Three‐Dimensional P‐Wave Velocity Structure Around the Xiaojiang Fault Zone and Its Tectonic Implications

Abstract

As the southeastern margin of the Tibetan Plateau, the Xiaojiang fault zone plays an important role in the lateral escape of Tibetan Plateau materials. In this work, the 3-D P-wave velocity structure of the Xiaojiang fault zone and its surrounding areas was imaged by using travel time data from permanent seismic stations and a dense temporal seismic array. The results show that in the upper and middle crust, the Xiaojiang fault zone is mainly of low velocity, while high velocity anomalies exist to its east. In the middle and lower crust, a low velocity anomaly is shown in the central portion of the Xiaojiang fault zone, and high velocity anomalies exist both in the northern and southern areas. The northern high velocity anomaly near Panzhihua extends from lower crust to the surface, and the southern one in the lower crust extends to the upper mantle. We consider that the low velocity anomaly in the central portion of the Xiaojiang fault zone is caused by high temperature derived from upper mantle. The high anomalies in the northern area are most likely related to the late Paleozoic mantle plume activity which caused a large number of basic and ultrabasic mantle materials intruding into the crust. These high-velocity anomalies, with greater mechanical strength, play a certain impediment to southward escaping of Tibetan Plateau materials, and lead to the rapid uplift of the northern sub-block of the Sichuan-Yunnan active block. The north-dipping high velocity anomaly in the southern area is a further barrier for southward extrusion of the Sichuan-Yunnan active block, leading to strong deformation and intense seismicity within the low-velocity area in the overlaying upper and middle crust. Meantime, this high-velocity block contributes to stabilization of the upper mantle, which makes mantle materials of its southern region less affected by the southward escape of Tibetan Plateau materials.