J. Mechie

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

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Publications (129)366.18 Total impact

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    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
    Tectonics 07/2014; · 3.49 Impact Factor
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    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.
    05/2014: pages 29-52; , ISBN: 978-94-017-8871-7
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    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. The resulting velocity models show a north–south contrast in crustal seismic velocities in the Pamir, with very low P velocities (5.7–5.9 km/s at 15–30 km depth), coupled with relatively low vp/vs (<1.70), at mid-crustal levels in the southern part of the orogen. At sub-Moho depths, we image an arcuate high-velocity (8.2–8.6 km/s) slab dipping south in the eastern Pamir and east in the Pamirʼs southwest, underlying the intermediate-depth earthquakes. On top of the high-velocity slab and just above the onset of deep seismicity, between a depth of 60 to 100 km, very low compressional wavespeeds (around 7.1 km/s) and high vp/vs ratios (⩾1.80) attest to subducted crustal rocks. Additionally, we carried out 2D numerical thermomechanical modeling of the continental collision in the Pamir, focusing on the fate of the crust and mantle lithosphere of the Asian and Indian plates. Seismic velocities were computed from the modeling results, and the resulting images were compared with the velocity distributions obtained from seismic traveltimes. Combining tomography and modeling results, we infer that a substantial amount of crustal material is pulled down beneath the Pamir by cold mantle lithosphere to depths of at least 80–100 km. From there on, only lower crust and mantle lithosphere continue their subduction, and earthquakes occur inside the lower crustal layer probably due to metamorphic reactions. The buoyant Asian upper and middle crust does not penetrate deeper into the mantle, but pools at this depth level, from where it might eventually exhume or relaminate.
    Earth and Planetary Science Letters 12/2013; 384:165–177. · 4.72 Impact Factor
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    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.
  • J. Mechie, R. Kind
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    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.
    Tectonophysics 10/2013; · 2.68 Impact Factor
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    ABSTRACT: This study is a part of the Cyprus Arc project and comprises a 300 km long seismic wide-angle reflection/refraction (WRR) profile between Salt Lake and Anamur in central Turkey and a 45 km long seismic profile in Southern Cyprus which were completed in spring 2010. The locations of the profiles were determined to be more or less perpendicular to the complex tectonic structure of the Eastern Mediterranean which is dominated by the Cyprus Arc. The Cyprus Arc is widely accepted as a subduction zone which was active beneath Cyprus until the early Miocene. The seismic measurements comprised two onshore shots and airgun shots offshore that were recorded by 143 three-component and 101 one-component instruments. Stations were averagely spaced with 1.25 km along the whole length of the N-S trending profiles in Turkey and Southern Cyprus. In this study, crustal velocity structure models along the profiles were derived by using finite-difference ray tracing. The models were further refined using forward modeling to generate synthetic seismograms for individual shot gathers. Thus by varying the velocity structure the theoretical times and amplitudes of the various arrivals could be matched to the observed times and amplitudes. Additionally, 2-D gravity modelling was done by using the obtained crustal model to generate theoretical gravity data and comparing these data with the observed gravity data. Key words: Controlled source seismology, central Anatolia, Cyprus arc, ray tracing
    04/2013;
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    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.
    Tectonics. 04/2013;
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    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.
    04/2013;
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    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.
    Geophysical Research Letters 01/2013; 40. · 3.98 Impact Factor
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    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.
    Earth and Planetary Science Letters 01/2013; · 4.72 Impact Factor
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    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.
    Journal of Geophysical Research 01/2013; 118(4):1438-1457. · 3.17 Impact Factor
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    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.
    Tectonophysics 01/2013; 609:234–249. · 2.68 Impact Factor
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    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.
    Geophysical Research Letters 11/2012; 39(22):22302-. · 3.98 Impact Factor
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    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'.
