Hot Spring Gas Geochemical Characteristics and Geological Implications of the Northern Yadong-Gulu Rift in the Tibetan Plateau

To reveal the heat source and its formation mechanism of the northern Yadong-Gulu rift (YGR), we analyzed the helium isotope, carbon isotope (δ¹³C_CO2), and CO₂/³He and CH₄/³He ratios of hot spring gases for tracing the source of volatiles and discussing their geological significance. The results show the following: helium is mainly derived from the crust, and the radioactive decay of the thicker crust and granites provided more ⁴He to the low helium isotopes; thermal decomposition of carbonate rocks is the main source of CO₂; CH₄ may be of organic origin. To sum up, the gas geochemical characteristics of hot springs in the northern YGR indicate that the volatiles are mainly derived from the crust. The crust/mantle heat flow ratios (q_c/q_m) calculated by helium isotopes cover a range of 0.84–1.48, suggesting that the heat is mainly contributed by the crust. The crustal origin gas and heat flow demonstrates that the heat source beneath the northern YGR is formed by the process of interior crust. Combined with geophysical data, we suggest that the stress heat caused by the collision of the Indo-Eurasian plate and the radiant heating of the crust lead to the heat source (partial melting) and provide heat for thermal activities.

1 Introduction

The collision orogeny between India and Eurasia is one of the most important geological events since the Cenozoic, which led to the uplift of the Tibetan Plateau (TP). The dynamics of plateau uplifting has attracted wide attention, especially the crust–mantle structure in the collision zone (Himalayan terrane and Lhasa terrane) (Bourjot and Romanowicz, 1992; Nelson et al., 1996; Wu et al., 2005; Xu et al., 2015). The development of nearly north–south extensional rift systems in the central and southern TP is related to the dynamics of plateau uplift, and they are considered to be formed by the collapse of the orogenic plateau or regional stress changes (England and Houseman, 1989; Molnar et al., 1993; Yin and Harrison, 2000). The Yadong-Gulu rift (YGR) is the largest north–south trending rift in the central TP, which runs through the Himalayan terrane and Lhasa terrane, so it is an ideal place to study the deep structure of the collision area of the TP.

The YGR developed a large number of hot springs, and the tectonic activity is strong, which provides favorable conditions for the migration of gas from depth. Gases volatile from hot spring in the fault zone are closely related to the deep geological structure (de Moor et al., 2013; Italiano et al., 2013; Kulongoski et al., 2013). They are a mixture of gases from different sources, and their composition will significantly change due to the different physical and chemical properties of various gas components, different geological backgrounds, and different ways to enter hot springs from their sources (Wang et al., 1992). Helium is an ideal tracer in hot spring gases because it is chemically inert and its abundance is changed only by radioactive decay and physical processes during migration (Farley and Neroda, 1998). Helium has two isotopic nuclides, ³He and ⁴He. ³He mainly derives from the mantle, while ⁴He is mainly produced by the decay of uranium and thorium in the crust. The helium isotope is expressed as ³He/⁴He, and it varies greatly with different sources (Poreda and Craig, 1989). Generally, helium includes the atmospheric source, the mantle source, and the crustal source, and their helium isotopes are 1 Ra, 8 Ra, and 0.02 Ra, respectively (Ra is atmospheric ³He/⁴He, Ra = 1.4 × 10⁻⁶) (Sano and Wakita, 1985). Hot spring helium isotopes have been successfully used for studying the deep geological structure in fault zones and volcanic areas (Mutlu et al., 2008; Klemperer et al., 2013). In addition, CO₂ is usually the main component of hot spring volatiles. However, the concentration and isotopes of CO₂ are easily affected by geophysical and chemical reactions in the process of migration. Therefore, δ¹³C_CO2 and CO₂/³He are usually combined to explore the source of CO₂ (Zhou et al., 2020). Moreover, CH₄/³He is also used as an index to trace the source of gas (Tao et al., 2005).

