Plio-Pleistocene fluvial dynamics in the pro-foreland basins of Taiwan: Thermochronological constraints and tectonic implications from the syn-orogenic deposits

In order to tackle the relationship between the exhumed Taiwan Orogeny and the associated sediment routing systems, new results from zircon fission track (ZFT) dating, calcareous nannofossil biostratigraphy, and the occurrence of the diagnostic magnetic mineral pyrrhotite in modern and ancient syn-orogenic sediments from pro-foreland basins in western Taiwan are reported. The ZFT age spectra of modern river sediments correspond well to the thermochronometric ages of exposed bedrock and indicate a rapidly exhumed source with a young ZFT peak age of 1.7 Myr in central Taiwan, which has not been widely reported. ZFT results from the Pleistocene successions show diachronous deposition of totally-reset zircons from south to north, accompanied by a trend of younging up-section. The progression of these ZFT signals in conjunction with changes in petrography and pyrrhotite occurrence suggest transient developments of sediment routing systems in central Taiwan and the importance of paleo sediment routing systems. In the Dadu River, totally-reset zircons did not appear until 1–0.45 Myr, when the flux of thermally mature material became prominent. The course of the Choshui river migrated during the past 1 Myr and the reorganization of the river reflects activity of frontal thrusts in Pleistocene times. Exhumation rates recorded by detrital sediments of the Choshui River catchment have been consistently higher than those of the Dadu River, suggesting inherited heterogeneity of exhumation in the orogen.