Crustal structure across Longmenshan fault belt from passive source seismic profiling (original) (raw)
Related papers
Regional Flow in the Lower Crust and Upper Mantle under the Southeastern Tibetan Plateau
International Journal of Geosciences, 2011
Seismic tomography reveals an "R-shape" regional flow constrained between the depths of 50 to 80 km in the Southeastern Tibetan Plateau (STP) which demonstrates some of the differences revealed by the magnetotelluric (MT) soundings in some areas. The "R-shape" flow could be present in both the lower crust and uppermost mantle, but not in the lower crust above the Moho discontinuity. Lateral flow has been imaged under the Qiangtang and Songpan-Ganzi blocks while two channel flows have been revealed beneath the south part of the STP with the eastward lateral flow from the Qiangtang block separating into two channel flows. One branch turns southwards at the south Qiangtang block, along the Bangong-Nujiang fault reaching to the Indochina block, and another is across the Songpan-Ganzi block (fold system) which then separates into northward and southward parts. The northward branch is along the edge of the north Sichuan basin reaching to the Qingling fault and the southward channel turns south along the Anninghe fault, then turns eastward along the margins of the south Sichuan basin. Our study suggests that the crustal deformation along the deep, large sutures (such as the Longmen Shan fault zone) is maintained by dynamic pressure from the regional flow intermingled with the hot upwelling asthenosphere. The material in the lower crust and uppermost mantle flowing outward from the center of the plateau is buttressed by the old, strong lithosphere that underlies the Sichuan basin, pushing up on the crust above and maintaining steep topography through dynamic pressure. We therefore consider that the "R-shape" regional flow played a key role in the crustal deformation along the deep suture zones of the Bangong-Nujiang, the Longmen-Shan faults, and other local heavily faulted zones.
Science in China Series D, 2004
The reflecting events from Moho and other interfaces within the crust are recognized from the wavefield characteristics of P-and S-wave for the 480km long wide-angle seismic profile between Peigu Tso and Pumoyong Tso. Then, seismic crustal structures of P-and S-wave velocities and Poisson ratio under the nearly east-west profile in southern Tibet are interpreted by fitting the observed traveltimes with the calculated ones by forward modelling. Our interpreting results demonstrate that the crustal thickness varies remarkably in the east-west direction, showing a pattern that the crust could be divided into three parts bounded by the west of Dingri and the east of Dinggyê, respectively, where the depth of Moho is about 71km for the western part, about 76km for the middle and about 74km for the eastern. There is one lower velocity layer (LVL) at the bottom of the upper crust with depth of 20 30 km. One of the distinct features is that the thickness of LVL abruptly thins from 24km on the west to 6km on the east. The other is that the velocity variation in the crust along east-west direction for both P-and S-wave displays a feature as quasi-periodic variation. The lower velocity (compared to the average value for the continent of the globe) in the lower crust and three sets of north-southward active normal faults are probably attributed to the coupling process of material delamination in the lower crust, crustal thicking and east-westward escape of the crustal material accompanied with the continental collision between India and Eurasia Plate.
Geophysical Journal International, 1991
This paper addresses the velocity structure of the crust and upper mantle beneath southern China with special emphasis on the Tibet region. Waveform data from 48 earthquakes as recorded on the WWSSN and GDSN are used in this detailed forward modelling study. Constraints on the upper crustal section are derived from modelling local Love waves in the time domain applying the mode-sum modelling technique. Lower crustal constraints are derived by modelling the PnI-wavetrain with the reflectivity method.-An average crustal thickness of 70 km is obtained beneath the Tibetan Plateau with a modest increase of velocity with depth. The lithospheric and upper mantle structure is deduced from modelling S and SS triplication waveform data and relative traveltimes by applying a combination of WKBJ and generalized ray methods. S-SS seismograms chosen with bounce-points directly under Tibet allow remote sensing of this inaccessible region. The resulting model is an averaged 1-D model where corrections for lateral variation have been applied. We conclude that the upper mantle structure in the entire region is basically shield-like below 200km (SNA). However, the velocity of the lithosphere is abnormally slow, roughly 5 per cent beneath Tibet. The model for Tibet derived does not have a distinct lid, and has a positive velocity gradient in the crust, suggesting crustal shortening. A preliminary velocity model for southeastern China is also suggested.
