KINEMATICS AND TIMING OF THE SAN JUAN FAULT IN NORTHERN CASCADIA: IMPLICATIONS FOR EOCENE TERRANE ACCRETION OF SILETZIA

The terrane-bounding San Juan fault on southern Vancouver Island, Canada is traditionally thought to have accommodated the accretion of the Pacific Rim terrane to Wrangellia. We present results of detailed structural mapping coupled with stratigraphic and biostratigraphic analysis of overlying strata that suggest the San Juan fault was reactivated to accommodate significant along-strike gradients in shortening during the Eocene accretion of Siletzia to the North American margin. We map the San Juan fault zone as an ~1 km wide brittle fault zone with a 10-20 m fault core that displays a scaly fabric and several m-scale synthetic faults at acute angles to the main fault. Kinematic analyses based on P- and T-axes derived from kinematic inversions of fault slickenline measurements from brittle slip surfaces within the fault zone indicate that left-lateral slip occurred on the E-W striking western 50 km of the fault zone. This fault zone is unconformably overlain by an ~200 m thick package of relatively undeformed clastic marine shelf and slope sediments that are not offset by left-lateral slip. Stratigraphic correlation and strontium isotope ages of foraminifers from these sediments indicate Eocene / Oligocene depositional ages and bracket the timing of left-lateral slip on the San Juan fault as pre-Oligocene. Pre-Oligocene left-lateral slip on the San Juan fault is consistent with SW—NE compression documented in the region during Eocene Siletzia terrane accretion. This suggests that Siletzia acted as an indenter during terrane accretion, with the San Juan fault accommodating a lateral strain gradient via left-lateral strike-slip faulting.