The Control Effect of Strike-slip Faults on Geothermal Resources: A Case Study of the Middle-high Temperature Hot Spring in Xianshuihe Fault Zone, the Eastern of Qinghai-Tibet Plateau

doi:10.21203/rs.3.rs-2488310/v1

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The Control Effect of Strike-slip Faults on Geothermal Resources: A Case Study of the Middle-high Temperature Hot Spring in Xianshuihe Fault Zone, the Eastern of Qinghai-Tibet Plateau

https://doi.org/10.21203/rs.3.rs-2488310/v1

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Based on analysis of the fault structure of the Xianshuihe Fault Zone (XFZ) in combination with the analysis of hydrogen and oxygen isotopes and hydrochemical characteristics of hot springs, the coupling mechanism of the strike slip fault and hot springs was studied. The XFZ is located in the high-temperature geothermal area in the eastern part of the Qinghai–Tibet Plateau. The results have shown that the hot spring in the XFZ is in strong alignment with the mechanical segment of the fault. This can be divided into three geothermal zones with pronounced heterogeneity in terms of isotopic composition and hydrochemical characteristics. Among them, the water δD-δ¹⁸O composition of hot springs in the Daofu hydrothermal area was the separation, the water δD-δ¹⁸O composition of hot springs in the Kangding hydrothermal area was relatively enrichment, and the water δD-δ¹⁸O composition of hot springs in the Moxi hydrothermal area was the enrichmentest. This has highlighted the difference in recharge sources (elevation) in the three hydrothermal areas. The water chemistry of the hot springs reflects significant differences in the mechanism of water–rock action among the three hydrothermal areas. The water chemistry of the hot springs in the Daofu and Kangding hydrothermal areas is predominantly influenced by the dissolution of silicate rocks, while that in the Moxi hydrothermal area is mainly influenced by the dissolution of carbonate rocks and evaporite salts. The analysis suggests that the large strike-slip faults are the main channels for deep hydrothermal upwelling and control the heat source of the geothermal zone on the western Sichuan Plateau. Meanwhile, the structures on both sides of the large faults constitute local water circulation channels.

Hot springs

Geothermal belt

Strike-slip faults

Xianshuihe Fault

Qinghai–Tibet Plateau

The eastern Qinghai–Tibet Plateau is located at the western end of the Mediterranean–Himalayan tectonic belt across Eurasia, which is one of the most famous geothermal anomaly zones worldwide (Rybach and Muffler, 1981). Here, the collision boundary between the Indian and Eurasian plates occurs. Despite the high heat flow under the action of strong tectonic movements, it is one of the most important geothermal regions in China (Bai et al., 2010; Jiang et al., 2016). At present, more than 200 hot springs have been discovered in the area (Luo et al., 2016), most of which are medium–high temperature hot springs. Many geothermal drilling wells encountered high temperature geothermal energy, among which the well head temperature of ZK02 geothermal well in Yulin Village, Kangding was 198 ℃. The bottom hole temperatures reached 208 ℃ at 300 m in depth (Tang et al., 2017). These medium–high temperature hot springs are distributed along strike-slip fault zones such as the Jinshajiang, Dege–Xiangcheng, Ganzi–Litang, and Xianshuihe faults (Tang et al., 2017; Zhang et al., 2017; Zhang et al., 2021).

Strike-slip faults have always been the focus of tectonic geological research. They have a series of geometric characteristics that are different from those of other faults. The structural space and plane distribution morphology are complex (Billi et al., 2010; Zhao et al., 2012). They are an important migration channel and storage space for underground fluids and often control the occurrence conditions of underground fluids (Nemčok et al., 2002). Oil and gas exploration practices have shown that strike-slip faults are closely related to oil and gas resources. They can serve as an important pathway for oil and gas migration and as a shield for oil and gas accumulation, which leads to the formation of fault-related reservoirs (Sylvester, 1988; Moeck et al., 2009). Compared with underground cold-water flow and oil and gas systems, there has been less research on the law of circulation and occurrence of underground hot water in strike-slip fault zones. The circulation and occurrence of underground hot water under the control of the strike-slip fault related to the deep tectonic setting are highly complex at larger temporal and spatial scales.

The surface heat display of the XFZ is relatively strong, and a substantial number of medium–high temperature hot springs are distributed along the fault zone (Tang et al., 2017; Zhang et al., 2017; Zhang et al., 2021). The distribution of geothermal resources is irregular along the fault strike (Luo et al., 2016; Zhang et al., 2017; Wang et al., 2017; Li et al., 2020). The geothermal water system of the Xianshuihe fault is controlled by the strike-slip characteristics of faults (Zhang et al., 2017; Zhang et al., 2021). The NE–SW trending tensional faults and joint dense zones control the development of the surface drainage system and deep circulation of regional groundwater. Some research has been carried out on the mode of occurrence, heat sources, formation mechanisms, and the main controlling factors in the XFZ. However, the geothermal water cycle in deep and large strike-slip faults is a highly complicated process. The occurrence state, heat sources, and genesis mechanisms of hot springs in geothermal systems need to be further examined.

Based on the research need and research in the field, this study selected hot springs in the XFZ as the research object. Through remote sensing interpretation of the XFZ combined with hydrochemical analysis of surface hot springs, the hydrodynamic mechanisms of the geothermal water of strike-slip faults, the formation conditions of hot water, and the genesis of deep geothermal water in the deep strike-slip fault zone were examined. It is necessary to establish a conceptual model for controlling the occurrence of geothermal energy using deep and large strike-slip faults. This study has considerable theoretical importance for the in-depth study of the formation process and evolution of the geothermal water system of the deep strike-slip fault but also has strategic importance and practical value for the development and use of large-scale geothermal water resources in the eastern part of the Qinghai–Tibet Plateau.

