INITIATION AND CENOZOIC EVOLUTION OF THE CASCADIA FOREARC BASIN: MULTIDISCIPLINARY EVIDENCE FROM THE COOS BAY AREA CENOZOIC STRATIGRAPHY, SOUTHWESTERN OREGON (Invited Presentation)

New investigations of the Coos Bay forearc basin in SW Oregon have established a revised chronology and kinematic history of basin development and its relation to the rest of the Cascadia forearc following the accretion of the Siletzia forearc basement. Detrital zircon ages provide refined stratal correlations suggesting that the Oregon Paleogene basin was a single depositional area punctuated by local and regional unconformities. We propose that deposition occurred approximately 25 km farther W-SW than previous models, west of the offshore N-S Fulmar Fault mapped as coincident with the gravity/magnetic boundary of Siletzia. This westward extension is based on offshore well data and Eel River and Newport basin seismic analogs. Onset of Cascadia subduction is estimated at 49-46 Ma followed by southern Cascadia volcanic arc eruptions at 40 Ma. The Paleogene phase of the Cascadia forearc basin (48 Ma to 31 Ma) is characterized by northward prograding deposition (present day coordinates) constrained by margin-parallel extension (Tillamook magmatic episode) resulting in successive depositional sequences with dominantly northward paleocurrents. A change in regional tectonics during late Oligocene (30 to 25? Ma) resulted in SW-NE compression (present day coordinates) expressed in the Coos Bay area by the NW-SE trending South Slough Syncline and Coos Bay Coal Field fold and thrust faulting. This deformation caused uplift of the southern Coast Range that propagated northward progressively separating the forearc into subregional Neogene depocenters by right-lateral strike/slip faults and clockwise rotation driven by Basin/Range westward extension and San Andreas transform system northward compression. Significant Neogene deformation events result in a mid-Miocene (17-15 Ma) unconformity between the Tarheel and Empire formations, and a late Miocene-Pliocene (<6 Ma) post-Empire unconformity. New paleomagnetic data from the Cape Arago section suggests approximately 70^o (+/-25^o) of post-Eocene clockwise rotation. Paleocurrents from magnetic and field indicators imply a coastline oriented roughly north south prior to tectonic rotation.