Hot spring gases have also been widely applied for exploring the deep geological structure in the TP, such as in the Tengchong volcanic area in Yunnan Province (Wang et al., 1993; Xu et al., 2004; Zhao et al., 2012; Cheng et al., 2014), the Jinshajiang-Red River fault zone (Zhou et al., 2020), the Litang fault zone (Zhou et al., 2017), and the Xianshuihe-Anninghe fault zone (Xu et al., 2021). It is found that the change of hot spring gases is related to the deep heat source or the activity of fault zone. In recent years, some scholars have tried to explore the material sources (gas, fluid) and deep structures under the YGR by using hot spring gases (Zhao et al., 1998; Yokoyama et al., 1999; Zhao et al., 2002; Zhang et al., 2017), but the heat source and its formation mechanism are still under debate (Yokoyama et al., 1999; Liu et al., 2014; Zhang et al., 2017; Xie et al., 2021).

In this study, 17 hot spring gas samples were collected from the northern YGR. We analyzed the helium and carbon isotopes, CO₂/³He ratios, and CH₄/³He ratios for tracing the source of hot spring gases and discussing their geological significance to reveal the heat source and its formation mechanism of the northern YGR.

2 Geological Settings

The Tibetan Plateau is a complex tectonic assemblage system (Yin and Harrison, 2000). It contains five relatively stable blocks, which are the Himalayan block, Lhasa block, Qiangtang block, Songpan-Ganzi block, and Kunlun-Qilian block from south to north (Pan et al., 2012). In the southern TP, there are six junior nearly north–south trending rifts (Armijo et al., 1986; Yin, 2010), and their extensional age is from the Middle Miocene to the late Miocene (Zhang et al., 2020). The YGR is the largest rift in the TP, with a total length of nearly 600 km, which spans the Himalayan terrane and the Lhasa terrane (Figure 1A). The YGR can be divided into the southern, middle, and northern segments during research (Zhang et al., 2021). Our study area is mainly in the northern YGR, and it is a graben composed of two NE trending normal faults. The faults extend from Yangbajing to Sangxiong, and the northern end of the faults is cut off by a NW trending strike-slip fault (Jiali fault) (Figure 1B).

FIGURE 1

FIGURE 1. (A) Simplified tectonic map of the Tibetan Plateau (modified from Yin and Harrison (2000)); (B) geological map of the northern Yadong-Gulu rift (modified from Zhang et al. (2017)) and hot spring gas sampling sites (the black dots in (B)).

Due to the difference of basement and sedimentary caprocks in the Lhasa terrane, it can be divided into three subterranes with the boundaries of Shiquanhe-Nancuo melange belt (SNMB) and Luobadui-Milashan fault (LMF) (Zhu et al., 2011; Zhang et al., 2019). The northern YGR crosses the northern and central parts of the Lhasa block. The northern Lhasa has a young crust, the sedimentary caprock is a Lower Cretaceous volcanic sedimentary sequence, and the east is covered by Triassic–Jurassic strata (Zhu et al., 2013). The Middle–Upper Triassic strata are mainly composed of slate, sandstone, and radiolarian chert (Pan et al., 2006). The Precambrian basement (Zhang et al., 2012b; Dong et al., 2011), overlying Carboniferous–Permian metamorphic sedimentary rocks, and Lower Cretaceous volcanic sedimentary sequences are developed in the central Lhasa terrane, accompanied by a small amount of Ordovician, Silurian, and Triassic sedimentary rocks (Zhu et al., 2013). After collision, the Lhasa block developed multi-stage magmatism and widely developed Meso-Cenozoic igneous rocks, mainly Paleocene Gangdise granite and Linzizong volcanic rock distributed in the southern Lhasa (Wu et al., 2008).

