The late Pleistocene reorganization of sediment routing systems at the pro-foreland basins of Taiwan Orogeny: thermochronological constraints and tectonic implications from the syn-orogenic deposits
In order to tackle the fast exhumation of the Taiwan Orogeny to the adjacent sedimentary basins, we applied zircon fission track (ZFT) analysis on syn-orogenic sediments of present-day river sands and the Pleistocene sedimentary samples from the pro-foreland of western Taiwan. The ZFT age spectrum of the modern Dadu River corresponds well to the exposed bed rock with the reset zircon ages of 3-4 Myr and partially-reset zircon ages of 10-40 Myr. The age spectrum of the modern Choshui River, however, indicates a fast exhumed source with young peak age of 1.7 Myr which has not been widely explored and reported. The Pleistocene successions in the Dadu and the Choshui catchments both show younging upwards signals which mirrored the exhumation of the sourced terranes. Nevertheless, the variations of ZFT age population in conjunction with the petrography and magnetic mineral tracer indicated very complex developments of sediment routing systems within each catchment for the past 1 Myr. The knowledge of the sediment routing systems has great influences on our interpretations for the exhumation history. The present-day basin-wide exhumation rates were estimated as 1.1 mm/yr for the Choshui River and 0.9 mm/yr for the Dadu River. The maximum exhumation rates were estimated as 3.2 mm/yr for the Choshui River and 2.0 mm/yr for the Dadu River. In general, the exhumation rates increased since 1 Myr with variations and the reorganization of the rivers reflected the exhumation history of different tectonic provinces. The difference of exhumation rates across the Dadu and Choshui catchments suggested along-strike variation of exhumation in central Taiwan. In addition, the discrepancies between the Tamshui and Dadu catchments through time suggested the waning of exhumation in northern Taiwan, possibly resulting from the flipping of subduction polarity in a regional scale.