UNRAVELING SPATIAL AND TEMPORAL DRIVERS OF INCISION IN SOUTHERN TAIWAN: A STORY OF TECTONICS AND TYPHOONS

The island of Taiwan is a youthful mountain range (~5myr) formed from the collision of the Philippine Sea plate and the Eurasian plate. Southward propagating uplift makes the southern portion of the Central Range an ideal place to study the beginning stages of fluvial response to uplift. In comparison, watersheds to the north are thought to have already reached steady-state of equilibrium between uplift and erosion due to a longer duration of uplift and landscape response. This research will investigate the drivers of incision in southern Taiwan and test the hypothesis that incision rates will match uplift rates up to a point when critical hillslope gradients are reached and a threshold is crossed. After this point, mass wasting processes are expected to provide too much sediment for the river to evacuate, leading to the blanketing of the river channel and slowing of bedrock erosion. These expected temporal changes will be tested by replacing space for time, assuming a northward propagating progression of uplift and landscape response.

In addition to recent rapid uplift and related seismic activity, Taiwan has a wet tropical climate which causes rapid erosion rates speeding up the timescale of fluvial adjustment to tectonic forces. Monsoonal rains, tropical storms and infrequent, but large Typhoon events trigger large mass-wasting events that can cause significant aggradation in rivers slowing bedrock incision. In addition to spatial variations, this research will look for temporal changes in incision rates to investigate if past glacial-interglacial and finer climate changes influenced river incision rates due to changes in storm activity and related mass wasting events that would influence bed armoring.

We will use a combination of geomorphic mapping and luminescence dating of strath terraces to investigate temporal and spatial changes in incision rates. Twenty-eight samples have been collected from drainages spanning a 90 km N-S transect of southern Taiwan from the east and west side of the Central Range. Initial results indicate that strath terraces ranging from 4 - 350 m above the modern channel range in age from 1.2-150 ka and incision rates ranging from 0.3-2.5 mm/kyr. Spatial and temporal patterns in incision rates are just starting to emerge and will be presented.