Seismic structure and deformation features beneath the Yinchuan-Hetao graben, NW China

Abstract

The Yinchuan-Hetao (YC-HT) graben is the boundary zone between the Ordos and the Alax blocks in NW China, which shows tectonic patterns distinct from the Haiyuan arcuate and Liupanshan thrust belts to the south. As a Cenozoic rift, the YC-HT graben originated from back-arc spreading related to westward subduction of the western Pacific, but its deformation and development may have been more influenced by the extension of the northeastern Tibetan Plateau (NETP) since the Quaternary. To investigate the dynamics of deep deformation of the YC-HT graben and adjacent regions, we apply the receiver function method to obtain the seismic structure in this area. Our results show that the depth of the Moho beneath the YC-HT graben is ∼50 km and significantly greater than that beneath the Alxa and Ordos blocks. The crust of the Ordos block is characterized by a layered structure, and the intracrustal structure below the YC-HT graben has undergone significant deformation. The Vp/Vs ratio in a local area of the western Hetao graben is as high as ∼1.9. By combining our observations with previous geophysical and geological studies, we speculate that the influence of NETP expansion has reached the YC-HT graben at depth. Meanwhile, the upper and lower crust experienced different deformation modes in this area, which manifested as elastic deformation in the upper crust and plastic deformation in the lower crust.