Sample records for subducted african lithosphere

  1. Subducting Plate Breakup by Plume-Lithosphere Interaction (United States)

    Koptev, A.; Gerya, T.; Jolivet, L.; Leroy, S. D.


    We use a 3D high-resolution thermo-mechanical modeling to investigate the impact of active mantle plume on a subducting lithospheric plate. Initial model setup consists of an overriding continental lithosphere and subducting lithospheric plate including oceanic and continental lithosphere. A mantle plume thermal anomaly has been initially seeded at the bottom of the model box underneath the continental segment of subducting plate. Mantle plume impingement on lithospheric bottom leads to thinning of continental lithosphere and decompressional melting of both lithospheric and sublithospheric mantle along stretched trench-parallel zone. Further continental breakup is followed by opening of an oceanic basin separating a newly formed microcontinent from the main subducting continent. Despite continuous push applied at the boundary of subducting plate, plume-induced oceanic basin opens during several Myrs reaching several hundred kilometers wide. Cooling of the mantle plume and beginning of collision between the separated microcontinent and the overriding continental plate lead to gradual closure of newly formed oceanic basin that gets further involved into subduction and collision. The final stage sees continental subduction of main body of subducting plate and simultaneous tectonic exhumation of the upper crust of the subducted microcontinent. This scenario involving a plume-induced rifting of a microcontinent away from main body of subducted plate can be compared to the Mesozoic-Cenozoic development of the African plate characterized by the consecutive separation of the Apulian microcontinent and Arabian plate (in the Jurassic and the Neogene, respectively) during subduction of Neo-Tethys oceanic lithosphere beneath the Eurasian margin.

  2. Evidence for retrograde lithospheric subduction on Venus (United States)

    Sandwell, David T.; Schubert, Gerald


    Annular moats and outer rises around large Venus coronas such as Artemis, Latona, and Eithinoha are similar in arcuate planform and topography to the trenches and outer rises of terrestrial subduction zones. On earth, trenches and outer rises are modeled as the flexural response of a thin elastic lithosphere to the bending moment of the subducted slab; this lithospheric flexure model also accounts for the trenches and outer rises outboard of the major coronas on Venus. Accordingly, it is proposed that retrograde lithospheric subduction may be occurring on the margins of the large Venus coronas while compensating back-arc extension is occurring in the expanding coronas interiors. Similar processes may be taking place at other deep arcuate trenches or chasmata on Venus such as those in the Dali-Diana chasmata area of aestern Aphrodite Terra.

  3. Convective Removal of Continental Margin Lithosphere at the Edges of Subducting Oceanic Plates (United States)

    Levander, A.; Bezada, M. J.; Palomeras, I.; Masy, J.; Humphreys, E.; Niu, F.


    Although oceanic lithosphere is continuously recycled to the deeper mantle by subduction, the rates and manner in which different types of continental lithospheric mantle are recycled is unclear. Cratonic mantle can be chemically reworked and essentially decratonized, although the frequency of decratonization is unclear. Lithospheric mantle under or adjacent to orogenic belts can be lost to the deeper mantle by convective downwellings and delamination phenomena. Here we describe how subduction related processes at the edges of oceanic plates adjacent to passive continental margins removes the mantle lithosphere from beneath the margin and from the continental interior. This appears to be a widespread means of recycling non-cratonic continental mantle. Lithospheric removal requires the edge of a subducting oceanic plate to be at a relatively high angle to an adjacent passive continental margin. From Rayleigh wave and body wave tomography, and receiver function images from the BOLIVAR and PICASSO experiments, we infer large-scale removal of continental margin lithospheric mantle from beneath 1) the northern South American plate margin due to Atlantic subduction, and 2) the Iberian and North African margins due to Alboran plate subduction. In both cases lithospheric mantle appears to have been removed several hundred kilometers inland from the subduction zones. This type of ';plate-edge' tectonics either accompanies or pre-conditions continental margins for orogenic activity by thinning and weakening the lithosphere. These processes show the importance of relatively small convective structures, i.e. small subducting plates, in formation of orogenic belts.

  4. Subduction-driven recycling of continental margin lithosphere. (United States)

    Levander, A; Bezada, M J; Niu, F; Humphreys, E D; Palomeras, I; Thurner, S M; Masy, J; Schmitz, M; Gallart, J; Carbonell, R; Miller, M S


    Whereas subduction recycling of oceanic lithosphere is one of the central themes of plate tectonics, the recycling of continental lithosphere appears to be far more complicated and less well understood. Delamination and convective downwelling are two widely recognized processes invoked to explain the removal of lithospheric mantle under or adjacent to orogenic belts. Here we relate oceanic plate subduction to removal of adjacent continental lithosphere in certain plate tectonic settings. We have developed teleseismic body wave images from dense broadband seismic experiments that show higher than expected volumes of anomalously fast mantle associated with the subducted Atlantic slab under northeastern South America and the Alboran slab beneath the Gibraltar arc region; the anomalies are under, and are aligned with, the continental margins at depths greater than 200 kilometres. Rayleigh wave analysis finds that the lithospheric mantle under the continental margins is significantly thinner than expected, and that thin lithosphere extends from the orogens adjacent to the subduction zones inland to the edges of nearby cratonic cores. Taking these data together, here we describe a process that can lead to the loss of continental lithosphere adjacent to a subduction zone. Subducting oceanic plates can viscously entrain and remove the bottom of the continental thermal boundary layer lithosphere from adjacent continental margins. This drives surface tectonics and pre-conditions the margins for further deformation by creating topography along the lithosphere-asthenosphere boundary. This can lead to development of secondary downwellings under the continental interior, probably under both South America and the Gibraltar arc, and to delamination of the entire lithospheric mantle, as around the Gibraltar arc. This process reconciles numerous, sometimes mutually exclusive, geodynamic models proposed to explain the complex oceanic-continental tectonics of these subduction zones.

  5. The Geodynamics of Continental Lithosphere Entering a Subduction Zone (United States)

    Steedman, C. E.; Kaus, B. J.; Becker, T. W.; Okaya, D.; Wu, F. T.


    As deformation patterns resulting from subduction of a passive continental margin are insufficiently understood, here we perform 2-D numerical simulations to explore the effects of continental lithosphere entering a subduction zone. The model setup consists of a subduction zone in which the oceanic part of a passive continental margin initially subducts beneath an oceanic plate. A particle-based 2-D visco-elasto-plastic thermo-mechanical finite element code is employed to study the dynamics of the system. A novel new feature of the code is that the resolution of the model can be significantly increased in selected parts of the domain, which allows for self-consistent modelling of mantle-lithosphere interaction. In the present study this feature is employed to study how crustal scale deformation around the subduction zone is influenced by surface processes and by flow in the upper mantle. Using systematic 2-D numerical simulations, we explore the parameters that are dominant in controlling near- surface structures, both with regards to changes in topography over time, and subsurface features such as Moho undulations. The main parameters that have been varied are: the lithospheric density structure; the strength of the lower crust; the amounts of erosion; imposed pushing versus density-driven (slab-pull and ridge- push) convergence; the upper boundary condition (free surface versus free slip); rheology (non-Newtonian versus Newtonian, viscous, visco-elasto-plastic); and finally the effect of an imposed slab breakoff. In all cases we track surface uplift, subduction evolution and rock exhumation history. The results can be compared to evidence from areas such as Taiwan where continental subduction or convergence is thought to be happening. Preliminary results indicate that a low viscosity lower crust may contribute to crustal uplift.

  6. Using open sidewalls for modelling self-consistent lithosphere subduction dynamics

    NARCIS (Netherlands)

    Chertova, M.V.; Geenen, T.; van den Berg, A.; Spakman, W.


    Subduction modelling in regional model domains, in 2-D or 3-D, is commonly performed using closed (impermeable) vertical boundaries. Here we investigate the merits of using open boundaries for 2-D modelling of lithosphere subduction. Our experiments are focused on using open and closed (free

  7. Uplift of the Colorado Plateau due to lithosphere attenuation during Laramide low-angle subduction (United States)

    Spencer, J.E.


    The Colorado Plateau is blanketed by Phanerozoic marine and nonmarine strata as young as Cretaceous that are now exposed at elevations of about 2 km. Crustal thickening due to magmatism and horizontal crustal shortening was far less than necessary to cause this uplift, which is commonly attributed to the consequences of mantle lithosphere thinning and heating. The Colorado Plateau and the midcontinent region around Iowa consist of Precambrian bedrock overlain by a similar amount of Paleozoic platformal strata, and thus both regions once had similar lithospheric buoyancy. Mesozoic sedimentation increased the crustal thickness and lithospheric buoyancy of the Colorado Plateau relative to the midcontinent region. Backstripping calculations yield elevation without these sediments and lead to a calculated elevation difference between the two areas of about 1200 m, which represents unexplained plateau uplift. Review of constraints on uplift timing finds little support for a late Cenozoic uplift age and allows early to middle Cenozoic uplift, which is consistent with uplift mechanisms related to low-angle subduction that ended in the middle Cenozoic. Finite element heat flow calculations of low-angle subduction and lithosphere attenuation, using a range of initial lithosphere thicknesses and degree of attenuation, indicate that required uplift can result from tectonic removal of about 120 km of mantle lithosphere from an initially 200-km-thick lithosphere. This allows for partial preservation of North American mantle lithosphere with its distinctive isotopic signature in some late Cenozoic volcanic rocks and is consistent with normal Pn velocities in the uppermost mantle beneath the plateau.

  8. Geodynamic models of continental subduction and obduction of overriding plate forearc oceanic lithosphere on top of continental crust

    NARCIS (Netherlands)

    Edwards, Sarah J.; Schellart, Wouter P.; Duarte, Joao C.


    Continental subduction takes place in the final stage of subduction when all oceanic lithosphere is consumed and continental passive margin is pulled into the mantle. When the overriding plate is oceanic, dense forearc oceanic lithosphere might be obducted onto light continental crust forming an

  9. Structure and evolution of subducted lithosphere beneath the Sunda arc, Indonesia

    NARCIS (Netherlands)

    Widiyantoro, Sri; Hilst, R.D. van der


    Tomographic imaging reveals seismic anomalies beneath the Sunda island arc, Indonesia, that suggest that the lithospheric slab penetrates to a depth of at least 1500 kilometers. The Sunda slab forms the eastern end of a deep anomaly associated with the past subduction of the plate underlying the

  10. Tectonic implications of tomographic images of subducted lithosphere beneath northwestern South America

    NARCIS (Netherlands)

    Hilst, R.D. van der; Mann, P.


    We used seismic tomography to investigate the complex structure of the upper mantle below northwestern South America. Images of slab structure not delineated by previous seismicity studies help us to refine existing tectonic models of subducted Caribbean-Pacific lithosphere beneath the study area.

  11. Shallow and buoyant lithospheric subduction : causes and implications from thermo-chemical numerical modeling

    NARCIS (Netherlands)

    Hunen, Jeroen van


    Where two lithospheric plates converge on the Earth, one of them disappears into the mantle. The dominant driving mechanism for plate motion is regarded to be `slab pull': the subducted plate, the slab, exerts a pulling force on the attached plate at the surface. However, what has been puzzling

  12. Deformation Patterns and Subduction Behavior of Continental Lithosphere Entering a Trench (United States)

    Steedman, C. E.; Kaus, B. J.; Becker, T. W.; Okaya, D.


    We perform 2-D numerical simulations of continental lithosphere entering a subduction zone, to better understand deformation patterns resulting from subduction of a continental margin. The model consists of a subduction zone in which an attached slab drives subduction of a passive continental margin beneath an oceanic plate. A particle-based 2-D visco-elasto-plastic thermo-mechanical finite element code is employed to study the dynamics of the system. A novel feature of the code is that the resolution of the model can be significantly increased in selected parts of the domain, which allows for self-consistent modelling of mantle-lithosphere interaction. In the present study we employ this feature to study how lithospheric-scale deformation around and within the subduction zone is influenced by surface processes such as erosion, and by flow in the upper mantle. Using systematic 2-D numerical simulations, we explore the parameters that are dominant in controlling near- surface structures, both with regards to changes in topography and trench location, and subsurface features such as Moho undulations. The main parameters that have been varied are: the lithospheric density structure; the lithospheric age and temperature structure; the strength of the lower crust; the presence of a weak zone at the plate interface; the amounts of erosion; the upper boundary condition (free surface versus free slip); rheology (non-Newtonian versus Newtonian, viscous, visco-elasto-plastic); and finally the effect of an imposed slab breakoff. In all cases we track surface uplift, subduction evolution and rock exhumation history. We find that the strength of the overriding plate influences surface uplift and the shape of subsurface deformation, and that the density and thermal structure of the subducting plate affects trench motion. Denser slab roll back, and younger, lighter slabs advance, while neither slab rheology nor the presence of erosion greatly affect trench location. For all cases

  13. Breaking the oceanic lithosphere of a subducting slab: the 2013 Khash, Iran earthquake (United States)

    Barnhart, William D.; Hayes, Gavin P.; Samsonov, S.; Fielding, E.; Seidman, L.


    [1] Large intermediate depth, intraslab normal faulting earthquakes are a common, dangerous, but poorly understood phenomenon in subduction zones owing to a paucity of near field geophysical observations. Seismological and high quality geodetic observations of the 2013 Mw7.7 Khash, Iran earthquake reveal that at least half of the oceanic lithosphere, including the mantle and entire crust, ruptured in a single earthquake, confirming with unprecedented resolution that large earthquakes can nucleate in and rupture through the oceanic mantle. A rupture width of at least 55 km is required to explain both InSAR observations and teleseismic waveforms, with the majority of slip occurring in the oceanic mantle. Combining our well-constrained earthquake slip distributions with the causative fault orientation and geometry of the local subduction zone, we hypothesize that the Khash earthquake likely occurred as the combined result of slab bending forces and dehydration of hydrous minerals along a preexisting fault formed prior to subduction.

  14. Finding the last 200Ma of subducted lithosphere in tomography and incorporating it into plate reconstructions (United States)

    Suppe, J.; Wu, J.; Chen, Y. W.


    Precise plate-tectonic reconstruction of the Earth has been constrained largely by the seafloor magnetic-anomaly record of the present oceans formed during the dispersal of the last supercontinent since 200Ma. The corresponding world that was lost to subduction has been only sketchily known. We have developed methodologies to map in 3D these subducted slabs of lithosphere in seismic tomography and unfold them to the Earth surface, constraining their initial size, shapes and locations. Slab edges are commonly formed at times of plate reorganization (for example bottom edges typically record initiation of subduction) such that unfolded slabs fit together at times of reorganization, as we illustrate for the Nazca slab at 80Ma and the western Pacific slabs between Kamchatka and New Zealand at 50Ma. Mapping to date suggests that a relatively complete and decipherable record of lithosphere subducted over the last 200Ma may exist in the mantle today, providing a storehouse for new discoveries. We briefly illustrate our procedure for obtaining slab-constrained plate-tectonic models from tomography with our recent study of the Philippine Sea plate, whose motions and tectonic history have been the least known of the major plates because it has been isolated from the global plate and hotspot circuit by trenches. We mapped and unfolded 28 subducted slabs in the mantle under East Asia and Australia/Oceania to depths of 1200km, with a subducted area of 25% of present-day global oceanic lithosphere, and incorporated them as constraints into a new globally-consistent plate reconstruction of the Philippine Sea and surrounding East Asia, leading to a number of new insights, including: [1] discovery of a major (8000 km x 2500 km) set of vanished oceans that we call the East Asia Sea that existed between the Pacific and Indian Oceans, now represented by flat slabs in the lower mantle under present-day Philippine Sea, eastern Sundaland and northern Australia and [2] the Philippine Sea

  15. Plate coupling across the northern Manila subduction zone deduced from mantle lithosphere buoyancy (United States)

    Lo, Chung-Liang; Doo, Wen-Bin; Kuo-Chen, Hao; Hsu, Shu-Kun


    The Manila subduction zone is located at the plate boundary where the Philippine Sea plate (PSP) moves northwestward toward the Eurasian plate (EU) with a high convergence rate. However, historically, no large earthquakes greater than Mw7 have been observed across the northern Manila subduction zone. The poorly understood plate interaction between these two plates in this region creates significant issues for evaluating the seismic hazard. Therefore, the variation of mantle lithospheric buoyancy is calculated to evaluate the plate coupling status across the northern Manila subduction zone, based on recently published forward gravity modeling constrained by the results of the P-wave seismic crustal structure of the TAIGER (Taiwan Integrated Geodynamic Research) project. The results indicate weak plate coupling between the PSP and EU, which could be related to the release of the overriding PSP from the descending EU's dragging force, which was deduced from the higher elevation of the Luzon arc and the fore-arc basin northward toward the Taiwan orogen. Moreover, serpentinized peridotite is present above the plate boundary and is distributed more widely and thickly closer to offshore southern Taiwan orogen. We suggest that low plate coupling may facilitate the uplifting of serpentinized mantle material up to the plate boundary.

  16. The Two Subduction Zones of the Southern Caribbean: Lithosphere Tearing and Continental Margin Recycling in the East, Flat Slab Subduction and Laramide-Style Uplifts in the West (United States)

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


    The southern Caribbean plate boundary is a complex strike-slip fault system bounded by oppositely vergent subduction zones, the Antilles subduction zone in the east, and a currently locked Caribbean-South American subduction zone in the west (Bilham and Mencin, 2013). Finite-frequency teleseismic P-wave tomography images both the Atlanic (ATL) and the Caribbean (CAR) plates subducting steeply in opposite directions to transition zone depths under northern South America. Ps receiver functions show a depressed 660 discontinuity and thickened transition zone associated with each subducting plate. In the east the oceanic (ATL) part of the South American (SA) plate subducts westward beneath the CAR, initiating the El Pilar-San Sebastian strike slip system, a subduction-transform edge propagator (STEP) fault (Govers and Wortel, 2005). The point at which the ATL tears away from SA as it descends into the mantle is evidenced by the Paria cluster seismicity at depths of 60-110 km (Russo et al, 1993). Body wave tomography and lithosphere-asthenosphere boundary (LAB) thickness determined from Sp and Ps receiver functions and Rayleigh waves suggest that the descending ATL also viscously removes the bottom third to half of the SA continental margin lithospheric mantle as it descends. This has left thinned continental lithosphere under northern SA in the wake of the eastward migrating Antilles subduction zone. The thinned lithosphere occupies ~70% of the length of the El Pilar-San Sebastian fault system, from ~64oW to ~69oW, and extends inland several hundred kilometers. In northwestern SA the CAR subducts east-southeast at low angle under northern Colombia and western Venezuela. The subducting CAR is at least 200 km wide, extending from northernmost Colombia as far south as the Bucaramanga nest seismicity. The CAR descends steeply under Lake Maracaibo and the Merida Andes. This flat slab is associated with three Neogene basement cored, Laramide-style uplifts: the Santa Marta

  17. Lithospheric expression of cenozoic subduction, mesozoic rifting and the Precambrian Shield in Venezuela (United States)

    Masy, Jeniffer; Niu, Fenglin; Levander, Alan; Schmitz, Michael


    We have combined surface wave tomography with Ps and Sp receiver-function images based on common-conversion-point (CCP) stacking to study the upper mantle velocity structure, particularly the lithosphere-asthenosphere boundary (LAB), beneath eastern and central Venezuela. Rayleigh phase velocities in the frequency range of 0.01-0.05 Hz (20-100 s in period) were measured using the two-plane-wave method and finite-frequency kernels, and then inverted on a 0.5° × 0.5° grid. The phase velocity dispersion data at grid points were inverted for 1D shear velocity profiles using initial crust-mantle velocity models constructed from previous studies. The 3D velocity model and receiver-function images were interpreted jointly to determine the depth of the LAB and other upper mantle features. The tomographic images revealed two high velocity anomalies extending to more than ∼200 km depth. One corresponds to the top of the subducting Atlantic plate beneath the Serrania del Interior. The other anomaly is a highly localized feature beneath the Maturin Basin. The LAB depth varies significantly in the study region: It is located at ∼110 km depth beneath the Guayana Shield, and reaches ∼130 km at the northern edge of the Maturin Basin, which might be related to the downward flexural bending due to thrust loading of the Caribbean plate and pull from the subducting Atlantic plate. Immediately to the west, the lithosphere is thin (∼50-60 km) along the NE-SW trending Espino Graben from the Cariaco basin to the Orinoco River at the northern edge of the craton. The LAB in this region is the top of a pronounced low velocity zone. Westward, the lithosphere deepens to ∼80 km depth beneath the Barinas Apure Basin, and to ∼90 km beneath the Neogene Merida Andes and Maracaibo block. Both upper mantle velocity structure and lithosphere thickness correlate well with surface geology and are consistent with northern South American tectonics.

  18. Dehydration of subducting slow-spread oceanic lithosphere in the Lesser Antilles (United States)

    Paulatto, Michele; Laigle, Mireille; Galve, Audrey; Charvis, Philippe; Sapin, Martine; Bayrakci, Gaye; Evain, Mikael; Kopp, Heidrun


    Subducting slabs carry water into the mantle and are a major gateway in the global geochemical water cycle. Fluid transport and release can be constrained with seismological data. Here we use joint active-source/local-earthquake seismic tomography to derive unprecedented constraints on multi-stage fluid release from subducting slow-spread oceanic lithosphere. We image the low P-wave velocity crustal layer on the slab top and show that it disappears beneath 60–100 km depth, marking the depth of dehydration metamorphism and eclogitization. Clustering of seismicity at 120–160 km depth suggests that the slab’s mantle dehydrates beneath the volcanic arc, and may be the main source of fluids triggering arc magma generation. Lateral variations in seismic properties on the slab surface suggest that serpentinized peridotite exhumed in tectonized slow-spread crust near fracture zones may increase water transport to sub-arc depths. This results in heterogeneous water release and directly impacts earthquakes generation and mantle wedge dynamics. PMID:28691714

  19. Dehydration of subducting slow-spread oceanic lithosphere in the Lesser Antilles (United States)

    Paulatto, Michele; Laigle, Mireille; Galve, Audrey; Charvis, Philippe; Sapin, Martine; Bayrakci, Gaye; Evain, Mikael; Kopp, Heidrun


    Subducting slabs carry water into the mantle and are a major gateway in the global geochemical water cycle. Fluid transport and release can be constrained with seismological data. Here we use joint active-source/local-earthquake seismic tomography to derive unprecedented constraints on multi-stage fluid release from subducting slow-spread oceanic lithosphere. We image the low P-wave velocity crustal layer on the slab top and show that it disappears beneath 60-100 km depth, marking the depth of dehydration metamorphism and eclogitization. Clustering of seismicity at 120-160 km depth suggests that the slab's mantle dehydrates beneath the volcanic arc, and may be the main source of fluids triggering arc magma generation. Lateral variations in seismic properties on the slab surface suggest that serpentinized peridotite exhumed in tectonized slow-spread crust near fracture zones may increase water transport to sub-arc depths. This results in heterogeneous water release and directly impacts earthquakes generation and mantle wedge dynamics.

  20. The characteristics of mantle lithosphere buoyancy revealed from the northern Manila subduction zone to the active collision in Taiwan region (United States)

    Lo, Chung-Liang; Doo, Wen-Bin; Kuo-Chen, Hao; Hsu, Shu-Kun; Lin, Jing-Yi


    It has been widely studied on the complexity tectonic structure in the active Taiwan orogenesis, since the converging between the Philippine Sea plate (PSP) and the Eurasian plate (EU) along with the Manila subduction zone extended from the Philippine to offshore the southern Taiwan and the Ryukyu subduction zone in the east. Considering the separate contribution of the crust and the mantle lithosphere to the topography, we try to examine the mantle lithosphere buoyancy (Hm) behavior from the northern Manila subduction zone to the active collision in Taiwan region. In this study, we present several Hm profiles across the northern Manila subduction zone and the Taiwan island. In order to calculate the Hm, the crust structures are constrained by the forward gravity modeling, in which the density is provided from the multi-channel seismic data and on land seismic data (thanks to the Taiwan Integrated Geodynamic Research (TAIGER) project). The result shows that the Hm across the northern Manila subduction zone displays apparent undulations, and undulates more drastic approaching the north end of the subduction zone. It implies that the plate coupling between the PSP and the EU here is weak. The Hm across the southern Taiwan undulates still, but the amplitudes are smaller with relative gentle undulations. This reflects the contribution from the slab underneath while the initial collision occurs in south Taiwan. Into the central Taiwan, the Hm pattern behaves undulating mild comparing with that across the subduction zone because the slab structure effects not obvious. Besides, the Hm in the central Taiwan primarily is affects by both the thickening crust and high elevation caused by the strong lateral external compression stress.

  1. Understanding lithospheric stresses: systematic analysis of controlling mechanisms with applications to the African Plate (United States)

    Medvedev, Sergei


    Many mechanisms control the state of stress within Earth plates. First-order well-known mechanisms include stresses induced by lateral variations of lithospheric density structure, sublithospheric tractions, ridge push and subduction pull. In this study, we attempt to quantify the influence of these mechanisms to understand the origin of stresses in the lithosphere, choosing the African plate (TAP) as an example. A finite-element based suite, Proshell, was developed to combine several data sets, to estimate the gravitational potential energy (GPE) of the lithosphere and to calculate stresses acting on the real (non-planar) geometry of TAP. We introduce several quantitative parameters to measure the degree of fit between the model and observations. Our modelling strategy involves nine series of numerical experiments. We start with the simplest possible model and then, step by step, build it up to be a more physically realistic model, all the while discussing the influence of each additional component. The starting (oversimplified) model series (1) is based on the CRUST2 data set for the crust and a half-space-cooling approximation of the lithospheric mantle. We then describe models (series 2-5) that account for lithospheric mantle density heterogeneities to build a more reliable GPE model. The consecutive series involve basal traction from the convective mantle (series A, C) and the rheological heterogeneity of the TAP via variations in its effective elastic thickness (series B, C). The model quality reflects the increase in complexity between series with an improving match to observed stress regimes and directions. The most complex model (series D) also accounts for the bending stresses in the elastic lithosphere and achieves a remarkably good fit to observations. All of our experiments were based on the iteration of controlling parameters in order to achieve the best fit between modelled and observed stresses, always considering physically feasible values. This

  2. Teleseismic P wave tomography of South Island, New Zealand upper mantle: Evidence of subduction of Pacific lithosphere since 45 Ma (United States)

    Zietlow, Daniel W.; Molnar, Peter H.; Sheehan, Anne F.


    A P wave speed tomogram produced from teleseismic travel time measurements made on and offshore the South Island of New Zealand shows a nearly vertical zone with wave speeds that are 4.5% higher than the background average reaching to depths of approximately 450 km under the northwestern region of the island. This structure is consistent with oblique west-southwest subduction of Pacific lithosphere since about 45 Ma, when subduction beneath the region began. The high-speed zone reaches about 200-300 km below the depths of the deepest intermediate-depth earthquakes (subcrustal to ~200 km) and therefore suggests that ~200-300 km of slab below them is required to produce sufficient weight to induce the intermediate-depth seismicity. In the southwestern South Island, high P wave speeds indicate subduction of the Australian plate at the Puysegur Trench to approximately 200 km depth. A band with speeds ~2-3.5% lower than the background average is found along the east coast of the South Island to depths of ~150-200 km and underlies Miocene or younger volcanism; these low speeds are consistent with thinned lithosphere. A core of high speeds under the Southern Alps associated with a convergent margin and mountain building imaged in previous investigations is not well resolved in this study. This could suggest that such high speeds are limited in both width and depth and not resolvable by our data.

  3. Perovskite inclusions in deep mantle diamonds and the fate of subducted lithosphere (United States)

    Walter, Michael; Armstrong, Lora


    Sublithospheric diamonds are typically Type II, frequently exhibit complex zoning, and sometimes contain mineral inclusions that can potentially reveal deep mantle lithologies and petrologic processes. A considerable number of these diamonds contain inclusions with elemental stoichiometries consistent with transition zone (e.g. majoritic garnet, Ca-perovskite) and lower mantle phases (e.g. Mg-perovskite, Ca-perovskite, (Mg,Fe)-periclase) [1]. Ca-rich perovskites, some containing considerable CaTiO3 component, almost invariably have very low Mg contents, unlike what would be expected in solid lower mantle peridotitic or basaltic lithologies, but have elevated incompatible elements abundances that almost certainly indicate crystallization from a low-degree Ca-rich partial melt [2,3]. High-Ca majoritic garnets also have both major and trace element characteristics indicating the role of low-degree, Ca-rich partial melts [3,4], and in some cases calculated melts likely formed in subducted oceanic crust or lithosphere [3]. Given that diamond crystallized syngenetically with the inclusions, crystallization from carbonated melts is implicated. The reducing conditions expected in the ambient transition zone and lower mantle [5] could promote reduction of the carbonate component in slab-derived, carbonated (oxidized), partial melts. Reduction can lead to diamond and perovskite crystallization from the melt, possibly with H2O as a by-product through a reaction such as: CaCO3 (melt) + SiO2(melt-solid) + CH4(fluid-melt)= CaSiO3(melt-solid)) + 2H2O(melt) + 2Cdiamond Mg-perovskite could crystallize via a similar reaction involving the MgCO3 component of the melt. We speculate that when subducted slabs stall at the base of the transition zone, they may heat up and release low-degree carbonated melts [6]. Such melts may migrate, crystallize and metasomatize the ambient mantle. Trace element abundances in some kimberlites are remarkably similar to liquids that could have coexisted

  4. Plate Tectonic Consequences of competing models for the origin and history of the Banda Sea subducted oceanic lithosphere

    CERN Document Server

    Heine, Christian; McKay, Hamish; Müller, R Dietmar


    The Banda Arc, situated west of Irian Jaya and in the easternmost extension of the Sunda subduction zone system, reveals a characteristic bowl-shaped geometry in seismic tomographic images. This indicates that the oceanic lithosphere still remains attached to the surrounding continental margins of northern Australia and the Bird's Head microcontinent. Major controversies exist between authors proposing an allochthonous or autochthonous origin of the Bird's Head block. Either scenario has important implications for plate kinematic models aiming to reconstruct the tectonic evolution of the region and the late Jurassic seaoor spreading geometry of this now subducted Argo-Tanimbar-Seram (ATS) ocean basin. Wider implications affect the tectonic conguration of the Tethyan-Pacic realm, the distribution of plate boundaries as well as the shape and size of continental blocks which have been rifted off the northeastern Gondwana margin during the Late Jurassic and are now accreted to the SE Asia margin. We apply structu...

  5. Phase change in subducted lithosphere, impulse, and quantizing Earth surface deformations (United States)

    Bowin, C. O.; Yi, W.; Rosson, R. D.; Bolmer, S. T.


    The new paradigm of plate tectonics began in 1960 with Harry H. Hess's 1960 realization that new ocean floor was being created today and is not everywhere of Precambrian age as previously thought. In the following decades an unprecedented coming together of bathymetric, topographic, magnetic, gravity, seismicity, seismic profiling data occurred, all supporting and building upon the concept of plate tectonics. Most investigators accepted the premise that there was no net torque amongst the plates. Bowin (2010) demonstrated that plates accelerated and decelerated at rates 10-8 times smaller than plate velocities, and that globally angular momentum is conserved by plate tectonic motions, but few appeared to note its existence. Here we first summarize how we separate where different mass sources may lie within the Earth and how we can estimate their mass. The Earth's greatest mass anomalies arise from topography of the boundary between the metallic nickel-iron core and the silicate mantle that dominate the Earth's spherical harmonic degree 2 and 3 potential field coefficients, and overwhelm all other internal mass anomalies. The mass anomalies due to phase changes in olivine and pyroxene in subducted lithosphere are hidden within the spherical harmonic degree 4-10 packet, and are an order of magnitude smaller than those from the core-mantle boundary. Then we explore the geometry of the Emperor and Hawaiian seamount chains and the 60° bend between them that aids in documenting the slow acceleration during both the Pacific Plate's northward motion that formed the Emperor seamount chain and its westward motion that formed the Hawaiian seamount chain, but it decelerated at the time of the bend (46 Myr). Although the 60° change in direction of the Pacific Plate at of the bend, there appears to have been nary a pause in a passive spreading history for the North Atlantic Plate, for example. This, too, supports phase change being the single driver for plate tectonics and

  6. Formation and metasomatism of continental lithospheric mantle in intra-plate and subduction-related tectonic settings (United States)

    Ionov, Dmitri


    , major and trace element and isotope compositions of fertile lherzolites and thus cannot provide viable alternatives to the concept of melt extraction from pristine mantle as the major mechanism of CLM formation. Published data on xenoliths from andesitic volcanoes and on supra-subduction oceanic peridotites [4] show that the most common rocks in mantle wedge lithosphere are highly refractory harzburgites characterized by a combination of variable but generally high modal opx (18-30%) with very low modal cpx (1.5-3%). At a given olivine (or MgO) content, they have higher opx and silica, and lower cpx, Al and Ca contents than normal refractory peridotite xenoliths in continental basalts; the Mg-Si and Al-Si trends in those rocks resemble those in cratonic peridotites. These features may indicate either fluid fluxing during melting in the mantle wedge or selective post-melting metasomatic enrichments in silica to transform some olivine to opx. High oxygen fugacities and radiogenic Os-isotope compositions in those rocks may be related to enrichments by slab-derived fluids, but these features are not always coupled with trace element enrichments or patterns commonly attributed to "subduction zone metasomatism" deduced from studies of arc volcanic rocks and experiments. The valuable insights provided by experimental work and xenolith case studies are difficult to apply to many natural peridotite series because late-stage processes commonly overlap the evidence for initial melting. References: [1] Herzberg C., J. Petrol. 45: 2507 (2004). [2] Ionov D. & Sobolev A., GCA 72 (S1): A410 (2008). [3] Ionov D., Contrib. Miner. Petrol. (2007) [4] Ionov D., J. Petrol. doi: 10.1093/petrology/egp090 (2010)

  7. A common Pan-African Lithospheric Mantle (PALM) source for HIMU-like Pb-isotope signatures in circum-Mediterranean magmas (United States)

    Young, H. P.; Wang, Z.; Brandon, M. T.


    Isotopic compositions of widely distributed basaltic rocks of Europe and North Africa are clustered around a point that is displaced from modern MORB in 208Pb/204Pb vs. 206Pb/204Pb, pointing to the 'HIMU' component proposed by Zindler and Hart (1986). This observation was originally highlighted in an abstract by Cebria and Wilson (1995), who suggested that a reservoir of unknown origin exists in the convecting upper mantle of the Mediterranean and coin it the 'European asthenospheric reservoir' or EAR in order to distinguish it from the apparent influence of an additional 'lithospheric' component having a Sr-Nd isotope composition similar to continental crust that is observed in some, but not all, Cenozoic igneous rocks. While this study and most authors agree that the 'lithospheric' component in the model of Cebria and Wilson (1995) is crustal material associated with Cenozoic subduction, explanations for the origin of the HIMU-like EAR reservoir, however, are diverse, ranging from deep plumes to recently subducted slabs. These explanations are problematic. For example, neither plumes nor recent subduction are spatially broad enough to explain all of the EAR occurrences. Alternatively, we argue that both components (lithospheric and EAR) observed by Cebria and Wilson are lithospheric in origin. We propose that the origin of the HIMU-like Pb component is metasomatized sub-continental lithospheric mantle (SCLM). Comparison with synthetic evolution models of a veined mantle show the HIMU-like composition of European Cenozoic igneous rocks can be generated after ~500 Ma (Pilet et al., 2011). Major and trace element compositions of the European alkalic-basalts are similar to experimental melts of amphibole-pyroxenite veins in peridotite (a common feature of the SCLM) (Médard et al., 2006). A likely candidate for a veined 500 Ma SCLM in this region is the 'Pan-African' age terrane that is currently widely distributed from England to the Sahara as well as on the

  8. Geothermal structure of the eastern Black Sea basin and the eastern Pontides orogenic belt: Implications for subduction polarity of Tethys oceanic lithosphere

    Directory of Open Access Journals (Sweden)

    Nafiz Maden


    Full Text Available The numerical results of thermal modeling studies indicate that the lithosphere is cold and strong beneath the Black Sea basin. The thermal lithospheric thickness increases southward from the eastern Pontides orogenic belt (49.4 km to Black Sea basin (152.2 km. The Moho temperature increases from 367 °C in the trench to 978 °C in the arc region. The heat flow values for the Moho surface change between 16.4 mW m−2 in the Black Sea basin and 56.9 mW m−2 in the eastern Pontides orogenic belt. Along the southern Black Sea coast, the trench region has a relatively low geothermal potential with respect to the arc and back-arc region. The numerical studies support the existence of southward subduction beneath the Pontides during the late Mesozoic–Cenozoic.

  9. Crustal thickening in Gansu-Qinghai, lithospheric mantle subduction, and oblique, strike-slip controlled growth of the Tibet plateau (United States)

    Meyer, B.; Tapponnier, P.; Bourjot, L.; Métivier, F.; Gaudemer, Y.; Peltzer, G.; Shunmin, Guo; Zhitai, Chen


    Late-Cenozoic crustal shortening on NE sections between the Kunlun fault and the Hexi corridor are estimated to range between 100 and 200 km. In keeping with the inference of a deep crustal décollement and with the existence of Mid-Miocene to Pliocene plutonism and volcanism south of the Kunlun range, such values suggest that the lithospheric mantle of the Qaidam plunged obliquely into the asthenosphere south of that range to minimum depths of the order of 200-300 km. A minimum of ~150 km of shortening in the last ~10 Ma, consistent with the average age of the earliest volcanic-plutonic rocks just south of the Kunlun (~10.8 Ma) would imply average Late-Cenozoic rates of shortening and regional uplift in NE Tibet of at least ~15 mm yr-1 and ~0.2 mm yr-1, respectively. Such numbers are consistent with a cumulative sinistral offset and slip rate of at least ~200 km and ~2 cm yr-1, respectively, on the Altyn Tagh fault east of 88°E. The fault may have propagated more than 1000 km, to 102°E, in the last 10 Ma. Our study of ongoing tectonics in northeast Tibet is consistent with a scenario in which, while the Himalayas-Gangdese essentially `stagnated' above India's subducting mantle, much of Tibet grew by thickening of the Asian crust, as propagation of large, lithospheric, strike-slip shear zones caused the opposite edge of the plateau to migrate far into Asia. The Asian lithospheric mantle, decoupled from the crust, appears to have subducted southwards along the two Mesozoic sutures that cut Tibet north of the Gangdese, rather than to have thickened. The Bangong-Nujiang suture was probably reactivated earlier than the Jinsha-Kunlun suture, located farther north. Overall, the large-scale deformation bears a resemblance to plate tectonics at obliquely convergent margins, including slip-partioning along large strike-slip faults such as the Altyn Tagh and Kunlun faults. Simple mechanisms at the level of the lithospheric mantle are merely hidden by the broader distribution

  10. Volcanic Markers of the Post-Subduction Evolution of Baja California and Sonora, Mexico: Slab Tearing Versus Lithospheric Rupture of the Gulf of California (United States)

    Calmus, Thierry; Pallares, Carlos; Maury, René C.; Aguillón-Robles, Alfredo; Bellon, Hervé; Benoit, Mathieu; Michaud, François


    The study of the geochemical compositions and K-Ar or Ar-Ar ages of ca. 350 Neogene and Quaternary lavas from Baja California, the Gulf of California and Sonora allows us to discuss the nature of their mantle or crustal sources, the conditions of their melting and the tectonic regime prevailing during their genesis and emplacement. Nine petrographic/geochemical groups are distinguished: "regular" calc-alkaline lavas; adakites; magnesian andesites and related basalts and basaltic andesites; niobium-enriched basalts; alkali basalts and trachybasalts; oceanic (MORB-type) basalts; tholeiitic/transitional basalts and basaltic andesites; peralkaline rhyolites (comendites); and icelandites. We show that the spatial and temporal distribution of these lava types provides constraints on their sources and the geodynamic setting controlling their partial melting. Three successive stages are distinguished. Between 23 and 13 Ma, calc-alkaline lavas linked to the subduction of the Pacific-Farallon plate formed the Comondú and central coast of the Sonora volcanic arc. In the extensional domain of western Sonora, lithospheric mantle-derived tholeiitic to transitional basalts and basaltic andesites were emplaced within the southern extension of the Basin and Range province. The end of the Farallon subduction was marked by the emplacement of much more complex Middle to Late Miocene volcanic associations, between 13 and 7 Ma. Calc-alkaline activity became sporadic and was replaced by unusual post-subduction magma types including adakites, niobium-enriched basalts, magnesian andesites, comendites and icelandites. The spatial and temporal distribution of these lavas is consistent with the development of a slab tear, evolving into a 200-km-wide slab window sub-parallel to the trench, and extending from the Pacific coast of Baja California to coastal Sonora. Tholeiitic, transitional and alkali basalts of subslab origin ascended through this window, and adakites derived from the partial

  11. Continental crust subducted deeply into lithospheric mantle: the driving force of Early Carboniferous magmatism in the Variscan collisional orogen (Bohemian Massif) (United States)

    Janoušek, Vojtěch; Schulmann, Karel; Lexa, Ondrej; Holub, František; Franěk, Jan; Vrána, Stanislav


    relamination mechanisms. The presence of refractory light material rich in radioactive elements under the denser upper plate would eventually result in gravity-driven overturns in the thickened crust. The contaminated lithospheric mantle domains yielded, soon thereafter, ultrapotassic magmas whose major- and compatible-trace element signatures point to equilibration with the mantle peridotite, while their LILE contents and radiogenic isotope signatures are reminiscent of the subducted continental crust. This research was financially supported by the GAČR Project P210-11-2358 (to VJ) and Ministry of Education of the Czech Republic program LK11202 (to KS). Becker, H. 1996. Journal of Petrology 37, 785-810. Kotková, J. et al. 2011. Geology 39, 667-670. Massonne, H.-J. 2001. European Journal of Mineralogy 13, 565-570. Naemura, K. et al. 2009. Journal of Petrolology 50, 1795-1827. Schulmann, K., et al., 2014. Geology, in print. Vrána, S. 2013. Journal of Geosciences 58, 347-378. Zheng, Y. F. 2012. Chemical Geology 328, 5-48.

  12. Modification of an ancient subcontinental lithospheric mantle by continental subduction: Insight from the Maowu garnet peridotites in the Dabie UHP belt, eastern China (United States)

    Chen, Yi; Su, Bin; Chu, Zhuyin


    Orogenic mantle-derived peridotites commonly originate from the subcontinental lithospheric mantle (SCLM) and thus provide a key target to investigate the modification of the SCLM by a subducting slab. The Maowu ultramafic rocks from the Dabie ultrahigh-pressure (UHP) metamorphic belt have formerly been debated as representing cumulates or mantle-derived peridotites. Detailed petrological and geochemical data presented in this study provide new constraints on the origin and formation of the peridotites involving melt depletion in the ancient SCLM and deep crustal metasomatism. The Maowu garnet dunites have refractory bulk compositions characterized by high Mg# (91.9-92.0) and Ni (2537-2892 ppm) values and low Al2O3 (0.26-0.76 wt.%), CaO (0.05-0.32 wt.%), TiO2 (enrichment in incompatible elements. Mineral and whole-rock chemistry indicate that these veins represent metasomatic products between the wall dunites and silica-rich hydrous melts under UHP conditions. The veins show large variations in platinum-group element (PGE) signatures and Re-Os isotopes. The garnet-poor orthopyroxenite veins are characterized by low Al2O3 ( 6 wt.%) and S (99-306 ppm) contents and show melt-like PGE patterns and high 187Os/188Os ratios (up to 0.36910). These features, combined with the occurrence of interstitial sulfides in the garnet-rich orthopyroxenite veins, suggest that crust-derived sulfur-saturated silicate melts may have significantly modified the PGE signature and destroyed the Re-Os systematics of the SCLM. However, when the crust-derived silicate melts became sulfur-depleted, such melts would not significantly modify the PGE patterns, radiogenic Os-isotope compositions or the Re-depletion model ages of the SCLM. Consequently, deep crust-mantle interactions in continental subduction zones could induce high degrees of Os isotopic heterogeneity in the SCLM wedge.

  13. On the initiation of subduction (United States)

    Mueller, Steve; Phillips, Roger J.


    Estimates of shear resistance associated with lithospheric thrusting and convergence represent lower bounds on the force necessary to promote trench formation. Three environments proposed as preferential sites of incipient subduction are investigated: passive continental margins, transform faults/fracture zones, and extinct ridges. None of these are predicted to convert into subduction zones simply by the accumulation of local gravitational stresses. Subduction cannot initiate through the foundering of dense oceanic lithosphere immediately adjacent to passive continental margins. The attempted subduction of buoyant material at a mature trench can result in large compressional forces in both subducting and overriding plates. This is the only tectonic force sufficient to trigger the nucleation of a new subduction zone. The ubiquitous distribution of transform faults and fracture zones, combined with the common proximity of these features to mature subduction complexes, suggests that they may represent the most likely sites of trench formation if they are even marginally weaker than normal oceanic lithosphere.

  14. Lithospheric Stress Tensor from Gravity and Lithospheric Structure Models (United States)

    Eshagh, Mehdi; Tenzer, Robert


    In this study we investigate the lithospheric stresses computed from the gravity and lithospheric structure models. The functional relation between the lithospheric stress tensor and the gravity field parameters is formulated based on solving the boundary-value problem of elasticity in order to determine the propagation of stresses inside the lithosphere, while assuming the horizontal shear stress components (computed at the base of the lithosphere) as lower boundary values for solving this problem. We further suppress the signature of global mantle flow in the stress spectrum by subtracting the long-wavelength harmonics (below the degree of 13). This numerical scheme is applied to compute the normal and shear stress tensor components globally at the Moho interface. The results reveal that most of the lithospheric stresses are accumulated along active convergent tectonic margins of oceanic subductions and along continent-to-continent tectonic plate collisions. These results indicate that, aside from a frictional drag caused by mantle convection, the largest stresses within the lithosphere are induced by subduction slab pull forces on the side of subducted lithosphere, which are coupled by slightly less pronounced stresses (on the side of overriding lithospheric plate) possibly attributed to trench suction. Our results also show the presence of (intra-plate) lithospheric loading stresses along Hawaii islands. The signature of ridge push (along divergent tectonic margins) and basal shear traction resistive forces is not clearly manifested at the investigated stress spectrum (between the degrees from 13 to 180).

  15. Plume-induced subduction (United States)

    Gerya, T.; Stern, R. J.; Baes, M.; Sobolev, S. V.; Whattam, S. A.


    Dominant present-day subduction initiation mechanisms require acting plate forces and/or pre-existing zones of lithospheric weakness, which are themselves consequences of plate tectonics. In contrast, recently discovered plume-induced subduction initiation could have started the first subduction zone without pre-existing plate tectonics. Here, we investigate this new mechanism with high-resolution 3D numerical thermomechanical modeling experiments. We show that typical plume-induced subduction dynamics is subdivided into five different stages: (1) oceanic plateau formation by arrival of a mantle plume head; (2) formation of an incipient trench and a descending nearly-circular slab at the plateau margins; (3) tearing of the circular slab; (4) formation of several self-sustained retreating subduction zones and (5) cooling and spreading of the new lithosphere formed between the retreating subduction zones. At the final stage of plume-induced subduction initiation, a mosaic of independently moving, growing and cooling small oceanic plates heading toward individual retreating subduction zones forms. The plates are separated by spreading centers, triple junctions and transform faults and thus the newly formed multi-slab subduction system operates as an embryonic plate tectonic cell. We demonstrate that three key physical factors combine to trigger self-sustained plume-induced subduction: (1) old negatively buoyant oceanic lithosphere; (2) intense weakening of the lithosphere by plume-derived magmas; and (3) lubrication of the forming subduction interface by hydrated oceanic crust. We furthermore discuss that plume-induced subduction, which is rare at present day conditions, may have been common in the Precambrian time and likely started global plate tectonics on Earth.

  16. Kinematics of subduction and subduction-induced flow in the upper mantle

    NARCIS (Netherlands)

    Schellart, W. P.


    Results of fluid dynamical experiments are presented to model the kinematics of lithospheric subduction in the upper mantle. The experiments model a dense highviscosity plate (subducting lithosphere) overlying a less dense low-viscosity layer (upper mantle). The overriding lithosphere is not

  17. Seismicity and the subduction process (United States)

    Ruff, L.; Kanamori, H.


    There is considerable variation between subduction zones in the largest characteristic earthquake within each zone. Assuming that coupling between downgoing and upper plates is directly related to characteristic earthquake size, tests for correlations between variation in coupling and other physical features of subduction zones are conducted: the lateral extent and penetration depth of Benioff zones, age of subducting lithosphere, convergence rate, and back-arc spreading. Using linear multivariate regression, coupling is correlated with two variables: convergence rate and lithosphere age. Secondary correlations within the data set are penetration depth versus lithosphere age, and lateral extent versus convergence rate. Taken together, the observed correlations suggest a simple qualitative model where convergence rate and lithosphere age determine the horizontal and sinking rates, respectively, of slabs: these parameters influence the seismic coupling in the subduction zone. In the limit of a fast sinking rate and slow convergence rate, back-arc spreading occurs and thereby appears to be a passive process.

  18. Buoyant subduction on Venus: Implications for subduction around coronae (United States)

    Burt, J. D.; Head, J. W.


    Potentially low lithospheric densities, caused by high Venus surface and perhaps mantle temperatures, could inhibit the development of negative buoyancy-driven subduction and a global system of plate tectonics/crustal recycling on that planet. No evidence for a global plate tectonic system was found so far, however, specific features strongly resembling terrestrial subduction zones in planform and topographic cross-section were described, including trenches around large coronae and chasmata in eastern Aphrodite Terra. The cause for the absence, or an altered expression, of plate tectonics on Venus remains to be found. Slab buoyancy may play a role in this difference, with higher lithospheric temperatures and a tendency toward positive buoyancy acting to oppose the descent of slabs and favoring under thrusting instead. The effect of slab buoyancy on subduction was explored and the conditions which would lead to under thrusting versus those allowing the formation of trenches and self-perpetuating subduction were defined. Applying a finite element code to assess the effects of buoyant forces on slabs subducting into a viscous mantle, it was found that mantle flow induced by horizontal motion of the convergent lithosphere greatly influences subduction angle, while buoyancy forces produce a lesser effect. Induced mantle flow tends to decrease subduction angle to near an under thrusting position when the subducting lithosphere converges on a stationary overriding lithosphere. When the overriding lithosphere is in motion, as in the case of an expanding corona, subduction angles are expected to increase. An initial stage involved estimating the changes in slab buoyancy due to slab healing and pressurization over the course of subduction. Modeling a slab, descending at a fixed angle and heated by conduction, radioactivity, and the heat released in phase changes, slab material density changes due to changing temperature, phase, and pressure were derived.

  19. Late Miocene extensional systems in northern Tunisia and their relation with SE directed delamination of the African subcontinental mantle lithosphere (United States)

    Booth-Rea, Guillermo; Gaidi, Seif; Melki, Fetheddine; Pérez-Peña, Vicente; Marzougui, Wissem; Azañón, Jose Miguel; Galve, Jorge Pedro


    Recent work has proposed the delamination of the subcontinental mantle lithosphere under northern Tunisia during the late Miocene. This process is required to explain the present location of the Tunisian segment of the African slab, imaged by seismic tomography, hanging under the Gulf of Gabes to the south of Tunisia. Thus, having retreated towards the SE several hundred km from its original position under the Tellian-Atlas nappe contact that crops out along the north of Tunisia. However, no tectonic structures have been described which could be related to this mechanism of lithospheric mantle peeling. Here we describe for the first time extensional fault systems in northern Tunisia that strongly thinned the Tellian nappes, exhuming rocks from the Tunisian Atlas in the core of folded extensional detachments. Two normal fault systems with sub-orthogonal extensional transport occur. These were active during the late Miocene associated to the extrusion of 13 Ma granodiorite and 9 Ma rhyodacite in the footwall of the Nefza detachment. We have differentiated an extensional system formed by low-angle normal faults with NE- and SW-directed transport cutting through the Early to Middle Miocene Tellian nappen stack and a later system of low and high-angle normal faults that cuts down into the underlying Tunisian Atlas units with SE-directed transport, which root in the Nefza detachment. Both normal fault systems have been later folded and cut by thrusts during Plio-Quaternary NW-SE directed compression. These findings change the interpretation of the tectonic evolution of Tunisia that has always been framed in a transpressive to compressive setting, manifesting the extensional effects of Late Miocene lithospheric mantle delamination under northern Tunisia.

  20. Processes and consequences of deep subduction

    NARCIS (Netherlands)

    Rubie, David C.; Hilst, R.D. van der


    Subduction of slabs of oceanic lithosphere into the deep mantle involves a wide range of geophysical and geochemical processes and is of major importance for the physical and chemical evolution of the Earth. For example, subduction and subduction-related volcanism are major processes through

  1. The continental lithosphere

    DEFF Research Database (Denmark)

    Artemieva, Irina


    of the Royal Society of London. Series A, 360, 2475–2491.; Shapiro N.M., Ritzwoller M.H. 2002. Monte-Carlo inversion for a global shear velocity model of the crust and upper mantle. Geophysical Journal International 151, 1–18.] and lithospheric temperatures [Artemieva I.M., Mooney W.D., 2001. Thermal structure......The goal of the present study is to extract non-thermal signal from seismic tomography models in order to distinguish compositional variations in the continental lithosphere and to examine if geochemical and petrologic constraints on global-scale compositional variations in the mantle...... are consistent with modern geophysical data. In the lithospheric mantle of the continents, seismic velocity variations of a non-thermal origin (calculated from global Vs seismic tomography data [Grand S.P., 2002. Mantle shear-wave tomography and the fate of subducted slabs. Philosophical Transactions...

  2. Geochronology of diamond-bearing zircons from garnet peridotite in the North Qaidam UHPM belt, Northern Tibetan Plateau: A record of complex histories from oceanic lithosphere subduction to continental collision [rapid communication (United States)

    Song, Shuguang; Zhang, Lifei; Niu, Yaoling; Su, Li; Jian, Ping; Liu, Dunyi


    We report the results of a comprehensive study of zircons separated from the Lüliangshan peridotite massif within the 400-km-long North Qaidam UHPM belt, northern Tibetan Plateau, NW China. The peridotite massif is dominated by garnet lherzolite with minor amounts of interlayered garnet-bearing dunite and cross-cutting garnet pyroxenite dikes. Most zircons from the garnet lherzolite show rather complex zoning. One diamond and a few graphite inclusions are identified in some zircons by Raman spectroscopy. SHRIMP dating on these zircons show four major age groups: (a) 484-444 Ma (weighted mean age, 457 ± 22 Ma) for cores of most crystals, whose morphology and rare earth element (REE) systematics (i.e., very high [Lu / Sm] CN = 88-230) suggest a magmatic origin, consistent with the protolith being magmatic cumulate; (b) 435-414 Ma with a mean of 423 ± 5 Ma, which, given by mantle portions of zircon crystals, is interpreted to record the event of ultrahigh-pressure metamorphism (UHPM) at depths greater than 200 km in an Andean-type subduction zone; (c) 402-384 Ma (mean age 397 ± 6 Ma) for near-rim portions of zircon crystals; and (d) 368-349 Ma for outermost rims, which is interpreted as representing some post-orogenic thermal events. Inherited cores in two zircon crystals were identified using CL and found to be Proterozoic. Morphology and CL images show that zircons from dunite and garnet pyroxenite are of metamorphic origin. The mean age of dunite zircons is 420 ± 5 Ma, which overlaps the mantle age of the garnet lherzolite zircon (see (b) above). The mean age of garnet pyroxenite zircons is 399 ± 8 Ma, which overlaps ages of near-rim domains in garnet lherzolite zircons (see (c) above). Some garnet pyroxenite zircons also recorded a retrograde event at 358 ± 7 Ma. All these data suggest that the Lüliangshan garnet peridotite massif is not a fragment of ancient lithospheric mantle, but a peridotite body with long and complex histories from Early Ordovician

  3. Imaging Canary Island hotspot material beneath the lithosphere of Morocco and southern Spain (United States)

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


    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.

  4. The subduction dichotomy of strong plates and weak slabs (United States)

    Petersen, Robert I.; Stegman, Dave R.; Tackley, Paul J.


    A key element of plate tectonics on Earth is that the lithosphere is subducting into the mantle. Subduction results from forces that bend and pull the lithosphere into the interior of the Earth. Once subducted, lithospheric slabs are further modified by dynamic forces in the mantle, and their sinking is inhibited by the increase in viscosity of the lower mantle. These forces are resisted by the material strength of the lithosphere. Using geodynamic models, we investigate several subduction models, wherein we control material strength by setting a maximum viscosity for the surface plates and the subducted slabs independently. We find that models characterized by a dichotomy of lithosphere strengths produce a spectrum of results that are comparable to interpretations of observations of subduction on Earth. These models have strong lithospheric plates at the surface, which promotes Earth-like single-sided subduction. At the same time, these models have weakened lithospheric subducted slabs which can more easily bend to either lie flat or fold into a slab pile atop the lower mantle, reproducing the spectrum of slab morphologies that have been interpreted from images of seismic tomography.

  5. The African Plate: A history of oceanic crust accretion and subduction since the Jurassic

    NARCIS (Netherlands)

    Gaina, C.; Torsvik, T.H.; van Hinsbergen, D.J.J.; Medvedev, S.; Werner, S.C.; Labails, C.


    We present a model for the Jurassic to Present evolution of plate boundaries and oceanic crust of the African plate based on updated interpretation of magnetic, gravity and other geological and geophysical data sets. Location of continent ocean boundaries and age and geometry of old oceanic crust

  6. Dynamics of intraoceanic subduction initiation: 2D thermomechanical modeling (United States)

    Zhou, X.; Gerya, T.; LI, Z.; Stern, R. J.


    Intraoceanic subduction initiation occurs in previous weak zones which could be transform faults or old fracture zones, and concurrents with the change of plate motions. It is an important process to understand the beginning of plate tectonics. However, the dynamic process during (after) subduction initiation remain obscure. The process of suducting slabs move from down to downdip is also not revealed clearly. In order to obtain better understanding of the transitional process of subducting slab motion, we use finite difference and marker-in-cell methods to establish a series of self-sustainable subduction initiation models and explore many visco-plastic parameters to qualify the dynamical process of subduction initiation. The following parameters are systematic tested: (1) the age of the subducting slab; (2) friction coefficient of the mantle material; (3) the mantle potential temperature; (4) the age of the overriding slab. We find out the critical age of the oceanic lithosphere which can produce subduction initiation. And the age of subducting slab plays important roles during subduction initiation. The young subducting slab induces fast trench retreat and then trench begin to advance. For the old subducting slab, it induces relative slower trench retreat and then stop moving. The age of overriding slabs impacts coupling with the subducting slab. The friction coefficient of lithosphere also impacts the backarc spreading and subduction velocity. Stronger subducted plate gives lower subduction velocity and faster trench retreat velocity. The mantle potential temperature changes the critical age of subducted slabs.

  7. PdS and SdP Receiver Functions Image of the Lithosphere underneath the Southern African Regions

    DEFF Research Database (Denmark)

    Soliman, Mohammad Youssof Ahmad; Thybo, Hans; Levander, A


    to 350 km depth by Jordan (1975), has to be revealed in more detail, and a better understanding should yield new insight into the origin of Earth’s early continents. We have reassessed the data from the Kaapvaal seismic experiment for lithosphere structure by application of PdS receiver functions...

  8. The lithosphere-asthenosphere Italy and surroundings

    CERN Document Server

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


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

  9. Investigating and Imaging the Lithospheric Structure of the Westernmost Mediterranean Using S Receiver Functions (United States)

    Miller, Meghan S.; Butcher, Amber


    The Alboran System was created during the Neogene at the western edge of the Alpine-Himalayan orogenic belt, as the result of convergence between the European and African plates. This system includes the Gibraltar Arc, Rif-Betic chain, Atlas Mountains, and Alboran Sea. The evolution from ocean subduction to continental collision, particularly in complex three-dimensional settings such as this, is poorly understood. Advances in this subject are likely to come from multidisciplinary projects, such as PICASSO (Program to Investigate Convective Alboran Sea System Overturn): a study of the Alboran Sea, Atlas Mountains, and Gibraltar arc. Several models have been suggested to explain the tectonics of this system including: continental lithospheric delamination and drips, slab breakoff, and subducting slab rollback. Advances in defining the lithosphere - asthenosphere boundary (LAB) are crucial to understanding the geochemical and geodynamic evolution of the region. Seismic data from ~85 broadband instruments deployed in Morocco and Spain as part of the PICASSO project are being utilized to constrain lithospheric structure beneath this part of the Western Mediterranean via identification of S-to-p conversions from S receiver functions. A previous study indicates that the lithospheric thinning beneath the Atlas High may be the result of mantle upwelling induced thermal erosion, while a more recent imaging study suggests that the LAB could be at depths >200 km, tens of kilometers thicker than previous models. Our preliminary results indicate LAB depths down to ~100 - 110 km near the Straight of Gibraltar and as shallow as ~65 - 80 km under the Atlas High. The primary purpose of this project is to advance our understanding of the structure and evolution of the lithosphere - asthenosphere boundary (LAB) of the Atlas Mountains and surrounding areas.

  10. Does subduction polarity changes below the Alps? Inferences from analogue modelling

    NARCIS (Netherlands)

    Luth, S.; Willingshofer, E.; Sokoutis, D.; Cloetingh, S.


    The surface expression of a lateral polarity change of continental mantle lithosphere subduction has been studied by using lithosphere-scale physical models. Key parameters investigated were: the degree of lateral coupling between adjacent domains of opposing subduction polarity, the width of the

  11. Lithospheric processes

    Energy Technology Data Exchange (ETDEWEB)

    Baldridge, W. [and others


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

  12. Subduction zone decoupling/retreat modeling explains south Tibet (Xigaze) and other supra-subduction zone ophiolites and their UHP mineral phases (United States)

    Butler, Jared P.; Beaumont, Christopher


    The plate tectonic setting in which proto-ophiolite 'oceanic' lithosphere is created remains controversial with a number of environments suggested. Recent opinions tend to coalesce around supra-subduction zone (SSZ) forearc extension, with a popular conceptual model in which the proto-ophiolite forms during foundering of oceanic lithosphere at the time of spontaneous or induced onset of subduction. This mechanism is favored in intra-oceanic settings where the subducting lithosphere is old and the upper plate is young and thin. We investigate an alternative mechanism; namely, decoupling of the subducting oceanic lithosphere in the forearc of an active continental margin, followed by subduction zone (trench) retreat and creation of a forearc oceanic rift basin, containing proto-ophiolite lithosphere, between the continental margin and the retreating subduction zone. A template of 2D numerical model experiments examines the trade-off between strength of viscous coupling in the lithospheric subduction channel and net slab pull of the subducting lithosphere. Three tectonic styles are observed: 1) C, continuous subduction without forearc decoupling; 2) R, forearc decoupling followed by rapid subduction zone retreat; 3) B, breakoff of subducting lithosphere followed by re-initiation of subduction and in some cases, forearc decoupling (B-R). In one case (BA-B-R; where BA denotes backarc) subduction zone retreat follows backarc rifting. Subduction zone decoupling is analyzed using frictional-plastic yield theory and the Stefan solution for the separation of plates containing a viscous fluid. The numerical model results are used to explain the formation of Xigaze group ophiolites, southern Tibet, which formed in the Lhasa terrane forearc, likely following earlier subduction and not necessarily during subduction initiation. Either there was normal coupled subduction before subduction zone decoupling, or precursor slab breakoff, subduction re-initiation and then decoupling

  13. Horizontal mantle flow controls subduction dynamics. (United States)

    Ficini, E; Dal Zilio, L; Doglioni, C; Gerya, T V


    It is generally accepted that subduction is driven by downgoing-plate negative buoyancy. Yet plate age -the main control on buoyancy- exhibits little correlation with most of the present-day subduction velocities and slab dips. "West"-directed subduction zones are on average steeper (~65°) than "East"-directed (~27°). Also, a "westerly"-directed net rotation of the lithosphere relative to the mantle has been detected in the hotspot reference frame. Thus, the existence of an "easterly"-directed horizontal mantle wind could explain this subduction asymmetry, favouring steepening or lifting of slab dip angles. Here we test this hypothesis using high-resolution two-dimensional numerical thermomechanical models of oceanic plate subduction interacting with a mantle flow. Results show that when subduction polarity is opposite to that of the mantle flow, the descending slab dips subvertically and the hinge retreats, thus leading to the development of a back-arc basin. In contrast, concordance between mantle flow and subduction polarity results in shallow dipping subduction, hinge advance and pronounced topography of the overriding plate, regardless of their age-dependent negative buoyancy. Our results are consistent with seismicity data and tomographic images of subduction zones. Thus, our models may explain why subduction asymmetry is a common feature of convergent margins on Earth.

  14. The initiation of subduction: criticality by addition of water? (United States)

    Regenauer-Lieb, K; Yuen, D A; Branlund, J


    Subduction is a major process of plate tectonics; however, its initiation is not understood. We used high-resolution (less than 1 kilometer) finite-element models based on rheological data of the lithosphere to investigate the role played by water on initiating subduction. A solid-fluid thermomechanical instability is needed to drive a cold, stiff, and negatively buoyant lithosphere into the mantle. This instability can be triggered slowly by sedimentary loading over a time span of 100 million years. Our results indicate that subduction can proceed by a double feedback mechanism (thermoelastic and thermal-rheological) promoted by lubrication due to water.

  15. Thermal impact of magmatism in subduction zones (United States)

    Rees Jones, David W.; Katz, Richard F.; Tian, Meng; Rudge, John F.


    Magmatism in subduction zones builds continental crust and causes most of Earth's subaerial volcanism. The production rate and composition of magmas are controlled by the thermal structure of subduction zones. A range of geochemical and heat flow evidence has recently converged to indicate that subduction zones are hotter at lithospheric depths beneath the arc than predicted by canonical thermomechanical models, which neglect magmatism. We show that this discrepancy can be resolved by consideration of the heat transported by magma. In our one- and two-dimensional numerical models and scaling analysis, magmatic transport of sensible and latent heat locally alters the thermal structure of canonical models by ∼300 K, increasing predicted surface heat flow and mid-lithospheric temperatures to observed values. We find the advection of sensible heat to be larger than the deposition of latent heat. Based on these results we conclude that thermal transport by magma migration affects the chemistry and the location of arc volcanoes.

  16. Imaging the Lithospheric - Asthenosphere Boundary Structure of the Westernmost Mediterranean Using S Receiver Functions (United States)

    Butcher, A.; Miller, M. S.; Diaz Cusi, J.


    The Iberian microcontinent, in the westernmost portion of the Mediterranean is comprised of the Betic Cordillera Zone, the South Portuguese Zone, the Ossa-Morena Zone, the Central Iberian Zone, the Galicia-Tras Os Montes Zone, the West Asturian-Leonese Zone, and the Cantabrian Zone. These zones were created as a result of three primary stages of Iberian evolution, with the last being the collision of Iberia with in the Late Cretaceous. In northeastern Africa, Neogene convergence between the European and African plates created the Alboran System: comprised of the Gibraltar Arc, Rif-Betics, Atlas Mountains, and Alboran Sea. The primary purpose of this study is to advance our understanding of the structure and evolution of the lithosphere, as well as the lithosphere - asthenosphere boundary (LAB) of the Iberian microcontinent and surrounding areas. Of particular interest is improving our understanding of the evolution from ocean subduction to continental collision that has been taking place in the late stage convergence of this part of the Mediterranean., The region is a particularly complex three-dimensional settings and, several models have been suggested to explain the tectonics of this system including: continental lithospheric delamination and drips, slab breakoff, and subducting slab rollback. Here we use broadband seismic data from 272 broadband instruments deployed in Morocco and Spain as part of the PICASSO and IBERArray (Díaz, J., et al., 2009) projects to constrain lithospheric structure via identification of S-to-p conversions from S receiver functions (SRF). We use SRFs to image the characteristics and structure in terms of seismic velocity discontinuities, including the crust-mantle boundary (Moho) and the lithosphere-asthenosphere boundary (LAB) beneath the region. Our SRFs agree with previous work that suggests that the lithospheric thickness is shallow (~65 km) beneath the Atlas and thickest (~120 km) beneath the Rif. Additionally, LAB structures

  17. Metallogeny of subduction zones

    Directory of Open Access Journals (Sweden)

    Sorokhtin N. O.


    Full Text Available The paper deals with the multistage mechanism of the Earth's crust enrichment in ore elements in underthrust zones. The processes of metamorphism and the formation of hydrothermal solutions at pulling of the watered oceanic lithospheric plate into the subduction zone have been described. Some physical and chemical transformation regularities of structural-material complexes in these areas and mechanisms of the formation of ore deposits have been discussed. Spatio-temporal patterns of the localization of a number of endogenetic and exogenetic deposits have been described using metallogeny of the Ural and the Verkhoyansk-Kolyma Fold Belts as an example. It has been shown that in nature there are several effective mechanisms of the enrichment of the crust in ore minerals. One of them is the process of pulling into subduction zone of metalliferous sediments and ferromanganese crusts as well as seabed nodules, their metamorphic transformation, partial melting and transition of ore components into magmatic melts and mineralized fluids. In the future this leads to the release of ore material by magmas and hydrothermal solutions into the folded formations of island-arc and Andean types and the formation of igneous, metasomatic and hydrothermal deposits. Another, yet no less powerful natural mechanism of a conveyor enrichment of the crust in ore elements is the process of destruction and sedimentation of mineral deposits formed in the folded areas as well as the formation of placers and their transfer to the marginal parts of the continent. Later, during the collision of active and passive margins of two lithospheric plates, such as the collision of the Kolyma Massif with the eastern part of the Siberian craton in the middle of the Mesozoic there was a thrusting of a younger lithospheric plate over a more ancient one. As a result, the sedimentary sequences of the passive margin of the Siberian plate were submerged and partially melted by the basic magmas

  18. Ablative subduction - A two-sided alternative to the conventional subduction model (United States)

    Tao, Winston C.; O'Connell, Richard J.


    The plausibility of a two-sided fluid-based model of lithospheric subduction that is based upon current views of lithospheric structure is examined. In this model the viscous lower lithosphere flows downward, and the brittle upper lithosphere deforms in passive response. This process is potentially double-sided, since it is found that even a buoyant plate can be dragged downward by a dense descending neighbor. Thus an apparent overriding plate may be worn away by a process of viscous ablation, with the rate of ablation a function of plate buoyancy. This process, called 'ablative subduction,' makes it possible to simply interpret observations concerning slab profiles, interplate seismicity, back arc tectonics, and complex processes such as double subduction and subduction polarity reversal. When experiments modeling the evolution of simple fluid 'slabs' are performed, slab profile is found to be strongly influenced by ablation in the overriding plate. When ablation is weak, as when a buoyant continent borders the trench, deformable slabs adopt shallow Andean-style profiles.

  19. Seismic coupling and uncoupling at subduction zones (United States)

    Ruff, L.; Kanamori, H.


    Some of the correlations concerning the properties of subduction zones are reviewed. A quantitative global comparison of many subduction zones reveals that the largest earthquakes occur in zones with young lithosphere and fast convergence rates. Maximum earthquake size is directly related to the asperity distribution on the fault plane. This observation can be translated into a simple model of seismic coupling where the horizontal compressive stress between two plates is proportional to the ratio of the summed asperity area to the total area of the contact surface. Plate age and rate can control asperity distribution directly through the horizontal compressive stress associated with the vertical and horizontal velocities of subducting slabs. The basalt to eclogite phase change in the down-going oceanic crust may be largely responsible for the uncoupling of subduction zones below a depth of about 40 km.

  20. A satellite magnetic perspective of subduction zones, large igneous provinces, rifts, and diffuse plate boundary zones (United States)

    Purucker, M. E.; Whaler, K. A.


