Extensional Stress Field in the Central and Southern Qinghai-Tibetan Plateau and the Dynamic Mechanism of Geothermic Anomaly in Yangbajain

Abstract

The central and southern regions of the Qinghai-Tibetan plateau are famous geothermal active zones in the world. Temperature of boiling spring is as high as 93°C, heat flow value is up to 364 mW/m2 in some areas in the plateau. In the present analysis the relationships among the depth of lithospheric brittle fracture, stress field and mechanism of geothermal anomaly were investigated using the seismic dada from ISC and Chinese seismic net and other geophysical data. The results suggest that earthquakes about M6 frequently occur in the geothermal active zones, the seismicity is more active than the average inside the plateau. The events are distributed along the belts near N-S direction coinciding to that of fault-depression zones with intense geothermal activity. Events of intermediate hypocenters also appear in the high heat flow zone, the hypocenter depth stretches to about 100km. Normal fault type earthquakes due to the extensional stress field dominate the seismicity in the southern and central plateau, differing from the reverse or strike-slip faulting events dominating the seismicity in the low edges around the plateau, although the directions of the main compressive and extensional stress field coincide to those in the low edges. Extension along near E-W direction dominates the lithospheric stress field in and around the Yangbajain area. Such stress acts as deep as over 100km, to the bottom of lithosphere. Strong extensional stress field near E-W direction results in a series of large-scale normal faulting events or fractures near N-S direction in the geothermal anomalous zones in the plateau. The fractures are beneficial to the upwelling of heat flows with high temperature from the asthenosphere to the surface, and probably result in geothermal anomalous zones with high temperature in the plateau.