The Spatial and Temporal Patterns of Erosion Rate in the Southern Central Range of Taiwan

Abstract:

The oblique arc-continental collision coupled with high tectonic deformation rates and sub-tropical climate have made Taiwan an ideal area for studies of the interactions among tectonic, climate and surface processes. Numerous efforts have been made to derive the uplift pattern at different time scales; however, there is still a data gap at the millennial timescale. In-situ, cosmogenic ¹⁰Be concentrations in quartz from modern river sands address this timescale and can provide basin-wide erosion rates. We present here a study of the southern Central Range, combining a cosmogenic nuclide study with analysis of the main topographic characteristics. In this region, the mechanism of mountain building changes from accretionary wedge deformation to arc-continental collision with transient landscapes and complex uplift patterns.

To characterize the uplift pattern in the southern Central Range, we apply χ analysis to infer average channel steepness of each drainage basin, for comparison to the in-situ ¹⁰Be concentrations in modern river sands. The spatial pattern of average channel steepness correlates with the basin-wide erosion rate, both showing a gradual increase to the north. The correlation confirms that ¹⁰Be concentrations reflect erosion rates over the timescale of landscape evolution. We also identify several transient features from χ analysis, indicating drainage basin reorganization. These drainage area rearrangements are also reflected in the basin-wide erosion rates, which confirms transience in the landscape including the channel network geometry. We also tested the impact of an extreme event, typhoon Morakot, which impacted the study area in 2009 with heavy rainfall and several thousand landslides. We obtained sand samples from before and after the typhoon. We find that post-typhoon samples yield higher inferred erosion rates, suggesting dilution of the sediment by quartz with low concentrations of ¹⁰Be.