The XFZ is located at the northeast boundary of the Sichuan–Yunnan rhomboid block in the eastern Qinghai–Tibet Plateau (Fig. 1). It is a large seismic fault zone with strong activity and is one of the areas with the most intense geothermal activity (Tang et al., 2017; Li et al., 2018). The XFZ starts from Ganzi Donggu in the north, and extends east–south through Zhuwo, Luhuo, Daofu, Bamei, Kangding, and Moxi, before reaching Shimian (Weng et al., 1989), with a total length of approximately 440 km and an overall trend of 320°–330°. It is a deep and large fault protruding slightly toward the east–north direction (Bai et al., 1990; Xiong et al., 2010). Based on the geology, deep structure, geomorphology and seismicity of this fault zone (Wen et al., 1985; Long et al., 1986; Yang et al., 2007; Zhang et al., 2017; Sun et al., 2021), the XFZ is approximately divided into three segments, namely the NW segment which is known as the Luhuo–Daofu segment, the middle segment which is known as the Kangding segment, and the SE segment which is known as the Moxi segment. The NW segment has tending sinistral strike-slip motion with a relatively large strike-slip rate, and high frequency of strong earthquake activity. Meanwhile, the SE segment is dominated by tensional left-lateral strike-slip movement, with relatively weak fault activity and a small amount of strike-slip, but complex slip states (Allen et al., 1991; Peng et al., 2007).

The hot springs distributed in the XFZ are closely related to the segmental characteristics of the fault. The hot springs can be divided into three hydrothermal zones according to the segmental characteristics of the fault. The hot springs in the Daofu hydrothermal area are predominantly distributed on both sides of the main fault, with a few distributed on the secondary faults. The hot springs in the Kangding hydrothermal area are mainly distributed on the west side of the main fault of the Yala River fault, the Selaha fault, the Zedotang fault, and the intersection of the fractures. The hot springs in the Moxi hydrothermal area are predominantly distributed on the west side of the main fault of the Moxi fault and the secondary fractures and their intersection.

The geological backgrounds of the hot springs in different geothermal areas of the XFZ are different (Fig. 2a). The hot springs in the Daofu hydrothermal area are predominantly distributed in the Zhuwo Formation of the Upper Triassic (T₃zh) sandstone, slate, basic volcanic rocks, and limestone, as well as the Lower Weiguan Group of Devonian (Dwg¹) carbonaceous siliceous slate, phyllite, limestone, and quartz sandstone, and in the Middle and Upper Maoxian Group of Silurian (Smx^2–3) limestone, greenschist, and marble (Fig. 2b). The hot springs in the Kangding hydrothermal area are mainly distributed in the Zagunao Group of Middle Triassic (T₂z) quartz sandstone, slate, limestone, and in the late Yanshanian (γβ₅³) biotite granite, as well as in the middle Yanshanian (γο₅²) plagioclase granitic mixers (Fig. 2c). The hot springs in the Moxi hydrothermal area are predominantly distributed in the Middle and Upper Devonian (D_2 − 3) dolomite, siliceous dolomite, and slate, as well as in the middle and lower Permian (P_1 − 2) quartzite, schist, marble, and slate (Fig. 2d).

In this study, 25 samples of hot spring water were collected from the XFZ, and the hydrochemistry and H and O isotope test results are shown in Table 1. During the sampling process, the hot spring water sample was filtered through a 0.45µm filter film, and a sample bottle was filled with the water sample before it was then sealed in a polyethylene bottle. For the hot spring water samples used for the cation test (K⁺, Na⁺, Ca²⁺, Mg²⁺), NO₃⁻ was added after sampling on site and dropped until the PH value of the water sample was lower than 2. For the anion (Cl⁻, SO₄²⁻, F⁻) and H, O isotope tests of the hot spring water, the samples did not acidize. The hydrochemistry test was conducted at the State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences. The H and O isotopes were analyzed using a liquid water isotope instrument (IWA-35-EP), and the Vienna standard mean ocean water was used as the standard. The δD error was ± 0.6‰ and the δ¹⁸O error was ± 0.02‰. Anions and cations were analyzed using ion chromatography (ICS600). HCO₃⁻ was measured by titration. The water temperature and PH were measured on-site using portable instruments. Table 1 shows the water chemical analysis results for the hot spring of XFZ.

Based on remote sensing image data and geographic information data, digital image enhancement technology, information extraction, and the color, tone anomaly, microgeomorphic features, water system, and texture of the satellite remote sensing images, the geological structure of the XFZ was interpreted. Through field geological investigation and analysis of the fault activity of contrast, the fracture structure type, composition, and distribution rule was determined for the study area.

4.1 Hydrochemical characteristics

Several geothermal wells in the study area were drilled for high temperature geothermal energy, among which the wellhead temperature of the ZK02 geothermal well in Yulin Village of Kangding reached 198 ℃ and the bottom hole temperature reached 208 ℃ at 300 m deep (Tang et al., 2017). However, owing to the influence of the surface water and quaternary sediments, the temperature of the hot spring exposed on the surface was characterized by low temperatures. Table 1 shows that the hot spring water temperature in the Daofu geothermal area was approximately 40.12–74.46 ℃. These temperatures were the natural outfall, as was the case with the data presented below. The average water temperature was 53.07 ℃, the pH value was 6.3 ~ 8.0, which was slightly alkaline. The river water temperature was 6.48 ℃~15.59 ℃. The hot spring water temperature in the Kangding geothermal area ranged from 15.86 ℃ to 74.81 ℃. The average water temperature was 50.72 ℃, the pH was 6.61 ~ 8.97 with fresh or brackish water, and the river water temperature was 8.52 ℃ to 19.88 ℃. The hot spring water temperature in the Moxi hydrothermal area ranged from 40.52 ℃ to 70.02 ℃, with an average temperature of 50.72 ℃ and a pH value of 6.4 ~ 9.0, which was slightly alkaline water.

