Article

# Crustal Swave velocity beneath the northeastern Tibetan plateau inferred from teleseismic Pwave receiver functions

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## Abstract

The northeastern (NE) Tibetan Plateau is an ideal place for investigating the far field effects of collision between the Indian and Eurasian plates. By what ways the Tibetan crust is thickened and extended is a long-term debated issue for absence of convincing evidence for proving the existence of the inner-crustal low velocity zone and its role. Using teleseimic P wave data from the China Seismograph Network in Qinghai and Gansu provinces recorded from 2007 to 2009, the crustal S-wave velocity structure beneath the NE Tibet plateau is resolved. The receiver function waveform inversion technique (PROGRAM330) is used to analyze the crustal S-wave velocity structure below seismic stations. The receiver functions are estimated by a time-domain iterative deconvolution method of Ammon (1991) with four different Gaussian coefficients (α=1.0, 1.5, 2.0 and 2.5). Firstly, receiver functions, which are much similar in waveforms and within a narrow range of back-azimuth (less than 10°) and ray parameter (less than 0.002), are stacked to enhance the main signal characteristics. To maintain the intrinsical details of the receiver functions, they are stacked without normal moveout. Results of Tian et al. (2013) and Li et al. (2006) are used as the constraints in the inversion process to reduce the uncertainty. The results show thata relatively low velocity layer (LVL) exists between the upper and lower crust in the region between the East Kunlun fault and the Haiyuan fault. The depth of the LVL shallows northeastward from ~35 km to ~20 km along the surface movement direction, while the Moho uplifts. The variation in thickness of the upper crust is more obvious than the lower crust. The thickness of the lower crust (15~20 km) beneath the Kunlun-west Qinling orogenic belt is thinner than that beneath the adjacent Qilian block (25~30 km). Beneath the NE Tibetan plateau, as well as the Alashan and Ordos blocks, the S-wave velocity in the lower crust increases with depth. The whole crustal S-wave velocity increases with depth beneath the eastern part of the west Qinling orogenic belt, the Ordos and Alashan blocks. It can be concluded that the observed LVL in the NE Tibetan plateau can act as an intra-crustdecollement/detechment to decouple the deformation between the upper and lower crust. The geometry of the LVL and the Moho indicates that the NE Tibetan crust is growing northeastward, and is predominated by upper-crustal thickening at present. The lower crust of the Kunlun-west Qinling orognic belt may be more rigid than the adjacent Qilian block and thus has experienced less deformation and crustal thickening. The lower crust of the NE Tibetan plateau is normal and possess high viscosity, so that is not conducive to the flow of the lower crust.

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... C. Y. Wang et al., 2013;Y. P. Wang et al., 2021). The Ordos Block has a representative double-layer crustal structure characterized by the ancient crystalline basement disconformably underlain by the Mesoproterozoic to Phanerozoic sedimentary, reflecting as the highvelocity anomalies and low heat flow (S. Z. Li et al., 2019;C. Y. Wang et al., 2013;H. S. Zhang et al., 2015). The structural patterns and sedimentary related to the Proterozoic tectonics may have been well F I G U R E 1 (a). The location of the study area in China. (b) Geological map of the Ordos Block and its surrounding areas, modified after Tectonic map of China (Ren, 2003 Wang et al., 2021). Since the Mesozoic, the Ordos has experienced te ...
... Red shading represents the low-resistivity area. Zhang et al., 2015), but others argue that thickening occurs in the lower crust (M. J. Liu et al., 2008;M. ...
Article
The Ordos Block, located at the western edge of the North China Block, is a stable rigid block. It is characterized by inactive tectonic activity and rare earthquakes, while the south‐west margin of this block and adjacent areas present the opposite features. There is still considerable ambiguity with regard to the lithospheric mechanical strength in these areas. The effective elastic thickness (Te), widely used for comprehending the mechanism of cratonic deformation and lithospheric dynamics, is a proxy of the lithospheric strength. This study provided detailed two‐dimensional lithospheric Te spatial variations in the Ordos and adjacent areas by using coherence analysis of the topography and Bouguer anomaly. The result showed that the higher Te values in the Ordos Block varied from 80 to 100 km and implied that it is capable of strong rigidity. The lower Te values were distributed over the Qinling Block (20–45 km), the Qilian Block and Haiyuan arcuate tectonic region (20–40 km), as well as the Baryan‐Har Block (15–30 km). By comparing the Te values with the geophysical results, we believe that the higher Te value in the Ordos Block was supported not only by the crust, but also by the upper mantle. In contrast, the mechanical strength of the south‐west margin, especially the Baryan‐Har Block and western Qinling Block, was mainly borne by the brittle upper crust, but the tectonic activity and brittle failure likely resulted in a lower Te. Furthermore, we found that a large number of shallow earthquakes were located above low‐velocity and low‐resistivity layers, most of which probably occurred in the brittle upper crust with small mechanical strength. The mechanical strength in the Ordos Block was supported by the crust and the upper mantle, so it showed strong rigidity and less deformation. In contrast, it was borne by the upper crust in the south‐west margin, so the tectonic activity and brittle failure likely resulted in shallow earthquakes.
... Seismological methods are the most effective and promising methods for studying the interior structure and dynamics of theEarth (e.g., Mooney, 1989;Zhao, 2015). The seismic velocity structure of the crust beneath the QLOB and its adjacent areas has been investigated by several studies of active-source seismic profiling (e.g.,Zhang et al., 1985;Gao et al., 1995;Li et al., 2001;Li et al., 2002;Liu et al., 2006;Zhang et al., 2008;Zhang et al., 2013) and receiver-function analysis (e.g.,Li et al., 2006b;Pan and Niu, 2011;Feng et al., 2014;Liu et al., 2014;Li et al., 2015;Shen et al., 2015;Zhang et al., 2015b). The 2-D velocity models obtained by these studies show the existence of obvious low seismic velocity (low-V) anomalies in the lower crust beneath the QLOB (Zhang et al., 2013;Shen et al., 2015;Zhang et al., 2015b). ...
