THE BRIDGE RIVER-SHULAPS CULMINATION: A WINDOW INTO PERMO-TRIASSIC SUBDUCTION INITIATION, JURASSIC-CRETACEOUS COLLISION, AND ARC POLARITY IN THE CANADIAN CORDILLERA

The Bridge River-Hozameen terrane is a Mesozoic subduction complex located in the cryptic Insular-Intermontane Suture Zone (IISZ) of southern British Columbia and northern Washington state. It records the destruction of an ocean basin that experienced >170 million years of pelagic sedimentation between Mississippian and Middle Jurassic time. Despite its location within the key IISZ and its record of a large oceanic plate off the western margin of North America, a holistic tectonic model for the origin, age, and polarity of subduction-accretion in the Bridge River-Hozameen complex has not yet been established due to prescribed structural complexities, uncertain correlations with adjacent terranes, and a scarcity of geochronologic and provenance data that can correlate the subduction complex to either the outboard (Insular) or inboard (Intermontane) terranes during its infancy. Based on new and old geologic mapping, field observations, geochronology, and geochemistry, we present a tectonic model for the development of a west-facing forearc assemblage on the outboard (western) edge of the Intermontane superterrane. This west-facing forearc assemblage is composed of (1) the Bridge River-Hozameen subduction complex, (2) the Shulaps-Cadwallader forearc ophiolite, and (3) the Tyaughton-Methow forearc basin. The entire forearc assemblage is exposed in a large (>1500 km²) NW-plunging antiformal culmination where faults imbricate subduction complex rocks of the Bridge River-Hozameen terrane and branch upward into the subduction roof thrust zone that carries the overlying Shulaps ophiolite and Tyaughton-Methow forearc basin. Based on accretionary age trends, tapering of the forearc ophiolite mantle wedge, and trends in metamorphic grade within the subduction complex, we show that the Bridge River-Hozameen subduction zone was east-dipping, active between at least Late Triassic and Early Cretaceous time, and tied to the western edge of the Intermontane superterrane. The forearc assemblage was juxtaposed against the Insular superterrane during late Early Cretaceous time by sinistral faults and collided with the Insular terrane ca. 100 Ma during a regional plate kinematic shift from sinistral-oblique to strongly orthogonal convergence.