CENOZOIC STRUCTURAL JUXTAPOSITION OF THE SAN JUAN ISLANDS – WRANGELLIA AND THE NORTHWEST CASCADES – COAST PLUTONIC COMPLEX, NORTHWESTERN WASHINGTON AND SOUTHWEST BRITISH COLUMBIA

Rocks of the Wrangellia terrane (WT) and San Juan Islands (SJI) are separated from the Coast Plutonic Complex (CPC) and the northwest Cascades (NC) by a Cenozoic fault system that juxtaposes terranes with different lithologies and disparate metamorphic and structural histories. Paleo-Eocene clastic rocks provide a stratigraphic link between outboard and inboard terranes but a link during deposition of Upper Cretaceous clastic rocks cannot be discounted. Mesozoic layered rocks of the WT were weakly deformed during sub-greenschist facies regional metamorphism and overprinted by amphibolite-hornfels aureoles around Jurassic to Early Cretaceous plutons. In the CPC, Jurassic to Eocene plutons intrude metasedimentary and metavolcanic rocks penetratively deformed at upper-greenschist to amphibolite conditions. The terranes are separated by the Johnstone-Malaspina high-angle fault zone exposed on several islands northeast of Vancouver Island and traced south along narrow marine channels bordering the British Columbia mainland. In the SJI and western NC, sedimentary mélange and depositionally overlying Jura-Cretaceous sedimentary rocks experienced penetrative deformation at sub-blueschist and sub-greenschist conditions. Although similarities exist between the mélange successions, the overlying Jura-Cretaceous clastic rocks differ in lithology, provenance, stratigraphy, and structural history and do not provide a structural or stratigraphic link across a fault inferred beneath Rosario Strait in Washington. The SJI and WT were unconformably overlain by Upper Cretaceous clastic rocks and together were penetratively deformed and imbricated during top-to-the northeast thrusting in the Paleocene. The Paleocene thrust belt abruptly ends and does not extend east of the Rosario Strait fault and Upper Cretaceous clastic rocks overlying the CPC, which may correlate with those of the SJTB and WT, were not involved in the deformation. Folded Paleo-Eocene clastic rocks unconformably overlie the WT and SJI Paleocene thrust belt and the NC tectonites to the east. Right-lateral displacement on the Cenozoic fault system is limited to ~450 km if Late Cretaceous stratigraphic correlations across the structure stand but is open-ended if discounted. Right-lateral offset since the Paleo-Eocene can exceed 50 km.