Seismic reflection profiling of the Cheyenne belt Proterozoic suture in the Medicine Bow Mountains, southeastern Wyoming: A tie to geology

Abstract

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 suggest. 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.

Geophysical studies of crustal structure in the Rocky Mountain region: A review

“…In the Laramie Mountains, the COCORP profile crossed the Cheyenne belt and suggests that the Archean-Proterozoic suture dips 55°s outheast . The group from the University of Wyoming has recorded new seismic data recently in the Laramie Mountains (Gohl and Smithson, 1994;Speece et al, 1994;Templeton and Smithson, 1994). These data show a reflective lower crust in this area of Proterozoic outcrops in the absence of clear Moho reflections.…”

Geophysical constraints on the deep structure of the Cheyenne belt, southeastern Wyoming

“…SMZ = southern mylonite zone. RSZ = Rambler shear zone (after Templeton and Smithson, 1994). and discontinuous down to about 15 km. Some reflections on other lines parallel the frontal-thrust reflection but do not cut the basement unconformity.…”

“…This line (Fig. 5) imaged a series of moderately dipping, crisscrossing reflections (Templeton and Smithson, 1994). Interpretation of these events is based on the known geology, and the crisscrossing events are tied to geology.…”

“…The UW profile in the Medicine Bow Mountains was specifically designed to image the CB and follow it to depth in a profile that ran oblique to the strike of the CB and crossed its surface trace (Templeton and Smithson, 1994). In the pattern of crisscrossing reflections in the seismic profile (Fig.…”

“…HC is Horse Creek seismic line (Speece et al, 1994). CB is Cheyenne belt seismic line in Medicine Bow Mountains (Templeton and Smithson, 1994). x x = wide-angle recording lines and vibrator source points, = shot points into wide-angle recording spread (Gohl and Smithson, 1994).…”