J. Mechie

Helmholtz-Zentrum Potsdam - Deutsches GeoForschungsZentrum GFZ, Potsdam, Brandenburg, Germany

Are you J. Mechie?

Claim your profile

Publications (152)468.81 Total impact

  • C. Ramos · J. Mechie · M. Feng

    No preview · Article · Mar 2016 · Geophysical Journal International
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Subduction of buoyant continental lithosphere is one of the least understood plate-tectonic processes. Yet under the Pamir-Hindu Kush, at the northwestern margin of the India-Asia collision zone, unusual deep earthquakes and seismic velocity anomalies suggest subduction of Asian and Indian lithosphere. Here, we report new precise earthquake hypocenters, detailed tomographic images and earthquake source mechanisms, which allow distinguishing a narrow sliver of Indian lithosphere beneath the deepest Hindu Kush earthquakes and a broad, arcuate slab of Asian lithosphere beneath the Pamir. We suggest that this double subduction zone arises by contrasting modes of convergence under the Pamir and Hindu Kush, imposed by the different mechanical properties of the three types of lithosphere involved. While the buoyant northwestern salient of Cratonic India bulldozes into Cratonic Asia, forcing delamination and rollback of its lithosphere, India's thinned western continental margin separates from Cratonic India and subducts beneath Asia. This torn-off narrow plate sliver forms a prominent high-velocity anomaly down to the mantle transition zone. Our images show that its uppermost section is thinned or already severed and that intermediate depth earthquakes cluster at the neck connecting it to the deeper slab, providing a rare glimpse at the ephemeral process of slab break-off.
    Full-text · Article · Feb 2016 · Earth and Planetary Science Letters
  • Hui Qian · James Mechie · Haibing Li · Guangqi Xue · Heping Su · Xiang Cui
    [Show abstract] [Hide abstract]
    ABSTRACT: Earthquake location is essential when defining fault systems and other geological structures. Many methods have been developed to locate hypocenters within a 1D velocity model. In this study, a new approach, named MatLoc, has been developed which can simultaneously invert for the locations and origin times of the hypocenters and the velocity structure, from the arrival times of local earthquakes. Moreover, it can invert for layer boundary depths, such as Moho depths, which can be well constrained by the Pm and Pn phases. For this purpose, the package was developed to take into account reflected phases, e.g., the Pm phase. The speed of the inversion is acceptable due to the use of optimized matrix calculations. The package has been used to re-locate the Lushan earthquake series which occurred in Sichuan, China, from April 20 to April 22, 2013. The results obtained with the package show that the Lushan earthquake series defines the dip of the Guankou fault, on which most of the series occurred, to be 39° toward the NW. Further, the surface projection of the Lushan earthquake series is consistent with the regional tectonic strike which is about N45° E.
    No preview · Article · Sep 2015 · Journal of Seismology
  • R. Kind · X. Yuan · J. Mechie · F. Sodoudi
    [Show abstract] [Hide abstract]
    ABSTRACT: We used more than 40 000 S-receiver functions recorded by the USArray project to study the structure of the upper mantle between the Moho and the 410 km discontinuity from the Phanerozoic western United States to the cratonic central US. We obtained clear observations of downward velocity reductions in the uppermost mantle which are commonly interpreted as the lithosphere-asthenosphere boundary (LAB) in the western US and as the mid-lithospheric discontinuity (MLD) in the cratonic US. We observe the western LAB reaching partly to the mid-continental rift system underneath the cratonic crust. The MLD is surprisingly plunging steeply towards the west from the Rocky Mountains Front to about 200 km depth near the Sevier Thrust Belt. There is a significant break in the lithosphere at the Sevier Thrust Belt. We also observe a velocity reduction about 30 km above the 410 km discontinuity in the same region where in the western US the LAB is observed, but not in the cratonic US.
