Ridge subduction, eduction, and the neogene tectonics of southwestern north america

Abstract

Subduction of a spreading ridge beneath a continent, the ridge half-spreading rate being greater than the continent-ridge convergence rate, leads to the possibility of lateral flow of mantle material beneath the continent and its emergence at the continental margin. The emergence, here termed eduction, provides a mechanism for exhumation of formerly subducted blueschists. An example is provided by the overriding of the East Pacific Rise south of the Mendocino transform fault by North America (~30 Ma ago). The relative motion between the Pacific and North American plates being in the direction of the future San Andreas Fault, subduction melange will be educted along those parts of the margin having a more northerly trend than the relative motion once the ridge is subducted. Also, as North America moves southwards over the Mendocino transform, blueschists, under-plated on North America by the subducting Juan de Fuca plate, are carried south across the transform and educted by the emerging Pacific plate without being heated by the ridge. Emergence of Pacific plate from beneath the continental margin caused small-scale rifting and fragmentation of the margin. Formation of coastal sedimentary basins correlates with the expansion of the eduction regime as the Mendocino and Rivera triple junctions migrated along the continental margin. After coastal California joined the Pacific plate (inception of movement on San Andreas fault, ~5 Ma), eduction was restricted to that portion of the margin between the north end of the San Andreas system at Point Arena and the Mendocino transform (then both 450 km southeast of their present positions relative to North America). As coastal California moves northwest it traps educted Franciscan rocks in the wedge-shaped gap between the northern part of the San Andreas Fault and the former North American coast. The boundary between the North American and Pacific plates is the vertical San Andreas Fault near the surface but becomes a shallowly east-dipping discontinuity at depth. This explains the dearth of seismicity deeper than 15 km along the trace of the San Andreas Fault, and north of its northernmost surface exposure at Point Arena. Passage of the spreading East Pacific Rise beneath North America accounts for Neogene block faulting, uplift of the Colorado Plateau, and northeastward migration of bimodal volcanism across the southwest U.S.A. Southwestward movement of North America across the Mendocino transform shuts off Cascade volcanism and lifts up the Sierra Nevada block because the American plate moves from a cool subduction regime above the steeply dipping Juan de Fuca plate to a hot, eduction regime above the shallowly lying Pacific plate.