Yn-fng Feng, Xio-xu Zhng, Bo Zhng, Jing-to Liu, Yng-gng Wng, De-long Ji, Li-rong Ho,Zho-yu Kong

a Development and Research Center of China Geological Survey, Beijing 100037, China

b China University of Geosciences (Beijing), Beijing 100083, China

ABSTRACT

The Gonghe Basin, a Cenozoic down-warped basin, is located in the northeastern part of the Qinghai-Xizang (Tibetan) Plateau, and spread over important nodes of the transfer of multiple blocks in the central orogenic belt in the NWW direction. It is also called “Qin Kun Fork” and “Gonghe Gap”. The basin has a high heat flow value and obvious thermal anomaly. The geothermal resources are mainly hot dry rock and underground hot water. In recent years, the mechanism of geothermal formation within the basin has been controversial. On the basis of understanding the knowledge of predecessors, this paper proposes the geothermal formation mechanism of the “heat source–heat transfer–heat reservoir and caprock–thermal system” of the Gonghe Basin from the perspective of a geological background through data integrationintegrated research-expert, discussion-graph, compilation-field verification and other processes: (1) Heat source: geophysical exploration and radioisotope calculations show that the heat source of heat in the basin has both the contribution of mantle and the participation of the earth's crust, but mainly the contribution of the deep mantle. (2) Heat transfer: The petrological properties of the basin and the exposed structure position of the surface hot springs show that one transfer mode is the material of the mantle source upwells and invades from the bottom, directly injecting heat; the other is that the deep fault conducts the deep heat of the basin to the middle and lower parts of the earth's crust, then the secondary fracture transfers the heat to the shallow part. (3) Heat reservoir and caprock: First, the convective strip-shaped heat reservoir exposed by the hot springs on the peripheral fault zone of the basin; second, the underlying hot dry rock layered heat reservoir and the upper new generation heat reservoir and caprock in the basin revealed by drilling data. (4) Thermal system: Based on the characteristics of the “heat source-heat transfer-heat reservoir and caprock”, it is preliminarily believed that the Gonghe Basin belongs to the non-magmatic heat source hydrothermal geothermal system (type II21) and the dry heat geothermal system (type II22). Its favorable structural position and special geological evolutionary history have given birth to a unique environment for the formation of the geothermal system. There may be a cumulative effect of heat accumulation in the eastern part of the basin, which is expected to become a favorable exploration area for hot dry rocks.

Keywords:

Gonghe Basin

Geothermal formation mechanism

Qinghai-Xizang (Tibetan) Plateau

Heat source-heat transfer-heat reservoir and

caprock-thermal system

Hot dry rocks

1. Introduction

1.1. Regional geological background of the Gonghe Basin

The Gonghe Basin is located in the northeastern part of the Qinghai-Tibetan Plateau. It resembles a diamond shape and is small in the west. It has a total area of 24400 km2and is the third largest basin in Qinghai, China. The Yellow River flows from the southwest to the northeast along its short axis,and is embedded in the center of the basin with a cutting depth of 500–600 m. The basin is divided in two to form a drainage basin. It is customary to refer to the Tanggemu-Gonghe and its west part from the Yellow River as the West Basin, the Guinan-Shagou and its southeast part as the East Basin, and the Longyangxia Dam to the east as the Guide Basin. The Gonghe Basin mentioned in this paper is the generalized Gonghe Basin covering the West Basin, the East Basin and the Guide Basin.

The Gonghe Basin is spread over the important nodes of the transfer of multiple blocks (such as the West Qinling,Qilian Mountain, East Kunlun, Qaidam Block, and Oulong Brooke Blocks) in the central orogenic belt in the NWW direction (Zhang GW et al., 2004; Jiang CF et al., 2000), Shi BY et al. (1982) called it “Qin-Kun Fork” and “Gonghe Gap”.The basin is bounded by the Wahongshan fault and the Qaidam block (including the East Kunlun orogenic belt) in the west, and is adjacent to the West Qinling with the Duohemao fault zone as the bound in the east, and is adjacent to the Bayan Har-Songpan orogenic belt with the Mianlve-Animaqing suture zone as the bound in the south, and is adjacent to the Qilian orogenic belt with the Zongwulong-Qinghai Nanshan fault as the bound in the north (Fig. 1a, b).The basement of the basin was formed in the Triassic period.It began to accept sediments in the Early Cretaceous period and shaped in the Early Pleistocene period. It has experienced the complex process of basin formation-lake basin development and the expansion-basin fixing-interval uplift of the basin.

Fig. 1.Brief map of the geotectonic location in the Gonghe Basin: a-Geological map of the Gonghe Basin; b-modified from Zhang HF et al.,2006, Zheng JP et al., 2010.

1.2. The status of geothermal resources in the Gonghe Basin

The Gonghe Basin has a higher heat flow value and a significant thermal anomaly. The average geothermal gradient is (6.7 –6.8)℃/100 m (Yan WD et al., 2015). Geothermal resources are mainly hot dry rock and underground hot water.The hot dry rock resources are widely distributed in the basin basement and the buried depth is relatively shallow. Since 2013, China Geological Survey and Qinghai Province have jointly promoted the investigation of hot dry rock geothermal energy in key areas of Qinghai, and achieved a major breakthrough in the exploration of hot dry rock in China.Fourteen concealed hot dry rock masses were identified in the Gonghe Basin, and four hot dry rock masses were identified outside the basin, with a total area of 3092 km2. In the exploration holes carried out in one of the hot dry rock masses, the Qabqa hot dry rock, the temperature at the depth of the 3705 m at the hole bottom reached 236℃. There are two types of underground hot water resources: one is densely integrated in the form of hot springs and exposed in the deep faults around the basin. According to incomplete statistics,there are 6 hot springs over 60℃, the highest temperature can reach 93.5℃ (Fig. 2). The second one spreads throughout the basin in the form of layered heat reservoirs dominated by conduction heat, including the Lower Tertiary thermal reservoir and the Neogene thermal reservoir of the Quaternary.

