UPLIFT AND EROSION OF THE CASCADIA FOREARC HIGH AS INDICATED BY (U-TH)/HE APATITE DATING

Our objective is to use (U-Th)/He apatite dating to resolve the uplift and erosion history of the Cascadia forearc high, which corresponds to the coastal mountain range extending from N California to Vancouver Island. Geologic evidence indicates that this forearc high has grown by accretion and tectonic shortening associated with subduction of the Juan de Fuca plate over the last 35 m.y. (U-Th)/He apatite dating in the Olympic Mountains is consistent with steady state erosion in the Olympic Mountains since 14 Ma (Batt et al., JGR, 2001). Dating of 28 new samples along the length of the Cascadia forearc high indicates only modest erosion elsewhere (<3 km). We take this as evidence that much of the forearc high has only recently (<5 Ma) become emergent above sea level. Miocene cooling ages in NW Oregon are of particular significance. Preservation of Columbia River Basalts precludes erosional cooling as an explanation. Rather, we believe that these ages record gradual conductive cooling of the forearc high by subduction of the Juan de Fuca plate. Neogene (1-5 Ma) ages were encountered around Cape Mendocino and the King Range of NW California, but the geographic distribution suggests that fast erosion there is related to localized uplift around the Mendocino triple junction, rather than the Cascadia subduction system. At present we are completing our He dating, and also supplementing our study with some preliminary ¹⁰Be measurements of modern river sediments to estimate long-term regional-scale erosion rates, which are needed to constrain the timing of emergence of the forearc high.