地幔上涌对鄂尔多斯西缘岩石圈的改造:来自远震多尺度层析成像的证据

Rebuilding of the lithosphere beneath the western margin of Ordos:Evidence from multiscale seismic tomography

本文利用喜马拉雅二期科学探测台阵的678个地震台站及26个固定台站记录到的9,641个地震共约160000条远震P波走时数据,采用基于稀疏约束的多尺度层析成像方法,获得了鄂尔多斯西缘及邻区上地幔800km深度范围内P波速度结构.结果显示,在东经104°附近阿拉善地块与鄂尔多斯盆地间存在岩石圈深度的构造边界,这表明阿拉善地块与鄂尔多斯可能分别从属于不同的大地构造单元.以北纬38°线为界,鄂尔多斯地块西缘在岩石圈范围内南北存在明显的速度差异,鄂尔多斯南部上地幔200~300km深度范围显示为高速异常,而鄂尔多斯北部上地幔显示大面积的低速异常.这一现象表明,鄂尔多斯地块南北两部分经历了不同的构造演化过程.根据本文的结果可以进一步推断,由于青藏高原、阿拉善地块向东北方向推挤以及岩石圈的拆离引起的上地幔扰动导致了地幔上涌,上涌的热物质改造了鄂尔多斯西北缘地区的岩石圈,并使该区的岩石圈减薄.地幔上涌也可能是东经104°边界带和北纬38°构造带形成的深部动力学因素.

Probing the fine structure of the lithosphere-asthenosphere system is important for understanding the evolution of the western Ordos block.We perform high-resolution multi-scale teleseismic traveltime tomography with P-wave arrival time recorded by 678 stations of Himalayan Scientific Seismic Array and 26 stations of a regional seismic network in the western Ordos region.Totally 160,000 P-wave relative traveltimes from 9,641 teleseismic events recorded by 704 seismic stations are available.The CRUST1.0 model is used to correct the contribution of crust structure to the teleseismic relative travel times.We use the multi-scale seismic tomography method with sparsity constraint to obtain the P-wave velocity structure of the upper mantle to depth of 800 km.Our results show that there exist a clear boundary between Alxa and Ordosalong 104°E in upper mantle.Along the 38°N tectonic line,there is a different structure between south part and north part of Ordos in the upper mantle,of which the south shows a high-velocity feature and the north exhibits low-velocity.This feature can be so interpreted that the south part and north part of Ordos have different tectonic evolution processes.Furthermore,our tomography results suggest that the hot upper mantle upwelling material caused by the lithosphere detachment in northeastern Tibet might have rebuilt the north part of Ordos lithosphere and thinned the lithosphere in this area.Upwelling of the upper mantle may also be a dynamic factor at depth for forming the 104°E boundary and the 38°N tectonic line.