ROLE OF LITHOSPHERIC-SCALE WEDGE TECTONICS IN THE FORMATION OF THE SICHUAN FORELAND BASIN (Invited Presentation)

The Sichuan Basin juxtaposes the eastern margin of the Tibetan plateau along the west-dipping Longmen Shan thrust belt where shortening during the Triassic and Cenozoic has deformed the pre-Triassic passive continental margin sequence. Although the ongoing convergence along the eastern edge of the Tibetan plateau has resulted in >5 km relief between the Longmen Shan and Sichuan Basin, the basin lacks Cenozoic sedimentation except for <100s m Quaternary deposits. Corresponding to the boundary between the Longmen Shan and Sichuan Basin is a >20-km Moho offset as constrained by the receiver function method. In this study, we integrate surface geology and deep-crustal and lithospheric structures established by existing geological and geophysical studies to create a self-consistent structural model that can explain the correlation between the basin boundary and Moho offset. We propose that the Moho offset occurs along an east-dipping Cenozoic thrust zone that places the Sichuan Basin lithosphere over the eastern Tibet lithosphere. The inferred thrust merges westward with the west-dipping thrusts observed at the surface to form a lithospheric-scale, west-tapering thrust wedge. The unique loading structure of the wedge system would prevent subsidence of the Sichuan Basin, as it is partially supported by the eastern Tibet lithosphere from below. This interpretation is consistent with a pure-shear shortening model involving the entire eastern Tibetan lithosphere for the Cenozoic development of the eastern Tibetan plateau.