## 4 Discussion
### Formation Mechanism of Lushan Geothermal Belt
#### 4.1.1. Geothermal Water Recharge  
The surface river, Sha River, flows from west to east on the south side of the Checun-Xiatang fault, which is nearly parallel to Xiatang fault and 500-4000 m away from Xiatang fault. The river water is easy to infiltrate along the fault. In the west of Zhongtang geothermal field, there are mainly volcanic rocks, among which there are sandstone intercalations. The surface precipitation is easy to flow to Xiatang fault, and it infiltrates or flows into the underground storage space along the fracture zone and the south inclined main fault surface. In the north side of Zhongtang-Xiatang fault it is mainly composed of clastic rocks and locally carbonate rocks. The rock stratum tends to southeast. The surface precipitation can be stored along the rock layer to the main fault surface, or infiltrating along the main fault surface and both sides of the fracture zones to the water storage space. The atmospheric precipitation in the south side of Sha River flows into the river along the terrain, and infiltrate into the underground water storage space through Erlangmiao-Wentang fault and Shuimozhuang-Lishu fault along the fracture and secondary fault of granite. Groundwater  
On the north side of Xiatang fault, there are different thickness carbonate rocks. Dolomite with thickness of 100-150 m and limestone with thickness of 200-250 m were also found in the research area. 2-3 km to the south of the geothermal field, the marble of Archaeozoic was remained in the granite, distributed in patches, covering an area of 4-6 km\({}^{2}\). All of these may store karst groundwater and transport it to geothermal field through some channel. The water storage space of Lushan geothermal belt is mainly Checun-Xiatang fault. The fault had three stages of geological activities. In the early stage, it was formed at Yanshanian. The hanging wall dominated by granite on the south side thrusted against the football with huge nappe stress, forming a huge thick fracture zone of granite with a width of 500-1000 m. During this period, because of the compression fracture, there were few pores in the fracture zone, and the breccias of the fault breccia were relatively close with poor water capacity. The middle stage of the fault was at Cenozoic Era, and the main activity was tension activity. After the relaxation of thrust stress, the tension activity made the south plate of the fault fall, and the pores appeared in the fault zone. The late stage of the fault was at quaternary, the two plates of the fault made a left-lateral displacement, which was a torsional activity. Among the above three stages of activities, the space formed in the middle stage by the tensional activity was nearly 50 km long and 4000 m wide, which became the main water storage space of geothermal field. The heat source of Lushan geothermal field mainly comes from the deep heat energy of the crust, and magma heat and shear heat also provide part of the heat source. Deep Heat Energy of Crust  
The crust of Lushan area is more than 20 km thick. Checun-Xiatang deep fault and Liziyu-Banpoquan ductile shear zone are developed in and around the Lushan geothermal field. Two structural zones provide channels for the deep heat energy to be transported upward. The Checun-Xiatang fault is a deep fault reaching the upper mantle, causing large-scale rock fault. The material heat energy in the melting state of the upper mantle is transmitted upward along the shear zone and fracture zone of the fault for a long and nonstop time. The deep heat energy can be transmitted to the surface and overflowed along the secondary faults (mainly NW and NE faults), forming a geothermal area with a width of several hundred meters and a length of tens of kilometers. Liziyu-Banpoquan ductile shear zone passes through the Proterozoic granite, which are granitic mylonites with a width of 100-1000 m, inclination 177-225\({}^{\circ}\), and dip angle of 55-85\({}^{\circ}\). This ductile shear zone is as deep as the upper mantle.