Velocity Structure Of Crust And Upper Mantlein The Ordos Block And Surrounding Regions, North China

Ordos block and surrounding regions is a main zone of seismic activity inChina,and large earthquakes with serious damages were frequently occurred in thisarea. It is located between the uplifting Tibetan Plateau drivenby the India–Eurasiacollision and North China block with the lithosphere severe damaged by the westernsubducting Pacific slab. Thus, the activities and structures in this area arecharacterized by both collision and extensional tectonics. We determined the3Dvelocity structure down to400km depth beneath the Ordos block and surroundingregions using107768teleseismic P-wave arrival times, which were recorded by338seismic stations in our study region. Considering the complicated structure, theMultistage Fast-Marching Algorithm and Subspace Inversion Method were used inthe seismic tomography. Then the reactivation and thinning of the lithosphere ofNorth China Craton were discussedas well as the interaction between Ordos Blockand its surrounding regions.The lateral variations and vertical differences in crust and upper mantle of Ordosblock and surrounding regions strongly imply the complexity of the region’s deepvelocity structure. The crustal velocity structure in the study area is coherent withsurficial tectonic though it attenuates with increasing depth.A lithospheric root is imaged clearly beneath the Ordos block with an averagethicknessof250km, much larger than the periphery rift zone.The Ordos blocklithosphere is non-uniform in thickness with south larger than north, and almost thesame in east and west which is larger than the middle part. Lithospheric thinning ofthe periphery rift zone may occur.The India–Eurasia collision and the later sustained compression have resulted incrustal shortening and thickening within the Tibetan Plateau, and maybe the extrudingof crustal and mantle material towards the western NCC. The low-velocity zonebelow indicates that Liupanshan Orogen and Weihe Graben may be the narrow channel of the crustal and mantle material, and may cause stretch tension of Weihe riftzone and promote Ordos block counterclockwise rotation.Large range of low velocity distribution and the trend of local upwelling beneathOrdos block indicate that under the tremendous compressive stress of Tibet PlateauNEE, the extension of periphery rift zone maybe formed by counterclockwise rotationand overall uplift of the Ordos block.Velocity distributions of crust and upper mantle beneath the study areaaredistinctly inhomogeneous inboth horizontal direction and vertical direction, whichindicates that the eastern, central and western lithosphere of North China Craton maysuffer from different tectonic deformation processes.