    Large and intermediate-scale tectonic features such as subduction zones, large igneous provinces, rifts, and diffuse plate boundary zones are often seen to have a magnetic signature visible from the perspective of near-Earth magnetic field satellites such as CHAMP and Orsted. Why do these tectonic features have a magnetic signature, while others do not? A new model of the lithospheric field (MF-6, Maus et al., 2008) extending to spherical harmonic degree 120 (333 km wavelength) has been used to evaluate the magnetic state of the lithosphere under the assumption that the magnetization is either induced (with a seismic starting model), or remanent (with a minimum norm approach). Some of the features identified from these images include the Tethyan and NE Siberian diffuse plate boundary zones, the Red Sea rift, and Cretaceous rift basins developed on the West African shield. Almost without exception, subduction zones exhibit a magnetic signature, as do many large igneous provinces. In this talk we discuss some of the new insights this magnetic perspective provides, and speculate on the controls which determine whether tectonic features will be expressed magnetically.

  1. Subduction trench migration since the Cretaceous (United States)

    Williams, S.; Flament, N. E.; Müller, D.; Butterworth, N. P.


    Much of our knowledge about subduction zone processes is derived from analyzing present-day Earth. Several studies of contemporary plate motions have investigated the balance between retreating and advancing trenches and shown that subduction zone kinematics are sensitive to the choice of Absolute Plate Motion (APM) model (or "reference frame"). For past times, the absolute motions of the lithospheric plates relative to the Earth's deep interior over tens of millions of years are commonly constrained using observations from paleomagnetism and age-progressive seamount trails. In contrast, a reference frame linking surface plate motions to subducted slab remnants mapped from seismic tomography has recently been proposed. APM models derived using different methodologies, different subsets of hotspots, or differing assumptions of hotspot motion, have contrasting implications for parameters that describe the long term state of the plate-mantle system, such as the balance between advance and retreat of subduction zones, plate velocities, and net lithospheric rotation. Here we quantitatively compare the subduction zone kinematics, net lithospheric rotation and fit to hotspot trails derived the last 130 Myr for a range of alternative reference frames and a single relative plate motion model. We find that hotspot and tomographic slab-remnant reference frames yield similar results for the last 70 Myr. For the period between 130 and 70 Ma, when hotspot trails become scarce, hotspot reference frames yield a much more dispersed distribution of slab advance and retreat velocities, which is considered geodynamically less plausible. By contrast, plate motions calculated using the slab-remnant reference frame, or using a reference frame designed to minimise net rotation, yield more consistent subduction zone kinematics for times older than 70 Ma. Introducing the global minimisation of trench migration rates as a key criterion in the construction of APM models forms the foundation

  2. Plate tectonics on the Earth triggered by plume-induced subduction initiation. (United States)

    Gerya, T V; Stern, R J; Baes, M; Sobolev, S V; Whattam, S A


    Scientific theories of how subduction and plate tectonics began on Earth--and what the tectonic structure of Earth was before this--remain enigmatic and contentious. Understanding viable scenarios for the onset of subduction and plate tectonics is hampered by the fact that subduction initiation processes must have been markedly different before the onset of global plate tectonics because most present-day subduction initiation mechanisms require acting plate forces and existing zones of lithospheric weakness, which are both consequences of plate tectonics. However, plume-induced subduction initiation could have started the first subduction zone without the help of plate tectonics. Here, we test this mechanism using high-resolution three-dimensional numerical thermomechanical modelling. We demonstrate that three key physical factors combine to trigger self-sustained subduction: (1) a strong, negatively buoyant oceanic lithosphere; (2) focused magmatic weakening and thinning of lithosphere above the plume; and (3) lubrication of the slab interface by hydrated crust. We also show that plume-induced subduction could only have been feasible in the hotter early Earth for old oceanic plates. In contrast, younger plates favoured episodic lithospheric drips rather than self-sustained subduction and global plate tectonics.

  3. Subduction, back-arc spreading and global mantle flow (United States)

    Hager, B. H.; Oconnell, R. J.; Raefsky, A.


    It is pointed out that the subducted lithosphere associated with Benioff zones provides the only direct evidence about the flow in the earth's interior associated with plate motions. It is the primary objective of the present investigation to study the relation between the orientation of subducting lithosphere and the flow patterns (both local and global) near subduction zones. Most of the calculations conducted are based on simple flow models for radially symmetric, Newtonian viscous spheres. The investigation is concerned with the possibility that a simple model of global mantle flow could account for some features of subduction zones. It is found that such a model can account for the orientation of the seismic zones, and, in addition, also for features related to back-arc spreading and perhaps the maximum earthquake size.

  4. Reaction-induced rheological weakening enables oceanic plate subduction


    Hirauchi, Ken-ichi; Fukushima, Kumi; Kido, Masanori; Muto, Jun; Okamoto, Atsushi


    Earth is the only terrestrial planet in our solar system where an oceanic plate subducts beneath an overriding plate. Although the initiation of plate subduction requires extremely weak boundaries between strong plates, the way in which oceanic mantle rheologically weakens remains unknown. Here we show that shear-enhanced hydration reactions contribute to the generation and maintenance of weak mantle shear zones at mid-lithospheric depths. High-pressure friction experiments on peridotite goug...

  5. A thermo-mechanical model of horizontal subduction below an overriding plate

    NARCIS (Netherlands)

    Hunen, Jeroen van; Berg, A.P. van den; Vlaar, N.J.


    Subduction of young oceanic lithosphere cannot be explained by the gravitational driving mechanisms of slab pull and ridge push. This deficiency of driving forces can be overcome by obduction of an actively overriding plate, which forces the young plate either to subduct or to collide. This

  6. Reevaluating carbon fluxes in subduction zones, what goes down, mostly comes up. (United States)

    Kelemen, Peter B; Manning, Craig E


    Carbon fluxes in subduction zones can be better constrained by including new estimates of carbon concentration in subducting mantle peridotites, consideration of carbonate solubility in aqueous fluid along subduction geotherms, and diapirism of carbon-bearing metasediments. Whereas previous studies concluded that about half the subducting carbon is returned to the convecting mantle, we find that relatively little carbon may be recycled. If so, input from subduction zones into the overlying plate is larger than output from arc volcanoes plus diffuse venting, and substantial quantities of carbon are stored in the mantle lithosphere and crust. Also, if the subduction zone carbon cycle is nearly closed on time scales of 5-10 Ma, then the carbon content of the mantle lithosphere + crust + ocean + atmosphere must be increasing. Such an increase is consistent with inferences from noble gas data. Carbon in diamonds, which may have been recycled into the convecting mantle, is a small fraction of the global carbon inventory.

  7. Subduction zones seen by GOCE gravity gradients

    DEFF Research Database (Denmark)

    Švarc, Mario; Herceg, Matija; Cammarano, Fabio

    In this study, the GOCE (Gravity field and steady state Ocean Circulation Explorer) gradiometry data were used to study geologic structures and mass variations within the lithosphere in areas of known subduction zones. The advantage of gravity gradiometry over other gravity methods...... is that gradients are extremely sensitive to localized density contrasts within regional geological settings, which makes it ideally suited for detecting subduction zones. Second order gravity gradients of disturbing potential were extracted from global geopotential model, the fifth release GOCE model ‘EGM_TIM_RL05......’. In order to remove the signal which mainly corresponds to the gravity signal of the lower mantle, long wavelength part of the gravity signal was removed up to degree and order 60. Because the areas with notable topography differences coincide with subduction zones, topography correction was also performed...

  8. Constraining the hydration of the subducting Nazca plate beneath Northern Chile using subduction zone guided waves (United States)

    Garth, Tom; Rietbrock, Andreas


    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 (first motion dispersion observed at multiple stations, or the extended P-wave coda observed in arrivals from intermediate depth events within the Nazca plate. These signals can however be accurately 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

  9. Dynamics and Significance of Plume-Induced Subduction Initiation: Numerical Modeling (United States)

    Gerya, T.; Stern, R. J.; Baes, M.; Sobolev, S. V.; Whattam, S. A.


    How did the first subduction zone form? Most present-day subduction initiation mechanisms require acting plate forces and/or preexisting zones of lithospheric weakness, which are themselves the consequence of plate tectonics (Stern 2004). In contrast, spontaneous plume-induced subduction initiation - suggested on the basis of numerical thermo-mechanical experiments (Ueda et al., 2008) and supported by data re-interpretation of how subduction started in Late Cretaceous time around the Caribbean LIP (Whattam and Stern, 2014) - does not require pre-existing lithospheric fabric, such as are created by active plate tectonics and is viable for both stagnant lid and mobile/deformable lid conditions. Here, we present first results of high-resolution 3D numerical thermo-mechanical modeling of plume-induced subduction resulting from mechanical-magmatic interaction of an ascending thermal mantle plume with old, cold, dense oceanic lithosphere. We demonstrate that weakening of the strong lithosphere by plume-induced magmatism is the key factor enabling subduction initiation around the plume head. A large plume head is required to overcome ring confinement, and subduction initiation is further favored when plume activity and lithospheric weakening continues for several tens of Ma. We further discuss possible implications of this scenario for modern plate tectonics as well as for plate tectonics initiation in Precambrian time. ReferencesStern, R.J., 2004. Subduction initiation: spontaneous and induced. EPSL 226, 275-292.Ueda, K., Gerya, T., Sobolev, S.V., 2008. Subduction initiation by thermal-chemical plumes. PEPI 171, 296-312.Whattam, S.A., Stern, R. 2014. Late Cretaceous plume-induced subduction initiation along the southern margin of the Caribbean and NW South America: The first documented example with implications for the onset of plate tectonics. Gondwana Research, (accepted).

  10. Variability of lithospheric structure in the Baltic Shield (United States)

    Pedersen, Helle; Debayle, Eric; Maupin, Valérie


    We present the shear velocity structure down to 250km depth beneath the dense LAPNET array in northern Finland, located at the northern end of the Baltic Shield. We analysed phase velocity dispersion of fundamental mode Rayleigh waves, using data from 46 seismic broadband stations and almost 200 magnitude >6 events. The inversion of the dispersion curve shows a well resolved low velocity zone starting at approximately 150km depth, while the shear velocities above are typical for cratonic lithosphere. The comparison to other parts of the Baltic Shield show strong variability of the lithospheric structure. Immediately south of LAPNET, in an area dominated by paleaproterozoic rocks at surface, the lithosphere is fast to a depth of 225-250km, while cratonic lithosphere seems to be absent beneath southern Norway, in spite of Proterozoic age tectonic ages. The low velocity zone beneath northern Finland indicates that the lithosphere in this area is either modified at depth, for example through metasomatism, or that it is thinner than the more internal part of the Baltic shield. We suggest that the modification of the cratonic lithosphere beneath northern Finland is not related to continental breakup at the opening of the Atlantic Ocean, as the continental shelf continues north, beneath the Barents Sea. We rather favour the hypothesis that subduction and/or collision could potentially modify (by fluid injection) or remove (by erosion/dripping) otherwise stable cratonic lithosphere.

  11. Evolution of passive continental margins and initiation of subduction zones

    NARCIS (Netherlands)

    Cloetingh, Sierd


    The initiation of subduction is a key element in plate tectonic schemes for the evolution of the Earth's lithosphere. Nevertheless, up to present, the underlying mechanism has not been very well understood (e.g. Dickinson and Seely, 1979; Hager, 1980; Kanamori, 1980). The insight into the initiation

  12. Evolution of passive continental margins and initiation of subduction zones

    NARCIS (Netherlands)

    Cloetingh, S.A.P.L.


    The initiation of subduction is a key element in plate tectonic schemes for the evolution of the Earth's lithosphere. Nevertheless, up to present, the underlying mechanism has not been very well understood (e.g. Dickinson and Seely, 1979; Hager, 1980; Kanamori, 1980). The insight into the

  13. Surface deformation resulting from subduction and slab detachment

    NARCIS (Netherlands)

    Buiter, S.J.H.


    Convergence of lithospheric plates is accommodated at active margins by one plate moving beneath the other into the Earth's mantle. Changes in this subduction process may cause variations in the topography of the Earth's surface near a convergent plate margin. The focus of this thesis lies on

  14. Some consequences of the subduction of young slabs

    NARCIS (Netherlands)

    England, P.; Wortel, R.

    The negative buoyancy force exerted by a subducting oceanic slab depends on its descent velocity, and strongly on its age. For lithosphere close to thermal equilibrium, this force dominates by a large margin the resisting forces arising from friction on the plate boundary and compositional buoyancy.

  15. Where does subduction initiate and die? Insights from global convection models with continental drift (United States)

    Ulvrova, Martina; Williams, Simon; Coltice, Nicolas; Tackley, Paul


    Plate tectonics is a prominent feature on Earth. Together with the underlying convecting mantle, plates form a self-organized system. In order to understand the dynamics of the coupled system, subduction of the lithospheric plates plays the key role since it links the exterior with the interior of the planet. In this work we study subduction initiation and death with respect to the position of the continental rafts. Using thermo-mechanical numerical calculations we investigate global convection models featuring self-consistent plate tectonics and continental drifting employing a pseudo-plastic rheology and testing the effect of a free surface. We consider uncompressible mantle convection in Boussinesq approximation that is basaly and internaly heated. Our calculations indicate that the presence of the continents alterns stress distribution within a certain distance from the margins. Intra-oceanic subudction initiation is favorable during super-continent cycles while the initiation at passive continental margin prevails when continents are dispersed. The location of subduction initiation is additionally controlled by the lithospheric strength. Very weak lithosphere results in domination of intra-oceanic subduction initiation. The subduction zones die more easily in the vicinity of the continent due to the strong rheological contrast between the oceanic and continental lithosphere. In order to compare our findings with subduction positions through time recorded on Earth, we analyse subduction birth in global plate reconstruction back to 410 My.

  16. A major Early Miocene thermal pulse due to subduction segmentation and rollback in the western Mediterranean region (United States)

    Spakman, W.; Van Hinsbergen, D. J.; Vissers, R.


    Geological studies have shown that Eo-Oligocene subduction related high-pressure, low-temperature metasediments and peridotites of the Alboran region (Spain, Morocco) and the Kabylides (Algeria) experienced a major Early Miocene (~21 Ma) thermal pulse requiring asthenospheric temperatures at ~60 km depth. Despite earlier propositions, the cause of this thermal pulse is still controversial while also the paleogeographic origin of the Alboran and Kabylides units is debated. Here, we relate the thermal pulse to segmentation of the West Alpine-Tethyan slab under the SE Iberian margin (Baleares-Sardinia). We restore the Alboran rocks farther east than previously assumed, to close to the Balearic Islands, adjacent to Sardinia. We identify three major lithosphere faults, the NW-SE trending North Balearic Transform Zone (NBTZ) and the ~W-E trending Emile Baudot and North African transforms that accommodated the Miocene subduction evolution of slab segmentation, rollback, and migration of Alboran and Kabylides rocks to their current positions. The heat pulse occurred S-SE of the Baleares where slab segmentation along the NBTZ triggered radially outgrowing S-SW rollback opening a slab window that facilitated local ascent of asthenosphere below the rapidly extending Alboran-Kabylides accretionary prism. Subsequent slab rollback carried the Kabylides and Alboran domains to their present positions. Our new reconstruction is in line with tomographically imaged mantle structure and focuses attention on the crucial role of evolving subduction segmentation driving HT-metamorphism and subsequent extension, fragmentation, and dispersion of geological terrains.

  17. Geochemistry and geochronology of the mafic lavas from the southeastern Ethiopian rift (the East African Rift System): assessment of models on magma sources, plume-lithosphere interaction and plume evolution (United States)

    Shinjo, Ryuichi; Chekol, Takele; Meshesha, Daniel; Itaya, Tetsumaru; Tatsumi, Yoshiyuki


    Major and trace element and isotopic ratios (Sr, Nd and Pb) are presented for mafic lavas (MgO > 4 wt%) from the southwestern Yabello region (southern Ethiopia) in the vicinity of the East African Rift System (EARS). New K/Ar dating results confirm three magmatic periods of activity in the region: (1) Miocene (12.3-10.5 Ma) alkali basalts and hawaiites, (2) Pliocene (4.7-3.6 Ma) tholeiitic basalts, and (3) Recent (1.9-0.3 Ma) basanite-dominant alkaline lavas. Trace element and isotopic characteristics of the Miocene and Quaternary lavas bear a close similarity to ocean island basalts that derived from HIMU-type sublithospheric source. The Pliocene basalts have higher Ba/Nb, La/Nb, Zr/Nb and 87Sr/86Sr (0.70395-0.70417) and less radiogenic Pb isotopic ratios (206Pb/204Pb = 18.12-18.27) relative to the Miocene and Quaternary lavas, indicative of significant contribution from enriched subcontinental lithospheric mantle in their sources. Intermittent upwelling of hot mantle plume in at least two cycles can explain the magmatic evolution in the southern Ethiopian region. Although plumes have been originated from a common and deeper superplume extending from the core-mantle boundary, the diversity of plume components during the Miocene and Quaternary reflects heterogeneity of secondary plumes at shallower levels connected to the African superplume, which have evolved to more homogeneous source.

  18. Rapid conversion of an oceanic spreading center to a subduction zone inferred from high-precision geochronology. (United States)

    Keenan, Timothy E; Encarnación, John; Buchwaldt, Robert; Fernandez, Dan; Mattinson, James; Rasoazanamparany, Christine; Luetkemeyer, P Benjamin


    Where and how subduction zones initiate is a fundamental tectonic problem, yet there are few well-constrained geologic tests that address the tectonic settings and dynamics of the process. Numerical modeling has shown that oceanic spreading centers are some of the weakest parts of the plate tectonic system [Gurnis M, Hall C, Lavier L (2004) Geochem Geophys Geosys 5:Q07001], but previous studies have not favored them for subduction initiation because of the positive buoyancy of young lithosphere. Instead, other weak zones, such as fracture zones, have been invoked. Because these models differ in terms of the ages of crust that are juxtaposed at the site of subduction initiation, they can be tested by dating the protoliths of metamorphosed oceanic crust that is formed by underthrusting at the beginning of subduction and comparing that age with the age of the overlying lithosphere and the timing of subduction initiation itself. In the western Philippines, we find that oceanic crust was less than ∼1 My old when it was underthrust and metamorphosed at the onset of subduction in Palawan, Philippines, implying forced subduction initiation at a spreading center. This result shows that young and positively buoyant, but weak, lithosphere was the preferred site for subduction nucleation despite the proximity of other potential weak zones with older, denser lithosphere and that plate motion rapidly changed from divergence to convergence.

  19. Lithospheric buoyancy and continental intraplate stresses (United States)

    Zoback, M.L.; Mooney, W.D.


    gravitational potential energy by taking a vertical integral over the computed lithosphere density. Our computed values suggest that the thick roots beneath cratons lead to strong negative potential energy differences relative to surrounding regions, and hence exert compressive stresses superimposed on the intraplate stresses derived from plate boundary forces. Forces related to this lithosphere structure thus may explain the dominance of reverse-faulting earthquakes in cratons. Areas of high elevation and a thin mantle lid (e.g., western U.S. Basin and Range, East African rift, and Baikal rift) are predicted to be in extension, consistent with the observed stress regime in these areas.

  20. Noble gases recycled into the mantle through cold subduction zones (United States)

    Smye, Andrew J.; Jackson, Colin R. M.; Konrad-Schmolke, Matthias; Hesse, Marc A.; Parman, Steve W.; Shuster, David L.; Ballentine, Chris J.


    Subduction of hydrous and carbonated oceanic lithosphere replenishes the mantle volatile inventory. Substantial uncertainties exist on the magnitudes of the recycled volatile fluxes and it is unclear whether Earth surface reservoirs are undergoing net-loss or net-gain of H2O and CO2. Here, we use noble gases as tracers for deep volatile cycling. Specifically, we construct and apply a kinetic model to estimate the effect of subduction zone metamorphism on the elemental composition of noble gases in amphibole - a common constituent of altered oceanic crust. We show that progressive dehydration of the slab leads to the extraction of noble gases, linking noble gas recycling to H2O. Noble gases are strongly fractionated within hot subduction zones, whereas minimal fractionation occurs along colder subduction geotherms. In the context of our modelling, this implies that the mantle heavy noble gas inventory is dominated by the injection of noble gases through cold subduction zones. For cold subduction zones, we estimate a present-day bulk recycling efficiency, past the depth of amphibole breakdown, of 5-35% and 60-80% for 36Ar and H2O bound within oceanic crust, respectively. Given that hotter subduction dominates over geologic history, this result highlights the importance of cooler subduction zones in regassing the mantle and in affecting the modern volatile budget of Earth's interior.

  1. The lithosphere-asthenosphere system of the Periadriatic region: a geophysical perspective (United States)

    Romanelli, Fabio; Brandmayr, Enrico; Panza, Giuliano


    We present a multiscale 3D model of the crust and upper mantle of the Periadriatic region showing how the Adriatic plate, the Northern indent of the African promontory, is involved in the Apennines, Alpine, and Dinarides subduction zones, respectively surrounding its western, northern, and eastern margins. The model is obtained through the ensemble of cellular models expressed in terms of shear waves velocity (Vs), thickness and density of the layers, to a depth of 350 km. These physical properties are obtained by means of advanced non-linear inversion techniques, such as the "hedgehog" inversion method of group and phase velocity dispersion curves for the determination of Vs and the non-linear inversion of gravity data by means of the method GRAV3D. The model obtained at the scale of 1°x1° is analysed along selected sections perpendicular to the orogenic complexes of the study area (Apennines, Alps, Dinarides) and it confirms the existence of deep structural asymmetries between E- and W-directed subduction zones. The asymmetry found between the almost vertical Apenninic subduction and the Alpine-Dinaric subduction, which is in turn characterized by a low dip angle, can be ascribed to an eastward mantle flow taking place in the low velocity zone (LVZ) that characterizes the top of the very shallow asthenosphere beneath the Tyrrhenian basin. The high-resolution model obtained for the Alpine region at a scale of 0.5°x0.5° enlightens the extreme variability of the crustal thickness as well the small scale heterogeneities in the upper mantle beneath the study area. The density model clearly shows that the subducting lithosphere turns out to be less dense than the surrounding mantle. A temperature model of the mantle layers is obtained by means of an advanced conversion technique of Vs to temperature that takes in account variable chemical composition and bulk water content. The superposition of different geodynamic mechanisms in the mentioned areas is coherent with

  2. To what depth can continental crust be subducted: numerical predictions and critical observations (United States)

    Gerya, T.; Faccenda, M.


    We performed systematic two-dimensional numerical modeling of continental collision associated with subduction of the lithospheric mantle. Results of our experiments suggest that two contrasting modes of lithospheric subduction below an orogen can exist: one-sided and double-sided. One-sided subduction brings continental crust subducting atop the slab to the contact with hot asthenosperic mantle wedge below the overriding plate. This can result in strong heating, partial melting and rheological weakening of the crust triggering its delamination from subducting mantle lithosphere in form of compositionally buoyant structures (cold plumes) propagating away from subducting plate, passing through the hot mantle wedge, underplating the overriding lithosphere and producing large amount of relatively felsic syn-orogenic magmas at sub-lithospheric depths. One-sided subduction of the buoyant continental crust can also result in a transient "hot channel effect" triggering formation and exhumation of coesite- and diamond- bearing rocks metamorphosed at 700 to 900oC. Anomalously high temperature is caused by intense viscous and radiogenic heating in the channel composed of deeply subducted radiogenic upper-crustal rocks. Low effective viscosity of the channel subsequent to increased temperature and partial melting permits profound mixing of mantle and crustal rocks. The hot channel exists during few million years only but rapidly produces and exhumes large amounts of ultrahigh-pressure, high-temperature rocks within the orogen. Double-sided subduction can follow the one-sided mode at later stages of orogeny when significant rheological coupling between two plates occurs during the collision. In this case the orogen is characterized by double- verging structure, the layer of subducting continental crust is embedded between two negatively buoyant lithospheric slabs and delamination of the crust does not occur. This mode of subduction can bring crustal rocks from the bottom of an

  3. Kinematics of Late Cretaceous subduction initiation in the Neo-Tethys Ocean reconstructed from ophiolites of Turkey, Cyprus, and Syria


    Maffione, Marco; van Hinsbergen, Douwe J.J.; de Gelder, Giovanni I.N.O.; van der Goes, Freek C.; Morris, Antony


    Formation of new subduction zones represents one of the cornerstones of plate tectonics, yet both the kinematics and geodynamics governing this process remain enigmatic. A major subduction initiation event occurred in the Late Cretaceous, within the Neo-Tethys Ocean between Gondwana and Eurasia. Suprasubduction zone ophiolites (i.e., emerged fragments of ancient oceanic lithosphere formed at suprasubduction spreading centers) were generated during this subduction event and are today distribut...

  4. Lithospheric Response of the Anatolian Plateau in the Realm of the Black Sea and the Eastern Mediterranean (United States)

    Ergun, Mustafa


    The Eastern Mediterranean and the Middle East make up the southern boundary of the Tethys Ocean for the last 200 Ma by the disintegration of the Pangaea and closure of the Tethys Ocean. It covers the structures: Hellenic and Cyprus arcs; Eastern Anatolian Fault Zone; Bitlis Suture Zone and Zagros Mountains. The northern boundary of the Tethys Ocean is made up the Black Sea and the Caspian Sea, and it extends up to Po valley towards the west (Pontides, Caucasus). Between these two zones the Alp-Himalayan orogenic belt is situated where the Balkan, Anatolia and the Iran plateaus are placed as the remnants of the lost Ocean of the Tethys. The active tectonics of the eastern Mediterranean is the consequences of the convergence between the Africa, Arabian plates in the south and the Eurasian plate in the north. These plates act as converging jaws of vise forming a crustal mosaic in between. The active crustal deformation pattern reveals two N-S trending maximum compression or crustal shortening syntaxes': (i) the eastern Black Sea and the Arabian plate, (ii) the western Black Sea and the Isparta Angle. The transition in young mountain belts, from ocean crust through the agglomeration of arc systems with long histories of oceanic closures, to a continental hinterland is well exemplified by the plate margin in the eastern Mediterranean. The boundary between the African plate and the Aegean/Anatolian microplate is in the process of transition from subduction to collision along the Cyprus Arc. Since the Black Sea has oceanic lithosphere, it is actually a separate plate. However it can be considered as a block, because the Black Sea is a trapped oceanic basin that cannot move freely within the Eurasian Plate. Lying towards the northern margin of orogenic belts related to the closure of the Tethys Ocean, it is generally considered to be a result of back-arc extension associated with the northward subduction of the Tethyan plate to the south. Interface oceanic lithosphere at

  5. Kinematics of subduction and plate convergence under Taiwan and its geomorphic, geodetic and seismic expressions (United States)

    Suppe, J.; Carena, S.; Kanda, R. V.; Wu, Y.; Huang, H.; Wu, J. E.


    Deciphering the kinematics of ongoing subduction and rapid plate convergence under Taiwan is neither trivial nor straightforward. A 3D synthesis of diverse constraints is required, for example tomography, geodesy, tectonic geomorphology, stress inversion, and Philippine Sea plate motions. Eurasian-Philippine Sea plate convergence is ~90mm/y in a mildly oblique 300° azimuth relative to the ~NS nearly vertically subducting Eurasian mantle lithosphere which extends to ~500km depth. If all the current plate convergence were consumed in subduction of Eurasian mantle, the subduction flexural hinge would migrate westward at ~80mm/y, which is fast relative to the ~30mm/y long-term slip rate on the Taiwan main detachment that represents the Eurasian subduction interface under the Taiwan Central Mountains. If this fast simple subduction were occurring, subduction would too quickly outrun the mountain belt in conflict with data. Instead we estimate that subduction of Eurasian lithosphere is proceeding at ~50mm/y with the remaining ~40mm/y convergence at a lithospheric level consumed by secondary subduction above and to the east of the main plate interface. This secondary subduction is largely transient deformation that is most obvious under the Coastal Range, which represents the deforming western margin of the Philippine Sea plate during the last ~1-1.5 Ma. The thrust faults of the Coastal Range function as subduction faults with the long-term net motion of their footwalls moving largely down relative to their only slowly uplifting hanging walls, with a net secondary subduction of ~40-50km in the last ~1-1.5Ma as estimated from seismic tomography and other data. In addition we find evidence for ongoing subduction of the eastern Central Mountains of Taiwan. The crest of the mountains coincides with the western edge of the migrating plate flexure, a band of extensional geodetic strain coincides with the flexure, and an extensional stress state in the upper 5-10km coincides

  6. Mantle convection, tectonics and the evolution of the Tethyan subduction zone (United States)

    Jolivet, Laurent; Sternai, Pietro; Menant, Armel; Faccenna, Claudio; Becker, Thorsten; Burov, Evguenii


    side of Africa from the Jurassic until the collision in the Oligocene, and even afterward when Arabia formed by opening of the Red Sea and the Gulf of Aden. This also suggests a dominant role of an underlying flow at large scale, dragging and mechanically eroding plates and breaking them into fragments, then passively carried. Only during a short period of the Late Cretaceous did the situation change drastically with the obduction event giving the large ophiolitic nappes observed from Oman to Turkey. This obduction event has never been really explained. It has been shown to be coeval with faster plate velocities and more active formation of oceanic crust globally, which in turn suggests a link with deep mantle convection. We discuss this succession of events and propose to relate them with the basal drag induced by convective mantle flow below the African continental lithosphere. We discuss the effects of convection on crustal deformation at different scales from deep convection related to plumes and subduction zones to more local mantle flow due to slab retreat and tearing.

  7. Isabella Anomaly: Lithospheric drip, delamination or fragment of the Farallon plate? (United States)

    Forsyth, D. W.; Rau, C. J.


    The Isabella Anomaly or Central Valley Anomaly in California is perhaps the best known example of a high seismic velocity anomaly that has been interpreted as a lithospheric instability. High P and S velocities extend to a depth of at least 150 km and perhaps to several hundred km in a nearly cylindrical region 100-150 km across. The amplitude of the anomaly in the upper 200 km is similar to that of the subducted Gorda plate. This anomaly has been variously interpreted as a convective drip or as a remnant of the lithosphere delaminated from beneath the eastern Sierra Nevada. We suggest instead that the Isabella anomaly may represent a fragment of the subducted Farallon plate that is still attached to the Pacific lithosphere. Directly seaward of the anomaly is the fossil Monterrey microplate, which is a remnant of the Farallon plate that was left when subduction ceased before the spreading center itself subducted. The microplate was then incorporated into the Pacific plate, but it is not clear how much of the subducting slab remained attached to the surface microplate. New Rayleigh wave tomographic images of Baja California show that there are still fragments of the Farallon plate remaining attached to the unsubducted Guadelupe and Magdelena microplate remnants, with anomalies extending down to at least 150 km. The geometry of these anomalies in relationship to the microplates is very similar to that of the Isabella anomaly. A major question with this interpretation is whether a bit of oceanic lithosphere extending down into the asthenosphere could be dragged along with the surface microplate/Pacific plate for 20 Ma since subduction ceased. Another anomaly similar to the Isabella anomaly begins in the shallow mantle beneath the northern end of San Francisco bay and dips to the west - another candidate for a lithospheric drip or convective instability?

  8. Effect of the Earth's rotation on subduction processes (United States)

    Levin, B. W.; Rodkin, M. V.; Sasorova, E. V.


    The role played by the Earth's rotation is very important in problems of physics of the atmosphere and ocean. The importance of inertia forces is traditionally estimated by the value of the Rossby number: if this parameter is small, the Coriolis force considerably affects the character of movements. In the case of convection in the Earth's mantle and movements of lithospheric plates, the Rossby number is quite small; therefore, the effect of the Coriolis force is reflected in the character of movements of the lithospheric plates. Analysis of statistical data on subduction zones verifies this suggestion.

  9. Late Pan-African and early Mesozoic brittle compressions in East and Central Africa: lithospheric deformation within the Congo-Tanzania Cratonic area (United States)

    Delvaux, D.; Kipata, M. L.; Macheyeki, A. S.


    Tectonic reconstructions leading to the formation of the Central-African part of Gondwana have so far not much taken into account constraints provided by the evolution of brittle structures and related stress field. This is largely because little is known on continental brittle deformation in Equatorial Africa before the onset of the Mesozoic Central-African and Late Cenozoic East-African rifts. We present a synthesis of fault-kinematic data and paleostress inversion results from field surveys covering parts of Tanzania, Zambia and the Democratic Republic of Congo. It is based on investigations along the eastern margin of the Tanzanian craton, in the Ubendian belt between the Tanzanian craton and Bangweulu block, in the Lufilian Arc between the Kalahari and Congo cratons and along the Congo intracratonic basin. Paleostress tensors were computed for a substantial database by interactive stress tensor inversion and data subset separation, and the relative succession of major brittle events established. Two of them appear to be of regional importance and could be traced from one region to the other. The oldest one is the first brittle event recorded after the paroxysm of the Terminal Pan-African event that led to the amalgamation Gondwana at the Precambrian-Cambrian transition. It is related to compressional deformation with horizontal stress trajectories fluctuating from an E-W compression in Central Tanzania to NE-SW in the Ubende belt and Lufilian Arc. The second event is a transpressional inversion with a consistent NW-SE compression that we relate to the far-field effects of the active margin south of Gondwana during the late Triassic - early Jurassic.

  10. Breaking the shell: Initiating plate tectonic-like subduction on Europa (United States)

    Bland, Michael T.; McKinnon, William B.


    Europa’s prominent bands have been proposed to form by a seafloor-spreading-like mechanism involving complete separation of Europa’s lithosphere and the emplacement of fresh ice from below [Prockter et al. 2002]. This formation mechanism poses a challenge for Europa’s strain balance: extensional rifting at bands must be offset by lithospheric shortening elsewhere, yet few obvious contractional features have been observed. Kattenhorn and Prockter [2014] suggested that extension on Europa is accommodated by subduction of the lithosphere at linear, tabular zones termed subsumption bands. Subduction of Europa’s lithosphere implicitly requires that lithospheric-scale thrust faults can develop. This contrasts with previous numerical modeling, which found that lithospheric shortening is instead primarily accommodated by folding or passive thickening [Bland and McKinnon 2012, 2013]. Here we reevaluate the conditions required to form large-scale thrust faults using a numerical model of lithospheric shortening on Europa that includes realistic localization of brittle failure (non-associated plasticity). In the absence of strain weakening (wherein brittle failure decreases the subsequent yield strength) essentially all shortening results in folding or thickening, consistent with previous results. With moderate strain weakening, deformation becomes localized within fault-like zones for surface temperatures ≤100 K; however, the resulting surface deformation suggests a complex interplay between folding and faulting. Only if the ice shell weakens very easily does faulting dominate. Large-scale faults preferentially form at cold surface temperatures and high heat fluxes. Cold temperatures promote faulting (as opposed to folding), and high heat fluxes result in a thinner lithosphere, which is more easily subducted. The subsumption bands identified by Kattenhorn and Prockter [2014] are at a relatively high latitude (cold temperature), and are associated with putative

  11. Neotectonic fault detection and lithosphere structure beneath SW of High Atlas (Morocco) (United States)

    Timoulali, Youssef; Radi, Said; Azguet, Roumaissae; Bachaoui, Mostapha


    subduction zone marking the limit between SW of High Atlas and the western Meseta. A second high velocity body, dipping north beneath the Hercynian Tichka Massif is detected. This positive velocity anomaly can be interpreted as an old subduction zone marking the limit between Meseta Domain and West African Craton. The occurrence of tholeiitic and alkaline magmatism respectively in the Essaouira basin zone and in the Hercynian Ticka Massif zone leads us to conclude that remains of subduction exist in these zones. The negative lithospheric anomalies are interpreted as heat or hot asthenospheric material upwelling from depth crossing the main crustal structures in SW High Atlas. The negative lithospheric anomalies are interpreted as hot asthenospheric material or heat coming from depth to replace the detached crust. Based on our analysis of local P wave velocities and profiles, the crustal thickness beneath SW of High Atlas region varies from 30 km to 45 km in central and western part.

  12. Subduction Mode Selection During Slab and Mantle Transition Zone Interaction: Numerical Modeling (United States)

    Shi, Yanan; Wei, Dongping; Li, Zhong-Hai; Liu, Ming-Qi; Liu, Mengxue


    Global seismic tomography of the subduction zones shows that the subducting slabs could either stagnate around the 660-km discontinuity, or penetrate into the lower mantle. The stagnating slabs also have various morphologies. These are directly related to the interaction between the subducting slabs and the mantle transition zone (MTZ), the dynamics of which are still debated. Using a 2-D thermo-mechanical model, we systematically investigated the modes of subduction in the mantle transition zone and explored the key constraints of various subduction styles. Four basic subduction modes are obtained in the numerical experiments, including one with slab penetrating through the 660-km discontinuity and three other modes with slab stagnating in the MTZ (i.e. folding, lying and rolling-back). The numerical models indicate that the age of subducting oceanic plate, the thickness of overriding continental lithosphere and the convergence velocity play crucial roles in the dynamics of subducting slab and MTZ interaction. In general, the young subducting slab favors the penetration or folding mode, whereas the old subducting slab tends to result in lying or rolling-back mode, although other parameters can also affect. Our models also show a strong correlation between the subduction mode selection and dip angle of the slab tip when reaching the 660-km phase boundary.

  13. Lithospheric thinning in the Eastern Indian Craton: Evidence for lithospheric delamination below the Archean Singhbhum Craton? (United States)

    Mandal, Prantik


    We herein present shear velocity structure extending down to 300 km depth below the Archean Singhbhum-Odisha Craton (SOC) and Proterozoic Chotanagpur granitic-gneissic terrain (CGGT), which has been obtained through the inversion modeling of P-receiver functions. We use three-component broadband recordings of 200 teleseismic earthquakes (30° ≤ ∆ ≤ 90°) from a 15 station seismic network that has been operational in the Eastern Indian shield since February 2013. We obtain the thinnest crust of 35 km overlying a thin lithosphere of 78 km, below the region near south Singhbhum shear zone, which could be attributed to the 1.6 Ga plume activity associated with Dalma volcanic. However, the thickest crust of 47 km overlying a thin lithosphere of 81 km is noticed below the region near the Singhbhum granite of 3.6 Ga. This thinning of lithosphere could be attributed to the delamination of lithospheric root due to the Himalayan orogeny with a shortening rate of 2 cm/year. This delamination model in SOC gets further support from the densification of the lower crust, which could result from repeated episodes of basaltic underplating associated with episodes related to Dalma ( 1.6 Ga) and Rajmahal ( 117 Ma) volcanisms. This led to relatively more mafic, heterogeneous and deformed crustal structure in SOC as well as EGMB (with an average crustal Vs of 4.0 km/s) in comparison to that in CGGT (with an average crustal Vs of 3.9 km/s), as seen through our modeling results. The thickest lithosphere of 100 km is observed in the southwestern SOC as well as northeastern CGGT. We also notice that a sharp and flat Moho in CGGT, which could be attributed to thermal reactivation and large volume melting of the mafic cratonic crust during the late Archean subduction process and associated volcanism episodes. This model gets further support from the estimated 169 km thick lower Vs zone in the upper mantle below CGGT. Our modeling results also support a northward subduction of Archean

  14. Experimental and observational evidence for plume-induced subduction on Venus (United States)

    Davaille, A.; Smrekar, S. E.; Tomlinson, S.


    Why Venus lacks plate tectonics remains an unanswered question in terrestrial planet evolution. There is observational evidence for subduction--a requirement for plate tectonics--on Venus, but it is unclear why the features have characteristics of both mantle plumes and subduction zones. One explanation is that mantle plumes trigger subduction. Here we compare laboratory experiments of plume-induced subduction in a colloidal solution of nanoparticles to observations of proposed subduction sites on Venus. The experimental fluids are heated from below to produce upwelling plumes, which in turn produce tensile fractures in the lithosphere-like skin that forms on the upper surface. Plume material upwells through the fractures and spreads above the skin, analogous to volcanic flooding, and leads to bending and eventual subduction of the skin along arcuate segments. The segments are analogous to the semi-circular trenches seen at two proposed sites of plume-triggered subduction at Quetzalpetlatl and Artemis coronae. Other experimental deformation structures and subsurface density variations are also consistent with topography, radar and gravity data for Venus. Scaling analysis suggests that this regime with limited, plume-induced subduction is favoured by a hot lithosphere, such as that found on early Earth or present-day Venus.

  15. Major and trace element chemistry of pyrope garnets from the El Kseibat area (Algeria): implications for lithosphere features in the north-eastern part of the West African Craton (United States)

    Kahoui, Mohammed; Griffin, William L.; Mahdjoub, Yamina; Belousova, Elena A.; Chabane, Manseur; Kaminsky, Felix V.


    Thirty-eight (39) pyrope garnets from the El Kseibat area (north-eastern part of the West African Craton or WAC, Algeria) have been studied using major and trace element chemistry to obtain information on the lithospheric mantle composition and thermal properties. All pyrope grains are found in Cretaceous and Quaternary alluvial sediments. They are well rounded and have undergone a long history of transportation, like the diamonds of Djebel Aberraz placer from the El Kseibat area (Kahoui et al., 2008). 37 of the studied garnets are lherzolitic in composition (including five high-Ti peridotitic, or G11, according to Grutter et al., 2004) and two grains are harzburgitic (G10), very close to the borderline with lherzolitic garnets. The plot of the garnet grains, in the Zr/Y vs. Y/Ga diagram (Griffin et al. 1998), shows that our pyropes fall within the Archon/Proton field consistent with the Eglab Shield geology which is a Proton with some Archaean relics. Some pyrope grains fall outside the fields of Archons and Protons. It seems that there are two layers in the mantle, one shallower and one deeper, like in the Lac de Gras area, Slave Craton, Canada (Griffin et al., 1999). The thermobarometry of garnet with the use of Ni-thermometry and Cr-barometry (Griffin et al., 1989) and the calibration by Ryan et al. (1996) gives T(Ni) = 790-1392°C and P(Cr) = 26.3 - 49.8 Kb. The geotherm corresponding and calculated using heat flow constraints is high (45-50 mW/m2), with nearly all garnets lying in the graphite field; this geotherm indicates a sampling interval of ca 100-170 km. Based on trace element distribution, garnets can be divided in three groups: (1) depleted low Sr, (2) high Zr, low Ti, Sr and (3) high Zr, Ti. The diagram Ti vs. Zr indicates that group 1 may be considered as depleted, but group 2 and 3 show a wide range-intensity of metasomatic processes: phlogopite-metasomatism being typical of group 2, and melt-metasomatism, typical of group 3. There are four types

  16. Dynamics of interplate domain in subduction zones: influence of rheological parameters and subducting plate age

    Directory of Open Access Journals (Sweden)

    D. Arcay


    Full Text Available The properties of the subduction interplate domain are likely to affect not only the seismogenic potential of the subduction area but also the overall subduction process, as it influences its viability. Numerical simulations are performed to model the long-term equilibrium state of the subduction interplate when the diving lithosphere interacts with both the overriding plate and the surrounding convective mantle. The thermomechanical model combines a non-Newtonian viscous rheology and a pseudo-brittle rheology. Rock strength here depends on depth, temperature and stress, for both oceanic crust and mantle rocks. I study the evolution through time of, on one hand, the brittle-ductile transition (BDT depth, zBDT, and, on the other hand, of the kinematic decoupling depth, zdec, simulated along the subduction interplate. The results show that both a high friction and a low ductile strength at the asthenospheric wedge tip shallow zBDT. The influence of the weak material activation energy is of second order but not negligible. zBDT becomes dependent on the ductile strength increase with depth (activation volume if the BDT occurs at the interplate decoupling depth. Regarding the interplate decoupling depth, it is shallowed (1 significantly if mantle viscosity at asthenospheric wedge tip is low, (2 if the difference in mantle and interplate activation energy is weak, and (3 if the activation volume is increased. Very low friction coefficients and/or low asthenospheric viscosities promote zBDT = zdec. I then present how the subducting lithosphere age affects the brittle-ductile transition depth and the kinematic decoupling depth in this model. Simulations show that a rheological model in which the respective activation energies of mantle and interplate material are too close hinders the mechanical decoupling at the down-dip extent of the interplate

  17. The lithosphere structure beneath the central Mediterranean from S receiver functions (United States)

    Bianchi, Irene; Miller, Meghan; Piana Agostinetti, Nicola; O'Driscoll, Leland


    The last piece of Mesozoic oceanic lithosphere from the Neo-Tethys Ocean is being consumed beneath Eurasia in the Central Mediterranean area, squeezed by the continuing convergence of Africa with the Eurasian plate. Convergence between Africa and Eurasia has been ongoing since the Cenozoic, forming a series of arcuate shaped subduction zones, and producing the current complex plate boundary that strikes across the Mediterranean area. Moreover, geophysical imaging depicts a variable lithospheric structure related to remnants of both oceanic and continental lithosphere within this convergent margin, which contributes to its complexity. In fact, the subduction/collision of blocks with different rheologies and thicknesses (e.g. continental or oceanic) has resulted in complex setting that includes accretionary wedges, orogenesis, and formation of an intricate back-arc/fore-arc/trench system. In order to shed light on these tectonic structures, we provide observations and interpretations of the lithospheric structure of the central Mediterranean via S receiver functions analysis. Teleseismic observations recorded at permanent and temporary seismic stations have been employed to produce images of the lithospheric discontinuities with tens of kilometers lateral resolution. We illustrate the feasibility of the lithosphere-asthenosphere boundary detection on a regional scale, and detect the occurrence of deeper seismic discontinuities due both to positive and negative seismic velocity jumps.

  18. Deep vs. shallow expressions of continental cratons: Can cratonic roots be destroyed by subduction? (United States)

    Perry-Houts, J.; Calo, M.; Eddy, C. L.; Guerri, M.; Holt, A.; Hopper, E.; Tesoniero, A.; Romanowicz, B. A.; Becker, T. W.; Wagner, L. S.


    Cratons are parts of continents that have remained tectonically quiescent over billion-year timescales. Although cratonic lithosphere has the stabilizing properties of chemical buoyancy and high viscosity, it can still be destroyed. The best known example of a missing cratonic root is beneath the eastern North China Craton (NCC). Despite strong evidence for the past existence of a craton in northern China, high heat flow, Mesozoic basin formation, extensive seismicity, and the lack of a fast seismic root imply that the deep cratonic lithosphere is missing. The mechanism for the lithospheric root loss is a source of much debate. Many mechanisms have been proposed, among them: shearing of the lithospheric root by asthenospheric flow induced by the Indo-Eurasian collision; ponding of the Pacific slab in the transition zone acting as a source of fluids that enable hydrous weakening; and thermal erosion due to the corner-flow upwelling of hot, deep material. It is generally agreed that the influence of subduction is key, both from the temporal coincidence of subduction with increased tectonomagmatic activity on the craton and from the spatial correlation of lithospheric loss adjacent to the Pacific trench. We investigate how cratons extend to depth through comparison between seismic signatures of the cratonic lithosphere in the upper mantle and surficial evidence of cratonic boundaries. We examine global and regional tomography, as well as receiver-function constraints on lithospheric thickness in the NCC. We define craton boundaries at the surface through analyses on crust and lithospheric mantle ages and kimberlite locations. We aim to identify regions where the fast cratonic root has been lost or altered beneath Archean and Proterozoic crust and in particular place constraints on the extent of the remaining cratonic root beneath North China. Given the common emphasis on the role of subduction as a driving force for the root loss beneath the eastern NCC, we focus on

  19. Formation and stability of a double subduction system: a numerical study (United States)

    Pusok, Adina E.; Stegman, Dave


    al., 2016] capable of simulating lithospheric deformation while simultaneously taking mantle flow and an internal free surface into account. We start from a single subduction setup, where subduction is already initiated (mature) and we stress the system by controlling the convergence rate of the system (i.e. imposing influx/outflux boundary conditions). Under certain conditions, a second subduction may develop and transform into a stable double subduction system. Preliminary results suggest that the fate of the incipient secondary subduction depends on internal factors (i.e. buoyancy and rheology), but also on the dynamics of the primary subduction zone and the boundary conditions (i.e. convergence rate).

  20. Geodynamics and The Evolution of Continental Lithosphere (United States)

    Hamilton, W. B.

    R.L. ArmstrongSs (1991) posthumous paper demonstrated that, contrary to conven- tional (and still dominant) geochemical assumption, isotopic data require most con- tinental material to have been fractionated into crust early in Earth history and to have been variably recycled since through the mantle. Recent information confirms predictions implicit in his recycling model. Archean cratonic crust mostly lacks a thick, mafic basal layer and is in bulk more felsic than Proterozoic and younger crust, whereas Archean lithospheric mantle is much more refractory than younger litho- sphere, which becomes markedly more enriched in fusible components with decreas- ing age. The oldest rocks in all Archean cratons are felsic migmatites, plus abundant but subordinate ultramafic, mafic, and anorthositic rocks. Ion-microprobe ages of zir- cons in these gneisses typically scatter from a maximum (currently 4.4 Ga in Yilgarn, and 4.1-3.6 Ga in other cratons) to minima near ages of overlying supracrustal rocks or of remobilization into late domiform batholiths. The gneisses were near wet-solidus temperature for long periods, the felsic protolith may have been mostly fractionated into crust by 4.4 Ga, and these ancient gneisses may dominate middle and lower Archean crust. Setting of formation is unknown, although steep REE patterns sug- gest derivation, by hydrous magmatic fractionation or partial melting, complementary to garnet-rich rocks that comprise geophysical mantle. Waterlaid supracrustal rocksU-° first quartzite, then mostly mafic and ultramafic lavaUwere deposited on the basement ° rocks after 3.5 Ga, and were deformed by gravitational rise of domiform batholiths and sinking of dense synclines. Indicators of plate-tectonic rifting and convergence are widespread in terrains younger than 2 Ga but are wholly lacking in the Archean. Plate tectonics is driven by subduction, which is enabled by top-down cooling of light asthenosphere to form dense oceanic lithosphere. Hinges

  1. Reconstructions of subducted ocean floor along the Andes: a framework for assessing Magmatic and Ore Deposit History (United States)

    Sdrolias, M.; Müller, R.


    The South American-Antarctic margin has been characterised by numerous episodes of volcanic arc activity and ore deposit formation throughout much of the Mesozoic and Cenozoic. Although its Cenozoic subduction history is relatively well known, placing the Mesozoic arc-related volcanics and the emplacement of ore bodies in their plate tectonic context remains poorly constrained. We use a merged moving hotspot (Late Cretaceous- present) and palaeomagnetic /fixed hotspot (Early Cretaceous) reference frame, coupled with reconstructed spreading histories of the Pacific, Phoenix and Farallon plates to understand the convergence history of the South American and Antarctic margins. We compute the age-area distribution of oceanic lithosphere through time, including subducting oceanic lithosphere and estimate convergence rates along the margin. Additionally, we map the location and migration of spreading ridges along the margin and relate this to processes on the overriding plate. The South American-Antarctic margin in the late Jurassic-early Cretaceous was dominated by rapid convergence, the subduction of relatively young oceanic lithosphere (Verdes" in southern South America. The speed of subduction increased again along the South American-Antarctic margin at ~105 Ma after another change in tectonic regime. Newly created crust from the Farallon-Phoenix ridge continued to be subducted along southern South America until the cessation of the Farallon-Phoenix ridge in the latest Cretaceous / beginning of the Cenozoic. The age of the subducting oceanic lithosphere along the South American-Antarctic margin has increased steadily through time.

  2. Impact-driven subduction on the Hadean Earth (United States)

    O'Neill, C.; Marchi, S.; Zhang, S.; Bottke, W.


    Impact cratering was a dominant geologic process in the early Solar System that probably played an active role in the crustal evolution of the young terrestrial planets. The Earth's interior during the Hadean, 4.56 to 4 billion years ago, may have been too hot to sustain plate tectonics. However, whether large impacts could have triggered tectonism on the early Earth remains unclear. Here we conduct global-scale tectonic simulations of the evolution of the Earth through the Hadean eon under variable impact fluxes. Our simulations show that the thermal anomalies produced by large impacts induce mantle upwellings that are capable of driving transient subduction events. Furthermore, we find that moderate-sized impacts can act as subduction triggers by causing localized lithospheric thinning and mantle upwelling, and modulate tectonic activity. In contrast to contemporary subduction, the simulated localized subduction events are relatively short-lived (less than 10 Myr) with relatively thin, weak plates. We suggest that resurgence in subduction activity induced by an increased impact flux between 4.1 and 4.0 billion years ago may explain the coincident increase in palaeointensity of the magnetic field. We further suggest that transient impact-driven subduction reconciles evidence from Hadean zircons for tectonic activity with other lines of evidence consistent with an Earth that was largely tectonically stagnant from the Hadean into the Archaean.

  3. Lithospheric thermal evolution and dynamic mechanism of destruction of the North China Craton (United States)

    Li, Zian; Zhang, Lu; Lin, Ge; Zhao, Chongbin; Liang, Yingjie


    The dynamic mechanism for destruction of the North China Craton (NCC) has been extensively discussed. Numerical simulation is used in this paper to discuss the effect of mantle upward throughflow (MUT) on the lithospheric heat flux of the NCC. Our results yield a three-stage destruction of the NCC lithosphere as a consequence of MUT variation. (1) In Late Paleozoic, the elevation of MUT, which was probably caused by southward and northward subduction of the paleo-Asian and paleo-Tethyan oceans, respectively, became a prelude to the NCC destruction. The geological consequences include a limited decrease of the lithospheric thickness, an increase of heat flux, and a gradual enhancement of the crustal activity. But the tectonic attribute of the NCC maintained a stable craton. (2) During Late Jurassic-Early Cretaceous, the initial velocity of the MUT became much faster probably in response to subduction of the Pacific Ocean; the conductive heat flux at the base of the NCC lithosphere gradually increased from west to east; and the lithospheric thickness was significantly decreased. During this stage, the heat flux distribution was characterized by zonation and partition, with nearly horizontal layering in the lithosphere and vertical layering in the underlying asthenosphere. Continuous destruction of the NCC lithosphere was associated with the intense tectono-magmatic activity. (3) From Late Cretaceous to Paleogene, the velocity of MUT became slower due to the retreat of the subducting Pacific slab; the conductive heat flux at the base of lithosphere was increased from west to east; the distribution of heat flux was no longer layered. The crust of the western NCC is relatively hotter than the mantle, so-called as a `hot crust but cold mantle' structure. At the eastern NCC, the crust and the mantle characterized by a `cold crust but hot mantle.' The western NCC (e.g., the Ordos Basin) had a tectonically stable crust with low thermal gradients in the lithosphere; whereas

  4. Kinematics of Late Cretaceous subduction initiation in the Neo-Tethys Ocean reconstructed from ophiolites of Turkey, Cyprus, and Syria (United States)

    Maffione, Marco; van Hinsbergen, Douwe J. J.; de Gelder, Giovanni I. N. O.; van der Goes, Freek C.; Morris, Antony


    Formation of new subduction zones represents one of the cornerstones of plate tectonics, yet both the kinematics and geodynamics governing this process remain enigmatic. A major subduction initiation event occurred in the Late Cretaceous, within the Neo-Tethys Ocean between Gondwana and Eurasia. Suprasubduction zone ophiolites (i.e., emerged fragments of ancient oceanic lithosphere formed at suprasubduction spreading centers) were generated during this subduction event and are today distributed in the eastern Mediterranean region along three E-W trending ophiolitic belts. Several models have been proposed to explain the formation of these ophiolites and the evolution of the associated intra-Neo-Tethyan subduction zone. Here we present new paleospreading directions from six Upper Cretaceous ophiolites of Turkey, Cyprus, and Syria, calculated by using new and published paleomagnetic data from sheeted dyke complexes. Our results show that NNE-SSW subduction zones were formed within the Neo-Tethys during the Late Cretaceous, which we propose were part of a major step-shaped subduction system composed of NNE-SSW and WNW-ESE segments. We infer that this subduction system developed within old (Triassic?) lithosphere, along fracture zones and perpendicular weakness zones, since the Neo-Tethyan spreading ridge formed during Gondwana fragmentation would have already been subducted at the Pontides subduction zone by the Late Cretaceous. Our new results provide an alternative kinematic model of Cretaceous Neo-Tethyan subduction initiation and call for future research on the mechanisms of subduction inception within old (and cold) lithosphere and the formation of metamorphic soles below suprasubduction zone ophiolites in the absence of nearby spreading ridges.

  5. 3D isotropic shear wave velocity structure of the lithosphere-asthenosphere system underneath the Alpine-Mediterranean Mobile belt (United States)

    El-Sharkawy, Amr; Weidle, Christian; Christiano, Luigia; Lebedev, Sergei; Meier, Thomas


    The Alpine-Mediterranean mobile belt is, tectonically, one of the most complicated and active regions in the world. Since the Mesozoic, collisions between Gondwana-derived continental blocks and Eurasia, due to the closure of a number of rather small ocean basins, have shaped the Mediterranean geology. During the late Mesozoic, it was dominated by subduction zones (e.g., in Anatolia, the Dinarides, the Carpathians, the Alps, the Apennines, and the Betics), which inverted the extensional regime, consuming the previously formed oceanic lithosphere, the adjacent passive continental margins and presumably partly also continental lithosphere. The location, distribution, and evolution of these subduction zones were mainly controlled by the continental or oceanic nature, density, and thickness of the lithosphere inherited from the Mesozoic rift after the European Variscan Orogeny. Despite the numerous studies that have attempted to characterize the lithosphere-asthenosphere structure in that area, details of the lithospheric structure and dynamics, as well as flow in the asthenosphere are, however, poorly known. A 3D shear-wave velocity structure of the lithosphere-asthenosphere system in the Mediterranean is investigated using new tomographic images obtained from surface wave tomography. An automated algorithm for inter-station phase velocity measurements is applied here to obtain both Rayleigh and Love fundamental mode phase velocities. We utilize a database consisting of more than 4000 seismic events recorded by more than 2000 broadband seismic stations within the area, provided by the European Integrated Data Archive (WebDc/EIDA) and IRIS. Moreover, for the first time, data from the Egyptian National Seismological Network (ENSN), recorded by up to 25 broad band seismic stations, are also included in the analysis. For each station pair, approximately located on the same great circle path, the recorded waveforms are cross correlated and the dispersion curves of

  6. The Gutenberg Discontinuity: Melt at the Lithosphere-Asthenosphere Boundary (United States)

    Schmerr, Nicholas


    The lithosphere-asthenosphere boundary (LAB) beneath ocean basins separates the upper thermal boundary layer of rigid, conductively cooling plates from the underlying ductile, convecting mantle. The origin of a seismic discontinuity associated with this interface, known as the Gutenberg discontinuity (G), remains enigmatic. High-frequency SS precursors sampling below the Pacific plate intermittently detect the G as a sharp, negative velocity contrast at 40- to 75-kilometer depth. These observations lie near the depth of the LAB in regions associated with recent surface volcanism and mantle melt production and are consistent with an intermittent layer of asthenospheric partial melt residing at the lithospheric base. I propose that the G reflectivity is regionally enhanced by dynamical processes that produce melt, including hot mantle upwellings, small-scale convection, and fluid release during subduction.

  7. Magmatism in Lithosphere Delamination process inferred from numerical models (United States)

    Göǧüş, Oǧuz H.; Ueda, Kosuke; Gerya, Taras


    The peel away of the oceanic/continental slab from the overlying orogenic crust has been suggested as a ubiquitous process in the Alpine-Mediterranean orogenic region (e.g. Carpathians, Apennines, Betics and Anatolia). The process is defined as lithospheric delamination where a slab removal/peel back may allow for the gradual uprising of sub-lithospheric mantle, resulting in high heat flow, transient surface uplift/subsidence and varying types of magma production. Geodynamical modeling studies have adressed the surface response to the delamination in the context of regional tectonic processes and explored wide range of controlling parameters in pre-syn and post collisional stages. However, the amount and styles of melt production in the mantle (e.g. decompression melting, wet melting in the wedge) and the resulting magmatism due to the lithosphere delamination remains uncertain. In this work, by using thermomechanical numerical experiments, designed in the configuration of subduction to collision, we investigated how melting in the mantle develops in the course of delamination. Furthermore, model results are used to decipher the distribution of volumetric melt production, melt extraction and the source of melt and the style of magmatism (e.g. igneous vs. volcanic). The model results suggest that a broad region of decompression melting occurs under the crust, mixing with the melting of the hydrated mantle derived by the delaminating/subducting slab. Depending on the age of the ocean slab, plate convergence velocity and the mantle temperature, the melt production and crust magmatism may concentrate under the mantle wedge or in the far side of the delamination front (where the subduction begins). The slab break-off usually occurs in the terminal stages of the delamination process and it may effectively control the location of the magmatism in the crust. The model results are reconciled with the temporal and spatial distribution of orogenic vs. anorogenic magmatism in

  8. The lithospheric stress field from joint modeling of lithosphere and mantle circulation using constraints from the latest global tomography models (United States)

    Wang, X.; Holt, W. E.; Ghosh, A.


    An understanding of the lithospheric stress field is important because these stresses are one indication of processes within the Earth's interior. In order to calculate the lithosphere stress field it is necessary to take into account the effects of lithosphere structure and topography along with coupling with 3-D mantle flow. We separate these effects into two parts: (1) contributions from topography and lithosphere structure are calculated by computing the stresses associated with gravitational potential energy (GPE) differences, and (2) stresses associated with mantle tractions are computed using the latest tomography models. The contributions from GPE and tractions are then combined to obtain model estimates of the lithospheric stress field, strain rate field, and surface velocity field. We simultaneously use the World Stress Map, the Global Strain Rate Model, and the No-Net-Rotation (NNR) surface velocity vectors to constrain models. We systematically test the latest global tomography models (SEMum [Lekic and Romanowicz, 2011], S40RTS [Ritsema et al., 2011], and S362ANI_PREM [Kustowski et al., 2008]) and the composite tomography model (SMEAN [Becker and Boschi, 2002]), along with the influence of different mantle radial viscosity models. We find that a coupled model with a weak viscosity channel, sandwiched between a strong lithosphere and strong lower mantle is best able to match the observational constraints, although there is a slight difference in stress field among the different tomography models. There is considerable evidence that the contributions from shallow versus deeper sources vary dramatically over the surface of the globe. We quantify these relative contributions as a function of position on the globe and systematically compare the results of different tomography models. Subduction zones are dominated by the effects of GPE differences, whereas within many of the plate interiors the contributions from mantle flow dominate.

  9. Kinematics of Late Cretaceous subduction initiation in the Neo-Tethys Ocean reconstructed from ophiolites of Turkey, Cyprus, and Syria (United States)

    Maffione, Marco; van Hinsbergen, Douwe; de Gelder, Giovanni; van der Goes, Freek; Morris, Antony


    Formation of new subduction zones represents one of the cornerstones of plate tectonics, yet both the kinematics and geodynamics governing this process remain enigmatic. A major subduction initiation event occurred in the Late Cretaceous, within the Neo-Tethys Ocean between Gondwana and Eurasia. Supra-subduction zone (SSZ) ophiolites (i.e., emerged fragments of ancient oceanic lithosphere accreted at supra-subduction spreading centers) were generated during this subduction event, and are today distributed in the eastern Mediterranean region along three E-W trending ophiolitic belts. Current models associate these ophiolite belts to simultaneous initiation of multiple, E-W trending subduction zones at 95 Ma. Here we report paleospreading direction data obtained from paleomagnetic analysis of sheeted dyke sections from seven Neo-Tethyan ophiolites of Turkey, Cyprus, and Syria, demonstrating that these ophiolites formed at NNE-SSW striking ridges parallel to the newly formed subduction zones. This subduction system was step-shaped and composed of NNE-SSW and ESE-WNW segments. The eastern subduction segment invaded the SW Mediterranean, leading to a radial obduction pattern similar to the Banda arc. Emplacement age constraints indicate that this subduction system formed close to the Triassic passive and paleo-transform margins of the Anatolide-Tauride continental block. Because the original Triassic-Jurassic Neo-Tethyan spreading ridge must have already subducted below the Pontides before the Late Cretaceous, we infer that the Late Cretaceous Neo-Tethyan subduction system started within ancient lithosphere, along NNE-SSW oriented fracture zones and faults parallel to the E-W trending passive margins. This challenges current concepts suggesting that subduction initiation occurs along active intra-oceanic plate boundaries.

  10. 3D geodynamic models for the development of opposing continental subduction zones: The Hindu Kush-Pamir example (United States)

    Liao, Jie; Gerya, Taras; Thielmann, Marcel; Webb, A. Alexander G.; Kufner, Sofia-Katerina; Yin, An


    The development of opposing continental subduction zones remains scantly explored in three dimensions. The Hindu Kush-Pamir orogenic system at the western end of the Himalayan orogen provides a rare example of continental collision linked to two opposing intra-continental subduction zones. The subducted plates feature a peculiar 3D geometry consisting of two distinct lithospheric fragments with different polarities, subduction angles and slab-curvatures beneath the Hindu Kush and Pamir, respectively. Using 3D geodynamic modeling, we simulate possible development of two opposing continental subduction zones to understand the dynamic evolution of the Hindu Kush-Pamir orogenic system. Our geodynamic model reproduces the major tectonic elements observed: (1) the deeper subduction depth, the steeper dip angle and the southward offset of the Hindu Kush subduction zone relative to the Pamir naturally occur if convergence direction of the subducting Indian plate and dip-direction of the Hindu Kush subduction zone match. (2) The formation of the highly asymmetrically curved Pamir region and the south-dipping subduction is promoted by the initial geometry of the indenting Indian lithosphere together with the existence of a major strike-slip fault on the eastern margin of the Pamir region. (3) Subduction of only the lower continental crust during continental collision can occur if the coupling between upper and lower crusts is weak enough to allow a separation of these two components, and that (4) the subduction of mainly lower crust then facilitates that conditions for intermediate-depth seismicity can be reached. (5) The secondary tectonic features modeled here such as strike-slip-fault growth, north-northwest striking extension zone, and lateral flow of the thickened ductile upper crust are comparable to the current tectonics of the region. (6) Model results are further compared to the potentially similar orogenic system, i.e., the Alpine orogen, in terms of the curved

  11. The lithosphere-asthenosphere boundary beneath the South Island of New Zealand (United States)

    Hua, Junlin; Fischer, Karen M.; Savage, Martha K.


    Lithosphere-asthenosphere boundary (LAB) properties beneath the South Island of New Zealand have been imaged by Sp receiver function common-conversion point stacking. In this transpressional boundary between the Australian and Pacific plates, dextral offset on the Alpine fault and convergence have occurred for the past 20 My, with the Alpine fault now bounded by Australian plate subduction to the south and Pacific plate subduction to the north. Using data from onland seismometers, especially the 29 broadband stations of the New Zealand permanent seismic network (GeoNet), we obtained 24,971 individual receiver functions by extended-time multi-taper deconvolution, and mapped them to three-dimensional space using a Fresnel zone approximation. Pervasive strong positive Sp phases are observed in the LAB depth range indicated by surface wave tomography. These phases are interpreted as conversions from a velocity decrease across the LAB. In the central South Island, the LAB is observed to be deeper and broader to the northwest of the Alpine fault. The deeper LAB to the northwest of the Alpine fault is consistent with models in which oceanic lithosphere attached to the Australian plate was partially subducted, or models in which the Pacific lithosphere has been underthrust northwest past the Alpine fault. Further north, a zone of thin lithosphere with a strong and vertically localized LAB velocity gradient occurs to the northwest of the fault, juxtaposed against a region of anomalously weak LAB conversions to the southeast of the fault. This structure could be explained by lithospheric blocks with contrasting LAB properties that meet beneath the Alpine fault, or by the effects of Pacific plate subduction. The observed variations in LAB properties indicate strong modification of the LAB by the interplay of convergence and strike-slip deformation along and across this transpressional plate boundary.

  12. 3D Numerical modelling of topography development associated with curved subduction zones (United States)

    Munch, Jessica; Ueda, Kosuke; Burg, Jean-Pierre; May, Dave; Gerya, Taras


    Curved subduction zones, also called oroclines, are geological features found in various places on Earth. They occur in diverse geodynamic settings: 1) single slab subduction in oceanic domain (e.g. Sandwich trench in the Southern Atlantic); 2) single slab subduction in continental domain, (e.g. Gibraltar-Alboran orocline in the Western Mediterranean) 3); multi-slab subduction (e.g. Caribbean orocline in the South-East of the Gulf of Mexico). These systems present various curvatures, lengths (few hundreds to thousands of km) and ages (less than 35 Ma for Gibraltar Alboran orocline, up to 100 Ma for the Caribbean). Recent studies suggested that the formation of curved subduction systems depends on slab properties (age, length, etc) and may be linked with processes such as retreating subduction and delamination. Plume induced subduction initiation has been proposed for the Caribbean. All of these processes involve deep mechanisms such as mantle and slab dynamics. However, subduction zones always generate topography (trenches, uplifts, etc), which is likely to be influenced by surface processes. Hence, surface processes may also influence the evolution of subduction zones. We focus on different kinds of subduction systems initiated by plume-lithosphere interactions (single slab subduction/multi-slab subduction) and scrutinize their surface expression. We use numerical modeling to examine large-scale subduction initiation and three-dimensional slab retreat. We perform two kinds of simulations: 1) large scale subduction initiation with the 3D-thermomechanical code I3ELVIS (Gerya and Yuen, 2007) in an oceanic domain and 2) large scale subduction initiation in oceanic domain using I3ELVIS coupled with a robust new surface processes model (SPM). One to several retreating slabs form in the absence of surface processes, when the conditions for subduction initiation are reached (c.f. Gerya et al., 2015), and ridges occur in the middle of the extensional domain opened by slab

  13. Reconciling the Isabella Anomaly with Lithosphere Delamination and Basin & Range Extension (United States)

    Straub, J.; Hu, J.; Zhou, Q.; Liu, L.


    The Isabella anomaly is a high-seismic velocity region located below the Sierra Nevada, and its presence has been explained through lithosphere delamination occurring within the past 10-15 Ma and an accreted oceanic micro-plate. Both the lack of consensus on the origin of this high velocity structure and its relation to putative lithospheric delamination and surface uplift requires more careful geodynamic studies. While earlier models of lithosphere delamination neglected the nearby subduction, we attempt to explain this anomaly in the context of delamination related to the subducted slab of the Farallon plate. We design a two-dimensional high-resolution model that extends from the west coast of the United States to the Colorado Plateau, including both the Sierra Nevada and Basin and Range regions. The Basin and Range experienced accelerated extension towards the northwest since 15 Ma, resulting in a topographical pattern with alternating subsidence and uplift; the mechanisms for both the deformation and topography of the region are presently debated. Preliminary results from our 2-D slab-lithosphere interaction model indicate that the foundering of an earlier accreted Farallon slab causes the surface to uplift and the overriding plate to extend. The model can potentially reproduce the main tectonic characteristics of the region, offering new insights on the evolution of the lithosphere-mantle system.

  14. Subduction processes related to the Sea of Okhotsk (United States)

    Zabarinskaya, Ludmila P.; Sergeyeva, Nataliya


    It is obviously important to study a role of subduction processes in tectonic activity within the continental margins. They are marked by earthquakes, volcanic eruptions, tsunami and other natural disasters hazardous to the people,plants and animals that inhabit such regions. The northwest part of the Sea of Okhotsk including the northern part of Sakhalin Island and the Deryugin Basin is the area of the recent intensive tectonic movements. The geological and geophysical data have made it possible to construct the geodynamic model of a deep structure of a lithosphere for this region. This geodynamic model has confirmed the existence of the ophiolite complex in the region under consideration. It located between the North Sakhalin sedimentary basin and the Deryugin basin. The Deryugin basin was formed on the side of an ancient deep trench after subducting the Okhotsk Sea Plate under Sakhalin in the Late Cretaceous-Paleogene. The North Sakhalin Basin with oil and gas resources was formed on the side of back-arc basin at that time. Approximately in the Miocene period the subduction process, apparently, has stopped. The remains of the subduction zone in the form of ophiolite complex have been identified according to geological and geophysical data. On a surface the subduction zone is shown as deep faults stretched along Sakhalin.