... The seismic velocity structure of the crust beneath the QLOB and its adjacent areas has been investigated by several studies of active-source seismic profiling (e.g.,Zhang et al., 1985;Gao et al., 1995;Li et al., 2001;Li et al., 2002;Liu et al., 2006;Zhang et al., 2008;Zhang et al., 2013) and receiver-function analysis (e.g.,Li et al., 2006b;Pan and Niu, 2011;Feng et al., 2014;Liu et al., 2014;Li et al., 2015;Shen et al., 2015;Zhang et al., 2015b). The 2-D velocity models obtained by these studies show the existence of obvious low seismic velocity (low-V) anomalies in the lower crust beneath the QLOB (Zhang et al., 2013;Shen et al., 2015;Zhang et al., 2015b). However, these 2-D velocity models could not reveal reliably the 3-DFig. ...
... Crustal thickness, Vp/Vs ratio and velocity are related to the rock composition and stress state, which can provide constraints on continental tectonic evolution (Christensen, 1996;Ji et al., 2009). The simultaneous inversion of receiver functions (RFs) with multiple frequencies can reduce the non-uniqueness of inversion results compared with simple-frequency receiver function inversion (Zhang et al., 2015;Li et al., 2017). In addition, the joint inversion of RF and surface wave dispersion can further reduce the non-uniqueness of RF inversion and increase the resolution of surface wave dispersion inversion (Julià et al., 2000;Hu et al., 2005). ...
... The depth of this interface corresponds to the shallow part of a known lower-velocity layer, which has been identified in seismic datasets in nearby areas (e.g., Cui et al., 1995;Li et al., 2014). The lower-velocity layer occurs at a depth of 20-30 km beneath NE Tibet, and it is generally considered to be a décollement layer (e.g., Zhang et al., 2015). Thus, the interface that we identified may be part of the same crustal décollement. ...
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We reprocessed a high-resolution deep seismic reflection profile to reveal the detailed crustal structure of the junction between the Qilian Shan-Nan Shan thrust belt and the Hexi Corridor basin in the NE Tibetan Plateau. The profile shows that the Moho discontinuity is characterized by complex dislocations and overlapping geometries. Restoration of the deformed Moho discontinuity reveals 50.2 km (35.5%) of Cenozoic lower-crustal shortening along the seismic profile. Compression of the lower crust produces crustal thickening of ∼12.8 km. This demonstrates that lower-crustal shortening plays a significant role in topographic uplift and crustal thickening at the NE margin of the Tibetan Plateau. Thus, we propose a double-layered shortening mechanism, with decoupled deformation of upper crust and lower crust. The proposed deformation mechanism differs from previous crustal shortening models for the northern, eastern, and northeastern Tibetan Plateau. Our model eliminates the need for lower-crustal flow to explain topographic uplift along the NE margin of the Tibetan Plateau.
... The depth of this interface corresponds to the shallow part of a known lower-velocity layer, which has been identified in seismic datasets in nearby areas (e.g., Cui et al., 1995;Li et al., 2014). The lower-velocity layer occurs at a depth of 20-30 km beneath NE Tibet, and it is generally considered to be a décollement layer (e.g., Zhang et al., 2015). Thus, the interface that we identified may be part of the same crustal décollement. ...
Article
Full-text available
We reprocessed a high-resolution deep seismic reflection profile to reveal the detailed crustal structure of the junction between the Qilian Shan-Nan Shan thrust belt and the Hexi Corridor basin in the NE Tibetan Plateau. The profile shows that the Moho discontinuity is characterized by complex dislocations and overlapping geometries. Restoration of the deformed Moho discontinuity reveals 50.2 km (35.5%) of Cenozoic lower-crustal shortening along the seismic profile. Compression of the lower crust produces crustal thickening of ∼12.8 km. This demonstrates that lower-crustal shortening plays a significant role in topographic uplift and crustal thickening at the NE margin of the Tibetan Plateau. Thus, we propose a double-layered shortening mechanism, with decoupled deformation of upper crust and lower crust. The proposed deformation mechanism differs from previous crustal shortening models for the northern, eastern, and northeastern Tibetan Plateau. Our model eliminates the need for lower-crustal flow to explain topographic uplift along the NE margin of the Tibetan Plateau.
... However, a high-resolution seismic-reflection survey revealed that the crust and mantle architecture of east Tibet is not compatible with the existence of a lower-crustal or upper-mantle channel (Gao et al., 2013). The low Poisson's ratio and seismic wave velocity structures along the northeastern margin of the plateau obtained from other geophysical methods also contradict the crustal-flow model (Pan and Niu, 2011;Zhang et al., 2012Zhang et al., , 2015. ...
Article
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The way that far-field stresses and deformation propagated eastward in response to the growth and extrusion of the northeastern Tibetan Plateau remains a crucial scientific issue. This paper focuses on the Bailongjiang and Hanan faults, which are the easternmost part of the East Kunlun fault in northeast Tibet. Based on new field geological investigations, structural data, satellite imagery interpretation, and optically stimulated luminescence and 14C dating results, this paper presents the structural geometry and neotectonic activities of the two faults. The ∼200-km-long Bailongjiang fault, bounding the Bayan Har block in northeast Tibet, consists of two segments. Along the western segment, late Pleistocene lacustrine-facies deposits and Holocene activities were discovered in a great fault scarp. The left-slip rate of the fault is estimated to be ∼1.73−2.61 mm/yr, with an elapsed time of ∼2205 yr after a catastrophic paleoseismic event greater than M 7.2 ruptured the fault. The eastern segment splits into two branches and shows a positive flower structure where a pull-apart basin developed, filled with ∼200-m-thick mudstone and argillaceous siltstone, which record the mid-late Miocene deformation of the Bailongjiang fault. The Hanan fault features reverse faulting caused by NNW-SSE compression in the late Cenozoic. The two faults, together with the Maqên-Maqu-Tazang fault, confine the area of a strip block, the eastward extrusion of which was accommodated by thrusting due to the resistance of the stable Bikou massif since the late Cenozoic, which led to decreasing slip rates along the easternmost part of the Kunlun fault.