Fig. 2.Distribution of major faults and hot springs in the Gonghe Basin and surrounding areas (after Zhang SQ et al., 2016).

1.3. Disputes over the geothermal formation mechanism in the Gonghe Basin

The geothermal energy source mainly comes from the inshell melt, the Cenozoic volcanic rocks, the deep heat of the Great Rift Valley, the presence of highly radioactive materials, and the structural collisional shear heat. Different scholars hold different views on the geothermal sources and formation mechanisms of the Gonghe Basin. Representative understanding mainly has the following aspects:

(i) Sun ZX et al. (2011); Xue JQ et al. (2013); Yan WD et al. (2015) and Zhang SQ et al. (2016) considered that the geothermal energy source in the Gonghe Basin is hot dry rock, based on the fact that the basin is in the transitional slope from the Yellow River uplift which is shallow in the west and deep in the east to the Tanggemu depression (Fig. 5),the basement depth is 1200–1400 m, and the lithology is the Indosinian-Yanshanian intermediate-acidic rock mass and the Triassic clastic rocks and carbonate rocks. The upper Quaternary system is a thermal reservoir cap with a thickness of 530–600 m. The lower Neogene fine sandstone and glutenite are thermal reservoirs with a buried depth of 600–1400 m, which can form a basin-type layered heat reservoir. The geothermal gradient of the basin is twice that of the normal crust, which is a geothermal anomaly zone, and the heat source is related to granite.

(ii) Wang B et al. (2010) believe that the Gonghe Basin is a Cenozoic fault basin with a geothermal geologic background of well-conducting and fault convective heat reservoir, and forms the Qin-Kun joint geothermal girdle in the SN direction together with the fault type geothermal distribution in the tectonic magmatic belt of the eastern and western sides. Therefore, the geothermal heat reservoir in the Gonghe Basin is a basin-conducting layered heat reservoir and a fault convective strip-shaped heat reservoir.

(iii) Li BX et al. (2013); Li LG et al. (2017) and Zhang C et al. (2018) argued that the high temperature heat flow anomalies in the Gonghe Basin may be due to the combined action of high-area background heat flow, mainly related to the radioactive heat producing the thickened crust and the cooling of shallow magma chambers.

(iv) Yan WD et al. (2013) argued that there is no continental rift in the Gonghe Basin. The basement of the basin is the Mesozoic Indosinian volcanic rocks, and the existence of high-level radioactive materials has not been found in the geological survey. The geothermal energy accumulation in the basin may be closely related to the heat flow anomalies of the crust and the mantle, which is conductive geothermal.

(v) Zhang SQ et al. (2016) believed that the hot dry rock of the Guide Hot Springs may belong to the hot dry rock controlled by the semi-flower-like strike-slip fault. There is a small magma chamber in the deep end, and the magma moves upward along the fault plane, heating crystalline rock series(medium-deep metamorphism and granite) to form hot dry rocks.

Analysis of the scholars' views above, will inevitably raise and consider the following questions:

(i) Where does the heat source of the Gonghe Basin originate? If the Triassic granite in the basement of the Gonghe Basin is a hot dry rock, can the heat be stored up to now after about 200 Ma? If the geothermal formation mechanism is heat producing by the radioactive isotopes of the thickened crust, can the radioactive isotopes generate such a large amount of heat? If you agree with generating heat by radioactive isotopes, should there be abundant geothermal resources in other parts of the Qinghai-Tibetan Plateau? Is the high geothermal flow value derived from the mantle, the Cenozoic rock mass or the deep magma chamber?

(ii) What is the heat conducting and transferring route in the Gonghe Basin? Is the fault structure the only channel?

(iii) Is the heat storage in the Gonghe Basin a conductive layered heat reservoir and a fault convective strip heat storage? Is the caprock enough to sustain such a large amount of heat?

(iv) How is the thermal system of the Gonghe Basin formed?

Based on this, the authors discussed the geothermal formation mechanism of the Gonghe Basin.

2. Methods

2.1. Research basis and process

(i) Comprehensively collecting and digesting pre-existing materials and literature journals related to regional geological surveys, geophysical surveys, hydrological-engineeringenvironmental and a geothermal resources survey and exploration, carrying out data integration and comprehensive research work, and referring to, and learning from, foreign mature and advanced exploration methods and techniques of hot dry rocks (Thorsten A et al., 2014; Donald W et al., 2012;Cash D et al., 2013), we hope that these successful experiences will be instructive for the analysis of geothermal resources in the Gonghe Basin. Among the geological work completed by predecessors in the region, there is less integration with geothermal resources. To explore the geothermal formation mechanism of the Gonghe Basin, it is necessary to consider all factors. For example, on the basis of the main trace element content and dating data of the rock mass around the Gonghe Basin (Zhang HF et al., 2006), the radioactivity of the rock mass can be calculated (Zhang BT et al., 2010), and based on the comparison and reference of the origin of the hot dry rock in the Cooper Basin, Australia(Meixner T et al., 2010; Siégel C et al., 2014), it can be evaluated whether radioactive heat in the Gonghe Basin can generate enough heat (Rybach L and Buntebarth G, 1981).

