EXHUMATION OF THE OREGON CASCADES: TESTING THE ROLE OF LITHOSPHERIC AND SURFACE PROCESSES ON OROGEN EVOLUTION

A current challenge to the tectonics community is understanding the interactions between surface and lithospheric processes, and the relative contribution of each factor to the development of topography. Resolving the linkages between climate, erosion, and tectonics has implications for understanding geologic problems in deep time and in the future, including the development of natural hazards such as landslides and the supply of sediment into drainage networks. Studies yield conflicting results as to which factor drives mountain range development, with some supporting structural drivers and others supporting links between long-term erosion rates and precipitation rates. The Cascade Range, NW USA is an ideal setting to assess how these drivers may guide rock uplift patterns, as structural and modern precipitation patterns have a unique along-strike spatial distribution. Orographic precipitation uniformly incises the western flank, broadly N-S compression in northern Oregon transitions to generally E-W extension in the south, and magmatism is segmented along the arc. New single-sample multi-proxy geo- and thermochronologic methods were applied at seven sample locations along the Western Cascades range. At each location, a unique geologic relationship between incised Cenozoic plutons capped by basalts provides the opportunity to reconstruct the pluton exhumation pathway from crystallization to surface exposure. New and existing U-Pb geochronologic data reveal pluton crystallization ages between ~12–23 Ma. New apatite and zircon (U-Th/He) results from this study reveal ages between 3-17 Ma. New ⁴⁰Ar/³⁹Ar geochronologic results define the eruption age of basalts that unconformably overlie the plutons, further bracketing the timing and magnitude of exhumation. Existing low-temperature apatite and zircon (U-Th)/He thermochronologic results from Washington document uniform ~6-10 Ma exhumation ages that were initially attributed to focused orographic precipitation and erosion. The diversity of ages in Oregon differ from results in Washington, suggesting the timing of exhumation along the Cascades was variable. These results may be more consistent with processes that demonstrate along-strike variability, such as tectonics or magmatic processes.