The hot spring water in the study area contained a variety of elements, and the main cations were Na⁺ and Ca²⁺. However, their contents were different. The Na⁺ and Ca²⁺ content of the hot springs water in the Daofu hydrothermal zone ranged from 104.51 mg/L to 536.19 mg/L with an average of 234.66 mg/L, and 49.4 mg/L to 130.4 mg/L with an average of 70.50 mg/L, respectively. The Na⁺ and Ca²⁺ contents of the hot spring water of the Kangding hydrothermal zone ranged from 155.27mg/L to 371.70mg/L with an average of 266.27mg/L to 5.6mg/L to 238.9mg/L with an average of 79.91mg/L. The Na⁺ and Ca²⁺content of the hot spring water of the Moxi hydrothermal zone ranged from 38.90 mg/L to 295.60 mg/L with an average of 173.12 mg/L, and 1.23 mg/L to 117.30 mg/L with an average of 65.10 mg/L, respectively.

According to Shukalev's classification, where anions greater than 20% of milligram equivalents are invxaysolved in naming, the hot springs of XFZ can be divided into six chemical types, namely Na-HCO₃, Na·Ca-HCO₃, Na-HCO₃·Cl, Na·Ca-HCO₃·Cl, Na·Ca-HCO₃·SO₄, and Na-HCO₃·SO₄·Cl. A Piper diagram (Fig. 3) was drawn using Origin software to analyze the ionic components of the hot spring water in the XFZ. The results have shown that there are substantial differences in the chemical types of the hot spring water in the three hydrothermal areas. The hydrochemical types of the Daofu hydrothermal active area are Na-HCO₃ and Na·Ca-HCO₃, while the hydrochemical types of Kangding hydrothermal area are mainly Na-HCO₃, Na·Ca-HCO₃, and Na-HCO₃·Cl. The hydrochemical types of the Moxi hydrothermal zone are Na-HCO₃, Na·Ca-HCO₃, Na·Ca-HCO₃·SO₄,Na·Ca-HCO₃· Cl, and Na-HCO₃·SO₄·Cl.

4.2 H and O isotope characteristics

The H and O isotopic composition of the hot spring water can strongly reflect the source, recharge, water–rock interaction, and intensity of water alternation of hot spring water. It is an effective indicator for understanding the circulation path of hot spring water. Craig (1961) summarized the numerical variation relationship of δD and δ¹⁸O in meteoric precipitation in different regions worldwide and proposed the Global Meteoric Water Line equation δD = δ¹⁸O + 10 (Global Meteoric Water Line, GMWL). Since then, researchers have proposed a series of atmospheric precipitation lines for various regions. In this study, the atmospheric precipitation line equation of the eastern margin of the Qinghai–Tibet Plateau proposed by Wang et al. (2012) was selected, that is δD = 8.41δ¹⁸O + 16.72 (Local, LMWL) and the global Meteoric Water Line equation: δD = δ¹⁸O + 10. Based on the test results, a straight-line diagram of the δD-δ¹⁸O relationship of the hot spring water samples in the XFZ was established (Fig. 4).

Figure 4 shows that the H and O isotope values of the hot spring water samples in XFZ are close to those of the GMWL and LMWL. This indicates that the source of the hot spring recharge in this fault zone is meteoric precipitation.

4.3 Hot water recharge elevation

The XFZ is located on the eastern margin of the Qinghai–Tibet Plateau, and the elevation difference changed substantially. The δ¹⁸O and δD of the hot springs were negatively correlated with the exposed height (Zhang et al., 2021). The H and O isotope values were affected by the elevation effect and gradually reduced in value with increasing precipitation height. Therefore, this effect can be used to estimate the recharge height of the hot spring (Clark and Fritz, 1997). Given the slight oxygen drift in the hot spring water sample, which leads to the fractionation of oxygen isotopes, it is more effective to use δD as a calculation index in estimating the elevation of the hot spring recharge.

H = (δS − δP)/K + h (1)

where H is the replenishment elevation of the hot spring in units of m, δS is the δD value of the sampling point of the hot spring water (‰), δP is the δD value of precipitation (‰), and K is the δD isotope gradient value of precipitation in the study area (‰/km). H is the altitude of the hot spring water sampling point (m). The δD isotope gradient in The Sichuan–Tibetan–Guizhou region of China is − 26‰/km (Yu et al., 1984), and the δD value of precipitation in the Western Sichuan Plateau is approximately − 90‰. The relationship between the δD value of precipitation and altitude H in Sichuan, Guizhou, and Tibet in China is as follows (Zhu et al., 2008; Zhou et al., 2010):

δD = − 0.026H − 30.2 (2)

The elevation effect formula for the δD of precipitation in China (Zhou et al., 2010) is as follows:

δD = − 0.03H_ALT − 27 (3)

In the formula, H_ALT is the recharge elevation for the hot springs.

According to formulas (1), (2), and (3), the recharge elevation of the hot spring on the XFZ was calculated (Table 2).

4.4 Reservoir temperature and circulation depth

The commonly used geothermal temperature scales can be divided into two main categories, namely the cationic temperature scale and SiO₂ temperature scale. The cationic geothermal temperature scale is established using the relationship between the ratio of cations in the chemical components of hot spring water and the temperature. The cations are less affected by dilution and boiling effects in the absence of external ion mixing. Giggenbach (1988) proposed the Na–K–Mg triangle diagram method to evaluate the water–rock equilibrium state and temperature. The hot spring water is divided into three zones, namely fully equilibrium water, partially equilibrium water and immature water, and whether it is suitable for cationic geothermal temperature scale is determined. The water–rock reaction diagram of the study area in Fig. 5 shows that most of the water sample points in the study area were located in the immature water area in the lower right corner of the triangle diagram. Only three water sample points were located in the partial equilibrium water area. This indicates that most of the water sample points did not reach the water–rock equilibrium state. This is likely because the hot water mixed with the shallow cold water during the rising process or the groundwater did not fully react with the surrounding rock in thermal storage. Therefore, the cationic geothermal temperature scale was not suitable for estimating the thermal storage temperature in the study area. The hot water temperature is below 110°C, and it is generally chalcedony that controls the silica content in hot spring water (Fournier and Rowe, 1966), calculated according to the method by (Arnórsson, 1983)

T = 1032/[4.969 − lg(C_S )] − 273.15 (4)

In Eq. (4), C_S represents the mass concentration of SiO₂ in water, and the thermal storage temperature of the Daofu section in the study area of this study was calculated to be 103–166°C, the thermal storage temperature of the Kangding section was 70–120°C, and the thermal storage temperature of the Moxi section was 60–120°C. Given that some points in the hot spring water samples from the Kangding section were not sampled, the thermal storage temperature of the Kangding section was supplemented with other studies to become 70–300°C (Bian et al., 2018; Jiang et al., 2022).