(ii) Studying the deep structure of the basin (Chang H et al., 2009; He RQ et al., 2017; Long ZY et al., 2009; Sun ZX et al., 2011), Earthquake (Chang BQ et al., 1997; Zhang M et al., 2000; Li X et al., 1996) and surface structure characteristics (Sun YG et al., 2007), analyzing the geological background information of geothermal resources, and compiling the geological background map of the geothermal source in the Gonghe Basin, comprehensively expresses the geothermal-related strata, rocks, structures and various geological anomalies, providing a strong and reliable evidence for exploring geothermal genesis.

(iii) Organizing several expert seminars on magmatic rocks, active structures, geophysical exploration and geothermal resources in the Gonghe Basin, and inviting experts to guide and check the progress of the work, the contents of maps and the geothermal mechanism, and deepening the research results, reaching and completing the content of the maps.

(iv) Organizing the field research and verification of geological key points in the Republican Basin, enriching and supplementing the geological background of geothermal resources, further revising and perfecting the relevant maps,and making new knowledge and new judgments more convincing, and making the content of the drawings more objective and practical.

(v) Based on the above work, the formation mechanism of geothermal “heat source –heat transfer–heat reservoir and caprock–thermal system” in the Gonghe Basin is proposed from the perspective of geological background, and the paper is summarized and compiled.

2.2. Analysis of the geothermal formation mechanism in the Gonghe Basin

The terrestrial heat-flow (referred to as heat flow) is the most direct surface display of the thermodynamic process inside the earth, reflecting the thermal state and energy balance of the lithosphere. It is a necessary parameter for evaluating the potential of geothermal resources and has important social and economic value. The terrestrial heat-flow value of the Gonghe Basin is 136.6 mW/m2, which is much higher than the average heat flux of the northeastern margin of the Qinghai-Tibetan Plateau (74–75 mW/m2) (Jiang GZ et al., 2016). How is such rich geothermal energy formed? The sources, transmission channels and storage conditions of the geothermal formation in the Gonghe Basin are discussed and analyzed one by one. Based on this, the thermal system is studied and analyzed, and this is used as the initial understanding of the geothermal formation mechanism in the basin.

3. Heat source

The source of geothermal energy in the basin is mainly deep mantle.

3.1. Analysis of geophysical exploration data

Xu ZQ et al. (2004) ; Zhang XT et al. (2007); Jiang M.,(2009) and Yan WD et al. (2013) analyzed and judged based on the Qinghai-Tibetan Plateau seismic tomography profile and the Yushu-Gonghe seismic tomography profile analysis:There is a low-speed belt with a width of 150 km in the upper mantle of the eastern Kunlun block where the Gonghe Basin is located. It is associated with the mantle plume characterized by large low-speed anomalies in the deep mantle of the Bayan Har block. The low-speed belt extends to the crust, and forms a heat flow anomaly zone in the Gonghe Basin and in different parts of the surrounding area 1–40 km below the surface (Fig. 3), resulting in the formation of abundant underground thermal energy resources mainly composed of hot dry rock and underground hot water in the shallow part of the basin. According to the magnetotelluric inversion results of He RQ et al. (2017), the Gonghe Basin has an upper crust high-conductivity layer between 5–18 km deep in the crust,with a resistivity of 4.4–7.8 Ωm and a thickness of 5 to 13.5 km,which is possibly the heat source of high temperature hot water.

Fig. 3.Velocity section in shallow crust of Yushu-Gonghe (modified from Jiang M, 2009; Yan WD et al., 2013).

According to Huang WC et al. (2000), when the Indian plate collided with the Asian plate, there was a geological event in which the asthenosphere escaped eastward between the Bangong-Nujiang suture zone, Jinshajiang suture zone,and Kunlun fault zone. According to Deng JF and Mo XX(2004, unpublished data), when the asthenosphere of the Qinghai-Tibetan Plateau is extruded eastward, there should be greater flux along the Ailaoshan-Red River Belt and the Qinling Mountains, namely the “asthenosphere channel”,which has been confirmed by GPS measurements. The heat source of the Gonghe Basin may be related to the asthenosphere channel of Qinling.

Based on the above geophysical exploration data, the geothermal energy of the Gonghe Basin may be the result of the thermal conduction of a deep mantle.

3.2. Radioisotope calculation

3.2.1. Calculation of the radioactive heat generation rate of rock

The radioactive heat generation rate of rock refers to the radioactive heat generated by radioactive elements contained in a unit volume (or unit weight) of rock during the decay process per unit time. It is one of the basic parameters describing the thermal physical properties of rock, and it is also an indispensable important parameter for the analysis of the thermal structure of lithosphere. Its magnitude is decisive for distinguishing the crust and mantle distribution of surface heat flow. The radioactive heat generation rate of rocks depends on the content of U, Th and K. The order of importance of the three elements in the history of geothermal evolution is Th, U and K (Morgan P, 1985). The heat generation amount of the first two elements accounts for 80%–90% of the total amount (Lachenbruch AH and Bunker CM, 1971). The relevant parameter data for calculating the radioactive heat generation rate of rocks is listed in Table 1.

Table 1.Radioactive heat generation rate and related constants of U, Th, and K (after Zhang BT et al., 2010).

Rybach (1976) proposed a formula for calculating the radioactive heat generation rate of rocks:

Zhang BT et al. (2010) derived the calculation formula of the radioactive heat generation rate of rock by three different factors according to the relevant parameters:

Then, according to the time (t) of formation rock (rock mass or ore body) formation, the current measured radioactive heat generation rate is corrected to obtain a correction formula that is more in line with the geological actual paleo-radiation heat generation rate (QA*):

In formulas (1)–(7),QAis the radioactive heat generation rate of rock,QA* is the paleo-radiothermal heat generation rate;ρis the density of the corresponding rock, the unit is g/cm3;CU,CThand CKare respectively U, Th and K content in the rock, the unit of U and Th is 10-6, the unit of K is %;HGU is the unit of radioactive heat generation rate (1HGU =4.1868 × 10-7J/m3·s or 1 × 10-13Cal/cm3·s);NU,NTh,NKare correction coefficients for U, Th, and K, respectively.

