Distribution characteristics of typical geological relics in the Western Sichuan Plateau

3.1.1 Spatial distribution laws of hot spring geological relics

The geological relics of hot springs in the WSP are mainly distributed along the fault zone. The formation of hot springs is closely related to heat sources, aquifers, and hot water channels. Therefore, the deep fault zone of the WSP provides a constant source of heat for the hot springs (deep to the ground to facilitate the formation of heat sources, such as the Anning River fault and the Xianshuihe fault), and the intersection of faults (especially, the intersection of low-order sub-branched faults and high-order sub-main faults) provides a suitable method for the hot springs to emerge. From the perspective of spatial morphology, the hot springs of the WSP are mainly along the Anning River. The main fault zone, the Longmenshan fault zone, and the Xianshuihe fault zone developed [7].

Hot springs in the WSP developed along the river valley. The tectonic movements in the WSP are frequent, and the morphology of the land surface caused by the long-term large structural uplift is strongly undercut, so that the hot water layer can be widely exposed inside the deeper gullies. At the same time, the water level in the aquifer is lower and the static pressure in the upper layer is smaller. Both provide powerful conditions for the formation of the hot spring channel, which makes up the hot spring outcrop along the valley. Overall, the hot springs of the WSP are characterized by rivers and valleys along the Dadu River, the Yalong River, the Xiaojin River, and the Dajin River (Figure 2 and Table 3).

Figure 2

Analysis of the distribution pattern of hot spring geological relics.

3.1.2 Cause analysis of hot springs in the WSOP

Based on the field investigation, this study investigated possible causes of the formation of the following hot springs in the area by combining our findings with previous research: Rubu (Daocheng County), Huashuiwan, Hongge (Yanbian County), Hailuogou, Erqiao (Kangding County), and Hongba (Jiulong County).

Rubu Hot Spring, Daocheng County: the hot spring mainly outcrops from the strata of the Triassic Lanashan Formation, Indosinian adamellite, and the Quaternary alluvial system. Influenced by the deep Rubu fault, the basement rocks of the area are deeply cut and the rock mass is broken, which plays a decisive role in connecting with the deep heat source. Regarding hydrochemical composition, the Rubuchaka Hot Spring consists of a heavily carbonated-sodium fluoride type natural mineral water and has very low salinity. Anions present are mainly HCO³⁻ and SO₄²⁻, while cations are dominated by Na⁺. Furthermore, it contains K⁺, Mg²⁺, Ca²⁺, and NH₄⁺, and is medium-alkaline with a pH value of 7.7 and temperature of ∼70°C. The hot spring is mainly replenished by atmospheric precipitation and the melting of ice and snow from the mountainous areas. The deep heat source is likely accompanied by the heat released during the cooling of granite. The deep heat storage is formed by the heating action of the infiltration water. Under the action of high head pressure of aquifers, it flows upward along the fracture zone or fractures, and finally discharges under the artesian conditions, thus forming the Rubuchaka Hot Spring [26].

Huashuiwan Hot Spring: The hot spring is located in the transitional zone between the Sichuan Basin and the WSP and in anticlinal axis of the Wuzhong Mountain near the southwest plunging end. Coal reserves in the Upper Triassic Xujiahe Formation mainly outcrop in this area where they occur within the larger Longmenshan fault zone. The hot spring is mainly supplied by atmospheric precipitation through vertical infiltration along the joint fractures and lateral infiltration of the rivers. The hot spring water is generally of the Cl–SO₄–Na-type, with salinity of 10.09/L and containing many trace elements, such as Li, Sr, B, H₂S, F, and I. The temperature of the water is 68°C and the pH value is 7.1. The geothermal energy was formed by heating paleoseawater by the heat-temperature earth’s core during geological changes of sea and land in the ancient Sichuan Basin [27].

Hongge Hot Spring, Yanbian County: The hot spring is located in Hongge Town, Yanbian County, at the eastern edge of Hongge Basin in the transitional zone between the WSP and the Yunnan–Guizhou Plateau, adjacent to the Yayang River, which is a small tributary of the Jinsha River. The hot spring contains radium metaborate, radon, silicon, and hydrogen sulfide and ranges from 55°C to 56°C. The heat of the hot spring is mainly provided by the deep and large Xigeda fault. In addition, the interbedded layers of sand, mud, and shale at Xigeda fault serve as suitable thermal insulation layers, and ample precipitation, surface water, and karst water in this zone jointly interact, thus forming the hot spring [28].

