## 4 Discussion
### Formation Mechanism of Lushan Geothermal Belt
#### 4.1.1. Geothermal Water Recharge
The huge heat energy of the melting material in the upper mantle is transmitted upward along the shear zone, forming another deep heat transfer zone 2-3 km south of Xiatang fault. Lushan geothermal belt is right sandwiched between the two deep heat transfer zones. Magmatic Heat Sources  
Granites developed in the Lushan geothermal field were formed in the Middle Proterozoic, Late Proterozoic, and Early Cretaceous. The Proterozoic granite is a ductile shear zone formed in deep environment, which contains a large amount of thermal energy. The early Cretaceous granite is the youngest intrusive rock in Lushan and its surrounding areas because of its relatively new age. It is possible that it is still in a melting state in the deep underground and has great thermal energy. The thermal energy preserved by the above two types of granites is transported upward along the North-South boundary faults of the geothermal field, which plays a certain role in the temperature rise of cold water in the geothermal field. Shear Structural Heat  
The north and south boundaries faults of Lushan geothermal field all developed traces left by shear activities. In the late stage of Checun-Xiatang fault, the thermal energy generated by the shear activity was large enough to heat the recharge water in the geothermal field. In the southern boundary fault of geothermal field, the strike slip shear would make the minerals produce friction and generate heat energy, which would accumulate in the deep and transmit upward to form heat source. Water Gushing Structure  
The heat conduction structure of Lushan geothermal field is a complex fault system, which is composed of Checun-Xiatang fault in the north boundary, Erlangmiao-Wentang fault in the south boundary, and the NE and NW secondary faults oblique to the two main faults. As mentioned before, the Checun-Xiatang fault experienced three stages of geological activities, in which the early compressive activities formed fracture zones, the middle tension activities formed water storage spaces, and the late torsional and left-lateral displacement activities were the activities of hot water being \"squeezed out\" and rising to the surface. The left-lateral displacement only distorted the strata on the north side of the fault, and the fault was not obvious. The granite on the south side of the fault was prone to fracture with other minerals because of the development of crystal-like phenocryst. Under the shear action, it was easy to form a NW trending fault, while the early NE thrust was easy to form NE fault. The joint of these two groups of faults was right the best channel for the rise of geothermal water. Because the Checun-Xiatang fault faces south with a dip angle of 75-85\({}^{\circ}\), the location of hot spring artesian flow is to the south of the fault surface, which indicates that when the heated water stored in the fracture zone rises to the surface, it will overflow in the south of the fault. In Shangtang field, broom structure was developed, that is, the NE fault converges to the NE and spreads to the SW. With the simultaneous action of the NW fault, geothermal water gushed out in a plane shape with an area of 4000 m\({}^{2}\), which was obviously caused by the faults. Zhongtang and Wentang field were not far away. The intersection of NE and NW faults made the geothermal water rise along the joint position of the two sets of faults to form hot springs, so the two gushing out points of Zhongtang and Wentang hot springs were actually one hot spring that was formed by a single set of faults system. There was a large-scale northwest fault in Xiatang field, namely Yangzhuang fault, while the earlier NE fault was relatively small, the faults joint and interaction position was deep underground, therefore the geothermal water mainly rose along the northwest fault. There were two kinds of lithologies in the south of Shentang field, early Cretaceous granite and Majiahe formation andesite. The faults were mainly developed in granite.