PLUME-MODIFIED OROGENY - AN EXAMPLE FROM THE WESTERN UNITED-STATES

Plate tectonic theory implies that orogeny at convergent margins resul ts from several processes, including subduction of oceanic crust, subd uction of aseismic ridges, accretion of terranes, and continental coll ision. Each of these processes involves the consumption of an oceanic tract, and each has its own characteristic style of tectonothermal act ivity, An additional, potentially important, orogenic process has been largely overlooked. Oceanic regions contain chains of volcanic island s formed at hotspots, which are generally considered to reflect the si tes of rising plumes, In a hotspot reference frame, where active conti nental margins advance and override a plume, the plume's buoyant swell may profoundly change the tectonothermal expression of subduction and hence orogenic processes at the continental margin. The Late Cretaceo us-Tertiary evolution of the western United States may be an example o f this process, which has been overlooked in the de development of oro genic paradigms.