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 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 San Juan fault accommodated strain resulting from the
accretion of this terrane. This result does not support hypotheses that
brittle slip along the San Juan fault directly accommodated earlier
accretion of the Pacific Rim terrane to Wrangellia, instead, it offsets
the older accretionary boundary between these two terranes.