Modern Stress Fields In Western Himalayan Syntaxis And Southern Tibet And Numerical Modeling Of Double-Sided Subduction And Low-Velocity Body

The convergence and collision of India and Eurasia plate formed the Tibetan-Himalayan orogen,which is one of the most significant geological events since the Cenozoic and affects the regional tectonics,and global climate and environment.The continental collision has created complex deep lithospheric structures in the Tibetan-Himalayan orogen,especially resulting in the different surface pattern and deep structure between the interior and margin of the orogen.Therefore,the identification of deep lithospheric structures at different locations of the Tibetan-Himalayan orogen is a key factor for understanding the modes of continental collision and the mechanisms of plateau growth and uplift.Bearing these in mind,I employed various methods such as focal mechanism solution,stress field analysis,and numerical simulation to explore the lithospheric structures,geodynamics and seismogenic mechanisms in the Pamir-Hindu Kush region of the western Himalayan syntaxis and the southern part of the Tibetan Plateau.The Pamir-Hindu Kush region at the western end of the Tibetan-Himalayan orogen is one of the most active regions on the globe with strong seismicity and deformation and provides a window to evaluate continental collision linked to two intra-continental subduction zones with different polarities.The seismicity and seismic tomography data show a steep northward subducting slab beneath the Hindu Kush and southward subducting slab under the Pamir.Here,I collect 3988 seismic catalogs to compute seismicity images and 926 earthquake events to invert focal mechanism solutions and stress field with a view to characterize the subducting slabs under the Pamir-Hindu Kush region.The results define two distinct seismic zones:a steep one beneath the Hindu Kush and a broad one beneath the Pamir.Deep and intermediate-depth earthquakes are mainly distributed in the Hindu Kush region which is controlled by thrust faulting,whereas the Pamir is dominated by strike-slip stress regime with shallow and intermediate-depth earthquakes.The area where the maximum principal stress axis is vertical in the southern Pamir corresponds to the location of a high-conductivity low-velocity region that contributes to the seismogenic processes in this region.I interpret the two distinct seismic zones to represent a double-sided subduction system where the Hindu Kush zone represents the northward subduction of the Indian plate,and the Pamir zone shows southward subduction of the Eurasian plate.A transition fault is inferred in the region between the Hindu Kush and the Pamir which regulates the opposing directions of motion of the Indian and Eurasian plates.Besides,a subduction model in the Pamir-Hindu Kush region based on Stokes fluid is established by ASPECT,which realizes the transformation of the subducting slab from the Indian to the Eurasian during the subduction process.The research shows that the occurrence of subducting slab transition has specific conditions,which is that the lithosphere has a low strain rate,and the strain rate at the collision site reaches 10^-11s^-1.This also indicates a strong uplift in the Pamir region occurred after the southward subduction of Eurasian plate.The southern part of the Tibetan-Himalayan Orogen in this research includes the south Qiangtang,Lhasa and Himalayan blocks.Previous multidisciplinary work studied the deep lithospheric structure in this region and identified the extensive distribution of high-conductivity and low-velocity bodies（LVBs）at the crustal depth.These LVBs are distributed interphase with normal layers and used as evidence to propose a channel flow model.In this study,I first used the acquired seismic waveform data to invert the focal mechanism solution in the southern Tibetan Plateau,and then combined with the published data to calculate the stress field state in the region.The results show that the study area is dominated by normal fault type and strike-slip type earthquakes,and the amounts of the them are almost the same;at depth,the earthquakes are mainly developed in the upper and middle crust.These indicate that under the overall strike-slip geological background in the southern part of the Tibetan Plateau,there are several spatially discontinuous regions with normal fault stress states.Combined with other geophysical data,I suggest that these regions correspond to high-conductivity and low-velocity bodies distributed in the upper and middle crust.The discontinuities of these regions imply the discontinuities of the ductile rheological layer within the crust,and therefore it is unlikely to trigger large-scale lower crustal flows throughout the entire southern Tibetan Plateau.In order to study the influence of high-conductivity low-velocity bodies that exist widely in the Tibetan Plateau on lithospheric deformation,a generalized high-conductivity low-velocity body model is set up.The results show that the high-conducting low-velocity body is related to the occurrence of earthquakes and the formation of active faults.The energy release inside the high-conducting low-velocity body triggers earthquakes,which cause surface rupture to form faults and rupture first from the brittle domain.The area where the high-conducting low-velocity body is located has a slow uplift rate and forms the subsidence basin in the compression zone.According to the seismic velocity data,five high-conducting and low-velocity bodies are also set in the upper crust of the southern Tibetan Plateau.The results show that the presence of high-conducting low-velocity bodies can change the stress state in the lithosphere.The maximum principal stress axis of the brittle domain above the high-conducting low-velocity body shows a high dip angle or even vertical orientation,and when the low-velocity body is large in scale,it will affect the stress state of the crust and lithospheric mantle below it.Influenced by the high-conducting low-velocity body,a large strain rate of about 3×10^-15s^-1(0.95×10^-7yr^-1)exists at the surface above a low-velocity body at the junction of south and north Tibet,and it is supported by observational geophysical data.