Velocity and azimuthal anisotropy structures beneath the Dianzhong Block and its vicinity, SE Tibetan Plateau, revealed by eikonal equation-based traveltime tomography

Abstract

The Dianzhong Block (DZB) located on the southeastern margin of the Tibetan Plateau is an important tectonic zone characterized by the intense flow of crustal material. Although it is widely accepted that materials originating from the Tibetan Plateau migrate southeastward in the DZB and its vicinity, the flow mechanism and subsurface structures remain controversial. Here, we image the subsurface velocity and azimuthal anisotropy structures beneath the DZB and its surroundings by using eikonal equation-based tomography to study the crustal flow and deformation mechanisms. In this study, we collect high-quality seismic data from 290 broadband seismic stations and manually pick a total of 31,981 first arrivals. Our tomographic results display prominent velocity anomalies and complex azimuthal anisotropy patterns. In the upper crust, our results reveal predominant high-velocity anomalies, which are related to the Emeishan large igneous province (ELIP) and the stable Simao Block. We find strong low-velocity anomalies beneath the DZB in the middle–lower crust. The low-velocity anomalies extend southward from the Xiaojinhe Fault (XJHF) to the Red River Fault (RRF) and then rotate southeastward to the east of the Xiaojiang Fault (XJF). In addition, the inverted fast velocity directions generally follow the strike of the low-velocity anomalies. Based on these findings, we consider the existence of middle–lower crustal flow inside the DZB. Three distinct features of azimuthal anisotropy in the middle-upper crust are found around the XJF reflecting complex deformation, which may indicate different factors controlling the deformation of the XJF from the north to the south. In the lower crust and uppermost mantle, the azimuthal anisotropy beneath the RRF is relatively complex, exhibiting two patterns: one perpendicular and another parallel to the fault.