The temperature of hot springs is often controlled by the geothermal heat in which they circulate. The depth of hot water circulation can be estimated using Eq. (5):

Z = T_grad (T − T₀ ) + Z₀ (5)

Where Z₀ is the circulation depth (m), T_grad is the geothermal warming rate, which is taken as 20.4 m/°C (Zhang et al., 2021; Zhang et al., 2021), T is the thermal storage temperature (°C), T₀ is the average annual temperature of the recharge area, which is taken as 7°C, and Z₀ is the depth of the normal temperature zone, which is taken as 30 m. The circulation depth of the study area is calculated to be 1.1 km–6.0 km, which is consistent with that of previous studies (Zhang et al., 2017; Li et al., 2020).

4.5 Structural interpretation of XFZ

The XFZ shows a highly striking linear topography shape in the satellite remote sensing images (Fig. 6a). This is generally distributed in an arc direction toward the NE microridges. The remote sensing interpretation marks of this fault zone are clear, with wide valleys and depressions being formed along the fault zone (Fig. 6b) and the shore of the lake being controlled by the fault and having a straight line (Fig. 6c). The ridge and drainage system adjacent to the fault are twisted and deformed.

The XFZ generally shows strong compressive properties, and the shear properties are strengthened from the southeast to the northwest. Therefore, the mechanical characteristics of the XFZ were segmented and divided into three sections. The Daofu section west of Bamei town of Daofu County is extremely straight-line in shape (Fig. 7a) with a long and narrow fault valley. The fault strike is N55°W, the ridge and drainage system on both sides of the main fault show regular “elbow” twist, with sinistral shear properties. This section of the XFZ is composed of an pronounced major fault and lateral dispersion of the small faults. The Kangding section from Bamei town to Kangding city comprised of three diagonally dependent faults (Fig. 7b, c). From SW to NE there are the Zheduotang fault, Selaha fault, and Yala River faults. The fault surface is in the shape of a slow wave, the shores of seismic lakes controlled by these faults are rectilinear, and drainage dislocations have occurred. The overall characteristics of the fault are torsional compression properties, which are primarily torsional. The length of the Moxi section (Kangding–Tianwan) is approximately 40 km, and the fault has a smooth wavy shape along the strike, showing compression and torsion properties and predominantly compressional properties (Fig. 7d). In general, the torsion property of the XFZ increased from southeast to northwest, while the pressure property gradually decreased. This is because the strike of the southeast segment of the fault zone is deflected from NW to NNW, and the increase in lateral pressure from the sinestral movement has led to the obstruction of the east-trending horizontal slip of the southwest wall of the southeast segment of the fault and the conversion to a vertical motion component.

5.1 Analysis of water chemical genesis

5.1.1 Water–rock interaction

The water chemistry components of hot spring water are influenced by water–rock interactions in hot water systems, a process that predominantly includes the dissolution and precipitation of minerals, redox reactions of ions, and ion exchange interactions. We can discern the process of subsurface fluid water–rock interaction based on the chemical composition and composition of the hot spring water (Farid et al., 2015; Fu et al., 2018).

As shown in Fig. 8a, the Na⁺ /HCO₃⁻ of the hot spring water samples in the study area fell on both sides of the 1:1 dissolution line. This is the ideal molar concentration ratio for silicate rock dissolved in carbonate fluid equilibrium. The dissolution of silicate rock makes Na⁺ in excess relative to Cl⁻. Figure 8b and 8c show that the ratios of Na⁺ /Cl⁻ and (Na⁺ +K⁺)/Cl⁻ are above the 1:1 dissolution line. This indicates that the groundwater Na⁺, K⁺, and Cl⁻ are mainly derived from the dissolution of salt rocks. Meanwhile, the excess Na⁺ and K⁺ may be related to the dissolution or ion exchange of silicate rocks such as sodium feldspar and potassium feldspar.

Ca²⁺ and Mg²⁺ in natural waters are predominantly derived from the dissolution of carbonate rocks, weathering of silicate rocks, and evaporite salts (gypsum). Figure 8e shows that gypsum dissolution is an important process affecting the chemical composition of the hot spring water in the XFZ. Most of the hot spring water samples from the three hydrothermal areas were distributed above the gypsum dissolution line. This indicated that gypsum dissolution is not the only source of Ca²⁺ but may also be influenced by the dissolution of Ca-bearing silicates, dissolution of carbonate rocks, and ion exchange effects (Farid et al., 2015; Fu et al., 2018). The mean values of the Ca²⁺ /Mg²⁺ mg equivalent concentration ratios of the hot spring water in the three hydrothermal areas were 3.40, 7.48, and 5.91, respectively. This indicated that the influence of silicate rock or evaporite dissolution on the chemical fraction of hot spring water in the three hydrothermal zones was dominant. The SO₄²⁻ content of the hot spring water from the Daofu and Kangding hydrothermal area was relatively low, with mean values of 0.25 mEq/L and 0.75 mEq/L, respectively. Meanwhile, the SO₄²⁻ content of the Moxi hydrothermal area was 2.12 mEq/L, indicating that the chemical components of the hot spring water from the Daofu and Kangding hydrothermal areas were less affected by evaporite dissolution than those from the Moxi hydrothermal area. By analyzing the milligram-equivalent concentrations of Ca²⁺ +Mg²⁺ versus HCO₃⁻ and Ca²⁺ +Mg²⁺ versus HCO₃⁻ +SO₄²⁻, the recharge sources for Ca²⁺, Mg²⁺, and SO₄²⁻ in hot spring water can be further understood (Li et al., 2015). Figures 8f and 8g show that the hot spring water samples in the Daofu and Kangding hydrothermal areas are distributed below the 1:1 dissolution line with a large offset, indicating that K⁺, Ca²⁺, Na⁺, Mg²⁺ and SO₄²⁻ in the hot spring water in the Daofu and Kangding hydrothermal areas are predominantly derived from the dissolution of silicate rocks. Although the hot spring water samples from the Moxi hydrothermal area were all below the 1:1 dissolution line, most of the water sample points were near the 1:1 dissolution line with a small offset. This indicated that the main ions in the hot spring water from the Moxi hydrothermal area originated from the dissolution of carbonate rocks, and the low content of Ca²⁺ +Mg²⁺ compared to HCO₃⁻ +SO₄²⁻ may be influenced by ion exchange (Fig. 8h). The excess HCO₃⁻ + SO₄²⁻ in the hot spring water from the three hydrothermal areas is not only due to the dissolution of minerals, but also from the dissolution of HCO₃⁻ + SO₄²⁻ which may originate from deep fluids in addition to the dissolution of minerals (Kagoshima et al., 2015; Zhang et al., 2021).

