Plio-Pleistocene fluvial dynamics in the pro-foreland basins of Taiwan Orogeny: thermochronological constraints and tectonic implications from the syn-orogenic deposits
Description
In order to tackle the fast exhumation of the Taiwan Orogeny to the adjacent sedimentary basins, we report new results of zircon fission track (ZFT), calcareous nannofossil biostratigraphy, and diagnostic magnetic mineral tracer on syn-orogenic sediments of present-day river sands and Plio-Pleistocene sedimentary samples from pro-foreland basins of western Taiwan. ZFT age spectrum of the modern Dadu River corresponds well to exposed bed rock with reset zircon ages of 3-4 Myr and partially-reset zircon ages of 10-40 Myr. 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 sourced terranes. Nevertheless, variations of ZFT age population in conjunction with petrography and magnetic mineral tracer indicated very complex developments of sediment routing systems within each catchment for the past 1 Myr. Knowledge towards then-current sediment routing systems has great influences on our interpretations for the exhumation history. 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, exhumation rates increased since 1 Myr with variations and the reorganization of the Choshui Rivers reflected the exhumation history of different tectonic provinces. The difference of exhumation rates across the Dadu and Choshui catchments suggests along-strike variation of exhumation in central Taiwan. In addition, the discrepancies between the Tamshui and Dadu catchments through time suggests the waning of exhumation in northern Taiwan, possibly resulting from the flipping of subduction polarity in a regional scale.