Hailuogou Hot Spring: The hot spring features slightly mineralized brackish water in the type of HCO3–Na·Ca with multiple mineral compositions. The water is weakly alkaline and slightly mineralized with the pH value ranging from 7.8 to 7.9. The hot spring comprises multiple elements and compositions, such as F, H₂S, H₂SiO₃, Sr, Fe, Ba, and Mn, and the temperature of water flowing out of the hot spring vent is 89–91.5°C. Hailuogou Hot Spring outcrops on a rhombic uplifting block in Minya Konka to the west of the large Moxi regional fault (southern section of the deeper and larger Xianshuihe fault), where there are intense neotectonic movements. Moreover, the faults and structural fractures are well developed and the faults mostly cut into the deep crust, which forms a suitable channel for the upward seepage of geothermal energy. In addition, there is abundant atmospheric precipitation, a wide distribution of glacial snow fields, and sufficient groundwater supply in this area, all of which provide superior geological and hydrogeological conditions for the formation of hot mineral water [29].

Erqiao Hot Springs, Kangding County: The hot springs are exposed on the west bank of the Yarra River, whose morphology is mainly controlled by the Yarragou fault. Rock masses around the vents are broken and fractures have developed. The vents of the hot spring are covered by Quaternary sediments. The hot spring water boils at about 32–56°C and flows out of Quaternary sediments on the west bank of Yarra River and has an odor of H₂S. The hot spring in the study area is mainly supplied by atmospheric precipitation and snow melting. The huge heat generated by the deformation of the Xianshuihe fault is the main heat source for intense hydrothermal activities in the study area, which commonly constitutes the heat sources for hydrothermal activities in the study area together with energy caused by the decay of residual trace radioactive elements in granite and the local melting body [33].

Hongba Hot Spring, Jiulong County: the hot spring of the HCO₃–Na type with high salinity is located in the northwest of Hongba Township and distributed along Moxigou. Repeatedly inherited and activated under the influence of multiple tectonic movements in the later period, the Zhenggou fault in the zone is characterized by multistage activities and squeezing first and then extension. Moreover, it deeply cuts the basement and is a heat conduction fault with inheritance, which plays an important role in connecting the deep heat source. In addition to the normal geothermal gradient of groundwater during the deep circulation in the earth’s crust, the heat of Hongba Hot Spring also comes from the heat carried by diorite during the upwelling and invasion, and the heat generated by the radioactive elements in the granite in mixed diorite. However, atmospheric precipitation and melting of ice and snow are the main water sources [30].

Based on the above analysis, the frequent tectonic function of the WSP reserves ample heat sources for the formation of hot springs (e.g., geological heat transfer in the fault zone, thermal cooling in the Yanshanian granite, new tectonic movements, mechanical heat generated by earthquakes, geothermal gradient warming, and chemical energy generated by radioactive element metamorphosis). In the case of certain precipitation supplements [8], hot springs are formed by geothermal heating, and under the promotion of structural force and geothermal circulation, through well-developed migration channels (fracture zones, tectonic movements, and earthquakes). The role of damage to the earth’s crust, and finally the formation of hot springs on the surface of the outcrop take place [9,10].

3.2.1 Distribution laws of geological relics in canyon geomorphology

The overall layout of canyon-like geological relics is controlled by the action of water flow and tectonic action, showing the characteristics of agglomeration distribution at the intersection of river and fault. The tectonic movements in the WSP are frequent, and the tectonic action forms fault ruptures and a large number of joints, which provide a material basis for the formation of canyon geomorphology. At the same time, tectonic uplift and climate cycles will also change the erosion datum [11], accelerating the flow of water. The ecliptic effect provides an external force for the formation of typical canyons such as steep slopes and waterfalls. The geological relics of the canyons in this area have obvious distribution patterns at the junction of the Dadu River, Qingyi River, Minjiang River, and the Xianshuihe fault zone–Anninghe fault zone–Longmenshan fault zone (Figure 3 and Table 4).