The medium-acid intrusive rock is easy to enrich radioactive heat generating elements and is the most important radioactive heat generation rock in the crust. The data on the radioactive heat generation rate and the paleoradiothermal heat generation rate (QA*) of the medium-acid intrusive rocks in the various periods of the Gonghe Basin are shown in Table 2.

It can be seen from Table 2: (1) The difference between the paleo-radiation heat generation rate and the radioactive heat generation rate of the rocks in the Gonghe Basin is between 0.03 and 0.17, and the change is greater than 3%. It is necessary to correct the paleo-radiation heat generation rate of the intrusive rock mass. (2) The current radioactive heat generation rate of the western Triassic medium-acid rock mass is 0.89 –3.56 μW/m3, with an average value of 1.87 μW/m3(n=21); the current radioactive heat generation rate of the Triassic intermediate-acidic rock mass in the eastern basin is 2.06 –4.60 μW/m3, and the average value is 2.62 μW/m3(n=17), which is higher than that in the west, and is basically equal to the average heat generation rate of granite in North Tibet (65 Ma) 2.6 μW/m3(Shen XJ et al., 1990). If the radioactive heat generation rate of the early Jurassic granodiorite (J1γδ) is accumulated, the value may be higher.Combined with the analysis of the geothermal heat values of 136.6 mW/m2and 123.1 mW/m2(Jiang GZ et al., 2016) in the western and eastern parts of the Gonghe Basin (Jiang GZ et al., 2016), it is known that: (1) more of the geothermal heat in the western part of the Gonghe Basin may be from the deep mantle, but not the middle and lower crust; (2) The crust rocks in the eastern part of the basin are more acidic (γδ,ηoandγ), and the formation age is also relatively new (the oldest age is 237 Ma, no Middle Triassic (T2) rock mass, and there is early Jurassic granodiorite (J1γδ) exposed), and the nearsurface radioactive element enrichment layer is less affected by denudation and flattening.

3.2.2. Analysis of geothermal heat flux in the basin

The commonly observed terrestrial heat-flow density (Q)is the sum of the mantle heat flow density (Qm) and the crustal heat flow density (Qc):Q=Qm+Qc. The crustal heat flow mainly comes from the heat generated by the radioactive decay of rocks in the earth's crust, and the calculation method isQc=ΣAi×Di. Wherein AiandDirepresent the radioactive heat generation rate of the i-th layer and the thickness of the layer, respectively.

The value of the heat generation rate in the deep crust is critical to the calculation accuracy of the lithospheric thermal structure. At present, there are two main ideas for estimating the heat generation rate in the deep part of the basin,especially the middle and lower crust. First, based on the radioactive heat generation rate of various types of rocks in the system study area and its periphery, establishing the crustal structure and rock composition model of the crust in the study area according to geophysics, geology, geochemical and other materials to estimate the heat generation rate of the deep crust rocks by the weighted average of the lithology percentage. Second, based on the basic principle about the crustal wave velocity, the greater the wave velocity, the more basic components in the material composition, and the lower the heat generation rate. Rybach L and Buntebarth G (1981)used empirically tested rock wave velocity and heat generation rate values to establish an empirical relationship between P-wave velocityVpand the heat generation rate A,which was used by a large number of researchers: lnA=B-2.17×Vp, whereBis a constant term, 12.6 for the Phanerozoic rocks and 13.7 for the Precambrian rocks. Because of the lack of corresponding data in the study area, we use the second method. Wherein the Vp data is from Zhang Z et al. (2011).

For the sedimentary caprock of the shallow surface, the corresponding heat generation rate should be calculated by systematic sampling and testing U, Th and40K content, but there is no corresponding data for the Gonghe Basin, so the summary value of the heat flow for the sedimentary caprock of Zhang C et al. (2018) is used; and for the deep crust and lithospheric mantle below the sedimentary caprock, it is estimated based on the seismic velocity data and the crustal composition research results in the study area. The calculation results of terrestrial heat-flow density are shown in Table 3.

Table 2.Data andcalculation results of theradioactive heat generation rate (QA) andthe paleo-radiationheat generation rate (QA＊) of medium-acidintrusive rocks in the Gonghe Basin.

Table 3.Calculation results of terrestrial heat-flow density in the Gonghe Basin (after Zhang ZJ et al., 2011; Jiang GZ et al., 2016).

It can be seen from Table 3 that the heat generation rate of the upper crust (excluding the sedimentary layer) in the western part of the Republic Basin is 1.57 μW/m3, and the heat generation rate of the middle and lower crust is 0.34 μW/m3and 0.14 μW/m3, respectively. The crustal heat generation rate generally decreases exponentially with depth,which is consistent with the general rule that the radioactive elements U, Th and40K in the crust are enriched in the shallow crust. The terrestrial heat-flow density in the western part of the Gonghe Basin is 136.6 mW/m2, in which the crustal heat flow density is 49.85 mW/m2, and the mantle heat flow density are 86.75 mW/m2, accounting for 36.49% and 63.51% of the terrestrial heat-flow density, respectively. The terrestrial heat-flow density in the eastern part of the basin is 123.1 mW/m2, in which the crustal heat flow density is 47.46 mW/m2, and the mantle heat flow density is 75.64 mW/m2,which account for 38.55% and 61.45% of the terrestrial heatflow density, respectively. The results of the terrestrial heatflow density analysis show that the heat source of the Gonghe Basin has both the contribution of mantle and the participation of the earth's crust, but mainly the contribution is of the deep mantle and more obvious in the west. It should be noted that theVp data obtained from the seismic section of the Guide Basin is used to estimate the crustal heat flow because the parameters of the seismic section in the western part of the Gonghe Basin are not found.