  15. Collapse of passive margins by lithospheric damage and plunging grain size (United States)

    Mulyukova, Elvira; Bercovici, David


    The collapse of passive margins has been proposed as a possible mechanism for the spontaneous initiation of subduction. In order for a new trench to form at the junction between oceanic and continental plates, the cold and stiff oceanic lithosphere must be weakened sufficiently to deform at tectonic rates. Such rates are especially hard to attain in the cold ductile portion of the lithosphere, at which the mantle lithosphere reaches peak strength. The amount of weakening required for the lithosphere to deform in this tectonic setting is dictated by the available stress. Stress in a cooling passive margin increases with time (e.g., due to ridge push), and is augmented by stresses present in the lithosphere at the onset of rifting (e.g., due to drag from underlying mantle flow). Increasing stress has the potential to weaken the ductile portion of the lithosphere by dislocation creep, or by decreasing grain size in conjunction with a grain-size sensitive rheology like diffusion creep. While the increasing stress acts to weaken the lithosphere, the decreasing temperature acts to stiffen it, and the dominance of one effect or the other determines whether the margin might weaken and collapse. Here, we present a model of the thermal and mechanical evolution of a passive margin, wherein we predict formation of a weak shear zone that spans a significant depth-range of the ductile portion of the lithosphere. Stiffening due to cooling is offset by weakening due to grain size reduction, driven by the combination of imposed stresses and grain damage. Weakening via grain damage is modest when ridge push is the only source of stress in the lithosphere, making the collapse of a passive margin unlikely in this scenario. However, adding even a small stress-contribution from mantle drag results in damage and weakening of a significantly larger portion of the lithosphere. We posit that rapid grain size reduction in the ductile portion of the lithosphere can enable, or at least

  16. Subduction of a buoyant plateau at the Manila Trench: Tomographic evidence and geodynamic implications (United States)

    Fan, Jianke; Zhao, Dapeng; Dong, Dongdong


    We determined P-wave tomographic images by inverting a large number of arrival-time data from 2749 local earthquakes and 1462 teleseismic events, which are used to depict the three-dimensional morphology of the subducted Eurasian Plate along the northern segment of the Manila Trench. Dramatic changes in the dip angle of the subducted Eurasian Plate are revealed from the north to the south, being consistent with the partial subduction of a buoyant plateau beneath the Luzon Arc. Slab tears may exist along the edges of the buoyant plateau within the subducted plate induced by the plateau subduction, and the subducted lithosphere may be absent at depths greater than 250 km at ˜19°N and ˜21°N. The subducted buoyant plateau is possibly oriented toward NW-SE, and the subducted plate at ˜21°N is slightly steeper than that at ˜19°N. These results may explain why the western and eastern volcanic chains in the Luzon Arc are separated by ˜50 km at ˜18°N, whereas they converge into a single volcanic chain northward, which may be related to the oblique subduction along the Manila Trench caused by the northwestern movement of the Philippine Sea Plate. A low-velocity zone is revealed at depths of 20-200 km beneath the Manila Accretionary Prism at ˜22°N, suggesting that the subduction along the Manila Trench may stop there and the collision develops northward. The Taiwan Orogeny may originate directly from the subduction of the buoyant plateau, because the initial time of the Taiwan Orogeny is coincident with that of the buoyant plateau subduction.

  17. Viscous Dissipation and Criticality of Subducting Slabs (United States)

    Riedel, Mike; Karato, Shun; Yuen, Dave


    Rheology of subducting lithosphere appears to be complicated. In the shallow part, deformation is largely accomodated by brittle failure, whereas at greater depth, at higher confining pressures, ductile creep is expected to control slab strength. The amount of viscous dissipation ΔQ during subduction at greater depth, as constrained by experimental rock mechanics, can be estimated on the basis of a simple bending moment equation [1,2] 2ɛ˙0(z) ∫ +h/2 2 M (z) = h ṡ -h/2 4μ(y,z)y dy , (1) for a complex multi-phase rheology in the mantle transition zone, including the effects of a metastable phase transition as well as the pressure, temperature, grain-size and stress dependency of the relevant creep mechanisms; μ is here the effective viscosity and ɛ˙0(z) is a (reference) strain rate. Numerical analysis shows that the maximum bending moment, Mcrit, that can be sustained by a slab is of the order of 1019 Nm per m according to Mcrit˜=σp ∗h2/4, where σp is the Peierl's stress limit of slab materials and h is the slab thickness. Near Mcrit, the amount of viscous dissipation grows strongly as a consequence of a lattice instability of mantle minerals (dislocation glide in olivine), suggesting that thermo-mechanical instabilities become prone to occur at places where a critical shear-heating rate is exceeded, see figure. This implies that the lithosphere behaves in such cases like a perfectly plastic solid [3]. Recently available detailed data related to deep seismicity [4,5] seems to provide support to our conclusion. It shows, e.g., that thermal shear instabilities, and not transformational faulting, is likely the dominating mechanism for deep-focus earthquakes at the bottom of the transition zone, in accordance with this suggested "deep criticality" model. These new findings are therefore briefly outlined and possible implications are discussed. References [1] Riedel, M. R., Karato, S., Yuen, D. A. Criticality of Subducting Slabs. University of Minnesota

  18. Multiple subduction imprints in the mantle below Italy detected in a single lava flow (United States)

    Nikogosian, Igor; Ersoy, Özlem; Whitehouse, Martin; Mason, Paul R. D.; de Hoog, Jan C. M.; Wortel, Rinus; van Bergen, Manfred J.


    Post-collisional magmatism reflects the regional subduction history prior to collision but the link between the two is complex and often poorly understood. The collision of continents along a convergent plate boundary commonly marks the onset of a variety of transitional geodynamic processes. Typical responses include delamination of subducting lithosphere, crustal thickening in the overriding plate, slab detachment and asthenospheric upwelling, or the complete termination of convergence. A prominent example is the Western-Central Mediterranean, where the ongoing slow convergence of Africa and Europe (Eurasia) has been accommodated by a variety of spreading and subduction systems that dispersed remnants of subducted lithosphere into the mantle, creating a compositionally wide spectrum of magmatism. Using lead isotope compositions of a set of melt inclusions in magmatic olivine crystals we detect exceptional heterogeneity in the mantle domain below Central Italy, which we attribute to the presence of continental material, introduced initially by Alpine and subsequently by Apennine subduction. We show that superimposed subduction imprints of a mantle source can be tapped during a melting episode millions of years later, and are recorded in a single lava flow.

  19. Effect of a weak layer at the base of an oceanic plate on subduction dynamics (United States)

    Carluccio, Roberta; Kaus, Boris


    The plate tectonics model relies on the concept of a relatively rigid lithospheric lid moving over a weaker asthenosphere. In this frame, the lithosphere asthenosphere boundary (LAB) is a first-order discontinuity that accommodates differential motions between tectonic plates and the underlying mantle. Recent seismic studies have revealed the existence of a low velocity and high electrical conductivity layer at the base of subducting tectonic plates. This thin layer has been interpreted as being weak and slightly buoyant and was suggested to affect the dynamics of subducting plates. However, geodynamically, the role of a weak layer at the base of the lithosphere remains poorly studied, especially at subduction zones. Therefore, we here use numerical models to investigate the first-order effects of a weak buoyant layer at the LAB on subduction dynamics. We employ both 2-D and 3-D models in which the slab and mantle are either linear viscous or have a more realistic temperature-dependent visco-elastic-plastic rheology. Results show that a weak layer affects the dynamics of the plates, foremost by increasing the subduction speed. The impact of this effect depends on the thickness of the layer and the viscosity contrast between the mantle and the weak layer. For moderate viscosity contrasts (1000), it can also change the morphology of the subduction itself, perhaps because this changes the overall effective viscosity contrast between the slab the and the mantle. For thinner layers, the overall effect is reduced. Yet, if seismological observations are correct that suggests that this layer is 10 km thick and partially molten, such that the viscosity is 1000 times lower than that of the mantle, our models suggest that this effect should be measurable. Some of our models also show a pile-up of weak material in the bending zone of the subducting plate, consistent with recent seismological observations.

  20. Geodynamic inversion to constrain the rheology of the lithosphere: What is the effect of elasticity? (United States)

    Baumann, Tobias; Kaus, Boris; Thielmann, Marcel


    the rheological parameters of these models with viscoplastic geodynamic inversions. We focus on a typical intra-oceanic subduction system as well as a typical scenario of subduction of an oceanic plate underneath a continental arc. Baumann, T. S. & Kaus, B. J. P., 2015. Geodynamic inversion to constrain thenon-linear rheology of the lithosphere, Geophys. J. Int., 202(2), 1289-1316. Burov, E. B. & Diament, M., 1995. The effective elastic thickness (Te) of continental lithosphere: What does it really mean?, J. Geophys. Res., 100, 3905-3927. Burov, E. B. & Watts, A. B., 2006. The long-term strength of continental lithosphere : jelly sandwich or crème brûlée?, GSA today, 16(1), 4-10. Burov, E. B., 2007. Crust and Lithosphere Dynamics: Plate Rheology and Mechanics, in Treatise Geophys., vol. 6, chap. 3, pp. 99-151, ed. Watts, A. B., Elsevier. Goetze, C. & Evans, B., 1979. Stress and temperature in the bending lithosphere as constrained by experimental rock mechanics, Geophys. J. Int., 59(3), 463-478. Jackson, J., 2002. Strength of the continental lithosphere: Time to abandon the jelly sandwich?, GSA today, 12(9), 4-9. Maggi, A., Jackson, J. A., McKenzie, D., & Priestley, K., 2000a. Earthquake focal depths, effective elastic thickness, and the strength of the continental lithosphere, Geology, 28, 495-498. Watts, A. B., 2001. Isostasy and Flexure of the Lithosphere, Cambridge University Press.

  1. Geoid anomalies in the vicinity of subduction zones (United States)

    Mcadoo, D. C.


    In the considered investigation, attention is given to the line source model, a surface source model, an application of the model, and a model of the thermal lithosphere associated with marginal basins. It is found that undulations in the altimetrically observed geoid of the southwest Pacific are strongly controlled by positive density anomalies in the subducting slabs of the region and the effects of elevation of the geotherm in behind arc lithosphere (corresponding to young marginal basins). Finer details of slab geometry do not obviously manifest themselves in the observed geoid. Such gravitational effects are quite attenuated at sea level and are apparently mixed with crustal effects, oceanographic noise, etc. It appears that slabs in global composite may contribute substantially to intermediate and long wavelength portions (down to spherical harmonic degree 3 or 4) of the earth's gravity field.

  2. Plume-subduction interaction forms large auriferous provinces. (United States)

    Tassara, Santiago; González-Jiménez, José M; Reich, Martin; Schilling, Manuel E; Morata, Diego; Begg, Graham; Saunders, Edward; Griffin, William L; O'Reilly, Suzanne Y; Grégoire, Michel; Barra, Fernando; Corgne, Alexandre


    Gold enrichment at the crustal or mantle source has been proposed as a key ingredient in the production of giant gold deposits and districts. However, the lithospheric-scale processes controlling gold endowment in a given metallogenic province remain unclear. Here we provide the first direct evidence of native gold in the mantle beneath the Deseado Massif in Patagonia that links an enriched mantle source to the occurrence of a large auriferous province in the overlying crust. A precursor stage of mantle refertilisation by plume-derived melts generated a gold-rich mantle source during the Early Jurassic. The interplay of this enriched mantle domain and subduction-related fluids released during the Middle-Late Jurassic resulted in optimal conditions to produce the ore-forming magmas that generated the gold deposits. Our study highlights that refertilisation of the subcontinental lithospheric mantle is a key factor in forming large metallogenic provinces in the Earth's crust, thus providing an alternative view to current crust-related enrichment models.The lithospheric controls on giant gold deposits remain unclear. Here, the authors show evidence for native gold in the mantle from the Deseado Massif in Patagonia demonstrating that refertilisation of the lithospheric mantle is key in forming metallogenic provinces.

  3. A fictitious domain method for fluid/solid coupling applied to the lithosphere/asthenosphere interaction. (United States)

    Cerpa, Nestor; Hassani, Riad; Gerbault, Muriel


    A large variety of geodynamical problems can be viewed as a solid/fluid interaction problem coupling two bodies with different physics. In particular the lithosphere/asthenosphere mechanical interaction in subduction zones belongs to this kind of problem, where the solid lithosphere is embedded in the asthenospheric viscous fluid. In many fields (Industry, Civil Engineering,etc.), in which deformations of solid and fluid are "small", numerical modelers consider the exact discretization of both domains and fit as well as possible the shape of the interface between the two domains, solving the discretized physic problems by the Finite Element Method (FEM). Although, in a context of subduction, the lithosphere is submitted to large deformation, and can evolve into a complex geometry, thus leading to important deformation of the surrounding asthenosphere. To alleviate the precise meshing of complex geometries, numerical modelers have developed non-matching interface methods called Fictitious Domain Methods (FDM). The main idea of these methods is to extend the initial problem to a bigger (and simpler) domain. In our version of FDM, we determine the forces at the immersed solid boundary required to minimize (at the least square sense) the difference between fluid and solid velocities at this interface. This method is first-order accurate and the stability depends on the ratio between the fluid background mesh size and the interface discretization. We present the formulation and provide benchmarks and examples showing the potential of the method : 1) A comparison with an analytical solution of a viscous flow around a rigid body. 2) An experiment of a rigid sphere sinking in a viscous fluid (in two and three dimensional cases). 3) A comparison with an analog subduction experiment. Another presentation aims at describing the geodynamical application of this method to Andean subduction dynamics, studying cyclic slab folding on the 660 km discontinuity, and its relationship

  4. Lithosphere-asthenosphere system in the Mediterranean region in the framework of polarized plate tectonics

    CERN Document Server

    Raykova, Reneta Blagoeva; Doglioni, Carlo


    Velocity structure of the lithosphere-asthenosphere system, to the depth of about 350 km, is obtained for almost 400 cells, sized 1 degree by 1 degree in the Mediterranean region. The models are obtained by the following sequence of methods and tools: surface-wave dispersion measurements and collection; 2D tomography of dispersion relations; non-linear inversion of cellular dispersion relations; smoothing optimization method to select a preferred model for each cell. The 3D velocity model, that satisfies Occam razor principle, is obtained as a juxtaposition of selected cellular models. The reconstructed picture of the lithosphere-asthenosphere system evidences the, globally well known, asymmetry between the W- and E-directed subduction zones, attributed to the westward drift of the lithosphere relative to the mantle. Different relationship between slabs and mantle dynamics cause strong compositional differences in the upper mantle, as shown by large variations of seismic waves velocity, consistent with Polari...

  5. Reaction-induced rheological weakening enables oceanic plate subduction. (United States)

    Hirauchi, Ken-Ichi; Fukushima, Kumi; Kido, Masanori; Muto, Jun; Okamoto, Atsushi


    Earth is the only terrestrial planet in our solar system where an oceanic plate subducts beneath an overriding plate. Although the initiation of plate subduction requires extremely weak boundaries between strong plates, the way in which oceanic mantle rheologically weakens remains unknown. Here we show that shear-enhanced hydration reactions contribute to the generation and maintenance of weak mantle shear zones at mid-lithospheric depths. High-pressure friction experiments on peridotite gouge reveal that in the presence of hydrothermal water, increasing strain and reactions lead to an order-of-magnitude reduction in strength. The rate of deformation is controlled by pressure-solution-accommodated frictional sliding on weak hydrous phyllosilicate (talc), providing a mechanism for the 'cutoff' of the high peak strength at the brittle-plastic transition. Our findings suggest that infiltration of seawater into transform faults with long lengths and low slip rates is an important controlling factor on the initiation of plate tectonics on terrestrial planets.

  6. Thermal implications of the cessation of subduction in the Sierra Nevada and Baja- California arcs (United States)

    Erkan, K.; Blackwell, D. D.


    The thermal regime in the extinct Sierra Nevada arc has undergone substantial transformation as a result of the cessation of subduction in the last 30 My. The dynamic mechanism of cooling in the arc has been replaced by re-equilibration of the lithosphere toward continental averages. Preliminary 1D thermal models reveal that the effect of asthenospheric heating at the bottom do not lead to credible changes in the lithospheric temperatures for a 30 My years period in terms of surface manifestations. In the Great Valley, neither the topography nor the surface heat flow show considerable variations before and after the cessation of subduction. On the other hand, Sierra Nevada has experienced substantial uplift in the post subduction California. In the Sierra Nevada, the surface heat flow lags deep temperature response but the topography responds directly to temperatures at depth. As the end of subduction migrated north with the Mendocino triple junction, the dynamic equilibrium between the cold Sierra Nevada and Basin and Range has been upset and the high temperatures of Basin and Range started to invade the Sierra Nevada lithosphere. Our 2D thermal model reveals that conductive heating of the Sierran lithosphere by the hot Basin and Range lithosphere could be the dominant source for the tilted uplift in the Sierra Nevada. The heating from the bottom is likely not very effective in Sierra Nevada as no uplift is observed in the neighboring Great Valley region. The thermal uplift due to Basin and Range heating is substantial at the east edge of the Sierra Nevada and decreases rapidly toward the west. The uplift as far as 100 km toward west could be accommodated by flexural bending of the cold Sierra Nevada lithosphere as the eastern edge thermally expanded. A similar thermo-tectonic scenario could explain the westward tilted Baja-California peninsular ranges which were part of the same tectonic setting of the Sierra Nevada during subduction. We also investigated the

  7. Plume-induced roll back subduction around Venus large coronae (United States)

    Davaille, A.; Smrekar, S. E.; Tomlinson, S. M.


    On Venus, possible subduction trenches are mainly associated with large coronae, eventhough the latter are thought to be produced by hot mantle plumes. The mechanism of assocation between subduction and plume has long remained elusive. However, we recently observe the same association in laboratory experiments on thermal convection in colloidal aqueous dispersions of silica nanoparticles, which deform in the Newtonian regime at low solid particle fraction φp, and transition to strain-rate weakening, plasticity, elasticity, and brittle properties as φp increases. Hence, a dense skin akin to a planetary lithosphere grows on the surface when the system is dried from above. When a hot plume rises under the skin, the latter undergoes a flexural deformation which puts it under tension. Cracks then develop, sometimes using pre-existing weaknesses. Plume material (being more buoyant that the laboratory lithosphere) upwells through the cracks and spreads as a axisymmetric gravity current above the broken denser skin. The latter bends and sinks under the conjugate action of its own weight and the plume gravity current. The brittle character of the top experimental lithosphere forbids it to deform viscously to accomodate the sinking motions. Instead, the plate continues to tear as a sheet of paper would do upon intrusion. Several slabs are therefore produced, associated with trenches localized along partial circles on the plume, and strong roll-back is always observed. Depending on the lithospheric strength, roll-back can continue and triggers a complete resurfacing, or it stops when the plume stops spreading. Scalings derived from the experiments suggest that a weaker lithosphere than that present on Earth today is required for such a convective regime. We identified two candidates on Venus. At Artemis and Quetzelpetlatl Coronae, the radar image observations and subsurface density variations inferred from modeling the gravity and topography agree with the predictions from

  8. The interplay between subduction and lateral extrusion : A case study for the European Eastern Alps based on analogue models

    NARCIS (Netherlands)

    van Gelder, I. E.; Willingshofer, E.; Sokoutis, D.; Cloetingh, S. A.P.L.


    A series of analogue experiments simulating intra-continental subduction contemporaneous with lateral extrusion of the upper plate are performed to study the interference between these two processes at crustal levels and in the lithospheric mantle. The models demonstrate that intra-continental

  9. The potential influence of subduction zone polarity on overriding plate deformation, trench migration and slab dip angle

    NARCIS (Netherlands)

    Schellart, W. P.


    A geodynamic model exists, the westward lithospheric drift model, in which the variety of overriding plate deformation, trench migration and slab dip angles is explained by the polarity of subduction zones. The model predicts overriding plate extension, a fixed trench and a steep slab dip for

  10. Seismic anisotropy and mantle flow below subducting slabs (United States)

    Walpole, Jack; Wookey, James; Kendall, J.-Michael; Masters, T.-Guy


    Subduction is integral to mantle convection and plate tectonics, yet the role of the subslab mantle in this process is poorly understood. Some propose that decoupling from the slab permits widespread trench parallel flow in the subslab mantle, although the geodynamical feasibility of this has been questioned. Here, we use the source-side shear wave splitting technique to probe anisotropy beneath subducting slabs, enabling us to test petrofabric models and constrain the geometry of mantle fow. Our global dataset contains 6369 high quality measurements - spanning ∼ 40 , 000 km of subduction zone trenches - over the complete range of available source depths (4 to 687 km) - and a large range of angles in the slab reference frame. We find that anisotropy in the subslab mantle is well characterised by tilted transverse isotropy with a slow-symmetry-axis pointing normal to the plane of the slab. This appears incompatible with purely trench-parallel flow models. On the other hand it is compatible with the idea that the asthenosphere is tilted and entrained during subduction. Trench parallel measurements are most commonly associated with shallow events (source depth < 50 km) - suggesting a separate region of anisotropy in the lithospheric slab. This may correspond to the shape preferred orientation of cracks, fractures, and faults opened by slab bending. Meanwhile the deepest events probe the upper lower mantle where splitting is found to be consistent with deformed bridgmanite.

  11. Why lithospheric extension separated the Aegean from Turkey (United States)

    Ring, U.; Gessner, K.; Thomson, S. N.; Markwitz, V.


    The Aegean Sea region in the eastern Mediterranean is one of the classic and best-studied extensional provinces. Inspired by recent 3D geodynamic models of laterally heterogeneous accretion during rollback we discuss the nature of the transition from the Aegean Sea basin (Hellenides) into the Anatolian plateau of west Turkey (Anatolides). The Hellenides and Anatolides experienced similar rates of convergence, but display remarkable differences in lithospheric structure. Whereas the Aegean is characterized by sustained high-pressure metamorphism followed by slab retreat since c. 60 Ma, a south verging greenschist-facies thrust-and-fold belt formed in the Anatolides since c. 45 Ma. Fission-track contour maps show that since c. 24 Ma extension in both regions evolved differently. Gravity data, earthquake locations and seismic velocity anomalies highlight a N-S oriented subvertical boundary in the upper mantle between a fast slab below the Aegean and a slow asthenospheric region below west Turkey, the West Anatolia Transfer Zone (WATZ). Our data support the hypothesis that the WATZ developed as a result of laterally inhomogeneous convergence along the boundary of the Adriatic and Anatolian lithospheres. 3D numerical simulations of laterally inhomogeneous convergence predict a similar evolution, where two distinct domains develop along strike: a region of distributed shortening where the systems gets congested by a microcontinent (Anatolides), and a region of extension associated with rollback of the active subduction zone (Hellenides). Strike-slip deformation concentrates perpendicular to the boundary of the two domains (WATZ). The numerical simulations also predict other salient features of regional geology and geodynamics, including the origin of a lithospheric window below west Turkey, local ocean floor topography, and the formation of the North Anatolian Fault zone. We argue that the seemingly complex tectonic evolution of the Aegean-Anatolian portion of the

  12. Extensive decarbonation of continuously hydrated subducting slabs (United States)

    Arzilli, Fabio; Burton, Mike; La Spina, Giuseppe; Macpherson, Colin G.


    CO2 release from subducting slabs is a key element of Earth's carbon cycle, consigning slab carbon either to mantle burial or recycling to the surface through arc volcanism, however, what controls subducted carbon's fate is poorly understood. Fluids mobilized by devolatilization of subducting slabs play a fundamental role in the melting of mantle wedges and in global geochemical cycles [1]. The effect of such fluids on decarbonation in subducting lithologies has been investigated recently [2-5], but several thermodynamic models [2-3], and experimental studies [6] suggest that carbon-bearing phases are stable at sub-arc depths (80-140 km; 2.6-4.5 GPa), implying that this carbon can be carried to mantle depths of >140 km. This is inconsistent with observations of voluminous CO2 release from arc volcanoes [7-10], located above slabs that are at 2.6-4.5 GPa pressure. The aim of this study is to re-evaluate the role of metamorphic decarbonation, showing if decarbonation reactions could be feasible at sub-arc depths combined with a continuous hydration scenario. We used the PerpleX software combined with a custom-designed algorithm to simulate a pervasive fluid infiltration characterized by "continuous hydration" combined with a distillation model, in which is possible to remove CO2 when decarbonation occurs, to obtain an open-system scenario. This is performed by repeatedly flushing the sediment with pure H2O at 0.5, 1.0 or 5 wt.% until no further decarbonation occurs. Here we show that continuous hydrated of sediment veneers on subducting slabs by H2O released from oceanic crust and serpentinised mantle lithosphere [11-13], produces extensive slab decarbonation over a narrow, sub-arc pressure range, even for low temperature subduction pathways. This explains the location of CO2-rich volcanism, quantitatively links the sedimentary composition of slab material to the degree of decarbonation and greatly increases estimates for the magnitude of carbon flux through the arc

  13. Geophysical detection of relict metasomatism from an Archean (approximately 3.5 Ga) subduction zone. (United States)

    Chen, Chin-Wu; Rondenay, Stéphane; Evans, Rob L; Snyder, David B


    When plate tectonics started on Earth has been uncertain, and its role in the assembly of early continents is not well understood. By synthesizing coincident seismic and electrical profiles, we show that subduction processes formed the Archean Slave craton in Canada. The spatial overlap between a seismic discontinuity and a conductive anomaly at approximately 100 kilometers depth, in conjunction with the occurrence of mantle xenoliths rich in secondary minerals representative of a metasomatic front, supports cratonic assembly by subduction and accretion of lithospheric fragments. Although evidence of cratonic assembly is rarely preserved, these results suggest that plate tectonics was operating as early as Paleoarchean times, approximately 3.5 billion years ago (Ga).

  14. Trench dynamics: Effects of dynamically migrating trench on subducting slab morphology and characteristics of subduction zones systems (United States)

    Yoshida, Masaki


    Understanding the mechanisms of trench migration (retreat or advance) is crucial to characterizing the driving forces of Earth's tectonics plates, the origins of subducting slab morphologies in the deep mantle, and identifying the characteristics of subduction zones systems, which are among the fundamental issues of solid Earth science. A series of numerical simulations of mantle convection, focusing on plate subduction in a three-dimensional (3-D) regional spherical shell coordinate system, was performed to examine subduction zone characteristics, including geodynamic relationships among trench migration, back-arc stress, and slab morphology. The results show that a subducting slab tends to deflect around the base of the mantle transition zone and form a sub-horizontal slab because its front edge (its 'toe') is subject to resistance from the highly viscous lower mantle. As the sub-horizontal slab starts to penetrate into the lower mantle from its 'heel,' the toe of the slab is drawn into the lower mantle. The results for models with dynamically migrating trenches suggest that trench retreat is the dynamically self-consistent phenomenon in trench migration. The reason for this is that the strong lateral mantle flow that is generated as a sequence of events leading from corner flow at the subduction initiation to return flow of the formation of a sub-horizontal slab in the shallower part of mantle wedge produces the retreat of the subducting slab. In fact, a 'mantle suction force,' which is generated in the mantle wedge to fill space left by the retreating subducting plate, is enhanced by the subsequent trench retreat. Even when upwelling flow with significant positive buoyancy originates just above a mantle phase boundary at a depth of 410 km (as inferred from independent seismic tomographic, geodynamic, geochemical, and mineral physics), reaches the base of the overriding plate, and the overriding plate is slightly thinned, lithospheric stress tends to be

  15. Deformation of the central Andes (15-27 deg S) derived from a flow model of subduction zones (United States)

    Wdowinski, Shimon; O'Connell, Richard J.


    A simple viscous flow model of a subduction zone is used to calculate the deformation within continental lithosphere above a subducting slab. This formulation accounts for two forces that dominate the deformation in the overriding lithosphere: tectonic forces and buoyancy forces. Numerical solutions, obtained by using a finite element technique, are compared with observations from the central Andes (15-27 deg S). The model predicts the observed deformation pattern of extension in the forearc, compression in the Western Monocline (corresponding to magmatic activity), extension in the Altiplano, compression in the Eastern Monocline and Subandes, and no deformation in the Brazilian Shield. By comparing the calculated solutions with the large-scale tectonic observations, the forces that govern the deformation in the central Andes are evaluated. The approximately constant subduction velocity in the past 26 million years suggests that the rate of crustal shortening in the Andes has decreased with time due to the thickening of the crust.

  16. The role of frictional strength on plate coupling at the subduction interface

    KAUST Repository

    Tan, Eh


    At a subduction zone the amount of friction between the incoming plate and the forearc is an important factor in controlling the dip angle of subduction and the structure of the forearc. In this paper, we investigate the role of the frictional strength of sediments and of the serpentinized peridotite on the evolution of convergent margins. In numerical models, we vary thickness of a serpentinized layer in the mantle wedge (15 to 25km) and the frictional strength of both the sediments and serpentinized mantle (friction angle 1 to 15, or static friction coefficient 0.017 to 0.27) to control the amount of frictional coupling between the plates. With plastic strain weakening in the lithosphere, our numerical models can attain stable subduction geometry over millions of years. We find that the frictional strength of the sediments and serpentinized peridotite exerts the largest control on the dip angle of the subduction interface at seismogenic depths. In the case of low sediment and serpentinite friction, the subduction interface has a shallow dip, while the subduction zone develops an accretionary prism, a broad forearc high, a deep forearc basin, and a shallow trench. In the high friction case, the subduction interface is steep, the trench is deeper, and the accretionary prism, forearc high and basin are all absent. The resultant free-air gravity and topographic signature of these subduction zone models are consistent with observations. We believe that the low-friction model produces a geometry and forearc structure similar to that of accretionary margins. Conversely, models with high friction angles in sediments and serpentinite develop characteristics of an erosional convergent margin. We find that the strength of the subduction interface is critical in controlling the amount of coupling at the seismogenic zone and perhaps ultimately the size of the largest earthquakes at subduction zones. © 2012. American Geophysical Union. All Rights Reserved.

  17. Lithosphere Removal in the Central Andes: Reconciling Seismic Images and Elevation History (United States)

    Henderson, O.; Currie, C. A.


    Shortening of the upper crust should be accompanied by thickening of the deeper lithosphere. However, for many orogens, including the central Andes, geophysical and geological observations indicate that mantle lithosphere is spatially heterogeneous, being anomalously thin or absent. Seismic studies of the central Andes suggest that mantle lithosphere is locally removed. Also, present day elevations of the central Andean Plateau have been explained by rapid removal of mantle lithosphere over the last 10 Ma. Yet, the geological record is innately incomplete, and seismic tomography and receiver functions can offer only a present day snapshot of the subsurface. None of these techniques provides concrete insight into the physical processes responsible for current Andean elevations (3-5 km). A 2D plane-strain thermo-mechanical code, SOPALE, is used to examine the deep lithospheric dynamics connected to mantle lithosphere removal within a subduction zone setting, such as the Andes. Three models have been tested: removal by viscous dripping, by delamination, and a model with no removal. The removal models contain a high density eclogite root, creating a contrast between mantle lithosphere and mantle material. For the viscous drip models, mantle lithosphere is removed within 2.5-5 Myrs, descending subvertically through the mantle, causing subsequent surface rebound. Prior to this rebound, surface topography subsides locally over the dense root. This subsidence is influenced by crustal rheology, where weaker crustal rheologies produce deep, narrower basins (25-75 km wide, ~1 km deep), and stronger crustal rheologies produce shallow, broader basins (300-400 km wide, ~0.5 km deep). Delamination, which involves the coherent removal of mantle lithosphere along the Moho, affects a larger region, and is reflected in broader basins that extend into the back-arc. In all models, the deep lithosphere dynamics have an appreciable effect on surface topography, therefore, removal events

  18. Crustal and upper mantle structure of the Anatolian plate: Imaging the effects of subduction termination and continental collision with seismic techniques (United States)

    Delph, Jonathan R.

    The neotectonic evolution of the eastern Mediterranean is intimately tied to interactions between the underthrusting/subducting slab along the southern margin of Anatolia and the overriding plate. The lateral variations in the subduction zone can be viewed as a temporal analogue of the transition between continuous subduction and subduction termination by continent-continent collision. By investigating the lateral variations along this subduction zone in the overriding plate, we can gain insight into the processes that precede continent collision. This dissertation summarizes the results of three studies that focus on different parts of the subduction margin: 1) In the west, where the development of a slab tear represents the transition between continuous and enigmatic subduction, 2) In the east, where continent-continent collision between the Arabian and Eurasian Plate is leading to the development of the third largest orogenic plateau on earth after complete slab detachment, and 3) In central Anatolia, where the subducting slab is thought to be in the processes of breaking up, which is affecting the flow of mantle material leading to volcanism and uplift along the margin. In the first study, we interpret that variations in the composition of material in the downgoing plate (i.e. a change from the subduction of oceanic material to continental material) may have led to the development of a slab tear in the eastern Aegean. This underthrusting, buoyant continental fragment is controlling overriding plate deformation, separating the highly extensional strains of western Anatolia from the much lower extensional strains of central Anatolia. Based on intermediate depth seismicity, it appears that the oceanic portion of the slab is still attached to this underthrusting continental fragment. In the second study, we interpret that the introduction of continental lithosphere into the north-dipping subduction zone at the Arabian-Eurasian margin led to the rollback and

  19. Back-arc Extension: Critical Analisys of Subduction-related and Non Subduction-related Driving Mechanisms (United States)

    Mantovani, E.; Viti, M.; Babbucci, D.; Tamburelli, C.; Albarello, D.

    It is argued that the opening of back arc basins can hardly be explained as an effect of subduction related forces, since this kind of interpretation has not yet provided plausible explanations for several major features of such processes in the world. In particular, it is not clear why back arc extension occurs in some subduction zones and not in others, why extension ceased in zones where subduction has remained active, why the arcs associated with back arc basins are often characterized by a strongly curved shape, why arc-trench-back arc systems do not develop along the entire length of consuming borders and why no significant correlation can be recognized between any parameter of subduction processes and the occurrence of back arc extension. In addition, modelling experiments indicate that the magnitude of the tensional stress induced in the overriding plate by subduction-related forces is significantly lower than the lithospheric strength. These problems are discussed, in particular, for three subduction-related interpretations, the "slab-pull", the "corner flow" and the "sea an- chor" models, which seem to be the most quoted in literature. It is then argued that possible solutions of the above problems may be provided by the extrusion model, which postulates that back arc basins are generated by the forced separation of the arc from the overriding plate, along a sector of the consuming border. This separa- tion is generally caused by the oblique indentation of strong and buoyant structures against the accretionary belt. In this view, subduction and back arc extension are not causally linked one to the other, but rather represent simultaneous effects of the lateral migration of the arc, driven by plate convergence. It is pointed out that the conditions required for the occurrence of this kind of mechanism may be recognized in the tec- tonic contexts where back arc basins developed in the wake of arc-trench migrating systems. On the other hand, in the zones

  20. Lithospheric thickness jumps at the S-Atlantic continental margins from satellite gravity data and modelled isostatic anomalies (United States)

    Shahraki, Meysam; Schmeling, Harro; Haas, Peter


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

  1. Are diamond-bearing Cretaceous kimberlites related to shallow-angle subduction beneath western North America? (United States)

    Currie, C. A.; Beaumont, C.


    The origin of deep-seated magmatism (in particular, kimberlites and lamproites) within continental plate interiors remains enigmatic in the context of plate tectonic theory. One hypothesis proposes a relationship between kimberlite occurrence and lithospheric subduction, such that a subducting plate releases fluids below a continental craton, triggering melting of the deep lithosphere and magmatism (Sharp, 1974; McCandless, 1999). This study provides a quantitative evaluation of this hypothesis, focusing on the Late Cretaceous- Eocene (105-50 Ma) kimberlites and lamproites of western North America. These magmas were emplaced along a corridor of Archean and Proterozoic lithosphere, 1000-1500 km inboard of the plate margin separating the subducting Farallon Plate and continental North America Plate. Kimberlite-lamproite magmatism coincides with tectonic events, including the Laramide orogeny, shut-down of the Sierra Nevada arc, and eastward migration of volcanism, that are commonly attributed to a change in Farallon Plate geometry to a shallow-angle trajectory (1999; Sharp, W.E., Earth Planet. Sci. Lett., v.21, pp.351-354, 1974.

  2. Lithospheric Stress and Geodynamics: History, Accomplishments and Challenges (United States)

    Richardson, R. M.


    The kinematics of plate tectonics was established in the 1960s, and shortly thereafter the Earth's stress field was recognized as an important constraint on the dynamics of plate tectonics. Forty years ago the 1976 Chapman Conference on the Stress in the Lithosphere, which I was fortunate to attend as a graduate student, and the ensuing 1977 PAGEOPH Stress in the Earth publication's 28 articles highlighted a range of datasets and approaches that established fertile ground for geodynamic research ever since. What are the most useful indicators of stress? Do they measure residual or tectonic stresses? Local or far field sources? What role does rheology play in concentrating deformation? Great progress was made with the first World Stress Map in 1991 by Zoback and Zoback, and the current version (2016 release with 42,348 indicators) remains a tremendous resource for geodynamic research. Modeling sophistication has seen significant progress over the past 40 years. Early applications of stress to dynamics involved simple lithospheric flexure, particularly at subduction zones, Hawaii, and continental foreland basin systems. We have progressed to full 3-D finite element models for calculating the flexure and stress associated with loads on a crust and mantle with realistic non-linear viscoelastic rheology, including frictional sliding, low-temperature plasticity, and high-temperature creep. Initial efforts to use lithospheric stresses to constrain plate driving forces focused on a "top-down" view of the lithosphere. Such efforts have evolved to better include asthenosphere-lithosphere interactions, have gone from simple to complicated rheologies, from 2-D to 3-D, and seek to obtain a fully thermo-mechanical model that avoids relying on artificial boundary conditions to model plate dynamics. Still, there are a number of important issues in geodynamics, from philosophy (when are more complicated models necessary? can one hope to identify "the" answer with modeling, or only

  3. Implications for metal and volatile cycles from the pH of subduction zone fluids. (United States)

    Galvez, Matthieu E; Connolly, James A D; Manning, Craig E


    The chemistry of aqueous fluids controls the transport and exchange-the cycles-of metals and volatile elements on Earth. Subduction zones, where oceanic plates sink into the Earth's interior, are the most important geodynamic setting for this fluid-mediated chemical exchange. Characterizing the ionic speciation and pH of fluids equilibrated with rocks at subduction zone conditions has long been a major challenge in Earth science. Here we report thermodynamic predictions of fluid-rock equilibria that tie together models of the thermal structure, mineralogy and fluid speciation of subduction zones. We find that the pH of fluids in subducted crustal lithologies is confined to a mildly alkaline range, modulated by rock volatile and chlorine contents. Cold subduction typical of the Phanerozoic eon favours the preservation of oxidized carbon in subducting slabs. In contrast, the pH of mantle wedge fluids is very sensitive to minor variations in rock composition. These variations may be caused by intramantle differentiation, or by infiltration of fluids enriched in alkali components extracted from the subducted crust. The sensitivity of pH to soluble elements in low abundance in the host rocks, such as carbon, alkali metals and halogens, illustrates a feedback between the chemistry of the Earth's atmosphere-ocean system and the speciation of subduction zone fluids via the composition of the seawater-altered oceanic lithosphere. Our findings provide a perspective on the controlling reactions that have coupled metal and volatile cycles in subduction zones for more than 3 billion years(7).

  4. Constraining the near-surface response to lithospheric reorientation: Structural thermochronology along the TRANSALP geophysical transect (United States)

    Glotzbach, Christoph; Büttner, Lukas; Ehlers, Todd


    Tomographic analyses of the lithosphere structure underneath the Alps suggest a complex geodynamic history (e.g. Lippitsch et al. 2003), indicating, among other things, switches in the direction of subduction. A subduction polarity switch is proposed to have occurred in Miocene times between the Central and Eastern Alps (e.g. Lippitsch et al. 2003; Handy et al. 2015). In the Western and Central Alps SE-directed subduction of European continental lithosphere occurs, whereas NW-directed subduction of Adriatic lithosphere occurs further east (e.g. Kissling et al. 2006). The subducted slab steepens at the transition to the Eastern Alps, roughly at the position of the TRANSALP geophysical profile (S. Germany to N. Italy). This lithospheric reorientation was pre-dated by slab breakoff and also involves the delamination of the lower lithosphere, both processes producing distinct long-wavelength deformation (e.g. Gerya et al. 2004). Thermochronological data can be used to study the surface response to such a long-wavelength deformation. We present new apatite and zircon (U-Th)/He ages of 23 samples collected along 210 km of the TRANSALP profile. The samples were collected along a balanced cross section the TRANSALP profile (e.g. Lüschen et al. 2004) across individual structures that can be tied to deeper, seismically imaged, structures. The thermochronometer ages provide a record of exhumation related to both crustal shortening and post deformation erosional exhumation. Interpretation of the data is in progress and being used to discriminate between competing kinematic/geometric models, and the timing of major fault activity. Variations in exhumation along the section will also unravel the timing and shape of possible long-wavelength rock uplift event(s). References Gerya, T.V., Yuen, D.A., Maresch, W.V. 2004. Thermomechanical modelling of slab detachment. Earth Planet. Sci. Lett. 226, 101-116. Handy, M.R., Ustaszewski, K., Kissling, E. 2015. Reconstructing the Alps

  5. Post-collisional magmatism in the central East African Orogen: The Maevarano Suite of north Madagascar (United States)

    Goodenough, K.M.; Thomas, Ronald J.; De Waele, B.; Key, R.M.; Schofield, D.I.; Bauer, W.; Tucker, R.D.; Rafahatelo, J.-M.; Rabarimanana, M.; Ralison, A.V.; Randriamananjara, T.


    Late tectonic, post-collisional granite suites are a feature of many parts of the Late Neoproterozoic to Cambrian East African Orogen (EAO), where they are generally attributed to late extensional collapse of the orogen, accompanied by high heat flow and asthenospheric uprise. The Maevarano Suite comprises voluminous plutons which were emplaced in some of the tectonostratigraphic terranes of northern Madagascar, in the central part of the EAO, following collision and assembly during a major orogeny at ca. 550 Ma. The suite comprises three main magmatic phases: a minor early phase of foliated gabbros, quartz diorites, and granodiorites; a main phase of large batholiths of porphyritic granitoids and charnockites; and a late phase of small-scale plutons and sheets of monzonite, syenite, leucogranite and microgranite. The main phase intrusions tend to be massive, but with variably foliated margins. New U-Pb SHRIMP zircon data show that the whole suite was emplaced between ca. 537 and 522 Ma. Geochemically, all the rocks of the suite are enriched in the LILE, especially K, and the LREE, but are relatively depleted in Nb, Ta and the HREE. These characteristics are typical of post-collisional granitoids in the EAO and many other orogenic belts. It is proposed that the Maevarano Suite magmas were derived by melting of sub-continental lithospheric mantle that had been enriched in the LILE during earlier subduction events. The melting occurred during lithospheric delamination, which was associated with extensional collapse of the East African Orogen. ?? 2009 Natural Environment Research Council.

  6. Dry Juan de Fuca slab revealed by quantification of water entering Cascadia subduction zone (United States)

    Canales, J. P.; Carbotte, S. M.; Nedimović, M. R.; Carton, H.


    Water is carried by subducting slabs as a pore fluid and in structurally bound minerals, yet no comprehensive quantification of water content and how it is stored and distributed at depth within incoming plates exists for any segment of the global subduction system. Here we use seismic data to quantify the amount of pore and structurally bound water in the Juan de Fuca plate entering the Cascadia subduction zone. Specifically, we analyse these water reservoirs in the sediments, crust and lithospheric mantle, and their variations along the central Cascadia margin. We find that the Juan de Fuca lower crust and mantle are drier than at any other subducting plate, with most of the water stored in the sediments and upper crust. Variable but limited bend faulting along the margin limits slab access to water, and a warm thermal structure resulting from a thick sediment cover and young plate age prevents significant serpentinization of the mantle. The dryness of the lower crust and mantle indicates that fluids that facilitate episodic tremor and slip must be sourced from the subducted upper crust, and that decompression rather than hydrous melting must dominate arc magmatism in central Cascadia. Additionally, dry subducted lower crust and mantle can explain the low levels of intermediate-depth seismicity in the Juan de Fuca slab.

  7. The Terminal Stage of Subduction: the Hindu Kush Slab Break-off (United States)

    Kufner, S. K.; Schurr, B.; Sippl, C.; Yuan, X.; Ratschbacher, L.; Akbar, A. S. M.; Ischuk, A.; Murodkulov, S.; Schneider, F.; Mechie, J.; Tilmann, F. J.


    The terminal stage of subduction arrives when the ocean basin is closed and the continental margin arrives at the trench. The opposite forces of the sinking slab and buoyant continent ultimately leads to break-off of the subducted slab. This process, although common in geological history, is rarely observed, because it is short-lived. Here we report new precise earthquake hypocenters, detailed tomographic images and earthquake source mechanisms from the Hindu Kush region in Central Asia, which hint at continental subduction and plate necking. Our images provide a rare glimpse at the ephemeral process of slab break-off: the Hindu Kush slablet in its uppermost section is thinned or already severed and that intermediate depth earthquakes cluster at the neck connecting it to the deeper slab. From a strain rate analysis, we deduce that the deep portion of the slab is in the process of detaching from the shallower fragment at much higher rates than the current convergence rate at the surface. The increased strain rate might arise as the buoyant continental crust, which is dragged into the subduction system in its terminal stage, resists subduction, whereas the earlier subducted mantle lithosphere pulls from underneath.

  8. Connecting the Surface and the Deep: Evolving Role of Subduction Zone Fluids Through Time (United States)

    Galvez, Matthieu Emmanuel


    The speciation of aqueous fluids controls the transport and exchange of metals and volatile elements on Earth. Subduction zones are the most important geodynamic setting for this fluid-mediated chemical exchange. Characterizing the ionic speciation and pH of fluids equilibrated with rocks at subduction zone conditions has been a major challenge in Earth science. I will first present thermodynamic predictions of fluid-rock equilibria that tie together models of mineralogy and fluid speciation along a range of model P-T paths. The pH of fluids in subducted crustal lithologies is uniform and confined to a mildly alkaline range, controlled by rock volatile and chlorine contents. In contrast, the pH of mantle wedge fluids exhibits marked sensitivity to minor variations in rock chemistry. These variations may be caused by intramantle differentiation, or by infiltration of fluids enriched in alkali components extracted from the subducted crust. The sensitivity of pH to carbon, alkali and halogens illustrates a top-down control of Earth's atmosphere - ocean chemistry on the speciation of subduction zone fluids via the hydrothermally altered oceanic lithosphere. These results provide a perspective on the physicochemical mechanisms that have coupled metal and volatile cycles in subduction zones for over 2.5 billion years.

  9. Fossil intermediate-depth earthquakes in subducting slabs linked to differential stress release (United States)

    Scambelluri, Marco; Pennacchioni, Giorgio; Gilio, Mattia; Bestmann, Michel; Plümper, Oliver; Nestola, Fabrizio


    The cause of intermediate-depth (50-300 km) seismicity in subduction zones is uncertain. It is typically attributed either to rock embrittlement associated with fluid pressurization, or to thermal runaway instabilities. Here we document glassy pseudotachylyte fault rocks—the products of frictional melting during coseismic faulting—in the Lanzo Massif ophiolite in the Italian Western Alps. These pseudotachylytes formed at subduction-zone depths of 60-70 km in poorly hydrated to dry oceanic gabbro and mantle peridotite. This rock suite is a fossil analogue to an oceanic lithospheric mantle that undergoes present-day subduction. The pseudotachylytes locally preserve high-pressure minerals that indicate an intermediate-depth seismic environment. These pseudotachylytes are important because they are hosted in a near-anhydrous lithosphere free of coeval ductile deformation, which excludes an origin by dehydration embrittlement or thermal runaway processes. Instead, our observations indicate that seismicity in cold subducting slabs can be explained by the release of differential stresses accumulated in strong dry metastable rocks.

  10. Age of the subducting Pacific slab beneath East Asia and its geodynamic implications (United States)

    Liu, Xin; Zhao, Dapeng; Li, Sanzhong; Wei, Wei


    We study the age of the subducting Pacific slab beneath East Asia using a high-resolution model of P-wave tomography and paleo-age data of ancient seafloor. Our results show that the lithosphere age of the subducting slab becomes younger from the Japan Trench (∼130 Ma) to the slab's western edge (∼90 Ma) beneath East China, and the flat (stagnant) slab in the mantle transition zone (MTZ) is the subducted Pacific plate rather than the proposed Izanagi plate which should have already collapsed into the lower mantle. The flat Pacific slab has been in the MTZ for no more than ∼10-20 million years, considerably less than the age of the big mantle wedge beneath East Asia (>110 million years). Hence, the present flat Pacific slab in the MTZ has contributed to the Cenozoic destruction of the East Asian continental lithosphere with extensive intraplate volcanism and back-arc spreading, whereas the destruction of the North China Craton during the Early Cretaceous (∼140-110 Ma) was caused by the subduction of the Izanagi (or the Paleo-Pacific) plate.

  11. High-pressure creep of serpentine, interseismic deformation, and initiation of subduction. (United States)

    Hilairet, Nadege; Reynard, Bruno; Wang, Yanbin; Daniel, Isabelle; Merkel, Sebastien; Nishiyama, Norimasa; Petitgirard, Sylvain


    The supposed low viscosity of serpentine may strongly influence subduction-zone dynamics at all time scales, but until now its role could not be quantified because measurements relevant to intermediate-depth settings were lacking. Deformation experiments on the serpentine antigorite at high pressures and temperatures (1 to 4 gigapascals, 200 degrees to 500 degrees C) showed that the viscosity of serpentine is much lower than that of the major mantle-forming minerals. Regardless of the temperature, low-viscosity serpentinized mantle at the slab surface can localize deformation, impede stress buildup, and limit the downdip propagation of large earthquakes at subduction zones. Antigorite enables viscous relaxation with characteristic times comparable to those of long-term postseismic deformations after large earthquakes and slow earthquakes. Antigorite viscosity is sufficiently low to make serpentinized faults in the oceanic lithosphere a site for subduction initiation.

  12. Investigating the Lithospheric Structure of Southern Madagascar (United States)

    Tilmann, Frederik; Yuan, Xiaohui; Rümpker, Georg; Gerard, Rambolamana; Elisa, Rindraharisaona; Priestley, Keith


    The island of Madagascar occupies a key region in both the assembly and the multi-stage breakup of Gondwanaland, itself part of the super-continent Pangaea. Madagascar consists of an amalgamation of continental material, with the oldest rocks being of Archaean age. Its ancient fabric is characterised by several shear zones, some of them running oblique to the N-S trend, in particular in the south of the island. More recently during the Neogene, moderate volcanism has occurred in the Central and Northern part of the island, and there are indications of uplift throughout Eastern Madagascar over the last 10 Ma. Although Madagascar is now located within the interior of the African plate and far away from major plate boundaries (> 1000 km from the East African rift system and even further from the Central and South-West Indian Ridges), its seismic activity indicates that some deformation is taking place, and present-day kinematic models based on geodetic data and earthquake moment tensors in the global catalogues identify a diffuse N-S-oriented minor boundary separating two microplates, which appears to pass through Madagascar. In spite of the presence of Archaean and Proterozoic rocks continent-wide scale studies indicate a thin lithosphere (Madagascar, but are based on sparse data. We are operating a ENE-WSW oriented linear array of 25 broadband stations in southern Madagascar, extending from coast to coast and sampling the sedimentary basins in the west as well as the metamorphic rocks in the East, cutting geological boundaries seen at the surface at high angle. The array crosses the prominent Bongolava-Ranotsara shear zone which is thought to have been formed during Gondwanaland assembly, although this interpretation has recently been questioned. The array recorded the magnitude 5.3 earthquake of January 25, 2013 which occurred just off its western edge. In addition, in May 2013 we have deployed 25 short period sensors in the eastern part of the study area, where

  13. Boron cycling in subduction zones


    Palmer, Martin R.


    Subduction zones are geologically dramatic features, with much of the drama being driven by the movement of water. The “light and lively” nature of boron, coupled with its wide variations in isotopic composition shown by the different geo-players in this drama, make it an ideal tracer for the role and movement of water during subduction. The utility of boron ranges from monitoring how the fluids that are expelled from the accretionary prism influence seawater chemistry, to the subduction of c...

  14. Softening trigerred by eclogitization, the first step toward exhumation during continental subduction (United States)

    Jolivet, Laurent; Raimbourg, Hugues; Labrousse, Loïc; Avigad, Dov; Leroy, Yves; Austrheim, Håkon; Andersen, Torgeir B.


    Direct observation of peak pressure deformation in exhumed subduction channels is difficult because little evidence of this deformation survives later syn-exhumation deformation. Most ultrahigh-pressure parageneses are found in continental derived metamorphic rocks making continental subduction the best context to observe peak pressure deformation. Whereas many studies have enlightened the main driving parameters of exhumation such as buoyancy forces, low viscosity in the subduction channel, overburden removal by erosion and normal faulting, a basic question is seldom considered: why is a tectonic unit disconnected from the descending lithosphere and why does it start its way towards the surface? This event, seminal to exhumation processes, must involve some deformation and decoupling of the exhumed slice from the descending slab at peak pressure conditions or close to it. Our field observations in the Bergen arc show that Caledonian eclogitization and later amphibolitization of a granulitic terrane was achieved with a consistent component of simple shear compatible with the sense of the Caledonian subduction. Thus, the sequence of deformation preserved in the Bergen Arc documents the decoupling of subducted crustal material from the descending slab at the onset of exhumation. This observation suggests that deformation in the subduction channel is largely controlled by kinematic boundary conditions, i.e. underthrusting of the subducting slab. In this context of simple shear, metamorphic reactions assisted by fracturating, fluid infiltration and ductile deformation lower the resistance of rocks and allow the localisation of shear zones and the decoupling of buoyant tectonic units from the subducting slab. These tectonic units can then be incorporated into the channel circulation and start their upward travel.

  15. Blueschist facies pseudotachylytes from Corsica: First account of fossil earthquakes from a subduction complex (United States)

    Andersen, T. B.; Austrheim, H.


    Pseudotachylytes (PST) are products of deformation at extreme slip-rates along faults or in impact structures. Fault-plane PSTs are considered to represent fossil earthquakes. Tectonics in subduction zones, generate >80% of the seismic energy. Earthquake rocks should therefore also be common in old subduction complexes. Blueschist terrains are formed in the upper 15 to 50 km by tectonic burial in accreationary complexes and subduction zones. In spite of the very common earthquakes recorded from present-day subduction complexes, we are unaware of previous accounts of fossil earthquakes from exhumed subduction complexes. With a working hypothesis predicting fossil earthquakes to be preserved in subduction complexes, we have re-examined parts of the Alpine blueschist-eclogite terrain in Corsica. Within blueschist facies ophiolite gabbro and peridotite of Cape Corse, we discovered a number of faults decorated with ultra-fine fault rocks including PSTs. Detailed probe and SEM-studies reveal that some of the PSTs have quench textures proving the former presence of a melt. Quenched minerals, including fassaitic pyroxene are found as spherulites and dendrites. Fassaite has previously been described from UHP complexes and from PSTs formed in HP experiments. Other devitrification minerals include glaucophane, barroisite and zoisite as well as pumpellyite and albite. Our hypothesis -- fossil earthquakes in the form of frictional heat generated PST and ultra-cataclasite may be preserved within subduction complexes -- has been confirmed. Whilst most models view the properties of subducted lithosphere as a function of temperature, it is increasingly recognized that the rheological properties of rocks depend on their metamorphic status and importantly on fluids. Fluids from dehydration reactions in subduction complexes may reduce the effective stress to allow rapid brittle failure, which in turn may produce frictional heating and additional dehydration. It is, however, also

  16. Matching Lithosphere velocity changes to the GOCE gravity signal (United States)

    Braitenberg, Carla


    Authors: Carla Braitenberg, Patrizia Mariani, Alberto Pastorutti Department of Mathematics and Geosciences, University of Trieste Via Weiss 1, 34100 Trieste Seismic tomography models result in 3D velocity models of lithosphere and sublithospheric mantle, which are due to mineralogic compositional changes and variations in the thermal gradient. The assignment of density is non-univocal and can lead to inverted density changes with respect to velocity changes, depending on composition and temperature. Velocity changes due to temperature result in a proportional density change, whereas changes due to compositional changes and age of the lithosphere can lead to density changes of inverted sign. The relation between velocity and density implies changes in the lithosphere rigidity. We analyze the GOCE gradient fields and the velocity models jointly, making simulations on thermal and compositional density changes, using the velocity models as constraint on lithosphere geometry. The correlations are enhanced by applying geodynamic plate reconstructions to the GOCE gravity field and the tomography models which places today's observed fields at the Gondwana pre-breakup position. We find that the lithosphere geometry is a controlling factor on the overlying geologic elements, defining the regions where rifting and collision alternate and repeat through time. The study is carried out globally, with focus on the conjugate margins of the African and South American continents. The background for the study can be found in the following publications where the techniques which have been used are described: Braitenberg, C., Mariani, P. and De Min, A. (2013). The European Alps and nearby orogenic belts sensed by GOCE, Boll. Bollettino di Geofisica Teorica ed Applicata, 54(4), 321-334. doi:10.4430/bgta0105---- Braitenberg, C. and Mariani, P. (2015). Geological implications from complete Gondwana GOCE-products reconstructions and link to lithospheric roots. Proceedings of 5th

  17. Thermobarometric and fluid expulsion history of subduction zones (United States)

    Ernst, W. G.


    Phanerozoic, unmetamorphosed, weathered, and altered lithotectonic complexes subjected to subduction exhibit the prograde metamorphic facies sequence: zeolite → prehnite-pumpellyite → glaucophane schist → eclogite. Parageneses reflect relatively high-P trajectories, accompanied by semicontinuous devolatilization. The thermal evolution of convergent plate junctions results in early production of high-rank blueschists, high-P amphibolites, and eclogues at depth within narrow subduction zones while the hanging wall lithosphere is still hot. Protracted underflow drains heat from the nonsubducted plate and, even at profound depths, generates very low-T/high-P parageneses. Inclusion studies suggest that two-phase immiscible volatiles (liquid H2O, and gaseous high-hydrocarbons, CH4 and CO2) are evolved in turn during progressive metamorphism of the subducted sections. Expulsion of pore fluids and transitions from weathered and altered supracrustal rocks to zeolite facies assemblages release far more fluid than the better understood higher-grade transformations. Many blueschist parageneses, such as those of the internal Western Alps, have been partially overprinted by later greenschist and/or epidote-amphibolite facies assemblages. Alpine-type postblueschist metamorphic paths involved fairly rapid, nearly adiabatic decompression; some terranes even underwent modest continued heating and fluid evolution during early stages of ascent. Uplift probably occurred as a consequence of the underthrusting of low-density island arc or microcontinental crust along the convergent plate junction, resulting in marked deceleration or cessation of lithospheric underflow, decoupling, and nearly isothermal rise of the recrystallized subduction complex. Other, less common blueschist terranes, such as the eastern Franciscan belt of western California, preserve metamorphic aragonite and other high-P minerals, and lack a low-pressure overprint; physical conditions during retrogression

  18. Offshore Southern California lithospheric velocity structure from noise cross-correlation functions (United States)

    Bowden, D. C.; Kohler, M. D.; Tsai, V. C.; Weeraratne, D. S.


    A new shear wave velocity model offshore Southern California is presented that images plate boundary deformation including both thickening and thinning of the crustal and mantle lithosphere at the westernmost edge of the North American continent. The Asthenospheric and Lithospheric Broadband Architecture from the California Offshore Region Experiment (ALBACORE) ocean bottom seismometer array, together with 65 stations of the onshore Southern California Seismic Network, is used to measure ambient noise correlation functions and Rayleigh wave dispersion curves which are inverted for 3-D shear wave velocities. The resulting velocity model defines the transition from continental lithosphere to oceanic, illuminating the complex history and deformation in the region. A transition to the present-day strike-slip regime between the Pacific and North American Plates resulted in broad deformation and capture of the now >200 km wide continental shelf. Our velocity model suggests the persistence of the uppermost mantle volcanic processes associated with East Pacific Rise spreading adjacent to the Patton Escarpment, which marks the former subduction of Farallon Plate underneath North America. The most prominent of these seismic structures is a low-velocity anomaly underlying the San Juan Seamount, suggesting ponding of magma at the base of the crust, resulting in thickening and ongoing adjustment of the lithosphere due to the localized loading. The velocity model also provides a robust framework for future earthquake location determinations and ground-shaking simulations for risk estimates.

  19. The dominant surface-topography contributions of individual subduction parameters (United States)

    Crameri, Fabio; Lithgow-Bertelloni, Carolina; Tackley, Paul


    It is no secret, not any longer, that dynamic processes below the plate exert a significant contribution to the elevation of the plate at the surface (e.g., Flament et al., 2013). We have therefore studied* the individual impact each and every major subduction parameter has on surface topography. This allows us to qualitatively compare the different sources amongst each other, and to quantify their actual potential to vertically deflect the surface. The gained knowledge from this compilation is crucial: We might finally be able to link the directly-observable surface topography to the dynamics (buoyancy, rheology, and geometry) of the subduction system. *This study is made possible by the efficient convection code StagYY (Tackley 2008), the largely-automated post-processing and visualisation toolbox StagLab (, and crucial model developments (Crameri and Tackley, 2015; Crameri et al., G-cubed, submitted, Crameri and Lithgow-Bertelloni, Tectonophysics, submitted). REFERENCES 
Flament, N., M. Gurnis, and R. D. Müller (2013), A review of observations and models of dynamic topography, Lithosphere, 5(2), 189-210. Crameri, F., and P. J. Tackley (2015), Parameters controlling dynamically self-consistent plate tectonics and single-sided subduction in global models of mantle convection, J. Geophys. Res. Solid Earth, 120(5), 3680-3706. Crameri, F., C. R. Lithgow-Bertelloni, and P. J. Tackley (submitted), The dynamical control of subduction parameters on surface topography, Geochem. Geophys. Geosyst. Crameri, F., and C. R. Lithgow-Bertelloni (submitted), Dynamic Mantle-Transition-Zone Controls on Upper-Plate Tilt, Tectonophysics. Tackley, P.J (2008) Modelling compressible mantle convection with large viscosity contrasts in a three- dimensional spherical shell using the yin-yang grid. Physics of the Earth and Planetary Interiors 171(1-4), 7-18.

  20. Geologic signature of early Tertiary ridge subduction in Alaska (United States)

    Bradley, Dwight C.; Kusky, Timothy M.; Haeussler, Peter J.; Goldfarb, Richard J.; Miller, Marti L.; Dumoulin, Julie A.; Nelson, Steven W.; Karl, Susan M.


    ridge subduction, and changes in the strength of the prism as it was heated and then cooled. In this model, events in the Alaskan interior would have taken place above more distal, deeper parts of the slab window. Extensional (or transtensional) basin subsidence was driven by the two subducting plates that each exerted different tractions on the upper plate. The magmatic lull along the arc presumably marks a time when hydrated lithosphere was not being subducted beneath the arc axis. The absence of a subducting slab also may explain uplift of the Brooks Range and North Slope: Geodynamic models predict that longwavelength uplift of this magnitude will take place far inboard from Andean-type margins when a subducting slab is absent. Precise correlations between events in the accretionary prism and the Alaskan interior are hampered, however, by palinspastic problems. During and since the early Tertiary, margin-parallel strike-slip faulting has offset the near-trench plutonic belt - i.e., the very basis for locating the triple junction and slab window - from its backstop, by an amount that remains controversial.Near-trench magmatism began at 61 Ma at Sanak Island in the west but not until 51 Ma at Baranof Island, 2200 km to the east. A west-to-east age progression suggests migration of a trench-ridge-trench triple junction, which we term the Sanak-Baranof triple junction. Most workers have held that the subducted ridge separated the Kula and Farallon plates. As a possible alternative, we suggest that the ridge may have separated the Kula plate from another oceanic plate to the east, which we have termed the Resurrection plate.

  1. Seismic evidence of the lithosphere-asthenosphere boundary beneath Izu-Bonin area (United States)

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


    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

  2. Dynamic Topography during Flat Subduction: Subsidence or Uplift? (United States)

    Davila, F. M.; Lithgow-Bertelloni, C. R.


    Since the first studies on dynamic topography and basin evolution, low-dipping subduction has been related to intracontinental, long-wavelength and high-amplitude subsidence, whereas retreating to normal subduction systems to uplift. This was proposed to explain the Cretaceous-early Cenozoic topographic evolution of the western US. However, modern flat-slab and slab-retreating segments of South America do not record such a subsidence and uplift patterns. For example, the flat slab of Peru at ˜10°SL, related to the subduction of the Nazca Ridge, underlies an elevated promontory known as the Fitzcarrald Arch. The Argentine flat-slab at ˜31°SL associated to the subduction of the Juan Fernandez Ridge underlies a high-elevated intermontane system known as the Pampean broken foreland. Both upwarping features are younger than 7 Ma and contemporaneous with the arrival of flat subduction to these segments. In order to shed light into this controversy, we calculate dynamic topography along the Andean flat-slab segments using the Hager and O'Connell (1981) instantaneous flow formulation, an accurate reconstruction of the slab geometry along the central Andes and a density contrast between the flat slabs and the country mantle close to zero (△δ≈0) in order to simulate a buoyant oceanic lithosphere. We demonstrate that dynamic subsidence develops only at the leading edge of flat subduction, where the slabs plunge >30°, whereas the flatter slabs reproduce minor or no dynamic topography signals. These results agree with geological and geophysical proxies. Along the Argentine Plains, the account for a accumulated relief of ˜200 m, which might be considered as an "observed dynamic subsidence" signal (given that no tectonic activity has been recorded in this region since the Cretaceous to explain this surface topography). This gives a ˜0.03 mm/yr dynamic subsidence rate that are curiously similar to the exhumations estimated by low-temperature thermochronology along the

  3. Constraints on the depth and thermal history of cratonic lithosphere from peridotite xenoliths, xenocrysts and seismology (United States)

    Mather, Kathy A.; Pearson, D. Graham; McKenzie, Dan; Kjarsgaard, Bruce A.; Priestley, Keith


    the southern African lithosphere. We find very similar estimates for the heat flow and thickness of the lithosphere between SW Namibia (off-craton) and Bultfontein (on-craton). This supports suggestions of a cratonic thermal regime and equivalent lithospheric thickness across that region of southern Africa at the time of kimberlite sampling, with concurrent local thermal disturbance evident in Namibia. Complimentary, novel, seismically-obtained geotherm estimates show that the lithosphere in Namibia is now significantly thinner than the estimate at 70 Ma obtained from xenolith thermobarometry.

  4. Laboratory models of the thermal evolution of the mantle during rollback subduction. (United States)

    Kincaid, C; Griffiths, R W


    The subduction of oceanic lithosphere plays a key role in plate tectonics, the thermal evolution of the mantle and recycling processes between Earth's interior and surface. Information on mantle flow, thermal conditions and chemical transport in subduction zones come from the geochemistry of arc volcanoes, seismic images and geodynamic models. The majority of this work considers subduction as a two-dimensional process, assuming limited variability in the direction parallel to the trench. In contrast, observationally based models increasingly appeal to three-dimensional flow associated with trench migration and the sinking of oceanic plates with a translational component of motion (rollback). Here we report results from laboratory experiments that reveal fundamental differences in three-dimensional mantle circulation and temperature structure in response to subduction with and without a rollback component. Without rollback motion, flow in the mantle wedge is sluggish, there is no mass flux around the plate and plate edges heat up faster than plate centres. In contrast, during rollback subduction flow is driven around and beneath the sinking plate, velocities increase within the mantle wedge and are focused towards the centre of the plate, and the surface of the plate heats more along the centreline.

  5. Postcollisional mafic igneous rocks record crust-mantle interaction during continental deep subduction. (United States)

    Zhao, Zi-Fu; Dai, Li-Qun; Zheng, Yong-Fei


    Findings of coesite and microdiamond in metamorphic rocks of supracrustal protolith led to the recognition of continental subduction to mantle depths. The crust-mantle interaction is expected to take place during subduction of the continental crust beneath the subcontinental lithospheric mantle wedge. This is recorded by postcollisional mafic igneous rocks in the Dabie-Sulu orogenic belt and its adjacent continental margin in the North China Block. These rocks exhibit the geochemical inheritance of whole-rock trace elements and Sr-Nd-Pb isotopes as well as zircon U-Pb ages and Hf-O isotopes from felsic melts derived from the subducted continental crust. Reaction of such melts with the overlying wedge peridotite would transfer the crustal signatures to the mantle sources for postcollisional mafic magmatism. Therefore, postcollisonal mafic igneous rocks above continental subduction zones are an analog to arc volcanics above oceanic subduction zones, providing an additional laboratory for the study of crust-mantle interaction at convergent plate margins.

  6. Subduction and Slab Advance at Orogen Syntaxes: Predicting Exhumation Rates and Thermochronometric Ages with Numerical Modeling (United States)

    Nettesheim, Matthias; Ehlers, Todd A.; Whipp, David M.


    The change in plate boundary orientation and subducting plate geometry along orogen syntaxes may have major control on the subduction and exhumation dynamics at these locations. Previous work documents that the curvature of subducting plates in 3D at orogen syntaxes forces a buckling and flexural stiffening of the downgoing plate. The geometry of this stiffened plate region, also called indenter, can be observed in various subduction zones around the world (e.g. St. Elias Range, Alaska; Cascadia, USA; Andean syntaxis, South America). The development of a subducting, flexurally stiffened indenter beneath orogen syntaxes influences deformation in the overriding plate and can lead to accelerated and focused rock uplift above its apex. Moreover, the style of deformation in the overriding plate is influenced by the amount of trench or slab advance, which is the amount of overall shortening not accommodated by underthrusting. While many subduction zones exhibit little to no slab advance, the Nazca-South America subduction and especially the early stages of the India-Eurasia collision provide end-member examples. Here, we use a transient, lithospheric-scale, thermomechanical 3D model of an orogen syntaxis to investigate the effects of subducting a flexurally stiffened plate geometry and slab advance on upper plate deformation. A visco-plastic upper-plate rheology is used, along with a buckled, rigid subducting plate. The free surface of the thermomechanical model is coupled to a landscape evolution model that accounts for erosion by fluvial and hillslope processes. The cooling histories of exhumed rocks are used to predict the evolution of low-temperature thermochronometer ages on the surface. With a constant overall shortening for all simulations, the magnitude of slab advance is varied stepwise from no advance, with all shortening accommodated by underthrusting, to full slab advance, i.e. no motion on the megathrust. We show that in models where most shortening is

  7. Subduction of hydrated basalt of the oceanic crust: Implications for recycling of water into the upper mantle and continental growth (United States)

    Rapp, R. P.


    Subduction zones are presently the dominant sites on Earth for recycling and mass transfer between the crust and mantle; they feed hydrated basaltic oceanic crust into the upper mantle, where dehydration reactions release aqueous fluids and/or hydrous melts. The loci for fluid and/or melt generation will be determined by the intersection of dehydration reaction boundaries of primary hydrous minerals within the subducted lithosphere with slab geotherms. For metabasalt of the oceanic crust, amphibole is the dominant hydrous mineral. The dehydration melting solidus, vapor-absent melting phase relationships; and amphibole-out phase boundary for a number of natural metabasalts have been determined experimentally, and the pressure-temperature conditions of each of these appear to be dependent on bulk composition. Whether or not the dehydration of amphibole is a fluid-generating or partial melting reaction depends on a number of factors specific to a given subduction zone, such as age and thickness of the subducting oceanic lithosphere, the rate of convergence, and the maturity of the subduction zone. In general, subduction of young, hot oceanic lithosphere will result in partial melting of metabasalt of the oceanic crust within the garnet stability field; these melts are characteristically high-Al2O3 trondhjemites, tonalites and dacites. The presence of residual garnet during partial melting imparts a distinctive trace element signature (e.g., high La/Yb, high Sr/Y and Cr/Y combined with low Cr and Y contents relative to demonstrably mantle-derived arc magmas). Water in eclogitized, subducted basalt of the oceanic crust is therefore strongly partitioned into melts generated below about 3.5 GPa in 'hot' subduction zones. Although phase equilibria experiments relevant to 'cold' subduction of hydrated natural basalts are underway in a number of high-pressure laboratories, little is known with respect to the stability of more exotic hydrous minerals (e.g., ellenbergite) and

  8. Zooming into the Hindu Kush slab break-off: A rare glimpse on the terminal stage of subduction (United States)

    Kufner, Sofia-Katerina; Schurr, Bernd; Haberland, Christian; Zhang, Yong; Saul, Joachim; Ischuk, Anatoly; Oimahmadov, Ilhomjon


    The terminal stage of subduction sets in when the continental margin arrives at the trench and the opposite forces of the sinking slab and buoyant continent extend and ultimately sever the subducted lithosphere. This process, although common in geological history, is short-lived, and therefore rarely observed. The deep seismicity under the Hindu Kush (Central Asia), including the 2015 Mw 7.5 event, is a rare case that testifies to this process. Here, we use new seismological data to create a high resolution picture of slab break-off and infer its dynamics. High precision earthquake locations and tomographic images show subduction of continental crust down to ∼180 km. A large dataset of source mechanisms indicates sub-vertical extension in the entire slab but a strain rate analysis showed that the deeper seismogenic portion of the slab, below the subducted crust, extends at higher rates (∼40 km/Ma). Most Mw > 7 earthquakes between 1983-2015, relocated relative to our new well-constrained earthquake catalog, cluster in a small volume below 180 km, and indicate shearing on an overturned interface. A slip model for the latest 2015 Mw 7.5 event suggests that it ruptured into a seismic gap on this interface. From this configuration we conclude that a horizontal slab tear develops along-strike of the Hindu Kush seismic zone at the base of the subducted continental crust. Below the subducted crust, the deepest and also largest earthquakes (180-265 km) are likely associated with deformation in the mantle lithosphere. From the seismicity distribution and the rupture mechanisms we further deduce that the dominant deformation mechanism in this deeper portion of the slab changes along-strike from simple to pure shear. The fastest detachment rates and largest earthquakes occur during the simple shear dominated stage. Earthquakes in the upper part (60-180 km), above the rapidly extending slab, might be triggered by processes related to the subduction of crustal rocks.