    Geophysical Journal International 08/2012; · 2.85 Impact Factor
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    ABSTRACT: 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 has no problems with multiples like P receiver functions. Only two major discontinuities are observed in the entire area down to about 300km 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 70-80km in large parts of westernmost North America. East of the Rocky Mountains its depth is generally between 90 and 110km. Regions with LAB depths down to about 140km 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 at the east coast. However, in the central cratonic part of the USA the tomography LAB is near 200km depth. At this depth no discontinuity is seen in the S receiver functions. The negative signal near 100km depth in the central part of the USA is interpreted by Yuan and Romanowicz (2010) or 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). We are comparing our results from North America with other regions (South Africa, South America, Europe).
    Solid Earth Discussions 04/2012; 4(1):4996-.
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    ABSTRACT: The Pamir and Hindu Kush regions in Central Asia host the most active zone of intracontinental seismic activity at intermediate depths (up to nearly 300km) in the world, which is still poorly understood in terms of its detailed structure and, most importantly, its origin. Being situated far from any typical subduction zone setting and displaying a change in its polarity along strike, this seismically active zone has been interpreted in numerous ways, e.g. as a single slab of Indian lithosphere originally subducted northwards which was subsequently overturned in its eastern part or as two adjacent subduction zones of opposing polarity. Several key questions concerning this region, among them the nature of subducted material (oceanic or continental?), the mechanism behind the generation of these intermediate-depth earthquakes and the region's tectonic framework have not been answered as of yet. As the seismological subpart of the TIPAGE project, we deployed a network of 40 seismometer stations for a total duration of two years (2008-2010) in Tajikistan and southern Kyrgyzstan, covering the Pamir mountains and surroundings. Complemented with two more temporary deployments and additional data from several permanent networks in adjacent areas, this constitutes a seismic dataset of unprecedented station density for this part of Central Asia. Showing the distribution of more than 9,500 earthquakes located with a highly precise double-difference method based on the cross-correlation of individual traces, fault plane solutions for shallow and deep earthquakes as well as preliminary results from traveltime tomography, we can resolve the exact geometry of the deep seismic zone and draw further constraints on the tectonic processes active in the region. The S-shaped region of intermediate-depth seismicity is clearly subdivided into two separate parts, hence termed Hindu Kush and Pamir seismic zones. The Hindu Kush seismic zone strikes due east-west at a latitude of about 36.4°N. Depth sections show that earthquakes extend from depths of 50 to around 250 km along a planar, steeply northward-dipping structure. Earthquakes are most frequent in the depth range from 180 to 220 km, whereas there is a seismic "gap" at about 150 km depth. Intriguingly, the Hindu Kush seismic zone features a small-scale reversal of dip polarity in its lower part (beneath the 150km gap) towards its eastern termination. The Pamir seismic zone forms an arc, the strike of which varies by 90 degrees from north-south at its southwestern end (where it borders the Hindu Kush seismic zone) to east-west at its eastern termination. The dip direction of the structure changes from due east to due south from west to east. Seismic activity outlines a narrow (10-15 km) Wadati-Benioff zone displaying a constant dip of about 50 degrees all along its extent. Whereas seismic activity ceases at depths of 130-150 km in the east, the south-western part of the zone shows earthquakes reaching depths of up to 240 km that outline a vertical structure beneath 150 km depth. All along the arc, the upper termination of seismic activity is found at depths of 60-80 km, leaving a gap to shallow seismic activity which is confined to the uppermost 20-25 km. Intermediate-depth earthquakes in the eastern Pamir and the low-velocity zone they are situated within can be linked with shallow seismic activity along the Main Pamir Thrust (MPT) further north, which implies ongoing intracontinental southward subduction, presumably of continental material, in the Alai Valley. Tracing the surface expression of the deep earthquakes along the Pamir arc, towards the southwest, proves complicated. It is possible that the western part of the Pamir seismic zone is linked to the ongoing east-west compression in the Tajik Depression seen by GPS, but evidence is scarce. Shallow seismic activity concentrates along the Pamir's northern rim (MPT) and in its western part, wheras the eastern Pamir is seismically quiet. Fault-plane solutions obtained by moment tensor inversion show predominance of sinistral strike-slip events with presumably northeast-southwest striking rupture planes in the western Pamir, whereas the MPT mostly shows thrusting events along east-west trending rupture planes. Analysis of P and T axes shows a general trend of north-south compression and east-west extension throughout the Pamir, which is consistent with GPS observations.