    No preview · Article · Mar 2015 · Solid Earth Discussions
  • Source
    M. Feng · P. Kumar · J. Mechie · W. Zhao · R. Kind · H. Su · G. Xue · D. Shi · H. Qian
    [Show abstract] [Hide abstract]
    ABSTRACT: For a period of about 1 yr between the summers of 2010 and 2011, 25 broad-band seismographs were deployed in a roughly linear array across the eastern end of the Qaidam basin and the Qilian Shan in the northeastern Tibetan plateau. This region is probably the most suitable place to study the ongoing convergence interaction between the high Tibetan plateau and the main Asian continental plate. Low-frequency P receiver function analysis of the data provides an image of the crust and mantle down to 700 km depth. In addition to the Moho at 45-65 km depth beneath the profile, the 410 and 660 km discontinuities bounding the mantle transition zone can be identified at 400-410 and 650-660 km depths, respectively. A possible increase in temperature in the upper mantle thought to exist beneath the northern part of the high Tibetan plateau is thus confined to this part of the plateau and lower upper-mantle temperatures similar to those beneath southern Tibet occur beneath the Qaidam basin and Qilian Shan. When higher frequencies are included in the P receiver function analysis, a positive Ps converter dipping down to the south from 70-75 km depth at 37.9°N to about 110 km depth at 36°N is imaged. As this feature is only seen in high-frequency images and not in the low-frequency image, it is modelled as the positive Ps conversion from the base of an approximately 5-km-thick anisotropic layer at the top of the Asian mantle lithosphere which is currently subducting. This south-dipping converter continues to the south on the INDEPTH IV profile. S receiver function analysis completes the image of the structure below the Qilian Shan profile with the identification of the lithosphere-asthenosphere boundary (LAB). The LAB of the Asian Plate is identified for a reference slowness of 6.4 s deg-1 at 12-14 s (105-125 km depth) between 38 and 41°N below the northern part of the S receiver function profile. To the south it increases in depth such that it is at about 19 s (170 km depth) between 34 and 35°N at the southern end of the profile. The LAB of the Asian Plate occurs at similar depths on the INDEPTH IV profile at the latitudes where the INDEPTH IV and Qilian Shan profiles overlap. As on the INDEPTH IV profile to the south, between 34 and 35°N at the southern end of the Qilian Shan profile there is evidence from the S receiver functions for the LAB of a separate Tibetan Plate. © The Authors 2014. Published by Oxford University Press on behalf of The Royal Astronomical Society.
    Full-text · Article · Oct 2014 · Geophysical Journal International
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Based on a 2 year seismic record from a local network, we characterize the deformation of the seismogenic crust of the Pamir in the northwestern part of the India‐Asia collision zone. We located more than 6000 upper crustal earthquakes in a regional 3‐D velocity model. For 132 of these events, we determined source mechanisms, mostly through full waveform moment tensor inversion of locally and regionally recorded seismograms. We also produced a new and comprehensive neotectonic map of the Pamir, which we relate to the seismic deformation. Along Pamir's northern margin, where GPS measurements show significant shortening, we find thrust and dextral strike‐slip faulting along west to northwest trending planes, indicating slip partitioning between northward thrusting and westward extrusion. An active, north‐northeast trending, sinistral transtensional fault system dissects the Pamir's interior, connecting the lakes Karakul and Sarez, and extends by distributed faulting into the Hindu Kush of Afghanistan. East of this lineament, the Pamir moves northward en bloc, showing little seismicity and internal deformation. The western Pamir exhibits a higher amount of seismic deformation; sinistral strike‐slip faulting on northeast trending or conjugate planes and normal faulting indicate east‐west extension and north‐south shortening. We explain this deformation pattern by the gravitational collapse of the western Pamir Plateau margin and the lateral extrusion of Pamir rocks into the Tajik‐Afghan depression, where it causes thin‐skinned shortening of basin sediments above an evaporitic décollement. Superposition of Pamir's bulk northward movement and collapse and westward extrusion of its western flank causes the gradual change of surface velocity orientations from north‐northwest to due west observed by GPS geodesy. The distributed shear deformation of the western Pamir and the activation of the Sarez‐Karakul fault system may ultimately be caused by the northeastward propagation of India's western transform margin into Asia, thereby linking deformation in the Pamir all the way to the Chaman fault in the south in Afghanistan. New crustal seismicity, focal mechanism data from local network for the PamirNew comprehensive neotectonic map for the PamirSeismic deformation is dominated by N‐S shortening and westward extrusion
