GEOLOGIC SIGNATURE OF PALEOGENE RIDGE SUBDUCTION AND SLAB WINDOW FORMATION BENEATH THE CENTRAL NORTH AMERICAN CORDILLERA

Plate kinematic models for the Pacific basin imply that the Kula-Farallon ridge system was subducting beneath the western margin of North America from Late Cretaceous to Eocene time. Ridge subduction results in formation of a slab window (gap) between the two diverging slabs at depth. In modern settings the migration of a ridge-trench-trench triple junction and the opening of a slab window disrupts the magmatic and structural environment of the overriding plate. In the Pacific Northwest, several aspects of Paleogene geology are inconsistent with generation by normal subduction processes but are predicted by a model involving ridge subduction. These features include felsic forearc intrusions and volcanics on Vancouver Island, within-plate geochemical signature of the Challis-Kamloops volcanic belt from south-central British Columbia to Montana, the presence of adakites (possible slab melts) in southern British Columbia, and cessation of magmatism in the Coast Batholith. Structural features in the region which are also predicted by a ridge subduction model include development of near-trench fold-and-thrust belts, trench-perpendicular crustal-scale extension in the backarc, and a syn-extensional to strike-slip emplacement style for backarc volcanism. The unusual structural and magmatic trends are almost exclusively early Eocene in age. Kinematic reconstructions of the subducting Kula-Farallon spreading-ridge system and its slab window suggest that the window migrated northwards through the Paleogene and was present beneath Idaho, Montana and southern British Columbia in the Early Eocene. This position also coincides with a sharp contrast in the distribution of volcanism within the Challis-Kamloops belt. A slab window model accounts for the observed magmatic and structural trends in the region, as well as providing a heat source to account for the high Paleogene heat flow observed in the Pacific Northwest. Meshing this study to a recently proposed tectonic model, which indicates likely interaction of a previously unrecognised plate along the Paleogene North American subduction zone, would require that the Pacific Northwest slab window is bounded on its north by the Resurrection plate, but does not compromise the suitability of the ridge subduction model to the region.