GEOLOGIC INTERPRETATION OF CRUSTAL REFLECTION PROFILE ACROSS THE CHEYENNE BELT, PROTEROZOIC SUTURE, SE WYOMING AND NORTHERN COLORADO

We are reporting on the first crustal seismic reflection line recorded on land in the US in many years, crossing the exposure of a Precambrian suture – the Cheyenne Belt. The seismic line targeting this Archean-Proterozoic boundary in southern Wyoming starts at the exposed Archean supracrustal rocks of the Rawlins uplift, proceeds south across the top of the Sierra Madre, perpendicularly crossing the Cheyenne Belt, going south in Colorado through the Farwell Mtn area and ends in Park Range. The seismic data were collected in Fall 1999.

The stacked data and the preliminary interpretation show abundant criss-crossing reflections in the northern portion of the profile that we associated with strongly deformed supracrustal rocks of the upper 20 km of Archean crust; blurred Moho at 13 s (about 39 km) under Archean part of the profile; events at 10 s steeply dipping south that could be a possible remnant of deep Archean thrusts associated with docking of an exotic Archean terrane. A dipping event just to the south of the surface trace at the Cheyenne Belt starting at about 2 s on unmigrated data is paralleled by other events and could be projected to the surface at the Cheyenne belt area. This event could possibly be the Cheyenne belt itself or a younger Proterozoic fault cutting the Cheyenne belt suture. The Cheyenne Belt could either continue to Moho as steeply dipping suture dissected by Proterozoic thrusts or could be represented by the moderately south dipping thrusts themselves. Two groups of events, dipping north and south, are projected at the surface at the Farwell Mtn area. We interpret them as a cryptic manifestation of a Proterozoic suture. A strong reflection dipping north at the very southern end of the profile could be associated with the Homestake suture zone. Moho reflections are either absent or are very weak in the south perhaps due to reworked lower crust in Colorado.