservations into line with measurements of foreign events, and enable one to make more reliable estimates of yields of underground nuclear explosions, wherever the explosion occurs

Upper mantle seismic structure beneath southwest Africa from finite-frequency P- and S-wave tomography

DEFF Research Database (Denmark)

Soliman, Mohammad Youssof Ahmad; Yuan, Xiaohui; Tilmann, Frederik

2015-01-01

We present a 3D high-resolution seismic model of the southwestern Africa region from teleseismic tomographic inversion of the P- and S- wave data recorded by the amphibious WALPASS network. We used 40 temporary stations in southwestern Africa with records for a period of 2 years (the OBS operated...... for 1 year), between November 2010 and November 2012. The array covers a surface area of approximately 600 by 1200 km and is located at the intersection of the Walvis Ridge, the continental margin of northern Namibia, and extends into the Congo craton. Major questions that need to be understood......, probably related to surficial suture zones and the presence of fertile material. A shallower depth extent of the lithospheric plate of ∼100 km was observed beneath the ocean, consistent with plate-cooling models. In addition to tomographic images, the seismic anisotropy measurements within the upper mantle...

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

Update on the Search for Chemical Interactions Between the Core and Mantle

Science.gov (United States)

Walker, R. J.

2017-12-01

Recent tomographic studies provide strong geophysical evidence for deep mantle upwellings, commonly referred to as "plumes", rising from the core-mantle boundary to regions underlying some ocean island basalt occurrences. Nevertheless, the existence of plumes and their association with ocean islands remains questioned by some. In addition, the occurrence and extent of chemical exchange between the core and lowermost mantle remains essentially un-constrained. If some plumes rise from the core-mantle boundary and there has been some level of chemical interaction between the core and mantle at some point in time, then it is possible that plumes could contain a unique chemical or isotopic fingerprint that is characteristic of the core. There is currently no strong evidence supporting this possibility. The short-lived 182Hf→182W (t½ = 9 m.y.) system has been proposed as a geochemical tool for detecting possible core-mantle interactions. Mass balance constraints suggest the 182W/184W and W concentration of the core are 200 ppm lower and 20 times higher, respectively, than the bulk silicate Earth. Recent discovery of negative correlations between 182W/184W and 3He/4He in ocean island basalts (OIB) from Hawaii and Samoa suggests that these volcanic systems may access a primordial component inside the Earth with W-He isotopic characteristics broadly consistent with the core. However, direct contribution of metal from the outer core to a rising plume is inconsistent with the concentrations of highly siderophile elements (HSE) in the isotopically anomalous lavas. In order for the isotopically anomalous W and He to be tied to the core, a transfer mechanism for isotopic signal, other than metal infiltration into the mantle is needed, as is a present day storage site for the signal. The possible existence of one or more basal magma oceans at some points in Earth history present opportunity for isotopic exchange between the lowermost mantle and core, without collateral

Oceanic provenance of lithospheric mantle beneath Lower Silesia (SW Poland) and the two kinds of its "Fe-metasomatism"

Science.gov (United States)

Puziewicz, Jacek; Matusiak-Małek, Magdalena; Ntaflos, Theodoros; Kukuła, Anna; Ćwiek, Mateusz

2016-04-01

Our recent studies (Puziewicz et al. 2015, IJES 104:1913-1924, and references therein) show that the subcontinental lithospheric mantle (SCLM) beneath Lower Silesia (SW Poland) and neighbouring part of Upper Lusatia (SE Germany) is dominated by harzburgites. Part of them contain small amounts of clinopyroxene which, despite its primary textural appearance, is a late addition to the protoliths which are residues after extensive (up to 30 %) partial melting. This clinopyroxene was added to the harzburgites in Cenozoic times by alkaline basaltic melts migrating upwards from their asthenospheric sources during rifting in the Variscan foreland of the Alpine-Carpathian chain. The pre-rifting history of the SCLM beneath the region is thus recorded in the olivine and orthopyroxene. The forsterite content in olivine divides the Lower Silesian harzburgites into two groups: A (olivine Fo 90.5 - 92.0), and B (olivine Fo 84.0 - 90.0; for data see Puziewicz et al. 2015, op. cit.). The Al content in orthopyroxene is low and similar in both A and part of B harzburgites, called B1 in the following. The orthopyroxene occurring in the B1 harzburgites contains typically 0.05 - 0.10 atoms of Al per formula unit (corresponding to 0.5 - 2.5 wt. % Al2O3), although slightly lower (down to 0.02 a pfu) and slightly higher (up to 0.13 a pfu) Al contents occur in subordinate number of samples. The Al content in the B1 orthopyroxene is not correlated with forsterite content in coexisting olivine. The B2 harzburgites occur only in one site (Księginki). They contain orthopyroxene which Al content exhibits negative correlation with forsterite content in coexisting olivine. The most Al -rich orthopyroxene (0.24 atoms of Al pfu, corresponding to ca. 5.7 wt % Al2O3) coexists with olivine Fo 86.5 in Księginki. The low contents of Al in orthopyroxene is specific for the Lower Silesian/Upper Lusatian domain of European lithospheric mantle. The Al-poor mantle domain below Lower Silesia and upper

Seismic properties of the upper mantle beneath Lanzarote (Canary Islands): Model predictions based on texture measurements by EBSD

Science.gov (United States)

Vonlanthen, Pierre; Kunze, Karsten; Burlini, Luigi; Grobety, Bernard

2006-12-01

We present a petrophysical analysis of upper mantle xenoliths, collected in the Quaternary alkali basalt fields (Series III and IV) from the island of Lanzarote. The samples consist of eight harzburgite and four dunite nodules, 5 to 15 cm in size, and exhibit a typical protogranular to porphyroclastic texture. An anomalous foliation resulting from strong recovery processes is observed in half of the specimens. The lattice preferred orientations (LPO) of olivine, orthopyroxene and clinopyroxene were measured using electron backscatter diffraction (EBSD). In most samples, olivine exhibits LPOs intermediate between the typical single crystal texture and the [100] fiber texture. Occasionally, the [010] fiber texture was also observed. Simultaneous occurrence of both types of fiber textures suggests the existence of more than one deformation regime, probably dominated by a simple shear component under low strain rate and moderate to high temperature. Orthopyroxene and clinopyroxene display a weaker but significant texture. The LPO data were used to calculate the seismic properties of the xenoliths at PT conditions obtained from geothermobarometry, and were compared to field geophysical data reported from the literature. The velocity of P-waves (7.9 km/s) obtained for a direction corresponding to the existing seismic transect is in good agreement with the most recent geophysical interpretation. Our results are consistent with a roughly W-E oriented fastest P-wave propagation direction in the uppermost mantle beneath the Canary Islands, and with the lithosphere structure proposed by previous authors involving a crust-mantle boundary at around 18 km in depth, overlaid by intermediate material between 11 and 18 km.

Crustal and Upper Mantle Velocity Structure beneath Northwestern South America revealed by the CARMArray

Science.gov (United States)

Miao, W.; Cornthwaite, J.; Levander, A.; Niu, F.; Schmitz, M.; Dionicio, V.; Nader-Nieto, M. F.

2017-12-01

The Caribbean plate (CAR) is a fragment of the Farallon plate heavily modified by igneous processes that created the Caribbean large igneous province (CLIP) between 110 and 80 Ma.The CAR collided with and initiated subduction beneath northwestern South America plate (SA) at about 60-55 Ma as a narrow flat-slab subduction zone with an accretionary prism offshore, but no volcanic arc. Large scale regional tomography suggests that 1000 km of the CAR has been subducted (Van Benthem et al., 2013, JGR). The flat slab has caused Laramide-style basement uplifts of the Merida Andes, Sierra de la Perija, and Santa Marta ranges with elevations >5 km. The details of subduction geometry of the CAR plate beneath northeastern Colombia and northwestern Venezuela are complicated and remain unclear. The region of slab steepening lies below the triangular Maracaibo block (Bezada et al, 2010, JGR), bounded by major strike slip faults and currently escaping to the north over the CAR. Geodetic data suggests the this region has the potential for a magnitude 8+ earthquake (Bilham and Mencin, 2013, AGU Abstract). To better understand the subduction geometry, we deployed 65 broadband (BB) stations across northeastern Colombia and northwestern Venezuela in April of 2016. The 65 stations interweave with the 32 existing Colombian and Venezuelan BB stations, forming a 2-D array (hereafter referred to as CARMArray) with a station spacing of 35-100 km that covers an area of 600 km by 400 km extending from the Caribbean coast in Colombia to the interior plains of Venezuela. With data from the first year of operation, we have measured the Rayleigh wave phase velocities and Z/H ratios in the period range of 8-40 s using both ambient noise and earthquake data recorded by the CARMArray. We also generated Ps receiver functions from waveform data of teleseismic events recorded by the array. We then jointly inverted the three datasets to construct a 3-D S-wave velocity model beneath the array. We will

Structure in the lowermost mantle from seismic anisotropy

Science.gov (United States)

Walpole, J.; Wookey, J. M.; Nowacki, A.; Walker, A.; Kendall, J. M.; Masters, G.; Forte, A. M.

2017-12-01

Anisotropy is well established in D'' and places important constraints on the nature and dynamics of this elusive region. We present the results of a recent study probing anisotropy in D'', over a large area, using shear wave splitting on core-reflected ScS phases. Our dataset contains laterally continuous coverage beneath a large swath of east Asia - extending about 3000 km along the CMB - from south-east Asia to the north-east Pacific. The centre of this area represents a large down-welling core for subduction that has occurred over several super-continent cycles. In the centre of this region we observe a clear VSV}>V{SH fabric, in direct conflict with the prevailing view that fast, `cold', regions are associated with VSH}>V{SV fabric. Furthermore, systematic rotation of the fast axis traces out an apparent dome-like feature extending over thousands of km, albeit complicated by some short-scale variability. The dataset also samples regions where slab material may be actively impinging on the CMB; and a region corresponding to the edge of the Pacific LLSVP. We interpret our results in light of a combined computational geodynamic-petrofabric-seismic study designed to test the possibility that anisotropy is caused by the lattice preferred orientation of post-perovskite. We take into account the important finite-frequency effects of wave propagation in our synthetics by using the SPECFEM3D_GLOBE code; this can lead to drastically different results when compared to the less accurate ray theory.

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.

Slab melting and magma formation beneath the southern Cascade arc

Science.gov (United States)

Walowski, Kristina J.; Wallace, Paul J.; Clynne, Michael A.; Rasmussen, D.J.; Weis, D.

2016-01-01

The processes that drive magma formation beneath the Cascade arc and other warm-slab subduction zones have been debated because young oceanic crust is predicted to largely dehydrate beneath the forearc during subduction. In addition, geochemical variability along strike in the Cascades has led to contrasting interpretations about the role of volatiles in magma generation. Here, we focus on the Lassen segment of the Cascade arc, where previous work has demonstrated across-arc geochemical variations related to subduction enrichment, and H-isotope data suggest that H2O in basaltic magmas is derived from the final breakdown of chlorite in the mantle portion of the slab. We use naturally glassy, olivine-hosted melt inclusions (MI) from the tephra deposits of eight primitive (MgO>7 wt%) basaltic cinder cones to quantify the pre-eruptive volatile contents of mantle-derived melts in this region. The melt inclusions have B concentrations and isotope ratios that are similar to mid-ocean ridge basalt (MORB), suggesting extensive dehydration of the downgoing plate prior to reaching sub-arc depths and little input of slab-derived B into the mantle wedge. However, correlations of volatile and trace element ratios (H2O/Ce, Cl/Nb, Sr/Nd) in the melt inclusions demonstrate that geochemical variability is the result of variable addition of a hydrous subduction component to the mantle wedge. Furthermore, correlations between subduction component tracers and radiogenic isotope ratios show that the subduction component has less radiogenic Sr and Pb than the Lassen sub-arc mantle, which can be explained by melting of subducted Gorda MORB beneath the arc. Agreement between pMELTS melting models and melt inclusion volatile, major, and trace element data suggests that hydrous slab melt addition to the mantle wedge can produce the range in primitive compositions erupted in the Lassen region. Our results provide further evidence that chlorite-derived fluids from the mantle portion of the

Morphological indicators of a mascon beneath Ceres' largest crater, Kerwan

Science.gov (United States)

Bland, Michael T.; Ermakov, Anton; Raymond, Carol A.; Williams, David A.; Bowling, Tim J.; Preusker, F.; Park, Ryan S.; Marchi, Simone; Castillo-Rogez, Julie C.; Fu, R.R.; Russell, Christopher T.

2018-01-01

Gravity data of Ceres returned by the National Aeronautics and Space Administration's Dawn spacecraft is consistent with a lower density crust of variable thickness overlying a higher density mantle. Crustal thickness variations can affect the long‐term, postimpact modification of impact craters on Ceres. Here we show that the unusual morphology of the 280 km diameter crater Kerwan may result from viscous relaxation in an outer layer that thins substantially beneath the crater floor. We propose that such a structure is consistent with either impact‐induced uplift of the high‐density mantle beneath the crater or from volatile loss during the impact event. In either case, the subsurface structure inferred from the crater morphology is superisostatic, and the mass excess would result in a positive Bouguer anomaly beneath the crater, consistent with the highest‐degree gravity data from Dawn. Ceres joins the Moon, Mars, and Mercury in having basin‐associated gravity anomalies, although their origin may differ substantially.

Morphological Indicators of a Mascon Beneath Ceres's Largest Crater, Kerwan

Science.gov (United States)

Bland, M. T.; Ermakov, A. I.; Raymond, C. A.; Williams, D. A.; Bowling, T. J.; Preusker, F.; Park, R. S.; Marchi, S.; Castillo-Rogez, J. C.; Fu, R. R.; Russell, C. T.

2018-02-01

Gravity data of Ceres returned by the National Aeronautics and Space Administration's Dawn spacecraft is consistent with a lower density crust of variable thickness overlying a higher density mantle. Crustal thickness variations can affect the long-term, postimpact modification of impact craters on Ceres. Here we show that the unusual morphology of the 280 km diameter crater Kerwan may result from viscous relaxation in an outer layer that thins substantially beneath the crater floor. We propose that such a structure is consistent with either impact-induced uplift of the high-density mantle beneath the crater or from volatile loss during the impact event. In either case, the subsurface structure inferred from the crater morphology is superisostatic, and the mass excess would result in a positive Bouguer anomaly beneath the crater, consistent with the highest-degree gravity data from Dawn. Ceres joins the Moon, Mars, and Mercury in having basin-associated gravity anomalies, although their origin may differ substantially.

Heterogeneous seismic anisotropy in the transition zone and uppermost lower mantle: evidence from South America, Izu-Bonin and Japan

Science.gov (United States)

Lynner, Colton; Long, Maureen D.

2015-06-01

Measurements of seismic anisotropy are commonly used to constrain deformation in the upper mantle. Observations of anisotropy at mid-mantle depths are, however, relatively sparse. In this study we probe the anisotropic structure of the mid-mantle (transition zone and uppermost lower mantle) beneath the Japan, Izu-Bonin, and South America subduction systems. We present source-side shear wave splitting measurements for direct teleseismic S phases from earthquakes deeper than 300 km that have been corrected for the effects of upper mantle anisotropy beneath the receiver. In each region, we observe consistent splitting with delay times as large as 1 s, indicating the presence of anisotropy at mid-mantle depths. Clear splitting of phases originating from depths as great as ˜600 km argues for a contribution from anisotropy in the uppermost lower mantle as well as the transition zone. Beneath Japan, fast splitting directions are perpendicular or oblique to the slab strike and do not appear to depend on the propagation direction of the waves. Beneath South America and Izu-Bonin, splitting directions vary from trench-parallel to trench-perpendicular and have an azimuthal dependence, indicating lateral heterogeneity. Our results provide evidence for the presence of laterally variable anisotropy and are indicative of variable deformation and dynamics at mid-mantle depths in the vicinity of subducting slabs.

3-D Upper-Mantle Shear Velocity Model Beneath the Contiguous United States Based on Broadband Surface Wave from Ambient Seismic Noise

Science.gov (United States)

Xie, Jun; Chu, Risheng; Yang, Yingjie

2018-05-01

Ambient noise seismic tomography has been widely used to study crustal and upper-mantle shear velocity structures. Most studies, however, concentrate on short period (structure on a continental scale. We use broadband Rayleigh wave phase velocities to obtain a 3-D V S structures beneath the contiguous United States at period band of 10-150 s. During the inversion, 1-D shear wave velocity profile is parameterized using B-spline at each grid point and is inverted with nonlinear Markov Chain Monte Carlo method. Then, a 3-D shear velocity model is constructed by assembling all the 1-D shear velocity profiles. Our model is overall consistent with existing models which are based on multiple datasets or data from earthquakes. Our model along with the other post-USArray models reveal lithosphere structures in the upper mantle, which are consistent with the geological tectonic background (e.g., the craton root and regional upwelling provinces). The model has comparable resolution on lithosphere structures compared with many published results and can be used for future detailed regional or continental studies and analysis.

Hot upwelling conduit beneath the Atlas Mountains, Morocco

Science.gov (United States)

Sun, Daoyuan; Miller, Meghan S.; Holt, Adam F.; Becker, Thorsten W.

2014-11-01

The Atlas Mountains of Morocco display high topography, no 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. However, the existence, shape, and physical properties of an associated mantle anomaly are debated. Here we use seismic waveform analysis from a broadband deployment and geodynamic modeling to define the physical properties and morphology of the anomaly. The imaged low-velocity structure extends to ~200 km beneath the Atlas and appears ~350 K hotter than the ambient mantle with possible partial melting. It includes a lateral conduit, which suggests that the Quaternary volcanism arises from the upper mantle. Moreover, the shape and temperature of the imaged anomaly indicate that the unusually high topography of the Atlas Mountains is due to active mantle support.

Fine crustal and uppermost mantle S-wave velocity structure beneath the Tengchong volcanic area inferred from receiver function and surface-wave dispersion: constraints on magma chamber distribution

Science.gov (United States)

Li, Mengkui; Zhang, Shuangxi; Wu, Tengfei; Hua, Yujin; Zhang, Bo

2018-03-01

The Tengchong volcanic area is located in the southeastern margin of the collision zone between the Indian and Eurasian Plates. It is one of the youngest intraplate volcano groups in mainland China. Imaging the S-wave velocity structure of the crustal and uppermost mantle beneath the Tengchong volcanic area is an important means of improving our understanding of its volcanic activity and seismicity. In this study, we analyze teleseismic data from nine broadband seismic stations in the Tengchong Earthquake Monitoring Network. We then image the crustal and uppermost mantle S-wave velocity structure by joint analysis of receiver functions and surface-wave dispersion. The results reveal widely distributed low-velocity zones. We find four possible magma chambers in the upper-to-middle crust and one in the uppermost mantle. The chamber in the uppermost mantle locates in the depth range from 55 to 70 km. The four magma chambers in the crust occur at different depths, ranging from the depth of 7 to 25 km in general. They may be the heat sources for the high geothermal activity at the surface. Based on the fine crustal and uppermost mantle S-wave velocity structure, we propose a model for the distribution of the magma chambers.

Deformation and hydration state of the lithospheric mantle beneath the Styrian Basin (Pannonian Basin, Eastern Austria)

Science.gov (United States)

Aradi, L. E.; Hidas, K.; Kovács, I. J.; Klébesz, R.; Szabo, C.

2016-12-01

the shear wave delay time observed on the surface could be generated in the lithospheric mantle in addition to the contribution of the assumed NW-SE asthenospheric flow beneath the Styrian Basin.

Imaging Lithospheric-scale Structure Beneath Northern Altiplano in Southern Peru and Northern Bolivia

Science.gov (United States)

Kumar, A.; Wagner, L. S.; Beck, S. L.; Zandt, G.; Long, M. D.

2014-12-01

The northern Altiplano plateau of southern Peru and northern Bolivia is one of the highest topographic features on the Earth, flanked by Western and Eastern Cordillera along its margin. It has strongly influenced the local and far field lithospheric deformation since the early Miocene (Masek et al., 1994). Previous studies have emphasized the importance of both the crust and upper mantle in the evolution of Altiplano plateau (McQuarrie et al., 2005). Early tomographic and receiver function studies, south of 16° S, show significant variations in the crust and upper mantle properties in both perpendicular and along strike direction of the Altiplano plateau (Dorbath et. al., 1993; Myers et al., 1998; Beck and Zandt, 2002). In order to investigate the nature of subsurface lithospheric structure below the northern Altiplano, between 15-18° S, we have determined three-dimensional seismic tomography models for Vp and Vs using P and S-wave travel time data from two recently deployed local seismic networks of CAUGHT and PULSE. We also used data from 8 stations from the PERUSE network (PERU Subduction Experiment). Our preliminary tomographic models show a complex variation in the upper mantle velocity structure with depth, northwest and southeast of lake Titicaca. We see the following trend, at ~85 km depth, northwest of lake Titicaca: low Vp and Vs beneath the Western Cordillera, high Vs beneath the Altiplano and low Vp and Vs beneath the Eastern Cordillera. This low velocity anomaly, beneath Eastern Cordillera, seems to coincide with Kimsachata, a Holocene volcano in southern Peru. At depth greater than ~85 km: we find high velocity anomaly beneath the Western Cordillera and low Vs beneath the Altiplano. This high velocity anomaly, beneath Western Cordillera, coincides with the well-located Wadati-Benioff zone seismicity and perhaps represents the subducting Nazca slab. On the southeast of lake Titicaca, in northern Bolivia, we see a consistently high velocity anomaly

Lateral temperature variations at the core-mantle boundary deduced from the magnetic field

Science.gov (United States)

Bloxham, Jeremy; Jackson, Andrew

1990-01-01

Recent studies of the secular variation of the earth's magnetic field over periods of a few centuries have suggested that the pattern of fluid motion near the surface of earth's outer core may be strongly influenced by lateral temperature variations in the lowermost mantle. This paper introduces a self-consistent method for finding the temperature variations near the core surface by assuming that the dynamical balance there is geostrophic and that lateral density variations there are thermal in origin. As expected, the lateral temperature variations are very small. Some agreement is found between this pattern and the pattern of topography of the core-mantle boundary, but this does not conclusively answer to what extent core surface motions are controlled by the mantle, rather than being determined by processes in the core.

Sr-Nd evidence of paleoproterozoic mantle metasomatism in the lithospheric mantle beneath northeastern Brazil

International Nuclear Information System (INIS)

Hollanda, M.H.B.M.; Pimentel, M.M.; Jardim de Sa, E.F

2001-01-01

discuss about mantle metasomatism against crustal contamination. The difficulty in commenting about this question taking into consideration Proterozoic mantle-derived plutonic rocks is related to non-uniqueness in interpreting the common enriched signatures, since that are similar to geochemical signature of crustal rocks. In this study, the data were carefully treated for filtering out the effects of crustal contamination to recognise the nature of their mantle source and obtain a picture of the lithospheric mantle chemical at Proterozoic time (au)

Constitution and structure of earth's mantle

DEFF Research Database (Denmark)

Zunino, Andrea; Khan, Amir; Cupillard, Paul

2016-01-01

the inaccessible parts of the Earth. Computation of physical properties using thermodynamic models is described and discussed, and an application of the joint inverse methodology is illustrated in a case study where mantle composition and thermal state beneath continental Australia is determined directly from...

Seismic attenuation structure beneath Nazca Plate subduction zone in southern Peru

Science.gov (United States)

Jang, H.; Kim, Y.; Clayton, R. W.

2017-12-01

We estimate seismic attenuation in terms of quality factors, QP and QS using P and S phases, respectively, beneath Nazca Plate subduction zone between 10°S and 18.5°S latitude in southern Peru. We first relocate 298 earthquakes with magnitude ranges of 4.0-6.5 and depth ranges of 20-280 km. We measure t*, which is an integrated attenuation through the seismic raypath between the regional earthquakes and stations. The measured t* are inverted to construct three-dimensional attenuation structures of southern Peru. Checkerboard test results for both QP and QS structures ensure good resolution in the slab-dip transition zone between flat and normal slab subduction down to a depth of 200 km. Both QP and QS results show higher attenuation continued down to a depth of 50 km beneath volcanic arc and also beneath the Quimsachata volcano, the northernmost young volcano, located far east of the main volcanic front. We also observe high attenuation in mantle wedge especially beneath the normal subduction region in both QP and QS (100-130 in QP and 100-125 in QS) and slightly higher QP and QS beneath the flat-subduction and slab-dip transition regions. We plan to relate measured attenuation in the mantle wedge to material properties such as viscosity to understand the subduction zone dynamics.

Upper mantle low velocity heterogeneities beneath NE China revealed by source- and receiver-side converted waves

Science.gov (United States)

Guan, Z.; Niu, F.

2017-12-01

Common-conversion-point (CCP) stacking of receiver function is a powerful tool in mapping upper mantle heterogeneities. However, reverberations from shallow boundaries with large velocity contrast could contaminate the imaging profiles severely. Applying the refined Slowness Weighted CCP (SWCCP) stacking technique (Guan and Niu, 2017) on NECESSArray data, we eliminated the multiple effects and systematically imaged the upper mantle low velocity heterogeneities in NE China where there exist rich unconsolidated sediments. The SWCCP profiles reveal a 350 km low velocity heterogeneity which is possibly associated with the Changbai Mountain volcanism and interpreted as a negatively buoyant silicate melt lying atop of the 410 km discontinuity. Besides, the imaging results are also suggestive of a sporadic 580-620 km low velocity heterogeneity locating in the easternmost part of NE China with a velocity contrast comparable with the 660-km discontinuity. In addition, between 42º N and 45º N, we also found a double 660-km discontinuity at the two sides of the localized depression in the longitudinal range of 128º E to 131º E. On the other hand, we gathered USArray and Alaska regional array seismic data of deep earthquakes occurring beneath NE China and the surrounding areas and employed stacking technique to study the source side S-to-P conversions. The source-side stacking also showed a strong S-to-P conversion at 600 km deep, consistent with the SWCCP stacks. Meanwhile, we also confirmed the double 660-km discontinuity feature from the source-side conversions. The receiver- and source-side observations provide strong constraints on these low velocity anomalies that may offer insights on the subduction dynamics of the Pacific plate.

Tomographic and Geodynamic Constraints on Convection-Induced Mixing in Earth's Deep Mantle

Science.gov (United States)

Hafter, D. P.; Forte, A. M.; Bremner, P. M.; Glisovic, P.

2017-12-01

Seismological studies reveal two large low-shear-velocity provinces (LLSVPs) in the lowermost mantle (e.g., Su et al. 1994; Wang & Wen 2007; He & Wen 2012), which may represent accumulations of subducted slabs at the CMB (Tan & Gurnis 2005; Christensen & Hoffman 1994) or primordial material generated in the early differentiation of Earth (e.g. Li et al. 2014). The longevity or stability of these large-scale heterogeneities in the deep mantle depends on the vigor and spatial distribution of the convective circulation, which is in turn dependent on the distribution of mantle buoyancy and viscosity (e.g. Glisovic & Forte 2015). Here we explore the state of convective mixing in the mantle using the ASPECT convection code (Kronbichler et al. 2012). A series of experiments are conducted to consider the geochemical and dynamical contributions of LLSVPs to deep-mantle upwellings and corresponding plume-sourced volcanism. The principal feature of these experiments is the use of particle tracers to track geochemical changes in the LLSVPs and mantle plumes in addition to identifying those parts of the mantle that may remain unmixed. We employ 3-D mantle density anomalies derived from joint inversions of seismic, geodynamic and mineral physics constraints and geodynamically-constrained viscosity distributions (Glisovic et al. 2015) to ensure that the predicted flow fields yield a good match to key geophysical constraints (e.g. heat flow, global gravity anomalies and plate velocities).

3D Crust and Uppermost Mantle Structure beneath Tian Shan Region from ambient noise and earthquake surface waves

Science.gov (United States)

Xiao, X.; Wen, L.

2017-12-01

As a typical active intracontinental mountain range in Central Asia, Tian Shan Mt serves as the prototype in studying geodynamic processes and mechanism of intracontinental mountain building. We study 3D crust and the uppermost mantle structure beneath Tian Shan region using ambient noise and earthquake surface waves. Our dataset includes vertical component records of 62 permanent broadband seismic stations operated by the Earthquake Administration of China. Firstly, we calculate two-year stacked Cross-Correlation Functions (CCFs) of ambient noise records between the stations. The CCFs are treated as the Empirical Green's Functions (EGFs) of each station pair, from which we measured phase velocities of fundamental-mode Rayleigh wave in the period of 3-40 s using a frequency-time analysis method. Secondly, we collect surface wave data from tele-seismic events with Mw > 5.5 and depth shallower than 200 km and measure phase velocities of the fundamental-mode of Rayleigh wave in the period of 30-150 s using a two-station method. Finally, we combine the phase velocity measurements from ambient noise and earthquake surface waves, obtain lateral isotropic phase velocity maps at different periods based on tomography and invert a 3D Vsv model of crust and uppermost mantle down to about 150 km using a Monte Carlo Inversion method. We will discuss our inversion results in detail, as well as their implications to the tectonics in the region.

Petrological constraints on melt generation beneath the Asal Rift (Djibouti) using quaternary basalts

Science.gov (United States)

Pinzuti, Paul; Humler, Eric; Manighetti, Isabelle; Gaudemer, Yves

2013-08-01

The temporal evolution of the mantle melting processes in the Asal Rift is evaluated from the chemical composition of 56 new lava flows sampled along 10 km of the rift axis and 9 km off-axis (i.e., erupted within the last 620 kyr). Petrological and primary geochemical results show that most of the samples of the inner floor of the Asal Rift are affected by plagioclase accumulation. Trace element ratios and major element compositions corrected for mineral accumulation and crystallization show a symmetric pattern relative to the rift axis and preserved a clear signal of mantle melting depth variations. While FeO, Fe8.0, Zr/Y, and (Dy/Yb)N decrease from the rift shoulders to the rift axis, SiO2, Na/Ti, Lu/Hf increase and Na2O and Na8.0 are constant across the rift. These variations are qualitatively consistent with shallow melting beneath the rift axis and deeper melting for off-axis lava flows. Na8.0 and Fe8.0 contents show that beneath the rift axis, melting paths are shallow, from 81 ± 4 to 43 ± 5 km. These melting paths are consistent with adiabatic melting in normal-temperature fertile asthenosphere, beneath an extensively thinned mantle lithosphere. On the contrary, melting on the rift shoulders (from 107 ± 7 to 67 ± 8 km) occurred beneath thicker lithosphere, requiring a mantle solidus temperature 100 ± 40°C hotter. In this geodynamic environment, the calculated rate of lithospheric thinning appears to be 4.0 ± 2.0 cm yr-1, a value close to the mean spreading rate (2.9 ± 0.2 cm yr-1) over the last 620 kyr.

Imaging of Upper-Mantle Upwelling Beneath the Salton Trough, Southern California, by Joint Inversion of Ambient Noise Dispersion Curves and Receiver Functions

Science.gov (United States)

Klemperer, S. L.; Barak, S.

2016-12-01

We present a new 2D shear-wave velocity model of the crust and upper-mantle across the Salton Trough, southern California, obtained by jointly inverting our new dataset of receiver functions and our previously published Rayleigh-wave group-velocity model (Barak et al., G-cubed, 2015), obtained from ambient-noise tomography. Our results show an upper-mantle low-velocity zone (LVZ) with Vs ≤4.2 km/s extending from the Elsinore Fault to the Sand Hills Fault, that together bracket the full width of major San Andreas dextral motion since its inception 6 Ma b.p., and underlying the full width of low topography of the Imperial Valley and Salton Trough. The lateral extent of the LVZ is coincident with the lateral extent of an upper-mantle anisotropic region interpreted as a zone of SAF-parallel melt pockets (Barak & Klemperer, Geology, 2016). The shallowest part of the LVZ is 40 km depth, coincident with S-receiver function images. The western part of the LVZ, between the Elsinore and San Jacinto faults (the region of greatest modern dextral slip), appears to continue to significantly greater depth; but a puzzling feature of our preliminary models is that the eastern part of the LVZ, from the San Jacinto Fault to the Sand Hills Fault, appears to be underlain by more-normalvelocity upper mantle (Vs ≥ 4.5 km/s) below 75 km depth. We compare our model to the current SCEC community models CVM-H and CVM-S, and to P-wave velocity models obtained by the active-source Salton Sea Imaging Project (SSIP). The hypothesized lower-crustal low-velocity zone beneath the Salton Trough in our previous model (Barak et al., G-cubed, 2015), there interpreted as a region of partial melt, is not supported by our new modeling. Melt may be largely absent from the lower crust of the Salton trough; but appears required in the upper mantle at depths as shallow as 40 km.

Universal single grain amphibole thermobarometer for mantle rocks - preliminary calibration.

Science.gov (United States)

Ashchepkov, Igor

2017-04-01

SCLM and the third group refer to richterites form the depleted manle peridotites. In SCLM beneat the Sytykanskaya they are more frequent and trace through all the mantle layers. In SCLM beneat the Aykhal they mostly are from the lower and in Komsomolskaya from the middle SCLM parts. In Daldyn field rare amdphibles from Dalnaya are Fe- enriched pargasites belonging to the Ilm bearing peridotites in middle SCLM part as well as in SCLM beneath thr Udachnaya. But there are Fe- low amphiboles substitutng the orthopyroxenes. In Zarnitsa the Cr - hornblendes occur in shallow garnet pyroxenites. One deep seated richterite substitute garnet grains. Rare amphiboles were detedted in Mirninsky filed in Internatiolnaya pipe and reffer to the resorbed and deformed granets from the Garnet -Spinel facies and from 4.0 GPa boundary. Amphiboles are frequent in the SCLM from the northern part of Siberian craton. In SCLM beneath the Kharmai the Fe- encriched varietes are from the Moho boundary. Common Cr-pargasite occurs to 3 GPa in Obnazhennay, pipe, Kharamai field In mantle SCLM beneath Obnazhennaya pipe and circum Anabr region friquent Cr- pargasies and horblendes refer to the relatively hot branch of mantle lithosphere and probably corresponds to the Triassic mantle reactivation. Mantle Cr- hornbleneds occurs on most upper part of the mantle column beneath Quaternary mujeritic Bartoy vocanoes in Transbaikal. The pargasites and kaersutites in this locality refer to more heated conditions and could be found to 2.0 GPa. Grant RFBR 16.-05-000860

P-wave velocity structure beneath the northern Antarctic Peninsula

Science.gov (United States)

Park, Y.; Kim, K.; Jin, Y.

2010-12-01

We have imaged tomographically the tree-dimensional velocity structure of the upper mantle beneath the northern Antarctic Peninsula using teleseismic P waves. The data came from the seven land stations of the Seismic Experiment in Patagonia and Antarctica (SEPA) campaigned during 1997-1999, a permanent IRIS/GSN station (PMSA), and 3 seismic stations installed at scientific bases, Esperanza (ESPZ), Jubany (JUBA), and King Sejong (KSJ), in South Shetland Islands. All of the seismic stations are located in coast area, and the signal to noise ratios (SNR) are very low. The P-wave model was inverted from 95 earthquakes resulting in 347 ray paths with P- and PKP-wave arrivals. The inverted model shows a strong low velocity anmaly beneath the Bransfield Strait, and a fast anomaly beneath the South Shetland Islands. The low velocity anomaly beneath the Bransfield might be due to a back arc extension, and the fast velocity anomaly beneath the South Shetland Islands could indicates the cold subducted slab.

Mantle Flow Across the Baikal Rift Constrained With Integrated Seismic Measurements

Science.gov (United States)

Lebedev, S.; Meier, T.; van der Hilst, R. D.

2005-12-01

The Baikal Rift is located at the boundary of the stable Siberian Craton and deforming central Mongolia. The origin of the late Cenozoic rifting and volcanism are debated, as is the mantle flow beneath the rift zone. Here we combine new evidence from azimuthally-anisotropic upper-mantle tomography and from a radially-anisotropic inversion of interstation surface-wave dispersion curves with previously published shear-wave-splitting measurements of azimuthal anisotropy across the rift (Gao et al. 1994). While our tomographic model maps isotropic and anisotropic shear-velocity heterogeneity globally, the inversion of interstation phase-velocity measurements produces a single, radially-anisotropic, shear-velocity profile that averages from the rift to 500 km SE of it. The precision and the broad band (8-340 s) of the Rayleigh and Love wave curves ensures high accuracy of the profile. Tomography and shear-wave splitting both give a NW-SE fast direction (perpendicular to the rift) in the vicinity of the rift, changing towards W-E a few hundred kilometers from it. Previously, this has been interpreted as evidence for mantle flow similar to that beneath mid-ocean ridges, with deeper vertical flow directly beneath the rift also proposed. Our radially anisotropic profile, however, shows that while strong anisotropy with SH waves faster than SV waves is present in the thin lithosphere and upper asthenosphere beneath and SE of the rift, no anisotropy is required below 110 km. The tomographic model shows thick cratonic lithosphere north of the rift. These observations suggest that instead of a flow diverging from the rift axis in NW and SE directions, the most likely pattern is the asthenospheric flow in SE direction from beneath the Siberian lithosphere and across the rift. Possible driving forces of the flow are large-scale lithospheric deformation in East Asia and the draining of asthenosphere at W-Pacific subduction zones; a plume beneath the Siberian craton also cannot be

One billion year-old Mid-continent Rift leaves virtually no clues in the mantle

Science.gov (United States)

Bollmann, T. A.; Frederiksen, A. W.; van der Lee, S.; Wolin, E.; Revenaugh, J.; Wiens, D.; Darbyshire, F. A.; Aleqabi, G. I.; Wysession, M. E.; Stein, S.; Jurdy, D. M.

2017-12-01

We measured the relative arrival times of more than forty-six thousand teleseismic P waves recorded by seismic stations of Earthscope's Superior Province Rifting Earthscope Experiment (SPREE) and combined them with a similar amount of such measurements from other seismic stations in the larger region. SPREE recorded seismic waves for two and a half years around the prominent, one billion year-old Mid-continent Rift structure. The curvilinear Mid-continent Rift (MR) is distinguished by voluminous one billion year-old lava flows, which produce a prominent gravity high along the MR. As for other seismic waves, these lava flows along with their underplated counterpart, slightly slow down the measured teleseismic P waves, on average, compared to P waves that did not traverse structures beneath the Mid-continent Rift. However, the variance in the P wave arrival times in these two groups is nearly ten times higher than their average difference. In a seismic-tomographic inversion, we mapped all measured arrival times into structures deep beneath the crust, in the Earth's mantle. Beneath the crust we generally find relatively high P velocities, indicating relatively cool and undeformable mantle structures. However, the uppermost mantle beneath the MR shows several patches of slightly decreased P velocities. These patches are coincident with where the gravity anomalies peak, in Iowa and along the northern Minnesota/Wisconsin border. We will report on the likelihood that these anomalies are indeed a remaining mantle-lithospheric signature of the MR or whether these patches indirectly reflect the presence of the lava flows and their underplated counterparts at the crust-mantle interface. Other structures of interest and of varying depth extent in our tomographic image locate at 1) the intersection of the Superior Craton with the Penokean Province and the Marshfield Terrane west of the MR in southern Minnesota, 2) the intersection of the Penokean, Yavapai, and Mazatzal Terranes

Tomography images of the Alpine roots and surrounding upper mantle

Science.gov (United States)

Plomerova, Jaroslava; Babuska, Vladislav

2017-04-01

Teleseismic body-wave tomography represents powerful tool to study regional velocity structure of the upper mantle and to image velocity anomalies, such as subducted lithosphere plates in collisional zones. In this contribution, we recapitulate 3D models of the upper mantle beneath the Alps, which developed at a collision zone of the Eurasian and African plates. Seismic tomography studies indicate a leading role of the rigid mantle lithosphere that functioned as a major stress guide during the plate collisions. Interactions of the European lithosphere with several micro-plates in the south resulted in an arcuate shape of this mountain range on the surface and in a complicated geometry of the Alpine subductions in the mantle. Early models with one bended lithosphere root have been replaced with more advanced models showing two separate lithosphere roots beneath the Western and Eastern Alps (Babuska et al., Tectonophysics 1990; Lippitsch et al., JGR 2003). The standard isotropic velocity tomography, based on pre-AlpArray data (the currently performed passive seismic experiment in the Alps and surroundings) images the south-eastward dipping curved slab of the Eurasian lithosphere in the Western Alps. On the contrary, beneath the Eastern Alps the results indicate a very steep northward dipping root that resulted from the collision of the European plate with the Adriatic microplate. Dando et al. (2011) interpret high-velocity heterogeneities at the bottom of their regional tomographic model as a graveyard of old subducted lithospheres. High density of stations, large amount of rays and dense ray-coverage of the volume studied are not the only essential pre-requisites for reliable tomography results. A compromise between the amount of pre-processed data and the high-quality of the tomography input (travel-time residuals) is of the high importance as well. For the first time, the existence of two separate roots beneath the Alps has been revealed from carefully pre

Mantle mixing and thermal evolution during Pangaea assembly and breakup

Science.gov (United States)

Rudolph, M. L.; Li, M.; Zhong, S.; Manga, M.

2016-12-01

Continents insulate the underlying mantle, and it has been suggested that the arrangement of the continents can have a significant effect on sub-continental mantle temperatures. Additionally, the dispersal or agglomeration of continents may affect the efficacy of continental insulation, with some studies suggesting warming of 100K beneath supercontinents. During the most recent supercontinent cycle, Pangaea was encircled by subduction, potentially creating a `curtain' of subducted material that may have prevented mixing of the sub-Pangaea mantle with the sub-Panthalassa mantle. Using 3D spherical shell geometry mantle convection simulations, we quantify the effect of insulation by continents and supercontinents. We explore the differences in model predictions for purely thermal vs. thermochemical convection, and we use tracers to quantify the exchange of material between the sub-oceanic to the sub-continental mantle.

Archean crust-mantle geochemical differentiation

Science.gov (United States)

Tilton, G. R.

Isotope measurements on carbonatite complexes and komatiites can provide information on the geochemical character and geochemical evolution of the mantle, including the sub-continental mantle. Measurements on young samples establish the validity of the method. These are based on Sr, Nd and Pb data from the Tertiary-Mesozoic Gorgona komatiite and Sr and Pb data from the Cretaceous Oka carbonatite complex. In both cases the data describe a LIL element-depleted source similar to that observed presently in MORB. Carbonatite data have been used to study the mantle beneath the Superior Province of the Canadian Shield one billion years (1 AE) ago. The framework for this investigation was established by Bell et al., who showed that large areas of the province appear to be underlain by LIL element-depleted mantle (Sr-85/Sr-86=0.7028) at 1 AE ago. Additionally Bell et al. found four complexes to have higher initial Sr ratios (Sr-87/Sr-86=0.7038), which they correlated with less depleted (bulk earth?) mantle sources, or possibly crustal contamination. Pb isotope relationships in four of the complexes have been studied by Bell et al.

Archean crust-mantle geochemical differentiation

Science.gov (United States)

Tilton, G. R.

1983-01-01

Isotope measurements on carbonatite complexes and komatiites can provide information on the geochemical character and geochemical evolution of the mantle, including the sub-continental mantle. Measurements on young samples establish the validity of the method. These are based on Sr, Nd and Pb data from the Tertiary-Mesozoic Gorgona komatiite and Sr and Pb data from the Cretaceous Oka carbonatite complex. In both cases the data describe a LIL element-depleted source similar to that observed presently in MORB. Carbonatite data have been used to study the mantle beneath the Superior Province of the Canadian Shield one billion years (1 AE) ago. The framework for this investigation was established by Bell et al., who showed that large areas of the province appear to be underlain by LIL element-depleted mantle (Sr-85/Sr-86=0.7028) at 1 AE ago. Additionally Bell et al. found four complexes to have higher initial Sr ratios (Sr-87/Sr-86=0.7038), which they correlated with less depleted (bulk earth?) mantle sources, or possibly crustal contamination. Pb isotope relationships in four of the complexes have been studied by Bell et al.

Seismological Imaging of Melt Production Regions Beneath the Backarc Spreading Center and Volcanic Arc, Mariana Islands

Science.gov (United States)

Wiens, Douglas; Pozgay, Sara; Barklage, Mitchell; Pyle, Moira; Shiobara, Hajime; Sugioka, Hiroko

2010-05-01

We image the seismic velocity and attenuation structure of the mantle melt production regions associated with the Mariana Backarc Spreading Center and Mariana Volcanic Arc using data from the Mariana Subduction Factory Imaging Experiment. The passive component of this experiment consisted of 20 broadband seismographs deployed on the island chain and 58 ocean-bottom seismographs from June, 2003 until April, 2004. We obtained the 3D P and S wave velocity structure of the Mariana mantle wedge from a tomographic inversion of body wave arrivals from local earthquakes as well as P and S arrival times from large teleseismic earthquakes determined by multi-channel cross correlation. We also determine the 2-D attenuation structure of the mantle wedge using attenuation tomography based on local and regional earthquake spectra, and a broader-scale, lower resolution 3-D shear velocity structure from inversion of Rayleigh wave phase velocities using a two plane wave array analysis approach. We observe low velocity, high attenuation anomalies in the upper mantle beneath both the arc and backarc spreading center. These anomalies are separated by a higher velocity, lower attenuation region at shallow depths (< 80 km), implying distinct magma production regions for the arc and backarc in the uppermost mantle. The largest magnitude anomaly beneath the backarc spreading center is found at shallower depth (25-50 km) compared to the arc (50-100 km), consistent with melting depths estimated from the geochemistry of arc and backarc basalts (K. Kelley, pers. communication). The velocity and attenuation signature of the backarc spreading center is narrower than the corresponding anomaly found beneath the East Pacific Rise by the MELT experiment, perhaps implying a component of focused upwelling beneath the spreading center. The strong velocity and attenuation anomaly beneath the spreading center contrasts strongly with preliminary MT inversion results showing no conductivity anomaly in the

Three-dimensional crust and upper mantle structure at the Nevada test site

International Nuclear Information System (INIS)

Taylor, S.R.

1983-01-01

The three-dimensional crust and upper mantle structure at the Nevada Test Site (NTS) is derived by combining teleseismic P wave travel time residuals with Pn source time terms. The NTS time terms and relative teleseismic residuals are calculated by treating the explosions as a network of 'receivers' which record 'shots' located at the surrounding stations. Utilization of the Pn time terms allows for better crustal resolution than is possible from teleseismic information alone. Average relative teleseismic P wave residuals show a consistent progression of positive (late arrivals) to negative residuals from east to west across the NTS. However, Pn time terms beneath Rainier Mesa are at least 0.3 and 0.5 s less than those beneath Pahute Mesa and Yucca Flat, respectively, indicating the presence of high-velocity crustal material or crustal thinning beneath Rainier Mesa. The time terms at Pahute Mesa are surprisingly uniform, and the largest time terms and residuals are observed in the northwest and southern parts of Yucca Flat. The Pn time terms show a slight correlation with the working-point velocity at the shot point for Pahute Mesa and Yucca Flat, indicating that part of the observed lateral variations are caused by shallow effects of the upper crust. Three-dimensional inversion of the travel time residuals suggests that Yucca Flat is characterized by low-velocity anomalies confined to the upper crust, Rainer Mesa by very high velocities in the upper and middle crust, and Pahute Mesa by a high-velocity anomaly extending through the crust and into the upper mantle. Relatively low velocities are observed in the lower crust beneath the Timber Mountain caldera south of Pahute Mesa with no expression in the upper mantle. These observed differences in velocity beneath the Tertiary Silent Canyon and Timber Mountain calderas may be related to their magma volume and mode of enrichment from a mantle-derived magma source

Complex structure of the lithospheric slab beneath the Banda arc, eastern Indonesia depicted by a seismic tomographic model

Directory of Open Access Journals (Sweden)

Sri Widiyantoro

2011-10-01

Full Text Available Seismic tomography with a non-linear approach has been successfully applied to image the P-wave velocity structure beneath the Banda arc in detail. Nearly one million compressional phases including the surfacereflected depth phases pP and pwP from events within the Indonesian region have been used. The depth phases have been incorporated in order to improve the sampling of the uppermantle structure, particularly below the Banda Sea in the back-arc regions. For the model parameterization, we have combined a highresolution regional inversion with a low-resolution global inversion to allow detailed images of slab structures within the study region and to minimize the mapping of distant aspherical mantle structure into the volume under study. In this paper, we focus our discussion on the upper mantle and transition zone structure beneath the curved Banda arc. The tomographic images confirm previous observations of the twisting of the slab in the upper mantle, forming a spoon-shaped structure beneath the Banda arc. A slab lying flat on the 660 km discontinuity beneath the Banda Sea is also well imaged. Further interpretations of the resulting tomograms and seismicity data support the scenario of the Banda arc subduction rollback.

Shear-wave splitting observations of mantle anisotropy beneath Alaska

Science.gov (United States)

Bellesiles, A. K.; Christensen, D. H.; Entwistle, E.; Litherland, M.; Abers, G. A.; Song, X.

2009-12-01

Observations of seismic anisotropy were obtained from three different PASSCAL broadband experiments throughout Alaska, using shear-wave splitting from teleseismic SKS phases. The MOOS (Multidisciplinary Observations Of Subduction), BEAAR (Broadband Experiment Across the Alaska Range), and ARCTIC (Alaska Receiving Cross-Transects for the Inner Core) networks were used along with selected permanent broadband stations operated by AEIC (Alaska Earthquake Information Center) to produce seismic anisotropy results for the state of Alaska along a north south transect from the active subduction zone in the south, through continental Alaska, to the passive margin in the north. The BEAAR network is in-between the ARCTIC and MOOS networks above the subducting Pacific Plate and mantle wedge and shows a tight ~90 degree rotation of anisotropy above the 70km contour of the subducting plate. The southern stations in BEAAR yield anisotropy results that are subparallel to the Pacific Plate motion as it subducts under North America. These stations have an average fast direction of -45 degrees and 1.03 seconds of delay on average. The MOOS network in south central Alaska yielded similar results with an average fast direction of -30 degrees and delay times of .9 seconds. In the north portion of the BEAAR network the anisotropy is along strike of the subduction zone and has an average fast direction of 27 degrees with an average delay time of 1.4 seconds, although the delay times above the mantle wedge range from 1 to 2.5 seconds and are directly correlated to the length of ray path in the mantle wedge. This general trend NE/SW is seen in the ARCTIC stations to the north although the furthest north stations are oriented more NNE compared to those in BEAAR. The average fast direction for the ARCTIC network is 40 degrees with an average delay time of 1.05 seconds. These results show two distinct orientations of anisotropy in Alaska separated by the subducting Pacific Plate.

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.

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.

Slab melting beneath the Cascades Arc driven by dehydration of altered oceanic peridotite

Science.gov (United States)

Walowski, Kristina J; Wallace, Paul J.; Hauri, E.H.; Wada, I.; Clynne, Michael A.

2015-01-01

Water is returned to Earth’s interior at subduction zones. However, the processes and pathways by which water leaves the subducting plate and causes melting beneath volcanic arcs are complex; the source of the water—subducting sediment, altered oceanic crust, or hydrated mantle in the downgoing plate—is debated; and the role of slab temperature is unclear. Here we analyse the hydrogen-isotope and trace-element signature of melt inclusions in ash samples from the Cascade Arc, where young, hot lithosphere subducts. Comparing these data with published analyses, we find that fluids in the Cascade magmas are sourced from deeper parts of the subducting slab—hydrated mantle peridotite in the slab interior—compared with fluids in magmas from the Marianas Arc, where older, colder lithosphere subducts. We use geodynamic modelling to show that, in the hotter subduction zone, the upper crust of the subducting slab rapidly dehydrates at shallow depths. With continued subduction, fluids released from the deeper plate interior migrate into the dehydrated parts, causing those to melt. These melts in turn migrate into the overlying mantle wedge, where they trigger further melting. Our results provide a physical model to explain melting of the subducted plate and mass transfer from the slab to the mantle beneath arcs where relatively young oceanic lithosphere is subducted.

Experimental investigation of the stability of Fe-rich carbonates in the lower mantle

Science.gov (United States)

Boulard, E.; Menguy, N.; Auzende, A.; Benzerara, K.; Bureau, H.; Antonangeli, D.; Corgne, A.; Morard, G.; Siebert, J.; Perrillat, J.; Guyot, F. J.; Fiquet, G.

2011-12-01

Carbonates are the main C-bearing minerals that are transported deep in the Earth's mantle via subduction of the oceanic lithosphere [1]. The fate of carbonates at mantle conditions plays a key role in the deep carbon cycle. Decarbonation, melting or reduction of carbonates will affect the extent and the way carbon is recycled into the deep Earth. To clarify the fate of carbonates in the deep mantle, high-pressure high-temperature experiments were carried out up to 105 GPa and 2850 K on oxide assemblages of (Mg,Fe)O + CO2. The presence of Fe(II) in starting materials induces redox reactions from which Fe(II) is oxidized and a part of the carbon is reduced. This leads to an assemblage of magnetite, diamonds, and carbonates or, pressure depending, their newly discovered Fe(III)-bearing high-pressure polymorphs based on a silicate-like chemistry with tetrahedrally coordinated carbon [2]. Our results show the possibility for carbon to be recycled in the lowermost mantle and provide evidence of a possible coexistence of reduced and oxidized carbon at lower mantle conditions. [1] Sleep, N. H., and K. Zahnle (2001) J. Geophys. Res.-Planets 106(E1), 1373-1399. [2] Boulard et al. (2011) PNAS, 108, 5184-5187.

Sn-wave velocity structure of the uppermost mantle beneath the Australian continent

Science.gov (United States)

Wei, Zhi; Kennett, Brian L. N.; Sun, Weijia

2018-06-01

We have extracted a data set of more than 5000 Sn traveltimes for source-station pairs within continental Australia, with 3-D source relocation using Pn arrivals to improve data consistency. We conduct tomographic inversion for S-wave-speed structure down to 100 km using the Fast Marching Tomography (FMTOMO) method for the whole Australian continent. We obtain a 3-D model with potential resolution of 3.0° × 3.0°. The new S-wave-speed model provides strong constraints on structure in a zone that was previously poorly characterized. The S velocities in the uppermost mantle are rather fast, with patterns of variation generally corresponding to those for Pn. We find strong heterogeneities of Swave speed in the uppermost mantle across the entire continent of Australia with a close relation to crustal geological features. For instance, the cratons in the western Australia usually have high S velocities (>4.70 km s-1), while the volcanic regions on the eastern margin of Australia are characterized by low S velocities (<4.40 km s-1). Exploiting an equivalent Pn inversion, we also determine the Vp/Vs ratios across the whole continent. We find that most of the uppermost mantle has Vp/Vs between 1.65 and 1.85, but with patches in central Australia and in the east with much higher Vp/Vs ratios. Distinctive local anomalies on the eastern margin may indicate the positions of remnants of mantle plumes.

Clustering of arc volcanoes caused by temperature perturbations in the back-arc mantle.

Science.gov (United States)

Lee, Changyeol; Wada, Ikuko

2017-06-29

Clustering of arc volcanoes in subduction zones indicates along-arc variation in the physical condition of the underlying mantle where majority of arc magmas are generated. The sub-arc mantle is brought in from the back-arc largely by slab-driven mantle wedge flow. Dynamic processes in the back-arc, such as small-scale mantle convection, are likely to cause lateral variations in the back-arc mantle temperature. Here we use a simple three-dimensional numerical model to quantify the effects of back-arc temperature perturbations on the mantle wedge flow pattern and sub-arc mantle temperature. Our model calculations show that relatively small temperature perturbations in the back-arc result in vigorous inflow of hotter mantle and subdued inflow of colder mantle beneath the arc due to the temperature dependence of the mantle viscosity. This causes a three-dimensional mantle flow pattern that amplifies the along-arc variations in the sub-arc mantle temperature, providing a simple mechanism for volcano clustering.

Analysis of PKP scattering using mantle mixing simulations and axisymmetric 3D waveforms

Science.gov (United States)

Haugland, Samuel M.; Ritsema, Jeroen; van Keken, Peter E.; Nissen-Meyer, Tarje

2018-03-01

The scattering of PKP waves in the lower mantle produces isolated signals before the PKIKP phase. We explore whether these so-called PKIKP precursors can be related to wave scattering off mid ocean ridge basalt (MORB) fragments that have been advected in the deep mantle throughout geologic time. We construct seismic models of small-scale (>20 km) heterogeneity in the lower mantle informed by mantle mixing simulations from Brandenburg et al. (2008) and generate PKIKP precursors using 3D, axisymmetric waveform simulations up to 0.75 Hz. We consider two end-member geodynamic models with fundamentally different distributions of MORB in the lower mantle. Our results suggest that the accumulation of MORB at the base of the mantle is a viable hypothesis for the origin of PKP scattering. We find that the strength of the PKIKP precursor amplitudes is consistent with P wave speed heterogeneity of 0.1-0.2%, as reported previously. The radial distribution of MORB has a profound effect on the strength of PKIKP precursors. Simulation of PKIKP precursors for models with an increasing MORB concentration in the lowermost 500 km of the mantle appears to reproduce most accurately the strength of PKIKP precursors in Global Seismic Network waveforms. These models assume that MORB has an excess density of at least 7%. Additional simulations of more complex geodynamic models will better constrain the geodynamic conditions to explain the significant variability of PKP scattering strength.

