The formation mechanism of the typical hot dry rocks in Hunan Province, China

Calculated by using the pipeline model, the burial depths of the 150 and 180°C hot dry rocks (HDRs) are 3.20–3.56 and 3.92–4.36 km in the Reshuiwei geothermal field, respectively, while those in the Huitang geothermal field are 3.13–3.50 and 3.84–4.28 km, respectively. Based on the analysis of the geophysical and terrestrial heat flow data, rock thermal generation rates, and the hydrochemical thermometric scale data, we summarized the formation mechanisms of the HDRs in Hunan Province: (i) the boundary deep faults in the welding area have made the intersection part of the deep faults an open tectonic environment, which is beneficial to the upwelling of deep-mantle-sourced thermal matter. (ii) The southeastern Hunan Province is the front region of the West Pacific subduction and retreat. Lithospheric thinning in this area is significant since the Mesozoic era, especially in the Dongting Lake basin, which was formed during extension and depression. This kind of regional lithospheric thinning is favorable for the lateral and upward migration and the intrusion of the deep-mantle-sourced hot substances. (iii) Rocks with high radioactive heat production rates are important crustal thermal sources for the formation of HDRs. Besides that, plutons with similar geological conditions are favorable for the formation of HDR resources. These plutons include the Baimashan pluton and its surrounding areas, and the eastern Dongting Lake plain. Moreover, the intermediate and acid granites distributed in the Chenzhou-Linwu deep fault are also part of the plutons, and the area where the Chenzhou-Linwu deep fault convergent with the Changde-Anren deep fault.