Crustal Mechanics of Cordilleran Foreland Deformation: A Regional and Scale-Model Approach

Abstract

Foreland Rockies deformation was caused by compressive stresses transmitted tangentially through the continental basement and the overlying geosynclinal prism from the Pacific continental margin. The stresses probably originated from relative overriding of that ocean block by the continental block as a result of drag on the bottoms of the blocks by convection cells in the mantle. Dissipation of these stresses by deformation has altered through time from the continental margin to the foreland, depending on the ability of the geosyncline to transmit the stress. Through Mississippian time, Pacific-margin deformation continually thickened the geosynclinal prism. Permo-Pennsylvanian stress transmission through the thickened prism caused deformation of the Ancestral Rocky Mountain foreland. A change in Mesozoic time to hydraulic stresses while batholiths developed in the geosyncline halted foreland deformation by relieving tangential stresses. Stress transmission resulting from Late Cretaceous solidification of the batholiths caused Laramide breakup of the foreland. Tertiary development of northwest oblique yield at the continental margin (San Andreas and Basin-Rang structure) again dissipated tangential stress, halting Laramide breakup. A west-northwest left-lateral superimposed on this compression, and caused by greater resistance of the Canadian shield to compression, caused weakening and deformation of the foreland. Eastward movement of the Colorado Plateau block caused the to be accentuated in and diminished in Colorado. This caused crust-thick, drag fold-slabs in Wyoming. North-south compression between the Wyoming couple and the Colorado Plateau caused the Uinta uplift. Eastward jamming of the Colorado Plateau against the Mid-Continent caused the Front Range uplift. Release along the south margin was by left-lateral yield on the Wichita lineament. The deformation has been duplicated in models using similar stresses. The crust was flexed into combinations of three basic configurations: (1) a slab compressed tangentially into a sine curve with negative parts subsiding as positive parts rose; (2) overlapping slabs caused by failure of the mutual limb of this sine curve; and (3) isostatically collapsed slabs. Basin-margin depression by surrounding uplifts caused the Rock Springs uplift. Isostatic collapse of overlapping slabs during and after compression caused the high south rim of the Sweetwater uplift. Post-compression rheid crustal thinning, failure of the mutual limb (fold-thrusting), late removal of basin sediments, and the effect of the left-lateral have caused collapse. Bounding structures are aggregates of monoclines, upthrusts, sheared-out and overturned limbs, basin-block wells, second-order faults and folds, isostatically reversed thrusts, and several types of collapse structure. The mechanical functions of these diverse structures can be isolated.