GEOLOGICAL CONSTRAINTS FOR LATE JURASSIC-EOCENE CORDILLERAN OROGENY IN THE WESTERN U.S.A.: REVIEW AND ACTUALISTIC COMPARISONS

Geological data from the Late Jurassic-Eocene North American Cordillera (NAC) document 100 Myr of continuous eastward subduction of the Farallon plate, producing an arc-trench system, high hinterland plateau, retroarc thrust belt, and foreland basin disrupted by basement uplifts. Proterozoic-Devonian strata from foreland to hinterland are correlative and span footwall and hanging wall domains of the NAC thrust belt. Detrital zircon (DZ) data demonstrate North American provenance for all these units, falsifying models requiring an intercontinental suture along the Lewis, Paris-Willard, Sheeprock, Canyon Range, and Wheeler Pass thrust systems. Sedimenticlastic Jurassic-Cretaceous foreland basin strata have westward provenance, volcanic ash, and near-depo-age DZs throughout, attesting to continuous airborne and overland fluvial transport of detritus from an active Cordilleran magmatic arc and thrust belt. Bedrock and detrital thermochronologic data show exhumation of thrust belt sources from ca. 160 Ma onward. Rand-Orocopia-Pelona schists and arclogite xenoliths in southern California and the Arizona transition zone require forearc and sub-arc sources and a kinematic process of transfer into their present retroarc locations. This process is lacking in a westward subducting collisional system, but obvious in an eastward subducting cordilleran system in which slab subduction angle was demonstrably variable. Comparison of the NAC with well-known cordilleran (Andes) and collisional (Himalayan) orogenic belts is instructive: The Himalaya is a poor match for the NAC; it lacks an arc, accommodated >1000 km of shortening since 60 Ma, and is eroded down to >20 km depths over 30-50% of its present surface. Despite deep erosion, the Indus-Yarlung suture is well represented in outcrop. The foreland basin is filled with metamorphiclastic and sedimenticlastic detritus and lacks volcanic and depo-age components. In contrast the central Andes contain an active magmatic arc, high hinterland plateau, eastward verging thrust belt, and a foreland basin locally ruptured by intraforeland uplifts. Like the NAC, the central Andes lack a Cenozoic suture zone despite a shallow depth (<8 km) of erosion, and the foreland basin contains sedimenticlastic and volcaniclastic sediment with ubiquitous depo-age DZs.