青藏高原东部P波速度结构及其对高原隆升的启示

A P-wave velocity study beneath the eastern region of Tibetan Plateau and its implication for plateau growth

利用青藏高原东部的高密度流动台站数据和位于研究区范围内固定台站的同期数据,采用远震体波走时层析成像的方法,反演获得了青藏高原东部及其周边高分辨的壳幔P波速度异常结构,结果显示:研究区的速度异常主要集中在地壳及上地幔顶部.高原内部强烈变形的区域存在深达200 km左右的低速异常,而高原周边克拉通性质的块体下方存在深达200~300 km的高速异常,高、低速异常和构造块体有较好的对应关系.在400 km以下区域,速度异常的幅值和范围都很小,可认为其速度值与全球平均模型一致,不存在明显的高速俯冲物质和低速地幔上涌物质.青藏高原东部的隆升成因主要由浅部的低强度岩石圈变形所导致,由于同时受到印度板块推挤和周边刚性块体的制约,青藏高原下方低强度岩石圈的水平缩短吸收印度板块的推挤作用,垂直向伸展导致高原隆升.

The eastern region of Tibetan Plateau（ERTP） and surrounding regions have gentle to moderate slope in topography,contrasting to the steep margins in the north and south. Many geodynamic models have been proposed to explain plateau growth in the east part of Tibetan Plateau and no consensus have yet been reached. In part, this is because of lack of detailed tomography images of the deep structure, which is essential for understanding the deep dynamics that govern the evolution of ERTP.Two stages of Chin Array have been conducted from 2011 to 2015. Nearly 1000 temporary seismic stations, with an average spacing ~40 km, were installed in ERTP and its adjacent regions. In addition, there was 297 temporary stations deployed in eastern margin of Tibetan Plateau during September 2006 to July 2009. Additional data from 271 permanent stations in 2007-2015 were also included. The overall station distribution densely covers the northeastern, eastern and southeastern margins of Tibetan Plateau.We imaged high-resolution 3 D P-wave velocity structures of the crust and upper mantle beneath ERTP and its surrounding regions through teleseismic traveltime tomography. The horizontal and vertical resolutions are 0.8° and 80 km,respectively. The imaged velocity anomalies correlate well with geological blocks. Slow velocity anomalies, extending down to ~200 km, are found beneath the Tibetan Plateau. Fast anomalies associated with cratonic blocks around the plateau are imaged within 200-300 km depth. Below 400 km there are less velocity anomalies, indicating a normal mantle close to the global average. Neither fast subducted slabs nor slow mantle upwellings are obeserved beneath the study region. The plateau growth is mainly caused by the deformation of the weak lithosphere at the shallow depth. Surrounded by the Indian plate and surrounding rigid blocks, the weak lithosphere beneath Tibetan Plateau is horizontally shortened and vertically stretched. The horizontal shortening helps accommodate the ongoing collision between the Indian and Asian plates and the plateau growth is resulted from the vertical stretching.