Abstract The Sichuan basin (SB) in southwestern China is a key region to understand the closure of the Paleo‐Tethys Ocean. Based on sedimentary petrographic, dense mineral, and detrital zircon U‐Pb geochronology data from two outcrop sections located in northwestern SB, a new compositional transition level is documented within the Upper Triassic Xujiahe Formation. Below this level, the strata contain pure quartzose sandstone, ultra‐stable dense minerals, and detrital zircon age populations of 230–290, 430–460, and 850–1,200 Ma. Above this level, the strata contain lithic‐rich recycled‐orogen detritus, abundant unstable dense minerals, and detrital zircon age clusters of 210–290, 440, 650–850, and 1,750–1,950 Ma. The compositional changes signify a change in provenance from the southern South China block (SCB) to the Longmenshan thrust belt (LTB) and Songpan‐Ganzi terrane and probably reflect transition from distal foredeep to proximal foredeep deposits of the foreland basin system associated with the LTB. Combined with the depositional characteristics, regional distribution of stratigraphic intervals, and a major unconformity within the Upper Triassic strata, a simple foreland basin evolution model is constructed. The timing of provenance change along the western margin of the SCB was probably during Late Triassic instead of between Middle and Late Triassic time as commonly believed. Commencement of unroofing of the Songpan‐Ganzi terrane and the LTB may have occurred during Norian time. Models for Paleo‐Tethys Ocean closure should incorporate coeval deformation and erosion of the Songpan‐Ganzi basin and LTB during the Late Triassic.
Tian et al. (Wed,) studied this question.