OROGEN AND REACH SCALE PATTERNS IN ROCK STRENGTH, BEDROCK RIVER MORPHOLOGY, AND SEDIMENT COVER ACROSS THE TAIWAN CENTRAL RANGE (Invited Presentation)

The strength of potential orogen-scale feedbacks between surface and deep Earth processes is sensitive to how rock strength influences bedrock river incision. Rock strength varies across orogens, where metamorphic grade, lithology, fabric, and accumulated damage at the surface depends on the subsurface burial and exhumation history. These rock properties influence both the erodibility of in-channel bedrock and the grain size of sediment supplied to the channel, leading to a complex suite of local and non-local lithologic controls on channel morphology. Quantifying rock strength controls on channel morphology at the orogen scale is difficult, as rock strength can vary at a range of scales and measuring channel morphology and sediment grain size is difficult in steep landscapes. Unmanned aerial vehicle (UAV) surveys enable mapping at high enough resolution to capture channel geometry and sediment cover, while at large enough spatial scales to account for local heterogeneity.

Here, we focus on the Taiwan Central range, which has a west to east gradient in metamorphic grade and exhumation depth, and a south to north gradient in uplift rate. Paired UAV photogrammetry and field surveys span 60 km of bedrock river channel in 21 reaches, and sample systematically across both gradients and all formations and lithologies present in the range. We directly measured channel morphology and sediment cover in each surveyed channel and tributary from 1-5 cm resolution 3D models and orthoimagery and used these data to analyze patterns in clast mobility. At the reach scale, channel slope, width, and depth vary at different length scales in response to rock strength controls. This results in initial motion thresholds that are spatially variable, implying that sediment transport timescales could be buffered by local storage at lithologic boundaries. Across the orogen, we observe similarly high ranges of variability in channel morphology. However, the size of hillslope derived coarse sediment systematically increases with metamorphic grade, which can lead to otherwise similar morphologies eroding at very different time scales. While tectonic processes and surface processes can act at different length scales and time scales, data rich approaches like these can help disentangle systematic variations.