    Full-text · Article · Jul 2014 · Tectonics
  • [Show abstract] [Hide abstract]
    ABSTRACT: In this chapter we report on the deep structure of the Dead Sea Transform (DST) as derived from geophysical observations and numerical modelling, calibrated by geological and geodynamic evidence. We use seismics, seismology and gravity to study the crust and lithosphere of the Dead Sea Transform (DST) system. These observations are integrated with 3D thermo-mechanical modelling of the evolution of the DST through time to understand the deeper structure of the DST. The three seismic profiles crossing the DST from the Mediterranean in the West to the Jordan highlands in the East show an increase in Moho depth from about 25 km to about 35 km; with only minor topography. This depth increase of about 10 km of the Moho from West to East is also found in tomographic images using regional and teleseismic events, which shows additionally a N – S trending thickening of the crust under the Arava/Araba Fault (AF). In the Dead Sea Basin (DSB) proper the imaging of the Moho is complicated by the presence of the Lisan Salt dome. From these results and other evidence we conclude that the Dead Sea basin is a mostly upper crustal feature with a decoupling zone at about 20 km depth. Using SKS waves we find below the Moho under the DST a narrow, ca. 20 km wide, vertical decoupling zone reaching into the mantle, representing the boundary layer between the African and Arabian plates. This observation agrees with the results from the study of surface waves that also show a region of reduced S-velocities under the DST, reaching down into the lithosphere. Whereas the lithosphere thins gradually east of the DST from N to S from ca. 80 to ca. 67 km, below about 120 km depth little structure can be observed in tomographic images. The abovementioned observational constraints can all be fitted with the classical pull-apart model, if the lithosphere was thermally eroded to 80 km thickness about 20 Ma ago, combined with weak rheologies for crust and upper mantle. The most likely explanation of the features described is thus a thinning of the lithosphere around the DST in the Late Cenozoic, likely following by rifting and spreading of the Red Sea.
    No preview · Chapter · May 2014
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: An inclined zone of intermediate-depth seismicity beneath the Pamir orogen in Central Asia has been interpreted as southward subduction of a slab of Asian lithosphere. However, it is not known whether Asian lithosphere subducts intact or only partially. We used arrival times of shallow and intermediate-depth earthquakes, recorded with a temporary (2008-2010) seismic network in this region, to invert for 3D models of seismic velocities in an attempt to answer this question. With local seismicity reaching depths of up to 240 km, the deep structure of the Pamir could be illuminated with high resolution.
    Full-text · Article · Dec 2013 · Earth and Planetary Science Letters
  • [Show abstract] [Hide abstract]
    ABSTRACT: In this study three new maps of Moho depths beneath the Arabian plate and margins are presented. The first map is based on the combined gravity model, EIGEN 06C, which includes data from satellite missions and ground-based studies, and thus covers the whole region between 31°E and 60°E and between 12°N and 36°N. The second map is based on seismological and ground-based gravity data while the third map is based only on seismological data. Both these maps show gaps due to lack of data coverage especially in the interior of the Arabian plate. Beneath the interior of the Arabian plate the Moho lies between 32 and 45 km depth below sea level. There is a tendency for higher Pn and Sn velocities beneath the northeastern parts of the plate interior with respect to the southwestern parts of the plate interior. Across the northern, destructive margin with the Eurasian plate, the Moho depths increase to over 50 km beneath the Zagros mountains. Across the conservative western margin, the Dead Sea Transform (DST), Moho depths decrease from almost 40 km beneath the highlands east of the DST to about 21–23 km under the southeastern Mediterranean Sea. This decrease seems to be modulated by a slight depression in the Moho beneath the southern DST. The constructive southwestern and southeastern margins of the Arabian plate also show the Moho shallowing from the plate interior towards the plate boundaries. A comparison of the abruptness of the Moho shallowing between the margins of the Arabian plate, the conjugate African margin at 26°N and several Atlantic margins shows a complex picture and suggests that the abruptness of the Moho shallowing may reflect fundamental differences in the original structure of the margins.
