LARAMIDE CRUSTAL DETACHMENT IN THE ROCKIES: CORDILLERAN SHORTENING OF FLUID-WEAKENED CRUST

How is shortening accommodated during low-angle subduction in Andean orogens? Proposed mechanisms include (1) whole-scale lithospheric buckling and (2) crustal bucking above a lower crustal detachment. The National Science Foundation/EarthScope Bighorn Project combined passive and active seismic imaging with restorable structural modeling to address this question by determining the Laramide (Late Cretaceous-Paleogene) evolution of the crust and Moho in north-central Wyoming.

Our seismic results show that the Laramide NNW-trending Bighorn Arch is discordant to the structure of the underlying Moho, whose more regionally-planar surface is not involved in folds that parallel the Laramide structural grain and is not cut by major Laramide faults. Restorable kinematic models of the Wind River and Bighorn arches show that a near-horizontal detachment fault at ~30 km depth provides an excellent fit to observed arch geometries. The similar depths of the intervening Wind River and Bighorn basins are also consistent with lateral movement over a near-horizontal detachment. The 120 km spacings between the Wind River, Owl Creek, and Bighorn arches are consistent with crustal buckling above a lower-crustal detachment, not lithospheric buckling, which would generate arch spacings of ~400 km.

The near-zero critical taper in the allochthon above the sub-horizontal Laramide detachment suggests that a zone of extreme weakness developed in the lower crust. We hypothesize that hydration of the lower crust during low-angle subduction of the Farallon Plate during the Laramide Orogeny caused retrograde metamorphism and/or increased fluid pressure that facilitated detachment of the upper crust. We speculate that low-angle subduction suppressed the fluid-consuming processes that characterize higher-angle subduction, allowing subducted fluids to be transported far into the continental interior and then escape into overlying foreland crust.