PENROSE MEDAL: CONTINENTAL PLATE TECTONICS AND SOME LONG-STANDING CONTROVERSIES SURROUNDING THE TECTONIC HISTORY OF WESTERN NORTH AMERICA

Theories concerning the rich Cenozoic tectonic history of Western North America intricately depend upon how one visualizes the subducting Farallon slab. Starting from the spreading record preserved in the Pacific plate, an array of subduction scenarios have been proposed to explain the complex time-space patterns observed in continental magmatism, deformation, and elevation. Model differences arise from differing assumptions concerning slab dip, plate driving mechanisms, the nature of spreading centers and fracture zones, and processes surrounding spreading center subduction. A late Cretaceous low angle subducting slab and its mid Cenozoic removal can explain many seeming geologic anomalies, including the cessation of Sierran magmatism, Laramide deformations, the partial absence of old continental mantle lithosphere, flare-ups of ignimbrite eruption, and the wetting of the over-riding plate. The widely accepted description of spreading centers as passive cracks implies that they will quickly disintegrate upon subduction, leaving growing holes where the slab once lay. Both spreading center subduction and the steepening of a low angle slab can be described as "slab removal events" that abruptly expose the cool, wet continental underside to hot asthenosphere, causing uplift and bursts of surface volcanism. Finally, fracture zones should be viewed as the strongest areas within oceanic plates, not as lines along which a slab will likely break.