Abstract
Topography within continental arcs exerts a first-order control on global and regional climate and functions as a surficial expression of mantle and lithospheric processes along the plate margin. In the Oregon Cascade arc, debate persists about the timing of uplift, with workers proposing conflicting models of either "early" pre-Miocene uplift or "late" mid-Miocene to present uplift. Since each model prescribes distinct geodynamic interpretations and regional tectono-climatic linkages, additional work is necessary to refine the elevation history of the arc. Here we reconstruct the paleotopography of the Oregon Cascades by measuring delta D values of volcanic glass secondary hydration water (delta D (sub glass) ) for 22 Oligocene and Miocene ( nearly equal 33 - 11 Ma) samples collected from the lee of the range. From the early Oligocene to early Miocene, delta D (sub glass) values proximal to the arc decrease by 30 to 40 per mille, a shift opposite in sign to that predicted by observed regional warming over this interval. We calculate isotopic change across the range (i.e., Delta delta D values) by subtracting an existing paleocoastal precipitation delta D datum from coeval leeward delta D (sub glass) values. To quantify paleoelevation change, we find elevations that are consistent with measured Delta delta D values using an existing 1-D thermodynamic Rayleigh distillation model, which controls for the influence of climatic change on delta D (sub glass) values. Results show that surface elevations monotonically increased by over one km from the early Oligocene to early Miocene, attaining above-modern elevations of nearly equal 2 - 2.6 km by the mid-Miocene. Surface uplift is broadly synchronous with independent evidence of leeward aridification, implying the establishment of an orographic rainshadow by the late Oligocene. In combination with existing geochemical, structural, and thermochronologic data, we attribute Oligocene and early Miocene uplift to magmatic additions to the crust during a phase of high volcanic flux.