Origin of the Songpan–Garzê terrane, Tibetan Plateau: a perspective from the tectonic evolution of the Palaeo-Tethys Ocean

Abstract The Songpan–Garzê terrane is the largest Triassic remnant flysch basin on Earth and formed as the Palaeo-Tethys Ocean closed during the final amalgamation of the Pangaea supercontinent. However, the origin of the Songpan–Garzê terrane is highly controversial. A synthesis of the tectonic evolution of the Palaeo-Tethys Ocean and its branches surrounding the Songpan–Garzê terrane is presented, which clarifies the nature and relationships among the many Palaeo-Tethys sutures. Provenance analyses suggest that branches of the Palaeo-Tethys near the Songpan–Garzê terrane closed before the Early Triassic. In contrast, the main Palaeo-Tethys Ocean (Longmu Co-Shuanghu) did not close until the beginning of the Late Triassic. This study argues against the Songpan–Garzê terrane being a remnant ocean basin, and proposes that it was a back-arc basin of the main Palaeo-Tethys Ocean. It initially underwent extension by the combined effects of the main Palaeo-Tethys Ocean subduction and the Emeishan mantle plume in the Late Permian, and subsequently developed into a back-arc basin in the Triassic, into which huge turbiditie units were deposited derived from all surrounding terranes and orogens. The final closure of the main Palaeo-Tethys Ocean in the beginning of the Late Triassic and subsequent continent–continent collision led to basin inversion in the Late Triassic.