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

The San Juan fault, 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 San Juan fault'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 San Juan fault 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 San Juan fault. 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 package of clastic marine shelf and s...