In fact, the thermal energy inside the Earth mainly comes from the deep part, which has become the consensus of the current earth science community (Luo ZH et al., 2006). Due to the inaccessibility of the Earth's interior, based on the substantial differences between radioactive elements in the Earth's layers, scholars used to focus on the accumulation of the disintegration heat of radioactive elements in the earth's crust as the main source of thermal energy. Even now, there are still many scholars that regard the disintegration heat of radioactive elements in the earth's crust as the main driving force of geological processes (Husson L and Moretti I, 2002).However, three-quarters of the world's magma activity is concentrated in the mid-ocean ridge (Best MG, 2003),convincing us that thermal energy comes from deep in the earth, even if the flow of the lower crust also depends on the injection of deep heat or volatiles (Dan MK and Jackson J,2002). In summary, geothermal in the Gonghe Basin may be mainly derived from the deep mantle.

4. Heat transfer

Heat injecting by the upwelling and underplating of the mantle source material and heat conduction of the Cenozoic fault.

4.1. Analysis of petrology evidence in the basin: the mantle source material upwells and invades from the bottom to inject heat

Studies have shown that the intraplate environment is also an important place for crustal growth (Condie KC, 2000).After the formation of the continental crust and lithospheric mantle, it is possible to re-inject the convective mantle material and transform the original continental crust and lithospheric mantle. The convective mantle material injected into the continent refers to the basaltic magma separated from the convective mantle, often referred to as a primary or new continental crust material. Jin ZM et al. (1996) proposed that magma underplating refers to the process or effect of intrusion or addition of basic magma (basaltic melt) from the upper mantle melting to the bottom of the lower crust. The aforementioned section 4 has proved that the heat in the Gonghe Basin is mainly derived from the deep mantle, but what evidence can we provide in petrology?

According to a survey conducted by Qi SS et al. (2012),the Duohemao Formation (K1d) in the eastern part of the Gonghe Basin is a typical combination of alkaline basalt +continental red moraine rock in the Early Cretaceous. The lithology is a grey-purple block layered-almond olive basalt,basaltic andesite, volcanic breccia with a grey-purple thick layered conglomerate, gray-purple-blue-gray medium-thick layered pebbly coarse sandstone and gray-purple thin layered mudstone. The volcanic rocks are mainly the alkaline series.Upon comprehensive analysis by petrology and geochemistry,they believed that the magmatic activity product in the internal extensional environment of the continental plate is a volcanic rock formed in the background of the extensional tectonic structure similar to the continental rift environment.Fan LY et al. (2007) of the Guangzhou Institute of Geochemistry, Chinese Academy of Sciences believe that the basic volcanic rocks were formed in the Early Cretaceous(K1), and were alkaline basalts with OIB properties, and the genesis is closely related to the de-rooting action of the lithosphere after the collision. Hu XJ et al. (2012) used the40Ar/39Ar age results of the entire rock to determine the formation of the Duofutun based volcanic rocks was 96.21 Ma±2.10 Ma, which was the early period of the Late Cretaceous (K2); they believed that the magmetism associated with the Duofutun volcanic rocks originated from the remote continental tectonic effect caused by plate boundary dynamics, that is, the early collision between the Indian plate and Eurasia (100–85 Ma), due to the rigid characteristics of the northern Tibetan Plateau, the stress is transmitted to the northeastern margin of the plateau, resulting in the tectonic disturbance of the North South composite tectonic belt that occurred in the western part of western Qinling, inducing the partial melting of the mantle in the asthenosphere to form the primary magma of the Duofu volcanic rock. Guo AL et al.(2007a, b, c) discovered the Miocene (N1) sodium-based volcanic rock series in the Duofutun area in the eastern part of the Gonghe Basin, belonging to the intracontinental volcanic rocks similar to OIB. Zheng JP et al. (2010) carried out mineral and zircon LAM-ICPMS and LAM-MC-ICPMS analyses of this basalt, and obtained the age value of –14 Ma and the dark inclusions in the rock mass. Cai PJ et al. (2014)discovered the volcanic rock interlayer above the Linxia Formation of the Neogene in the eastern part of Guide County. This information provides important clues for analyzing geothermal sources in the Gonghe Basin! In particular, the discovery of the Cretaceous and Miocene basic volcanic rocks undoubtedly tell us: (1) the deep mantle source in the eastern basins in Cretaceous and Miocene upwells and invades from the bottom to inject heat. (2) There have been several magmatic activities in the eastern part of the basin before the Miocene period (N1), and there may be a cumulative effect of heat accumulation. (3) There should be contemporaneous intrusive rocks under the basic volcanic rocks of different ages, and it is recommended to pay more attention in the exploration of hot dry rocks.

