Heat generation effects from shear friction along Xianshui river strike-slip fault in western Sichuan, China

Abstract

The Xianshui river fault zone is located in the western Sichuan Plateau, on the eastern edge of the Himalayan Geothermal Belt; numerous hot springs are exposed at the surface and distributed along the fault zone, forming an area of high-temperature hydrothermal activity. The genetic mechanisms of these high-temperature geothermal systems is currently unclear, which poses an urgent scientific problem in the field of geothermal research This study uses numerical simulation methods based on geological, hydrological, geothermal, and geophysical data to study the effects of shear frictional heat on high-temperature hydrothermal activity in the research area under the conditions of different slip velocities, parameters of thermal materials, and strain rates of the Xianshui river left-slip fault zone. The results show that shear frictional heat is the main contributor of the local thermal anomalies in the high-temperature hydrothermal system of the Xianshui river fault zone. The local high temperature generated by shear friction causes the crust to partially melt at a depth of 20–25 km; this provides the heat source for the striped geothermal system, which is controlled by the Xianshui river fault zone. In addition, the surface heat flow generated by shear friction was measured at 10 Ma, along a two-dimensional section across the Xianshui river fault in an E–W direction, and it was found to be approximately 5–50 mW/m². Furthermore, the nonlinear relationship between thermal conductivity, thermal capacity, shear stress, and temperature gives the shear frictional heat generation process "positive" and "negative" feedback effects, which maintain the dynamic equilibrium.

Introduction

The high-temperature geothermal belt in western Sichuan is one of the most active geothermal regions in China. It is located in the Tethys tectonic region, which lies in the eastern part of the Himalayan Geothermal Belt (HGB) (Fig. 1a) and is an area with intense tectonic activity. Investigation has indicated that the average value of the heat flow is higher than 80 mW/m². Recent results of hydrogeochemistry research and drilling work (Cao et al., 2006; Guo et al., 2017; Jiao et al., 2018; Luo et al., 2017) have proven the existence of numerous high-temperature geothermal systems in the west Sichuan high-temperature geothermal belt, which can be subdivided into the Batang, Litang, and Kangding geothermal belts, according to the distribution of hot springs from the west to the east (Tang et al., 2017). All the types of high-temperature surface geothermal manifestations that occur within China can be found in this region, including boiling springs, boiling fountains, steam vents, steaming ground, and a hydrothermal exploration zone (Liao, 1990a; Liao, 1990b), which are distributed with banding structures.

High-temperature geothermal systems are closely related to volcanic and seismic activities and can act as superficial supports and constraints for corresponding geodynamic processes. Therefore, defining the origin of high-temperature geothermal systems is not only important for conducting geothermal and hydrogeological groundwork, but it is also beneficial for resolving geodynamic scientific issues. In addition, modeling and analyzing the generation process of geothermal systems is important for the evaluation and development of geothermal resources. Furthermore, the modern generation of hydrothermal mineralization has an evident relation to geothermal systems. Thus, the results of associated studies can be used as a reference in order to predict the presence of metal and mineral resources.

Although the distribution of the springs in the studied area has been thoroughly explored and detailed, the generation of the geothermal system process has not been completely apprehended. Thus, exploring the heat source is the key to analyzing the problems associated with the generation of geothermal systems. The high crustal heat beneath the HGB has been explained and discussed (Bird, 1978; England et al., 1992; Hochstein and Regenauer-Lieb, 1998). The exposed volcanic rocks, generated more than 30 Ma ago, having anomalously low ³He/⁴He ratios of geothermal gases that were collected from several high-temperature systems within the HGB show that the high heat flow in the HGB (including the western Sichuan geothermal systems) has an intra-crustal origin, and is not related to the intrusions of sub-crustal melts that have been occurring during the collisional period (Giggenbach et al., 1983). Research (Regenauer-Lieb, 1992) has shown that plastic deformation of the ductile crust may also have played a role in forming the HGB and the enriched radioactive elements in crust due to the crustal thickening during collision may have increased the radioactive heat flow. The uplift of the thickening crust related to the isostatic compensation has led to the elevation of the isothermal surface and increased the heat flow background value. It has also been proposed that shear heating occurs in the crustal thrust interface between the Indian and Asian plates beneath the HGB(Bird, 1978; England et al., 1992; Shi and Zhu, 1993). Furthermore, heat can be stored within magma — in the form of latent heat when partial melting is induced by fault shear frictional heat — and can affect the heat flow to some extent. Heat from the late-magmatic intrusion (<10 Ma) and released melting latent heat generated from magma solidification can also be considered. Overall, the heat source is mostly located within the crust in the western Sichuan high-temperature geothermal systems.

The focus of this study is the Xianshui river high-temperature geothermal systems, controlled by the Xianshui river strike-slip fault, where the Yulingong in Kangding, Yuemachaqu, and Qimeikechaqu in Daofu hot springs are located along the fault. The well was drilled within Kangding City during 2013, which has shown that temperatures at the top and bottom of the well reach 140 ℃ and 208 ℃ at 267 m, respectively; this classifies the springs as high-temperature systems and implies that the Kangding geothermal systems are possibly significant latent heat resources. Studies on the hydrogeochemical and isotopic characteristics of hot springs and gases in the Kangding geothermal area have also been conducted, and it has been found that the reservoir temperature reaches approximately 260 °C (Guo et al., 2017; Luo et al., 2017). Furthermore, the heat-storage value of the Kangding geothermal systems has also been estimated (Liao, 2018; Wang et al., 2019).

Nevertheless, it is currently unclear how a geothermal system such as Kangding is controlled by the Xianshui river strike-slip fault; therefore, this study conducts quantitative simulations of temperature evolution beneath the Xianshui river strike-slip fault using COMSOL Multiphysics software to elucidate this. The contributions of mantle heat flow, radioactive heat flow, shear frictional heat flow, block uplift heat flow, and melting latent heat flow to surface heat flow, are also respectively calculated. The simulations consider several important developments: first, fault geometry inferred from analysis of historical seismic events since 1870 is used to constrain the model structure; second, different kinematic histories of the Xianshui river strike-slip fault are tested; third, high-temperature and pressure experimental petrology physics data are used to assist in understanding the important feedback processes between temperature, shear heat generation, and the thermal structure. As a typical representation of the western Sichuan geothermal belt, the modeling results for the Kangding geothermal system may provide an insight into the deep dynamic mechanism and thermal controlling structures of the system and other analogous high-temperature geothermal systems in the Tethys tectonic region.