    04/2012;
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    ABSTRACT: Recent receiver function (RF) results show a southward-dipping low velocity zone (LVZ) in the mantle south of the Main Pamir Thrust (MPT). It is clearly observed in a north-south cross section by two parallel running velocity contrasts, the lower one with negative, the upper one with positive amplitudes. The thickness of the LVZ is approximately 15 km. It is resolved in the longitudinal interval ranging from 38N to 39.3N and in depth from 80 km to 150 km. The inclination is steepening southwards from 25 • to 65 • and its location coincides with the Wadati-Benioff zone formed by the hypocenters of the intermediate-depth earthquakes in this region. The LVZ crops out at the MPT and thus connects the crustal seismic activity at the MPT with the seismic activity in the upper mantle. The analyzed data were collected within the framework of the Tien Shan Pamir Geodynamic program (TIPAGE) from 2008 to 2010. In the first year, we were running 24 stations in the eastern Pamir, forming the 350 km long north-south main TIPAGE seismic profile. The profile was elongated northwards with data collected in a temporary seismic experiment in the Ferghana Valley, which ran from 2009 to 2010. In the second year, we rearranged the deployment in order to get an equally distributed areal network with a station spacing of approximately 40 km. Further data of permanent stations in western Tajikistan were provided by the Geophysical Survey of Tajikistan. A diagonal north-west to south-east cross sections of migrated RFs from the areal network shows the same main features as the north-south profile, indicating an arcuate configuration of the subduction. While in the eastern Pamir the LVZ is dipping due south, further west the dipping direction is bending south-east, following the S-shape formed by the epicenters of the mantle seismicity. In addition, S-RFs are calculated and migrated along the main TIPAGE profile. Results for the lateral variation of the Moho depth gained by P-and S-RFs verify each other. Crustal thickness, which is determined by direct conversions and crustal multiples of P-RFs varies along the profile from 75 km beneath the southern Pamir to 64 km beneath the Tien Shan and then decreases to 45 km towards the Ferghana Valley. 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. In synthetic tests it is shown, that for the observed dipping structures and the given ray geometry the T-component shows a better signal than the Q-component. This is observed in the real data as well.
    EGU General Assembly 2012; 04/2012
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    ABSTRACT: The over 7000m high peaks of the Trans-Alai and the on average over 4000m elevated Pamir plateau is in some way the mirrored equivalent of the Himalaya and Tibet plateau on the northwestern promontory of India-Eurasia collision. Current shortening across the Trans Alai of 13mm/a takes up about 1/3 of India-Eurasia convergence, only little less than across the Himalaya and the highest rate localized far away from a plate boundary. Accumulated Cenozoic shortening reaches also a magnitude similar to the adjacent Himalaya-Tibet system, yet was accommodated over about half the meridional width. There are other marked differences between the two systems. The Pamir is presumably thrusted over Eurasia rather than India and instead of a foreland basin another major orogen, the Tien Shan, stands between its frontal thrust and stable Eurasia. The Pamir and adjacent Hindu Kush feature vigorous intermediate depth (100-300km) mantle seismicity, an attribute absent beneath all other major continental orogens. We will report on a new earthquake data set collected during a field campaign between 2008 and 2010, when we operated a network of 40 seismic stations across the southern Tien Shan and Pamir mountain ranges in Kyrgyzstan and Tajikistan. This is the first modern, digital and dense seismic network in the region. From more than 6000 well located earthquakes approximately two third are crustal. To derive robust source mechanisms and additional constraint on event depths, we use full waveform inversion of our local temporary and regional permanent seismic station recordings for events with magnitudes > 3.5. We use relocated hypocenters and fault plane solutions to describe the deformation pattern in the crust. Seismicity clusters in several well defined known and unknown structures. The Main Pamir Thrust (MPT) on the northern perimeter of the Pamir is clearly outlined by a string of earthquakes with thrust mechanisms, with the eastern part of the Alai valley more active than the western part. The largest earthquake we recorded, an M6.7, occurred at MPT's north-easternmost point, where the Alai valley closes and the Pamir collides with the Tien Shan. This is a region of significant structural complexity, where the MPT fans out in a series of northeast trending orographic features. The mainshock shows an almost pure thrust mechanism with one steeper (55°) and one more shallow (38°) nodal plane. The aftershock seismicity displays two lineaments forming a hockey stick like feature that tightly follows the orographic relief. The "blade" strikes approximately 85° in agreement with one nodal plane of the mainshock double couple. A cross section through the aftershocks reveals that the steeper, south-dipping nodal plane is the fault plane. The earthquake probably ruptured the very tip of the Pamir frontal thrust where it reaches the surface in the southern margin of the Alai valley. On the Pamir plateau deformation is mainly extensional with dextral components. One seismically active zone crosses the entire plateau from Lake Karakul to the Wakhan corridor of Afghanistan. High seismicity is also detected along the deeply incised valleys of the western Pamir.
