Exhumation Timing in the Oregon Cascade Range Decoupled From Deformation, Magmatic, and Climate Patterns

Abstract

AbstractThe Cascade Range in the Pacific Northwest, USA, developed as an Eocene to recent volcanic arc along the Pacific/North American ocean‐continent subduction zone. The volcanic arc is characterized by temporally and spatially variable magmatism. Crustal deformation that accompanied development of the arc transitions from N‐S transpression in the Yakima Fold Belt and Puget lowland (western Washington, northern Oregon) to generally E‐W transtension in the south (central and southern Oregon). Orography focuses precipitation along the western flank of the Cascade Range, whereas the eastern flank is relatively arid. We use the unique spatial variation in deformation, magmatic history, and orographic precipitation to investigate the contributions from tectonic and surface processes to rock uplift. We reconstruct the exhumation pathway of plutonic rocks throughout the Cascade Range by exploiting the unique juxtaposition of basalt‐capped ridges above valleys exposing exhumed Cenozoic plutons. Multiple bedrock geochronometers and thermochronometers constrain the thermal history in six catchments along the Cascade arc from southern Oregon to southern Washington. U‐Pb geochronology defines circa 10–24 Ma pluton crystallization ages. Apatite and zircon (U‐Th)/He thermochronology ages range from circa 8–23 Ma. 40Ar/39Ar geochronology defines circa 5–8 Ma basalt emplacement. Our results reveal spatially and temporally variable shallow exhumation in the southern Washington and Oregon Cascades and that the timing of exhumation is earlier than circa 6–12 Ma exhumation previously reported in the Washington Cascades. Spatially and temporally variable exhumation highlights that crustal deformation plays a significant role in rock uplift in addition to erosional mass removal in concert with orographic precipitation.