基于P波三重震相研究青藏高原上地幔速度结构及其动力学意义

青藏高原因其复杂的结构和演化历史，一直都是研究大陆碰撞、构造运动及其动力学的热点区域。本文采用三重震相波形拟合技术，基于中国地震观测台网和大型流动台阵记录到的某地震P波垂向记录，获得了包括拉萨、南羌塘和松潘甘孜地块在内的青藏高原上地幔P波速度结构。结果表明:①拉萨和南羌塘地块下方地幔过渡带存在高速异常，推测是俯冲的印度板片滞留体，过渡带底部的板片残余温度较低，使得660-km相变滞后约3～8km。而松潘甘孜地块下方过渡带同样存在高速异常，可能是欧亚岩石圈发生拆沉进入地幔过渡带所致。这说明印度板块俯冲作用的影响已经到达地幔过渡带，其俯冲前缘位于班公怒江缝合带附近。②从拉萨、南羌塘到松潘甘孜地块，200km之上的地幔岩石圈高速盖层速度由南向北逐渐减小，松潘甘孜地块则出现盖层缺失。推测受小规模地幔对流或者热不稳定性的影响，在南羌塘和松潘甘孜地块，增厚的欧亚岩石圈发生拆沉作用，岩石圈被减薄和弱化，造成羌塘地块上地幔低速和松潘甘孜地块上地幔高速盖层缺失。拆沉的冷的欧亚岩石圈可能部分停留在410-km上方，使得410-km抬升约10km，部分沉入地幔过渡带，表现为松潘甘孜地块地幔过渡带中存在高速异常。低温造成660-km下沉约8km，导致地幔过渡带增厚。 

Upper Mantle Velocity Structure and Its Dynamic Implications beneath the Tibetan Plateau from Triplicated Seismic P Waveforms

Due to its complex structure and evolution history, Tibetan plateau has always been a focused region for study continental collision, tectonic movement and its geodynamics. Based on the the vertical P wave records from a selected earthquake recorded by CDSN (Chinese Digital Seismic Network) and temporary seismic stations, the high resolution upper mantle structures beneath the Tibetan Plateau including the Lhasa Terrane,south Qiangtang terrane and Songpan-Ganzi terrane are obtained by triplication waveform fitting technique. Our results show that: (1) The high velocity anomaly observed at the bottom of MTZ (mantle transition zone) beneath Lhasa and south Qiangtang terranes are interpreted as the remnants of the subducted Indian slab,and these cold material depressed the post-spinel phase transition of 660km discontinuity by 3~8km. The high velocity anomaly can also be observed beneath Songpan-Ganzi terrane, which may be caused by the delamination of thickened Eurasian lithosphere. It suggests that the influence of Indian slab subduction has reached the MTZ, and the subduction front beneath the Tibetan Plateau is located near the Bangong-Nujiang suture. (2) From Lhasa, south Qiangtang to Songpan-Ganzi terranes, the upper-mantle velocities over 200km decreases gradually along the N-S profile, and the high-velocity lid even disappears in Songpan-Ganzi terrane. Influenced by the small-scale mantle convection or thermal instability, the thickened Eurasian lithosphere beneath south Qiangtang and Songpan-Ganzi terranes was delaminated, and the lithosphere was thinned and weakened, resulting in the low velocity of upper mantle in Qiangtang and the absence of high-velocity lid in Songpan-Ganzi terrane. Some of the delaminated lithosphere maybe now sitting atop of the 410km discontinuity, causing the uplift of 410km discontinuity by about 10km and others may have penetrated into the MTZ, characterized by the high velocity in the lower MTZ beneath Songpan-Ganzi Terrane. Low temperature causes the depression of 660km discontinuity by about 8km, resulting in the thickening of the MTZ.