A Hypothesis on Concealed Magmatism in Southern Tibet

Abstract

High-temperature geothermal resources are known along extensional grabens in southern Tibet where a well reached 329.8 ºC at a depth of 1850 m and seismological studies detected a partially molten layer at a depth form 15 to 18 km. Nevertheless, neither Plio-Pleistocene volcanoes nor Plio-Pleistocene volcanic rocks are observed at the surface, suggesting that magmatism is concealed without any eruptions in southern Tibet. A new 40Ar/39Ar age, 10 Ma, obtained on a possible volcanic edifice in Yanyi also supports this idea. A special tectonic situation in Tibet is that the deeper zone is constrained by contraction tectonics due to continental collision and the shallower zone is constrained by extension tectonics due to gravity collapse. A hypothesis is proposed on the density and stress profiles on the special tectonic regime in Tibet. According to the analysis, the buoyancy-driven magma rise is constrained by the low density host rocks at the shallow depth due to extension tectonics and the pumping type magma rise is difficult to occur by the high tectonic stress at the greater depth due to contraction tectonics. Therefore, the buoyancy-driven magma bodies could be only expected along the grabens at a several kilometer depth in southern Tibet.