UPLIFT, GRAVITATIONAL COLLAPSE, THICK-SKINNED DEFORMATION, AND SLAB FAILURE MAGMATISM DURING CRETACEOUS-TERTIARY OROGENY IN THE NORTH AMERICAN CORDILLERA

The Cretaceous-Tertiary Cordilleran orogeny was likely caused by failed westerly-directed subduction of the segmented western margin of North America beneath an arc-bearing superterrane, and not in a back-arc setting as suggested in older models. Based on widespread deposition of intraplatformal gravels directly beneath the foredeep, the collision, which generated the Sevier-Rocky Mountain fold-thrust belt and its foreland basin, started at about 124 Ma and continued until slab failure at about 75-70 Ma.

During failure, the outermost portion of the North American craton, and likely all of the transitional crust with its rift deposits, remained attached to the oceanic anchor and were subducted. The failure of the North American plate led to (1) shutdown of the thin-skinned thrusting in the fold-thrust belt; (2) uplift, exhumation and gravitational collapse in the hinterland belt and in the Coast plutonic complex; (3) linear belts of slab break-off magmatism within the Canadian and Sonoran segments of the orogen; and (4) thick-skinned basement-involved folding and thrusting within the Great Basin segment, located between the Lewis and Clark lineament on the north and the Sonoran segment to the south.

The thick-skinned folds, thrust faults, and basins characteristic of Laramide deformation formed due to diachronous propogation of the tearing within the subducting plate coupled with rapid isostatic uplift of the collision zone. The uplift led to a strong coupling between upper and lower plates due to frictional forces, such that compression was partitioned through the entire thickness of the lower plate (NorAm) crust within the Great Basin segment. This area may have had little magmatism because the lithospheric mantle necked slowly, but hadn't broken, so asthenosphere couldn't rise high enough to create a substantial quantity of melt; whereas both the Canadian and Sonoran segments had torn rapidly and adibiatically-generated magmas were able to flood the collision zone, reduce the frictional forces between plates, and interact with the crust to produce a linear belt of magmatism extending from Wyoming to northern Canada and from Arizona southward through western Mexico. These features demonstrate both the longitudinal variations involved in slab break-off as well as the variations related to speed of break-off.