Seismic anisotropy of the mantle lithosphere beneath the Swedish National Seismological Network (SNSN)

Czech Academy of Sciences Publication Activity Database

Eken, T.; Plomerová, Jaroslava; Roberts, R.; Vecsey, Luděk; Babuška, Vladislav; Shomali, H.; Bodvarsson, R.

2010-01-01

Roč. 480, č. 1-4 (2010), s. 241-258 ISSN 0040-1951 R&D Projects: GA AV ČR IAA300120709; GA AV ČR(CZ) KJB300120605 Institutional research plan: CEZ:AV0Z30120515 Keywords : Baltic Shield * mantle lithosphere * seismic anisotropy * domains and their boundaries in the mantle Subject RIV: DC - Siesmology, Volcanology, Earth Structure Impact factor: 2.509, year: 2010

Crustal and upper mantle velocity structure of the Salton Trough, southeast California

Science.gov (United States)

Parsons, T.; McCarthy, J.

1996-01-01

This paper presents data and modelling results from a crustal and upper mantle wide-angle seismic transect across the Salton Trough region in southeast California. The Salton Trough is a unique part of the Basin and Range province where mid-ocean ridge/transform spreading in the Gulf of California has evolved northward into the continent. In 1992, the U.S. Geological Survey (USGS) conducted the final leg of the Pacific to Arizona Crustal Experiment (PACE). Two perpendicular models of the crust and upper mantle were fit to wide-angle reflection and refraction travel times, seismic amplitudes, and Bouguer gravity anomalies. The first profile crossed the Salton Trough from the southwest to the northeast, and the second was a strike line that paralleled the Salton Sea along its western edge. We found thin crust (???21-22 km thick) beneath the axis of the Salton Trough (Imperial Valley) and locally thicker crust (???27 km) beneath the Chocolate Mountains to the northeast. We modelled a slight thinning of the crust further to the northeast beneath the Colorado River (???24 km) and subsequent thickening beneath the metamorphic core complex belt northeast of the Colorado River. There is a deep, apparently young basin (???5-6 km unmetamorphosed sediments) beneath the Imperial Valley and a shallower (???2-3 km) basin beneath the Colorado River. A regional 6.9-km/s layer (between ???15-km depth and the Moho) underlies the Salton Trough as well as the Chocolate Mountains where it pinches out at the Moho. This lower crustal layer is spatially associated with a low-velocity (7.6-7.7 km/s) upper mantle. We found that our crustal model is locally compatible with the previously suggested notion that the crust of the Salton Trough has formed almost entirely from magmatism in the lower crust and sedimentation in the upper crust. However, we observe an apparently magmatically emplaced lower crust to the northeast, outside of the Salton Trough, and propose that this layer in part

Water Content in the SW USA Mantle Lithosphere: FTIR Analysis of Dish Hill and Kilbourne Hole Pyroxenites

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Gibler, Robert; Peslier, Anne H.; Schaffer, Lillian Aurora; Brandon, Alan D.

2014-01-01

Kilbourne Hole (NM, USA) and Dish Hill (CA, USA) mantle xenoliths sample continental mantle in two different tectonic settings. Kilbourne Hole (KH) is located in the Rio Grande rift. Dish Hill (DH) is located in the southern Mojave province, an area potentially affected by subduction of the Farallon plate beneath North America. FTIR analyses were obtained on well characterized pyroxenite, dunite and wehrlite xenoliths, thought to represent crystallized melts at mantle depths. PUM normalized REE patterns of the KH bulk-rocks are slightly LREE enriched and consistent with those of liquids generated by 6% melting of a spinel peridotite source. Pyroxenite pyroxenes have no detectable water but one DH wehrlite, which bulk-rock is LREE enriched, has 4 ppm H2O in orthopyroxene and 2 Ga. The Farallon subduction appears to have enriched in water the southwestern United States lithospheric mantle further east than DH, beneath the Colorado plateau.

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.

Mantle roots of the Emeishan plume: an evaluation based on teleseismic P-wave tomography

Directory of Open Access Journals (Sweden)

C. He

2017-11-01

Full Text Available The voluminous magmatism associated with large igneous provinces (LIPs is commonly correlated to upwelling plumes from the core–mantle boundary (CMB. Here we analyse seismic tomographic data from the Emeishan LIP in southwestern China. Our results reveal vestiges of delaminated crustal and/or lithospheric mantle, with an upwelling in the upper mantle beneath the Emeishan LIP rather than a plume rooted in the CMB. We suggest that the magmatism and the Emeishan LIP formation might be connected with the melting of delaminated lower crustal and/or lithospheric components which resulted in plume-like upwelling from the upper mantle or from the mantle transition zone.

Structure of the Crust Beneath Cameroon, West Africa, from the Joint Inversion of Rayleigh Wave Group Velocities and Receiver Functions

International Nuclear Information System (INIS)

Tokam, Alain-Pierre K.; Tabod, Charles T.; Nyblade, Andrew A.; Jordi Julia; Wiens, Douglas A.; Pasyanos, Michael E.

2009-09-01

The joint inversion of Rayleigh wave group velocities and receiver functions was carried out to investigate the crustal and uppermost mantle structures beneath Cameroon. This was achieved using data from 32 broadband seismic stations installed for 2 years across Cameroon. The Moho depth estimates reveal that the Precambrian crust is variable across the country and shows some significant differences compared to other similar geologic units in East and South Africa. These differences suggest that the setting of the Cameroon Volcanic Line (CVL) and the eastward extension of the Benue Trough have modified the crust of the Panafrican mobile belt in Cameroon by thinning beneath the Rift area and CVL. The velocity models obtained from the joint inversion show at most stations, a layer with shear wave velocities ≥ 4.0 km/s, indicating the presence of a mafic component in the lower crust, predominant beneath the Congo Craton. The lack of this layer at stations within the Panafrican mobile belt may partly explain the crustal thinning observed beneath the CVL and rift area. The significant presence of this layer beneath the Craton, results from the 2100 Ma magmatic events at the origin of the emplacement of swarms of mafic dykes in the region. The CVL stations are underlain by a crust of 35 km on average except near Mt-Cameroon where it is about 25 km. The crustal thinning observed beneath Mt. Cameroon supported by the observed positive gravity anomalies here, suggests the presence of dense astenospheric material within the lithosphere. Shear wave velocities are found to be slower in the crust and uppermost mantle beneath the CVL than the nearby tectonic terrains, suggesting that the origin of the line may be an entirely mantle process through the edge-flow convection process. (author)

Upper Mantle Dynamics of Bangladesh by Splitting Analyzes of Core Refracted SKS and SKKS Waves

Science.gov (United States)

Tiwari, A. K.; Bhushan, K.; Eken, T.; Singh, A.

2017-12-01

New shear wave splitting measurements are obtained from hitherto less studied Bengal Basin using core refracted SKS and SKKS phases. Splitting parameters, time delays (δt) and fast polarization directions (Φ) were estimated through analysis of 64 high-quality waveforms (≥ 2.5 signal to noise ratio) from 29 earthquakes with magnitude ≥5.5 recorded at eight seismic stations deployed over Bangladesh. We found no evidence of splitting which indicates azimuthal isotropy beneath the region. Null measurements can be explained by near vertical axis of anisotropy or by the presence of multiple anisotropic layers with different fast polarization directions, where combined effect results in null. We consider that the presence of partial melts within the upper mantle due to Kerguelen mantle plume activities may be the potential geodynamic cause for observed null measurements. It locally perturbed mantle convection flow beneath the region and reoriented the lattice preferred orientation of the upper mantle mineral mainly olivine as this disabled the core refracted SKS and SKKS phases to scan the anisotropic characteristics of the region, and hence null measurements are obtained.

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.

Average Potential Temperature of the Upper Mantle and Excess Temperatures Beneath Regions of Active Upwelling

Science.gov (United States)

Putirka, K. D.

2006-05-01

The question as to whether any particular oceanic island is the result of a thermal mantle plume, is a question of whether volcanism is the result of passive upwelling, as at mid-ocean ridges, or active upwelling, driven by thermally buoyant material. When upwelling is passive, mantle temperatures reflect average or ambient upper mantle values. In contrast, sites of thermally driven active upwellings will have elevated (or excess) mantle temperatures, driven by some source of excess heat. Skeptics of the plume hypothesis suggest that the maximum temperatures at ocean islands are similar to maximum temperatures at mid-ocean ridges (Anderson, 2000; Green et al., 2001). Olivine-liquid thermometry, when applied to Hawaii, Iceland, and global MORB, belie this hypothesis. Olivine-liquid equilibria provide the most accurate means of estimating mantle temperatures, which are highly sensitive to the forsterite (Fo) contents of olivines, and the FeO content of coexisting liquids. Their application shows that mantle temperatures in the MORB source region are less than temperatures at both Hawaii and Iceland. The Siqueiros Transform may provide the most precise estimate of TpMORB because high MgO glass compositions there have been affected only by olivine fractionation, so primitive FeOliq is known; olivine thermometry yields TpSiqueiros = 1430 ±59°C. A global database of 22,000 MORB show that most MORB have slightly higher FeOliq than at Siqueiros, which translates to higher calculated mantle potential temperatures. If the values for Fomax (= 91.5) and KD (Fe-Mg)ol-liq (= 0.29) at Siqueiros apply globally, then upper mantle Tp is closer to 1485 ± 59°C. Averaging this global estimate with that recovered at Siqueiros yields TpMORB = 1458 ± 78°C, which is used to calculate plume excess temperatures, Te. The estimate for TpMORB defines the convective mantle geotherm, and is consistent with estimates from sea floor bathymetry and heat flow (Stein and Stein, 1992), and

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

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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.

Response of mantle transition zone thickness to plume buoyancy flux

Science.gov (United States)

Das Sharma, S.; Ramesh, D. S.; Li, X.; Yuan, X.; Sreenivas, B.; Kind, R.

2010-01-01

The debate concerning thermal plumes in the Earth's mantle, their geophysical detection and depth characterization remains contentious. Available geophysical, petrological and geochemical evidence is at variance regarding the very existence of mantle plumes. Utilizing P-to-S converted seismic waves (P receiver functions) from the 410 and 660 km discontinuities, we investigate disposition of these boundaries beneath a number of prominent hotspot regions. The thickness of the mantle transition zone (MTZ), measured as P660s-P410s differential times (tMTZ), is determined. Our analyses suggest that the MTZ thickness beneath some hotspots correlates with the plume strength. The relationship between tMTZ, in response to the thermal perturbation, and the strength of plumes, as buoyancy flux B, follows a power law. This B-tMTZ behavior provides unprecedented insights into the relation of buoyancy flux and excess temperature at 410-660 km depth below hotspots. We find that the strongest hotspots, which are located in the Pacific, are indeed plumes originating at the MTZ or deeper. According to the detected power law, even the strongest plumes may not shrink the transition zone by significantly more than ~40 km (corresponding to a maximum of 300-400° excess temperature).

Plate Tectonic Cycling and Whole Mantle Convection Modulate Earth's 3He/22Ne Ratio

Science.gov (United States)

Dygert, N. J.; Jackson, C.; Hesse, M. A.; Tremblay, M. M.; Shuster, D. L.; Gu, J.

2016-12-01

3He and 22Ne are not produced in the mantle or fractionated by partial melting, and neither isotope is recycled back into the mantle by subduction of oceanic basalt or sediment. Thus, it is a surprise that large 3He/22Ne variations exist within the mantle and that the mantle has a net elevated 3He/22Ne ratio compared to volatile-rich planetary precursor materials. Depleted subcontinental lithospheric mantle and mid-ocean ridge basalt (MORB) mantle have distinctly higher 3He/22Ne compared to ocean island basalt (OIB) sources ( 4-12.5 vs. 2.5-4.5, respectively) [1,2]. The low 3He/22Ne of OIBs approaches chondritic ( 1) and solar nebula values ( 1.5). The high 3He/22Ne of the MORB mantle is not similar to solar sources or any known family of meteorites, requiring a mechanism for fractionating He from Ne in the mantle and suggesting isolation of distinct mantle reservoirs throughout geologic time. We model the formation of a MORB source with elevated and variable 3He/22Ne though diffusive exchange between dunite channel-hosted basaltic liquids and harzburgite wallrock beneath mid-ocean ridges. Over timescales relevant to mantle upwelling beneath spreading centers, He may diffuse tens to hundreds of meters into wallrock while Ne is relatively immobile, producing a regassed, depleted mantle lithosphere with elevated 3He/22Ne. Subduction of high 3He/22Ne mantle would generate a MORB source with high 3He/22Ne. Regassed, high 3He/22Ne mantle lithosphere has He concentrations 2-3 orders of magnitude lower than undegassed mantle. To preserve the large volumes of high 3He/22Ne mantle required by the MORB source, mixing between subducted and undegassed mantle reservoirs must have been limited throughout geologic time. Using the new 3He/22Ne constraints, we ran a model similar to [3] to quantify mantle mixing timescales, finding they are on the order of Gyr assuming physically reasonable seafloor spreading rates, and that Earth's convecting mantle has lost >99% of its primordial

Alkaline and carbonatite metasomatism of lithospheric mantle beneath SW Poland- Pilchowice case

Science.gov (United States)

Ćwiek, Mateusz; Matusiak-Małek, Magdalena; Puziewicz, Jacek; Ntaflos, Theodoros

2014-05-01

The Cenozoic basanites from Pilchowice (SW Poland) form volcanic plug located exactly at Intra- Sudetic Fault. These basanites belong to the Polish part of the Central European Volcanic Province and contain numerous, usually small (pfu and mg# from 0.915- 0.920 . One xenolith contains clinopyroxene with abundant spongy rims. Primary clinopyroxene is very rare and Al-enriched (mg# 0.92, 0.17 atoms of Al pfu). The spinel is Cr enriched (cr# 0.46-0.68) and is usually associated with clinopyroxene. Orthopyroxene is depleted in REE compared to primitive mantle. Orthopyroxene from majority of xenoliths are strongly LREE depleted ((La/Lu)N = 0.03-0.21). All studied peridotites contain clinopyroxene which is enriched (2 to 70 times) in REE compared to primitive mantle. Clinopyroxene patterns show relative low HREE concentration ((La/Lu)N = 4.75- 19.99), moreover patterns from three samples are convex- upward shaped with inflection point on Nd ((La/Nd)N = 0.36-0.96). Clinopyroxene- poor lithology, high cr# in spinel and LREE- depleted nature of orthopyroxene suggest that upper mantle sampled by Pilchowice basanite is a restite after partial melting. The LREE enriched composition of clinopyroxene suggest that peridotites were metasomatised. Clinopyroxene convex- upward shaped REE plots with inflection point on Nd is typical for metasomatism related with alkaline melt. On the other hand very low ratios of Ti/ Eu (24.8- 738.9) and high (La/ Yb)N (3.5- 17) ratio (Coltorti, 1999) suggest that the metasomatic agent was either a mixture of alkaline silicate melt with carbonatite or peridotite reaction with two independent agents is recorded. This study is a part of MSc thesis of the first author and was possible thanks to the project NCN 2011/03/B/ST10/06248 of Polish National Centre for Science. Coltorti, M., Bonadiman, C., Hinton, R. W., Siena, F. & Upton, B. G. J. (1999). Carbonatite metasomatism of the oceanic upper mantle: Evidence from clinopyroxenes and glasses in

Upper-mantle fabrics beneath the Northern Apennines revealed by seismic anisotropy

Czech Academy of Sciences Publication Activity Database

Munzarová, Helena; Plomerová, Jaroslava; Babuška, Vladislav; Vecsey, Luděk

2013-01-01

Roč. 14, č. 4 (2013), s. 1156-1181 ISSN 1525-2027 R&D Projects: GA AV ČR IAA300120709; GA ČR GAP210/12/2381 Institutional support: RVO:67985530 Keywords : body-wave anisotropy * Northern Apennines * upper mantle Subject RIV: DC - Siesmology, Volcanology, Earth Structure Impact factor: 3.054, year: 2013

Layered crust-mantle transition zone below a large crustal intrusion in the Norwegian-Danish basin

DEFF Research Database (Denmark)

Sandrin, Alessandro; Nielsen, Lars; Thybo, Hans

2009-01-01

the lowermost crust (7.7 km/s) and the uppermost mantle (7.9-8.0 km/s). The seismic data show a "ringing" Moho below the western part of the intrusion. The coda trailing the main PmP reflection is about 1.0 s long and is composed of 4-5 wavelets. We demonstrate that this feature may be explained by a layered......The crust and uppermost mantle below the large positive gravity anomaly in the central part of the Norwegian-Danish Basin, the Silkeborg Gravity High (SGH), is investigated using controlled source seismic data. A more than 80 km long and ~ 20 km thick intrusion is interpreted. The seismic velocity...... transition zone between 30 and 35 km depth, where high-velocity layers of mantle affinity (7.9-8.05 km/s) alternate with layers of typical lower crustal velocity (6.7-7.3 km/s). The characteristics of this layering, which causes thePmP coda, are modelled by matching synthetic seismograms to the observed data...

Probability density functions for radial anisotropy: implications for the upper 1200 km of the mantle

Science.gov (United States)

Beghein, Caroline; Trampert, Jeannot

2004-01-01

The presence of radial anisotropy in the upper mantle, transition zone and top of the lower mantle is investigated by applying a model space search technique to Rayleigh and Love wave phase velocity models. Probability density functions are obtained independently for S-wave anisotropy, P-wave anisotropy, intermediate parameter η, Vp, Vs and density anomalies. The likelihoods for P-wave and S-wave anisotropy beneath continents cannot be explained by a dry olivine-rich upper mantle at depths larger than 220 km. Indeed, while shear-wave anisotropy tends to disappear below 220 km depth in continental areas, P-wave anisotropy is still present but its sign changes compared to the uppermost mantle. This could be due to an increase with depth of the amount of pyroxene relative to olivine in these regions, although the presence of water, partial melt or a change in the deformation mechanism cannot be ruled out as yet. A similar observation is made for old oceans, but not for young ones where VSH> VSV appears likely down to 670 km depth and VPH> VPV down to 400 km depth. The change of sign in P-wave anisotropy seems to be qualitatively correlated with the presence of the Lehmann discontinuity, generally observed beneath continents and some oceans but not beneath ridges. Parameter η shows a similar age-related depth pattern as shear-wave anisotropy in the uppermost mantle and it undergoes the same change of sign as P-wave anisotropy at 220 km depth. The ratio between dln Vs and dln Vp suggests that a chemical component is needed to explain the anomalies in most places at depths greater than 220 km. More tests are needed to infer the robustness of the results for density, but they do not affect the results for anisotropy.

Seismic characteristics of central Brazil crust and upper mantle: A deep seismic refraction study

Science.gov (United States)

Soares, J.E.; Berrocal, J.; Fuck, R.A.; Mooney, W.D.; Ventura, D.B.R.

2006-01-01

A two-dimensional model of the Brazilian central crust and upper mantle was obtained from the traveltime interpretation of deep seismic refraction data from the Porangatu and Cavalcante lines, each approximately 300 km long. When the lines were deployed, they overlapped by 50 km, forming an E-W transect approximately 530 km long across the Tocantins Province and western Sa??o Francisco Craton. The Tocantins Province formed during the Neoproterozoic when the Sa??o Francisco, the Paranapanema, and the Amazon cratons collided, following the subduction of the former Goia??s ocean basin. Average crustal VP and VP/VS ratios, Moho topography, and lateral discontinuities within crustal layers suggest that the crust beneath central Brazil can be associated with major geological domains recognized at the surface. The Moho is an irregular interface, between 36 and 44 km deep, that shows evidences of first-order tectonic structures. The 8.05 and 8.23 km s-1 P wave velocities identify the upper mantle beneath the Porangatu and Cavalcante lines, respectively. The observed seismic features allow for the identification of (1) the crust has largely felsic composition in the studied region, (2) the absence of the mafic-ultramafic root beneath the Goia??s magmatic arc, and (3) block tectonics in the foreland fold-and-thrust belt of the northern Brasi??lia Belt during the Neoproterozoic. Seismic data also suggested that the Bouguer gravimetric discontinuities are mainly compensated by differences in mass distribution within the lithospheric mantle. Finally, the Goia??s-Tocantins seismic belt can be interpreted as a natural seismic alignment related to the Neoproterozoic mantle domain. Copyright 2006 by the American Geophysical Union.

Interrelation between rifting, faulting, sedimentation, and mantle serpentinization during continental margin formation

Science.gov (United States)

Rupke, L.; Schmid, D. W.; Perez-Gussinye, M.; Hartz, E. H.

2013-12-01

We explore the conditions under which mantle serpentinization may take place during continental rifting with 2D thermotectonostratigraphic basin models. The basic concept follows the idea that the entire extending continental crust has to be brittle for crustal scale faulting and mantle serpentinization to occur. The new model tracks the rheological evolution of the continental crust and allows for kinetically controlled mantle serpentinization processes. The isostatic and latent heat effects of the reaction are fully coupled to the structural and thermal solutions. A systematic parameter study shows that a critical stretching factor exists for which complete crustal embrittlement and serpentinization occurs. Sedimentation shifts this critical stretching factor to higher values as both deeper burial and the low thermal conductivity of sediments lead to higher crustal temperatures. Serpentinization reactions are therefore only likely in settings with low sedimentation rates and high stretching factors. In addition, we find that the rate of sediment supply has first order controls on the rheology of the lower crust, which may control the overall margin geometry. We further test these concepts in ideas in a case study for the Norwegian margin. In particular, we evaluate whether the inner lower crustal bodies (LCB) imaged beneath the More and Voring margin could be serpentinized mantle. For this purpose we reconstruct multiple 2D transects through a 3D data set. This reconstruction of the Norwegian margin shows that serpentinization reactions are indeed possible and likely during the Jurassic rift phase. Predicted present-day thicknesses and locations of partially serpentinized mantle rocks fit well to information on LCBs from seismic and gravity data. We conclude that some of the inner LCBs beneath the Norwegian margin may, in fact, be partially serpentinized mantle.

A Lower-Crust or Mantle Source for Mineralizing Fluids Beneath the Olympic Dam IOCG Deposit, Australia: New Evidence From Magnetotelluric Sounding

Science.gov (United States)

Heinson, G.

2005-12-01

The iron-oxide-copper-gold (IOCG) Olympic Dam (OD) deposit, situated along the margin of the Proterozoic Gawler Craton, South Australia, is the world's largest uranium deposit, and sixth largest copper deposit; it also contains significant reserves of gold, silver and rare-earth elements (REE). Gaining a better understanding of the mechanisms for genesis of the economic mineralisation is fundamental for defining exploration models in similar crustal-settings. To delineate crustal structures that may constrain mineral system fluid pathways, coincident deep crustal seismic and magnetotelluric (MT) transects were obtained along a 220 km section that crosses OD and the major crustal boundaries. We present results from 58 long-period (10-104 s) MT sites, with site spacing of 5 to 10 km. A 2D inversion of all MT data to a depth of 100 km shows four notable features: (a) sedimentary cover sequences with low resistivity (1000 Ω.m) Archaean crustal core, from a more conductive crust to the north (typically <500 Ω.m); (c) to the north of OD, the crust to about 20 km is quite resistive (~1000 Ω.m), but the lower crust is much more conductive (<100 Ω.m); and (d) beneath OD, we image a low-resistivity region (<100 Ω.m) throughout the crust, coincident with a seismically transparent region. We argue that the cause of the low-resistivity and low-reflectivity region beneath OD may be due to the upward movement of crustal-volatiles that have deposited conductive graphite mineralisation along grain boundaries, simultaneously annihilating acoustic impedance boundaries. The source of the volatiles may be from the mantle-degassing or retrograde metamorphism of the lower crust associated with Proterozoic crustal deformation.

Mantle melting and melt refertilization beneath the Southwest Indian Ridge: Mineral composition of abyssal peridotites

Science.gov (United States)

Chen, Ling; Zhu, Jihao; Chu, Fengyou; Dong, Yan-hui; Liu, Jiqiang; Li, Zhenggang; Zhu, Zhimin; Tang, Limei

2017-04-01

As one of the slowest spreading ridges of the global ocean ridge system, the Southwest Indian Ridge (SWIR) is characterized by discontinued magmatism. The 53°E segment between the Gallieni fracture zone (FZ) (52°20'E) and the Gazelle FZ (53°30'E) is a typical amagmatic segment (crustal thickness 1cm) Opx, and Mg-rich mineral compositions akin to harzburgite xenoliths that sample old continental lithospheric mantle (Kelemen et al., 1998). Melt refertilization model shows that Group 2 peridotites were affected by an enriched low-degree partial melt from the garnet stability field. These results indicate that depleted mantle which experiences ancient melting event are more sensitive to melt refertilization, thus may reduce the melt flux, leading to extremely thin crust at 53°E segment. This research was granted by the National Basic Research Programme of China (973 programme) (grant No. 2013CB429705) and the Fundamental Research Funds of Second Institute of Oceanography, State Oceanic Administration (JG1603, SZ1507). References: Johnson K T M, Dick H J B, Shimizu N. Melting in the oceanic upper mantle: An ion microprobe study of diopsides in abyssal peridotites[J]. Journal of Geophysical Research, 1990, 95(B3):2661-2678. Kelemen P B, Hart S R, Bernstein S. Silica enrichment in the continental upper mantle via melt/rock reaction[J]. Earth & Planetary Science Letters, 1998, 164(1-2):387-406. Zhou H, Dick H J. Thin crust as evidence for depleted mantle supporting the Marion Rise.[J]. Nature, 2013, 494(7436):195-200.

Formation of heterogeneous magmatic series beneath North Santorini, South Aegean island arc

DEFF Research Database (Denmark)

Bailey, John C; Jensen, E.S.; Hansen, A.

2008-01-01

magma formation beneath North Santorini throughout its 500 ka history is attributed to variable transfer of sedimentary components - either terrigenous or pelagic, as bulk sediments or high-temperature partial melts rather than fluids or low-temperature partial melts - from a rupture zone...... in the subducted slab to the overlying mantle. The three main magmatic series followed independent paths of assimilation of upper crustal materials during fractional crystallization. Assimilation was more pronounced at the basaltic stage. The long-lived histories of the three main magmatic series imply repetitive...... melting of isolated mantle regions, ascent of magmas through independent feeder systems, and their residence in separate crustal magma chambers....

Seismic structure of the western U.S. mantle and its relation to regional tectonic and magmatic activity

Science.gov (United States)

Schmandt, Brandon

Vigorous convective activity in the western U.S. mantle has long been inferred from the region's widespread intra-plate crustal deformation, volcanism, and high elevations, but the specific form of convective activity and the degree and nature of lithospheric involvement have been strongly debated. I design a seismic travel-time tomography method and implement it with seismic data from the EarthScope Transportable Array and complementary arrays to constrain three-dimensional seismic structure beneath the western U.S. Tomographic images of variations in compressional velocity, shear velocity, and the ratio of shear to compressional velocity in the western U.S. mantle to a depth of 1000 km are produced. Using these results I investigate mantle physical properties, Cenozoic subduction history, and the influence of small-scale lithospheric convection on regional tectonic and magmatic activity, with particular focus on southern California and the Pacific Northwest. This dissertation includes previously published co-authored material. Chapter II presents a travel-time tomography method I designed and first implemented with data from southern California and the surrounding southwestern U.S. The resulting images provide a new level of constraint on upper mantle seismic anomalies beneath the Transverse Ranges, southern Great Valley, Salton Trough, and southwestern Nevada volcanic field. Chapter III presents tomographic images of the western U.S. mantle, identifies upper mantle volumes where partial melt is probable, and discusses implications of the apparently widespread occurrence of gravitational instabilities of continental lithsophere and the complex geometry and buoyancy of subducted ocean lithosphere imaged beneath the western U.S. In Chapter IV, tomography images are used in conjunction with geologic constraints on major transitions in crustal deformation and magmatism to construct a model for Pacific Northwest evolution since the Cretaceous. Accretion in the Pacific

A detailed map of the 660-kilometer discontinuity beneath the izu-bonin subduction zone.

Science.gov (United States)

Wicks, C W; Richards, M A

1993-09-10

Dynamical processes in the Earth's mantle, such as cold downwelling at subduction zones, cause deformations of the solid-state phase change that produces a seismic discontinuity near a depth of 660 kilometers. Observations of short-period, shear-to-compressional wave conversions produced at the discontinuity yield a detailed map of deformation beneath the Izu-Bonin subduction zone. The discontinuity is depressed by about 60 kilometers beneath the coldest part of the subducted slab, with a deformation profile consistent with the expected thermal signature of the slab, the experimentally determined Clapeyron slope of the phase transition, and the regional tectonic history.

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.

Mantle upwellings and convective instabilities revealed by seismic tomography and helium isotope geochemistry beneath eastern Africa

Science.gov (United States)

Montagner, Jean-Paul; Marty, Bernard; Stutzmann, Eléonore; Sicilia, Déborah; Cara, Michel; Pik, Raphael; Lévêque, Jean-Jacques; Roult, Geneviève; Beucler, Eric; Debayle, Eric

2007-11-01

The relationship between intraplate volcanism and continental tectonics has been investigated for North and East Africa using a high resolution three-dimensional anisotropic tomographic model derived from seismic data of a French experiment ``Horn of Africa'' and existing broadband data. The joint inversion for seismic velocity and anisotropy of the upper 400 km of the mantle, and geochemical data reveals a complex interaction between mantle upwellings, and lithosphere. Two kinds of mantle upwellings can be distinguished: The first one, the Afar ``plume'' originates from deeper than 400 km and is characterized by enrichment in primordial 3He and 3He/4He ratios higher than those along mid-ocean ridges (MOR). The second one, associated with other Cenozoic volcanic provinces (Darfur, Tibesti, Hoggar, Cameroon), with 3He/4He ratios similar to, or lower than MOR, is a consequence of shallower upwelling. The presumed asthenospheric convective instabilities are oriented in an east-west direction, resulting from interaction between south-north asthenospheric mantle flow, main plume head and topography on the base of lithosphere.

Small scatterers in the lower mantle observed at German broadband arrays

Science.gov (United States)

Thomas, C.; Weber, M.; Wicks, C.W.; Scherbaum, F.

1999-01-01

Seismograms of earthquakes from the South Pacific recorded at a German broadband array and network show precursors to PKPdf. These precursors mainly originate from off-path scattering of PKPab or a nearby PKPbc to P (for receiver-side scattering) or from scattering of P to PKPab or PKPbc on the PKPdf path (for source-side scattering). Standard array processing techniques based on plane wave approximations (such as vespagram or frequency-wavenumber analysis) are inadequate for investigating these precursors since scattered waves cannot be approximated as plane waves for arrays and networks larger than 300 x 300 km for short-period waves. We therefore develop a migration method to estimate the location of scatterers in the mantle, at the core-mantle boundary and at the top of the outer core. With our method we are able to find isolated scatterers at the source side and the receiver side, although the depth of the scatterer is not well constrained. However, from looking at the first possible arrival time of precursors at different depth and the region where scattering can take place (scattering volume), we believe that the location of the scatterers is in the lowermost mantle. Since we have detected scatterers in regions where ultralow-velocity zones have been discovered recently, we think that the precursor energy possibly originates from scattering at partial melt at the base of the mantle. Comparing results from broadband and band-pass-filtered data the detection of small-scale structure of the ultralow-velocity zones becomes possible. Copyright 1999 by the American Geophysical Union.

Subduction to the lower mantle – a comparison between geodynamic and tomographic models

Directory of Open Access Journals (Sweden)

T. W. Becker

2012-11-01

Full Text Available It is generally believed that subduction of lithospheric slabs is a major contribution to thermal heterogeneity in Earth's entire mantle and provides a main driving force for mantle flow. Mantle structure can, on the one hand, be inferred from plate tectonic models of subduction history and geodynamic models of mantle flow. On the other hand, seismic tomography models provide important information on mantle heterogeneity. Yet, the two kinds of models are only similar on the largest (1000 s of km scales and are quite different in their detailed structure. Here, we provide a quantitative assessment how good a fit can be currently achieved with a simple viscous flow geodynamic model. The discrepancy between geodynamic and tomography models can indicate where further model refinement could possibly yield an improved fit. Our geodynamical model is based on 300 Myr of subduction history inferred from a global plate reconstruction. Density anomalies are inserted into the upper mantle beneath subduction zones, and flow and advection of these anomalies is calculated with a spherical harmonic code for a radial viscosity structure constrained by mineral physics and surface observations. Model viscosities in the upper mantle beneath the lithosphere are ~1020 Pas, and viscosity increases to ~1023 Pas in the lower mantle above D". Comparison with tomography models is assessed in terms of correlation, both overall and as a function of depth and spherical harmonic degree. We find that, compared to previous geodynamic and tomography models, correlation is improved, presumably because of advances in both plate reconstructions and mantle flow computations. However, high correlation is still limited to lowest spherical harmonic degrees. An important ingredient to achieve high correlation – in particular at spherical harmonic degree two – is a basal chemical layer. Subduction shapes this layer into two rather stable hot but chemically dense "piles

Seismic structure of the European upper mantle based on adjoint tomography

Science.gov (United States)

Zhu, Hejun; Bozdağ, Ebru; Tromp, Jeroen

2015-04-01

We use adjoint tomography to iteratively determine seismic models of the crust and upper mantle beneath the European continent and the North Atlantic Ocean. Three-component seismograms from 190 earthquakes recorded by 745 seismographic stations are employed in the inversion. Crustal model EPcrust combined with mantle model S362ANI comprise the 3-D starting model, EU00. Before the structural inversion, earthquake source parameters, for example, centroid moment tensors and locations, are reinverted based on global 3-D Green's functions and Fréchet derivatives. This study consists of three stages. In stage one, frequency-dependent phase differences between observed and simulated seismograms are used to constrain radially anisotropic wave speed variations. In stage two, frequency-dependent phase and amplitude measurements are combined to simultaneously constrain elastic wave speeds and anelastic attenuation. In these two stages, long-period surface waves and short-period body waves are combined to simultaneously constrain shallow and deep structures. In stage three, frequency-dependent phase and amplitude anomalies of three-component surface waves are used to simultaneously constrain radial and azimuthal anisotropy. After this three-stage inversion, we obtain a new seismic model of the European curst and upper mantle, named EU60. Improvements in misfits and histograms in both phase and amplitude help us to validate this three-stage inversion strategy. Long-wavelength elastic wave speed variations in model EU60 compare favourably with previous body- and surface wave tomographic models. Some hitherto unidentified features, such as the Adria microplate, naturally emerge from the smooth starting model. Subducting slabs, slab detachments, ancient suture zones, continental rifts and backarc basins are well resolved in model EU60. We find an anticorrelation between shear wave speed and anelastic attenuation at depths agreement with previous global attenuation studies

Density heterogeneity of the upper mantle beneath Siberia from satellite gravity and a new regional crustal model

DEFF Research Database (Denmark)

Herceg, Matija; Thybo, Hans; Artemieva, Irina

2013-01-01

We present a new regional model for the density structure of the upper mantle below Siberia. The residual mantle gravity anomalies are based on gravity data derived from the GOCE gravity gradients and geopotential models, with crustal correction to the gravity field being calculated from a new...... on regional and global crustal models. We analyze how uncertainties and errors in the crustal model propagate from crustal densities to mantle residual gravity anomalies and the density model of the upper mantle. The new regional density model for the Siberian craton and the West Siberian Basin complements...... regional crustal model. This newly compiled database on the crustal seismic structure, complemented by additional constraints from petrological analysis of near-surface rocks and lower crustal xenoliths, allows for a high-resolution correction of the crustal effects as compared to previous studies based...

Structure of the crust and upper mantle beneath the Balearic Islands (Western Mediterranean)

Science.gov (United States)

Banda, E.; Ansorge, J.; Boloix, M.; Córdoba, D.

1980-09-01

Data are presented from deep seismic sounding along the strike of the Balearic Islands carried out in 1976. The interpretation of the data gives the following results: A sedimentary cover of 4 km around Ibiza to 7 km under Mallorca overlies the crystalline basement. This basement with a P-wave velocity of 6.0 km/s at the top reaches a depth of at least 15 km under Ibiza and 17 km under Mallorca with an increase to 6.1 km/s at these depths. The crust-mantle boundary lies at a depth of 20 km and 25 km, respectively. A well documented upper-mantle velocity of 7.7 km/s is found along the entire profile. The Moho rises to a depth of 20 km about 30 km north of Mallorca and probably continues rising towards the center of the North Balearic Sea. The newly deduced crustal structure together with previously determined velocity-depth sections in the North Balearic Sea as well as heat flow and aeromagnetic data can be interpreted as an extended rift structure caused by large-scale tensional processes in the upper mantle. The available data suggest that the entire zone from the eastern Alboran Sea to the area north of the Balearic Islands represents the southeastern flank of this rift system. In this model the provinces of Spain along the east coast would represent the northwestern rift flank.

Upper mantle flow in the western Mediterranean

Energy Technology Data Exchange (ETDEWEB)

Panza, G F [Dipartimento di Scienze della Terra, Universita degli Studi di Trieste, Trieste (Italy) and Abdus Salam International Centre for Theoretical Physics, Trieste (Italy); Raykova, R [Geophysical Institute of BAS, Sofia (Bulgaria) and Istituto Nazionale di Geofisica e Vulcanologia, Sezione Bologna, Bologna (Italy); Carminati, E; Doglioni, C [Dipartimento di Scienze della Terra, Universita degli Studi di Trieste, Trieste (Italy)

2006-07-15

Two cross-sections of the western Mediterranean Neogene-to-present backarc basin are presented, in which geological and geophysical data of the Transmed project are tied to a new shear-wave tomography. Major results are i) the presence of a well stratified upper mantle beneath the older African continent, with a marked low-velocity layer between 130-200 km of depth; ii) the dilution of this layer within the younger western Mediterranean backarc basin to the north, and iii) the easterly raising of a shallower low-velocity layer from about 140 km to about 30 km in the Tyrrhenian active part of the backarc basin. These findings suggest upper mantle circulation in the western Mediterranean backarc basin, mostly easterly-directed and affecting the boundary between upper asthenosphere (LVZ) and lower asthenosphere, which undulates between about 180 km and 280 km. (author)

Upper mantle flow in the western Mediterranean

International Nuclear Information System (INIS)

Panza, G.F.; Raykova, R.; Carminati, E.; Doglioni, C.

2006-07-01

Two cross-sections of the western Mediterranean Neogene-to-present backarc basin are presented, in which geological and geophysical data of the Transmed project are tied to a new shear-wave tomography. Major results are i) the presence of a well stratified upper mantle beneath the older African continent, with a marked low-velocity layer between 130-200 km of depth; ii) the dilution of this layer within the younger western Mediterranean backarc basin to the north, and iii) the easterly raising of a shallower low-velocity layer from about 140 km to about 30 km in the Tyrrhenian active part of the backarc basin. These findings suggest upper mantle circulation in the western Mediterranean backarc basin, mostly easterly-directed and affecting the boundary between upper asthenosphere (LVZ) and lower asthenosphere, which undulates between about 180 km and 280 km. (author)

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.

Global Discontinuity Structure of the Mantle Transition Zone from Finite-Frequency Tomography of SS Precursors

Science.gov (United States)

Guo, Z.; Zhou, Y.

2017-12-01

We report global structure of the 410-km and 660-km discontinuities from finite-frequency tomography using frequency-dependent traveltime measurements of SS precursors recorded at the Global Seismological Network (GSN). Finite-frequency sensitivity kernels for discontinuity depth perturbations are calculated in the framework of traveling-wave mode coupling. We parametrize the global discontinuities using a set of spherical triangular grid points and solve the tomographic inverse problem based on singular value decomposition. Our global 410-km and 660-km discontinuity models reveal distinctly different characteristics beneath the oceans and subduction zones. In general, oceanic regions are associated with a thinner mantle transition zone and depth perturbations of the 410-km and 660-km discontinuities are anti-correlated, in agreement with a thermal origin and an overall warm and dry mantle beneath the oceans. The perturbations are not uniform throughout the oceans but show strong small-scale variations, indicating complex processes in the mantle transition zone. In major subduction zones (except for South America where data coverage is sparse), depth perturbations of the 410-km and 660-km discontinuities are correlated, with both the 410-km and the 660-km discontinuities occurring at greater depths. The distributions of the anomalies are consistent with cold stagnant slabs just above the 660-km discontinuity and ascending return flows in a superadiabatic upper mantle.

Investigating Ultra-low Velocity Zones beneath the Southwestern Pacific

Science.gov (United States)

Carson, S. E.; Hansen, S. E.; Garnero, E.

2017-12-01

The core mantle boundary (CMB), where the solid silicate mantle meets the liquid iron-nickel outer core, represents the largest density contrast on our planet, and it has long been recognized that the CMB is associated with significant structural heterogeneities. One CMB structure of particular interest are ultra low-velocity zones (ULVZs), laterally-varying, 5-50 km thick isolated patches seen in some locations just above the CMB that are associated with increased density and reduced seismic wave velocities. These variable characteristics have led to many questions regarding ULVZ origins, but less than 40% of the CMB has been surveyed for the presence of ULVZs given limited seismic coverage of the lowermost mantle. Therefore, investigations that sample the CMB with new geometries are critical to further our understanding of ULVZs and their potential connection to other deep Earth processes. The Transantarctic Mountains Northern Network (TAMNNET), a 15-station seismic array that was recently deployed in Antarctica, provides a unique dataset to further study ULVZ structure with new and unique path geometry. Core-reflected ScP and PcP phases from the TAMNNET dataset particularly well sample the CMB in the vicinity of New Zealand in the southwestern Pacific, providing coverage between an area to the north where ULVZ structure has been previously identified and another region to the south, which shows no ULVZ evidence. By identifying and modeling pre- and post-cursor ScP and PcP energy, we are exploring a new portion of the CMB with a goal of better understanding potential ULVZ origins. Our study area also crosses the southern boundary of the Pacific Large Low Shear Velocity Province (LLSVP); therefore, our investigations may allow us to examine the possible relationship between LLSVPs and ULVZs.

Crustal structure beneath Beijing and its surrounding regions derived from gravity data

Science.gov (United States)

Jiang, Wenliang; Zhang, Jingfa; Lu, Xiaocui; Lu, Jing

2011-06-01

In this paper we use gravity data to study fine crustal structure and seismogenic environment beneath Beijing and its surrounding regions. Multi-scale wavelet analysis method is applied to separating gravity fields. Logarithmic power spectrum method is also used to calculate depth of gravity field source. The results show that the crustal structure is very complicated beneath Beijing and its surrounding areas. The crustal density exhibits laterally inhomogeneous. There are three large scale tectonic zones in North China, i.e., WNW-striking Zhangjiakou-Bohai tectonic zone (ZBTZ), NE-striking Taihang piedmont tectonic zone (TPTZ) and Cangxian tectonic zone (CTZ). ZBTZ and TPTZ intersect with each other beneath Beijing area and both of them cut through the lithosphere. The upper and middle crusts consist of many small-scale faults, uplifts and depressions. In the lower crust, these small-scale tectonic units disappear gradually, and they are replaced by large-scale tectonic units. In surrounding regions of Beijing, ZBTZ intersects with several other NE-striking tectonic units, such as Cangxian uplift, Jizhong depression and Shanxi Graben System (SGS). In west of Taihangshan uplift, gravity anomalies in upper and middle crusts are correlated with geological and topographic features on the surface. Compared with the crust, the structure is comparatively simple in uppermost mantle. Earthquakes mainly occurred in upper and middle crusts, especially in transitional regions between high gravity anomaly and low gravity anomaly. Occurrence of large earthquakes may be related to the upwelling of upper mantle and asthenosphere heat flow materials, such as Sanhe earthquake ( M S8.0) and Tangshan earthquake ( M S7.8).

Formation and modification of chromitites in the mantle

Science.gov (United States)

Arai, Shoji; Miura, Makoto

2016-11-01

Podiform chromitites have long supplied us with unrivaled information on various mantle processes, including the peridotite-magma reaction, deep-seated magmatic evolution, and mantle dynamics. The recent discovery of ultrahigh-pressure (UHP) chromitites not only sheds light on a different aspect of podiform chromitites, but also changes our understanding of the whole picture of podiform chromitite genesis. In addition, new evidence was recently presented for hydrothermal modification/formation chromite/chromitite in the mantle, which is a classical but innovative issue. In this context, we present here an urgently needed comprehensive review of podiform chromitites in the upper mantle. Wall-rock control on podiform chromitite genesis demonstrates that the peridotite-magma reaction at the upper mantle condition is an indispensable process. We may need a large system in the mantle, far larger than the size of outcrops or mining areas, to fulfill the Cr budget requirement for podiform chromitite genesis. The peridotite-magma reaction over a large area may form a melt enriched with Na and other incompatible elements, which mixes with a less evolved magma supplied from the depth to create chromite-oversaturated magma. The incompatible-element-rich magma trapped by the chromite mainly precipitates pargasite and aspidolite (Na analogue of phlogopite), which are stable under upper mantle conditions. Moderately depleted harzburgites, which contain chromite with a moderate Cr# (0.4-0.6) and a small amount of clinopyroxene, are the best reactants for the chromitite-forming reaction, and are the best hosts for podiform chromitites. Arc-type chromitites are dominant in ophiolites, but some are of the mid-ocean ridge type; chromitites may be common beneath the ocean floor, although it has not yet been explored for chromitite. The low-pressure (upper mantle) igneous chromitites were conveyed through mantle convection or subduction down to the mantle transition zone to form

Isotopic characterisation of the sub-continental lithospheric mantle beneath Zealandia, a rifted fragment of Gondwana

DEFF Research Database (Denmark)

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

2013-01-01

The greater New Zealand region, known as Zealandia, represents an amalgamation of crustal fragments accreted to the paleo-Pacific Gondwana margin and which underwent significant thinning during the subsequent split from Australia and Antarctica in the mid-Cretaceous following opening of the Tasma...... Sea and the Southern Ocean. We present Sr, Nd and Pb isotopes and laser ablation trace element data for a comprehensive suite of clinopyroxene separates from spinel peridotite xenoliths (lherzolite to harzburgite) from the sub-continental lithospheric mantle across southern New Zealand...... composition, age or geographical separation. These isotopic compositions indicate that the sub-continental lithospheric mantle under southern New Zealand has a regionally distinct and pervasive FOZO to HIMU – like signature. The isotopic signatures are also similar to those of the alkaline magmas...... that transported the xenoliths and suggest that most of the HIMU signature observed in the volcanics could be derived from a major source component in the sub-continental lithospheric mantle. Trace element abundances in clinopyroxene are highly heterogeneous and vary from LREE-enriched, relatively flat and MORB...

Dynamo Tests for Stratification Below the Core-Mantle Boundary

Science.gov (United States)

Olson, P.; Landeau, M.

2017-12-01

Evidence from seismology, mineral physics, and core dynamics points to a layer with an overall stable stratification in the Earth's outer core, possibly thermal in origin, extending below the core-mantle boundary (CMB) for several hundred kilometers. In contrast, energetic deep mantle convection with elevated heat flux implies locally unstable thermal stratification below the CMB in places, consistent with interpretations of non-dipole geomagnetic field behavior that favor upwelling flows below the CMB. Here, we model the structure of convection and magnetic fields in the core using numerical dynamos with laterally heterogeneous boundary heat flux in order to rationalize this conflicting evidence. Strongly heterogeneous boundary heat flux generates localized convection beneath the CMB that coexists with an overall stable stratification there. Partially stratified dynamos have distinctive time average magnetic field structures. Without stratification or with stratification confined to a thin layer, the octupole component is small and the CMB magnetic field structure includes polar intensity minima. With more extensive stratification, the octupole component is large and the magnetic field structure includes intense patches or high intensity lobes in the polar regions. Comparisons with the time-averaged geomagnetic field are generally favorable for partial stratification in a thin layer but unfavorable for stratification in a thick layer beneath the CMB.

Seismic evidence of the lithosphere-asthenosphere boundary beneath Izu-Bonin area

Science.gov (United States)

Cui, H.; Gao, Y.; Zhou, Y.

2016-12-01

The lithosphere-asthenosphere boundary (LAB), separating the rigid lithosphere and the ductile asthenosphere layers, is the seismic discontinuity with the negative velocity contrast of the Earth's interior [Fischer et al., 2010]. The LAB has been also termed the Gutenberg (G) discontinuity that defines the top of the low velocity zone in the upper mantle [Gutenberg, 1959; Revenaugh and Jordan, 1991]. The seismic velocity, viscosity, resistivity and other physical parameters change rapidly with the depths across the boundary [Eaton et al., 2009]. Seismic detections on the LAB in subduction zone regions are of great help to understand the interactions between the lithosphere and asthenosphere layers and the geodynamic processes related with the slab subductions. In this study, the vertical broadband waveforms are collected from three deep earthquake events occurring from 2000 to 2014 with the focal depths of 400 600 km beneath the Izu-Bonin area. The waveform data is processed with the linear slant stack method [Zang and Zhou, 2002] to obtain the vespagrams in the relative travel-time to slowness domain and the stacked waveforms. The sP precursors reflected on the LAB (sLABP), which have the negative polarities with the amplitude ratios of 0.17 0.21 relative to the sP phases, are successfully extracted. Based on the one-dimensional modified velocity model (IASP91-IB), we obtain the distributions for six reflected points of the sLABP phases near the source region. Our results reveal that the LAB depths range between 58 and 65 km beneath the Izu-Bonin Arc, with the average depth of 62 km and the small topography of 7 km. Compared with the results of the tectonic stable areas in Philippine Sea [Kawakatsu et al., 2009; Kumar and Kawakatsu, 2011], the oceanic lithosphere beneath the Izu-Bonin Arc shows the obvious thinning phenomena. We infer that the lithospheric thinning is closely related with the partial melting, which is caused by the volatiles continuously released

Seismic Evidence for the North China Plate Underthrusting Beneath Northeastern Tibet and its Implications for Plateau Growth

Science.gov (United States)

Ye, Z.; Gao, R.; Li, Q.; Zhang, H.

2016-12-01

The effects of India-Asia collision and the subsequent interaction between the two continents on northeastern Tibet (NE Tibet), i.e., the tectonic transition zone between the Tibetan plateau and the North China craton (NCC) for example, remain uncertain due to inadequate geophysical data coverage in NE Tibet. Here in this research, based on new dataset collected from a dense linear array of 38 broadband seismograph stations, we applied seismic receiver functions (Sp and Ps converted waves) to imaging the lithospheric structure and shear wave splitting (XKS waves) to inspecting the anisotropy in the lithosphere and upper mantle beneath NE Tibet. The seismic array traverses NE Tibet to the westernmost NCC (Alxa block) in an SSW-NNE direction. The lithosphere-asthenosphere boundary (LAB) is clearly defined and appears as a south-dipping interface that runs continuously from the Alxa interior to the Qilian orogen on the S-wave receiver function images. Shear wave splitting measurements show significant lateral variations of seismic anisotropy across NE Tibet. Under joint constraints from both the lithospheric structure imaging and the regional anisotropic regime, combined with previous studies and through a thorough analysis/comparison/integration, we finally constructed a comprehensive lithospheric model of NE Tibet. The model tells that the NCC lithospheric mantle has been persistently underthrust beneath the Qilian orogen in response to on-going convergence/compression between the interior Tibetan plateau and the NCC. This process forms the syntectonic crustal thrust. The regional anisotropic regime can be well accommodated in our interpretation. The lithospheric model summarized here can be well accommodated in a scenario of northeastward migration of stepwise/multiple Aisan mantle lithosphere underthrusting beneath the Tibetan plateau. The multiple Aisan lithospheric blocks underthrust the plateau stepwise in small scale. Our results provide a new section from

Upper mantle structure under western Saudi Arabia from Rayleigh wave tomography and the origin of Cenozoic uplift and volcanism on the Arabian Shield

Energy Technology Data Exchange (ETDEWEB)

Park, Y; Nyblade, A; Rodgers, A; Al-Amri, A

2007-11-09

The shear velocity structure of the shallow upper mantle beneath the Arabian Shield has been modeled by inverting new Rayleigh wave phase velocity measurements between 45 and 140 s together with previously published Rayleigh wave group velocity measurement between 10 and 45 s. For measuring phase velocities, we applied a modified array method that minimizes the distortion of raypaths by lateral heterogeneity. The new shear velocity model shows a broad low velocity region in the lithospheric mantle across the Shield and a low velocity region at depths {ge} 150 km localized along the Red Sea coast and Makkah-Madinah-Nafud (MMN) volcanic line. The velocity reduction in the upper mantle corresponds to a temperature anomaly of {approx}250-330 K. These finding, in particular the region of continuous low velocities along the Red Sea and MMN volcanic line, do not support interpretations for the origin of the Cenozoic plateau uplift and volcanism on the Shield invoking two separate plumes. When combined with images of the 410 and 660 km discontinuities beneath the southern part of the Arabian Shield, body wave tomographic models, a S-wave polarization analysis, and SKS splitting results, our new model supports an interpretation invoking a thermal upwelling of warm mantle rock originating in the lower mantle under Africa that crosses through the transition zone beneath Ethiopia and moves to the north and northwest under the eastern margin of the Red Sea and the Arabian Shield. In this interpretation, the difference in mean elevation between the Platform and Shield can be attributed to isostatic uplift caused by heating of the lithospheric mantle under the Shield, with significantly higher region along the Red Sea possibly resulting from a combination of lithosphere thinning and dynamic uplift.

Deep mantle seismic heterogeneities in Western Pacific subduction zones

Science.gov (United States)

Bentham, H. L. M.; Rost, S.

2012-04-01

In recent years array seismology has been used extensively to image the small scale (~10 km) structure of the Earth. In the mantle, small scale structure likely represents chemical heterogeneity and is essential in our understanding of mantle convection and especially mantle mixing. As subduction is the main source of introducing crustal material into the Earth's mantle, it is of particular interest to track the transport of subducted crust through the mantle to resolve details of composition and deformation of the crust during the subduction process. Improved knowledge of subduction can help provide constraints on the mechanical mixing process of crustal material into the ambient mantle, as well as constraining mantle composition and convection. This study uses seismic array techniques to map seismic heterogeneities associated with Western Pacific subduction zones, where a variety of slab geometries have been previously observed. We use seismic energy arriving prior to PP, a P-wave underside reflection off the Earth's surface halfway between source and receiver, to probe the mantle for small-scale heterogeneities. PP precursors were analysed at Eielson Array (ILAR), Alaska using the recently developed Toolkit for Out-of-Plane Coherent Arrival Tracking (TOPCAT) algorithm. The approach combines the calculated optimal beampower and an independent semblance (coherency) measure, to improve the signal-to-noise ratio of coherent arrivals. 94 earthquakes with sufficient coherent precursory energy were selected and directivity information of the arrivals (i.e. slowness and backazimuth) was extracted from the data. The scattering locations for 311 out-of-plane precursors were determined by ray-tracing and minimising the slowness, backazimuth and differential travel time misfit. Initial analyses show that deep scattering (>1000 km) occurs beneath the Izu-Bonin subduction zone, suggesting that subducted crust does continue into the lower mantle in this location. Other

Deep Subducction in a Compressible Mantle: Observations and Theory

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King, S. D.

2017-12-01

Our understanding of slab dynamics is primarily based on the results of numerical models of subduction. In such models coherent, cold slabs are clearly visible from the surface of the Earth to the core mantle boundary. In contrast, fast seismic anomalies associated with cold subducted slabs are difficult to identify below 1500-2000 km in tomographic models of Earth's mantle. One explanation for this has been the resolution, or lack thereof, of seismic tomography in the mid-mantle region; however in this work I will explore the impact of compressibility on the dynamics of subducting slabs, specifically shear heating of the slab and latent heat of phase transformations. Most geodynamic models of subduction have used an incompressible formulation, thus because subducted slabs are assumed to be cold and stiff, the primary means of thermal equilibration is conduction. With an assumed sinking velocity of approximately 0.1 m/yr, a subducted slab reaches the core-mantle boundary in approximately 30 Myrs—too fast for significant conductive cooling of the downgoing slab. In this work I consider a whole-mantle geometry and include both phase transformations with associated latent heat and density changes from the olivine-wadsleyite-ringwoodite-bridgmanite system and the pyroxene-garnet system. The goal of this work is to understand both the eventual fate and thermal evolution of slabs beneath the transition zone.

