CYCLIC BEHAVIOR IN CORDILLERAN OROGENIC SYSTEMS

Cordilleran orogenic systems, such as those in the western Americas, form in regions of significant upper plate shortening where oceanic lithosphere subducts beneath continental lithosphere. These systems are characterized by extreme crustal thickening and high regional elevation, calc-alkaline magmatic arcs, large retroarc thrust belts and foreland basins, and forearc-trench zones that range between highly erosive (sediment-starved) and accretionary (sediment-rich). We posit that Cordilleran orogenic systems operate in a cyclic fashion, and that predictable relationships exist among diverse processes including retroarc thrusting, magmagenesis by upper plate melting and creation of dense garnet pyroxenite (eclogite) residues, removal of these residues by gravitational foundering (or delamination), and forearc erosion/accretion. A typical cycle lasts 25-50 Myr, and is highlighted by periodic high-flux events (HFE’s) in the magmatic arc composed of melts derived chiefly from underthrusted foreland lithosphere and continental lower crust (beneath the thrust belt hinterland and arc). These HFE’s exhibit highly evolved isotopic compositions and apparent magma flux rates up to an order of magnitude greater than normal ‘background’ flux rates. Concomitant with the HFE, dense eclogitic residues form beneath the arc and hinterland. Build-up of this residual material beneath the arc impedes rapid underthrusting and upper crustal shortening. The eclogitic material accumulates until it reaches critical mass and founders into the upper mantle, simultaneously relieving the room problem beneath the arc and hinterland and allowing renewal of rapid underthrusting and rejuvenation of the cycle. The association of rapidly shortening thrust belts (e.g., Central Andes) with higher frequency cycles suggests that cycle frequency is modulated by overall convergence and retroarc shortening rates, as well as melt fertility of the underthrusting continental plate. Our model makes testable predictions about numerous phenomena in Cordilleran systems, including migration and subsidence in the retroforeland basin, accumulation of intermontane hinterland basins, shortening and surface uplift/subsidence rates, and extension in the forearc and hinterland.