CORDILLERAN ARC MOBILITY AND ACCRETIONARY TECTONICS DUE TO DEGREE-2 MANTLE CONVECTION

Deducing mechanisms responsible for the advance and retreat of magmatic arcs is fundamental to understanding accretionary tectonics and the evolution of continental lithosphere. Nonetheless, first-order explanations of such changes in large-scale and long-term magmatic arc evolution remain elusive. Here we present evidence suggesting that the evolution of the Cordilleran magmatic arc system was dominantly controlled by long-lived, degree-2 (spherical harmonic) mantle convection. Established just prior to breakup of supercontinent Pangea, degree-2 mantle convection is characterized by two antipodal upwellings beneath the African and Pacific plates that are bisected by meridional downwelling. Radiogenic isotopes suggest the development of this convection pattern led to both the rifting of Pangea and retreat of the Cordilleran arcs away from Pangea and towards the emerging downwelling surrounding the former supercontinent. Once established, the meridional “subduction girdle” remained the locus of Cordilleran ocean arcs whereas the continual westward migration of North and South America led to the apparent advance of subduction zones, inversion of the young backarc ocean basins, eventual arc-continent collision, and subduction polarity reversals. The classic continental arc commenced when the continent arrived at the subduction girdle and the ocean arcs accreted to the continental margin. The isotopic patterns associated with apparent arc retreat and advance occur along the entire Cordilleran system, supporting the model that slab dynamics are controlled to first-order by long-wavelength mantle convection.