Thermo-kinematic evolution of the Taiwan oblique-collision mountain belt as revealed by zircon fission track dating

Based on analyses of about 970 zircon grain fission-track (FT) ages from 44 clastic rock samples collected from six transects and two pooled ages of apatite concentrates separated from a diabase body, the thermo-kinematic evolution of the Taiwan mountain belt since the last orogenic phase (the Penglai Orogeny) has been delineated for the first time. As a consequence of the active Penglai Orogeny since late Tertiary times, pre-orogenic FTs in detrital zircons and apatites have been subjected to varying degrees of annealing by geothermal heating. The spatial boundary between the partial and complete resetting of zircons coincides well with that corresponding to the 260°C isotherm between the greenschist facies and prehnite–pumpellite facies defined previously by crystallinity of potassic micas. The age distribution along each cross-section exhibits younger ages from the mountain front toward the rear plate boundary between the Eurasian and the Philippine Sea plates, suggesting asymmetric cooling and exhumation. Along the strike direction (roughly north to south) of the regional structure, the age distribution reflects southward propagation of the arc–continent collision and subsequent uplift-and-denudation. Spatially, the width of the zircon complete-reset zone gradually narrows down along the strike of the regional structure, reflecting the southward propagation of the arc–continent collision and subsequent uplift-and-denudation. Temporally, zircon FT ages for the western margin of the completely reset zone are progressively younger from 5–6 Ma in the northern part to ca. 2 Ma in the south-central part, then resume to ca. 6 Ma for the southern end, where collision is only in its initial stage. The previously tectonized pre-Tertiary Peikang Basement High on the Asian continental margin plays an important role in defining the uplift-and-cooling history and shaping the major salient-and-recess structure and neotectonics of the mountain belt.