Gravity deep structure and dynamics of Yishu fault zone revealed by land-sea gravity observation

The Yishu fault zone is the main part of the Tanlu fault zone, located between the Luxi Uplift, Jiaonan Uplift, and the Sulu Orogen. The tectonic environment is highly complex in this region, where the seismic activities are the most intense within the Tanlu fault zone. Studying the distribution of crustal thickness and density structure in this region is of significant practical importance for understanding the seismic mechanisms in the Yishu fault zone. Based on measured gravity data, we utilized regional gravity isostatic analysis and crustal density structure inversion methods to explore and analyze the deep structural characteristics and dynamic significance of the Yishu fault zone and its adjacent areas. The results indicate that the Yishu fault zone, as a lithospheric-scale tectonic boundary, exhibits different gravity anomaly distribution characteristics across the various blocks in the study area. The northern part of this fault zone is in an unbalanced state, exhibiting more frequent tectonic activity. The Xuhuai block and Jiaonan Uplift at the southern end of the Luxi Uplift demonstrate an upward uplift trend. A widespread distribution of high-density anomalies is observed in the lower crust beneath the Yishu fault zone and the Luxi Uplift, likely due to the upwelling of high-density materials from the upper mantle, which causes crustal compression and uplift. It may also reflect the metamorphic rocks formed by high-temperature and high-pressure metamorphism in the deep crust of the Yishu fault zone. Furthermore, earthquakes in the study area are frequently concentrated in the crustal density transition zone and regions where faults intersect. The intersection of the Yishu fault zone with the Mengshan front fault and the Cangni fault creates a deep seismogenic environment with moderate to strong earthquakes.