... However, our results reveal a positive correlation between the crustal thickness and the thickness of the upper and middle crust (V s < 3.7 km/s) beneath the NETP (Figures 9a-9c). The upper and middle crust beneath the NETP is marked by a thick low-velocity layer (V s < 3.7 km/s) compared to the Alxa block and Ordos block with a thin crust, which seems to suggest that the crustal thickening in the NETP is concentrated in the upper-middle crust Zhang et al., 2015], inconsistent with the vertically coherent deformation model. ...
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The International Deep Profiling of Tibet and the Himalaya Phase IV (INDEPTH IV) active source seismic profile in northeast Tibet extends 270 km roughly north-south across the Songpan-Ganzi terrane, the predominantly strike-slip North Kunlun Fault (along the Kunlun suture), the East Kunlun Mountains, and the south Qaidam Basin. Refraction, reflection, and gravity modeling provide constraints on the velocity and density structure down to the Moho. The central Qaidam Basin resembles average continental crust, whereas the Songpan-Ganzi terrane and East Kunlun Mountains exhibit thickened, lower-velocity crust also characteristic of southern Tibet. The crustal thickness changes from 70 km beneath the Songpan-Ganzi terrane and East Kunlun Mountains to 50 km beneath the Qaidam Basin. This jump in crustal thickness is located ∼100 km north of the North Kunlun Fault and ∼45 km north of the southern Kunlun-Qaidam boundary, farther north than previously suggested, ruling out a Moho step caused by a crustal-penetrating North Kunlun Fault. The Qaidam Moho is underlain by crustal velocity material (6.8-7.1 km/s) for ∼45 km near the crustal thickness transition. The southernmost 10 km of the Qaidam Moho are underlain by a 70 km reflector that continues to the south as the Tibetan Moho. 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 stronger Qaidam crust.
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Two end member models of how the high elevations in Tibet formed are (i) continuous thickening and widespread viscous flow of the crust and mantle of the entire plateau and (ii) time-dependent, localized shear between coherent lithospheric blocks. Recent studies of Cenozoic deformation, magmatism, and seismic structure lend support to the latter. Since India collided with Asia ∼55 million years ago, the rise of the high Tibetan plateau likely occurred in three main steps, by successive growth and uplift of 300- to 500-kilometer-wide crustal thrust-wedges. The crust thickened, while the mantle, decoupled beneath gently dipping shear zones, did not. Sediment infilling, bathtub-like, of dammed intermontane basins formed flat high plains at each step. The existence of magmatic belts younging northward implies that slabs of Asian mantle subducted one after another under ranges north of the Himalayas. Subduction was oblique and accompanied by extrusion along the left lateral strike-slip faults that slice Tibet's east side. These mechanisms, akin to plate tectonics hidden by thickening crust, with slip-partitioning, account for the dominant growth of the Tibet Plateau toward the east and northeast.
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Many seismic and magnetotelluric experiments within Tibet provide proxies for litho-spheric temperature and lithology, and hence rheology. Most data have been collected between c. 888E and 958E in a corridor around the Lhasa–Golmud highway, but newer experiments in western Tibet, and inversions of seismic data utilizing wave-paths transiting the Tibetan Plateau, support a substantial uniformity of properties broadly parallel to the principal Cenozoic and Mesozoic sutures, and perpendicular to the modern NNE convergence direction. These data require unusually weak zones in the crust at different depths throughout Tibet at the present day. In southern Tibet these weak zones are in the upper crust of the Tethyan Himalaya, the middle crust in the southern Lhasa terrane, and the middle and lower crust in the northern Lhasa terrane. In northern Tibet, north of the Banggong–Nujiang suture, the middle and probably the lower crust of both the Qiangtang and Songpan–Ganzi terranes are unusually weak. The Indian uppermost mantle is cold and seismogenic beneath the Tethyan Himalaya and the southern-most Lhasa terrane, but is probably overlain by a northward thickening zone of Asian mantle beneath the northern Lhasa terrane. Beneath northern Tibet the upper mantle has not been replaced by subducting Indian and Asian lithospheres, and is warmer than to the south. These inferred ver-tical strength profiles all have minima in the crust, thereby permitting, though not actually requir-ing, some form of channelized flow at the present day. Using the simplest parameterization of channel-flow models, I infer that a Poiseuille-type flow (flow between stationary boundaries) parallel to India –Asia convergence is occurring throughout much of southern Tibet, and a com-bination of Couette (top-driven, between moving boundaries) and Poiseuille lithospheric flow, perpendicular to lithospheric shortening, is active in northern Tibet. Explicit channel-flow models that successfully replicate much of the large-scale geophysical behaviour of Tibet need refinement and additional model complexity to capture the full details of the temporal and spatial variation of the India– Asia collision. 'Only fools, or unusually insightful individuals not yet recog-nized to be ahead of their time, would doubt that the warmth of the lower crust in regions of extension makes decoupling of upper crust and upper mantle more likely there than in other settings. Thus, the real challenge to understanding how such decoupling or coupling occurs will require study of regions of intracontinental crustal shortening' (Molnar 2000).