The hot springs in the Daofu hydrothermal area are predominantly exposed in Triassic slates, sandstones, and granites. The hot springs in the Kangding hydrothermal area are mainly exposed in the Permian and Triassic sandstones, slates, and granites. The dissolution of silicate minerals in the surrounding rocks affects the chemical components in the hot spring water (Zhang et al., 2021), which is consistent with the findings of the previous analysis. The hot springs in the Moxi hydrothermal area are mainly exposed in Devonian dolomite, siliceous dolomite, Permian schist, tuff, and dacite. The hot springs in the southern part of the Moxi hydrothermal area are generally characterized by the mixing of hot water from the primary thermal reservoir by the secondary thermal reservoir of Permian crystalline tuffs. This may explain why some of the hot springs in this area are exposed in the schist. However, the chemical composition of the hot spring water is predominantly influenced by the dissolution of carbonate rocks.

5.1.2 Ion exchange effect

Figure 8a shows that the ratio of milligram equivalent concentrations of Na⁺ and Cl⁻ in the hot spring water was substantially larger than 1. The Na⁺ of the water samples may be related to the ion exchange between the hot spring water and surrounding rocks, in addition to the dissolution from atmospheric precipitation and salt rock. In this study, the relationship between the chlor-alkali index (CAI) CAI-1 (CAI-1 = (CI⁻ -Na⁺ -K⁺ )/Cl⁻ ) and CAI-2 (CAI-2 = (Cl⁻ -Na⁺ -K⁺ )/(SO₄²⁻ + HCO₃⁻ + CO₃²⁻ +NO³⁻)) was used to verify the direction of ion exchange in the hot spring water samples (Schoeller, 1977). When Ca²⁺ (Mg²⁺) in the water sample is exchanged with Na⁺ and K⁺ in the surrounding rock, the concentration of Na⁺ and K⁺ in the water sample will increase. Meanwhile, the content of Ca²⁺ (Mg²⁺ ) in the surrounding rock will decrease, and CAI-1 and CAI-2 will be negative. Conversely, when Na⁺ and K⁺ in the water sample is exchanged with Ca²⁺ (Mg²⁺⁾) in the aquifer minerals, the concentration of Na⁺ and K⁺ in the water sample will decrease, and the content of Ca²⁺ (Mg ²⁺) will increase, and CAI-1 and CAI-2 will be positive. The milligram equivalent concentration relationship between Ca²⁺ +Mg²⁺ -HCO₃⁻ -SO₄²⁻ and Na⁺ +K⁺ -Cl⁻ in the water samples can be used to characterize the degree to which water samples are affected by ion exchange (Farid et al., 2015; Pan et al., 2019). When the ratio of Ca²⁺ +Mg²⁺ -HCO₃⁻ -SO₄²⁻ to Na⁺ +K⁺ -Cl⁻ is close to − 1, this indicates that the ion exchange of water samples is present in the subsurface environment (Ma et al., 2013).

The relationship graph of CA1-1 and CAI-2 of hot spring water in the Xianshui River Fault zone (Fig. 9) shows that the CAI-1 and CAI-2 of the water samples are negative. This indicates that ion exchange occurs between Ca²⁺ and Mg²⁺ in the hot spring water and the Na⁺ and K⁺ of surrounding rocks (2(Ca²⁺ +Mg²⁺ )+(Na⁺, K⁺ )X = 2(Na⁺ +K⁺ )+(Ca²⁺ +Mg²⁺ )X₂). The (Ca²⁺ +Mg²⁺ -SO₄²⁻ -HCO₃⁻)-(Na⁺ +K⁺ -Cl⁻) relationship diagram in Fig. 10 shows that the hot spring points in the Daofu, Kangding, and Moxi hydrothermal areas are distributed along straight lines with slopes of − 0.949 (R² = 0.963), − 0.763 (R² = 0.405) and − 0.801 (R² = 0.742), respectively. This indicates that ion exchange effects have a greater influence on the geochemical characteristics of hot springs in the hydrothermal areas of Daofu and Moxi, with a smaller influence on the hot springs in the hydrothermal area of Kangding.

5.2 Genetic model of hot spring

5.2.1 Heat resource of the hot springs

The great uplift of the Tibetan Plateau since the Cenozoic is the result of the continuous eastward escape of middle and lower crustal material within the plateau in the form of continuous crustal flow (King et al., 1997; Clark et al., 2010). In recent years, many geophysical datasets have shown that low-speed and high-conductivity layers developed in the middle–lower crust of the Songpan–Ganzi block, with a plastic flow and magmatic activity environment (Bai et al., 2010). Partially molten high-temperature materials have provided a deep thermal dynamic basis for the hydrothermal activities of the surface temperature and hot springs (Zhang et al., 2017). Therefore, it is likely that the geothermal heat source in the study area predominantly comes from molten magma in the middle and upper crust of the lower part of the block.

