The Depositional Record Of Southwestern Upper Yangtze Area During Triassic And Its Restriction On The Tectonic Framework Of Basin And Range

The tectono-magmatic thermal event of Emeishan Large Igneous Province (ELIP) and the formation of Sanjiang Orogen caused by closure of Paleo-Tethys ocean along the western margin of the Upper Yangtze area during Neopaleozoic to early Mesozoic shaped the unique patterns of basin-mountain system. In this paper, sandstone petrology, geochemistry, and detrital zircon U-Pb ages are integrated to investigate the provenance of Triassic strata from the southwestern Sichuan Basin, SW China. Based on this, combined the sedimentological data, the geological survey and borehole data, the correlations of cross sections, we try to establish the tectonic frameworks of basin-range during Triassic in sothwestern upper Yangtze area. The main conclusions are listed as follows:The detrital components of Sandstone in southwestern Sichuan is volcanic clastic mainly and suggests that the provenance of Early Triassic strata was from a nearly source of volcanic rock. The middle Triassic strata were a set of carbonate platform deposits, as a clastic rocks interlayer, which is deposited on the passive continental margin. The Late Triassic strata were a typical foreland basin with a continental containing coal clastic sedimentary, sourced from the recycled sedimentary materials, indicating that the Late Triassic samples all recycled orogen source.Six sandstone samples from Lower to Upper Triassic were collected for detrital zircon U-Pb dating. Detrital zircons from Lower Triassic samples yield U-Pb ages of ca 261 Ma is consistent with the Emeishan mantle plume activity event. Middle Triassic samples exhibite a detrital zircon U-Pb age distribution pattern with major age groups at 257Ma,500-650Ma,710-880Ma,900-1000Ma, are consistent with the age distribution pattern of ELIP, the pre-Cambrian to Paleozoic plutons and recycled clastic rocks within Yangtze craton. Later Triassic samples shows the populations of 210-245Ma,260-280Ma,410-460Ma,710-880Ma,1600-1800Ma,2400-2600Ma, are consistent with the age distribution pattern of the Archaean to Triassic plutons and recycled clastic rocks within and outer Yangtze craton, indicates multiple-sourced components.Geochemical characteristics of sandstones from Lower Triassic show low maturity, low weathering of major elements, and are similar ratios of relative trace elements to those High Ti basalt of ELIP and pre-Triassic recycled clastic rocks within Yangtze craton. Middle samples exhibit low maturity, low weathering of major elements, and are similar ratios of relative trace elements to those High Ti basalt of Emeishan LIP, basement of Qinling and Proterozoic-early Paleozoic recycled clastic rocks within Yangtze craton. Later Triassic samples show high-maturity of major elements, and are similar ratios of relative trace elements to the recycled sediments sourced from Qinling orogen, Songpan-ganzi fold belt, Yangtze craton, and Jiangnan-xuefeng thrust belt.Combined the reported sedimentological data, sandstone petrology, geochemistry, and detrital zircon U-Pb ages data, we propose that the Lower Triassic sedimentary rocks shed from the ELIP, Khamdian paleo-land and the Yangtze craton, with no exterior Yangtze craton provenances. The Middle Triassic sedimentary rocks sourced from the Khamdian paleo-land and Chuanzhong paleo-uplift within the Yangtze craton. The later Triassic sedimentary rocks exhibited multiple provenances mainly shed from Qinling Orogen and khamdian paleo-land during Carnian, from Qinling Orogen and khamdian paleo-land, Yidun arc complex and uplifting Jiangnan-xuefeng thrust belt in Norian, and from Sanjiang orogen, Songpan-ganzi fold belt, Qinling Orogen, Jiangnan-xuefeng thrust belt during Rhaetian. All above suggest that the southwestern Sichuan area started to receive materials from the exterior Yangtze craton.The analysis of Triassic deposition characteristic and provenance in the southwestern Upper Yangtze area suggest that the southwestern Yangtze area as the passive continental margin mainly received the materials from Khamdian paleo-land and Chuanzhong paleo-uplift within the Yangtze craton during Early-Middle Triassic. During Carnian of Later Triassic, the Qinling Orogen started to provide materials for the southwestern Sichuan area. In Norian, the Sanjiang orogen which comprised of Qiangtang block, Zhongzan massif, Yidun arc complex, started to collide with Yangtze block, while the uplifting Jiangnan-xuefeng thrust belt began to supply materials for the southwestern Sichuan area which deposited as the foreland basin. During Rhaetian, Sanjiang Orogen, uplifted Songpan-ganzi fold belt, Qinling Orogen and Jiangnan-xuefeng thrust belt provided materials together for the southwestern Sichuan area. The southwestern Upper Yangtze area evolved from passive continental margin to foreland basin started at Norian.