Conjecture with water and rheological control for subducting slab in the mantle transition zone

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Fumiko Tajima

2015-01-01

Full Text Available Seismic observations have shown structural variation near the base of the mantle transition zone (MTZ where subducted cold slabs, as visualized with high seismic speed anomalies (HSSAs, flatten to form stagnant slabs or sink further into the lower mantle. The different slab behaviors were also accompanied by variation of the “660 km” discontinuity depths and low viscosity layers (LVLs beneath the MTZ that are suggested by geoid inversion studies. We address that deep water transport by subducted slabs and dehydration from hydrous slabs could affect the physical properties of mantle minerals and govern slab dynamics. A systematic series of three-dimensional numerical simulation has been conducted to examine the effects of viscosity reduction or contrast between slab materials on slab behaviors near the base of the MTZ. We found that the viscosity reduction of subducted crustal material leads to a separation of crustal material from the slab main body and its transient stagnation in the MTZ. The once trapped crustal materials in the MTZ eventually sink into the lower mantle within 20–30 My from the start of the plate subduction. The results suggest crustal material recycle in the whole mantle that is consistent with evidence from mantle geochemistry as opposed to a two-layer mantle convection model. Because of the smaller capacity of water content in lower mantle minerals than in MTZ minerals, dehydration should occur at the phase transformation depth, ∼660 km. The variation of the discontinuity depths and highly localized low seismic speed anomaly (LSSA zones observed from seismic P waveforms in a relatively high frequency band (∼1 Hz support the hypothesis of dehydration from hydrous slabs at the phase boundary. The LSSAs which correspond to dehydration induced fluids are likely to be very local, given very small hydrogen (H+ diffusivity associated with subducted slabs. The image of such local LSSA zones embedded in HSSAs may not

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

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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.

Seismic Evidence for Lower Mantle Plume Under the Yellowstone Hotspot

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Nelson, P.; Grand, S.

2017-12-01

The mantle plume hypothesis for the origin of intraplate volcanism has been controversial since its inception in the 1970s. The hypothesis proposes hot narrow upwelling of rock rooted at the core mantle boundary (CMB) rise through the mantle and interact with the base of the lithosphere forming linear volcanic systems such as Hawaii and Yellowstone. Recently, broad lower mantle (>500 km in diameter) slow velocity conduits, most likely thermochemical in origin, have been associated with some intraplate volcanic provinces (French and Romanowicz, 2015). However, the direct detection of a classical thin thermal plume in the lower mantle using travel time tomography has remained elusive (Anderson and Natland, 2014). Here we present a new shear wave tomography model for the mantle beneath the western United States that is optimized to find short wavelength, sub-vertical structures in the lower mantle. Our approach uses carefully measured SKS and SKKS travel times recorded by dense North American seismic networks in conjunction with finite frequency kernels to build on existing tomography models. We find the presence of a narrow ( 300 km diameter) well isolated cylindrically shaped slow anomaly in the lower most mantle which we associate with the Yellowstone Hotspot. The conduit has a 2% reduction in shear velocity and is rooted at the CMB near the California/Arizona/Nevada border. A cross sectional view through the anomaly shows that it is slightly tilted toward the north until about 1300 km depth where it appears to weaken and deflect toward the surficial positon of the hotspot. Given the anomaly's strength, proximity to the Yellowstone Hotspot, and morphology we argue that a thermal plume interpretation is the most reasonable. Our results provide strong support for a lower mantle plume origin of the Yellowstone hotspot and more importantly the existence of deep thermal plumes.

Radial anisotropy of the North American upper mantle based on adjoint tomography with USArray

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Zhu, Hejun; Komatitsch, Dimitri; Tromp, Jeroen

2017-10-01

We use seismic data from USArray to image the upper mantle underneath the United States based on a so-called `adjoint tomography', an iterative full waveform inversion technique. The inversion uses data from 180 regional earthquakes recorded by 4516 seismographic stations, resulting in 586 185 frequency-dependent measurements. Three-component short-period body waves and long-period surface waves are combined to simultaneously constrain deep and shallow structures. The transversely isotropic model US22 is the result of 22 pre-conditioned conjugate-gradient iterations. Approximate Hessian maps and point-spread function tests demonstrate good illumination of the study region and limited trade-offs among different model parameters. We observe a distinct wave-speed contrast between the stable eastern US and the tectonically active western US. This boundary is well correlated with the Rocky Mountain Front. Stable cratonic regions are characterized by fast anomalies down to 250-300 km, reflecting the thickness of the North American lithosphere. Several fast anomalies are observed beneath the North American lithosphere, suggesting the possibility of lithospheric delamination. Slow wave-speed channels are imaged beneath the lithosphere, which might indicate weak asthenosphere. Beneath the mantle transition zone of the central US, an elongated north-south fast anomaly is observed, which might be the ancient subducted Farallon slab. The tectonically active western US is dominated by prominent slow anomalies with magnitudes greater than -6 per cent down to approximately 250 km. No continuous lower to upper mantle upwellings are observed beneath Yellowstone. In addition, our results confirm previously observed differences between oceans and continents in the anisotropic parameter ξ = (βh/βv)2. A slow wave-speed channel with ξ > 1 is imaged beneath the eastern Pacific at depths from 100 to 200 km, reflecting horizontal shear within the asthenosphere. Underneath continental

Mantle structure and tectonic history of SE Asia

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Hall, Robert; Spakman, Wim

2015-09-01

Seismic travel-time tomography of the mantle under SE Asia reveals patterns of subduction-related seismic P-wave velocity anomalies that are of great value in helping to understand the region's tectonic development. We discuss tomography and tectonic interpretations of an area centred on Indonesia and including Malaysia, parts of the Philippines, New Guinea and northern Australia. We begin with an explanation of seismic tomography and causes of velocity anomalies in the mantle, and discuss assessment of model quality for tomographic models created from P-wave travel times. We then introduce the global P-wave velocity anomaly model UU-P07 and the tectonic model used in this paper and give an overview of previous interpretations of mantle structure. The slab-related velocity anomalies we identify in the upper and lower mantle based on the UU-P07 model are interpreted in terms of the tectonic model and illustrated with figures and movies. Finally, we discuss where tomographic and tectonic models for SE Asia converge or diverge, and identify the most important conclusions concerning the history of the region. The tomographic images of the mantle record subduction beneath the SE Asian region to depths of approximately 1600 km. In the upper mantle anomalies mainly record subduction during the last 10 to 25 Ma, depending on the region considered. We interpret a vertical slab tear crossing the entire upper mantle north of west Sumatra where there is a strong lateral kink in slab morphology, slab holes between c.200-400 km below East Java and Sumbawa, and offer a new three-slab explanation for subduction in the North Sulawesi region. There is a different structure in the lower mantle compared to the upper mantle and the deep structure changes from west to east. What was imaged in earlier models as a broad and deep anomaly below SE Asia has a clear internal structure and we argue that many features can be identified as older subduction zones. We identify remnants of slabs

Lithology and temperature: How key mantle variables control rift volcanism

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Shorttle, O.; Hoggard, M.; Matthews, S.; Maclennan, J.

2015-12-01

Continental rifting is often associated with extensive magmatic activity, emplacing millions of cubic kilometres of basalt and triggering environmental change. The lasting geological record of this volcanic catastrophism are the large igneous provinces found at the margins of many continents and abrupt extinctions in the fossil record, most strikingly that found at the Permo-Triassic boundary. Rather than being considered purely a passive plate tectonic phenomenon, these episodes are frequently explained by the involvement of mantle plumes, upwellings of mantle rock made buoyant by their high temperatures. However, there has been debate over the relative role of the mantle's temperature and composition in generating the large volumes of magma involved in rift and intra-plate volcanism, and even when the mantle is inferred to be hot, this has been variously attributed to mantle plumes or continental insulation effects. To help resolve these uncertainties we have combined geochemical, geophysical and modelling results in a two stage approach: Firstly, we have investigated how mantle composition and temperature contribute to melting beneath Iceland, the present day manifestation of the mantle plume implicated in the 54Ma break up of the North Atlantic. By considering both the igneous crustal production on Iceland and the chemistry of its basalts we have been able to place stringent constraints on the viable temperature and lithology of the Icelandic mantle. Although a >100°C excess temperature is required to generate Iceland's thick igneous crust, geochemistry also indicates that pyroxenite comprises 10% of its source. Therefore, the dynamics of rifting on Iceland are modulated both by thermal and compositional mantle anomalies. Secondly, we have performed a global assessment of the mantle's post break-up thermal history to determine the amplitude and longevity of continental insulation in driving excess volcanism. Using seismically constrained igneous crustal

Unusually thickened crust beneath the Emeishan large igneous province detected by virtual deep seismic sounding

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Liu, Zhen; Tian, Xiaobo; Chen, Yun; Xu, Tao; Bai, Zhiming; Liang, Xiaofeng; Iqbal, Javed; Xu, Yigang

2017-11-01

The Emeishan Large Igneous Province (ELIP) in southwest China represents the erosional remnant of a vast basalt field emplaced during the Permian Period. Spanning 0.25 million km2, the ELIP occupies a relatively small area relative to other Large Igneous Provinces (LIPs) such as the Siberian Traps and Ontong Java Plateau. The original volume of an ancient LIP can be constrained from estimates of its intrusive component. We used virtual deep seismic sounding (VDSS) to detect the boundary between the crust and the upper mantle (Moho) beneath the ELIP. A strong set of reflections from depths of 60-70 km indicate an unusually thick crust having a P-wave velocity of 7.0-7.4 km/s located beneath the inner zone of the ELIP. A high-velocity lower crustal body (HVLCB) of this thickness may have been formed by ponding magmas derived from the Emeishan mantle plume and associated fractionated materials. Combined images of crustal structure allow re-estimation of Emeishan magmatic volume. With a total estimated volume of 1.76-3.2 × 106 km3, the ELIP appears to have been a typical sized plume-generated LIP relative to other global examples.

Constraints on mantle melt geometries from body wave attenuation in the Salton Trough and Snake River Plain

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Byrnes, J. S.; Bezada, M.

2017-12-01

Melt can be retained in the mantle at triple junctions between grain boundaries, be spread in thin films along two-grain boundaries, or be organized by shear into elongate melt-rich bands. Which of these geometries is most prevalent is unknown. This ambiguity makes the interpretation of anomalous seismic velocities and quality factors difficult, since different geometries would result in different mechanical effects. Here, we compare observations of seismic attenuation beneath the Salton Trough and the Snake River Plain; two regions where the presence of melt has been inferred. The results suggest that seismic attenuation is diagnostic of melt geometry. We measure the relative attenuation of P waves from deep focus earthquakes using a time-domain method. Even though the two regions are underlain by comparably strong low-velocity anomalies, their attenuation signature is very different. The upper mantle beneath the Salton Trough is sufficiently attenuating that the presence of melt must lower Qp, while attenuation beneath the Snake River Plain is not anomalous with respect to surrounding regions. These seemingly contradictory results can be reconciled if different melt geometries characterize each region. SKS splitting from the Salton Trough suggests that melt is organized into melt-rich bands, while this is not the case for the Snake River Plain. We infer that beneath the Snake River Plain melt is retained at triple junctions between grain boundaries, a geometry that is not predicted to cause seismic attenuation. More elongate geometries beneath the Salton Trough may cause seismic attenuation via the melt-squirt mechanism. In light of these results, we conclude that prior observations of low seismic velocities with somewhat high quality factors beneath the East Pacific Rise and Southern California suggest that melt does not organize into elongate bands across much of the asthenosphere.

Trust but Verify: a spot check for the new stratified model of upper mantle anisotropy beneath North America

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Levin, V. L.; Yuan, H.

2011-12-01

A newly developed 3D model of shear wave velocity and anisotropy beneath the North American continent (Yuan et al, 2011) offers a Solomonic solution to the long-standing dispute regarding the provenance of seismic anisotropy, with directional dependency of wave speed placed into both the lithosphere and the asthenosphere. However, due to its continent-wide coverage, the new model has lateral resolution on the scale of 500 km and is expected to average, and thus misrepresent, structure in regions with abrupt lateral changes in properties. The north-eastern US, especially along the coast, presents an example of such complex region. One of the earliest cases for stratified anisotropy was built on data from this part of North America (Levin et al., 1999), and also this is a region with significant, and enigmatic, lateral changes in isotropic velocity (van der Lee and Nolet, 1997; Nettles and Dziewonski, 2008). A decade since the initial studies of the region were performed, we have vastly more data that facilitate a new look at the seismic anisotropy parameters of the upper mantle beneath this region. We use shear wave splitting observations and anisotropy-aware receiver functions to develop high-quality constraints on the vertical and lateral variation in attributes of anisotropy, which we then compare (and contrast) with structure predicted for this region by the Yuan et al. (2011) model. Our goals are both to test the new model in one place, and to develop a strategy for such testing. Our primary data set comes from one of the longest-operating broad-band stations, HRV (Harvard, MA). Here, P wave receiver functions (PRFs) confirm the presence of features previously associated with the LAB and a mid-lithosphere discontinuity by Rychert et al. (2007). Notably, both features have very significant anisotropic components, with likely orientation of anisotropic symmetry axes being ~130SE or ~220SW. Similar symmetry is seen in PRFs constructed for other nearby sites

Identifying regions of strong scattering at the core-mantle boundary from analysis of PKKP precursor energy

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Rost, S.; Earle, P.S.

2010-01-01

We detect seismic scattering from the core-mantle boundary related to the phase PKKP (PK. KP) in data from small aperture seismic arrays in India and Canada. The detection of these scattered waves in data from small aperture arrays is new and allows a better characterization of the fine-scale structure of the deep Earth especially in the southern hemisphere. Their slowness vector is determined from array processing allowing location of the heterogeneities at the core-mantle boundary using back-projection techniques through 1D Earth models. We identify strong scattering at the core-mantle boundary (CMB) beneath the Caribbean, Patagonia and the Antarctic Peninsula as well as beneath southern Africa. An analysis of the scattering regions relative to sources and receivers indicates that these regions represent areas of increased scattering likely due to increased heterogeneities close to the CMB. The 1. Hz array data used in this study is most sensitive to heterogeneity with scale lengths of about 10. km. Given the small size of the scatterers, a chemical origin of the heterogeneities is likely. By comparing the location of the fine-scale heterogeneity to geodynamical models and tomographic images, we identify different scattering mechanisms in regions related to subduction (Caribbean and Patagonia) and dense thermo chemical piles (Southern Africa). ?? 2010 Elsevier B.V.

Mantle Serpentinization near the Central Mariana Trench Constrained by Ocean Bottom Surface Wave Observations

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Cai, C.; Wiens, D. A.; Lizarralde, D.; Eimer, M. O.; Shen, W.

2017-12-01

We investigate the crustal and uppermost mantle seismic structure across the Mariana trench by jointly inverting Rayleigh wave phase and group velocities from ambient noise and longer period phase velocities from Helmholtz tomography of teleseismic waveforms. We use data from a temporary deployment in 2012-2013, consisting of 7 island-based stations and 20 broadband ocean bottom seismographs, as well as data from the USGS Northern Mariana Islands Seismograph Network. To avoid any potential bias from the starting model, we use a Bayesian Monte-Carlo algorithm to invert for the azimuthally-averaged SV-wave velocity at each node. This method also allows us to apply prior constraints on crustal thickness and other parameters in a systematic way, and to derive formal estimates of velocity uncertainty. The results show the development of a low velocity zone within the incoming plate beginning about 80 km seaward of the trench axis, consistent with the onset of bending faults from bathymetry and earthquake locations. The maximum depth of the velocity anomaly increases towards the trench, and extends to about 30 km below the seafloor. The low velocities persist after the plate is subducted, as a 20-30 km thick low velocity layer with a somewhat smaller velocity reduction is imaged along the top of the slab beneath the forearc. An extremely low velocity zone is observed beneath the serpentine seamounts in the outer forearc, consistent with 40% serpentinization in the forearc mantle wedge. Azimuthal anisotropy results show trench parallel fast axis within the incoming plate at uppermost mantle depth (2%-4% anisotropy). All these observations suggest the velocity reduction in the incoming plate prior to subduction results from both serpentinized normal faults and water-filled cracks. Water is expelled from the cracks early in subduction, causing a modest increase in the velocity of the subducting mantle, and moves upward and causes serpentinization of the outer forearc

Kinematics and dynamics of the East Pacific Rise linked to a stable, deep-mantle upwelling

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Rowley, David B.; Forte, Alessandro M.; Rowan, Christopher J.; Glišović, Petar; Moucha, Robert; Grand, Stephen P.; Simmons, Nathan A.

2016-01-01

Earth’s tectonic plates are generally considered to be driven largely by negative buoyancy associated with subduction of oceanic lithosphere. In this context, mid-ocean ridges (MORs) are passive plate boundaries whose divergence accommodates flow driven by subduction of oceanic slabs at trenches. We show that over the past 80 million years (My), the East Pacific Rise (EPR), Earth’s dominant MOR, has been characterized by limited ridge-perpendicular migration and persistent, asymmetric ridge accretion that are anomalous relative to other MORs. We reconstruct the subduction-related buoyancy fluxes of plates on either side of the EPR. The general expectation is that greater slab pull should correlate with faster plate motion and faster spreading at the EPR. Moreover, asymmetry in slab pull on either side of the EPR should correlate with either ridge migration or enhanced plate velocity in the direction of greater slab pull. Based on our analysis, none of the expected correlations are evident. This implies that other forces significantly contribute to EPR behavior. We explain these observations using mantle flow calculations based on globally integrated buoyancy distributions that require core-mantle boundary heat flux of up to 20 TW. The time-dependent mantle flow predictions yield a long-lived deep-seated upwelling that has its highest radial velocity under the EPR and is inferred to control its observed kinematics. The mantle-wide upwelling beneath the EPR drives horizontal components of asthenospheric flows beneath the plates that are similarly asymmetric but faster than the overlying surface plates, thereby contributing to plate motions through viscous tractions in the Pacific region. PMID:28028535

Titanates of the lindsleyite-mathiasite (LIMA) group reveal isotope disequilibrium associated with metasomatism in the mantle beneath Kimberley (South Africa)

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Giuliani, Andrea; Woodhead, Jon D.; Phillips, David; Maas, Roland; Davies, Gareth R.; Griffin, William L.

2018-01-01

Radiogenic isotope variations unrelated to radiogenic ingrowth are common between minerals found in metasomatised mantle xenoliths entrained in kimberlite, basalts and related magmas. As the metasomatic minerals are assumed to have been in isotopic equilibrium originally, such variations are typically attributed to contamination by the magma host and/or interaction with mantle fluids during or before xenolith transport to surface. However, the increasing evidence of metasomatism by multiple, compositionally distinct fluids permeating the lithospheric mantle, coeval with specific magmatic events, suggests that isotopic disequilibrium might be a consequence of discrete, though complex, metasomatic events. Here we provide clear evidence of elemental and Sr isotope heterogeneity between coeval Ti-rich LIMA (lindsleyite-mathiasite) minerals at the time of their formation in the mantle. LIMA minerals occur in close textural association with clinopyroxene and phlogopite in low-temperature (∼800-900 °C), strongly metasomatised mantle xenoliths from the ∼84 Ma Bultfontein kimberlite (South Africa). Previous U/Pb dating of the LIMA phases was used to argue that each xenolith recorded a single event of LIMA crystallisation at ∼180-190 Ma, coeval with the emplacement of Karoo magmas. SEM imaging reveals that up to four types of LIMA phases coexist in each xenolith, and occasionally in a single LIMA grain. Major element and in situ Sr isotope analyses of the different LIMA types show that each phase has a distinct elemental composition and initial 87Sr/86Sr ratio (e.g., 0.7068-0.7086 and 0.7115-0.7129 for two LIMA types in a single xenolith; 0.7053-0.7131 across the entire sample suite). These combined age and isotopic constraints require that multiple fluids metasomatised these rocks at broadly the same time (i.e. within a few thousands to millions of years), and produced similar mineralogical features. Elemental and isotopic variations between different LIMA types

Neogene subduction beneath Java, Indonesia: Slab tearing and changes in magmatism

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Cottam, Michael; Hall, Robert; Cross, Lanu; Clements, Benjamin; Spakman, Wim

2010-05-01

Java is a Neogene calc-alkaline volcanic island arc formed by the northwards subduction of the Indo-Australian Plate beneath Sundaland, the continental core of SE Asia. The island has a complex history of volcanism and displays unusual subduction characteristics. These characteristics are consistent with the subduction of a hole in the down going slab that was formed by the arrival of a buoyant oceanic plateau at the trench. Subduction beneath Java began in the Eocene. However, the position and character of the calc-alkaline arc has changed over time. An older Paleogene arc ceased activity in the Early Miocene. Volcanic activity resumed in the Late Miocene producing a younger arc to the north of the older arc, and continues to the present day. An episode of Late Miocene thrusting at about 7 Ma is observed throughout Java and appears to be linked to northward movement of the arc. Arc rocks display typical calc-alkaline characteristics and reflect melting of the mantle wedge and subducted sediments associated with high fluid fluxes. Between West Java and Bali the present arc-trench gap is unusually wide at about 300 km. Seismicity identifies subducted Indian Ocean lithosphere that dips north at about 20° between the trench and the arc and then dips more steeply at about 60-70° from 100 to 600 km depth. In East Java there is gap in seismicity between about 250 and 500 km. Seismic tomography shows that this gap is not an aseismic section of the subduction zone but a hole in the slab. East Java is also unusual in the presence of K-rich volcanoes, now inactive, to the north of the calc-alkaline volcanoes of the active arc. In contrast to the calc-alkaline volcanism of the main arc, these K-rich melts imply lower fluid fluxes and a different mantle source. We suggest that all these observations can be explained by the tearing of the subducting slab when a buoyant oceanic plateau arrived at the trench south of East Java at about 8 Ma. With the slab unable to subduct

Velocity Models of the Upper Mantle Beneath the MER, Somali Platform, and Ethiopian Highlands from Body Wave Tomography

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Hariharan, A.; Keranen, K. M.; Alemayehu, S.; Ayele, A.; Bastow, I. D.; Eilon, Z.

2016-12-01

The Main Ethiopian Rift (MER) presents a unique opportunity to improve our understanding of an active continental rift. Here we use body wave tomography to generate compressional and shear wave velocity models of the region beneath the rift. The models help us understand the rifting process over the broader region around the MER, extending the geographic region beyond that captured in past studies. We use differential arrival times of body waves from teleseismic earthquakes and multi-channel cross correlation to generate travel time residuals relative to the global IASP91 1-d velocity model. The events used for the tomographic velocity model include 200 teleseismic earthquakes with moment magnitudes greater than 5.5 from our recent 2014-2016 deployment in combination with 200 earthquakes from the earlier EBSE and EAGLE deployments (Bastow et al. 2008). We use the finite-frequency tomography analysis of Schmandt et al. (2010), which uses a first Fresnel zone paraxial approximation to the Born theoretical kernel with spatial smoothing and model norm damping in an iterative LSQR algorithm. Results show a broad, slow region beneath the rift with a distinct low-velocity anomaly beneath the northwest shoulder. This robust and well-resolved low-velocity anomaly is visible at a range of depths beneath the Ethiopian plateau, within the footprint of Oligocene flood basalts, and near surface expressions of diking. We interpret this anomaly as a possible plume conduit, or a low-velocity finger rising from a deeper, larger plume. Within the rift, results are consistent with previous work, exhibiting rift segmentation and low-velocities beneath the rift valley.

Role of mantle dynamics in rebuilding the Tianshan Orogenic Belt in NW China: A seismic tomographic investigation

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He, Chuansong; Santosh, M.

2018-05-01

The Tianshan orogenic belt, Junggar terrane and Altai terrane are located at the southwestern part of the Central Asian Orogenic Belt (CAOB). Here, we investigate the velocity structure beneath the Xinjiang region in NW China, which includes the Tarim terrane, Tianshan orogenic belt, Junggar terrane and Altai terrane with a view to evaluate the mantle dynamics based on teleseismic data recorded by 103 seismic stations. Our tomographic results show both high and low velocity perturbations beneath the Tianshan orogenic belt. We suggest that the high velocity perturbations beneath this orogenic belt might represent the northward subducted lithosphere of the Tarim Basin and the southward subducted lithosphere of the Junggar Basin. The low velocity structure beneath the Tianshan orogenic belt might represent asthenosphere upwelling that triggered the extensive magmatism which contributed to rebuilding of the Tianshan orogenic belt.

D" anisotropy and slip systems in post-perovskite

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Nowacki, Andy; Wookey, James; Kendall, J.-Michael

2010-05-01

The lowermost few hundred kilometres of the Earth's mantle-known as D″-form the boundary between it and the core below, control the Earth's convective system, and are the site of probable large thermochemical heterogeneity. Seismic observations of D″ show a large (~2%) increase in S-wave velocity and significant seismic anisotropy (the variation of wave speed with direction) are present in many parts of the region. On the basis of continuous regions of fast shear velocity (V S) anomalies in global models, it is also proposed as the resting place of subducted slabs, notably the Farallon beneath North America. The MgSiO3-post-perovskite mineral phase is the most compelling explanation for observations of anisotropy, though an outstanding question is how post-perovskite and other mineral phases may deform to produce this: different mechanisms are possible. With knowledge either of mantle flow or which slip system is responsible for causing deformation, we can determine the other with the seismic anisotropy which is created. We investigate the dynamics at the CMB beneath North America using differential shear wave splitting in S and ScS phases from earthquakes of magnitude MW > 5.5 in South and Central America, Hawaii the Mid-Atlantic Ridge and East Pacific Rise. They are detected on ~500 stations in North America, giving ~700 measurements of anisotropy in D″. We achieve this by correcting for anisotropy in the upper mantle (UM) beneath both the source and receiver. The measurements cover three regions beneath western USA, the Yucatan peninsula and Florida. In each case, two different, crossing ray paths are used, so that the style of anisotropy can be constrained-only one azimuth cannot distinguish differing cases. Our results showing ~1% anisotropy dependent on azimuth are not consistent with transverse isotropy with a vertical symmetry axis (VTI) anywhere. The same but with a tilted axis is possible (TTI) and would be consistent with inclusions of seismically

Seismic constraints on magma evolution beneath Mount Baekdu (Changbai) volcano from transdimensional Bayesian inversion of ambient noise data

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Kim, Seongryong; Tkalčić, Hrvoje; Rhie, Junkee

2017-07-01

The magmatic process of continental intraplate volcanism (CIV) is difficult to understand due to heterogeneous interactions with the crust and the lithospheric upper mantle. Mount Baekdu (Changbai) volcano (MBV) is one of the prominent CIVs in northeast Asia that has shown a complex history of eruptions and associated magmatic structures. In addition, the relationship between the crustal magmatic structures and upper mantle phenomena are enigmatic due to the lack of consistent seismic constraints for the lithospheric structure. To enhance comprehensive understanding of the MBV magma evolution, we image the lithospheric structure beneath the MBV and surrounding regions using ambient noise data and the following two approaches: (1) multiple measures of ambient noise dispersion are acquired through different methods and (2) a transdimensional Bayesian inversion method is utilized to obtain unbiased results in joint analysis of the multiple data sets. The estimated Earth structure shows a thick crust ( 40 km) and a crustal anomaly with relatively high S wave velocity in the depth range 20-40 km. This type of structure extends to 100 km north from the MBV and is accompanied by the shallow and rapid S wave velocity decrease beneath the mantle lid ( 80 km). Through a comparison with previous P wave models, we interpret this structure as a consequence of compositional partitioning by mafic underplating and overlying cooled felsic layers as a result of fractional crystalization.

Helium as a tracer for fluids released from Juan de Fuca lithosphere beneath the Cascadia forearc

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McCrory, Patricia A.; Constantz, James E.; Hunt, Andrew G.; Blair, James Luke

2016-01-01

The ratio between helium isotopes (3He/4He) provides an excellent geochemical tracer for investigating the sources of fluids sampled at the Earth's surface. 3He/4He values observed in 25 mineral springs and wells above the Cascadia forearc document a significant component of mantle-derived helium above Juan de Fuca lithosphere, as well as variability in 3He enrichment across the forearc. Sample sites arcward of the forearc mantle corner (FMC) generally yield significantly higher ratios (1.2-4.0 RA) than those seaward of the corner (0.03-0.7 RA). The highest ratios in the Cascadia forearc coincide with slab depths (40-45 km) where metamorphic dehydration of young oceanic lithosphere is expected to release significant fluid and where tectonic tremor occurs, whereas little fluid is expected to be released from the slab depths (25-30 km) beneath sites seaward of the corner.Tremor (considered a marker for high fluid pressure) and high RA values in the forearc are spatially correlated. The Cascadia tremor band is centered on its FMC, and we tentatively postulate that hydrated forearc mantle beneath Cascadia deflects a significant portion of slab-derived fluids updip along the subduction interface, to vent in the vicinity of its corner. Furthermore, high RA values within the tremor band just arcward of the FMC, suggest that the innermost mantle wedge is relatively permeable.Conceptual models require: (1) a deep fluid source as a medium to transport primordial 3He; (2) conduits through the lithosphere which serve to speed fluid ascent to the surface before significant dilution from radiogenic 4He can occur; and (3) near lithostatic fluid pressure to keep conduits open. Our spatial correlation between high RA values and tectonic tremor provides independent evidence that tremor is associated with deep fluids, and it further suggests that high pore pressures associated with tremor may serve to keep fractures open for 3He migration through ductile upper mantle and lower crust.

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.

Comment on `Banana-doughnut kernels and mantle tomography' by van der Hilst and de Hoop

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Montelli, R.; Nolet, G.; Dahlen, F. A.

2006-12-01

E debbasi considerare come non è cosa più difficile a trattare, né più dubbia a riuscire, nè più pericolosa a maneggiare, che farsi capo ad introdurre nuovi ordini. Perché lo introduttore ha per nimici tutti quelli che delli ordini vecchi fanno bene, et ha tepidi defensori tutti quelli che delli ordini nuovi farebbono bene.† Machiavelli, Il Principe The claim by van der Hilst and de Hoop that finite-frequency (FF) inversion of seismic traveltimes does not result in measurable improvements in tomographic images is misguided, and based upon a biased selection of images in the upper mantle, where wave front healing effects are indeed small, and where our models are generally poorly resolved because we primarily used teleseismic waves that travel steeply in the upper mantle; and upon an improper application of statistics to the better-resolved anomalies in the lower mantle. If station corrections for long-period P waves are computed using ray theory, as we do, unmodelled FF effects may be responsible for slow anomalies of up to 0.3 per cent beneath very small island stations, but these effects are negligible for larger islands such as Reunion and Kerguelen. The presence of a plume beneath these islands is the most probable explanation for the observed low velocities.

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

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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

Mantle Attenuation Estimated from Regional and Teleseismic P-waves of Deep Earthquakes and Surface Explosions

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Ichinose, G.; Woods, M.; Dwyer, J.

2014-03-01

We estimated the network-averaged mantle attenuation t*(total) of 0.5 s beneath the North Korea test site (NKTS) by use of P-wave spectra and normalized spectral stacks from the 25 May 2009 declared nuclear test (mb 4.5; IDC). This value was checked using P-waves from seven deep (580-600 km) earthquakes (4.8 test, which confirms the equality with the sum of t*(u) and t*(d). We included constraints on seismic moment, depth, and radiation pattern by using results from a moment tensor analysis and corner frequencies from modeling of P-wave spectra recorded at local distances. We also avoided finite-faulting effects by excluding earthquakes with complex source time functions. We assumed ω2 source models for earthquakes and explosions. The mantle attenuation beneath the NKTS is clearly different when compared with the network-averaged t* of 0.75 s for the western US and is similar to values of approximately 0.5 s for the Semipalatinsk test site within the 0.5-2 Hz range.

Upper Mantle Seismic Anisotropy Beneath West Antarctica from Shear Wave Splitting Analysis of POLENET/ANET Data

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Accardo, N.; Wiens, D. A.; Hernandez, S.; Aster, R. C.; Nyblade, A.; Anandakrishnan, S.; Huerta, A. D.; Wilson, T. J.

2011-12-01

We constrain azimuthal anisotropy in the Antarctic upper mantle using shear wave splitting parameters obtained from teleseismic SKS, SKKS, and PKS phases recorded at 30 broad-band seismometers deployed in West Antarctica, and the Transantarctic Mountains as a part of POLENET/ANET. The first seismometers were deployed in late 2007 and additional seismometers were deployed in 2008 and 2009. The seismometers generally operate year-round using solar power, insulated boxes, and either rechargeable AGM or primary lithium batteries. We used an eigenvalue technique to linearize the rotated and shifted shear wave particle motions and determine the best splitting parameters. Robust windows around the individual phases were chosen using the Teanby cluster-analysis algorithm. We visually inspected all results and assigned a quality rating based on factors including signal-to-noise ratios, particle motions, and error contours. The best results for each station were then stacked to get an average splitting direction and delay time. The delay times range from 0.33 to 1.33 s, but generally average about 1 s. We conclude that the splitting results from anisotropy in the upper mantle, since the large splitting times cannot be accumulated in the relatively thin crust (20-30 km) of the region. Overall, fast directions in West Antarctica are at large angles to the direction of Antarctic absolute plate motion in either hotspot or no-net rotation frameworks, showing that the anisotropic fabric does not result from shear associated with the motion of Antarctica over the mantle. The West Antarctic fast directions are also much different than those found in East Antarctica by previous studies. We suggest that the East Antarctic splitting results from anisotropy frozen into the cold cratonic continental lithosphere, whereas West Antarctic splitting is related to Cenozoic tectonism. Stations within the West Antarctic Rift System (WARS), a region of Cenozoic extension, show fast directions

Mantle Upwellings Below the Ibero-Maghrebian Region with a Common Deep Source from P Travel-time Tomography

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Civiero, C.; Custodio, S.; Silveira, G. M.; Rawlinson, N.; Arroucau, P.

2017-12-01

The processes responsible for the geodynamical evolution of the Ibero-Maghrebian domain are still enigmatic. Several geophysical studies have improved our understanding of the region, but no single model has been accepted yet. This study takes advantage of the dense station networks deployed from France in the north to Canary Islands and Morocco in the south to provide a new high-resolution P-wave velocity model of the structure of the upper-mantle and top of the lower mantle. These images show subvertical small-scale upwellings below Atlas Range, Canary Islands and Central Iberia that seem to cross the transition zone. The results, together with geochemical evidence and a comparison with previous global tomographic models, reveal the ponding or flow of deep-plume material beneath the transition zone, which seems to feed upper-mantle "secondary" pulses. In the upper mantle the plumes, in conjunction with the subduction-related upwellings, allow the hot mantle to rise in the surrounding zones. During its rising, the mantle interacts with horizontal SW slab-driven flow which skirts the Alboran slab and connects with the mantle upwelling below Massif Central through the Valencia Trough rift.

Crustal and uppermost mantle structure and deformation in east-central China

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Li, H.; Yang, X.; Ouyang, L.; Li, J.

2017-12-01

We conduct a non-linear joint inversion of receiver functions and Rayleigh wave dispersions to obtain the crustal and upper mantle velocity structure in east-central China. In the meanwhile, the lithosphere and upper mantle deformation beneath east-central China is also evaluated with teleseismic shear wave splitting measurements. The resulting velocity model reveals that to the east of the North-South Gravity Lineament, the crust and the lithosphere are significantly thinned. Furthermore, three extensive crustal/lithospheric thinning sub-regions are clearly identified within the study area. This indicates that the modification of the crust and lithosphere in central-eastern China is non-uniform due to the heterogeneity of the lithospheric strength. Extensive crustal and lithospheric thinning could occur in some weak zones such as the basin-range junction belts and large faults. The structure beneath the Dabie orogenic belt is complex due to the collision between the North and South China Blocks during the Late Paleozoic-Triassic. The Dabie orogenic belt is generally delineated by a thick crust with a mid-crust low-velocity zone and a two-directional convergence in the lithospheric scale. Obvious velocity contrast exhibits in the crust and upper mantle at both sides of the Tanlu fault, which suggests the deep penetration of this lithospheric-scale fault. Most of our splitting measurements show nearly E-W trending fast polarization direction which is slightly deviating from the direction of plate motion. The similar present-day lithosphere structure and upper mantle deformation may imply that the eastern NCC and the eastern SCB were dominated by a common dynamic process after late Mesozoic, i.e., the westward subduction of Pacific plate and the retreat of the subduction plate. The westward subduction of the Philippine plate and the long-range effects of the collision between the Indian plate and Eurasia plate during Cenozoic may have also contributed to the present

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

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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.

Fossil plume head beneath the Arabian lithosphere?

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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

Melt migration modeling in partially molten upper mantle

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Ghods, Abdolreza

The objective of this thesis is to investigate the importance of melt migration in shaping major characteristics of geological features associated with the partial melting of the upper mantle, such as sea-floor spreading, continental flood basalts and rifting. The partial melting produces permeable partially molten rocks and a buoyant low viscosity melt. Melt migrates through the partially molten rocks, and transfers mass and heat. Due to its much faster velocity and appreciable buoyancy, melt migration has the potential to modify dynamics of the upwelling partially molten plumes. I develop a 2-D, two-phase flow model and apply it to investigate effects of melt migration on the dynamics and melt generation of upwelling mantle plumes and focusing of melt migration beneath mid-ocean ridges. Melt migration changes distribution of the melt-retention buoyancy force and therefore affects the dynamics of the upwelling plume. This is investigated by modeling a plume with a constant initial melt of 10% where no further melting is considered. Melt migration polarizes melt-retention buoyancy force into high and low melt fraction regions at the top and bottom portions of the plume and therefore results in formation of a more slender and faster upwelling plume. Allowing the plume to melt as it ascends through the upper mantle also produces a slender and faster plume. It is shown that melt produced by decompressional melting of the plume migrates to the upper horizons of the plume, increases the upwelling velocity and thus, the volume of melt generated by the plume. Melt migration produces a plume which lacks the mushroom shape observed for the plume models without melt migration. Melt migration forms a high melt fraction layer beneath the sloping base of the impermeable oceanic lithosphere. Using realistic conditions of melting, freezing and melt extraction, I examine whether the high melt fraction layer is able to focus melt from a wide partial melting zone to a narrow region

Box Tomography: first application to the imaging of upper-mantle shear velocity and radial anisotropy structure beneath the North American continent

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Clouzet, P.; Masson, Y.; Romanowicz, B.

2018-06-01

The EarthScope Transpotable Array (TA) deployment provides dense array coverage throughout the continental United States and with it, the opportunity for high-resolution 3-D seismic velocity imaging of the stable part of the North American (NA) upper mantle. Building upon our previous long-period waveform tomographic modeling, we present a higher resolution 3-D isotropic and radially anisotropic shear wave velocity model of the NA lithosphere and asthenosphere. The model is constructed using a combination of teleseismic and regional waveforms down to 40 s period and wavefield computations are performed using the spectral element method both for regional and teleseismic data. Our study is the first tomographic application of `Box Tomography', which allows us to include teleseismic events in our inversion, while computing the teleseismic wavefield only once, thus significantly reducing the numerical computational cost of several iterations of the regional inversion. We confirm the presence of high-velocity roots beneath the Archean part of the continent, reaching 200-250 km in some areas, however the thickness of these roots is not everywhere correlated to the crustal age of the corresponding cratonic province. In particular, the lithosphere is thick (˜250 km) in the western part of the Superior craton, while it is much thinner (˜150 km) in its eastern part. This may be related to a thermomechanical erosion of the cratonic root due to the passage of the NA plate over the Great Meteor hotspot during the opening of the Atlantic ocean 200-110 Ma. Below the lithosphere, an upper-mantle low-velocity zone (LVZ) is present everywhere under the NA continent, even under the thickest parts of the craton, although it is less developed there. The depth of the minimum in shear velocity has strong lateral variations, whereas the bottom of the LVZ is everywhere relatively flat around 270-300 km depth, with minor undulations of maximum 30 km that show upwarping under the thickest

The South Tibetan Tadpole Zone: Ongoing density sorting at the Moho beneath the Indus-Tsangpo suture zone (and beneath volcanic arcs?)

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Kelemen, Peter; Hacker, Bradley

2016-04-01

Some Himalayan cross-sections show Indian crust thrust beneath Tibetan crust, with no intervening mantle wedge (e.g., Powell & Conaghan 73), others indicate thickening of both crustal sections, juxtaposed along a steep suture (e.g., Dewey & Burke 73), and many combine features of both end-members (e.g., Argand 24). To understand crustal scale structure and related phenomena, we focus on data from an area in southern Tibet at 28-30°N, 84-91°E. 21st century observations in this area show a horizontal Moho at ca 80 km depth, extending from thickened Indian crust, across a region where Tibetan crust is interpreted to overlie Indian crust, into thickened Tibetan crust (Zhao et al 01; Monsalve et al 08; Wittlinger et al 09; Nabelek et al 09; Kind et al 02; Schulte-Pelkum et al 05; Shi et al 15). About half the subducted Indian crustal volume is present, whereas the other half is missing (Replumaz et al 10). Vp/Vs indicates the alpha-beta quartz transition is at ca 50 km depth (Sheehan et al 13). Miocene lavas include primitive andesites probably formed by interaction of crustal material with mantle peridotite at > 1000°C (Turner et al 93; Williams et al 01, 04; Chung et al 05). Thermobarometry of xenoliths in a 12.7 Ma dike records ~ 1100°C at 2.2-2.6 GPa and 920°C at 1.7 GPa (Chan et al 09). Biotite-rich pyroxenites among the xenoliths, similar to those in central Tibet (Hacker et al 00) and the Pamirs (Hacker et al 05), may form via reaction of hot crustal lithologies and mantle peridotite (e.g., Sekine & Wyllie 82, 83). These data, taken together, indicate Miocene to present day temperatures exceeding 800°C from ca 50 km depth to the Moho, unlike thermal models with a hot mid-crust and cold Moho (McKenzie & Priestley 08, Craig et al 12, Wang et al 13; Nabelek & Nabelek 14), and despite the observation of numerous, near-Moho earthquakes (Chen & Molnar 83; Chen & Yang 04; Monsalve et al 06; Priestley et al 08; Craig et al 12) interpreted by many as brittle failure

Array-Based Receiver Function Analysis of the Subducting Juan de Fuca Plate Beneath the Mount St. Helens Region and its Implications for Subduction Geometry and Metamorphism

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Mann, M. E.; Abers, G. A.; Creager, K. C.; Ulberg, C. W.; Crosbie, K.

2017-12-01

Mount St. Helens (MSH) is unusual as a prolific arc volcano located 50 km towards the forearc of the main Cascade arc. The iMUSH (imaging Magma Under mount St. Helens) broadband deployment featured 70 seismometers at 10-km spacing in a 50-km radius around MSH, spanning a sufficient width for testing along-strike variation in subsurface geometry as well as deep controls on volcanism in the Cascade arc. Previous estimates of the geometry of the subducting Juan de Fuca (JdF) slab are extrapolated to MSH from several hundred km to the north and south. We analyze both P-to-S receiver functions and 2-D Born migrations of the full data set to locate the upper plate Moho and the dip and depth of the subducting slab. The strongest coherent phase off the subducting slab is the primary reverberation (Ppxs; topside P-to-S reflection) from the Moho of the subducting JdF plate, as indicated by its polarity and spatial pattern. Migration images show a dipping low velocity layer at depths less than 50 km that we interpret as the subducting JdF crust. Its disappearance beyond 50 km depth may indicate dehydration of subducting crust or disruption of high fluid pressures along the megathrust. The lower boundary of the low velocity zone, the JdF Moho, persists in the migration image to depths of at least 90 km and is imaged at 74 km beneath MSH, dipping 23 degrees. The slab surface is 68 km beneath MSH and 85 km beneath Mount Adams volcano to the east. The JdF Moho exhibits 10% velocity contrasts as deep as 85 km, an observation difficult to reconcile with simple models of crustal eclogitization. The geometry and thickness of the JdF crust and upper plate Moho is consistent with similar transects of Cascadia and does not vary along strike beneath iMUSH, indicating a continuous slab with no major disruption. The upper plate Moho is clear on the east side of the array but it disappears west of MSH, a feature we interpret as a result of both serpentinization of the mantle wedge and a

The Universal Cpx Jd-Di barometer for mantle peridotite eclogite and pyroxenites and it using for the mantle petrology

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Ashchepkov, Igor

2015-04-01

The Jd-Di exchange in clinopyroxenes used for the calibration of pyroxene barometer (Ashchepkov, 2000;2002; Ashchepkov et al 2010;2011;2012) was transformed to make one universal equation for mantle peridotite eclogites and pyroxenites. The original barometer (Ashchepkov, 2002) calibrated on pressures produced by Opx barometry (McGregor , 1974) was transformed (Ashchepkov et al ., 2004; 2010; 2011) to satisfy the increasing data bases for the mantle xenoliths and experimental values 530 in peridotitic and 650 in elcogitic systems . The obtained difference Pd =Pcpx- Pexp were studied for the dependence on each component and their combination . Instead of the common activities we used the temperature-dependent empirical equations. The three separate equations for the common peridotites, pyroxenites and eclogites (Ashchepkov et al., 2010) were checked and complex To and Al-Na-Fe dependent universal coefficients were received. The KD is determined as follows: KD=Na/AlCr*Mg/Ca The logarithmic dependence between P and KD was transformed to a linear one. Final pressure equations are: AlCr=(Al-0.01) *((T-600)/700)**0.75+Cr*(ToK-100)/1000+(4*Ti-0.0125)/ (T0-801)*650 +0.55*((Fe-0.23) *(T0-900)/10000-K) P=0.26*(5+12*(Al+0.30*Na)KD* ToK**0.75 /(1+Fe+ Fe*(ToK-600)/1000)-ln(1273/ ToK))*40*(7*Na-Al-15*Ti+10*Cr+Mg/4)+7.5*Si-20*( Al*Na*Mg/Ca/(Al-2*Ti+Na-2*Fe/(Fe+Mg))+50*(Na+0.1*Al-2*Ti+0.05*Mg-0.22*Ca-0.7*Na)/Ca). Obtained equation in combination with the (Nimis,Taylor, 2000) thermometer allow to reconstruct position of the magma feeder systems of the alkali basaltic magma withing the mantle diapirs in modern platforms like in Vitim plateau (Ashchepkov et al., 2011) and now was applicated to reconstruct the deep seated magma conduits beneath the mountain collision systems, island arcs ocean plateaus etc. This equation allows to receive the positions of the major groups of eclogites mantle sections and to find out the regularities of their behavior. The Fe rich eclogites commonly

Structure and seismicity of the upper mantle using deployments of broadband seismographs in Antarctica and the Mariana Islands

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Barklage, Mitchell

. We investigate seismic velocity structure of the upper mantle across the Central Mariana subduction system using data from the 2003-2004 Mariana Subduction Factory Imaging Experiment. This 11-month experiment consisted of 20 broadband seismic stations deployed on islands and 58 semi-broadband ocean bottom seismographs deployed across the forearc, island arc, and back-arc spreading center. We determine Vp and Vp/Vs structure on a three dimensional grid using over 25,000 local travel time observations as well as over 2000 teleseismic arrival times determined by waveform cross correlation. The mantle wedge is characterized by a region of low velocity and high Vp/Vs beneath the forearc, an inclined zone of low velocity underlying the volcanic front, and a broad region of low velocity beneath the back-arc spreading center. The slow velocity anomalies are strongest at roughly 20-30 km depth in the forearc, 60-70 km depth beneath the volcanic arc, and 20-30 km beneath the back-arc spreading center. The slow velocity anomalies beneath the arc and back-arc appear as separate and distinct features in our images, with a small channel of connectivity occurring at approximately 75 km depth. The subducting Pacific plate is characterized by high seismic velocities. An exception occurs in the forearc beneath the big blue seamount and at the top of the slab at roughly 80 km depth where slow velocities are observed. We interpret the forearc anomalies to represent a region of large scale serpentinization of the mantle whereas the arc and back-arc anomalies represent regions of high temperature with a small amount of increased water content and/or melt and constrain the source regions in the mantle for arc and back-arc lavas. We investigate the double seismic zone (dsz) beneath the Central Mariana Arc using data from a land-sea array of 58 ocean bottom seismographs and 20 land seismographs deployed during 2003-2004. Nearly 600 well-recorded earthquakes were located using a P and S wave

Gravity inversion of deep-crust and mantle interfaces in the Three Gorges area

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Wang Jian

2012-11-01

Full Text Available To better understand the heterogeneity of deep-crust and mantle interfaces in the region of the Three Gorges, China, we used the Parker-Oldenburg iterative inversion method to invert existing Bouguer gravity data from the Three Gorges area (1 : 500000, a new gravity map of the Three Gorges Dam (1 : 200000, and the results of deep seismic soundings. The inversion results show a Moho depth of 42 km between Badong and Zigui and the depth of the B2 lower-crustal interface beneath the Jianghan Plain and surrounding areas at 21–25 km. The morphology of crustal interfaces and the surface geology present an overpass structure. The mid-crust beneath the Three Gorges Dam is approximately 9 km thick, which is the thinnest in the Three Gorges area and may be related to the shallow low-density body near the Huangling anticline. The upper crust is seismogenic, and there is a close relationship between seismicity and the deep-crust and mantle interfaces. For example, the M5. 1 Zigui earthquake occurred where the gradients of the Moho and the B2 interface are the steepest, showing that deep structure has a very important effect on regional seismicity.

Depth variations of the 410 and 520 km-discontinuities beneath Asia and the Pacific from PP precursors

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Schäfer, J.; Wölbern, I.; Rümpker, G.

2009-06-01

We investigate depth variations of the 410 and 520 km-discontinuities beneath Asia and the Pacific which serve as examples for a continental and an oceanic region, respectively. The depths are derived from travel-time differences between the PP-phase and its precursors that are reflected at the discontinuities. After accounting for differences in average crustal thickness, we find that the depth of the ‘410’ is rather uniform but larger than expected beneath both regions with a value of approximately 418 km. Signals from the ‘520’ are slightly less pronounced. However, while the average depth of the ‘520’ beneath Asia is about 519 km, we obtain a value of about 531.5 km for the Pacific. Here, the depression of the discontinuities can be explained in view of thermal anomalies in relation to mantle plumes. For Asia, however, the observations seem to require a more complex pattern of thermal anomalies possibly complemented by variations in chemical composition.

Shear wave splitting and upper mantle deformation in French Polynesia: Evidence for small-scale heterogeneity related to the Society hotspot

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Russo, R. M.; Okal, E. A.

1998-07-01

We determined shear wave splitting parameters at four island sites in French Polynesia: Tiputa (TPT) on Rangiroa in the Tuamotu archipelago; Papeete (PPT) on Tahiti in the Society Islands; Tubuai (TBI) in the Cook-Austral island chain; and Rikitea (RKT) on Mangareva in the Gambier Islands. We also examined splitting at Pitcairn (PTCN) on Pitcairn Island; because of the short time of operation of PTCN, our results there are preliminary. We find substantial differences in splitting, most likely caused by variable upper mantle deformation beneath the five stations. At TPT the fast split shear wave (ϕ) direction is N66°W±4°, parallel to the current Pacific-hotspots relative motion (APM) vector; the delay time between fast and slow waves is 1.3±0.2 s. At PPT, on Tahiti, we could detect no splitting despite many clear SKS observations. At TBI, on Tubuai we detected splitting with a delay time of 1.1±0.1 s and a ϕ direction midway between the local APM direction and the fossil spreading direction (N86°W±2°), as locally indicated by the nearby Austral Fracture Zone. At RKT in the Gambier Islands, ϕ trends N53°W±6°, 16° clockwise of the local APM azimuth, and delay time at RKT is 1.1±0.1 s. Results at PTCN include ϕ near N38°W±9° and a delay time of 1.1±0.3 s. These different results imply variable upper mantle deformation beneath the five sites. We interpret splitting at TPT and, possibly, RKT as indicative of asthenospheric flow or shear in the APM direction beneath the stations. At PPT, azimuthal isotropy indicates deformed upper mantle with a vertical symmetry axis, or absence of strong or consistently oriented mantle deformation fabric beneath Tahiti. Either effect could be related to recent hotspot magmatism on Tahiti. At TBI, splitting may be complicated by juxtaposition of different lithospheric thicknesses along the nearby Austral Fracture Zone, resulting in perturbation of asthenospheric flow. The absence of splitting related to fossil

Upper Mantle Discontinuities Underneath Central and Southern Mexico

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Perez-Campos, X.; Clayton, R. W.

2011-12-01

Central and southern Mexico are affected by the subduction of Cocos plate beneath North American plate. The MesoAmerican Subduction Experiment (MASE) and the Veracruz-Oaxaca (VEOX) project have mapped the geometry of the Cocos slab. It is characterized in central Mexico by a shallow horizontal geometry up to ~300 km from the trench, then it dives steeply (70°) into the mantle, to its apparent end at 500 km depth. In contrast, some 400 km to the south, the slab subducts smoothly, with a dip angle of ~26° to a depth of 150 km. We use receiver functions from teleseismic events, recorded at stations from MASE, VEOX, and the Servicio Sismológico Nacional (SSN, Mexican National Seismological Service) to map the upper mantle discontinuities and properties of the transition zone in central and southern Mexico. We also use data from the Mapping the Rivera Subduction Zone (MARS) Experiment to get a complete picture of the subduction regime in central Mexico and compare the mantle transition zone in a slab tear regime. The 410 discontinuity shows significant variation in topography in central Mexico, particularly where the slab is expected to reach such depth. The 660 discontinuity shows a smoother topography, indicating that the slab does not penetrate this far down. The results will be compared with a ridge regime in the Gulf of California.

Constraining the composition and thermal state of the mantle beneath Europe from inversion of long-period electromagnetic sounding data

DEFF Research Database (Denmark)

Khan, Amir; Connolly, J.A.D.; Olsen, Nils

2006-01-01

We reexamine the problem of inverting C responses, covering periods between 1 month and 1 year collected from 42 European observatories, to constrain the internal structure of the Earth. Earlier studies used the C responses, which connect the magnetic vertical component and the horizontal gradient...... of the horizontal components of electromagnetic variations, to obtain the conductivity profile of the Earth's mantle. Here, we go beyond this approach by inverting directly for chemical composition and thermal state of the Earth, rather than subsurface electrical conductivity structure. The primary inversion...... of geophysical data for compositional parameters, planetary composition, and thermal state is feasible. The inversion indicates most probable lower mantle geothermal gradients of similar to 0.58 K/km, core mantle boundary temperatures of similar to 2900 degrees C, bulk Earth molar Mg/Si ratios of similar to 1...

Sediment Trapping by Emerged Channel Bars in the Lowermost Mississippi River during a Major Flood

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Bo Wang

2015-11-01

Full Text Available The formation of channel bars has been recognized as the most significant sediment response to the highly trained Mississippi River (MR. However, no quantitative study exists on the dynamics of emerged channel bars and associated sediment accumulation in the last 500-kilometer reach of the MR from the Gulf of Mexico outlet, also known as the lowermost Mississippi River. Such knowledge is especially critical for riverine sediment management to impede coastal land loss in the Mississippi River Delta. In this study, we utilized a series of satellite images taken from August 2010 to January 2012 to assess the changes in surface area and volume of three large emerged channel bars in the lowermost MR following an unprecedented spring flood in 2011. River stage data were collected to develop a rating curve of surface areas detected by satellite images with flow conditions for each of the three bars. A uniform geometry associated with the areal change was assumed to estimate the bar volume changes. Our study reveals that the 2011 spring flood increased the surface area of the bars by 3.5% to 11.1%, resulting in a total surface increase of 7.3%, or 424,000 m2. Based on the surface area change, we estimated a total bar volume increase of 4.4%, or 1,219,900 m3. This volume increase would be equivalent to a sediment trapping of approximately 1.0 million metric tons, assuming a sediment bulk density of 1.2 metric tons per cubic meter. This large quantity of sediment is likely an underestimation because of the neglect of subaqueous bar area change and the assumption of a uniform geometry in volume estimation. Nonetheless, the results imply that channel bars in the lowermost MR are capable of capturing a substantial amount of sediment during floods, and that a thorough assessment of their long-term change can provide important insights into sediment trapping in the lowermost MR as well as the feasibility of proposed river sediment diversions.