  9. From rifting to subduction: the role of inheritance in the Wilson Cycle (United States)

    Beaussier, Stéphane; Gerya, Taras; Burg, Jean-Pierre


    The Wilson Cycle entails that oceans close and reopen. This cycle is a fundamental principle in plate tectonics, inferring continuity from divergence to convergence and that continental rifting takes place along former suture zones. This view questions the role of inherited structures at each stage of the Wilson Cycle. Using the 3D thermo-mechanical code, I3ELVIS (Gerya and Yuen 2007) we present a high-resolution continuous model of the Wilson cycle from continental rifting, breakup and oceanic spreading to convergence and spontaneous subduction initiation. Therefore, all lateral and longitudinal structures of the lithospheres are generated self-consistently and are consequences of the initial continental structure, tectono-magmatic inheritance and material rheology. In the models, subduction systematically initiates off-ridge and is controlled by the convergence-induced swelling of the ridge. Geometry and dynamics of the developing off-ridge subduction is controlled by four main factors: (1) the obliquity of the ridge with respect to the convergence direction; (2) fluid-induced weakening of the oceanic crust; (3) irregularity of ridge and margins inherited from rifting and spreading; (4) strain localization at transform faults formed during ocean floor spreading. Further convergence can lead to obduction of the oceanic crust and segments of ridge after the oceanic lithosphere is entrained into subduction. We show that the main parameters controlling the occurrence and geometry of obducted ophiolite are the convergence rate and the inherited structure of the passive margins and ridge. Our numerical experiments results show the essential role played by inheritance during the Wilson Cycle and are consistent with nature observations such as the tectonic history of the Oman subduction-obduction system. REFERENCES Gerya, T. V., and D. A. Yuen. 2007: "Robust Characteristics Method for Modelling Multiphase Visco-Elasto-Plastic Thermo-Mechanical Problems, Physics of the

  10. Earth's evolving subcontinental lithospheric mantle: inferences from LIP continental flood basalt geochemistry (United States)

    Greenough, John D.; McDivitt, Jordan A.


    Archean and Proterozoic subcontinental lithospheric mantle (SLM) is compared using 83 similarly incompatible element ratios (SIER; minimally affected by % melting or differentiation, e.g., Rb/Ba, Nb/Pb, Ti/Y) for >3700 basalts from ten continental flood basalt (CFB) provinces representing nine large igneous provinces (LIPs). Nine transition metals (TM; Fe, Mn, Sc, V, Cr, Co, Ni, Cu, Zn) in 102 primitive basalts (Mg# = 0.69-0.72) from nine provinces yield additional SLM information. An iterative evaluation of SIER values indicates that, regardless of age, CFB transecting Archean lithosphere are enriched in Rb, K, Pb, Th and heavy REE(?); whereas P, Ti, Nb, Ta and light REE(?) are higher in Proterozoic-and-younger SLM sources. This suggests efficient transfer of alkali metals and Pb to the continental lithosphere perhaps in association with melting of subducted ocean floor to form Archean tonalite-trondhjemite-granodiorite terranes. Titanium, Nb and Ta were not efficiently transferred, perhaps due to the stabilization of oxide phases (e.g., rutile or ilmenite) in down-going Archean slabs. CFB transecting Archean lithosphere have EM1-like SIER that are more extreme than seen in oceanic island basalts (OIB) suggesting an Archean SLM origin for OIB-enriched mantle 1 (EM1). In contrast, OIB high U/Pb (HIMU) sources have more extreme SIER than seen in CFB provinces. HIMU may represent subduction-processed ocean floor recycled directly to the convecting mantle, but to avoid convective homogenization and produce its unique Pb isotopic signature may require long-term isolation and incubation in SLM. Based on all TM, CFB transecting Proterozoic lithosphere are distinct from those cutting Archean lithosphere. There is a tendency for lower Sc, Cr, Ni and Cu, and higher Zn, in the sources for Archean-cutting CFB and EM1 OIB, than Proterozoic-cutting CFB and HIMU OIB. All CFB have SiO2 (pressure proxy)-Nb/Y (% melting proxy) relationships supporting low pressure, high % melting

  11. Diapir versus along-channel ascent of crustal material during plate convergence: Constrained by the thermal structure of subduction zones (United States)

    Liu, Ming-Qi; Li, Zhong-Hai; Yang, Shao-Hua


    Subduction channel processes are crucial for understanding the material and energy exchange between the Earth's crust and mantle. Crustal rocks can be subducted to mantle depths, interact with the mantle wedge, and then exhume to the crustal depth again, which is generally considered as the mechanism for the formation of ultrahigh-pressure metamorphic rocks in nature. In addition, the crustal rocks generally undergo dehydration and melting at subarc depths, giving rise to fluids that metasomatize and weaken the overlying mantle wedge. There are generally two ways for the material ascent from subarc depths: one is along subduction channels; the other is through the mantle wedge by diapir. In order to study the conditions and dynamics of these contrasting material ascent modes, systematic petrological-thermo-mechanical numerical models are constructed with variable thicknesses of the overriding and subducting continental plates, ages of the subducting oceanic plate, as well as the plate convergence rates. The model results suggest that the thermal structures of subduction zones control the thermal condition and fluid/melt activity at the slab-mantle interface in subcontinental subduction channels, which further strongly affect the material transportation and ascent mode. The thick overriding continental plate and the low-angle subduction style induced by young subducting oceanic plate both contribute to the formation of relatively cold subduction channels with strong overriding mantle wedge, where the along-channel exhumation occurs exclusively to result in the exhumation of HP-UHP metamorphic rocks. In contrast, the thin overriding lithosphere and the steep subduction style induced by old subducting oceanic plate are the favorable conditions for hot subduction channels, which lead to significant hydration and metasomatism, melting and weakening of the overriding mantle wedge and thus cause the ascent of mantle wedge-derived melts by diapir through the mantle wedge

  12. Origin and consequences of western Mediterranean subduction, rollback, and slab segmentation (United States)

    van Hinsbergen, Douwe J. J.; Vissers, Reinoud L. M.; Spakman, Wim


    The western Mediterranean recorded subduction rollback, slab segmentation and separation. Here we address the questions of what caused Oligocene rollback initiation, and how its subsequent evolution split up an originally coherent fore arc into circum-southwest Mediterranean segments. We kinematically reconstruct western Mediterranean geology from subduction initiation to present, using Atlantic plate reconstructions as boundary condition. We test possible reconstructions against remnants of subducted lithosphere imaged by seismic tomography. Transform motion between Africa and Iberia (including the Baleares) between 120 and 85 Ma was followed by up to 150 km convergence until 30 Ma. Subduction likely initiated along the transform fault that accommodated pre-85 Ma translation. By the 30 Ma inception of rollback, up to 150 km of convergence had formed a small slab below the Baleares. Iberia was disconnected from Sardinia/Calabria through the North Balearic Transform Zone (NBTZ). Subduction below Sardinia/Calabria was slightly faster than below the Baleares, the difference being accommodated in the Pyrenees. A moving triple junction at the trench-NBTZ intersection formed a subduction transform edge propagator fault between the Baleares and Calabria slab segments. Calabria rolled back eastward, whereas the Baleares slab underwent radial (SW-S-SE) rollback. After Kabylides-Africa collision, the western slab segment retreated toward Gibraltar, here reconstructed as the maximum rollback end-member model, and a Kabylides slab detached from Africa. Opening of a slab window below the NBTZ allowed asthenospheric rise to the base of the fore arc creating high-temperature metamorphism. Western Mediterranean rollback commenced only after sufficient slab-pull was created from 100 to 150 km of slow, forced subduction before 30 Ma.

  13. Seismic anisotropy in the Hellenic subduction zone: Effects of slab segmentation and subslab mantle flow (United States)

    Evangelidis, C. P.


    The segmentation and differentiation of subducting slabs have considerable effects on mantle convection and tectonics. The Hellenic subduction zone is a complex convergent margin with strong curvature and fast slab rollback. The upper mantle seismic anisotropy in the region is studied focusing at its western and eastern edges in order to explore the effects of possible slab segmentation on mantle flow and fabrics. Complementary to new SKS shear-wave splitting measurements in regions not adequately sampled so far, the source-side splitting technique is applied to constrain the depth of anisotropy and to densify measurements. In the western Hellenic arc, a trench-normal subslab anisotropy is observed near the trench. In the forearc domain, source-side and SKS measurements reveal a trench-parallel pattern. This indicates subslab trench-parallel mantle flow, associated with return flow due to the fast slab rollback. The passage from continental to oceanic subduction in the western Hellenic zone is illustrated by a forearc transitional anisotropy pattern. This indicates subslab mantle flow parallel to a NE-SW smooth ramp that possibly connects the two subducted slabs. A young tear fault initiated at the Kefalonia Transform Fault is likely not entirely developed, as this trench-parallel anisotropy pattern is observed along the entire western Hellenic subduction system, even following this horizontal offset between the two slabs. At the eastern side of the Hellenic subduction zone, subslab source-side anisotropy measurements show a general trench-normal pattern. These are associated with mantle flow through a possible ongoing tearing of the oceanic lithosphere in the area. Although the exact geometry of this slab tear is relatively unknown, SKS trench-parallel measurements imply that the tear has not reached the surface yet. Further exploration of the Hellenic subduction system is necessary; denser seismic networks should be deployed at both its edges in order to achieve

  14. The Lithospheric Structure of Madagascar (United States)

    Wysession, M. E.; Pratt, M. J.; Andriampenomanana Ny Ony, F. S. T.; Tsiriandrimanana, R.; Nyblade, A.; Aleqabi, G. I.; Shore, P.; Tucker, R.; Wiens, D. A.; Rambolamanana, G.


    The lithosphere of Madagascar is revealed for the first time from a combination of studies using data from the 2011-2013 MACOMO (Madagascar, the Comoros, and Mozambique) broadband seismic array from the IRIS PASSCAL program (funded by NSF, with additional data from the RHUM-RUM and Madagascar Seismic Profile projects). Methods include seismicity locations, body-wave receiver functions, Pn tomography, body-wave tomography, and ambient-noise and two-plane-wave earthquake surface-wave analyses. Madagascar's crustal thickness varies greatly, from 20 to 45 km, in good agreement with its past tectonic history of rifting from the mainland and having India break away to the north. The crust is thickest along the central spine of the island, along a ridge of mountains, but unusually high elevations suggest some amount f thermal buoyancy in the mantle. Crust is also thick along the east coast, where archean terranes were severed from India. Crust is thinnest along the west coast, where thick sedimentary basins up to 8 km thick are found on top of unusually thinned basement crust (about 12 km thick), a remnant of rifting away from Africa 130-160 Ma ago. Madagascar has an unusually high level of intraplate seismicity, with 918 earthquakes located during the 2-year period. Seismicity shows interesting correlations with paleotectonic features, but much is located in the central regions of the island, associated with normal faulting along several graben structures. This region also corresponds to the central of three regions within Madagascar (north, central, and southwest) that display strong lithospheric seismic low-velocity anomalies that underlie regions of current or recent volcanic activity. Surface waves show that these low-velocity zones (LVZs) extending down into the asthenosphere, and body-wave tomography shows them extending even deeper. Pn tomography shows that the width of the central LVZ is only about 100-200 km in diameter at the top of the mantle, indicative of

  15. Reevaluating plate driving forces from 3-D models of subduction (United States)

    Stegman, D. R.; Freeman, J.; Schellart, W. P.; Moresi, L.; May, D.; Turnbull, R.


    Subducting lithospheric slabs mechanically attached to tectonic plates provide the main driving force for surface plate motion. Numerical models historically simulate slab dynamics as a 2-D process and further simplify the problem into either a density driven model (no heat transfer) or a corner-flow problem (thermal convection) [Christensen, 2001; Enns et al., (in revision); van Keken, 2003]. Recent 3-D global models of density driven flow incorporating a history of plate motion (Conrad and Lithgow-Bertelloni, 2002) have succussfully ruled out slab "suction" (basal shear traction induced by downward flow of the slabs) as a major driving force, but exact partitioning of the remaining forces acting on the slab remain unconstrained. A survey of trenches around the world reveals that over half of the slabs presently subducted in the upper mantle have a discontinuous edge (either a slab tip on a young slab, or the side edge of a slab with finite width) around which mantle can flow: prime examples being slabs in the Mediterranean and Carribean. However, even slabs with a wide lateral extent (and where a 2-D approximation may seem appropriate), show signs of having 3-D complexity. For example, on the surface Tonga appears relatively symmetric, but when the history of subduction is considered, the slab has a twisted, 3-D structure due to significant eastward retreat of just the northern part of an originally N-S oriented trench edge. Similarly the widest slabs, South American and Kamchatka, show seismic anisotropy attributed to trench parallel mantle flow (Russo and Silver, 1994; Peyton, et al., 2001, respectively), while the Aleutian trench has oblique subduction varying in magnitude from west to east, and medium width Central American slab likely has a slab window allowing 3-D flow (Johnston and Thorkelson, 1997). Recent laboratory experiments of subduction have demonstrated the full complexity of flow occuring in 3-D geometry (Kincaid and Griffiths, 2003; Schellart

  16. A discussion of numerical subduction initiation (United States)

    Buiter, Susanne; Ellis, Susan


    In nature, subduction can initiate in various ways: Shortening can localise at oceanic transform faults, extinct spreading centres, or inherited passive margin faults; or, alternatively, subduction can be triggered from existing subduction systems by along-strike trench propagation, polarity reversals, or trench jumps. Numerical studies that specifically address subduction initiation have highlighted the roles of sediment loading, rheological strength contrasts, strain softening, and continental topographic gradients, among others. Usually, however, numerical models that aim to investigate subduction dynamics prefer to bypass the subduction initiation phase and its complexities, and focus instead on the stages during which the slab is descending into the mantle. However, even in these models, subduction still needs to begin. It is disturbingly easy to define initial model geometries that do not result in subduction. The specific combination of initial model geometries and values for rheological parameters that successfully initiates subduction has even been referred to as 'the sweet spot' in model space. One cause of subduction initiation failure is when the subducting and overriding plates lock, resulting in either indentation or severe dragging downwards of the overriding plate. This may point to a difficulty in maintaining a weak subduction interface during model evolution. A second factor that may cause difficulties is that initial model geometry and stresses need to balance, as otherwise the first model stages may show spurious deformation associated with reaching equilibrium. A third requirement that may cause problems is that the surface needs to have sufficient displacement freedom to allow the overriding plate to overthrust the subducting plate. That also implies an exclusion of sharp corners in the subduction interface near the surface. It is the interplay of subduction interface geometry, interface strength and subducting plate rheology that determines

  17. Compression-extension transition of continental crust in a subduction zone: A parametric numerical modeling study with implications on Mesozoic-Cenozoic tectonic evolution of the Cathaysia Block. (United States)

    Zuo, Xuran; Chan, Lung Sang; Gao, Jian-Feng


    The Cathaysia Block is located in southeastern part of South China, which situates in the west Pacific subduction zone. It is thought to have undergone a compression-extension transition of the continental crust during Mesozoic-Cenozoic during the subduction of Pacific Plate beneath Eurasia-Pacific Plate, resulting in extensive magmatism, extensional basins and reactivation of fault systems. Although some mechanisms such as the trench roll-back have been generally proposed for the compression-extension transition, the timing and progress of the transition under a convergence setting remain ambiguous due to lack of suitable geological records and overprinting by later tectonic events. In this study, a numerical thermo-dynamical program was employed to evaluate how variable slab angles, thermal gradients of the lithospheres and convergence velocities would give rise to the change of crustal stress in a convergent subduction zone. Model results show that higher slab dip angle, lower convergence velocity and higher lithospheric thermal gradient facilitate the subduction process. The modeling results reveal the continental crust stress is dominated by horizontal compression during the early stage of the subduction, which could revert to a horizontal extension in the back-arc region, combing with the roll-back of the subducting slab and development of mantle upwelling. The parameters facilitating the subduction process also favor the compression-extension transition in the upper plate of the subduction zone. Such results corroborate the geology of the Cathaysia Block: the initiation of the extensional regime in the Cathaysia Block occurring was probably triggered by roll-back of the slowly subducting slab.

  18. Some geophysical and geochemical consequences of slab serpentinization at subduction zones (United States)

    Phipps Morgan, J.; Ruepke, L. H.; Ranero, C.; Hort, M.


    Here we explore the potential impact of slab serpentinization and deserpentinization processes on arc-melting and on water, carbon-dioxide, U, Pb, and noble gas recycling into the deep mantle. We examine the consequences of a scenario in which bend-faulting between the outer rise and trench axis creates the conduits for seawater to reach and react with cold lithospheric mantle to serpentinize it. Water penetration to serpentinize the slab-lithosphere will be inhibited by thick sediments (e.g. Cascades) or thick oceanic crust (subducting oceanic plateaus), while subducting long-offset fracture zones will be especially serpentine-rich because they serpentinized at both the spreading center and subduction zone. If this process occurs, then the incoming lithosphere will typically contain ~500m of altered sediments, ~6 km of partially hydrated oceanic crust, and ~20-55km of partially serpentinized slab mantle. Possible regional geophysical consequences of this scenario are: (1) Fracture Zones preferentially become tears in subducting slabs because they are relatively serpentine rich, thus they deserpentinize more. (2) If so, then their greater deserpentinization should produce greater sub-arc water release which leads to greater arc melting above subducted fracture zones. (3) Regions of little serpentinization will be correlated with flat subduction, lower volumes of slab-water release, and relatively low rates of arc-volcanism. Our thermomechanical modelling implies, depending upon a slab's age and subduction rate, between 30-90% of the slab's chemically bound water is likely to survive sub-arc dehydration to transport its water into the deeper mantle. Possible global geochemical consequences of this scenario are: (1) At current subduction rates, 0.5-1.5 oceans of water would be recycled past the arc-melting region into the deeper mantle during the past Ga. (2) Since 0.3%, 1%, and 3% of the exosphere's Ne, Ar, and Xe are dissolved in the oceans, this implies that at

  19. Deformation in the continental lithosphere (United States)

    The Physical Properties of Earth Materials Committee, a technical committee of AGU's Tectonophysics Section, is organizing a dinner/colloquium as part of the Fall Meeting in San Francisco, Calif. This event will be held Monday, December 3rd, in the Gold Rush Room of the Holiday Inn Golden Gateway Hotel at 1500 Van Ness St. There will be a no-host bar from 6:30 to 7:30 P.M., followed by dinner from 7:30 to 8:30 P.M. Paul Tapponnier will deliver the after-dinner talk, “Large-Scale Deformation Mechanisms in the Continental Lithosphere: Where Do We Stand?” It will start at 8:30 P.M. and a business meeting will follow at 9:30 P.M.

  20. Crossing the Iberian Plate from the Bay of Biscay to the Alboran Sea: a lithospheric geotransect (United States)

    Carballo, Alberto; Fernandez, Manel; Torne, Montserrat; Jiménez-Munt, Ivone; Vergés, Jaume; Pedreira, David; Díaz, Jordi; Villaseñor, Antonio


    A ~1000-km-long lithospheric transect running from the North-Iberian Margin to the Neogene Alboran Basin (W- Mediterranean) is investigated. The main goal is to image the lateral changes in the crustal and lithospheric structure occurring in: i) the North-Iberian margin, whose deformation in Alpine times gave rise to the uplift of the Cantabrian Mountains related to incipient subduction; ii) the Spanish Meseta, characterized by the presence of Cenozoic basins on top of a Variscan basement with weak Alpine deformation in the Spanish Central System and localized Neogene-Quaternary deep volcanism; and iii) the Betic-Alboran system related to the roll-back of the Ligurian-Tethyan domain. The modeling approach based on the LitMod package combines potential fields, elevation, thermal, seismic and petrological data under a self-consistent scheme. The crustal structure is mainly constrained by active and passive seismic experiments whereas the upper mantle is constrained by tomography models. The results highlight the lateral variations in the topography of the lithosphere-asthenosphere boundary (LAB), suggesting a strong lithospheric mantle strain below the Cantabrian and Betic mountain belts. The LAB depth ranges from 160 km beneath the Cantabrian Mountains to 110-90 km beneath Iberia Meseta deepenly again to values of 190 km beneath Betic Mountain. The Spanish Central System, with elevations higher than 1400 m, has no noticeable signature on the LAB depth. We have considered three lithospheric mantle compositions: a predominantly average Phanerozoic in the continental mainland, and two more fertile PUM (primitive upper mantle) compositions in the oceanic domains of Cantabrian and Mediterranean seas, and in the Calatrava volcanic field. These compositional differences allowed us to reproduce the main trends of the geophysical observables as well as the inferred P-, Pn- and S-wave seismic velocities from tomography models and seismic experiments available in the study

  1. The Magnetic Field of the Earth's Lithosphere (United States)

    Thébault, Erwan; Purucker, Michael; Whaler, Kathryn A.; Langlais, Benoit; Sabaka, Terence J.


    The lithospheric contribution to the Earth's magnetic field is concealed in magnetic field data that have now been measured over several decades from ground to satellite altitudes. The lithospheric field results from the superposition of induced and remanent magnetisations. It therefore brings an essential constraint on the magnetic properties of rocks of the Earth's sub-surface that would otherwise be difficult to characterize. Measuring, extracting, interpreting and even defining the magnetic field of the Earth's lithosphere is however challenging. In this paper, we review the difficulties encountered. We briefly summarize the various contributions to the Earth's magnetic field that hamper the correct identification of the lithospheric component. Such difficulties could be partially alleviated with the joint analysis of multi-level magnetic field observations, even though one cannot avoid making compromises in building models and maps of the magnetic field of the Earth's lithosphere at various altitudes. Keeping in mind these compromises is crucial when lithospheric field models are interpreted and correlated with other geophysical information. We illustrate this discussion with recent advances and results that were exploited to infer statistical properties of the Earth's lithosphere. The lessons learned in measuring and processing Earth's magnetic field data may prove fruitful in planetary exploration, where magnetism is one of the few remotely accessible internal properties.

  2. The effects of the overriding plate thermal state on the slab dip in an ocean-continent subduction system

    CERN Document Server

    Roda, Manuel; Spalla, Maria Iole; 10.1016/j.crte.2011.01.005


    To evaluate the effects of variations in the thermal state of the overriding plate on the slab dip in an ocean-continent subduction system, a 2-D finite element thermo-mechanical model was implemented. The lithosphere base was located at the depth of the 1600 K isotherm. Numerical simulations were performed while taking into account four different initial thicknesses for the oceanic lithosphere (60, 80, 95 and 110 km) and five different thicknesses of the overriding plate, as compared in terms of the continental-oceanic plate thickness ratio (100, 120, 140, 160 and 200% of the oceanic lithosphere thickness). The results of numerical modeling indicate that a high variability of the subducting plate geometry occurs for an oceanic lithosphere thickness ranging from 60 to 80 km, while the variability decreases where the oceanic plates are thicker (95 and 110 km). Furthermore, the slab dip strongly depends on the thermal state of the overriding plate, and, in particular, the slab dip decreases with the increase in...

  3. Subduction Controls of Hf and Nd Isotopes in Lavas of the Aleutian Island Arc

    Energy Technology Data Exchange (ETDEWEB)

    Yogodzinski, Gene; Vervoort, Jeffery; Brown, Shaun Tyler; Gerseny, Megan


    The Hf and Nd isotopic compositions of 71 Quaternary lavas collected from locations along the full length of the Aleutian island arc are used to constrain the sources of Aleutian magmas and to provide insight into the geochemical behavior of Nd and Hf and related elements in the Aleutian subduction-magmatic system. Isotopic compositions of Aleutian lavas fall approximately at the center of, and form a trend parallel to, the terrestrial Hf-Nd isotopic array with {var_epsilon}{sub Hf} of +12.0 to +15.5 and {var_epsilon}{sub Nd} of +6.5 to +10.5. Basalts, andesites, and dacites within volcanic centers or in nearby volcanoes generally all have similar isotopic compositions, indicating that there is little measurable effect of crustal or other lithospheric assimilation within the volcanic plumbing systems of Aleutian volcanoes. Hafnium isotopic compositions have a clear pattern of along-arc increase that is continuous from the eastern-most locations near Cold Bay to Piip Seamount in the western-most part of the arc. This pattern is interpreted to reflect a westward decrease in the subducted sediment component present in Aleutian lavas, reflecting progressively lower rates of subduction westward as well as decreasing availability of trench sediment. Binary bulk mixing models (sediment + peridotite) demonstrate that 1-2% of the Hf in Aleutian lavas is derived from subducted sediment, indicating that Hf is mobilized out of the subducted sediment with an efficiency that is similar to that of Sr, Pb and Nd. Low published solubility for Hf and Nd in aqueous subduction fluids lead us to conclude that these elements are mobilized out of the subducted component and transferred to the mantle wedge as bulk sediment or as a silicate melt. Neodymium isotopes also generally increase from east to west, but the pattern is absent in the eastern third of the arc, where the sediment flux is high and increases from east to west, due to the presence of abundant terrigenous sediment in the

  4. Secular Changes in Lithospheric Diamonds from the Archean to the Proterozoic (United States)

    Shirey, S. B.; Richardson, S. H.; Aulbach, S.; Pearson, G.


    Geochronologic study of lithospheric diamonds uses the Sm-Nd and Rb-Sr systems on silicate inclusions of peridotitic (harzbugitic and lherzolitic) and eclogitic parageneses and Re-Os on sulfide inclusions also of peridotitic (pentlendite-rich) and eclogitic (pyrrhotite-rich) parageneses. While sulfide and silicate inclusions rarely occur in the same diamond, they usually occur in the diamond population of each kimberlite. More than 20 kimberlites on 4 cratons have geochronology on both their sulfide and silicate inclusion-bearing diamonds. This global dataset shows interesting systematic differences with age from the Archean to Proterozoic and can be used to test for secular changes in deep mantle processes and geodynamics. With few exceptions, harzburgitic silicate and sulfide inclusions are Paleo- to Mesoarchean and rare in the Proterozoic, in accord with the xenolith record. This suggests that the depletion registered in the harzburgitic diamonds was associated with the igneous processes that created or at least followed closely on the creation of stable cratonic lithosphere. The evidence for relatively shallow final equilibration of many mantle peridotite suites [1,2] indicates that early tectonic forces had enough lateral component to be capable of thickening the lithosphere into the diamond stability field regardless of the initial depletion of the mantle in a plume or a subduction wedge. Eclogitic sulfide inclusions are common from the Archean throughout the Proterozoic. Given the diverse surficial geochemical signatures present in these inclusions, in their diamond hosts, and in the diamond-hosting lithologies, this suggests that subduction-accretion was a major tectonic process operating on Earth at least since the Mesoarchean. The Re/Os of some Archean eclogitic sulfides is lower than Re/Os of Proterozoic eclogitic sulfides which potentially links the creation of Archean sulfides to the creation of more primitive, higher MgO oceanic lithosphere typical of

  5. Melting carbonated epidote eclogites: carbonatites from subducting slabs (United States)

    Poli, Stefano


    Current knowledge on the solidus temperature for carbonated eclogites suggests that carbonatitic liquids should not form from a subducted oceanic lithosphere at sub-arc depth. However, the oceanic crust includes a range of gabbroic rocks, altered on rifts and transforms, with large amounts of anorthite-rich plagioclase forming epidote on metamorphism. Epidote disappearance with pressure depends on the normative anorthite content of the bulk composition; we therefore expect that altered gabbros might display a much wider pressure range where epidote persists, potentially affecting the solidus relationships. A set of experimental data up to 4.6 GPa, and 1000 °C, including new syntheses on mafic eclogites with 36.8 % normative anorthite, is discussed to unravel the effect of variable bulk and volatile compositions in model eclogites, enriched in the normative anorthite component ( An 37 and An 45). Experiments are performed in piston cylinder and multianvil machines. Garnet, clinopyroxene, and coesite form in all syntheses. Lawsonite was found to persist at 3.7 GPa, 750 °C, with both dolomite and magnesite; at 3.8 GPa, 775-800 °C, fluid-saturated conditions, epidote coexists with kyanite, dolomite, and magnesite. The anhydrous assemblage garnet, omphacite, aragonite, and kyanite is found at 4.2 GPa, 850 °C. At 900 °C, a silicate glass of granitoid composition, a carbonatitic precipitate, and Na-carbonate are observed. Precipitates are interpreted as evidence of hydrous carbonatitic liquids at run conditions; these liquids produced are richer in Ca compared to experimental carbonatites from anhydrous experiments, consistently with the dramatic role of H2O in depressing the solidus temperature for CaCO3. The fluid-absent melting of the assemblage epidote + dolomite, enlarged in its pressure stability for An-rich gabbros, is expected to promote the generation of carbonatitic liquids. The subsolidus breakdown of epidote in the presence of carbonates at depths

  6. First results of high-resolution modeling of Cenozoic subduction orogeny in Andes (United States)

    Liu, S.; Sobolev, S. V.; Babeyko, A. Y.; Krueger, F.; Quinteros, J.; Popov, A.


    The Andean Orogeny is the result of the upper-plate crustal shortening during the Cenozoic Nazca plate subduction beneath South America plate. With up to 300 km shortening, the Earth's second highest Altiplano-Puna Plateau was formed with a pronounced N-S oriented deformation diversity. Furthermore, the tectonic shortening in the Southern Andes was much less intensive and started much later. The mechanism of the shortening and the nature of N-S variation of its magnitude remain controversial. The previous studies of the Central Andes suggested that they might be related to the N-S variation in the strength of the lithosphere, friction coupling at slab interface, and are probably influenced by the interaction of the climate and tectonic systems. However, the exact nature of the strength variation was not explored due to the lack of high numerical resolution and 3D numerical models at that time. Here we will employ large-scale subduction models with a high resolution to reveal and quantify the factors controlling the strength of lithospheric structures and their effect on the magnitude of tectonic shortening in the South America plate between 18°-35°S. These high-resolution models are performed by using the highly scalable parallel 3D code LaMEM (Lithosphere and Mantle Evolution Model). This code is based on finite difference staggered grid approach and employs massive linear and non-linear solvers within the PETSc library to complete high-performance MPI-based parallelization in geodynamic modeling. Currently, in addition to benchmark-models we are developing high-resolution (Paleozoic-Cenozoic sediments above the uppermost crust in the Subandean Ranges. Future work will be focused on the origin of different styles of deformation and topography evolution in Altiplano-Puna Plateau and Central-Southern Andes through 3D modeling of large-scale interaction of subducting and overriding plates.

  7. Lithospheric architecture of the South-Western Alps revealed by multiparameter teleseismic full-waveform inversion (United States)

    Beller, S.; Monteiller, V.; Operto, S.; Nolet, G.; Paul, A.; Zhao, L.


    The Western Alps, although being intensively investigated, remains elusive when it comes to determining its lithospheric structure. New inferences on the latter are important for the understanding of processes and mechanisms of orogeny needed to unravel the dynamic evolution of the Alps. This situation led to the deployment of the CIFALPS temporary experiment, conducted to address the lack of seismological data amenable to high-resolution seismic imaging of the crust and the upper mantle. We perform a 3-D isotropic full-waveform inversion (FWI) of nine teleseismic events recorded by the CIFALPS experiment to infer 3-D models of both density and P- and S-wave velocities of the Alpine lithosphere. Here, by FWI is meant the inversion of the full seismograms including phase and amplitude effects within a time window following the first arrival up to a frequency of 0.2 Hz. We show that the application of the FWI at the lithospheric scale is able to generate images of the lithosphere with unprecedented resolution and can furnish a reliable density model of the upper lithosphere. In the shallowest part of the crust, we retrieve the shape of the fast/dense Ivrea body anomaly and detect the low velocities of the Po and SE France sedimentary basins. The geometry of the Ivrea body as revealed by our density model is consistent with the Bouguer anomaly. A sharp Moho transition is followed from the external part (30 km depth) to the internal part of the Alps (70-80 km depth), giving clear evidence of a continental subduction event during the formation of the Alpine Belt. A low-velocity zone in the lower lithosphere of the S-wave velocity model supports the hypothesis of a slab detachment in the western part of the Alps that is followed by asthenospheric upwelling. The application of FWI to teleseismic data helps to fill the gap of resolution between traditional imaging techniques, and enables integrated interpretations of both upper and lower lithospheric structures.

  8. The lithospheric mantle below southern West Greenland

    DEFF Research Database (Denmark)

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


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

  9. Modeling mantle circulation and density distributions in subduction zones: Implications for seismic studies (United States)

    Kincaid, C. R.; Druken, K. A.; Griffiths, R. W.; Long, M. D.; Behn, M. D.; Hirth, G.


    Subduction of ocean lithosphere drives plate tectonics, large-scale mantle circulation and thermal-chemical recycling processes through arcs. Seismologists have made important advances in our ability to map circulation patterns in subduction zones though anisotropy data/methods and in providing detailed images of mantle density fields. Increasingly, seismic and geodynamic disciplines are combining to extend our understanding of time varying subduction processes and associated vertical mass and energy fluxes. We use laboratory experiments to characterize three-dimensional flow fields in convergent margins for a range in plate forcing conditions and background, buoyancy-driven flow scenarios. Results reveal basic patterns in circulation, buoyant flow morphologies and density distributions that have implications for reconciling seismic data with mantle convection models. Models utilize a glucose working fluid with a temperature dependent viscosity to represent the upper 2000km of the mantle. Subducting lithosphere is modeled with a Phenolic plate and back-arc extension is produced using Mylar sheets. We recreate basic subduction styles observed in previous dynamic subduction models using simplified, kinematic forcing. Slab plate segments, driven by hydraulic pistons, move with various combinations of downdip, rollback and steepening motion. Neutral density finite strain markers are distributed throughout the fluid and used as proxies for tracking the evolution of olivine alignment through space and time in the evolving flow fields. Particle image velocimetry methods are also used to track time varying 3D velocity fields for use in directly calculating anisotropy patterns. Results show that complex plate motions (rollback, steepening, back-arc extension) in convergent margins produce relatively simple anisotropy patterns (e.g., trench-normal alignments) and underscore the importance of initial strain marker orientations on alignment patterns in the wedge. Results also

  10. Vertically Integrated Rheology of Deforming Oceanic Lithosphere (United States)

    Mishra, J. K.; Gordon, R. G.


    The tectonics of the oceans have traditionally been modeled in terms of rigid plates interacting at narrow boundaries. The now well-documented existence of diffuse oceanic plate boundaries, across which relative motion is distributed over hundreds to thousands of kilometers, demonstrates the need for a different approach to understanding the tectonics and geodynamics of a substantial fraction of oceanic lithosphere. A model that has usefully been applied to diffuse zones of continental deformation is that of a thin viscous sheet of fluid obeying a power-law rheology. The model has few adjustable parameters, typically a power-law exponent, n, and the Argand number [England & McKenzie, 1982], which is a measure of the size of buoyancy forces caused by the deformation, and which can be neglected for deformation of oceanic lithosphere. In prior investigations of a thin sheet of power-law fluid for continental regions, most studies have found that the most appropriate power-law exponent is ≈3 [e.g., England & Molnar 1991, 1997], but a value as large as ≈10 has been recently suggested by Dayem et al. [2009]. Because the rheology of oceanic lithosphere differs significantly from that of continental lithosphere, the most appropriate exponent may be larger than 3, and should in some sense be an appropriately weighted average between the properties of the upper lithosphere, which deforms brittlely and semi-brittlely, and for which the power-law exponent is n → ∞, and the lower lithosphere, which deforms by dislocation glide [Goetze 1978; Evans & Goetze 1979; Ratteron et al. 2003; Dayem et al. 2009; Mei et al. 2010], which obeys an exponential law, and by dislocation creep for which n≈3 [Sonder & England, 1986]. To estimate the appropriate power-law exponent consistent with laboratory experiments we determine strain rate as a function of applied end load on the lithosphere for various ages of lithosphere. We find that a power-law fluid well approximates the

  11. Building a Subduction Zone Observatory (United States)

    Gomberg, Joan S.; Bodin, Paul; Bourgeois, Jody; Cashman, Susan; Cowan, Darrel; Creager, Kenneth C.; Crowell, Brendan; Duvall, Alison; Frankel, Arthur; Gonzalez, Frank; Houston, Heidi; Johnson, Paul; Kelsey, Harvey; Miller, Una; Roland, Emily C.; Schmidt, David; Staisch, Lydia; Vidale, John; Wilcock, William; Wirth, Erin


    Subduction zones contain many of Earth’s most remarkable geologic structures, from the deepest oceanic trenches to glacier-covered mountains and steaming volcanoes. These environments formed through spectacular events: Nature’s largest earthquakes, tsunamis, and volcanic eruptions are born here.

  12. POLARIS: Portable Observatories for Lithospheric Analysis and Research Investigating Seismicity - New Opportunities in Canada (United States)

    Cassidy, J. F.; Adams, J.; Asudeh, I.; Atkinson, G.; Bostock, M. G.; Eaton, D. W.; Ferguson, I. J.; Snyder, D.; Unsworth, M.


    POLARIS is a multi-institutional 10M project recently funded by the Canadian Foundation for Innovation, Provincial Governments and Universities, and private industry across Canada. This project is providing new, state-of-the-art portable geophysical observatories for research into lithospheric structure, continental dynamics, and earthquake hazards in Canada. When completed, POLARIS infrastructure will comprise 90 three-component broadband seismographs and 30 magnetotelluric (MT) mobile field systems and complementary satellite telemetry data acquistion. Over the initial four year installation of this project, the seismograph network will be deployed as three subarrays of 30 instruments each. The MT instruments will be used in shorter-term deployments at each of the three seismic subarrays for lithospheric imaging, and continuous recording at selected elements for deep-mantle imaging. The initial scientific objectives include: 1) three-dimensional detailed mapping of the asthenosphere and upper mantle of the Slave Province in Canada's north to assist the emerging diamond industry; 2) mapping lithospheric structure and earthquake hazards in the heavily populated area of southern Ontario (identifying zones of crustal weakness, obtaining accurate earthquake parameters, and ground motion attenuation studies). 3) mapping the structure and earthquake hazards over the Cascadia subduction zone in southwest British Columbia (subducting oceanic plate, site-response in urban areas, identifying active crustal faults, and seismic attenuation studies). The latter array will provide new opportunities into research involving slab seismicity, including focal mechanisms, attenuation studies, and detailed structural studies. Further research initiatives that will be possible include: testing Rapid Warning Systems for ground shaking in the urban areas of Canada; developing new fine-scale imaging techniques using the scattered wavefield; and investigating geomagnetically induced

  13. Evidences for recent plume-induced subduction, microplates and localized lateral plate motions on Venus (United States)

    Davaille, Anne; Smrekar, Suzanne


    Using laboratory experiments and theoretical modeling, we recently showed that plumes could induce roll-back subduction around large coronae. When a hot plume rises under a brittle and visco-elasto-plastic skin/lithosphere, the latter undergoes a flexural deformation which puts it under tension. Radial cracks and rifting of the skin then develop, sometimes using pre-existing weaknesses. Plume material upwells through the cracks (because it is more buoyant) and spreads as a axisymmetric gravity current above the broken denser skin. The latter bends and sinks under the combined force of its own weight and that of the plume gravity current. However, due to the brittle character of the upper part of the experimental lithosphere, it cannot deform viscously to accomodate the sinking motions. Instead, the plate continues to tear, as a sheet of paper would do upon intrusion. Several slabs are therefore produced, associated with trenches localized along partial circles on the plume, and strong roll-back is always observed. Depending on the lithospheric strength, roll-back can continue and triggers a complete resurfacing, or it stops when the plume stops spreading. Two types of microplates are also observed. First, the upwelling plume material creates a set of new plates interior to the trench segments. These plates move rapidly and expand through time, but do not subduct.. In a few cases, we also observe additional microplates exterior to the trenches. This happens when the subducting plate contains preexisting heterogeneities (e.g. fractures) and the subducted slab is massive enough for slab pull to become efficient and induce horizontal plate motions. Scalings derived from the experiments suggest that Venus lithosphere is soft enough to undergo such a regime. And indeed, at least two candidates can be identified on Venus, where plume-induced subduction could have operated. (1) Artemis Coronae is the largest (2300 km across) coronae on Venus and is bounded over 270° of

  14. Westernmost Mediterranean Mantle Tomography: Slab Rollback and Delaminated Atlas Lithosphere (United States)

    Bezada, M. J.; Humphreys, E.


    We present a new velocity model for the upper mantle in the westernmost Mediterranean including the Iberian Peninsula and northern Morocco. Our imaging improves over previous efforts by taking advantage of the data generated by the PICASSO, IberArray, TopoMed and connected seismograph deployments and by using a new methodology that includes finite-frequency effects and iterative ray tracing, utilizes local earthquakes in addition to teleseismic events and includes constraints from surface wave analyses. We image a subducted slab as a high velocity anomaly located under the Alboran Sea and southern Spain that extends to the bottom of the transition zone. The anomaly has an arcuate shape at most depths and reaches the surface beneath Gibraltar but not under southern Spain. The N-S oriented Gibraltar and E-W oriented southern Spain segments of the slab appear to be separated by a vertical tear or "slab gap". Under the Atlas Mountains in northern Morocco we image low velocities to depths of over 200 km and a high-velocity body at depths of 300-450 km beneath the Middle Atlas, which we tentatively interpret as delaminated lithosphere.

  15. Thermal buoyancy on Venus - Underthrusting vs subduction (United States)

    Burt, Jeffrey D.; Head, James W.


    The thermal and buoyancy consequences of the subduction endmember are modeled in an attempt to evaluate the conditions distinguishing underthrusting and subduction. Thermal changes in slabs subducting into the Venusian mantle with a range of initial geotherms are used to predict density changes and, thus, slab buoyancy. Based on a model for subduction-induced mantle flow, it is then argued that the angle of the slab dip helps differentiate between underthrusting and subduction. Mantle flow applies torques to the slab which, in combination with torques due to slab buoyancy, act to change the angle of slab dip.

  16. Record of mid-Archaean subduction from metamorphism in the Barberton terrain, South Africa. (United States)

    Moyen, Jean-François; Stevens, Gary; Kisters, Alexander


    Although plate tectonics is the central geological process of the modern Earth, its form and existence during the Archaean era (4.0-2.5 Gyr ago) are disputed. The existence of subduction during this time is particularly controversial because characteristic subduction-related mineral assemblages, typically documenting apparent geothermal gradients of 15 degrees C km(-1) or less, have not yet been recorded from in situ Archaean rocks (the lowest recorded apparent geothermal gradients are greater than 25 degrees C km(-1)). Despite this absence from the rock record, low Archaean geothermal gradients are suggested by eclogitic nodules in kimberlites and circumstantial evidence for subduction processes, including possible accretion-related structures, has been reported in Archaean terrains. The lack of spatially and temporally well-constrained high-pressure, low-temperature metamorphism continues, however, to cast doubt on the relevance of subduction-driven tectonics during the first 1.5 Gyr of the Earth's history. Here we report garnet-albite-bearing mineral assemblages that record pressures of 1.2-1.5 GPa at temperatures of 600-650 degrees C from supracrustal amphibolites from the mid-Archaean Barberton granitoid-greenstone terrain. These conditions point to apparent geothermal gradients of 12-15 degrees C-similar to those found in recent subduction zones-that coincided with the main phase of terrane accretion in the structurally overlying Barberton greenstone belt. These high-pressure, low-temperature conditions represent metamorphic evidence for cold and strong lithosphere, as well as subduction-driven tectonic processes, during the evolution of the early Earth.

  17. Fluid and deformation regime of an advancing subduction system at Marlborough, New Zealand. (United States)

    Wannamaker, Philip E; Caldwell, T Grant; Jiracek, George R; Maris, Virginie; Hill, Graham J; Ogawa, Yasuo; Bibby, Hugh M; Bennie, Stewart L; Heise, Wiebke


    Newly forming subduction zones on Earth can provide insights into the evolution of major fault zone geometries from shallow levels to deep in the lithosphere and into the role of fluids in element transport and in promoting rock failure by several modes. The transpressional subduction regime of New Zealand, which is advancing laterally to the southwest below the Marlborough strike-slip fault system of the northern South Island, is an ideal setting in which to investigate these processes. Here we acquired a dense, high-quality transect of magnetotelluric soundings across the system, yielding an electrical resistivity cross-section to depths beyond 100 km. Our data imply three distinct processes connecting fluid generation along the upper mantle plate interface to rock deformation in the crust as the subduction zone develops. Massive fluid release just inland of the trench induces fault-fracture meshes through the crust above that undoubtedly weaken it as regional shear initiates. Narrow strike-slip faults in the shallow brittle regime of interior Marlborough diffuse in width upon entering the deeper ductile domain aided by fluids and do not project as narrow deformation zones. Deep subduction-generated fluids rise from 100 km or more and invade upper crustal seismogenic zones that have exhibited historic great earthquakes on high-angle thrusts that are poorly oriented for failure under dry conditions. The fluid-deformation connections described in our work emphasize the need to include metamorphic and fluid transport processes in geodynamic models.

  18. The viscosity of Earth's lower mantle inferred from sinking speed of subducted lithosphere

    NARCIS (Netherlands)

    Čížková, H.; van den Berg, A.P.; Spakman, W.; Matyska, C.


    The viscosity of the mantle is indispensable for predicting Earth's mechanical behavior at scales ranging from deep mantle material flow to local stress accumulation in earthquakes zones. But, mantle viscosity is not well determined. For the lower mantle, particularly, only few constraints result

  19. Late Miocene Pacific plate kinematic change explained with coupled global models of mantle and lithosphere dynamics (United States)

    Stotz, I. L.; Iaffaldano, G.; Davies, D. R.


    The timing and magnitude of a Pacific plate motion change within the past 10 Ma remains enigmatic, due to the noise associated with finite-rotation data. Nonetheless, it has been hypothesized that this change was driven by the arrival of the Ontong Java Plateau (OJP) at the Melanesian arc and the consequent subduction polarity reversal. The uncertainties associated with the timing of this event, however, make it difficult to quantitatively demonstrate a dynamical association. Here, we first reconstruct the Pacific plate's absolute motion since the mid-Miocene (15 Ma), at high-temporal resolution, building on previous efforts to mitigate the impact of finite-rotation data noise. We find that the largest change in Pacific plate-motion direction occurred between 10 and 5 Ma, with the plate rotating clockwise. We subsequently develop and use coupled global numerical models of the mantle/lithosphere system to test hypotheses on the dynamics driving this change. These indicate that the arrival of the OJP at the Melanesian arc, between 10 and 5 Ma, followed by a subduction polarity reversal that marked the initiation of subduction of the Australian plate underneath the Pacific realm, were the key drivers of this kinematic change.

  20. Seismic imaging of a mid-lithospheric discontinuity beneath Ontong Java Plateau (United States)

    Tharimena, Saikiran; Rychert, Catherine A.; Harmon, Nicholas


    Ontong Java Plateau (OJP) is a huge, completely submerged volcanic edifice that is hypothesized to have formed during large plume melting events ∼90 and 120 My ago. It is currently resisting subduction into the North Solomon trench. The size and buoyancy of the plateau along with its history of plume melting and current interaction with a subduction zone are all similar to the characteristics and hypothesized mechanisms of continent formation. However, the plateau is remote, and enigmatic, and its proto-continent potential is debated. We use SS precursors to image seismic discontinuity structure beneath Ontong Java Plateau. We image a velocity increase with depth at 28 ± 4 km consistent with the Moho. In addition, we image velocity decreases at 80 ± 5 km and 282 ± 7 km depth. Discontinuities at 60-100 km depth are frequently observed both beneath the oceans and the continents. However, the discontinuity at 282 km is anomalous in comparison to surrounding oceanic regions; in the context of previous results it may suggest a thick viscous root beneath OJP. If such a root exists, then the discontinuity at 80 km bears some similarity to the mid-lithospheric discontinuities (MLDs) observed beneath continents. One possibility is that plume melting events, similar to that which formed OJP, may cause discontinuities in the MLD depth range. Plume-plate interaction could be a mechanism for MLD formation in some continents in the Archean prior to the onset of subduction.

  1. Mantle hydration and Cl-rich fluids in the subduction forearc (United States)

    Reynard, Bruno


    In the forearc region, aqueous fluids are released from the subducting slab at a rate depending on its thermal state. Escaping fluids tend to rise vertically unless they meet permeability barriers such as the deformed plate interface or the Moho of the overriding plate. Channeling of fluids along the plate interface and Moho may result in fluid overpressure in the oceanic crust, precipitation of quartz from fluids, and low Poisson ratio areas associated with tremors. Above the subducting plate, the forearc mantle wedge is the place of intense reactions between dehydration fluids from the subducting slab and ultramafic rocks leading to extensive serpentinization. The plate interface is mechanically decoupled, most likely in relation to serpentinization, thereby isolating the forearc mantle wedge from convection as a cold, potentially serpentinized and buoyant, body. Geophysical studies are unique probes to the interactions between fluids and rocks in the forearc mantle, and experimental constrains on rock properties allow inferring fluid migration and fluid-rock reactions from geophysical data. Seismic velocities reveal a high degree of serpentinization of the forearc mantle in hot subduction zones, and little serpentinization in the coldest subduction zones because the warmer the subduction zone, the higher the amount of water released by dehydration of hydrothermally altered oceanic lithosphere. Interpretation of seismic data from petrophysical constrain is limited by complex effects due to anisotropy that needs to be assessed both in the analysis and interpretation of seismic data. Electrical conductivity increases with increasing fluid content and temperature of the subduction. However, the forearc mantle of Northern Cascadia, the hottest subduction zone where extensive serpentinization was first demonstrated, shows only modest electrical conductivity. Electrical conductivity may vary not only with the thermal state of the subduction zone, but also with time for

  2. Finite-frequency P-wave tomography of the Western Canada Sedimentary Basin: Implications for the lithospheric evolution in Western Laurentia (United States)

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


    The lithosphere beneath the Western Canada Sedimentary Basin has potentially undergone Precambrian subduction and collisional orogenesis, resulting in a complex network of crustal domains. To improve the understanding of its evolutionary history, we combine data from the USArray and three regional networks to invert for P-wave velocities of the upper mantle using finite-frequency tomography. Our model reveals distinct, vertically continuous high (> 1%) velocity perturbations at depths above 200 km beneath the Precambrian Buffalo Head Terrane, Hearne craton and Medicine Hat Block, which sharply contrasts with those beneath the Canadian Rockies (lithosphere is substantially thinner beneath the adjacent Buffalo Head Terrane (160 km) and Medicine Hat Block (200 km). These findings are consistent with earlier theories of tectonic assembly in this region, which featured distinct Archean and Proterozoic plate convergences between the Hearne craton and its neighboring domains. The highly variable, bimodally distributed craton thicknesses may also reflect different lithospheric destruction processes beneath the western margin of Laurentia.

  3. Water in the Cratonic Mantle Lithosphere (United States)

    Peslier, A. H.


    The fact that Archean and Proterozoic cratons are underlain by the thickest (>200 km) lithosphere on Earth has always puzzled scientists because the dynamic convection of the surrounding asthenosphere would be expected to delaminate and erode these mantle lithospheric "keels" over time. Although density and temperature of the cratonic lithosphere certainly play a role in its strength and longevity, the role of water has only been recently addressed with data on actual mantle samples. Water in mantle lithologies (primarily peridotites and pyroxenites) is mainly stored in nominally anhydrous minerals (olivine, pyroxene, garnet) where it is incorporated as hydrogen bonded to structural oxygen in lattice defects. The property of hydrolytic weakening of olivine [4] has generated the hypothesis that olivine, the main mineral of the upper mantle, may be dehydrated in cratonic mantle lithospheres, contributing to its strength. This presentation will review the distribution of water concentrations in four cratonic lithospheres. The distribution of water contents in olivine from peridotite xenoliths found in kimberlites is different in each craton (Figure 1). The range of water contents of olivine, pyroxene and garnet at each xenolith location appears linked to local metasomatic events, some of which occurred later then the Archean and Proterozoic when these peridotites initially formed via melting. Although the low olivine water contents ( 6 GPa at the base of the Kaapvaal cratonic lithosphere may contribute to its strength, and prevent its delamination, the wide range of those from Siberian xenoliths is not compatible with providing a high enough viscosity contrast with the asthenophere. The water content in olivine inclusions from Siberian diamonds, on the other hand, have systematically low water contents (water contents. The olivine inclusions, however, may have been protected from metasomatism by their host diamond and record the overall low olivine water content of

  4. Modeling Geodynamic Mobility of Anisotropic Lithosphere (United States)

    Perry-Houts, J.; Karlstrom, L.


    The lithosphere is often idealized as a linear, or plastic layer overlying a Newtonian half-space. This approach has led to many insights into lithospheric foundering that include Rayligh-Taylor drips, slab-style delaminations, and small scale convection in the asthenosphere. More recent work has begun to quantify the effect of anisotropic lithosphere viscosity on these same phenomena. Anisotropic viscosity may come about due to stratigraphic deposition in the upper crust, dike/sill emplacement in the mid crust, or volcanic underplating at the Moho related to arcs or plumes. Anisotropic viscosity is also observed in the mantle, due to preferential orientation of olivine grains during flow. Here we extend the work of Lev & Hager (2008) on modeling anisotropic lithospheric foundering to investigate the effects of anisotropic regions which vary in size, magnitude, and orientation. We have extended Aspect, a modern geodynamic finite element code with a large developer and user base, to model exotic constitutive laws with an arbitrary fourth order tensor in place of the viscosity term. We further implement a material model to represent a transverse isotropic medium, such as is expected in a layered, or fractured lithosphere. We have validated our implementation against previous results, and analytic solutions, reproducing the result that horizontally oriented anisotropy tends to inhibit drips, and produce longer-wavelength instabilities. We expect that increased lateral extent of anisotropic regions will exaggerate this effect, to a limit at which the effect will plateau. Varying lithosphere thickness, and mantle anisotropy anisotropy may produce similar behavior. The implications of this effect are significant to lithospheric foundering beneath arcs and hotspots, possibly influencing the recycling of eclogite, production of silicic magmas, and dynamic topography.

  5. Multi-dimensional Crustal and Lithospheric Structure of the Atlas Mountains of Morocco by Magnetotelluric Imaging (United States)

    Kiyan, D.; Jones, A. G.; Fullea, J.; Ledo, J.; Siniscalchi, A.; Romano, G.


    The PICASSO (Program to Investigate Convective Alboran Sea System Overturn) project and the concomitant TopoMed (Plate re-organization in the western Mediterranean: Lithospheric causes and topographic consequences - an ESF EUROSCORES TOPO-EUROPE project) project were designed to collect high resolution, multi-disciplinary lithospheric scale data in order to understand the tectonic evolution and lithospheric structure of the western Mediterranean. The over-arching objectives of the magnetotelluric (MT) component of the projects are (i) to provide new electrical conductivity constraints on the crustal and lithospheric structure of the Atlas Mountains, and (ii) to test the hypotheses for explaining the purported lithospheric cavity beneath the Middle and High Atlas inferred from potential-field lithospheric modeling. We present the results of an MT experiment we carried out in Morocco along two profiles: an approximately N-S oriented profile crossing the Middle Atlas, the High Atlas and the eastern Anti-Atlas to the east (called the MEK profile, for Meknes) and NE-SW oriented profile through western High Atlas to the west (called the MAR profile, for Marrakech). Our results are derived from three-dimensional (3-D) MT inversion of the MT data set employing the parallel version of Modular system for Electromagnetic inversion (ModEM) code. The distinct conductivity differences between the Middle-High Atlas (conductive) and the Anti-Atlas (resistive) correlates with the South Atlas Front fault, the depth extent of which appears to be limited to the uppermost mantle (approx. 60 km). In all inverse solutions, the crust and the upper mantle show resistive signatures (approx. 1,000 Ωm) beneath the Anti-Atlas, which is the part of stable West African Craton. Partial melt and/or exotic fluids enriched in volatiles produced by the melt can account for the high middle to lower crustal and uppermost mantle conductivity in the Folded Middle Atlas, the High Moulouya Plain and the

  6. Defining Incipient Subduction by Detecting Serpentenised Mantle in the Regional Magnetic Field (United States)

    Pires, Rui; Clark, Stuart; Reis, Rui


    Keywords: Subduction initiation, Incipient Subduction, Active Margins, Southeast Asia, Mantle wedge The mechanisms of subduction initiation are poorly understood. One idea is to look for incipient subduction zones in the present day and see what features are common in these zones. However, incipient subduction zones are very difficult to detect and debate surrounds particular cases as to whether they qualify as incipient or not. In the analysis conducted in this work, we use the signal of the presence of a mantle wedge in the magnetic anomaly field as an indicator of incipient subduction. Each subduction zone exhibits variations in the particular responses of the system, such as slab-dip angle, maximum earthquake depths and volcanism to various parameters. So far, attempts to reduce the system to a dominate controlling parameter have failed, probably as a result of the limited number of cases and the large variety of controlling parameters. Parameters such as down-going and overriding plate morphology and velocity, mantle flow, the presence of plumes or not, sediment transport into the trench are a few of the parameters that have been studied in the literature. However, one of the characteristics associated with a subduction zones is the presence of a mantelic wedge as a result of the partial melt of the subducting plate and the development of a mantle wedge between the subducting plate and the overriding plate. The wedge is characterised by the presence of water (coming from sediments in the down-going plate) as well as lower temperatures (because the wedge is between two relatively cold lithospheres). As a results a serpentinized mantle wedge is formed that contains hydrous minerals, of which magnetite is an example, that alter the composition and properties of this region. According to Blakely (2005), this region exhibits both higher magnetic susceptibility and lower densities than the surrounding medium. We analysed five active margin boundaries located

  7. Flexural modeling of the elastic lithosphere at an ocean trench: A parameter sensitivity analysis using analytical solutions (United States)

    Contreras-Reyes, Eduardo; Garay, Jeremías


    The outer rise is a topographic bulge seaward of the trench at a subduction zone that is caused by bending and flexure of the oceanic lithosphere as subduction commences. The classic model of the flexure of oceanic lithosphere w (x) is a hydrostatic restoring force acting upon an elastic plate at the trench axis. The governing parameters are elastic thickness Te, shear force V0, and bending moment M0. V0 and M0 are unknown variables that are typically replaced by other quantities such as the height of the fore-bulge, wb, and the half-width of the fore-bulge, (xb - xo). However, this method is difficult to implement with the presence of excessive topographic noise around the bulge of the outer rise. Here, we present an alternative method to the classic model, in which lithospheric flexure w (x) is a function of the flexure at the trench axis w0, the initial dip angle of subduction β0, and the elastic thickness Te. In this investigation, we apply a sensitivity analysis to both methods in order to determine the impact of the differing parameters on the solution, w (x). The parametric sensitivity analysis suggests that stable solutions for the alternative approach requires relatively low β0 values (<15°), which are consistent with the initial dip angles observed in seismic velocity-depth models across convergent margins worldwide. The predicted flexure for both methods are compared with observed bathymetric profiles across the Izu-Mariana trench, where the old and cold Pacific plate is characterized by a pronounced outer rise bulge. The alternative method is a more suitable approach, assuming that accurate geometric information at the trench axis (i.e., w0 and β0) is available.

  8. The lithospheric structure beneath southeast Tibet revealed by P and S receiver functions (United States)

    Yang, Haiyan; Peng, Hengchu; Hu, Jiafu


    Yunnan is located at the margin of southeast Tibet, where dramatic tectonic activities occur. In this study, we calculated the P and S receiver functions by the deconvolution of three-component seismic data from 48 permanent broad-band stations deployed in Yunnan region. In order to improve signal-noise ratios of the receiver functions, we move-out corrected receiver functions to a reference epicentral distance of 67°, and then stacked them to one trace for each station. Finally, the stacked traces were converted to the depth domain to obtain the crustal and lithospheric thicknesses beneath each station. In southwestern Yunnan (at the west side of the Jinshajiang-Red River Fault), the crustal thicknesses from the P-wave receiver functions (PRFs) and from the S-wave receiver functions (SRFs) are in the ranges of 30-36 km, and of 33-39 km, respectively. But in northwestern Yunnan, the crustal thicknesses from PRFs and SRFs are from 66 to 69 km and from 63 to 66 km, respectively. Our results show that the crustal thicknesses in Yunnan from PRFs and SRFs are consistent, with a maximum deviation of 3 km; and increase gradually from ∼30 km in the south to ∼68 km in the northwest. Besides, the lithospheric thickness from PRFs is also similar to that from SRFs, with the largest difference of 15-20 km in southeastern Yunnan. At the west side of the Jinshajiang-Red River Fault in western Yunnan, it is only about 80-100 km, and increases to 140-150 km regionally in northern and southeastern Yunnan. The thinned lithosphere extends eastward from western Yunnan to eastern Yunnan. We attribute the thinned lithosphere to the upwelling of hot upper mantle materials associated with the eastward subduction of the Indian plate.

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


    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

  10. Sedimentary halogens and noble gases within Western Antarctic xenoliths: Implications of extensive volatile recycling to the sub continental lithospheric mantle (United States)

    Broadley, Michael W.; Ballentine, Chris J.; Chavrit, Déborah; Dallai, Luigi; Burgess, Ray


    Recycling of marine volatiles back into the mantle at subduction zones has a profound, yet poorly constrained impact on the geochemical evolution of the Earth's mantle. Here we present a combined noble gas and halogen study on mantle xenoliths from the Western Antarctic Rift System (WARS) to better understand the flux of subducted volatiles to the sub continental lithospheric mantle (SCLM) and assess the impact this has on mantle chemistry. The xenoliths are extremely enriched in the heavy halogens (Br and I), with I concentrations up to 1 ppm and maximum measured I/Cl ratios (85.2 × 10-3) being ∼2000 times greater than mid ocean ridge basalts (MORB). The Br/Cl and I/Cl ratios of the xenoliths span a range from MORB-like ratios to values similar to marine pore fluids and serpentinites, whilst the 84Kr/36Ar and 130Xe/36Ar ratios range from modern atmosphere to oceanic sediments. This indicates that marine derived volatiles have been incorporated into the SCLM during an episode of subduction related metasomatism. Helium isotopic analysis of the xenoliths show average 3He/4He ratios of 7.5 ± 0.5 RA (where RA is the 3He/4He ratio of air = 1.39 × 10-6), similar to that of MORB. The 3He/4He ratios within the xenoliths are higher than expected for the xenoliths originating from the SCLM which has been extensively modified by the addition of subducted volatiles, indicating that the SCLM beneath the WARS must have seen a secondary alteration from the infiltration and rise of asthenospheric fluids/melts as a consequence of rifting and lithospheric thinning. Noble gases and halogens within these xenoliths have recorded past episodes of volatile interaction within the SCLM and can be used to reconstruct a tectonic history of the WARS. Marine halogen and noble gas signatures within the SCLM xenoliths provide evidence for the introduction and retention of recycled volatiles within the SCLM by subduction related metasomatism, signifying that not all volatiles that survive

  11. The geomorphic record of deep lithospheric deformation (United States)

    Clark, M. K.


    Where deformation of the upper crust is minimal or absent, a young history of topographic change can be related to active or recent processes occurring in the lower crust and mantle lithosphere. Geomorphic systems, fluvial systems in particular, are sensitive indicators of topographic change manifested by the erosional/depositional record, drainage pattern perturbations and landscape form. Evolution of landscape form over geologically relevant timescales (Myr) is commonly derived from low-temperature (form. Examples from the eastern Tibetan Plateau and the Sierra Nevada, California illustrate how geomorphic evolution of mountain ranges and plateaus may be related to lower crustal flow and mantle lithosphere removal. In these cases, a combination of multi-disciplinary data is used to delineate a holistic view of lithospheric evolution from the surface to the upper mantle that can be used to quantify rates of deformation and improve our understanding of deformation mechanisms. Future directions in linking surface and deep lithospheric processes will be aided by new thermochronometric techniques and further development of quantitative laws that govern surface processes, which will allow us to better understand landscape response to tectonic forcing. In turn, these advances may be applied for a more complete understanding of lithospheric dynamics.

  12. Crustal magmatism and lithospheric geothermal state of western North America and their implications for a magnetic mantle (United States)

    Wang, Jian; Li, Chun-Feng


    The western North American lithosphere experienced extensive magmatism and large-scale crustal deformation due to the interactions between the Farallon and North American plates. To further understand such subduction-related dynamic processes, we characterize crustal structure, magmatism and lithospheric thermal state of western North America based on various data processing and interpretation of gravimetric, magnetic and surface heat flow data. A fractal exponent of 2.5 for the 3D magnetization model is used in the Curie-point depth inversion. Curie depths are mostly small to the north of the Yellowstone-Snake River Plain hotspot track, including the Steens Mountain and McDermitt caldera that are the incipient eruption locations of the Columbia River Basalts and Yellowstone hotspot track. To the south of the Yellowstone hotspot track, larger Curie depths are found in the Great Basin. The distinct Curie depths across the Yellowstone-Snake River Plain hotspot track can be attributed to subduction-related magmatism induced by edge flow around fractured slabs. Curie depths confirm that the Great Valley ophiolite is underlain by the Sierra Nevada batholith, which can extend further west to the California Coast Range. The Curie depths, thermal lithospheric thickness and surface heat flow together define the western edge of the North American craton near the Roberts Mountains Thrust (RMT). To the east of the RMT, large Curie depths, large thermal lithospheric thickness, and low thermal gradient are found. From the differences between Curie-point and Moho depth, we argue that the uppermost mantle in the oceanic region is serpentinized. The low temperature gradients beneath the eastern Great Basin, Montana and Wyoming permit magnetic uppermost mantle, either by serpentinization/metasomatism or in-situ magnetization, which can contribute to long-wavelength and low-amplitude magnetic anomalies and thereby large Curie-point depths.

  13. Impact of Mantle Wind on Subducting Plate Geometry and Interplate Pressure: Insights From Physical Modelling. (United States)

    Boutelier, D.; Cruden, A. R.


    New physical models of subduction investigate the impact of large-scale mantle flow on the structure of the subducted slab and deformation of the downgoing and overriding plates. The experiments comprise two lithospheric plates made of highly filled silicone polymer resting on a model asthenosphere of low viscosity transparent silicone polymer. Subduction is driven by a piston that pushes the subducting plate at constant rate, a slab-pull force due to the relative density of the slab, and a basal drag force exerted by flow in the model asthenosphere. Large-scale mantle flow is imposed by a second piston moving at constant rate in a tunnel at the bottom of the experiment tank. Passive markers in the mantle track the evolution of flow during the experiment. Slab structure is recorded by side pictures of the experiment while horizontal deformation is studied via passive marker grids on top of both plates. The initial mantle flow direction beneath the overriding plate can be sub-horizontal or sub-vertical. In both cases, as the slab penetrates the mantle, the mantle flow pattern changes to accommodate the subducting high viscosity lithosphere. As the slab continues to descend, the imposed flow produces either over- or under-pressure on the lower surface of the slab depending on the initial mantle flow pattern (sub-horizontal or sub-vertical respectively). Over-pressure imposed on the slab lower surface promotes shallow dip subduction while under-pressure tends to steepen the slab. These effects resemble those observed in previous experiments when the overriding plate moves horizontally with respect to a static asthenosphere. Our experiments also demonstrate that a strong vertical drag force (due to relatively fast downward mantle flow) exerted on the slab results in a decrease in strain rate in both the downgoing and overriding plates, suggesting a decrease in interplate pressure. Furthermore, with an increase in drag force deformation in the downgoing plate can switch

  14. Structure of the Lithosphere-Asthenosphere Boundary Onshore and Offshore the California Continental Margin from Three-Dimensional Seismic Anisotropy (United States)

    Gomez, C. D.; Escobar, L., Sr.; Rathnayaka, S.; Weeraratne, D. S.; Kohler, M. D.


    The California continental margin, a major transform plate boundary in continental North America, is the locus of complex tectonic stress fields that are important in interpreting both remnant and ongoing deformational strain. Ancient subduction of the East Pacific Rise spreading center, the rotation and translation of tectonic blocks and inception of the San Andreas fault all contribute to the dynamic stress fields located both onshore and offshore southern California. Data obtained by the ALBACORE (Asthenospheric and Lithospheric Broadband Architecture from the California Offshore Region Experiment) and the CISN (California Integrated Seismic Network) seismic array are analyzed for azimuthal anisotropy of Rayleigh waves from 80 teleseismic events at periods 16 - 78 s. Here we invert Rayleigh wave data for shear wave velocity structure and three-dimensional seismic anisotropy in the thee regions designated within the continental margin including the continent, seafloor and California Borderlands. Preliminary results show that seismic anisotropy is resolved in multiple layers and can be used to determine the lithosphere-asthenosphere boundary (LAB) in offshore and continental regions. The oldest seafloor in our study at age 25-35 Ma indicates that the anisotropic transition across the LAB occurs at 73 km +/- 25 km with the lithospheric fast direction oriented WNW-ESE, consistent with current Pacific plate motion direction. The continent region west of the San Andreas indicates similar WNW-ESE anisotropy and LAB depth. Regions east of the San Andreas fault indicate NW-SE anisotropy transitioning to a N-S alignment at 80 km depth north of the Garlock fault. The youngest seafloor (15 - 25 Ma) and outer Borderlands indicate a more complex three layer fabric where shallow lithospheric NE-SW fast directions are perpendicular with ancient Farallon subduction arc, a mid-layer with E-W fast directions are perpendicular to remnant fossil fabric, and the deepest layer

  15. Surface and tectonic consequences of plume-lithosphere interactions in continents: insights from modelling approach based on realistic representation of lithosphere. (United States)

    Burov, E.; Guillou-Frottier, L.


    Plume-Lithosphere Interactions (PLI) are conditioned by plume dynamics but also by complex (visco-elasto- plastic) lithosphere rheology, structure and regional intraplate stress field. In continents, PLI are often identified within extensional or compression geodynamic contexts, near boundaries between younger plates (e.g., orogenic) and older stable plates (e.g., cratons), which represent important geometrical and thermo- rheological barriers that affect plume head emplacement (e.g., Archean West Africa, East Africa, Pannonian - Carpathian system). We address these problems by considering a free-surface thermo-dynamically coupled (continuous phase transforms) thermo-mechanical numerical model of PLI that treats stratified elasto-viscous-plastic (EVP) continental blocks of contrasting properties submitted to regional compression and extension. The results show that: (1) topographic response to PLI is highly different from the predictions of classical convection models, in particular, the commonly expected long-wavelength uplift is short-lived and is replaced by mainly bi-harmonic deformation of 'tectonic- style' (poly-phase basins and uplifts) characterized by two short wavelengths (50-100 and 200-400 km). (2) tectonic deformation due to far-field forces, such as folding, may interact with lithospheric response to PLI, sometimes in a very complex way; (3) in presence of intra-plate boundaries or blocks, plume head flattening is highly asymmetric and can be blocked from one side by older (and colder) lithospheric block, which leads to mechanical decoupling of crust from the mantle lithosphere and cab be accompanied by localized faulting at the margin; (4) the return flow from the plume head results in sub- vertical down-thrusting (delamination) of the lithosphere below the margin, producing vertical cold 'subduction like' boundary that can be traced down to the 400 km depth; (5) plume head flattening and migration towards the younger plate results in concurrent

  16. On the Yield Strength of Oceanic Lithosphere (United States)

    Jain, Chhavi; Korenaga, Jun; Karato, Shun-ichiro


    The yield strength of oceanic lithosphere determines the mode of mantle convection in a terrestrial planet, and low-temperature plasticity in olivine aggregates is generally believed to govern the plastic rheology of the stiffest part of lithosphere. Because, so far, proposed flow laws for this mechanism exhibit nontrivial discrepancies, we revisit the recent high-pressure deformation data of Mei et al. (2010) with a comprehensive inversion approach based on Markov chain Monte Carlo sampling. Our inversion results indicate that the uncertainty of the relevant flow law parameters is considerably greater than previously thought. Depending on the choice of flow law parameters, the strength of oceanic lithosphere would vary substantially, carrying different implications for the origin of plate tectonics on Earth. To reduce the flow law ambiguity, we suggest that it is important to establish a theoretical basis for estimating macroscopic stress in high-pressure experiments and also to better utilize marine geophysical observations.

