THE UNGLUING OF CONTINENTAL CRATONS: A SOLUTION TO THE PARADOX OF LARAMIDE CRUSTAL DETACHMENT

Why do basement-involved foreland thrust belts extend farther into cratons where Cordilleran orogens undergo low-angle subduction? Some tectonic models explain this association by invoking shear from below during low-angle subduction, perhaps aided by a down-going oceanic plateau or a protruding cratonic keel. But in the Wyoming Rockies, Laramide shortening directions (~N65E, according to folds, fabrics and faults) are discordant to plate convergence directions (~N25E). Moreover, shortening in the Laramide foreland continued long after the postulated plateau’s center moved through Wyoming.

The congruencies of Laramide timing and shortening directions in Wyoming with slip in the adjoining Cordilleran Thrust Belt indicate a shared motive stress. A push from the west explains the top-to-the-ENE crustal detachment at ~30 km depth indicated by the miss-match between Laramide arch and Moho geometries, as recently documented by the seismic experiments and structural modeling of the NSF/EarthScope Bighorn Project. Thus, the ~ENE-directed compression that drove Laramide crustal detachment in Wyoming probably originated in the Cordilleran orogenic welt.

But why did Laramide lower-crustal detachment develop in Wyoming’s mostly Archean craton and not in the craton adjacent to the Canadian Thrust Belt to the north? Low-angle subduction places a slab beneath the craton’s mantle lithosphere and should cool the overlying craton, making it more resistant to deformation. We suggest that the subducted slab contributed fluids which, when thwarted from their usual roles of fluxing arc magmatism and aiding corner flow, leaked upward into the craton and unglued it. Fluids probably pooled in the lower crust because layer-parallel, constrictional shortening in all horizontal directions within the detached crust would have greatly reduced upper-crustal permeability. The detachment’s considerable weakness, as indicated by the Laramide’s very low critical-taper angle, probably resulted from retrograde metamorphism to phyllosilicate-rich lower crust and/or increased fluid pressure. This process may have been analogous to that responsible for today’s episodic tremor and slip (ETS) events. Thus, the Laramide was probably driven by Cordilleran compression aided by subduction dewatering that unglued the craton.