Characteristics and Genesis of the Yangbajing Geothermal Field, Tibet

Abstract

The non-vocanic high-temperature Yangbajing Geothermal Field is located close to the convergent collision zone between the Indian and Eurasia plates. The Yangbajing convective system consists of shallow and deep reservoirs. The shallow reservoir is in unconsolidated Quaternary alluvium and altered Himalayan granite. Its temperature ranges from 150 to 165?C at depths between 180 and 280 m. The shallow sodium chloride thermal water is a mixture of deep thermal water and cold groundwater. The deep reservoir is in a slip-fault zone of the Nyainqentanglha core complex and fractured Himalayan granite. The deep reservoir can be subdivided into two parts: the upper part, with a temperature of 251?C, is between depths of 950 and 1350 m, while the lower part, below a depth of 1850 m, has temperatures as high as 329?C. No significant permeability has yet been encountered in the lower part. The deep thermal water is sodium chloride also, with a salinity of 2.8 g/l. Carbon dioxide is the major noncondensable gas in both reservoirs. The Yangbajing Geothermal Field is in an active part of a large-scale slip-fault zone of a metamorphic core complex. Multiple tectonic activities caused tensional uplift and Proterozoic strata detachment and emplacement, which formed the core complex and a ductile shear-belt. It allowed an anatectic magma to rise and mix with the core complex to create a shallow magmatic heat source in the crust. Meteoric water is warmed as it flows down along tensional fractures and fissures of the core complex.