Deformation of crust and upper mantle in central Tibet caused by the northward subduction and slab tearing of the Indian lithosphere: New evidence based on shear wave splitting measurements

摘要

Shear-wave splitting provides insight into geodynamic processes such as lithospheric deformation and upper mantle flow. This study presents shear wave splitting parameters determined from XKS (SKS, SKKS and PKS) and Pms phases (from receiver functions) recorded by the 2D seismic array, SANDWICH, deployed in central Tibet from November 2013 to April 2016. The XKS splitting measurements show a generally strong anisotropy with an average of 1.3 s, that even includes 17 stations with delay times no less than 2.0 s. Interestingly, significantly weak splitting is also observed between Nam Tso and Siting Tso near 90 degrees E. Spatial coherence analysis of the splitting parameters indicates an upper asthenospheric source as the primary cause of seismic anisotropy in the study region. The regionally dominant ENE-WSW oriented upper-mantle anisotropy can emerge from the corner flow in the overlying mantle wedge induced by the subduction of the Indian lithospheric slab. The crustal anisotropy obtained through Pms splitting analysis might reflect lattice preferred orientation of anisotropic minerals formed by the ENE oriented middle-lower crustal flow, with a minor contribution to the strong anisotropy. The weak splitting observations in the east are likely caused by the nearly vertical alpha-axis of olivine induced by upwelling asthenosphere at a slab tear in the Indian lithosphere. (C) 2019 Elsevier B.V. All rights reserved.