Interior Structure Of The Tibetan Plateau Revealed By Seismic Imaging

The continental collision between India plate and Asia plate has contributed to the growth of the Tibetan Plateau,which is the most prominent uplift worldwide since Cenozoic.Deformation mechanism beneath Tibet is the key factor for understanding the mode of continental collision and the uplift of the plateau.The plateau elevation and tectonics as well as volcanic rocks observed in Tibet provided directive evidence to the surface deformation.Therefore,the subsurface deformation and structure of Tibet plays a crucial role in investigating post-collisional effects.Crust and upper mantle structures are thus an indispensable part to comprehend the motion of materials which comes from the convergence of India and Asia.As one of the most effective technique to probe the subsurface structure,seismic tomography illuminates the earth‘s interior by imposing constraints on the composition of various layers within the earth.In addition,seismic tomography also advances the cognition of the tectonic evolution and mechanism of strong earthquakes as well as the deep geodynamic features.In this study,both the adjoint waveform tomography and the regional double-difference travel-time tomography were applied to acquire the multi-scale deep structure of the Tibetan plateau.Our seismic data included waveform of large earthquakes and travel-time of local earthquakes,which was recorded either by large-scale temporary seismic observations or permanent stations.We present a new 3D radial anisotropy seismic model ?TP2019? of Tibet,which using a frequency-dependent waveform tomographic technique and 3 large-scale dense arrays from ChinArray.Based on the high-performance compute,the total computational time of our inversion is nearly 836000 cpu hours.The new images show obvious east-west geometrical change of the underthrusting India lithosphere,which extends to Pamir in western Tibet while only reaches north Lhasa in central and eastern Tibet.A significant feature of our model is a sub-vertical subduction of Indian mantle lithosphere following convective removal of thickened Tibet lithosphere in north Lhasa,which may relate to the wide spread magmatism and rapid uplift of central Tibet in late Miocene.Lithosphere delamination beneath Longmenshan fault belt might contribute to the surface evolution between eastern Tibet and surrounding rigid blocks.Crustal structure including P wave velocity,S wave velocity and Poisson‘s ratio of the northeastern Tibetan Plateau was retrieved from double-difference travel-time tomography.In this research,we collected almost 500 thousand travel-time data from 2009 to 2017 and the results shows that seismic velocity at shallow depth coincides well with sedimentary thickness.Tectonic earthquakes in the northeastern Tibet mainly occurred in the Hexi Corridor after relocation,which is a low Poisson‘s ratio region.An acid crust beneath the northeastern Tibetan plateau indicates partial melts might not happen and crust thickening mainly took place in the middle or lower crust.Tomographic images from wave equation-based waveform tomography and ray theory-based travel-time tomography reveals that the seismic structure of crust and upper mantle beneath Yangtze block and Ordos block share a vertically coherent deformation mode and act as a rigid block of the northeastern Tibetan Plateau.The distinct lateral heterogeneity indicates strong deformation probably occurred within Alxa.The low velocity from middle-crust to upper mantle underneath Qilian orogen of northeastern Tibetan Plateau may relate to the interaction between Alxa and the Tibetan Plateau.Haiyuan fault was not the northeastern boundaries of the lateral expansion of Tibet,even if it may cut throughout the whole crust.Owing to the power of dense array and advanced seismic tomographic methods,we obtained multi-scale seismic structure of Tibet.Our new seismic model leads to new insights about the lateral expansion pattern of deep materials beneath Tibet and should provide tighter seismic constraints on deformation mechanism induced by continental collision.