HIGH EROSION RATES IN TAIWAN FROM APATITE AND ZIRCON FISSION TRACK AGES

The active arc-continent collision in Taiwan presents an opportunity to measure erosion rates in an actively uplifting region where the wet, sub-tropical climate leads to high erosion rates. In addition, the oblique orientation of the colliding margin and arc imply a southward propagation of the collision so that the transient evolution of the eroding mountain belt can also be observed. The evolution of erosion can be observed from the early stages of emergence above sea level in southern Taiwan to a mature steady state in central and northern Taiwan. We document this evolution with low temperature thermochronometers, including new apatite fission track ages and previously published zircon fission track ages. Thermochronometer data are interpreted through one dimensional thermal models to correct for perturbations to the geothermal gradient by high erosion rates. Fission track ages from Miocene and Eocene meta-sandstones are consistently unreset (ages older than deposition) in southernmost Taiwan and consistently reset (ages less than stratigraphic age) at distances of 60 km (apatite) and 100 km (zircon) north of the southern tip of Taiwan. We interpret this as reflecting progressive erosion towards the north.

The progressive onset of collision from north to south permits us to estimate erosion rates from the fission track data using a variety of methods. The following methods are applied: (1) Modeling the ages directly, using the time of onset of erosion from the collision propagation rate; (2) elevation vs. age relationship over a 1200m section; (3) distance between first northward occurrence of reset apatite and reset zircon. All methods consistently give erosion rates of 4 to 6 mm/yr. The occurrence of reset apatite ages only 60 km north of the southern tip of Taiwan suggests that these high rates of erosion initiate quickly (<100kyrs) after emergence above sealevel.