The Cheyenne belt, a broad mylonitic shear zone, marks a 1.8--1.7 Ga suture between Archean craton to the north and a Proterozoic island arc to the south. A University of Wyoming seismic reflection profile across the Medicine Bow Mountains imaged the steeply dipping Cheyenne belt and crosscutting Laramide faults. By combining surface geology with reflection data from a line that trended oblique to the Cheyenne belt suture and Laramide thrusts, the true subsurface geometry of both could be resolved. The suture zone dips 60¿ southeast to depths of at least 9 to 14 km. A 5- to 10-km thick zone of reflectors project to the surface near mapped Laramide faults and have a true structural dip of 45¿. The imbricate thrusts within this zone cut the Cheyenne belt near 9 km depth. Some weaker events persist throughout the Archean lower crust; however, no clear Moho is observed. The subsurface geometry of the Cheyenne belt suggests that Archean crust is not absent at depth just south of the southernmost splay of the suture as Nd isotope studies might suggests.

Alternatively, the Proterozoic hanging wall thickens such that rising plutons may form completely within the hanging wall and show only Proterozoic geochemical signatures. The complex reflectivity pattern observed in the Cheyenne belt line is caused mainly by the multideformational history of the area and the oblique orientation of the line. Ties between reflections and outcrop show that fault-related lithologic contrasts and deformation within the fault zones generated reflectors at midcrustal depths during the Proterozoic. Later Laramide brittle faulting add to, rather than erased, the reflectivity. ¿ American Geophysical Union 1994