Figure 3

Analysis of the distribution of geological relics in the canyon type.

The geological relics of the canyons generally show the distribution pattern of accumulation in the middle and lower reaches of the Yalong River, Dadu River, Qingyi River, and Minjiang River. Combined with the 1:1 million geological maps in Sichuan Province, most of the associated areas of canyon landforms are sedimentary rocks such as sandstone, carbonate, conglomerate, and limestone. These rock types easily form valley landforms, such as rock sills, falls, waterfalls, and isolated peaks under the action of flow erosion, which also provide favorable geological conditions for the formation of valley landforms. At the same time, it can be inferred from the concentrated distribution of the canyon geological relics (the change in the erosion base level caused by the climate cycle and the change in the erosion base level lead to the intensification of the headstream erosion, which leads to the tendency of the river terrace and canyon landform to gather in the upper reaches of the river) that their formation is less affected by the climate cycle, and is mainly dominated by tectonics.

3.2.2 Cause analysis of canyon geomorphology in the WSP

In this study, the valley relic sites were all located in river valleys and simple valley geological relics were not involved; therefore, a cause analysis was performed using Jinkou River Canyon as a typical representative. Jinkou River Canyon is located in the lower reaches of Dadu River and in the transitional zone between the western edge of Sichuan Basin and Hengduan Mountains. The regional outcropping strata are composed of the Presinian system, Sinian system, Paleozoic Erathem, and Quaternary system, described as follows. The thickness of Presinian system is more than 6,000 m, interlaced with dolomite of 500–900 m thick; the Sinian dolomite is ∼1,000 m thick; the thickness of carbonate rocks in the Paleozoic Erathem is about 1,000 m; and the total thickness of carbonate rock in the area is 2,500–3,000 m. These carbonate rocks form narrow gorges with steep cliffs on both sides of the river and the valley depth is much larger than the valley width due to water flow and strong erosion. Furthermore, multigrade denudation surfaces and multistage terraces are formed in this area. The former is divided into six levels from top to bottom, i.e., Level 1 with the elevation of 3,200–3,300 m is represented by the Dawa Mountain; Level 2 is at the elevation of 2,800–2,900 m, represented by the adjacent Haiziping; Level 3 is at the elevation of 2,360–2,540 m and represented by Lanbaoping; Level 4 at the elevation of 1,750–2,050 m is represented by the Huangmu District; Level 5 at the elevation of 1,500–1,600 m is represented by Fangmaping; Level 6 is at 1,200–1,250 m elevation. Most of the canyons are formed by fluvial incision on the above denudation surfaces. For example, the Shunshui River canyon downstream of Chaping is formed by stream trenching on the Level 6 denudation surface, while there are 8-level terraces at the exit of Jinkou Grand Canyon. These illustrate that the area has experienced tectonic uplift many times. Based on the previous quantitative dating of this area, the average rising speed of the Earth’s crust in Jinkou Grand Canyon area since 1 Ma BP has been calculated to be 1.2 mm/a. The entire Jinkou River Canyon is the result of the joint action of tectonic uplift and fluvial incision, and is mainly controlled by lithology and tectonic movement.

Based on the research of Liu Yong et al., at present, the WSP is still in the process of uplift, with an average uplift rate of about 0.2–0.3 m/km. However, since the Pleistocene, the uplift rates of Zagunao River and Xianshui River have been 0.37 and 0.39 m/km, respectively. Therefore, we infer that the WSP has experienced frequent tectonism, and especially since the Quaternary period, it has experienced a great deal of tectonic uplift [12]. During this period, it has also experienced many glacial–interglacial cycles [13]. The superimposition of tectonic uplift and climate cycles resulted in hydration and metasomatism reactions, as well as a series of physical weathering when the river flowed through rock and soil mass including sandstone and carbonate rock that are easily eroded. Finally, the spectacular canyon geomorphology of the WSP was shaped after years of water erosion [14].