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We apply ambient noise tomography to significant seismic data resources in a region including the northeastern Tibetan plateau, the Ordos block and the Sichuan basin. The seismic data come from about 160 stations of the provincial broadband digital seismograph networks of China. Ambient noise cross-correlations are performed on the data recorded between 2007 and 2009 and high quality inter-station Rayleigh phase velocity dispersion curves are obtained between periods of 6 s to 35 s. Resulting Rayleigh wave phase velocity maps possess a lateral resolution between 100 km and 200 km. The phase velocities at short periods (<20 s) are lower in the Sichuan basin, the northwest segment of the Ordos block and the Weihe graben, and outline sedimentary deposits. At intermediate and long periods (>25 s), strong high velocity anomalies are observed within the Ordos block and the Sichuan basin and low phase velocities are imaged in the northeastern Tibetan plateau, reflecting the variation of crustal thickness from the Tibetan plateau to the neighboring regions in the east. Crustal and uppermost mantle shear wave velocities vary strongly between the Tibetan plateau, the Sichuan basin and the Ordos block. The Ordos block and the Sichuan basin are dominated by high shear wave velocities in the crust and uppermost mantle. There is a triangle-shaped low velocity zone located in the northeastern Tibetan plateau, whose width narrows towards the eastern margin of the plateau. No low velocity zone is apparent beneath the Qinling orogen, suggesting that mass may not be able to flow eastward through the boundary between the Ordos block and the Sichuan basin in the crust and uppermost mantle. Key wordsphase velocity-Ordos block-ambient noise tomography-crustal structure CLC numberP315.2
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Using the measurements of similar to 726 GPS stations around the Tibetan Plateau, we determine the rigid rotation of the entire plateau in a Eurasia-fixed reference frame which can be best described by an Euler vector of (24.38 degrees +/- 0.42 degrees N, 102.37 degrees +/- 0.42 degrees E, 0.7096 degrees +/- 0.0206 degrees/Ma). The rigid rotational component accommodates at least 50% of the northeastward thrust from India and dominates the eastward extrusion of the northern plateau. After removing the rigid rotation to highlight the interior deformation within the plateau, we find that the most remarkable interior deformation of the plateau is a "glacier-like flow&apos;&apos; zone which starts at somewhere between the middle and western plateau, goes clockwise around the Eastern Himalayan Syntaxis (EHS), and ends at the southeast corner of the plateau with a fan-like front. The deformation feature of the southern plateau, especially the emergence of the flow zone could be attributed to an eastward escape of highly plastic upper crustal material driven by a lower crust viscous channel flow generated by lateral compression and gravitational buoyancy at the later developmental stage of the plateau. The first-order feature of crustal deformation of the northeastern plateau can be well explained by a three-dimensional elastic half-space dislocation model with rates of dislocation segments comparable to the ones from geological observations. In the eastern plateau, although GPS data show no significant convergence between the eastern margin of the plateau and the Sichuan Basin, a small but significant compressional strain rate component of similar to 10.5 +/- 2.8 nstrain/yr exists in a relatively narrow region around the eastern margin. In addition, a large part of the eastern plateau, northeast of the EHS, is not undergoing shortening along the northeastward convergence direction of the EHS but is stretching.
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Since 1958, about ninety seismic refraction/wide angle reflection profiles, with a cumulative length of more than sixty thousand kilometers, have been completed in mainland China. We summarize the results in the form of (1) a new contour map of crustal thickness, (2) fourteen representative crustal seismic velocity–depth columns for various tectonic units, and, (3) a Pn velocity map. We found a north–south-trending belt with a strong lateral gradient in crustal thickness in central China. This belt divides China into an eastern region, with a crustal thickness of 30–45 km, and a western region, with a thickness of 45–75 km. The crust in these two regions has experienced different evolutionary processes, and currently lies within distinct tectonic stress fields. Our compilation finds that there is a high-velocity (7.1–7.4 km/s) layer in the lower crust of the stable Tarim basin and Ordos plateau. However, in young orogenic belts, including parts of eastern China, the Tianshan and the Tibetan plateau, this layer is often absent. One exception is southern Tibet, where the presence of a high-velocity layer is related to the northward injection of the cold Indian plate. This high-velocity layer is absent in northern Tibet. In orogenic belts, there usually is a low-velocity layer (LVL) in the crust, but in stable regions this layer seldom exists. The Pn velocities in eastern China generally range from 7.9 to 8.1 km/s and tend to be isotropic. Pn velocities in western China are more variable, ranging from 7.7 to 8.2 km/s, and may display azimuthal anisotropy.
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The Cenozoic collision between the Indian and Asian continents formed the Tibetan plateau, beginning about 70 million years ago. Since this time, at least 1,400 km of convergence has been accommodated by a combination of underthrusting of Indian and Asian lithosphere, crustal shortening, horizontal extrusion and lithospheric delamination. Rocks exposed in the Himalaya show evidence of crustal melting and are thought to have been exhumed by rapid erosion and climatically forced crustal flow. Magnetotelluric data can be used to image subsurface electrical resistivity, a parameter sensitive to the presence of interconnected fluids in the host rock matrix, even at low volume fractions. Here we present magnetotelluric data from the Tibetan-Himalayan orogen from 77 degrees E to 92 degrees E, which show that low resistivity, interpreted as a partially molten layer, is present along at least 1,000 km of the southern margin of the Tibetan plateau. The inferred low viscosity of this layer is consistent with the development of climatically forced crustal flow in Southern Tibet.
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The crust beneath the northeastern (NE) Tibetan Plateau records far field effects of collision and convergence occurring between the Indian and Eurasian plates. A better structural understanding of the crust beneath NE Tibet can improve our understanding of Cenozoic deformation resulting from the India–Eurasia collision. Taking advantage of the relatively dense coverage in most areas in NE Tibet except for the Qaidam basin by regional seismic networks of Gansu and Qinghai Provinces, we isolate receiver functions from the teleseismic P wave data recorded from 2007 to 2009 and resolve the spatial distribution of crustal thickness and Vp/Vs ratio beneath NE Tibet from H–κ scanning. Our results can be summarized as: (1) NE Tibet is characterized by ~ 60-km-thick crust beneath the Nan Shan, Qilian Shan thrust belts and the Anyemaqen Shan, and 45–50 km-thick crust beneath the Tarim basin, the Alashan depression and the Ordos basin; the crust thins gradually from west to east in addition to the previously observed pronounced thinning from south to north; (2) the crust of NE Tibet exhibits a relatively lower Vp/Vs ratio of 1.72 than the north China block and a decrease in average crustal Vp/Vs ratio with increasing crustal thickness; and (3) the crustal thicknesses are less than the values predicted by the simple isostatic model of the whole Tibetan plateau wherein the elevation is larger than 3.0 km. Our observations can be explained by the hypothesis that deformation occurring in NE Tibet is predominated by upper-crustal thickening or lower-crust extrusion.
