ROUTING OF SYNOROGENIC SEDIMENT IN COLLISIONAL OROGENS

Recent advances in detrital geochronology have led to a revived use of the sedimentary record to study active margin tectonism. Using data from the U.S. Cordillera, southeastern Asia, and southern Gondwana, we will explore the sedimentary response to orogenesis using detrital geochronology to compare sediment transport routes from source to sink. In the accreted terranes of the Blue Mountains (U.S. Cordillera), Jurassic-Cretaceous strata are interpreted to be coeval with accretion to Laurentia and contain U-Pb detrital zircon populations that are largely devoid of continent-age zircon. This suggests either (a) continental sediment was partitioned during orogenesis and unable to enter accretion-proximal basins, (b) volcaniclastic material diluted the continental sedimentary signal, or (c) the timing of accretion is suspect, and strata may in fact not be synorogenic. In contrast, detrital zircon geochronology of South China Sea sediments yields evidence that continental sediment was directly routed via turbidite systems from mainland China into orogen-adjacent, deepwater basins, with little evidence for active Cenozoic accretion that is ongoing on nearby Taiwan.

Synorogenic strata associated with the Carboniferous-Triassic Gondwanide Orogeny (South America, Africa, Antarctica, Australia) provide an additional pathway for orogenic-sourced sediment to be routed into sedimentary basins. Detrital zircon and sandstone petrography reveal a high proportion of volcaniclastic material in the Gondwanide foreland basin that would have been separated from the active margin by the rising Gondwanide hinterland and fold belt. The presence of this volcanic material requires a significant component of aerial transport across orogenic drainage divides. Together, these observations suggest that: (a) sedimentary linking between allochtonous terranes and continent is not a direct proxy for the timing of accretion, (b) the dominant sediment source may not necessarily represent the most proximal source, and (c) detrital zircon provenance reflects more than ancient, subaqueous drainages.