    No preview · Article · Dec 2013 · Tectonophysics
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The northeastern boundary of the Tibetan high plateau is marked by a 2 km topographic drop and a coincident rapid change in crustal thickness. Surface tectonics are dominated by the Kunlun strike-slip fault system and adjacent Kunlun concealed thrust. The main objective of the current study is to map lateral variations of seismic anisotropy parameters in this region along the linear INDEPTH IV array in order to investigate the link between surface and internal deformation in the context of crust and mantle structure. To achieve this aim, we performed Minimum-Transverse-Energy based SKS splitting measurements using 23 stations of the INDEPTH IV array deployed across the northeastern margin of Tibet. Average fast polarization directions and splitting time delays are obtained by averaging stacked misfit surfaces of all an-alyzed events at each station. The agreement of fast directions with the strikes of major active strike-slip faults and strike-slip focal mechanisms, but not with fossil structures such as the Jinsha suture, implies that the anisotropy records lithospheric petrofabric formed by recent deformation within the lithosphere rather than representing frozen-in anisotropy or shear within the asthenosphere due to absolute plate motion. The dis-tribution of large splitting delays throughout the northern plateau suggests that defor-mation is distributed rather than focused onto narrow shear zones associated with the Kunlun strike-slip faults. The drop in splitting delays toward the Qaidam is then a natural consequence of the much lower degree of deformation there.
    Full-text · Dataset · Dec 2013
  • J. Mechie · R. Kind
    [Show abstract] [Hide abstract]
    ABSTRACT: A 700 km deep seismic velocity cross-section beneath the Lhasa to Golmud transect across the Tibetan plateau is presented. In contrast to the first version of this cross-section, which comprised an 800 km wide swath centred on the Lhasa to Golmud transect, due to the recent proliferation of publications concerning the mantle structure beneath Tibet, this study is based only on seismic profiles which either run along or cross the transect and arrays or studies which at least partly cover the transect. The results from the recent INDEPTH IV project indicate that the crustal thickness change from 70 km beneath the Songpan-Ganzi terrane and Kunlun mountains to 54 km beneath the Qaidam basin is located about 100 km north of the Kunlun Fault and almost 45 km north of the North Kunlun Thrust. The Qaidam basin Moho is underlain by crustal velocity material for almost 45 km and the apparently overlapping crustal material may represent Songpan-Ganzi lower crust underthrusting or flowing northward beneath the Qaidam basin Moho. Thus the high Tibetan plateau may be thickening northward into south Qaidam as its weak, thickened lower crust is injected beneath the stronger Qaidam crust. Beneath the crust, high-velocity, dense, cold Indian lithospheric mantle extends northwards until about the Banggong-Nujiang suture. Northwards, Asian lithospheric mantle is overlain by a low-velocity, less dense, warm Tibetan plate consisting of an upper lithospheric and a lower asthenospheric part. The apparent northwards deepening of the 410 and 660 km discontinuities by about 20 km implies that the upper mantle beneath north Tibet is slower, less dense and warmer than under south Tibet, in agreement with the observed uppermost mantle velocities. This, in turn, could provide some of the isostatic support for the high elevations in the north where the crust is somewhat thinner than in the southern plateau.
    No preview · Article · Oct 2013 · Tectonophysics
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Exhumation of ultra-high pressure metamorphic rocks testifies that the continental crust can subduct to significant depth into the mantle despite its buoyancy. However, direct observation of ongoing subduction of continental crust is rare. The Pamir is regarded as a possible place of active continental subduction because of the intermediate-depth seismicity, crustal xenoliths and estimates of crustal shortening versus convergence rates. Here we present for the first time receiver function images from a passive-source seismic array traversing the Tien Shan and the Pamir plateau showing southward subduction of Eurasian continental crust. In the eastern Pamir, we observe a southerly dipping 10–15 km thick low-velocity zone (LVZ) that extends from 50 km depth near the base of the crust to more than 150 km depth with a dip angle increasing to subvertical. While the upper- and mid-crustal material seems to be shortened and incorporated into the Pamir, the lower Eurasian crust detaches and subducts. In its deeper part (>80 km) the LVZ envelopes the intermediate-depth earthquakes. Our observations imply that the complete arcuate intermediate depth seismic zone beneath the Pamir traces a slab of subducting Eurasian continental lower crust.
