AN INTRACONTINENTAL RIFTING TO OROGENY: THE SONGPAN–GARZÊ TERRANE, NORTHEASTERN TIBETAN PLATEAU, CHINA

A rifting with continuous rift-depression and then closing in the continental tectonics, or the rifting followed with oceanic basin and then subduction-colliding in the plate tectonics, is a greatly challenged topic. The formation and tectonic setting as the intracontinental rifting or the suture zone in the plate tectonics, Songpan-Garzê Terrane (SGT) in northeastern Tibetan Plateau, as part of the Paleo-Tethyan tectonic terrain, is hotly debated in last several decades. SGT was considered as a deep marine during Middle-Late Triassic based on the turbidity sedimentary sequences. However, whether it was a remnant ocean of Paleo-Tethys or back-arc basin, even a rift basin was controverted. Different interpretations of the Songpan-Garzê Terrane nature lead to different tectonic reconstructions between Gondwana and Laurasia continents.

Here we focus on Permian evolution and combine with the sedimentology, geochemistry and geochronology to decipher the mystery of SGT. We have discovered Middle-Late Permian conglomerate, sandstone and mudstone in SGT, which displayed fining-upward sedimentary sequences and indicated deepening water environment. Detrital zircons from Lower Permian show a distinct age cluster with 720-850 Ma which indicate simple source from Yangtze Block. Considering the OIB-type Late Permian basalt (263-256 Ma) in SGT same as Emeishan flood basalt (265-258 Ma), we advocated that Songpan-Garzê Terrane originated from an intracontinental rifting within Yangtze Block during Middle-Late Permian, and developed to comparative deep water until Late Triassic (Carnian-Noria). Subsequent orogeny and deformation of SGT initiated coevally with or little earlier than latest Triassic (Noria-Rhaetian) Sichuan foreland basin based on the detrital zircon provenance. This crustal structure and evolution are comparable with Pyrenean orogeny in west European continent.