3.3.1 Distribution law of glacier geological relics

It is found that the glacial geological relics in Sichuan Province are mainly distributed along the Dadu River and the upper reaches of the Jinsha, Minjiang, and Yalong rivers. There are 24 samples of glacial relics in the distribution map of geological relics of the Sichuan Provincial Land Department. Among them, 23 are in the WSP and only one, located at the boundary between the WSP and the surrounding mountains, in the non-WSP. Considering the mapping errors comprehensively, the distribution of glacial geological relics in the WSP is very typical (Figure 4 and Table 5).

Figure 4

Analysis of the distribution of glacial geological relics.

The distribution of glacial geological relics decreases from west to east. There are seven glacial relics along the watershed of Jinsha River and Yalong River, seven along the watershed of Yalong River and Dadu River, three along the watershed of Dadu River and Minjiang River, and one along the watershed of Minjiang River and Jialing River, which is also consistent with the decreasing trend of elevation of the WSP from west to east.

The development of glaciers is closely related to topography, precipitation, and temperature. Since the Jurassic Yanshan Movement, the WSP has also experienced the Himalayan movement. The continuous uplift of the earth’s crust has created a series of extremely high mountains, such as the great snow mountain, Sharuli mountain, and Qionglai mountain. As a result, many mountainous areas are prominent above the glacier balance line [15] and have created a low-temperature condition for the development of glaciers (i.e., the peaks and ridges of these extreme mountains are often watersheds between the basins). These landscapes have also provided a wide area for ice and snow accumulation site leading to glacial development. In addition, being affected by the southeast monsoon, the high-humidity atmosphere from the ocean flows into the area from south to north, providing abundant material supply conditions for glacier development [16]. At the same time, since the Quaternary period, the WSP area has experienced many glacial cycles, which also provide temperature conditions for the formation of glaciers.

3.3.2 Cause analysis of glacier geological relics

Through field investigation, this study selected Hailuogou, Mt. Siguniang, and Xiaoxiangling glaciers for cause analysis by combining with theoretical research.

Hailuogou Glacier: Modern glacier in the Hailuogou Valley is located in the main peak of Minya Konka that belongs to the middle part of the Daxue Mountain. Under the compression of the Indosinian movement and Yanshan movement, a series of SN-trended structures were formed in the Daxue Mountain, which was uplifted largely again due to the very strong Neoid movement in the Quaternary period. In the middle part of the Daxue Mountain, the Minya Konka, with an altitude of 7,556 m, and the surrounding alpine areas with an altitude higher than 5,000 m were formed, which provided temperature conditions for the formation of the glacier. In terms of geographical conditions, the area is obliquely connected with the NE-trended Longmen Mountain in the north and adjacent to Qinghai–Tibet Plateau in the northwest. Moreover, it borders the SN-trended Hengduan Mountain system in the south and the SN-trended big and small Liangshan Mountains on the western margin of Sichuan Basin in the east. The NE-trended high mountain system in the north turns the northwest cold airflow to the south, which directly affects the area. The SN-trended Hengduan Mountain system and its deep valleys in the south form a corner with the SN-trended structural mountain system and deep valleys in Minya Konka. Under the effects of southeast monsoon, the air with high humidity at sea enters the area continuously along the mountains and valleys, which guarantees the water source for glacier formation, thus finally forming the current Hailuogou Glacier [31].

Glacier of Mt. Siguniang: Mt. Siguniang is located at the junction of Wenchuan County and Xiaojin County in Sichuan Province, which is in the middle-south section of Qionglai Mountain. Mt. Siguniang lies in the transitional zone between the eastern margin of Hengduan Mountain and Sichuan Basin, which belongs to a monsoon humid climate area. The southeast slope of the mountain is the windward slope of southeast warm and humid airflow, with abundant rainfall. Based on the classification of the existing four-level cirques and analysis of multistage glacial drift, it is shown that there have been many glaciations in this area since the Quaternary period. Climate change and the high altitude of Mt. Siguniang provide favorable low-temperature conditions for the development of the glacier, thus finally forming the Glacier of Mt. Siguniang [32].

