DEXTRAL SHEAR, CLOCKWISE ROTATION, AND FAULTING IN THE CASCADIA FOREARC

Large clockwise rotation of western Oregon observed in geodetic and paleomagnetic data has been interpreted to be the result of microplate rotation, distributed dextral shear, or some combination of both. A detailed geologic and geophysical transect from the coast to Portland, OR provides an opportunity to reexamine the role of distributed shear in producing large clockwise rotations of the Oregon forearc. The new surveys reveal 12 km of post-35 Ma right lateral displacement on the NNW-trending Gales Creek Fault (GCF) west of Portland. The mapping also reveals sinistral offsets of the GCF by WNW-trending antithetic faults from the Coast Range, in the style of domino faulting as described by Freund (1974). This pattern has been observed in SW Washington (Wells and Coe, 1985) and along the Cascadia slope offshore (Goldfinger et al, 1992), where the faults are thought to accommodate clockwise rotation. We interpret the GCF as the eastern boundary of a 60 km-wide Coast Range rotation domain, which is transected by several, large WNW sinistral faults forming fault-bounded “dominoes” about 70 km long and 20-40 km across. The largest of these is the Tillamook Bay-Yamhill River fault zone (TBYRF), which forms the northern margin of Tillamook Bay and coincides with 20 km of sinistral separation on the top of the 41.6 Ma Tillamook Volcanics (TV). For a 40 km spacing of faults, this displacement would accommodate 21° of rotation, about 47% of the 45° observed in the TV. The TBYRF and similar WNW structures probably originated as normal faults, as they are parallel to numerous dikes visible on aeromagnetic maps that fed Tillamook and Yachats volcanism in the Coast Range. The TBYRF then rotated clockwise, becoming a sinistral reverse fault, and finally a dextral fault, based on fault slip data. N-S faults offshore may form the western boundary of the Coast Range domain and separate it from the slope domain described by Goldfinger et al. In the Willamette Valley, 6 km of post-15 Ma dextral slip on the GCF accounts for 10°, or 44% of the rotation measured in Columbia River Basalt. Domino faulting, likely driven by oblique subduction, may accommodate the observed westward increase in rotation toward the plate boundary. Today’s geodetic rotation rate matches the permanent rotation from paleomagnetism and suggests the entire fault system may be active.