At about 65 Ma, the Indo-Eurasian plate collided and the Indian landmass subducted beneath the Eurasian plate, resulting in the increase of crustal thickness of the southern TP (England and Houseman, 1989; Yin and Harrison, 2000; Wu et al., 2008; Pan et al., 2012), and the thickest area appeared in the Lhasa terrane, at about 80 km (Zhao et al., 1993; Zhao et al., 2001; Kind et al., 2002; Zhang et al., 2011; Li et al., 2014). The heat flow values of Yangbajing, Yangyingxiang, Laduogang, and Naqu in Lhasa are 108 m/Wm², 264 m/Wm², 338 m/Wm², and 319 m/Wm², respectively (Teng et al., 2019), which are higher than the average of Chinese mainland (63 m/Wm²) (Hu et al., 2000; Jiang et al., 2016; Jiang et al., 2019). Previous geophysical studies have found that many shallow low velocity and/or high conductivity zones are distributed in the crust of the Lhasa terrane, such as Yangbajing and Dangxiong, which are considered to be granitic partial melting bodies with a depth of 15–25 km and a thickness of 20 km (Brown et al., 1996; Kind et al., 1996; Nelson et al., 1996; Zhao et al., 2001).

The TP is rich in geothermal resources, and the hot springs in the Lhasa block are concentrated in the north–south trending rifts. In the northern YGR, hot springs are developed in Yangbajing, Ningzhong, Gulu, Naqu, and other places, with spring temperatures ranging from 23 to 87°C. The Yangbajing geothermal field is one of the non-volcanic high temperature geothermal fields in Tibet, with downhole temperatures as high as 330°C (Guo, 2012; He et al., 2012; Yuan et al., 2014; Zhang et al., 2015; Wang et al., 2022). The pH of hot spring water in the study area (such as Yangbajing, Ningzhong, and Gulu) is mostly between 7 and 9, showing a slightly alkaline property (He et al., 2012; Yuan et al., 2014; Wang et al., 2018; Guo et al., 2019). Besides, the main hydrochemical types of hot springs are mostly transitional HCO₃-Na and Cl·HCO₃-Na, while Yangbajing geothermal water is of Cl-Na type (Liu et al., 2014; Wang et al., 2017b).

3 Sampling and Analysis

Seventeen hot spring gas samples were collected from eight sites along the northern YGR (Figure 1B). Hot spring gas collection used the drainage gas extraction method: we used a sodium glass bottle with low helium permeability as a collection container, filled the bottle with the corresponding hot spring water, and put the bottle mouth into the hot spring water, and then the gas replaced the water. To avoid air pollution, we stopped collecting when the gas filled 2/3 of the bottle volume, and then we sealed the bottle mouth with a rubber stopper and kept the bottle mouth down during transportation and storage. In order to prevent the leakage of rare gases, gas samples were analyzed within 14 days after collection.

All the gas samples were tested for gas composition and isotopes in the Oil and Gas Resources Research Center of Northwest Institute of Ecological Environment and Resources, CAS. Conventional gas components were analyzed by an MAT271 Gas Mass Spectrometer with a relative standard deviation less than 0.01% and a precision of ±0.1%. The test conditions are as follows: EI ion source, ionization energy 86 eV, emission current 40 μA, ion source temperature 95°C, SIM scanning mode, and injection volume 1 ml. The abundance and isotopes of noble gases were analyzed by a VG5400 Static Vacuum Noble Gas Mass Spectrometer, and the test condition is a high voltage ion source of 9 kV and a trap current of 800 μA. Before and after analyzing the gas sample, the internal standard gas collected from the top of Gaolan Shan in Lanzhou is first tested for its noble gas components and isotopes. δ¹³C_CO2 was analyzed with a stable isotope ratio mass spectrometer (Thermo Fisher Scientific Delta Plus XP), and the reference value is the Pee Dee Belemnite (PDB) standard.

4 Results

4.1 Chemical Compositions of Hot Spring Volatile Gases

Gas chemical compositions are listed in Table 1. In these hot springs, the temperature of spring water ranges from 10 to 88°C. Hot spring gases are mainly composed of N₂ and CO₂, which account for 14.53%–90.40% and 4.30%–89.74% of the total gas content, respectively. The contents of Ar and O₂ are relatively low, ranging from 0.19% to 1.03% and 0.44% to 17.52%, respectively. We found that the Ar content of Sangxiong is higher than the atmospheric value (∼0.93%), indicating that there may be Ar from deep sources. The N₂/Ar ratio is a reliable index to judge whether the gas derives from the atmosphere. The atmospheric N₂/Ar is about 84, and the water-soluble gas N₂/Ar of saturated air is about 38. The N₂/Ar ratio of hot spring gas in the northern YGR is close to that of saturated air dissolved in water and atmosphere, which is the result of infiltration of surface water into the ground and participates in the geothermal water cycle, while the N₂/Ar ratio of Ningzhong 2 is relatively higher, suggesting that there may be N₂ addition from deep structures (Kita et al., 1993). The contents of H₂, He, and CH₄ are very low, which are four orders of magnitude lower than those of the main gas components (N₂, CO₂).

