Xiaofeng Liang

Lithospheric and upper mantle structure of the northeastern Tibetan Plateau

[1]xa0We use receiver functions calculated for data collected by the INDEPTH-IV seismic array to image the three-dimensional geometry of the crustal and upper mantle velocity discontinuities beneath northeastern Tibet. Our results indicate an average crustal thickness of 65 to 70xa0km in northern Tibet. In addition, we observe a 20xa0km Moho offset beneath the northern margin of the Kunlun Mountains, a 10xa0km Moho offset across the Jinsha River Suture and gently northward dipping Moho beneath the Qaidam Basin. A region in the central Qiangtang Terrane with higher than normal crustal Vp/Vs ratio of ∼1.83 can be the result of the Eocene magmatic event. In the Qiangtang Terrane, we observe a significant lithospheric mantle discontinuity beneath the Bangong-Nujiang Suture at 80xa0km depth which dips ∼10° to the north, reaching ∼120xa0km depth. We interpret this feature as either a piece of Lhasa Terrane or remnant oceanic slab underthrust below northern Tibet. We detect a ∼20xa0km depression of the 660-km discontinuity in the mantle transition zone beneath the northern Lhasa Terrane in central Tibet, which suggests this phase transition has been influenced by a dense and/or cold oceanic slab. A modest ∼10xa0km depression of the 410-km discontinuity located beneath the northern Qiangtang Terrane may be the result of localized warm upwelling associated with small-scale convection induced by the penetration of the sinking Indian continental lithosphere into the transition zone beneath the central Tibetan Plateau.

Indian mantle corner flow at southern Tibet revealed by shear wave splitting measurements

[1]xa0We constrain anistropic seismic structure in the southernmost Tibet and the Tethyan Himalays using SKS and SKKS phases recorded from a temporary seismic network, operated from July 2004 to August 2005 in conjunction with the Hi-Climb project. Shear wave splitting is not detected at 16 stations, most of which are located near and to the north of the Indus-Yalong suture (IYS). For the first time anisotropy with an N-S fast direction and 0.4 ∼ 1 s delay times is observed to the region about 50 km south of the IYS. This weak anisotropy correlates with the flat part of the subducting Indian lithosphere and could be caused by northward asthenospheric flow and shear at the base of the Indian lithosphere. The null measurements in the vicinity of the IYS are most likely the result of a vertical asthenospheric flow, the corner flow induced by the subvertical subduction of the Indian lithosphere just south of the Bangong-Nujiang suture (BNS). Observations of the systematic changes in mantle anisotropy from southern Tibet to the central Tibet provide evidence for different mantle flow fields between the Indian and the Eurasian asthenospheric mantle, which are separated by a subvertical Indian lithosphere at the BNS.

Crustal and mantle velocity models of southern Tibet from finite frequency tomography

[1]xa0Using traveltimes of teleseismic body waves recorded by several temporary local seismic arrays, we carried out finite-frequency tomographic inversions to image the three-dimensional velocity structure beneath southern Tibet to examine the roles of the upper mantle in the formation of the Tibetan Plateau. The results reveal a region of relatively high P and S wave velocity anomalies extending from the uppermost mantle to at least 200 km depth beneath the Higher Himalaya. We interpret this high-velocity anomaly as the underthrusting Indian mantle lithosphere. There is a strong low P and S wave velocity anomaly that extends from the lower crust to at least 200 km depth beneath the Yadong-Gulu rift, suggesting that rifting in southern Tibet is probably a process that involves the entire lithosphere. Intermediate-depth earthquakes in southern Tibet are located at the top of an anomalous feature in the mantle with a low Vp, a high Vs, and a low Vp/Vs ratio. One possible explanation for this unusual velocity anomaly is the ongoing granulite-eclogite transformation. Together with the compressional stress from the collision, eclogitization and the associated negative buoyancy force offer a plausible mechanism that causes the subduction of the Indian mantle lithosphere beneath the Higher Himalaya. Our tomographic model and the observation of north-dipping lineations in the upper mantle suggest that the Indian mantle lithosphere has been broken laterally in the direction perpendicular to the convergence beneath the north-south trending rifts and subducted in a progressive, piecewise and subparallel fashion with the current one beneath the Higher Himalaya.

Earthquake distribution in southern Tibet and its tectonic implications

[1]xa0A temporary 37-station seismic array was operated in southern Tibet from June 2004 to August 2005 close to Xigaze along a traverse from Tangra Yum Co rift in the west to Yadong-Gulu rift in the east. This lateral array of the international Hi-CLIMB project recorded a total of 885 local earthquakes during the 14-month deployment. Hypocenter locations of these events were obtained using HYPOINVERSE2000, and about half of them were relocated by the joint hypocenter determination method. Lateral variations in crustal seismic P and S wave velocities beneath the array were obtained simultaneously as part of the station corrections during the relocation procedure. While earthquakes were scattered in the region, more than 250 earthquakes were clustered within a small area at about 50 km north of the Indus-Yalu Suture and west of the Pumqu-Xianza rift. Crustal and uppermost mantle earthquakes are concentrated beneath the Himalayan crest. Projection of all earthquakes to a north–south profile shows that the subducting Indian plate passes the Indus-Yalu Suture below the Lhasa Terrane. While earthquakes are concentrated in the upper crust and upper mantle beneath the Himalayas, seismicity is mostly restricted to the upper crust to the north, which is consistent with a ductile middle/lower crust beneath southern Tibet. About 20 deep crustal and uppermost mantle events (at depths of 50–70 km) were observed to be associated with the subducting Indian lithosphere. Our relocation results of these local earthquakes in southern Tibet support the popular “jelly-sandwich” model for the rheology of continental lithosphere in which the strong seismogenic layers of upper curst and uppermost mantle are separated by a weak lower crust.

