Active faulting and tectonics of the Ningxia‐Hui Autonomous Region, China

Abstract

Strike‐slip, thrust, and normal faulting all seem to play an active role in the tectonics of Ningxia. In the southernmost part of the region a major left‐lateral strike‐slip fault enters the region from the neighboring Gansu province to the west and trends about S65°E. This fault is very clear on Landsat imagery and on aerial photos, and the portion in eastern Gansu and Ningxia broke in the Haiyuan earthquake of December 16, 1920. Displacements of 5–10 m caused by that earthquake are clear in numerous localities and accord with a revised value of the seismic moment of 1.2×1021N m. The eastern end of the Haiyuan fault terminates in a narrow south trending fold and thrust zone. Several other similar, north to northwest trending fold and thrust belts are present in the area about 50–200 km northeast of the Haiyuan fault and divide it into small, apparently relatively undeformed blocks 10–40 km in dimensions. The geometry of the structures in the fold and thrust zones and the apparently shallow depths at the time of deformation suggest that current deformation is similar to that that occurred in the fold and thrust belt of the Idaho‐Wyoming Rocky Mountains. North of this area, both the Helan Shan (a horst) and the Yinchuan graben are bounded by clear, active northerly trending normal faults, in some cases with right‐lateral strike‐slip components. The overall deformation, hence, seems to include dominant components of east‐west left‐lateral strike‐slip movement, northeast‐southwest crustal shortening, and northwest‐southeast extension. We interpret the extension as a response to a northeast directed force applied to the Ordos block and both this northeast directed force and the left‐lateral slip on the Haiyuan fault to the eastward displacement of material on the northeast edge of the Tibetan plateau with respect to Eurasia north of it.