New data on the evolution of the Tan-Lu fault belt: constraints from geological-geophysical surveys in the southern segment

The Tan-Lu fault is a well-known active fault belt in eastern China that has been the focus of geologic studies over the past 40 years. Since the late 1990s, numerous geophysical and geological investigations of this dislocation zone have been carried out by Chinese oil companies, as well as by universities. However, its deep structure, active periods of slip, and fault mechanism remain obscure. This study focuses on the deep structures within the Jiashan-Lujiang segment of the Tan-Lu fault belt, using high-precision geophysical tools, including magnetotelluric and magnetic sounding, and artificial seismic exploration using active source methods. Our results suggest that this segment is composed of several sub-faults. The southern part of the Tan-Lu fault belt, along the Jiashan-Lujiang sub-fault, can be divided into two parts on the basis of contrasting geological features. The Chihe-Taihu sub-fault is taken as the boundary between the two. The region east of the Chihe-Taihu sub-fault is dominated by strike-slip activity along several sub-faults. Only the Jiashan-Lujiang sub-fault is exposed at the surface, forming a large, positive flower structure, the result of late Middle Jurassic to early Late Jurassic strike-slip movement along the dislocation zone. Three sub-faults are present in Dingyuan County, two of which disappear in the southern Hefei Basin. Only the Chihe-Taihu sub-fault extends to the eastern edge of this basin, creating a half-graben depression that formed during the Early Cretaceous. Our results indicate that the present-day deep structure of the southern portion of the Tan-Lu fault zone is the result of a combination of strike-slip and extensional tectonics.