The Rocky Mountain Front forms the eastern edge of the North American Cordillera and represents significant Laramide deformation of the continental basement 1,500 km from the nearest coeval plate margin. COCORP deep seismic profiles were recorded across the northern part of the front to investigate its structure and the influence of the Archean-Proterozoic crustal boundary, expressed in the nearby Medicine Bow Mountains as the Nash Fork-Mullen Creek (NFMC) shear zone.

Four COCORP profiles totaling 180 km transect the Denver basin, Laramie Mountains, and Laramie basin. West-dipping (20°) reflections beneath the mountains truncate basement events and project to key frontal faults, suggesting that the northern front has a structure of shallow, en echelon basement thrusts. A steep northwest dip for the NFMC shear zone is indicated by equivocal truncations and diffractions in basement beneath the east edge of the Laramie basin. Alternatively, a band of events with apparent southward dip under the mountains may be sideswiped from the shear zone, which, together with a predominant southeast-dipping seismic basement fabric, suggests a moderately steep southeast-dipping shear zone. The second interpretation is favored. Continuous reflections at 15.5 to 17.0 sec east of the mountains may indicate a Moho depth of 48 km, while the deepest events on other lines are shallower (11 to 13 sec).

Thus, COCORP profiling and nearby refraction surveys suggest crustal thinning to the northwest across the Archean-Proterozoic boundary which also controlled the segmentation of the northern Rocky Mountain front. On a regional scale, crustal thinning may be partly responsible for the greater diversity of the Laramide in Wyoming.

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