The Change From a Marine to a Continental Setting in the Late Triassic of the Western Yangtze Craton: Constraints From the Sedimentary Record and Paleogeographic Significance

Abstract

In the Late Triassic, the western Yangtze Craton was located within the oblique subduction and collision setting of the South and North China Blocks as the Paleo-Tethys Ocean retreated westward. The Upper Triassic Xujiahe Formation constitutes well-preserved sedimentary records from the marine–continental transition. However, there are spatiotemporal discrepancies in provenance, and the timing of the marine–continental transition remains vague, thereby restricting our understanding of the tectonosedimentary history of the craton. Based on field and core observations, lithic assemblages, detrital zircon U–Pb ages, gravel composition and seismic data, six major provenance areas are identified: the Songpan-Ganzi Terrane, the Longmenshan Thrust Belt, the South Qinling Orogenic Belt, the Micang-Dabashan Thrust Belt, the Jiangnan-Xuefeng Thrust Belt and the old crust of the Kangdian region. Marine sediments deposited during the early formation of the Longmenshan Foreland Basin were sourced from the South Qinling Orogenic Belt and were primarily deposited as distal braided-river deltas. During later development of the Longmenshan Foreland Basin, the northern and central Longmenshan Thrust Belt was uplifted and formed fan deltas in northwestern Sichuan. Distal braided-river deltas in the northeast originated from the South Qinling Orogenic Belt. During the Dabashan Uplift period, the Longmenshan Thrust Belt had been fully uplifted, the sea completely receded, and terrestrial lacustrine basin deposits developed. The uplift of the South Qinling Orogenic Belt forced the Micang-Dabashan Thrust Belt into a thrust–nappe phase that produced braided-river delta deposits. This spatiotemporal basin–orogen coupling highlights the importance of using lithology, stratigraphy and sedimentary facies characteristics to reconstruct the closure of the Paleo-Tethys Ocean and the Late Triassic tectonic-sedimentary evolution of East Asia tectonic domain, and offers a framework for interpreting similar geologic settings.