4.2. The location of an exposed springs shows that the Cenozoic faults become a good channel for heat transfer

4.2.1. Intensive distribution of hot springs on the Cenozoic fault zone

There are three near SN-to-right strike-slip active fault zones and hot spring dense distribution belts on the east and west sides and the middle part of the Gonghe Basin (Fig. 2),which are sandwiched between the two main bound faults in the NWW direction, East Kunlun and West Qinling. The fault zone on the west side of the basin is distributed along the Wulan-Xinghai Hot Spring in the Ela Mountain. There is Ulanba Yinggeli Hot Spring (42.5℃), Xinghai Qinggenhe Hot Spring (30.0℃) and Qinggenhe SE9 km Hot spring(62℃), Xinghai hot spring (61℃) and so on along the fault zone. The middle Waligongshan fault zone is distributed along the Dangjiasi-Guomaying line. The Dangjiasi-Qunqiang concealed fault belt extends along the Waligongshan uplift belt from the north and south along the NNW. The hot springs along this tectonic belt have the highest temperatures. From north to south, there are Qunaihai Hot Springs (86℃),Hydrothermal Springs (93.5℃), and Xinjie Hot Springs(64℃). The Waligongshan fault zone on the east side is distributed along the Dangjiasi-Guomaying-Duohemao line.There are Duohemao, Dangjiasi and Qunqiang concealed fault extending along the Guomaying-Waligongshan uplift zone from the north and south along the NNW, distributed with Tongren Lancai Hot Spring (69℃), Tongren Qukuhu Hot Spring (48.5℃) and Xibusha Hot Spring (44℃). The abovementioned tectonic belt has a deep fracture and deep cut, and the Quaternary new tectonic activity is strong, which provides a good channel for deep thermal fluid convective migration(Sun ZX et al., 2011).

In addition, we learned through comprehensive research and mapping work: (1) For hydrothermal geothermal resources, the deep faults conduct heat from the deep basin to the middle and shallow parts, and then the secondary faults transfer the heat to the shallow parts (Fig. 4). (2) The intersection of multiple faults is often where the most densely distributed hot springs are found. (3) The Cenozoic faults near the EW and near SN directions in the basin and the activated NW-SE faults are favorable thermal conduction structures,followed by the NE-SW Cenozoic faults.

4.2.2. Many hot springs exposed on the contact surface of the surface uplift and depression

Combined with the analysis of previous data, the planar structure of the Gonghe Basin is generally represented by one uplift and three depressions (Fig. 5). One uplift is the Yellow River uplift, and three depressions are the Tanggemu depression, the Guide depression and the Guinan depression,which are located on the northwest, east and south sides of the Yellow River uplift. Except for the Guide depression, the secondary faults and depressions are developed in the Tanggemu depression and the Guinan depression, and the boundary is the fault structure. The Yellow River uplift area is located in the widening part of the basin, and the bulge area is located in the southwestern margin of the basin. The basin is characterized by a slope to the northeast, and the deepest is located at the Shazhuyu River. The hot springs inside the basin are not only controlled by fault structures, but also distributed in or near the contact zone between the Yellow River uplift and the surrounding depressions, indicating that the contact zone between the uplift zone and the depression zone in the basin is a favorable part of the hot spring exposure. This also seems to remind us that the contact zone between the uplift zone and the depression zone may be an important potential area for geothermal resources.

Fig. 4.Schematic diagram of the structure of the Gonghe Basin in Qinghai, China.

Fig. 5.Schematic diagram of the structure of the Gonghe basin, China (after Sun ZX et al., 2011; Zhang SQ et al., 2016).

5. Heat reservoir and caprock

Convective strip heat reservoir, hot dry rock layered heat reservoir and Cenozoic heat reservoir and caprock.

5.1. Convective strip heat storage around the basin

According to the previous survey data, there are hot spring dense integrated belts exposed along the deep and large faults around the Gonghe Basin. There are 6 hot springs over 60℃,and the highest temperature can reach 93.5℃. Among them,is the Wahongshan fault zone on the west side, where the spring water temperature is 30–62℃; in the Qinghai Nanshan fault zone on the north side, the spring water temperature is about 40℃; in the Duohemao fault zone on the east side, the spring water temperature is 48 –69℃ (Fig. 2). These hot springs constitute a convective strip heat reservoir.

5.2. Hot dry rock type layered heat reservoir underlying the basin

According to the results of borehole exploration in the Qiabqia area of the Gonghe Basin, it is complete granite(normally a high-speed stratum) 1000 m below the underground, and the temperature is generally high; the basin heat flow is abnormal, and the temperature has a linear relationship with the depth, indicating that the basin thermal energy is conductive geothermal. There are hot dry rock resources in the deep (Yan WD et al., 2013; Li LG et al., 2017; Sun ZX et al., 2011; Xue JQ et al., 2013; Zhang SQ et al., 2016).

According to DR1 drilling data, the depth of the granite(T3γδ on the exploration hole histogram, T3γ in the text report)is 1354 m, and the rock temperature reaches 87℃ when the hole depth is 1445 m (Zhao GF et al., 2016). According to DR2 drilling data, the depth of the granite (T3γδ on the exploration hole histogram, T3γ in the text report) is 1441 m,and the rock temperature reaches 112℃ when the hole depth is 1850 m. According to DR3 drilling data, the depth of the granite (T3γδ on the column of the exploration hole, ηγ in the text report, the era is unknown) is 1360 m, when the hole depth is 1620 m, the rock temperature reaches 107℃; when the hole depth is 3000 m, the rock temperature is up to 180℃.According to the DR4 drilling data, the depth of the granite(T3γδ on the column of the exploration hole, γ in the text report, the era is unknown) is 1400 m, when the hole depth is 3086 m, the rock temperature reaches 179℃; when the hole depth is 3080 m, the rock temperature is 182℃. It can be seen that the Gonghe Basin is rich in hot dry rock resources and has a high temperature and shallow burial, which is expected to become a favorable exploration area for hot dry rocks. In view of the importance of hot dry rock exploration for geothermal resources, it is recommended to conduct mineralogical, petrological and geochemical research on the granite samples in the borehole. On the one hand, it can help us accurately grasp the era and lithology of the hot dry rock in the Gonghe Basin. On the other hand, it has important scientific guiding significance for the exploration of hot dry rock exploration.

