The Origin of High-Enthalpy Geothermal of Non-Volcanic Environment---As a Case Study of Yangbajing Geothermal Field at Qinghai-Tibet Plateau

Among global high-enthalpy geothermal resources, geothermal fields within Tibet are located in non-volcanic environment only. Results of the PTt (pressure-temperature-time) trajectory calculation of the plateau uplifting gave a comparatively satisfactory explanation for the formation and for the depth of burial of high-enthalpy resources-the reason why volcanic environment is not formed. The method of PTt trajectory reconstruction used in the study of metamorphic rocks can be adopted for study of thermal evolution of young orogenic belt. For lack of petrographic PT data recorded the influence of uplifting in Qinghai-Tibet plateau, we brought such data from Caledonides orogenic belt of North Europe for the calculation. The results show that any double-layered block can make the earth crust ¿heating¿ and originate high-enthalpy geothermal activities near earth surface, if only it uplifts at a high speed. The calculation of Qinghai-Tibet plateau uplifting model showed that: along with the increase of uplifting speed, the temperature of earth crust increase and the depth to high-temperature front increases; and along with the increase of uplifting duration, the same results occurred also. A new type of high-enthalpy geothermic-the uplifting -thick crust type-is being put forward.