Cenozoic magmatism and plate tectonics in western North America: Have we got it wrong?

Abstract

ABSTRACT The current tectonic framework for understanding Cenozoic magmatism in western North America was laid out in a series of influential papers in the early days of the plate-tectonics revolution. These ideas, largely developed through deductive analysis, were so revolutionary yet seemingly self-evident that they quickly passed from hypothesis to axiom. These include the following. (1) Inboard and outboard sweeps of magmatism resulted from shallowing and then rapid steepening of a subducted slab. (2) The Oligocene–Miocene ignimbrite flareup resulted from sinking and rollback of a shallow slab. (3) Late Cenozoic basaltic magmatism resulted from opening of a slab window. (4) The current Cascade arc is the remnant of a much more continuous ancestral arc that ran the length of western North America. When tested against current databases of igneous rock ages and chemical analyses, these conjectures largely fail; some are clearly contradicted, whereas others are possible but ad hoc and unfalsifiable. Ironically, the plate-tectonics revolution nicely explains plate-boundary magmatism in much of the world but is less successful in western North America, where many of these links were first developed. It is time for a second revolution.