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Topography extracted from swath profiles along the northern, southern, and eastern margins of the Tibetan Plateau show two end-member morphologies: steep, abrupt margins and long-wavelength, low-gradient margins. Because the lack of significant upper crustal shortening across much of the eastern plateau margin implies that the crustal thickening occurs mainly in the deep crust, we compare regional topographic gradients surrounding the plateau to model results for flux of a Newtonian fluid through a lower crustal channel of uniform thickness. For an assumed 15-km-thick channel, we estimate a viscosity for the lower crust of 1018 Pa · s beneath the low-gradient margins, 1021 Pa · s beneath the steep margins, and an upper bound of 1016 Pa · s beneath the plateau. These results indicate that the large-scale morphology of the eastern plateau reflects fluid flow within the underlying crust; crustal material flows around the strong crust of the Sichuan and Tarim Basins, creating broad, gentle margins, and “piles up” behind the basins creating narrow, steep margins. These results imply that this portion of the Eurasian crust was heterogeneous, but largely weak, even prior to construction of the Tibetan Plateau.
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We present S receiver functions and SKS splitting measurements from the China Seismograph Network located in the Qinghai and Gansu provinces. Teleseismic data are used to interpret the structure of the lithosphere-asthenosphere boundary (LAB) and upper-mantle deformation beneath the northeastern Tibetan Plateau (NETP) in regions north of the east Kunlun Fault. Based on our observations, the LAB lies at a depth of 125-135 km beneath the northeastern Songpan-Ganzi block and the west Qinling orogen, between 145 and 175 km beneath the Kunlun and Qilian orogen, and deepens below the Qaidam Basin (175-190 km), Ordos Craton (170 km) and Alashan platform (200 km). The NETP is characterized by a nearly uniform fast NW-SE S-wave direction. These observations are different from those to the south of the Kunlun Fault where fast S directions are rotating clockwise from the inner plateau. The change in fast directions across the Kunlun Fault implies a sudden variation of upper-mantle deformation. Shear wave splitting delay times vary from 0.8 to 1.9 s. Data from beneath regions north of the Kunlun-Ayimaqin suture showed that delay time was positively correlated with lithospheric thickness with an increase of 0.7 s per 100 km. This indicates that the anisotropy may develop in the uppermost mantle, such as the lithosphere, beneath the NETP.
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Cenozoic Qaidam basin, the largest active intermountain basin inside Tibet, figures importantly in the debates on the history and mechanism of Tibetan plateau formation during the Cenozoic Indo-Asian collision. To determine when and how the basin was developed, we conducted detailed geologic mapping and analyses of a dense network of seismic reflection profiles from the southern Qilian Shan-Nan Shan thrust belt and northern Qaidam basin. Our geologic observations provide new constraints on the timing and magnitude of Cenozoic crustal thickening in northern Tibet. Specifically, our work shows that the southernmost part of the Qilian Shan-Nan Shan thrust belt and contractional structures along the northern margin of Qaidam basin were initiated in the Paleocene-early Eocene (65-50 Ma), during or immediately after the onset of the Indo-Asian collision. This finding implies that stress was transferred rapidly through Tibetan lithosphere to northern Tibet from the Indo-Asian convergent front located >1000 km to the south. The development of the thrust system in northern Qaidam basin was driven by motion on the Altyn Tagh fault, as indicated by its eastward propagation away from the Altyn Tagh fault. The eastward lengthening of the thrust system was spatially and temporally associated with eastward expansion of Qaidam basin, suggesting thrust loading was the main control on the basin formation and evolution. The dominant structure in northern Qaidam basin is a southwest-tapering triangle zone, which started to develop since the Paleocene and early Eocene (65-50 Ma) and was associated with deposition of an overlying southwest-thickening, growth-strata sequence. Recognition of the triangle zone and its longevity in northern Qaidam basin explains a long puzzling observation that Cenozoic depocenters have been located consistently along the central axis of the basin. This basin configuration is opposite to the prediction of classic foreland-basin models that require the thickest part of foreland sediments deposited along basin edges against basin-bounding thrusts. Restoration of balanced cross sections across the southern Qilian Shan-Nan Shan thrust belt and northern Qaidam basin suggests that Cenozoic shortening strain is highly inhomogeneous, varying from similar to 20% to >60%, both vertically in a single section and from section to section across the thrust belt. The spatially variable strain helps explain the conflicting paleomagnetic results indicating different amounts of Cenozoic rotations in different parts of Qaidam basin. The observed crustal shortening strain also implies that no lower-crustal injection or thermal events in the mantle are needed to explain the current elevation (similar to 3000-3500 m) and crustal thickness (45-50 km) of northern Qaidam basin and the southern Qilian Shan-Nan Shan thrust belt. Instead, thrusting involving continental crystalline basement has been the main mechanism of plateau construction across northern Qaidam basin and the southern Qilian Shan-Nan Shan region.
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Study on the electricity structure along a magnetotelluric (MT) sounding profile on the northeastern margin of Qinghai-Tibet Plateau indicates that four crustal blocks can be de-termined from southwest to northeast, namely Bayan Har block (BH), Qin-Qi block (QQ), Hai-yuan block (HY) or the North-South seismotectonic belt and Ordos block (OD). The BH, QQ and OD blocks display a similar electricity structure of the crust. The upper crust represents a high- resistivity layer and the upper part of lower crust represents a low-resistivity layer with the resis-tivity increasing gradually with depth from the lower part of lower crust to the upper mantle. The electricity structure of the crust in these three blocks is similar to that in the complete blocks on the southern and eastern margins of the Qinghai-Tibet Plateau and belongs to normal electricity layering of the crust in slightly deformed or complete intracontinental blocks. The crust in HY block as a boundary zone has been significantly deformed, hence its electricity layering was de-stroyed and the structure was complex and the block became a recent tectonically active and great seismo-active region. The contact belts between the blocks on the northeastern margin of Qinghai-Tibet Plateau exhibit both upthrusting outward and strike-slip movement different from those on the southern and eastern margins of the plateau. The genesis of the low-resistivity layer in the crust is analyzed and the thickness of the lithosphere is estimated in the paper.
