MAGMATISM AND DEFORMATION DURING THE TRANSITION FROM THE CRETACEOUS-PALEOCENE NORTH CASCADES ARC TO THE ANCESTRAL CASCADES ARC

Profound changes in magmatism and deformation occurred from 65–45 Ma in the Pacific Northwest during the transition from the NW-trending, Cretaceous North Cascades arc to the N-S-trending Cascadia system. Shortly after incorporation and metamorphism of sediments to >30 km depth in the North Cascades crystalline core, a magmatic lull lasted from ca. 60–50 Ma. Subduction of young oceanic lithosphere and encroachment of a large oceanic plateau (Siletzia) are inferred to have caused low-angle subduction during the lull. The major structural response to Siletzia collision began at ~51–50 Ma in the crystalline core and adjacent fore-arc basin, and continued at least until 48 Ma. Dextral strike along the eastern margin of the North Cascades arc during the lull in part overlapped temporally with postulated large-scale transport of “Baja British Columbia”. Voluminous magmatism farther to the east in the Kamloops – Challis belt was accompanied by E-W extension in metamorphic core complexes and supra-detachment basins and grabens, and plutonism migrated southwestward across NE Washington. These events suggest that low-angle subduction was followed by slab rollback and breakoff during accretion of Siletzia. A profound magmatic flare-up was associated with rollback and breakoff between ca. 49.4 Ma and 45 Ma, and included bimodal volcanism near the eastern edge of Siletzia, intrusion of granodioritic to granitic plutons in the crystalline core of the North Cascades, and voluminous, dominantly NE-striking dikes intruded over a >1250 km² area in the Cascades. Transtension during and shortly before the flare-up led to >300 km of total offset on dextral strike-slip faults, formation of a strike-slip basin, and subhorizontal ductile stretching and rapid exhumation of 8-10 kb metamorphic rocks in the North Cascades crystalline core. By ca. 45 Ma, subduction was ere-stablished outboard of Siletzia, and strike-slip faulting was localized on the north-striking Straight Creek – Fraser River fault. These events culminated in the establishment of the modern Cascadia convergent margin.