Thermal Structure and Rheology of the Upper Mantle beneath Guangdong and Hainan Provinces, China

The geotherm and rheology of the upper mantle beneath Guangdong and Hainan Provinces are derived from the detailed study of spinel lherzolite and garnet websterite xenoliths from these areas. The constructed upper mantle geotherm is higher than the oceanic geotherm, but is slightly lower than those for the upper mantle beneath eastern China constructed by Xu et al. (1995) and for southeastern Australia constructed by O'Reilly et al. (1985). The high geotherm is considered to be related to the upper mantle diapir that occurred in this area. The crust mantle boundary defined by this geotherm is about 33 km deep, indicating the underplating of upper mantle materials. The rheologic features of the xenoliths show that the upper mantle beneath these areas was subjected to a relatively strong deformation process, which might be related to upper mantle diapir. During this process the differential stress might reach up to 50 MPa. As a result, high strain rate, low viscosity zones occurred at the depth of 65 km. These zones are believed to be upper mantle shear zone. The lithosphere asthenosphere boundary was deduced to be at the depth of about 85 km. All these results coincide well with the available geophysical data.