Mantle temperature under drifting deformable continents during the supercontinent cycle

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Yoshida, Masaki

2013-04-01

The thermal heterogeneity of the Earth's mantle under the drifting continents during a supercontinent cycle is a controversial issue in earth science. Here, a series of numerical simulations of mantle convection are performed in 3D spherical-shell geometry, incorporating drifting deformable continents and self-consistent plate tectonics, to evaluate the subcontinental mantle temperature during a supercontinent cycle. Results show that the laterally averaged temperature anomaly of the subcontinental mantle remains within several tens of degrees (±50 °C) throughout the simulation time. Even after the formation of the supercontinent and the development of subcontinental plumes due to the subduction of the oceanic plates, the laterally averaged temperature anomaly of the deep mantle under the continent is within +10 °C. This implies that there is no substantial temperature difference between the subcontinental and suboceanic mantles during a supercontinent cycle. The temperature anomaly immediately beneath the supercontinent is generally positive owing to the thermal insulation effect and the active upwelling plumes from the core-mantle boundary. In the present simulation, the formation of a supercontinent causes the laterally averaged subcontinental temperature to increase by a maximum of 50 °C, which would produce sufficient tensional force to break up the supercontinent. The periodic assembly and dispersal of continental fragments, referred to as the supercontinent cycle, bear close relation to the evolution of mantle convection and plate tectonics. Supercontinent formation involves complex processes of introversion, extroversion or a combination of these in uniting dispersed continental fragments, as against the simple opening and closing of individual oceans envisaged in Wilson cycle. In the present study, I evaluate supercontinent processes in a realistic mantle convection regime. Results show that the assembly of supercontinents is accompanied by a

Travel-time Tomography of the Upper Mantle using Amphibious Array Seismic Data from the Cascadia Initiative and EarthScope

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Cafferky, S.; Schmandt, B.

2013-12-01

Offshore and onshore broadband seismic data from the Cascadia Initiative and EarthScope provide a unique opportunity to image 3-D mantle structure continuously from a spreading ridge across a subduction zone and into continental back-arc provinces. Year one data from the Cascadia Initiative primarily covers the northern half of the Juan de Fuca plate and the Cascadia forearc and arc provinces. These new data are used in concert with previously collected onshore data for a travel-time tomography investigation of mantle structure. Measurement of relative teleseismic P travel times for land-based and ocean-bottom stations operating during year one was completed for 16 events using waveform cross-correlation, after bandpass filtering the data from 0.05 - 0.1 Hz with a second order Butterworth filter. Maps of travel-time delays show changing patterns with event azimuth suggesting that structural variations exist beneath the oceanic plate. The data from year one and prior onshore travel time measurements were used in a tomographic inversion for 3-D mantle P-velocity structure. Inversions conducted to date use ray paths determined by a 1-D velocity model. By meeting time we plan to present models using ray paths that are iteratively updated to account for 3-D structure. Additionally, we are testing the importance of corrections for sediment and crust thickness on imaging of mantle structure near the subduction zone. Low-velocities beneath the Juan de Fuca slab that were previously suggested by onshore data are further supported by our preliminary tomographic inversions using the amphibious array data.

Chemical trends in ocean islands explained by plume–slab interaction

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Dannberg, Juliane; Gassmöller, Rene

2018-04-01

Earth's surface shows many features, of which the genesis can be understood only through their connection with processes in Earth's deep interior. Recent studies indicate that spatial geochemical patterns at oceanic islands correspond to structures in the lowermost mantle inferred from seismic tomographic models. This suggests that hot, buoyant upwellings can carry chemical heterogeneities from the deep lower mantle toward the surface, providing a window to the composition of the lowermost mantle. The exact nature of this link between surface and deep Earth remains debated and poorly understood. Using computational models, we show that subducted slabs interacting with dense thermochemical piles can trigger the ascent of hot plumes that inherit chemical gradients present in the lowermost mantle. We identify two key factors controlling this process: (i) If slabs induce strong lower-mantle flow toward the edges of these piles where plumes rise, the pile-facing side of the plume preferentially samples material originating from the pile, and bilaterally asymmetric chemical zoning develops. (ii) The composition of the melt produced reflects this bilateral zoning if the overlying plate moves roughly perpendicular to the chemical gradient in the plume conduit. Our results explain some of the observed geochemical trends of oceanic islands and provide insights into how these trends may originate.

Geophysical Investigation of Upper Mantle Anomalies of the Australian-Antarctic Ridge

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Park, S. H.; Choi, H.; Kim, S. S.; Lin, J.

2017-12-01

upper mantle beneath the AAR may thus be manifested as shallow axial depths and prominent negative gravity anomalies. Here we present exploratory geophysical data analyses of the AAR to estimate the spatial variation of the Southern Ocean mantle.

Slab Penetration vs. Slab Stagnation: Mantle Reflectors as an Indicator

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Okeler, A.; Gu, Y. J.; Schultz, R.; Contenti, S. M.

2011-12-01

Subducting oceanic lithosphere along convergent margins may stagnate near the base of the upper mantle or penetrate into the lower mantle. These dynamic processes cause extensive thermal and compositional variations, which can be observed in terms of impedance contrast (reflectivity) and topography of mantle transition zone (MTZ) discontinuities, i.e., 410- and 660-km discontinuities. In this study, we utilize ~ 15000 surface-reflected shear waves (SS) and their precursory arrivals (S410S and S660S) to analyze subduction related deformations on mantle reflectivity structure. We apply pre-stack, time-to-depth migration technique to SS precursors, and move weak underside reflections using PREM-predicted travel-time curves. Common Mid-point gathers are formed to investigate structure under the western Pacific, south America, and Mediterranean convergent boundaries. In general, mantle reflectivity structures are consistent with previous seismic tomography models. In regions of slab penetration (e.g., southern Kurile arc, Aegean Sea), our results show 1) a substantial decrease in S660S amplitude, and 2) strong lower mantle reflector(s) at ~ 900 km depth. These reflective structures are supported by zones of high P and S velocities extending into the lower mantle. Our 1-D synthetic simulations suggest that the decreasing S660S amplitudes are, at least partially, associated with shear wave defocusing due to changes in reflector depth (by ±20 km) within averaging bin. Assuming a ~500 km wide averaging area, a dipping reflector with 6-8 % slope can reduce the amplitude of a SS precursor by ~50%. On the other hand, broad depressions with strong impedance contrast at the base of the MTZ characterize the regions of slab stagnation, such as beneath the Tyrrhenian Sea and northeastern China. For the latter region, substantial topography on the 660-km discontinuity west of the Wadati-Benioff zone suggests that the stagnant part of the Pacific plate across Honshu arc is not

Seismic Investigations of the Crust and Upper Mantle Structure in Antarctica and Madagascar

Science.gov (United States)

Ramirez, Cristo

In the three studies that form this dissertation, seismic data from Antarctica and Madagascar have been analyzed to obtain new insights into crustal structure and mantle flow. Until recently, there have been little seismic data available from these areas for interrogating Earth structure and processes. In Antarctica, I analyzed datasets from temporary deployments of broadband seismic stations in both East and West Antarctica. In Madagascar, I analyzed data from a temporary network of broadband stations, along with data from three permanent stations. The seismic data have been processed and modeled using a wide range of techniques to characterize crust and mantle structure. Crustal structure in the East Antarctic Craton resembles Precambrian terrains around the world in its thickness and shear wave velocities. The West Antarctic Rift System has thinner crust, consistent with crustal thickness beneath other Cretaceous rifts. The Transantarctic Mountains show thickening of the crust from the costal regions towards the interior of the mountain range, and high velocities in the lower crust at several locations, possibly resulting from the Ferrar magmatic event. Ross Island and Marie Byrd Land Dome have elevated crustal Vp/Vs ratios, suggesting the presence of partial melt and/or volcaniclastic material within the crust. The pattern of seismic anisotropy in Madagascar is complex and cannot arise solely due to mantle flow from the African superplume, as previously proposed. To explain the complex pattern of anisotropy, a combination of mechanisms needs to be invoked, including mantle flow from the African superplume, mantle flow from the Comoros hotspot, small scale upwelling in the mantle induced by lithospheric delamination, and fossil anisotropy in the lithospheric mantle along Precambrian shear zones.

Influence of mantle viscosity structure and mineral grain size on fluid migration pathways in the mantle wedge.

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Cerpa, N. G.; Wada, I.; Wilson, C. R.; Spiegelman, M. W.

2016-12-01

We develop a 2D numerical porous flow model that incorporates both grain size distribution and matrix compaction to explore the fluid migration (FM) pathways in the mantle wedge. Melt generation for arc volcanism is thought to be triggered by slab-derived fluids that migrate into the hot overlying mantle and reduce its melting temperature. While the narrow location of the arcs relative to the top of the slab ( 100±30 km) is a robust observation, the release of fluids is predicted to occur over a wide range of depth. Reconciling such observations and predictions remains a challenge for the geodynamic community. Fluid transport by porous flow depends on the permeability of the medium which in turn depends on fluid fraction and mineral grain size. The grain size distribution in the mantle wedge predicted by laboratory derived laws was found to be a possible mechanism to focusing of fluids beneath the arcs [Wada and Behn, 2015]. The viscous resistance of the matrix to the volumetric strain generates compaction pressure that affects fluid flow and can also focus fluids towards the arc [Wilson et al, 2014]. We thus have developed a 2D one-way coupled Darcy's-Stokes flow model (solid flow independent of fluid flow) for the mantle wedge that combines both effects. For the solid flow calculation, we use a kinematic-dynamic approach where the system is driven by the prescribed slab velocity. The solid rheology accounts for both dislocation and diffusion creep and we calculate the grain size distribution following Wada and Behn [2015]. In our fluid flow model, the permeability of the medium is grain size dependent and the matrix bulk viscosity depends on solid shear viscosity and fluid fraction. The fluid influx from the slab is imposed as a boundary condition at the base of the mantle wedge. We solve the discretized governing equations using the software package TerraFERMA. Applying a range of model parameter values, including slab age, slab dip, subduction rate, and fluid

Electrical structure beneath the Hangai Dome, Mongolia, from magnetotelluric data

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Comeau, Matthew; Käufl, Johannes; Becken, Michael; Kuvshinov, Alexey; Demberel, Sodnomsambuu; Sukhbaatar, Usnikh; Batmagnai, Erdenechimeg; Tserendug, Shoovdor; Nasan, Ochir

2017-04-01

The Hangai Dome in west-central Mongolia is an unusual high-elevation intra-continental plateau located far from tectonic plate boundaries and characterized by dispersed, low-volume, basaltic volcanism. This region is an ideal natural laboratory for studying intra-continental orogenic and magmatic processes resulting from crust-mantle interactions. The processes responsible for developing the Hangai Dome remain unexplained, due in part to a lack of high resolution geophysical data over the area. Here we present newly acquired broadband (0.008 - 3,000 s) magnetotelluric (MT) data from a large-scale ( 200 x 450 km) and high resolution (site spacing > 5 km) survey across the Hangai Dome. A total of 125 sites were collected and include full MT sites and telluric-only sites where inter-station transfer functions were computed. The MT data are used to generate an electrical resistivity model of the crust and upper mantle below the Hangai Dome. The model shows that the lower crust ( 30 - 50 km; below the brittle-ductile transition zone) beneath the Hangai Dome contains anomalous discrete pockets of low-resistivity ( 30 ohm-m) material that indicate the presence of local accumulations of fluids and/or low-percent partial melts. These anomalous regions appear to be spatially associated with the surface expressions of past volcanism, hydrothermal activity, and an increase in heat flow. They also correlate with observed crustal low-density and low-velocity anomalies. However they are in contrast to some geochemical and petrological studies which show long-lived crustal melt storage is impossible below the Hangai due to limited crustal assimilation and crustal contamination, arguing for a single parent-source at mantle depths. The upper mantle ( 6%) at this location. The results are consistent with modern geochemical and geophysical data, which show a thin lithosphere below the Hangai region. Furthermore the results agree with geodynamic models that require a low-heat flux

Mantle helium along the Newport-Inglewood fault zone, Los Angeles basin, California: A leaking paleo-subduction zone

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Boles, J. R.; Garven, G.; Camacho, H.; Lupton, J. E.

2015-07-01

Mantle helium is a significant component of the helium gas from deep oil wells along the Newport-Inglewood fault zone (NIFZ) in the Los Angeles (LA) basin. Helium isotope ratios are as high as 5.3 Ra (Ra = 3He/4He ratio of air) indicating 66% mantle contribution (assuming R/Ra = 8 for mantle), and most values are higher than 1.0 Ra. Other samples from basin margin faults and from within the basin have much lower values (R/Ra geothermal gradients, and is modeled as truncated by a proposed major, potentially seismically active, décollement beneath the LA basin. Our results demonstrate that the NIFZ is a deep-seated fault directly or indirectly connected with the mantle. Based on a 1-D model, we calculate a maximum Darcy flow rate q ˜ 2.2 cm/yr and a fault permeability k ˜ 6 × 10-17 m2 (60 microdarcys), but the flow rates are too low to create a geothermal anomaly. The mantle leakage may be a result of the NIFZ being a former Mesozoic subduction zone in spite of being located 70 km west of the current plate boundary at the San Andreas fault.

What drives the Tibetan crust to the South East Asia? Role of upper mantle density discontinuities as inferred from the continental geoid anomalies

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Rajesh, S.

2012-04-01

The Himalaya-Tibet orogen formed as a result of the northward convergence of India into the Asia over the past 55 Ma had caused the north south crustal shortening and Cenozoic upliftment of the Tibetan plateau, which significantly affected the tectonic and climatic framework of the Asia. Geodetic measurements have also shown eastward crustal extrusion of Tibet, especially along major east-southeast strike slip faults at a slip rate of 15-20 mm a-1 and around 40 mm a-1. Such continental scale deformations have been modeled as block rotation by fault boundary stresses developed due to the India-Eurasia collision. However, the Thin Sheet model explained the crustal deformation mechanism by considering varying gravitational potential energy arise out of varying crustal thickness of the viscous lithosphere. The Channel Flow model, which also suggests extrusion is a boundary fault guided flow along the shallow crustal brittle-ductile regime. Although many models have proposed, but no consensus in these models to explain the dynamics of measured surface geodetic deformation of the Tibetan plateau. But what remains conspicuous is the origin of driving forces that cause the observed Tibetan crustal flow towards the South East Asia. Is the crustal flow originated only because of the differential stresses that developed in the shallow crustal brittle-ductile regime? Or should the stress transfer to the shallow crustal layers as a result of gravitational potential energy gradient driven upper mantle flow also to be accounted. In this work, I examine the role of latter in the light of depth distribution of continental geoid anomalies beneath the Himalaya-Tibet across major upper mantle density discontinuities. These discontinuity surfaces in the upper mantle are susceptible to hold the plastic deformation that may occur as a result of the density gradient driven flow. The distribution of geoid anomalies across these density discontinuities at 220, 410 and 660 km depth in the

Geochemistry of the mantle beneath the Rodriguez Triple Junction and the South-East Indian Ridge

International Nuclear Information System (INIS)

Michard, A.; Montigny, R.; Schlich, R.

1986-01-01

Rare earth element abundances and SR, Nd, Pb isotope compositions have been measured on zero-age dredge samples from the Rodriguez Triple Junction (RTJ) and the South-East Indian Ridge (SEIR). Along the SEIR, the geochemical ''halo'' of the St. Paul hot spot has a half-width of about 400 km and the data may be fairly well accounted for by a binary mixing between an Indian MORB-type component ( 87 Sr/ 86 Sr=0.7028, 143 Nd/ 144 Nd=0.51304, 206 Pb/ 204 Pb=17.8) and the plume type St. Paul component (0.7036, 0.5129 and 18.7 respectively). The alignment of the lead isotope data is particularly good with age of 1.95+-0.13 Ga and Th/U source value of 3.94. One sample dredged on the ridge 60 km southeast of St. Paul bears a definite Kertguelen isotopic signature. The RTJ has distinctive geochemical properties which contrast with those of the adjacent ridge segments. Low 206 Pb/ 204 Pb ratios which plots to the left of the geochron, rather high 208 Pb/ 204 Pb and 87 Sr/ 86 Sr ratios (17.4, 37.4, and 0.7031 respectively) a striking isotopic homogeneity, and variable LRE/HREE fractionation with (LA/S)sub(N) 0.3-0.8 make this triple junction an anomalous site. The geochemical properties of the Indian Ocean basalts have been examined using a three-component mantle model involving (a) a normal MORB-type source though to represent the depleted upper mantle matrix, (b) an OIB-type source of uncertain parentage (recycled oceanic crust), and (c) a component with low μ, Low Sm/Nd, high Rb/Sr (time-averaged value) which is tentatively assigned to ancient hydrothermal and abyssal sediments recycled in the mantle. The high 208 Pb/ 204 Pb and 87 Sr/ 86 Sr ratios typical of the Dupal anomaly are likely due to the widespread distribution of this latter component in the basalt source from this area, including that for MORBs. (orig.)

Geochemistry of the mantle beneath the Rodriguez Triple Junction and the South-East Indian Ridge

Science.gov (United States)

Michard, A.; Montigny, R.; Schlich, R.

1986-05-01

Rare earth element abundances and Sr, Nd. Pb isotope compositions have been measured on zero-age dredge samples from the Rodriguez Triple Junction (RTJ) and the South-East Indian Ridge (SEIR), Along the SEIR. the geochemical "halo" of the St. Paul hot spot has a half-width of about 400 km and the data may be fairly well accounted for by a binary mixing between an Indian MORB-type component ( 87Sr/ 86Sr = 0.7028. 143Nd/ 144Nd = 0.51304. 206Pb/ 204Pb = 17.8) and the plume-type St. Paul component (0.7036, 0.5129, and 18.7 respectively). The alignment of the lead isotope data is particularly good with an apparent age of 1.95 ± 0.13 Ga and Th/U source value of 3.94. One sample dredged on the ridge 60 km southeast of St. Paul bears a definite Kerguelen isotopic signature. The RTJ has distinctive geochemical properties which contrast with those of the adjacent ridge segments. Low 206Pb/ 204Pb ratios which plots to the left of the geochron, rather high 208Pb/ 204Pb and 87Sr/ 87Sr ratios (17.4. 37.4, and 0.7031 respectively), a striking isotopic homogeneity, and variable LREE/HREE fractionation with (La/Sm) N, = 0.3-0.8 make this triple junction an anomalous site. The geochemical properties of the Indian Ocean basats have been examined using a three-component mantle model involving (a) a normal MORB-type source though to represent the depleted upper mantle matrix, (b) an OIB-type source of uncertain parentage (recycled oceanic crust?), and (c) a component with low μ. low Sm/Nd. high Rb/Sr (time-averaged value) which is tentatively assigned to ancient hydrothermal and abyssal sediments recycled in the mantle. The high 208Pb/ 204Pb and 87Sr/ 86Sr ratios typical of the Dupal anomaly are likely due to the widespread distribution of this latter component in the basalt source from this area. including that for MORBs.

Birch's Mantle

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Anderson, D. L.

2002-12-01

Francis Birch's 1952 paper started the sciences of mineral physics and physics of the Earth's interior. Birch stressed the importance of pressure, compressive strain and volume in mantle physics. Although this may seem to be an obvious lesson many modern paradoxes in the internal constitution of the Earth and mantle dynamics can be traced to a lack of appreciation for the role of compression. The effect of pressure on thermal properties such as expansivity can gravitational stratify the Earth irreversibly during accretion and can keep it chemically stratified. The widespread use of the Boussinesq approximation in mantle geodynamics is the antithesis of Birchian physics. Birch pointed out that eclogite was likely to be an important component of the upper mantle. Plate tectonic recycling and the bouyancy of oceanic crust at midmantle depths gives credence to this suggestion. Although peridotite dominates the upper mantle, variations in eclogite-content may be responsible for melting- or fertility-spots. Birch called attention to the Repetti Discontinuity near 900 km depth as an important geodynamic boundary. This may be the chemical interface between the upper and lower mantles. Recent work in geodynamics and seismology has confirmed the importance of this region of the mantle as a possible barrier. Birch regarded the transition region (TR ; 400 to 1000 km ) as the key to many problems in Earth sciences. The TR contains two major discontinuities ( near 410 and 650 km ) and their depths are a good mantle thermometer which is now being exploited to suggest that much of plate tectonics is confined to the upper mantle ( in Birch's terminology, the mantle above 1000 km depth ). The lower mantle is homogeneous and different from the upper mantle. Density and seismic velocity are very insensitive to temperature there, consistent with tomography. A final key to the operation of the mantle is Birch's suggestion that radioactivities were stripped out of the deeper parts of

Mantle wedge structure beneath the Yamato Basin, southern part of the Japan Sea, revealed by long-term seafloor seismic observations

Science.gov (United States)

Shinohara, M.; Nakahigashi, K.; Yamashita, Y.; Yamada, T.; Mochizuki, K.; Shiobara, H.

2016-12-01

The Japanese Islands are located at subduction zones where Philippine Sea (PHS) plate subducts from the southeast beneath the Eurasian plate and the Pacific plate descends from the east beneath the PHS and Eurasian plates and have a high density of seismic stations. Many seismic tomography studies using land seismic station data were conducted to reveal the seismic structure. These studies discussed the relationship between heterogeneous structures and the release of fluids from the subducting slab, magma generation and movement in the subduction zone. However, regional tomography using the land station data did not have a sufficient resolution to image a deep structure beneath the Japan Sea.To obtain the deep structure, observations of natural earthquakes within the Japan Sea are essential. Therefore, we started the repeating long-term seismic observations using ocean bottom seismometers(OBSs) in the Yamato Basin from 2013 to 2016. We apply travel-time tomography method to the regional earthquake and teleseismic arrival-data recorded by OBSs and land stations. In this presentation, we will report the P and S wave tomographic images down to a depth of 300 km beneath the southern part of the Japan Sea. This study was supported by "Integrated Research Project on Seismic and Tsunami Hazards around the Sea of Japan" conducted by the Ministry of Education, Culture, Sports, Science, and Technology (MEXT) of Japan.

Inferring global upper-mantle shear attenuation structure by waveform tomography using the spectral element method

Science.gov (United States)

Karaoǧlu, Haydar; Romanowicz, Barbara

2018-06-01

We present a global upper-mantle shear wave attenuation model that is built through a hybrid full-waveform inversion algorithm applied to long-period waveforms, using the spectral element method for wavefield computations. Our inversion strategy is based on an iterative approach that involves the inversion for successive updates in the attenuation parameter (δ Q^{-1}_μ) and elastic parameters (isotropic velocity VS, and radial anisotropy parameter ξ) through a Gauss-Newton-type optimization scheme that employs envelope- and waveform-type misfit functionals for the two steps, respectively. We also include source and receiver terms in the inversion steps for attenuation structure. We conducted a total of eight iterations (six for attenuation and two for elastic structure), and one inversion for updates to source parameters. The starting model included the elastic part of the relatively high-resolution 3-D whole mantle seismic velocity model, SEMUCB-WM1, which served to account for elastic focusing effects. The data set is a subset of the three-component surface waveform data set, filtered between 400 and 60 s, that contributed to the construction of the whole-mantle tomographic model SEMUCB-WM1. We applied strict selection criteria to this data set for the attenuation iteration steps, and investigated the effect of attenuation crustal structure on the retrieved mantle attenuation structure. While a constant 1-D Qμ model with a constant value of 165 throughout the upper mantle was used as starting model for attenuation inversion, we were able to recover, in depth extent and strength, the high-attenuation zone present in the depth range 80-200 km. The final 3-D model, SEMUCB-UMQ, shows strong correlation with tectonic features down to 200-250 km depth, with low attenuation beneath the cratons, stable parts of continents and regions of old oceanic crust, and high attenuation along mid-ocean ridges and backarcs. Below 250 km, we observe strong attenuation in the

Textures in spinel peridotite mantle xenoliths using micro-CT scanning: Examples from Canary Islands and France

Science.gov (United States)

Bhanot, K. K.; Downes, H.; Petrone, C. M.; Humphreys-Williams, E.

2017-04-01

Spinel pyroxene-clusters, which are intergrowths of spinel, orthopyroxene and clinopyroxene in mantle xenoliths, have been investigated through the use of micro-CT (μ-CT) in this study. Samples have been studied from two different tectonic settings: (1) the northern Massif Central, France, an uplifted and rifted plateau on continental lithosphere and (2) Lanzarote in the Canary Islands, an intraplate volcanic island on old oceanic lithosphere. μ-CT analysis of samples from both locations has revealed a range of spinel textures from small Lanzarote are regions that have experienced significant lithospheric thinning. This process provides a mechanism where the sub-solidus reaction of olivine + garnet = orthopyroxene + clinopyroxene + spinel is satisfied by providing a pathway from garnet peridotite to spinel peridotite. We predict that such textures would only occur in the mantle beneath regions that show evidence of thinning of the lithospheric mantle. Metasomatic reactions are seen around spinel-pyroxene clusters in some Lanzarote xenoliths, so metasomatism post-dated cluster formation.

Deep Mantle Origin for the DUPAL Anomaly?

Science.gov (United States)

Ingle, S.; Weis, D.

2002-12-01

Twenty years after the discovery of the Dupal Anomaly, its origin remains a geochemical and geophysical enigma. This anomaly is associated with the Southern Hemisphere oceanic mantle and is recognized by basalts with geochemical characteristics such as low 206Pb/204Pb and high 87Sr/86Sr. Both mid-ocean ridge basalts (MORB) and ocean island basalts (OIB) are affected, despite originating from melting at different depths and of different mantle sources. We compile geochemical data for both MORB and OIB from the three major oceans to help constrain the physical distribution and chemical composition of the Dupal Anomaly. There is a clear decrease in 206Pb/204Pb and an increase in 87Sr/86Sr with more southerly latitude for Indian MORB and OIB; these correlations are less obvious in the Atlantic and non-existent in the Pacific. The average* 143Nd/144Nd for Pacific and Atlantic OIB is 0.5129, but is lower for Indian OIB (0.5128). Interestingly, Pacific, Atlantic and Indian OIB all have 176Hf/177Hf averages of 0.2830. Indian MORB also record this phenomenon of low Nd with normal Hf isotopic compositions (Chauvel and Blichert-Toft, EPSL, 2001). Hf isotopes appear, therefore, to be a valid isotopic proxy for measuring the presence and magnitude of the Dupal Anomaly at specific locations. Wen (EPSL, 2001) reported a low-velocity layer at the D'' boundary beneath the Indian Ocean from which the Dupal Anomaly may originate. This hypothesis may be consistent with our compilations demonstrating that the long-lived Dupal Anomaly does not appear to be either mixing efficiently into the upper mantle or spreading to other ocean basins through time. We suggest that the Dupal source could be continually tapped by upwelling Indian Ocean mantle plumes. Plumes would then emplace pockets of Dupal material into the upper mantle and other ascending plumes might further disperse this material into the shallow asthenosphere. This could explain both the presence of the Dupal signature in MORB

Depth-Dependent Earthquake Properties Beneath Long-Beach, CA: Implications for the Rheology at the Brittle-Ductile Transition Zone

Science.gov (United States)

Inbal, A.; Clayton, R. W.; Ampuero, J. P.

2015-12-01

Except for a few localities, seismicity along faults in southern California is generally confined to depths shallower than 15 km. Among faults hosting deep seismicity, the Newport-Inglewood Fault (NIF), which traverses the Los-Angeles basin, has an exceptionally mild surface expression and low deformation rates. Moreover, the NIF structure is not as well resolved as other, less well instrumented faults because of poor signal-to-noise ratio. Here we use data from three temporary dense seismic arrays, which were deployed for exploration purposes and contain up to several thousands of vertical geophones, to investigate the properties of deep seismicity beneath Long-Beach (LB), Compton and Santa-Fe Springs (SFS). The latter is located 15 km northeast of the NIF, presumably above a major detachment fault underthrusting the basin.Event detection is carried out using a new approach for microseismic multi-channel picking, in which downward-continued data are back-projected onto the volume beneath the arrays, and locations are derived from statistical analysis of back-projection images. Our technique reveals numerous, previously undetected events along the NIF, and confirms the presence of an active shallow structure gently dipping to the north beneath SFS. Seismicity characteristics vary along the NIF strike and dip. While LB seismicity is uncorrelated with the mapped trace of the NIF, Compton seismicity illuminates a sub-vertical fault that extends down to about 20 km. This result, along with the reported high flux of mantle Helium along the NIF (Boles et al., 2015), suggests that the NIF is deeply rooted and acts as a major conduit for mantle fluids. We find that the LB size distribution obeys the typical power-law at shallow depths, but falls off exponentially for events occurring below 20 km. Because deep seismicity occurs uniformly beneath LB, this transition is attributed to a reduction in seismic asperity density with increasing depth, consistent with a transition

A possible mechanism for earthquakes found in the mantle wedge of the Nazca subduction zone

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Warren, L. M.; Chang, Y.; Prieto, G. A.

2017-12-01

Beneath Colombia, the Cauca cluster of intermediate-depth earthquakes extends for 200 km along the trench (3.5°N-5.5°N, 77.0°W-75.3°W) and, with 58 earthquakes per year with local magnitude ML >= 2.5, has a higher rate of seismicity than the subduction zone immediately to the north or south. By precisely locating 433 cluster earthquakes from 1/2010-3/2014 with data from the Colombian National Seismic Network, we found that the earthquakes are located both in a continuous Nazca plate subducting at an angle of 33°-43° and in the overlying mantle wedge. The mantle wedge earthquakes (12% of the earthquakes) form two isolated 40-km-tall columns extending perpendicular to the subducting slab. Using waveform inversion, we computed focal mechanisms for 69 of the larger earthquakes. The focal mechanisms are variable, but the intraslab earthquakes are generally consistent with an in-slab extensional stress axis oriented 25° counterclockwise from the down-dip direction. We suggest that the observed mantle wedge earthquakes are the result of hydrofracture in a relatively cool mantle wedge. This segment of the Nazca Plate is currently subducting at a normal angle, but Wagner et al. (2017) suggested that a flat slab slowly developed in the region between 9-5.9 Ma and persisted until 4 Ma. During flat slab subduction, the overlying mantle wedge typically cools because it is cut off from mantle corner flow. After hydrous minerals in the slab dehydrate, the dehydrated fluid is expelled from the slab and migrates through the mantle wedge. If a cool mantle wedge remains today, fluid dehydrated from the slab may generate earthquakes by hydrofracture, with the mantle wedge earthquakes representing fluid migration pathways. Dahm's (2000) model of water-filled fracture propagation in the mantle wedge shows hydrofractures propagating normal to the subducting slab and extending tens of km into the mantle wedge, as we observe.

The crust and mantle beneath the Siberian provinces: a preliminary model based on new receiver function analysis

DEFF Research Database (Denmark)

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

2012-01-01

The new receiver function (RF) study complements the existing seismic data on the crustal and upper mantle structure at the margins of the Siberian craton and the West Siberian Basin. So far, RF studies of Siberia have been largely restricted to the Baikal rift zone (Gao et al., 2004; Liu and Gao......, 2006; Anan'in et al., 2009). However, available seismic data allow to apply the RF approach to other tectonic structures of the region. We calculate the RF using the LQT method (Vinnik, 1977; Kind et al. 1995) in the version by Yuan et al. (1997). This method involves rotating the earth...... the deconvolved signals using the appropriate moveout corrections which account for the dependence of Ps arrivals on P wave slowness. The results of RF analysis of the crustal and mantle structure are interpreted in terms of tectonic and geodynamic evolution of different provinces of Siberia that range from...

Silica-enriched mantle sources of subalkaline picrite-boninite-andesite island arc magmas

Science.gov (United States)

Bénard, A.; Arculus, R. J.; Nebel, O.; Ionov, D. A.; McAlpine, S. R. B.

2017-02-01

compositions extracted from these hybrid sources are higher in normative quartz and hypersthene (i.e., they have a more silica-saturated character) in comparison with basalts derived from prior melt-depleted asthenospheric mantle beneath ridges. These primary arc melts range from silica-rich picrite to boninite and high-Mg basaltic andesite along a residual spinel harzburgite cotectic. Silica enrichment in the mantle sources of arc-related, subalkaline picrite-boninite-andesite suites coupled with the amount of water and depth of melting, are important for the formation of medium-Fe ('calc-alkaline') andesite-dacite-rhyolite suites, key lithologies forming the continental crust.

PARVULARUGOGLOBIGERINA EUGUBINA TYPE-SAMPLE AT CCESELLI (ITALY: PLANKTIC FORAMINIFERAL ASSEMBLAGE AND LOWERMOST DANIAN BIOSTRATIGRAPHIC IMPLICATIONS

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IGNACIO ARENILLAS

2000-11-01

Full Text Available The Parvularugoglobigerina eugubina Biozone (lowermost Danian was defined at Gubbio (Italy to precisely define the Cretaceous/Paleogene (K/P boundary. It was defined by the total range of Pv. eugubina, but this small morphospecies presents some taxonomic problems. The Pv. eugubina holotype and the planktic foraminiferal assemblage of the Pv. eugubina type-sample at Ceselli (Ceselli 3 were revised to precise the biostratigraphic position of this biozone. Of the 21 morphospecies identified in Ceselli 3, 14 are early Paleocene species and 7 are possible Cretaceous survivors of the K/P boundary extinction event. To clarify the lowermost Danian bizonation, it was necessary to taxonomically revise Pv. eugubina and Pv. longiapertura, which have both been identified in this sample. Following the definition of Pv. eugubina and the original definition of the nominal biozone, the base of Pv. eugubina Biozone should be placed at the first appearance datum of the eponymous species and not at the first appearance datum of Pv. longiapertura. 

Water, lithium and trace element compositions of olivine from Lanzo South replacive mantle dunites (Western Alps): New constraints into melt migration processes at cold thermal regimes

Science.gov (United States)

Sanfilippo, Alessio; Tribuzio, Riccardo; Ottolini, Luisa; Hamada, Morihisa

2017-10-01

Replacive mantle dunites are considered to be shallow pathways for extraction of mantle melts from their source region. Dunites offer a unique possibility to unravel the compositional variability of the melts produced in the upper mantle, before mixing and crystal fractionation modify their original signature. This study includes a quantification of H2O, Li and trace elements (Ni, Mn, Co, Sc, V, Ti, Zr, Y and HREE) in olivine from large replacive dunite bodies (>20 m) within a mantle section exposed in the Western Italian Alps (Lanzo South ophiolite). On the basis of olivine, clinopyroxene and spinel compositions, these dunites were previously interpreted to be formed by melts with a MORB signature. Variations in Ni, Mn, Co and Ca contents in olivine from different dunite bodies suggested formation by different melt batches. The variable H2O and Li contents of these olivines agree with this idea. Compared to olivine from residual peridotites and olivine phenocrysts in MORB (both having H2O 1 ppm), the Lanzo South dunite olivine has high H2O (18-40 ppm) and low Li (0.35-0.83 ppm) contents. Geochemical modelling suggests that the dunite-forming melts were produced by low melting degrees of a mixed garnet-pyroxenite-peridotite mantle source, with a contribution of a garnet pyroxenite component variable from 20 to 80%. The Lanzo dunites experienced migration of melts geochemically enriched and mainly produced in the lowermost part of the melting region. Extraction of enriched melts through dunite channels are probably characteristic of cold thermal regimes, where low temperatures and a thick mantle lithosphere inhibit mixing with melts produced at shallower depths.

Carbonate metasomatism and CO2 lithosphere-asthenosphere degassing beneath the western Mediterranean: An integrated model arising from petrological and geophysical data

International Nuclear Information System (INIS)

Frezzotti, Maria Luce; Peccerillo, Angelo; Panza, Giuliano

2009-03-01

We present an integrated petrological, geochemical, and geophysical model that offers an explanation for the present-day anomalously high non-volcanic deep (mantle derived) CO 2 emission in the Tyrrhenian region. We investigate how decarbonation or melting of carbonate-rich lithologies from a subducted lithosphere may affect the efficiency of carbon release in the lithosphere-asthenosphere system. We propose that melting of sediments and/or continental crust of the subducted Adriatic-Ionian (African) lithosphere at pressure greater than 4 GPa (130 km) may represent an efficient mean for carbon cycling into the upper mantle and into the exosphere in the Western Mediterranean area. Melting of carbonated lithologies, induced by the progressive rise of mantle temperatures behind the eastward retreating Adriatic-Ionian subducting plate, generates low fractions of carbonate-rich (hydrous-silicate) melts. Due to their low density and viscosity, such melts can migrate upward through the mantle, forming a carbonated partially molten CO 2 -rich mantle recorded by tomographic images in the depth range from 130 to 60 km. Upwelling in the mantle of carbonate-rich melts to depths less than 60 - 70 km, induces massive outgassing of CO 2 . Buoyancy forces, probably favored by fluid overpressures, are able to allow migration of CO 2 from the mantle to the surface, through deep lithospheric faults, and its accumulation beneath the Moho and within the lower crust. The present model may also explain CO 2 enrichment of the Etna active volcano. Deep CO 2 cycling is tentatively quantified in terms of conservative carbon mantle flux in the investigated area. (author)

Thermal Coupling Between the Ocean and Mantle of Europa: Implications for Ocean Convection

Science.gov (United States)

Soderlund, Krista M.; Schmidt, Britney E.; Wicht, Johannes; Blankenship, Donald D.

2015-11-01

Magnetic induction signatures at Europa indicate the presence of a subsurface ocean beneath the cold icy crust. The underlying mantle is heated by radioactive decay and tidal dissipation, leading to a thermal contrast sufficient to drive convection and active dynamics within the ocean. Radiogenic heat sources may be distributed uniformly in the interior, while tidal heating varies spatially with a pattern that depends on whether eccentricity or obliquity tides are dominant. The distribution of mantle heat flow along the seafloor may therefore be heterogeneous and impact the regional vigor of ocean convection. Here, we use numerical simulations of thermal convection in a global, Europa-like ocean to test the sensitivity of ocean dynamics to variations in mantle heat flow patterns. Towards this end, three end-member cases are considered: an isothermal seafloor associated with dominant radiogenic heating, enhanced seafloor temperatures at high latitudes associated with eccentricity tides, and enhanced equatorial seafloor temperatures associated with obliquity tides. Our analyses will focus on convective heat transfer since the heat flux pattern along the ice-ocean interface can directly impact the ice shell and the potential for geologic activity within it.

New constraints on the crustal structure beneath northern Tyrrhenian Sea

Science.gov (United States)

Levin, V. L.; Park, J. J.

2009-12-01

We present new seismological data on the seismic structure beneath the Tyrrhenian Sea between Corsica and the coast of Italy. Teleseismic receiver functions from two Tyrrhenian islands (Elba and Gorgona) identify clear P-to-S mode-converted waves from two distinct interfaces, at ~20 and ~45 km depth. Both interfaces are characterized by an increase of seismic wavespeed with depth. Using a summation of direct and multiply-reflected body waves within the P wave coda we estimate the mean ratio of compressional and shear wave speeds above the 45 km interface to be 1.75-1.80. Using reflectivity computations in 1D layered models we develop a model of seismic wavespeed distribution that yields synthetic seismograms very similar to those observed. We apply a Ps-multiple summation procedure to the synthetic waveforms to further verify the match between observed and predicted wavefields. The lower layer of our model, between 20 and 45 km, has Vp ~ 7.5 km/sec, a value that can be ascribed to either very fast crustal rocks or very slow upper mantle rocks. The Vp/Vs ratio is ~1.8 in this intermediate layer. On the basis of a well-constrained downward increase in seismic wave speed beneath this second layer, we interpret it as the magmatically reworked lower crust, a lithology that has been proposed to explain high-Vp layers in the crustal roots of island-arc terranes and volcanically altered continental margins, as well as lower-crustal high-Vp features sometimes seen beneath continental rifts. The presence of a thick layer of high-Vp, but crustal, lithology beneath the Tyrrhenian Sea differs considerably from previous estimates that interpreted the interface at ~20 km as the Moho. Our new interpretation obviates a need for a crustal thickness change of over 20 km at the crest of the Apennines orogen. We propose an alteration in the properties of the lower crust instead. We argue that ongoing convergent subduction of the Adriatic lithospehre is not required beneath northern

Cenozoic mantle composition evolution of southern Tibet indicated by Paleocene ( 64 Ma) pseudoleucite phonolitic rocks in central Lhasa terrane

Science.gov (United States)

Qi, Yue; Gou, Guo-Ning; Wang, Qiang; Wyman, Derek A.; Jiang, Zi-Qi; Li, Qiu-Li; Zhang, Le

2018-03-01

The question of whether continental subduction processes in collisional orogenic belts can trigger wide-spread mantle metesomatism and crustal material recycling remains unresolved. Miocene (25-8 Ma) ultrapotassic rocks in southern Tibet are the only mantle-derived magmatic rocks emplaced after the collision between India and Asia and they have been linked to the onset of east-west extensional stresses as the surface uplift of the Tibetan Plateau reached near-maximum elevation. However, their petrogenesis remains highly controversial, particularly the issue of whether their extremely enriched Sr-Nd isotopic characteristics were related to metasomatism derived from subducted Indian continental materials during the Cenozoic. Here we report on a Paleocene silicate-unsaturated, pseudoleucite phonolitic dike, in the Rongniduo area of central Lhasa terrane. In-situ SIMS (secondary ion mass spectrometry) apatite U-Pb age indicates the dike was generated at 64.1 ± 4.2 Ma, which slightly predates the age of initial India and Asia collision (about 55-50 Ma). This is the oldest age yet reported for ultrapotassic rocks in southern Tibet. Samples from this dike have distinctly more depleted Sr-Nd (whole rock: (87Sr/86Sr)i = 0.7064 to 0.7062, εNd(t) = - 1.5 to 0.4; in situ apitite: (87Sr/86Sr)i = 0.7059 to 0.7060, εNd(t) = - 2.0 to 0.4) isotopic compositions, than those of Miocene (25-8 Ma) ultrapotassic rocks in the central Lhasa terrane ((87Sr/86Sr)i = 0.7106 to 0.7399, εNd(t) = - 10.6 to - 18.5). Our new data provides important constraints on pre-collisional mantle characteristics beneath the Lhasa terrane. We suggest that these 64 Ma pseudoleucite phonolitic rocks were derived from the enriched lithospheric mantle metasomatized by subducted Tethyan oceanic materials in response to Neo-Tethyan slab roll-back. As a consequence, the younger Miocene ultrapotassic rocks, which display different geochemical compositions from the pre-collisional ultrapotassic rocks, were most

Deformation and fluid-enhanced annealing in subcontinental lithospheric mantle beneath the Pannonian Basin (Styrian Basin, Eastern Austria)

Science.gov (United States)

Aradi, Laszlo; Hidas, Károly; János Kovács, István; Tommasi, Andrea; Garrido, Carlos; Szabó, Csaba

2017-04-01

In the Carpathian-Pannonian region, xenolith-bearing Neogene alkali basaltic volcanism occurred in five volcanic fields [1], from which the Styrian Basin Volcanic Field (SBVF) is the westernmost one. In this study, we present new petrographic and crystal preferred orientation (CPO) data, and structural hydroxyl ("water") contents of upper mantle xenoliths from 12 volcanic outcrops across the SBVF. The studied xenoliths are mostly coarse granular hydrous spinel lherzolites. Amphiboles, replacing pyroxenes and spinels, are present in almost every sample. The peridotites are highly annealed, olivines and pyroxenes show no significant amount of intragranular deformation. Despite the annealed texture of the peridotites, olivine CPO is unambiguous and varies between [010]-fiber, orthogonal and [100]-fiber symmetry. The CPO of pyroxenes is coherent with coeval deformation with olivine. The fabric and CPO of amphiboles suggest postkinematic epitaxial overgrowth on the precursor pyroxenes. The structural hydroxyl content of the studied xenoliths exhibits rather high, equilibrium values, up to 10, 290 and 675 ppm in olivine, ortho- and clinopyroxene, respectively. The olivines contain more structural hydroxyl in the annealed xenoliths than in the more deformed ones. The xenoliths show equilibrium temperatures from 850 to 1100 °C, which corresponds to lithospheric mantle depths between 30 and 60 km. Equilibrium temperatures show correlation with the varying CPO symmetries and grain size: coarser grained xenoliths with [100]-fiber and orthorhombic symmetry appear in the high temperature (>1000 °C) xenoliths, which is characteristic for asthenospheric environments [2]. Most of the samples display transitional CPO symmetry between [010]-fiber and orthogonal, which indicate lithospheric deformation under varying stress field from transtensional to transpressional settings [3], probably related to the Miocene evolution of the Pannonian Basin, during which varying compressive and

Crustal thickness and Moho sharpness beneath the Midcontinent rift from receiver functions

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Moikwathai Moidaki

2013-02-01

Full Text Available The Mesoproterozoic Midcontinent rift (MCR in the central US is an approximately 2000 km long, 100 km wide structure from Kansas to Michigan. During the 20-40 million years of rifting, a thick (up to 20 km layer of basaltic lava was deposited in the rift valleys. Quantifying the effects of the rifting and associated volcanic eruptions on the structure and composition of the crust and mantle beneath the MCR is important for the understanding of the evolution of continental lithosphere. In this study we measure the crustal thickness (H, and the sharpness of the Moho (R at about 24 portable and permanent stations in Iowa, Kansas, and South Dakota by stacking Pto- S converted waves (PmS and their multiples (PPmS and PSmS. Under the assumption that the crustal mean velocity in the study area is the same as the IASP91 earth model, we find a significantly thickened crust beneath the MCR of about 53 km. The crustal Vp/Vs ratios increases from about 1.80 off rift to as large as 1.95 within the rift, which corresponds to an increase of Poisson’s ratio from 0.28 to 0.32, suggesting a more mafic crust beneath the MCR. The R measurements are spatially variable and are relatively small in the vicinity of the MCR, indicating the disturbance of the original sharp Moho by the rifting and magmatic intrusion and volcanic eruption.

Imaging the Mediterranean upper mantle by p- wave travel time tomography

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A. Morelli

1997-06-01

Full Text Available Travel times of P-waves in the Euro-Mediterranean region show strong and consistent lateral variations, which can be associated to structural heterogeneity in the underlying crust and mantle. We analyze regional and tele- seismic data from the International Seismological Centre data base to construct a three-dimensional velocity model of the upper mantle. We parameterize the model by a 3D grid of nodes -with approximately 50 km spacing -with a linear interpolation law, which constitutes a three-dimensional continuous representation of P-wave velocity. We construct summary travel time residuals between pairs of cells of the Earth's surface, both inside our study area and -with a broader spacing -on the whole globe. We account for lower mantle heterogeneity outside the modeled region by using empirical corrections to teleseismic travel times. The tomo- graphic images show generai agreement with other seismological studies of this area, with apparently higher detail attained in some locations. The signature of past and present lithospheric subduction, connected to Euro- African convergence, is a prominent feature. Active subduction under the Tyrrhenian and Hellenic arcs is clearly imaged as high-velocity bodies spanning the whole upper mantle. A clear variation of the lithospheric structure beneath the Northem and Southern Apennines is observed, with the boundary running in correspon- dence of the Ortona-Roccamonfina tectonic lineament. The western section of the Alps appears to have better developed roots than the eastern, possibly reflecting à difference in past subduction of the Tethyan lithosphere and subsequent continental collision.

Anisotropic Signature of the Afar plume in the Upper Mantle.

Science.gov (United States)

Sicilia, D.; Montagner, J.; Debayle, E.; Leveque, J.; Cara, M.; Lepine, J.

2002-12-01

Plumes remain enigmatic geological objects and it is still unclear how they are formed and whether they act independently from plate tectonics. The role of plumes in mantle dynamics can be investigated by studying their interaction with lithosphere and crust and their perturbations on flow pattern in the mantle. The flow pattern can be derived from seismic anisotropy. An anisotropic surface wave tomography in the Horn of Africa was performed. The choice of the experiment in the Horn of Africa is motivated by the the presence of the Afar hotspot, one of the biggest continental hotspot. In the framework of the mantle degree 2 pattern, the Afar hotspot is the antipode of the Pacific superswell, but its origin at depth and its geodynamic importance are still debated. Data were collected from the permanent IRIS and GEOSCOPE networks and from the PASSCAL experiment in Tanzania and Saudi Arabia. We completed our data base with a French deployment of portable broadband stations surrounding the Afar Hotspot. Path average phase velocities are obtained by using a method based on a least-squares minimization (Beucler et al.,2002). A correction of the data is applied according to the a priori 3SMAC model (Nataf and Ricard, 1996). 3D-models of velocity, radial and azimuthal anisotropies are inverted for. Down to 250km, low velocities are found beneath the Red Sea, the Gulf of Aden, the South East of the Tanzania Craton, the Afar hotspot. High velocities are present in the eastern Arabia and the Tanzania Craton. These results are in agreement with the isotropic model of Debayle et al. (2002). The anisotropy model beneath Afar displays a complex pattern. The azimuthal anisotropy shows that the Afar plume might be interpreted as feeding other hotspots in central Africa. Deeper in the asthenosphere, a wide stem of positive radial anisotropy (VSH > VSV) comes up, where we might expect the reverse sign. The same observation was made below Iceland (Gaherty, 2001) and Hawaii (Montagner

Geodynamic Constraints on the Sources of Seismic Anisotropy Beneath Madagascar

Science.gov (United States)

Rajaonarison, T. A.; Stamps, D. S.; Fishwick, S.

2017-12-01

The rheological structure of the lithosphere-asthenosphere system controls the degree in which the mantle drives surface motions. Seismic anisotropy is a proxy to infer information about previous tectonic events imprinted in lithospheric structures and/or asthenospheric flow pattern in regions absent of active volcanism, however, distinguishing between the shallow and deeper sources, respectively, remains ambiguous. Madagascar is an ideal natural laboratory to study the sources of anisotropy and the rheological implications for lithosphere-asthenosphere system because 1) active volcanism is minimal or absent, 2) there are well-exposed tectonic fabrics for comparison, and 3) numerous geological and geophysical observations provides evidence of present-day tectonic activities. Recent studies suggest new seismic anisotropy observations in southern Madagascar are sourced from both fossilized lithospheric structure and asthenospheric flow driven by rigid lithospheric plate motion. In this work we compare geodynamic simulations of the lithosphere-asthenosphere system with seismic anisotropy data set that includes all of Madagascar. We use the numerical code Advanced Solver for Problems in Earth's ConvecTion (ASPECT) to calculate instantaneous deformation in the lithosphere and edge-driven convective flow in the asthenosphere accounting for variations in buoyancy forces and temperature dependent viscosity. The initial temperature conditions are based on interpretations from high resolution regional surface wave tomography. We assume visco-plastic rheology for a uniform crust, dislocation creep for a laterally varying mantle lithospheric structure, and diffusion creep for the asthenosphere. To test for the source of anisotropy we compare our velocity solution azimuths with azimuths of anisotropy at 25 km depth intervals. Calculated asthenospheric flow aligns with measured seismic anisotropy with a 15° WRMS at 175 km depth and possibly down to 250 km suggesting the

Imaging a Remnant Slab Beneath Southeastern US: New Results from Teleseismic, Finite-frequency Tomography.

Science.gov (United States)

Biryol, C. B.; Wagner, L. S.; Fischer, K. M.; Hawman, R. B.

2014-12-01

Our new results from teleseismic, finite-frequency, body-wave tomography analysis reveal a relatively steep east-dipping fast velocity anomaly beneath the Southeastern US. The resolving power of our dataset is good enough to retrieve major mantle anomalies, such as this fast velocity body, owing to the dense receiver coverage provided by US Transportable Array (TA) and the SouthEastern Suture of the Appalachian Margin Experiment (SESAME). Various resolution and recovery tests demonstrate the robustness of this anomaly in our tomographic model between the depths of 60 and 660 km. Our images reveal that the dip of this structure decreases significantly in the mantle transition zone where it terminates. We also observe major gaps in the lateral continuity of this structure. Based on the amplitude, location and geometry of the velocity perturbation, we interpret this anomaly as remnant subducted lithosphere, suspended in the upper mantle after a subduction phase as young as 100-110 Ma or as old as 1Ga. Basic calculations and evaluations on the geometry and location of this anomaly help us to narrow down the origin of this slab to the Farallon flat-slab subduction in the west and Grenville Subduction during assembly of supercontinent Rodinia. Our images reveal possible mechanisms that would allow this slab to remain in the upper mantle without sinking into deeper mantle for such extended periods of time. We believe the flat geometry of the slab near the transition zone and the fragmented nature provide important clues about processes that could delay/resist the sinking while providing necessary time for it to transform into a more neutrally buoyant state. In this respect, we believe our results have broad implications for subduction processes and piece-meal slab failure, as well as tectonic implications for characteristics of former subduction zones that help shape North American Plate.

Shear wave velocities in the upper mantle of the Western Alps: new constraints using array analysis of seismic surface waves

Science.gov (United States)

Lyu, Chao; Pedersen, Helle A.; Paul, Anne; Zhao, Liang; Solarino, Stefano

2017-07-01

It remains challenging to obtain absolute shear wave velocities of heterogeneities of small lateral extension in the uppermost mantle. This study presents a cross-section of Vs across the strongly heterogeneous 3-D structure of the western European Alps, based on array analysis of data from 92 broad-band seismic stations from the CIFALPS experiment and from permanent networks in France and Italy. Half of the stations were located along a dense sublinear array. Using a combination of these stations and off-profile stations, fundamental-mode Rayleigh wave dispersion curves were calculated using a combined frequency-time beamforming approach. We calculated dispersion curves for seven arrays of approximately 100 km aperture and 14 arrays of approximately 50 km aperture, the latter with the aim of obtaining a 2-D vertical cross-section of Vs beneath the western Alps. The dispersion curves were inverted for Vs(z), with crustal interfaces imposed from a previous receiver function study. The array approach proved feasible, as Vs(z) from independent arrays vary smoothly across the profile length. Results from the seven large arrays show that the shear velocity of the upper mantle beneath the European plate is overall low compared to AK135 with the lowest velocities in the internal part of the western Alps, and higher velocities east of the Alps beneath the Po plain. The 2-D Vs model is coherent with (i) a ∼100 km thick eastward-dipping European lithosphere west of the Alps, (ii) very high velocities beneath the Po plain, coherent with the presence of the Alpine (European) slab and (iii) a narrow low-velocity anomaly beneath the core of the western Alps (from the Briançonnais to the Dora Maira massif), and approximately colocated with a similar anomaly observed in a recent teleseismic P-wave tomography. This intriguing anomaly is also supported by traveltime variations of subvertically propagating body waves from two teleseismic events that are approximately located on

Grain-Size Dynamics Beneath Mid-Ocean Ridges: Implications for Permeability and Melt Extraction

Science.gov (United States)

Turner, A. J.; Katz, R. F.; Behn, M. D.

2014-12-01

The permeability structure of the sub-ridge mantle plays an important role in how melt is focused and extracted at mid-ocean ridges. Permeability is controlled by porosity and the grain size of the solid mantle matrix, which is in turn controlled by the deformation conditions. To date, models of grain size evolution and mantle deformation have not been coupled to determine the influence of spatial variations in grain-size on the permeability structure at mid-ocean ridges. Rather, current models typically assume a constant grain size for the whole domain [1]. Here, we use 2-D numerical models to evaluate the influence of grain-size variability on the permeability structure beneath a mid-ocean ridge and use these results to speculate on the consequences for melt focusing and extraction. We construct a two-dimensional, single phase model for the steady-state grain size beneath a mid-ocean ridge. The model employs a composite rheology of diffusion creep, dislocation creep, dislocation accommodated grain boundary sliding, and a brittle stress limiter. Grain size is calculated using the "wattmeter" model of Austin and Evans [2]. We investigate the sensitivity of the model to global variations in grain growth exponent, potential temperature, spreading-rate, and grain boundary sliding parameters [3,4]. Our model predicts that permeability varies by two orders of magnitude due to the spatial variability of grain size within the expected melt region of a mid-ocean ridge. The predicted permeability structure suggests grain size may promote focusing of melt towards the ridge axis. Furthermore, the calculated grain size structure should focus melt from a greater depth than models that exclude grain-size variability. Future work will involve evaluating this hypothesis by implementing grain-size dynamics within a two-phase mid-ocean ridge model. The developments of such a model will be discussed. References: [1] R. F. Katz, Journal of Petrology, volume 49, issue 12, page 2099

Seismic images of the transition zone: is Hawaiian volcanism produced by a secondary plume from the top of the lower mantle?

Science.gov (United States)

Cao, Q.; van der Hilst, R. D.; Shim, S.; De Hoop, M. V.