Crustal structures across the western Weihe Graben, North China: Implications for extrusion tectonics at the northeast margin of Tibetan Plateau

The stable Ordos Plateau, extensional Weihe Graben, and Qinling orogenic belt are located at the northeast margin of the Tibetan Plateau. They have been thought to play different roles in the eastward expanding of the Tibetan Plateau. Peking University deployed a linear seismic array across the western end of the Weihe Graben to investigate the crustal structures of the tectonic provinces of this structure. Receiver function analyses revealed low-to-moderate Poissons ratios and anticorrelations between Poissons ratios and topography beneath the Qinling Orogen. These features may indicate a tectonic thickening of the felsic upper crust by folding and thrusting within the Qinling Orogen. We observed a strong horizontal negative signal at the midcrust beneath the Ordos Plateau which may indicate a low-velocity zone. This observation would suggest the stable cratonic Ordos Plateau had been modified due to the compression between the Tibetan Plateau and the Ordos Plateau. We also observed an abrupt 4u2009km Moho offset across the Weihe Fault, changing from ~44u2009km beneath the Ordos Plateau to ~40u2009km beneath the Qinling Orogen. We conclude that the Weihe Fault is a lithosphere-scale fault/shear zone, which extends into the upper mantle beneath the Weihe Graben. It acts as the major boundary separating the stable Ordos Plateau and the active Qinling Orogen.

Delamination of southern Puna lithosphere revealed by body wave attenuation tomography

The southern Puna Plateau has been proposed to result from a major Pliocene delamination event that has previously been inferred from geochemical, geological, and some preliminary geophysical data. Seventy-five seismic stations were deployed across the southern Puna Plateau in 2007–2009 by scientists from the U.S., Germany, Chile, and Argentina to test the delamination model for the region. The Puna passive seismic stations were located between 25 and 28°S. Using the seismic waveform data collected from the PUNA experiment, we employ attenuation tomography methods to resolve both compressional and shear quality factors (Qp and Qs, respectively) in the crust and uppermost mantle. The images clearly show a high-Q Nazca slab subducting eastward beneath the Puna plateau and another high-Q block with a westward dip beneath the Eastern Cordillera. We suggest that the latter is a piece of delaminated South American lithosphere. A significant low-Q zone lies between the Nazca slab and the South American lithosphere and extends southward from the northern margin of the seismic array at 25°S before vanishing around 27.5°S. This low-Q zone extends farther west in the crust and uppermost mantle at the southern end of the seismic array. The low-Q zone reaches ~100u2009km depth beneath the northern part of the array but only ~50u2009km depth in the south. Lateral variations of the low-Q zone reflect the possible mechanism conversion between mantle upwelling related to delamination and dehydration. The depth of the Nazca slab as defined by Q images decreases from north to south beneath the plateau, which is consistent with the steep-flat transition of the angle of the subducting slab as defined by previous earthquake studies.

SANDWICH: A 2D Broadband Seismic Array in Central Tibet

ABSTRACT The tectonic processes that formed the Tibetan plateau have been a significant topic in earth science, but images of the subducting Indian continental lithosphere (ICL) are still not clear enough to reveal detailed continental collision processes. Seismological methods are the primary ways to obtain images of deep crust and upper‐mantle structures. However, previous temporary seismic stations have been unevenly distributed over central Tibet. The Institute of Geology and Geophysics, Chinese Academy of Sciences, has initiated a 2D broadband seismic network in central Tibet across the Bangong–Nujiang suture to fill in gaps among earlier north–south linear profiles for the purpose of detecting the lateral variation of the northern end of the subducting ICL. The health status for each station has been checked at each scheduled service trip. The noise level analysis shows a quiet background in central Tibet, with low cultural noise. Preliminary earthquake locations indicate that they are crustal and broadly distributed rather than only occurring along major faults, suggesting a diffused deformation in the conjugated strike‐slip fault zone. Preliminary receiver function analysis shows a complicated crust with significant east–west lateral variations.

Limited southward underthrusting of the Asian lithosphere and material extrusion beneath the northeastern margin of Tibet, inferred from teleseismic Rayleigh wave tomography

The northeastern margin of Tibet is of key importance in understanding the uplift and expansion of Tibetan Plateau. In this research, we perform Rayleigh wave tomography from 20xa0s to 167xa0s using the data recorded by China Digital Seismic Network and several portable seismic arrays in Northeastern Tibet and adjacent areas. Our resultant 3-D shear wave velocity model exhibits strong lateral variations in the lithosphere. High shear wave velocities are observed in the mantle of the cratonic Ordos and Yangtze Blocks, as well as the East Qinling Orogen. The Qilian Orogen is underlain by a ∼120xa0km thick lithosphere of intermediate velocities and a low-velocity layer of asthenospheric mantle below the depth of 120xa0km. Low velocities are characteristic in the middle-to-lower crust of the Songpan-Ganzi Terrane and the West Qinling Orogen, possibly representing the crustal flow penetrating the Kunlun Fault. An uppermost mantle low-velocity zone is found in the same location, indicating either the asthenosphere upwelling or the transitional crust-mantle boundary layer. Our observations have two tectonic implications. First, the large-scale eastward Tibetan material extrusion into Eastern China through the East Qinling Orogen is unlikely to be an ongoing process above 200xa0km depth; second, the southward subduction of the North China Craton toward Northeastern Tibet may not have fully developed in this region. Furthermore, we propose that the difference in the lithospheric strength between the Qaidam Block and the West Qinling Orogen is a critical element to control the tectonics in the northeastern margin of Tibet.