Pn Velocity And Anisotropic Uppermost-Mantle Tomography In And Around The Shanxi Rift Zone

In this study we manually pick 25304 high-quality Pn arrival-time data from seismograms of natural earthquakes with magnitudes greater than 2. 0 in and around the Shanxi rift zone and non-natural earthquakes that occurred in Shenmu and Fugu counties, Shaanxi Province with magnitudes greater than 3.0, recorded at the Chinese digital permanent seismic stations during 2008 to 2019. Through the inversion of these arrival times, we establish a high-resolution Pn velocity and anisotropy model of the uppermost mantle beneath the Shanxi rift zone and adjacent areas. Our results show the existence of strong lateral heterogeneities in the study region. Prominent low Pn velocities are observed under the Datong volcano and Xinding basin, Taihangshan orogen belt, southern parts of North China Basin and parts of Luliangshan orogen belt, whereas obvious high velocities are imaged under the Taiyuan, northern Yuncheng, Linfen, Datong, and North China Basins and the Ordos block. The low Pn velocities and fast direction of the uppermost mantle beneath the Datong volcano generally present a scattered structure centered on the volcano, combined with the imaging results of teleseismic events, suggesting that the magma of the Datong volcano may be related to the mantle upwelling, magmatic underplating or thermal erosion could lead to the lithosphere destruction and the activation of the North China Craton. This inference is consistent with the thermal-chemical erosion model of North China Craton destruction. The Pn velocity model of the uppermost mantle beneath the Shanxi rift zone is closely related to surface geology, and the fast directions of Pn anisotropy are consistent with the trend of the geological structure and the fast directions of SKS anisotropy. These results suggest that the deformation of the Shanxi rift is dominated by simple shear, and its formation and evolution process are closely related to the movement of the upper mantle materials.