The Cretaceous crustal shortening and thickening of the South Qiangtang Terrane and implications for proto-Tibetan Plateau formation

摘要

The timing and magnitude of deformation across the central Tibetan Plateau, including the South Qiangtang Terrane (SQT), are poorly constrained but feature prominently in geodynamic models of the Tibetan Plateau formation. The Ejiu fold and thrust belt (EFTB), which is located in the SQT, provides valuable records of the Mesozoic-Cenozoic deformation history of the central Tibetan Plateau. Here we integrate geochronology of volcanic rocks, low-temperature thermochronology, geologic mapping and a balanced cross section to resolve the deformation history of the SQT. Geochronologic data suggest that major deformation that initiated in the early Cretaceous continued until at least 80 Ma and ceased by ~40Ma. The balanced cross section resolves ~66 km upper crustal shortening (34%) mainly during the Cretaceous Qiangtang-Lhasa collision. However, the Cenozoic crustal shortening is not well constrained because of a lack of successive Cenozoic strata. We also discussed whether the observed crustal shortening can account for the modern crustal thickness and elevation in the SQT. Our observations indicate that crustal shortening and thickening within the central Tibetan Plateau was mostly accomplished during the Cretaceous Lhasa-Qiangtang collision. A thick crust could be maintained since the Cretaceous due to slow erosion rates since ~40 Ma. Minor Late Cenozoic shortening also contributed to a small amount of crustal thickening in the central Tibetan Plateau. However, close to modern >4700m elevation was finally attained by lithospheric mantle foundering in the Qiangtang Terrane at ~25 Ma.