Reconstructing the ancestral Yellowstone plume from accreted seamounts and its relationship to flat-slab subduction

Recent geodynamic analyses have emphasized the relationship between modem flat-slab subduction zones and the overriding of buoyant oceanic crust. Although most models for the evolution of the Late Mesozoic-Cenozoic Laramide orogeny in the southwestern United States involve flat-slab subduction, the mechanisms proposed are controversial. An examination of the geological evolution of the 60-50-Ma Crescent terrane of the Coast Ranges indicates that it was formed in a shallowing-upward Loihi-type oceanic setting culminating in the eruption of subaerial lavas. Plate reconstructions indicate that the Crescent terrane was emplaced into ca. 20-Ma crust, and the presence of subaerial lavas implies an uplift due to the plume of ca. 4.2 km, which we use to calculate a minimum buoyancy flux of 1.1 Mg s(-1), similar to that of the modem Yellowstone plume. Published paleomagnetic data indicate that the Crescent terrane was formed at a paleolatitude similar to that of the Yellowstone plume. The Crescent seamount was accreted within 5 My of the cessation of plume magmatism. Plate reconstructions indicate that it would have originated about 750 kin to the west of the North American plate margin if it developed above a fixed Yellowstone plume, and are therefore consistent with the recorded very short interval between its formation and tectonic emplacement. We interpret the Crescent terrane as due to the ancestral Yellowstone plume. Such a plume would have generated an elongate swell and related plateau that would have been overridden by the North American margin. Taken together, the relationship between flat-slab and overriding of oceanic plateau in Laramide times would have been analogous to the relationship between modem Andean flat-slab subduction zones and the Juan Fernandez and Nazca oceanic plateaus. (C) 2003 Elsevier Science B.V. All rights reserved.