    Full-text · Article · Aug 2013 · Earth and Planetary Science Letters
  • [Show abstract] [Hide abstract]
    ABSTRACT: Exhumation of ultra-high pressure metamorphic rocks testifies that the continental crust can subduct to greater depth in the mantle despite its buoyancy. However, direct observation of ongoing subduction of continental crust is rare. The Pamir is regarded as a possible place of active continental subduction because of the observed intermediate-depth seismicity, findings of crustal xenoliths from upper mantle depths and estimates of high cenozoic convergence for this region that could hardly be accommodated by crustal deformation alone. Here we present receiver function results from the seismological part of the Tien Shan Pamir Geodynamic program (TIPAGE). In a high resolution north-south cross section along the main TIPAGE profile, we observe a southerly dipping thin (with a thickness of 11 km) low-velocity zone (LVZ) that starts from the base of the crust and extends to a depth of more than 150 km with an increasing dip angle to subvertical. A diagonal northwest to southeast cross section shows that towards the western Pamir the dip direction of the LVZ bends to the southeast resulting in an arcuate subduction configuration of Eurasian lithosphere beneath the Pamir. In both profiles, the LVZ identified with receiver functions appears to envelope the intermediate-depth earthquakes of the Pamir Hindu-Kush seismic zone. For imaging of the dipping interface a migration procedure is used and tested that accounts for the inclination of the conversion layers. Migrated cross sections of Q- and T-components of the P-RFs are compared. The crustal thickness is determined and mapped for this region by stacking direct Ps and multiple PpPs and PpSs phases. At the most places in the Pamir, it is ranging between 65 km and 75 km, while the greatest Moho depths of around 80 km are observed at the upper end of the LVZ. The surrounding areas namely the Tajik Depression, the Ferghana and Tarim Basins show Moho depths of around 40 to 45 km giving an estimate of the pre-collisional crustal thickness of the former Basin area that was overthrusted by the Pamir.
    No preview · Article · Apr 2013
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The Pamir Mountains form a complex orographic node north of the western Himalayan Syntaxis. Due to the Pamir's remote location, crustal tectonics of the region is not well studied. We report new data on distribution and kinematics of crustal earthquakes in the Pamir and its surroundings. Our data set stems from a deployment of seismometers between 2008-2010 that covered the SW Tien Shan, Pamir and Tajik basin. We detected and carefully relocated several thousand crustal earthquakes that are confined to the uppermost 20 km of the crust and thereby clearly separated from Pamir's unique intermediate depth seismicity. For the larger earthquakes (M<3) we use both full waveform inversion and first motion polarities to determine source mechanisms. A string of earthquakes outlines the thrust system along the northern Pamir's perimeter. In the east, where the Pamir collides with the Tien Shan, the M6.7 Nura earthquake activated several faults. Whereas the main shock shows almost pure reverse faulting on a south dipping thrust, many aftershocks also show sinistral strike-slip faulting along a NE striking lineamnet. In the centre, where the Pamir overthrusts the intramontane Alai valley, micro-seismicity recedes southward from the Frontal and Trans Alai thrust systems. The largest of these earthquakes show mostly strike-slip mechanisms. Further west, where the Pamir thrust system bends southward, earthquakes show thrust mechanisms again with strikes following the oroclinal structures. Inside the Pamir a NE striking lineament runs from the eastern end of Lake Sarez across Lake Kara Kul to the Pamir thrust system. Source mechanisms along the lineament are sinistral strike slip and transtensional. This lineament approximately separates the deeply incised western Pamir, which shows significant seismic deformation, from the relatively aseismic eastern Pamir. In the western Pamir earthquakes cluster along approximately the Vanch valley and near Lake Sarez. Diffuse seismicity is also visible beneath the SW Pamir's basement domes. Source mechanisms exhibit mostly sinistral strike slip faulting on NE striking or conjugate planes indicating north-south compression and east-west extension. At the Pamir's western margin, where the mountains merge into the Tajik basin's fold and thrust belt, we observe numerous earthquakes with mechanisms exhibiting EW slip on subhorizontal planes. We interpret this as movement along the Jurassic evaporite decollement that detaches the sedimentary section from the basement. Our data indicate that in the western Pamir NS compression is accommodated by westward escape, i.e. the western Pamir is pushed into the Tajik depression ontop of a weak evaporite detachment. This is in accordance with the observed GPS displacement vectors rotating anticlockwise from NS to EW when traversing from the eastern Pamir into the Tajik depression.
