DETERMINING THE AGE OF UPLIFT OF THE CASCADE MOUNTAIN RANGE USING DETRITAL ZIRCON GEOCHRONOLOGY

The Cascade Range is the volcanic arc associated with the Cascadia subduction zone, extending northward from northern California through Oregon and Washington to southern British Columbia. The uplift of the Cascade Range in Washington State had a dramatic impact on regional ecosystems and global climate by creating continent-scale rain shadow effects, enhancing regional aridity, and providing a topographic anchor to the Cordilleran ice sheet during glacial ages. Despite their importance, however, the chronology of Cascade Range uplift remains poorly understood, with proposed ages for the uplift ranging from Eocene to Pliocene. Prior to the onset of uplift, paleo rivers would have drained westwards across Washington State; uplift would have disrupted these fluvial systems and separated river drainages on both sides of the mountain Cascade Range. Here, we use U-Pb dating of detrital zircons to compare the provenance of Paleogene and Neogene sedimentary units east and west of the Cascade Range. Our samples are derived from the Chuckanut Formation (60-51 Ma), Puget Group (Eocene) and Seattle Basin (Oligocene to Miocene) west of the Cascade Range, the Swauk Basin (60-51 Ma), Chiwaukum Graben (Eocene to Oligocene), and Ellensburg Formation (16 Ma) to the east. We will present preliminary results from our 46 samples from both sides of the Cascade Range and propose a schematic paleogeographic reconstruction of Cenozoic river drainages, discussed in light of Cascade Range uplift history.