Lithospheric electrical structure of Zhenghe-Dapu fault in South China determined by multi-geophysical observations: Implications for geothermal mechanism

Abstract

The Zhenghe-Dapu fault (ZDF), situated in the Cathaysia Block of South China, has played a major role in the tectono-magmatic and geothermal activities in the region since Mesozoic-Cenozoic. However, the regional topography restricts the availability of surface heat flow data, hindering lithospheric thermal structure studies. Therefore, we used a broadband and long-period magnetotelluric profile with 33 points across the Cathaysia Block and ZDF to obtain a 3-D electrical conductivity model. Tectonic zones, such as the Jiangshao fault and ZDF exhibit high-resistivity (>1000 Ωm) blocks, interpreted as remnants of the cratonic lithosphere; low-conductivity areas (<100 Ωm) indicate lithosphere modified by materials derived from the deep mantle. A high-conductivity body (<10 Ωm) in the lower crust (~20–30 km) beneath the ZDF coincides with spatial position of low magnetic anomalies, high gravity, and low S-wave velocity. Moho depth (~34 km) is significantly greater than that in the adjacent areas (~28–32 km). Integrated geophysical observations suggest that the high-conductivity body may represent mantle-derived melt fluids ascending along faults, leading to the partial melting of lower crustal rocks. Mantle intrusion led to the thickening of the lower crust, that previously underwent regional thinning, ultimately resulting in the formation of a substantial amount of Mesozoic granite bodies, the radiogenic heat of which constitutes a major heat source. Since the Cenozoic, asthenospheric thermal material upwelling, magmatic underplating, and volcanic activity have provided heat to the upper crust. The radiogenic heat from the Mesozoic granites and Cenozoic magmatic-hydrothermal activities together contributed to the regional thermal anomaly, as supported by our resistivity model.