50 MILLION YEARS OF FAULT SLIP IN THE CASCADIA FOREARC (Invited Presentation)

To better understand the deformation history of the Cascadia forearc and its role in active faulting, we measured the geometry and kinematics of exposed faults encountered during systematic geologic mapping of the Oregon (OR) and Washington (WA) Coast Ranges. The study area is outlined by Siletzia, the oceanic basalt terrane that accreted to the North American margin ca. 50 Ma, where the timing of faults is well-constrained by isotopic ages and the orientation by paleomagnetic measurement of mostly clockwise rotations (CWR). We focused on faults in volcanic rocks, including Siletzia (50 Ma; ~70° CWR), the Tillamook episode (42-35 Ma; ~46° CWR), and the Columbia River Basalt Group (CRBG, 16-12 Ma; ~20-16° CWR). Within these units, we measured fault plane orientation, slip vector (slickenlines), slip sense, displacement, gouge width, and cross cutting relations from 330 faults at 150 stations between Roseburg, OR and the Puget Lowland, WA and measured hundreds of dike azimuths. We calculated principal strain axes and composite fault plane solutions where appropriate (method of Marrett and Allmendinger, 1990), but fault length and displacement amount were not always clear in the forested terrain. Our results document three distinct deformation episodes (corrected for CWR): 1) Siletzia Accretion: ENE-directed folding, thrusting, and block rotation 50-48 Ma in southern OR, slightly later in WA; 2) Tillamook Rifting: post accretion relaxation and NW-SE-directed, margin-parallel extension, SW-trending dike swarms, and mafic forearc magmatism 42-35 Ma in OR and WA, perhaps related to ridge- or hotspot-trench interactions; and 3) Forearc Migration: Neogene to Quaternary ~N-S shortening and NW-SE oriented right-lateral shear in CRBG and younger strata (< 16 Ma) that we relate to northward migration and CWR of the forearc. Fault-slip data from paleoseismic trenches across Holocene surface-rupturing faults from Gales Creek, OR to Seattle, WA also record N-S shortening and dextral shear, consistent with well-bore breakouts and earthquake focal mechanisms. Some faults show evidence of prolonged activity throughout the Cenozoic or reactivation in the Neogene. For example, the NE-oriented Corvallis fault is parallel to accretion-related thrusts in Siletzia but displays an 11-km-long lidar scarp suggesting Holocene activity.