Gas geochemical evidence for the India-Asia lithospheric transition boundary near the Karakorum fault in western Tibet

Abstract

The lithospheric structure of the Tibetan Plateau plays a key role in explaining the underlying plateau formation mechanism(s) and growth history. Although many studies have focused on the Moho discontinuity, lithosphere boundary, and lithospheric structure of central and eastern Tibet, there are few corresponding constraints for western Tibet. In this contribution, we systematically investigated the lithosphere in western Tibet by analyzing the He and C isotopes of seven hydrothermal spring systems along the southeastern segment of the Karakorum fault. We found a high Rc/Ra ratio (1.298) for springs along the Karakorum fault, suggesting that the fault plays a key role as a lithospheric-scale conduit for fluid uplift after mantle degassing. In addition, we identified two different origins of the gases at thermal springs, which may reflect two distinct mantle sources. We infer that the locations of these springs reflect the boundary between the Indian and Asian mantles, which is consistent with the geophysics-based interpretations of a “mantle suture” along the Karakorum fault. Our work confirms the boundary between the Indian and Asian mantle, which provides geochemical support to the model of the subduction of Indian lithosphere.