Program

The Songpan-Ganzi Complex (SGC) of the Tibetan Plateau forms the largest continuous exposure of Triassic strata on earth and is composed of thick, widespread deep-water oceanic clastic gravity-flow deposits that were subjected to intense post-depositional contractile deformation. Sediment provenance and basin filling history of the Songpan-Ganzi turbidites is an important and rich record of tectonism associated with evolution and closure of the Eastern Paleo-Tethys Ocean. However, the origin of those deposits remains controversial and ambiguous. In this study, we constrain the sediment provenance of the Middle–Upper Triassic turibidites from Central SGC and turbidites of the adjacent Yidun terrane by using detrital zircon U-Pb geochronology. The results show that the turbidite detrital zircon ages mainly consists of 5 populations: 240–310 Ma, 400–480 Ma, 750–1000 Ma, 1700–2000 Ma and 2300–2600 Ma. These results indicate that the East Kunlun and Qinling-Dabie (EKQD) belts and North China block served as major sources for the sediment to the north and central areas of the SGC. The dominance of Paleozoic age zircon crystals in the Upper Triassic turbidites implies that the EKQD orogenic belts probably underwent a phase of rapid uplift and exhumation during Late Triassic time, producing a relatively high-relief landscape with widespread exposure of the Paleozoic rocks. In contrast, turbidites of southern SGC area and Yidun Group of the central Yidun terrane were likely derived from rocks of the Yidun terrane. The similarity among detrital zircon age populations of the southern SGC and Yidun Group turbidites, which yield significant age peaks at ca. 1850 Ma and subordinate peaks at Paleozoic and Neoproterozic ages, favor a scenario with close association of Songpan-Ganzi and Yidun terranes by at least Middle Triassic time. Furthermore, regional comparison of detrital zircon ages from Mesozoic strata suggests that the subsequent deformation and uplift of both SGC and Yidun terrane Triassic turbidite strata fed detritus into the Sichuan foreland basin to the east and the Qamdo basin to the west since latest Triassic time. This deformation event may be coeval with or possibly predated the Late Triassic phase of shortening within Longmenshan thrust belt.