5-2 Orographic precipitation controls relief production in response to uplift across the southern Cascadia forearc

Session: Advances in Mountain Hydrology: Connecting Cryosphere, Surface, and Subsurface Processes

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Authors:

Abstract:

Interpreting uplift histories from landscapes requires models that sufficiently capture the long-term effects of spatiotemporally variable climate and rock uplift over timescales relevant to the development of topography. Along the Cascadia forearc, topography co-varies with uplift rates measured geodetically (10¹ yr) and inferred from marine terraces (10⁴ - 10⁵ yr) and with rates of fluvial incision and erosion. Although these variations may reflect ongoing differential rock uplift along the forearc, differences in rock mass quality across lithostratigraphic terranes and variations in runoff, (both mean discharge and annual variability) are expected to modulate the relationship between erosion rate and topography. Here, we exploit a densely sampled data set of ¹⁰Be erosion rates along with analysis of channel steepness from ~90 watersheds spanning the Cascadia forearc between ~47˚ to 40˚N. Our results reveal that erosion rates across the Klamath and Siskiyou Mountains in the southern forearc (south of ~43˚N) range from to ~200–300 m/Myr in watersheds draining the western forearc but decrease systematically, eastward away from the plate boundary to ~40–50 m/Myr. Watersheds along the western Klamath and Siskiyou Mountains receive high rates (up to 4 m/yr) of mean annual precipitation (MAP), whereas more subdued topography in the eastern forearc receives MAP between ~0.7–1 m/yr. Comparing watersheds with smooth channel profiles and draining bedrock of similar rock mass quality, we observe differences in the scaling between channel steepness and erosion rate across the forearc that depend on the magnitude of MAP. By calibrating a stochastic-threshold stream power model against local runoff records spanning ~40 years, we account for variations in erosional efficiency driven by spatially variable precipitation and predict erosion rates across the southern forearc. We test these predictions against fluvial incision rates determined from dated Late Pleistocene strath terraces along the Illinois and Rogue Rivers ranging in age from 30 ka to 500 ka. Our results compare favorably and suggest that calibrated stochastic-threshold stream power models capture salient aspects of the adjustment of landscape topography to tectonic forcing over timescales of 10⁵ – 10⁶ yr.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-10666

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