Article
S receiver functions from 51 permanent broad-band stations are used to investigate the structures of the crust and lithosphere beneath eastern Tibet and Sichuan Basin. By stacking the S receiver functions, the Sp converted phases from the Moho and the lithosphere–asthenosphere boundary (LAB) are clearly visible. The Moho depth ranges from 40 to 50 km beneath Sichuan Basin and from 55 to 60 km on the west side of the Longmenshan (LMS) fault. The largest depth is 70 km, occurring at the northern Sichuan–Yunnan terrane. The LAB depth beneath Sichuan Basin ranges from 130 to 170 km, while that on the west side of LMS ranges from 100 to 120 km. From the east to the west across LMS, there is a sudden rise of the LAB, followed by a gradual dip. Western Sichuan Basin is found to be surrounded by thinner lithosphere, extending northeastward along the LMS and southward to Indochina. The thinner lithosphere may indicate that the lower part of eastern Tibet lithosphere is heated and delaminated by the underlying hot asthenospheric flow, thus reflecting the size and direction of the flow. It appears that asthenospheric flow from Tibet is resisted by the cold rigid Sichuan Basin, thus branching into northeastward and southeastward parts. The complicated topography of the lithosphere likely results from this dynamic process.Research Highlights► We investigate the crustal and lithospheric structures in eastern Tibet. ► A thinner lithosphere surrounds western Sichuan Basin. ► The eastern Tibet lithosphere is delaminated by the asthenospheric flow. ► The thinner lithosphere is consistent with a lower density within it.
Article
We analyzed thousands of receiver-function data recorded by 154 national and regional seismic stations to study the crustal structure beneath the Ordos plateau and the NE margin of the Tibetan plateau. Moho depth and average crustal Vp/Vs ratio were measured at each station. The Ordos plateau and the Trans-North China Orogen east of the Ordos are underlain by a moderately thick crust of ~ 42 km. The Weihe Graben lying at the southern edge of the Ordos plateau has a thin crust of ~ 30 km, while its southern neighbor, the Qinling orogenic belt shows a thick crust extending to as much as 45 km deep. The Moho depth beneath the NE margin of the Tibetan plateau varies from 55 to 65 km. We found a remarkable contrast between the Tibetan and Ordos plateaus in the measured Poisson's ratio: the Ordos plateau is featured by a high Poisson's ratio while the Tibetan margin has a very low Poisson's ratio. In general, mafic lower crustal rocks have a higher Poisson's ratio than felsic ones. The measured low Poisson's ratio beneath the NE margin of the Tibetan plateau thus indicates that the crustal column beneath the margin is rather felsic, which seems to be inconsistent with a scenario of an inflated crust due to extrusion of lower crust material from the Tibetan plateau to the margin.Research Highlights► Crustal thickness and Vp/Vs of the Ordos plateau and NE Tibetan plateau were estimated. ► The Ordos is underlain by a moderately thick crust with an intermediate composition. ► Crust beneath the NE Tibetan plateau varies is thick with a low Vp/Vs ratio. ► The low Vp/Vs ratio is inconsistent with the lower-crust flow model proposed for crustal thickening. ► Seismicity around the Ordos plateau is controlled by crustal and lithosphere structure.
Article
Fieldwork complemented by SPOT image analysis throws light on current crustal shortening processes in the ranges of northeastern Tibet (Gansu and Qinghai provinces, China). The ongoing deformation of Late-Pleistocene bajada aprons in the forelands of the ranges involves folding, at various scales, and chiefly north-vergent, seismogenic thrusts. The most active thrusts usually break the ground many kilometres north of the range-fronts, along the northeast limbs of growing, asymmetric ramp-anticlines. Normal faulting at the apex of other growing anticlines, between the range fronts and the thrust breaks, implies slip on blind ramps connecting distinct active décollement levels that deepen southwards. The various patterns of uplift of the bajada surfaces can be used to constrain plausible links between contemporary thrusts downsection. Typically, the foreland thrusts and décollements appear to splay from master thrusts that plunge at least 15–20 km down beneath the high ranges. Plio-Quaternary anticlinal ridges rising to more than 3000 m a.s.l. expose Palaeozoic metamorphic basement in their core. In general, the geology and topography of the ranges and forelands imply that structural reliefs of the order of 5–10 km have accrued at rates of 1–2 mm yr⁻¹ in approximately the last 5 Ma.
Article
In the last decade, several international joint projects were conducted in the Tibetan Plateau by Chinese, American and French geophysicists and geologists. In the present review, the results from vertical reflections, wide-angle reflections and broadband digital seismic recordings are reviewed and compared. Constraints for the dynamics of continent-continent collision from the lithospheric structures, seismicity, focal mechanism and anisotropy are discussed.The velocities ofPn,Sn, [(P)]\bar P , [(S)]\bar S were accurately determined by using their travel times from local events. They evidenced that the uppermost mantle underneath the Tibetan Plateau was similar to that of the ordinary continental mantle.The reflection profile from INDEPTH-I furnishes convincing evidence that the Indian crust penetrates into the Tibetan lower crust. The results from teleseismic waveform inversion reveal that the Moho discontinuity dips northwards, and an offset of Moho occurs near Bangong suture.The fact that materials within the Tibetan Plateau escape laterally has been proposed by several authors. Recent data and studies provide further convincing evidence that eastward mass transfer does occur, and their paths and natures are investigated.Some authors suggested that the large strike slip faults (Kun Lun, Xianshuihe) in the eastern plateau may be related to the lateral extrusion. However, most of the strike slips are left-lateral, and extrusion could not occur without right-lateral strike slips. Recent observations of the focal mechanisms and geological structure indicate that the earthquakes in the Yanshiping-Changdu belt are left-lateral strike slip. It is the southeast zone of the left-lateral slip faults in the eastern Tibetan plateau. Geological and seismological evidence show that the Bencuo-Jiali belt is the only large right-lateral fault in the eastern plateau. It was proposed that the present eastward extrusion occurs between the Yangshiping-Changdu left-lateral strike slip and the Bencuo-Jiali right-lateral strike slip. The other left-lateral strike slips north of the Yangshiping-Changdu belt are considered to be the fossils of the ancient flow paths.
