Miocene ridge impingement and the spawning of secondary ridges off Oregon, Washington and British Columbia

Abstract

The East Pacific Rise is the only extant portion of the South Pacific Spreading Center, which began to impinge on the North American plate margin ~30 Ma ago. The North Pacific Spreading Center, offset ~1500 km to the west on the Mendocino transform fault, remained active until ~9 Ma ago when it, too, impinged on the trench off the Oregon—Washington margin. The zones of mantle upwelling associated with the spreading centers have since migrated inland to their present sites beneath the Cascade and Garibaldi volcanic chains and the Rio Grande Rift, maintaining the ~1500 km offset that they have had since the Early Tertiary. Impingement of the North Pacific Spreading Center on the trench at anomaly 5 time juxtaposed the Pacific and American plates and the continuing relative motion between these plates across the north—south boundary has been accommodated by secondary spreading at the Gorda—Juan de Fuca—Explorer ridge system. These obliquely spreading ridges, fed by lateral mantle flow originating at the zone of mantle upwelling beneath the Cascade belt, are migrating away from the North American coast at half the Pacific—America velocity, and no subduction is occurring along the continental margin, the Juan de Fuca sea floor being an integral part of the American plate. This scenario is consistent with the fact that the sea floor along the continental margin has the same age as that which bears the last north—south anomaly on the Pacific plate (anomaly 5) preceding initiation of spreading at the Juan de Fuca and Explorer ridges (a fortuitous coincidence in models involving continued subduction of the Juan de Fuca sea floor). This model implies that the rate of motion between the Pacific and American plates is 8 cm/a. The Blanco fracture zone formed as a transform fault at the south end of the Juan de Fuca ridge and was initially collinear with the North American margin south of Cape Mendocino. The North Pacific Spreading Center impinged on this portion of the margin somewhat later than north of Cape Mendocino, and the Gorda ridge began to migrate northwestwards. Interaction among the Pacific plate south of the Mendocino fracture zone, the American plate north of the Blanco fracture zone, and the growing Gorda plate caused clockwise rotation of the Gorda plate, faulting of the Juan de Fuca sea floor and counterclockwise rotation of the Blanco fracture zone. The mantle upwelling that was formerly the site of the North Pacific Spreading Center is now situated beneath Oregon, Washington and western British Columbia and is responsible for high heat flow, basaltic volcanism, low P n velocities, high upper mantle conductivity and crustal rifting in this region.