Graduate Thesis Or Dissertation
 

Geomorphic Evidence for Differential Rock Uplift Across the Southern Cascadia Forearc

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  • The nature of upper plate deformation along the Cascadia subduction zone (CSZ) is poorly understood. Systematic covariation among topographic relief, geodetically determined uplift rates, decadal to millennial erosion rates, and the frequency of episodic tremor and slip (ETS) along the Cascadia forearc suggest a genetic association between forearc topography and plate boundary deformation. Although spatial variations in uplift are commonly attributed to the inboard position of the locked plate interface, the association of high topographic relief and rapid erosion across the boundary between the central Oregon Coast Ranges and the Klamath mountains suggests that ongoing rock uplift may contribute to this signal. Here, I test this hypothesis using a combination of stream profile analysis, observations of fluvial terraces, and existing determinations of watershed-average erosion rate. In the Rogue River watershed, these analyses reveal systematic spatial patterns in channel steepness (a measure of channel gradient normalized for contributing basin area) that delineate a western block characterized by high channel steepness and high local relief (up to 1.5 km) from an eastern portion characterized by lower relief and gentler channels. The boundary between these two domains is a sharp, linear mountain front that trends NNE and is approximately coincident with a bedrock fault zone within the Western Klamath Terrane. East of this structure, herein referred to as the Eight Dollar Fault, fluvial networks are characterized by low-gradient, alluviated valleys, whereas western channels are decorated with flights of strath terraces and perched gravels attesting to recent incision. Longitudinal profiles of the trunk streams of the Rogue and Illinois Rivers exhibit a broad steepening across this transition that is manifest as a convex longitudinal profile. Knickpoints are observed throughout the watershed, but are not systematically associated with the elevation or drainage area of the convexities along trunk channels. Comparison of channel steepness values to existing data on erosion rate suggest that these differences in landscape morphology, channel profile steepness, and geomorphic character of channels are associated with sustained high rates of rock uplift and erosion in the western Klamath mountains. These results suggest that the western Klamath mountains are actively uplifting relative to the eastern forearc. Permanent strain associated with active deformation within the southern Cascadia forearc may be a non-negligible component geodetically measured uplift rates in the region.
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file details vonDassowW2019.pdf 2019-01-24 Pubblico Scaricare