New insights on the Triassic tectonic development of South China from the detrital zircon provenance of Nanpanjiang turbidites

The tectonically complex geology of southeast Asia provides a unique window on the mechanics of continental collision, and the ways in which orogens are assembled through a combination of steady-state subduction and the closure of ocean basins. The Triassic turbidite system of the Nanpanjiang basin has long been accepted as a sedimentary archive of the collision between the South China and Indochina blocks. Paleocurrent restoration and the arrangement of deep-water architectural elements together suggest that the turbidite system was filled primarily from east to west, consistent with axial flow in deep water roughly parallel to intrabasinal faults. These observations and new U-Pb ages and Hf isotope data for detrital zircons obtained from Middle Triassic turbidites and approximately correlative Ladinian floodplain deposits suggest that the sediments were not derived from the hypothesized collisional orogen, but for the most part came from a poorly preserved arc associated with the convergent plate boundary of southern China. Hafnium isotopic data for dated euhedral zircons of Paleozoic age more closely match the pattern expected for steady-state subduction of oceanic lithosphere than for continental collision. Our data lead to a new model for the Triassic tectonic evolution of South China and draw attention to the value of dating detrital grains within the sediment delivery system as well as at the final site of turbidite deposition.