  17. Investigation of mantle kinematics beneath the Hellenic-subduction zone with teleseismic direct shear waves (United States)

    Confal, Judith M.; Eken, Tuna; Tilmann, Frederik; Yolsal-Çevikbilen, Seda; Çubuk-Sabuncu, Yeşim; Saygin, Erdinc; Taymaz, Tuncay


    The subduction and roll-back of the African plate beneath the Eurasian plate along the arcuate Hellenic trench is the dominant geodynamic process in the Aegean and western Anatolia. Mantle flow and lithospheric kinematics in this region can potentially be understood better by mapping seismic anisotropy. This study uses direct shear-wave splitting measurements based on the Reference Station Technique in the southern Aegean Sea to reveal seismic anisotropy in the mantle. The technique overcomes possible contamination from source-side anisotropy on direct S-wave signals recorded at a station pair by maximizing the correlation between the seismic traces at reference and target stations after correcting the reference stations for known receiver-side anisotropy and the target stations for arbitrary splitting parameters probed via a grid search. We obtained splitting parameters at 35 stations with good-quality S-wave signals extracted from 81 teleseismic events. Employing direct S-waves enabled more stable and reliable splitting measurements than previously possible, based on sparse SKS data at temporary stations, with one to five events for local SKS studies, compared with an average of 12 events for each station in this study. The fast polarization directions mostly show NNE-SSW orientation with splitting time delays between 1.15 s and 1.62 s. Two stations in the west close to the Hellenic Trench and one in the east show N-S oriented fast polarizations. In the back-arc region three stations exhibit NE-SW orientation. The overall fast polarization variations tend to be similar to those obtained from previous SKS splitting studies in the region but indicate a more consistent pattern, most likely due to the usage of a larger number of individual observations in direct S-wave derived splitting measurements. Splitting analysis on direct shear waves typically resulted in larger split time delays compared to previous studies, possibly because S-waves travel along a longer path

  18. PICASSO: Lithosphere Structure in the Western Mediterranean from Ps Receiver Functions and Rayleigh Wave Tomography (United States)

    Palomeras, I.; Thurner, S.; Levander, A.; Humphreys, E.; Miller, M. S.; Carbonell, R.; Gallart, J.


    The western Mediterranean is a diffuse plate boundary separating the African and Eurasian plates. Cenozoic deformation is centered on the Gibraltar arc and Alboran Sea, and occupies a wide area from the southern Iberian Massif in Spain to the Atlas Mountains in Morocco. We present a model of the lithospheric structure of this region derived from Rayleigh wave tomography and Ps receiver functions, using data from the PICASSO (Program to Investigate Convective Alboran Sea System Overturn) linear broadband array of ~100 seismographs. This array is deployed from central Spain to the Morocco-Algerian border. We complement these data with some of that recorded by IberArray, an areal broadband array, operated by the Spanish seismological community, covering the same region with a uniform 50 km x 50 km grid of stations. Rayleigh phase velocities have been measured from 20-167s period using the two-plane-wave method to remove complications due to multi-pathing, and finite-frequency kernels to improve lateral resolution. The phase velocities were inverted for 1D structure on a 0.25 by 0.25 degree grid. Ps receiver functions at 1Hz and 2Hz were calculated for the same area using water-level and time-domain iterative deconvolution, and were then CCP stacked. The Rayleigh wave shear velocity model, jointly interpreted with the discontinuity structure from the CCP stack, shows the first-order lithospheric structure, and the lithosphere-asthenosphere boundary (LAB). From north to south along the PICASSO profile: The lithosphere is ~120 km thick beneath the Iberian Massif, where it has the highest shear velocity, 4.45 km/s. To the south the lithosphere thins dramatically beneath the Betic Mountains to ~85 km, and then varies in thickness and decreases in velocity beneath the Alboran Sea and Gibraltar Arc. The thinnest lithosphere, ~60 km, is observed beneath the Rif mountains and Middle Atlas, with a low velocity feature (4.2 km/s) at ~60 km depth beneath a site of Late Cenozoic

  19. A recent deep earthquake doublet in light of long-term evolution of Nazca subduction. (United States)

    Zahradník, J; Čížková, H; Bina, C R; Sokos, E; Janský, J; Tavera, H; Carvalho, J


    Earthquake faulting at ~600 km depth remains puzzling. Here we present a new kinematic interpretation of two Mw7.6 earthquakes of November 24, 2015. In contrast to teleseismic analysis of this doublet, we use regional seismic data providing robust two-point source models, further validated by regional back-projection and rupture-stop analysis. The doublet represents segmented rupture of a ∼30-year gap in a narrow, deep fault zone, fully consistent with the stress field derived from neighbouring 1976-2015 earthquakes. Seismic observations are interpreted using a geodynamic model of regional subduction, incorporating realistic rheology and major phase transitions, yielding a model slab that is nearly vertical in the deep-earthquake zone but stagnant below 660 km, consistent with tomographic imaging. Geodynamically modelled stresses match the seismically inferred stress field, where the steeply down-dip orientation of compressive stress axes at ∼600 km arises from combined viscous and buoyant forces resisting slab penetration into the lower mantle and deformation associated with slab buckling and stagnation. Observed fault-rupture geometry, demonstrated likelihood of seismic triggering, and high model temperatures in young subducted lithosphere, together favour nanometric crystallisation (and associated grain-boundary sliding) attending high-pressure dehydration as a likely seismogenic mechanism, unless a segment of much older lithosphere is present at depth.

  20. Li isotopic constraints from the Erro-Tobbio serpentinites on Alpine subduction processes (United States)

    Chu, M.-F.; Scambelluri, M.; Griffin, W. L.; O'Reilly, S. Y.; Pearson, N. J.


    Subduction zones represent a unique feature of the dynamic Earth and provide important constraints on how plate tectonics works. Subduction of serpentinized oceanic lithosphere, characterized by releasing water into the mantle wedge via dehydration, i.e. breakdown of hydrous minerals, plays a critical role in not only the generation of continental crust but also the Earth's water cycle. To track the recycling of water or fluid released by subduction, the stable isotope system of Li, a lithophile and mobile element, shows its high potential because 7Li, relative to 6Li, preferentially moves into the fluid phase when fractionation occurs. Here we present new Li abundance and isotopic compositions of the Erro-Tobbio serpentinized peridotite complex, a remnant mantle slice of the Alpine subduction. Our data indicate that most of the serpentinized ultramafic rocks have uniform Li concentrations, around 1 ppm. These rocks, however, show variable Li isotopic compositions. Among them, the high-pressure antigorite-bearing metaperidotites, formed under a low-strain condition, have a limited range in Li isotopic values, with δ7Li = +1.5 to +4.0, similar to those of serpentinized peridotites. In contrast, apparently heavier Li isotopes, up to +10.0, are observed in the high-pressure serpentinite mylonites that also contain antigorite but formed in high-strain domains. We note that O-H isotope ratios of the high-pressure ultramafic rocks reported in previous study (Früh-Green et al., 2001, Contrib. Mineral Petrol. 141: 145-159) show insignificant variations between the low- and high-strain domains. This demonstrates the superiority of Li isotope than conventional stable isotope systems in offering critical information about fluid-releasing processes in subduction zones. Moreover, our new data unsupport the general assumption that fluid released from subducted slabs is in favor of extracting 7Li than 6Li, thus capable of forming the much lighter δ7Li values observed in

  1. Geodynamical Analysis of Plate Reconstructions based on Subduction History Models (United States)

    Quevedo, L. E.; Butterworth, N. P.; Matthews, K. J.; Morra, G.; Müller, R. D.


    We present a novel method to produce global subduction history models from plate reconstructions and use their predicted geodynamic behaviour as a quality metric for the physical consistency of absolute motions. We show that modelled slabs constructed by advecting material into the mantle according to absolute and relative plate motions given by a particular reconstruction are better correlated with the present day slab dips observed in mantle tomography than instantaneous kinematic quantities like present convergence rate. A complete simulation incorporating lithospheric thickness derived from oceanic age and a rheological model of the lithosphere was run using the Boundary Element Method-based software BEMEarth to infer the global pattern of mantle flow. The predicted plate motion orientations in the form of Euler pole location for the present day and mid-Cretaceous (125 Ma) were compared with the kinematic model for a set of rheologies and mantle structures, and found to be a robust and efficient indicator of the physical consistency of kinematic reconstructions based on their effect on the balance of plate driving forces. As an application example, during the Early Cretaceous, the predicted motion of the Farallon plate was found to be more consistent with the regional geology of the Western North American Cordillera system than the instantaneous motion suggested by a reconstruction at 125 Ma based on sparse hotspot track data on the Pacific Plate. This suggests that a methodology based on forward geodynamic modellling could be used to predict absolute plate motions in reconstructions for times that are ill-constrained by observations constraining absolute plate motions.

  2. Experimental Determination of Chloritoid Stability in Subducting Oceanic Crust (United States)

    Forneris, J.; Holloway, J. R.


    Dehydration of subducting oceanic lithosphere is the key process for understanding arc magma generation and transport of H2O into the mantle. To establish when and how H2O may be released from the slab into the overlying mantle it is necessary to determine the stability of hydrous phases in the subducting lithosphere. In the past 10 years, experimental investigations of phase relationships in basaltic compositions representing the crustal component of the slab have led to controversial results. Results obtained by Schmidt and Poli (1998) and Pawley and Holloway (1993) on basaltic compositions under H2O saturated conditions showed the potential importance of hydrous phases other than amphibole (such as chloritoid, epidote and lawsonite) in the dehydration process. However, these results are in disagreement with the experiments of Liu et al. (1996), which showed that no hydrous phases are stable beyond the amphibole breakdown reaction at or above 650° C. In our study, piston-cylinder experiments were conducted between 2.2 GPa and 2.8 GPa at 650° C. The starting material consisted of a natural basaltic glass with blueschist/eclogite seeds and H2O. Samples were pressure-sealed in a thick-walled silver capsule with a gold lining designed to prevent hydrogen diffusion in long-duration experiments. The oxygen fugacity was fixed at or near Ni+NiO. These experiments have been focused on determining the stability field of chloritoid by running long-duration experiments (up to 1 month). Our results are in agreement with results by Liu et al. (1996): Chloritoid appears in short-duration runs (144 hours or less at 2.6 GPa and 650° C) but is not present in longer-duration experiments (696 hours or more under the same conditions). The amphiboles obtained in our run products have a glaucophane composition and seem to be stable up to higher pressures (at least 2.6 GPa) than the more calcic amphiboles obtained by the three other groups. Epidote/zoisite is present up to at least

  3. Characterizing Seismic Anisotropy across the Peruvian Flat-Slab Subduction Zone: Shear Wave Splitting from PULSE (United States)

    Eakin, C. M.; Long, M. D.; Beck, S. L.; Wagner, L. S.; Tavera, H.


    Although 10% of subduction zones worldwide today exhibit shallow or flat subduction, we are yet to fully understand how and why these slabs go flat. An excellent study location for such a problem is in Peru, where the largest region of flat-subduction currently exists, extending ~1500 km in length (from 3 °S to 15 °S) and ~300 km in width. Across this region we investigate the pattern of seismic anisotropy, an indicator for past and/or ongoing deformation in the upper mantle. To achieve this we conduct shear wave splitting analyzes at 40 broadband stations from the PULSE project (PerU Lithosphere and Slab Experiment). These stations were deployed for 2+ years across the southern half of the Peruvian flat-slab region. We present detailed shear wave splitting results for deep and teleseismic events, making use of a wide variety of available phases that sample the upper mantle directly beneath the stations (such as SKS, SKKS, PKS, sSKS, SKiKS, ScS and local/direct S). We analyze the variability of our results with respect to initial polarizations and ray paths, as well as spatial variability between stations as the underlying slab morphology changes. Preliminary results show predominately NW-SE fast polarizations (trench oblique to sub-parallel) over the flat-slab region east of Lima. These results are consistent with observations of more complex multi-layered anisotropy beneath a nearby permanent station (NNA). Further south, towards the transition to steeper subduction, the splitting pattern becomes increasingly dominated by null measurements. Over to the east however, beyond Cuzco, where the mantle wedge might begin to play a role, we record fast polarizations quasi-parallel to the local slab contours. We carefully evaluate the different possible source locations within the subduction zone for this seismic anisotropy and observe increasing evidence for distinct anisotropy within the slab as well as the sub-slab mantle.

  4. Why Do We Need 3-d Numerical Models of Subduction? (United States)

    Morra, G.; Faccenna, C.; Funiciello, F.; Giardini, D.; Regenauer-Lieb, K.

    We use a set of 2-D and 3-D numerical fluid dynamic experiments, modeled with different strain rate dependent rheologies (viscous, visco-plastic, power law) to ana- lyze the long-term dynamics of the subduction of an oceanic slab into an iso-viscous or stratified mantle. For the lithosphere a fluid dynamic approach has been bench- marked with our previous solid mechanical approach with the aim of overcoming the coherency problem of fluid dynamic calculations. The solid mechanical dichotomy Sstrong before failure and weak where it failsT has been cast into a specialized non- & cedil;linear fluid rheology. Analog 2-D and 3-D experiments are finally compared with the numerical experiments. 2-D numerical experiments are considered with and without free surface to investigate the limitations induced by a closed top boundary. The effect of asymmetric boundary conditions (with and without overriding plate) is analyzed with respect to the possibility of trench retreat. We clearly state the importance for the free surface analysis. 2-D experiments have inherent weaknesses: first they provide an unrealistic simulation of mantle flow (suppression of toroidal flow), second they give rise to the Sclosed boxT problem (interaction of the slab with a boundary, i.e. & cedil;660 km and the left and right box boundaries). 3-D numerical experiments permit to overcome these problems. A natural analysis of the behavior of the mantle flow during subduction and the three-dimensional behavior of the slab is thus possible. Physical observables like trench retreat and toroidal and poloidal flow are compared with the results of our companion analog 3-D experiments.

  5. Recovering lateral variations in lithospheric strength from bedrock motion data using a coupled ice sheet-lithosphere model

    National Research Council Canada - National Science Library

    Berg, J. van den; Wal, R.S.W. van de; Oerlemans, J


    A vertically integrated two-dimensional ice flow model was coupled to an elastic lithosphere-Earth model to study the effects of lateral variations in lithospheric strength on local bedrock adjustment...

  6. Coupled Global Models of Mantle and Lithosphere Dynamics: Identifying the Forces Governing Pacific Plate Motions since the mid–Miocene

    DEFF Research Database (Denmark)

    Stotz Canales, Ingo Leonardo


    at the Melanesian arc. Furthermore, we demonstrate, for the first time, that the sub–Pacific asthenosphere features a significant component of pressure–driven flow (i.e., Poiseuille), and that this accounts for more than half of the Pacific plate motion over at least the past 15 Myr. Our results highlight the power......Almost all surface processes, including motions of the lithospheric plates, can be related to underlying mantle circulation. Geological expressions across Earth’s surface, such as the Andean mountain range in South America, the ancient cratons of Australia and the African continent’s unusual.......e. the direction and magnitude) of lithospheric plates. In this thesis, we have developed novel coupled global numerical models of mantle and lithosphere dynamics and, subsequently, used these to test hypotheses on the force–balance governing motion of the Pacific plate since the mid–Miocene. These coupled models...

  7. Lithospheric Flexural Modeling of Iapetus' Equatorial Ridge (United States)

    Zheng, W.; Ip, W.-H.; Teng, L. S.


    Iapetus, which is one of Saturn's ball-shaped satellites, has some unique features in the Solar System. This satellite has a mean radius of 735 km, and there is an approximately 20-kilometer-high mountain lying precisely on its equator. The mountain is known as an "equatorial ridge" since it makes Iapetus appear walnut shaped. The origin of the equatorial ridge is attributed to several hypotheses, including different endogenesis and exogenesis processes. In this work, we attempted to construct a flexural model of the equatorial ridge using elastic lithosphere theory. The equatorial ridge is treated as a linear load which exerts uniform force on Iapetus' hard shell (i.e. elastic lithosphere of Iapetus). To calculate the deflection of surface, we use the Digital Terrain Model (DTM) data of Iapetus' leading side published by Giese et al. (2008). Giese et al. also pointed out that the elastic lithospheric thickness of Iapetus must exceed 100 km to support the ridge without deflecting. However, we found possible evidence in the DTM data that implied deflection. There are two sites of surface depression on the northern side of the equatorial ridge. The few-kilometer deflection implies a thinner lithosphere than previous suggested. Assume that the thickness of elastic lithosphere is only 5% below of the radius of Iapetus, so the flat-Earth and one-plate condition could adapt to the flexure model of Iapetus. Based on analysis of the distance between a bulge and the ridge, the calculated lithospheric thickness is 6-10 km. The new result seems controversial, but the modeled surface profile is highly consistent with numerical ridge DTM profile extracted from Giese et al. (2008). Thinner lithosphere also supports the contraction model proposed by Sandwell and Schubert (2010) since the bucking harmonic degree increases. In the other hand, the transformation layer between hard shell and plastic inner core may need constraint on thermal history or crystal form of ice. In

  8. Silicothermal fluid: A novel medium for mass transport in the lithosphere (United States)

    Wilkinson, J. J.; Nolan, J.; Rankin, A. H.


    New experimental data from synthetic fluid-inclusion studies in the system K2O-CO2-SiO2-H2O (KCSH) show that a potassic, silica-rich (≈ 90 wt% SiO2) fluid can coexist immiscibly with a supercritical, alkaline, aqueo-carbonic fluid and quartz from temperatures as low as 300 °C to more than 750 °C at relatively low geologic pressures (<200 MPa). This type of fluid phase may form in a range of geologic environments, including carbonatite complexes, alkaline subvolcanic-plutonic systems, and subduction zones. With a probable polymerized (silica-rich, melt-like) structure, such SiO2-rich fluids, if they form in the lithosphere, are likely to be important in the mobilization and transport of silica and large ion lithophile elements (e.g., K, Cs, Ba) and metals of economic significance (e.g., Au, Ag, U).

  9. The story of a craton from heart to margins: illuminating cratonic lithosphere with Rayleigh wave phase velocities in Eastern Canada (United States)

    Petrescu, L.; Darbyshire, F. A.; Gilligan, A.; Bastow, I. D.; Totten, E. J.


    Cratons are Precambrian continental nuclei that are geologically distinct from modern continental regions and are typically underlain by seismically fast lithospheric roots (keels) to at least 200 km depth. Both plate and non-plate tectonic origin theories such as stacking of subducted slabs or multiple mantle plume underplating have been proposed to explain keel growth.Eastern Canada is an ideal continental region to investigate cratonization processes and the onset of plate tectonics. It comprises part of the largest Archean craton in the world, the Superior Province, flanked by a ~1.1 Ga Himalayan-scale orogenic belt, the Grenville Province, and the 500-300 Ma old Appalachian orogenic province, following the same general SW-NE axial trend. The region is also cross-cut by the Great Meteor Hotspot track, providing an excellent opportunity to study the interaction of hotspot tectonism with progressively younger lithospheric domains.We investigate the lithospheric structure of Precambrian Eastern Canada using teleseismic earthquake data recorded at permanent and temporary networks. We measure interstation dispersion curves of Rayleigh wave phase velocities between ~15 and 220 s, and compare the results to standard continental and cratonic reference models. We combine the dispersion curves via a tomographic inversion which solves for isotropic phase velocity heterogeneity and azimuthal anisotropy across the region at a range of periods. The phase velocity maps indicate variations in lithospheric properties from the heart of the Superior craton to the SE Canadian coast.The new regional-scale models will help to understand the processes that generated, stabilized and reworked the cratonic roots through their billion-year tectonic history. We investigate how surface tectonic boundaries relate to deeper lithospheric structural changes, and consider the effects of the multiple Wilson cycles that affected Laurentia.

  10. Lithospheric structure of the Western Alps as seen by full-waveform inversion of CIFALPS teleseismic data (United States)

    Beller, Stephen; Monteiller, Vadim; Operto, Stéphane; Nolet, Guust; Paul, Anne; Zhao, Liang


    Full-waveform inversion (FWI) is a powerful but constitutionally intensive technique that aims to recover 3D multiparameter images of the subsurface by minimising the waveform difference between the full recorded and modelled seismograms. This method has recently been adapted and successfully applied in lithospheric settings by tackling teleseismic waveform modelling with hybrid methods. For each event, a global scale simulation is performed once and for all to store the wavefield solutions on the edges of the lithospheric target. Then, for each modelling involved in the FWI process, these global scale solutions are injected within the lithospheric medium from the boundaries. We present the results of the application of teleseismic FWI to the data acquired by the CIFALPS experiment that was conducted in the Western Alps to gain new insights its lithospheric structure and geodynamic evolution of the alpine range. Nine teleseismic events were inverted to infer 3D models of density, P-wave velocity and S-wave velocity of the crust and the upper-mantle down to 200 km depth. Our models show clear evidences of continental subduction during the alpine orogeny. They outline a dipping European Moho down to 75 km depth and finely delineate the geometry of the Ivrea body at the suture between European and Adriatic plates. Deeper, in the mantle a slow S-wave velocity anomaly might indicate the location of the European slab detachment. Overall, FWI models give access to new seismic images that fill the resolution gap between smooth tomographic model and sharp receiver function images of the lithosphere and enable integrated interpretations of crustal and upper-mantle structures.

  11. Estimating lithospheric properties at Atla Regio, Venus (United States)

    Phillips, Roger J.


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

  12. Geochemical and Isotopic Evidence for Melting and Erosion of Wyoming Craton Mantle Lithosphere Prior to 48 Ma (United States)

    Duke, G. I.; Carlson, R. W.; Frost, C. D.


    Trace-element geochemistry of Cretaceous-Tertiary Great Plains igneous rocks supports isotopic data that reveal a sequence of digestion of lithospheric mantle followed by intrusion of dominantly asthenospheric magmas. Multiple Archean, Proterozoic, and Phanerozoic subduction events beneath the Wyoming craton concentrated Ba and K within the underlying mantle lithosphere, resulting in earliest Cretaceous-Tertiary lithospheric melts with fingerprints of high K, high Ba/Nb and negative epsilon-Nd, but low U, Th, total REE, and less extreme values of LREE/HREE. Youngest (Eocene-Oligocene) magmas were kimberlite and carbonatite, with high U, Th, LREE, extremely high LREE/HREE, and positive epsilon-Nd, but with high-T xenoliths from depths of only 150 km (Carlson et al., 1999). Importantly, in the entire Wyoming craton, the Homestead kimberlite is the only one of K-T age that has transported a diamond—a single micro-diamond discovered. The shallow low-T to high-T xenolith transition, lack of diamonds, and changing magma geochemistry, suggest that a significant portion of the mantle lithosphere beneath the Wyoming Archean craton must have been consumed prior to the ≤48 Ma kimberlite eruptions. In contrast, the earliest phase of Cretaceous magmatism in Arkansas was explosive diamond-containing lamproite (~102 Ma) with a Proterozoic lithospheric isotopic signature (Lambert et al., 1995). In Arkansas, there was no earlier subalkalic magmatism, and no evidence of slow digestion of the mantle lithosphere, although later magmatism trended toward higher positive epsilon-Nd values (i.e. larger asthenospheric component). Removal by melting of a significant portion of the Wyoming mantle lithosphere during late Cretaceous-early Tertiary magmatism, along with heating, may have helped promote lithospheric “relaxation” related to extension further west between 53 Ma and 49 Ma, followed by more facile penetration by asthenospheric magmas, an idea proposed to explain the time

  13. Subduction dynamics: Constraints from gravity field observations (United States)

    Mcadoo, D. C.


    Satellite systems do the best job of resolving the long wavelength components of the Earth's gravity field. Over the oceans, satellite-borne radar altimeters such as SEASAT provide the best resolution observations of the intermediate wavelength components. Satellite observations of gravity contributed to the understanding of the dynamics of subduction. Large, long wavelength geoidal highs generally occur over subduction zones. These highs are attributed to the superposition of two effects of subduction: (1) the positive mass anomalies of subducting slabs themselves; and (2) the surface deformations such as the trenches convectively inducted by these slabs as they sink into the mantle. Models of this subduction process suggest that the mantle behaves as a nonNewtonian fluid, its effective viscosity increases significantly with depth, and that large positive mass anomalies may occur beneath the seismically defined Benioff zones.

  14. Dehydration reactions in subducting oceanic crust: implications for arc volcanism (United States)

    Forneris, J. F.; Holloway, J. R.


    In subduction zones, oceanic lithosphere progressively dehydrates as it sinks deep into the underlying mantle. Fluids released from the subducting slab are thought to trigger partial melting in the overlying mantle wedge, leading to the formation of volcanic arcs. Experiments were conducted in the ranges of 2.2--3.4 GPa (70 to 100 km) and 625--750^oC to determine the dehydration reactions that control fluid release from the basaltic layer of the subducting slab. The experimental duration was typically one month, although some experiments were replicated with a shorter run duration (one to two weeks) in order to identify potentially metastable phases. A mixture of a natural mid-ocean ridge basalt glass and mineral seeds was used as the starting material. Oxygen fugacity was buffered within ±1.3 log units of nickel-bunsenite (NiNiO). The results obtained indicate that the transformation of a hydrated eclogite into a nominally dry eclogite occurs through the decomposition of three hydrous phases: amphibole, lawsonite, and zoisite. Chloritoid, a mineral described as an H_2O carrier in previous experimental studies, is found to be metastable in the examined pressure-temperature (P-T) range and therefore should not be involved in the global fluid release from the basaltic crust. A detailed chemical analysis reveals that amphiboles are sodic-calcic (barroisite) at low pressures (2.2 to 2.4 GPa), but become sodic (glaucophane) with increasing pressure. This observation is the first experimental confirmation of the high-pressure stability of glaucophane in metabasalt compositions. At pressures above the stability field of amphibole, zoisite/clinozoisite becomes the stable hydrous phase at temperatures above 645^oC, whereas lawsonite is stable at lower temperatures. H_2O contents of eclogitic assemblages have been estimated based on modal abundance of minerals calculated from electron microprobe analyses. These results indicate that a slab following an intermediate

  15. Subduction induced mantle flow: Length-scales and orientation of the toroidal cell (United States)

    Király, Ágnes; Capitanio, Fabio A.; Funiciello, Francesca; Faccenna, Claudio


    Subduction-induced mantle circulation plays an important role in the dynamics of convergent margins. Different components of the flow, i.e. toroidal and poloidal, provide relevant driving forces for back-arc basin formation, overriding plate deformation, curvature of subduction zones and volcanic activity. Here, we investigate on the emergence and controls on the toroidal component of the subduction-induced mantle flow by means of numerical modeling. To characterize the toroidal cell's three-dimensional flow, size and length-scales and its disposing factors, we test separately a series of lithospheric and mantle parameters, such as the density difference and viscosity ratio between the slab and the mantle, the width of the slab, as opposed to the size, the stratification and the rheology of the mantle. Out of the tested parameters, the numerical results show that the strength of the flow depends on the mantle viscosity and the magnitude of the slab pull force, that is slab-mantle density difference and the mantle thickness, however the characteristic length, axis and the shape of the toroidal cell are almost independent of the slab's properties and mainly depend on the thickness of the convecting mantle.

  16. Geodynamic evolution of the lithosphere beneath the Eastern Anatolia region: Constraints from geodynamic modeling (United States)

    Memis, Caner; Hakan Gogus, Oguz; Pysklywec, Russell; Keskin, Mehmet; Celal Sengor, A. M.; Topuz, Gultekin


    The east Anatolian orogenic plateau is characterized by an average elevation of 2 km, and is delimited by the Bitlis-Zagros collision zone to the south and the Pontide arc to the north. Stratigraphic evidence suggests that the high plateau attained its current elevation since the Serravallian (about 12 million years ago), but probably did not reach its present height until at least the latest Pliocene. While the crustal shortening following the Arabia-Eurasia collision in the south enabled its relatively rapid rise and regional tectonic evolution, the presumed removal of the downgoing slab beneath east Anatolia has potentially played a significant role in this geodynamic configuration. According to the proposed scenario, the northward subducting slab of Neo-Tethys peels away from the overlying crust similar to the lithospheric delamination model. In this work, we performed a series of lithospheric removal models by varying rheological, physical and mechanical properties by using 2D numerical geodynamic experiments, (e.g. plate convergence rate, crustal thickness, mantle lithosphere yield-stress). Our model results show that the average amount of delamination hinge motion is maximum (18 km/my) when the lower crustal rheology is felsic granulite. The slab break-off only occurs at lower convergence rates (≤ 2 cm/yr), and is imposed on the margin of delaminating mantle lithosphere. The surface uplift takes place above the asthenospheric column (or plateau gap) through isostatic and thermal support of asthenospheric upwelling, and varies dependent on the width of the asthenospheric column. However; with higher plate convergence rates (≥3 cm/yr), the asthenospheric column does not widen enough and the continental collision occurs rather than delamination/peeling away. In this case, the average uplift appears in the central section of the crust, and this exceeds a surface elevation of 3 km. All model results are consistent with the observations from the Eastern

  17. Surface imprint of toroidal flow at retreating slab edges: The first geodetic evidence in the Calabrian subduction system (United States)

    Palano, M.; Piromallo, C.; Chiarabba, C.


    Dense GPS observations can help Earth scientists to capture the surface imprint of mantle toroidal flow at slab edges. We document this process in the Calabrian subduction system, where the Ionian slab rollback took place during the past 30 Ma, following a stepwise process driven by migration of lithospheric tearing. We found rotation rates of 1.29°/Ma (counterclockwise) and 1.74°/Ma (clockwise), for poles located close to the northern and southern slab edges, respectively. These small-scale, opposite rotations occur along complex sets of active faults representing the present-day lithospheric expression of the tearing processes affecting the southeastward retreating Ionian slab at both edges. The observed rotations are likely still young and the process more immature at the northern tear, where it is unable to reorient mantle fabric and therefore is unseen by SKS splitting.

  18. Mantle Wedge formation during Subduction Initiation: evidence from the refertilized base of the Oman ophiolitic mantle (United States)

    Prigent, C.; Guillot, S.; Agard, P.; Godard, M.; Lemarchand, D.; Ulrich, M.


    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. The latter one is supported by studies on volcanic sequences whereas studies dealing on the mantle section do not involve a significant influence of subduction processes on its structure and composition. We herein focus on basal peridotites from all along the ophiolite strike in order to decipher and characterize potential fluid/melt transfers relate to subduction processes. Samples were taken across the basal banded unit directly overlying the amphibolitic/granulitic metamorphic sole which represents an accreted part of the lower plate. We carried out a petrological, structural and geochemical study on these rocks and their constitutive minerals. Our results show that basal peridotites range from lherzolites to highly depleted harzburgites in composition. Clinopyroxenes (cpx) display melt impregnation textures and co-crystallized with HT/HP amphiboles (amph), spinels and sulfurs. Major and trace elements of the constitutive minerals indicate that these minerals represent trapped incremental partial melt after hydrous melting. Different cpx-bearing lithologies then result from varying degrees of partial melting and melt extraction. Combined with Boron isotopic data, we demonstrate that fluids responsible for hydrous melting of these ophiolitic basal peridotites are subduction-related, most likely derived from dehydration of the metamorphic sole during its formation in subduction initiation. From these observations and thermal constraints, we interpret the occurrence of these basal lherzolites as representing a freezing front developed by thermal re-equilibration (cooling) during subduction processes: subduction-related hydrous partial melts were

  19. Strike-slip earthquakes in the oceanic lithosphere: Observations of exceptionally high apparent stress (United States)

    Choy, George; McGarr, A.


    of τa for the two types. Secondly, many of the intraplate events have focal mechanisms with the T-axes that are normal to the nearest ridge crest or subduction zone and P-axes that are normal to the proximate transform fault. These observations suggest that forces associated with the reorganization of plate boundaries play an important role in causing high-τa earthquakes inside oceanic plates. Extant transform boundaries may be misaligned with current plate motion. To accommodate current plate motion, the pre-existing plate boundaries would have to be subjected to large horizontal transform push forces. A notable example of this is the triple junction near which the second large aftershock of the 1992 April Cape Mendocino, California, sequence occurred. Alternatively, subduction zone resistance may be enhanced by the collision of a buoyant lithosphere, a process that also markedly increases the horizontal stress. A notable example of this is the Aleutian Trench near which large events occurred in the Gulf of Alaska in late 1987 and the 1998 March Balleny Sea M= 8.2 earthquake within the Antarctic Plate.

  20. Constraints on the mantle and lithosphere dynamics from the observed geoid with the effect of visco-elasto-plastic rheology in the upper 300 km (United States)

    Osei Tutu, Anthony; Steinberger, Bernhard; Rogozhina, Irina; Sobolev, Stephan


    Over the past decades rheological properties of the Earth's mantle and lithosphere have been extensively studied using numerical models calibrated versus a range of surface observations (e.g., free-air-gravity anomaly/geoid, dynamic topography, plate velocity, etc.).The quality of model predictions however strongly depends on the simplifying assumptions, spatial resolution and parameterizations adopted by numerical models. The geoid is largely (Hager & Richards, 1989) determined by both the density anomalies driving the mantle flow and the dynamic topography at the Earth surface and the core-mantle boundary. This is the effect of the convective processes within the Earth's mantle. The remainder is mostly due to strong heterogeneities in the lithospheric mantle and the crust, which also need to be taken into account. The surface topography caused by density anomalies both in the sub-lithospheric mantle and within the lithosphere also depends on the lithosphere rheology. Here we investigate the effects of complex lithosphere rheology on the modelled dynamic topography, geoid and plate motion using a spectral mantle flow code (Hager & O'Connell, 1981) considering radial viscosity distribution and a fully coupled code of the lithosphere and mantle accounting for strong heterogeneities in the upper mantle rheology in the 300 km depths (Popov & Sobolev, 2008). This study is the first step towards linking global mantle dynamics with lithosphere dynamics using the observed geoid as a major constraint. Here we present the results from both codes and compare them with the observed geoid, dynamic topography and plate velocities from GPS measurements. This method allows us to evaluate the effects of plate rheology (e.g., strong plate interiors and weak plate margins) and stiff subducted lithosphere on these observables (i.e. geoid, topography, plate boundary stresses) as well as on plate motion. This effort will also serve as a benchmark of the two existing numerical methods

  1. Atmosphere-ocean-lithosphere interactions during the Great Oxidation Event: insights from zircon δ18O (United States)

    Spencer, C. J.; Partin, C. A.; Kirkland, C.; Shiels, C.; Raub, T. D.; Kinny, P.


    The Great Oxidation Event (GOE) records a precipitous atmospheric oxygen rise, perhaps by as much as three to four orders of magnitude within a few million years. The timescale of the GOE is primarily constrained by the rapid loss of mass-independently fractionated sulfur isotopes. The drastic surface changes associated with the GOE are reflected by the appearance of marine sulfate and manganese deposits, as well as increased redox-sensitive trace metal abundances in banded iron formations and shale. Each of these manifestations is recorded at the atmosphere-lithosphere or atmosphere-ocean interface. However, how the GOE affected the lithosphere beyond the atmosphere interface has received little attention to date. We present zircon δ18O data from Paleoproterozoic sedimentary successions in Western Australia and Canada that display a step-change from the isotopically distinct reservoir with high δ18O that was incorporated into subduction zone magmas. One likely candidate is marine sulfate evaporite deposits, which appear with the GOE. The incorporation of this enriched δ18O reservoir would have facilitated the step change seen in the zircon δ18O record. This signal may also be present to a much lower degree associated with the "whiffs" of atmospheric oxygen prior to the GOE.

  2. Geochemical evidence for subduction in the early Archaean from quartz-carbonate-fuchsite mineralization, Isua Supracrustal Belt, West Greenland

    DEFF Research Database (Denmark)

    Pope, Emily Catherine; Rosing, Minik Thorleif; Bird, Dennis K.

    of this metasomatic-tectonic relationship requires that 1) Phanerozoic orogenic Au-deposits form in subduction-zone environments, and 2) the geochemical similarity of Precambrian orogenic deposits to their younger counterparts is the result of having the same petrogenetic origin. Hydrogen and oxygen isotope...... compositions of fuchsite and quartz from auriferous mineralization in the ca. 3.8 Ga Isua Supracrustal Belt (ISB) in West Greenland, in conjunction with elevated concentrations of CO2, Cr, Al, K and silica relative to protolith assemblages, suggest that this mineralization shares a common petro-tectonic origin......, are the result of seawater-derived fluids liberated from subducting lithosphere interacting with ultramafic rocks in the mantle wedge and lower crust, before migrating up crustal-scale vertical fracture zones. Thus, the presence of quartz-carbonate-fuchsite mineralization in the Isua supracrustal belt and other...

  3. Helium as a tracer for fluids released from Juan de Fuca lithosphere beneath the Cascadia forearc (United States)

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


    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.

  4. Mantle transition zone beneath the central Tien Shan: Lithospheric delamination and mantle plumes (United States)

    Kosarev, Grigoriy; Oreshin, Sergey; Vinnik, Lev; Makeyeva, Larissa


    We investigate structure of the mantle transition zone (MTZ) under the central Tien Shan in central Asia by using recordings of seismograph stations in Kyrgyzstan, Kazakhstan and adjacent northern China. We apply P-wave receiver functions techniques and evaluate the differential time between the arrivals of seismic phases that are formed by P to SV mode conversion at the 410-km and 660-km seismic boundaries. The differential time is sensitive to the thickness of the MTZ and insensitive to volumetric velocity anomalies above the 410-km boundary. Under part of the southern central Tien Shan with the lowest S wave velocity in the uppermost mantle and the largest thickness of the crust, the thickness of the MTZ increases by 15-20 km relative to the ambient mantle and the reference model IASP91. The increased thickness is a likely effect of low (about - 150 K) temperature. This anomaly is indicative of delamination and sinking of the mantle lithosphere. The low temperature in the MTZ might also be a relic of subduction of the oceanic lithosphere in the Paleozoic, but this scenario requires strong coupling and coherence between structures in the MTZ and in the lithosphere during plate motions in the last 300 Myr. Our data reveal a reduction of thickness of the MTZ of 10-15 km under the Fergana basin, in the neighborhood of the region of small-scale basaltic volcanism at the time near the Cretaceous-Paleogene boundary. The reduced thickness of the MTZ is the effect of a depressed 410-km discontinuity, similar to that found in many hotspots. This depression suggests a positive temperature anomaly of about 100-150 K, consistent with the presence of a thermal mantle plume. A similar depression on the 410-km discontinuity is found underneath the Tarim basin.

  5. Geodynamical evolution of the Southern Carpathians: inferences from computational models of lithospheric gravitational instability (United States)

    Lorinczi, Piroska; Houseman, Gregory


    The Carpathians are a geologically young mountain chain which, together with the Alps and the Dinarides, surround the extensional Pannonian and Transylvanian basins of Central Europe. The tectonic evolution of the Alpine-Carpathian-Pannonian system was controlled by convergence between the Adriatic and European plates, by the extensional collapse of thickened Alpine crust and by the retreat of the Eastern Carpathians driven by either a brief episode of subduction or by gravitational instability of the continental lithospheric mantle. The Southeast corner of the Carpathians has been widely studied due to its strong seismic activity. The distribution and rate of moment release of this seismic activity provides convincing evidence of a mantle drip produced by gravitational instability of the lithospheric mantle developing beneath the Vrancea region now. The question of why gravitational instability is strongly evident beneath Vrancea and not elsewhere beneath the Southern Carpathians is unresolved. Geological and geophysical interpretations of the Southern Carpathians emphasise the transcurrent deformation that has dominated recent tectonic evolution of this mountain belt. We use computational models of gravitational instability in order to address the question of why the instability appears to have developed strongly only at the eastern end of this mountain chain. We use a parallelised 3D Lagrangean-frame finite deformation algorithm, which solves the equations of momentum and mass conservation in an incompressible viscous fluid, assuming a non-linear power-law that relates deviatoric stress and strain-rate. We consider a gravitationally unstable system, with a dense mantle lithosphere overlying a less dense asthenosphere, subject to boundary conditions which simulate the combination of shear and convergence that are thought to have governed the evolution of the South Carpathians. This program (OREGANO) allows 3D viscous flow fields to be computed for spatially

  6. Tectonics and volcanism of Eastern Aphrodite Terra: No subduction, no spreading (United States)

    Hansen, Vicki L.; Keep, Myra; Herrick, Robert R.; Phillips, Roger J.


    Eastern Aphrodite Terra is approximately equal in size to the western North American Cordillera, from Mexico to Alaska. Its size and unique landforms make it an important area for understanding the tectonics of Venus, yet models for its formation are diametrically opposed. This region is part of the Equatorial Highlands, which was proposed as a region of lithospheric thinning, isostatic uplift, and attendant volcanism. Eastern Aphrodite Terra is dominated by circular structures within which deformation and volcanism are intimately related. These structures are marked by radial and concentric fractures, and volcanic flows that emanate from a central vent, as well as from concentric fracture sets. Cross-cutting relations between flows and concentric fracture sets indicate that outer concentric fracture sets are younger than inner fracture sets. The circular structures are joined by regional northeast- to east-trending fractures that dominantly postdate formation of the circular structures. We propose that the circular structures 'grow' outward with time. Although these structures probably represent addition of crust to the lithosphere, they do not represent significant lithospheric spreading or convergence, and the region does not mark the boundary between two distinct tectonic plates. This region is not easily explained by analogy with either terrestrial midocean rifts or subduction zones. It is perhaps best explained by upwelling of magma diapirs that blister the surface, but do not cause significant lithospheric spreading. Further study of the structural and volcanic evolution of this region using Magellan altimetry and SAR data should lead to better understanding of the tectonic evolution of this region.

  7. Sr-Nd-Hf-Pb isotopic evidence for modification of the Devonian lithospheric mantle beneath the Chinese Altai (United States)

    Yu, Yang; Sun, Min; Huang, Xiao-Long; Zhao, Guochun; Li, Pengfei; Long, Xiaoping; Cai, Keda; Xia, Xiaoping


    Intensive Devonian felsic magmatism is recorded within the southwestern Mongolian collage system of the Central Asian Orogenic Belt (CAOB). The voluminous magmas have isotopic compositions of juvenile materials from the mantle, thus manifesting significant mantle-crust interaction and continental growth at this time. Here, we present systematic Sr-Nd-Hf-Pb isotopic data for the Devonian mafic intrusions in the Chinese Altai, a key region within the southwestern Mongolian collage system to decipher the evolution of the mantle during this important tectonothermal event. The Keketuohai gabbro (409 ± 5 Ma) and type I mafic dykes (376 ± 5 Ma) within the Habahe complex have high (87Sr/86Sr)i, (206Pb/204Pb)t, (207Pb/204Pb)t and (208Pb/204Pb)t ratios, and decoupled Nd-Hf isotopic compositions; e.g., low εNd(t) values (- 2.5 to + 5.4) combined with high εHf(t) (+ 2.6 to + 15.1) values. These rocks have low Ba/La and high La/Yb and Th/Yb ratios, and are enriched in Pb, the light rare earth elements (LREE) and Th. They formed from magmas generated from the depleted lithospheric mantle metasomatised by hydrous melts from subducted sediments. In comparison, the gabbroic samples from the Habahe complex (369 ± 3 Ma) are enriched in the LREE, Th and Ba and have high La/Yb, Th/Yb and Ba/La ratios. They do not show significant Pb anomalies, and have depleted isotopic compositions that include low initial 87Sr/86Sr, 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb ratios and high εNd(t) (+ 7.4 to + 7.8) and εHf(t) (+ 13.4 to + 15.3) values. These rocks are thought to have formed from magmas derived from the lithospheric mantle metasomatised by hydrous melts from subducted oceanic crust. The type II mafic dykes within the Habahe complex are depleted in the LREE and Th, have high Ba/La ratios, and are enriched in Pb, Ba, Sr, and U. They have positive εNd(t) (+ 7.6 to + 8.1) and εHf(t) (+ 14.1 to + 15.4) values, high initial 87Sr/86Sr, 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb ratios

  8. Origin and dynamics of depositionary subduction margins (United States)

    Vannucchi, Paola; Morgan, Jason P.; Silver, Eli; Kluesner, Jared W.


    Here we propose a new framework for forearc evolution that focuses on the potential feedbacks between subduction tectonics, sedimentation, and geomorphology that take place during an extreme event of subduction erosion. These feedbacks can lead to the creation of a “depositionary forearc,” a forearc structure that extends the traditional division of forearcs into accretionary or erosive subduction margins by demonstrating a mode of rapid basin accretion during an erosive event at a subduction margin. A depositionary mode of forearc evolution occurs when terrigenous sediments are deposited directly on the forearc while it is being removed from below by subduction erosion. In the most extreme case, an entire forearc can be removed by a single subduction erosion event followed by depositionary replacement without involving transfer of sediments from the incoming plate. We need to further recognize that subduction forearcs are often shaped by interactions between slow, long-term processes, and sudden extreme events reflecting the sudden influences of large-scale morphological variations in the incoming plate. Both types of processes contribute to the large-scale architecture of the forearc, with extreme events associated with a replacive depositionary mode that rapidly creates sections of a typical forearc margin. The persistent upward diversion of the megathrust is likely to affect its geometry, frictional nature, and hydrogeology. Therefore, the stresses along the fault and individual earthquake rupture characteristics are also expected to be more variable in these erosive systems than in systems with long-lived megathrust surfaces.

  9. Origin and dynamics of depositionary subduction margins (United States)

    Vannucchi, Paola; Morgan, Jason P.; Silver, Eli A.; Kluesner, Jared W.


    Here we propose a new framework for forearc evolution that focuses on the potential feedbacks between subduction tectonics, sedimentation, and geomorphology that take place during an extreme event of subduction erosion. These feedbacks can lead to the creation of a "depositionary forearc," a forearc structure that extends the traditional division of forearcs into accretionary or erosive subduction margins by demonstrating a mode of rapid basin accretion during an erosive event at a subduction margin. A depositionary mode of forearc evolution occurs when terrigenous sediments are deposited directly on the forearc while it is being removed from below by subduction erosion. In the most extreme case, an entire forearc can be removed by a single subduction erosion event followed by depositionary replacement without involving transfer of sediments from the incoming plate. We need to further recognize that subduction forearcs are often shaped by interactions between slow, long-term processes, and sudden extreme events reflecting the sudden influences of large-scale morphological variations in the incoming plate. Both types of processes contribute to the large-scale architecture of the forearc, with extreme events associated with a replacive depositionary mode that rapidly creates sections of a typical forearc margin. The persistent upward diversion of the megathrust is likely to affect its geometry, frictional nature, and hydrogeology. Therefore, the stresses along the fault and individual earthquake rupture characteristics are also expected to be more variable in these erosive systems than in systems with long-lived megathrust surfaces.


    Directory of Open Access Journals (Sweden)

    A. I. Tatarkin


    Full Text Available Interaction of the lithosphere and the biosphere with a view of socioeconomic activity by a human being has the most complete effect in the process of mineral resource utilization. It is typical of mining and metal production complexes (MMC of the Ural, which are basic for the regional economy. A research methodology related to anthropogenic transformation of environment caused by MMCs, including techniques, principles of construction, forms, means and approaches to the scientific knowledge has been developed.

  11. Lithospheric structure of Iberia and Morocco using finite-frequency Rayleigh wave tomography from earthquakes and seismic ambient noise (United States)

    Palomeras, I.; Villaseñor, A.; Thurner, S.; Levander, A.; Gallart, J.; Harnafi, M.


    We present a new 3-D shear velocity model of the western Mediterranean from the Pyrenees, Spain, to the Atlas Mountains, Morocco, and the estimated crustal and lithospheric thickness. The velocity model shows different crustal and lithospheric velocities for the Variscan provinces, those which have been affected by Alpine deformation, and those which are actively deforming. The Iberian Massif has detectable differences in crustal thickness that can be related to the evolution of the Variscan orogen in Iberia. Areas affected by Alpine deformation have generally lower velocities in the upper and lower crust than the Iberian Massif. Beneath the Gibraltar Strait and surrounding areas, the crustal thickness is greater than 50 km, below which a high-velocity anomaly (>4.5 km/s) is mapped to depths greater than 200 km. We identify this as a subducted remnant of the NeoTethys plate referred to as the Alboran and western Mediterranean slab. Beneath the adjacent Betic and Rif Mountains, the Alboran slab is still attached to the base of the crust, depressing it, and ultimately delaminating the lower crust and mantle lithosphere as the slab sinks. Under the adjacent continents, the Alboran slab is surrounded by low upper mantle shear wave velocities (Vs lateral flow induced by the Alboran slab.

  12. Transient thermal regimes in the Sierra Nevada and Baja California extinct outer arcs following the cessation of Farallon subduction (United States)

    Erkan, Kamil; Blackwell, David


    We examine the thermal relaxation of the Sierra Nevada and Baja California extinct outer arc blocks following the progressive cessation of Farallon subduction under western North America beginning at ˜30 Ma. Being parts of the same outer arc until the inland jump of the San Andreas transform fault at ˜5 Ma, these two regions show many similarities in their geology, geomorphology, rigid body behavior, and their relatively low seismicity. In the thermal model, we combine results of different geophysical and geophysical studies to constrain the thermal state and geometry of the outer arcs before the cessation of subduction and then model the postsubduction temperature responses in these regions using the results of the tectonic reconstructions. A well-constrained regional thermal model of these blocks using the results of many earlier studies in these regions confirms that the present low heat flow values in these regions are the remnants of the very cold outer arc thermal regime of the subduction zone even as long as 30 Ma after cessation of subduction. Thus the entire Pacific boundary of the North American plate is still in a transient thermal state. The calculated low lithospheric temperatures in the Sierra Nevada and Peninsular blocks correlate very well with their rigid body behavior obtained from geodetic studies, and seismogenic layer thicknesses obtained from seismological studies. This is in contrast with the fact that both regions are surrounded by intense deformation associated with the western North America intraplate and extraplate motions. These low-temperature islands play important roles in the present interaction of the North American and Pacific plates and contribute to the broad deformation of the transform boundary. The thermal relaxation of the extinct outer arcs includes both vertical heating from the underlying asthenosphere and the lateral heating from the extinct back arc (Basin and Range), which has remained as a high heat flow region after

  13. Lithospheric Thickness Modeled from Long Period Surface Wave Dispersion

    Energy Technology Data Exchange (ETDEWEB)

    Pasyanos, M E


    The behavior of surface waves at long periods is indicative of subcrustal velocity structure. Using recently published dispersion models, we invert surface wave group velocities for lithospheric structure, including lithospheric thickness, over much of the Eastern Hemisphere, encompassing Eurasia, Africa, and the Indian Ocean. Thicker lithosphere under Precambrian shields and platforms are clearly observed, not only under the large cratons (West Africa, Congo, Baltic, Russia, Siberia, India), but also under smaller blocks like the Tarim Basin and Yangtze craton. In contrast, it is found that remobilized Precambrian structures like the Saharan Shield and Sino-Korean Paraplatform do not have well-established lithospheric keels. The thinnest lithospheric thickness is found under oceanic and continental rifts, as well as along convergence zones. We compare our results to thermal models of continental lithosphere, lithospheric cooling models of oceanic lithosphere, lithosphere-asthenosphere boundary (LAB) estimates from S-wave receiver functions, and velocity variations of global tomography models. In addition to comparing results for the broad region, we examine in detail the regions of Central Africa, Siberia, and Tibet. While there are clear differences in the various estimates, overall the results are generally consistent. Inconsistencies between the estimates may be due to a variety of reasons including lateral and depth resolution differences and the comparison of what may be different lithospheric features.

  14. What can we learn from lithosphere-scale models of passive margins? (United States)

    Scheck-Wenderoth, Magdalena; Maystrenko, Yuriy; Hirsch, Katja K.


    flow increases in the opposite direction: from the ocean to the continent. We find that the thermal field at this older passive margin is controlled by the thickness of the crystalline crust and related radiogenic heat production. References Maystrenko, Y., Scheck-Wenderoth, M. (2009): Density contrasts in the upper mantle and lower crust across the continent-ocean transition: constraints from 3-D gravity modelling at the Norwegian margin. Geophysical Journal International, 179, 536-548. doi: 10.1111/j.1365-246X.2009.04273.x. Hirsch, K. K.; Bauer, K.; Scheck-Wenderoth, M. (2009): Deep structure of the western South African passive margin - Results of a combined approach of seismic, gravity and isostatic investigations., Tectonophysics, 470, 1-2, 57-70. Scheck-Wenderoth, M., Maystrenko Y., 2008. How warm are passive continental margins? A 3D lithosphere-scale study from the Norwegian margin. Geology, 36(5): 419-422.

  15. Deep structure and historical earthquakes in the Calabrian subduction zone (Southern Italy): preliminary results from multi-channel seismic reflection profiles (United States)

    Gallais, F.; Gutscher, M.; Torelli, L.; Polonia, A.; Riminucci, F.


    The Calabrian subduction zone is located in the complex Central Mediterranean area. This subduction is characterized by the presence of deep earthquakes under the Tyrrhenian Sea down to 500 km depth. The Tethyan remnant Ionian slab descends towards the NW at a dip of about 70° and is associated with an active volcanic arc (the Aeolian Islands). Recently reported GPS and seismicity studies suggest that the subduction of the Ionian lithosphere beneath the Calabrian Arc may be locally still active, though at very slow rates (thrust earthquakes, characteristic of active subduction zone, suggests that if subduction is active, the fault plane may be locked since the instrumental period. To seek evidence of continuous tectonic activity of the Calabrian system, we present preliminary results from reprocessed 96-channels seismic reflection profiles (French Archimede cruise, 1997) offshore Sicily. This analysis permits to recognize a well-defined stratigraphy in the Ionian Abyssal Plain, this stratigraphy becomes difficult to follow under the deformed Calabrian Prism. But the joint interpretation with the reprocessed PM01 profile (French PRISMED cruise, 1994) helps constrain this interpretation and to image some characteristic structures of an accretionary wedge (fore/back-thrusts, basal decollement...). This study also include interpretation of a more recent Italian seismic cruise (Calamare, 2008) and CROP profiles. This work will help to prepare a future cruise proposal (CIRCEE, to be submitted in January 2010) to study the Calabrian subduction with OBS, MCS seismic, heat-flow measurements and sediment coring. The goals are : 1/ to image the deep structure of this subduction zone, 2/ to characterize its thermal state to deduce a geometry of the seismogenic part of the plate interface and add new constraints on seismic risk linked with the Calabrian subduction.

  16. Numerical modeling of the deformations associated with large subduction earthquakes through the seismic cycle (United States)

    Fleitout, L.; Trubienko, O.; Garaud, J.; Vigny, C.; Cailletaud, G.; Simons, W. J.; Satirapod, C.; Shestakov, N.


    A 3D finite element code (Zebulon-Zset) is used to model deformations through the seismic cycle in the areas surrounding the last three large subduction earthquakes: Sumatra, Japan and Chile. The mesh featuring a broad spherical shell portion with a viscoelastic asthenosphere is refined close to the subduction zones. The model is constrained by 6 years of postseismic data in Sumatra area and over a year of data for Japan and Chile plus preseismic data in the three areas. The coseismic displacements on the subduction plane are inverted from the coseismic displacements using the finite element program and provide the initial stresses. The predicted horizontal postseismic displacements depend upon the thicknesses of the elastic plate and of the low viscosity asthenosphere. Non-dimensionalized by the coseismic displacements, they present an almost uniform value between 500km and 1500km from the trench for elastic plates 80km thick. The time evolution of the velocities is function of the creep law (Maxwell, Burger or power-law creep). Moreover, the forward models predict a sizable far-field subsidence, also with a spatial distribution which varies with the geometry of the asthenosphere and lithosphere. Slip on the subduction interface does not induce such a subsidence. The observed horizontal velocities, divided by the coseismic displacement, present a similar pattern as function of time and distance from trench for the three areas, indicative of similar lithospheric and asthenospheric thicknesses and asthenospheric viscosity. This pattern cannot be fitted with power-law creep in the asthenosphere but indicates a lithosphere 60 to 90km thick and an asthenosphere of thickness of the order of 100km with a burger rheology represented by a Kelvin-Voigt element with a viscosity of 3.1018Pas and μKelvin=μelastic/3. A second Kelvin-Voigt element with very limited amplitude may explain some characteristics of the short time-scale signal. The postseismic subsidence is

  17. Full-waveform seismic tomography of the Vrancea, Romania, subduction region (United States)

    Baron, Julie; Morelli, Andrea


    The Vrancea region is one of the few locations of deep seismicity in Europe. Seismic tomography has been able to map lithospheric downwelling, but has not been able yet to clearly discriminate between competing geodynamic interpretations of the geological and geophysical evidence available. We study the seismic structure of the Vrancea subduction zone, using adjoint-based, full-waveform tomography to map the 3D vP and vS structure in detail. We use the database that was built during the CALIXTO (Carpathian Arc Lithosphere X-Tomography) temporary experiment, restricted to the broadband sensors and local intermediate-depth events. We fit waveforms with a cross-correlation misfit criterion in separate time windows around the expected P and S arrivals, and perform 17 iterations of vP and vS model updates (altogether, requiring about 16 million CPU hours) before reaching stable convergence. Among other features, our resulting model shows a nearly vertical, high-velocity body, that overlaps with the distribution of seismicity in its northeastern part. In its southwestern part, a slab appears to dip less steeply to the NW, and is suggestive of ongoing - or recently concluded - subduction geodynamic processes. Joint inversion for vP and vS allow us to address the vP/vS ratio distribution, that marks high vP/vS in the crust beneath the Focsani sedimentary basin - possibly due to high fluid pressure - and a low vP/vS edge along the lower plane of the subducting lithosphere, that in other similar environment has been attributed to dehydration of serpentine in the slab. In spite of the restricted amount of data available, and limitations on the usable frequency pass-band, full-waveform inversion reveals its potential to improve the general quality of imaging with respect to other tomographic techniques - although at a sensible cost in terms of computing resources. Our study also shows that re-analysis of legacy data sets with up-to-date techniques may bring new, useful

  18. Magnetolluric Imaging of the Lithosphere beneath the Tibetan Plateau: A Synoptic View (United States)

    Dong, H.; Wei, W.; Jin, S.; Ye, G.; Jing, J.; Zhang, L.; Jones, A. G.; Xie, C.; Yin, Y.


    To better understand the collision and convergence process between India and Asia continent, magnetotelluric (MT) data have been used to image crustal electrical resistivity and to estimate the rheological structure in the south, east and north margin of Tibetan Plateau in the last decades. However, most of these MT data were collected as isolated profiles and insufficient to convey a synoptic view of the Plateau. Recently, the MT data coverage is greatly improved from the 1 by 1 degree SINOPROBE MT array data from 2011-2013. In this study, a three-dimensional (3D) geo-electrical model of the crust and uppermost mantle is derived from the combined broadband and long period array dataset. The resulting model features broadly resistive structure in the upper crust, with the exception of small scale conductors lining along the Yarlung-Zangbo Suture (YZS) and Bangong-Nujiang Suture (BNS). The lower crust consists of widely spread high conductive structures beneath Lhasa and Qiangtang Block except for the southeast part of Tibetan Plateau. Instead of the previous proposed homogenous high conductive layer, these lower crustal conductors are generally N-S directed and perpendicular to the above Cenozoic and Mesozoic sutures. While the lower crustal structures probably indicate partial melts induced by radiogenic heat from the thickened crust, the conductor along the suture zones in the upper crust might imply pathways for deep fluids into the shear zones of YZS and BNS. One of the most prominent feature revealed is the upper mantle conductive sturcture in the central part of Tibet Plateau (85-92°E) accompanied by two resistive regions in the southeast and southwest part of the Plateau. While the southwest resistive zone is broadly consistent with the "flat subduction" region of Indian lithosphere revealed by seismic receiver function study, the conductive middle part of Tibetan Plateau might represent the upper mantle melts induced by the water carried by the steeper

  19. The seismic cycle at subduction thrusts: 1. Insights from laboratory models

    KAUST Repository

    Corbi, F.


    Subduction megathrust earthquakes occur at the interface between the subducting and overriding plates. These hazardous phenomena are only partially understood because of the absence of direct observations, the restriction of the instrumental seismic record to the past century, and the limited resolution/completeness of historical to geological archives. To overcome these restrictions, modeling has become a key-tool to study megathrust earthquakes. We present a novel model to investigate the seismic cycle at subduction thrusts using complementary analog (paper 1) and numerical (paper 2) approaches. Here we introduce a simple scaled gelatin-on-sandpaper setup including realistic tectonic loading, spontaneous rupture nucleation, and viscoelastic response of the lithosphere. Particle image velocimetry allows to derive model deformation and earthquake source parameters. Analog earthquakes are characterized by “quasi-periodic” recurrence. Consistent with elastic theory, the interseismic stage shows rearward motion, subsidence in the outer wedge and uplift of the “coastal area” as a response of locked plate interface at shallow depth. The coseismic stage exhibits order of magnitude higher velocities and reversal of the interseismic deformation pattern in the seaward direction, subsidence of the coastal area, and uplift in the outer wedge. Like natural earthquakes, analog earthquakes generally nucleate in the deeper portion of the rupture area and preferentially propagate upward in a crack-like fashion. Scaled rupture width-slip proportionality and seismic moment-duration scaling verifies dynamic similarities with earthquakes. Experimental repeatability is statistically verified. Comparing analog results with natural observations, we conclude that this technique is suitable for investigating the parameter space influencing the subduction interplate seismic cycle.

  20. A subduction zone reference frame based on slab geometry and subduction partitioning of plate motion and trench migration

    NARCIS (Netherlands)

    Schellart, W. P.


    The geometry of subducted slabs that interact with the transition zone depends critically on the partitioning of the subduction velocity (v S⊥) at the surface into its subducting plate motion component (vSP⊥) and trench migration component (vT⊥). Geodynamic models of progressive subduction

  1. The earthquake cycle in subduction zones (United States)

    Melosh, H. J.; Fleitout, L.


    A simplified model of a subduction zone is presented, which incorporates the mechanical asymmetry induced by the subducted slab to anchor the subducting plate during post-seismic rebound and thus throw most of the coseismic stream release into the overthrust plate. The model predicts that the trench moves with respect to the deep mantle toward the subducting plate at a velocity equal to one-half of the convergence rate. A strong extensional pulse is propagated into the overthrust plate shortly after the earthquake, and although this extension changes into compression before the next earthquake in the cycle, the period of strong extension following the earthquake may be responsible for extensional tectonic features in the back-arc region.

  2. Insights into the lithospheric architecture of Iberia and Morocco from teleseismic body-wave attenuation (United States)

    Bezada, Maximiliano J.


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

  3. A numerical reference model for themomechanical subduction

    DEFF Research Database (Denmark)

    Quinquis, Matthieu; Chemia, Zurab; Tosi, Nicola


    response to systematic variations in input parameters, numerical studies often start from a 'reference' subduction model. However, the reference model often varies between different numerical studies, making it difficult to compare results directly. We aim therefore to define a numerical reference model......, and initial temperature distribution. We will show results of the evolution and dynamics of the subduction reference model using different numerical codes: a finite element code, SULEC, and two finite difference codes, YACC and FDcon....

  4. Controls on continental strain partitioning above an oblique subduction zone, Northern Andes (United States)

    Schütt, Jorina M.; Whipp, David M., Jr.


    Strain partitioning is a common process at obliquely convergent plate margins dividing oblique convergence into margin-normal slip on the plate-bounding fault and horizontal shearing on a strike-slip system parallel to the subduction margin. In subduction zones, strain partitioning in the upper continental plate is mainly controlled by the shear forces acting on the plate interface and the strength of the continental crust. The plate interface forces are influenced by the subducting plate dip angle and the obliquity angle between the normal to the plate margin and the convergence velocity vector, and the crustal strength of the continent is strongly affected by the presence or absence of a volcanic arc, with the presence of the volcanic arcs being common at steep subduction zones. Along the ˜7000 km western margin of South America the convergence obliquity, subduction dip angles and presence of a volcanic arc all vary, but strain partitioning is only observed along parts of it. This raises the questions, to what extent do subduction zone characteristics control strain partitioning in the overriding continental plate, and which factors have the largest influence? We address these questions using lithospheric-scale 3D numerical geodynamic experiments to investigate the influence of subduction dip angle, convergence obliquity, and weaknesses in the crust owing to the volcanic arc on strain partitioning behavior. We base the model design on the Northern Volcanic Zone of the Andes (5° N - 2° S), characterized by steep subduction (˜ 35°), a convergence obliquity between 31° -45° and extensive arc volcanism, and where strain partitioning is observed. The numerical modelling software (DOUAR) solves the Stokes flow and heat transfer equations for a viscous-plastic creeping flow to calculate velocity fields, thermal evolution, rock uplift and strain rates in a 1600 km x 1600 km box with depth 160 km. Subduction geometry and material properties are based on a

  5. Three-dimensional thermal structure and seismogenesis in the Tohoku and Hokkaido subduction system (United States)

    van Keken, P. E.; Kita, S.; Nakajima, J.; Bengtson, A. K.; Hacker, B. R.; Abers, G. A.


    The Northern Japan arc is characterized by fast subduction of old oceanic lithosphere. The high density instrumentation and high seismicity make this an ideal natural laboratory to study the interplay between subduction zone dynamics, dehydration, migration of fluids, and seismogenesis. In this study we use high resolution finite element models to predict the thermal structure of the subduction slab below Tohoku (Northern Honshu) and Hokkaido. These models allow us to predict the pressure, temperature and mineralogy of the subducted crust and mantle. We use these models to predict the (p,T) conditions of earthquakes that are relocated with a precision of around 1 km by double difference techniques. Below Northern Hokkaido and Tohoku we find that the earthquake activity is strong in crust and the uppermost mantle for temperatures seismic moment. The strongest 3D variations in this arc occur below southern Hokkaido. This 200 km wide region is characterized by a change in trench geometry, anomalously low heatflow and an anomalous velocity structure in the mantle wedge. Tomographic imaging suggest that continental crust is subducted to significant depth, thereby insulating the subducting slab from the hot mantle wedge at least at intermediate depths. The thermal insulation is also suggested by the deepening of the earthquakes in the slab (Kita et al., EPSL, 2010). This region may be characterized by active crustal erosion which would lead to a further blanketing of the crust by a sedimentary layer. Further modifications in thermal structure are possible due to the 3D wedge flow that is generated by the along-arc variations in trench geometry. We quantitatively verify the relative importance of these processes using 2D and 3D dynamical models. Without the seismically imaged crustal structure the earthquake temperatures are significantly elevated compared to the Tohoku and (northern) Hokkaido sections. If we take the modified crustal structure into account we find a (p

  6. Crustal and lithospheric imaging of the Atlas Mountains of Morocco inferred from magnetotelluric data (United States)

    Kiyan, D.; Jones, A. G.; Fullea, J.; Hogg, C.; Ledo, J.; Sinischalchi, A.; Campanya, J.; Picasso Phase II Team


    The Atlas System of Morocco is an intra-continental mountain belt extending for more than 2,000 km along the NW African plate with a predominant NE-SW trend. The System comprises three main branches: the High Atlas, the Middle Atlas, and the Anti Atlas. We present the results of a very recent multi-institutional magnetotelluric (MT) experiment across the Atlas Mountains region that started in September, 2009 and ended in February, 2010, comprising acquisition of broadband and long-period MT data. The experiment consisted of two profiles: (1) a N-S oriented profile crossing the Middle Atlas through the Central High Atlas to the east and (2) a NE-SW profile crossing the western High Atlas towards the Anti Atlas to the west. The MT measurements are part of the PICASSO (Program to Investigate Convective Alboran Sea System Overturn) and the concomitant TopoMed (Plate re-organization in the western Mediterranean: Lithospheric causes and topographic consequences - an ESF EUROCORES TOPO-EUROPE project) projects, to develop a better understanding of the internal structure and evolution of the crust and lithosphere of the Atlas Mountains. The MT data have been processed with robust remote reference methods and submitted to comprehensive strike and dimensionality analysis. Two clearly depth-differentiated strike directions are apparent for crustal (5-35 km) and lithospheric (50-150 km) depth ranges. These two orientations are roughly consistent with the NW-SE Africa-Eurasia convergence acting since the late Cretaceous, and the NNE-SSW Middle Atlas, where Miocene to recent Alkaline volcanism is present. Two-dimensional (2-D) smooth electrical resistivity models were computed independently for both 50 degrees and 20 degrees E of N strike directions. At the crustal scale, our preliminary results reveal a middle to lower-crustal conductive layer stretching from the Middle Atlas southward towards the High Moulouya basin. The most resistive (and therefore potentially thickest

  7. Lithospheric low-velocity zones associated with a magmatic segment of the Tanzanian Rift, East Africa (United States)

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


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

  8. Comparing the effects of rheology on the dynamics and topography of 3D subduction-collision models (United States)

    Pusok, Adina E.; Kaus, Boris; Popov, Anton


    Most of the major mountain belts and orogenic plateaus are found within the overlying plate of active or fossil subduction and/or collision zones. It is well known that they evolve differently from one another as the result of specific combinations of surface and mantle processes. The differences among the structures and evolutions of mountain belts arise for several reasons, such as different strengths of materials, different amounts of regional isostatic compensation, and different mechanisms by which forces are applied to the convergence plates. All these possible controlling factors can change with space and time. Of all the mountain belts and orogenic plateaus, the most striking example is the India-Asia collision zone, which gave rise to the Himalayas and the Tibetan Plateau, the largest region of elevated topography and anomalously thick crust on Earth. Understanding the formation and evolution of such a highly elevated region has been the focus of many tectonic and numerical models. While some of these models (i.e. thin sheet model) have successfully illustrated some of the basic physics of continental collision, none can simultaneously represent active processes such as subduction, underthrusting, channel flow or extrusion, for which fully 3D models are required. Here, we employed the 3D code LaMEM to investigate the role that subduction, continental collision and indentation play on lithosphere dynamics at convergent margins, and the implications they have for the Asian tectonics. Our model setup resembles a simplified tectonic map of the India-Asia collision zone and we performed long-term 3D simulations to analyse the dynamics and the conditions under which large topographic plateaus, such as the Tibetan Plateau can form in an integrated lithospheric and upper-mantle scale model. Results of models with linear viscous rheologies show different modes between the oceanic subduction side (continuous subduction, trench retreat and slab roll-back) and the

  9. Detailed Structure and Thickness of Upper Mantle Discontinuities in the Tonga Subduction Zone From Regional Broadband Arrays (United States)

    Tibi, R.; Wiens, D. A.