2011-12-01

The Hawaiian hotspot is often attributed to hot material rising from depth in the mantle, but efforts to detect a thermal plume seismically have been inconclusive. Most tomographic models reveal anomalously low wavespeeds beneath Hawaii, but the depth extent of this structure is not well known. S or P data used in traveltime inversions are associated with steep rays to distant sources, which degrades depth resolution, and surface wave dispersion does not have sufficient sensitivity at the depths of interest. To investigate pertinent thermal anomalies we mapped depth variations of upper mantle discontinuities using precursors of the surface-reflected SS wave. Instead of stacking the data over geographical bins, which leads to averaging of topography and hence loss of spatial resolution, we used a generalized Radon transform (GRT) to detect and map localized elasticity contrasts in the transition zone (Cao et al., PEPI, 2010). We apply the GRT to produce 3D image volumes beneath a large area of the Pacific Ocean, including Hawaii and the Hawaii-Emperor seamount chain (Cao et al., Science, 2011). The 3D image volumes reveal laterally continuous interfaces near 410 and 660 km depths, that is, the traditional boundaries of the transition zone, but also suggest (perhaps intermittent) scatter horizons near 300-350, 520-550, and 800-1000 km depth. The upper mantle appears generally hot beneath Hawaii, but the most conspicuous topographic (and probably thermal) anomalies are found west of Hawaii. The GRT images reveal a 800 km wide uplift of the 660 discontinuity just west of Hawaii, but there is no evidence for a corresponding localized depression of the 410 discontinuity. This expression of the 410 and 660 km topographies is consistent with some existed geodynamical modeling results, in which a deep-rooted mantle plume impinging on the transition zone, creating a broad pond of hot material underneath endothermic phase change at 660 km depth, and with secondary plumes

Mesoproterozoic and Paleoproterozoic subcontinental lithospheric mantle domains beneath southern Patagonia: Isotopic evidence for its connection to Africa and Antarctica

Czech Academy of Sciences Publication Activity Database

Mundl, A.; Ntaflos, T.; Ackerman, Lukáš; Bizimis, M.; Bjerg, E. A.; Hauzenberger, Ch. A.

2015-01-01

Roč. 43, č. 1 (2015), s. 39-42 ISSN 0091-7613 Institutional support: RVO:67985831 Keywords : lithospheric mantle * Mesoproterozoic * Paleoproterozoic Subject RIV: DD - Geochemistry Impact factor: 4.548, year: 2015

Crust and uppermost-mantle structure of Greenland and the Northwest Atlantic from Rayleigh wave group velocity tomography

Science.gov (United States)

Darbyshire, Fiona A.; Dahl-Jensen, Trine; Larsen, Tine B.; Voss, Peter H.; Joyal, Guillaume

2018-03-01

The Greenland landmass preserves ˜4 billion years of tectonic history, but much of the continent is inaccessible to geological study due to the extensive inland ice cap. We map out, for the first time, the 3-D crustal structure of Greenland and the NW Atlantic ocean, using Rayleigh wave anisotropic group velocity tomography, in the period range 10-80 s, from regional earthquakes and the ongoing GLATIS/GLISN seismograph networks. 1-D inversion gives a pseudo-3-D model of shear wave velocity structure to depths of ˜100 km with a horizontal resolution of ˜200 km. Crustal thickness across mainland Greenland ranges from ˜25 km to over 50 km, and the velocity structure shows considerable heterogeneity. The large sedimentary basins on the continental shelf are clearly visible as low velocities in the upper ˜5-15 km. Within the upper continental basement, velocities are systematically lower in northern Greenland than in the south, and exhibit a broadly NW-SE trend. The thinning of the crust at the continental margins is also clearly imaged. Upper-mantle velocities show a clear distinction between typical fast cratonic lithosphere (Vs ≥4.6 km s-1) beneath Greenland and its NE margin and anomalously slow oceanic mantle (Vs ˜4.3-4.4 km s-1) beneath the NW Atlantic. We do not observe any sign of pervasive lithospheric modification across Greenland in the regions associated with the presumed Iceland hotspot track, though the average crustal velocity in this region is higher than that of areas to the north and south. Crustal anisotropy beneath Greenland is strong and complex, likely reflecting numerous episodes of tectonic deformation. Beneath the North Atlantic and Baffin Bay, the dominant anisotropy directions are perpendicular to the active and extinct spreading centres. Anisotropy in the subcontinental lithosphere is weaker than that of the crust, but still significant, consistent with cratonic lithosphere worldwide.

Detailed Configuration of the Underthrusting Indian Lithosphere Beneath Western Tibet Revealed by Receiver Function Images

Science.gov (United States)

Xu, Qiang; Zhao, Junmeng; Yuan, Xiaohui; Liu, Hongbing; Pei, Shunping

2017-10-01

We analyze the teleseismic waveform data recorded by 42 temporary stations from the Y2 and ANTILOPE-1 arrays using the P and S receiver function techniques to investigate the lithospheric structure beneath western Tibet. The Moho is reliably identified as a prominent feature at depths of 55-82 km in the stacked traces and in depth migrated images. It has a concave shape and reaches the deepest location at about 80 km north of the Indus-Yarlung suture (IYS). An intracrustal discontinuity is observed at 55 km depth below the southern Lhasa terrane, which could represent the upper border of the eclogitized underthrusting Indian lower crust. Underthrusting of the Indian crust has been widely observed beneath the Lhasa terrane and correlates well with the Bouguer gravity low, suggesting that the gravity anomalies in the Lhasa terrane are induced by topography of the Moho. At 20 km depth, a midcrustal low-velocity zone (LVZ) is observed beneath the Tethyan Himalaya and southern Lhasa terrane, suggesting a layer of partial melts that decouples the thrust/fold deformation of the upper crust from the shortening and underthrusting in the lower crust. The Sp conversions at the lithosphere-asthenosphere boundary (LAB) can be recognized at depths of 130-200 km, showing that the Indian lithospheric mantle is underthrusting with a ramp-flat shape beneath southern Tibet and probably is detached from the lower crust immediately under the IYS. Our observations reconstruct the configuration of the underthrusting Indian lithosphere and indicate significant along strike variations.

Digging Deep: Is Lunar Mantle Excavated Around the Imbrium Basin?

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Klima, R. L.; Bretzfelder, J.; Buczkowski, D.; Ernst, C. M.; Greenhagen, B. T.; Petro, N. E.; Shusterman, M. L.

2017-12-01

The Moon has experienced over a dozen impacts resulting in basins large enough to have excavated mantle material. With many of those basins concentrated on the lunar near side, and extensive regolith mixing since the lunar magma ocean crystallized, one might expect that some mantle material would have been found among the lunar samples on Earth. However, so far, no mantle clasts have been definitively identified in lunar samples [1]. From orbit, a number of olivine-bearing localities, potentially sourced from the mantle, have been identified around impact basins [2]. Based on analysis of near-infrared (NIR) and imaging data, [3] suggest that roughly 60% of these sites represent olivine from the mantle. If this is the case and the blocks are coherent and not extensively mixed into the regolith, these deposits should be ultramafic, containing olivine and/or pyroxenes and little to no plagioclase. In the mid-infrared, they would thus exhibit Christiansen features at wavelengths in excess of 8.5 μm, which has not been observed in global studies using the Diviner Lunar Radiometer [4]. We present an integrated study of the massifs surrounding the Imbrium basin, which, at over 1000 km wide, is large enough to have penetrated through the lunar crust and into the mantle. These massifs are clearly associated with the Imbrium basin-forming impact, but existing geological maps do not distinguish between whether they are likely ejecta or rather uplifted from beneath the surface during crustal rebound [5]. We examine these massifs using vis, NIR and Mid IR data to determine the relationships between and the bulk mineralogy of local lithologies. NIR data suggest that the massifs contain exposures of four dominant minerals: olivine, Mg-rich orthopyroxene, a second low-Ca pyroxene, and anorthite. Mid IR results suggest that though many of these massifs are plagioclase-rich, portions of some may be significantly more mafic. We will present our growing mineralogical map of the

Dehydration of chlorite explains anomalously high electrical conductivity in the mantle wedges.

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Manthilake, Geeth; Bolfan-Casanova, Nathalie; Novella, Davide; Mookherjee, Mainak; Andrault, Denis

2016-05-01

Mantle wedge regions in subduction zone settings show anomalously high electrical conductivity (~1 S/m) that has often been attributed to the presence of aqueous fluids released by slab dehydration. Laboratory-based measurements of the electrical conductivity of hydrous phases and aqueous fluids are significantly lower and cannot readily explain the geophysically observed anomalously high electrical conductivity. The released aqueous fluid also rehydrates the mantle wedge and stabilizes a suite of hydrous phases, including serpentine and chlorite. In this present study, we have measured the electrical conductivity of a natural chlorite at pressures and temperatures relevant for the subduction zone setting. In our experiment, we observe two distinct conductivity enhancements when chlorite is heated to temperatures beyond its thermodynamic stability field. The initial increase in electrical conductivity to ~3 × 10(-3) S/m can be attributed to chlorite dehydration and the release of aqueous fluids. This is followed by a unique, subsequent enhancement of electrical conductivity of up to 7 × 10(-1) S/m. This is related to the growth of an interconnected network of a highly conductive and chemically impure magnetite mineral phase. Thus, the dehydration of chlorite and associated processes are likely to be crucial in explaining the anomalously high electrical conductivity observed in mantle wedges. Chlorite dehydration in the mantle wedge provides an additional source of aqueous fluid above the slab and could also be responsible for the fixed depth (120 ± 40 km) of melting at the top of the subducting slab beneath the subduction-related volcanic arc front.

Relationships between seismic wave-Speed, density, and electrical conductivity beneath Australia from seismology, mineralogy, and laboratory-based conductivity profiles

DEFF Research Database (Denmark)

Khan, A.; Koch, S.; Shankland, T. J.

2015-01-01

We present maps of the three-dimensional density (ρ), electrical conductivity (σ), and shear-wave speed (VS) structure of the mantle beneath Australia and surrounding ocean in the depth range of 100–800 km. These maps derived from stochastic inversion of seismic surface-wave dispersion data...... shear-wave speeds, low densities, and high conductivities. This trend appears to continue to depths well below 300 km. The slow-fast shear-wave speed distribution found here is also observed in independent seismic tomographic models of the Australian region, whereas the coupled slow-fast shear......-wave speed, low-high density, and high-low electrical conductivity distribution has not been observed previously. Toward the bottom of the upper mantle at 400 km depth marking the olivine ⃗ wadsleyite transformation (the “410–km” seismic discontinuity), the correlation between VS, ρ, and σ weakens...

Upper Mantle Responses to India-Eurasia Collision in Indochina, Malaysia, and the South China Sea

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Hongsresawat, S.; Russo, R. M.

2016-12-01

We present new shear wave splitting and splitting intensity measurements from SK(K)S phases recorded at seismic stations of the Malaysian National Seismic Network. These results, in conjunction with results from Tibet and Yunnan provide a basis for testing the degree to which Indochina and South China Sea upper mantle fabrics are responses to India-Eurasia collision. Upper mantle fabrics derived from shear wave splitting measurements in Yunnan and eastern Tibet parallel geodetic surface motions north of 26°N, requiring transmission of tractions from upper mantle depths to surface, or consistent deformation boundary conditions throughout the upper 200 km of crust and mantle. Shear wave splitting fast trends and surface velocities diverge in eastern Yunnan and south of 26°N, indicating development of an asthenospheric layer that decouples crust and upper mantle, or corner flow above the subducted Indo-Burma slab. E-W fast shear wave splitting trends southwest of 26°N/104°E indicate strong gradients in any asthenospheric infiltration. Possible upper mantle flow regimes beneath Indochina include development of olivine b-axis anisotropic symmetry due to high strain and hydrous conditions in the syntaxis/Indo-Burma mantle wedge (i.e., southward flow), development of strong upper mantle corner flow in the Indo-Burma wedge with olivine a-axis anisotropic symmetry (i.e., westward flow), and simple asthenospheric flow due to eastward motion of Sundaland shearing underlying asthenosphere. Further south, shear-wave splitting delay times at Malaysian stations vary from 0.5 seconds on the Malay Peninsula to over 2 seconds at stations on Borneo. Splitting fast trends at Borneo stations and Singapore trend NE-SW, but in northern Peninsular Malaysia, the splitting fast polarization direction is NW-SE, parallel to the trend of the Peninsula. Thus, there is a sharp transition from low delay time and NW-SE fast polarization to high delay times and fast polarization directions that

Sulfide enrichment at an oceanic crust-mantle transition zone: Kane Megamullion (23°N, MAR)

Science.gov (United States)

Ciazela, Jakub; Koepke, Juergen; Dick, Henry J. B.; Botcharnikov, Roman; Muszynski, Andrzej; Lazarov, Marina; Schuth, Stephan; Pieterek, Bartosz; Kuhn, Thomas

2018-06-01

The Kane Megamullion oceanic core complex located along the Mid-Atlantic Ridge (23°30‧N, 45°20‧W) exposes lower crust and upper mantle directly on the ocean floor. We studied chalcophile elements and sulfides in the ultramafic and mafic rocks of the crust-mantle transition and the mantle underneath. We determined mineralogical and elemental composition and the Cu isotope composition of the respective sulfides along with the mineralogical and elemental composition of the respective serpentines. The rocks of the crust-mantle transition zone (i.e., plagioclase harzburgite, peridotite-gabbro contacts, and dunite) overlaid by troctolites are by one order of magnitude enriched in several chalcophile elements with respect to the spinel harzburgites of the mantle beneath. Whereas the range of Cu concentrations in spinel harzburgites is 7-69 ppm, the Cu concentrations are highly elevated in plagioclase harzburgites with a range of 90-209 ppm. The zones of the peridotite-gabbro contacts are even more enriched, exhibiting up to 305 ppm Cu and highly elevated concentrations of As, Zn, Ga, Sb and Tl. High Cu concentrations show pronounced correlation with bulk S concentrations at the crust-mantle transition zone implying an enrichment process in this horizon of the oceanic lithosphere. We interpret this enrichment as related to melt-mantle reaction, which is extensive in crust-mantle transition zones. In spite of the ubiquitous serpentinization of primary rocks, we found magmatic chalcopyrites [CuFeS2] as inclusions in plagioclase as well as associated with pentlandite [(Fe,Ni)9S8] and pyrrhotite [Fe1-xS] in polysulfide grains. These chalcopyrites show a primary magmatic δ65Cu signature ranging from -0.04 to +0.29 ‰. Other chalcopyrites have been dissolved during serpentinization. Due to the low temperature (enrichment, increased sulfide modes, and potentially formation of small sulfide deposits could be expected globally along the petrological Moho.

Glaciation control of melting rates in the mantle: U-Th systematics of young basalts from Southern Siberia and Central Mongolia

Science.gov (United States)

Rasskazov, S.; Chebykin, E.

2012-04-01

Eastern Sayans, Siberia and Hangay, Central Mongolia are mountainous uplifts effected by Quaternary volcanism, but only the former area was covered by glaciers that were as thick as 500 m. Glaciation time intervals were marked by moraines and sub-glacial hyaloclastite-bearing volcanic edifices, whereas interglacial ones were exhibited by sub-aerial "valley" flows and cinder cones. To estimate temporal variations of maximum rates of melting and mantle upwelling in the glacial and glacial-free areas, we measured radionuclides of the U-Th system for 74 samples of the Middle-Late Pleistocene through Holocene basalts by ICP-MS technique (Chebykin et al. Russian Geol. Geophys. 2004. 45: 539-556) using mass-spectrometer Agilent 7500ce. The obtained U-Th isochron ages for the Pleistocene volcanic units in the age interval of the last 400 Kyr are mostly consistent with results of K-Ar dating. The measured (230Th/238U) ratios for the Holocene basalts from both areas are within the same range of 1.08-1.16 (parentheses denote units of activity), whereas the 50 Kyr lavas yield, respectively, the higher and lower initial (230Th0/238U) ratios (1.18-1.46 and 1.05-1.13). This discrepancy demonstrates contrast maximum rates of melting in conventional garnet peridotite sources. We suggest that this dynamical feature was provided by the abrupt Late Pleistocene deglaciation that caused the mantle decompression expressed by the earlier increasing melting beneath Eastern Sayans than beneath Hangay. In the last 400 Kyr, magmatic liquids from both Eastern Sayans and Hangay showed the overall temporal decreasing (230Th0/238U) (i.e. relative increasing rates of melting and upwelling of the mantle) with the systematically lower isotopic ratios (i.e. increased mantle activity) in the former area than in the latter. The 400 Kyr phonotephrites in Hangay showed elevated concentrations of Th (6-8 ppm) and Th/U (3.7-3.9). The high (230Th0/238U) (4.3-6.0) reflected slow fractional melting

Deformation of Tibetan Crust and Mantle and the Uplift of the Plateau: Insights from Broadband Surface Waves

Science.gov (United States)

Agius, M. R.; Lebedev, S.

2013-12-01

results show that the prominent high-velocity anomalies in the upper mantle are most consistent with the presence of subducted Indian lithosphere beneath large portions of Tibet. Estimated thermal anomalies within the high-velocity features match those expected for subducted India. The morphology of India's subduction beneath Tibet is complex and shows pronounced west-east variations. Beneath eastern and northeastern Tibet, in particular, the subducted Indian lithosphere appears to have subducted, at a shallow angle, hundreds of km NNE-wards. Azimuthal anisotropy beneath Tibet is distributed in multiple layers with different fast-propagations directions, which accounts for the complexity of published shear-wave splitting observations. The fast directions within the mid-lower crust are parallel to the extensional components of the current strain rate field at the surface, consistent with similar deformation through the entire crust, rather than channel flow. Anisotropy within the asthenosphere beneath northeastern Tibet (sandwiched between the Tibetan lithosphere above and the subducted Indian lithosphere below) indicates SSW-NNE flow, parallel to the direction of motion of the Indian Plate, including its subducted leading edge.

Two magma bodies beneath the summit of Kilauea Volcano unveiled by isotopically distinct melt deliveries from the mantle

Science.gov (United States)

Pietruszka, Aaron J.; Heaton, Daniel E.; Marske, Jared P.; Garcia, Michael O.

2015-01-01

The summit magma storage reservoir of Kīlauea Volcano is one of the most important components of the magmatic plumbing system of this frequently active basaltic shield-building volcano. Here we use new high-precision Pb isotopic analyses of Kīlauea summit lavas—from 1959 to the active Halema‘uma‘u lava lake—to infer the number, size, and interconnectedness of magma bodies within the volcano's summit reservoir. From 1971 to 1982, the 206Pb/204Pb ratios of the lavas define two separate magma mixing trends that correlate with differences in vent location and/or pre-eruptive magma temperature. These relationships, which contrast with a single magma mixing trend for lavas from 1959 to 1968, indicate that Kīlauea summit eruptions since at least 1971 were supplied from two distinct magma bodies. The locations of these magma bodies are inferred to coincide with two major deformation centers identified by geodetic monitoring of the volcano's summit region: (1) the main locus of the summit reservoir ∼2–4 km below the southern rim of Kīlauea Caldera and (2) a shallower magma body 4 km3 of lava erupted), must therefore be sustained by a nearly continuous supply of new melt from the mantle. The model results show that a minimum of four compositionally distinct, mantle-derived magma batches were delivered to the volcano (at least three directly to the summit reservoir) since 1959. These melt inputs correlate with the initiation of energetic (1959 Kīlauea Iki) and/or sustained (1969–1974 Mauna Ulu, 1983-present Pu‘u ‘Ō‘ō and 2008-present Halema‘uma‘u) eruptions. Thus, Kīlauea's eruptive behavior is partly tied to the delivery of new magma batches from the volcano's source region within the Hawaiian mantle plume.

Temporal geochemical trends in northern Luzon arc lavas (Philippines): implications on metasomatic processes in the island arc mantle

International Nuclear Information System (INIS)

Maury, R.C.; Bellon, H.; Jacques, D.; Defant, J.; Joron, J.L.; Mcdermott, F.; Vidal, Ph.

1998-01-01

Neogene and Quaternary lavas from Batan, Babuyan de Claro, Camiguin and Calayan islands (northern Luzon arc) display temporal increases in incompatible elements including Cs, Rb, Ba, K, La, Ce, Th, U, Ta, Hf and Zr from volcanoes older than 3 Ma to younger ones. These enrichments occur either within a single island (Batan) or within an island group (from Calayan to Camiguin and Babuyan). We show that these enrichments result from incompatible element input into the mantle wedge rather than from partial melting or fractionation effects. The fact that highly incompatible elements display temporal enrichment patterns in Batan lavas whatever their chemical properties indicates that hydrous fluids are not the only metasomatic agents operating in the mantle wedge and that slab-derived melts (adakitic magmas) may also be involved. The coupled temporal variation patterns of large ion lithophile elements and Sr-Nd isotopes suggest that the metasomatic budgets beneath the southern group of islands are mainly controlled by hydrous fluids inputs. In contrast, young Batan lavas likely derive from a mantle source mostly metasomatized by adakitic magmas. (authors)

The effects of magmatic redistribution of heat producing elements on the lunar mantle evolution inferred from numerical models that start from various initial states

Science.gov (United States)

Ogawa, Masaki

2018-02-01

To discuss how redistribution of heat producing elements (HPEs) by magmatism affects the lunar mantle evolution depending on the initial condition, I present two-dimensional numerical models of magmatism in convecting mantle internally heated by incompatible HPEs. Mantle convection occurs beneath a stagnant lithosphere that inhibits recycling of the HPE-enriched crustal materials to the mantle. Magmatism is modeled by a permeable flow of magma generated by decompression melting through matrix. Migrating magma transports heat, mass, and HPEs. When the deep mantle is initially hot with the temperature TD around 1800 K at its base, magmatism starts from the beginning of the calculated history to extract HPEs from the mantle. The mantle is monotonously cooled, and magmatism ceases within 2 Gyr, accordingly. When the deep mantle is initially colder with TD around 1100 K, HPEs stay in the deep mantle for a longer time to let the planet be first heated up and then cooled only slightly. If, in addition, there is an HPE-enriched domain in the shallow mantle at the beginning of the calculation, magma continues ascending to the surface through the domain for more than 3 Gyr. The low TD models fit in with the thermal and magmatic history of the Moon inferred from spacecraft observations, although it is not clear if the models are consistent with the current understanding of the origin of the Moon and its magnetic field. Redistribution of HPEs by magmatism is a crucial factor that must be taken into account in future studies of the evolution of the Moon.

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

Picrite "Intelligence" from the Middle-Late Triassic Stikine arc: Composition of mantle wedge asthenosphere

Science.gov (United States)

Milidragovic, D.; Zagorevski, A.; Weis, D.; Joyce, N.; Chapman, J. B.

2018-05-01

Primitive, near-primary arc magmas occur as a volumetrically minor ≤100 m thick unit in the Canadian Cordillera of northwestern British Columbia, Canada. These primitive magmas formed an olivine-phyric, picritic tuff near the base of the Middle-Late Triassic Stuhini Group of the Stikine Terrane (Stikinia). A new 40Ar/39Ar age on hornblende from a cross-cutting basaltic dyke constrains the tuff to be older than 221 ± 2 Ma. An 87Sr/86Sr isochron of texturally-unmodified tuff samples yields 212 ± 25 Ma age, which is interpreted to represent syn-depositional equilibration with sea-water. Parental trace element magma composition of the picritic tuff is strongly depleted in most incompatible trace elements relative to MORB and implies a highly depleted ambient arc mantle. High-precision trace element and Hf-Nd-Pb isotopic analyses indicate an origin by mixing of a melt of depleted ambient asthenosphere with ≤2% of subducted sediment melt. Metasomatic addition of non-conservative incompatible elements through melting of subducted Panthalassa Ocean floor sediments accounts for the arc signature of the Stuhini Group picritic tuff, enrichment of light rare earth elements (LREE) relative to heavy rare earth elements (HREE) and high field strength elements (HFSE), and anomalous enrichment in Pb. The inferred Panthalassan sediments are similar in composition to the Neogene-Quaternary sediments of the modern northern Cascadia Basin. The initial Hf isotopic composition of the picritic tuff closely approximates that of the ambient Middle-Late Triassic asthenosphere beneath Stikinia and is notably less radiogenic than the age-corrected Hf isotopic composition of the Depleted (MORB) Mantle reservoir (DM or DMM). This suggests that the ambient asthenospheric mantle end-member experienced melt depletion (F ≤ 0.05) a short time before picrite petrogenesis. The mantle end-member in the source of the Stuhini Group picritic tuff is isotopically similar to the mantle source of

Structural Heterogeneities in Southeast Tibet: Implications for Regional Flow in the Lower Crust and Upper Mantle

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Zhi Wang

2012-01-01

Full Text Available Our seismic study together with the MT analysis reveal a “R-shape” flow existing in both the lower crust and uppermost mantle, which suggests the crustal deformation along the deep, large sutures (such as the Longmen Shan fault and the Anninghe Fault under the southeastern Tibetan Plateau is maintained by dynamic pressure from the regional flow intermingled with the hot upwelling asthenosphere. The material in the lower crust and uppermost mantle flowing outward from the center of the plateau is buttressed by the old, strong lithosphere that underlies the Sichuan basin, pushing up on the crust above and maintaining steep orogenic belt through dynamic pressure. We therefore consider that the “R-shape” regional flow played a key role in the crustal deformation along the deep suture zones of the Bangong-Nujiang, the Longmen-Shan faults, and other local heavily faulted zones beneath the southeastern Tibetan Plateau.

From the Atlas to the Variscan Core of Iberia: Progress on the Knowledge of Mantle Anisotropy from SKS Splitting

Science.gov (United States)

Diaz Cusi, J.; Grevemeyer, I.; Thomas, C.; Harnafi, M.

2012-12-01

The data provided by the dense Iberarray broad-band seismic network deployed in the framework of the large-scale TopoIberia project, as well as from permanent broad-band stations operating in Morocco, Portugal and Spain has allowed to get a large scale view of the anisotropic properties of the mantle beneath the western termination of the Mediterranean region and its transition to the Atlantic ocean. In this contribution we will gather the previously presented results with the analysis of the data provided by IberArray stations in the central part of Iberia, broad-band OBSs deployments in the Alboran Sea and the Gulf of Cadiz and new seismic networks deployed in the High Atlas and the Moroccan Meseta. The High Atlas has been investigated using data from a broad-band network installed by the Univ. of Munster with a primary focus on the study of the properties of the deep mantle. Additionally, up to 10 Iberarray stations have been shifted southward to complete the survey along the Atlas and to investigate the Moroccan Meseta. In agreement with the results presented by the Picasso team along a profile crossing the Atlas northward, the anisotropy observed in this area is small (0.6 - 0.9 s) with a fast polarization direction (FPD) oriented roughly E-W. It is important to note that there is a very significant number of high quality events without evidence for anisotropy. This may be the result of the combined effect of two or more anisotropic layers or of the presence of a large vertical component of flow in the upper mantle. Moving northwards, the first TopoIberia-Iberarray deployment in the Betics-Alboran zone has evidenced a spectacular rotation of the FPD along the Gibraltar arc following the curvature of the Rif-Betic chain, from roughly N65E beneath the Betics to close to N65W beneath the Rif chain. To complete this image, we have now processed data from two OBS deployments in the Alboran Sea and Gulf of Cadiz installed by Geomar as part of the TopoMed project

Is the track of the Yellowstone hotspot driven by a deep mantle plume? -- Review of volcanism, faulting, and uplift in light of new data

Science.gov (United States)

Pierce, Kenneth L.; Morgan, Lisa A.

2009-01-01

Geophysical imaging of a tilted mantle plume extending at least 500 km beneath the Yellowstone caldera provides compelling support for a plume origin of the entire Yellowstone hotspot track back to its inception at 17 Ma with eruptions of flood basalts and rhyolite. The widespread volcanism, combined with a large volume of buoyant asthenosphere, supports a plume head as an initial phase. Estimates of the diameter of the plume head suggest it completely spanned the upper mantle and was fed from sources beneath the transition zone, We consider a mantle–plume depth to at least 1,000 km to best explain the large scale of features associated with the hotspot track. The Columbia River–Steens flood basalts form a northward-migrating succession consistent with the outward spreading of a plume head beneath the lithosphere. The northern part of the inferred plume head spread (pancaked) upward beneath Mesozoic oceanic crust to produce flood basalts, whereas basalt melt from the southern part intercepted and melted Paleozoic and older crust to produce rhyolite from 17 to 14 Ma. The plume head overlapped the craton margin as defined by strontium isotopes; westward motion of the North American plate has likely "scraped off" the head from the plume tail. Flood basalt chemistries are explained by delamination of the lithosphere where the plume head intersected this cratonic margin. Before reaching the lithosphere, the rising plume head apparently intercepted the east-dipping Juan de Fuca slab and was deflected ~ 250 km to the west; the plume head eventually broke through the slab, leaving an abruptly truncated slab. Westward deflection of the plume head can explain the anomalously rapid hotspot movement of 62 km/m.y. from 17 to 10 Ma, compared to the rate of ~ 25 km/m.y. from 10 to 2 Ma.

Garnet Signatures in Geophysical and Geochemical Observations: Insights into the Thermo-Petrological Structure of Oceanic Upper Mantle

Science.gov (United States)

Grose, C. J.; Afonso, J. C.

2013-12-01

We have developed new physically comprehensive thermal plate models of the oceanic lithosphere which incorporate temperature- and pressure-dependent heat transport properties and thermal expansivity, melting beneath ridges, hydrothermal circulation near ridge axes, and insulating oceanic crust. These models provide good fits to global databases of seafloor topography and heat flow, and seismic evidence of thermal structure near ridge axes. We couple these thermal plate models with thermodynamic models to predict the petrology of oceanic lithosphere. Geoid height predictions from our models suggest that there is a strong anomaly in geoid slope (over age) above ~25 Ma lithosphere due to the topography of garnet-field mantle. A similar anomaly is also present in geoid data over fracture zones. In addition, we show that a new assessment of a large database of ocean island basalt Sm/Yb systematics indicates that there is an unmistakable step-like increase in Sm/Yb values around 15-20 Ma, indicating the presence of garnet. To explain this feature, we have attempted to couple our thermo-petrological models of oceanic upper mantle with an open system, non-modal, dynamic melting model with diffusion kinetics to investigate trace element partitioning in an ascending mantle column.

Radial viscous fingering of hot asthenosphere within the Icelandic plume beneath the North Atlantic Ocean

Science.gov (United States)

Schoonman, C. M.; White, N. J.; Pritchard, D.

2017-06-01

The Icelandic mantle plume has had a significant influence on the geologic and oceanographic evolution of the North Atlantic Ocean during Cenozoic times. Full-waveform tomographic imaging of this region shows that the planform of this plume has a complex irregular shape with significant shear wave velocity anomalies lying beneath the lithospheric plates at a depth of 100-200 km. The distribution of these anomalies suggests that about five horizontal fingers extend radially beneath the fringing continental margins. The best-imaged fingers lie beneath the British Isles and beneath western Norway where significant departures from crustal isostatic equilibrium have been measured. Here, we propose that these radial fingers are generated by a phenomenon known as the Saffman-Taylor instability. Experimental and theoretical analyses show that fingering occurs when a less viscous fluid is injected into a more viscous fluid. In radial, miscible fingering, the wavelength and number of fingers are controlled by the mobility ratio (i.e. the ratio of viscosities), by the Péclet number (i.e. the ratio of advective and diffusive transport rates), and by the thickness of the horizontal layer into which fluid is injected. We combine shear wave velocity estimates with residual depth measurements around the Atlantic margins to estimate the planform distribution of temperature and viscosity within a horizontal asthenospheric layer beneath the lithospheric plate. Our estimates suggest that the mobility ratio is at least 20-50, that the Péclet number is O (104), and that the asthenospheric channel is 100 ± 20 km thick. The existence and planform of fingering is consistent with experimental observations and with theoretical arguments. A useful rule of thumb is that the wavelength of fingering is 5 ± 1 times the thickness of the horizontal layer. Our proposal has been further tested by examining plumes of different vigor and planform (e.g. Hawaii, Cape Verde, Yellowstone). Our results

Multidimensional Mantle Convection Models in Eastern Anatolia, the North Arabian Platform, and Caucasus Region

Science.gov (United States)

Sengul Uluocak, E.; Shahnas, H.; Pysklywec, R.; Gogus, O.; Eken, T.

2017-12-01

Eastern Anatolia, the North Arabian Platform, and Caucasus regions show many features of collisional tectonics with different convergence rates and shortening from south to north. The volcanism, sediment provenience, and thermochronological data suggest that the shortening and exhumation in the Greater Caucasus started during the Eocene-Oligocene synchronously with the collision between Arabia-Bitlis-Pötürge Massif in the south. Previous works indicate that the uplift (up to 2 km) in Eastern Anatolia related to upwelling mantle following the deformation of the Arabian oceanic lithosphere ( 11 Ma) during the ongoing Greater Caucasus closure is the dominant tectonic processes in the center of the region. However, there is no integrated geodynamic model that explains the deformation mechanisms of the region -and their possible interactions with each other -under the dynamic forces. In this study, we use multidimensional mantle-lithosphere convection/deformation models to quantify the geodynamic processes as constrained by the geological/geophysical observations in the region. For the models, seismic studies provide the high-resolution images of the upwelling mantle beneath Eastern Anatolia and the presence -and the locations- of the seismically fast structures associated with the relic/subducted slabs at varying depths such as the Bitlis slab in the south, and the Pontide and Kura slabs in the north. Fast polarization directions observed from splitting analyses exhibit an overall NE-SW oriented mantle anisotropy and a comparison between Pn and SKS derived fast wave azimuths indicates a crust-mantle coupling most likely implying vertically coherent deformation to the north of the study area. For the geodynamic models, we modify the mantle and lithosphere rheology as well as the thermal state. We interpret the estimated uplift and subsidence anomalies related to lithospheric variations (ranging from 54 km to 211 km) and subducting slab behavior with observed

Imaging magma plumbing beneath Askja volcano, Iceland

Science.gov (United States)

Greenfield, Tim; White, Robert S.

2015-04-01

and velocity structure. The 2014-15 Bárðarbunga dyke intrusion has provided a 45 km long, distributed source of large earthquakes which are well located and provide accurate arrival time picks. Together with long-term background seismicity these provide excellent illumination of the Askja volcano from all directions. We find a pronounced low-velocity anomaly beneath the caldera at a depth of ~7 km. The anomaly is ~10% slower than the initial best fitting 1D model and has a Vp/Vs ratio higher than the surrounding crust, suggesting the presence of increased temperature or partial melt. The body is unlikely to be entirely melt as S-waves are still detected at stations directly above the anomaly. This low-velocity body is slightly deeper than the depth range suggested by InSAR and GPS studies of a deflating source beneath Askja. Beneath the main low-velocity zone a region of reduced velocities extends into the lower crust and is coincident with deep seismicity. This is suggestive of a high temperature channel into the lower crust which could be a pathway for melt rising from the mantle.

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

Complex Crustal Structure Beneath Western Turkey Revealed by 3D Seismic Full Waveform Inversion (FWI)

Science.gov (United States)

Cubuk-Sabuncu, Yesim; Taymaz, Tuncay; Fichtner, Andreas

2016-04-01

We present a 3D radially anisotropic velocity model of the crust and uppermost mantle structure beneath the Sea of Marmara and surroundings based on the full waveform inversion method. The intense seismic activity and crustal deformation are observed in the Northwest Turkey due to transition tectonics between the strike-slip North Anatolian Fault (NAF) and the extensional Aegean region. We have selected and simulated complete waveforms of 62 earthquakes (Mw > 4.0) occurred during 2007-2015, and recorded at (Δ Technological Research Council of Turkey (TUBITAK Project No: ÇAYDAG-114Y066), and EU-HORIZON-2020: COST Actions: Earth System Science and Environmental Management: ES1401 - Time Dependent Seismology (TIDES).

Coexisting contraction-extension consistent with buoyancy of the crust and upper mantle in North-Central Italy

CERN Document Server

Aoudia, A; Ismail-Zadeh, A T; Panza, G F; Pontevivo, A

2002-01-01

The juxtaposed contraction and extension observed in the crust of the Italian Apennines and elsewhere has, for a long time, attracted the attention of geoscientists and is a long-standing enigmatic feature. Several models, invoking mainly external forces, have been put forward to explain the close association of these two end-member deformation mechanisms clearly observed by geophysical and geological investigations. These models appeal to interactions along plate margins or at the base of the lithosphere such as back-arc extension or shear tractions from mantle flow or to subduction processes such as slab roll back, retreat or pull and detachment. We present here a revisited crust and upper mantle model that supports delamination processes beneath North-Central Italy and provides a new background for the genesis and age of the recent magmatism in Tuscany. Although external forces must have been important in the building up of the Apennines, we show that internal buoyancy forces solely can explain the coexist...

Mantle enrichment by volatiles as the Nazca plate subducts beneath the Payenia backarc of the Southern Volcanic Zone, Argentina

DEFF Research Database (Denmark)

Brandt, Frederik Ejvang

The thesis is a contribution towards the understanding of the generation of the source mantle for magmas related to the subduction of the Nazca plate under South America with an emphasis on the geochemistry of the volatiles Cl, F, S, H2O and CO2. The study presents analytical data for tephra, min...

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

Tomographic Imaging of the Seismic Structure Beneath the East Anatolian Plateau, Eastern Turkey

Science.gov (United States)

Gökalp, Hüseyin

2012-10-01

The high level of seismic activity in eastern Turkey is thought to be mainly associated with the continuing collision of the Arabian and Eurasian tectonic plates. The determination of a detailed three-dimensional (3D) structure is crucial for a better understanding of this on-going collision or subduction process; therefore, a body wave tomographic inversion technique was performed on the region. The tomographic inversion used high quality arrival times from earthquakes occurring in the region from 1999 to 2001 recorded by a temporary 29 station broadband IRIS-PASSCAL array operated by research groups from the Universities of Boğaziçi (Turkey) and Cornell (USA). The data was inverted and consisted of 3,114 P- and 2,298 S-wave arrival times from 252 local events with magnitudes ( M D) ranging from 2.5 to 4.8. The stability and resolution of the results were qualitatively assessed by two synthetic tests: a spike test and checkerboard resolution test and it was found that the models were well resolved for most parts of the imaged domain. The tomographic inversion results reveal significant lateral heterogeneities in the study area to a depth of ~20 km. The P- and S-wave velocity models are consistent with each other and provide evidence for marked heterogeneities in the upper crustal structure beneath eastern Turkey. One of the most important features in the acquired tomographic images is the high velocity anomalies, which are generally parallel to the main tectonic units in the region, existing at shallow depths. This may relate to the existence of ophiolitic units at shallow depths. The other feature is that low velocities are widely dispersed through the 3D structure beneath the region at deeper crustal depths. This feature can be an indicator of the mantle upwelling or support the hypothesis that the Anatolian Plateau is underlain by a partially molten uppermost mantle.

Variable crustal thickness beneath Thwaites Glacier revealed from airborne gravimetry, possible implications for geothermal heat flux in West Antarctica

Science.gov (United States)

Damiani, Theresa M.; Jordan, Tom A.; Ferraccioli, Fausto; Young, Duncan A.; Blankenship, Donald D.

2014-12-01

Thwaites Glacier has one of the largest glacial catchments in West Antarctica. The future stability of Thwaites Glacier's catchment is of great concern, as this part of the West Antarctic Ice Sheet has recently been hypothesized to already be en route towards collapse. Although an oceanic trigger is thought to be responsible for current change at the grounding line of Thwaites Glacier, in order to determine the effects of this coastal change further in the interior of the West Antarctic Ice Sheet it is essential to also better constrain basal conditions that control the dynamics of fast glacial flow within the catchment itself. One major contributor to fast glacial flow is the presence of subglacial water, the production of which is a result of both glaciological shear heating and geothermal heat flux. The primary goal of our study is to investigate the crustal thickness beneath Thwaites Glacier, which is an important contributor to regional-scale geothermal heat flux patterns. Crustal structure is an indicator of past tectonic events and hence provides a geophysical proxy for the thermal status of the crust and mantle. Terrain-corrected Bouguer gravity disturbances are used here to estimate depths to the Moho and mid-crustal boundary. The thin continental crust we reveal beneath Thwaites Glacier supports the hypothesis that the West Antarctic Rift System underlies the region and is expressed topographically as the Byrd Subglacial Basin. This rifted crust is of similar thickness to that calculated from airborne gravity data beneath neighboring Pine Island Glacier, and is more extended than crust in the adjacent Siple Coast sector of the Ross Sea Embayment. A zone of thinner crust is also identified near the area's subaerial volcanoes lending support to a recent interpretation predicting that this part of Marie Byrd Land is a major volcanic dome, likely within the West Antarctic Rift System itself. Near-zero Bouguer gravity disturbances for the subglacial highlands

Upper mantle dynamics of Bangladesh by splitting analysis of core-mantle refracted SKS, PKS, and SKKS phases

Science.gov (United States)

Tiwari, Ashwani Kant; Bhushan, Kirti; Eken, Tuna; Singh, Arun

2018-06-01

New shear wave splitting measurements are obtained from the Bengal Basin using core-mantle refracted SKS, PKS, and SKKS phases. The splitting parameters, namely time delays (δ t) and fast polarization directions (ϕ), were estimated through analysis of 54 high-quality waveforms (⩾ 2.5 signal to noise ratio) from 30 earthquakes with magnitude ⩾ 5.5 recorded at ten seismic stations deployed over Bangladesh. No evidence of splitting was found, which indicates azimuthal isotropy beneath the region. These null measurements can be explained by either vertically dipping anisotropic fast axes or by the presence of multiple horizontal anisotropic layers with different fast polarization directions, where the combined effect results in a null characterization. The anisotropic fabric preserved from rifting episodes of Antarctica and India, subduction-related dynamics of the Indo-Burmese convergence zone, and northward movement of the Indian plate creating shear at the base of the lithosphere can explain the observed null measurements. The combined effect of all these most likely results in a strong vertical anisotropic heterogeneity, creating the observed null results.

Asthenospheric counterflows beneath the moving lithosphere of Central and East Asia in the past 90 Ma: volcanic and tomographic evidence

Science.gov (United States)

Rasskazov, Sergei; Chuvashova, Irina; Kozhevnikov, Vladimir

2015-04-01

Asthenospheric counterflows, accompanied motions of the lithosphere in Central and East Asia, are defined on basis of spatial-temporal activity of mantle sources [Rasskazov et al., 2012; Rasskazov, Chuvashova, 2013; Chuvashova, Rasskazov, 2014] and the tomographic model of the Rayleigh wave group velocities [Kozhevnikov et al., 2014]. The opposite fluxes are defined relative to centers of convective instability (low-velocity anomalies), expressed by thinning of the mantle transition layer under Southwestern Gobi (44 °N, 95 °E) and Northern Baikal (52 °N, 108 °E). Cretaceous-Paleogene volcanic fields in Southern Gobi are shifted eastwards relative to the former anomaly over 600 km with the opposite sub-lithospheric flux at depths of 150-300 km. Likewise, the Late Tertiary Vitim volcanic field is shifted relative to the latter anomaly over 100-200 km. We suggest that the Gobi and Baikal asthenospheric counterflows contributed to the rollback mechanism of downgoing slab material from the Pacific under the eastern margin of Asia in the Cretaceous-Paleogene and Early-Middle Miocene. The east-west Gobi reverse flux, caused by differential block motions in front of the Indo-Asian convergence, resulted in the oblique Honshu-Korean flexure of the Pacific slab that propagated beneath the continental margin, while the Japan Sea was quickly opening at about 15 Ma. The Baikal N60°W reverse flux, originated due to oncoming traffic between Eurasia and the Pacific plate, entailed the formation of the Baikal Rift Zone and direct Hokkaido Amur slab flexure [Rasskazov et al., 2004]. The study is supported by the Russian Foundation for Basic Research (Grant 14-05-31328). References Chuvashova I.S., Rasskazov S.V. Magmatic sources in the mantle of the evolving Earth. Irkutsk: Publishing House of the Irkutsk State University, 2014. 310 p. (in Russian) Kozhevnikov V.M., Seredkina A.I., Solovei O.A. 3D mantle structure of Central Asia from Rayleigh wave group velocity dispersion

The latest geodynamics in Asia: Synthesis of data on volcanic evolution, lithosphere motion, and mantle velocities in the Baikal-Mongolian region

Directory of Open Access Journals (Sweden)

Sergei Rasskazov

2017-07-01

Full Text Available From a synthesis of data on volcanic evolution, movement of the lithosphere, and mantle velocities in the Baikal-Mongolian region, we propose a comprehensive model for deep dynamics of Asia that assumes an important role of the Gobi, Baikal, and North Transbaikal transition-layer melting anomalies. This layer was distorted by lower-mantle fluxes at the beginning of the latest geodynamic stage (i.e. in the early late Cretaceous due to avalanches of slab material that were stagnated beneath the closed fragments of the Solonker, Ural-Mongolian paleoceans and Mongol-Okhotsk Gulf of Paleo-Pacific. At the latest geodynamic stage, Asia was involved in east–southeast movement, and the Pacific plate moved in the opposite direction with subduction under Asia. The weakened upper mantle region of the Gobi melting anomaly provided a counterflow connected with rollback in the Japan Sea area. These dynamics resulted in the formation of the Honshu-Korea flexure of the Pacific slab. A similar weakened upper mantle region of the North Transbaikal melting anomaly was associated with the formation of the Hokkaido-Amur flexure of the Pacific slab, formed due to progressive pull-down of the slab material into the transition layer in the direction of the Pacific plate and Asia convergence. The early–middle Miocene structural reorganization of the mantle processes in Asia resulted in the development of upper mantle low-velocity domains associated with the development of rifts and orogens. We propose that extension at the Baikal Rift was caused by deviator flowing mantle material, initiated under the moving lithosphere in the Baikal melting anomaly. Contraction at the Hangay orogen was created by facilitation of the tectonic stress transfer from the Indo-Asian interaction zone due to the low-viscosity mantle in the Gobi melting anomaly.

Active and fossil mantle flows in the western Alpine region unravelled by seismic anisotropy analysis and high-resolution P wave tomography

Science.gov (United States)

Salimbeni, Simone; Malusà, Marco G.; Zhao, Liang; Guillot, Stéphane; Pondrelli, Silvia; Margheriti, Lucia; Paul, Anne; Solarino, Stefano; Aubert, Coralie; Dumont, Thierry; Schwartz, Stéphane; Wang, Qingchen; Xu, Xiaobing; Zheng, Tianyu; Zhu, Rixiang

2018-04-01

The anisotropy of seismic velocities in the mantle, when integrated with high-resolution tomographic models and geologic information, can be used to detect active mantle flows in complex plate boundary areas, providing new insights on the impact of mantle processes on the topography of mountain belts. Here we use a densely spaced array of temporary broadband seismic stations to analyze the seismic anisotropy pattern of the western Alpine region, at the boundary between the Alpine and Apenninic slabs. Our results are supportive of a polyphase development of anisotropic mantle fabrics, possibly starting from the Jurassic to present. Geophysical data presented in this work, and geologic evidence taken from the literature, indicate that: (i) fossil fabrics formed during Tethyan rifting may be still preserved within the Alpine and Apenninic slabs; (ii) mantle deformation during Apenninic slab rollback is not compensated by a complete toroidal flow around the northern tip of the retreating slab; (iii) the previously observed continuous trend of anisotropy fast axes near-parallel to the western Alpine arc is confirmed. We observe that this arc-parallel trend of fast axes is located in correspondence to a low velocity anomaly in the European upper mantle, beneath regions of the Western and Ligurian Alps showing the highest uplift rates. We propose that the progressive rollback of the Apenninic slab, in the absence of a counterclockwise toroidal flow at its northern tip, induced a suction effect at the scale of the supraslab mantle. The resulting mantle flow pattern was characterized by an asthenospheric counterflow at the rear of the unbroken Western Alps slab and around its southern tip, and by an asthenospheric upwelling, mirrored by low P wave velocities, that would have favored the topographic uplift of the Alpine belt from the Mont Blanc to the Mediterranean sea.

The 2016 Case for Mantle Plumes and a Plume-Fed Asthenosphere (Augustus Love Medal Lecture)

Science.gov (United States)

Morgan, Jason P.

2016-04-01

discrepancies between idealized plume/hotspot models and geochronological observations will also be briefly discussed. A further consequence of the existence of strong deep mantle plumes is that hot plume material should preferentially pond at the base of the lithosphere, draining towards and concentrating beneath the regions where the lithosphere is thinnest, and asthenosphere is being actively consumed to make new tectonic plates - mid-ocean ridges. This plume-fed asthenosphere hypothesis makes predictions for the structure of asthenosphere flow and anisotropy, patterns of continental edge-volcanism linked to lateral plume drainage at continental margins, patterns of cratonic uplift and subsidence linked to passage from hotter plume-influenced to cooler non-plume-influenced regions of the upper mantle, and variable non-volcanic versus volcanic modes of continental extension linked to rifting above '~1425K cool normal mantle' versus 'warm plume-fed asthenosphere' regions of upper mantle. These will be briefly discussed. My take-home message is that "Mantle Plumes are almost certainly real". You can safely bet they will be part of any successful paradigm for the structure of mantle convection. While more risky, I would also recommend betting on the potential reality of the paradigm of a plume-fed asthenosphere. This is still a largely unexplored subfield of mantle convection. Current observations remain very imperfect, but seem more consistent with a plume-fed asthenosphere than with alternatives, and computational and geochemical advances are making good, falsifiable tests increasingly feasible. Make one!

Alkaline lavas from southern Mendoza, Argentina, extend the Patagonian DUPAL mantle field to the north

Science.gov (United States)

Soager, N.; Holm, P. M.; Llambias, E.

2010-12-01

The lavas sampled around Río Colorado ~37°S at the border of Mendoza and Neuquén provinces, Argentina, define an OIB-like end-member composition for the Pleistocene and Holocene activity in the Payún Matrú volcanic field. Although positioned in the far back-arc of the Andes, only a few lavas show signs of involvement of slab fluids or crustal contamination such as relatively high LILEs relative to Nb. The very low La/Nb (~0.66) and Zr/Nb (~5) and high U/Pb (0.3-0.4) of the end-member composition clearly distinguish the source from normal MORB mantle, while high Ba/Nb (~10) and K/Nb (370-400) compared to FOZO and HIMU type OIBs suggest an EM type of mantle. Overall, the trace element patterns of the Río Colorado lavas are similar to the central and north Patagonian intraplate basalts and to South Atlantic E-MORB affected by the Discovery plume and the LOMU component (le Roux et al., 2002, EPSL 203). The isotopic composition of the Río Colorado component has a 206Pb/204Pb = 18.4, 207Pb/204Pb = 15.58, 208Pb/204Pb = 38.3, 87Sr/86Sr = 0.70353 and 143Nd/144Nd = 0.51285. This composition overlaps the central and north Patagonian intraplate basalts in Pb-isotopic space but is slightly less enriched in Sr and Nd-isotopes. It is distinctly different from the FOZO like composition of the south Patagonian intraplate basalts and the nearby Juan Fernandéz plume but similar to the South Atlantic N-MORB and MORB from the southern Chile Ridge segment 4 (Sturm et al., 1999, JGR 104) described as DUPAL type. The DUPAL-MORB type isotopic composition and the plume-like trace element patterns of the Río Colorado lavas suggest the presence of a weak plume beneath the area. The eruption of the large Payún Matrú volcano and the gigantic Pleistocene flood basalts also calls for a thermal anomaly to produce these melts during a weakly compressive tectonic regime with no significant addition of slab fluids. This was supported by Burd et al. (2008, Abstr., 7th Int. Sym. And. Geo

Density Anomalies in the Mantle and the Gravitational Core-Mantle Interaction

Science.gov (United States)

Kuang, Weijia; Liu, Lanbo

2003-01-01

Seismic studies suggest that the bulk of the mantle is heterogeneous, with density variations in depth as well as in horizontal directions (latitude and longitude). This density variation produces a three- dimensional gravity field throughout the Earth. On the other hand, the core density also varies in both time and space, due to convective core flow. Consequently, the fluid outer core and the solid mantle interact gravitationally due to the mass anomalies in both regions. This gravitational core-mantle interaction could play a significant role in exchange of angular momentum between the core and the mantle, and thus the change in Earth's rotation on time scales of decades and longer. Aiming at estimating the significance of the gravitational core-mantle interaction on Earth's rotation variation, we introduce in our MoSST core dynamics model a heterogeneous mantle, with a density distribution derived from seismic results. In this model, the core convection is driven by the buoyancy forces. And the density variation is determined dynamically with the convection. Numerical simulation is carried out with different parameter values, intending to extrapolate numerical results for geophysical implications.

Uranium in mantle processes

International Nuclear Information System (INIS)

Cortini, M.

1984-01-01

(1) Metasomatism is an effective process in the mantle. It controls the distribution of U, Th and Pb in the mantle before the onset of magma formation. (2) Radioactive disequilibria demonstrate that magma formation is an open-system very fast process in which Ra, U and Th are extracted in large amounts from a mantle source that is geochemically distinct from the mantle fraction from which the melt is formed. (3) Because the enrichment of U, Th and Ra in the magma is so fast, the concept of mineral-melt partition coefficient is not valid for these elements during magma formation. (4) Metasomatism seems to generally produce an increase in μ and a decrease in K of the metasomatized mantle region. (5) Magma formation at oceanic ridges and islands seems to generally produce a decrease in K, in its mantle source region. (6) The major source of U, Th, Ra and Pb in a magma probably is the metasomatic mantle component. Instead, the major source of Sr and Nd in a magma is the non-metasomatic, more 'refractory' mantle component. (7) This proposed model is testable. It predicts isotopic disequilibrium of Pb between coexisting minerals and whole rocks, and a correlation of Pb with Th isotopes. (author)

The uniquely high-temperature character of Cullinan diamonds: A signature of the Bushveld mantle plume?

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Korolev, N. M.; Kopylova, M.; Bussweiler, Y.; Pearson, D. G.; Gurney, J.; Davidson, J.

2018-04-01

The mantle beneath the Cullinan kimberlite (formerly known as "Premier") is a unique occurrence of diamondiferous cratonic mantle where diamonds were generated contemporaneously and shortly following a mantle upwelling that led to the formation of a Large Igneous Province that produced the world's largest igneous intrusion - the 2056 Ma Bushveld Igneous Complex (BIC). We studied 332 diamond inclusions from 202 Cullinan diamonds to investigate mantle thermal effects imposed by the formation of the BIC. The overwhelming majority of diamonds come from three parageneses: (1) lithospheric eclogitic (69%), (2) lithospheric peridotitic (21%), and (3) sublithospheric mafic (9%). The lithospheric eclogitic paragenesis is represented by clinopyroxene, garnet, coesite and kyanite. Main minerals of the lithospheric peridotitic paragenesis are forsterite, enstatite, Cr-pyrope, Cr-augite and spinel; the sublithospheric mafic association includes majorite, CaSiO3 phases and omphacite. Diamond formation conditions were calculated using an Al-in-olivine thermometer, a garnet-clinopyroxene thermometer, as well as majorite and Raman barometers. The Cullinan diamonds may be unique on the global stage in recording a cold geotherm of 40 mW/m2 in cratonic lithosphere that was in contact with underlying convecting mantle at temperatures of 1450-1550 °C. The studied Cullinan diamonds contain a high proportion of inclusions equilibrated at temperatures exceeding the ambient 1327 °C adiabat, i.e. 54% of eclogitic diamonds and 41% of peridotitic diamonds. By contrast, ≤ 1% of peridotitic diamond inclusions globally yield equally high temperatures. We propose that the Cullinan diamond inclusions recorded transient, slow-dissipating thermal perturbations associated with the plume-related formation of the 2 Ga Bushveld igneous province. The presence of inclusions in diamond from the mantle transition zone at 300-650 km supports this view. Cullinan xenoliths indicative of the thermal state of

Linear Array Ambient Noise Adjoint Tomography Reveals Intense Crust-Mantle Interactions in North China Craton

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Zhang, Chao; Yao, Huajian; Liu, Qinya; Zhang, Ping; Yuan, Yanhua O.; Feng, Jikun; Fang, Lihua

2018-01-01

We present a 2-D ambient noise adjoint tomography technique for a linear array with a significant reduction in computational cost and show its application to an array in North China. We first convert the observed data for 3-D media, i.e., surface-wave empirical Green's functions (EGFs) to the reconstructed EGFs (REGFs) for 2-D media using a 3-D/2-D transformation scheme. Different from the conventional steps of measuring phase dispersion, this technology refines 2-D shear wave speeds along the profile directly from REGFs. With an initial model based on traditional ambient noise tomography, adjoint tomography updates the model by minimizing the frequency-dependent Rayleigh wave traveltime delays between the REGFs and synthetic Green functions calculated by the spectral-element method. The multitaper traveltime difference measurement is applied in four-period bands: 20-35 s, 15-30 s, 10-20 s, and 6-15 s. The recovered model shows detailed crustal structures including pronounced low-velocity anomalies in the lower crust and a gradual crust-mantle transition zone beneath the northern Trans-North China Orogen, which suggest the possible intense thermo-chemical interactions between mantle-derived upwelling melts and the lower crust, probably associated with the magmatic underplating during the Mesozoic to Cenozoic evolution of this region. To our knowledge, it is the first time that ambient noise adjoint tomography is implemented for a 2-D medium. Compared with the intensive computational cost and storage requirement of 3-D adjoint tomography, this method offers a computationally efficient and inexpensive alternative to imaging fine-scale crustal structures beneath linear arrays.

