Crustal structure and deformation beneath eastern and northeastern Tibet revealed by P-wave receiver functions

Abstract

The present-day Tibetan crust records the shallow response of the Cenozoic continental collision between the Indian and Eurasian plates. An analysis of the deep crustal structure beneath eastern and northeastern Tibet is of vital significance for studying the geodynamic processes of crustal thickening and expansion of the Tibetan Plateau. We herein provide detailed images of the crustal structure of eastern and northeastern Tibet and the adjacent Sichuan Basin using teleseismic P-wave receiver function (P-RF) data from a NW–SE-trending linear seismic array. Our P-RF imaging result reveals distinct structural features of the study region, including marked lateral variations in the depth to basement beneath the Songpan–Ganzi block and the Sichuan Basin, a seismically slow mid-lower crust beneath the Songpan–Ganzi block and a low-velocity anomaly just above the Moho around the easternmost Kunlun fault area, and obvious Moho offsets near the boundaries of tectonics blocks. These structural features may reflect various crustal responses within the continental interior to the India–Eurasia collision at the plate margin. The rigid crust of the Sichuan Basin might have wedged into the Tibetan crust in the Longmenshan area, which probably facilitated crustal thickening and enabled channelized mid-lower crustal flow in the Songpan–Ganzi block to the west. Being a pre-existing tectonic boundary, the Kunlun fault could have acted as a focus of heating and hot mantle upwelling associated with the deep processes of the Indian plate underthrusting and subduction, possibly resulting in localized weakening and modification of the lower crust around this fault area. The observed significant differences in the crustal structure of eastern and northeastern Tibet suggest that crustal shortening in this region may have been absorbed by not only vertical thickening in the interiors of the tectonic blocks but also complex local deformation along the boundary zones.