Owing to compression of the Indian Ocean plate, the Tibetan Plateau is characterized by strong crustal strike-slip and contractive deformation. This is accompanied by the eastward escape of tectonics, forming three NW-trending strike-slip faults in the eastern margin of the Qinghai–Tibet Plateau, namely the Xianshuihe, Litang, and Jinshajiang faults (Xu et al., 2003; Zhang et al., 2003). The formation of strike-slip faults may be the result of crustal flow formed by molten magma in the deep middle and upper crust (Bai et al., 2010). Therefore, it is likely that its control depth is more than 100 km from the bottom of the lithosphere. This can result in high-temperature heat flow upwelling along active faults and deep fissures through the lithosphere to the surface, forming a hypergeothermal anomaly area.

5.2.2 Analysis of hot spring water recharge

The δ¹⁸O in the Daofu hydrothermal area ranged from − 19.07‰ to − 16.45‰, with an average of − 18.16‰, and δD ranged from − 143.73‰ to − 125.43‰, with an average value of − 136.58‰. The δ¹⁸O in the Kangding hydrothermal area ranged from − 18.69‰ to − 15.07‰, with an average of − 17.14‰, and the δD ranged from − 137.90‰ to − 109.57‰, with an average value of − 128.08‰. The δ¹⁸O in Moxi hydrothermal area ranged from − 15.26‰ to − 11.07‰, with an average of − 13.23‰, and the δD ranged from − 110.57‰ to − 76.23‰, with an average of − 92.81‰. In the δD-δ¹⁸O diagram, the three hydrothermal areas had pronounced zoning characteristics. Overall, the H and O isotope compositions of the XFZ from the SE segment in the Moxi geothermal area to the NW segment in the Daofu geothermal area gradually became relatively separation. This indicated that the sources of atmospheric precipitation recharge in the three geothermal areas are different.

In the XFZ, the H and O isotopes of some hot springs in the Daofu and Kangding geothermal areas exhibited a slight O drift phenomenon, and the O drift phenomenon in the Kangding geothermal area was more obvious than that in other Daofu geothermal areas. The ¹⁸O drift often occurs in high-temperature geothermal systems. Under high-temperature conditions, a δ¹⁸O isotope imbalance occurs between the geothermal fluid and surrounding rock, and oxygen isotope exchange occurs between the geothermal fluid and δ¹⁸O enrichment rocks and minerals This results in higher δ¹⁸O in geothermal water than in atmospheric precipitation (Craig, 1963). Oxygen isotope drift can be used as an important indicator to determine the deep temperatures of geothermal systems. The larger the ¹⁸O drift, the higher the deep temperature of the hot springs. Therefore, the geothermal water in the Kangding hydrothermal area has a longer cycle path or a longer time of water–rock interaction with the surrounding rock. The heat storage temperature of the geothermal water in the Kangding hydrothermal active area is higher than that in other hydrothermal active areas.

Combined with chemical analysis of the spring water, the geological conditions, and geomorphic features of the hot spring determined, it has been shown that the recharge of the spring water in the study area predominantly comes from the atmospheric precipitation in the peripheral mountains, and the minority may also be replenished by surface water and meltwater (Fig. 11).

5.2.3 Segmentation of XFZ and hydrochemical characteristics of hot springs

The hydrochemical characteristics of the three hydrothermal zones were different. The water chemistry types of the hot springs in the Daofu and Kangding hydrothermal zones were relatively simple. The main cations in the Daofu hydrothermal zone were Na⁺, Ca²⁺ and HCO₃⁻. The main cations in the Kangding hydrothermal zone were Na⁺ and Ca²⁺ and the anions were HCO₃⁻ and Cl⁻. Meanwhile, the water chemistry types in the Moxi hydrothermal zone were more complex. The main cations were Na⁺ and Ca²⁺ and the anions were HCO₃⁻, SO₄²⁻ and Cl.

The chemical composition of the hot spring water in the Daofu and Kangding hydrothermal areas was predominantly influenced by the dissolution of silicate rocks. Meanwhile, the chemical composition of the hot spring water in the Moxi hydrothermal area was mainly influenced by the dissolution of carbonate rocks and evaporite salts. There was a more pronounced positive ion exchange between the hot spring water and the surrounding rocks in the Daofu and Moxi hydrothermal areas. However, no pronounced ion exchange occurred in the hot springs in the Kangding hydrothermal zone. The degree of ion exchange between the groundwater and the surrounding rocks was higher in the Daofu than in the Moxi hydrothermal area.

5.2.4 Controlling effect of faults on geothermal water

The large strike-slip faults in the eastern part of the Qinghai–Tibet Plateau are the main channels of deep hydrothermal upwelling, controlling the heat source in the geothermal area (Tang et al., 2017). The structures derived from the two sides of the large faults constitute local water circulation channels, and are the source of concentrated heat and water transport to the heat storage. They have the dynamic characteristics of horizontal convection replenishment from the mountains on both sides. The horizontal permeability of heat storage is the main factor controlling the fluidity of hot water in a hot field. Therefore, the spatial configuration of structures derived from large faults and regional lithological combinations play a decisive role in the formation of geothermal fields. The spatial configuration determines whether the hot field is closed or open and the corresponding pressure characteristics, whereas the plane configuration controls the distribution range and temperature of the heat-carrying fluid.

The spatial configuration of the geothermal field in the XFZ determines that the hot spring and shallow geothermal field are controlled by the mechanical properties of the strike-slip fault. The fault zone of the Daofu segment is a linear segment of the strike-slip fault with relatively simple mechanical properties and hot springs distributed along both sides of the main fault. The hot water recharge channel is predominantly a secondary fault on both sides of the main fault. The fault zone of the Kangding segment comprises three faults that are approximately inclined to each other. The fault zone is mainly torsional and exhibits complex mechanical properties. It is likely that the flower-like structure slip faults in this area extend deep underground, which is conducive to the conduction of deep heat sources. The main recharge channels for hot water are secondary faults and permeable strata. The faults in the Moxi section are predominantly compressional. The hot springs in the area are mainly distributed in the hanging wall of the fault, and the geothermal water circulation is not deep. The main recharge channels for the hot water are permeable strata.