TABLE 1

TABLE 1. Chemical composition of hot spring gases in the northern Yadong-Gulu rift.

4.2 Isotopic Compositions of Volatile Gases

4.2.1 Helium Isotopes

Helium isotopes are expressed as R/Ra (R is the ³He/⁴He value of the sample and Ra is the atmospheric ³He/⁴He, Ra = 1.4 × 10⁻⁶). The measured helium isotopes cover a range of 0.11–0.93 Ra (Table 2). In order to deduct the contamination from air, we use the ⁴He/²⁰Ne ratio to correct R/Ra and express it as Rc/Ra, which ranges from 0.10 to 0.87 Ra. Figure 2 shows the correlation between ⁴He/²⁰Ne ratios. We found that the helium isotopes in Ningzhong appear near the air source, indicating that they may be affected by air contamination, while other gas samples show the characteristics of crustal origin.

TABLE 2

TABLE 2. Isotopic compositions of He and CO₂ of hot spring gases in the northern Yadong-Gulu rift.

FIGURE 2

FIGURE 2. Diagram of Rc/Ra versus ⁴He/²⁰Ne. The Rc/Ra and ⁴He/²⁰Ne ratios’ end-members are as follows: air helium: Rc/Ra = 1, ⁴He/²⁰Ne = 0.318; crustal helium: Rc/Ra = 0.02, ⁴He/²⁰Ne = 1000; mantle helium: Rc/Ra = 8, ⁴He/²⁰Ne = 1000 (Sano and Wakita, 1985; Sano and Marty, 1995). Data of the Tengchong volcanic area and Jinshajiang-Red River fault zones are cited from Wang et al. (1993) and Zhou et al. (2020), respectively.

4.2.2 Carbon Isotopes (δ¹³C_CO2)

Generally, CO₂ in hot spring gas in the geothermal area mainly includes three sources: the mantle source, the source of carbonate decomposition, and the source of decomposed organic matter in sediments. Their carbon isotopes are from −8 ‰ to 4‰, ∼0‰, and less than −10‰, respectively (Dai et al., 1996; Xu et al., 2012). δ¹³C_CO2 of hot spring gas in the northern YGR ranges from −11.2 ‰ to 0.1‰ (versus PDB) (Table 2), indicating that CO₂ is mainly derived from mantle degassing and the decomposition of carbonate rock, while Gulu 1 and Sangxiong CO₂ may be organic sources.

4.3 CO₂/³He and CH₄/³He Ratios

Generally, mantle fluid has a CO₂/³He ratio of about 1.5 × 10⁹ (Marty and Jambon, 1987), while the CO₂/³He value between 10¹² and 10¹⁴ is of crustal origin (limestone and sedimentary organic matter) (Sano and Marty, 1995). In our study, CO₂/³He ratios cover a range of 6.3 × 10⁸ and 1.8 × 10¹¹ (Table 2). We noted that the CO₂/³He value of Sangxiong is lower than that of the mantle, which may be affected by secondary processes (fractionation or calcite precipitation). The CO₂/³He values of Luoma 1 are close to that of the mantle (1.5 × 10⁹), while the higher CO₂/³He values of other hot spring gases indicate that they are derived from the crust. The CH₄/³He value can show the potential source of methane (Botz et al., 1999). The CH₄/³He ratio ranges from 0.2 × 10⁸ to 3.8 × 10⁹ (Table 2), which is larger than the value of the mantle, indicating that the gas is mainly derived from the crust (Botz et al., 1999).