Crustal structure and deformation of the SE Tibetan plateau revealed by receiver function data
2012
We analyze a large amount of receiver function data recorded by regional seismic networks of the China Earthquake Administration to estimate crustal structure and deformation beneath the southeast margin of the Tibetan plateau and its surrounding areas. We develop a comprehensive analysis method that facilitates robust extraction of azimuthal seismic anisotropy from receiver function data. The method includes an estimate of fast polarization direction and splitting time by a joint analysis of radial and transverse receiver function data, and an evaluation of measurement reliability by statistical and harmonic analysis. We find significant seismic anisotropy with a splitting time of 0.5-0.9 s beneath the SE margin of the Tibetan plateau. Both the splitting time and fast polarization direction are comparable to those estimated from SKS/SKKS data, suggesting that crustal anisotropy is the main cause of shear wave splitting of the SKS/SKKS wave. This also suggests that deformation in the upper mantle is either weak or predominantly vertical, and is obviously different from the one in the crust. A vertical flow in the upper mantle, combined with the observation of a thin lithosphere beneath the study area, leads to the inference that part of the mantle lithosphere may have been removed and is descending into deep mantle. Stations located in the surrounding areas, on the other hand, exhibit very little to no crustal anisotropy. The estimated Moho depth and Vp/Vs ratio also show a distinct difference between the SE Tibetan plateau and the surrounding regions. Stations on the Tibetan plateau have a Vp/Vs ratio of $ 1.79, which is substantially higher than those measured at the Yunnan-Guizhou (Yungui) plateau ($ 1.69). Our observations here are consistent with the scenario that the SE Tibet has been built by lower crustal flow. They also suggest that the mantle lithosphere beneath the margin may have been mechanically decoupled from the upper crust.
Vertical crustal motions across Eastern Tibet revealed by topography-dependent seismic tomography
Scientific reports, 2017
Using a topography-dependent tomographic scheme, the seismic velocity structure of the Eastern Tibetan Plateau, including the uplifted Longmenshan (LMS) orogenic belt, is accurately imaged in spite of the extreme topographic relief in the LMS region and thick sedimentary covers in the neighbouring Sichuan Basin. The obtained image shows a high-resolution upper crustal structure on a 500 km-long profile that is perpendicular to the LMS. The image clearly shows that the crystalline basement was uplifted within the LMS orogenic belt, and that the neighbouring Songpan-Ganzi Terrane was covered by a thick flysch belt, with evidence of near-surface thrust faults caused by convergence between Eastern Tibet and the Sichuan Basin. The indication that the lower crust beneath the LMS was folded and pushed upwards and the upper crust was removed by exhumation, supports the concept of a lower crustal channel flow beneath Eastern Tibet. The image also reveals that the destructive Wenchuan earthqu...
Science in China Series D, 2002
We recognized 6 sets of reflecting P-and S-wave events from Moho and other interfaces within the crust, respectively, with the wide-angle seismic data acquired from 510 km-long Selincuo-Ya'anduo profile in the northern Tibet, fitted the observed events with forward modeling, and interpreted crustal structure of P-and S-wave velocities and Poisson's ratio under the profile. The results demonstrate that the crustal structure between Yarlungzangbo and Bangong-Nujiang sutures changes abruptly, and the crust is the thickest at the middle part of the profile with thickness of 80 km or more. The "down-bowing" Moho is the striking feature for the crustal variation along the west-east direction. The Moho uplifts with steps, and the uplifting rate westward is greater than that eastward. The heterogeneity of P-and S-wave velocities exists both vertically and horizontally, and one lower velocity layer (LVL) exists with the depth range of 27-34 km and the thickness range of 5-7 km. For the upper crust, Poisson's ratio is the lowest at the middle part of the profile; for the lower crust, the Poisson's ratio at the east segment is lower than that at west segment, which means that the crustal rigidity for the upper crust is different from the lower crust, and the lower crust under the east segment of the profile is more ductile. We infer that the substance in the lower crust endured eastward flow along with the collision between Eurasian and Indian plates, and the "down-bowing" Moho is attributable to the multi-phase E-W tectonic processes.
