Paper No. 88-17
Presentation Time: 9:00 AM-1:00 PM
EMERGENT THRESHOLD LANDSCAPE DYNAMICS IN THE NORTHERN ALASKA RANGE (USA)
The interplay of tectonic rock uplift, valley incision, and landslide erosion shapes Earth’s mountainous topography. Where ~30° hillslope angles endure above river channel networks despite rapid bedrock exhumation and downcutting, landslide erosion is hypothesized to balance rates of differential incision and approximately maintain the “threshold” hillslope angle as relief grows over time. Few studies explicitly test this hypothesis, however, and conflicting studies demonstrate that mass wasting-derived coarse sediment can subdue incision and suppress relief production on 104-year timescales. How, then, do feedbacks among these processes persist to generate high-relief threshold landscapes? We explore this question where the Totatlanika River drains the northern Alaska Range, an active continental orogen that rises in far-field response to 5 cm/yr Yakutat microplate–southern Alaska collision. Combining field observations, luminescence dating, topographic and image analysis, we quantify rates and patterns of topographic growth, bedrock incision, landslide erosion, and fluvial adjustments that mediate process feedbacks. Maximum relief of 650–700 m occurs where the Totatlanika River cuts through the schist-cored Jumbo and Rex anticlines. There, hillslope angles approach ~30°, landslide scar density increases, coarse bedload (D90) triples based on remote measurement of >4000 in-channel boulders, and co-located two-to-fourfold channel slope increases and width decreases reflect stream power enhancement. This enhancement appears sufficient to mobilize bedload and cut into bedrock over 104-year timescales; luminescence ages (10–70 ka) of sand on boulder-mantled strath terraces 36–70 m above the river yield incision rates of 1–5 mm/yr across the Rex anticline, whereas the lack of terraces and nearly continuous bedrock landslides across the Jumbo anticline may indicate even faster rates. We are quantifying thresholds for sediment motion to understand the conditions that promote the spatial covariation of Totatlanika River incision rate/efficiency, topographic relief, hillslope angle, and landslide distribution that collectively indicate an emerging threshold landscape in the northern Alaska Range.