Electrical structures of the Yinchuan Basin and adjacent area, western North China Craton, inferred from magnetotelluric imaging

Abstract

• The Alxa and Ordos blocks are characterized by simple, laterally uniform, layered electrical structures. • The East Helanshan and Yellow River faults correspond to major electrical boundaries beneath the Yinchuan Basin. • Electrically conductive crustal weak zone is the main driver of earthquakes in the Yinchuan Basin. The Yinchuan Basin is located in the contact zone between the Alxa block and the Ordos block, North China Craton . The Yinchuan Basin also lies within the northern segment of the North-South seismic zone in China, which has experienced frequent earthquakes, including the M 8.0 Pingluo earthquake in 1739. In this study, two magnetotelluric profiles that cross the Yinchuan Basin and the adjacent Helanshan Uplift were used to obtain a 3-D image of the crustal electrical structure in this region. The results show that the electrical structure of the Yinchuan Basin and its surrounding areas differ not only in the east–west direction but also from north to south. The Yinchuan Basin and Helanshan Uplift comprise a typical basin-range tectonic feature , with a high-resistivity body in the mountain range and a low-resistivity body in the basin. Within this region, the East Helanshan and Yellow River faults appear as significant large-scale electrical boundaries along the east and west edges of a seismically imaged “flower” structure within the deep part of the Yinchuan Basin. The Yinchuan Fault is believed to extend northward as a buried fault, and here we speculate that it may be the master fault of the “flower” structure. The epicenter of the 1739 Pingluo earthquake falls along the boundary between high and low resistivity bodies. The low-resistivity body, within the Yinchuan Basin, extends to a depth of more than 50 km and is regarded as a weak zone; we speculate that the Pingluo earthquake was triggered by the interaction of multiple seismogenic faults within that weak zone. In addition, the alternating high and low-resistivity bodies in Yinchuan Basin and Helanshan Uplift, along which deformation is accommodated between the stable Alxa and Ordos blocks, likely defines the geodynamic framework of this earthquake. Comparing the electrical structure from the southwestern and northeastern margins of the Ordos block, we can conclude that the different tectonic frameworks can explain different kinematic properties within these active fault zones.