Crust and upper mantle velocity structure beneath the Ordos Block and its tectonic significance

The Ordos Block in the western North China Craton (NCC) occupies an important position in the tectonic evolution and deep earthquake environment research in China continent. In previous seismic tomography studies, the sparse distribution of seismic stations resulted in a relatively low lateral resolution of 3D structure of crust and upper mantle in the Ordos Block. In this paper, a dense broadband seismograph network was established, including the permanent seismic stations of the regional networks and the temporary seismic stations funded by the "Destruction of the North China Craton" project and the North China Scientific Seismic Array. Based on the waveform data recorded by 338 seismic stations in the Ordos Block and surrounding regions, we used FMTOMO traveltime tomography method to determine the high-resolution P-wave velocity structures of the crust and upper mantle down to 400 km depth, and analyzed the relationship between deep structure and a range of geodynamic issues, such as the volcanic and strong earthquake activities and the destruction of the NCC. The tectonic implications of the lateral and the vertical variations of P-wave velocity in the crust and upper mantle structures are described as follow. The transverse variation and vertical difference of velocity structure of crust and uppermost mantle indicate significant correlations between deep tectonics and surface geological setting. The thick lithosphere with positive velocity anomaly beneath the Ordos block suggests that the Ordos block did not suffer extensive destruction. The velocity anomaly distribution of the Taihang orogenic belt in the central NCC is characterized by discontinuous strip anomalies, which is significantly different from that of the North China Plain and Ordos block. The low-velocity anomalies deeper than 400 km might be related to the lithosphere destruction in the eastern NCC. A low-velocity columnar anomaly down to 400 km depth beneath the Datong volcanic groups indicates that the magmatism of the Datong volcanic group might originate from the mantle transitional zone, due to the upwelling and expulsion of basaltic magma caused by the subduction of the Pacific plate. The low-velocity zone in approximately EW direction in the upper mantle between the Ordos Block and the Sichuan basin suggests the possible existence of a narrow outflow channel of the asthenosphere. In summary, the low-velocity anomaly down to the upper mantle in the Ordos Block and surrounding regions indicates that the north-eastward compressive stress of the Tibetan Plateau and the westward subduction of the Pacific plate may induce the migration and upwelling of mantle materials, causing the overall uplift of the Ordos Block and the formation of extensional fault belt in surrounding regions. The vertical upwelling and lateral movement of the deep materials provide the stress conditions for the preparation and occurrence of strong earthquakes resulting in frequent occurrence of strong earthquakes in the regions around the Ordos Block. © 2018, Science Press. All right reserved.