    Full-text · Article · Apr 2013
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We present new seismicity images based on a two-year seismic deployment in the Pamir and SW Tien Shan. A total of 9532 earthquakes were detected, located, and rigorously assessed in a multistage automatic procedure utilizing state-of-the-art picking algorithms, waveform cross-correlation, and multi-event relocation. The obtained catalog provides new information on crustal seismicity and reveals the geometry and internal structure of the Pamir-Hindu Kush intermediate-depth seismic zone with improved detail and resolution. The relocated seismicity clearly defines at least two distinct planes: one beneath the Pamir and the other beneath the Hindu Kush, separated by a gap across which strike and dip directions change abruptly. The Pamir seismic zone forms a thin (approximately 10 km width), curviplanar arc that strikes east-west and dips south at its eastern end and then progressively turns by 90° to reach a north-south strike and a due eastward dip at its southwestern termination. Pamir deep seismicity outlines several streaks at depths between 70 and 240 km, with the deepest events occurring at its southwestern end. Intermediate-depth earthquakes are clearly separated from shallow crustal seismicity, which is confined to the uppermost 20-25 km. The Hindu Kush seismic zone extends from 40 to 250 km depth and generally strikes east-west, yet bends northeast, toward the Pamir, at its eastern end. It may be divided vertically into upper and lower parts separated by a gap at approximately 150 km depth. In the upper part, events form a plane that is 15-25 km thick in cross section and dips sub-vertically north to northwest. Seismic activity is more virile in the lower part, where several distinct clusters form a complex pattern of sub-parallel planes. The observed geometry could be reconciled either with a model of two-sided subduction of Eurasian and previously underthrusted Indian continental lithosphere or by a purely Eurasian origin of both Pamir and Hindu Kush seismic zones, which necessitates a contortion and oversteepening of the latter.
    Full-text · Article · Apr 2013 · Journal of Geophysical Research Atmospheres
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: 1] We use ambient-noise tomography to map regional differences in crustal Rayleigh-wave group velocities with periods of 8–40 s across north Tibet using the International Deep Profiling of Tibet and the Himalaya phase IV arrays (132 stations, deployed for 10–24 months). For periods of 8–24 s (sensitive to midcrustal depths of ~5–30 km), we observe striking velocity changes across the Bangong-Nujiang and Jinsha suture zones as well as the Kunlun-Qaidam boundary. From south to north, we see higher velocities beneath the Lhasa terrane, lower velocities beneath the Qiangtang, higher velocities in the Songpan-Ganzi and Kunlun terranes, and the lowest velocities beneath the Qaidam Basin. Maps at periods of 34 and 40 s (sensitive to the middle and lower crust at depths of ~30– 60 km) do not show evidence of changes across those boundaries. Any differences between the Tibetan terrane lower crusts that were present at accretion have been erased or displaced by Cenozoic processes and replaced almost ubiquitously by uniformly low velocities.
    Full-text · Article · Mar 2013 · Geophysical Research Letters
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Knowledge of the rock types and pressure-temperature conditions at crustal depths in an active orogeny is key to understanding the mechanism of mountain building and its associated modern deformation, erosion and earthquakes. Seismic-wave velocities by themselves generally do not have the sensitivity to discriminate one rock type from another or to decipher the P-T conditions at which they exist. But laboratory-measured ratios of velocities of P to S waves (Vp/Vs) have been shown to be effective. Results of 3-D Vp and Vp/Vs tomographic imaging based on dense seismic arrays in the highly seismic environment of Taiwan provides the first detailed Vp/Vs structures of the orogen. The sharp reduction in the observed Vp/Vs ratio in the felsic core of the mountain belts implies that the α-β quartz transition temperature is reached at a mean depth of 24 ± 3 km. The transition temperature is estimated to be 750 ± 25°C at this depth, yielding an average thermal gradient of 30 ± 3°C/km.