    Recordings of deep Tonga earthquakes from two arrays of 12 broadband seismographs each in the Fiji and Tonga islands are stacked and searched for reflections and conversions from upper mantle discontinuities in the Tonga subduction zone. The arrays operated as part of the Seismic Arrays in Fiji and Tonga (SAFT) experiment from July 2001 to August 2002. In comparison with the commonly used teleseismic approaches, the short path lengths for the regional data provide smaller Fresnel zones and high frequency content for precise mapping of discontinuity topography and sharpness. This is particularly important for a subduction zone, where variations in temperature and water content may be expected which should cause changes in the elevation and sharpness of the discontinuities. We studied the phases s410p, P660p and S660p. To enhance these low-amplitude phases, deconvolved seismograms from each event/array pair are aligned on the maximum amplitude of the direct P wave and subsequently slant-stacked. For the 410-km discontinuity, the results show no systematic variations in depth with distance to the cold slab. The 660-km discontinuity varies between 656 and 714 km in depth. For the southern and central parts of the subduction zone, the largest depths occur in the core of the Tonga slab. For the northern part, two separate depressions of the 660 are observed. These anomalies are interpreted as being induced by the active, steeply subducting Tonga deep zone and a subhorizontally lying remnant of subducted lithosphere from the fossil Vityaz trench, respectively. Interpreting the deflections of the 660 in terms of local temperatures implies a thermal anomaly at 660 km depth of -800 to -1200oK for the Tonga slab, and -600 to -950oK for the piece of the Vityaz lithosphere. Except for the southern region where it thickens, the Tonga slab seems to penetrate the 660 with little deformation. Waveform modeling susggests that both the 410 and 660 discontinuities are sharp. The 410

  10. Are mid-lithospheric discontinuities (MLDs) caused by layers of frozen-in melts? (United States)

    Rader, E. L.; Frost, D. A.; Cheng, C.; YU, C.; Menard, J.; Emry, E.; Schmerr, N. C.


    A Mid-Lithospheric Discontinuity (MLD) has been observed at a depth of ~90 km within the continental lithosphere by a wide variety of seismic techniques, including shear wave anisotropy, P and S receiver functions, active-source seismic experiments, SS and PP precursors, and ScS reverberation phases. Proposed geological mechanisms for explaining the MLD include the presence of melt, relict subduction zone accretionary structures, or an abrupt change in composition from frozen-in melts and metasomatism. During the 2013 CIDER summer workshop, our working group began an investigation of the MLD by compiling a seismological database of where the interface has been observed, methods used to identify it, and characteristics (depth, velocity contrast, gradients) of the observations. Additionally, we created a petrological database of xenoliths that originated immediately above, below, and within the vicinity of the seismologically observed structure. We find that regions of thick continental lithosphere, exhibit evidence for an abrupt decrease in seismic shear velocity (~5% over 10-20 km) at 60-160 km depth, with the majority of observations within ~80-90 km. We used the petrological and seismological constraints to then assess the feasibility of a metasomatic and frozen-in melt layer associated with the MLD. We used shear wave velocity data combined with compiled petrological data to determine which hydrous minerals are most likely to have an impact on the shear velocity at the MLD. Preliminary calculations at 1GPa show that by increasing the composition of hydrous minerals, such as phlogopite, by 5-20% in the rock, noticeable decreases in shear velocity are produced. To further test the feasibility of this explanation, we used standard thermobarometry methods to determine xenolith formation depth for samples containing amphibole and phlogopite, comparing them to the expected MLD pressure range (~2-5 GPa; 60-160 km). We find that only 25% of amphibole samples formed

  11. Lithospheric mantle deformation in the Yerer-Tullu Wellel Volcanotectonic Lineament: A study of peridotite xenoliths (United States)

    Trestrail, K. R.; Rooney, T. O.


    Although a great deal is known about crustal extensional processes in the East African Rift System (EARS), questions remain as to the impact of extension on the continental lithospheric mantle. The northernmost portion of the EARS is the Main Ethiopian Rift (MER), which is divided into three sectors: the Northern, (NMER), Central (CMER), and the Southern Main Ethiopian Rift (SMER). The NMER-CMER transition coincides with the Yerer-Tullu Wellel volcanotectonic lineament (YTVL), and may represent a terrain boundary along which extension was directed during the Cenozoic. Here we present petrographic data from peridotite xenoliths contained within ~6 Ma lavas recovered along the western portion of the YTVL near Nekempte, Ethiopia to examine the characteristics of the lithospheric mantle, and how peridotite has responded to extensional strain. Our preliminary results show that the Nekempte xenoliths are deformed spinel-bearing lherzolites, which previous studies have constrained to the spinel lherzolites field at 1.1 GPa in the lithospheric mantle. Petrographic examination of these xenoliths reveals two generations of crystals. These generations are defined by both size and texture; the first generation (coarse crystals) being up to 4 mm in diameter and the second generation (fine crystals) being less than 1 mm in diameter. The samples contain two generations of olivine (Ol I and Ol II) and orthopyroxene (Opx I and Opx II), one generation of clinopyroxene (Cpx I), spinel (Spl I), amphibole (Amp), and many fluid inclusion trails. We compare these xenoliths to samples from Injibara, Lake Tana Province, approximately 200 km north of Nekempte on the Ethiopian Plateau, a region with less focused extensional strain. Xenoliths from Injibara are also spinel-bearing lherzolites, however they exhibit less strain and metasomatic textures in comparison to xenoliths from along the YTVL. Based on the petrographic and textural analyses performed, we propose that the larger scale

  12. Global equivalent magnetization of the oceanic lithosphere (United States)

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


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

  13. Lithospheric Delamination or Relict Slab Beneath the Former North American Cratonic Margin in Idaho and Oregon? New Constraints From Seismic Tomography. (United States)

    Stanciu, A. C.; Russo, R. M.; Mocanu, V. I.; VanDecar, J. C.; Hongsresawat, S.; Bremner, P. M.; Torpey, M. E.; Panning, M. P.


    We present a new high-resolution P-wave velocity model of the upper mantle beneath the former passive margin of the North American craton in Oregon and Idaho. We identify high velocity anomalies in the central part of the model and low velocity anomalies to the northwest and southeast. Our results derive from an integrated data set of teleseismic P waves recorded at 145 broadband stations, 85 deployed between 2011 and 2013 as part of the IDOR Passive experiment, and 60 USArray-TA stations. We determined 15,000 travel-times using multi-channel cross-correlation (VanDecar and Crosson, 1990). Phanerozoic tectonic events that affected upper mantle seismic structure here include subduction of Farallon and Juan de Fuca lithosphere, accretion of Blue Mountains terranes, Sevier and Laramide orogenies, Idaho batholith formation, Yellowstone and Columbia River volcanism, and Basin and Range extension. Our results indicate a high P-wave velocity anomaly located beneath the Idaho Batholith in west-central Idaho traceable down to 150-200 km depth. A similar anomaly identified by Schmandt and Humphrey (2011) beneath Washington and Montana was interpreted as a slab remnant from the accretion of Siletzia to North America. Alternatively, the fast Vp anomalies are delaminated North American craton lithosphere. Thickened lithosphere may have formed during Farallon subduction, terrane collision and accretion. Crust as much as 55 km thick present during Late Cretaceous (Foster et al., 2001; Gaschnig et al., 2011) is potentially indicative of lithospheric thickening leading to delamination. To the southeast, upper mantle low velocity anomalies occur beneath the Western Snake River Plain. We associate these low velocities with high temperatures generated by the Yellowstone mantle plume system. We observe a low velocity anomaly beneath the Wallowa Mountains starting at 150-200 km extending to depths below the resolution of our model.

  14. Hydration of marginal basins and compositional variations within the continental lithospheric mantle inferred from a new global model of shear and compressional velocity

    DEFF Research Database (Denmark)

    Tesoniero, Andrea; Auer, Ludwig; Boschi, Lapo


    VP/VS and not extremely low VS at ∼150 km depth, consistently with presence of water. Most pronounced anomalies are located in the Sea of Japan, in the back-arc region of the Philippine Sea, and in the South China Sea. Our results indicate the effectiveness of slab-related processes to hydrate...... the mantle and suggest an important role of Pacific plate subduction also for the evolution of the South China Sea. We detect lateral variations in composition within the continental lithospheric mantle. Regions that have been subjected to rifting, collisions, and flood basalt events are underlain...

  15. Modelling the interplate domain in thermo-mechanical simulations of subduction: Critical effects of resolution and rheology, and consequences on wet mantle melting (United States)

    Arcay, Diane


    The present study aims at better deciphering the different mechanisms involved in the functioning of the subduction interplate. A 2D thermo-mechanical model is used to simulate a subduction channel, made of oceanic crust, free to evolve. Convergence at constant rate is imposed under a 100 km thick upper plate. Pseudo-brittle and non-Newtonian behaviours are modelled. The influence of the subduction channel strength, parameterized by the difference in activation energy between crust and mantle (ΔEa) is investigated to examine in detail the variations in depth of the subduction plane down-dip extent, zcoup . First, simulations show that numerical resolution may be responsible for an artificial and significant shallowing of zcoup if the weak crustal layer is not correctly resolved. Second, if the age of the subducting plate is 100 Myr, subduction occurs for any ΔEa . The stiffer the crust is, that is, the lower ΔEa is, the shallower zcoup is (60 km depth if ΔEa = 20 kJ/mol) and the hotter the fore-arc base is. Conversely, imposing a very weak subduction channel (ΔEa > 135 J/mol) leads there to an extreme mantle wedge cooling and inhibits mantle melting in wet conditions. Partial kinematic coupling at the fore-arc base occurs if ΔEa = 145 kJ/mol. If the incoming plate is 20 Myr old, subduction can occur under the conditions that the crust is either stiff and denser than the mantle, or weak and buoyant. In the latter condition, cold crust plumes rise from the subduction channel and ascend through the upper lithosphere, triggering (1) partial kinematic coupling under the fore-arc, (2) fore-arc lithosphere cooling, and (3) partial or complete hindrance of wet mantle melting. zcoup then ranges from 50 to more than 250 km depth and is time-dependent if crust plumes form. Finally, subduction plane dynamics is intimately linked to the regime of subduction-induced corner flow. Two different intervals of ΔEa are underlined: 80-120 kJ/mol to reproduce the range of slab

  16. Estimation of seismic velocity in the subducting crust of the Pacific slab beneath Hokkaido, northern Japan by using guided waves (United States)

    Shiina, T.; Nakajima, J.; Toyokuni, G.; Kita, S.; Matsuzawa, T.


    A subducting crust contains a large amount of water as a form of hydrous minerals (e.g., Hacker et al., 2003), and the crust plays important roles for water transportation and seismogenesis in subduction zones at intermediate depths (e.g., Kirby et al., 1996; Iwamori, 2007). Therefore, the investigation of seismic structure in the crust is important to understand ongoing physical processes with subduction of oceanic lithosphere. A guided wave which propagates in the subducting crust is recorded in seismograms at Hokkaido, northern Japan (Shiina et al., 2014). Here, we estimated P- and S-wave velocity in the crust with guided waves, and obtained P-wave velocity of 6.6-7.3 km/s and S-wave velocity of 3.6-4.2 km/s at depths of 50-90 km. Moreover, Vp/Vs ratio in the crust is calculated to be 1.80-1.85 in that depth range. The obtained P-wave velocity about 6.6km/s at depths of 50-70 km is consistent with those estimated in Tohoku, northeast Japan (Shiina et al., 2013), and this the P-wave velocity is lower than those expected from models of subducting crustal compositions, such as metamorphosed MORB model (Hacker et al., 2003). In contrast, at greater depths (>80 km), the P-wave velocity marks higher velocity than the case of NE Japan and the velocity is roughly comparable to those of the MORB model. The obtained S-wave velocity distribution also shows characteristics similar to P waves. This regional variation may be caused by a small variation in thermal regime of the Pacific slab beneath the two regions as a result of the normal subduction in Tohoku and oblique subduction in Hokkaido. In addition, the effect of seismic anisotropy in the subducting crust would not be ruled out because rays used in the analysis in Hokkaido propagate mostly in the trench-parallel direction, while those in Tohoku are sufficiently criss-crossed.

  17. Lower slab boundary in the Japan subduction zone (United States)

    Tonegawa, Takashi; Hirahara, Kazuro; Shibutani, Takuo; Fujii, Naoyuki


    We have successfully detected the lower boundary of a subducting slab. The successive imaging of the lower slab boundary beneath northeastern (NE) Japan is attained by receiver function (RF) depth conversion analysis using a recent 3D tomographic velocity model. We use waveforms from 249 teleseismic events collected by Hi-net and J-array short-period stations in NE Japan. RFs are calculated through frequency domain division of radial components by vertical ones with a water level of 0.001 and a 1.0 Hz low-pass Gaussian filter. Assuming that all later phases in the radial RFs are due to Ps phases converted at discontinuities beneath stations, we calculate depth-converted RFs, mapped onto the cross-section with the CCP (common conversion point) stacking. In a cross section, the slab surface and the oceanic Moho can be imaged down to 120 km depth. For the greater depths, the RF amplitudes corresponding to them cannot be seen, because, in the oceanic crust, basalt would be completely metamorphosed to eclogite below this depth. The lower boundary of the Pacific slab can also be traced down to 200 km depth or more. It is parallel to the slab surface and the oceanic Moho, and the thickness between the slab surface and the lower boundary is ˜ 80 km. Finally, we estimate a top-to-bottom slab velocity model that explains the RFs observed at broadband stations with the synthetic RFs. This model exhibits a 13% velocity reduction downwards the lower slab boundary, which would relatively sharp for the base of the thermal boundary layer. Therefore, this sharp discontinuity is presumably considered to be the subducting G (Gutenberg) discontinuity that is formed by the change of the amount of H 2O (water), meaning that the G discontinuity is the chemical boundary at the bottom of the oceanic lithosphere. The G discontinuity depth is controlled by the potential temperature of the asthenospheric mantle beneath the mid-ocean ridge, and hence the observed thickness of 80 km, i.e. the

  18. A Review of Recent Developments in the Study of Regional Lithospheric Electrical Structure of the Asian Continent (United States)

    Zhang, Letian


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

  19. Imaging the Ionian Sea subducting slab panels and faults to control present day motion in the Hellenic-Aegean region (United States)

    Sachpazi, Maria


    The Hellenic-Mediterranean subduction system characterized by its fast overriding upper plate, fast trench retreat and its most rapidly extending Corinth Rift has been the target of several conceptual models on slab dynamics and lithosphere extension. Using teleseismic waves conversions on a dense 2-D seismic array -installed in the frame of Thales Was Right project- from Crete to the North Aegean coast through central Greece, a high-resolution imaging of the Hellenic slab and the overlying Aegean plate lithospheric mantle has been acquired. The subducting slab top appears segmented into panels 30- 50km wide by SW-NE along dip faults to at least 100km depth. Intermediate-depth Mw>6 earthquakes are located on those faults which implies that they are seismically active at 70 km depth. Smaller magnitude earthquakes of the upper Benioff zone commonly related to dehydration processes of the descending slab, are also resolved to be clustered along these faults. These faults are likely inherited structures of the oceanic lithosphere and sites of preferred hydration. Their revealed relation with this specific seismicity provides high-resolution insight validating dehydration embrittlement. RF imaging on 4 OBS sites has allowed to resolve the depth and geometry of the updip offshore part of the slab, the thrust interplate boundary. The observations support a trenchward continuation of the slab faults and correlation with the similarly segmented thrusting contact of the Mediterranean Ridge accretionary wedge over the upper plate. The slab faults may control the location and size of major historical megathrust earthquakes a hypothesis that has been strengthened by the study of the Mw 6.8 14.02.2008 earthquake, the first large instrumental interplate earthquake offshore SW Peloponnesus. New high-resolution imaging resolves the Aegean plate lithospheric mantle and shows the presence of a significant heterogeneity on top of the presently subducting slab, never imaged before. It

  20. Subduction zone and crustal dynamics of western Washington; a tectonic model for earthquake hazards evaluation (United States)

    Stanley, Dal; Villaseñor, Antonio; Benz, Harley


    The Cascadia subduction zone is extremely complex in the western Washington region, involving local deformation of the subducting Juan de Fuca plate and complicated block structures in the crust. It has been postulated that the Cascadia subduction zone could be the source for a large thrust earthquake, possibly as large as M9.0. Large intraplate earthquakes from within the subducting Juan de Fuca plate beneath the Puget Sound region have accounted for most of the energy release in this century and future such large earthquakes are expected. Added to these possible hazards is clear evidence for strong crustal deformation events in the Puget Sound region near faults such as the Seattle fault, which passes through the southern Seattle metropolitan area. In order to understand the nature of these individual earthquake sources and their possible interrelationship, we have conducted an extensive seismotectonic study of the region. We have employed P-wave velocity models developed using local earthquake tomography as a key tool in this research. Other information utilized includes geological, paleoseismic, gravity, magnetic, magnetotelluric, deformation, seismicity, focal mechanism and geodetic data. Neotectonic concepts were tested and augmented through use of anelastic (creep) deformation models based on thin-plate, finite-element techniques developed by Peter Bird, UCLA. These programs model anelastic strain rate, stress, and velocity fields for given rheological parameters, variable crust and lithosphere thicknesses, heat flow, and elevation. Known faults in western Washington and the main Cascadia subduction thrust were incorporated in the modeling process. Significant results from the velocity models include delineation of a previously studied arch in the subducting Juan de Fuca plate. The axis of the arch is oriented in the direction of current subduction and asymmetrically deformed due to the effects of a northern buttress mapped in the velocity models. This

  1. Crustal structure of the Carpathian orogen from receiver function analysis: how craton subduction and active delamination affect the crust (United States)

    Petrescu, Laura; Tataru, Dragos; Grecu, Bogdan


    The Carpathian arc is an uncommon curved collisional system, involving the subduction of the Eastern European craton and the Proterozoic Moesian platform beneath younger European microplates. The Cenozoic collision led to the closure of the Tethys Oceanic basin, portions of which are actively breaking off or delaminating beneath the orogen, generating deep mantle earthquakes. Neogene volcanism, possibly related to subduction slab roll-back, also formed a band of presently extinct volcanoes in the back-arc region. The Carpathian embayment is thus an ideal laboratory to investigate crustal processes related to subduction of cratonic material, multiple plate junctions and active delamination. To better understand how the crustal structure changes from the Eastern European cratonic foreland, across the curved subduction zone, to the younger European microplates, we analyse teleseismic earthquakes recorded at broadband seismic stations located across eastern and southern Carpathians, in Romania and Moldova. We processed data from permanent seismic networks (The Romanian National Seismic Network) as well as data from temporary deployments such as CALIXTO (Carpathian Arc Lithosphere X-Tomography) and SCP (South Carpathian Project). Using extended multi-taper spectral division, we compute and analyse radial and transverse receiver functions. Energy on the transverse component may be an indicator of crustal anisotropy or the existence of intracrustal dipping interfaces. Using phase-weighted H-k stacking of receiver functions, we estimate the crustal thickness and the bulk crustal Poisson's ratio as well as the seismic sharpness of the Moho discontinuity. Furthermore, we invert receiver functions to obtain the S-wave velocity structure of the crust and upper mantle beneath individual stations, which provide concurrent information on the Moho nature. Our results provide a better understanding of crustal structure across complex collisional systems involving the subduction of

  2. P-wave tomography of Northeast Asia: Constraints on the western Pacific plate subduction and mantle dynamics (United States)

    Ma, Jincheng; Tian, You; Liu, Cai; Zhao, Dapeng; Feng, Xuan; Zhu, Hongxiang


    A high-resolution model of 3-D P-wave velocity structure beneath Northeast Asia and adjacent regions is determined by using 244,180 arrival times of 14,163 local and regional earthquakes and 319,857 relative travel-time residuals of 9988 teleseismic events recorded at ∼2100 seismic stations in the study region. Our tomographic results reveal the subducting Pacific slab clearly as a prominent high-velocity anomaly from the Japan Trench to the North-South Gravity lineament (NSGL) in East China. The NSGL is roughly coincident with the western edge of the stagnant Pacific slab in the mantle transition zone (MTZ). The subducting Pacific slab has partly sunk into the lower mantle beneath Northeast China, but under the Sino-Korean Craton the slab lies horizontally in the MTZ. The NSGL, as an important tectonic line in Mainland China, is marked by sharp differences in the surface topography, gravity anomaly, crustal and lithospheric thickness and mantle seismic velocity from the east to the west. These features of the NSGL and large-scale hot and wet upwelling in the big mantle wedge (BMW) in the east of the NSGL are all related to the subduction processes of the Western Pacific plate. The Changbai intraplate volcanic group is underlain by a striking low-velocity anomaly from the upper MTZ and the BMW up to the surface, and deep earthquakes (410-650 km depths) occur actively in the subducting Pacific slab to the east of the Changbai volcano. We propose that the Changbai volcanic group is caused by upwelling of hot and wet asthenospheric materials and active convection in the BMW. The formation of other volcanic groups in the east of the NSGL is also associated with the subduction-driven corner flow in the BMW.

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

    NARCIS (Netherlands)

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


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

  4. Intrinsic and Extrinsic Factors in Subduction Dynamics (United States)

    Billen, Magali; Arredondo, Katrina


    Since the realization that tectonic plates sink into the mantle, in a process we now call subduction, our understanding of this process has improved dramatically through the combined application of observations, theory and modeling. During that time independent research groups focusing on different aspects of subduction have identified factors with a significant impact on subduction, such as three-dimensionality, slab rollback, rheology of the slab and mantle and magnitude of phase changes. However, as each group makes progress we often wonder how these different factors interact as we all strive to understand the real world subduction system. These factors can be divided in two groups: intrinsic factors, including the age of the slab, its thermal structure, composition, and rheology, and extrinsic factors including others forces on plates, overall mantle flow, structure of the overriding plate, rheology of the mantle and phase changes. In addition, while modeling has been a powerful tool for understanding subduction, all models make important (but often necessary) approximations, such as using two dimensions, imposed boundary conditions, and approximations of the conservation equations and material properties. Here we present results of a study in which the "training wheels" are systematically removed from 2D models of subduction to build a more realistic model of subduction and to better understand how combined effects of intrinsic and extrinsic factors contribute to the dynamics. We find that a change from the Boussinesq to the extended Boussinesq form of the conservation equations has a dramatic effect on slab evolution in particular when phase changes are included. Allowing for free (dynamically-driven) subduction and trench motion is numerically challenging, but also an important factor that allows for more direct comparison to observations of plate kinematics. Finally, compositional layering of the slab and compositionally-controlled phase changes also have

  5. The Southern Tyrrhenian subduction system: recent evolution and neotectonic implications

    Directory of Open Access Journals (Sweden)

    A. Argnani


    Full Text Available Geological and geophysical data have been integrated with the aim of presenting a new evolutionary model for the Southern Tyrrhenian and adjacent regions. The Southern Tyrrhenian backarc basin opened within a plate convergence regime because of sinking and rollback of the oceanic Ionian lithosphere. On the basis of seismological observations, I infer that the sinking slab was torn apart on either side in the last 2 Ma and this process controlled the neotectonics of the Southern Apennines - Tyrrhenian region. On the north-eastern side the slab broke off from NW to SE and this process triggered volcanism and NW-SE extension along the Eastern Tyrrhenian margin, and strike-slip tectonics along NW-SE trending faults in Northern Calabria. On the south-western side the slab broke off from W to E along the Aeolian Island alignment, although the tear has currently been reoriented along the NNW-SSE Malta escarpment. During its sinking the subducted slab also detached from the overriding plate, favouring the wedging of the asthenosphere between the two plates and the regional uplift of the Calabrian arc and surroundings. This regional uplift promoted gravitational instability within the orogenic wedge, particularly towards low topography areas; the large-scale sliding of the Calabrian arc towards the Ionian basin can be the cause of CW rotation and graben formation in Calabria. Also the E-dipping extensional faults of the Southern Apennines can be related to accommodation of vertical motions within the fold-and-thrust belt. The pattern of recent seismicity reflects this neotectonics where crustal-scale gravity deformation within the orogenic wedge is responsible for extensional earthquakes in Calabria and the Southern Apennines, whereas Africa plate convergence can account for compressional earthquakes in Sicily.

  6. Subduction controls the distribution and fragmentation of Earth’s tectonic plates. (United States)

    Mallard, Claire; Coltice, Nicolas; Seton, Maria; Müller, R Dietmar; Tackley, Paul J


    The theory of plate tectonics describes how the surface of Earth is split into an organized jigsaw of seven large plates of similar sizes and a population of smaller plates whose areas follow a fractal distribution. The reconstruction of global tectonics during the past 200 million years suggests that this layout is probably a long-term feature of Earth, but the forces governing it are unknown. Previous studies, primarily based on the statistical properties of plate distributions, were unable to resolve how the size of the plates is determined by the properties of the lithosphere and the underlying mantle convection. Here we demonstrate that the plate layout of Earth is produced by a dynamic feedback between mantle convection and the strength of the lithosphere. Using three-dimensional spherical models of mantle convection that self-consistently produce the plate size–frequency distribution observed for Earth, we show that subduction geometry drives the tectonic fragmentation that generates plates. The spacing between the slabs controls the layout of large plates, and the stresses caused by the bending of trenches break plates into smaller fragments. Our results explain why the fast evolution in small back-arc plates reflects the marked changes in plate motions during times of major reorganizations. Our study opens the way to using convection simulations with plate-like behaviour to unravel how global tectonics and mantle convection are dynamically connected.

  7. Testing the intraplate origin of mega-earthquakes at subduction margins

    Directory of Open Access Journals (Sweden)

    Prosanta K. Khan


    Full Text Available The disastrous Mw 9.3 (seismic moment 1.0×1030 dyn/cm earthquake that struck northwest Sumatra on 26 December 2004 and triggered ∼30 m high tsunami has rejuvenated the quest for identifying the forcing behind subduction related earthquakes around the world. Studies reveal that the strongest part (elastic core of the oceanic lithosphere lie between 20 and 60 km depth beneath the upper (∼7 km thick crustal layer, and compressive stress of GPa order is required to fail the rock-layers within the core zone. Here we present evidences in favor of an intraplate origin of mega-earthquakes right within the strong core part (at the interface of semi-brittle and brittle zone, and propose an alternate model exploring the flexing zone of the descending lithosphere as the nodal area for major stress accumulation. We believe that at high confining pressure and elevated temperature, unidirectional cyclic compressive stress loading in the flexing zone results in an increase of material yield strength through strain hardening, which transforms the rheology of the layer from semi-brittle to near-brittle state. The increased compressive stress field coupled with upward migration of the neutral surface (of zero stress fields under non-coaxial deformation triggers shear crack. The growth of the shear crack is initially confined in the near-brittle domain, and propagates later through the more brittle crustal part of the descending oceanic lithosphere in the form of cataclastic failure.

  8. Recovering lateral variationin lithospheric strength from bedrock motion data using a coupled ice sheet-lithosphere model

    NARCIS (Netherlands)

    van de Berg, W.J.|info:eu-repo/dai/nl/304831611; van de Wal, R.S.W.|info:eu-repo/dai/nl/101899556; Oerlemans, J.|info:eu-repo/dai/nl/06833656X


    A vertically integrated two-dimensional ice flow model was coupled to an elastic lithosphere-Earth model to study the effects of lateral variations in lithospheric strength on local bedrock adjustment. We used a synthetic bedrock profile and a synthetic climate to model a characteristic ice sheet

  9. Recovering lateral variations in lithospheric strength from bedrock motion data using a coupled ice sheet-lithosphere model

    NARCIS (Netherlands)

    Berg, J. van den; Wal, R.S.W. van de; Oerlemans, J.


    A vertically integrated two-dimensional ice flow model was coupled to an elastic lithosphere-Earth model to study the effects of lateral variations in lithospheric strength on local bedrock adjustment. We used a synthetic bedrock profile and a synthetic climate to model a characteristic ice sheet

  10. Seismic Structure of the Subducted Cocos Plate (United States)

    Clayton, R. W.; Davis, P. M.; Perez-Campos, X.


    The Meso-American Subduction Experiment (MASE) was designed to determine the critical parameters to necessary to simulate the subduction process in Central Mexico . A preliminary analysis of the data shows a 200km section of the slab that is subhorizontal and to within the resolution of the receiver functions it underplates the continental crust with no intervening asthenosphere. This is an interesting situation because the short-term (GPS) and long-term (geologic) strain measurements show almost no compressive strain in this region. This would imply that the crust is decoupled from the subducting slab. Near the coast, the receiver functions show that the slab cuts through the crust at an approximately a 15-degree angle, and under the Trans-Mexican Volcanic Belt the slab becomes detached from the crust, but its geometry at depth is not yet determined from the receiver functions, but a well-developed mantle wedge is apparent from the attenuation of regional earthquakes.

  11. Diffusion creep of fine-grained garnetite: Implications for the flow strength of subducting slabs (United States)

    Wang, Zichao; Ji, Shaocheng


    Creep experiments were performed on synthetic fine-grained garnetite to investigate the flow strength of the Earth's subducting slabs. Experiments were conducted at temperatures (T) of 1373-1543 K and total pressure (P) of 0.1 MPa in controlled atmospheres of fO2 =10-17-10-8 MPa. The mechanical data indicate a grain-size sensitive diffusion flow and the creep behavior can be described by an equation of the form: FD1 ɛ.=(5.32±3.10)×10-6Td2.5±0.3fO20σ1.1±0.2exp(-347±46kJ/molRT) where T in Kelvin, d in meter, σ and fO2 in MPa. Based on the diffusivities (D) calculated from creep and diffusion experiments, we proposed that grain boundary diffusion is the dominant mechanism for high temperature creep of the fine-grained garnetite. Normalized creep strength of the garnetite is found to be comparable to those of feldspar and olivine in diffusion creep regime, suggesting that garnetite may not form a strong layer in the subducted oceanic lithosphere if it deforms by grain boundary diffusion creep.

  12. Imaging the Middle America subduction zone with body waves extracted from ambient noise by seismic interferometry (United States)

    Vargas, W.; Brown, L. D.; Cabolova, A.; Quiros, D. A.; Chen, C.


    Subduction zones have long been a prime target for seismic imaging with a variety of active and passive methodologies. Here we report an attempt to use seismic interferometry to extract body waves (P and S) from ambient noise recorded during a broadband experiment in southwestern Mexico for reflection imaging of the crust and subducting Cocos plate. The Middle America Subduction Experiment (MASE; Kim et al., 2010) included a quasi linear array of 100 broadband seismic instruments deployed at a nominal spacing of 6 km which continuously recorded for up to 30 months. Our focus was on using cross-correlation and autocorrelation of ambient noise along this array to 1) determine if useful body waves could be extracted, 2) assess which conditions were most favorable for such extraction, and 3) evaluate whether these waves could be used to image deep lithospheric structure, with particular interest in the seismogenic zone. While surface wave tomography using cross-correlation techniques have found widespread success in mapping crustal structure, examples of body wave imaging of crustal targets using this approach are still very few. In our analysis, we have found it necessary to suppress the surface wave energy to enhance body waves from virtual sources. Our pre-processing sequence includes bias removal, bandpass filtering, deconvolution (spectral whitening), and sign- bit conversion. The resulting data windows are cross-correlated and stacked until useful signals are apparent. The virtual shot gathers thus far produced show clear Rayleigh and Pg waves, with weaker but distinct Sg phases. We have also found arrivals with hyperbolic travel times that match those expected for deep reflections. Crustal imaging is limited by the large station spacing, which results in relatively few stations at sub-critical offsets. However several apparent reflections from sub-Moho depths suggest that key elements of the subduction process can be imaged using reflections derived from ambient

  13. Metamorphic zirconology of continental subduction zones (United States)

    Chen, Ren-Xu; Zheng, Yong-Fei


    Zircon is widely used to date geological events and trace geochemical sources in high-pressure (HP) to ultrahigh-pressure (UHP) metamorphic rocks of continental subduction zones. However, protolith zircons may be modified by three different types of metamorphic recrystallization via mechanisms of solid-state transformation, metasomatic alteration and dissolution reprecipitation; new zircon growth may be induced by dehydration reactions below the wet solidus of crustal rocks (metamorphic zircon) or peritectic reactions above the wet solidus (peritectic zircon). As a consequence, there are different origins of zircon domains in high-grade metamorphic rocks from collisional orogens. Thus, determining the nature of individual zircon domains is substantial to correct interpretation of their origin in studies of isotopic geochronology and geochemical tracing. We advocate an integrated study of zircon mineragraphy (internal structure and external morphology), U-Pb ages, mineral inclusions, trace elements, and Lu-Hf and O isotope compositions. Only in this way we are in a position to advance the simple zircon applications to metamorphic zirconology, enabling discrimination between the different origins of zircon and providing constraints on the property of fluid activity at subduction-zone conditions. The metamorphic recrystallization of protolith zircons and the new growth of metamorphic and peritectic zircons are prominent in HP to UHP metamorphic rocks of collisional orogens. These different types of recrystallized and grown zircons can be distinguished by their differences in element and isotope compositions. While the protolith nature of metamorphosed rocks dictates water availability, the P-T conditions of subduction zones dictate the property of subduction-zone fluids. The fluids of different properties may be produced at different positions of subducting and exhuming crustal slices, and they may physically and chemically mix with each other in continental

  14. Electrical conductivity imaging in the western Pacific subduction zone (United States)

    Utada, Hisashi; Baba, Kiyoshi; Shimizu, Hisayoshi


    Oceanic plate subduction is an important process for the dynamics and evolution of the Earth's interior, as it is regarded as a typical downward flow of the mantle convection that transports materials from the near surface to the deep mantle. Recent seismological study showed evidence suggesting the transportation of a certain amount of water by subduction of old oceanic plate such as the Pacific plate down to 150-200 km depth into the back arc mantle. However it is not well clarified how deep into the mantle the water can be transported. The electromagnetic induction method to image electrical conductivity distribution is a possible tool to answer this question as it is known to be sensitive to the presence of water. Here we show recent result of observational study from the western Pacific subduction zone to examine the electrical conductivity distribution in the upper mantle and in the mantle transition zone (MTZ), which will provide implications how water distributes in the mantle. We take two kinds of approach for imaging the mantle conductivity, (a) semi-global and (b) regional induction approaches. Result may be summarized as follows: (a) Long (5-30 years) time series records from 8 submarine cables and 13 geomagnetic observatories in the north Pacific region were analyzed and long period magnetotelluric (MT) and geomagnetic deep sounding (GDS) responses were estimated in the period range from 1.7 to 35 days. These frequency dependent response functions were inverted to 3-dimensional conductivity distribution in the depth range between 350 and 850 km. Three major features are suggested in the MTZ depth such as, (1) a high conductivity anomaly beneath the Philippine Sea, (2) a high conductivity anomaly beneath the Hawaiian Islands, and (3) a low conductivity anomaly beneath and in the vicinity of northern Japan. (b) A three-year long deployment of ocean bottom electro-magnetometers (OBEM's) was conducted in the Philippine Sea and west Pacific Ocean from 2005

  15. Thermochronology of the Sulu ultrahigh-pressure metamorphic terrane: Implications for continental collision and lithospheric thinning (United States)

    Liu, Li-Ping; Li, Zheng-Xiang; Danišík, Martin; Li, Sanzhong; Evans, Noreen; Jourdan, Fred; Tao, Ni


    The thermal history of the Dabie-Sulu orogenic belt provides important constraints on the collision process between the South China and North China blocks during the Mesozoic, and possible lithospheric thinning event(s) in the eastern North China Block. This study reports on the thermal evolution of the Sulu ultrahigh-pressure metamorphic (UHP) terrane using zircon U-Pb geochronology and multiple thermochronology methods such as mica and hornblende 40Ar/39Ar, zircon and apatite fission track, and zircon and apatite (U-Th)/He dating. 40Ar/39Ar and zircon (U-Th)/He data show that the UHP terrane experienced accelerated cooling during 180-160 Ma. This cooling event could be interpreted to have resulted from extensional unroofing of an earlier southward thrusting nappe, or, more likely, an episode of northward thrusting of the UHP rocks as a hanging wall. A subsequent episode of exhumation took place between ca. 125 Ma and 90 Ma as recorded by zircon (U-Th)/He data. This event was more pronounced in the northwest section of the UHP terrane, whereas in the southeast section, the zircon (U-Th)/He system retained Jurassic cooling ages of ca. 180-160 Ma. The mid-Cretaceous episode of exhumation is interpreted to have resulted from crustal extension due to the removal of thickened, enriched mantle. A younger episode of exhumation was recorded by apatite fission track and apatite (U-Th)/He ages at ca. 65-40 Ma. Both latter events were linked to episodic thinning of lithosphere along the Sulu UHP terrane in an extensional environment, likely caused by the roll-back of the Western Pacific subduction system.

  16. Mantle Subduction and Uplift of Intracontinental Mountains: A Case Study from the Chinese Tianshan Mountains within Eurasia. (United States)

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


    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.

  17. Lithosphere Deformation Modelling of the Italian Peninsula: A Tool for Seismic Hazard Assessment (United States)

    Jiménez-Munt, I.; Pagani, M.; Marcellini, A.; Sabadini, R.


    Large uncertainties of input data for seismic hazard assessment, like the incompleteness of the historical catalogues, magnitude estimation uncertainties and low reliability of epicentral location forces the adoption of additional information for the characterization of seismicity. Lithosphere deformation measuring and modelling techniques were largely increased and improved during the last decade; the detail and the reliability of the deformation models presently available make possible the development of methodologies for constraining the geodynamical evolution of active areas and particularly of their long term seismotectonic behaviour. In the present work we describe and test a procedure to create a probabilistic hazard source model for some areas of the Italian peninsula by integrating crustal deformation modelling results with historical evidences of earthquake occurrence. This procedure uses a Bayesian approach with geophysical input to define the earthquake occurrence behaviour: historical earthquake data characterize the sample likelihood function while strain derived occurrence rates define prior distribution parameters. Modelled strain rates are calculated by means of a finite element model based on the thin shell scheme of peninsular Italy and central Mediterranean that simulates the effects of Africa-Eurasia convergence and subduction underneath the Calabrian Arc on lithospheric deformation. The computed horizontal velocities and strain rates are compared with their geodetic counterparts retrieved by a network of permanent GPS receivers in the area. In order to reproduce the clockwise rotation in southern Italy from the NNW direction of the NOTO site (south eastern Sicily), envisaging the motion of the Africa plate, to the NNE direction of GPS sites in Calabria and Matera, the effects of the Calabrian subduction must be completed by the effects of the counter-clockwise rotation of the Adria microplate. This kinematics of Adria reproduces the NNE motion

  18. How inheritance, geochemical and geophysical properties of the lithospheric mantle influence rift development and subsequent collision (United States)

    Picazo, Suzanne; Chenin, Pauline; Müntener, Othmar; Manatschal, Gianreto; Karner, Garry; Johnson, Christopher


    . Conversely to a classical subduction where the oceanic lithosphere being subducted produces a mobile component that contributes to the formation of long-lived volcanic arcs, a hyper-extended rifted system and small oceanic basins (Journal of Geophysical Research, 108(B1):2029. Picazo, S., Müntener, O., Manatschal, G., Bauville, A., Karner, G., & Johnson, C. (2016). Mapping the nature of mantle domains in Western and Central Europe based on clinopyroxene and spinel chemistry: Evidence for mantle modification during an extensional cycle. Lithos, 266, 233-263.

  19. Influence of the subducting plate velocity on the geometry of the slab and migration of the subduction hinge

    NARCIS (Netherlands)

    Schellart, Wouter P.


    Geological observations indicate that along two active continental margins (East Asia and Mediterranean) major phases of overriding plate extension, resulting from subduction hinge-retreat, occurred synchronously with a reduction in subducting plate velocity. In this paper, results of fluid

  20. Stress orientations in subduction zones and the strength of subduction megathrust faults. (United States)

    Hardebeck, Jeanne L


    Subduction zone megathrust faults produce most of the world's largest earthquakes. Although the physical properties of these faults are difficult to observe directly, their frictional strength can be estimated indirectly by constraining the orientations of the stresses that act on them. A global investigation of stress orientations in subduction zones finds that the maximum compressive stress axis plunges systematically trenchward, consistently making an angle of 45° to 60° with respect to the subduction megathrust fault. These angles indicate that the megathrust fault is not substantially weaker than its surroundings. Together with several other lines of evidence, this implies that subduction zone megathrusts are weak faults in a low-stress environment. The deforming outer accretionary wedge may decouple the stress state along the megathrust from the constraints of the free surface. Copyright © 2015, American Association for the Advancement of Science.

  1. Stress orientations in subduction zones and the strength of subduction megathrust faults (United States)

    Hardebeck, Jeanne L.


    Subduction zone megathrust faults produce most of the world’s largest earthquakes. Although the physical properties of these faults are difficult to observe directly, their frictional strength can be estimated indirectly by constraining the orientations of the stresses that act on them. A global investigation of stress orientations in subduction zones finds that the maximum compressive stress axis plunges systematically trenchward, consistently making a 45°-60° angle to the subduction megathrust fault. These angles indicate that the megathrust fault is not substantially weaker than its surroundings. Together with several other lines of evidence, this implies that subduction zone megathrusts are weak faults in a low-stress environment. The deforming outer accretionary wedge may decouple the stress state along the megathrust from the constraints of the free surface.

  2. Imprints of a Pan-African transpressional orogen superimposed on an inferred Grenvillian accretionary belt in central East Antarctica (United States)

    Ferraccioli, Fausto; Seddon, Samuel; Finn, Carol; Bell, Robin; Wu, Guochao; Jordan, Tom


    The Gamburtsev Subglacial Mountains in interior East Antarctica are underlain by 50-60 km thick crust imaged by gravity and seismic models (Ferraccioli et al., 2011; An et al., 2015). In contrast, the composite Archean to Mesoproterozoic Mawson craton that occupies the Wilkes and Terre Adelie sector of East Antarctica typically features only 40-45 km thick crust (Aitken et al., 2014). Over 200 km thick and seismically fast lithosphere underlies the Gamburtsev Province, as typically observed over Precambrian lithosphere that has not been substantially reworked during Phanerozoic subduction or collision. Satellite and airborne magnetic data indicate that the Gamburtev Province is sandwiched in between distinct Precambrian lithospheric blocks including the Ruker, Princess Elizabeth Land, Vostok, Nimrod (Goodge and Finn, 2010), South Pole and Recovery provinces. Ferraccioli et al., (2011) proposed that a segment of a stalled orogen (i.e. an orogen where widespread orogenic collapse and root delamination has not occurred) is preserved in the Gamburtsev Province and further hypothesised that its origin relates to widespread accretionary and subsequent collisional events at ca 1 Ga, linked to the assembly of the Rodinia supercontinent. However, recent passive seismic interpretations (An et al., 2015) indicate that crustal thickening may relate instead to Pan-African age assembly of Greater India, East Antarctica and Australia within Gondwana (at ca 550 Ma). Here we interpret a set of enhanced magnetic and gravity images, depth to magnetic and gravity sources and preliminary 2D and 3D forward and inverse models to characterise in detail the crustal architecture of the Gamburtsev Province. Enhanced aeromagnetic images reveal a system of subglacial faults that segment the Gamburtsev Province into three distinct geophysical domains, the northern, central and southern domains. Apparent offsets in high-frequency magnetic anomalies within the central domain are interpreted here

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

    DEFF Research Database (Denmark)

    Artemieva, Irina


    The purpose of this chapter is to provide a summary of geophysical data on the structure of the stable continental lithosphere and its evolution since the Archean. Here, the term lithosphere is used to define the outer layer of the Earth which includes the crust and uppermost mantle, forms...... the roots of the continents, and moves together with continental plates. Depending on geophysical techniques (and physical properties measured), the lithosphere has different practical definitions. Most of them (i.e., seismic, electrical) are on the basis of a sharp change in temperature-dependent physical...

  4. 3D instantaneous dynamics modeling of present-day Aegean subduction (United States)

    Glerum, Anne; Spakman, Wim; van Hinsbergen, Douwe; Pranger, Casper


    To study the sensitivity of surface observables to subduction and mantle flow, i.e. the coupling of crustal tectonics and the underlying mantle dynamics, we have developed 3D numerical models of the instantaneous crust-mantle dynamics of the eastern Mediterranean. These models comprise both a realistic crust-lithosphere system and the underlying mantle. The focus for this presentation lies on the regional crustal flow response to the present-day Aegean subduction system. Our curved model domain measures 40°x40°x2900km with the Aegean subduction system taken as the geographic center. Model set-ups are based on geological and geophysical data of the eastern Mediterranean. We first create a 3D synthetic geometry of the crust-lithosphere system in a stand-alone program, including the present-day configuration of the plates in the region and crust and lithosphere thickness variations abstracted from Moho and LAB maps (Faccenna et al., 2014, Carafa et al., 2015). In addition we construct the geometry of the Aegean slab from a seismic tomography model (UU-P07; Amaru, 2007) and earthquake hypocenters (NCEDC, 2014). Geometries are then imported into the finite element code ASPECT (Kronbichler et al., 2012) using specially designed plugins. The mantle initial temperature conditions can include deviations from an adiabatic profile obtained from conversion of the UU-P07 seismic velocity anomalies to temperature anomalies using a depth-dependent scaling (Karato, 2008). We model compressible mantle flow for which material properties are obtained from thermodynamics P-T lookup-tables (Perple_X, Connolly, 2009) in combination with nonlinear viscoplastic rheology laws. Sublithospheric flow through the lateral model boundaries is left free via open boundary conditions (Chertova et al., 2012), while plate motion is prescribed at the model sides in terms of relative as well as absolute plate motion velocities (e.g. Doubrovine et al., 2012). So far, we used a free-slip surface, but

  5. Subduction and volatile recycling in Earth's mantle (United States)

    King, S. D.; Ita, J. J.; Staudigel, H.


    The subduction of water and other volatiles into the mantle from oceanic sediments and altered oceanic crust is the major source of volatile recycling in the mantle. Until now, the geotherms that have been used to estimate the amount of volatiles that are recycled at subduction zones have been produced using the hypothesis that the slab is rigid and undergoes no internal deformation. On the other hand, most fluid dynamical mantle flow calculations assume that the slab has no greater strength than the surrounding mantle. Both of these views are inconsistent with laboratory work on the deformation of mantle minerals at high pressures. We consider the effects of the strength of the slab using two-dimensional calculations of a slab-like thermal downwelling with an endothermic phase change. Because the rheology and composition of subducting slabs are uncertain, we consider a range of Clapeyron slopes which bound current laboratory estimates of the spinel to perovskite plus magnesiowustite phase transition and simple temperature-dependent rheologies based on an Arrhenius law diffusion mechanism. In uniform viscosity convection models, subducted material piles up above the phase change until the pile becomes gravitationally unstable and sinks into the lower mantle (the avalanche). Strong slabs moderate the 'catastrophic' effects of the instabilities seen in many constant-viscosity convection calculations; however, even in the strongest slabs we consider, there is some retardation of the slab descent due to the presence of the phase change.

  6. Earth's oldest mantle fabrics indicate Eoarchaean subduction. (United States)

    Kaczmarek, Mary-Alix; Reddy, Steven M; Nutman, Allen P; Friend, Clark R L; Bennett, Vickie C


    The extension of subduction processes into the Eoarchaean era (4.0-3.6 Ga) is controversial. The oldest reported terrestrial olivine, from two dunite lenses within the ∼3,720 Ma Isua supracrustal belt in Greenland, record a shape-preferred orientation of olivine crystals defining a weak foliation and a well-defined lattice-preferred orientation (LPO). [001] parallel to the maximum finite elongation direction and (010) perpendicular to the foliation plane define a B-type LPO. In the modern Earth such fabrics are associated with deformation of mantle rocks in the hanging wall of subduction systems; an interpretation supported by experiments. Here we show that the presence of B-type fabrics in the studied Isua dunites is consistent with a mantle origin and a supra-subduction mantle wedge setting, the latter supported by compositional data from nearby mafic rocks. Our results provide independent microstructural data consistent with the operation of Eoarchaean subduction and indicate that microstructural analyses of ancient ultramafic rocks provide a valuable record of Archaean geodynamics.

  7. Earthquake nucleation in weak subducted carbonates

    NARCIS (Netherlands)

    Kurzawski, Robert M.; Stipp, Michael; Niemeijer, André R.; Spiers, Chirstopher J.; Behrmann, Jan H.

    Ocean-floor carbonate- and clay-rich sediments form major inputs to subduction zones, especially at low-latitude convergent plate margins. Therefore, knowledge of their frictional behaviour is fundamental for understanding plate-boundary earthquakes. Here we report results of mechanical tests

  8. Global correlations between maximum magnitudes of subduction zone interface thrust earthquakes and physical parameters of subduction zones

    NARCIS (Netherlands)

    Schellart, W. P.; Rawlinson, N.


    The maximum earthquake magnitude recorded for subduction zone plate boundaries varies considerably on Earth, with some subduction zone segments producing giant subduction zone thrust earthquakes (e.g. Chile, Alaska, Sumatra-Andaman, Japan) and others producing relatively small earthquakes (e.g.

  9. Dynamics of intraoceanic subduction initiation : 1. Oceanic detachment fault inversion and the formation of supra-subduction zone ophiolites

    NARCIS (Netherlands)

    Maffione, Marco; Thieulot, Cedric|info:eu-repo/dai/nl/270177493; van Hinsbergen, Douwe J.J.|info:eu-repo/dai/nl/269263624; Morris, Antony; Plümper, Oliver|info:eu-repo/dai/nl/37155960X; Spakman, Wim|info:eu-repo/dai/nl/074103164

    Subduction initiation is a critical link in the plate tectonic cycle. Intraoceanic subduction zones can form along transform faults and fracture zones, but how subduction nucleates parallel to mid-ocean ridges, as in e.g., the Neotethys Ocean during the Jurassic, remains a matter of debate. In

  10. Shallow and deep lithosphere slabs beneath the Dinarides from teleseismic tomography as the result of the Adriatic lithosphere downwelling (United States)

    Šumanovac, Franjo; Markušić, Snježana; Engelsfeld, Tihomir; Jurković, Klaudia; Orešković, Jasna


    The study area covers the Dinarides and southwestern part of the Pannonian basin as the marginal zone between the Adriatic microplate (African plate) and the Pannonian tectonic segment (Eurasian plate). We created a three-dimensional seismic velocity model to 450 km depth using teleseismic tomography. Our travel-time dataset was collected by means of 40 seismic stations from the ORFEUS database and Croatian Seismological Survey database. A set of 90 teleseismic earthquakes were selected in the time range 2014-2015, and relative P-wave travel-time residuals were calculated. For the first time the seismic P-wave velocity model of a relatively high resolution on the entire Dinaridic mountain belt was obtained. Based on this model, a more reliable insight in the relations of the lithosphere plates has been achieved. We imaged a fast velocity anomaly extending underneath the entire Dinaridic mountain belt which indicates cold, rigid materials. The anomaly is steeply sloping towards the northeast and directly indicates the sinking of the Adriatic microplate underneath the Pannonian tectonic segment. In the Northern Dinarides the anomaly extends to the depth of 250 km, whereas in the Southern Dinarides it covers greater depths, up to 450 km. The shallow Adriatic slab extends along the External Dinarides, while the deep Adriatic slab extends beneath the Internal Dinarides and ophiolite zones in the area of central and southern Dinarides. Different slab depths are interpreted as the faster convergence of the plate in the southern Dinarides than in the northern, or the convergence of the plates had started in the southern part and systematically developed to the north.

  11. The lithosphere of the Antarctic continent: new insights from satellite gravity gradient data (United States)

    Ferraccioli, Fausto; Ebbing, Jorg; Pappa, Folker; Kern, Michael; Forsberg, Rene


    gravity data arguably provides one the clearest large-scale views to date of the potential extent of the Archean to Mesoproterozoic Terre Adelie Craton, and clearly shows the contrast wrt to the crust and lithosphere underlying both the Wilkes Subglacial Basin to the east and the Sabrina Subglacial Basin to the west. This finding corroborates and also augments recent independent interpretations of aeromagnetic and airborne gravity data over the region, suggesting that the Mawson Continent is a composite lithospheric-scale entity, which was affected by several Paleoproterozoic and Mesoproterozoic orogenic events (Aitken et al., 2016). Thick crust is clearly imaged beneath the Transantarctic Mountains, the Terre Adelie Craton, the Gamburtsev Subglacial Mountains and also Eastern Dronning Maud Land, in particular beneath the recently proposed region of the Tonian Oceanic Arc Superterrane (Jacobs et al., 2015). The GIA and GIU components help delineate the edges of several of these lithospheric provinces, both in West and East Antarctica. One of the largest and previously unknown lithospheric-scale features discovered in East Antarctica from the satellite gravity gradient images is a linear feature that appears to cut across East Antarctica, potentially extending from the area of the Lutzow Holm Complex on the Indian side of East Antarctica right across the continent to South Pole. We name this feature the Trans East Antarctic Shear Zone and propose that it represents a major lithospheric scale shear zone and possibly a major suture zone that separates the Gamburtsev Province from the Eastern Dronning Maud Land Province and also appears to form the southern boundary of the composite Recovery Province. We infer based on geological data in the Lutzow Holm Complex region and formerly adjacent segments of India and Madagascar and eastern Africa that it may represent a major hitherto unrecongnised Pan-African age suture zone related to the assembly of the Gondwana supercontinent

  12. Implications of a-priori constraints in transdimensional Bayesian inversion for continental lithospheric layering (United States)

    Roy, Corinna; Calo, Marco; Bodin, Thomas; Romanowicz, Barbara


    Competing hypotheses for the formation and evolution of continents are highly under debate, including the theory of underplating by hot plumes or accretion by shallow subduction in continental or arc settings. In order to support these hypotheses, documenting structural layering in the cratonic lithosphere becomes especially important. Studies of seismic-wave receiver function data have detected a structural boundary under continental cratons at 100-140 km depths, which is too shallow to be consistent with the lithosphere-asthenosphere boundary, as inferred from seismic tomography and other geophysical studies. This leads to the conclusion that 1) the cratonic lithosphere may be thinner than expected, contradicting tomographic and other geophysical or geochemical inferences, or 2) that the receiver function studies detect a mid-lithospheric discontinuity rather than the LAB. Recent studies (Bodin et al., 2015, Calò et al. 2016) confirmed the presence of a structural boundary under the North American craton at 100-140 km depths by taking advantage of the power of a trans-dimensional Monte Carlo Markov chain (TMCMC). They generated probabilistic 1D radially shear wave velocity profiles for selected stations in North America by jointly inverting 2 different data types (PS Receiver Functions, surface wave dispersion for Love and Rayleigh waves), which sample different volumes of the Earth and have different sensitivities to 
structure. In fact, they found at least one, and in some cases several additional mid-lithospheric discontinuities (MLD) at intermediate depths in the stable part of the craton. Such discontinuities are not present in the active western part of the US. However, in their Bayesian approach, they made two major assumptions: First, they fixed the Vp/Vs ratio to a constant, averaged value for crust and mantle. Second, they added constraints on the crustal discontinuity depths in the prior distribution of the shear wave velocity. Given these strong

  13. Lithospheric structure beneath the northeastern Tibetan Plateau and the western Sino-Korea Craton revealed by Rayleigh wave tomography (United States)

    Li, Yonghua; Pan, Jiatie; Wu, Qingju; Ding, Zhifeng


    A new 3-D shear wave velocity model of the northeastern (NE) Tibetan Plateau and western Sino-Korea Craton is presented. The model is based on Rayleigh waves recorded at 650 portable stations deployed in the region. Interstation phase and group velocity dispersions for more than 18 000 paths were estimated using the two-station method and then inverted to produce phase velocity maps for 10-80 s period and group velocity maps for 10-60 s period. Local 1-D shear wave velocity profiles for each 0.5° × 0.5°grid node were obtained by inverting Rayleigh wave dispersions obtained in this study together with previously published Rayleigh wave group velocities between 60 and 145 s and then assembled into a 3-D shear wave velocity model. The images obtained reveal an obvious mid-crustal low-velocity zone (LVZ) and low-velocity anomaly in the upper mantle beneath the Songpan-Ganzi terrane. The mid-crustal LVZ can be explained by the presence of partial melting, most likely from asthenospheric upwelling. The existence of a relatively weak mid-crustal LVZ beneath the Qilian orogeny is also confirmed. This LVZ is likely caused by the presence of deep crustal fluids. The Yinchuan-Hetao graben is characterized by relatively low velocities that extend to at least 200 km below the Earth's surface, strongly contrasting with the seismically fast lithosphere of the Ordos and Alax blocks. The model presented here shows the presence of a relatively thick and high-velocity lithosphere beneath the Ordos and NE Alxa blocks, as well as evidence of thinning of the lithosphere beneath the southwestern Alxa and Qilian blocks, indicating that the Alxa block is not subducting beneath NE Tibet.

  14. Passive seismoacoustic imaging from the seafloor to the lithosphere: Methods and applications to New Zealand and Ascension Island (United States)

    Ball, Justin S.

    potentially associated with subducted Pacific lithosphere and a relict Eocene passive margin, and a low-speed zone that correlates with Cenozoic surface volcanism.

  15. Lithospheric structure across the California Continental Borderland from receiver functions

    National Research Council Canada - National Science Library

    Reeves, Zachary; Lekić, Vedran; Schmerr, Nicholas; Kohler, Monica; Weeraratne, Dayanthie


    .... We map variations in present‐day lithospheric structure across the region using Ps and Sp receiver functions at permanent stations of the Southern California Seismic Network as well as ocean bottom seismometer (OBS...

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

    Digital Repository Service at National Institute of Oceanography (India)

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

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

  17. MONA LISA - Deep seismic investigations of the lithosphere in the southeastern North Sea (United States)

    MONA LISA Working Group


    The MONA LISA collaborative project has collected 1112 km of seismic normal-incidence reflection data (recorded to 26 s) and wide-angle data from 26 onshore and 2 offshore locations along 4 profiles in the southeastern North Sea. The seismic data clearly image structures in the crust and uppermost mantle that may be related to Caledonian collision and Late Palaeozoic to Mesozoic rifting and basin formation. For the first time, dipping and subhorizontal reflections from the mantle have been observed to 24 s twt in seismic normal-incidence reflection sections. Strong sub-horizontal reflections are observed on two perpendicular profiles at c. 21 s twt, which is near the base of the lithosphere. These deep reflections are sharper than expected from a thermal-rheological transition suggesting that they are generated at lithological or tectonic interfaces. The Moho is reflective along all four profiles. Particularly strong and sharp normal-incidence and wide-angle reflections were recorded from the Moho south of the Caledonian Deformation Front. The normal-incidence reflection sections show unusually low intra-crustal reflectivity. Lower crustal, subhorizontal reflectivity is only observed west of the Central Graben and in a short profile segment at the base of the interpreted Caledonian Deformation Front. The crustal suture between Avalonia and Baltica is interpreted to be south- to west-dipping. SSW- and N-dipping reflections to 20 s twt are observed from the uppermost mantle. Two models of Tornquist Sea subduction may explain these events: either southward subduction and later extensional reactivation or northward subduction with later extension along a SSW-dipping shear zone. The crust is c. 5 km thinner below the Central Graben than below the surrounding parts of the Mid North Sea-Ringkøbing-Fyn High. The lower crust is reflective on the western flank of the Central Graben. East-dipping reflections from the lower crust and the upper mantle to the east of the graben

  18. Philippine Sea and East Asian plate tectonics since 52 Ma constrained by new subducted slab reconstruction methods (United States)

    Wu, Jonny; Suppe, John; Lu, Renqi; Kanda, Ravi


    We reconstructed Philippine Sea and East Asian plate tectonics since 52 Ma from 28 slabs mapped in 3-D from global tomography, with a subducted area of ~25% of present-day global oceanic lithosphere. Slab constraints include subducted parts of existing Pacific, Indian, and Philippine Sea oceans, plus wholly subducted proto-South China Sea and newly discovered "East Asian Sea." Mapped slabs were unfolded and restored to the Earth surface using three methodologies and input to globally consistent plate reconstructions. Important constraints include the following: (1) the Ryukyu slab is ~1000 km N-S, too short to account for ~20° Philippine Sea northward motion from paleolatitudes; (2) the Marianas-Pacific subduction zone was at its present location (±200 km) since 48 ± 10 Ma based on a >1000 km deep slab wall; (3) the 8000 × 2500 km East Asian Sea existed between the Pacific and Indian Oceans at 52 Ma based on lower mantle flat slabs; (4) the Caroline back-arc basin moved with the Pacific, based on the overlapping, coeval Caroline hot spot track. These new constraints allow two classes of Philippine Sea plate models, which we compared to paleomagnetic and geologic data. Our preferred model involves Philippine Sea nucleation above the Manus plume (0°/150°E) near the Pacific-East Asian Sea plate boundary. Large Philippine Sea westward motion and post-40 Ma maximum 80° clockwise rotation accompanied late Eocene-Oligocene collision with the Caroline/Pacific plate. The Philippine Sea moved northward post-25 Ma over the northern East Asian Sea, forming a northern Philippine Sea arc that collided with the SW Japan-Ryukyu margin in the Miocene (~20-14 Ma).

  19. Petrogenesis of fertile mantle peridotites from the Monte del Estado massif (southwest Puerto Rico): a preserved section of Proto-Caribbean oceanic lithospheric mantle? (United States)

    Marchesi, Claudio; Jolly, Wayne T.; Lewis, John F.; Garrido, Carlos J.; Proenza, Joaquín. A.; Lidiak, Edward G.


    The Monte del Estado massif is the largest and northernmost serpentinized peridotite belt in southwest Puerto Rico. It is mainly composed of spinel lherzolite and minor harzburgite with variable clinopyroxene modal abundances. Mineral and whole rock major and trace element compositions of peridotites coincide with those of fertile abyssal peridotites from mid ocean ridges. Peridotites lost 2-14 wt% of relative MgO and variable amounts of CaO by serpentinization and seafloor weathering. HREE contents in whole rock indicate that the Monte del Estado peridotites are residues after low to moderate degrees (2-15%) of fractional partial melting in the spinel stability field. However, very low LREE/HREE and MREE/HREE in clinopyroxene cannot be explained by melting models of a spinel lherzolite source and support that the Monte del Estado peridotites experienced initial low fractional melting degrees (~ 4%) in the garnet stability field. The relative enrichment of LREE in whole rock is not due to secondary processes but probably reflects the capture of percolating melt fractions along grain boundaries or as microinclusions in minerals, or the presence of exotic micro-phases in the mineral assemblage. We propose that the Monte del Estado peridotite belt represents a section of ancient Proto-Caribbean (Atlantic) lithospheric mantle originated by seafloor spreading between North and South America in the Late Jurassic-Early Cretaceous. This portion of oceanic lithospheric mantle was subsequently trapped in the forearc region of the Greater Antilles paleo-island arc generated by the northward subduction of the Caribbean plate beneath the Proto-Caribbean ocean. Finally, the Monte del Estado peridotites belt was emplaced in the Early Cretaceous probably as result of the change in subduction polarity of the Greater Antilles paleo-island arc without having been significantly modified by subduction processes.

  20. Receiver Function Imaging of Crustal and Lithospheric Structure Beneath the Jalisco Block and Western Michoacan, Mexico. (United States)

    Reyes Alfaro, G.; Cruz-Atienza, V. M.; Perez-Campos, X.; Reyes Dávila, G. A.


    We used a receiver function technique for imaging western Mexico, a unique area with several active seismic and volcanic zones like the triple junction of Rivera, Cocos and North American plates and the Colima volcano complex (CVC), the most active in Mexico. Clear images of the distribution of the crust and the lithosphere-asthenosphere boundary are obtained using P-to-S receiver functions (RF) from around ~80 broadband stations recorded by the Mapping the Rivera Subduction Zone (MARS), the Colima Volcano Deep Seismic Experiment (CODEX) and a local network (RESCO) that allowed us to considerably increase the teleseismic database used in the project. For imaging, we constructed several 2-D profiles of depth transformed RFs to delineate the seismic discontinuities of the region. Low seismic velocities associated with the Michoacan-Guanajuato and the Mascota-Ayutla-Tapalpa volcanic fields are also observed. Most impressive, a large and well delineated magma body 100 km underneath CVC is recognized along a surely related depression of the moho discontinuity just above it. We bring more tools for a better understanding of the deep processes that ultimately control eruptive behavior in the region.

  1. Low heat flow from young oceanic lithosphere at the Middle America Trench off Mexico [rapid communication (United States)

    Minshull, Timothy A.; Bartolomé, Rafael; Byrne, Siobhán; Dañobeitia, Juanjo


    Seismic reflection profiles across the Middle America Trench at 20°N show a high amplitude bottom simulating reflector interpreted as marking a phase transition between methane hydrate and free gas in the pore space of both accreted and trench sediments. We determine the depth of the hydrate-gas phase boundary in order to estimate the geothermal gradient and hence the heat flow beneath the trench and the frontal part of the accretionary wedge which overlies the downgoing plate. After correction for sedimentation, heat flow values in the trench and through the accretionary wedge are only about half of the values predicted by plate cooling models for the 10 Ma subducting lithosphere. There is no systematic correlation between heat flow in the accretionary wedge and distance from the trench. A comparison with heat flow predicted by a simple analytical model suggests that there is little shear heating from within or beneath the wedge, despite the high basal friction suggested by the large taper angle of the wedge. The geothermal gradient varies systematically along the margin and is negatively correlated with the frontal slope of the wedge. Some local peaks may be attributed to channelised fluid expulsion.

  2. Magsat to CHAMP: Magnetic Satellite Explorations of Lithospheric Anomalies over Kursk, Bangui and the Antarctic (United States)

    Kim, H.; Taylor, Patrick T.; vonFrese, R. R.; Kim, J. W.


    We compare crustal magnetic anomaly maps over the Kursk (Russia) and Bangui (Central African Republic) isolated anomalies and the Antarctic derived from the Magsat, \\Orsted and CHAMP satellite fields. We wish to demonstrate how progress in satellite magnetic missions has improved the recovery of the crustal magnetic field. The 6-month long Magsat mission of 25 years ago generated two major methods of processing satellite magnetic anomaly data for lithospheric studies. The first was a global perspective using spherical harmonics that emphasize the more regional and global lithospheric fields. However, these fields commonly do not resolve local anomaly features in any detail. Therefore a second procedure involved the use of the individual satellite orbit or track data to recover small-scale anomalies on a regional scale. We present results over prominent magnetic anomalies such as Kursk, Bangui and the large Antarctic continent that demonstrate how the various analysis methods affect the recovery of crustal anomalies. The more recent \\Orsted and CHAMP missions are successfully recording data with an improved accuracy and with full spatial and temporal coverage. We show and interpret the total magnetic intensity anomaly maps over these areas from all three satellite magnetometer data sets.

  3. Flat-slab subduction, whole crustal faulting, and geohazards in Alaska: Targets for Earthscope (United States)

    Gulick, S. P.; Pavlis, T. L.; Bruhn, R. L.; Christeson, G. L.; Freymueller, J. T.; Hansen, R. A.; Koons, P. O.; Pavlis, G. L.; Roeske, S.; Reece, R.; van Avendonk, H. J.; Worthington, L. L.


    Crustal structure and evolution illuminated by the Continental Dynamics ST. Elias Erosion and tectonics Project (STEEP) highlights some fundamental questions about active tectonics processes in Alaska including: 1) what are the controls on far field deformation and lithospheric stabilization, 2) do strike slip faults extend through the entire crust and upper mantle and how does this influence mantle flow, and 3) how does the transition from “normal” subduction of the Pacific along the Aleutians to flat slab subduction of the Yakutat Terrane beneath southeast and central Alaska to translation of the Yakutat Terrane past North American in eastern Alaska affect geohazard assessment for the north Pacific? Active and passive seismic studies and geologic fieldwork focusing on the Yakutat Terrane show that the Terrane ranges from 15-35 km thick and is underthrusting the North American plate from the St. Elias Mountains to the Alaska Range (~500 km). Deformation of the upper plate occurs within the offshore Pamplona Zone fold and thrust belt, and onshore throughout the Robinson Mountains. Deformation patterns, structural evolution, and the sedimentary products of orogenesis are fundamentally influenced by feedbacks with glacial erosion. The Yakutat megathrust extends beneath Prince William Sound such that the 1964 Mw 9.2 great earthquake epicenter was on this plate boundary and jumped to the adjacent Aleutian megathrust coseismically; this event illuminates the potential for transitional tectonic systems to enhance geohazards. The northern, southern, and eastern limits of the Yakutat microplate are strike-slip faults that, where imaged, appear to cut the entire crustal section and may allow for crustal extrusion towards the Bering Sea. Yakutat Terrane effects on mantle flow, however, have been suggested to cross these crustal features to allow for far-field deformation in the Yukon, Brooks Range, and Amerasia Basin. From the STEEP results it is clear that the Yakutat

  4. Dynamic Linkages Between the Transition Zone & Surface Plate Motions in 2D Models of Subduction (United States)

    Arredondo, K.; Billen, M. I.


    While slab pull is considered the dominant force controlling plate motion and speed, its magnitude is controlled by slab behavior in the mantle, where tomographic studies show a wide range of possibilities from direct penetration to folding, or stagnation directly above the lower mantle (e.g. Fukao et al., 2009). Geodynamic studies have investigated various parameters, such as plate age and two phase transitions, to recreate observed behavior (e.g. Běhounková and Cízková, 2008). However, past geodynamic models have left out known slab characteristics that may have a large impact on slab behavior and our understanding of subduction processes. Mineral experiments and seismic observations have indicated the existence of additional phase transitions in the mantle transition zone that may produce buoyancy forces large enough to affect the descent of a subducting slab (e.g. Ricard et al., 2005). The current study systematically tests different common assumptions used in geodynamic models: kinematic versus free-slip boundary conditions, the effects of adiabatic heating, viscous dissipation and latent heat, compositional layering and a more complete suite of phase transitions. Final models have a complete energy equation, with eclogite, harzburgite and pyrolite lithosphere compositional layers, and seven composition-dependent phase transitions within the olivine, pyroxene and garnet polymorph minerals. Results show important feedback loops between different assumptions and new behavior from the most complete models. Kinematic models show slab weakening or breaking above the 660 km boundary and between compositional layers. The behavior in dynamic models with a free-moving trench and overriding plate is compared to the more commonly found kinematic models. The new behavior may have important implications for the depth distribution of deep earthquakes within the slab. Though the thermodynamic parameters of certain phase transitions may be uncertain, their presence and

  5. Subduction zone earthquakes and stress in slabs (United States)

    Vassiliou, M. S.; Hager, B. H.


    Simple viscous fluid models of subducting slabs are used to explain observations of the distribution of earthquakes as a function of depth and the orientation of stress axes of deep (greater than 300 km) and intermediate (70-300 km) earthquakes. Results suggest the following features in the distribution of earthquakes with depth: (1) an exponential decrease from shallow depths down to 250 to 300 km, (2) a minimum near 250 to 300 km, and (3) a deep peak below 300 km. Many shallow subducting slabs show only the first characteristic, while deeper extending regions tend to show all three features, with the deep peak varying in position and intensity. These data, combined with the results on the stress orientations of various-depth earthquakes, are consistent with the existence of a barrier of some sort at 670-km depth and a uniform viscosity mantle above this barrier.

  6. The hidden simplicity of subduction megathrust earthquakes (United States)

    Meier, M.-A.; Ampuero, J. P.; Heaton, T. H.


    The largest observed earthquakes occur on subduction interfaces and frequently cause widespread damage and loss of life. Understanding the rupture behavior of megathrust events is crucial for earthquake rupture physics, as well as for earthquake early-warning systems. However, the large variability in behavior between individual events seemingly defies a description with a simple unifying model. Here we use three source time function (STF) data sets for subduction zone earthquakes, with moment magnitude Mw ≥ 7, and show that such large ruptures share a typical universal behavior. The median STF is scalable between events with different sizes, grows linearly, and is nearly triangular. The deviations from the median behavior are multiplicative and Gaussian—that is, they are proportionally larger for larger events. Our observations suggest that earthquake magnitudes cannot be predicted from the characteristics of rupture onsets.

  7. Subduction related fluids fractionate Nb/Ta (United States)

    Salters, V. J.; Bizimis, M.; Sachi-Kocher, A.; Taylor, R.; Savov, I. P.; Stern, C. R.