A mysterious giant ichthyosaur from the lowermost Jurassic of Wales

Directory of Open Access Journals (Sweden)

Jeremy E. Martin

2015-12-01

Full Text Available Ichthyosaurs rapidly diversified and colonised a wide range of ecological niches during the Early and Middle Triassic period, but experienced a major decline in diversity near the end of the Triassic. Timing and causes of this demise and the subsequent rapid radiation of the diverse, but less disparate, parvipelvian ichthyosaurs are still unknown, notably because of inadequate sampling in strata of latest Triassic age. Here, we describe an exceptionally large radius from Lower Jurassic deposits at Penarth near Cardiff, south Wales (UK the morphology of which places it within the giant Triassic shastasaurids. A tentative total body size estimate, based on a regression analysis of various complete ichthyosaur skeletons, yields a value of 12–15 m. The specimen is substantially younger than any previously reported last known occurrences of shastasaurids and implies a Lazarus range in the lowermost Jurassic for this ichthyosaur morphotype.

Upper mantle fluids evolution, diamond formation, and mantle metasomatism

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Huang, F.; Sverjensky, D. A.

2017-12-01

During mantle metasomatism, fluid-rock interactions in the mantle modify wall-rock compositions. Previous studies usually either investigated mineral compositions in xenoliths and xenocrysts brought up by magmas, or examined fluid compositions preserved in fluid inclusions in diamonds. However, a key study of Panda diamonds analysed both mineral and fluid inclusions in the diamonds [1] which we used to develop a quantitative characterization of mantle metasomatic processes. In the present study, we used an extended Deep Earth Water model [2] to simulate fluid-rock interactions at upper mantle conditions, and examine the fluids and mineral assemblages together simultaneously. Three types of end-member fluids in the Panda diamond fluid inclusions include saline, rich in Na+K+Cl; silicic, rich in Si+Al; and carbonatitic, rich in Ca+Mg+Fe [1, 3]. We used the carbonatitic end-member to represent fluid from a subducting slab reacting with an excess of peridotite + some saline fluid in the host environment. During simultaneous fluid mixing and reaction with the host rock, the logfO2 increased by about 1.6 units, and the pH increased by 0.7 units. The final minerals were olivine, garnet and diamond. The Mg# of olivine decreased from 0.92 to 0.85. Garnet precipitated at an early stage, and its Mg# also decreased with reaction progress, in agreement with the solid inclusions in the Panda diamonds. Phlogopite precipitated as an intermediate mineral and then disappeared. The aqueous Ca, Mg, Fe, Si and Al concentrations all increased, while Na, K, and Cl concentrations decreased during the reaction, consistent with trends in the fluid inclusion compositions. Our study demonstrates that fluids coming from subducting slabs could trigger mantle metasomatism, influence the compositions of sub-lithospherc cratonic mantle, precipitate diamonds, and change the oxygen fugacity and pH of the upper mantle fluids. [1] Tomlinson et al. EPSL (2006); [2] Sverjensky, DA et al., GCA (2014

Carbon isotope composition of CO2-rich inclusions in cumulate-forming mantle minerals from Stromboli volcano (Italy)

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Gennaro, Mimma Emanuela; Grassa, Fausto; Martelli, Mauro; Renzulli, Alberto; Rizzo, Andrea Luca

2017-10-01

We report on measurements of concentration and carbon isotope composition (δ13CCO2) of CO2 trapped in fluid inclusions of olivine and clinopyroxene crystals separated from San Bartolo ultramafic cumulate Xenoliths (SBX) formed at mantle depth (i.e., beneath a shallow Moho supposed to be at 14.8 km). These cumulates, erupted about 2 ka ago at Stromboli volcano (Italy), have been already investigated by Martelli et al. (2014) mainly for Sr-Nd isotopes and for their noble gases geochemistry. The concentration of CO2 varies of one order of magnitude from 3.8·10- 8 mol g- 1 to 4.8·10- 7 mol g- 1, with δ13C values between - 2.8‰ and - 1.5‰ vs V-PDB. These values overlap the range of measurements performed in the crater gases emitted at Stromboli (- 2.5‰ residence within the volcano plumbing system. Such δ13C values are higher than those commonly reported for MORB-like upper mantle (- 8 ÷ - 4‰) and likely reflect the source contamination of the local mantle wedge by CO2 coming from the decarbonation of the sediments carried by the subducting Ionian slab with a contribution of organic carbon up to 7%.

Petrogenesis of oceanic kimberlites and included mantle megacrysts: the Malaitan alnoite

International Nuclear Information System (INIS)

Neal, C.R.

1988-01-01

The study of unambiguous suboceanic mantle was facilitated by the occurrence of anomalous kimberlite-type intrusives on Malaita in the Solomon Islands. The pseudo-kimberlites were termed alnoites, and are basically mica lamprophyres with melilite in the ground mass. Alnoitic magmas were explosively intruded into the Ontong Java Plateau (OJP) 34 Ma ago. The OJP is a vastly overthickened portion of the Pacific plate which now abuts the Indo-Australian plate. Malaita is considered to be the obducted leading edge of the OJP. Initial diapiric upwelling beneath the OJP produced the proto-alnoite magma. After impingement on the rigid lithosphere, megacrysts fractionation occurred, with augites precipitating first, representing the parental magma. Sea water-altered oceanic crust, which underplated the OJP, was assimilated by the proto-alnoite magma during megacrysts fractionation

Measurement of Rayleigh wave Z/H ratio and joint inversion for a high-resolution S wave velocity model beneath the Gulf of Mexico passive margin

Science.gov (United States)

Miao, W.; Li, G.; Niu, F.

2016-12-01

Knowledge on the 3D sediment structure beneath the Gulf of Mexico passive margin is not only important to explore the oil and gas resources in the area, but also essential to decipher the deep crust and mantle structure beneath the margin with teleseismic data. In this study, we conduct a joint inversion of Rayleigh wave ellipticity and phase velocity at 6-40 s to construct a 3-D S wave velocity model in a rectangular area of 100°-87° west and 28°-37° north. We use ambient noise data from a total of 215 stations of the Transportable Array deployed under the Earthscope project. Rayleigh wave ellipticity, or Rayleigh wave Z/H (vertical to horizontal) amplitude ratio is mostly sensitive to shallow sediment structure, while the dispersion data are expected to have reasonably good resolution to uppermost mantle depths. The Z/H ratios measured from stations inside the Gulf Coastal Plain are distinctly lower in comparison with those measured from the inland stations. We also measured the phase velocity dispersion from the same ambient noise dataset. Our preliminary 3-D model is featured by strong low-velocity anomalies at shallow depth, which are spatially well correlated with Gulf Cost, East Texas, and the Lower Mississippi basins. We will discuss other features of the 3-D models once the model is finalized.

Isotope geochemistry of xenoliths from East Africa. Implications for development of mantle reservoirs and their interaction

Energy Technology Data Exchange (ETDEWEB)

Cohen, R S; O' Nions, R K [Cambridge Univ. (UK). Dept. of Earth Sciences; Dawson, J B [Sheffield Univ. (UK). Dept. of Geology

1984-05-01

Pd, Nd and Sr isotope analyses together with U, Pb, Sm, Nd, Rb and Sr concentrations have been obtained for separated phases of lherzolite and bulk rock mafic granulite xenoliths in Recent volcanics from Tanzania. A garnet lherzolite from the Lashaine vent has yielded the least radiogenic Pb (/sup 206/Pb//sup 204/PB=15.55) and Nd (/sup 143/Nd//sup 144/Nd=0.51127; epsilon/sup 0/sub(Nd)=-26.7) isotope compositions recorded so far for an ultramafic xenolith, and /sup 87/Sr//sup 86/Sr=0.83604. The Pb isotope compositions of the mafic granulites are variable 15.77mantle part of the continental lithosphere beneath Tanzania has components that have undergone a complex history that includes major chemical fractionations ca. 2.0 Ga ago. A phlogopite-amphibole vein from the Pello Hill sample has Sr, Nd and Pb isotope compositions similar to those of mid-ocean ridge basalts, indicating both a young emplacement age for the vein material and a source which had an isotopic signature characteristic of depleted mantle. The Sr, Nd and Pb isotope systematics of ultramafic xenoliths do not conform with those of MORB, particularly in terms of their Pb-Sr, and Nd-Pb relationship. In this regard they are similar to some ocean islands and could be a viable source material for some ocean island basalts at least. The mantle part of the continental lithosphere is as likely to contain recycled components derived from the continental crust as are other regions of mantle. If the mantle part of continental lithosphere is invoked as a source for ocean islands, it does not negate the possibility that substantial recycled components are involved.

Physical mineralogy of (Ca,Al)-rich silicate phases of the Earth's mantle. Geodynamic implications

International Nuclear Information System (INIS)

Gautron, Laurent

2008-01-01

bottom heating of the big domes observed in the cross sections of the mantle obtained by seismic tomography. The possible relation between our results from mineral physics and the volume of 'hot' materials present at the bottom of the mantle, is also discussed. The second silicate phase bearing Ca and Al presented in this thesis is the new high pressure phase named CAS phase of composition CaAl 4 Si 2 O 11 . After many experimental studies performed at high pressure on basaltic crust assemblage, it is now commonly accepted that the CAS phase is one of the main mineral phases present in the oceanic crust (Mid-Ocean Ridge Basalt, MORB) subducted to the lowermost lower mantle. The CAS phase is shown to be one of the last solid residual phases (with Ca-perovskite) when the oceanic crust is partially molten, as expected when this crust reaches the D'' region. Here, we show that the CAS phase bears an iso-symmetrical transition where some silicon atoms adopt a coordination 5, in the trigonal bipyramidal site (2 face-sharing tetrahedra). The implications of such intermediate coordination (between coordinations 4 and 6) is discussed in terms of diffusion processes, diffusion creep deformation, viscosity: it appears that the formation of SiO 5 groups strongly favours the deformation properties of these materials, and then enhances their transport properties. It is clear that the coordination of silicon atoms could have a strong direct effect on the dynamic processes occurring in the deep mantle. With the two studies presented in this thesis, we see that experimental mineral physics can provide essential data for models in geodynamics, thermal behaviour and in seismology. Seismic waves give data about the structure of the deep Earth and the density profile, while experimental geodynamics reproduce the rheological behaviour of the mantle with appropriate fluids and a bottom heating: it is then important to provide complementary data about the Earth materials. The study of the CAS

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

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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

Present mantle flow in North China Craton constrained by seismic anisotropy and numerical modelling

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Qu, W.; Guo, Z.; Zhang, H.; Chen, Y. J.

2017-12-01

North China Carton (NCC) has undergone complicated geodynamic processes during the Cenozoic, including the westward subduction of the Pacific plate to its east and the collision of the India-Eurasia plates to its southwest. Shear wave splitting measurements in NCC reveal distinct seismic anisotropy patterns at different tectonic blocks, that is, the predominantly NW-SE trending alignment of fast directions in the western NCC and eastern NCC, weak anisotropy within the Ordos block, and N-S fast polarization beneath the Trans-North China Orogen (TNCO). To better understand the origin of seismic anisotropy from SKS splitting in NCC, we obtain a high-resolution dynamic model that absorbs multi-geophysical observations and state-of-the-art numerical methods. We calculate the mantle flow using a most updated version of software ASPECT (Kronbichler et al., 2012) with high-resolution temperature and density structures from a recent 3-D thermal-chemical model by Guo et al. (2016). The thermal-chemical model is obtained by multi-observable probabilistic inversion using high-quality surface wave measurements, potential fields, topography, and surface heat flow (Guo et al., 2016). The viscosity is then estimated by combining the dislocation creep, diffusion creep, and plasticity, which is depended on temperature, pressure, and chemical composition. Then we calculate the seismic anisotropy from the shear deformation of mantle flow by DREX, and predict the fast direction and delay time of SKS splitting. We find that when complex boundary conditions are applied, including the far field effects of the deep subduction of Pacific plate and eastward escaping of Tibetan Plateau, our model can successfully predict the observed shear wave splitting patterns. Our model indicates that seismic anisotropy revealed by SKS is primarily resulting from the LPO of olivine due to the shear deformation from asthenospheric flow. We suggest that two branches of mantle flow may contribute to the

Continental basalts record the crust-mantle interaction in oceanic subduction channel: A geochemical case study from eastern China

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Xu, Zheng; Zheng, Yong-Fei

2017-09-01

the crustal metasomatism through melt-peridotite reaction at the slab-mantle interface in oceanic subduction channels. Continental basalts of Mesozoic to Cenozoic ages from eastern China are used as a case example to illustrate the above petrogenetic mechanism. Subduction of the paleo-Pacific oceanic slab beneath the eastern edge of Eurasian continent in the Early Mesozoic would have transferred the crustal signatures into the mantle sources of these basalts. This process would be associated with rollback of the subducting slab at that time, whereas the partial melting of metasomatites takes place mainly in the Late Mesozoic to Cenozoic to produce the continental basalts. Therefore, OIB-like continental basalts are also the product of subduction-zone magmatism though they occur in intraplate settings.

Crustal and upper mantle shear velocities of Iberia, the Alboran Sea, and North Africa from ambient noise and ballistic finite-frequency Rayleigh wave tomography

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Palomeras, I.; Villasenor, A.; Thurner, S.; Levander, A.; Gallart, J.; mimoun, H.

2013-12-01

The complex Mesozoic-Cenozoic Alpine deformation in the western Mediterranean extends from the Pyrenees in northern Spain to the Atlas Mountains in southern Morocco. The Iberian plate was accreted to the European plate in late Cretaceous, resulting in the formation of the Pyrenees. Cenozoic African-European convergence resulted in subduction of the Tethys oceanic plate beneath Europe. Rapid Oligocene slab rollback from eastern Iberia spread eastward and southward, with the trench breaking into three segments by the time it reached the African coast. One trench segment moved southwestward and westward creating the Alboran Sea, floored by highly extended continental crust, and building the encircling Betics Rif mountains comprising the Gibraltar arc, and the Atlas mountains, which formed as the inversion of a Jurassic rift. A number of recent experiments have instrumented this region with broad-band arrays (the US PICASSO array, Spanish IberArray and Siberia arrays, the University of Munster array), which, including the Spanish, Portuguese, and Moroccan permanent networks, provide a combined array of 350 stations having an average interstation spacing of ~60 km. Taking advantage of this dense deployment, 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). Approximately 50,000 stations pairs were used to measure the phase velocity from ambient noise and more than 160 teleseismic events to measure phase velocity for longer periods. The inversion of the phase velocity dispersion curves provides a 3D shear velocity for the crust and uppermost mantle. Our results show differences between the various tectonic regions that extend to upper mantle depths (~200 km). In Iberia we obtain, on average, higher upper mantle shear velocities in the western Variscan region than in the younger eastern part. We map high upper mantle velocities (>4.6 km/s) beneath the

An unusual taphocoenosis of a sea urchin and a rectally inserted turriform gastropod from the lowermost Paleocene of Stevs klint, Denmark

DEFF Research Database (Denmark)

Milan, Jesper; Rasmussen, Bo Wilhelm; Vallon, Lothar H.

2015-01-01

A specimen of the common irregular echinoid Echinocorys sulcata (Goldfuss, 1826), recovered from the lowermost Paleocene Stevns Klint Formation, at Stevns Klint, Denmark, is of note in revealing a perfect external mold of the turriform gastropod Cerithiella fenestrata (Ravn, 1902) in the anal ope...

The 2007 Nazko, British Columbia, earthquake sequence: Injection of magma deep in the crust beneath the Anahim volcanic belt

Science.gov (United States)

Cassidy, J.F.; Balfour, N.; Hickson, C.; Kao, H.; White, Rickie; Caplan-Auerbach, J.; Mazzotti, S.; Rogers, Gary C.; Al-Khoubbi, I.; Bird, A.L.; Esteban, L.; Kelman, M.; Hutchinson, J.; McCormack, D.

2011-01-01

On 9 October 2007, an unusual sequence of earthquakes began in central British Columbia about 20 km west of the Nazko cone, the most recent (circa 7200 yr) volcanic center in the Anahim volcanic belt. Within 25 hr, eight earthquakes of magnitude 2.3-2.9 occurred in a region where no earthquakes had previously been recorded. During the next three weeks, more than 800 microearthquakes were located (and many more detected), most at a depth of 25-31 km and within a radius of about 5 km. After about two months, almost all activity ceased. The clear P- and S-wave arrivals indicated that these were high-frequency (volcanic-tectonic) earthquakes and the b value of 1.9 that we calculated is anomalous for crustal earthquakes but consistent with volcanic-related events. Analysis of receiver functions at a station immediately above the seismicity indicated a Moho near 30 km depth. Precise relocation of the seismicity using a double-difference method suggested a horizontal migration at the rate of about 0:5 km=d, with almost all events within the lowermost crust. Neither harmonic tremor nor long-period events were observed; however, some spasmodic bursts were recorded and determined to be colocated with the earthquake hypocenters. These observations are all very similar to a deep earthquake sequence recorded beneath Lake Tahoe, California, in 2003-2004. Based on these remarkable similarities, we interpret the Nazko sequence as an indication of an injection of magma into the lower crust beneath the Anahim volcanic belt. This magma injection fractures rock, producing high-frequency, volcanic-tectonic earthquakes and spasmodic bursts.

A study of experimental simulation of mantle metasomatism by the proton microprobe

International Nuclear Information System (INIS)

Sie, S.H.; Suter, G.F.; Sweeney, R.J.; Green, D.H.

1991-01-01

The chemistry of melts and fluids in the Earth's mantle is essential to understand the processes that generate them and the source areas from which they derive. The characterisation of these phases is particularly relevant with regard to the geochemical changes which would occur in a mantle subjected to the percolation of fluids (for example fluids that derive from a hydrated subducting slab to influence basic geochemistry in subduction zones) and small degree melts which percolate into a relatively cool mantle beneath continents. The development of a technique in the Geology Department of University of Tasmania, of trapping and isolating these small degree melts and fluids in pre-stressed (fractured) olivine disks inserted into run capsules is described. Little success is reported with the analysis of subsurface inclusions in olivine containing trace amounts (e.g. up to 1000 ppm) of elements of interest. This is primarily due to the fact that olivine is a heavy absorber of secondary X-rays principally a function of its higher Fe content. However, some success was achieved in the analysis of small surface melt inclusions where corrections had to be made for the overlap of the beam on the encapsulating olivine. The results carry large uncertainties (20%), primarily due to the smallness of the sample hence the large contribution of underlying olivine, and also of surrounding olivine when the beam is larger than the sample or when the beam drifts off the sample. An example of such measurements is described. Garnets in the peridotite were also analysed and this enabled the calculation of melt-garnet partition coefficients. 5 refs., 2 tabs

Dynamical geochemistry of the mantle

Directory of Open Access Journals (Sweden)

G. F. Davies

2011-09-01

Full Text Available The reconciliation of mantle chemistry with the structure of the mantle inferred from geophysics and dynamical modelling has been a long-standing problem. This paper reviews three main aspects. First, extensions and refinements of dynamical modelling and theory of mantle processing over the past decade. Second, a recent reconsideration of the implications of mantle heterogeneity for melting, melt migration, mantle differentiation and mantle segregation. Third, a recent proposed shift in the primitive chemical baseline of the mantle inferred from observations of non-chondritic ¹⁴²Nd in the Earth. It seems most issues can now be resolved, except the level of heating required to maintain the mantle's thermal evolution.

A reconciliation of refractory trace elements and their isotopes with the dynamical mantle, proposed and given preliminary quantification by Hofmann, White and Christensen, has been strengthened by work over the past decade. The apparent age of lead isotopes and the broad refractory-element differences among and between ocean island basalts (OIBs and mid-ocean ridge basalts (MORBs can now be quantitatively accounted for with some assurance.

The association of the least radiogenic helium with relatively depleted sources and their location in the mantle have been enigmatic. The least radiogenic helium samples have recently been recognised as matching the proposed non-chondritic primitive mantle. It has also been proposed recently that noble gases reside in a so-called hybrid pyroxenite assemblage that is the result of melt from fusible pods reacting with surrounding refractory peridotite and refreezing. Hybrid pyroxenite that is off-axis may not remelt and erupt at MORs, so its volatile constituents would recirculate within the mantle. Hybrid pyroxenite is likely to be denser than average mantle, and thus some would tend to settle in the D" zone at the base of the mantle, along with some old subducted

Mantle Subduction and Uplift of Intracontinental Mountains: A Case Study from the Chinese Tianshan Mountains within Eurasia.

Science.gov (United States)

Li, Jinyi; Zhang, Jin; Zhao, Xixi; Jiang, Mei; Li, Yaping; Zhu, Zhixin; Feng, Qianwen; Wang, Lijia; Sun, Guihua; Liu, Jianfeng; Yang, Tiannan

2016-06-29

The driving mechanism that is responsible for the uplift of intracontinental mountains has puzzled geologists for decades. This study addresses this issue by using receiver function images across the Chinese Tianshan Mountains and available data from both deep seismic profiles and surface structural deformation. The near-surface structural deformation shows that the Tianshan crust experienced strong shortening during the Cenozoic. The receiver function image across the Tianshan Mountains reveals that the lithosphere of the Junggar Basin to the north became uncoupled along the Moho, and the mantle below the Moho subducted southwards beneath the northern part of the Tianshan Mountains, thereby thickening the overlying crust. Similar deep structures, however, are not observed under the Tarim Basin and the adjacent southern Tianshan Mountains. This difference in the deep structures correlates with geomorphological features in the region. Thus, a new model of mantle subduction, herein termed M-type subduction, is proposed for the mountain-building processes in intracontinental compressional settings. The available geomorphological, geological and seismic data in the literatures show that this model is probably suitable for other high, linear mountains within the continent.

Lateral variations in upper-mantle seismic anisotropy in the Pacific from inversion of a surface-wave dispersion dataset

Science.gov (United States)

Eddy, C. L.; Ekstrom, G.; Nettles, M.; Gaherty, J. B.

2017-12-01

We present a three-dimensional model of the anisotropic velocity structure of the Pacific lithosphere and asthenosphere. The presence of seismic anisotropy in the oceanic upper mantle provides information about the geometry of flow in the mantle, the nature of the lithosphere-asthenosphere boundary, and the possible presence of partial melt in the asthenosphere. Our dataset consists of fundamental-mode dispersion for Rayleigh and Love waves measured between 25-250 s with paths crossing the Pacific Ocean. We invert the phase anomaly measurements directly for three-dimensional anisotropic velocity structure. Our models are radially anisotropic and include the full set of elastic parameters that describe azimuthal variations in velocity (e.g. Gc, Gs). We investigate the age dependence of seismic velocity and radial anisotropy and find that there are significant deviations from the velocities predicted by a simple oceanic plate cooling model. We observe strong radial anisotropy with vsh > vsv in the asthenosphere of the central Pacific. We investigate the radial anisotropy in the shallow lithosphere, where previous models have reported conflicting results. There is a contrast in both upper-mantle isotropic velocities and radial anisotropy between the Pacific and Nazca plates, across the East Pacific Rise. We also investigate lateral variations in azimuthal anisotropy throughout the Pacific upper mantle and find that there are large areas over which the anisotropy fast axis does not align with absolute plate motion, suggesting the presence of small-scale convection or pressure-driven flow beneath the base of the oceanic plate.

Mantle dynamics following supercontinent formation

Science.gov (United States)

Heron, Philip J.

This thesis presents mantle convection numerical simulations of supercontinent formation. Approximately 300 million years ago, through the large-scale subduction of oceanic sea floor, continental material amalgamated to form the supercontinent Pangea. For 100 million years after its formation, Pangea remained relatively stationary, and subduction of oceanic material featured on its margins. The present-day location of the continents is due to the rifting apart of Pangea, with supercontinent dispersal being characterized by increased volcanic activity linked to the generation of deep mantle plumes. The work presented here investigates the thermal evolution of mantle dynamics (e.g., mantle temperatures and sub-continental plumes) following the formation of a supercontinent. Specifically, continental insulation and continental margin subduction are analyzed. Continental material, as compared to oceanic material, inhibits heat flow from the mantle. Previous numerical simulations have shown that the formation of a stationary supercontinent would elevate sub-continental mantle temperatures due to the effect of continental insulation, leading to the break-up of the continent. By modelling a vigorously convecting mantle that features thermally and mechanically distinct continental and oceanic plates, this study shows the effect of continental insulation on the mantle to be minimal. However, the formation of a supercontinent results in sub-continental plume formation due to the re-positioning of subduction zones to the margins of the continent. Accordingly, it is demonstrated that continental insulation is not a significant factor in producing sub-supercontinent plumes but that subduction patterns control the location and timing of upwelling formation. A theme throughout the thesis is an inquiry into why geodynamic studies would produce different results. Mantle viscosity, Rayleigh number, continental size, continental insulation, and oceanic plate boundary evolution are

Shock Compression and Melting of an Fe-Ni-Si Alloy: Implications for the Temperature Profile of the Earth's Core and the Heat Flux Across the Core-Mantle Boundary

Science.gov (United States)

Zhang, Youjun; Sekine, Toshimori; Lin, Jung-Fu; He, Hongliang; Liu, Fusheng; Zhang, Mingjian; Sato, Tomoko; Zhu, Wenjun; Yu, Yin

2018-02-01

Understanding the melting behavior and the thermal equation of state of Fe-Ni alloyed with candidate light elements at conditions of the Earth's core is critical for our knowledge of the region's thermal structure and chemical composition and the heat flow across the liquid outer core into the lowermost mantle. Here we studied the shock equation of state and melting curve of an Fe-8 wt% Ni-10 wt% Si alloy up to 250 GPa by hypervelocity impacts with direct velocity and reliable temperature measurements. Our results show that the addition of 10 wt% Si to Fe-8 wt% Ni alloy slightly depresses the melting temperature of iron by 200-300 (±200) K at the core-mantle boundary ( 136 GPa) and by 600-800 (±500) K at the inner core-outer core boundary ( 330 GPa), respectively. Our results indicate that Si has a relatively mild effect on the melting temperature of iron compared with S and O. Our thermodynamic modeling shows that Fe-5 wt% Ni alloyed with 6 wt% Si and 2 wt% S (which has a density-velocity profile that matches the outer core's seismic profile well) exhibits an adiabatic profile with temperatures of 3900 K and 5300 K at the top and bottom of the outer core, respectively. If Si is a major light element in the core, a geotherm modeled for the outer core indicates a thermal gradient of 5.8-6.8 (±1.6) K/km in the D″ region and a high heat flow of 13-19 TW across the core-mantle boundary.

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.

Heat transfer correlations in mantle tanks

DEFF Research Database (Denmark)

Furbo, Simon; Knudsen, Søren

2005-01-01

on calculations with a CFD-model, which has earlier been validated by means of experiments. The CFD-model is used to determine the heat transfer between the solar collector fluid in the mantle and the walls surrounding the mantle in all levels of the mantle as well as the heat transfer between the wall...... transfer correlations are suitable as input for a detailed simulation model for mantle tanks. The heat transfer correlations determined in this study are somewhat different from previous reported heat transfer correlations. The reason is that this study includes more mantle tank designs and operation......Small solar domestic hot water systems are best designed as low flow systems based on vertical mantle tanks. Theoretical investigations of the heat transfer in differently designed vertical mantle tanks during different operation conditions have been carried out. The investigations are based...

A new conceptual model for whole mantle convection and the origin of hotspot plumes

Science.gov (United States)

Yoshida, Masaki

2014-08-01

A new conceptual model of mantle convection is constructed for consideration of the origin of hotspot plumes, using recent evidence from seismology, high-pressure experiments, geodynamic modeling, geoid inversion studies, and post-glacial rebound analyses. This conceptual model delivers several key points. Firstly, some of the small-scale mantle upwellings observed as hotspots on the Earth's surface originate at the base of the mantle transition zone (MTZ), in which the Archean granitic continental material crust (TTG; tonalite-trondhjemite-granodiorite) with abundant radiogenic elements is accumulated. Secondly, the TTG crust and the subducted oceanic crust that have accumulated at the base of MTZ could act as thermal or mechanical insulators, leading to the formation of a hot and less viscous layer just beneath the MTZ; which may enhance the instability of plume generation at the base of the MTZ. Thirdly, the origin of some hotspot plumes is isolated from the large low shear-wave velocity provinces (LLSVPs) under Africa and the South Pacific. I consider that the conceptual model explains why almost all the hotspots around Africa are located above the margins of the African LLSVP. Because a planetary-scale trench system surrounding a “Pangean cell” has been spatially stable throughout the Phanerozoic, a large amount of the oceanic crustal layer is likely to be trapped in the MTZ under the Pangean cell. Therefore, under Africa, almost all of the hotspot plumes originate from the base of the MTZ, where a large amount of TTG and/or oceanic crusts has accumulated. This conceptual model may explain the fact that almost all the hotspots around Africa are located on margins above the African LLSVP. It is also considered that some of the hotspot plumes under the South Pacific thread through the TTG/oceanic crusts accumulated around the bottom of the MTZ, and some have their roots in the South Pacific LLSVP while others originate from the MTZ. The numerical simulations

Seismic velocity structure of the crust and shallow mantle of the Central and Eastern United States by seismic surface wave imaging

Science.gov (United States)

Pollitz, Fred; Mooney, Walter D.

2016-01-01

Seismic surface waves from the Transportable Array of EarthScope's USArray are used to estimate phase velocity structure of 18 to 125 s Rayleigh waves, then inverted to obtain three-dimensional crust and upper mantle structure of the Central and Eastern United States (CEUS) down to ∼200 km. The obtained lithosphere structure confirms previously imaged CEUS features, e.g., the low seismic-velocity signature of the Cambrian Reelfoot Rift and the very low velocity at >150 km depth below an Eocene volcanic center in northwestern Virginia. New features include high-velocity mantle stretching from the Archean Superior Craton well into the Proterozoic terranes and deep low-velocity zones in central Texas (associated with the late Cretaceous Travis and Uvalde volcanic fields) and beneath the South Georgia Rift (which contains Jurassic basalts). Hot spot tracks may be associated with several imaged low-velocity zones, particularly those close to the former rifted Laurentia margin.

Evolution of the earliest mantle caused by the magmatism-mantle upwelling feedback: Implications for the Moon and the Earth

Science.gov (United States)

Ogawa, M.

2017-12-01

The two most important agents that cause mantle evolution are magmatism and mantle convection. My earlier 2D numerical models of a coupled magmatism-mantle convection system show that these two agents strongly couple each other, when the Rayleigh number Ra is sufficiently high: magmatism induced by a mantle upwelling flow boosts the upwelling flow itself. The mantle convection enhanced by this positive feedback (the magmatism-mantle upwelling, or MMU, feedback) causes vigorous magmatism and, at the same time, strongly stirs the mantle. I explored how the MMU feedback influences the evolution of the earliest mantle that contains the magma ocean, based on a numerical model where the mantle is hot and its topmost 1/3 is partially molten at the beginning of the calculation: The evolution drastically changes its style, as Ra exceeds the threshold for onset of the MMU feedback, around 107. At Ra 107, however, the mantle remains compositionally more homogeneous in spite of the widespread magmatism, and the deep mantle remains hotter than the shallow mantle, because of the strong convective stirring caused by the feedback. The threshold value suggests that the mantle of a planet larger than Mars evolves in a way substantially different from that in the Moon does. Indeed, in my earlier models, magmatism makes the early mantle compositionally stratified in the Moon, but the effects of strong convective stirring overwhelms that of magmatism to keep the mantle compositionally rather homogeneous in Venus and the Earth. The MMU feedback is likely to be a key to understanding why vestiges of the magma ocean are so scarce in the Earth.

Discharge controls on the sediment and dissolved nutrient transport flux of the lowermost Mississippi River: Implications for export to the ocean and for delta restoration

Science.gov (United States)

Allison, Mead A.; Pratt, Thad C.

2017-12-01

Lagrangian longitudinal surveys and fixed station data are utilized from the lowermost Mississippi River reach in Louisiana at high and low discharge in 2012-2013 to examine the changing stream power, sediment transport capacity, and nitrate conveyance in this backwater reach of the river. Nitrate appears to remain conservative through the backwater reach at higher discharges (>15,000 m3/s), thus, nitrate levels supplied from the catchment are those exported to the Gulf of Mexico, fueling coastal hypoxia. At lower discharges, interaction with fine sediments and organic matter stored on the bed due to estuarine and tidal processes, likely elevates nitrate levels prior to entering the Gulf: a further 1-2 week long spike in nitrate concentrations is associated with the remobilization of this sediments during the rising discharge phase of the Mississippi. Backwater characteristics are clearly observed in the study reach starting at river kilometer 703 (Vicksburg) in both longitudinal study periods. Stream power at the lowermost station is only 16% of that at Vicksburg in the high discharge survey, and 0.6% at low flow. The high-to-low discharge study differential in unit stream power at a station increases between Vicksburg and the lowermost station from a factor of 3 to 47-50 times. At high discharge, ∼30% of this energy loss can be ascribed to the removal of water to the Atchafalaya at Old River Control. Suspended sediment flux decreases downstream in the studied reach in both studies: the lowermost station has 75% of the flux at Vicksburg in the high discharge study, and 0.9% in the low discharge study. The high discharge values, given that this study was conducted during the highest rising hydrograph of the water year, are augmented by sediment resuspended from the bed that was deposited in the previous low discharge phase. Examination of this first detailed field observation studies of the backwater phenomenon in a major river, shows that observed suspended

Spin Transition in the Lower Mantle: Deep Learning and Pattern Recognition of Superplumes from the Mid-mantle and Mid-mantle Slab Stagnation

Science.gov (United States)

Yuen, D. A.; Shahnas, M. H.; De Hoop, M. V.; Pysklywec, R.

2016-12-01

The broad, slow seismic anomalies under Africa and Pacific cannot be explained without ambiguity. There is no well-established theory to explain the fast structures prevalent globally in seismic tomographic images that are commonly accepted to be the remnants of fossil slabs at different depths in the mantle. The spin transition from high spin to low spin in iron in ferropericlase and perovskite, two major constituents of the lower mantle can significantly impact their physical properties. We employ high resolution 2D-axisymmetric and 3D-spherical control volume models to reconcile the influence of the spin transition-induced anomalies in density, thermal expansivity, and bulk modulus in ferropericlase and perovskite on mantle dynamics. The model results reveal that the spin transition effects increase the mixing in the lower regions of mantle. Depending on the changes of bulk modulus associated with the spin transition, these effects may also cause both stagnation of slabs and rising plumes at mid-mantle depths ( 1600 km). The stagnation may be followed by downward or upward penetration of cold or hot mantle material, respectively, through an avalanche process. The size of these mid-mantle plumes reaches 1500 km across with a radial velocity reaching 20 cm/yr near the seismic transition zone and plume heads exceeding 2500 km across. We will employ a deep-learning algorithm to formulate this challenge as a classification problem where modelling/computation aids in the learning stage for detecting the particular patterns.The parameters based on which the convection models are developed are poorly constrained. There are uncertainties in initial conditions, heterogeneities and boundary conditions in the simulations, which are nonlinear. Thus it is difficult to reconstruct the past configuration over long time scales. In order to extract information and better understand the parameters in mantle convection, we employ deep learning algorithm to search for different

The Earth's mantle and geoneutrinos

International Nuclear Information System (INIS)

Fiorentini, Giovanni; Fogli, Gian Luigi; Lisi, Eligio; Mantovani, Fabio; Rotunno, Anna Maria; Xhixha, Gerti

2013-01-01

The KamLAND and Borexino experiments have observed, each at ∼4σ level, signals of electron antineutrinos produced in the decay chains of thorium and uranium in the Earth's crust and mantle (Th and U geoneutrinos). Various pieces of geochemical and geophysical information allow an estimation of the crustal geoneutrino flux components with relatively small uncertainties. The mantle component may then be inferred by subtracting the estimated crustal flux from the measured total flux. We find that crust-subtracted signals show hints of a residual mantle component, emerging at ∼2.4σ level by combining the KamLAND and Borexino data. The inferred mantle flux, slightly favoring scenarios with relatively high Th and U abundances, within ∼1σ uncertainties is comparable to the predictions from recent mantle models

Mapping the upper mantle beneath North American continent with joint inversion of surface-wave phase and amplitude

Science.gov (United States)

Yoshizawa, K.; Hamada, K.

2017-12-01

A new 3-D S-wave model of the North American upper mantle is constructed from a large number of inter-station phase and amplitude measurements of surface waves. A fully nonlinear waveform fitting method by Hamada and Yoshizawa (2015, GJI) is applied to USArray for measuring inter-station phase speeds and amplitude ratios of the fundamental-mode Rayleigh and Love waves. We employed the seismic events from 2007 - 2014 with Mw 6.0 or greater, and collected a large-number of inter-station phase speed data (about 130,000 for Rayleigh and 85,000 for Love waves) and amplitude ratio data (about 75,000 for Rayleigh waves) in a period range from 30 to 130 s for fundamental-mode surface waves. Typical inter-station distances are mostly in a range between 300 and 800 km, which can be of help in enhancing the lateral resolution of a regional tomography model. We first invert Rayleigh-wave phase speeds and amplitudes simultaneously for phase speed maps as well as local amplification factors at receiver locations. The isotropic 3-D S-wave model constructed from these phase speed maps incorporating both phase and amplitude data exhibits better recovery of the strength of velocity perturbations. In particular, local tectonic features characterized by strong velocity gradients, such as Rio Grande Rift, Colorado Plateau and New Madrid Seismic Zone, are more enhanced than conventional models derived from phase information only. The results indicate that surface-wave amplitude, which is sensitive to the second derivative of phase speeds, can be of great help in retrieving small-scale heterogeneity in the upper mantle. We also obtain a radial anisotropy model from the simultaneous inversions of Rayleigh and Love waves (without amplitude information). The model has shown faster SH wave speed anomalies than SV above the depth of 100 km, particularly in tectonically active regions in the western and central U.S., representing the effects of current and former tectonic processes on

Velocity structure around the 410 km discontinuity beneath the East China Sea based on the waveform modeling method

Science.gov (United States)

Li, W.; Cui, Q.; Gao, Y.; Wei, R.; Zhou, Y.; Yu, J.

2017-12-01

The 410 km discontinuity is the upper boundary of the mantle transition zone. Seismic detections on the structure and morphology of the 410 km discontinuity are helpful to understand the compositions of the Earth's interior and the relevant geodynamics. In this paper, we select the broadband P waveforms of an intermediate earthquake that occurred in the Ryukyu subduction zone and retrieved from the China Digital Seismograph Network, and study the fine velocity structure around the 410 km discontinuity by matching the observed triplicated waveforms with the theoretical ones. Our results reveal that (1) the 410 km discontinuity beneath the East China Sea is mostly a sharp boundary with a small-scale uplift of 8-15 km and a gradient boundary up to 20 km in the most southern part, and (2) there exist a low velocity layer atop the 410 km discontinuity with the thickness of 50-62 km and P-wave velocity decrease of 0.5%-1.5%, and (3) a high velocity anomaly with P-wave decrease of 1.0%-3.0% below 440 km. Combining with the previous topographic results in this area, we speculate that the high velocity anomaly is relevant to the stagnancy of the western Pacific slab in the mantle transition zone, the decomposition of phase E in the slab results in the increase of water content, which would cause the uplift of the 410 km discontinuity, and the low velocity layer atop the discontinuity should be related to the partial melting of the mantle peridotite induced by the dehydration of the hydrous minerals.

Dynamics of upper mantle rocks decompression melting above hot spots under continental plates

Science.gov (United States)

Perepechko, Yury; Sorokin, Konstantin; Sharapov, Victor

2014-05-01

temperature (THS) > 1900oC asthenolens size ~700 km. When THS = of 2000oC the maximum melting degree of the primitive mantle is near 40%. An increase in the TB > 1900oC the maximum degree of melting could rich 100% with the same size of decompression melting zone (700 km). We examined decompression melting above the HS having LHS = 100 km - 780 km at a TB 1850- 2100oC with the thickness of lithosphere = 100 km.It is shown that asthenolens size (Lln) does not change substantially: Lln=700 km at LHS = of 100 km; Lln= 800 km at LHS = of 780 km. In presence of asymmetry of large HS the region of advection is developed above the HS maximum with the formation of asymmetrical cell. Influence of lithospheric plate thicknesses on appearance and evolution of asthenolens above the HS were investigated for the model stepped profile for the TB ≤ of 1750oS with Lhs = 100km and maximum of THS =2350oC. With an increase of TB the Lln difference beneath lithospheric steps is leveled with retention of a certain difference to melting degrees and time of the melting appearance a top of the HS. RFBR grant 12-05-00625.

Mantle wedge serpentinization effects on slab dips

Directory of Open Access Journals (Sweden)

Eh Tan

2017-01-01

Full Text Available The mechanical coupling between a subducting slab and the overlying mantle wedge is an important factor in controlling the subduction dip angle and the flow in mantel wedge. This paper investigates the role of the amount of mantle serpentinization on the subduction zone evolution. With numerical thermos-mechanical models with elasto-visco-plastic rheology, we vary the thickness and depth extent of mantle serpentinization in the mantle wedge to control the degree of coupling between the slab and mantle wedge. A thin serpentinized mantle layer is required for stable subduction. For models with stable subduction, we find that the slab dip is affected by the down-dip extent and the mantle serpentinization thickness. A critical down-dip extent exists in mantle serpentinization, determined by the thickness of the overriding lithosphere. If the down-dip extent does not exceed the critical depth, the slab is partially coupled to the overriding lithosphere and has a constant dip angle regardless of the mantle serpentinization thickness. However, if the down-dip extent exceeds the critical depth, the slab and the base of the overriding lithosphere would be separated and decoupled by a thick layer of serpentinized peridotite. This allows further slab bending and results in steeper slab dip. Increasing mantle serpentinization thickness will also result in larger slab dip. We also find that with weak mantle wedge, there is no material flowing from the asthenosphere into the serpentinized mantle wedge. All of these results indicate that serpentinization is an important ingredient when studying the subduction dynamics in the mantle wedge.

Mantle Convection on Modern Supercomputers

Science.gov (United States)

Weismüller, J.; Gmeiner, B.; Huber, M.; John, L.; Mohr, M.; Rüde, U.; Wohlmuth, B.; Bunge, H. P.

2015-12-01

Mantle convection is the cause for plate tectonics, the formation of mountains and oceans, and the main driving mechanism behind earthquakes. The convection process is modeled by a system of partial differential equations describing the conservation of mass, momentum and energy. Characteristic to mantle flow is the vast disparity of length scales from global to microscopic, turning mantle convection simulations into a challenging application for high-performance computing. As system size and technical complexity of the simulations continue to increase, design and implementation of simulation models for next generation large-scale architectures is handled successfully only in an interdisciplinary context. A new priority program - named SPPEXA - by the German Research Foundation (DFG) addresses this issue, and brings together computer scientists, mathematicians and application scientists around grand challenges in HPC. Here we report from the TERRA-NEO project, which is part of the high visibility SPPEXA program, and a joint effort of four research groups. TERRA-NEO develops algorithms for future HPC infrastructures, focusing on high computational efficiency and resilience in next generation mantle convection models. We present software that can resolve the Earth's mantle with up to 1012 grid points and scales efficiently to massively parallel hardware with more than 50,000 processors. We use our simulations to explore the dynamic regime of mantle convection and assess the impact of small scale processes on global mantle flow.

Attenuation of short-period P, PcP, ScP, and pP waves in the earth's mantle

International Nuclear Information System (INIS)

Bock, G.; Clements, J.R.

1982-01-01

The parameter t* (ratio of body wave travel time to the average quality factor Q) was estimated under various assumptions of the nature of the earthquake sources for short-period P, PcP, and ScP phases originating from earthquakes in the Fiji-Tonga region and recorded at the Warramunga Seismic Array at Tennant Creek (Northern Territory, Australia). Spectral ratios were calculated for the amplitudes of PcP to P and of pP to P. The data reveal a laterally varying Q structure in the Fiji-Tonga region. The high-Q lithosphere descending beneath the Tonga Island arc is overlain above 350 km depth by a wedgelike zone of high attenuation with an average Q/sub α/ between 120 and 200 at short periods. The upper mantle farther to the west of the Tonga island arc is less attenuating, with Q/sub α/, between 370 and 560. Q/sub α/ is about 500 in the upper mantle on the oceanic side of the subduction zone. The t* estimates of this study are much smaller than estimates from the free oscillation model SL8. This can be partly explained by regional variations of Q in the upper mantle. If no lateral Q variations occur in the lower mantle, a frequency-dependent Q can make the PcP and ScP observations consistent with model SL8. Adopting the absorption band model to describe the frequency dependence of Q, the parameter tau 2 , the cut-off period of the high-frequency end of the absorption band, was determined. For different source models with finite corner frequencies, the average tau 2 for the mantle is between 0.01 and 0.10 s (corresponding to frequencies between 16 and 1.6 Hz) as derived from the PcP data, and between 0.06 and 0.12 s (2.7 and 1.3 Hz), as derived from the ScP data

Crust-mantle contribution to Andean magmatism

International Nuclear Information System (INIS)

Ruiz, J; Hildreth, W; Chesley, J

2001-01-01

There has long been great interest in quantifying the contributions of the continental crust to continental arc magmas, such as those of the Andes using osmium isotopes (Alves et al., 1999; Borg et al., 2000; Brandon et al., 1996; McInnes et al., 1999). In general, Andean volcanic rocks of all compositions show relatively low Sr-isotope ratios and positive to mildly negative epsilon Nd values. Nonetheless, in the Southern Volcanic Zone of central Chile, basalt-andesite-dacite volcanoes along the Quaternary volcanic front were shown (by Hildreth and Moorbath, 1988) to have latitudinally systematic chemical variations, as well as a monotonic increase in 87Sr/Sr86 from ca. 0.7035 to 0.7055 and a decrease in epsilon Nd values from ca. +3 to -1. The isotopic variations correlate with basement elevation of the volcanic edifices and with Bouguer gravity anomalies, both of which are thought to reflect along-arc variations in thickness and average age of the underlying crust. Volcanoes with the most evolved isotopic signatures were fed through the thickest crust. Correlation of chemical and isotopic variations with crustal thickness was interpreted to be caused by Melting (of deep-crustal host rocks), Assimilation, Storage, and Homogenization (MASH) of mantle-derived magmas in long-lived lower-crustal reservoirs beneath each center prior to eruption. We have now determined Os-isotope ratios for a sample suite from these volcanoes (33-36 S lat.), representing a range of crustal thickness from ca. 60-35 km. The samples range in MgO from ca. 8-4% and in SiO2 from 51-57%. The most evolved eruptive products occur above the thickest crust and have 87Sr/86Sr ratios of 0.7054 and epsilon Nd values of -1.5. The 187Os/188Os ratios correlate with the other isotopic systems and with crustal thickness. Volcanoes on the thinnest crust have 187Os/188Os ratios of 0.18-0.21. Those on the thickest crust have 187Os/188Os ratios as high as 0.64. All the Os values are much too radiogenic to

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)

Mantle dynamics and Cretaceous magmatism in east-central China: Insight from teleseismic tomograms

Science.gov (United States)

Jiang, Guoming; Zhang, Guibin; Zhao, Dapeng; Lü, Qingtian; Li, Hongyi; Li, Xinfu

2015-11-01

Both the rich mineralization in the Lower Yangtze Block (LYB) and the post-collisional mafic rocks in the Dabie Orogen (DBO) are closely related to the Cretaceous magmatism in east-central China. Various geodynamic models have been proposed for explaining the mechanism of the Cretaceous magmatism, but these models are controversial and even contradictory with each other, especially on the mechanism of adakites. A unified geodynamic model is required for explaining the magmatism in east-central China, in particular, the spatial and temporal correlations of magmatic activity in the DBO and that in the LYB. For this purpose, we apply teleseismic tomography to study P-wave velocity structure down to 800 km depth beneath east-central China. A modified multiple-channel cross-correlation method is used to collect 28,805 high-quality P-wave arrival-time data from seismograms of distant earthquakes recorded by permanent seismic stations and our temporary stations in the study region. To remove the influence of crustal heterogeneity on the mantle tomography, we used the CRUST1.0 model to correct the teleseismic relative residuals. Our tomography revealed distinct high-velocity (high-V) anomalies beneath the DBO and two flanks of the LYB, and low-velocity (low-V) anomalies above the high-V zones. Combining our tomographic images with previous geological, geochemical and geophysical results, we infer that these high-V and low-V anomalies reflect the detached lithosphere and upwelling asthenospheric materials, respectively, which are associated with the Late Mesozoic dynamic process and the Cretaceous magmatism. We propose a double-slab subduction model that a ridge subduction yielded the adakitic rocks in the LYB during 150-135 Ma and the subsequent Pacific Plate subduction played a crucial role in not only the formation of igneous rocks in the LYB but also remelting of the subducted South China Block beneath the DBO during 135-101 Ma.

Seismic attenuation in the African LLSVP estimated from PcS phases

Science.gov (United States)

Liu, Chujie; Grand, Stephen P.

2018-05-01

Seismic tomography models have revealed two broad regions in the lowermost mantle marked by ∼3% slower shear velocity than normal beneath the south central Pacific and southern Africa. These two regions are known as large-low-shear-velocity provinces (LLSVP). There is debate over whether the LLSVPs can be explained by purely thermal variations or whether they must be chemically distinct from normal mantle. Elastic properties alone, have been unable to distinguish the thermal from chemical interpretations. Anelastic structure, however, can help discriminate among models of the LLSVPs since intrinsic attenuation is more sensitive to temperature than to chemical variations. Here we estimate Qμ (the shear wave quality factor) in the African LLSVP using PcS waves generated from a Scotia Arc earthquake, recorded by broadband seismometers deployed in Southern Africa during the Kaapvaal experiment. The upward leg of the PcS waves sweeps from normal mantle into the African LLSVP across the array. We use the spectral ratio (SR) and instantaneous frequency matching (IFM) techniques to measure the differential attenuation (Δt*) between waves sampling the African LLSVP and the waves that sample normal lower mantle. Using both methods for estimating Δt* we find that PcS waves sampling the LLSVP are more attenuated than the waves that miss the LLSVP yielding a Δt* difference of more than 1 s. Using the Δt* measurements we estimate the average Qμ in the LLSVP to be about 110. Using a range of activation enthalpy (H*) estimates, we find an average temperature anomaly within the LLSVP ranging from +250 to +800 K. Our estimated temperature anomaly range overlaps previous isochemical geodynamic studies that explain the LLSVP as a purely thermal structure although the large uncertainties cannot rule out chemical variations as well.

North American Crust and Upper Mantle Structure Imaged Using an Adaptive Bayesian Inversion

Science.gov (United States)

Eilon, Z.; Fischer, K. M.; Dalton, C. A.

2017-12-01

We present a methodology for imaging upper mantle structure using a Bayesian approach that incorporates a novel combination of seismic data types and an adaptive parameterization based on piecewise discontinuous splines. Our inversion algorithm lays the groundwork for improved seismic velocity models of the lithosphere and asthenosphere by harnessing increased computing power alongside sophisticated data analysis, with the flexibility to include multiple datatypes with complementary resolution. Our new method has been designed to simultaneously fit P-s and S-p converted phases and Rayleigh wave phase velocities measured from ambient noise (periods 6-40 s) and earthquake sources (periods 30-170s). Careful processing of the body wave data isolates the signals from velocity gradients between the mid-crust and 250 km depth. We jointly invert the body and surface wave data to obtain detailed 1-D velocity models that include robustly imaged mantle discontinuities. Synthetic tests demonstrate that S-p phases are particularly important for resolving mantle structure, while surface waves capture absolute velocities with resolution better than 0.1 km/s. By treating data noise as an unknown parameter, and by generating posterior parameter distributions, model trade offs and uncertainties are fully captured by the inversion. We apply the method to stations across the northwest and north-central United States, finding that the imaged structure improves upon existing models by sharpening the vertical resolution of absolute velocity profiles and offering robust uncertainty estimates. In the tectonically active northwestern US, a strong velocity drop immediately beneath the Moho connotes thin (<70 km) lithosphere and a sharp lithosphere-asthenosphere transition; the asthenospheric velocity profile here matches observations at mid-ocean ridges. Within the Wyoming and Superior cratons, our models reveal mid-lithospheric velocity gradients indicative of thermochemical cratonic

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.

S-to-P Conversions from Mid-mantle Slow Scatterers in Slab Regions: Observations of Deep/Stagnated Oceanic Crust?

Science.gov (United States)

He, Xiaobo; Zheng, Yixian

2018-02-01

The fate of a subducted slab is a key ingredient in the context of plate tectonics, yet it remains enigmatic especially in terms of its crustal component. In this study, our efforts are devoted to resolve slab-related structures in the mid-mantle below eastern Indonesia, the Izu-Bonin region, and the Peru area by employing seismic array analysing techniques on high-frequency waveform data from F-net in Japan and the Alaska regional network and the USArray in North America. A pronounced arrival after the direct P wave is observed in the recordings of four deep earthquakes (depths greater than 400 km) from three subduction systems including the Philippines, the Izu-Bonin, and the Peru. This later arrival displays a slightly lower slowness compared to the direct P wave and its back-azimuth deviates somewhat from the great-circle direction. We explain it as an S-to-P conversion at a deep scatterer below the sources in the source region. In total, five scatterers are seen at depths ranging from 930 to 1500 km. Those scatterers appear to be characterised by an 7 km-thick low-velocity layer compared to the ambient mantle. Combined evidence from published mineral physical analysis suggests that past subducted oceanic crust, possibly fragmented, is most likely responsible for these thin-layer compositional heterogeneities trapped in the mid-mantle beneath the study regions. Our observations give a clue to the potential fate of subducted oceanic crust.

Mantle-cell lymphoma.

Science.gov (United States)

Barista, I; Romaguera, J E; Cabanillas, F

2001-03-01

During the past decade, mantle-cell lymphoma has been established as a new disease entity. The normal counterparts of the cells forming this malignant lymphoma are found in the mantle zone of the lymph node, a thin layer surrounding the germinal follicles. These cells have small to medium-sized nuclei, are commonly indented or cleaved, and stain positively with CD5, CD20, cyclin D1, and FMC7 antibodies. Because of its morphological appearance and a resemblance to other low-grade lymphomas, many of which grow slowly, this lymphoma was initially thought to be an indolent tumour, but its natural course was not thoroughly investigated until the 1990s, when the BCL1 oncogene was identified as a marker for this disease. Mantle-cell lymphoma is a discrete entity, unrelated to small lymphocytic or small-cleaved-cell lymphomas.

Large-scale compositional heterogeneity in the Earth's mantle

Science.gov (United States)

Ballmer, M.

2017-12-01

Seismic imaging of subducted Farallon and Tethys lithosphere in the lower mantle has been taken as evidence for whole-mantle convection, and efficient mantle mixing. However, cosmochemical constraints point to a lower-mantle composition that has a lower Mg/Si compared to upper-mantle pyrolite. Moreover, geochemical signatures of magmatic rocks indicate the long-term persistence of primordial reservoirs somewhere in the mantle. In this presentation, I establish geodynamic mechanisms for sustaining large-scale (primordial) heterogeneity in the Earth's mantle using numerical models. Mantle flow is controlled by rock density and viscosity. Variations in intrinsic rock density, such as due to heterogeneity in basalt or iron content, can induce layering or partial layering in the mantle. Layering can be sustained in the presence of persistent whole mantle convection due to active "unmixing" of heterogeneity in low-viscosity domains, e.g. in the transition zone or near the core-mantle boundary [1]. On the other hand, lateral variations in intrinsic rock viscosity, such as due to heterogeneity in Mg/Si, can strongly affect the mixing timescales of the mantle. In the extreme case, intrinsically strong rocks may remain unmixed through the age of the Earth, and persist as large-scale domains in the mid-mantle due to focusing of deformation along weak conveyor belts [2]. That large-scale lateral heterogeneity and/or layering can persist in the presence of whole-mantle convection can explain the stagnation of some slabs, as well as the deflection of some plumes, in the mid-mantle. These findings indeed motivate new seismic studies for rigorous testing of model predictions. [1] Ballmer, M. D., N. C. Schmerr, T. Nakagawa, and J. Ritsema (2015), Science Advances, doi:10.1126/sciadv.1500815. [2] Ballmer, M. D., C. Houser, J. W. Hernlund, R. Wentzcovitch, and K. Hirose (2017), Nature Geoscience, doi:10.1038/ngeo2898.

High Resolution Seismic Images of Transition Zone Discontinuities beneath the Hawaii-Emperor Seamount Chain

Science.gov (United States)

Cao, Q.; Wang, P.; van der Hilst, R. D.; Shim, S.