    Full-text · Article · Nov 2012 · Geophysical Research Letters
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: From the S-wave data collected along a 270-km-long profile spanning the Kunlun mountains in NE Tibet, 14595 Sg phase arrivals and 21 SmS phase arrivals were utilized to derive a whole-crustal S velocity model and, together with a previously derived P velocity model, a Poisson's ratio (σ) model beneath the profile. The final tomogram for the upper 10-15 km of the crust reveals the lower velocities associated with the predominantly Neogene-Quaternary sediments of the Qaidam basin to the north and the higher velocities associated with the predominantly Palaeozoic and Mesozoic upper crustal sequences of the Songpan-Ganzi terrane and Kunlun mountains to the south. This study finds no evidence that the Kunlun mountains are involved in large-scale northward overriding of the Qaidam basin along a shallow south-dipping thrust. The σ in the upper 10-15 km of the crust are often lower than 0.25, indicating a preponderance of quartz-rich rocks in the upper crust beneath the profile. Below 10-15 km depth, the remainder of the crust down to the Moho has an average σ of 0.24 beneath the Songpan-Ganzi terrane and Kunlun mountains and 0.25 below the Qaidam basin. These low σ are similar to other low σ found along other profiles in the northeastern part of the plateau. Assuming an isotropic situation and no significant variation in σ between 10-15 km depth and the Moho, then the lower crust between 25-30 km depth below sea level and the Moho with P velocities varying from 6.6 km s-1 at the top to around 6.9 km s-1 at the base and σ of 0.24-0.25 should comprise intermediate granulites in the upper part transitioning to granulite facies metapelites in the lower part. As the pre-Cenozoic Qaidam basin crust has probably not lost any of its lower crust during the present Himalayan orogenic cycle in the Cenozoic and only has a σ of 0.245-0.25, then it appears that the pre-Cenozoic Qaidam basin crust involved in the collision is more felsic and thus weaker and more easily deformable than normal continental crust with a global average σ of 0.265-0.27 and the Tarim and Sichuan basin crusts. This situation then probably facilitates the collision and promotes the formation of new high plateau crust at the NE margin of Tibet. South of the Qaidam basin, the crust of the Songpan-Ganzi terrane and Kunlun mountains has an even lower average crustal σ of 0.23-0.24 and is thus presumably even weaker and more easily deformable than the crust beneath the Qaidam basin. This then supports the hypothesis of Karplus et al. that 'the high Tibetan Plateau may be thickening northward into south Qaidam as its weak, thickened lower crust is injected beneath stronger Qaidam crust'.
    Full-text · Article · Aug 2012 · Geophysical Journal International
  • [Show abstract] [Hide abstract]
    ABSTRACT: Clear S-to-P converted waves from the crust–mantle boundary (Moho) and lithosphere–asthenosphere boundary (LAB) have been observed on the eastern part of the Dead Sea Basin (DSB), and are used for the determination of the depth of the Moho and the LAB. A temporary network consisting of 18 seismic broad-band stations was operated in the DSB region as part of the DEad Sea Integrated REsearch project for 1.5 years beginning in September 2006. The obtained Moho depth (∼35 km) from S-to-P receiver functions agrees well with the results from P-to-S receiver functions and other geophysical data. The thickness of the lithosphere on the eastern part of the DSB is about 75 km. The results obtained here support and confirm previous studies, based on xenolith data, geodynamic modeling, heat flow observations, and S-to-P receiver functions. Therefore, the lithosphere on the eastern part of the DSB and along Wadi Araba has been thinned in the Late Cenozoic, following rifting and spreading of the Red Sea. The thinning of the lithosphere occurred without a concomitant change in the crustal thickness and thus an upwelling of the asthenosphere in the study area is invoked as the cause of the lithosphere thinning.
    No preview · Article · Jul 2012 · Arabian Journal of Geosciences
  • Source
    Prakash Kumar · Rainer Kind · Xiaohui Yuan · James Mechie

    Full-text · Article · May 2012 · Seismological Research Letters

Publication Stats

4k Citations
468.81 Total Impact Points

Institutions

  • 1996-2015
    • Helmholtz-Zentrum Potsdam - Deutsches GeoForschungsZentrum GFZ
      • Division of Geophysical Deep Sounding
      Potsdam, Brandenburg, Germany
  • 1992
    • Karlsruhe Institute of Technology
      • Geophysical Institute
      Carlsruhe, Baden-Württemberg, Germany
  • 1987
    • University of Jordan
      `Ammān, Amman, Jordan