Evidence for Whole Mantle Convection Driving Cordilleran Tectonics

Abstract

AbstractDeducing mechanisms for advance and retreat of magmatic arcs is fundamental to understanding accretionary tectonics and the evolution of continents. However, first‐order explanations of large spatial and long temporal changes in magmatic arcs remain elusive. We present isotopic evidence that Cordilleran magmatic arc systems were controlled by spherical harmonic degree‐2 mantle convection and characterized by two antipodal upwellings bisected by a meridional downwelling. Once established, the meridional “subduction girdle” drives hemispheric slab rollback and remains the locus of Cordilleran oceanic arcs. Continual westward migration of the North and South American continents led to arc advancement and consumption of back‐arc basins, culminating in arc‐continent collisions and reversals of subduction polarity. Continental arcs initiated diachronously as North and South America arrived at the subduction girdle and oceanic arcs were accreted. Systematic patterns in radiogenic isotopes along the Cordilleran system support that slab dynamics are controlled, to first order, by long‐wavelength mantle convection.