5 Discussion

5.1 Origin of Volatiles in the Northern YGR

5.1.1 He

Helium in the northern YGR shows obvious crustal source (Rc/Ra < 1; Figure 2), which may be related to the extremely thick crust and the extensive development of granites in the Lhasa terrane. At about 55 Ma, the Indian plate collided with the Lhasa terrane, and the TP began to uplift (Zhu et al., 2015). With the continuous convergence of plates, the Indian plate subducted below the lower crust of Lhasa terrane, and the crustal thickness of Lhasa terrane was twice the thickness of normal crust (Zhang et al., 2014). In the crust, abundant ^{235, 238}U and ²³²Th will generate ⁴He by α-radioactive decay, and the in situ ⁴He concentration will gradually accumulate with the increase of time (Zhou and Ballentine, 2006). ⁴He will be released from rocks and minerals under the tectonic activities of fault zone. As shown in Figure 2, the helium isotopes of hot spring gas in the YGR decrease with the increase of ⁴He/²⁰Ne ratios, indicating the continuous addition of crustal helium in the process of helium transport from underground to the surface. Therefore, the thicker crustal thickness will produce more radiogenic helium, resulting in low helium isotopes, such that the helium isotopes of the Jinshajiang-Red River fault zone, which cover a range of 0.04 Ra to 0.62 Ra, show a crustal helium (Figure 2; Zhou et al., 2020), just like the helium characteristic in this study. A similar inference has been reported in the Karakoram fault zone on the TP (Klemperer et al., 2013). In addition, the Lhasa terrane had multi-stage magmatic intrusion (Ji et al., 2009; Zhang et al., 2014) in the process of amalgamation, convergence, and compression with the Qiangtang terrane and the Indian subcontinent. Magmatic rocks from the Mesozoic to the Miocene are developed in the YGR (Zhu et al., 2009; Zhu et al., 2011; Sun et al., 2015), mainly granites. Moreover, feldspathic granites in the late early Cretaceous are also developed outside the rift zone, such as Naqu (Sun et al., 2015). Granites contain more U and Th elements, so the radioactive decay of these two elements is one of the main sources of crustal helium in the YGR.

5.1.2 CO₂ and CH₄

In a geothermal system, the source of CO₂ may not be accurately determined just by δ¹³C_CO2 because the phase separation of CO₂ between vapor and liquid will lead to the fractionation of isotopes, and it makes the gas CO₂ rich in light isotopes and has a more negative carbon isotope value (Barry et al., 2013). The CO₂/³He ratio is another reliable index to judge the source of CO₂. However, this index is also affected by phase separation in hydrothermal systems in the following two cases: on the one hand, the temperature is larger than 100 °C, and on the other hand, some kind of gas is supersaturated. The fractionation is mainly manifested in the difference of solubility between CO₂ and He in water, and He is more inclined to release from the liquid than CO₂. Therefore, the gas CO₂/³He ratio can only represent the minimum estimated value of the original value (Barry et al., 2013). Except for the hot spring gas of Sangxiong and Luoma 1, the lowest CO₂/³He ratio of the northern YGR is higher than that of the mantle, indicating that CO₂ is mainly derived from the crust. Combining the CO₂/³He ratio with δ¹³C_CO2, the results show that CO₂ mainly comes from the decomposition of limestone, which is related to the lithostratigraphy of Lhasa terrane. From the middle and late early Cretaceous to the early late Cretaceous, extensive transgression occurred in Tibet. The marine strata covered most of the Lhasa terrane, and the lithofacies were composed of siliceous detritus and carbonate interbeds (Zhang et al., 2004; Zhang et al., 2012a). Therefore, CO₂ is mainly formed by thermal decarbonization of carbonate. In addition, the CO₂/³He ratio of Sangxiong hot spring is lower than that of the mantle, and the content of CO₂ is very low, which may be due to the loss of CO₂ in the process of gas migration. Barry et al. (2013) suggested that the precipitation of calcite will lead to the decrease of fluid CO₂/³He ratio. de Leeuw et al. (2010) also reported that the precipitation of calcite results in the decrease of fluid CO₂/³He ratio and δ¹³C_CO2 at the same time. It is speculated that the CO₂/³He ratio in the gas will be lower. Therefore, we think that the loss of CO₂ in Sangxiong hot spring may be related to the precipitation of calcite.

