Deep Concealed Coastal Fault of Dagang Area, North China: Geophysical Characteristics from Potential Field Data and its Geological Implications

摘要

The coastal fault of Dagang is an important tectonic transition zone in the Dagang area, North China. The former speculations of the existence of the coastal fault are mainly from geology studies. In this paper, we investigate the characteristics of Bouguer gravity anomaly and aeromagnetic anomaly, and provide geophysical evidences of the coastal fault. The upward continuation, directional derivative, wavelet analysis and 3D principle component analysis results of the gravity anomaly and reduction to pole magnetic anomaly are used to extract the coastal fault feature. The main indications of the coastal fault can be summarized with three aspects: (1) linear gravity gradient be (2) anomaly transition zone and (3) beard shaped magnetic anomalies. All these geophysical characteristics indicate the existence of a fault along the costal of Dagang area. Based on the deep fault system of Dagang area, a geodynamic model is given which shows two tectonic evolution stages with the stress field changed from NS compressive stress field of the Triassic to early-middle Jurassic to the NW-SE extensional stress field of the late Jurassic to Paleogene, and the coastal fault of Dagang also changed from the left-lateral strike-slip to the dextral strike-slip. The coastal fault of Dagang controlled the formation of the Qikou sag with regional deep faults. It also is the main uplifting channel of the magma and is the main control factor of the igneous rocks in this region.