(1) XFZ exhibits strong compressional properties and can be divided into three sections according to its mechanical characteristics. The distribution of hot springs in the XFZ is in strong agreement with the mechanical segmentation of the fault and can be divided into three hydrothermal areas. The isotopic components of the three hydrothermal areas formed pronounced sub-bands, and there were significant differences in the water chemistry types and mechanisms of water–rock interaction. The water δD-δ18O composition is the separation in the Dafu hot springs, relatively enrichment in the Kangding hydrothermal area, and the enrichmentest in the Moxi hot springs. This generally shows that the isotopic composition gradually becomes enrichmenter from the Daofu hydrothermal area to the Moxi hydrothermal area, showing the differences in the recharge sources (elevation) of the three hot water areas. The three hydrothermal areas also have substantial differences in water chemistry types, with the Dafu and Kangding hydrothermal areas being relatively simple. The Moxi hydrothermal area is relatively complex in terms of water chemistry types.

(2) The chemical composition of the hot spring water in the Daofu and Kangding hydrothermal areas is predominantly influenced by the dissolution of silicate rocks. Meanwhile, the chemical composition of the hot spring water in the Moxi hydrothermal area is mainly influenced by the dissolution of carbonate rocks and evaporite salts. Positive ion exchange occurs between the hot spring water and the surrounding rocks in the Daofu and Moxi hydrothermal areas, and more pronounced ion exchange occurs in the Daofu and Moxi hydrothermal areas. However, no pronounced ion exchange occurs in the hot springs in the Kangding hydrothermal area. The degree of ion exchange between the groundwater and the surrounding rocks is higher in the Daofu than in the Moxi hydrothermal area.

(3) The large strike-slip faults in the eastern part of the Qinghai–Tibet Plateau are the main channels for deep hydrothermal upwelling and they control the heat source in the geothermal area of the eastern margin of the Qinghai–Tibet Plateau. Here, the structures on both sides of the large faults constitute local water circulation channels and are the source of concentrated heat and water transport to the heat storage. The spatial configuration of structures derived from large faults and regional lithology combinations also play a decisive role in the formation of geothermal fields. Therefore, mechanical segmentation of strike-slip faults controls the emergence of hot springs and shallow geothermal energy.

Acknowledgements

The authors are grateful to Prof. Hou Lanjie and Prof Xu Chonghui from Southwest University of Science and Technology for initial work and help. They also acknowledge the car driver Huocheng for his help in field work.

Competing interests

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Funding

This work was supported by the National Key R&D Program of China (No. 2021YFB1507302，No. 2021YFB1507300). This study was also partially supported by the National Natural Science Foundation of China (grant no. 41672206) and the State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation (Chengdu University of Technology) (PLC20210203).

Authors' contributions

ZX, ZY, JY, HY and WF planned study strategy, performed field work and sampling, all scientific experiments and measurements, analysed and interpreted the experiments’ data, made all the data tables, performed the geological modelling. CP and ZH planned study strategy, supervised laboratory operations, analysed and interpreted the experiments’ data. LW and TJ analysed and interpreted the experiments’ data, contributed in writing the manuscript. All authors read and approved the final manuscript.

Availability of data and materials

Not applicable

Corresponding author

Correspondence to ZhaoXueqin ([email protected]).

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Table. 1 Chemical characteristics of hot spring water in xianshuihe fault zone