CH₄/³He values show that CH₄ is derived from the crust. CH₄ in nature includes biogenic CH₄ and abiogenic CH₄, mainly biogenic CH₄. Biogenic methane includes two sources: microbial methane production and decomposition of sedimentary organic matter (Schoell, 1988). In addition to the abiogenic CH₄ derived from the mantle, in the strongly alkaline (pH > 10) and middle–high temperature (>150°C) hydrothermal system dominated by serpentine, abiogenic CH₄ can be synthesized by FTT reaction (Horita and Berndt, 1999; Fu et al., 2007) on the one hand, which needs to be mediated by H₂; on the other hand, it can be formed by hydration of water and olivine (Oze, 2005; Miura et al., 2011; Suda et al., 2014). However, there is no sufficient condition for the synthesis of abiotic methane because hot springs in the northern YGR have granites, sandstones, and sand conglomerates as thermal reservoirs (Liu et al., 2014), and the pH value of the hot springs is less than 10 (He et al., 2012; Yuan et al., 2014; Guo et al., 2019). Therefore, we suggested that CH₄ in the northern YGR may be of biogenic origin.

5.2 Helium in the Northern YGR and Its Geological Significance

Hot spring volatiles show great difference in geochemical characteristics in different geological background due to the difference of deep heat source. The TP is rich in geothermal resources, and its heat flows are higher than the continental mean heat flow (Jiang et al., 2016; Jiang et al., 2019), indicating a high thermal anomaly. The northern YGR and Tengchong volcanic area are all located in the Lhasa block of the TP. The heat flow value of the former is up to 319 m/Wm² (in Naqu), while that of the latter is 80–150 m/Wm² (Hu et al., 2000). With the increase of distance from the volcanic area, the heat flow value decreases gradually (Sun et al., 2016). The Tengchong volcanic area has experienced many volcanic eruptions since the late Miocene (Wang et al., 2007), and its high heat flow is considered related to lithosphere thinning caused by asthenosphere upwelling (Sun et al., 2016). The helium isotope of Tengchong hot spring is close to 8 Ra (Figure 2), which indicates that hot spring gas mainly derives from mantle magmatic degassing, and this is also proved by subsequent studies on hot spring gas (Ren et al., 2005; Xu et al., 2012). However, the helium isotope of the northern YGR hot spring is less than 1 Ra, which is mainly of crustal origin. Different from the Tengchong geothermal area, the YGR is a typical non-volcanic high temperature geothermal area, and Quaternary volcanic activity has not been found (Dor, 2003). To understand the heat structure of the YGR, we calculated the crust/mantle heat flow ratio by using helium isotopes and expressed it as q_c/q_m. q_c/q_m values range from 0.84 to 1.48, indicating that the crustal heat flow is the main contributor of the heat flow (Wang, 2000; Tang et al., 2017), which is consistent with the previously reported geothermal structure of the hot crust and cold mantle in southern Tibet (Shi and Zhu, 1993). Therefore, we suggested that the heat of the northern YGR geothermal system may be contributed by the interior of the crust although other scholars suggested that there is partial melting in the boundary between the mantle and the lower crust or the upwelling of molten magma from mantle wedges beneath the northern YGR (Yokoyama et al., 1999; Zhang et al., 2017).

