Cyclicity in Cordilleran orogenic systems

Abstract

Cordilleran orogenic systems, such as the modern Andes, are long belts of deformation and magmatism that are associated with the subduction of oceanic plates beneath continental ones. Although the oceanic plates have been thought to control the evolution of such systems, a number of processes operating in the upper continental plates have not been fully accounted for. The western American Cordilleras, for example, display a 25-50 million year (Myr) cycle of linked upper-plate processes. In a typical cycle, as the two plates converge and a magmatic arc forms, most of the continental crust shortens by thrusting behind the arc, whereas the lowermost continental lithosphere is shoved beneath the arc a process that fuels episodic high-flux magmatism in the arc and simultaneously generates dense melt residues. On reaching a critical mass, these residues sink into the mantle, creating space beneath the arc and setting the stage for renewal of the cycle. This alternative model explains key features of Cordilleran systems, such as cyclical trends in the flux and composition of magma supplied to the upper plate, and the foundering of arc roots. © 2009 Macmillan Publishers Limited.