UPPER PLATE CONTROLS ON THE EXPRESSION OF MOUNTAIN BUILDING IN CORDILLERAN OROGENIC SYSTEMS (Invited Presentation)

Upper plates in contractional ocean-continent subduction systems exhibit substantial variations in the expression of mountain building, including in structural style. Most workers consider these variations to result from changes in plate boundary geodynamics (e.g., slab dip) and/or spatial variations in the strength of the upper plate. In both the North and South American Cordilleras, thin-skinned thrusts occur where mechanically weak, bedding-parallel weaknesses were available during thrust initiation, whereas thick-skinned or hybrid thrust systems occur elsewhere. Given that thrusts ramp up-section toward the foreland, stratigraphically deeper detachments are activated near thrust belt hinterlands and shallower detachments toward forelands. With progressive shortening, thrust wedges grow and frontal detachments step-down to deeper stratigraphic levels, if available. In much of the North American Cordilleran retroarc, the pre-thrusting strength of the upper continental plate is a relict of the Neoproterozoic and Paleozoic rift and passive margin, with >15 km thick, pre-thrusting strata tapering craton-ward to <4 km thick. Along-strike to the south and toward the foreland, the marginal succession was thinner prior to orogenesis, promoting basement-involvement and/or reactivation of older crustal weaknesses. The South American Cordillera did not possess a rift and passive margin succession in the upper crust prior to orogenesis, but thin-skinned thrusts are co-located with >3 km thick, pre-thrusting backarc basin strata. Where pre-thrusting strata were thinner, older rift faults were inverted, and basement-involved thrusts formed where the upper crust lacked sub-horizontal detachment horizons. In both regions, progressive orogenesis resulted in heating of the lower crust, reducing its strength, and limiting the importance of the pre-thrusting strength of the upper crust. Though we implicate 3D strength contrasts of the upper plate in controlling the expression of orogenesis, changes in plate boundary geodynamics may enhance plate coupling and/or change the thermal structure of the upper plate, promoting thrust belt propagation into regions where fewer undeformed stratigraphic detachment horizons are available and resulting in along- and across-strike changes in structural style.