Crustal structure beneath SE Tibet revealed by receiver functions

Abstract:

To interpret the geodynamic processes of the Tibetan plateau uplift, several mechanisms have been proposed, such as the rigid block extrusion or mid-lower crustal flow model. Recently the presences of mid-lower crustal ductile zones (exhibited as low velocity zones) were detected by various geophysical methods in SE Tibet. However, because of the relatively low spatial resolution of previous studies, the detailed variations of crustal structure as well as the relations between surface faults and ductile zones are poorly understood. Here, we utilized broadband waveform data recorded by a newly deployed dense seismic array (ChinaArray) in SE Tibet. We produced high-resolution images of crustal structure along two profiles by common-conversion-point (CCP) stacking from 4566 Ps receiver functions. The most conspicuous features are as follows: (1) the Moho depth changes abruptly across the Red-River fault (~10 km offset from ~38 to 48 km) and also the Lijiang-Xiaojinhe fault (~13 km offset from ~64 to ~51 km); (2) The low velocity zones are only visible beneath the Chuandian block, which are bounded by the Red-River fault and the Xiaojiang fault. These two faults may restrict the lateral extensions of the low velocity zones; (3) Discontinuous low velocity zones are observed beneath the Qiangtang block and the Yangtze craton respectively. As a suture zone between the Qiangtang block and the Yangtze craton, the Lijiang-Xiaojinhe fault may cut through the whole crust. We propose that the southeastward extrusion of the Qiangtang block is resisted by the Yangtze craton beneath Lijiang-Xiaojinhe fault.