2009-12-01

Taking advantage of the abundance of natural sources (earthquakes) in western Pacific subduction zones and the many seismograph stations in the Americas, we use inverse scattering - a generalized Radon transform - of SS precursors to image the transition zone discontinuities underneath Hawaii and the Hawaii-Emperor seamount chain. The GRT makes use of scattering theory and extracts structural information from broad band data windows that include precursors to SS (which are the specular reflections at the discontinuities that form the main arrivals) as well as non-specular scattered energy (which is often discarded as noise). More than 150,000 seismograms (from the IRIS Data Management Center) are used to form a 3-D image of the transition zone discontinuities beneath the central Pacific. In addition to clear signals near 410, 520, and 660 km depth, the data also reveal scatter interfaces near 370 km dept and between 800-1000 km depth, which may be regional, laterally intermittent scatter horizons. Our images reveal a conspicuous uplift of the 660 discontinuity in a region of 800km in diameter to the west of the active volcanoes of Hawaii. No correspondent localized depression of the 410 discontinuity is found. Instead, we find a smaller scale anomaly suggesting that the 410 discontinuity is locally elevated in the same region. This may indicate the presence of melt or minor chemical constitutes. The lack of correlation between and differences in lateral length scale of the topographies of the 410 and 660 km discontinuities are also consistent with a deep-mantle plume impinging on the transition zone, creating a pond of hot material underneath 660 discontinuity, and with secondary plumes connecting to the present-day hotspot at Earth’s surface. Our observations suggest that more complicated plume morphology and plume dynamics within the Earth's mantle should be taken into account to describe the plumes and, in particular, mass transport across the transition zone

A >100 Ma Mantle Geochemical Record: Retiring Mantle Plumes may be Premature

Science.gov (United States)

Konter, J. G.; Hanan, B. B.; Blichert-Toft, J.; Koppers, A. A.; Plank, T.; Staudigel, H.

2006-12-01

Hotspot volcanism has long been attributed to mantle plumes, but in recent years suggestions have been made that plate tectonic processes, such as extension, can account for all hotspot tracks. This explanation involves a profoundly less dynamic lower mantle, which justifies a critical evaluation before the plume model is dismissed. Such an evaluation has to involve a wide range of geochemical, geological, and geophysical techniques, broadly investigating the products of volcanism as well as the underlying lithosphere and mantle. We argue here that the combined geological record and geochemistry of intraplate volcanoes holds some important clues that help us decide between models of plume-like upwelling versus passive upwelling with lithospheric extension. The best of these integrated datasets can be obtained from the long seamount chains in the Pacific Ocean. A new combined dataset of trace element and isotopic compositions, along with modern 40Ar/39Ar ages from seamounts in the Gilbert Ridge, Tokelau chain, and West Pacific Seamount Province (WPSP) provides a record of current to Cretaceous volcanism in the South Pacific. We have reconstructed the eruptive locations of the seamounts using a range of absolute plate motion models, including some models with hotspot motion and others that use the Indo-Atlantic hotspot reference frame. Our results show that the backtracked locations consistently form clusters (300km radius) around the active ends of the Macdonald, Rurutu and Rarotonga hotspot chains, while closely matching their distinct C-HIMU and C-EM1 signatures. The oldest WPSP seamounts (older than 100 Ma) form the only exception and backtrack, with larger uncertainty, to north of Rarotonga. Therefore, the mantle currently underlying the Cook-Austral islands has produced volcanoes in three geochemically distinct areas for at least 100 m.y. Furthermore, we find the shortest mantle residence time, 0.6 Ga, for a source of mixed recycled DMM and an EM1-like

Petrology and deformation style of lithospheric mantle beneath the Heldburg Dike swarm (Central Germany) subset of Central European Volcanic Province

Science.gov (United States)

Kukuła, Anna; Puziewicz, Jacek; Hidas, Károly; Ntaflos, Theodoros; Matusiak-Małek, Magdalena; Milke, Ralf

2017-04-01

-30 % of melting of primitive mantle, which was overprinted by silicate and/or carbonatite metasomatism. The xenolith 3140 seems not to be affected by metasomatic overprint. Based on the EBSD analyses of 15 xenoliths, olivine grains are characterized by relatively strong CPO (crystal preferred orientation) with J indices 4.4 - 13.3, and they have orthorhombic (8 xenoliths) or [100]-fiber CPO (6 xenoliths) symmetries except for one [010]-fiber symmetry observed in group B (Tommasi et al., 1999). Pyroxenes have weaker CPO and the distribution of their crystallographic axes is inconsistent with their coeval deformation with olivine. We propose that their CPO postdates that of olivine, hence strongly support a later origin for pyroxenes. Funding. This study was possible thanks to the project NCN UMO-2014/15/B/ST10/00095 of Polish National Centre for Science to JP Tommasi, A., B. Tikoff, and A. Vauchez (1999). Upper mantle tectonics: three-dimensional deformation, olivine crystallographic fabrics and seismic properties, Earth Planet Sc Lett,168, 173-186. Upton, B.G.J., Downes, H., Kirstein, L.A., Bonadiman, C., Hill, P.G., Ntaflos, T. (2011). The lithospheric mantle and lower crust-mantle relationships under Scotland: a xenolithic perspective. J Geol Soc, 168, 873-886.

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

The origin of high-Mg magmas in Mt Shasta and Medicine Lake volcanoes, Cascade Arc (California): higher and lower than mantle oxygen isotope signatures attributed to current and past subduction

Science.gov (United States)

Martin, E.; Bindeman, I.; Grove, T. L.

2011-11-01

We report the oxygen isotope composition of olivine and orthopyroxene phenocrysts in lavas from the main magma types at Mt Shasta and Medicine Lake Volcanoes: primitive high-alumina olivine tholeiite (HAOT), basaltic andesites (BA), primitive magnesian andesites (PMA), and dacites. The most primitive HAOT (MgO > 9 wt%) from Mt. Shasta has olivine δ18O (δ18OOl) values of 5.9-6.1‰, which are about 1‰ higher than those observed in olivine from normal mantle-derived magmas. In contrast, HAOT lavas from Medicine Lake have δ18OOl values ranging from 4.7 to 5.5‰, which are similar to or lower than values for olivine in equilibrium with mantle-derived magmas. Other magma types from both volcanoes show intermediate δ18OOl values. The oxygen isotope composition of the most magnesian lavas cannot be explained by crustal contamination and the trace element composition of olivine phenocrysts precludes a pyroxenitic mantle source. Therefore, the high and variable δ18OOl signature of the most magnesian samples studied (HAOT and BA) comes from the peridotitic mantle wedge itself. As HAOT magma is generated by anhydrous adiabatic partial melting of the shallow mantle, its 1.4‰ range in δ18OOl reflects a heterogeneous composition of the shallow mantle source that has been influenced by subduction fluids and/or melts sometime in the past. Magmas generated in the mantle wedge by flux melting due to modern subduction fluids, as exemplified by BA and probably PMA, display more homogeneous composition with only 0.5‰ variation. The high-δ18O values observed in magnesian lavas, and principally in the HAOT, are difficult to explain by a single-stage flux-melting process in the mantle wedge above the modern subduction zone and require a mantle source enriched in 18O. It is here explained by flow of older, pre-enriched portions of the mantle through the slab window beneath the South Cascades.

Deformation, static recrystallization, and reactive melt transport in shallow subcontinental mantle xenoliths (Tok Cenozoic volcanic field, SE Siberia)

Science.gov (United States)

Tommasi, Andréa; Vauchez, Alain; Ionov, Dmitri A.

2008-07-01

Partial melting and reactive melt transport may change the composition, microstructures, and physical properties of mantle rocks. Here we explore the relations between deformation and reactive melt transport through detailed microstructural analysis and crystallographic orientation measurements in spinel peridotite xenoliths that sample the shallow lithospheric mantle beneath the southeastern rim of the Siberian craton. These xenoliths have coarse-grained, annealed microstructures and show petrographic and chemical evidence for variable degrees of reaction with silicate melts and fluids, notably Fe-enrichment and crystallization of metasomatic clinopyroxene (cpx). Olivine crystal preferred orientations (CPO) range from strong to weak. [010]-fiber patterns, characterized by a point concentration of [010] normal to the foliation and by dispersion of [100] in the foliation plane with a weak maximum parallel to the lineation, predominate relative to the [100]-fiber patterns usually observed in lithospheric mantle xenoliths and peridotite massifs. Variations in olivine CPO patterns or intensity are not correlated with modal and chemical compositions. This, together with the analysis of microstructures, suggests that reactive melt percolation postdated both deformation and static recrystallization. Preferential crystallization of metasomatic cpx along (010) olivine grain boundaries points to an influence of the preexisting deformation fabrics on melt transport, with higher permeability along the foliation. Similarity between orthopyroxene (opx) and cpx CPO suggests that cpx orientations may be inherited from those of opx during melt-rock reaction. As observed in previous studies, reactive melt transport does not weaken olivine CPO and seismic anisotropy in the upper mantle, except in melt accumulation domains. In contrast, recovery and selective grain growth during static recrystallization may lead to development of [010]-fiber olivine CPO and, if foliations are

Asymmetric three-dimensional topography over mantle plumes.

Science.gov (United States)

Burov, Evgueni; Gerya, Taras

2014-09-04

The role of mantle-lithosphere interactions in shaping surface topography has long been debated. In general, it is supposed that mantle plumes and vertical mantle flows result in axisymmetric, long-wavelength topography, which strongly differs from the generally asymmetric short-wavelength topography created by intraplate tectonic forces. However, identification of mantle-induced topography is difficult, especially in the continents. It can be argued therefore that complex brittle-ductile rheology and stratification of the continental lithosphere result in short-wavelength modulation and localization of deformation induced by mantle flow. This deformation should also be affected by far-field stresses and, hence, interplay with the 'tectonic' topography (for example, in the 'active/passive' rifting scenario). Testing these ideas requires fully coupled three-dimensional numerical modelling of mantle-lithosphere interactions, which so far has not been possible owing to the conceptual and technical limitations of earlier approaches. Here we present new, ultra-high-resolution, three-dimensional numerical experiments on topography over mantle plumes, incorporating a weakly pre-stressed (ultra-slow spreading), rheologically realistic lithosphere. The results show complex surface evolution, which is very different from the smooth, radially symmetric patterns usually assumed as the canonical surface signature of mantle upwellings. In particular, the topography exhibits strongly asymmetric, small-scale, three-dimensional features, which include narrow and wide rifts, flexural flank uplifts and fault structures. This suggests a dominant role for continental rheological structure and intra-plate stresses in controlling dynamic topography, mantle-lithosphere interactions, and continental break-up processes above mantle plumes.

Water in Earth's mantle: Hydrogen analysis of mantle olivine, pyroxenes and garnet using the SIMS

Science.gov (United States)

Kurosawa, Masanori; Yurimoto, Hisayoshi; Sueno, Shigeho

1993-01-01

Hydrogen (or water) in the Earth's interior plays a key role in the evolution and dynamics of the planet. However, the abundance and the existence form of the hydrogen have scarcely been clear in practice. Hydrogen in the mantle was incorporated in the interior during the formation of the Earth. The incorporated hydrogen was hardly possible to concentrate locally inside the Earth considering its high mobility and high reactivity. The hydrogen, preferably, could be distributed homogeneously over the mantle and the core by the subsequent physical and chemical processes. Therefore, hydrogen in the mantle could be present in the form of trace hydrogen in nominally anhydrous mantle minerals. The hydrogen and the other trace elements in mantle olivines, orthopyroxenes, clinopyroxenes, and garnets were determined using secondary ion mass spectrometry (SIMS) for elucidating (1) the exact hydrogen contents, (2) the correlation between the hydrogen and the other trace elements, (3) the dependence of the hydrogen contents on the depth, and (4) the dependence of the whole rock water contents on the depth.

Thermal Stratification in Vertical Mantle Tanks

DEFF Research Database (Denmark)

Knudsen, Søren; Furbo, Simon

2001-01-01

It is well known that it is important to have a high degree of thermal stratification in the hot water storage tank to achieve a high thermal performance of SDHW systems. This study is concentrated on thermal stratification in vertical mantle tanks. Experiments based on typical operation conditions...... are carried out to investigate how the thermal stratification is affected by different placements of the mantle inlet. The heat transfer between the solar collector fluid in the mantle and the domestic water in the inner tank is analysed by CFD-simulations. Furthermore, the flow pattern in the vertical mantle...

Gondwana subduction-modified mantle domain prevents magmatic seafloor generation in the Central Indian Ridge

Science.gov (United States)

Morishita, T.; Nakamura, K.; Senda, R.; Suzuki, K.; Kumagai, H.; Sato, H.; Sato, T.; Shibuya, T.; Minoguchi, K.; Okino, K.

2013-12-01

The creation of oceanic crust at mid-ocean ridges is essential to understanding the genesis of oceanic plate and the evolution of the Earth. Detailed bathymetric measurements coupled with dense sample recovery at mid-ocean ridge revealed a wide range of variations in the ridge and seafloor morphologies, which cannot be simply explained by a spreading rate, but also by ridge geometry, mantle compositions and thermal structure (Dick et al., 2003 Nature; Cannat et al. 2006 Geology). It is now widely accepted that very limited magmatic activity with tectonic stretching generates oceanic core complex and/or smooth seafloor surface in the slow to ultraslow-spreading ridges, where serpentinized peridotite and gabbros are expected to be exposed associated with detachment faults (Cann et al., 1997 Nature; Cannat et al., 2006), although magmatism might be an essential role for the formation of oceanic core complexes (Buck et al., 2005 Nature; Tucholke et al 2008 JGR). A rising question is why magmatic activity is sometimes prevented during the oceanic plate formation. Ancient melting domain, that are too refractory to melt even in adiabatically upwelling to the shallow upper mantle, might cause the amagmatic spreading ridges (Harvey et al., 2006 EPSL, Liu et al.,2008 Nature). Its origin and effect on seafloor generations are, however, not well understood yet. We report an oceanic hill as an example of an ancient subduction-modified mantle domain, probably formed at continental margin of the Gondwanaland~Pangea supercontinent, existing beneath the Central Indian Ridge. This domain is the most likely to have prevented magmatic seafloor generation, resulting in creation of very deep oceanic valley and serpentine diaper (now the studied oceanic hill) at the present Central Indian ridge.

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

3-D structure of the crust and uppermost mantle at the junction between the Southeastern Alps and External Dinarides from ambient noise tomography

Science.gov (United States)

Guidarelli, Mariangela; Aoudia, Abdelkrim; Costa, Giovanni

2017-12-01

We use ambient noise tomography to investigate the crust and the uppermost mantle structure beneath the junction between the Southern Alps, the Dinarides and the Po Plain. We obtained Rayleigh wave empirical Green's functions from cross-correlation of vertical component seismic recordings for three years (2010-2012) using stations from networks in Italy, Slovenia, Austria, Croatia, Serbia and Switzerland. We measure group and phase velocity dispersion curves from the reconstructed Rayleigh waves in the period range 5-30 and 8-37 s, respectively, and we invert the surface wave velocities for tomographic images on a grid of 0.1° × 0.1°. After the tomography, the group velocities are then inverted to compute the 3-D shear wave velocity model of the crust and the upper mantle beneath the region. Our shear wave velocity model provides the 3-D image of the structure in the region between Northeastern Italy, Slovenia and Austria. The velocity variations at shallow depths correlate with known geological and tectonic domains. We find low velocities below the Po Plain, the northern tip of the Adriatic and the Pannonian Basin, whereas higher velocities characterize the Alpine chain. The vertical cross-sections reveal a clear northward increase of the crustal thickness with a sharp northward dipping of the Moho that coincides at the surface with the leading edge of the Alpine thrust front adjacent to the Friuli Plain in Northeastern Italy. This geometry of the Moho mimics fairly well the shallow north dipping geometry of the decollement inferred from permanent GPS velocity field where high interseismic coupling is reported. From the northern Adriatic domain up to the Idrija right lateral strike-slip fault system beneath Western Slovenia, the crustal thickness is more uniform. Right across Idrija fault, to the northeast, and along its strike, we report a clear change of the physical properties of the crust up to the uppermost mantle as reflected by the lateral distribution

Large-scale retreat and advance of shallow seas in Southeast Asia driven by mantle flow

Science.gov (United States)

Zahirovic, Sabin; Flament, Nicolas; Dietmar Müller, R.; Seton, Maria; Gurnis, Michael

2016-04-01

The Indonesian islands and surrounding region represent one of the most submerged, low-lying continental areas on Earth. Almost half of this region, known as Sundaland, is presently inundated by a shallow sea. The role of mantle convection in driving long-wavelength topography and vertical motion of the lithosphere in this region has largely been ignored when interpreting regional stratigraphic sections, despite a consensus that Southeast Asia presently situated on a "dynamic topography low" resulting from long-term post-Pangea subduction. However, dynamic topography is typically described as a temporally and spatially transient process, implying that Sundaland may have experienced significant vertical motions in the geological past, and thus must be considered when interpreting relative sea level changes and the paleogeographic indicators of advancing and retreating shallow seas. Although the present-day low regional elevation has been attributed to the massive volume of oceanic slabs sinking in the mantle beneath Southeast Asia, a Late Cretaceous to Eocene regional unconformity indicates that shallow seas retreated following regional flooding during the mid-Cretaceous sea level highstand. During the Eocene, less than one fifth of Sundaland was submerged, despite global sea level being ~200 m higher than at present. The regional nature of the switch from marine to terrestrial environments, that is out-of-sync with eustatic sea levels, suggests that broad mantle-driven dynamic uplift may have led to the emergence of Sundaland in the Late Cretaceous and Paleocene. We use numerical forward modelling of plate tectonics and mantle convection, and compare the predicted trends of dynamic topography with evidence from regional paleogeography and eustasy to determine the extent to which mantle-driven vertical motions of the lithosphere have influenced regional basin histories in Southeast Asia. A Late Cretaceous collision of Gondwana-derived terranes with Sundaland choked

Delamination of lithospheric mantle evidenced by Cenozoic potassic rocks in Yunnan, SW China: A contribution to uplift of the Eastern Tibetan Plateau

Science.gov (United States)

Chen, Bei; Long, Xiaoping; Wilde, Simon A.; Yuan, Chao; Wang, Qiang; Xia, Xiaoping; Zhang, Zhaofeng

2017-07-01

) suggest that they were generated by partial melting of an ancient K-rich crust. The petrogenesis of the potassic intrusive rocks with their uniform Nd model ages (TDM = 968-1168 Ma) indicates that the lithospheric mantle beneath Yunnan was enriched by metasomatism related to ancient oceanic subduction. Considering the widespread and coeval nature of potassium-rich rocks in Eastern Tibet, we consider that delamination of metasomatized lithospheric mantle beneath Yunnan in the Eocene was a result of the India-Asia collision, which triggered partial melting of both the lithospheric mantle and the K-rich crust, and contributes to uplift of the Eastern Tibetan Plateau.

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.

Oxygen isotope geochemistry of the lassen volcanic center, California: Resolving crustal and mantle contributions to continental Arc magmatism

Science.gov (United States)

Feeley, T.C.; Clynne, M.A.; Winer, G.S.; Grice, W.C.

2008-01-01

center of the LVC. Conversely, the ??18O values of the rhyodacitic rocks suggest that the continental crust in the melt generation zones beneath the LVC has been substantially modified by intrusion of mantle-derived basaltic magmas, with the degree of hybridization ranging on a molar oxygen basis from approximately 60% at distances up to 12 km from the center of the system to 97% directly beneath the focus region. These results demonstrate on a relatively small scale the strong influence that intrusion of mantle-derived mafic magmas can have on modifying the composition of pre-existing continental crust in regions of melt production. Given this result, similar, but larger-scale, regional trends in magma compositions may reflect an analogous but more extensive process wherein the continental crust becomes progressively hybridized beneath frontal arc localities as a result of protracted intrusion of subduction-related basaltic magmas. ?? The Author 2008. Published by Oxford University Press. All rights reserved.

The role of plumes in mantle helium fluxes

International Nuclear Information System (INIS)

Kellogg, L.H.; Wasserburg, G.J.

1990-01-01

We present a simple model of 3 He and 4 He transport in the mantle using the appropriate rates of mass and species transfer and 4 He production. Previous workers have shown the presence of excess 3 He in hotspots such as Hawaii and Iceland and inferred that these hotspots tap a source with a higher 3 He/ 4 He ratio than the source region of mid-ocean ridge basalts (MORB). Hotspot ocean islands probably originate over upwelling plumes which carry material from the lower mantle to the upper mantle. Melting at hotspots and at mid-ocean ridges degasses the mantle of volatiles such as helium. The upper mantle is outgassed largely of helium due to melting at mid-ocean ridges and hotspots. We postulate that the excess 3 He seen in MORB originates in material that was carried from the lower mantle in plumes but not completely outgassed at hotspots. This helium is incoporated into the depleted upper mantle. Assuming that the upper mantle is in a quasi-steady-state with respect to helium, a simple model balancing 3 He and 4 He fluxes in the upper mantle indicates that the hotspots significantly outgas the lower mantle of 3 He. The concentration of 4 He in the plume source reservoir is 2-3 orders of magnitude lower than the concentration in carbonaceous chondrites. The residence time of helium in the upper mantle depends on the outgassing efficiency at hotspots, since the hotspots may outgas some upper mantle material which has been entrained in the plumes. The residence time of He in the upper mantle is about 1.4x10 9 yr. We conclude that the efficiency of outgassing of He from plumes is high and that the plumes dominate the present 3 He loss to the atmosphere. The 4 He in the less depleted layer of the mantle is not trapped ''primordial'' but is predominantly from in situ decay of U and Th in the depleted layer over ≅ 1.4x10 9 yr. The 4 He in the lower mantle is dominantly from in situ decay of U and Th over 4.4x10 9 yr. (orig./WL)

Nd-isotopes in selected mantle-derived rocks and minerals and their implications for mantle evolution

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Basu, A.R.; Tatsumoto, M.

1980-01-01

The Sm-Nd systematics in a variety of mantle-derived samples including kimberlites, alnoite, carbonatite, pyroxene and amphibole inclusions in alkali basalts and xenolithic eclogites, granulites and a pyroxene megacryst in kimberlites are reported. The additional data on kimberlites strengthen our earlier conclusion that kimberlites are derived from a relatively undifferentiated chondritic mantle source. This conclusion is based on the observation that the e{open}Nd values of most of the kimberlites are near zero. In contrast with the kimberlites, their garnet lherzolite inclusions show both time-averaged Nd enrichment and depletion with respect to Sm. Separated clinopyroxenes in eclogite xenoliths from the Roberts Victor kimberlite pipe show both positive and negative e{open}Nd values suggesting different genetic history. A whole rock lower crustal scapolite granulite xenolith from the Matsoku kimberlite pipe shows a negative e{open}Nd value of -4.2, possibly representative of the base of the crust in Lesotho. It appears that all inclusions, mafic and ultramafic, in kimberlites are unrelated to their kimberlite host. The above data and additional Sm-Nd data on xenoliths in alkali basalts, alpine peridotite and alnoite-carbonatites are used to construct a model for the upper 200 km of the earth's mantle - both oceanic and continental. The essential feature of this model is the increasing degree of fertility of the mantle with depth. The kimberlite's source at depths below 200 km in the subcontinental mantle is the most primitive in this model, and this primitive layer is also extended to the suboceanic mantle. However, it is clear from the Nd-isotopic data in the xenoliths of the continental kimberlites that above 200 km the continental mantle is distinctly different from their suboceanic counterpart. ?? 1980 Springer-Verlag.

Seismic velocity structure of the crust and upper mantle beneath the Texas-Gulf of Mexico margin from joint inversion of Ps and Sp receiver functions and surface wave dispersion

Science.gov (United States)

Agrawal, M.; Pulliam, J.; Sen, M. K.

2013-12-01

The seismic structure beneath Texas Gulf Coast Plain (GCP) is determined via velocity analysis of stacked common conversion point (CCP) Ps and Sp receiver functions and surface wave dispersion. The GCP is a portion of a ocean-continental transition zone, or 'passive margin', where seismic imaging of lithospheric Earth structure via passive seismic techniques has been rare. Seismic data from a temporary array of 22 broadband stations, spaced 16-20 km apart, on a ~380-km-long profile from Matagorda Island, a barrier island in the Gulf of Mexico, to Johnson City, Texas were employed to construct a coherent image of the crust and uppermost mantle. CCP stacking was applied to data from teleseismic earthquakes to enhance the signal-to-noise ratios of converted phases, such as Ps phases. An inaccurate velocity model, used for time-to-depth conversion in CCP stacking, may produce higher errors, especially in a region of substantial lateral velocity variations. An accurate velocity model is therefore essential to constructing high quality depth-domain images. To find accurate velocity P- and S-wave models, we applied a joint modeling approach that searches for best-fitting models via simulated annealing. This joint inversion approach, which we call 'multi objective optimization in seismology' (MOOS), simultaneously models Ps receiver functions, Sp receiver functions and group velocity surface wave dispersion curves after assigning relative weights for each objective function. Weights are computed from the standard deviations of the data. Statistical tools such as the posterior parameter correlation matrix and posterior probability density (PPD) function are used to evaluate the constraints that each data type places on model parameters. They allow us to identify portions of the model that are well or poorly constrained.

Highly CO2-supersaturated melts in the Pannonian lithospheric mantle - A transient carbon reservoir?

Science.gov (United States)

Créon, Laura; Rouchon, Virgile; Youssef, Souhail; Rosenberg, Elisabeth; Delpech, Guillaume; Szabó, Csaba; Remusat, Laurent; Mostefaoui, Smail; Asimow, Paul D.; Antoshechkina, Paula M.; Ghiorso, Mark S.; Boller, Elodie; Guyot, François

2017-08-01

Subduction of carbonated crust is widely believed to generate a flux of carbon into the base of the continental lithospheric mantle, which in turn is the likely source of widespread volcanic and non-volcanic CO2 degassing in active tectonic intracontinental settings such as rifts, continental margin arcs and back-arc domains. However, the magnitude of the carbon flux through the lithosphere and the budget of stored carbon held within the lithospheric reservoir are both poorly known. We provide new constraints on the CO2 budget of the lithospheric mantle below the Pannonian Basin (Central Europe) through the study of a suite of xenoliths from the Bakony-Balaton Highland Volcanic Field. Trails of secondary fluid inclusions, silicate melt inclusions, networks of melt veins, and melt pockets with large and abundant vesicles provide numerous lines of evidence that mantle metasomatism affected the lithosphere beneath this region. We obtain a quantitative estimate of the CO2 budget of the mantle below the Pannonian Basin using a combination of innovative analytical and modeling approaches: (1) synchrotron X-ray microtomography, (2) NanoSIMS, Raman spectroscopy and microthermometry, and (3) thermodynamic models (Rhyolite-MELTS). The three-dimensional volumes reconstructed from synchrotron X-ray microtomography allow us to quantify the proportions of all petrographic phases in the samples and to visualize their textural relationships. The concentration of CO2 in glass veins and pockets ranges from 0.27 to 0.96 wt.%, higher than in typical arc magmas (0-0.25 wt.% CO2), whereas the H2O concentration ranges from 0.54 to 4.25 wt.%, on the low end for estimated primitive arc magmas (1.9-6.3 wt.% H2O). Trapping pressures for vesicles were determined by comparing CO2 concentrations in glass to CO2 saturation as a function of pressure in silicate melts, suggesting pressures between 0.69 to 1.78 GPa. These values are generally higher than trapping pressures for fluid inclusions

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

Subduction and volcanism in the Iberia-North Africa collision zone from tomographic images of the upper mantle

Science.gov (United States)

Villaseñor, Antonio; Chevrot, Sébastien; Harnafi, Mimoun; Gallart, Josep; Pazos, Antonio; Serrano, Inmaculada; Córdoba, Diego; Pulgar, Javier A.; Ibarra, Pedro

2015-11-01

New tomographic images of the upper mantle beneath the westernmost Mediterranean suggest that the evolution of the region experienced two subduction-related episodes. First subduction of oceanic and/or extended continental lithosphere, now located mainly beneath the Betics at depths greater than 400 km, took place on a NW-SE oriented subduction zone. This was followed by a slab-tear process that initiated in the east and propagated to the west, leading to westward slab rollback and possibly lower crustal delamination. The current position of the slab tear is located approximately at 4°W, and to the west of this location the subducted lithosphere is still attached to the surface along the Gibraltar Arc. Our new P-wave velocity model is able to image the attached subducted lithosphere as a narrow high-velocity body extending to shallow depths, coinciding with the region of maximum curvature of the Gibraltar Arc, the occurrence of intermediate-depth earthquakes, and anomalously thick crust. This thick crust has a large influence in the measured teleseismic travel time residuals and therefore in the obtained P-wave tomographic model. We show that removing the effects of the thick crust significantly improves the shallow images of the slab and therefore the interpretations based on the seismic structure.

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

Traveltime dispersion in an isotropic elastic mantle: strong lower-mantle signal in differential-frequency residuals

Science.gov (United States)

Schuberth, Bernhard S. A.; Zaroli, Christophe; Nolet, Guust

2015-12-01

We study wavefield effects of direct P- and S-waves in elastic and isotropic 3-D seismic structures derived from the temperature field of a high-resolution mantle circulation model. More specifically, we quantify the dispersion of traveltime residuals caused by diffraction in structures with dynamically constrained length scales and magnitudes of the lateral variations in seismic velocities and density. 3-D global wave propagation is simulated using a spectral element method. Intrinsic attenuation (i.e. dissipation of seismic energy) is deliberately neglected, so that any variation of traveltimes with frequency can be attributed to structural effects. Traveltime residuals are measured at 15, 22.5, 34 and 51 s dominant periods by cross-correlation of 3-D and 1-D synthetic waveforms. Additional simulations are performed for a model in which 3-D structure is removed in the upper 800 km to isolate the dispersion signal of the lower mantle. We find that the structural length scales inherent to a vigorously convecting mantle give rise to significant diffraction-induced body-wave traveltime dispersion. For both P- and S-waves, the difference between long-period and short-period residuals for a given source-receiver pair can reach up to several seconds for the period bands considered here. In general, these `differential-frequency' residuals tend to increase in magnitude with increasing short-period delay. Furthermore, the long-period signal typically is smaller in magnitude than the short-period one; that is, wave-front healing is efficient independent of the sign of the residuals. Unlike the single-frequency residuals, the differential-frequency residuals are surprisingly similar between the `lower-mantle' and the `whole-mantle' model for corresponding source-receiver pairs. The similarity is more pronounced in case of S-waves and varies between different combinations of period bands. The traveltime delay acquired in the upper mantle seems to cancel in these differential

Mantle flow influence on subduction evolution

Science.gov (United States)

Chertova, Maria V.; Spakman, Wim; Steinberger, Bernhard

2018-05-01

The impact of remotely forced mantle flow on regional subduction evolution is largely unexplored. Here we investigate this by means of 3D thermo-mechanical numerical modeling using a regional modeling domain. We start with simplified models consisting of a 600 km (or 1400 km) wide subducting plate surrounded by other plates. Mantle inflow of ∼3 cm/yr is prescribed during 25 Myr of slab evolution on a subset of the domain boundaries while the other side boundaries are open. Our experiments show that the influence of imposed mantle flow on subduction evolution is the least for trench-perpendicular mantle inflow from either the back or front of the slab leading to 10-50 km changes in slab morphology and trench position while no strong slab dip changes were observed, as compared to a reference model with no imposed mantle inflow. In experiments with trench-oblique mantle inflow we notice larger effects of slab bending and slab translation of the order of 100-200 km. Lastly, we investigate how subduction in the western Mediterranean region is influenced by remotely excited mantle flow that is computed by back-advection of a temperature and density model scaled from a global seismic tomography model. After 35 Myr of subduction evolution we find 10-50 km changes in slab position and slab morphology and a slight change in overall slab tilt. Our study shows that remotely forced mantle flow leads to secondary effects on slab evolution as compared to slab buoyancy and plate motion. Still these secondary effects occur on scales, 10-50 km, typical for the large-scale deformation of the overlying crust and thus may still be of large importance for understanding geological evolution.

Survival of the primitive mantle reservoir?

Science.gov (United States)

Huang, S.; Jacobsen, S. B.; Mukhopadhyay, S.

2010-12-01

The high-3He lavas are thought to originate from a deep primitive mantle source that has not been much modified since the formation of Earth’s core. Comparison of 4He/3He in MORBs and plume lavas indicate that the plume sources must be a lower mantle feature, in agreement with most geophysical inferences. However, the lithophile element isotope systems of plume lavas are not primitive. The idea that the high-3He source is significantly less processed and more primitive than MORB source is clearly supported by mixing trends in plots of 4He/3He versus Sr, Nd and Pb isotope ratios, which have been extrapolated to an inferred 4He/3He of ~17,000 (~43x the atmospheric ratio), a mantle reservoir named PHEM (Primitive HElium Mantle). Slightly lower 4He/3He, ~15,000, were reported for Baffin Island picrites. Recently, Jackson et al. (2010) claimed that some Baffin Island and Greenland picrites with single-stage Pb model ages of ~4.5 Ga have low 4He/3He, and argued that “their source is the most ancient accessible reservoir in the Earth’s mantle, and it may be parental to all mantle reservoirs”. However, the available data are insufficient to make such a claim, and we suggest an alternative interpretation. Specially: 1. Four out of ten Baffin Island and Greenland picrites used by Jackson et al. (2010) have 4He/3He higher than average MORB value and all are far removed from the lowest measured value of 15,000. 2. Five Greenland picrites which cluster around the 4.50 Gyr geochron (Jackson et al., 2010) form a curved 207Pb*/206Pb*-4He/3He trend. This trend is best explained as a mixing line, implying that the single-stage Pb ages of these lavas are meaningless. 3. In a 207Pb*/206Pb*-4He/3He plot, Koolau lavas from Hawaii overlap with Baffin Island and Greenland picrites. If Baffin Island and Greenland picrites represent melts from the primitive mantle based on their Pb and He isotopes (Jackson et al., 2010), a similar argument can be applied to Koolau lavas. However, it

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.

Stagnant lids and mantle overturns: Implications for Archaean tectonics, magmagenesis, crustal growth, mantle evolution, and the start of plate tectonics

Directory of Open Access Journals (Sweden)

Jean H. Bédard

2018-01-01

Full Text Available The lower plate is the dominant agent in modern convergent margins characterized by active subduction, as negatively buoyant oceanic lithosphere sinks into the asthenosphere under its own weight. This is a strong plate-driving force because the slab-pull force is transmitted through the stiff sub-oceanic lithospheric mantle. As geological and geochemical data seem inconsistent with the existence of modern-style ridges and arcs in the Archaean, a periodically-destabilized stagnant-lid crust system is proposed instead. Stagnant-lid intervals may correspond to periods of layered mantle convection where efficient cooling was restricted to the upper mantle, perturbing Earth's heat generation/loss balance, eventually triggering mantle overturns. Archaean basalts were derived from fertile mantle in overturn upwelling zones (OUZOs, which were larger and longer-lived than post-Archaean plumes. Early cratons/continents probably formed above OUZOs as large volumes of basalt and komatiite were delivered for protracted periods, allowing basal crustal cannibalism, garnetiferous crustal restite delamination, and coupled development of continental crust and sub-continental lithospheric mantle. Periodic mixing and rehomogenization during overturns retarded development of isotopically depleted MORB (mid-ocean ridge basalt mantle. Only after the start of true subduction did sequestration of subducted slabs at the core-mantle boundary lead to the development of the depleted MORB mantle source. During Archaean mantle overturns, pre-existing continents located above OUZOs would be strongly reworked; whereas OUZO-distal continents would drift in response to mantle currents. The leading edge of drifting Archaean continents would be convergent margins characterized by terrane accretion, imbrication, subcretion and anatexis of unsubductable oceanic lithosphere. As Earth cooled and the background oceanic lithosphere became denser and stiffer, there would be an increasing

Mantle strength of the San Andreas fault system and the role of mantle-crust feedbacks

NARCIS (Netherlands)

Chatzaras, V.; Tikoff, B.; Newman, J.; Withers, A.C.; Drury, M.R.

2015-01-01

In lithospheric-scale strike-slip fault zones, upper crustal strength is well constrained from borehole observations and fault rock deformation experiments, but mantle strength is less well known. Using peridotite xenoliths, we show that the upper mantle below the San Andreas fault system

The mantle-plume model, its feasibility and consequences

NARCIS (Netherlands)

Calsteren, van P.W.C.

1981-01-01

High beat-flow foci on the Earth have been named ‘hot-spots’ and are commonly correlated with ‘mantle-plumes’ in the deep. A mantle plume may be described as a portion of mantle material with a higher heat content than its surroundings. The intrusion of a mantle-plume is inferred to be similar to

Crustal Structure and Mantle Transition Zone Thickness beneath the Central Mongolia from Teleseismic Receiver Functions

Science.gov (United States)

He, J.; Wu, Q.; Gao, M.; Munkhuu, U.; Demberel, S. G.

2013-12-01

The Mongolian Plateau (northern Asia) is situated between the Gobi-Altai range and the Siberian craton. In order to understand the crustal and mantle structure environmental characteristics, we use the teleseismic data recorded by 69 broadband stations located in the Central Mongolia(103.5°-111.5°E, 42°-50°N). The teleseismic events are selected from the global earthquakes between Aug. 2011 and Dec. 2013 with magnitude >5.5and the epicentral distance range from 30° to 95° to the center of the network. Lateral variations of the crustal thicknesses H and Vp/Vs ratios are obtained by using receiver function method. The crust thins gradually from northwest to southeast in the studying field. We found that the thinnest crust is ~37.5km in the southeast which is Gobi. The distribution of Vp/Vs ratios are between 1.68 and 1.84, which shows the heterogeneity. There are three high-anomaly areas: the Gobi range which is the Later Paleozoic Orogeny; the Khentei Mountains which is in the Jurassic-Cretaceous Reactive Continental Margin; the northwest area which is granite. Our research not only reveals the powerful evident of the crustal formation and evolution mechanism, but also provides some constraints on the mechanism of uplift of the Mongolian Plateau.This study was supported by the international cooperation project of the Ministry of Science and Technology of China (2011DFB20120).

Mantle geofluid and uranium ore-formation model

International Nuclear Information System (INIS)

Wu Jianhua; Liu Shuai; Yu Dagan; Zhang Bangtong

2005-01-01

Results of the recent research show that volcanic-type and granite-type uranium deposits have both early and late phases of uranium mineralization, and the early phase uranium mineralization is characterized by metallogenetic features of mantle fluids. This paper discusses the geofluids and related metallogenesis, as well as characteristics of early phase uranium mineralisation, and emphasizes, that the ΣCO 2 , U and H 2 O, that comprise the bulk of the ore-forming hot fluids, are originated from different sources, namely CO 2 comes from mantle fluids, U comes from country rocks the mantle fluids have passed during their ascending way, and H 2 O comes from mantle fluids and country rocks the mantle fluids have passed during their ascending way. (authors)

A New Comprehensive Model for Crustal and Upper Mantle Structure of the European Plate

Science.gov (United States)

Morelli, A.; Danecek, P.; Molinari, I.; Postpischl, L.; Schivardi, R.; Serretti, P.; Tondi, M. R.

2009-12-01

We present a new comprehensive model of crustal and upper mantle structure of the whole European Plate — from the North Atlantic ridge to Urals, and from North Africa to the North Pole — describing seismic speeds (P and S) and density. Our description of crustal structure merges information from previous studies: large-scale compilations, seismic prospection, receiver functions, inversion of surface wave dispersion measurements and Green functions from noise correlation. We use a simple description of crustal structure, with laterally-varying sediment and cristalline layers thickness and seismic parameters. Most original information refers to P-wave speed, from which we derive S speed and density from scaling relations. This a priori crustal model by itself improves the overall fit to observed Bouguer anomaly maps, as derived from GRACE satellite data, over CRUST2.0. The new crustal model is then used as a constraint in the inversion for mantle shear wave speed, based on fitting Love and Rayleigh surface wave dispersion. In the inversion for transversely isotropic mantle structure, we use group speed measurements made on European event-to-station paths, and use a global a priori model (S20RTS) to ensure fair rendition of earth structure at depth and in border areas with little coverage from our data. The new mantle model sensibly improves over global S models in the imaging of shallow asthenospheric (slow) anomalies beneath the Alpine mobile belt, and fast lithospheric signatures under the two main Mediterranean subduction systems (Aegean and Tyrrhenian). We map compressional wave speed inverting ISC travel times (reprocessed by Engdahl et al.) with a non linear inversion scheme making use of finite-difference travel time calculation. The inversion is based on an a priori model obtained by scaling the 3D mantle S-wave speed to P. The new model substantially confirms images of descending lithospheric slabs and back-arc shallow asthenospheric regions, shown in

Depleted subcontinental lithospheric mantle and its tholeiitic melt metasomatism beneath NE termination of the Eger Rift (Europe): the case study of the Steinberg (Upper Lusatia, SE Germany) xenoliths

Science.gov (United States)

Kukuła, Anna; Puziewicz, Jacek; Matusiak-Małek, Magdalena; Ntaflos, Theodoros; Büchner, Jörg; Tietz, Olaf

2015-12-01

The ca. 30 Ma Steinberg basanite occurs at the NE termination of the Eger (Ohře) Rift in the NW Bohemian Massif, Central Europe, and belongs to the Cenozoic alkaline Central European Volcanic Province. The basanite hosts a suite of mantle xenoliths, most of which are harzburgites containing relatively magnesian olivine (Fo 90.5-91.6) and Al-poor (0.04-0.13 a pfu) orthopyroxene (mg# 0.90-0.92). Some of these harzburgites also contain volumetrically minor clinopyroxene (mg# 0.92-0.95, Al 0.03-0.13 a pfu) and have U-shaped LREE-enriched REE patterns. The Steinberg harzburgites are typical for the Lower Silesian - Upper Lusatian domain of the European subcontinental lithospheric mantle. They represent residual mantle that has undergone extensive partial melting and was subsequently affected by mantle metasomatism by mixed carbonatite-silicate melts. The Steinberg xenolith suite comprises also dunitic xenoliths affected by metasomatism by melt similar to the host basanite, which lowered the Fo content in olivine to 87.6 %. This metasomatism happened shortly before xenolith entrainment in the erupting lava. One of the xenoliths is a wehrlite (olivine Fo 73 %, clinopyroxene mg# 0.83-0.85, subordinate orthopyroxene mg# 0.76-0.77). Its clinopyroxene REE pattern is flat and slightly LREE-depleted. This wehrlite is considered to be a tholeiitic cumulate. One of the studied harzburgites contains clinopyroxene with similar trace element contents to those in wehrlite. This type of clinopyroxene records percolation of tholeiitic melt through harzburgite. The tholeiitic melt might be similar to Cenozoic continental tholeiites occurring in the Central European Volcanic Province (e.g., Vogelsberg, Germany).

Thermoconvective waves in the earth's mantle

Science.gov (United States)

Birger, B. I.

1980-06-01

The thermoconvective instability of the Earth's mantle is analysed. The mantle is modelled as an infinite horizontal layer with a free upper surface, heated from below. The creep in the mantle is supposed to be transient when strains are small. This transient creep is described by Lomnitz's law modified by Jeffreys (1958a). It is shown that disturbances, in the form of thermoconvective waves with a period of 10 8 - 10 9y and wavelength of the order 10 3 km, can propagate through the mantle without attenuation. These waves induce oscillations of the Earth's surface. The pattern of flows differs greatly from that suggested by plate tectonics. An attempt is made to give a new explanation for the linear magnetic anomalies over oceanic ridges.

Isotopic and trace element geochemistry of the Seligdar magnesiocarbonatites (South Yakutia, Russia): Insights regarding the mantle evolution beneath the Aldan-Stanovoy shield

Science.gov (United States)

Doroshkevich, Anna G.; Prokopyev, Ilya R.; Izokh, Andrey E.; Klemd, Reiner; Ponomarchuk, Anton V.; Nikolaeva, Irina V.; Vladykin, Nikolay V.

2018-04-01

The Paleoproterozoic Seligdar magnesiocarbonatite intrusion of the Aldan-Stanovoy shield in Russia underwent extensive postmagmatic hydrothermal alteration and metamorphic events. This study comprises new isotopic (Sr, Nd, C and O) data, whole-rock major and trace element compositions and trace element characteristics of the major minerals to gain a better understanding of the source and the formation process of the carbonatites. The Seligdar carbonatites have high concentrations of P2O5 (up to 18 wt%) and low concentrations of Na, K, Sr and Ba. The chondrite-normalized REE patterns of these carbonatites display significant enrichments of LREE relative to HREE with an average La/Ybcn ratio of 95. Hydrothermal and metamorphic overprints changed the trace element characteristics of the carbonatites and their minerals. These alteration processes were responsible for Sr loss and the shifting of the Sr isotopic compositions towards more radiogenic values. The altered carbonatites are further characterized by distinct 18O- and 13C-enrichments compared to the primary igneous carbonatites. The alteration most likely resulted from both the percolation of crustal-derived hydrothermal fluids and subsequent metamorphic processes accompanied by interaction with limestone-derived CO2. The narrow range of negative εNd(T) values indicates that the Seligdar carbonatites are dominated by a homogenous enriched mantle source component that was separated from the depleted mantle during the Archean.

Does cement mantle thickness really matter?

OpenAIRE

Caruana, J.

2008-01-01

The thickness of the cement mantle around the femoral component of total hip replacements is a contributing factor to aseptic loosening and revision. Nevertheless, various designs of stems and surgical tooling lead to cement mantles of differing thicknesses. This thesis is concerned with variability in cement thickness around the Stanmore Hip, due to surgical approach, broach size and stem orientation, and its effects on stress and cracking in the cement. The extent to which cement mantle thi...

Sublithospheric flows in the mantle

Science.gov (United States)

Trifonov, V. G.; Sokolov, S. Yu.

2017-11-01

The estimated rates of upper mantle sublithospheric flows in the Hawaii-Emperor Range and Ethiopia-Arabia-Caucasus systems are reported. In the Hawaii-Emperor Range system, calculation is based on motion of the asthenospheric flow and the plate moved by it over the branch of the Central Pacific plume. The travel rate has been determined based on the position of variably aged volcanoes (up to 76 Ma) with respect to the active Kilauea Volcano. As for the Ethiopia-Arabia-Caucasus system, the age of volcanic eruptions (55-2.8 Ma) has been used to estimate the asthenospheric flow from the Ethiopian-Afar superplume in the northern bearing lines. Both systems are characterized by variations in a rate of the upper mantle flows in different epochs from 4 to 12 cm/yr, about 8 cm/yr on average. Analysis of the global seismic tomographic data has made it possible to reveal rock volumes with higher seismic wave velocities under ancient cratons; rocks reach a depth of more than 2000 km and are interpreted as detached fragments of the thickened continental lithosphere. Such volumes on both sides of the Atlantic Ocean were submerged at an average velocity of 0.9-1.0 cm/yr along with its opening. The estimated rates of the mantle flows clarify the deformation properties of the mantle and regulate the numerical models of mantle convection.

Constraining the hydration of the subducting Nazca plate beneath Northern Chile using subduction zone guided waves

Science.gov (United States)

Garth, Tom; Rietbrock, Andreas

2017-09-01

Guided wave dispersion is observed from earthquakes at 180-280 km depth recorded at stations in the fore-arc of Northern Chile, where the 44 Ma Nazca plate subducts beneath South America. Characteristic P-wave dispersion is observed at several stations in the Chilean fore-arc with high frequency energy (>5 Hz) arriving up to 3 s after low frequency (accounted for if dipping low velocity fault zones are included within the subducting lithospheric mantle. A grid search over possible LVL and faults zone parameters (width, velocity contrast and separation distance) was carried out to constrain the best fitting model parameters. Our results imply that fault zone structures of 0.5-1.0 km thickness, and 5-10 km spacing, consistent with observations at the outer rise are present within the subducted slab at intermediate depths. We propose that these low velocity fault zone structures represent the hydrated structure within the lithospheric mantle. They may be formed initially by normal faults at the outer rise, which act as a pathway for fluids to penetrate the deeper slab due to the bending and unbending stresses within the subducting plate. Our observations suggest that the lithospheric mantle is 5-15% serpentinised, and therefore may transport approximately 13-42 Tg/Myr of water per meter of arc. The guided wave observations also suggest that a thin LVL (∼1 km thick) interpreted as un-eclogitised subducted oceanic crust persists to depths of at least 220 km. Comparison of the inferred seismic velocities with those predicted for various MORB assemblages suggest that this thin LVL may be accounted for by low velocity lawsonite-bearing assemblages, suggesting that some mineral-bound water within the oceanic crust may be transported well beyond the volcanic arc. While older subducting slabs may carry more water per metre of arc, approximately one third of the oceanic material subducted globally is of a similar age to the Nazca plate. This suggests that subducting oceanic

Deformation and seismic anisotropy of the subcontinental lithospheric mantle in NE Spain: EBSD data on xenoliths from the Catalan Volcanic Zone

Science.gov (United States)

Fernández-Roig, Mercè; Galán, Gumer; Mariani, Elisabetta

2017-02-01

Mantle xenoliths in Neogene-Quaternary basaltic rocks related to the European Cenozoic Rift System serve to assess the evolution of the subcontinental lithospheric mantle beneath the Catalan Volcanic Zone in NE Spain. Crystallographic preferred orientations, major element composition of minerals, and temperature and pressure estimates have been used to this end. The mantle consists of spinel lherzolites, harzburgites and subordinate websterites. Protogranular microstructures are found in all peridotites and websterites, but lherzolites also display finer-grained porphyroclastic and equigranular microstructures. The dominant olivine deformation fabric is [010] fiber, but subordinate orthorhombic and [100]-fiber types are also present, especially in porphyroclastic and equigranular lherzolites. The fabric strength (J index = 10.12-1.91), equilibrium temperature and pressure are higher in xenoliths with [010]-fiber fabric and decrease in those with orthorhombic and [100]-fiber type. Incoherence between olivine and pyroxene deformation fabric is mostly found in porphyroclastic and equigranular lherzolites. Seismic anisotropy, estimated from the crystal preferred orientations, also decreases (AVp = 10.2-2.60%; AVs max = 7.95-2.19%) in porphyroclastic and equigranular lherzolites. The olivine [010]-fiber fabric points to deformation by simple shear or transpression which is likely to have occured during the development of late-Hercynian strike-slip shear zones, and to subsequent annealing during late Hercynian decompression, Permian and Cretaceous rifting. Also, it cannot be excluded that the percolation of mafic magmas during these extensional events provoked the refertilization of the lithospheric mantle. However, no clear relationship has been observed between fabric strength and mineral mode and composition. Later transtensional deformation during late Alpine orogenesis, at higher stress and decreasing temperature and pressure, transformed the earlier fabric into

Characteristics of Vertical Mantle Heat Exchangers for Solar Water Heaters

DEFF Research Database (Denmark)

Shah, Louise Jivan; Morrison, G.L.; Behnia, M.

1999-01-01

- The flow structure in vertical mantle heat exchangers was investigated using a full-scale tank designed to facilitate flow visualisation. The flow structure and velocities in the mantle were measured using a particle Image Velocimetry (PIV) system. A CFD simulation model of vertical mantle heat...... exchangers was also developed for detailed evaluation of the heat flux distribution over the mantle surface. Both the experimental and simulation results indicate that distribution of the flow around the mantle gap is governed by buoyancy driven recirculation in the mantle. The operation of the mantle...

MODELLING MANTLE TANKS FOR SDHW SYSTEMS USING PIV AND CFD

DEFF Research Database (Denmark)

Shah, Louise Jivan; Morrison, G.L.; Behnia, Masud

1999-01-01

Characteristics of vertical mantle heat exchanger tanks for SDHW systems have been investigated experimentally and theoretically using particle image velocimetry (PIV) and CFD modelling. A glass model of a mantle heat exchanger tank was constructed so that the flow distribution in the mantle could...... be studied using the PIV test facility. Two transient three-dimensional CFD-models of the glass model mantle tank were developed using the CFD-programmes CFX and FLUENT.The experimental results illustrate that the mantle flow structure in the mantle is complicated and the distribution of flow in the mantle...

Summary of northern Atlantic coastal plain hydrology and its relation to disposal of high-level radioactive waste in buried crystalline rock; a preliminary appraisal

Science.gov (United States)

Lloyd, O.B.; Larson, J.D.; Davis, R.W.

1985-01-01

Interpretation of available hydrologic data suggests that some areas beneath the Coastal Plain in the States of Delaware, Maryland, New Jersey, North Carolina, and Virginia might have some potential for the disposal of nuclear waste in crystalline rock that is buried beneath the Coastal Plain sediments. The areas of major interest occur where the top of the basement rock lies between 1,000 and 4,000 feet below sea level, the aquifer(s) immediately above the basement rock are saturated with saline water, confining material overlies the saline water bearing aquifer(s), and groundwater flow in the saline water aquifer(s) can be established. Preliminary data on (1) the distribution and thickness of the lowermost aquifers and confining beds, (2) the distribution of hydraulic conductivity in the lowermost aquifers, (3) estimated hydraulic heads and inferred direction of lateral groundwater flow for 1980, and (4) the distribution of saline water and brine, indicate eastern parts of the study area relatively best meet most of the criteria proposed for sediments that would overlie any potential buried crystalline-rock disposal site.

The ruthenium isotopic composition of the oceanic mantle

Science.gov (United States)

Bermingham, K. R.; Walker, R. J.

2017-09-01

The approximately chondritic relative, and comparatively high absolute mantle abundances of the highly siderophile elements (HSE), suggest that their concentrations in the bulk silicate Earth were primarily established during a final ∼0.5 to 1% of ;late accretion; to the mantle, following the cessation of core segregation. Consequently, the isotopic composition of the HSE Ru in the mantle reflects an amalgamation of the isotopic compositions of late accretionary contributions to the silicate portion of the Earth. Among cosmochemical materials, Ru is characterized by considerable mass-independent isotopic variability, making it a powerful genetic tracer of Earth's late accretionary building blocks. To define the Ru isotopic composition of the oceanic mantle, the largest portion of the accessible mantle, we report Ru isotopic data for materials from one Archean and seven Phanerozoic oceanic mantle domains. A sample from a continental lithospheric mantle domain is also examined. All samples have identical Ru isotopic compositions, within analytical uncertainties, indicating that Ru isotopes are well mixed in the oceanic mantle, defining a μ100Ru value of 1.2 ± 7.2 (2SD). The only known meteorites with the same Ru isotopic composition are enstatite chondrites and, when corrected for the effects of cosmic ray exposure, members of the Main Group and sLL subgroup of the IAB iron meteorite complex which have a collective CRE corrected μ100Ru value of 0.9 ± 3.0. This suggests that materials from the region(s) of the solar nebula sampled by these meteorites likely contributed the dominant portion of late accreted materials to Earth's mantle.

Mantle Circulation Models with variational data assimilation: Inferring past mantle flow and structure from plate motion histories and seismic tomography

Science.gov (United States)

Bunge, H.; Hagelberg, C.; Travis, B.

2002-12-01

EarthScope will deliver data on structure and dynamics of continental North America and the underlying mantle on an unprecedented scale. Indeed, the scope of EarthScope makes its mission comparable to the large remote sensing efforts that are transforming the oceanographic and atmospheric sciences today. Arguably the main impact of new solid Earth observing systems is to transform our use of geodynamic models increasingly from conditions that are data poor to an environment that is data rich. Oceanographers and meteorologists already have made substantial progress in adapting to this environment, by developing new approaches of interpreting oceanographic and atmospheric data objectively through data assimilation methods in their models. However, a similarly rigorous theoretical framework for merging EarthScope derived solid Earth data with geodynamic models has yet to be devised. Here we explore the feasibility of data assimilation in mantle convection studies in an attempt to fit global geodynamic model calculations explicitly to tomographic and tectonic constraints. This is an inverse problem not quite unlike the inverse problem of finding optimal seismic velocity structures faced by seismologists. We derive the generalized inverse of mantle convection from a variational approach and present the adjoint equations of mantle flow. The substantial computational burden associated with solutions to the generalized inverse problem of mantle convection is made feasible using a highly efficient finite element approach based on the 3-D spherical fully parallelized mantle dynamics code TERRA, implemented on a cost-effective topical PC-cluster (geowulf) dedicated specifically to large-scale geophysical simulations. This dedicated geophysical modeling computer allows us to investigate global inverse convection problems having a spatial discretization of less than 50 km throughout the mantle. We present a synthetic high-resolution modeling experiment to demonstrate that mid

Seismogenic Tectonic Environment of 1976 Great Tangshan Earthquake: Results from Dense Seismic Array Observations

Science.gov (United States)

LIU, Qiyuan; WANG, Jun; CHEN, Jiuhui; LI, Shuncheng; GUO, Biao

On July 28, 1976, the great Tangshan earthquake that shook the whole world took place in the Tangshan area of the Hebei Province, China. A big incomprehensible question is why such a tremendous earthquake took place in the Paleo-craton area in North China? It would be worth considering whether a similar event will reoccur in the Tangshan region. In this study, using the receiver function inversion technique and teleseismic P waveform data recorded at the Capital Circle Seismic network and our movable seismic array, we investigated the 3-D S-wave velocity structure of the crust and upper mantle down to 60 km beneath Tangshan area. Our results manifest that (1) the media beneath the Tangshan block cut by active faults are very different from the adjacent area, and all of the active faults surrounding the Tangshan block was through the whole crust; (2) in the upper and middle crust, there exist obvious heterogeneous low-velocity media beneath the Tangshan earthquake region; the crust-mantle boundary has an obvious block uplift and, in comparison with both sides, the top anomalous uplift of the upper mantle beneath the Tangshan block reaches to 10 km, and the upper mantle beneath has an anomalous heterogeneous structure; (4) beneath the Tangshan earthquake region, there are probably massive intrusions derived from the upper mantle, which form the low-velocity body in the upper and middle crust. Because of our results having much higher resolution than previous results, some new features of the crust and upper mantle velocity structure could be shown in this study; (5) the locations of destructive earthquakes are not random and are related closely to their deep structure of the crust and upper mantle. This provides a possibility of correctly estimating the location of destructive earthquakes. On the basis of our results, we discuss the dynamic genesis of the Tangshan earthquake. We consider that the main dynamic source for the Tangshan earthquake is the vertical

The basal part of the Oman ophiolitic mantle: a fossil Mantle Wedge?

