The active deformation field in subduction forearcs provides critical information about the stress and strain state of the upper plate and its potential for seismogenesis. However, these properties are challenging to quantify in most subduction systems, and in the northern Cascadia forearc, few faults have been identified that can be used to reconstruct the upper plate deformation field. Here we investigate the slip history of the Beaufort Range fault (BRF) on Vancouver Island. This fault was proposed to host the 1946 M7.3 Vancouver Island earthquake, but no surface rupture or evidence of Quaternary activity has been documented, and the stress and strain conditions that promoted this event are poorly understood. We provide the first evidence that the BRF is active, using newly-collected lidar to map topographic scarps along the fault system and to reconstruct slip vectors from offset geomorphic markers. Quaternary deposits and landforms that show increasing magnitude of displacement with age provide evidence for at least three M ~6.5-7.5 earthquakes since ~15 ka, with the most recent event occurring <3-4 ka. Kinematic inversions of offset geomorphic markers show that the BRF accommodates right-lateral transtension along a steeply NE-dipping fault. This fault geometry and kinematics are similar to those modeled for the 1946 earthquake, suggesting that the BRF is a candidate source fault for this event. We find that the kinematics of the BRF are consistent over decadal to millennial timescales, suggesting that this portion of the northern Cascadia forearc has accommodated transtension over multiple earthquake cycles.

Oceanic plates experience extensive normal faulting as they bend and subduct, enabling fracturing of the crust and upper mantle. Debate remains about the relative importance of pre-existing faults, plate curvature and other factors in controlling the extent and style of bending-related faulting. The subduction zone off the Alaska Peninsula is an ideal place to investigate controls on bending-related faulting as the orientation of abyssal-hill fabric with respect to the trench and plate curvature vary along the margin. Here we characterize bending faulting between longitudes 161°W and 155ºW using newly collected multibeam bathymetry data. We also use a compilation of seismic reflection data to constrain patterns of sediment thickness on the incoming plate. Although sediment thickness increases by over 1 km from 156°W to 160°W, most sediments were deposited prior to the onset of bending faulting and thus have limited impact on the expression of bend-related fault strikes and throws in bathymetry data. Where magnetic anomalies trend subparallel to the trench (<30°) west of ~156ºW, bending faulting parallels magnetic anomalies, implying bending faulting reactivates pre-existing structures. Where magnetic anomalies are highly oblique (>30°) to the trench east of 156ºW, no bending faulting is observed. Summed fault throws increase to the west, including where pre-existing structure orientations do not vary between 157-161ºW, suggesting that the increase in slab curvature directly influences fault throws. However, the westward increase in summed fault throws is more abrupt than expected for changes in slab bending alone, suggesting potential feedbacks between pre-existing structures, slab dip, and faulting.