Article
Songpan-Garze massif is located at the turning position of tectonics from the near west-east direction to the near north-south direction in the northeastern margin of Qinghai-Tibet Plateau, with Zoigê basin in the centre of the massif. In this paper, we build a crustal structure model of Zoigê basin and its surrounding folded orogenic belts using the deep seismic sounding data in this region. We also discuss structures and properties of the basement in Zoigê basin, tectonic relations between Zoigê upland basin and its surrounding folded orogenic belts, crustal deformation and thickening in the northeastern margin of Tibetan Plateau, and decoupling and relaxing processes in the crust. The results indicate that a special “Mesozoic basement” is formed of Triassic rocks with high density (2.65–2.75 g/cm3) and high velocity (5.6 km/s) in Zoigê basin. Songpan-Garze tectonic massif was transformed into two types of tectonic units with different crustal structures, i.e., relatively stable Zoigê upland basin and active folded orogenic belts around the basin, in the course of the crustal material of Tibetan Plateau flowing eastward and obstructed by surrounding stable blocks. The thickening of the crust in the northeastern margin of Tibetan Plateau mainly occurred in the mid and lower crust, and the structure characterized by low velocities and multiple reflectors obviously appears in the folded orogenic belts around Zoigê basin. It implies that the mid and lower crust underwent a strong tectonic deformation in the folded orogenic areas. The thickness of the crust is about 50 km in Zoigê basin and the folded orogenic belts at the both southern and northern sides of Zoigê basin. The “Mountain root” cannot be identified. It is inferred that during the later orogenic period the eastwards flowing deep materials moved clockwise along the relatively relaxing southern side around the eastern tectonic knot under the obstructing of surrounding rigid massifs, and it resulted in the strong stretching action of the folded orogenic belts around Zoigê basin.
Article
Barkam-Luqu-Gulang deep seismic sounding profile runs from north of Sichuan Province to south of Gansu Province. It is located at the northeastern edge of Tibetan Plateau and crosses eastern A’nyemaqên suture zone. The upper crust structures around eastern A’nyemaqên suture zone and its adjacent area are reconstructed based on the arrival times of refracted Pg and Sg waves by using finite difference method, ray tracing inversion, time-term method and travel-time curve analysis. The results show that the depth variation of basement along profile is very strong as indicated by Pg and Sg waves. The basement rose in Zoigê basin and depressed in eastern A’nyemaqên suture zone, and it gradually rose again northward and then depressed. The results also indicate that eastern A’nyemaqên suture zone behaves as inhomogeneous low velocity structures in the upper crust and is inclined toward the south. Hoh Sai Hu-Maqên fault, Wudu-Diebu fault and Zhouqu-Liangdang fault are characterized by low velocity distributions with various scales. The distinct variation in basement depth occurred near Hoh Sai Hu-Maqên fault and Zhouqu-Liangdang fault, which are main tectonic boundaries of A’nyemaqên suture zone. Wudu-Diebu fault, located at the depth variation zone of the basement, possibly has the same deep tectonic background with Zhouqu-Liangdang fault. The strongly depressed basement characterized by low velocity distribution and lateral inhomogeneity in A’nyemaqên suture zone implies crushed zone features under pinching action.
Article
New wide-angle reflection and refraction seismic data provide constraints on the structure of the upper lithosphere, and test models of its evolution to raise the northeastern part of the Tibetan Plateau. Amplitudes observed for reflections from the crust–mantle boundary are sufficiently large to suggest that there is no significant partial melt in the deep crust. The data show an increase of the crustal thickness between terranes from north of the Kun Lun Fault into the Qang Tang of central Tibet, and a contrast among their intracrustal images and compositions. In the north, P and S velocities are consistent with a dominantly felsic composition and show that only the upper crust thickened. South of the Kun Lun Fault a thicker crust made of two layers could result from the superposition of the originally thin crust of the Bayan Har terrane on the lower part of the crust of the domain to the north, which upper crust it shoved and thickened. Different modes of crustal thickening, either by thickening of individual layers or superpositions and imbrication among them appear to work jointly to raise the topography.
Article
Southeastern Tibet marks the site of presumed clockwise rotation of the crust due to the India–Eurasian collision and abutment against the stable Sichuan basin and South China block. Knowing the structure of the crust is a key to better understanding of crustal deformation and seismicity in this region. Here, we analyze recordings of teleseismic earthquakes from 25 temporary broadband seismic stations and one permanent station using the receiver function method. We find that the crustal thickness decreases gradually from the Tibetan Plateau proper to the Sichuan basin and Yangtze platform but that significant (intra-)crustal heterogeneity exists on shorter lateral scales (<1000 km). Most receiver functions reveal a time shift of ∼0.2 s in the direct P arrival and negative phases between 0 and 5 s after the first arrival. Inversion of the receiver functions yields S-velocity profiles marked by near-surface and intra-crustal low-velocity zones (IC-LVZs). The shallow low-velocity zones are consistent with the wide distribution of thick surface sedimentary layers. The IC-LVZ varies laterally in depth and strength; it becomes thinner toward the east and southeast and is absent in the Sichuan basin and the southern part of the Yangtze platform. Results from slant-stacking analysis show a concomitant decrease in crust thickness from ∼60 km in the Songpan-Ganze fold system to ∼46 km in the Sichuan basin and ∼40 km in the Yangtze platform. High Poisson's ratios (>0.30) are detected beneath the southeastern margin of Tibet but in the Sichuan basin and southeastern Yangtze platform the values are close to the global average. Combined with high regional heat flow and independent evidence for mid-crustal layers of high (electric) conductivity, the large intra-crustal S-wave velocity reduction (12–19%) and the intermediate-to-high average crustal Poisson's ratios are consistent with partial melt in the crust beneath parts of southeastern Tibet. These results could be used in support of deformation models involving intra-crustal flow, with the caveat that significant lateral variation in location and strength of this flow may occur.
