A Kinematic Model of Offshore Strike-Slip Faults in the Cascadia Accretionary Prism

Date: 4/24/2019

Time: 03:00 PM

Room: Puget Sound

The Juan de Fuca oceanic plate subducts obliquely relative to the trench offshore Oregon and Washington at a rate of ~4 cm/yr relative to North America. At the same time, the Oregon and SW Washington forearc translate northward as part of regional rotation. The net motion between the downgoing plate and the forearc is largely convergent, but a small component of obliquity remains. Unlike other subduction systems were slip partitioning is observed, there is no evidence of partitioning along large trench-parallel strike-slip faults in the terrestrial forearc of Cascadia. Therefore, trench-parallel deformation must be accommodated on oblique structures in the offshore prism and forearc. Bathymetric and seismic reflection surveys have identified up to nine vertical faults that cut obliquely across the accretionary prism. These left-lateral structures, which extend from the deformation front to the edge of the continental shelf and may continue into the forearc, are believed to accommodate the trench-parallel deformation. However, little is known about these faults. In this work, I reevaluate these offshore structures given new kinematic constraints from plate motions and forearc rotation. A three-dimensional elastic model of the Cascadian forearc and prism is constructed in Poly3D with a realistic geometry. The appropriate kinematic boundary conditions are applied such that forearc motion agrees with terrestrial GPS velocities, as well as updated estimates of Juan de Fuca plate motion. Stress-free boundary conditions are assigned to these shallow strike-slip faults to infer long-term slip rates that are consistent with regional driving forces. The model predicts slip rates to be 3-6 mm/yr, which is slightly smaller than what has been inferred from geological constraints. This model can also be used to estimate how oblique convergence is partitioned between strike-slip faults, forearc rotation, and slip obliquity on the megathrust across the margin.

Presenting Author: David A. Schmidt

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