The heat flow values of the geotectonic unit of the TP decrease gradually from south to north, and this regional anomaly relates to deep structure (Jin et al., 2019). The INDEPTH project found seismic bright spots at the subcrustal 15–18 km in southern Tibet (from the Tsangpo suture to the Dangxiong graben), which are considered to be partial melting in the crust (Brown et al., 1996; Kind et al., 1996; Nelson et al., 1996). Therefore, the heat source of the northern YGR may be the partially melted crust (Yuan et al., 2014). However, the genetic mechanism of partial melting in the crust is still controversial. On the TP, the overlapping shear friction heat generation of the lithosphere during the collision orogeny may induce local melting in the crust, resulting in the formation of abnormal heat flux in the crust (Wang et al., 2013; Jin et al., 2019). Early geophysical and geothermal spring geochemistry studies suggested that the partial melting is silicate magma (Brown et al., 1996; Kind et al., 1996; Li and Hou, 2005). Later, Bea (2012) pointed out that radiogenic heating of the crust is usually essential for the generation of large amounts of granitic magma. 3D thermomechanical modeling simulation suggested that radioactive heat and shear heating provide the heat source for partial melting in the middle crust in Tibet (Chen et al., 2019). The latest magnetotelluric study suggested that the partial melting of the middle crust of southern Tibet was formed by crustal radiant heat and strain heating in the Miocene (Xie et al., 2021). Other geophysical studies suggested that the genesis of the YGR is related to the asthenosphere upwelling and/or the tearing of the Indian plate lithosphere (Chen et al., 2015; Wang et al., 2017a), that is, the partial melting of the crust is formed by the upwelling of mantle material. The results of helium isotope, carbon isotope, and the ratio of gas composition to ³He in this study show that hot spring gas mainly derives from the crust, given the tensile age of the YGR is basically consistent with that of the melting of the middle crust, so we tend to suggest the partial melting of the YGR crust formed by the stress heat and radiation heat accumulated in the crust. Besides, the high heat flow of the Lhasa block is mainly concentrated in the north–south direction of Yangbajing–Lhasa–Naqu–Gudui area (Jiang et al., 2016), and geothermal activities are also developed in the north–south trending rift. Nábělek and Nábělek (2014) predicted the thickness (∼10–20 km) of the upper crust of brittle Tibet, which is consistent with the focal depth of the earthquake and the distribution of the crustal melting layer, indicating that the rift plays a role in connecting with the deep heat source. In addition, we have observed that high temperature hot springs are more developed in the rift than outside the rift, indicating that granite also plays an indispensable role in the supply of heat sources. Magmatic rocks are widely exposed in the rift, and the lithology is mainly granite. The radiative heat provided by granites may affect the shallow temperature of hot springs.

In a word, the helium isotopes of hot springs in the northern YGR indicate that the partial melting in the crust provides heat for the geothermal area, which was the stress heat during collision between India and Eurasia on the one hand and the radioactive heat of the crust on the other hand; the rift zone is the channel between the surface hot spring and the heat source; the local thermal difference of hot springs may be related to the distribution of granites.

6 Conclusion

We have analyzed the gas geochemical characteristics of hot springs in the north of Yadong-Gulu rift and found that the volatiles of hot springs are mainly derived from the crust. To further understand the geothermal structure of the Yadong-Gulu rift, we calculated the heat flow ratio of the crust to the mantle through helium isotopes and found that the heat flow is mainly contributed by the crust. Helium isotopes of hot springs indicate that the heat sources of hot springs are formed by crustal processes. Combined with the analysis of geophysical data, the results show that the tectonic heat and reflection heat of the middle crust of the Lhasa block are partially melted due to the collision of the Indo-Eurasian plate, and it provides heat for geothermal activity in the rift.

Data Availability Statement

The original contributions presented in the study are included in the article/Supplementary Material, further inquiries can be directed to the corresponding author.

Author Contributions

The idea for this article was provided by XW and XY. XY wrote the manuscript and drew the diagrams. XW, ZW, and GW guided and modified the work of this paper. HY conducted sample collection on-site. LL carried out the experimental work. TZ, XM, and SZ provided many suggestions for this article.

Funding

This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 41172133, 41831176, 41902028, and 41972030), the Second Tibetan Plateau Scientific Expedition and Research (STEP) Program (Grant No. 2019QZKK0707), the National Key R&D Program of China (Grant No. 2017YFA0604803), the Chinese Academy of Sciences Key Project (Grant No. XDB26020302), the CAS “Light of West China” Program, and the Key Laboratory Project of Gansu (Grant No. 1309RTSA041).

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Publisher’s Note

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.

Abbreviations

BNS, Bangong-Nujiang suture; ITS, Indus-Tsangpo suture; JF, Jiali fault; MBT, Main Boundary Thrust.

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