An analysis of the fault framework in southern Ningxia based on geophysical data

Southern Ningxia, located in a typical loess tableland area, hosts five semi-concealed - semi-exposed faults as boundaries of tectonic units. Based on the regional geotectonic conditions and outcrops, this study analyzed the distribution of the geophysical anomaly field in the study area, ascertained the distribution morphology of the concealed fault sections and the relationship between the faults, and established the fault framework in southern Ningxia. Based on the 1:200000 regional gravity and aeromagnetic data, this study extracted the weak signals of deep gravity and magnetic anomalies reflecting faults using both the multi-scale wavelet decomposition technique and the boundary recognition method and compared these signals with the deep faults depicted based on MT profiles. The results show that the five major faults in the study area are the boundaries of the significant gravity high anomaly zones in the detailed second-order wavelet field of gravity. The Niushoushan-Luoshan-Kongtongshan fault is the boundary between the north-south-trending long strip-shaped gravity anomalies and the north-west-trending flaky and banded gravity anomalies. This fault has typical dextral strike-slip characteristics and is the boundary fault between the Alxa microcontinent and the Ordos block. The Haiyuan fault is divided into Haiyuan faults Nos. 1 and 2 at depth. The No. 1 Haiyuan fault is concealed in the Haiyuan Basin and does not exhibit gravity anomalies. Moreover, the aeromagnetic anomaly field of this fault has significant zoning characteristics. The No. 2 Haiyuan fault is exposed at the northeastern feet of the northern and southern Mount Huashan and exhibits distinct characteristics of linear gravity anomalies but weak aeromagnetic anomalies. The two faults jointly constitute the composite boundary between the Early Paleozoic North Qilian Orogenic Belt and the Alxa microcontinent. Three faults in the Alxa microcontinent, namely the Tianjingshan fault, the Yantongshan-Yaoshan fault, and the fault at the eastern piedmont of Luoshan, are present as the northeastern boundary of the arcuate high-amplitude gravity anomaly zone.