CORDILLERAN AND FLUID CONTROLS ON LARAMIDE BASEMENT-INVOLVED FORELAND DEFORMATION (Invited Presentation)

The Colorado-Wyoming state line separates areas with different modes of Laramide shortening and post-Laramide extension. In the northern Rockies, the geometry and spacing of the anastomosing Laramide arches indicate lower-crustal detachment at ~30 km depth. Seismic data from the NSF/EarthScope Bighorn Project documented the miss-match between the Moho and the overlying Laramide arch, supporting crustal detachment. The low critical-taper angle in the detachment complex requires a very weak detachment. Post-Laramide extension is widely distributed in the northern Rockies, with small-displacement normal faults and extensional earthquakes throughout the region. The southern Rockies display similar fault types and chronologies but have more concentrated zones of Laramide and post-Laramide faulting as well as more pervasive magmatism. The relatively-undeformed Colorado Plateau of the southern Rockies has no analog to the north.

Laramide shortening directions for the orogen are radial, from NE-SW to the north to E-W to the south, suggesting that the causal stresses originated in the cordilleran welt to the west, not from direct contact between the plates, whose convergence direction was more NNE-SSW. Fluids from low-angle subduction appear to have weakened the continental lithosphere and allowed deformation deep into the craton. In the northern Rockies, fluids trapped in the lower crust below a constricting allochthon created a weak detachment, ungluing the cratonic layers. In the southern Rockies, hydration of the mantle lithosphere appears to have controlled deformation and magmatism. Along the Colorado Mineral Belt, Laramide magmatism may have pinned the crust of the Colorado Plateau to its mantle lithosphere. Basement-involved shortening in the southern Rockies was probably rooted in mantle deformation beneath the narrower welt of the Colorado Rockies, whose subsequent collapse spawned N-S magmatism and Rio Grande rifting.

Thus, Cordilleran compression and the weakening of cratonic lithosphere by subduction fluids controlled the formation of the Laramide Rockies, with lower crustal weakening allowing Laramide detachment in the northern Rockies and mantle weakening allowing more focused Laramide shortening and magmatism in the southern Rockies.