Tectonic and Climatic Forcing of Erosion rates in Taiwan across Timescales

Abstract:

Taiwan has served for several decades as a test-bed for study of the response of topography to tectonic and climatic forcing, allowing us to examine the important question of variability in erosion rates over space and time and their relationship to tectonic or climatic parameters. The extensive study of the region has provided a data base of thermochronometric ages and detrital cosmogenic isotope concentrations, both of which provide proxy measures of erosion rate, albeit over very different timescales. Is this study, we compare detrital zircon fission track (ZFT) ages and detrital cosmogenic radioactive nuclide (CRN) concentrations to each other and to topographic and climatic parameters, focusing on the eastern Central Range where catchments are dominantly orogen perpendicular. Detrital ZFT ages are dominantly unimodal and indicate erosion rates of 1 to 5 mm/yr. Inferred erosion rates correlate well with average steepness of the catchment rivers and the mean cross-orogen elevation, both of which vary systematically from south to north. This correlation indicates that the topography is consistent with erosion rates averaged over 1 to 2 Ma. There is only a weak correlation with mean annual precipitation (MAP) rate, but MAP does not show large variations across the studied catchments. Erosion rates averaged over a thousand year timescale are also available from CRN (¹⁰Be) concentrations in modern river sand from published and new data reported here. CRN-based erosion rates mostly overlap with ZFT-based erosion rates, indicating that erosion rates are stationary over these very different timescales. An exception is in southern Taiwan, which was affected by super- typhoon Morakot in 2009. Here we have obtained river sand samples that pre-date and post-date the 2009 event. We find that post-Morakot erosion rates are 2 to 5 times higher than pre-Morakot. This suggests significant dilution of ¹⁰Be in the sediment, presumably by sediment supplied by deep landslides. Taken together, these data suggest that erosion rates will stabilize to average tectonic and climatic conditions averaged over thousand year timescales, but will be affected over shorter timescales by extreme events.