A thermomechanical wedge model of Taiwan constrained by fission-track thermochronometry

We present 31 new apatite fission-track (AFT) ages for the island of Taiwan that, when combined with existing AFT and zircon fission-track (ZFT) data, provide regional spatial coverage of the island with respect to low-temperature thermochronometry. The overall pattern of ZFT and AFT ages in Taiwan exhibits unreset ages in the southern and western portions of the island and reset ages predominantly in the Central Range and eastern Taiwan. This pattern supports interpretations of the orogen kinematics as reflecting a crustal scale wedge with a southward propagating collision zone. In this model, new material is accreted to the wedge from the west and is transferred to the east with the greatest exhumation occurring along the eastern margin as recorded in the reset ages in the east and unreset ages in the west. The southward propagating collision is consistent with reset ages in the north, where erosional exhumation has been ongoing for longer, and unreset ages in the south, where the younger collision implies less time for erosional exhumation. Despite the variation in the age of the collision along the strike of the island, the widths of the AFT and ZFT reset age zones remain nearly constant between 23° 00′N to ∼ 24° 00′N and 23° 20′N to ∼ 24° 00′N, respectively, suggesting that the orogen is in an exhumational steady state over these regions with respect to the AFT and ZFT thermochronometers. We use the fission-track data in conjunction with observations of crustal structure, crustal fabric, and heat flow measurements to constrain a time-dependent, two-dimensional, thermomechanical model of orogen evolution. By accounting for the heat transfer, tectonic and erosion processes needed to predict AFT and ZFT ages, we are able to investigate the relationship between the measured ages and the tectonic characteristics of the orogen. With our model we conclude that: (1) roughly half of the material accretion in Taiwan occurs through underplating over an approximately 40 km wide region, (2) current average erosion rates are ∼ 3.3 mm/yr in the eastern Central Range and ∼ 2.3 mm/yr over the whole island, (3) the collision has been propagating southward at a rate between 20 and 51 km/Ma over the past 2–3 Ma, and (4) central Taiwan is in a topographic, thermal and exhumational steady state.