geothermal area	Sample number	Sample location	elevation（m）	temperature（℃）	δD （‰）	δ¹⁸O（‰）	Na⁺ /(mg/L)	k⁺ /(mg/L)	Mg²⁺ /(mg/L)	Ca²⁺ /(mg/L)	Cl^- /(mg/L)	S0₄^2- /(mg/L)	HCO₃^- /(mg/L)	Hydrochemical type
Daofu geothermal area	X1	Yinke hot spring	3691	52.43	-125.43	-16.45	168.81	12.92	18.48	58.9	15.50	5.92	926.2	Na·Ca-HCO₃
	X2	Yuqu hot spring	3699	40.12	-126.18	-16.27	173.7	14.96	20.17	75.4	20.41	19.96	875	Na·Ca-HO₃
	X3	Qimei hot spring 1 ^#	3889	52.59	-143.73	-19.03	190.5	12.01	13.24	66.28	10.83	10.24	854.1	Na·Ca-HCO₃
	X4	Dibai hot spring	3888	74.46	-141.87	-19.07	212.58	12.25	5.87	49.4	9.21	5.90	960.6	Na-HCO₃
	X5	Xinjianggou hot spring	3413	51.14	-143.31	-19.03	536.19	11.97	10.63	38.60	2.39	28.28	1640	Na-HCO₃
	X6	Longpugou hot spring	3431	45.49	-137.93	-18.27	256.35	9.87	7.37	74.5	16.26	2.57	1080.4	Na·Ca-HCO₃
	X7	Canglonggou hot spring	3534	55.26	-137.64	-19.01	104.51	14.02	25.19	130.4	3.13	12.29	866	Na·Ca-HCO₃
Kangding geothermal area	X8	Ximatang hot spring	2958	35.23	-122.36	-16.09	209.36	25.32	31.39	116.6	35.55	22.77	842.6	Na·Ca-HCO₃
	X9	Reshuitang hot spring	3630	46.45	-137.71	-18.25	281.44	19.88	0.56	47.2	39.47	1.87	745.2	Na-HCO₃
	X10	Xizhutang hot spring	3466	50.23	-135.90	-18.25	371.70	59.37	20.99	78	22.33	0.93	966.5	Na-HCO₃
	X11	Xiede hot spring	3651	15.86	-137.70	-18.39	196.40	10.21	22.69	84.3	0.51	0.46	640.2	Na·Ca-HCO₃
	X12	Kamacun hot spring	3670	40.40	-134.76	-18.16	155.27	21.43	32.52	68.6	2.40	9.72	876.5	Na-HCO₃
	X13	Zhonggu hot spring	3077	60.79	-122.50	-16.81	248.20	23.96	7.72	56.9	70.31	11.66	894.6	Na-HCO₃
	X14	Erdaoqiao hot spring	2570	45.46	-109.57	-15.07	176.81	17.68	40.84	238.9	42.59	239.05	1164	Na·Ca-HCO₃
	X15	Longtougou hot spring ^#	2956	72.17	-127.75	-16.38	502.5	54.91	33.20	48.42	228.30	26.53	1452	Na-HCO₃·Cl
	X16	Guanding hot spring ^#	3029	74.81	-115.37	-15.31	318.3	42.20	16.19	54.60	180.20	53.67	771.7	Na-HCO₃·Cl
	X17	Zheduotang hot spring	3232	65.79	-137.16	-18.69	202.68	2.69	0.72	5.6	10.27	6.87	420.5	Na-HCO₃
Moxi geothermal area	X18	Gonggashan hot spring ^#	2547	46.71	-110.57	-15.26	142	15.69	20.26	117.30	41.34	31.7	704.6	Na·Ca-HCO₃
	X19	Xinxing town hot spring	2257	41.59	-105.13	-14.84	295.6	9.18	11.27	23.12	98.09	18.74	750.4	Na-HCO₃
	X20	Gonghe hot spring ^#	1559	55.49	-88.63	-12.71	125.8	10.62	20.20	56.29	20.77	60.51	497.2	Na·Ca-HCO₃
	X21	Tianwan hot spring ^#	1175	57.19	-92.22	-13.27	140.2	13.93	6.14	61.75	12.98	256.70	262.3	Na·Ca-HCO₃·SO₄
	X22	Moxi hot spring 1	1598	64	-89.00	-12.75	156.1	10.29	19.18	54	20.88	67.04	567.6	Na·Ca-HCO₃
	X23	Gongyihai hot spring^#	2163	57.56	-107.87	-14.95	127.6	5.07	0.03	1.23	51.80	32.80	42.7	Na-HCO₃·SO₄·Cl
	X24	Wandong hot spring	1317	70.02	-76.62	-11.13	174.88	30.26	35.52	91.6	162.66	144.31	682.5	Na·Ca-HCO₃·Cl
	X25	Shiyuehe hot spring	1288	50.74	-76.23	-11.07	357	0.57	21.06	76.6	38.47	163.86	810.4	Na-HCO₃
	X26	Dareshui hot spring	1588	42.52	-89.00	-13.10	38.9	5.79	6.13	104	5	139	239.8	Na·Ca-HCO₃·SO₄

Note: the data marked with "#" are in the literature (Zhang et al., 2021)

Table. 2 Hot spring recharge elevation of Xianshuihe fault zone

geothermal area	Sample number	Sample location	δD/‰	Hot spring height (h/m)	Mean recharge elevation /m
Daofu geothermal area	X1	Yinke hot spring	-125.43	3691	3999.13
	X2	Yuqu hot spring	-126.18	3699	4029.36
	X3	Qimei hot spring 1 ^#	-143.73	3889	4737.69
	X4	Dibai hot spring	-141.87	3888	4669.00
	X5	Xinjianggou hot spring	-143.31	3413	4563.59
	X6	Longpugou hot spring	-137.93	3431	4371.86
	X7	Canglonggou hot spring	-137.64	3534	4395.54
Kangding geothermal area	X8	Ximatang hot spring	-122.36	2958	3641.97
	X9	Reshuitang hot spring	-137.71	3630	4430.11
	X10	Xizhutang hot spring	-135.90	3466	4308.92
	X11	Xiede hot spring	-137.70	3651	4436.74
	X12	Kamacun hot spring	-134.76	3670	4335.03
	X13	Zhonggu hot spring	-122.50	3077	3686.78
	X14	Erdaoqiao hot spring	-109.57	2570	3042.57
	X15	Longtougou hot spring^#	-127.75	2956	3839.39
	X16	Guanding hot spring ^#	-115.37	3029	3408.74
	X17	Zheduotang hot spring	-137.16	3232	4277.23
Moxi geothermal area	X18	Gonggashan hot spring ^#	-110.57	2547	3071.66
	X19	Xinxing town hot spring	-105.13	2257	2775.06
	X20	Gonghe hot spring ^#	-88.63	1559	1935.98
	X21	Tianwan hot spring^#	-92.22	1175	1939.92
	X22	Moxi hot spring 1	-89.00	1598	1962.58
	X23	Gongyihai hot spring^#	-107.87	2163	2844.43
	X24	Wandong hot spring	-76.62	1317	1413.92
	X25	Shiyuehe hot spring	-76.23	1288	1389.92
	X26	Dareshui hot spring	-89.00	1588	1959.25

Note: the data marked with "#" are in the literature (Zhang et al., 2021)

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The Control Effect of Strike-slip Faults on Geothermal Resources: A Case Study of the Middle-high Temperature Hot Spring in Xianshuihe Fault Zone, the Eastern of Qinghai-Tibet Plateau

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Abstract

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1. Introduction

2. Regional Geology

3. Data And Methods

4. Results

4.1 Hydrochemical characteristics

4.2 H and O isotope characteristics

4.3 Hot water recharge elevation

4.4 Reservoir temperature and circulation depth

4.5 Structural interpretation of XFZ

5. Discussion

5.1 Analysis of water chemical genesis

5.1.1 Water–rock interaction

5.1.2 Ion exchange effect

5.2 Genetic model of hot spring

5.2.1 Heat resource of the hot springs

5.2.2 Analysis of hot spring water recharge

5.2.3 Segmentation of XFZ and hydrochemical characteristics of hot springs

5.2.4 Controlling effect of faults on geothermal water

6. Conclusion

Declarations

References

Tables

Supplementary Files

Status:

Version 1