Article
We performed a receiver function analysis on teleseismic data recorded along two 550 km-long profiles crossing the northeastern Tibetan plateau. Results from time to depth migration, grid-search Vp/Vs determination and simulated annealing inversion of waveforms, reveal that the crust thickens from ∼50 km near the northern edge of the plateau to ∼80 km south of the Jinsha suture in the Qiang Tang block. Crustal thickening occurs in staircase fashion with steps located beneath the main, reactivated sutures. The Vp/Vs ratio, close to the global continental average does not suggest widespread partial melting but rather a more usual separation between an upper felsic and a lower mafic part within the northeastern Tibetan crust.
Article
Over the last three years, a major international effort has been made by the Sub-Commission on Earthquake Algorithms of the International Association of Seismology and the Physics of the Earth's Interior (IASPEI) to generate new global traveltime tables for seismic phases to update the tables of Jeffreys & Bullen (1940). The new tables are specifically designed for convenient computational use, with high-accuracy interpolation in both depth and range. The new iasp91 traveltime tables are derived from a radially stratified velocity model which has been constructed so that the times for the major seismic phases are consistent with the reported times for events in the catalogue of the International Seismological Centre (ISC) for the period 1964–1987. The baseline for the P-wave traveltimes in the iasp91 model has been adjusted to provide only a small bias in origin time for well-constrained events at the main nuclear testing sites around the world. For P-waves at teleseismic distances, the new tables are about 0.7s slower than the 1968 P-tables (Herrin 1968) and on average about 1.8–1.9 s faster than the Jeffreys & Bullen (1940) tables. For S-waves the teleseismic times lie between those of the JB tables and the results of Randall (1971). Because the times for all phases are derived from the same velocity model, there is complete consistency between the traveltimes for different phases at different focal depths. The calculation scheme adopted for the new iasp91 tables is that proposed by Buland & Chapman (1983). Tables of delay time as a function of slowness are stored for each traveltime branch, and interpolated using a specially designed tau spline which takes care of square-root singularities in the derivative of the traveltime curve at certain critical slownesses. With this representation, once the source depth is specified, it is straightforward to find the traveltime explicitly for a given epicentral distance. The computational cost is no higher than a conventional look-up table, but there is increased accuracy in constructing the traveltimes for a source at arbitrary depth. A further advantage over standard tables is that exactly the same procedure can be used for each phase. For a given source depth, it is therefore possible to generate very rapidly a comprehensive list of traveltimes and associated derivatives for the main seismic phases which could be observed at a given epicentral distance.
Article
GPS displacement vectors show that the crust in east Tibet is being squeezed in an easterly direction by the northward motion of the Indian plate, and the Sichuan Basin is resisting this stream and redirecting it mainly towards Indochina. The Longmen Shan, containing the steepest rise to the high plateau anywhere in Tibet, results from the strong interaction between the east Tibetan escape flow and the rigid Yangtze block (Sichuan Basin), but the kinematics and dynamics of this interaction are still the subject of some debates. We herein present results from a dense passive-source seismic profile from the Sichuan Basin into eastern Tibet in order to study the deep structure of this collision zone. Using P and S receiver function images we observe a sudden rise of the Lithosphere-Asthenosphere Boundary (LAB) from 120 to 150?km beneath the Sichuan Basin and from 70 to 80?km beneath eastern Tibet. In contrast, the depth of the crust-mantle boundary (Moho) increases from 36 to 40?km beneath the Sichuan Basin and from 55 to 60?km beneath eastern Tibet. The 410?km discontinuity is depressed below eastern Tibet by about 30?km, although the 660 remains at nearly the same depth throughout the LMS. From these observations, we conclude that the mode of collision that occurs between Tibet and the Sichuan Basin is very different to that found between India and Tibet. In southern Tibet, we observe in essence the subduction of the Indian plate, which penetrates northwards for several hundred kilometers under central Tibet. The very thin mantle part of the lithosphere beneath eastern Tibet may indicate delamination or removal of the bottom of the lithosphere by hot asthenospheric escape flow. This process leads to the exceptionally steep topography at the eastern Tibetan margin as a result of gravitational buoyancy. This view is supported by the very unusual depression of the 410?km discontinuity beneath eastern Tibet, which could be caused by the dynamics of the sub-vertical downward asthenospheric flow.
Article
We determine the 3-D shear wave speed variations in the crust and upper mantle in the southeastern borderland of the Tibetan Plateau, SW China, with data from 25 temporary broad-band stations and one permanent station. Interstation Rayleigh wave (phase velocity) dispersion curves were obtained at periods from 10 to 50 s from empirical Green’s function (EGF) derived from (ambient noise) interferometry and from 20 to 150 s from traditional two-station (TS) analysis. Here, we use these measurements to construct phase velocity maps (from 10 to 150 s, using the average interstation dispersion from the EGF and TS methods between 20 and 50 s) and estimate from them (with the Neighbourhood Algorithm) the 3-D wave speed variations and their uncertainty. The crust structure, parametrized in three layers, can be well resolved with a horizontal resolution about of 100 km or less. Because of the possible effect of mechanically weak layers on regional deformation, of particular interest is the existence and geometry of low (shear) velocity layers (LVLs). In some regions prominent LVLs occur in the middle crust, in others theymay appear in the lower crust. In some cases the lateral transition of shear wave speed coincides with major fault zones. The spatial variation in strength and depth of crustal LVLs suggests that the 3-D geometry of weak layers is complex and that unhindered crustal flow over large regions may not occur. Consideration of such complexity may be the key to a better understanding of relative block motion and patterns of seismicity.