EVIDENCE FOR SUBDUCTION ALONG THE LEECH RIVER FAULT AND IMPLICATIONS FOR CORDILLERAN TECTONICS

The Leech River Fault (LRF) on southern Vancouver Island is a terrane-bounding structure separating the Leech River Schist, a collection of metasedimentary schists part of the Pacific Rim Terrane, and the Crescent-Siletz Terrane, a large igneous province formerly part of the Farallon and Kula/Resurrection plates. The kinematics and relative timing of deformation along the LRF have significant implications for Eocene plate motions in the Cordillera, but the deformation history of this fault is still a subject of debate. We present field, microstructural, petrological, and geochronological observations that constrain the structural and metamorphic history of LRF. The LRF is defined by a 600-m wide mylonitic shear zone that is developed across the lithologic contact between the Leech River Schist (LRS) and Metchosin Basalt (MB). Foliation orientations, a steeply plunging stretching lineation, and kinematic indicators all suggest sinistral-oblique motion consistent with subduction of the MB beneath the LRS. Compositions of garnet in the LRS and amphibole in the MB record syn-kinematic prograde growth at peak temperatures of ~550 °C and pressures of approximately 700-850 MPa. The peak metamorphic conditions suggest elevated geotherms that are consistent with models of plate motions that position the Farallon-Kula/Resurrection ridge and Yellowstone hotspot in the Pacific Northwest in the early Eocene (~50 Ma). Detrital zircon U-Pb age distributions correlate with Mesozoic volcanic arcs, supporting Jurassic-Cretaceous deposition of the LRS as an accretionary complex in the near-trench environment. Precambrian detrital zircons show a lack of correlation with the provenance of the Baranof Schist in disagreement with the Baranof-Leech River hypothesis. Our observations establish the LRF as the downdip portion of a paleo-plate interface representing an Eocene subduction zone.