SUBDUCTION OF THE KULA-FARALLON / KULA-RESURRECTION RIDGE SYSTEM: IMPLICATIONS FOR THE EOCENE MAGMATIC AND TECTONIC HISTORY OF THE PACIFIC NORTHWEST

Subduction of the Kula-Farallon / Kula-Resurrection Ridge System (KFR) beneath western North America during the Eocene is constrained in space and time by ages of basalts in the WA-OR Coast Ranges and by ages of scattered centers of adakite and near-trench magmatism in WA and Vancouver Island. These data reveal: (1) a northward migration of magmatism that is slower (~32 km / Ma) than predicted on the basis of plate paleovelocity vectors, and: (2) repeated examples of age overlap among rocks at different latitudes. Both of these observations require left-stepping offsets of the KFR, which would have led to multiple ridge-trench intersections and the development of a series of fraternal slab windows (Thorkelson, 1996). Combining these geologic constraints on KFR location and offsets with published plate motion vectors makes it possible to reconstruct the evolving geometry of the KFR slab window beneath WA-OR-BC from ~60-35 Ma. When the effects of Basin and Range extension are accounted for, the NW-migrating KFR slab window provides an explanation for the anomalous width of the Challis “arc” in WA and for its subtle northwestward migration. Other Eocene phenomena in the Pacific NW that appear to correlate in space and time with passage of the KFR slab window include tholeiitic dike swarms in central WA, high-temperature metamorphism and unroofing in the core of the North Cascades, S- and A-type plutonism in WA and BC, and the development of sedimentary basins. Subsequent establishment of the modern Cascade arc at ~39 Ma correlates with a decrease in subduction rate and probable steepening of the subducting slab beneath OR/WA as the Farallon slab foundered. The KFR slab window may have facilitated this transition, serving as the leading edge along which slab rupture and foundering (roll-back) were initiated.