    Key differences between the chemical composition of terrestrial materials and those of meteorites have led to the suggestion that a `hidden’ high Nb/Ta reservoir exists in the Earth’s mantle. In order to test this hypothesis we must identify the processes that can create such a reservoir. It has been suggested that during subduction Nb is more refractory then Ta resulting in low Nb/Ta in the subducted slab, which then serves as a reservoir for the high Nb/Ta. Here we report high precision HFSE data on products of the subduction processes thought to fractionate Nb from Ta: boninites (hydrous melting), adakites (slab melting), oceanic island arc basalts and supra subduction zone peridotites. We developed a new method for the high precision determination of Nb, Ta, Zr, Hf concentrations based on a modified version of standard addition. All analyses were performed on a single collector ICPMS (ELEMENT 1), using Y and Yb as internal standards to correct for instrumental drift during the unspiked -spiked sample sequence. Concentrations are calculated using a York- type regression that accounts for all measured and propagated errors. Long-term reproducibility (multiple dissolutions and multiple spike solutions) for the standards BHVO-1, BIR-1 AGV-1 and BCR-1 are better than 0.8% (1s) for Nb/Ta and Zr/Hf ratios. The advantages of this method compared to previous methods are fast throughput, no column chemistry and low blanks. While the Zr/Hf ratios in subduction-related volcanics and ocean island basalts vary by less than a factor of two, the Nb/Ta ratio varies by a factor of four. Most of the Nb/Ta variation is observed in subduction related rocks. Samples with the highest Nb/Ta ratio (up to 19.5) are adakites from the Austral Volcanic Zone (Andes) which are thought to represent eclogitic melts from subducted oceanic crust which was most likely dehydrated. The lowest Nb/Ta (5) was found in boninites from Chichi-Jima, Bonin Island. Samples from Chichi-Jima and from the

  8. Diverse melanges of an ancient subduction complex

    Energy Technology Data Exchange (ETDEWEB)

    Lash, G.G.


    Three lithologically and structurally diverse melanges occur within an early Paleozoic (Early-Middle Ordovician) subduction complex in the central Appalachian orogen. Type I melange, characterized by horizons of variably deformed sandstone and scaly mudstone that alternate with coherent sandstone-rich sequences, is interpreted to reflect accretion-related deformation of water-saturated trench deposits. Type II melange, composed of exotic radiolaria-bearing mudstone clasts in a scaly mudstone matrix, can be explained by remobilization and mixing of inner-trench slope sediments. Type III melange is a poorly sorted polymict assemblage of native lithology clasts in a scaly mudstone matrix. Evidence of forceful injection of matrix mud into clasts and inferred discordant contacts between melange and surrounding bedded deposits suggest that the type III melange formed from mud diapirism. The close association of these melanges points out the diversity of tectonic and sedimentary processes previously documented from modern convergent margins that may be reflected in older subduction complexes.

  9. Volcanism and Subduction: The Kamchatka Region (United States)

    Eichelberger, John; Gordeev, Evgenii; Izbekov, Pavel; Kasahara, Minoru; Lees, Jonathan

    The Kamchatka Peninsula and contiguous North Pacific Rim is among the most active regions in the world. Kamchatka itself contains 29 active volcanoes, 4 now in a state of semi-continuous eruption, and I has experienced 14 magnitude 7 or greater earthquakes since accurate recording began in 1962. At its heart is the uniquely acute subduction cusp where the Kamchatka and Aleutian Arcs and Emperor Seamount Chain meet. Volcanism and Subduction covers coupled magmatism and tectonics in this spectacular region, where the torn North Pacific slab dives into hot mantle. Senior Russian and American authors grapple with the dynamics of the cusp with perspectives from the west and east of it, respectively, while careful tephrostratigraphy yields a remarkably precise record of behavior of storied volcanoes such as Kliuchevskoi and Shiveluch. Towards the south, Japanese researchers elucidate subduction earthquake processes with unprecedented geodetic resolution. Looking eastward, new insights on caldera formation, monitoring, and magma ascent are presented for the Aleutians. This is one of the first books of its kind printed in the English language. Students and scientists beginning research in the region will find in this book a useful context and introduction to the region's scientific leaders. Others who wish to apply lessons learned in the North Pacific to their areas of interest will find the volume a valuable reference.

  10. Regional differences in subduction ground motions

    CERN Document Server

    Beauval, Céline; Abrahamson, N; Theodulidis, N; Delavaud, E; Rodriguez, L; Scherbaum, F; Haendel, A


    A few ground-motion prediction models have been published in the last years, for predicting ground motions produced by interface and intraslab earthquakes. When one must carry out a probabilistic seismic hazard analysis in a region including a subduction zone, GMPEs must be selected to feed a logic tree. In the present study, the aim is to identify which models provide the best fit to the dataset M6+, global or local models. The subduction regions considered are Japan, Taiwan, Central and South America, and Greece. Most of the data comes from the database built to develop the new BCHydro subduction global GMPE (Abrahamson et al., submitted). We show that this model is among best-fitting models in all cases, followed closely by Zhao et al. (2006), whereas the local Lin and Lee (2008) is well predicting the data in Taiwan and also in Greece. The Scherbaum et al. (2009) LLH method prove to be efficient in providing one number quantifying the overall fit, but additional analysis on the between-event and within-ev...

  11. Global model for the lithospheric strength and effective elastic thickness (United States)

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


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

  12. Introduction to the structures and processes of subduction zones (United States)

    Zheng, Yong-Fei; Zhao, Zi-Fu


    Subduction zones have been the focus of many studies since the advent of plate tectonics in 1960s. Workings within subduction zones beneath volcanic arcs have been of particular interest because they prime the source of arc magmas. The results from magmatic products have been used to decipher the structures and processes of subduction zones. In doing so, many progresses have been made on modern oceanic subduction zones, but less progresses on ancient oceanic subduction zones. On the other hand, continental subduction zones have been studied since findings of coesite in metamorphic rocks of supracrustal origin in 1980s. It turns out that high-pressure to ultrahigh-pressure metamorphic rocks in collisional orogens provide a direct target to investigate the tectonism of subduction zones, whereas oceanic and continental arc volcanic rocks in accretionary orogens provide an indirect target to investigate the geochemistry of subduction zones. Nevertheless, metamorphic dehydration and partial melting at high-pressure to ultrahigh-pressure conditions are tectonically applicable to subduction zone processes at forearc to subarc depths, and crustal metasomatism is the physicochemical mechanism for geochemical transfer from the slab to the mantle in subduction channels. Taken together, these provide us with an excellent opportunity to find how the metamorphic, metasomatic and magmatic products are a function of the structures and processes in both oceanic and continental subduction zones. Because of the change in the thermal structures of subduction zones, different styles of metamorphism, metasomatism and magmatism are produced at convergent plate margins. In addition, juvenile and ancient crustal rocks have often suffered reworking in episodes independent of either accretionary or collisional orogeny, leading to continental rifting metamorphism and thus rifting orogeny for mountain building in intracontinental settings. This brings complexity to distinguish the syn-subduction

  13. Great earthquakes hazard in slow subduction zones (United States)

    Marcaillou, B.; Gutscher, M.; Westbrook, G. K.


    Research on the Sumatra-Andaman earthquake of 2004 has challenged two popular paradigms; that the strongest subduction earthquakes strike in regions of rapid plate convergence and that rupture occurs primarily along the contact between the basement of the overriding plate and the downgoing plate. Subduction zones presenting similar structural and geodynamic characteristics (slow convergence and thick wedges of accreted sediment) may be capable of generating great megathrust earthquakes (M>8.5) despite an absence of thrust type earthquakes over the past 40 years. Existing deep seismic sounding data and hypocenters are used to constrain the geometry of several key slow subduction zones (Antilles, Hellenic, Sumatra). This geometry forms the basis for numerical modelling of fore-arc thermal structure, which is applied to calculate the estimated width of the seismogenic portion of the subduction fault plane. The margins with the thickest accretionary wedges are commonly found to have the widest (predicted) seismogenic zone. Furthermore, for these margins there exists a substantial (20-60 km wide) region above the up-dip limit for which the contribution to tsunami generation is poorly understood. As the rigidity (mu) of these high-porosity sediments is low, co-seismic slip here can be expected to be slow. Accordingly, the contribution to seismic moment will be low, but the contribution to tsunami generation may be very high. Indeed, recent seismological data from Nankai indicate very low frequency shallow-thrust earthquakes beneath this portion of the accretionary wedge, long-considered to be "aseismic". We propose that thick accumulations of sediment on the downgoing plate and the presence of a thick accretionary wedge can increase the maximum size of the potential rupture fault plane in two ways; 1) by thermally insulating the downgoing plate and thereby increasing the total downdip length of the fault which can rupture seismically and 2) by "smoothing out" the

  14. Subduction of oceanic plate irregularities and seismicity distribution along the Mexican Subduction Zone (United States)

    Manea, Marina; Constantin Manea, Vlad; Gerya, Taras; Wong, Raul-Valenzuela; Radulian, Mircea


    It is known that oceanic plates morphology is not a simple one, but rather complicated by a series of irregularities as seamounts, fracture zones and mid-ocean ridges. These features present on the oceanic floor form part of the fabric of oceanic crust, and once formed they move together with the oceanic plates until eventually enter a subduction zone. Offshore Mexico the oceanic Cocos plate seafloor is littered with relatively small but numerous seamounts and seamount chains, and also large fracture zones. In this study we investigate the relationship between these oceanic irregularities located in the vicinity of the trench in Mexico and the distribution of subduction seismicity, including the rupture history of large subduction zone earthquakes. Since the interseismic locking degree is influenced by the rheological properties of crustal and mantle rocks, any variations along strike will result in significant changes in seismic behavior due to a change in frictional stability. Our preliminary study shows a direct relationship between the presence of seamounts chains on the incoming oceanic plate and the subduction seismicity distribution. We also found a clear relationship between the subduction of the Tehuantepec fracture zone (TFZ) and the low seismic activity in the region where this fracture zone intersects the trench. This region is also long term conspicuously quiet and considered a seismic gap where no significant large earthquake has occurred in more than 100 years. Using high-resolution three-dimensional coupled petrological-thermomechanical numerical simulations specifically tailored for the subduction of the Cocos plate in the region of TFZ we show that the weakened serpentinized fracture zone is partially scraped out in the forearc region because of its low strength and positive buoyancy. The presence of serpentinite in the fore arc apparently lowers the degree of interseismic locking, producing a seismic gap in southern Mexico.

  15. What's happening inside the subduction factory? (United States)

    Penniston-Dorland, S. C.; Bebout, G. E.; Gorman, J. K.; Piccoli, P. M.; Walker, R. J.


    Much research has focused on the inputs and outputs of the 'subduction factory,' however a variety of metamorphic processes occur within the subducting slab and at its interface with the mantle wedge that contribute to creating the mixed signals observed in arc magmas. Subduction-related metamorphic rocks from the Catalina Schist represent a range of metamorphic grades and provide a natural laboratory to investigate these processes. Hybrid rock types such as reaction zones or 'rinds' between mafic (crustal) and ultramafic (mantle) rocks have attracted recent interest since they have a different bulk chemistry and mineralogy compared to the original inputs to the subduction factory. Here we explore the mineralogical and geochemical differences between the metamorphic rocks, their reaction zones, and endmember subduction input lithologies over a range of metamorphic grades including lawsonite albite, lawsonite blueschist, and amphibolite facies (with peak T ranging from ~ 275 to ~ 750°C and peak P ranging up to ~1.1 GPa). The results shed light on chemical changes occurring within the subduction zone and the processes happening inside the 'subduction factory', including mass transfer of elements by both fluid infiltration and mechanical mixing. Elements commonly enriched in arc magmatic rocks, such as the LILE (e.g. Ba, K), are enriched in metamafic rocks at all metamorphic grades relative to likely MORB protoliths. These enrichments are interpreted as the product of metamorphic fluid infiltration. Many major- and trace-element concentrations in reaction rinds fall between those of metamafic blocks and surrounding ultramafic-rich mélange matrix (including TiO2, MgO, FeO, Al2O3, Zr, Ni and Cr). Spatial distributions of these elements within the rinds suggest that the intermediate concentrations may be due to mechanical mixing of crustal and mantle materials. Rind concentrations of the highly siderophile elements (HSE: including Os, Ir, Ru, Pt, Pd, Re) as well as

  16. Velocities of Subducted Sediments and Continents (United States)

    Hacker, B. R.; van Keken, P. E.; Abers, G. A.; Seward, G.


    The growing capability to measure seismic velocities in subduction zones has led to unusual observations. For example, although most minerals have VP/ VS ratios around 1.77, ratios 1.8 have been observed. Here we explore the velocities of subducted sediments and continental crust from trench to sub-arc depths using two methods. (1) Mineralogy was calculated as a function of P & T for a range of subducted sediment compositions using Perple_X, and rock velocities were calculated using the methodology of Hacker & Abers [2004]. Calculated slab-top temperatures have 3 distinct depth intervals with different dP/dT gradients that are determined by how coupling between the slab and mantle wedge is modeled. These three depth intervals show concomitant changes in VP and VS: velocities initially increase with depth, then decrease beyond the modeled decoupling depth where induced flow in the wedge causes rapid heating, and increase again at depth. Subducted limestones, composed chiefly of aragonite, show monotonic increases in VP/ VS from 1.63 to 1.72. Cherts show large jumps in VP/ VS from 1.55-1.65 to 1.75 associated with the quartz-coesite transition. Terrigenous sediments dominated by quartz and mica show similar, but more-subdued, transitions from ~1.67 to 1.78. Pelagic sediments dominated by mica and clinopyroxene show near-monotonic increases in VP/ VS from 1.74 to 1.80. Subducted continental crust that is too dry to transform to high-pressure minerals has a VP/ VS ratio of 1.68-1.70. (2) Velocity anisotropy calculations were made for the same P-T dependent mineralogies using the Christoffel equation and crystal preferred orientations measured via electron-backscatter diffraction for typical constituent phases. The calculated velocity anisotropies range from 5-30%. For quartz-rich rocks, the calculated velocities show a distinct depth dependence because crystal slip systems and CPOs change with temperature. In such rocks, the fast VP direction varies from slab-normal at

  17. “Imaging” the cross section of oceanic lithosphere: The development and future of electrical microresistivity logging through scientific ocean drilling (United States)

    Tominaga, Masako


    A detailed understanding of the architecture of volcanic and magmatic lithologies present within the oceanic lithosphere is essential to advance our knowledge of the geodynamics of spreading ridges and subduction zones. Undertaking sub-meter scale observations of oceanic lithosphere is challenging, primarily because of the difficulty in direct continuous sampling (e.g., by scientific ocean drilling) and the limited resolution of the majority of geophysical remote sensing methods. Downhole logging data from drillholes through basement formations, when integrated with recovered core and geophysical remote sensing data, can provide new insights into crustal accretion processes, lithosphere hydrogeology and associated alteration processes, and variations in the physical properties of the oceanic lithosphere over time. Here, we introduce an alternative approach to determine the formation architecture and lithofacies of the oceanic sub-basement by using logging data, particularly utilizing downhole microresistivity imagery (e.g. Formation MicroScanner (FMS) imagery), which has the potential to become a key tool in deciphering the high-resolution internal architecture of the intact upper ocean crust. A novel ocean crust lithostratigraphy model based on meticulously deciphered lava morphology determined by in situ FMS electrofacies analysis of holes drilled during Ocean Drilling Program legs (1) advances our understanding of ocean crust formation and accretionary processes over both time and space; and (2) allows the linking of local igneous histories deciphered from the drillholes to the regional magmatic and tectonic histories. Furthermore, microresistivity imagery can potentially allow the investigation of (i) magmatic lithology and architecture in the lower ocean crust and upper mantle; and, (ii) void space abundances in crustal material and the determination of complex lithology-dependent void geometries.

  18. Global patterns in Earth's dynamic topography since the Jurassic: the role of subducted slabs (United States)

    Rubey, Michael; Brune, Sascha; Heine, Christian; Rhodri Davies, D.; Williams, Simon E.; Dietmar Müller, R.


    We evaluate the spatial and temporal evolution of Earth's long-wavelength surface dynamic topography since the Jurassic using a series of high-resolution global mantle convection models. These models are Earth-like in terms of convective vigour, thermal structure, surface heat-flux and the geographic distribution of heterogeneity. The models generate a degree-2-dominated spectrum of dynamic topography with negative amplitudes above subducted slabs (i.e. circum-Pacific regions and southern Eurasia) and positive amplitudes elsewhere (i.e. Africa, north-western Eurasia and the central Pacific). Model predictions are compared with published observations and subsidence patterns from well data, both globally and for the Australian and southern African regions. We find that our models reproduce the long-wavelength component of these observations, although observed smaller-scale variations are not reproduced. We subsequently define geodynamic rules for how different surface tectonic settings are affected by mantle processes: (i) locations in the vicinity of a subduction zone show large negative dynamic topography amplitudes; (ii) regions far away from convergent margins feature long-term positive dynamic topography; and (iii) rapid variations in dynamic support occur along the margins of overriding plates (e.g. the western US) and at points located on a plate that rapidly approaches a subduction zone (e.g. India and the Arabia Peninsula). Our models provide a predictive quantitative framework linking mantle convection with plate tectonics and sedimentary basin evolution, thus improving our understanding of how subduction and mantle convection affect the spatio-temporal evolution of basin architecture.

  19. Global patterns in Earth's dynamic topography since the Jurassic: the role of subducted slabs

    Directory of Open Access Journals (Sweden)

    M. Rubey


    Full Text Available We evaluate the spatial and temporal evolution of Earth's long-wavelength surface dynamic topography since the Jurassic using a series of high-resolution global mantle convection models. These models are Earth-like in terms of convective vigour, thermal structure, surface heat-flux and the geographic distribution of heterogeneity. The models generate a degree-2-dominated spectrum of dynamic topography with negative amplitudes above subducted slabs (i.e. circum-Pacific regions and southern Eurasia and positive amplitudes elsewhere (i.e. Africa, north-western Eurasia and the central Pacific. Model predictions are compared with published observations and subsidence patterns from well data, both globally and for the Australian and southern African regions. We find that our models reproduce the long-wavelength component of these observations, although observed smaller-scale variations are not reproduced. We subsequently define geodynamic rules for how different surface tectonic settings are affected by mantle processes: (i locations in the vicinity of a subduction zone show large negative dynamic topography amplitudes; (ii regions far away from convergent margins feature long-term positive dynamic topography; and (iii rapid variations in dynamic support occur along the margins of overriding plates (e.g. the western US and at points located on a plate that rapidly approaches a subduction zone (e.g. India and the Arabia Peninsula. Our models provide a predictive quantitative framework linking mantle convection with plate tectonics and sedimentary basin evolution, thus improving our understanding of how subduction and mantle convection affect the spatio-temporal evolution of basin architecture.

  20. Subduction-related Late Carboniferous to Early Permian Magmatism in the Eastern Pontides, the Camlik and Casurluk plutons: Insights from geochemistry, whole-rock Sr-Nd and in situ zircon Lu-Hf isotopes, and U-Pb geochronology (United States)

    Karsli, Orhan; Dokuz, Abdurrahman; Kandemir, Raif


    Late Carboniferous to early Permian granitoid rocks represent a volumetrically minor component of the Eastern Pontide lithosphere, but they preserve useful information about the region's tectonomagmatic history. The Casurluk and Camlik plutons primarily consist of gabbro, gabbroic diorite, diorite, monzogabbro, monzodiorite and monzonite, which intrude early to middle Carboniferous granitic basement rocks in the region. In this study, we use in situ zircon U-Pb ages and Lu-Hf isotopic values, whole-rock Sr-Nd isotopic values, and mineral chemistry and geochemistry of these plutons to determine petrogenesis and crustal evolution; we also discuss geodynamic implications. LA-ICP-MS zircon U-Pb dating of magmatic zircons from the rocks suggests that the plutons were emplaced during the late Carboniferous to early Permian (302 Ma). The metaluminous and I-type intrusive rocks belong to the high-K calc-alkaline series. In addition, they are relatively enriched in light rare earth elements (LREEs) and large-ion lithophile elements (LILEs); they are depleted in heavy rare earth elements (HREEs) and high field strength elements (HFSEs), such as Nb and Ti. All of the samples have homogeneous initial ISr values (0.70675 to 0.70792) and low εNd (t) values (- 5.1 to - 3.3). Zircons from the rocks of both plutons have uniform negative to slightly positive εHf (t) values (- 3.5 to 1.4) and old Hf two-stage model ages (1323 to 1548 Ma), implying that they have the same source, as well as suggesting the involvement of old enriched lithospheric mantle materials during their magma genesis. These results, combined with the εHf (t) values and two-stage model ages, demonstrate that the primary magmas were derived from partial melting of old lithospheric mantle material metasomatized by subduction-related fluids. Considering other regional geological data from the Sakarya Zone where these plutons formed, we conclude that late Carboniferous to early Permian magmatism in the area

  1. From the North-Iberian Margin to the Alboran Basin: A lithosphere geo-transect across the Iberian Plate (United States)

    Carballo, A.; Fernandez, M.; Jiménez-Munt, I.; Torne, M.; Vergés, J.; Melchiorre, M.; Pedreira, D.; Afonso, J. C.; Garcia-Castellanos, D.; Díaz, J.; Villaseñor, A.; Pulgar, J. A.; Quintana, L.


    A ~ 1000-km-long lithospheric transect running from the North-Iberian Margin to the Alboran Basin (W-Mediterranean) is investigated. The main goal is to image the changes in the crustal and upper mantle structure occurring in: i) the North-Iberian margin, whose deformation in Alpine times gave rise to the uplift of the Cantabrian Mountains related to Iberia-Eurasia incipient subduction; ii) the Spanish Meseta, characterized by the presence of Cenozoic basins on top of a Variscan basement with weak Alpine deformation in the Central System, and localized Neogene-Quaternary deep volcanism; and iii) the Betic-Alboran system related to Africa-Iberia collision and the roll-back of the Ligurian-Tethyan domain. The modeling approach, combines potential fields, elevation, thermal, seismic, and petrological data under a self-consistent scheme. The crustal structure is mainly constrained by seismic data whereas the upper mantle is constrained by tomographic models. The results highlight the lateral variations in the topography of the lithosphere-asthenosphere boundary (LAB), suggesting a strong lithospheric mantle strain below the Cantabrian and Betic mountain belts. The LAB depth ranges from 180 km beneath the Cantabrian Mountains to 135-110 km beneath Iberia Meseta deepening again to values of 160 km beneath the Betic Cordillera. The Central System, with a mean elevation of 1300 m, has a negligible signature on the LAB depth. We have considered four lithospheric mantle compositions: a predominantly average Phanerozoic in the continental mainland, two more fertile compositions in the Alboran Sea and in the Calatrava Volcanic Province, and a hydrated uppermost mantle in the North-Iberian Margin. These compositional differences allowed us to reproduce the main trends of the geophysical observables as well as the inferred P- and S-wave seismic velocities from tomography models and seismic experiments available in the study transect. The high mean topography of Iberia can be

  2. Anisotropic Lithospheric layering in the North American craton, revealed by Bayesian inversion of short and long period data (United States)

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


    Competing hypotheses for the formation and evolution of continents are highly under debate, including the theory of underplating by hot plumes or accretion by shallow subduction in continental or arc settings. In order to support these hypotheses, documenting structural layering in the cratonic lithosphere becomes especially important. Studies of seismic-wave receiver function data have detected a structural boundary under continental cratons at 100-140 km depths, which is too shallow to be consistent with the lithosphere-asthenosphere boundary, as inferred from seismic tomography and other geophysical studies. This leads to the conclusion that 1) the cratonic lithosphere may be thinner than expected, contradicting tomographic and other geophysical or geochemical inferences, or 2) that the receiver function studies detect a mid-lithospheric discontinuity rather than the LAB. Recent studies (Bodin et al., 2015, Calo et al. 2016) confirmed the presence of a structural boundary under the north American craton at 100-140 km depths by taking advantage of the power of a trans-dimensional Monte Carlo Markov chain (TMCMC). They generated probabilistic 1D radially shear wave velocity profiles for selected stations in North America by jointly inverting 2 different data types (PS Receiver Functions, surface wave dispersion for Love and Rayleigh waves), which sample different volumes of the Earth and have different sensitivities to structure. The resulting 1D profiles include both isotropic and anisotropic discontinuities in the upper mantle (above 350 km depth). Here we extend this approach and include the vp/vs ratio as an unknown in the TMCMC algorithm to avoid artificial layers induced by multiples of the receiver functions. Additionally, we include SKS waveforms in the joint inversion and invert for azimuthal anisotropy to verify if layering in the anisotropic structure of the stable part of the North American continent involves significant changes in the direction of

  3. Seismological Insights on the Lithosphere-Asthenosphere Boundary (United States)

    Fischer, K. M.


    The paradigm of plate tectonics is fundamental to our understanding of the Earth, yet the question of what makes the lithosphere "plate-like" remains unanswered. As Earth's outer thermal boundary layer, the lithosphere derives its high viscosity largely from its cold temperatures, relative to the warmer asthenosphere. However, the roles of partial melt and volatiles in further reducing asthenospheric viscosity are still debated, even 90 years after Beno Gutenberg identified a widespread velocity decrease at 80 km depth which he later attributed to the top of a layer containing partial melt. Seismic phase conversions generated by a velocity gradient within the lithosphere-asthenosphere transition, combined with surface wave tomography, provide key constraints. At the base of thick, stable cratonic lithosphere, the absence of clear Sp conversions (except at the longest periods) is consistent with a shear velocity decrease over >60 km and a lithosphere-asthenosphere boundary (LAB) that reflects an increase in temperature alone. In contrast, strong Sp conversions from the LAB in tectonically active continental regions indicate a vertically-localized negative velocity gradient (discontinuities likely represent the remains of past plate collisions. The most widespread cratonic feature is a sub-horizontal negative velocity gradient, typically at depths of 70-90 km, that is explained by the volatile-rich products of now-solid partial melt that once ponded beneath the carbonated peridotite solidus.

  4. Control of high oceanic features and subduction channel on earthquake ruptures along the Chile-Peru subduction zone (United States)

    Contreras-Reyes, Eduardo; Carrizo, Daniel


    We discuss the earthquake rupture behavior along the Chile-Peru subduction zone in terms of the buoyancy of the subducting high oceanic features (HOF's), and the effect of the interplay between HOF and subduction channel thickness on the degree of interplate coupling. We show a strong relation between subduction of HOF's and earthquake rupture segments along the Chile-Peru margin, elucidating how these subducting features play a key role in seismic segmentation. Within this context, the extra increase of normal stress at the subduction interface is strongly controlled by the buoyancy of HOF's which is likely caused by crustal thickening and mantle serpentinization beneath hotspot ridges and fracture zones, respectively. Buoyancy of HOF's provide an increase in normal stress estimated to be as high as 10-50 MPa. This significant increase of normal stress will enhance seismic coupling across the subduction interface and hence will affect the seismicity. In particular, several large earthquakes (Mw ≥ 7.5) have occurred in regions characterized by subduction of HOF's including fracture zones (e.g., Nazca, Challenger and Mocha), hotspot ridges (e.g., Nazca, Iquique, and Juan Fernández) and the active Nazca-Antarctic spreading center. For instance, the giant 1960 earthquake (Mw = 9.5) is coincident with the linear projections of the Mocha Fracture Zone and the buoyant Chile Rise, while the active seismic gap of north Chile spatially correlates with the subduction of the Iquique Ridge. Further comparison of rupture characteristics of large underthrusting earthquakes and the locations of subducting features provide evidence that HOF's control earthquake rupture acting as both asperities and barriers. This dual behavior can be partially controlled by the subduction channel thickness. A thick subduction channel smooths the degree of coupling caused by the subducted HOF which allows lateral earthquake rupture propagation. This may explain why the 1960 rupture propagates

  5. Diapiric flow at subduction zones: a recipe for rapid transport. (United States)

    Hall, P S; Kincaid, C


    Recent geochemical studies of uranium-thorium series disequilibrium in rocks from subduction zones require magmas to be transported through the mantle from just above the subducting slab to the surface in as little as approximately 30,000 years. We present a series of laboratory experiments that investigate the characteristic time scales and flow patterns of the diapiric upwelling model of subduction zone magmatism. Results indicate that the interaction between buoyantly upwelling diapirs and subduction-induced flow in the mantle creates a network of low-density, low-viscosity conduits through which buoyant flow is rapid, yielding transport times commensurate with those indicated by uranium-thorium studies.

  6. Shear wave velocity structure in the lithosphere and asthenosphere across the Southern California continent and Pacific plate margin using inversion of Rayleigh wave data from the ALBACORE project. (United States)

    Price, A. C.; Weeraratne, D. S.; Kohler, M. D.; Rathnayaka, S.; Escobar, L., Sr.


    The North American and Pacific plate boundary is a unique example of past subduction of an oceanic spreading center which has involved oceanic plate capture and inception of a continental transform boundary that juxtaposes continental and oceanic lithosphere on a single plate. The amphibious ALBACORE seismic project (Asthenospheric and Lithospheric Broadband Architecture from the California Offshore Region Experiment) deployed 34 ocean bottom seismometers (OBS) on 15-35 Ma seafloor and offers a unique opportunity to compare the LAB in continental and oceanic lithosphere in one seismic study. Rayleigh waves were recorded simultaneously by our offshore array and 82 CISN network land stations from 2010-2011. Here we predict phase velocities for a starting shear wave velocity model for each of 5 regions in our study area and compare to observed phase velocities from our array in a least-squares sense that produces the best fit 1-D shear wave velocity structure for each region. Preliminary results for the deep ocean (seafloor 25-32 Ma) indicates high velocities reaching 4.5 km/s at depths of 50 km associated with the lithosphere for seafloor 25-32 Ma. A negative velocity gradient is observed below this which reaches a minimum of 4.0 km/s at 160 km depth. The mid-ocean region (age 13-25 Ma) indicates a slightly lower magnitude and shallower LVZ. The Inner Borderland displays the highest lithospheric velocities offshore reaching 4.8 km/s at 40 km depth indicating underplating. The base of the LVZ in the Borderland increases sharply from 4.0 km/s to 4.5 km/s at 80-150 km depth indicating partial melt and compositional changes. The LVZ displays a very gradual positive velocity gradient in all other regions such as the deep seafloor and continent reaching 4.5 km/s at 300 km depth. The deep ocean, Borderlands, and continental region each have unique lithospheric velocities, LAB depths, and LVZ character that indicate stark differences in mantle structure that occur on a

  7. Mineralogy of subducted clay and clay restite in the lower mantle (United States)

    Armstrong, L.; Skora, S. E.; Walter, M. J.


    Seismic tomography indicates that subducting oceanic lithosphere often penetrates the transition zone and eventually the lower mantle [e.g. 1, 2]. While mineralogical changes in the mafic and ultramafic portions of slabs have been well documented experimentally, the phase relations of overlying sediments at pressures above 25 GPa remain poorly studied. This is in part because sediments are expected to partially melt at sub-arc depth (P~2.5-4.5 GPa), and contribute to the genesis of arc magmas. Sediment restites left behind after the extraction of low pressure melts undergo major chemical changes, according to the melting reaction: Coe+Phen+Cpx+H2O = Grt+Ky+Melt [3]. However, sediments may not always melt depending on the thermal regime and volatile availability and composition [3]. Hence, chemically unmodified sediments as well as restites may be entrained to greater depths and contribute to compositional heterogeneity in the deep mantle. Indeed, mineral inclusions with compositions indicative of subducted sedimentary protoliths (CAS-phase; K-hollandite; stishovite) have been reported in 'ultradeep' diamonds and suggest that deep subduction and survival of sediments occurs to at least transition zone depths [4]. With this in mind, we have performed laser heated diamond anvil cell experiments at pressures of 8-80 GPa on two anhydrous glass starting materials: a marine clay and the restite that is left after 50% melt extraction of this clay at 3 GPa and 800 °C [3]. We chose to work with an anhydrous version of the marine clay given that the investigated pressure range exceeds that of phengite stability [5], and phengite is the only hydrous phase in subducted sediments at UHP conditions. The clay was heated along a P-T path representative of a cold subduction geotherm, whereas the clay restite was heated along a hotter subduction geotherm consistent with low pressure melting. Phases were identified by synchrotron X-ray micro-diffraction at beamline I15 of the Diamond

  8. Spatial distribution of random velocity inhomogeneities in the western part of Nankai subduction zone (United States)

    Takahashi, T.; Obana, K.; Yamamoto, Y.; Nakanishi, A.; Kodaira, S.; Kaneda, Y.


    In the Nankai trough, there are three seismogenic zones of megathrust earthquakes (Tokai, Tonankai and Nankai earthquakes). Lithospheric structures in and around these seismogenic zones are important for the studies on mutual interactions and synchronization of their fault ruptures. Recent studies on seismic wave scattering at high frequencies (>1Hz) make it possible to estimate 3D distributions of random inhomogeneities (or scattering coefficient) in the lithosphere, and clarified that random inhomogeneity is one of the important medium properties related to microseismicity and damaged structure near the fault zone [Asano & Hasegawa, 2004; Takahashi et al. 2009]. This study estimates the spatial distribution of the power spectral density function (PSDF) of random inhomogeneities the western part of Nankai subduction zone, and examines the relations with crustal velocity structure and seismic activity. Seismic waveform data used in this study are those recorded at seismic stations of Hi-net & F-net operated by NIED, and 160 ocean bottom seismographs (OBSs) deployed at Hyuga-nada region from Dec. 2008 to Jan. 2009. This OBS observation was conducted by JAMSTEC as a part of "Research concerning Interaction Between the Tokai, Tonankai and Nankai Earthquakes" funded by Ministry of Education, Culture, Sports, Science and Technology, Japan. Spatial distribution of random inhomogeneities is estimated by the inversion analysis of the peak delay time of small earthquakes [Takahashi et al. 2009], where the peak delay time is defined as the time lag from the S-wave onset to its maximal amplitude arrival. We assumed the von Karman type functional form for the PSDF. Peak delay times are measured from root mean squared envelopes at 4-8Hz, 8-16Hz and 16-32Hz. Inversion result can be summarized as follows. Random inhomogeneities beneath the Quaternary volcanoes are characterized by strong inhomogeneities at small spatial scale (~ a few hundreds meter) and weak spectral gradient

  9. Tsunami Hazard Assessment of Coastal South Africa Based on Mega-Earthquakes of Remote Subduction Zones (United States)

    Kijko, Andrzej; Smit, Ansie; Papadopoulos, Gerassimos A.; Novikova, Tatyana


    After the mega-earthquakes and concomitant devastating tsunamis in Sumatra (2004) and Japan (2011), we launched an investigation into the potential risk of tsunami hazard to the coastal cities of South Africa. This paper presents the analysis of the seismic hazard of seismogenic sources that could potentially generate tsunamis, as well as the analysis of the tsunami hazard to coastal areas of South Africa. The subduction zones of Makran, South Sandwich Island, Sumatra, and the Andaman Islands were identified as possible sources of mega-earthquakes and tsunamis that could affect the African coast. Numerical tsunami simulations were used to investigate the realistic and worst-case scenarios that could be generated by these subduction zones. The simulated tsunami amplitudes and run-up heights calculated for the coastal cities of Cape Town, Durban, and Port Elizabeth are relatively small and therefore pose no real risk to the South African coast. However, only distant tsunamigenic sources were considered and the results should therefore be viewed as preliminary.

  10. Orogen styles in the East African Orogen: A review of the Neoproterozoic to Cambrian tectonic evolution (United States)

    Fritz, H.; Abdelsalam, M.; Ali, K. A.; Bingen, B.; Collins, A. S.; Fowler, A. R.; Ghebreab, W.; Hauzenberger, C. A.; Johnson, P. R.; Kusky, T. M.; Macey, P.; Muhongo, S.; Stern, R. J.; Viola, G.


    The East African Orogen, extending from southern Israel, Sinai and Jordan in the north to Mozambique and Madagascar in the south, is the world´s largest Neoproterozoic to Cambrian orogenic complex. It comprises a collage of individual oceanic domains and continental fragments between the Archean Sahara-Congo-Kalahari Cratons in the west and Neoproterozoic India in the east. Orogen consolidation was achieved during distinct phases of orogeny between ∼850 and 550 Ma. The northern part of the orogen, the Arabian-Nubian Shield, is predominantly juvenile Neoproterozoic crust that formed in and adjacent to the Mozambique Ocean. The ocean closed during a protracted period of island-arc and microcontinent accretion between ∼850 and 620 Ma. To the south of the Arabian Nubian Shield, the Eastern Granulite-Cabo Delgado Nappe Complex of southern Kenya, Tanzania and Mozambique was an extended crust that formed adjacent to theMozambique Ocean and experienced a ∼650-620 Ma granulite-facies metamorphism. Completion of the nappe assembly around 620 Ma is defined as the East African Orogeny and was related to closure of the Mozambique Ocean. Oceans persisted after 620 Ma between East Antarctica, India, southern parts of the Congo-Tanzania-Bangweulu Cratons and the Zimbabwe-Kalahari Craton. They closed during the ∼600-500 Ma Kuungan or Malagasy Orogeny, a tectonothermal event that affected large portions of southern Tanzania, Zambia, Malawi, Mozambique, Madagascar and Antarctica. The East African and Kuungan Orogenies were followed by phases of post-orogenic extension. Early ∼600-550 Ma extension is recorded in the Arabian-Nubian Shield and the Eastern Granulite-Cabo Delgado Nappe Complex. Later ∼550-480 Ma extension affected Mozambique and southern Madagascar. Both extension phases, although diachronous,are interpreted as the result of lithospheric delamination. Along the strike of the East African Orogen, different geodynamic settings resulted in the evolution of

  11. Lithospheric structure models applied for locating the Romanian seismic events

    Directory of Open Access Journals (Sweden)

    V. Oancea


    Full Text Available The paper presents our attempts made for improving the locations obtained for local seismic events, using refined lithospheric structure models. The location program (based on Geiger method supposes a known model. The program is run for some seismic sequences which occurred in different regions, on the Romanian territory, using for each of the sequences three velocity models: 1 7 layers of constant velocity of seismic waves, as an average structure of the lithosphere for the whole territory; 2 site dependent structure (below each station, based on geophysical and geological information on the crust; 3 curves deseribing the dependence of propagation velocities with depth in the lithosphere, characterizing the 7 structural units delineated on the Romanian territory. The results obtained using the different velocity models are compared. Station corrections are computed for each data set. Finally, the locations determined for some quarry blasts are compared with the real ones.

  12. A lithospheric velocity model for the flat slab region of Argentina from joint inversion of Rayleigh wave phase velocity dispersion and teleseismic receiver functions (United States)

    Ammirati, Jean-Baptiste; Alvarado, Patricia; Beck, Susan


    In the central Andes, the Nazca plate displays large along strike variations in dip with a near horizontal subduction angle between 28 and 32°S referred to the Pampean flat slab segment. The upper plate above the Pampean flat slab has high rates of crustal seismicity and active basement cored uplifts. The SIEMBRA experiment, a 43-broad-band-seismic-station array was deployed to better characterize the Pampean flat slab region around 31°S. In this study, we explore the lithospheric structure above the flat slab as a whole and its relation to seismicity. We use the SIEMBRA data to perform a joint inversion of teleseismic receiver functions and Rayleigh wave phase velocity dispersion to constrain the shear wave velocity variations in the lithosphere. Our joint inversion results show: (1) the presence of several upper-plate mid-crustal discontinuities and their lateral extent that are probably related to the terrane accretion history; (2) zones of high shear wave velocity in the upper-plate lower crust associated with a weak Moho signal consistent with the hypothesis of partial eclogitization in the lower crust; (3) the presence of low shear-wave velocities at ˜100 km depth interpreted as the subducting oceanic crust. Finally, in order to investigate the relation of the lithospheric structure to seismicity, we determine an optimal velocity-depth model based on the joint inversion results and use it to perform regional moment tensor inversions (SMTI) of crustal and slab earthquakes. The SMTI for 18 earthquakes that occurred between 2007 and 2009 in the flat slab region below Argentina, indicates systematically shallower focal depths for slab earthquakes (compared with inversions using previous velocity models). This suggests that the slab seismicity is concentrated mostly between 90 and 110 km depths within the subducting Nazca plate's oceanic crust and likely related to dehydration. In addition, the slab earthquakes exhibit extensional focal mechanisms suggesting

  13. The South Sandwich "Forgotten" Subduction Zone and Tsunami Hazard in the South Atlantic (United States)

    Okal, E. A.; Hartnady, C. J. H.; Synolakis, C. E.


    While no large interplate thrust earthquakes are know at the "forgotten" South Sandwich subduction zone, historical catalogues include a number of events with reported magnitudes 7 or more. A detailed seismological study of the largest event (27 June 1929; M (G&R) = 8.3) is presented. The earthquake relocates 80 km North of the Northwestern corner of the arc and its mechanism, inverted using the PDFM method, features normal faulting on a steeply dipping fault plane (phi, delta, lambda = 71, 70, 272 deg. respectively). The seismic moment of 1.7*10**28 dyn*cm supports Gutenberg and Richter's estimate, and is 28 times the largest shallow CMT in the region. This event is interpreted as representing a lateral tear in the South Atlantic plate, comparable to similar earthquakes in Samoa and Loyalty, deemed "STEP faults" by Gover and Wortel [2005]. Hydrodynamic simulations were performed using the MOST method [Titov and Synolakis, 1997]. Computed deep-water tsunami amplitudes of 30cm and 20cm were found off the coast of Brazil and along the Gulf of Guinea (Ivory Coast, Ghana) respectively. The 1929 moment was assigned to the geometries of other know earthquakes in the region, namely outer-rise normal faulting events at the center of the arc and its southern extremity, and an interplate thrust fault at the Southern corner, where the youngest lithosphere is subducted. Tsunami hydrodynamic simulation of these scenarios revealed strong focusing of tsunami wave energy by the SAR, the SWIOR and the Agulhas Rise, in Ghana, Southern Mozambique and certain parts of the coast of South Africa. This study documents the potential tsunami hazard to South Atlantic shorelines from earthquakes in this region, principally normal faulting events.

  14. Rapid fore-arc extension and detachment-mode spreading following subduction initiation (United States)

    Morris, Antony; Anderson, Mark W.; Omer, Ahmed; Maffione, Marco; van Hinsbergen, Douwe J. J.


    Most ophiolites have geochemical signatures that indicate formation by suprasubduction seafloor spreading above newly initiated subduction zones, and hence they record fore-arc processes operating following subduction initiation. They are frequently underlain by a metamorphic sole formed at the top of the downgoing plate and accreted below the overlying suprasubduction zone lithosphere immediately following ophiolite formation. Paleomagnetic analyses of ophiolites can provide important insights into the enigmatic geodynamic processes operating in this setting via identification of tectonic rotations related to upper plate extension. Here we present net tectonic rotation results from the Late Cretaceous Mersin ophiolite of southern Turkey that document rapid and progressive rotation of ophiolitic rocks and their associated metamorphic sole. Specifically, we demonstrate that lower crustal cumulate rocks and early dykes intruded into the underlying mantle section have undergone extreme rotation around ridge-parallel, shallowly-plunging axes, consistent with oceanic detachment faulting during spreading. Importantly, later dykes cutting the metamorphic sole experienced rotation around the same axis but with a lower magnitude. We show that these rotations occurred via a common mechanism in a pre-obduction, fore-arc setting, and are best explained by combining (hyper)extension resulting from detachment-mode, amagmatic suprasubduction zone spreading in a fore-arc environment with a recently proposed mechanism for exhumation of metamorphic soles driven by upper plate extension. Available age constraints demonstrate that extreme rotation of these units was accommodated rapidly by these processes over a time period of <∼3 Myr, comparable with rates of rotation seen in oceanic core complexes in the modern oceans.

  15. Time-dependent geoid anomalies at subduction zones due to the seismic cycle (United States)

    Cambiotti, G.; Sabadini, R.; Yuen, D. A.


    We model the geoid anomalies excited during a megathrust earthquake cycle at subduction zones, including the interseismic phase and the contribution from the infinite series of previous earthquakes, within the frame of self-gravitating, spherically symmetric, compressible, viscoelastic Earth models. The fault cuts the whole 50 km lithosphere, dips 20°, and the slip amplitude, together with the length of the fault, are chosen in order to simulate an Mw = 9.0 earthquake, while the viscosity of the 170 km thick asthenosphere ranges from 1017 to 1020 Pa s. On the basis of a new analysis from the Correspondence Principle, we show that the geoid anomaly is characterized by a periodic anomaly due to the elastic and viscous contribution from past earthquakes and to the back-slip of the interseismic phase, and by a smaller static contribution from the steady-state response to the previous infinite earthquake cycles. For asthenospheric viscosities from 1017-1018 to 1019-1020 Pa s, the characteristic relaxation times of the Earth model change from shorter to longer timescales compared to the 400 yr earthquake recurrence time, which dampen the geoid anomaly for the higher asthenospheric viscosities, since the slower relaxation cannot contribute its whole strength within the interseismic cycle. The geoid anomaly pattern is characterized by a global, time-dependent positive upwarping of the geoid topography, involving the whole hanging wall and partially the footwall compared to the sharper elastic contribution, attaining, for a moment magnitude Mw = 9.0, amplitudes as high as 6.6 cm for the lowermost asthenospheric viscosities during the viscoelastic response compared to the elastic maximum of 3.8 cm. The geoid anomaly vanishes due to the back-slip of the interseismic phase, leading to its disappearance at the end of the cycle before the next earthquake. Our results are of importance for understanding the post-seismic and interseismic geoid patterns at subduction zones.

  16. Sabzevar Ophiolite, NE Iran: Progress from embryonic oceanic lithosphere into magmatic arc constrained by new isotopic and geochemical data (United States)

    Moghadam, Hadi Shafaii; Corfu, Fernando; Chiaradia, Massimo; Stern, Robert J.; Ghorbani, Ghasem


    The poorly known Sabzevar-Torbat-e-Heydarieh ophiolite belt (STOB) covers a large region in NE Iran, over 400 km E-W and almost 200 km N-S. The Sabzevar mantle sequence includes harzburgite, lherzolite, dunite and chromitite. Spinel Cr# (100Cr/(Cr + Al)) in harzburgites and lherzolites ranges from 44 to 47 and 24 to 26 respectively. The crustal sequence of the Sabzevar ophiolite is dominated by supra-subduction zone (SSZ)-type volcanic as well as plutonic rocks with minor Oceanic Island Basalt (OIB)-like pillowed and massive lavas. The ophiolite is covered by Late Campanian to Early Maastrichtian (~ 75-68 Ma) pelagic sediments and four plagiogranites yield zircon U-Pb ages of 99.9, 98.4, 90.2 and 77.8 Ma, indicating that the sequence evolved over a considerable period of time. Most Sabzevar ophiolitic magmatic rocks are enriched in Large Ion Lithophile Elements (LILEs) and depleted in High Field Strength Elements (HFSEs), similar to SSZ-type magmatic rocks. They (except OIB-type lavas) have higher Th/Yb and plot far away from mantle array and are similar to arc-related rocks. Subordinate OIB-type lavas show Nb-Ta enrichment with high Light Rare Earth Elements (LREE)/Heavy Rare Earth Elements (HREE) ratio, suggesting a plume or subcontinental lithosphere signature in their source. The ophiolitic rocks have positive εNd (t) values (+ 5.4 to + 8.3) and most have high 207Pb/204Pb, indicating a significant contribution of subducted sediments to their mantle source. The geochemical and Sr-Nd-Pb isotope characteristics suggest that the Sabzevar magmatic rocks originated from a Mid-Ocean Ridge Basalt (MORB)-type mantle source metasomatized by fluids or melts from subducted sediments, implying an SSZ environment. We suggest that the Sabzevar ophiolites formed in an embryonic oceanic arc basin between the Lut Block to the south and east and the Binalud mountains (Turan block) to the north, and that this small oceanic arc basin existed from at least mid-Cretaceous times

  17. Geochronological Constraints on the Exhumation and Emplacement of Subcontinental Lithospheric Mantle Peridotites in the Westernmost Mediterranean (United States)

    Garrido, Carlos J.; Hidas, Károly; Marchesi, Claudio; Varas-Reus, María Isabel; Booth-Rea, Guillermo


    Exhumation of subcontinental mantle peridotite in the Western Mediterranean has been attributed to different tectonic processes including pure extension, transpression, or alternating contractive and extensional processes related with continental subduction followed by extension, before final their contractive intracrustal emplacement. Any model trying to explain the exhumation and emplacement of subcontinental lithospheric mantle peridotites in the westernmost Mediterranean should take into account the available geochronological constraints, as well as the petrological and geochemical processes that lead to internal tectono-magmatic zoning so characteristic of the Betic and Rif orogenic peridotites. Different studies have suggested a Hercynian, Cenozoic-Mesozoic or an Alpine age for the late tectono-magmatic evolution and intra-crustal emplacement of Betic-Rif peridotites. The pervasive presence of Mesozoic U-Pb zircon ages in Ronda UHP and HP garnet pyroxenites does not support a Hercynian age for the intracrustal emplacement of the peridotite. A hyper-extended margin setting for is in good agreement with the Jurassic extensional event that pervasively affected ALKAPECA terrains (i.e. the Alboran, Kabylides, Peloritani, and Calabria domains) in the western Mediterranean due to the opening of the Piemonte-Ligurian Ocean. However, a Jurassic age and a passive margin tectonic setting do not account, among other observations, for the late Miocene thermochronological ages recorded in zircons rims (U-Pb) and garnets (Lu-Hf) in garnet pyroxenites from the Betic-Rif peridotites, the pervasive Miocene resetting of U-Pb zircon and monazite ages in the overlying Jubrique crustal section, the supra-subduction radiogenic signature of late pyroxenite intrusive dikes in the Ronda peridotite, and the arc tholeiitic affinity of late mantle-derived, gabbroic dykes intruding in the Ronda and Ojen plagioclase lherzolites. These data are more consistent with a supra-subduction

  18. Imaging of the subducted Kyushu-Palau Ridge in the Hyuga-nada region, western Nankai Trough subduction zone (United States)

    Yamamoto, Yojiro; Obana, Koichiro; Takahashi, Tsutomu; Nakanishi, Ayako; Kodaira, Shuichi; Kaneda, Yoshiyuki


    We performed 3D seismic tomography of the Hyuga-nada region, western Nankai subduction zone, to investigate the relationship of the subducted part of Kyushu-Palau Ridge (KPR) to coseismic rupture propagation, seismicity, and shallow very low frequency earthquakes. Combining active-source and passive-source data recorded both onshore and offshore, we imaged the deep slab from near the trough axis to the coastal area. Our results show the subducted KPR as a low-velocity belt oriented NW-SE extending down the plate boundary to around 30 km depth. At this depth, we suggest that the subducted KPR detaches from the slab and becomes underplated on the overriding continental plate. As the coseismic slip areas of past large earthquakes do not extend into the subducted KPR, we suggest that it may inhibit rupture propagation. The interior of the subducted KPR shows active intraslab seismicity with a wide depth distribution. Shallow very low frequency earthquakes are continuously active above the location of the subducted KPR, whereas they are intermittent to the northeast of the subducted KPR. Thus, the subducted KPR appears to be an important factor in coseismic rupture propagation and seismic phenomena in this region.

  19. Heterogeneity of Water Concentrations in the Mantle Lithosphere Beneath Hawaii (United States)

    Bizimis, M.; Peslier, A. H.; Clague, D.


    The amount and distribution of water in the oceanic mantle lithosphere has implications on its strength and of the role of volatiles during plume/lithosphere interaction. The latter plays a role in the Earth's deep water cycle as water-rich plume lavas could re-enrich an oceanic lithosphere depleted in water at the ridge, and when this heterogeneous lithosphere gets recycled back into the deep mantle. The main host of water in mantle lithologies are nominally anhydrous minerals like olivine, pyroxene and garnet, where hydrogen (H) is incorporated in mineral defects by bonding to structural oxygen. Here, we report water concentrations by Fourier transform infrared spectrometry (FTIR) on olivine, clino- and orthopyroxenes (Cpx & Opx) from spinel peridotites from the Pali vent and garnet pyroxenite xenoliths from Aliamanu vent, both part of the rejuvenated volcanism at Oahu (Hawaii). Pyroxenes from the Aliamanu pyroxenites have high water concentrations, similar to the adjacent Salt Lake Crater (SLC) pyroxenites (Cpx 400-500 ppm H2O, Opx 200 ppm H2O). This confirms that pyroxenite cumulates form water-rich lithologies within the oceanic lithosphere. In contrast, the Pali peridotites have much lower water concentrations than the SLC ones (10% modal Cpx and low spinel Cr# (0.09-0.10). The contrast between the two peridotite suites is also evident in their trace elements and radiogenic isotopes. The Pali Cpx are depleted in light REE, consistent with minimal metasomatism. Those of SLC have enriched light REE patterns and Nd and Hf isotopes consistent with metasomatism by alkaline melts. These observations are consistent with heterogeneous water distribution in the oceanic lithosphere that may be related to metasomatism, as well as relatively dry peridotites cross-cut by narrow (?) water-rich melt reaction zones.

  20. The melting of subducted banded iron formations (United States)

    Kang, Nathan; Schmidt, Max W.


    Banded iron formations (BIF) were common shelf and ocean basin sediments 3.5-1.8 Ga ago. To understand the fate of these dense rocks upon subduction, the melting relations of carbonated BIF were determined in Fe-Ca-(Mg)-Si-C-O2 at 950-1400 °C, 6 and 10 GPa, oxidizing (fO2 = hematite-magnetite, HM) and moderately reducing (fO2 ∼CO2-graphite/diamond, CCO) conditions. Solidus temperatures under oxidizing conditions are 950-1025 °C with H2O, and 1050-1150 °C anhydrous, but 250-175 °C higher at graphite saturation (values at 6-10 GPa). The combination of Fe3+ and carbonate leads to a strong melting depression. Solidus curves are steep with 17-20 °C/GPa. Near-solidus melts are ferro-carbonatites with ∼22 wt.% FeOtot, ∼48 wt% CO2 and 1-5 wt.% SiO2 at fO2 ∼ HM and ∼49 wt.% FeOtot, ∼20 wt% CO2 and 19-25 wt.% SiO2 at fO2 ∼ CCO . At elevated subduction geotherms, as likely for the Archean, C-bearing BIF could melt out all carbonate around 6 GPa. Fe-rich carbonatites would rise but stagnate gravitationally near the slab/mantle interface until they react with the mantle through Fe-Mg exchange and partial reduction. The latter would precipitate diamond and yield Fe- and C-rich mantle domains, yet, Fe-Mg is expected to diffusively re-equilibrate over Ga time scales. We propose that the oldest subduction derived diamonds stem from BIF derived melts.

  1. Revisiting the Ridge-Push Force Using the Lithospheric Geoid (United States)

    Richardson, R. M.; Coblentz, D. D.


    The geoid anomaly and driving force associated with the cooling oceanic lithosphere ("ridge push") are both proportional to dipole moment of the density-depth distribution, and allow a reevaluation of the ridge push force using the geoid. The challenge with this approach is to isolate the "lithospheric geoid" from the full geoid signal. Our approach is to use a band-pass spherical harmonic filter on the full geoid (e.g., EGM2008-WGS84, complete to spherical harmonic degree and order 2159) between orders 6 and 80. However, even this "lithospheric geoid" is noisy, and thus we average over 100 profiles evenly spaced along the global ridge system to obtain an average geoid step associated with the mid-ocean ridges. Because the positive ridge geoid signal is largest near the ridge (and to capture fast-spreading ridges), we evaluate symmetrical profiles extending ±45 m.y. about the ridge. We find an average ridge geoid anomaly of 4.5m, which is equivalent to a 10m anomaly for 100 m.y. old oceanic lithosphere. This geoid step corresponds to a ridge push force of ~2.4 x1012N/m for old oceanic lithosphere of 100 m.y., very similar to earlier estimates of ~2.5 x1012N/m based on simple half-space models. This simple half-space model also predicts constant geoid slopes of about 0.15 m/m.y. for cooling oceanic lithosphere. Our observed geoid slopes are consistent with this value for ages up to 40-50 m.y., but drop off to lower values at greater ages. We model this using a plate cooling model (with a thickness of the order of 125km) to fit the observation that the geoid anomaly and ridge driving force only increase slowly for ages greater than 40 m.y. (in contrast to the half-space model where the linear dependence on age holds for all ages). This reduction of the geoid slope results in a 20% decrease in the predicted ridge push force. This decrease is due to the combined effects of treating the oceanic lithosphere as a cooling plate (vs. a half-space), and the loss of geoidal

  2. The principal characteristics of the lithosphere of China

    Directory of Open Access Journals (Sweden)

    Tingdong Li


    Full Text Available The lithospheric structure of China and its adjacent area is very complex and is marked by several prominent characteristics. Firstly, China’s continental crust is thick in the west but thins to the east, and thick in the south but thins to the north. Secondly, the continental crust of the Qinghai–Tibet Plateau has an average thickness of 60–65 km with a maximum thickness of 80 km, whereas in eastern China the average thickness is 30–35 km, with a minimum thickness of only 5 km in the center of the South China Sea. The average thickness of continental crust in China is 47.6 km, which greatly exceeds the global average thickness of 39.2 km. Thirdly, as with the crust, the lithosphere of China and its adjacent areas shows a general pattern of thicker in the west and south, and thinner in the east and north. The lithosphere of the Qinghai–Tibet Plateau and northwestern China has an average thickness of 165 km, with a maximum thickness of 180–200 km in the central and eastern parts of the Tarim Basin, Pamir, and Changdu areas. In contrast, the vast areas to the east of the Da Hinggan Ling–Taihang–Wuling Mountains, including the marginal seas, are characterized by lithospheric thicknesses of only 50–85 km. Fourthly, in western China the lithosphere and asthenosphere behave as a “layered structure”, reflecting their dynamic background of plate collision and convergence. The lithosphere and asthenosphere in eastern China display a “block mosaic structure”, where the lithosphere is thin and the asthenosphere is very thick, a pattern reflecting the consequences of crustal extension and an upsurge of asthenospheric materials. The latter is responsible for a huge low velocity anomaly at a depth of 85–250 km beneath East Asia and the western Pacific Ocean. Finally, in China there is an age structure of “older in the upper layers and younger in the lower layers” between both the upper and lower crusts and between the


    Directory of Open Access Journals (Sweden)

    Lidia V. Solov’eva


    radiogenic osmium was supplied into the mantle lithosphere from the subduction zone (1.7–2.2 Ga, according to [Pernet et al., 2015]. In analyses of minerals in the pyroxenite-peridotite series from the Obnazhennaya pipe, data on the oxygen isotope geochemistry give evidence of an ancient subduction component (Fig. 26. It can be thus assumed that in the mantle lithosphere of the Birekte terrain, phlogopite-amphibole metasomatism took place due to fluids-melts ascending from the subduction zone about 1.8 Ga ago and correlates to the accretion of this block to the Siberian craton. The complex magmatic series of Phl-Ilm rocks formed later than the Mg pyroxenite-peridotite series. The more ancient ages of phlogopite (869–851 Ma from PhlIlm hyperbasites are somewhat higher than the most ancient dating of alkaline ultrabasic-carbonatite Tomtor massif (800 Ga, according to [Entin et al., 1990] and the time when the breakup of Rodinia began (825 Ga, according to [Li et al., 2008]. The difference may be explained by an advance occurrence of high-potassium, titanian, ferrous magmatites in the mantle lithosphere of the Birekte block as compared to their appearance on the surface. Phlogopite from xenoliths with subsolidus garnetization is significantly younger in age (500–600 Ma, may be, due to a loss of radiogenic argon caused by mica replacement. H2O, K, Ba, F and Cl were abundantly released during the replacement and supplied into the upper layers of the crust and mantle. The mantle high-potassium and high titanian Phl-Ilm series seems comagmatic with the surficial potassium ultramafites and mafites of the Siberian Platform and associated with the earlier episode of the Rodinia breakup.

  4. Detailed structure and sharpness of upper mantle discontinuities in the Tonga subduction zone from regional broadband arrays (United States)

    Tibi, Rigobert; Wiens, Douglas A.


    Recordings of deep Tonga earthquakes from two arrays of 12 broadband seismographs each in the Fiji and Tonga islands are stacked and searched for reflections and conversions from upper mantle discontinuities in the Tonga subduction zone. The arrays operated as part of the Seismic Arrays in Fiji and Tonga (SAFT) experiment from July 2001 to August 2002. In comparison with the commonly used teleseismic approaches, the short path lengths for the local data provide smaller Fresnel zones and high-frequency content for precise mapping of discontinuity topography and sharpness. To enhance the low-amplitude discontinuity phases s410p, P660p and S660p, deconvolved seismograms from each event/array pair are aligned on the maximum amplitude of the direct P wave and subsequently slant stacked. For the 410-km discontinuity, the results show no systematic variations in depth with distance to the cold slab. The 660-km discontinuity varies between 656 and 714 km in depth. For the southern and central parts of the subduction zone, the largest depths occur in the core of the Tonga slab. For the northern part, two separate depressions of the 660-km discontinuity are observed. These anomalies are interpreted as being induced by the active, steeply subducting Tonga deep zone and a subhorizontally lying remnant of subducted lithosphere from the fossil Vityaz trench, respectively. Interpreting the deflections of the 660-km discontinuity in terms of local temperatures implies a thermal anomaly of -800°K to -1200°K at 660 km depth. Except for the southern region where it may thicken, the width of the depressed 660-km discontinuity region implies that the Tonga slab seems to penetrate the 660-km discontinuity with little deformation. Waveform modeling suggests that both the 410- and 660-km discontinuities are sharp. The 660-km discontinuity is at most 2 km thick in many parts of the region, and a first-order discontinuity cannot be precluded. The 410-km discontinuity thickness shows

  5. Subduction of lower continental crust beneath the Pamir imaged by receiver functions from the seismological TIPAGE network (United States)

    Schneider, F. M.; Yuan, X.; Schurr, B.; Mechie, J.; Sippl, C.; Kufner, S.; Haberland, C. A.; Minaev, V.; Oimahmadov, I.; Gadoev, M.; Abdybachaev, U.; Orunbaev, S.


    As the northwestern promontory of the Tibetan Plateau, the Pamir forms an outstanding part of the India-Asia convergence zone. The Pamir plateau has an average elevation of more than 4000 m surrounded by peaks exceeding 7000 m at its northern, eastern and southern borders. The Pamir is thought to consist of the same collage of continental terranes as Tibet. However, in this region the Indian-Asian continental collision presents an extreme situation since, compared to Tibet, in the Pamir a similar amount of north-south convergence has been accommodated within a much smaller distance. The Pamir hosts a zone of intermediate depth earthquakes being the seismic imprint of Earth's most spectacular active intra-continental subduction zone. We present receiver function (RF) images from the TIPAGE seismic profile giving evidence that the intermediate depth seismicity is situated within a subducted layer of lower continental crust: We observe a southerly dipping 10-15 km thick low-velocity zone (LVZ), that starts from the base of the crust and extends to a depth of more than 150 km enveloping the intermediate depth earthquakes that have been located with high precision from our local network records. In a second northwest to southeast cross section we observe that towards the western Pamir the dip direction of the LVZ bends to the southeast following the geometry of the intermediate depth seismic zone. Our observations imply that the complete arcuate intermediate depth seismic zone beneath the Pamir traces a slab of subducting Eurasian continental lower crust. These observations provide important implications for the geodynamics of continental collision: First, it shows that under extreme conditions lower crust can be brought to mantle depths despite its buoyancy, a fact that is also testified by the exhumation of ultra-high pressure metamorphic rocks. Recent results from teleseismic tomography show a signal of Asian mantle lithosphere down to 600 km depth, implying a great

  6. Using glacial morphology to constrain the impact of the Chile active spreading ridge subduction in Central Patagonia (United States)

    Scalabrino, B.; Ritz, J. F.; Lagabrielle, Y.


    The Central Patagonian Cordillera is a unique laboratory to study interaction between oceanic and continental lithospheres during the subduction of an active spreading ridge beneath a continent. The subduction of the South Chile spreading Ridge, which separates the Nazca plate from the Antarctic plate, started ca. 15-14 Ma at the southern tip of Patagonia (55°S latitude). The northwards migration of the Chile Triple Junction induces the subduction of several segments especially around 46°S latitude. There, three segments subducted at ca. 6, 3 and 0.3 Ma, leading to the formation of a large asthenospheric slab-window beneath Central Patagonia. Contemporaneously, the Central Patagonia reliefs are undergoing major glacial events since at least 7 Ma. These events are evidenced to the east of the Central Patagonian morphotectonic front within perched relict surfaces. Inset in these perched glacial surfaces are found mid-Pleistocene glacial valleys, as the Lake General Carrera-Buenos Aires amphitheatre (LGCBA), which formed between 1.1 Ma and 16 ka. We used the relationships between the glacial valleys and the volcanism associated with the asthenospheric slab-window to better constraints the structural evolution of the Patagonian Cordillera related to the subduction of the Chili active spreading Ridge. The present work focused within two well-preserved perched flat surfaces named Meseta del Lago Buenos Aires and Meseta del Cerro Galera: (i) The meseta del Lago Buenos Aires defines a plateau made of interbedded units of tills and lavas dated between 12 Ma and 3 Ma. The top surface of the meseta, ˜2000 meters high is dated at 3 Ma, and is shaped by four NE-SW trending glacial lobes characterized with kettles, lineations and moraines. The glacial valleys are beheaded westwards and define perched valleys 200 to 400 meters higher than the western Cordillera. This suggests recent vertical movement along N160 extensive/transtensive corridor located between the morphotectonic

  7. GPS Monitoring of Subduction Zone Deformation in Costa Rica (United States)

    Lundgren, Paul


    The subduction of the Cocos plate beneath Costa Rica is among the highest convergence rates in the world. The high subduction rate and nearness of the Nicoya Peninsula, Costa Rica to the Middle America Trench (MAT) provide a unique opportunity to map variations in interseismic strain of the crust above the seismogenic zone in response to variations in seismic coupling.

  8. Slab detachment of subducted Indo-Australian plate beneath Sunda ...

    Indian Academy of Sciences (India)

    2007). We investigate the northward subduction of the. Indo-Australian plate along the eastern Sunda arc right from northwestern Sumatra, along Java to. Keywords. Slab detachment; subduction zone; Sunda arc; Indo-Australian slab; trench migration. J. Earth Syst. Sci. 120, No. 2, April 2011, pp. 193–204 c Indian Academy ...

  9. The Run-Up of Subduction Zones (United States)

    Riquelme, S.; Bravo, F. J.; Fuentes, M.; Matias, M.; Medina, M.


    Large earthquakes in subduction zones are liable to produce tsunamis that can cause destruction and fatalities. The Run-up is a geophysical parameter that quantifies damage and if critical facilities or population are exposed to. Here we use the coupling for certain subduction regions measured by different techniques (Potency and GPS observations) to define areas where large earthquakes can occur. Taking the slab 1.0 from the United States Geological Survey (USGS), we can define the geometry of the area including its tsunamigenic potential. By using stochastic earthquakes sources for each area with its maximum tsunamigenic potential, we calculate the numerical and analytical run-up for each case. Then, we perform a statistical analysis and calculate the envelope for both methods. Furthermore, we build an index of risk using: the closest slope to the shore in a piecewise linear approach (last slopecriteria) and the outputsfrom tsunami modeling. Results show that there are areas prone to produce higher run-up than others based on the size of the earthquake, geometrical constraints of the source, tectonic setting and the coast last slope. Based on these results, there are zones that have low risk index which can define escape routes or secure coastal areas for tsunami early warning, urban and planning purposes when detailed data is available.

  10. Satellite-derived geoid for the estimation of lithospheric cooling and ...

    Indian Academy of Sciences (India)

    Lithospheric cooling and basal heat flux anomalies over the northern Indian Ocean lithosphere 1679. • To study the progressive cooling of young-aged oceanic lithosphere from the divergent Mid-Oceanic type Carlsberg Ridge through the estimation of basalt heat flux anomalies and vertical temperature structure retrieved ...

  11. Mapping subduction interface coupling using magnetotellurics: Hikurangi margin, New Zealand (United States)

    Heise, W.; Caldwell, T. G.; Bannister, S.; Bertrand, E. A.; Ogawa, Y.; Bennie, S. L.; Ichihara, H.


    The observation of slow-slip, seismic tremor, and low-frequency earthquakes at subduction margins has provided new insight into the mechanisms by which stress accumulates between large subduction (megathrust) earthquakes. However, the relationship between the physical properties of the subduction interface and the nature of the controls on interplate seismic coupling is not fully understood. Using magnetotelluric data, we show in situ that an electrically resistive patch on the Hikurangi subduction interface corresponds with an area of increased coupling inferred from geodetic data. This resistive patch must reflect a decrease in the fluid or sediment content of the interface shear zone. Together, the magnetotelluric and geodetic data suggest that the frictional coupling of this part on the Hikurangi margin may be controlled by the interface fluid and sediment content: the resistive patch marking a fluid- and sediment-starved area with an increased density of small, seismogenic-asperities, and therefore a greater likelihood of subduction earthquake nucleation.

  12. Did high Neo-Tethys subduction rates contribute to early Cenozoic warming? (United States)

    Hoareau, G.; Bomou, B.; van Hinsbergen, D. J. J.; Carry, N.; Marquer, D.; Donnadieu, Y.; Le Hir, G.; Vrielynck, B.; Walter-Simonnet, A.-V.


    The 58-51 Ma interval was characterized by a long-term increase of global temperatures (+4 to +6 °C) up to the Early Eocene Climate Optimum (EECO, 52.9-50.7 Ma), the warmest interval of the Cenozoic. It was recently suggested that sustained high atmospheric pCO2, controlling warm early Cenozoic climate, may have been released during Neo-Tethys closure through the subduction of large amounts of pelagic carbonates and their recycling as CO2 at arc volcanoes. To analyze the impact of Neo-Tethys closure on early Cenozoic warming, we have modeled the volume of subducted sediments and the amount of CO2 emitted along the northern Tethys margin. The impact of calculated CO2 fluxes on global temperature during the early Cenozoic have then been tested using a climate carbon cycle model (GEOCLIM). We show that CO2 production may have reached up to 1.55 × 1018 mol Ma-1 specifically during the EECO, ~ 4 to 37 % higher that the modern global volcanic CO2 output, owing to a dramatic India-Asia plate convergence increase. The subduction of thick Greater Indian continental margin carbonate sediments at ~ 55-50 Ma may also have led to additional CO2 production of 3.35 × 1018 mol Ma-1 during the EECO, making a total of 85 % of the global volcanic CO2 outgassed. However, climate modeling demonstrates that timing of maximum CO2 release only partially fits with the EECO, and that corresponding maximum pCO2 values (750 ppm) and surface warming (+2 °C) do not reach values inferred from geochemical proxies, a result consistent with conclusions arising from modeling based on other published CO2 fluxes. These results demonstrate that CO2 derived from decarbonation of Neo-Tethyan lithosphere may have possibly contributed to, but certainly cannot account alone for early Cenozoic warming. Other commonly cited sources of excess CO2 such as enhanced igneous province volcanism also appear to be up to 1 order of magnitude below fluxes required by the model to fit with proxy data of pCO2 and

  13. Plume-subduction interaction in southern Central America: Mantle upwelling and slab melting (United States)

    Gazel, Esteban; Hoernle, Kaj; Carr, Michael J.; Herzberg, Claude; Saginor, Ian; den Bogaard, Paul van; Hauff, Folkmar; Feigenson, Mark; Swisher, Carl


    The volcanic front in southern Central America is well known for its Galapagos OIB-like geochemical signature. A comprehensive set of geochemical, isotopic and geochronological data collected on volumetrically minor alkaline basalts and adakites were used to better constrain the mantle and subduction magma components and to test the different models that explain this OIB signature in an arc setting. We report a migration of back-arc alkaline volcanism towards the northwest, consistent with arc-parallel mantle flow models, and a migration towards the southeast in the adakites possibly tracking the eastward movement of the triple junction where the Panama Fracture Zone intersects the Middle America Trench. The adakites major and trace element compositions are consistent with magmas produced by melting a mantle-wedge source metasomatized by slab derived melts. The alkaline magmas are restricted to areas that have no seismic evidence of a subducting slab. The geochemical signature of the alkaline magmas is mostly controlled by upwelling asthenosphere with minor contributions from subduction components. Mantle potential temperatures calculated from the alkaline basalt primary magmas increased from close to ambient mantle (~ 1380-1410 °C) in the Pliocene to ~ 1450 °C in the younger units. The calculated initial melting pressures for these primary magmas are in the garnet stability field (3.0-2.7 GPa). The average final melting pressures range between 2.7 and 2.5 GPa, which is interpreted as the lithosphere-asthenosphere boundary at ~ 85-90 km. We provide a geotectonic model that integrates the diverse observations presented here. The slab detached after the collision of the Galapagos tracks with the arc (~ 10-8 Ma). The detachment allowed hotter asthenosphere to flow into the mantle wedge. This influx of hotter asthenosphere explains the increase in mantle potential temperatures, the northwest migration in the back-arc alkaline lavas that tracks the passage of the


    Directory of Open Access Journals (Sweden)

    George Pararas-Carayannis


    Full Text Available The Colombia/Ecuador subduction zone is a region where high seismic stress is presently accumulating. Statistical probability studies and GPS measurements of crustal deformation indicate that the region has an increased potential to generate in the near future a major or great tsunamigenic earthquake similar to the 1979 or 1906. Although most of the major earthquakes along this margin usually generate local tsunamis, the recurrence of a great mega-thrust, inter-plate earthquake, similar in magnitude and rupture to the 1906 event (Mw=8.8, rupture 600 km., can generate a tsunami with destructive near and far-field impacts. To understand the potential for such destructive tsunami generation in this region, the present study examines and evaluates: a the controlling inter-plate coupling mechanisms of the tectonic regime of the margin – including lithospheric structure deformation, sea-floor relief and the subduction or accretion of highly folded, hydrated sediments along the seismogenic zone of southern Colombia/North Ecuador; b the seismo-dynamics and role in tsunami generation as affected by the Carnegie Ridge’s oblique subduction beneath the South American continent; and c the seismotectonic extensional processes in the vicinity of the Gulf of Guayaquil-Tumbes Basin and how the northwestward movement of the North Andes block away from the South American continent along the Dolores Guayaquil mega-thrust and the resulting strain rotation may cause sudden detachment, décollement and deformation, with the potential for local tsunami generation that may affect the Gulf of Guayaquil and other coastal areas along southern Ecuador.