Cause of Xiaoxiangling Glacier: Xiaoxiangling is the branch of the Snow Mt. on the eastern margin of Hengduan Mountain and belongs to the monsoon climate area with abundant rainfall. Since the Late Miocene to the Early Pliocene, the eastern margin of Qinghai–Tibet Plateau and Panzhihua–Xichang area have entered a rapid uplift and fluvial incision stage. It is estimated that the plateau was uplifted exceeding 2,000–2,500 m in this stage. Subsequently, scholars including Zhang Zonghu found that the third episode of the Daqingliangzi Movement in the Panzhihua–Xichang area also lifted Xiaoxiangling on a large scale, reaching the snowline, which allowed the formation of glaciers for the first time (large-scale high lateral moraine now visible in Xiaoxiangling). After that, Cheng Jianwu et al. studied the development of 4–5-level terraces along Anning River, and the ¹⁴C dating of Liu Weiming for Zhangla loess profile shows that Xiaoxiangling continues to experience uplift during this period. Wu Haibin et al. studied the Ganzi loess profile in the western Sichuan and found that the area experienced a significant cooling and dry–cold stage around 0.25 Ma BP, which offered the low-temperature conditions for the formation of Xiaoxiangling Glacier [34].

The WSP has experienced Himalayan movement since the Yanshanian movement in the Jurassic and the continuous uplift of the crust have created a series of extremely tall mountains, such as the Snow Mt., Shaluli Mountain, and Qionglai Mountain. These large mountains allow many peaks to protrude above the glacier equilibrium line [15]. This creates the low-temperature conditions for glacier development (the top and ridge of these extremely tall mountains are often the watersheds between basins), and also provides a wide area for ice and snow accumulation sites leading to glacier development. Since the Quaternary period, the WSP has experienced many glacial cycles, which also offer temperature conditions for the formation of the glaciers. In addition, under the influence of the southeast monsoon, the high-humidity air from the ocean constantly enters the area from south to north along the Sichuan Basin, structural mountain systems and their valleys, providing abundant material replenishment conditions for glacier development [16].

3.4.1 Distribution laws of rock and ore relics

Based on the classification criteria stipulated in the Specification for Geoheritage Investigation (DZ/T 0303-2017) issued by the Ministry of Natural Resources of the People’s Republic of China, rock and ore relic sites are defined as mainly important rock and ore producing areas, including four parts, i.e., typical deposit outcrops, typical mineral and rock naming place, mining relics, and meteorite craters and bodies. The geological relics in this study are all mine and ore relic sites, except for four typical mine outcrops; the silver, lead, and zinc polymetallic mine (Gacun, Baiyu County); the Hongyan serpentine deposit (Pengzhou City); the Gyabjeka lithium–beryllium–niobium–tantalum deposit (Kangding County); and the Guobayan marble deposit (Baoxing County). Through comparative analysis and nearest neighbor analysis, it can be concluded that the rock and ore geological relics in this area have the following characteristics.

The remains of rock ore are distributed along the tectonic belt. Morphologically, the geological relics of rock ore in this area are mainly distributed along the Anninghe fault zone, the Xianshuihe fault zone, and the Longmenshan fault zone. Their genesis is related to the dynamic and material conditions for metallogenic mineral deposits by the deep and large fault belt cutting through the mantle [17] (Figure 5 and Table 6).

Figure 5

Analysis of the distribution of rock ore geological relics.

The geological relics of rock ore in this area are distributed along the Anning River, Dadu River, Yalong River, Qingyi River, and Minjiang River. The mining of rock ore is closely related to human civilization. Most of the rock ore geological relics in this area are mining and metallurgical sites. Therefore, it is speculated that the reason for the distribution of relics along the river is mainly related to the outcrop of rocks and ores (flow erosion and tectonic effects are favorable for mineral deposits) and the large amount of industrial water needed for mining projects.

3.4.2 Cause analysis of rock and ore relics

Based on the study of Guobayan marble in Baoxing County and Gyabjeka lithium–beryllium–niobium–tantalum deposit in Kangding County, it can be obtained that rock mineralization is related to tectonic movement, volcanic movement, and sedimentation. The tectonic evolution of the WSP is complex. From the Early Paleozoic passive continental margin stage to the stage of Late Paleozoic–Triassic island sea activities which produced more edge structure, and through the Late Mesozoic back-arc orogenic stage to the Cenozoic intracontinental convergence–transform–strike–slip orogenic stage, a large number of tectonic movements have provided favorable conditions for producing rock metallogenic material [18,19,20].