Abstract

The Cheyenne belt (CB), a Precambrian suture separating the Archean Wyoming province from Proterozoic island-arc rocks to the south, is marked at the surface by a mylonite zone up to 7 km thick in the Sierra Madre and Medicine Bow Mountains. Geophysical data used to determine deep structure of the CB and surrounding crust include old crustal-refraction profiles, crustal-reflection profiles, several wide-angle seismic profiles, and Bouguer gravity anomalies. Laramide frontal faults that dip 30° to 45° westward to depths of 15 to 20 km offset Precambrian structures in seismic-reflection profiles. As determined from refraction profiles, crustal thickness changes from about 38 km in the northern Laramie Mountains to over 50 km in central Colorado, and Bouguer gravity anomalies show a corresponding decrease. The CB is imaged with moderate apparent dips on three seismic-reflection sections; true dip is about 60° southeast, and maximum depth in seismic sections is 9 to 14 km. Bouguer gravity anomalies are marked by a decrease to the south and an increased gradient across the CB. This is interpreted as a ramp in the Moho near the CB and a gently thickening crust southward toward Colorado. If so, this Moho ramp near the CB probably has persisted since Proterozoic suturing. Long-wavelength, Laramide crustal thickening and an overprinted, late-Laramide, crustal-upper mantle thermal event is evident in Colorado.

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