    AGU Fall Meeting Abstracts. 12/2011;
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    ABSTRACT: Seismological imaging has identified the Indian lithosphere penetrating underneath Tibet up to 500km to the north and to a depth of at least 200km along a front that is more than 1000km long. This is a classical case of continental subduction. In contrast, the collision of Tibet with the stable Tarim Basin in the north-west caused thickening of the Tibetan lithosphere to about 200km, whereas collision with the Sichuan Basin in the east caused thinning of the Tibetan lithosphere to about 70km. No sufficient seismic data on the mantle lithosphere have been available up to now at the boundary of Tibet to the Qaidam Basin, where subduction of the Asian lithosphere beneath Tibet was suggested. We report on results from a recent seismic passive source experiment in this region, which continued the series of INDEPTH experiments to the Qaidam Basin in the north-east. We used the S receiver function technique for data analysis, which is especially sensitive for observations of the lithosphere-asthenosphere boundary (LAB). As a surprising result, we found evidence that a newly identified relatively thin Tibetan lithosphere is overriding the flat subducting Asian lithosphere.
    AGU Fall Meeting Abstracts. 12/2011;
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    ABSTRACT: Numerous seismological experiments in central and southern Tibet have imaged the underthrusting Indian lithosphere and provided evidence of mid-crustal flow. A marked difference in mantle properties appears to occur in central Tibet, where body wave and Pn velocities become lower towards the north but with significant lateral variations. Surface wave studies tend to see velocities faster than the global average throughout Tibet in the deeper mantle (below ~150 km). Comparatively little seismological data had been collected in Northern Tibet. We combine data from the linear array of the INDEPTH-IV profile (2007-2008) and the ASCENT 2D deployment (2007-2009). We invert a high quality data set of more than 16,000 teleseismic P-wave arrival times and 3,100 teleseismic S-wave arrival times in a tomographic inversion to determine the velocity structure beneath NE Tibet in an area encompassing parts of the Qiangtang and Songpan-Ganzi terranes as well as the Kunlun Shan and part of the Qaidam. Major geological structures include the Kunlun Fault and the Jinsha and Bangong-Nujiang sutures. In total, 572 events at 80 stations were used for the P inversion, and 72 events for the S-inversion. Teleseismic regional tomography (ACH tomography) is most sensitive to lateral velocity contrasts, and in particular cannot resolve layer averages or very long range velocity variations; neither can it image variations in thickness or properties of the crust. In order to partially compensate for this, we utilise information from recent surface wave models, Acton et al (JGR, 2010) for the crust and shallow mantle, and a much extended version of the Priestley et al (JGR, 2006) model for the deeper mantle. The starting model for the body wave inversion combines both these models; in this way the well-established long wavelength background structure and absolute velocities provided by the surface waves are respected, but additional shorter range features can be imaged. For the P-wave starting model, we have translated the S-velocity variations into suitable P-velocities using a constant Vp/Vs ratio of 1.78. The resolution tests indicate that features more than a 100 km wide are reliably imaged throughout the study region; locally higher resolution is possible, but with significant smearing in the vertical direction. The resulting tomographic models show a number of large scale features. The lowest velocities are found below the Kunlun Shan, reaching depths of 300 km and more. There is a prominent high velocity zone extending across the south-west of the model below the Qiangtang terrane at approximately 130-220 km depth, whereas at shallower depths below the Qiangtang there appears to be more E-W variation. Further north, above 37°N, the velocity increases more smoothly with depth, and the high velocity zones are considerably less pronounced.
    AGU Fall Meeting Abstracts. 12/2011;

Publication Stats

2k Citations
366.18 Total Impact Points

Institutions

  • 1996–2013
    • Helmholtz-Zentrum Potsdam - Deutsches GeoForschungsZentrum GFZ
      Potsdam, Brandenburg, Germany
  • 1985–2007
    • Karlsruhe Institute of Technology
      • Geophysical Institute
      Karlsruhe, Baden-Wuerttemberg, Germany
  • 2004
    • Moscow Institute of Physics and Technology
      Moskva, Moscow, Russia