Drainage basin dynamics during the transition from early to mature orogeny in Southern Taiwan

Abstract The southern Central Range of Taiwan is in an early stage of orogeny and the landscape has not yet reached a steady state. To reveal the nature of the transient landscape and its evolution towards a steady state during orogeny, we conducted an in-situ 10Be cosmogenic nuclide study together with geomorphic analyses to characterize the spatial pattern of erosion rate at the scale of individual catchments. Sediment samples were collected at outlets of major basins, and within nested sub-basins. Basin-wide erosion rates were estimated from 10Be concentrations and basin mean 10Be production rates. The erosion rates of major basins show a northward increasing trend, which is consistent with the pattern of mean basin channel steepness, reflecting the early stage of orogeny and indicating that the landscape is evolving in response to tectonic forcing. However, nested samples reveal significant spatial variability in erosion rate at the sub-basin scale. These spatial variations are consistent with predictions of divide motion from χ analysis and other geomorphic metrics including cross-divide asymmetry. These suggest that the main divide is moving towards the west in southernmost Taiwan. We also found evidence of internal basin reorganization with several cases of growing basins at the expense of neighboring basins. These findings imply that even though tectonic forcing dominates the large-scale patterns, basin reorganization is a significant process affecting spatial patterns of erosion rate at the smaller scale, so that topographic steady state is likely to be found only for the largest characteristics of the landscape.