Deep crustal structure across northeastern Tibet from P receiver functions

Abstract

The uplift mechanism and deep crustal deformation of the northeast of the Tibetan plateau in response to the Cenozoic India-Asian collision are still poorly constrained. Previous studies linked the uplift mechanism of the northeast of Tibet to the southward underthrusting of the Asian mantle lithosphere and the northward subduction of the Indian plate; yet, the details of crustal properties and thickening mechanism beneath the northeast of Tibet remain controversial. To shed light on the uplift mechanism of the northeast of the Tibetan plateau, we present a detailed image of the crustal structure beneath northeastern Tibet using teleseismic P receiver functions from 54 temporary broadband seismic stations. The Moho beneath the Kunlun Shan reaches ∼66 km but abruptly shallows to ∼40 km at the conjunction of the Kunlun and Qaidam, marking the boundary of northern and central Tibet. The Moho has a convex shape beneath the Qaidam basin and gently undulates northward beneath the Qilian Shan, and substantially shallows to ∼48–50 km under the Alashan block without any Moho offset. The undulating Moho and the absence of any Moho disturbance suggest that crustal thickening and shortening is the main driving mechanism for the northeastern plateau uplift. The average Vp/Vs ratio (∼1.75) beneath northeastern Tibet is indicative of a felsic to intermediate composition without a prominent low-velocity zone or low-viscosity channel in the middle to lower crust. Thus, the moderate crustal Vp/Vs ratio excludes the crustal channel flow as a primary uplift mechanism, implying that the horizontal crustal shortening and vertical thickening is responsible for creating the thick crust of northeastern Tibet.