# Hydrogeochemistry of Fault-Related Hot Springs in the Qaidam Basin, China
## 2 Geological Structural Background of the Study Region  
The Qaidam Basin is the largest active inter-mountain basin on the Tibetan Plateau. It is a Meso-Cenozoic flexural basin that developed gradually under intense compression caused due to the long-term tectonic plate activities of the Qinghai-Tibet Plateau. The basin stretches 850 km from east to west and 150-300 km from north to south, with a total area of approximately 120,000 km\({}^{2}\) and an average elevation of approximately 3 km. In the middle of the Mesozoic Era, the uplift of the Kunlun Mountains caused the Qaidam region to gradually transition away from a marine environment. The frequent crustal movements led to the development of several faults and folds around the Qaidam region. The Qaidam Basin is one of the large-scale multi-energy terrestrial sedimentary basins in China, which has developed thick Cenozoic terrestrial sedimentary rocks. The formation and evolution of the Qaidam Basin were jointly controlled by the Qilian Mountains tectonic zone, the Altyn Tagh fault zone, and the East Kunlun fault belt. The region has witnessed an uplift in the Qinghai-Tibet Plateau (Kunlong Mountain), left-wing movement of the Altyn, and an evolutionary history of sedimentary filling, which is why it is considered a research hotspot by domestic as well as foreign scholars [14; 15; 16; 17]. The Qaidam Basin has been a large intracontinental depression with no foreland basin development since the Cenozoic period. Its sedimentary center is located on its central axis, with west-to-east migration [18]. The Cenozoic sedimentary strata are heavily covered in the central and southern parts of the basin, while they are exposed on the northern and western margins of the basin. The Cenozoic period witnessed exposure of the strata, with a total thickness of approximately 5271 m in the Dahonggou section located in the Qaidam Basin, which may be divided into the following six formations: the Lulehe formation, the Lower Ganchaigou formation, the Upper Ganchaigou formation, the Lower Youshashan formation, the Upper Youshashan formation, and the Shizigou formation. The groups are mostly in integrated contact with each other in the interior of the basin, with angular unconformity surface regions in the marginal areas of the basin [19; 20; 21; 22]. In recent years, different methods have been employed to explore the sedimentary and tectonic characteristics of the Qaidam Basin from different perspectives, and considerable progress has been achieved [23; 24; 25; 26]. The study region is located at the intersection of the Altyn Tagh fault zone and the East Kunlun fault belt, which belongs to the southwest rim of the Qaidam Basin (Figure 1b). The East Kunlun is located along the northern margin of the Qinghai-Xiang (Tibet) Plateau. It is a composite orogenic belt with multi-stage tectonic evolution [27; 28]. The belt may be divided into the northern East Kunlun tectonic belt, the central East Kunlun ophiolatry melange belt, the southern East Kunlun tectonic belt, and the Buqingshan-A'nyemaqen ophiolitic belt (from north to south) [29; 30; 31]. With the occurrence of large-scale left-slip activity in the Altyn Tagh fault zone on the west side of the East Kunlun during the Middle Cenozoic period, the western section of the East Kunlun close to this fault zone was greatly modified. Since the Cenozoic period, a basin-ridge interval tectonic pattern has developed that differs significantly from the eastern section of the East Kunlun. The Qaidam Basin's northern boundary is formed of the Altyn Tagh Fault. It is a massive sinistral strike-slip fracture zone in the Asian continent that truncates various geological units, controlling both geometric characteristics and the tectonic framework in northern Tibet and playing an important role in regulating the tectonic deformation caused by the India/Eurasia collision [32; 33]. The Altyn Tagh fault witnessed periods of strike-slip motion during the Cenozoic era, which were accompanied by an intense tectonic uplift.