New field and geophysical data about transpressional deformation: An improved model for the structural style of Sha Gou fault area in southern Ningxia, northeastern edge of Tibetan Plateau, China

Abstract

Situated near the northeastern edge of the Tibetan Plateau, the Sha Gou fault area (SGFA) represents an important area to understand Cenozoic regional interactions between northeastern (NE) Tibetan Plateau and southwestern (SW) Liupan Shan Basin. Within this region, several km-scale arcuate fault systems such as the Lijun fault and the Madong Shan piedmont fault accommodate the transpressive deformation, but their structural style is still controversial. Predecessors regarded these bow-like faults as southwest-dipping thrust systems with large-scale nappes, however, this opinion is not compatible with the new field and geophysical data. In this study, we aim to ascertain the structural pattern of this area based on the analyses of field observation data, interpretation of seismic reflection profiles and two-dimensional gravity forward modeling. According to the present fieldwork, the structural cleavages, low-angle slickenlines, sub-vertical fault surfaces, “ribbon effect” as well as the linear geomorphic features in plane view indicate that these faults belong to a strike-slip system. Newly acquired seismic profiles, characterized by distinct positive flower structures, strongly suggest that this area is again a strike-slip fault system rather than a thrusting system. Furthermore, gravity forward modeling results show that the reverse strike-slip model without large-scale thrust nappes is closer to the present structural style of this area. The gravity forward modeling is an effective method to identify km-scale thrust nappes, which could be used for reference by the structural style analysis in other similar regions. Based on these evidences, a new transpressional shear model for the structural pattern is proposed. The overall structural style of the SGFA is not characterized by large-scale thrust nappes, but is dominated by a transpressional shear system with significant strike-slip faulting. Such a new model can be an additional evidence for the tectonic escape that is going on in the area north of the Himalaya and is dominated by reverse strike-slip deformation. The stepwise morphology (i.e., the Pre-Cretaceous strata in the west of strike-slip faults are higher than those in the east successively) in this model is also caused by the uplift of the Tibetan Plateau. Furthermore, the strike-slip deformation indicates that the SW Liupan Shan Basin is a continental sub-basin segmented by several km-scale strike-slip faults rather than the imbricated style of purely contractional system in typical foreland basin.