Eocene Terrane Accretion in Northern Cascadia Recorded by Brittle Left-Lateral Slip on the San Juan Fault

The San Juan fault (SJF), on southern Vancouver Island, Canada, juxtaposes the oceanic Wrangellia and Pacific Rim terranes in the northern Cascadia forearc, and has been suggested to play a role in multiple Mesozoic-Cenozoic terrane accretion events. However, direct observations of the SJF's kinematics have not been documented and its exact role in accommodating strain arising from terrane accretion is unknown. To test if, how, and when the SJF accommodated accretion-related strain, we use geologic mapping, kinematic inversion of fault-plane slickenlines, and dating of marine sediments to constrain the timing and direction of brittle slip of the SJF. P- and T-axes from kinematic inversions indicate predominantly left-lateral slip. Left-lateral brittle faulting cross-cuts ∼51 Ma magmatic intrusions and foliation, providing a maximum age of brittle deformation. The fault zone is non-conformably overlain by a >300 m-thick sequence of clastic marine shelf and slope sediments that are not left-laterally offset. A strontium isotope age of foraminifers helps constrain the depositional age of the sediments to late Eocene–early Oligocene, bracketing left-lateral slip to the Eocene. Eocene left-lateral slip is temporally and kinematically consistent with regional southwest-northeast compression during accretion of the Siletzia ocean island plateau, suggesting brittle slip on the SJF accommodated strain resulting from the accretion of this terrane. This result does not support hypotheses that brittle slip along the SJF directly accommodated earlier accretion of the Pacific Rim terrane to Wrangellia, instead it offsets the older accretionary boundary between these two terranes.