西藏西部地区的远震P波层析成像研究

Structure of western Tibet from P wave teleseismic tomography

采用快速行进法正演计算了西藏西部地区的远震P波理论走时,随后基于该地区台阵提取到的相对到时残差利用子空间迭代算法反演得到该地区的地壳上地幔速度相对分布。层析成像结果显示:西藏西部地区在下地壳深度显示高速异常,上地幔深度该地区内部的高速异常范围从西往东逐渐减小,且研究区域东部存在高低速异常相间分布。据此认为印度板块在青藏高原内部近水平俯冲,西部俯冲范围较大,且俯冲过程中存在板块撕裂现象,撕裂的印度板块拆沉进入上地幔,而撕裂产生的间隙由于应力释放导致了西藏西部地区新生代裂谷的形成。

In this paper,the fast marching method(FMM)is used to calculate the theoretical travel time of the teleseismic P wave in our model.We inverted the relative travel-time residuals to obtain the relative velocity distribution of crust and upper mantle in western Tibet by subspace iterative algorithm.The results show that the high-velocity anomalies exhibit in the depth of lower crust of western Tibet,while in the depth of upper mantle,the range of the highvelocity anomaly gradually decreases from west to east.And there is an alternating distribution of high-and low-velocity anomalies in the east of our study area.Therefore it is considered that the Indian Plate subducted nearly horizontally within the Tibetan Plateau,and the subduction range in the west is larger.During the subduction process,the Indian Plate is being torn.Subsequently,the torn Indian Plate sank into the upper mantle,and the tearing gap affected the formation of Cenozoic rifts in western Tibet due to stress release.