Initial rifting to deep-water basin in the Songpan–Garzê Terrane, eastern Tethyan tectonic domain during Permian-Triassic 

Abstract

The initiation of a continental rift or the birth of an oceanic basin, and their long-term evolution, are challenges in the understanding of the evolution of regional and global tectonics. The tectonic setting prior to the Late Triassic in eastern Tethyan tectonic domain is poorly understood and hotly debated. Previous interpretations mainly regarded the Songpan-Garzê Terrane (SGT) as an oceanic basin filled with thick turbidite, which is a branch of eastern Paleo-Tethys during Middle-Late Triassic. Here, for the first time, the continuous Middle Permian – Upper Triassic comprehensive columns and cross-sections were reported in detail, and the geochemical and geochronological data of clastic rocks were systematically analyzed. Analysis results show that SGT initiated from continental rift controlled by normal faulting within Yangtze Block in Middle Permian, and developed to an intracontinental deep-water basin controlled by thermal subsidence associated with cooling of the crust and cessation of normal faulting during Triassic. The Triassic strata once known as “turbidite” have been reclassified based on Petrography and sedimentology: The lower Triassic strata consist of bottom conglomerate, coarse sandstone and upper interbedded greywake and silty slate, which indicate typical fluvial deposition. The middle Triassic strata composed mainly of thick limestone, calcareous sandstone and feldspar sandstone indicate relatively stable shallow sea or lake environment. The upper Triassic Carnian strata are dominated by thick massive sandstone and slate interbeds, and they are characterized by various sedimentary structures like asymmetrical ripple, flute cast, oblique bedding and normal-grade bedding formed in turbulent retrograde delta. Meanwhile sea level rise was induced by Carnian rainstorm, which together with adequate supply of sediment had increased the basin area. Then an intracontinental deep-water basin filled by carbonaceous slate, coal seam, pyrite bearing fine-grained sandstone, and a large number of plant fossils developed in anoxic reductive environment in the northeastern Songpan-Garzê tectonic belt in early Norian. Statistic of the detrital zircon age and paleocurrent accompanied by geochemical analysis of the clastic rocks provide important information about the protolith of detritus. In the north of the basin, detritus are dominantly derived from the Kunlun-Qilian- Qinling orogenic belt and the North China Plate, while in the south, they were mainly provided by the Yangtze Plate and the Yidun block. Based on the above, the Permian sedimentary record in Songpan-Garze terrane can be compared with the upper Maokou Formation and Emeishan large igneous province in the western Yangtze Plate. It can be concluded that they formed in the same extensional tectonic setting, which together constituted the large-scale rift system and volcanic eruption in the eastern margin of the Paleo-Tethys. It was controlled by extensional stress field until at least Carnian, and the tectonic inversion started at latest Late Triassic proved by sedimentary and magmatic records. This work was supported by a National Science Foundation (grant no. 92255302).