ALBERT

Description: We present direct observations of the structure of the tectonic plates beneath the Tibetan plateau using seismic waves converted at discontinuities beneath Tibet. By assembling sections of P-S and S-P receiver function sections we map the lithosphere-asthenosphere boundary (LAB), the Moho and the 410 and 660 discontinuities along a number of profiles that traverse the plateau. We have identified a Tibetan plate that is sandwiched between the Asian and Indian plates. The seismic velocities in the Tibetan plate are slower than in the Indian and Asian plates, suggesting a significant temperature difference and hence different rheological behaviour of the Tibetan plate. In the westernmost part of Tibet, the Asian plate is underthrusting beneath the Indian plate, while in central and eastern Tibet the Indian plate is underthrusting beneath the Tibetan plate. There are also indications of Asian plate subduction in northern Tibet that are connected with a thickening of the Tibetan plate. There is no indication of subduction at the boundary between eastern Tibet and the Sichuan Basin.

Description: The Tibetan Plateau is undergoing north–south shortening accompanied by west–east extension, as evidenced by the widespread development of north–south trending normal faults, grabens and rifts. While the mode of the north–south shortening has been the main focus of most international studies, knowledge of the deep structure beneath South Tibet is required for understanding the mechanism of the west–east extension. The onset of the north–south trending normal faulting is commonly taken as an indicator that the Tibetan Plateau was uplifted to a near-maximum elevation before entering a collapsing stage. Here we report on the receiver functions of a seismological experiment across the northern segment of the Yadong-Gulu Rift (YGR), one of the youngest rifts in South Tibet. The migrated receiver function images reveal that the YGR is a high-angle normal fault characterized by a 5-km Moho rise from its western to eastern flank, together with distinct differences in the crustal structure and intracrustal seismic conversion patterns between the two flanks. This highly asymmetric lithospheric structure suggests whole-crustal extension controlled by a simple/general shear rifting mechanism. This simple/general shear rifting in the YGR is attributed to an eastward (horizontal) shear at the base of the upper crust, as evidenced by the observed Tibetan GPS velocity field and our observation of shear wave splitting discrepancy among the upper crust, lower crust and lithospheric mantle. We propose that in the YGR, simple shear rifting accommodates the northward injection of the Indian lithosphere, which may suggest that the onset of the north–south normal faulting does not indicate gravitational collapse of the Tibetan lithosphere.

Description: 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 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 Lithophere-Asthenosphere Boundary (LAB) from 120 to 150 km beneath the Sichuan Basin and from 70 to 80 k 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.

Description: GPS displacement vectors show that Tibetan crust is squeezed in easterly direction by the northward motion of the Indian plate. The Sichuan Basin is resisting this stream and redirecting it towards Indochina. Seismic anisotropy shows that surface deformations continue to greater depth and indicate crust-mantle coupling. Here, we present results from a dense seismic receiver function profile from eastern Tibet across the Longmen-Shan Fault (LMS) into the Sichuan Basin. We find that the LMS extends down to at least 150 km and marks a sharp steplike boundary between the Tibetan and the Sichuan lithospheres (including a sharp step at the boundary between crust and mantle). That mode of collision between east Tibet and the Sichuan craton is thickening of the Tibetan lithosphere in contrast to subduction at the India-Tibet boundary. Furthermore we find that the mantle transition zone (MTZ, between 410 and 660 km depth in global average) is beneath the Sichuan Basin 30 km thicker (corresponding to 250-300 K lower temperature) than beneath the Sichuan Basin. The reason for this is unclear, as well as a possible connection between the change in MTZ thickness and the step of the lithosphere directly above. Receiver function data in eastern China make it unlikely that flat Pacific subduction is the cause of MTZ thickening below the Sichuan Basin, as it was indicated by tomography. We propose that thinning of the MTZ below eastern Tibet could be caused by dynamical suppression of the 410 discontinuity due to a vertical component of the eastern Tibetan escape flow.

Description: The fate of the colliding Indian and Asian tectonic plates below the Tibetan high plateau may be visualized by, in addition to seismic tomography, mapping the deep seismic discontinuities, like the crust-mantle boundary (Moho), the lithosphere-asthenosphere boundary (LAB), or the discontinuities at 410 and 660 km depth. We herein present observations of seismic discontinuities with the P and S receiver function techniques beneath central and western Tibet along two new profiles and discuss the results in connection with results from earlier profiles, which did observe the LAB. The LAB of the Indian and Asian plates is well-imaged by several profiles and suggests a changing mode of India-Asia collision in the east-west direction. From eastern Himalayan syntaxis to the western edge of the Tarim Basin, the Indian lithosphere is underthrusting Tibet at an increasingly shallower angle and reaching progressively further to the north. A particular lithospheric region was formed in northern and eastern Tibet as a crush zone between the two colliding plates, the existence of which is marked by high temperature, low mantle seismic wavespeed (correlating with late arriving signals from the 410 discontinuity), poor Sn propagation, east and southeast oriented global positioning system displacements, and strikingly larger seismic (SKS) anisotropy.