| Abstract:The large amount of surface uplift, deformation and strike-slip faulting around the southeast of Tibetan plateau margin is generally believed to be associated with the Cenozoic collision between Indian and Eurasian. However, details on the linkage between the surface deformation and the collision are not well understood, largely due to the lack of observations on subsurface deformation, especially in the deep crust and upper mantle. Studying the dynamics processes involoved in the deformation is one of the highlighted fields both in seismology and geodynamic field.We analyze a large amount of receiver function data recorded by regional seismic networks of the China Earthquake Administration (CEA), specifically, the provincial networks of the Sichuan, Yunnan, Guangxi, Guizhou, Guangdong and Hunan provinces. These data come from earthquakes occurring between July of2007and July of2010with broadband stations. We develop a comprehensive analysis method that facilitates the robust extraction of azimuthally 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 crustal seismic anisotropy with a splitting time of0.24-0.9s beneath the SE margin of the Tibetan plateau, and the Moho depth and Vp/Vs become weak from the SE Tibet plateau margin to southeast. These results shed new light on the deformation process of the Tibetan plateau and provide strong evidence for a lower crust flow 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. Mantle deformations are either weak or predominantly vertical and are obviously different from those seen in the crust. A vertically flow in the upper mantle, combined with the observation of a thin lithosphere beneath the area, leads to the inference that part of the mantle lithosphere may have delaminated and is descending into the 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 Tibetan plateau and the surrounding regions. These observations indicate the existence of a lower crustal flow may be present beneath the SE Tibetan plateau, and that the mantle lithosphere may have been mechanically decoupled from the upper crustal motions.In order to understand lithosphere deformation process associated with the uplift of the plateau and the relationship between the lower crust deformation and the lithosphere. We apply the finite frequency tomography method to the S waves data with differential travel times between pairs of stations in the inversion to eliminate traveltime anomalies resulting from heterogeneities outside the study area. We choose three different frequencies for ajoint inversion, there are0.1-0.5hz,0.05-0.1hz,0.02-0.05hz. To ensure the above assumption to be valid for a large-scale study area, we improve in selecting proper station pairs base on the traditional method. The stations loop in different events are designed and kept the surface distance between the neighboring stations pairs in loops within the shortest is larger than200km,300km,400km, respectively. We then conduct a joint inversion with three differential time data. Our results are consistent with previous tomography in terms of large-scale seismic anomalies, such as a high velocity anomaly beneath the Sichuan basin in the uppermost mantle and a high velocity anomaly in the transition zone that may be associated with the subducted Paleo-Pacific plate beneath the Yangtze craton. In addition to these known structures, we find relatively large-scale high velocity bodies inside the upper mantle beneath the SE margin of the Tibetan Plateau, along the longitude101°from the south to north. In particular, our images show continued high velocity bodies at～100km deep beneath latitude27°N, and further to the north, the latter high velocity body around32°N located at～350km, right below a large low velocity anomaly. Although other seismic observations are required to better constrain the nature of these high velocity structure, one possible scenario is that they may be drips or delaminated pieces of the continental lithosphere, as the consequence of the progressive uplift of the plateau.With the upwards systemic studies, we can get a comprehensive understanding the Southeast of Tibetan plateau. After the Indian-Eurasian collision, the lower crust flow beneath the SE Tibetan plateau margin was caused by the extrusion of Indian plate and the deformation of Eurasian crust. With this lower crust flow working, the Tibetan plateau is uplift. On the other hand, high density of the lower lithosphere detached from the upper lithosphere under downward pulling of gravity, which destroys the negative buoyancy, and then the surface of Tibetan plateau is uplifted with the positive buoyancy. In summary, uplift of the southeast borderland of the Tibetan Plateau was thought to have intensified since the late Tertiary as the whole and local deformation associated with the Indian-Eurasian collision accelerated in the region. Therefore, the uplift of Tibetan plateau is the result of joint action with multiple mechanisms, and multi-stage process with non-uniform and different velocity. |
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