  15. The nexus of soil radon and hydrogen dynamics and seismicity of the northern flank of the Kuril-Kamchatka subduction zone

    Directory of Open Access Journals (Sweden)

    O. P. Malysheva


    Full Text Available The comparison of kinematics and dynamic parameters of radon and molecular hydrogen concentration in subsoil air on the stations network at the Petropavlovsk-Kamchatsky geodynamic proving ground with seismicity of the northern flank of the Kuril-Kamchatka subduction zone was fulfilled in the period from July till August 2004. On the basis of correlation analysis of the regional seismicity and variations of radon flux density calculated using the data of gas-discharge counters of STS-6 type and SSNTDs it was shown that the radon mass transfer abnormal variations are conditioned by both regional seismicity in total and the subduction zone of proving ground. The azimuths of «geodeformation waves» coming to the registration points are calculated during clearly expressed anomaly beginnings, which coincide with directions to earthquake epicenters taking place at the same time. The geochemical anomalies recorded are presumptively deformative by nature and can be conditioned by processes of «quasi-viscous» flow of the lithosphere during rearrangement of tectonic stress fields of the subduction zone. The short-term (predicted time ? <14 days precursor of the earthquakes swarm was revealed in hydrogen dynamics on August, 4-5 (four earthquakes had M?5.3 and epicentral distance about 130 km from the Paratunka base station.

  16. Forced relative displacements of the core and mantle as the basic mechanism of secular changes of the Earth shape and lithosphere plates tectonics (United States)

    Barkin, Yury


    displacements of plates, first of all continental plates, between hemispheres in a geological time scale is carried out. Thus oscillations and displacements of the core control and organize a plume tectonics activity, and also a spreading activity without which motions of plates to the north would be impossible. This mechanism allows to give a logic explanation to observable tectonic processes and polar changes of geodynamic states of supercontinents observable at formation during geoevolution [10]. According to table 3 of the work [9] the horizontal latitudinal components (in a direction the south-north) of linear velocities of conditional epicentres of lithosphere plates (they correspond to calculated modelling positions of their centers of mass) are equal: 3.47 mm/yr for the Euroasian plate; 2.54 mm/yr for the African plate; 50.3 mm/yr for the Pacific plate; 83.8 mm/yr for the Australian plate; 48.3 mm/yr for the Indian plate; 26.8 mm/yr for the Arabian plate; 35.3 mm/yr for Philippine plate; 54.6 mm/yr for a plate the Cocos; 11.1 mm/yr for Juan de Fuka. For all specified 9 plates mentioned velocities speeds are positive and significant on values. Negative latitudional components of velocities have the American continents: -12.0 mm/yr (the Northern-American plate) and -9.8 mm/yr (the Southern-American plate). Also negative latitudinal components have velocities of the centers of mass of the small plates: -0.64 mm/yr (the Antarctic plate) and -1.2 mm/yr (Nasca). In another words the clearly expressed tendency of displacement of epicentres of the centers of mass of plates to the north exists in reality [9]. The specified displacements are observed with respect to geocentric reference system of coordinates HS2-NUVEL1 connected with hotspots. The obtained conclusion has a modelling character and does not consider some changes in positions of the conditional centers of plates because of the phenomena of spreading and subduction (i.e. here the conditional centers fastened to

  17. Satellite tidal magnetic signals constrain oceanic lithosphere-asthenosphere boundary

    DEFF Research Database (Denmark)

    Grayver, Alexander V.; Schnepf, Neesha R.; Kuvshinov, Alexey V.


    , there are no reports that these signals have been used to infer subsurface structure. We use satellite-detected tidal magnetic fields to image the global electrical structure of the oceanic lithosphere and upper mantle down to a depth of about 250 km. Themodel derived from more than 12 years of satellite data reveals...

  18. Upper mantle viscosity and lithospheric thickness under Iceland

    NARCIS (Netherlands)

    Barnhoorn, A.; Wal, W. van der; Drury, M.R.


    Deglaciation during the Holocene on Iceland caused uplift due to glacial isostatic adjustment. Relatively low estimates for the upper mantle viscosity and lithospheric thickness result in rapid uplift responses to the deglaciation cycles on Iceland. The relatively high temperatures of the upper

  19. Project Skippy explores the lithosphere and mantle beneath Australia

    NARCIS (Netherlands)

    Hilst, R.D. van der; Kennett, Brian; Christie, Doug; Grant, John


    A new project is probing the seismic structure of the lithosphere and mantle beneath Australia. The Skippy Project, named after the bush kangaroo, exploits Australia's regional seismicity and makes use of recent advances in digital recording technology to collect three-component broadband

  20. Lithospheric strength variations in Mainland China : Tectonic implications

    NARCIS (Netherlands)

    Deng, Yangfan; Tesauro, M.


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

  1. A new thermal and rheological model of the European lithosphere

    NARCIS (Netherlands)

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


    We present a new thermal and rheological model of the European lithosphere (10°W-35°E; 35°N-60°N), which is based on a combination of recently obtained geophysical models. To determine temperature distribution we use a new tomography model, which is principally improved by an a-priori correction of

  2. Lithospheric and atmospheric interaction on the planet Venus (United States)

    Volkov, Vladislav P.


    Lithospheric and atmospheric interaction in the planet Venus are discussed. The following subject areas are covered: (1) manifestation of exogenic processes using photogeological data; (2) the chemical composition and a chemical model of the troposphere of Venus; (3) the mineral composition of surface rock on Venus; and (4) the cycles of volatile components.

  3. Structure of the mantle lithosphere beneath the Siberian kimberlite pipes reconstructed by monomineral thermobarometry (United States)

    Ashchepkov, I. V.


    The original methods of the monomineral thermobarometry for clinopyroxene, garnet, ilmenite, chromite (Ashchepkov,2008) and orthopyroxene (Brey, Kohler, 1990- McGregor, 1974) thermobarometer allow to reconstruct the mantle columns. TP diagram for Udachnaya pipe suggests creation at least in tree stages of the melt percolation through the mantle column differing in Fe# and other parameters. The most high temperature (HT) (45 mvm-2) and Fe# rich refer to the last HT reactions with the protokimberlite melts formed the megacrystalline associations. Relict low temperature (LT) geotherm (35 45 mvm-2 and lower) is close to the conductive geotherm (Boyd et al., 1997). Most of ТР parameters for the minerals refer to the middle part of the geotherm (40- 45 mvm-2). Monomineral thermobarometry reconstructing the PTX values (Fe#; CrCpx, Cr-Ilm CaGar, TiChr) showing the high overlapping formed by the the melt percolation. The clinopyroxene growth in the mantle lithosphere in Daldyn, Akakite, Nakyn and Upper Muna are produced by the refertilization events under the influence of the protokimberlite melts. Their spreading in the lower part of mantle section of Garnet trend to subcalsic and pyroxenitic types is likely the result of submelting and heating of the mantle peridotites. Similar process for eclogites is responsible for the appearance of LT eclogites tracing subduction gradients and HT branches with the Ti- bearing associations corresponding to advective gradients. . For the larger pipes the scale of the perturbation is much higher then for smaller. The levels of the melt intrusions are reconstructed by the clotting of TP values inflections of TP paths and TiChr, CrIlm and Fe#. Ilmenite trends reveal the polybaric character of the fractionation and high degree interaction with the wall rock peridotites visible by CrIlm increase. The metasomatic associations differ in PTX diagrams by higher Cr and LT conditions the HT megacrystalls. The evident layered nature of the mantle

  4. Geomorphologic Indices for Transition from Subduction to Arc-Continent Collision in Sumba Island, Indonesia (United States)

    Authemayou, C.; Delcaillau, B.; Brocard, G. Y.; Molliex, S.; Nexer, M.; Pedoja, K.


    The Sumba Island lies in a key area to study the eastern Indonesia geodynamics. It is located in the Sunda-Banda fore-arc in the area of transition from subduction of the Indian oceanic lithosphere (W) beneath the Sunda-Banda arc to arc-continent collision between the Australian continental margin and the Sunda-Banda arc (E). East of the Sumba Island, the western boundary of the Savu basin originated during Middle Miocene by the southeastward slab retreat below the Sunda-Banda arc (Rigg and Hall, 2001). Previous studies has detected a global uplift of the island accommodating the Australian plate - South West Banda Arc convergence (Fleury et al., 2009). This uplift is associated with northeastward tilting and gravitational collapse to the South. Analyses of various geomorphic markers (perched low relief landscapes, rockyshore platform, coral reef terraces, drainages) and of morphometric indices allowed us to localise new structures (faults and folds), to determine the chronology of their activation and to better constraint the uplift history of Sumba Island. These results aim to understand the evolution of the deformation in Sumba Island in regard with its geodynamic context from Middle Miocene to present-day.

  5. Continuum-based 4D Plate Reconstructions: Linking Non-rigid Lithospheric Kinematics to Rigid Plate Motion (United States)

    Kneller, E. A.; Johnson, C. A.; Queffelec, T. A.; Nachtegaele, L.


    Non-rigid deformation in regions of continental extension and compression can lead to large lateral strain and changes in the shape and surface area of continental plates. This large lateral strain in turn leads to vertical strain in the lithosphere, which is a fundamental control on mechanical and thermal subsidence. Traditional plate reconstruction approaches only describe 2D changes in the shape of tectonic plates and do not include lateral strain gradients and vertical strain. Incorporating lateral and vertical strain into kinematic plate tectonic models is necessary for quantifying the past configuration of tectonic plates, modeling paleogeography and for linking subsidence and heat flow to lateral plate motion. Furthermore, traditional approaches are limited to describing processes at the surface of the Earth and cannot be used to investigate 3D slab kinematics. We build on previous work and overcome the limitations of traditional methods by developing an inverse non-rigid continuum-based plate reconstruction approach that links lateral plate tectonic motion to large-scale 4D deformation of continental plates and subducting slabs. We also describe how this approach can be implemented in open source 3D animation software that can be used to create extendable and easily maintained interactive tools. These tools allow the modeler to rapidly reconstruct deformation and map data and constrain plate models with 3D information. The methods presented in this work can improve paleogeographic reconstructions, help visualize complicated 4D deformation processes in a reconstructed framework, and constrain subsidence and lithospheric stretching, all of which are important for understanding thermal history and estimating heat flow in sedimentary basins.

  6. Deep subduction of hot young oceanic slab required by the Syros eclogites (United States)

    Flemetakis, Stamatis; Moulas, Evangelos; Kostopoulos, Dimitrios; Chatzitheodoridis, Elias


    calculated for a subduction velocity of 3 cm/yr, a subduction angle of 30° and an age of incoming lithosphere of ~20 Ma with a shear stress of 80 MPa at the slab-mantle interface [5]. The above are in excellent agreement with published isotopic work on zircons and garnets from Syros eclogites suggesting crystallisation from magmas derived from a depleted mantle at ~80 Ma and constraining the event of eclogitic metamorphism at ~55 Ma. Diffusion modelling of the garnet outermost rims suggests a brief heating pulse of only ~1,000 years at peak T. [1] Van der Molen (1981) Tectonophysics 73, 323-342 .[2] Koons and Thompson (1985) Chemical Geology 50, 3-30. [3] Baxter and Caddick (2013) Geology 41, 6, 643-646. [4] Poli et al. (2009) Earth and Planetary Science Letters 278, 350-360. [5] Peacock (1993) Geol. Soc. Am. Bull. 105, 684-694 .

  7. Petrological-thermomechanical modeling of Precambrian continental collision: geodynamical effects of subcontinental lithospheric mantle thickness (United States)

    Zakharov, Vladimir; Perchuk, Alexei; Zavyalov, Sergei; Sineva, Tamara; Gerya, Taras


    The Precambrian collision and orogeny remains enigmatic and contentious. Different tectonic styles of orogeny in the Precambrian compared to modern Earth are suggested by interpretations of geological, petrological and geochemical observations from Proterozoic and Archean orogenic belts. Here, we present results of 2D petrological-thermomechanical numerical modeling of continental collision at crustal thickness of 35 km and convergence rate of 5 cm/year with variable thickness of subcontinental lithospheric mantle (SCLM). The numerical experiments cover the range of SCLM thickness from 65 km to 165 km, the upper mantle temperature exceeded the modern temperature by 150 oC, and the radiogenic heat production of continental crust is 1.5 times higher than that at present. The numerical modeling has shown that in the case of SCLM thickness of 65 to 125 km the subduction terminates with slab break-off followed by the formation of a large igneous province in between the two continents instead of an orogenic belt. The time and the place of the slab break-off depend on SCLM thickness. The thinner it is, the earlier and the closer to the surface the slab breaks-off. For instance, the slab is detached in 10.3 m.y. at the depth 150 km when the model with SCLM of 115 km, whereas in the case of SCLM of 65 km the slab detaches in 5.1 m.y. almost near the very surface. In the latter case, the magmatic province is very large due to development at the both sides of the oceanic slab (instead of one side provinces in the other experiments). Continental collision with a very thick SCLM (of 165 km and more) proceeds without slab break-off and rather limited volcanism. This work was supported by the Russian Foundation for Basic Research, grant 13-05-01033 and by the Supercomputing Centre of Lomonosov Moscow State University.

  8. Mapping the Traces of the Assembly and Multistage Breakup of Gondwanaland in the Lithosphere of Madagascar (United States)

    Rindraharisaona, E. J.; Tilmann, F. J.; Yuan, X.; Rumpker, G.; Heit, B.; Rambolamanana, G.; Priestley, K. F.


    Madagascar is an ideal place to study the multistage assembly and break up of Gondwanaland, the supercontinent whose breakup also gave rise to most of present day continental regions. At the end of the Proterozoic the assembly of Gondwanaland has placed the Malagasy basement between the Antarctic, Dharwar, Arabo-Nubian and Nubian-Tanzanian cratons. The continental collision processes accompanying the assembly left their mark on the Malagasy basement, currently exposed in the Eastern two thirds of the island, in the form of metamorphic and mineral belts as well as massive ductile shear zones. During the Jurassic Madagascar, India and Seychelles were breaking up from African. Long after the breakaway of India and the Seychelles from Madagascar (Cretaceous time), volcanic activation has occurred in several locations of Madagascar mostly in the central and northern part (Neogene period). The surface traces of assembly and breakup processes have been studied extensively using geological methods in Madagascar but the imprint on the deep structure has so far not been studied in much detail. Between 2012 and 2014, 25 broadband stations were operated in the Southern Madagascar extending from East coast (Mananjary) to West coast (Toliary). The array crosses the Bongolava-Ranontsara shear zone, which is one of the major shear zones in Madagascar. In addition, between 2013 and 2014, 25 short period stations were deployed in the southeastern part of Madagascar. We will present preliminarily results of the lithosphere structure in the southern part of Madagascar based on surface wave dispersion analysis from both earthquakes and ambient noise combine with receiver function analysis. We will focus mostly on the contrast between the lithosphere structure in the eastern (Precambrian rocks) and the western (Sedimentary basins) parts of Madagascar.

  9. Cascadia subduction tremor muted by crustal faults (United States)

    Wells, Ray; Blakely, Richard J.; Wech, Aaron G.; McCrory, Patricia A.; Michael, Andrew


    Deep, episodic slow slip on the Cascadia subduction megathrust of western North America is accompanied by low-frequency tremor in a zone of high fluid pressure between 30 and 40 km depth. Tremor density (tremor epicenters per square kilometer) varies along strike, and lower tremor density statistically correlates with upper plate faults that accommodate northward motion and rotation of forearc blocks. Upper plate earthquakes occur to 35 km depth beneath the faults. We suggest that the faults extend to the overpressured megathrust, where they provide fracture pathways for fluid escape into the upper plate. This locally reduces megathrust fluid pressure and tremor occurrence beneath the faults. Damping of tremor and related slow slip caused by fluid escape could affect fault properties of the megathrust, possibly influencing the behavior of great earthquakes.

  10. Depositionary Margins: The Destruction and Renovation of Subduction Forearcs (United States)

    Vannucchi, P.; Morgan, J. P.; Silver, E. A.; Kluesner, J.


    A depositionary margin is a new framework for forearc evolution that focuses on the potential feedbacks between subduction tectonics, sedimentation, and geomorphology that take place during an extreme event of subduction erosion. These feedbacks can lead to the creation of a forearc structure that extends the traditional division of forearcs into accretionary or erosive subduction margins by demonstrating a mode of rapid basin accretion during an erosive event at a subduction margin. A depositionary mode of forearc evolution occurs when terrigenous sediments are deposited directly on the forearc while it is being removed from below by subduction erosion. In the most extreme case, an entire forearc can be removed by a single subduction erosion event followed by depositionary replacement without involving transfer of sediments from the incoming plate. We need to further recognize that subduction forearcs are often shaped by interactions between slow, long-term processes and sudden extreme events reflecting the sudden influences of large-scale morphological variations in the incoming plate. Both types of processes contribute to the large-scale architecture of the forearc, with extreme events associated with a replacive depositionary mode that rapidly creates sections of a typical forearc margin. The persistent upward diversion of the megathrust is likely to affect its geometry, frictional nature, and hydrogeology. Therefore, the stresses along the fault and individual earthquake rupture characteristics are also expected to be more variable in these erosive systems than in systems with long-lived megathrust surfaces.

  11. Subducted oceanic relief locks the shallow megathrust in central Ecuador (United States)

    Collot, Jean-Yves; Sanclemente, Eddy; Nocquet, Jean-Mathieu; Leprêtre, Angélique; Ribodetti, Alessandra; Jarrin, Paul; Chlieh, Mohamed; Graindorge, David; Charvis, Philippe


    Whether subducted oceanic reliefs such as seamounts promote seismic rupture or aseismic slip remains controversial. Here we use swath bathymetry, prestack depth-migrated multichannel seismic reflection lines, and wide-angle seismic data collected across the central Ecuador subduction segment to reveal a broad 55 km × 50 km, 1.5-2.0 km high, low height-to-width ratio, multipeaked, sediment-bare, shallow subducted oceanic relief. Owing to La Plata Island and the coastline being located, respectively, 35 km and 50-60 km from the trench, GPS measurements allow us to demonstrate that the subducted oceanic relief spatially correlates to a shallow, 80 km × 55 km locked interplate asperity within a dominantly creeping subduction segment. The oceanic relief geometrical anomaly together with its highly jagged topography, the absence of a subduction channel, and a stiff erosive oceanic margin are found to be long-term geological characteristics associated with the shallow locking of the megathrust. Although the size and level of locking observed at the subducted relief scale could produce an Mw >7+ event, no large earthquakes are known to have happened for several centuries. On the contrary, frequent slow slip events have been recorded since 2010 within the locked patch, and regular seismic swarms have occurred in this area during the last 40 years. These transient processes, together with the rough subducted oceanic topography, suggest that interplate friction might actually be heterogeneous within the locked patch. Additionally, we find that the subducted relief undergoes internal shearing and produces a permanent flexural bulge of the margin, which uplifted La Plata Island.

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

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


    extensive deformational regimes controlled the evolution of the basin. We suggest that the source of the fluids and melts, caused extensive annealing in the subcontinental lithospheric mantle, was the subducted Penninic-slab (e.g. [4]) below the Styrian Basin. The source of the high structural hydroxyl contents could be also this slab, which provided high H2O activity environment in the SCLM of the Styrian basin in a mantle-wedge-like setting. References: [1] Szabó, Cs. et al. 2004. Tectonophysics, 393(1), 119-137. [2] Blackman, D. K. et al. 2002. G3, 3, 1-24. [3] Tommasi, A. et al. 2000. J. of Geophys. Res.: Solid Earth, 105, 7893-7908. [4] Qorbani, E. et al 2015. Tectonophysics, 409, 96-108.

  13. Deformation cycles of subduction earthquakes in a viscoelastic Earth. (United States)

    Wang, Kelin; Hu, Yan; He, Jiangheng


    Subduction zones produce the largest earthquakes. Over the past two decades, space geodesy has revolutionized our view of crustal deformation between consecutive earthquakes. The short time span of modern measurements necessitates comparative studies of subduction zones that are at different stages of the deformation cycle. Piecing together geodetic 'snapshots' from different subduction zones leads to a unifying picture in which the deformation is controlled by both the short-term (years) and long-term (decades and centuries) viscous behaviour of the mantle. Traditional views based on elastic models, such as coseismic deformation being a mirror image of interseismic deformation, are being thoroughly revised.

  14. A record of spontaneous subduction initiation in the Izu-Bonin-Mariana arc

    NARCIS (Netherlands)

    Arculus, Richard J.; Ishizuka, Osamu; Bogus, Kara A.; Gurnis, Michael; Hickey-Vargas, Rosemary; Aljahdali, Mohammed H.; Bandini-Maeder, Alexandre N.; Barth, Andrew P.; Brandl, Philipp A.; Drab, Laureen; Do Monte Guerra, Rodrigo; Hamada, Morihisa; Jiang, Fuqing; Kanayama, Kyoko; Kender, Sev; Kusano, Yuki; Li, He; Loudin, Lorne C.; Maffione, Marco; Marsaglia, Kathleen M.; McCarthy, Anders; Meffre, Sebastién; Morris, Antony; Neuhaus, Martin; Savov, Ivan P.; Sena, Clara; Tepley, Frank J.; Van Der Land, Cees; Yogodzinski, Gene M.; Zhang, Zhaohui


    The initiation of tectonic plate subduction into the mantle is poorly understood. If subduction is induced by the push of a distant mid-ocean ridge or subducted slab pull, we expect compression and uplift of the overriding plate. In contrast, spontaneous subduction initiation, driven by subsidence

  15. Three-dimensional dynamic laboratory models of subduction with an overriding plate and variable interplate rheology

    NARCIS (Netherlands)

    Duarte, João C.; Schellart, Wouter P.; Cruden, Alexander R.


    Subduction zones are complex 3-D features in which one tectonic plate sinks underneath another into the deep mantle. During subduction the overriding plate (OP) remains in physical contact with the subducting plate and stresses generated at the subduction zone interface and by mantle flowforce the


    Directory of Open Access Journals (Sweden)

    Mikhail A. Goncharov


    drifting (Figures 26, and 27.By comparing the equations that describe the model of the northern drift of the lithosphere and the model of the core drift towards the North Pole, it is possible to establish a quantitative ‘bridge’ between the structures of meridional compression of the lithosphere and the core drifting structures.Conclusions based on the model of the northern drift of the lithosphere conform to many independent data and concepts, such as disturbance of the isostatic equilibrium of the Antarctica lithosphere and its high standing; the anomalously wide shelf of the Arctic ocean (Figure 28а and the increased thickness of the sediment cover, that is rich in hydrocarbons, in combination with the ultralow velocity of spreading in Gakkel Ridge; the approximately equal areas of Antarctica and the Arctic ocean as antipodes (Figure 28б; elongation (according to GPS data of the parallels in the Southern hemisphere, and their shortening in the Northern hemisphere (Figure 26; radial (relative to the South Pole rifts and other lineaments in Antarctica (Figures 29, and 30; the sub-concentric (relative to the same pole system of spreading around Antarctica, which develops northward into the submeridional system including three ‘trunks’ at a distance of about 90° (Figure 31.Due to the higher velocity of the northern drift of the lithosphere within the band with the middle meridian 100° E – 80° W, wherein the main mass of the continental lithosphere is concentrated and whose two ‘poles’ are marked by the axes of the African and Pacific superplumes (Figures 3, 4, 5, and 32, the following specific features have developed: maximum elongation of the Antarctic continent in the Southern (‘stretched’ hemisphere (Figure 28 б; maximum shortening of the Arctic ocean in the Northern (‘compressed’ hemisphere (Figure 28а; maximum spreading velocity in the SouthEastern Indian Ridge (Figure 33; maximum northern component of the horizontal displacements velocity


    Directory of Open Access Journals (Sweden)

    Mikhail A. Goncharov


    drifting (Figures 26, and 27.By comparing the equations that describe the model of the northern drift of the lithosphere and the model of the core drift towards the North Pole, it is possible to establish a quantitative ‘bridge’ between the structures of meridional compression of the lithosphere and the core drifting structures.Conclusions based on the model of the northern drift of the lithosphere conform to many independent data and concepts, such as disturbance of the isostatic equilibrium of the Antarctica lithosphere and its high standing; the anomalously wide shelf of the Arctic ocean (Figure 28а and the increased thickness of the sediment cover, that is rich in hydrocarbons, in combination with the ultralow velocity of spreading in Gakkel Ridge; the approximately equal areas of Antarctica and the Arctic ocean as antipodes (Figure 28б; elongation (according to GPS data of the parallels in the Southern hemisphere, and their shortening in the Northern hemisphere (Figure 26; radial (relative to the South Pole rifts and other lineaments in Antarctica (Figures 29, and 30; the sub-concentric (relative to the same pole system of spreading around Antarctica, which develops northward into the submeridional system including three ‘trunks’ at a distance of about 90° (Figure 31.Due to the higher velocity of the northern drift of the lithosphere within the band with the middle meridian 100° E – 80° W, wherein the main mass of the continental lithosphere is concentrated and whose two ‘poles’ are marked by the axes of the African and Pacific superplumes (Figures 3, 4, 5, and 32, the following specific features have developed: maximum elongation of the Antarctic continent in the Southern (‘stretched’ hemisphere (Figure 28 б; maximum shortening of the Arctic ocean in the Northern (‘compressed’ hemisphere (Figure 28а; maximum spreading velocity in the SouthEastern Indian Ridge (Figure 33; maximum northern component of the horizontal displacements velocity

  18. Behaviour of fluid mobile elements during subduction and exhumation of abyssal peridotites: Example of serpentinites from Cuba and Dominican Republic (United States)

    Deschamps, F.; Guillot, S.; Godard, M.; Chauvel, C.; Andreani, M.


    Seawater interaction with abyssal peridotites at the sea floor is an important process for chemical exchange between lithosperic mantle and ocean at slow-spreading ridge. Serpentinites from oceanic lithosphere are known to represent an important sink for fluid mobile element, notably boron. In parallel we know extreme enrichment in fluid mobile element (As, Sb, B, U, Li) in high-pressure serpentine minerals (antigorite) coming from part of mantle wedge hydrated by slab's fluids released (Himalaya, Deschamps et al., in prep.). In order to constrain geochemical behaviour of fluid mobile element hosted by serpentine during subduction processes, we examined abyssal serpentinites coming from the accretionary wedge of Greater Carribean (Cuba and Dominican Republic). They represent subducted Atlantic oceanic lithosphere, which have experienced low to high- metamorphism (greenschist to eclogite facies), before being exhumed. These rocks are an opportunity to understand the chemical mobility of fluid mobile element during prograde metamorphism occurring along subduction surface. Here we present bulk-rock and in-situ composition of serpentinites and their primary and alteration-related phase obtained on (LA-)HR-ICP-MS. Except for a few cumulates, serpentinites have depleted compositions in agreement with a refractory mantle, but also strong enrichment in U, Pb, Th, Sr, and other fluid mobile element (e.g., U up to 10xPM) indicating extensive fluid-rock intercation. In-situ composition allows the distinction of two groups of serpentine. Group 1 is characterized by relatively flat and depleted REE patterns (Yb ≈ 0.5xChondrite) reflecting formation after olivine, while group 2 formed after pyroxene is characterized by slightly higher HREE content (Yb ≈ 1xChondrite) but light LREE depleted patterns and displays higher content in Sc, V, Ti and Co. Both groups are strongly enriched in B (up to 120 ppm), and reach values already observed in serpentinites from mantle wedge

  19. The Elephants' Graveyard: Constraints from Mantle Plumes on the Fate of Subducted Slabs and Implications for the Style of Mantle Convection (United States)

    Lassiter, J. C.


    The style of mantle convection (e.g., layered- vs. whole-mantle convection) is one of the most hotly contested questions in the Geological Sciences. Geochemical arguments for and against mantle layering have largely focused on mass-balance evidence for the existence of "hidden" geochemical reservoirs. However, the size and location of such reservoirs are largely unconstrained, and most geochemical arguments for mantle layering are consistent with a depleted mantle comprising most of the mantle mass and a comparatively small volume of enriched, hidden material either within D" or within seismically anomalous "piles" beneath southern Africa and the South Pacific. The mass flux associated with subduction of oceanic lithosphere is large and plate subduction is an efficient driver of convective mixing in the mantle. Therefore, the depth to which oceanic lithosphere descends into the mantle is effectively the depth of the upper mantle in any layered mantle model. Numerous geochemical studies provide convincing evidence that many mantle plumes contain material which at one point resided close to the Earth's surface (e.g., recycled oceanic crust ± sediments, possibly subduction-modified mantle wedge material). Fluid dynamic models further reveal that only the central cores of mantle plumes are involved in melt generation. The presence of recycled material in the sources of many ocean island basalts therefore cannot be explained by entrainment of this material during plume ascent, but requires that recycled material resides within or immediately above the thermo-chemical boundary layer(s) that generates mantle plumes. More recent Os- isotope studies of mantle xenoliths from OIB settings reveal the presence not only of recycled crust in mantle plumes, but also ancient melt-depleted harzburgite interpreted to represent ancient recycled oceanic lithosphere [1]. Thus, there is increasing evidence that subducted slabs accumulate in the boundary layer(s) that provide the source

  20. South China Sea crustal thickness and lithosphere thinning from satellite gravity inversion incorporating a lithospheric thermal gravity anomaly correction (United States)

    Kusznir, Nick; Gozzard, Simon; Alvey, Andy


    The distribution of ocean crust and lithosphere within the South China Sea (SCS) are controversial. Sea-floor spreading re-orientation and ridge jumps during the Oligocene-Miocene formation of the South China Sea led to the present complex distribution of oceanic crust, thinned continental crust, micro-continents and volcanic ridges. We determine Moho depth, crustal thickness and continental lithosphere thinning (1- 1/beta) for the South China Sea using a gravity inversion method which incorporates a lithosphere thermal gravity anomaly correction (Chappell & Kusznir, 2008). The gravity inversion method provides a prediction of ocean-continent transition structure and continent-ocean boundary location which is independent of ocean isochron information. A correction is required for the lithosphere thermal gravity anomaly in order to determine Moho depth accurately from gravity inversion; the elevated lithosphere geotherm of the young oceanic and rifted continental margin lithosphere of the South China Sea produces a large lithosphere thermal gravity anomaly which in places exceeds -150 mGal. The gravity anomaly inversion is carried out in the 3D spectral domain (using Parker 1972) to determine 3D Moho geometry and invokes Smith's uniqueness theorem. The gravity anomaly contribution from sediments assumes a compaction controlled sediment density increase with depth. The gravity inversion includes a parameterization of the decompression melting model of White & McKenzie (1999) to predict volcanic addition generated during continental breakup lithosphere thinning and seafloor spreading. Public domain free air gravity anomaly, bathymetry and sediment thickness data are used in this gravity inversion. Using crustal thickness and continental lithosphere thinning factor maps with superimposed shaded-relief free-air gravity anomaly, we improve the determination of pre-breakup rifted margin conjugacy, rift orientation and sea-floor spreading trajectory. SCS conjugate margins

  1. Petrogenesis of nephelinites from the Tarim Large Igneous Province, NW China: Implications for mantle source characteristics and plume-lithosphere interaction (United States)

    Cheng, Zhiguo; Zhang, Zhaochong; Hou, Tong; Santosh, M.; Zhang, Dongyang; Ke, Shan


    The nephelinite exposed in the Wajilitage area in the northwestern margin of the Tarim large igneous province (TLIP), Xinjiang, NW China display porphyritic textures with clinopyroxene, nepheline and olivine as the major phenocryst phases, together with minor apatite, sodalite and alkali feldspar. The groundmass typically has cryptocrystalline texture and is composed of crystallites of clinopyroxene, nepheline, Fe-Ti oxides, sodalite, apatite, rutile, biotite, amphibole and alkali feldspar. We report rutile SIMS U-Pb age of 268 ± 30 Ma suggesting that the nephelinite may represent the last phase of the TLIP magmatism, which is also confirmed by the field relation. The nephelinite shows depleted Sr-Nd isotopic compositions with age-corrected 87Sr/86Sr and εNd(t) values of 0.70348-0.70371 and + 3.28 to + 3.88 respectively indicating asthenospheric mantle source. Based on the reconstructed primary melt composition, the depth of magma generation is estimated as 115-140 km and the temperatures of mantle melting as 1540-1575 °C. The hotter than normal asthenospheric mantle temperature suggests the involvement of mantle thermal plume. The Mg isotope values display a limited range of δ26Mg from - 0.35 to - 0.55‰, which are lower than the mantle values (- 0.25‰). The Mg isotopic compositions, combined with the Sr-Nd isotopes and major and trace element data suggest that the Wajilitage nephelinite was most likely generated by low-degree partial melting of the hybridized carbonated peridotite/eclogite source, which we correlate with metasomatism by subducted carbonates within the early-middle Paleozoic convergent regime. A plume-lithosphere model is proposed with slight thinning of the lithosphere and variable depth and degree of melting of the carbonated mantle during the plume-lithosphere interaction. This model also accounts for the variation in lithology of the TLIP.

  2. High- and low-Cr chromitite and dunite in a Tibetan ophiolite: evolution from mature subduction system to incipient forearc in the Neo-Tethyan Ocean (United States)

    Xiong, Qing; Henry, Hadrien; Griffin, William L.; Zheng, Jian-Ping; Satsukawa, Takako; Pearson, Norman J.; O'Reilly, Suzanne Y.


    The microstructures, major- and trace-element compositions of minerals and electron backscattered diffraction (EBSD) maps of high- and low-Cr# [spinel Cr# = Cr3+/(Cr3+ + Al3+)] chromitites and dunites from the Zedang ophiolite in the Yarlung Zangbo Suture (South Tibet) have been used to reveal their genesis and the related geodynamic processes in the Neo-Tethyan Ocean. The high-Cr# (0.77-0.80) chromitites (with or without diopside exsolution) have chromite compositions consistent with initial crystallization by interaction between boninitic magmas, harzburgite and reaction-produced magmas in a shallow, mature mantle wedge. Some high-Cr# chromitites show crystal-plastic deformation and grain growth on previous chromite relics that have exsolved needles of diopside. These features are similar to those of the Luobusa high-Cr# chromitites, possibly recycled from the deep upper mantle in a mature subduction system. In contrast, mineralogical, chemical and EBSD features of the Zedang low-Cr# (0.49-0.67) chromitites and dunites and the silicate inclusions in chromite indicate that they formed by rapid interaction between forearc basaltic magmas (MORB-like but with rare subduction input) and the Zedang harzburgites in a dynamically extended, incipient forearc lithosphere. The evidence implies that the high-Cr# chromitites were produced or emplaced in an earlier mature arc (possibly Jurassic), while the low-Cr# associations formed in an incipient forearc during the initiation of a new episode of Neo-Tethyan subduction at 130-120 Ma. This two-episode subduction model can provide a new explanation for the coexistence of high- and low-Cr# chromitites in the same volume of ophiolitic mantle.

  3. Characterizing Seismic Anisotropy across the Peruvian Flat-Slab Subduction Zone: Implications for the Dynamics of Flat-Slabs (United States)

    Eakin, Caroline; Long, Maureen; Beck, Susan; Wagner, Lara; Tavera, Hernando


    Although 10% of subduction zones worldwide today exhibit shallow or flat subduction, we are yet to fully understand how and why these slabs go flat. An excellent study location for such a problem is in Peru, where the largest region of flat-subduction currently exists, extending ~1500 km in length (from 3 °S to 15 °S) and ~300 km in width. Across this region we investigate the pattern of seismic anisotropy, an indicator for past and/or ongoing deformation in the upper mantle. To achieve this we conduct shear wave splitting analyzes at 40 broadband stations from the PULSE project (PerU Lithosphere and Slab Experiment). These stations were deployed for 2+ years across the southern half of the Peruvian flat-slab region. We present detailed shear wave splitting results for both teleseismic events (such as SKS, SKKS, PKS, sSKS) that sample the upper mantle column beneath the stations as well as direct S from local events that constrain anisotropy in the upper portion of the subduction zone. We analyze the variability of our results with respect to initial polarizations, ray paths, and frequency content as well as spatial variability between stations as the underlying slab morphology changes. Teleseismic results show predominately NW-SE fast polarizations (trench oblique to sub-parallel) over the flat-slab region east of Lima. These results are consistent with observations of more complex multi-layered anisotropy beneath a nearby permanent station (NNA) that suggests a trench-perpendicular fast direction in the lowest layer in the sub-slab mantle. Further south, towards the transition to steeper subduction, the splitting pattern becomes increasingly dominated by null measurements. Over to the east however, beyond Cuzco, where the mantle wedge might begin to play a role, we record fast polarizations quasi-parallel to the local slab contours. Local S results indicate the presence of weak (delay times typically less than 0.5 seconds) and heterogeneous supra

  4. Seismic anisotropy of the lithosphere/asthenosphere system beneath southern Madagascar (United States)

    Reiss, M. C.; Rumpker, G.; Tilmann, F. J.; Yuan, X.; Rindraharisaona, E. J.


    Madagascar is considered as a key region with respect to the assembly and break-up of the supercontinent Gondwana. Following the collision between East- and West-Gondwana (~700-650 Ma), its position was central to the Pan-African orogeny and later to the break-up between East-Africa, India and Antarctica. Today, Madagascar consists of different tectonic units; the eastern two thirds of the island are composed mainly of Precambian rocks, whereas the western part is dominated by sedimentary deposits. Southern Madagascar is characterized by several NS to NW-SE trending shear zones. To increase our understanding of these structures and related tectonic processes, we installed a dense temporary seismic network in southern Madagascar. It consisted of 50 stations, which were in operation for up to 2 years between 2012 and 2014. We present results from shear-wave splitting analyses to infer the seismic anisotropy of the lithosphere-asthenosphere system in response to deformational processes. The polarization of the fast shear wave and the delay time between the fast and slow waves provide constraints on the anisotropic fabric. For our study, we use core phases from up to 22 events. We first apply a conventional single-event splitting analysis by minimizing the transverse component. For stations that do not show a significant azimuthal dependence of the splitting parameters, we also apply a joint inversion involving all recorded waveforms from several events. Our results exhibit delay times between 0.4 and 1.5 s. In the center of the E-W profile, fast axes are mainly oriented NNW-SSE, whereas east of the Ranotsara zone, fast axes are oriented NE-SW. We apply full-waveform FD modeling to examine the effects of various anisotropic models of the crust and mantle. Our results indicate that recently proposed mantle flow models are insufficient to explain the small scale variations of splitting parameters observed along our profile. Our observations are best characterized by

  5. Subduction-stage P-T path of eclogite from the Sambagawa belt: Prophetic record for oceanic-ridge subduction (United States)

    Aoya, M.; Uehara, S.; Wallis, S. R.; Enami, M.


    The Sambagawa belt in SW Japan is a subduction-type high-P/T metamorphic belt. Subduction-stage P-T paths of its constituent rocks are important because they directly constrain physical conditions of the EarthOs interior at the time exhumation of high-P/T metamorphic rocks became feasible. Although a few examples of subduction-stage P-T paths for the Sambagawa rocks have been recognized, these are limited to relatively low-pressure regions (~10 kbar). To augment these data the subduction-stage P-T path of the Kotsu glaucophane (Gln) eclogite is derived. The tectonic significance of the derived and previously determined P-T paths is further examined using a new thermal model. By using compositions of matrix minerals and rims of porphyroblastic garnet (Grt), the peak-T conditions of the Kotsu Gln eclogite have been estimated as ~20 kbar/ 600° C. However, the dP/dT of the P-T path leading to the peak-T conditions is unknown. Petrological studies focusing on inclusion minerals in Grt show: (1) albite is absent as inclusions within Grt; (2) acmite (Acm) component of cpx decreased during growth of Grt; (3) Tschermakite (Ts) component of amphibole decreased and Gln component increased during growth of Grt; and (4) Grt-Cpx thermometry shows a temperature increase during growth of Grt. Along with mineral textures observed in the matrix, the Gln-formation reaction can be determined as: 4Acm + 2Ts + 2quartz + H2O (R) 2Gln + 2epidote + hematite. P-T curve of this reaction always has a large positive dP/dT (>7.1 kbar/100 ?C) with the Gln stability field on the high-P/T side. To cross this reaction curve into the Gln stability field during a rise in temperature, the Kotsu eclogite must trace a very steep subduction-type P-T path. Compilation of previously obtained subduction-stage P-T paths for the Sambagawa rocks along with the P-T path of the Kotsu Gln eclogite shows that the series of subduction-stage P-T paths are not distributed on a straight line starting from the origin

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


    Kumar, P.; Yuan, X.; Kind, R.; Mechie, J.


    The dense deployment of seismic stations so far in the western half of the United States within the USArray project provides the opportunity to study in greater detail the structure of the lithosphere-asthenosphere system. We use the S receiver function technique for this purpose, which has higher resolution than surface wave tomography, is sensitive to seismic discontinuities, and is free from multiples, unlike P receiver functions. Only two major discontinuities are observed in the entire a...

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

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


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

  8. Evolution of the Archaean crust by delamination and shallow subduction. (United States)

    Foley, Stephen F; Buhre, Stephan; Jacob, Dorrit E


    The Archaean oceanic crust was probably thicker than present-day oceanic crust owing to higher heat flow and thus higher degrees of melting at mid-ocean ridges. These conditions would also have led to a different bulk composition of oceanic crust in the early Archaean, that would probably have consisted of magnesium-rich picrite (with variably differentiated portions made up of basalt, gabbro, ultramafic cumulates and picrite). It is unclear whether these differences would have influenced crustal subduction and recycling processes, as experiments that have investigated the metamorphic reactions that take place during subduction have to date considered only modern mid-ocean-ridge basalts. Here we present data from high-pressure experiments that show that metamorphism of ultramafic cumulates and picrites produces pyroxenites, which we infer would have delaminated and melted to produce basaltic rocks, rather than continental crust as has previously been thought. Instead, the formation of continental crust requires subduction and melting of garnet-amphibolite--formed only in the upper regions of oceanic crust--which is thought to have first occurred on a large scale during subduction in the late Archaean. We deduce from this that shallow subduction and recycling of oceanic crust took place in the early Archaean, and that this would have resulted in strong depletion of only a thin layer of the uppermost mantle. The misfit between geochemical depletion models and geophysical models for mantle convection (which include deep subduction) might therefore be explained by continuous deepening of this depleted layer through geological time.

  9. Slab, drip, or peeling lithosphere: Teleseismic P-wave tomography and the Isabella anomaly of the southwestern Sierra Nevada, California (United States)

    Jones, C. H.; Reeg, H.; Zandt, G.; Gilbert, H. J.; Owens, T. J.; Stachnik, J. C.


    volumes are appropriate and the geometry compatible with such an origin. A slab origin is troubled if the slab is neutrally buoyant because of the odd coincidence of subsidence in the past 3-5 Ma, it is troubled if antibuoyant as the position so far east and a 60-70° dip ~20 Ma after subduction would seem difficult to maintain, and in either event, the Isabella anomaly is north of a position consistent with the subducted remains of the Monterey subplate. True delamination (i.e., peeling with minimal internal deformation) is unlikely as the volume of material is probably greater than what was present due east and no scar in the lithosphere has been preserved; however, some variation on delamination allowing considerable internal deformation might be possible. Identification of the robust and not-so-robust elements of tomography allows for a better test of hypotheses.

  10. Multiscale Architecture of a Subduction Complex and Insight into Large-scale Material Movement in Subduction Systems (United States)

    Wakabayashi, J.


    The >1000 km by >100 km Franciscan complex of California records >100 Ma of subduction history that terminated with conversion to a transform margin. It affords an ideal natural laboratory to study the rock record of subduction-interface and related processes exhumed from 10-70 km. The Franciscan comprises coherent and block-in-matrix (mélange) units forming a nappe stack that youngs structurally downward in accretion age, indicating progressive subduction accretion. Gaps in accretion ages indicate periods of non-accretion or subduction erosion. The Franciscan comprises siliciclastic trench fill rocks, with lesser volcanic and pelagic rocks and serpentinite derived from the downgoing plate, as well as serpentinite and felsic-intermediate igneous blocks derived as detritus from the upper plate. The Franciscan records subduction, accretion, and metamorphism (including HP), spanning an extended period of subduction, rather than a single event superimposed on pre-formed stratigraphy. Melanges (serpentinite and siliciclastic matrix) with exotic blocks, that include high-grade metamorphic blocks, and felsic-intermediate igneous blocks from the upper plate, are mostly/entirely of sedimentary origin, whereas block-in-matrix rocks formed by tectonism lack exotic blocks and comprise disrupted ocean plate stratigraphy. Mélanges with exotic blocks are interbedded with coherent sandstones. Many blocks-in-melange record two HP burial events followed by surface exposure, and some record three. Paleomegathrust horizons, separating nappes accreted at different times, appear restricted to narrow fault zones of structures, are accommodated by discrete faults or narrow shear zones, rather than by significant penetrative strain. Exhumation of Franciscan HP units, both coherent and mélange, was accommodated by significant extension of the overlying plate, and possibly extension within the subduction complex, with cross-sectional extrusion, and like subduction burial, took place at

  11. Strength, strain, and structure of the Australian continental lithosphere (United States)

    van der Hilst, R. D.; Simons, F. J.


    The diverse geologic makeup of Australia makes the continent an ideal candidate to investigate the relation of lithospheric strength to the age of the overlying crust. Australia is made up from Archean cratons, Proterozoic orogens and Phanerozoic formations in a west-to-east age decrease. Surrounded by active earthquake belts, Australia is ideally suited for regional seismic tomographic studies. High-quality seismic data sets from portable SKIPPY instruments have been inverted to yield detailed models of the three-dimensional wave speed structure, including its anisotropy. Australia's topography is subdued, which makes strength measurements using traditional admittance/coherence techniques between gravity and topography rather difficult. However, the development of advanced spectral techniques has made measurements of the relative strength of the lithosphere possible. In particular, the application of multitaper techniques for cross-spectral analysis has enabled us to study elastic thickness variations with location (and thus age) as well as azimuth (measuring strength anisotropy). Using our seismic wave speed model we have estimated the thickness of the high-velocity lid underlying the Australian continent and compared it to coherence estimates of its elastic thickness. The variations in seismic thickness within broad age divisions of the Australian continent are larger than the differences between the means over the age groups. This is especially surprising for the Australian Archean, whose high-velocity lid is far less pronounced than traditional evolution models would have suggested. Following a similar pattern, the mechanical strength of the lithosphere increases with age to first order only, and substantial strength differences exist within domains of equal crustal age. Seismically thicker continental keels are not necessarily mechanically stronger, and shallow mechanical strength does not appear to control the preservation of such keels. The seismic data set

  12. Sr, Nd, and Pb isotopes of ultramafic xenoliths in volcanic rocks of Eastern China: enriched components EMI and EMII in subcontinental lithosphere (United States)

    Tatsumoto, M.; Basu, A.R.; Wankang, H.; Junwen, W.; Guanghong, X.


    basalts. The EMII mantle domain may be present in the Chinese continental lithosphere just above the EMI domain of the basalt source at the lower part of the lithosphere. We argue that the ancient depleted continental lithosphere was metasomatized, imparting the EMI signature, in earlier times ( > 1000 m.y.), and U migrated upward, resulting in high Th U ratios in the lower portion of the lithosphere. Observed high Th U, Rb Sr, 87Sr 86Sr and ??208, low Sm Nd ratios, and a large negative ??Nd in phlogopite pyroxenite with a depleted mantle model age of 2.9 Ga, support our contention that metasomatized continental lower mantle lithosphere is the source for the EMI component. We also suggest that the EMII signature may have been introduced later (less than ??? 500 Ma) by another metasomatic event during the subduction of an oceanic plate, which was partially responsible for some of the observed inter-mineral isotopic disequilibria. ?? 1992.

  13. Spatial Patterns in Distribution of Kimberlites: Relationship to Tectonic Processes and Lithosphere Structure

    DEFF Research Database (Denmark)

    Chemia, Zurab; Artemieva, Irina; Thybo, Hans


    Since the discovery of diamonds in kimberlite-type rocks more than a century ago, a number of theories regarding the processes involved in kimberlite emplacement have been put forward to explain the unique properties of kimberlite magmatism. Geological data suggests that pre-existing lithosphere ...... broad limits and are, apparently controlled, by the past structure of the lithosphere and a "vigor" of lithosphere-mantle interaction, which caused kimberlite emplacements....

  14. Subduction initiation and Obduction: insights from analog models (United States)

    Agard, P.; Zuo, X.; Funiciello, F.; Bellahsen, N.; Faccenna, C.; Savva, D.


    Subduction initiation and obduction are two poorly constrained geodynamic processes which are interrelated in a number of natural settings. Subduction initiation can be viewed as the result of a regional-scale change in plate convergence partitioning between the set of existing subduction (and collision or obduction) zones worldwide. Intraoceanic subduction initiation may also ultimately lead to obduction of dense oceanic "ophiolites" atop light continental plates. A classic example is the short-lived Peri-Arabic obduction, which took place along thousands of km almost synchronously (within ~5-10 myr), from Turkey to Oman, while the subduction zone beneath Eurasia became temporarily jammed. We herein present analog models designed to study both processes and more specifically (1) subduction initiation through the partitioning of deformation between two convergent zones (a preexisting and a potential one) and, as a consequence, (2) the possible development of obduction, which has so far never been modeled. These models explore the mechanisms of subduction initiation and obduction and test various triggering hypotheses (i.e., plate acceleration, slab crossing the 660 km discontinuity, ridge subduction; Agard et al., 2007). The experimental setup comprises an upper mantle modelled as a low-viscosity transparent Newtonian glucose syrup filling a rigid Plexiglas tank and high-viscosity silicone plates. Convergence is simulated by pushing on a piston at one end of the model with plate tectonics like velocities (1-10 cm/yr) onto (i) a continental margin, (ii) a weakness zone with variable resistance and dip (W), (iii) an oceanic plate - with or without a spreading ridge, (iv) a subduction zone (S) dipping away from the piston and (v) an upper active continental margin, below which the oceanic plate is being subducted at the start of the experiment (as for the Oman case). Several configurations were tested over thirty-five parametric experiments. Special emphasis was

  15. Deformation of Marble, Quartzite, and Metabasalt during Subduction and its Aftermath (Invited) (United States)

    Whitney, D. L.; Teyssier, C.; Seaton, N. C.; Toraman, E.


    Field-based studies of naturally deformed minerals are a link between experimental studies of deformation mechanisms and models for the rheological behavior of the lithosphere. Of particular interest are field sites in which an array of rock types are present and in which field, outcrop, and microscale deformation features can be integrated to provide a comprehensive view of deformation history and syn-tectonic metamorphic reactions. An excellent example of such a site is the Sivrihisar Massif, Turkey, where interlayered marble, quartzite, and metabasalt in the NW part of the massif record conditions related to subduction metamorphism and deformation under high-P/T conditions (2 GPa, 75 km, and maintenance of low-T conditions during exhumation. Each rock type records subduction metamorphism in its texture and/or mineral assemblage, and each rock type varied in its response to overprinting by heat and later deformation. The dominant minerals in each rock type exhibit strong crystallographic preferred orientations (CPO): quartz in quartzite; calcite (former aragonite) in marble; omphacite, glaucophane, lawsonite in metabasalt. The overprinted rocks developed into L-tectonites, although calcite in marble lost both its CPO and shape-preferred orientation under greenschist facies conditions. In the NW, phengite-bearing marble records HP metamorphism in its texture (oriented, rod-shaped calcite pseudomorphs after aragonite); metabasalt has a HP mineral assemblage (e.g., lawsonite-omphacite-garnet-phengite ± glaucophane); and quartzite contains glaucophane + phengite + garnet ± omphacite ± lawsonite. In the south, the HP texture and white mica composition in marble were progressively obliterated by recrystallization at low T and P, and metabasalt interlayered with HP marble and quartzite is partially overprinted. At the P-T-deformation conditions under which the marble HP textures were obliterated, metabasalt shows no evidence for former HP conditions. In contrast, HP

  16. Deep electrical resistivity structure of Costa Rican Subduction Zone (United States)

    Worzewski, T.; Jegen, M.; Brasse, H.; Taylor, W.


    The water content and its distribution play an important role in the subduction process. Water is released from the subducting slab in a series of metamorphic reactions and the hydration of the mantle wedge may trigger the onset of melting, weakening and changes in the dynamics and thermal structure of subduction zones. However, the amount of water carried into the subduction zone and its distribution are not well constrained by existing data and are subject of vigorous current research in SFB574 (Volatiles and Fluids in Subduction Zones: Climate Feedback and Trigger Mechanisms for Natural Disasters). We will show numerical modeling studies which are used to determine the resolution and sensitivity of the MT response to fluids in the crust and subducting slab under the special condition of a coastal setting. In 2007-2008 we conducted a long-period magnetotelluric investigations in northwestern Costa Rica on- and offshore, where the Cocos Plate subducts beneath the Carribean plate. Eleven marine magnetotelluric Stations newly developed and constructed by IFM-GEOMAR and University of Kiel were deployed on the 200 km long marine extension of the profile for several months. We will present the data and its processing, as well as our attempts to eliminate motion induced noise observed on some stations on the cliffy shelf due to tidal waves hitting the shelf and trench parallel- and perpendicular currents. The marine profile was extended landwards by the Free University of Berlin over length of 160 kilometers with further 18 stations. We present preliminary modeling results of land data, which revealed interesting features, inter alia a possible image of fluid release from the downgoing slab in the forearc, as well as ongoing modeling of the combined on- and offshore data sets.

  17. Influence of the inherited lithospheric structure on the interaction between the Kenyan and Ethiopian rifts across the Turkana depression: analog and numerical models (United States)

    Corti, Giacomo; Brune, Sascha; Ranalli, Giorgio


    Rifting processes result from the application of extensional stresses to a pre-deformed, and thus already structured, anisotropic lithosphere; consequently, the pre-rift lithospheric rheological structure and its along-axis variations play a major role in controlling the evolution and architecture of continental rifts. The East African Rift is a classic example of this process. The rift system developed within a region that has experienced several deformation events, which have given rise to significant variations in the rheological structure of the lithosphere. These variations -in turn- have played a major role on rift evolution, as clearly testified by the localisation and propagation of major rift segments within weak Proterozoic mobile belts surrounding cratonic areas. Linkage and mechanical interaction between adjacent rift segments typically occurred in correspondence to transverse pre-existing fabrics, where structurally complex areas (transfer zones) allowed significant along-axis variations in subsidence of grabens and elevation of uplifted flanks. One of these complex areas is the Turkana depression where the Ethiopian and Kenyan rifts interact. The region is characterised by anomalous morphology and distribution of deformation with respect to the rift valleys in Kenya and Ethiopia. In this work we investigate whether these anomalies result from the presence of a pre-existing Mesozoic graben, transverse to the trend of the rift valleys and characterized by thin crust and lithosphere. To this aim, we integrate crustal-scale, isothermal analog experiments with lithospheric-scale, thermo-mechanical numerical models. The two different methodologies generate very similar results, reproducing the along-axis transition from narrow rift valleys in Ethiopia/Kenya to a distributed deformation within the Turkana depression. Modeling results indicate that this variation results from the inherited distribution of lithospheric strength and -in particular- from the

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

    NARCIS (Netherlands)

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

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

  19. The Ionian Abyssal Plain - closure of a remnant Mesozoic oceanic domain: subbottom structures, deep deformation and the Calabrian subduction zone (United States)

    Gallais, F.; Gutscher, M.; Graindorge, D.; Klaeschen, D.


    The Ionian Abyssal Plain (IAP), located in the Central Mediterranean area is a deep triangular shaped basin, surrounded by the Calabrian subduction zone to the NW, the Mediterranean Ridge to the NE and the Medina Ridges to the South. Available heat flow measurements show very low values under the Ionian Abyssal plain, suggesing a very old age of 180-200 Ma for the basin. The Bouguer gravity anomaly map shows only a strong positive anomaly in this region and the depth of the Moho is around 16 to 18 km corresponding to high refraction velocities of 8.1-8.2 km/s. The Ionian basin is interpreted as one of the oldest basins in the Mediterranean area, and is thought to represent a remnant part of the Mesozoic Tethyan ocean. Due to the complex relative motions of microplates and blocks, currently, the oceanic lithosphere of the Ionian basin is being simultaneously consumed by subduction to the NE beneath the Hellenic system and to the NW beneath Calabria. We present the most relevant lines of the Archimede multi-channel seismic cruise (1997, R/V Le Nadir) crossing the Ionian Abyssal Plain and the Calabrian subduction zone. Interpretation of this seismic dataset is based on correlation with published seismic data and with ESP results. Beneath the IAP, we identify a thick sedimentary cover (> 5km) from the Jurassic to the Plio-Quaternary in age, which overlies the oceanic basement. The Pre-Messinian sequences are affected by a set of NE/SW striking compressional faults with some syn-tectonic basins NW of these faults. These features are interpreted as a re-activated set of normal faults, possibly formed during rifting and/or subsequent accretion of oceanic crust. The orientation of the subbottom structures and the thickness of the Messinian deposits in the south-eastern part of the IAP may be linked with the presence of these faults and their activity through time. On the Calabrian side of the IAP, the Post-Messinian sequences are accreted to the Calabrian wedge. The weak

  20. SdP receiver function images of the lithosphere-asthenosphere boundary beneath the Western U.S using USArray data (United States)

    Miller, M. S.; Levander, A.; Xu, Y.; Jiang, M.; Collier, M.; Lee, C. A.


    S-wave receiver functions from the USArray transportable array data have been made to examine the lithosphere and upper mantle structure beneath the western U.S. The project was initiated at the CIDER 2008 workshop in Santa Barbara, CA. Our preliminary results image the volume between the west coast of the US and 107°W and between the Canadian and Mexican borders to approximately 200 km depth. The large area allows us to investigate the structure of a variety of tectonic regions: the San Andreas plate boundary region, Mendicino Triple Junction, Cascadia subduction zone, Sierra Nevada, Snake River Plane, Basin and Range, Colorado Plateau, and the Rocky Mountains. Data from 59 events between 2005 and 2008 at epicentral distances of 55 to 80 degrees from the 556 stations in the transportable array database were processsed. To start we depth converted, spatially repositioned, and common conversion point stacked the SdP receiver functions using a 1D reference model based on averages of Crust2.0 and MC35. We are currently recalculating the depth and lateral positioning of the receiver functions using linear tomography corrections for laterally variable structure. The depth converted and preliminary CCP stack of the SdP receiver functions image a number of interesting lithospheric features including its base, the lithosphere- asthenosphere boundary (LAB). The LAB is a negative amplitude feature that has significant topographic variation, and cannot be described as a single surface. In the Basin and Range and particularly in the southern Sierra Nevada drip region the SdP LAB depths agree well with those determined from PdS receiver functions (Levander et al., 2008), however elsewhere large discrepancies exist between LAB estimates from the two different types of receiver functions We see a particularly strong correlation between calculated equilibration pressures of 911 primitive basalt whole rock samples from across the western United States, extracted from the NAVDAT

  1. Evolution of the lithospheric mantle beneath Mt. Baekdu (Changbaishan): Constraints from geochemical and Sr-Nd-Hf isotopic studies on peridotite xenoliths in trachybasalt (United States)

    Park, Keunsu; Choi, Sung Hi; Cho, Moonsup; Lee, Der-Chuen


    well above the mantle array. The Lu-Hf and Sm-Nd model ages of residual clinopyroxenes yielded Early Proterozoic to Phanerozoic ages. No signature of Archean cratonic mantle was present. Therefore, Mt. Baekdu peridotite is residual lithospheric mantle that has undergone variable degrees of diachronous melt extraction and infiltration metasomatism involving subduction-related, fluid-bearing silicate melts. The predominance of Phanerozoic Hf model ages indicates that the lherzolites represent lithospheric mantle fragments newly accreted underneath the eastern NCC.

  2. LCS-1: a high-resolution global model of the lithospheric magnetic field derived from CHAMP and Swarm satellite observations (United States)

    Olsen, Nils; Ravat, Dhananjay; Finlay, Christopher C.; Kother, Livia K.


    We derive a new model, named LCS-1, of Earth's lithospheric field based on four years (2006 September-2010 September) of magnetic observations taken by the CHAMP satellite at altitudes lower than 350 km, as well as almost three years (2014 April-2016 December) of measurements taken by the two lower Swarm satellites Alpha and Charlie. The model is determined entirely from magnetic 'gradient' data (approximated by finite differences): the north-south gradient is approximated by first differences of 15 s along-track data (for CHAMP and each of the two Swarm satellites), while the east-west gradient is approximated by the difference between observations taken by Swarm Alpha and Charlie. In total, we used 6.2 mio data points. The model is parametrized by 35 000 equivalent point sources located on an almost equal-area grid at a depth of 100 km below the surface (WGS84 ellipsoid). The amplitudes of these point sources are determined by minimizing the misfit to the magnetic satellite 'gradient' data together with the global average of |Br| at the ellipsoid surface (i.e. applying an L1 model regularization of Br). In a final step, we transform the point-source representation to a spherical harmonic expansion. The model shows very good agreement with previous satellite-derived lithospheric field models at low degree (degree correlation above 0.8 for degrees n ≤ 133). Comparison with independent near-surface aeromagnetic data from Australia yields good agreement (coherence >0.55) at horizontal wavelengths down to at least 250 km, corresponding to spherical harmonic degree n ≈ 160. The LCS-1 vertical component and field intensity anomaly maps at Earth's surface show similar features to those exhibited by the WDMAM2 and EMM2015 lithospheric field models truncated at degree 185 in regions where they include near-surface data and provide unprecedented detail where they do not. Example regions of improvement include the Bangui anomaly region in central Africa, the west African

  3. Long term (since the late palaeogene) tectono-sedimentary evolution of the Lesser Antilles fore-arc at Marie-Galante Basin: a clue for geodynamical behavior at the subduction interfac (United States)

    Jean-Frederic, L.; DeMin, L.; Garrigou, J.; Münch, P.; Léticée, J. L.; Cornée, J. J.


    Oblique subduction of late cretaceous lithosphere of the Atlantic ocean beneath the thick (25km) crust of the Caribbean plate results in widespread deformation and vertical motions in the Lesser Antilles fore-arc. The present-day deformation includes a major transtensive left lateral fault system along the arc and several forearc transverse basins accommodating lengthening of the fore-arc northward. These deformations result from plate motion partitioning under increasing subduction obliquity from the Marie-Galante Basin (MGB) latitude (central Lesser Antilles) northward. Vertical motions in the fore-arc at a regional scale were interpreted as resulting from the effect of subducting ridges and reliefs. The present day uplift of the fore-arc islands acting since the late(?) Pleistocene is believed to attest for long wavelength bending of the plate under strongly coupled plate interface. Recent GPS data suggests a mostly uncoupled plate interface. To decipher between the models and to understand the long-term evolution of the Lesser Antilles forearc since the Late Palaeogene, we interpret high-resolution bathymetric and seismic data from the MGB, together with the onland geology of shallow water carbonate platforms. The tectonic pattern reveals both inherited and late Neogene structures (re)activated under multidirectional extensive tectonic. The sismo-stratigraphic interpretation of sedimentary deposit displays long-term drowning and flexing of the upper plate similar to that occurring under intensive tectonic erosion at the subduction interface. Several short term period of second order uplift can correlate with sweeping of subducting ridges or transient events at the plate interface. The evolution of the Lesser Antilles fore-arc since the Late Palaeogene is interpreted within the regional geodynamical evolution of the plate boundary following its last major reorganization: collision of the Bahamas Bank and inception of the Greater Antilles strike-slip fault zone.

  4. Thin lithosphere-asthenosphere boundary beneath Eastern Indian craton (United States)

    Shalivahan; Bhattacharya, Bimalendu B.; Rao, N. V. Chalapathi; Maurya, V. P.


    The lithosphere-asthenosphere boundary (LAB) separates the hard and rigid outer layer of the earth (lithosphere) and the weaker, hotter, and deeper part of the upper mantle (asthenosphere) and plays a pivotal role in plate tectonics. However, its definitive detection, especially beneath the cratons, is proving elusive. One of the geophysical tools used to map the LAB beneath the cratons is through magnetotelluric (MT) observations. The resistivity at boundary falls in the range of 5-25 Ω-m and can be explained by the presence of a small amount of water in the asthenosphere, possibly inducing partial melt. Here, we report thickness of the LAB in one of the oldest dated ancient cratons of India-Eastern Indian Craton (EIC) of ~ 3.3 Gyr, from MT studies. The two prominent phase-sensitive strike directions, one each for crust and mantle, and the presence of resistive continental lower crust act as a window to mantle in resolving deeper electrical conductivity structures beneath EIC. Our results show that the LAB beneath the EIC is at 95 km. The region is interesting as the electrical properties of the crust and mantle and the Moho depth are similar to those of the Slave Craton, Canada (~ 4.0 Gyr) but the depth of the LAB beneath the EIC is half that of the Slave craton. As cratonic signatures, depicted by ultrapotassic rocks from Gondwana coal fields close to EIC, are preserved at least till early Cretaceous (117 Ma) it is likely that Himalayan orogeny could have played a major role in delamination of the lithospheric roots of the EIC in addition to attendant seismicity.

  5. The lithosphere-asthenosphere boundary observed with USArray receiver functions (United States)

    Kumar, P.; Yuan, X.; Kind, R.; Mechie, J.


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

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

    Directory of Open Access Journals (Sweden)

    P. Kumar


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

  7. Modeling Plume-Triggered, Melt-Enabled Lithospheric Delamination (United States)

    Perry-Houts, J.; Humphreys, G.


    It has been suggested that arrival of the Yellowstone plume below North America triggered a lithospheric foundering event which aided the eruption of the Columbia River flood basalts. This hypothesis potentially accounts for some of the biggest mysteries related to the CRB's including their location as "off-track" plume volcanism; and the anomalous chemical signatures of the most voluminous units. The foundered lithosphere appears to be a remnant chunk of Farallon slab, which had been stranded beneath the Blue Mountains terrain since the accretion of Siletzia. If this is the case then the mechanisms by which this slab stayed metastable between Siletzia accretion and CRB time, and then so suddenly broke loose, is unclear. The addition of heat and mantle buoyancy supplied by the Yellowstone plume provides a clue, but the geodynamic process by which the slab was able to detach remains unclear.Efforts to model numerically the underlying processes behind delamination events have been gaining popularity. Typically, such models have relied on drastically weakened regions within the crust, or highly non-linear rheologies to enable initiation and propagation of lithosphere removal. Rather than impose such a weak region a priori, we investigated the role of mantle and crustal melt, generated by the addition of plume heat, as the source of such a rheologic boundary.We track melt generation and migration though geodynamic models using the Eulerian finite element code, ASPECT. Melt moves relative to the permeable, compacting, and viscously-deforming mantle using the approach of (Keller, et al. 2013) with the notable exception that ASPECT currently cannot model elasticity. Dike and sill emplacement is therefore still a work in progress. This work is still in the preliminary stages and results are yet inconclusive.

  8. Interactions of multi-scale heterogeneity in the lithosphere: Australia (United States)

    Kennett, B. L. N.; Yoshizawa, K.; Furumura, T.


    Understanding the complex heterogeneity of the continental lithosphere involves a wide variety of spatial scales and the synthesis of multiple classes of information. Seismic surface waves and multiply reflected body waves provide the main constraints on broad-scale structure, and bounds on the extent of the lithosphere-asthenosphere transition (LAT) can be found from the vertical gradients of S wavespeed. Information on finer-scale structures comes through body wave studies, including detailed seismic tomography and P-wave reflectivity extracted from stacked autocorrelograms of continuous component records. With the inclusion of deterministic large-scale structure and realistic medium-scale stochastic features fine-scale variations are subdued. The resulting multi-scale heterogeneity model for the Australian region gives a good representation of the character of observed seismograms and their geographic variations and matches the observations of P-wave reflectivity. P reflections in the 0.5-3.0 Hz band in the uppermost mantle suggest variations on vertical scales of a few hundred metres with amplitudes of the order of 1%. Interference of waves reflected or converted at sequences of such modest variations in physical properties produce relatively simple behaviour for lower frequencies, which can suggest simpler structures than are actually present. Vertical changes in the character of fine-scale heterogeneity can produce apparent discontinuities. In Central Australia a 'mid-lithospheric discontinuity' can be tracked via changes in frequency content of station reflectivity, with links to the broad-scale pattern of wavespeed gradients and, in particular, the gradients of radial anisotropy. Comparisons with xenolith results from southeastern Australia indicate a strong tie between geochemical stratification and P-wave reflectivity.

  9. Highly oxidising fluids generated during serpentinite breakdown in subduction zones. (United States)

    Debret, B; Sverjensky, D A


    Subduction zones facilitate chemical exchanges between Earth's deep interior and volcanism that affects habitability of the surface environment. Lavas erupted at subduction zones are oxidized and release volatile species. These features may reflect a modification of the oxidation state of the sub-arc mantle by hydrous, oxidizing sulfate and/or carbonate-bearing fluids derived from subducting slabs. But the reason that the fluids are oxidizing has been unclear. Here we use theoretical chemical mass transfer calculations to predict the redox state of fluids generated during serpentinite dehydration. Specifically, the breakdown of antigorite to olivine, enstatite, and chlorite generates fluids with high oxygen fugacities, close to the hematite-magnetite buffer, that can contain significant amounts of sulfate. The migration of these fluids from the slab to the mantle wedge could therefore provide the oxidized source for the genesis of primary arc magmas that release gases to the atmosphere during volcanism. Our results also show that the evolution of oxygen fugacity in serpentinite during subduction is sensitive to the amount of sulfides and potentially metal alloys in bulk rock, possibly producing redox heterogeneities in subducting slabs.

  10. Fluid flux and melting reactions in subduction zones (United States)

    Bouilhol, Pierre; Magni, Valentina; van Hunen, Jeroen; Kaislaniemi, Lars


    Understanding the metamorphic reactions that occurs within the slab is a must to constrain subduction zone processes. Slab dehydration reactions ultimately permit the mantle wedge to melt, by lowering its solidus, thus forming arcs above descending slabs. Alternatively the slab crust may cross its solidus in warm hydrated slabs. Moreover, slab dehydration allows chemical fractionation to occur between residual phases and transferred fluid phase, giving arc magmas part of their typical subduction zone chemical characteristics. To better comprehend such complex thermo-chemical open system, we are using a numerical model that reproduces the thermo-mechanical behaviour of a subducting slab and computes the thermodynamic equilibrium paragenesis at each P-T-X conditions of the system. Hence we generate a "paragenetic map" of a subduction system, allowing us to track the fate of water during dehydration and subsequent re-hydration or melting reactions. Here we highlight the role of dehydration and re-hydration reactions occurring in the slab's igneous crust and mantle and the mantle wedge for different slab configuration hence pres