Journal of Geophysical Research, 2005
1] A 500-km-long west-east wide-angle seismic profile from Selin Tso to Yaanduo in the northernmost Lhasa block of Tibet, acquired by the Sino-French joint seismic program in 1982, has been reinterpreted. We model the P and S wave velocity structure of the whole crust, while recognizing that in many places, uncertainties are large. A surprising but robust conclusion, consistent with previous interpretations of both this data set and other newer data from Tibet, is that Moho depth is about 60-65 km at 90°E but 75-80 km depth at 92.5°E. Our detailed interpretation uses multiple wide-angle P and S wave reflections from the crystalline basement and the Moho; but no Moho refractions are recognized. Along most of the profile, the crust may be crudely divided into an upper crust ($5-30 km depth, 5.0 < V p < 6.4 km/s), a middle crust ($33-45 km depth, 6.5 < V p < 6.8 km/s) and a lower crust (depths below 48km,7.0<Vp<7.4km/s).Thewest−to−eastincreaseincrustalthicknessisaccomplishedbya48 km, 7.0 < V p < 7.4 km/s). The west-to-east increase in crustal thickness is accomplished by a 48km,7.0<Vp<7.4km/s).Thewest−to−eastincreaseincrustalthicknessisaccomplishedbya50% thickening of the middle and lower crust. Larger vertical velocity gradients separate these three layers and bound them above (surficial and sedimentary rocks) and below (Moho transition zone). The most notable low-velocity zone in the crust lies at the base of the upper crustal layer. S wave velocity structure is less well constrained but parallels the P wave structure except that V p /V s ratios may decrease from west to east in the lower crust. Our data suggest considerable variation in structure along-strike of the Tibetan Plateau and show that interpretations of Tibet as a purely two-dimensional orogen are overly simplistic. The west-east increase in crustal thickness may occur across the Karakorum-Jiali fault system and be an indicator of lateral tectonic escape of the Qiangtang terrane.
Gondwana Research, 2013
In addition to crustal thickening, distinctly different mechanisms have been suggested to accommodate the huge convergences caused by the continental collision between India and Eurasia. As the transition zone between the two grand tectonic domains of Asia, the Tethys and the Pacific, east Tibet and its surrounding regions are the ideal places to study continental deformation. Pervasive rock deformation may produce anisotropy on the scale of seismic wavelengths; thus, seismic anisotropy provides insight into the deformation of the crust and mantle beneath tectonically active domains. In this study, we calculated receiver function pairs of radial-and transversecomponents at 98 stations located in Sichuan and Yunnan provinces, China. We selected 7423 pairs with high signal-to-noise ratio (SNR) and unambiguous Moho converted Ps phases (Pms) to measure the Pms splitting owing to the crustal anisotropy. Both the crustal thickness and the average crustal Vp/Vs ratio were calculated simultaneously by the H-k stacking method. The geodynamic implications were also investigated in relation to surface geological features, GPS velocities, absolute plate motion (APM), SKS/SKKS splitting, and other seismological observations. In addition to the fast polarization directions (FPDs) of the crustal anisotropy, we observed a conspicuous sharper clockwise rotation around the eastern Himalayan syntaxis than was revealed by GPS velocities. The distributed FPDs within and near the main active fault zones also favored the directions parallel to the faults. This implied that the deformation of a continuous medium revealed by GPS motions is a proxy for the deformation of the brittle shallow crust only, while the main active faults and the deep crustal interiors both play important roles in the deep deformation. Our results suggest that the deformation between the crust and upper mantle within the northernmost section of the Indochina block is decoupled due to the large difference in the directions between the observations related to the crust (GPS and crustal anisotropy) and mantle (APM and mantle anisotropy). Focusing on the transition zone between the plateau and the South China and Indochina blocks, we suggest that the motion of the Central Yunnan sub-block is a southeastward extrusion by way of tectonic escape. There is less deformation in the deep crust and the motion is controlled by the active boundary faults of the Ailaoshan-Red River shear zone to the west and the Xianshuihe-Xiaojiang fault to the east; the lower crustal flow within the plateau southeastward reached the Lijiang-Xiaojinhe fault, but further south it was obstructed by the Central Yunnan sub-block.