Science.gov (United States)

Prigent, Cécile; Guillot, Stéphane; Agard, Philippe; Godard, Marguerite; Chauvet, Alain; Dubacq, Benoit; Monié, Patrick; Yamato, Philippe

2014-05-01

Although the Oman ophiolite is classically regarded as being the direct analog of oceanic lithosphere created at fast spreading ridges, the geodynamic context of its formation is still highly debated. The other alternative end-member model suggests that this ophiolite entirely formed in a supra-subduction zone setting. Fluids involved in the hydration of the oceanic lithosphere and in the presence of a secondary boninitic and andesitic volcanism may provide a way to discriminate between these two interpretations: are they descending near-axis hydrothermal fluxes (first model) or ascending from a subducting slab (second model)? We herein focus on the base of the ophiolitic mantle in order to characterize the origin of fluids and decipher hydration processes. Samples were taken along hecto- to kilometre-long sections across the basal banded unit directly overlying the amphibolitic/granulitic metamorphic sole. We carried out a petrological, structural and geochemical study on these rocks and their constitutive minerals. Our results show that, unlike the generally refractory character of Oman harzburgites, all the basal mantle rocks display secondary crystallization of clinopyroxene and amphibole through metasomatic processes. The microstructures and the chronology of these secondary mineralizations (clinopyroxene, pargasitic amphibole, antigorite and then lizardite/chrysotile) suggest that these basal rocks have been affected by cooling from mantle temperatures (serpentines (B, Sr, Rb, Ba, As), are consistent with amphibolite-derived fluids (Ishikawa et al., 2005) and cannot be easily explained by other sources. Based on these observations, we propose a geodynamic model in which intense and continuous metasomatism of the cooling base of the ophiolitic mantle is due to the release of fluids coming from the progressive dehydration of underlying amphibolitic rocks. This process is compatible with the progressive subduction of the Arabian margin during the Upper

Atlas of the underworld: Slab remnants in the mantle, their sinking history, and a new outlook on lower mantle viscosity

Science.gov (United States)

van der Meer, Douwe G.; van Hinsbergen, Douwe J. J.; Spakman, Wim

2018-01-01

Across the entire mantle we interpret 94 positive seismic wave-speed anomalies as subducted lithosphere and associate these slabs with their geological record. We document this as the Atlas of the Underworld, also accessible online at www.atlas-of-the-underworld.org, a compilation comprising subduction systems active in the past 300 Myr. Deeper slabs are correlated to older geological records, assuming no relative horizontal motions between adjacent slabs following break-off, using knowledge of global plate circuits, but without assuming a mantle reference frame. The longest actively subducting slabs identified reach the depth of 2500 km and some slabs have impinged on Large Low Shear Velocity Provinces in the deepest mantle. Anomously fast sinking of some slabs occurs in regions affected by long-term plume rising. We conclude that slab remnants eventually sink from the upper mantle to the core-mantle boundary. The range in subduction-age versus - depth in the lower mantle is largely inherited from the upper mantle history of subduction. We find a significant depth variation in average sinking speed of slabs. At the top of the lower mantle average slab sinking speeds are between 10 and 40 mm/yr, followed by a deceleration to 10-15 mm/yr down to depths around 1600-1700 km. In this interval, in situ time-stationary sinking rates suggest deceleration from 20 to 30 mm/yr to 4-8 mm/yr, increasing to 12-15 mm/yr below 2000 km. This corroborates the existence of a slab deceleration zone but we do not observe long-term (> 60 My) slab stagnation, excluding long-term stagnation due to compositional effects. Conversion of slab sinking profiles to viscosity profiles shows the general trend that mantle viscosity increases in the slab deceleration zone below which viscosity slowly decreases in the deep mantle. This is at variance with most published viscosity profiles that are derived from different observations, but agrees qualitatively with recent viscosity profiles suggested

Effects of iron on the lattice thermal conductivity of Earth's deep mantle and implications for mantle dynamics.

Science.gov (United States)

Hsieh, Wen-Pin; Deschamps, Frédéric; Okuchi, Takuo; Lin, Jung-Fu

2018-04-17

Iron may critically influence the physical properties and thermochemical structures of Earth's lower mantle. Its effects on thermal conductivity, with possible consequences on heat transfer and mantle dynamics, however, remain largely unknown. We measured the lattice thermal conductivity of lower-mantle ferropericlase to 120 GPa using the ultrafast optical pump-probe technique in a diamond anvil cell. The thermal conductivity of ferropericlase with 56% iron significantly drops by a factor of 1.8 across the spin transition around 53 GPa, while that with 8-10% iron increases monotonically with pressure, causing an enhanced iron substitution effect in the low-spin state. Combined with bridgmanite data, modeling of our results provides a self-consistent radial profile of lower-mantle thermal conductivity, which is dominated by pressure, temperature, and iron effects, and shows a twofold increase from top to bottom of the lower mantle. Such increase in thermal conductivity may delay the cooling of the core, while its decrease with iron content may enhance the dynamics of large low shear-wave velocity provinces. Our findings further show that, if hot and strongly enriched in iron, the seismic ultralow velocity zones have exceptionally low conductivity, thus delaying their cooling.

Inferred rheological structure and mantle conditions from postseismic deformation following the 2010 Mw 7.2 El Mayor-Cucapah Earthquake

Science.gov (United States)

Dickinson-Lovell, Haylee; Huang, Mong-Han; Freed, Andrew M.; Fielding, Eric; Bürgmann, Roland; Andronicos, Christopher

2018-06-01

The 2010 Mw7.2 El Mayor-Cucapah earthquake provides a unique target of postseismic study as deformation extends across several distinct geological provinces, including the cold Mesozoic arc crust of the Peninsular Ranges and newly formed, hot, extending lithosphere within the Salton Trough. We use five years of global positioning system measurements to invert for afterslip and constrain a 3-D finite-element model that simulates viscoelastic relaxation. We find that afterslip cannot readily explain far-field displacements (more than 50 km from the epicentre). These displacements are best explained by viscoelastic relaxation of a horizontally and vertically heterogeneous lower crust and upper mantle. Lower viscosities beneath the Salton Trough compared to the Peninsular Ranges and other surrounding regions are consistent with inferred differences in the respective geotherms. Our inferred viscosity structure suggests that the depth of the Lithosphere/Asthenosphere Boundary (LAB) is ˜65 km below the Peninsular Ranges and ˜32 km beneath the Salton Trough. These depths are shallower than the corresponding seismic LAB. This suggests that the onset of partial melting in peridotite may control the depth to the base of the mechanical lithosphere. In contrast, the seismic LAB may correspond to an increase in the partial melt percentage associated with the change from a conductive to an adiabatic geotherm.

Magma Diversity in the Trans-Mexican Volcanic Belt: the role of Mantle Heterogeneities, Slab-derived Fluxes and Crustal Contamination.

Science.gov (United States)

Schaaf, P.; Valdez, G.; Siebe, C.; Carrasco, G.

2005-12-01

The Plio-Quaternary Trans-Mexican Volcanic Belt (TMVB) is related to subduction of the Cocos and Rivera plates underneath the North American plate. Non-parallelism of the magmatic arc with respect to the trench can be explained by oblique subduction and changes of dip angle. In this contribution we compare geochemical and Sr-Nd-Pb isotope data of five TMVB stratovolcanoes (from east to west: Colima Volcano, Nevado de Toluca, Popocatepetl, La Malinche, and Pico de Orizaba) and associated cinder cones. Volcanic products range in stratovolcanoes from andesites (e.g. Colima, Popocatepetl) to rhyolites (e.g. Pico de Orizaba), and from basalts to andesites in the monogenetic cones. Concentrations of incompatible elements correlate positively with Sr-Nd-Pb isotope ratios from east to west along the arc. 87Sr/86Sr, eNd, and 206Pb/204Pb range from 0.7034-0.7050, +6.9 to minus 1.8, and 18.57-18.78, respectively, displaying considerable differences. In the central TMVB, REE patterns of closely spaced high-Mg basaltic andesites differ substantially. This cannot be explained by fractional crystallization processes or differential partial melting of a homogeneous mantle source. Instead, it points towards small-scale mantle heterogeneities. LILE (e.g. Cs, Rb, Ba, Pb) and HFSE (e.g. Ta, Nb, Zr) display variations of orders in magnitude at different segments along the arc. These variations might correlate with amounts of slab-derived aqueous fluids and intensity of metasomatic reactions between the subducting lithosphere and the overlying mantle wedge. Isotopic ratios of mid-lower crustal xenoliths found in nearly all stratovolcano products reflect the nature of the underlying crust beneath the TMVB. Tertiary-Cretaceous plagiogranites (Colima), Cretaceous limestones (Popocatepetl), and Grenvillian quartzites (Pico de Orizaba)and their increasing radiogenic isotope ratios match well with the observed isotopic signatures of the stratovolcanoes. Moreover, elevated CO2 amounts in

The mantle cells lymphoma: a proposed treatment

International Nuclear Information System (INIS)

Chavez Martinez, Marlene Elizabeth

2012-01-01

A literature review was performed on mantle cells lymphoma in the therapeutic schemes. The literature that has been used is published in journals of medicine specializing in hematology, oncology, radiation therapy, molecular biology and internal medicine. The literature review was performed to propose a scheme of treatment according to Costa Rica. Epigenetic alterations have been revealed in patients with mantle lymphoma on current researches. The mantle lymphoma pathology has been described in various forms of clinical and histological presentation, stressing the importance of detailing the different methods and diagnostic reports. Working groups have proposed and developed various chemotherapy regimens and concluded that CHOP alone is without effect in mantle cell lymphoma unlike R-hyper-CVAD, CHOP / DHAP, high-dose Ara-C. Researchers have tried to develop new treatments based vaccines, use of modified viruses, specific monoclonal antibodies. The classic treatment has been triple intrathecal therapy. The central nervous system has been one of the most momentous sites of mantle cell lymphoma infiltration because poorer patient prognosis [es

[Cytostructure of the mantle zone in lymphatic tissue].

Science.gov (United States)

Bednár, B

1993-04-01

Four cellular layers of the follicular mantle zone in palatine tonsil lymphatic tissue were studied by electron microscopy after simultaneous immunophenotypical investigation. The first layer of the mantle zone consisting of small blastic cells was analogous to the small (centrocytoid) blastic layer of germinal centres. The second B monocytoid layer was lacking analogy in basic series of lymphocytes and seemed to be an independent morphological and probably functional unit. Plasmacytoid and clarocellular elements in outer layers of follicular mantle zone were in a way similar to T plasmacytoid and clarocellular components of Sézary syndrome infiltrates but considering transitional forms they had a local origin from incompletely transformed elements of B monocytoid layer. Inner follicular mantle zone was discussed as a source of incompletely transformed B lymphocytes for further mantle layers where their immunophenotypical modulation is taking place according to actual need. Outer mantle layers are aggressive against damaged epithelial and litoral structures and may be instrumental in a common reaction of B and T components.

Temperature Profile of the Upper Mantle

International Nuclear Information System (INIS)

Anderson, O.L.

1980-01-01

Following the procedure outlined by Magnitsky [1971], thermal profiles of the upper mantle are computed by deriving the thermal gradient from the seismic data given as dv/sub s//drho used along with the values of (dv/sub s//dT9/sub p/ and (dv/sub s//dP)/sub T/ of selected minerals, measured at high temperature. The resulting values of dT/dZ are integrated from 380 km upward toward the surface, where the integrating constant is taken from Akagi and Akimoto's work, T=1400 0 C at 380 km. The resulting geotherms for minerals are used to derive geotherms for an eclogite mantle and a lherzolite mantle, with and without partial melting in the low-velocity zone. The geotherms are all subadiabatic, and some are virtually isothermal in the upper mantle. Some are characterized by a large thermal hump at the lithosphere boundary

S-Wave Velocity Structure beneath Southwest North America from Seismogram Comparisons of the Mexico Earthquake on 22 June 1997

Directory of Open Access Journals (Sweden)

Bagus Jaya Santosa

2008-09-01

Full Text Available This research investigates earth structure beneath the Southwest North America landmass, especially between Mexico and California. Models based on S wave velocities for this area were obtained by carrying out seismogram fitting in time domain and three Cartesian components simultaneously. The data used is from an event, coded as C052297B that occurred in the state of Guerrero, Mexico and it was fitted to synthetic data computed with the GEMINI program at TS network stations. Earth model IASPEI91 and SPREM were used as input to create the synthetic data. Real and synthetic seismograms were subjected to a low-pass filter with a frequency corner of 20 mHz.Waveform analysis results show very unsystematic and strong deviations in the waveform, arrival times, amount of oscillation and the height of the wave amplitude. Discrepancies are met on S, Love, Rayleigh and ScS waves, where the stations epicentral distances are below 300. Deviation in analysis waveform because of the usage of model 1-D of SPREM and IASPEI91, because the 1-D was a kind of average value an elastic property at one particular depth of global earth. With the method of waveform analysis we can see how sensitive waveform is to structures within the layers of the Earth.To explain the discrepancies, a correction to the earth structure is essential. The corrections account for the thickness of the crust, speed gradient of bh, the coefficient for the bh and bv in the upper mantle for surface wave fitting, a small variation of the S speed structure at a layer under the upper mantle above 771 km for S wave fitting, and a small variation at the base the mantle layers for ScS wave fitting. At some stations, a correction for S speed structure have yielded P wave fitting. Results of this research indicate that the 1-D earth model obtained through seismogram fitting at every hypocenter-observation station pair is unique. The S-wave velocity on the upper mantle has strong negative anomalies. This

The bivalve Anopaea (Inoceramidae) from the Upper Jurassic-lowermost Cretaceous of Mexico

Science.gov (United States)

Zell, Patrick; Crame, J. Alistair; Stinnesbeck, Wolfgang; Beckmann, Seija

2015-07-01

In Mexico, the Upper Jurassic to lowermost Cretaceous La Casita and coeval La Caja and La Pimienta formations are well-known for their abundant and well-preserved marine vertebrates and invertebrates. The latter include conspicuous inoceramid bivalves of the genus Anopaea not formally described previously from Mexico. Anopaea bassei (Lecolle de Cantú, 1967), Anopaea cf. stoliczkai (Holdhaus, 1913), Anopaea cf. callistoensis Crame and Kelly, 1995 and Anopaea sp. are rare constituents in distinctive Tithonian-lower Berriasian levels of the La Caja Formation and one Tithonian horizon of the La Pimienta Formation. Anopaea bassei was previously documented from the Tithonian of central Mexico and Cuba, while most other members of Anopaea described here are only known from southern high latitudes. The Mexican assemblage also includes taxa which closely resemble Anopaea stoliczkai from the Tithonian of India, Indonesia and the Antarctic Peninsula, and Anopaea callistoensis from the late Tithonian to ?early Berriasian of the Antarctic Peninsula. Our new data expand the palaeogeographical distribution of the high latitude Anopaea to the Gulf of Mexico region and substantiate faunal exchange, in the Late Jurassic-earliest Cretaceous, between Mexico and the Antarctic Realm.

OPTIMISATION OF MANTLE TANKS FOR LOW FLOW SOLAR HEATING SYSTEMS

DEFF Research Database (Denmark)

Shah, Louise Jivan; Furbo, Simon

1996-01-01

A model, describing the heat transfer coefficients in the mantle of a mantle tank has been developed. The model is validated by means of measurements with varying operational conditions for different designed mantle tanks. The model has been implemented in an existing detailed mathematical...... with the programme and by means of tests of three SDHW systems with different designed mantle tanks. Based on the investigations design rules for mantle tanks are proposed. The model, describing the heat transfer coefficients in the mantle is approximate. In addition, the measurements have revealed...... that a temperature stratification in the hot water tank, above the mantle is built up. This phenomenon may be important, but it is not taken into calculation in the programme. Therefore, theoretical and practical work is continuing in order to make a more precise model for the whole mantle tank....

Role of mantle flow in Nubia-Somalia plate divergence

Science.gov (United States)

Stamps, D. S.; Iaffaldano, G.; Calais, E.

2015-01-01

Present-day continental extension along the East African Rift System (EARS) has often been attributed to diverging sublithospheric mantle flow associated with the African Superplume. This implies a degree of viscous coupling between mantle and lithosphere that remains poorly constrained. Recent advances in estimating present-day opening rates along the EARS from geodesy offer an opportunity to address this issue with geodynamic modeling of the mantle-lithosphere system. Here we use numerical models of the global mantle-plates coupled system to test the role of present-day mantle flow in Nubia-Somalia plate divergence across the EARS. The scenario yielding the best fit to geodetic observations is one where torques associated with gradients of gravitational potential energy stored in the African highlands are resisted by weak continental faults and mantle basal drag. These results suggest that shear tractions from diverging mantle flow play a minor role in present-day Nubia-Somalia divergence.

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

with Archean lithosphere at the southwestern of the Eastern Block, major the Trans-North China Orogen and western part of the Western Block. The lithospheric mantle beneath the northern part of the Eastern Block, central segment of the Trans-North China Craton and the eastern margin of the Western Block have experienced modification and replacement.

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

region of mechanically and chemically rejuvenated mantle that was likely emplaced during late Precambrian/early Cambrian rifting. These observations suggest that the lithospheric structure beneath the Reelfoot Rift may be an example of a global phenomenon in which MLDs act as weak zones that facilitate the removal of cratonic lithosphere that lies beneath.

Hf isotope evidence for a hidden mantle reservoir

DEFF Research Database (Denmark)

Bizzarro, Martin; Simonetti, A.; Stevenson, R.K.

2002-01-01

High-precision Hf isotopic analyses and U-Pb ages of carbonatites and kimberlites from Greenland and eastern North America, including Earth's oldest known carbonatite (3 Ga), indicate derivation from an enriched mantle source. This previously unidentified mantle reservoir-marked by an unradiogenic...... Hf isotopic composition and preserved in the deep mantle for at least 3 b.y.-may account for the mass imbalance in Earth's Hf-Nd budget. The Hf isotopic data presented here support a common mantle source region and genetic link between carbonatite and some oceanic-island basalt volcanoes....

Constraints on seismic anisotropy beneath the Appalachian Mountains from Love-to-Rayleigh wave scattering

Science.gov (United States)

Servali, A.; Long, M. D.; Benoit, M.

2017-12-01

The eastern margin of North America has been affected by a series of mountain building and rifting events that have likely shaped the deep structure of the lithosphere. Observations of seismic anisotropy can provide insight into lithospheric deformation associated with these past tectonic events, as well as into present-day patterns of mantle flow beneath the passive margin. Previous work on SKS splitting beneath eastern North America has revealed fast splitting directions parallel to the strike of the Appalachian orogen in the central and southern Appalachians. A major challenge to the interpretation of SKS splitting measurements, however, is the lack of vertical resolution; isolating anisotropic structures at different depths is therefore difficult. Complementary constraints on the depth distribution of anisotropy can be provided by surface waves. In this study, we analyze the scattering of Love wave energy to Rayleigh waves, which is generated via sharp lateral gradients in anisotropic structure along the ray path. The scattered phases, known as quasi-Love (QL) waves, exhibit amplitude behavior that depend on the strength of the anisotropic contrast as well as the angle between the propagation azimuth and the anisotropic symmetry axis. We analyze data collected by the dense MAGIC seismic array across the central Appalachians. We examine teleseismic earthquakes of magnitude 6.7 and greater over a range of backazimuths, and isolate surface waves at periods between 100 and 500 seconds. We compare the data to synthetic seismograms generated by the Princeton Global ShakeMovie initiative to identify anomalous QL arrivals. We find evidence significant QL arrivals at MAGIC stations, with amplitudes depending on propagation azimuth and station location. Preliminary results are consistent with a sharp lateral gradient in seismic anisotropy across the Appalachian Mountains in the depth range between 100-200 km.

The Earth's heterogeneous mantle a geophysical, geodynamical, and geochemical perspective

CERN Document Server

Khan, Amir

2015-01-01

This book highlights and discusses recent developments that have contributed to an improved understanding of observed mantle heterogeneities and their relation to the thermo-chemical state of Earth's mantle, which ultimately holds the key to unlocking the secrets of the evolution of our planet. This series of topical reviews and original contributions address 4 themes. Theme 1 covers topics in geophysics, including global and regional seismic tomography, electrical conductivity and seismic imaging of mantle discontinuities and heterogeneities in the upper mantle, transition zone and lower mantle. Theme 2 addresses geochemical views of the mantle including lithospheric evolution from analysis of mantle xenoliths, composition of the deep Earth and the effect of water on subduction-zone processes. Theme 3 discusses geodynamical perspectives on the global thermo-chemical structure of the deep mantle. Theme 4 covers application of mineral physics data and phase equilibrium computations to infer the regional-scale ...

SEM investigation of incandescent lamp mantle structure on durability

International Nuclear Information System (INIS)

Gerneke, D.; Lang, C.

2002-01-01

Full text: The incandescent mantle as used on pressure and non-pressure liquid fuel lamps has been in use for over 100 years. What remains unexplained is the way in which the resistance to mechanical shock and the decline in tensile strength with usage is experienced. It has been suggested that to improve durability it is necessary to continuously burn a new mantle for the first two to three hours. The known factors in mantle durability and mechanical strength are chemical composition and fabric weave. This study was undertaken to investigate the effects of burning time and temperature on thorium oxide mantles. The operating temperature of mantles on a range of kerosene pressure lamps was measured and found to be between 800 and 1100 deg C. Heat treatments of thorium based Coleman mantles were carried out in a laboratory furnace within these ranges of temperatures for periods ranging from 2 minutes to 2 hours. The mantles were then viewed in a LEO S440 analytical SEM. Results at 800 deg C show a distinct change in surface morphology with increasing exposure time. At the shorter times (2-5 minutes) the surface was relatively smooth. With increased time periods (15 - 120 minutes) the surface was observed to have a large lumpy structure. At 1100 deg C the difference in surface morphology was not apparent between the shortest and longest times. The surface appears much smoother and no lumpy structure was observed. This suggests that when a mantle is operated at the higher temperature of 1100 deg C the structure of the Thorium oxide is quickly transformed into the known stronger amorphous form. This is taken as the observed smooth structure seen in the SEM images of the 1100 deg C samples. Thus the mantle is expected to be more resistant to mechanical shock and have increased durability. Practical field test results confirm these observations. The mantle on a lamp that is operating efficiently, burns brightly, will far outlast a mantle on an inefficient lamp which bums

Radiation doses from radioactivity in incandescent mantles

International Nuclear Information System (INIS)

1985-01-01

Thorium nitrate is used in the production of incandescent mantles for gas lanterns. In this report dose estimates are given for internal and external exposure that result from the use of the incandescent mantles for gas lanterns. The collective, effective dose equivalent for all users of gas mantles is estimated to be about 100 Sv per annum in the Netherlands. For the population involved (ca. 700,000 persons) this is roughly equivalent to 5% to 10% of the collective dose equivalent associated with exposure to radiation from natural sources. The major contribution to dose estimates comes from inhalation of radium during burning of the mantles. A pessimistic approach results in individual dose estimates for inhalation of up to 0.2 mSv. Consideration of dose consequences in case of a fire in a storage department learns that it is necessary for emergency personnel to wear respirators. It is concluded that the uncontrolled removal of used gas mantles to the environment (soil) does not result in a significant contribution to environmental radiation exposure. (Auth.)

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.

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).

A mantle plume model for the Equatorial Highlands of Venus

Science.gov (United States)

Kiefer, Walter S.; Hager, Bradford H.

1991-01-01

The possibility that the Equatorial Highlands are the surface expressions of hot upwelling mantle plumes is considered via a series of mantle plume models developed using a cylindrical axisymmetric finite element code and depth-dependent Newtonian rheology. The results are scaled by assuming whole mantle convection and that Venus and the earth have similar mantle heat flows. The best model fits are for Beta and Atla. The common feature of the allowed viscosity models is that they lack a pronounced low-viscosity zone in the upper mantle. The shape of Venus's long-wavelength admittance spectrum and the slope of its geoid spectrum are also consistent with the lack of a low-viscosity zone. It is argued that the lack of an asthenosphere on Venus is due to the mantle of Venus being drier than the earth's mantle. Mantle plumes may also have contributed to the formation of some smaller highland swells, such as the Bell and Eistla regions and the Hathor/Innini/Ushas region.

Pillars of the Mantle

KAUST Repository

Pugmire, David

2017-07-05

In this work, we investigate global seismic tomographic models obtained by spectral-element simulations of seismic wave propagation and adjoint methods. Global crustal and mantle models are obtained based on an iterative conjugate-gradient type of optimization scheme. Forward and adjoint seismic wave propagation simulations, which result in synthetic seismic data to make measurements and data sensitivity kernels to compute gradient for model updates, respectively, are performed by the SPECFEM3D-GLOBE package [1] [2] at the Oak Ridge Leadership Computing Facility (OLCF) to study the structure of the Earth at unprecedented levels. Using advances in solver techniques that run on the GPUs on Titan at the OLCF, scientists are able to perform large-scale seismic inverse modeling and imaging. Using seismic data from global and regional networks from global CMT earthquakes, scientists are using SPECFEM3D-GLOBE to understand the structure of the mantle layer of the Earth. Visualization of the generated data sets provide an effective way to understand the computed wave perturbations which define the structure of mantle in the Earth.

Receptor units responding to movement in the octopus mantle.

Science.gov (United States)

Boyle, P R

1976-08-01

1. A preparation of the mantle of Octopus which is inverted over a solid support and which exposes the stellate ganglion and associated nerves is described. 2. Afferent activity can be recorded from stellar nerves following electrical stimulation of the pallial nerve. The latency and frequency of the phasic sensory response is correlated with the contraction of the mantle musculature. 3. It is proposed that receptors cells located in the muscle, and their activity following mantle contraction, form part of a sensory feedback system in the mantle. Large, multipolar nerve cells that were found between the two main layers of circular muscle in the mantle could be such receptors.

Vegetation and plant food reconstruction of lowermost Bed II, Olduvai Gorge, using modern analogs.

Science.gov (United States)

Copeland, Sandi R

2007-08-01

Vegetation and plant foods for hominins of lowermost Bed II, Olduvai Gorge were modeled by examining vegetation in modern habitats in northern Tanzania (Lake Manyara, Ngorongoro, Serengeti) that are analogous to the paleolandscape in terms of climate, land forms, and soil types, as indicated by previous paleoenvironmental studies of Olduvai. Plant species in the modern habitats were identified in a series of sample plots, and those known to be eaten by modern humans, chimpanzees, or baboons were considered potentially edible for early hominins. Within the 50-80 kyr deposition of lowermost Bed II, periods of drier climate were characterized by low lake stands and a broad eastern lacustrine plain containing a mosaic of springs, marsh, woodland, and edaphic grassland. Based on results of this study, plant food diversity in each of those habitats was relatively low, but the mosaic nature of the area meant that hominins could reach several different habitat types within short distances, with access to potential plant foods including marsh plants, grass grains, roots, shrub fruits, edible parts from palms, leafy herbaceous plants, and Acacia pods, flowers, and gum. Based on Manyara analogs, a greater variety of plant foods, such as tree fruits (e.g., Ficus, Trichilia) and the roots and fruits of shrubs (e.g., Cordia, Salvadora) would be expected further east along the rivers in the lacustrine terrace and alluvial fans. Interfluves of the alluvial fans were probably less wooded and offered relatively fewer varieties of plant foods, but there is sparse paleoenvironmental evidence for the character of Olduvai's alluvial fans, making the choice of appropriate modern analogs difficult. In the western side of the basin, based on modern analogs in the Serengeti, riverine habitats provided the greatest variety of edible plant food species (e.g., Acacia, Grewia, Justicia). If the interfluves were grassland, then a large variety of potentially edible grasses and forbs were present

Can Lower Mantle Slab-like Seismic Anomalies be Explained by Thermal Coupling Between the Upper and Lower Mantles?

NARCIS (Netherlands)

Cízková, H. (Hana); Cadek, O.; Berg, A.P. van den; Vlaar, N.J.

1999-01-01

Below subduction zones, high resolution seismic tomographic models resolve fast anomalies that often extend into the deep lower mantle. These anomalies are generally interpreted as slabs penetrating through the 660-km seismic discontinuity, evidence in support of whole-mantle convection. However,

Mantle ingredients for making the fingerprint of Etna alkaline magmas: implications for shallow partial melting within the complex geodynamic framework of Eastern Sicily

Science.gov (United States)

Viccaro, Marco; Zuccarello, Francesco

2017-09-01

Mantle ingredients responsible for the signature of Etnean Na- and K-alkaline magmas and their relationships with short-term geochemical changes of the erupted volcanic rocks have been constrained through a partial melting model that considers major, trace elements and water contents in the produced liquids. Characteristics of the Etnean source for alkaline magmas have been supposed similar to those of the mantle accessible at a regional scale, namely below the Hyblean Plateau. The assumption that the Etnean mantle resembles the one beneath the Hyblean Plateau is justified by the large geochemical affinities of the Etnean hawaiites/K-trachybasalts and the Hyblean hawaiites/alkali basalts for what concerns both trace elements and isotope systematics. We have modeled partial melting of a composite source constituted by two rock types, inferred by lithological and geochemical features of the Hyblean xenoliths: 1) a spinel lherzolite bearing metasomatic, hydrous phases and 2) a garnet pyroxenite in form of veins intruded into the spinel lherzolite. The partial melting modeling has been applied to each rock type and the resulting primary liquids have been then mixed in various proportions. These compositions have been compared with some Etnean alkaline magmas of the post ∼60 ka activity, which were firstly re-equilibrated to mantle conditions through mass balance calculations. Our results put into evidence that concentrations of major and trace elements along with the water obtained from the modeling are remarkably comparable with those of Etnean melts re-equilibrated at primary conditions. Different proportions of the spinel lherzolite with variable modal contents of metasomatic phases and of the garnet pyroxenite can therefore account for the signature of a large spectrum of Etnean alkaline magmas and for their geochemical variability through time, emphasizing the crucial role played by compositional small-scale heterogeneity of the source. These heterogeneities are

Mantle superplumes induce geomagnetic superchrons

Directory of Open Access Journals (Sweden)

Peter eOlson

2015-07-01

Full Text Available We use polarity reversal systematics from numerical dynamos to quantify the hypothesis that the modulation of geomagnetic reversal frequency, including geomagnetic superchrons, results from changes in core heat flux related to growth and collapse of lower mantle superplumes. We parameterize the reversal frequency sensitivity from numerical dynamos in terms of average core heat flux normalized by the difference between the present-day core heat flux and the core heat flux at geomagnetic superchron onset. A low-order polynomial fit to the 0-300 Ma Geomagnetic Polarity Time Scale (GPTS reveals that a decrease in core heat flux relative to present-day of approximately 30% can account for the Cretaceous Normal Polarity and Kiaman Reverse Polarity Superchrons, whereas the hyper-reversing periods in the Jurassic require a core heat flux equal to or higher than present-day. Possible links between GPTS transitions, large igneous provinces (LIPs, and the two lower mantle superplumes are explored. Lower mantle superplume growth and collapse induce GPTS transitions by increasing and decreasing core heat flux, respectively. Age clusters of major LIPs postdate transitions from hyper-reversing to superchron geodynamo states by 30-60 Myr, suggesting that superchron onset may be contemporaneous with LIP-forming instabilities produced during collapses of lower mantle superplumes.

The origin of volatiles in the Earth's mantle

Science.gov (United States)

Hier-Majumder, Saswata; Hirschmann, Marc M.

2017-08-01

The Earth's deep interior contains significant reservoirs of volatiles such as H, C, and N. Due to the incompatible nature of these volatile species, it has been difficult to reconcile their storage in the residual mantle immediately following crystallization of the terrestrial magma ocean (MO). As the magma ocean freezes, it is commonly assumed that very small amounts of melt are retained in the residual mantle, limiting the trapped volatile concentration in the primordial mantle. In this article, we show that inefficient melt drainage out of the freezing front can retain large amounts of volatiles hosted in the trapped melt in the residual mantle while creating a thick early atmosphere. Using a two-phase flow model, we demonstrate that compaction within the moving freezing front is inefficient over time scales characteristic of magma ocean solidification. We employ a scaling relation between the trapped melt fraction, the rate of compaction, and the rate of freezing in our magma ocean evolution model. For cosmochemically plausible fractions of volatiles delivered during the later stages of accretion, our calculations suggest that up to 77% of total H2O and 12% of CO2 could have been trapped in the mantle during magma ocean crystallization. The assumption of a constant trapped melt fraction underestimates the mass of volatiles in the residual mantle by more than an order of magnitude.Plain Language SummaryThe Earth's deep interior contains substantial amounts of volatile elements like C, H, and N. How these elements got sequestered in the Earth's interior has long been a topic of debate. It is generally assumed that most of these elements escaped the interior of the Earth during the first few hundred thousand years to create a primitive atmosphere, leaving the mantle reservoir nearly empty. In this work, we show that the key to this paradox involves the very early stages of crystallization of the mantle from a global magma ocean. Using numerical models, we show

Link between concentrations of sediment flux and deep crustal processes beneath the European Alps.

Science.gov (United States)

Garefalakis, Philippos; Schlunegger, Fritz

2018-01-09

Large sediment fluxes from mountain belts have the potential to cause megafans to prograde into the neighbouring sedimentary basins. These mechanisms have been documented based from numerical modelling and stratigraphic records. However, little attention has been focused on inferring temporal changes in the concentrations of supplied sediment from coarse-grained deposits. Here, we extract changes of this variable in the field from a Late Oligocene, c. 4 km-thick suite of fluvial conglomerates situated in the North Alpine foreland basin, which evolved in response to the tectonic and erosional history of the Alps. We measure a decrease in channel depths from >2 m to 20 cm from the base to the top of the suite. These constraints are used to calculate an increase in fan surface slopes from 1.0° based on the Shields criteria for sediment entrainment. We combine slope and bulk grain size data with the Bagnold equation for sediment transport to infer higher concentrations of the supplied sediment. We use these shifts to propose a change towards faster erosion and a steeper landscape in the Alpine hinterland, driven by mantle-scale processes beneath the Alps.

Traveltime Dispersion in an Isotropic Elastic Mantle: Dominance of the Lower Mantle Signal in Differential-frequency Time Residuals

Science.gov (United States)

Schuberth, B. S. A.; Zaroli, C.; Nolet, G.

2014-12-01

We study wavefield effects in elastic isotropic 3-D seismic structures derived from the temperature field of a high resolution mantle circulation model. More specifically, we quantify the structural dispersion of traveltime residuals of direct P- and S-waves in a model with realistic length-scales and magnitudes of the variations in seismic velocities and density. 3-D global wave propagation is simulated using a spectral element method, and traveltime residuals are measured in four different frequency bands by cross-correlation of 3-D and 1-D synthetic waveforms. Intrinsic (dissipative) attenuation is deliberately neglected, so that any variation of traveltimes with frequency can be attributed to structural effects. Additional simulations are performed for a model in which 3-D structure is removed in the upper 800 km to isolate the dispersion signal of the lower mantle. One question that we address is whether the structural length-scales inherent to a vigorously convecting mantle give rise to significant body-wave dispersion. In our synthetic dataset, the difference between long-period and short-period traveltime residuals generally increases with increasing short-period residual. However, we do not find an exact linear dependence, and in case of P-waves even non-monotonic behaviour. At largest short-period residuals, average dispersion is on the order of 2 s for both P- and S-waves and even larger when structure is confined to the lower mantle. Dispersion also appears to be asymmetric; that is, larger for negative than for positive residuals. The standard deviations of both P- and S-wave residuals also increase with increasing period and we discuss possible explanations for this behaviour. Overall, wavefield effects in both models are generally stronger for P-waves than for S-waves at the same frequencies. We also find that for certain combinations of periods, the difference between the respective residuals is very similar between the "whole mantle" and the "lower

Crust-mantle density distribution in the eastern Qinghai-Tibet Plateau revealed by satellite-derived gravity gradients

Science.gov (United States)

LI, Honglei; Fang, Jian; Braitenberg, Carla; Wang, Xinsheng

2015-04-01

tomography model. For example, the thickness of the uniform density blocks centered at140 km depth is as large as 60 km. Low-density crustal anomalies beneath the southern Lhasa and Songpan-Ganzi blocks in our model support the idea of weak lower crust and possible crustal flow, as a result of the thermal anomalies caused by the upwelling of hot deep materials. The weak lower crust may cause the decoupling of the upper crust and the mantle. These results are consistent with many other geophysical studies, confirming the effectiveness of the GOCE gravitational gradient data. Using these data in combination with other geodynamic constraints (e.g., gravity and seismic structure and preliminary reference Earth model), an improved dynamic model can be derived.

Potential Magma Chambers beneath the Tatun Volcanic Area, Taiwan: Results from Magnetotelluric Survey and Monitoring

Science.gov (United States)

Chen, C.

2013-12-01

Previous earthquakes analysis indicated existing seismicity anomaly beneath Tatun volcano, Taiwan, possibly caused by the fluid activity of the volcano. Helium isotope studies also indicated that over 60% of the fumarolic gases and vapors originated from deep mantle in the Tatun volcano area. The chemistry of the fumarolic gases and vapors and seismicity anomaly are important issues in view of possible magma chamber in the Tatun volcano, where is in the vicinity of metropolitan Taipei, only 15 km north of the capital city. In this study magnetotelluric (MT) soundings and monitoring were deployed to understand the geoelectric structures in the Tatun volcano as Electromagnetic methods are sensitive to conductivity contrasts and can be used as a supplementary tool to delineate reservoir boundaries. An anticline extending more than 10 km beneath the Chih-Shin-Shan and Da-You-Kan areas was recognized. Low resistivity at a shallow and highly porous layer 500m thick might indicate circulation of heated water. However, a high resistivity layer at depth between 2 and 6 km was detected. This layer could be associated with high micro-earthquakes zone. The characteristics of this layer produced by either the magma chamber or other geothermal activity were similar to that of some other active volcanic areas in the world. At 6 km underground was a dome structure of medium resistivity. This structure could be interpreted as a magma chamber in which the magma is possibly cooling down, as judged by its relatively high resistivity. The exact attributes of the magma chamber were not precisely determined from the limited MT soundings. At present, a joint monitors including seismic activity, ground deformation, volcanic gases, and changes in water levels and chemistry are conducted by universities and government agencies. When unusual activity is detected, a response team may do more ground surveys to better determine if an eruption is likely.

Mantle contamination and the Izu-Bonin-Mariana (IBM) 'high-tide mark': evidence for mantle extrusion caused by Tethyan closure

Science.gov (United States)

Flower, M. F. J.; Russo, R. M.; Tamaki, K.; Hoang, N.

2001-04-01

Western Pacific basins are characterized by three remarkable attributes: (1) complex kinematic histories linked to global-scale plate interactions; (2) DUPAL-like contaminated mantle; and (3) rapid post-Mesozoic rollback of the confining arc-trench systems. The coincidence of slab steepening, extreme arc curvature, and vigorous basin opening associated with the Mariana convergent margin suggests that rollback continues in response to an east-directed mantle 'wind'. Against a backdrop of conflicting kinematic and genetic interpretations we explore the notion that eastward asthenospheric flow driven by diachronous Tethyan closure caused stretching of eastern Eurasia and concomitant opening of western Pacific basins. Marking the eastern boundary of the latter, the Izu-Bonin-Mariana forearc may be regarded as a litho-tectonic 'high-tide mark' comprising igneous and metamorphic products from successive episodes (since ca. 45 Ma.) of arc sundering and backarc basin opening. The forearc also forms an isotopic boundary separating contaminated western Pacific mantle from the N-MORB Pacific Ocean reservoir. While the isotopic composition of western Pacific mantle resembles that feeding Indian Ocean hotspot and spreading systems, its spatial-temporal variation and the presence of subduction barriers to the south appear to preclude northward flow of Indian Ocean mantle and require an endogenous origin for sub-Eurasian contaminated mantle. It is concluded that the extrusion of Tethyan asthenosphere, contaminated by sub-Asian cratonic lithosphere, was a major cause of western Pacific arc rollback and basin opening. The model is consistent with paleomagnetic and geologic evidence supporting independent kinematic histories for constituent parts of the Philippine Sea and Sunda plates although interpretation of these is speculative. Compounded by effects of the Australia-Indonesia collision, late-Tethyan mantle extrusion appears to have produced the largest DUPAL domain in the

P-wave attenuation in the Pacific slab beneath northeastern Japan revealed by the spectral ratio of intraslab earthquakes

Science.gov (United States)

Shiina, Takahiro; Nakajima, Junichi; Matsuzawa, Toru

2018-05-01

We investigate P-wave attenuation, Qp-1, in the Pacific slab beneath northeastern (NE) Japan, adopting for the first time the spectral ratio technique for intraslab earthquakes. When seismograms of two earthquakes are recorded at a station and their ray paths to the station are largely overlapped, station-dependent amplification and structural effects on the overlapped rays can be canceled out from the ratio of the spectral amplitudes of the seismograms. Therefore, adopting the spectral ratio technique for intraslab earthquakes has a great advantage for the precise evaluation of Qp-1 in the slab because the structural effects above the slab, including the high-attenuation mantle wedge, are removed. For estimating the intraslab Qp-1, we determined corner frequency of the intraslab earthquakes using the S-coda wave spectral ratio as the first step. Then, we evaluated the inter-event path attenuation, Δt*, from the ratio of the spectral amplitudes of P waves. The obtained result shows that P-wave attenuation in the Pacific slab marks Qp-1 of 0.0015 (Qp of ∼670) at depths of 50-250 km. This indicates that the P-wave attenuation in the Pacific slab is weaker than that in the mantle wedge. The relatively high-Qp-1 is correlated with the distributions of intraslab earthquakes, suggesting that the P-wave amplitude is more attenuated around active seismicity zones in the slab. Therefore, our observations likely indicate the presence of fractures, hydrous minerals, and dehydrated fluid around seismogenic zones in the slab at intermediate depths.

Preface: Deep Slab and Mantle Dynamics

Science.gov (United States)

Suetsugu, Daisuke; Bina, Craig R.; Inoue, Toru; Wiens, Douglas A.

2010-11-01

We are pleased to publish this special issue of the journal Physics of the Earth and Planetary Interiors entitled "Deep Slab and Mantle Dynamics". This issue is an outgrowth of the international symposium "Deep Slab and Mantle Dynamics", which was held on February 25-27, 2009, in Kyoto, Japan. This symposium was organized by the "Stagnant Slab Project" (SSP) research group to present the results of the 5-year project and to facilitate intensive discussion with well-known international researchers in related fields. The SSP and the symposium were supported by a Grant-in-Aid for Scientific Research (16075101) from the Ministry of Education, Culture, Sports, Science and Technology of the Japanese Government. In the symposium, key issues discussed by participants included: transportation of water into the deep mantle and its role in slab-related dynamics; observational and experimental constraints on deep slab properties and the slab environment; modeling of slab stagnation to constrain its mechanisms in comparison with observational and experimental data; observational, experimental and modeling constraints on the fate of stagnant slabs; eventual accumulation of stagnant slabs on the core-mantle boundary and its geodynamic implications. This special issue is a collection of papers presented in the symposium and other papers related to the subject of the symposium. The collected papers provide an overview of the wide range of multidisciplinary studies of mantle dynamics, particularly in the context of subduction, stagnation, and the fate of deep slabs.

In-situ observation of Asian pollution transported into the Arctic lowermost stratosphere

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A. Roiger

2011-11-01

Full Text Available On a research flight on 10 July 2008, the German research aircraft Falcon sampled an air mass with unusually high carbon monoxide (CO, peroxyacetyl nitrate (PAN and water vapour (H₂O mixing ratios in the Arctic lowermost stratosphere. The air mass was encountered twice at an altitude of 11.3 km, ~800 m above the dynamical tropopause. In-situ measurements of ozone, NO, and NO_y indicate that this layer was a mixed air mass containing both air from the troposphere and stratosphere. Backward trajectory and Lagrangian particle dispersion model analysis suggest that the Falcon sampled the top of a polluted air mass originating from the coastal regions of East Asia. The anthropogenic pollution plume experienced strong up-lift in a warm conveyor belt (WCB located over the Russian east-coast. Subsequently the Asian air mass was transported across the North Pole into the sampling area, elevating the local tropopause by up to ~3 km. Mixing with surrounding Arctic stratospheric air most likely took place during the horizontal transport when the tropospheric streamer was stretched into long and narrow filaments. The mechanism illustrated in this study possibly presents an important pathway to transport pollution into the polar tropopause region.

Whole-mantle P-wave velocity structure and azimuthal anisotropy

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Yamamoto, Y.; Zhao, D.

2009-12-01

There are some hotspot volcanoes on Earth, such as Hawaii and Iceland. The mantle plume hypothesis was proposed forty years ago to explain hotspot volcanoes (e.g., Wilson, 1963; Morgan, 1971). Seismic tomography is a powerful technique to detect mantle plumes and determine their detailed structures. We determined a new whole-mantle 3-D P-wave velocity model (Tohoku model) using a global tomography method (Zhao, 2004, 2009). A flexible-grid approach with a grid interval of ~200 km is adopted to conduct the tomographic inversion. Our model shows that low-velocity (low-V) anomalies with diameters of several hundreds of kilometers are visible from the core-mantle boundary (CMB) to the surface under the major hotspot regions. Under South Pacific where several hotspots including Tahiti exist, there is a huge low-V anomaly from the CMB to the surface. This feature is consistent with the previous models. We conducted extensive resolution tests in order to understand whether this low-V anomaly shows a single superplume or a plume cluster. Unfortunately this problem is still not resolved because the ray path coverage in the mantle under South Pacific is not good enough. A network of ocean bottom seismometers is necessary to solve this problem. To better understand the whole-mantle structure and dynamics, we also conducted P-wave tomographic inversions for the 3-D velocity structure and azimuthal anisotropy. At each grid node there are three unknown parameters: one represents the isotropic velocity, the other two represent the azimuthal anisotropy. Our results show that in the shallow part of the mantle (Japan trench axis. In the Tonga subduction zone, the FVD is also perpendicular to the trench axis. Under the Tibetan region the FVD is NE-SW, which is parallel to the direction of the India-Asia collision. In the deeper part of the upper mantle and in the lower mantle, the amplitude of anisotropy is reduced. One interesting feature is that the FVD aligns in a radiated fashion

Circulation of carbon dioxide in the mantle: multiscale modeling

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Morra, G.; Yuen, D. A.; Lee, S.

2012-12-01

Much speculation has been put forward on the quantity and nature of carbon reservoirs in the deep Earth, because of its involvement in the evolution of life at the surface and inside planetary interiors. Carbon penetrates into the Earth's mantle mostly during subduction of oceanic crust, which contains carbonate deposits [1], however the form that it assumes at lower mantle depths is scarcely understood [2], hampering our ability to estimate the amount of carbon in the entire mantle by orders of magnitude. We present simulations of spontaneous degassing of supercritical CO2 using in-house developed novel implementations of the Fast-Multipole Boundary Element Method suitable for modeling two-phase flow (here mantle mineral and free CO2 fluid) through disordered materials such as porous rocks. Because the mutual interaction of droplets immersed either in a fluid or a solid matrix and their weakening effect to the host rock alters the strength of the mantle rocks, at the large scale the fluid phases in the mantle may control the creeping of mantle rocks [3]. In particular our study focuses on the percolation of supercritical CO2, estimated through the solution of the Laplace equation in a porous system, stochastically generated through a series of random Karhunen-Loeve decomposition. The model outcome is employed to extract the transmissivity of supercritical fluids in the mantle from the lowest scale up to the mantle scale and in combination with the creeping flow of the convecting mantle. The emerging scenarios on the global carbon cycle are finally discussed. [1] Boulard, E., et al., New host for carbon in the deep Earth. Proceedings of the National Academy of Sciences, 2011. 108(13): p. 5184-5187. [2] Walter, M.J., et al., Deep Mantle Cycling of Oceanic Crust: Evidence from Diamonds and Their Mineral Inclusions. Science, 2011. 334(6052): p. 54-57. [3] Morra, G., et al., Ascent of Bubbles in Magma Conduits Using Boundary Elements and Particles. Procedia Computer

Water Distribution in the Continental and Oceanic Upper Mantle

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Peslier, Anne H.

2015-01-01

Nominally anhydrous minerals such as olivine, pyroxene and garnet can accommodate tens to hundreds of ppm H2O in the form of hydrogen bonded to structural oxygen in lattice defects. Although in seemingly small amounts, this water can significantly alter chemical and physical properties of the minerals and rocks. Water in particular can modify their rheological properties and its distribution in the mantle derives from melting and metasomatic processes and lithology repartition (pyroxenite vs peridotite). These effects will be examined here using Fourier transform infrared spectrometry (FTIR) water analyses on minerals from mantle xenoliths from cratons, plume-influenced cratons and oceanic settings. In particular, our results on xenoliths from three different cratons will be compared. Each craton has a different water distribution and only the mantle root of Kaapvaal has evidence for dry olivine at its base. This challenges the link between olivine water content and survival of Archean cratonic mantle, and questions whether xenoliths are representative of the whole cratonic mantle. We will also present our latest data on Hawaii and Tanzanian craton xenoliths which both suggest the intriguing result that mantle lithosphere is not enriched in water when it interacts with melts from deep mantle upwellings (plumes).

Reconstructing mantle heterogeneity with data assimilation based on the back-and-forth nudging method: Implications for mantle-dynamic fitting of past plate motions

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Glišović, Petar; Forte, Alessandro

2016-04-01

The paleo-distribution of density variations throughout the mantle is unknown. To address this question, we reconstruct 3-D mantle structure over the Cenozoic era using a data assimilation method that implements a new back-and-forth nudging algorithm. For this purpose, we employ convection models for a compressible and self-gravitating mantle that employ 3-D mantle structure derived from joint seismic-geodynamic tomography as a starting condition. These convection models are then integrated backwards in time and are required to match geologic estimates of past plate motions derived from marine magnetic data. Our implementation of the nudging algorithm limits the difference between a reconstruction (backward-in-time solution) and a prediction (forward-in-time solution) on over a sequence of 5-million-year time windows that span the Cenozoic. We find that forward integration of reconstructed mantle heterogeneity that is constrained to match past plate motions delivers relatively poor fits to the seismic-tomographic inference of present-day mantle heterogeneity in the upper mantle. We suggest that uncertainties in the past plate motions, related for example to plate reorganization episodes, could partly contribute to the poor match between predicted and observed present-day heterogeneity. We propose that convection models that allow tectonic plates to evolve freely in accord with the buoyancy forces and rheological structure in the mantle could provide additional constraints on geologic estimates of paleo-configurations of the major tectonic plates.

Tracing Mantle Plumes: Quantifying their Morphology and Behavior from Seismic Tomography

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O'Farrell, K. A.; Eakin, C. M.; Jones, T. D.; Garcia, E.; Robson, A.; Mittal, T.; Lithgow-Bertelloni, C. R.; Jackson, M. G.; Lekic, V.; Rudolph, M. L.

2016-12-01

Hotspot volcanism provides a direct link between the deep mantle and the surface, but the location, depth and source of the mantle plumes that feed hotspots are highly controversial. In order to address this issue it is important to understand the journey along which plumes have travelled through the mantle. The general behavior of plumes in the mantle also has the potential to tell us about the vigor of mantle convection, net rotation of the mantle, the role of thermal versus chemical anomalies, and important bulk physical properties of the mantle such as the viscosity profile. To address these questions we developed an algorithm to trace plume-like features in shear-wave (Vs) seismic tomographic models based on picking local minima in velocity and searching for continuous features with depth. We apply this method to several of the latest tomographic models and can recover 30 or more continuous plume conduits that are >750 km long. Around half of these can be associated with a known hotspot at the surface. We study the morphology of these plume chains and find that the largest lateral deflections occur near the base of the lower mantle and in the upper mantle. We analyze the preferred orientation of the plume deflections and their gradient to infer large scale mantle flow patterns and the depth of viscosity contrasts in the mantle respectively. We also retrieve Vs profiles for our traced plumes and compare with velocity profiles predicted for different mantle adiabat temperatures. We use this to constrain the thermal anomaly associated with these plumes. This thermal anomaly is then converted to a density anomaly and an upwelling velocity is derived. We compare this to buoyancy fluxes calculated at the surface and use this in conjunction with our measured plume tilts/deflections to estimate the strength of the "mantle wind".

Lithospheric shear velocity structure of South Island, New Zealand, from amphibious Rayleigh wave tomography

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Ball, Justin S.; Sheehan, Anne F.; Stachnik, Joshua C.; Lin, Fan-Chi; Yeck, William L.; Collins, John A.

2016-05-01

We present a crust and mantle 3-D shear velocity model extending well offshore of New Zealand's South Island, imaging the lithosphere beneath the South Island as well as the 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 4 land-based and 29 ocean bottom instruments deployed off the South Island's east and west coasts in 2009-2010 by the Marine Observations of Anisotropy Near Aotearoa experiment with 28 land-based seismometers from New Zealand's permanent GeoNet array. Major features of our shear wave velocity (Vs) model include a low-velocity (Vs 50 km) beneath the central South Island exhibits strong spatial correlation with upper mantle earthquake hypocenters beneath the Alpine Fault. The ~400 km long low-velocity zone we image beneath eastern South Island and the inner Bounty Trough underlies Cenozoic volcanics and the locations of mantle-derived helium measurements, consistent with asthenospheric upwelling in the region.

A Comparative Study of Continental vs. Intraoceanic Arc Mantle Melting: Experimentally Determined Phase Relations of Hydrous, Primitive Melts

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Weaver, S.; Johnston, A.; Wallace, P. J.

2009-12-01

It is widely recognized that H2O and other volatiles play a crucial role in mantle melting in subduction zones. This work is a comparative study focused on determining the H2O-undersaturated, near-liquidus phase relations for two primitive subduction related compositions with the goal of determining the P-T-H2O conditions of mantle melting beneath arcs. These samples, JR-28, a calc-alkaline basalt from Volcan Jorullo, Mexico, and ID-16, a tholeiitic basalt from Okmok Volcano, Aleutian Islands, have major element compositions that indicate they are primary, mantle-derived melts. H2O-undersaturated piston cylinder experiments have been carried out at upper mantle pressures and temperatures (1.0-2.0 GPa and 1100-1350°C). The near-liquidus mineralogy of these two compositions has been mapped in P-T- H2O space in order to constrain the conditions under which these melts are multiply saturated with a mantle residue (lherzolite or harzburgite). Previous studies of dissolved volatiles in olivine-hosted melt inclusions have provided an estimate of pre-eruptive H2O-contents for JR-28 at ≥5 wt% H2O and experiments have been carried out accordingly. Preliminary results for JR-28 at 5 wt% H2O show olivine ± Cr-rich spinel on the liquidus at 1.0 GPa and enstatite as the liquidus phase at higher pressures (1.3 to 2.0 GPa). Ca-rich pyroxene appears in only one experiment 50°C below the liquidus at 1.5 GPa. These data show that JR-28 melts are multiply saturated with a harzburgite assemblage at ~1175°C and ~1.2 GPa at 5 wt% H2O. Experiments at 7 wt% H2O show similar results, although the olivine/Cr-spinel stability field expands at the expense of the enstatite stability field. Consequently, the olivine-enstatite cotectic is shifted to higher pressures and slightly cooler temperatures. The relatively high SiO2 content in the bulk rock (~52 wt% SiO2) supports the hypothesis that JR-28 last equilibrated with a depleted or harzburgite residue rather than a more fertile mantle