Velocity and Azimuthal Anisotropy Structures Beneath the Dianzhong Block and its Vicinity, Se Tibetan Plateau, Revealed by Eikonal Equation-Based Traveltime Tomography

The Dianzhong Block (DZB) located on the southeastern margin of the Tibetan Plateau acts as 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. The tomography results display prominent velocity anomalies and complex azimuthal anisotropy patterns. In the upper crust, our results reveal predominant high-velocity anomalies. Three distinct features of azimuthal anisotropy in the middle-upper crust are found around the Xiaojiang Fault (XJF), and they reflect the complex deformation. In addition, we detect that a low-velocity zone is in the middle-lower crust, that extends from the Xiaojinhe Fault (XJHF) to at least the intersection between the XJF and Red River Fault (RRF). In the lower crust and uppermost mantle, the azimuthal anisotropy beneath the RRF is relatively complex, and exhibits two patterns: one perpendicular and another parallel to the fault