INVESTIGATING ASYNCHRONOUS EXHUMATION TRENDS ALONG THE CASCADE RANGE IN WASHINGTON AND OREGON USING THERMOCHRONOLOGIC MODELING

The roles of tectonic and climate processes on rock and surface uplift remain unclear. The Oregon and Washington Cascades provide an excellent setting to explore this question, as the orographic precipitation gradient is consistent along-strike while the deformation style varies. The orographic precipitation gradient focuses rainfall on the western side of the range, and the eastern portion of the range is significantly drier. Deformation patterns differ along-strike, where broadly east-west extension in southern Oregon shifts to north-south shortening in southern Washington. This study uses new and existing results from multiple geo- and thermochronometers, including zircon U-Pb, apatite fission track, and apatite and zircon (U-Th)/He, to reconstruct the exhumation history of three Cenozoic plutons from intrusion to near-surface positions, before emplacement of overlying basalts in the Oregon and Washington Cascades. We applied HeFTy inverse modeling techniques at each site to constrain the timing of exhumation required by these data. Existing data from the central Washington location, near Granite Mountain (~20-16 Ma intrusion age), yield apatite (U-Th)/He cooling ages of 10.3-12.8 Ma. New modeling of these data suggests rapid cooling between 14-18 Ma. The site from northern Oregon near Mount Hood yields apatite (U-Th)/He ages between 2.9-4.0 Ma. New models suggest gradual cooling from emplacement at ~22.0 Ma to within ~500 m of the surface by ~2.0 Ma. The site in central Oregon yields zircon U-Pb crystallization ages of 11.1-11.7 Ma, and apatite and zircon U-Th/He ages between 11.1 and 13.3 Ma. Modeling suggests rapid cooling between 19-13 Ma. These preliminary results show non-synchronous onset and pace exhumation timing along the Cascades in Washington and Oregon. If climate and surface process are the main driver of exhumation in the Cascades, an mechanism is required to explain the asynchronous timing observed, despite the apparent uniform distribution of orographic precipitation on the western Cascades flank. Alternatively, along strike variations in deformation style and magmatic processes may be significant drivers of rock uplift.