5.3. Cenozoic layered heat reservoirs and caprocks in the basin

According to previous investigations and survey data, the layered heat reservoir dominated by heat conduction spreads throughout the Gonghe basin and can be divided into the Quaternary middle and lower Pleistocene heat reservoir and the Neogene heat reservoir. The heat reservoir is covered with a thick, stable and dense sedimentary caprock.

5.3.1. Quaternary middle and lower pleistocene heat reservoir and caprock

The lithology of the Quaternary Middle and Lower Pleistocene heat reservoirs is fine sand, medium coarse sand and gravel medium coarse sand. It is a good heat reservoir with a thickness of 200–583 m and a single well water inflow of 800 m3/d. Rich, good water quality, mineralization less than 1 g/L, water temperature 38 –46℃, heat reservoir temperature increase rate of 1–4℃/100 m. The caprock above the heat reservoir is a quaternary sub-clay and sub-sand with stable distribution and poor water permeability, and the depth is 100–200 m.

5.3.2. Neogene heat reservoir and caprock

The roof of the Neogene heat reservoir is buried at a depth of 669.0–718.3 m. The lithology is the Neogene medium coarse sandstone and glutenite. The heat reservoir thickness is 495.36–747.15 m and the water temperature are 82–84.2℃.According to the DR1 drilling data in Qabuqa, the thermal temperature increase rate of the heat reservoir is 4.6–5.3℃/100 m, and the single-well water output is 871.26–1002.15 m3/d. The water quality is good, and it is HCO3·CL-Na type water with a mineralization degree of 2.19–2.23 g/L. The lithology of the caprock above the heat reservoir is lower Pleistocene sub-clay, clay, and Neogene mudstone, sandy mudstone and the like with poor water permeability, and the thickness is 80–200 m.

6. Thermal system

The favorable structural position of the basin and the special geological evolution history have given birth to a unique geothermal formation environment.

The term geothermal system is widely used in related literatures at home and abroad. The meaning and classification of geothermal systems in different periods are different. In the 1980s, Rybach et al. (1984) proposed the concept of geothermal systems, arguing that "a system in which geothermal enrichment is sufficient to constitute an energy resource can be called a geothermal system." In recent years, Bin DZ et al. (2010) defines the geothermal system as“a relatively independent system of thermal energy storage,migration, and conversion.” It is believed that one geothermal system can be divided into multiple geothermal fields, and one geothermal field can also contain one or more geothermal systems. Wang JY et al. (2015) argued that “the geothermal system refers to a relatively independent geological unit in terms of heat and fluid circulation in which geothermal energy is gathered to an available level”. Based on this, Zhang Y et al. (2017) proposed that the geothermal system can be defined as a relatively independent geological unit, which is centered on the heat source and contains the geological elements and geological functions required for heat generation, migration,accumulation and preservation, the elements and functions constitute functional units that form thermal energy accumulation. “Geological elements” include sources(including heat and water sources), channels, reservoirs, and caprocks. “Geological functions” include heat transfer,storage, preservation, and loss. Based on the comprehensive analysis of the research results and understanding of geothermal systems at home and abroad, the geothermal system is divided into two classes, four sub-classes and eight types according to the geologic tectonic background, heat source type and heat occurrence mode of geothermal system development. (Table 4).

Table 4.Classification of geothermal systems (after Zhang Y et al., 2017).

Upon comprehensive analysis of the heat source, heat transfer, heat transfer and caprock characteristics of the Gonghe Basin, we believe that it belongs to the non-magmatic heat source hydrothermal geothermal system (II21) and the dry heat geothermal system (II22) of sedimentary basin. In the hydrothermal geothermal system (II21), the heat source is in close proximity to the heat reservoir, dominated by heat conduction, and there is local hydrothermal convection, most of which has a good caprock, and the caprock has a high geothermal gradient. The dry heat geothermal system (type II22) is often found in the deep part of the hydrothermal geothermal system, and exploration and development can be explored and attempted in areas with high thermal background values.

