Ongoing exhumation of the Taiwan orogenic wedge revealed by detrital apatite thermochronology: The impact of effective mineral fertility and zero-track grains

In clastic detritus derived from the erosion of young active orogens, the geologic interpretation of apatite fission-track (AFT) data is hampered by the occurrence of grains without spontaneous tracks and by contributions from multiple eroding sources with different apatite fertilities. Here, we use the Taiwan orogen as a test case to illustrate how to overcome these problems and deal with zero-track grains in detrital AFT studies following long-established principles of fission-track statistics. Our results support the effectiveness of Raman spectroscopy for a reliable identification of apatite grains. We found that zero-track grains are dominant in Taiwan modern sands, and generally contain enough U to provide useful AFT age constraints to exhumation. The absence of spontaneous tracks in most of these grains is due to a short residence time below the temperature of total AFT annealing rather than to low U concentration. Detrital AFT data shows that the extent of the Taiwan AFT reset zone previously inferred from bedrock data was partly underestimated. The revised AFT reset zone includes the southernmost part of the island and, when combined with published ZFT data, supports a scenario of southward progressing exhumation during arc-continent collision. The application of the Taiwan lesson to clastic detritus derived from the Himalaya resolves paradoxical interpretations of detrital fission-track data sets recently published in the framework of IODP Expedition 354, pointing to a major role of the Namche Barwa syntaxis as a source of apatite grains shed to the Bengal Fan.