The Gonghe Basin is located in the important junction area of the transfer of multiple blocks in the central orogenic belt. It is affected by the collision and extrusion environment of the Indian plate and the Eurasian plate. It is different from the Qinling and Kunlun orogenic belts in geological structure,magmatism and geomorphology. It is expressed in a unique form, and is also known as “the Gonghe Gap” and “Qin Kun Fork”. Its favorable tectonic position and geological evolution history are the basic prerequisites for heat source, heat transfer, heat reservoir and caprock. And the transmission,storage and preservation of heat have given birth to a unique and superior environment: (1) In Cretaceous (K), the Indian plate and the Eurasian plate began to collide, making the Zongwulong-Qinghai Nanshan orogenic belt and the East Kunlun orogenic belt adjust their positions through strike-slip faults in the background of the near-north-south extrusion to adapt to the new dynamic system. The NNW-trending dual right-lateral strike-slip and strike-slip faults, such as the Wahongshan-Hot Spring Fault, Hot Water-Riyueshan Fault and the Duohemao Fault, controlled the regional tectonic framework of the strike-slip pull-apart, strike-slip extrusion and strike-slip rotation in the Gonghe Basin (Hou KM et al.,1999; Zhang SQ, 2000). The strike-slip movement of the Duohemao fault caused rapid thermal thinning of the lithosphere and led to regional and local thermal anomalies,which contributed greatly to the geothermal resources in the eastern part of the basin. (2) In Paleogene-Neogene (E-N),with the northward movement and strong push of the Indian plate, the orogenic belts entered the uplifting orogeny period,and the difference between the basin and the surrounding orogenic belts increased significantly. The orogenic belts are strongly hedged toward the basin at the same time as the strike-slip, and the sedimentation rate is accelerated, which continues to expand and expand. The clastic sediments are as thick as 3000–7000 m, and the overall outline of the modern geological features of the basin is fixed. The Linxia Formation (N2l) conglomerate, the pebbly coarse sandstone and the Xianshuihe Formation (N2x) conglomerate and siltstone constitute a thermal reservoir, and the mudstone and sandy mudstone become caprocks. Due to violent tectonic movement, the fault zone at the edge of the basin is activated,and magmatic activity occurs in some areas, causing some heat to rise along the deep fault zone to the middle and lower parts of the crust. Geothermal resources in the eastern part of the Gonghe Basin may be closely related to this. (3) In the Quaternary Early Pleistocene (Qp1), the neotectonic movement activity is strong, the secondary fault of the fault zone develops, and the conduction heat reaches the shallow part of the basin; the Gonghe Formation river-lake sedimentary strata are widely distributed, forming the main thermal reservoir. The overlying Malan loess provides a good caprock for geothermal preservation. The hydrothermal activity in the shallow part of the basin is active and has become a prospective area for medium-low temperatures or medium-high temperature geothermal resources. (4) Since the Quaternary Pleistocene (Qp2), the basin entered the stage of intermittent subsidence, forming a significant layered geomorphic system, which further created conditions for geothermal storage.

7. Conclusions

(i) Heat source: The heat in the basin is mainly from the deep mantle. Geophysical prospecting data shows that: (1)The geothermal heat source in the Gonghe Basin is closely related to the low velocity body, and the geothermal energy is the result of deep heat conduction; (2) The heat source of the Gonghe Basin may be related to the Qinling asthenosphere channel. The results of radioisotope calculations show that the heat source of the Gonghe Basin has both the contribution of the mantle and the participation of the earth's crust, but mainly the contribution of the deep mantle, especially in the western part of the basin.

(ii) Heat Transfer: (1) The petrological properties of the basin show that the material of the mantle source upwells and invades from the bottom, directly injecting heat. (2) The exposed structure position of the surface hot springs shows that the deep faults conduct heat from the deep part of the basin to the middle and lower parts of the earth's crust, and then the secondary fracture transfers the heat to the shallow part. (3) The Cenozoic faults in the near-EW and near-SN directions of the basin (expressed as short spatial distribution and discontinuous cross section) and the activated NW-SE fault are favorable thermal conduction structures, followed by the NE-SW Cenozoic fault. (4) The contact zone between the fault intersection, the uplift zone and the depression zone in the basin is a favorable place for hot spring exposure, and may also be an important potential area for geothermal.

(iii) Heat reservoir and caprock: (1) There are several magmatic activities in the eastern part of the Gonghe Basin before Miocene (N1). There should be intrusive rocks of different ages underlying the volcanic rocks, and there may be a cumulative effect of heat accumulation, which is expected to become a favorable exploration area for hot dry rocks. (2) The Quaternary Lower Pleistocene heat reservoir and Neogene heat reservoir dominated by conduction heat spread throughout the Gonghe basin, and the upper cover has a stable and dense sedimentary layer.

(iv) Thermal system: (1) The Gonghe Basin belongs to the non-magmatic heat source hydrothermal geothermal system(Type II21) and the dry heat geothermal system (Type II22).The dry heat geothermal system often exists in the deep part of the hydrothermal geothermal system. Exploration and development can be explored in areas with high thermal background values. (2) The favorable structural position and special geological evolution history of the Gonghe Basin has given birth to a unique environment for the formation of the thermal system.

Geology is a historical science and a regional science. The exploration breakthrough of hot dry rock needs to proceed from the regional geological structure characteristics and regional evolutionary history processes, to carry out multidisciplinary joint warfare and in-depth research and exploration, in order to facilitate complementary professional advantages, technical methods, and effectively guide the exploration work. The existing understanding of the geothermal formation mechanism of the Gonghe Basin is limited to the preliminary rational understanding. It is necessary to explore the research from the depth of the complex sciences and the breadth of the earth system science,in order to extract a more profound theoretical system that reveals the actual and effective guidance of geothermal systems.

Acknowledgment

This paper was funded by the secondary project of the“National Series of Basic Geology Comprehensive Mapping(121201004000150013)” of the China Geological Survey. In the process of writing the article, the authors received strong support from China Geological Survey including, director Xueyi Xu and deputy director Lixia Xing of the Chief Engineer, director Aibing Hao and division director Eryong Zhang of the Hydrogeological Environmental Geological Survey Center, director Guiyi Xiao, deputy director Zhiyong Zhang, division director Xiaochang Mao of the Basic Department, and Director Yong Xu, secretary Haiqi Zhang,and Chief Engineer Xiaoping Hu, director Jianfeng Yang and deputy director Xuan Wu of the Development and Research Center. Professor Jinfu Deng, Changhou Zhang, Yimin Feng,Kexin Zhang, Shengsheng Qi, Senqi Zhang, Guiling Wang,Associate Professor Yongjun Di and Executive Editor-in-Chief Dr. Yan Yang gave careful guidance. Dr. Lei Fu, Wufu Li, Piaoluo Yu, Chuntao Wang and Cui Liu have provided valuable information, and the authors would like to express sincere gratitude! Thanks to the reviewers for their valuable comments and suggestions for revision!