LARAMIDE AND QUATERNARY-AGE FAULTING ALONG THE NORTHERN UNCOMPAHGRE PLATEAU, WESTERN COLORADO

Well-known Laramide-age structures of the northern Uncompahgre Plateau include NW-SE striking monoclines cored by NE-vergent, basement reverse faults. The Colorado National Monument displays classic monoclines and basement reverse faults. New EDMAP-funded fieldwork reveals that a series of WNW-ESE striking, left-lateral strike-slip faults links these monoclines and reverse faults. This kinematic pattern suggests that Laramide-age structures of the northern Uncompahgre Plateau form a regional left-lateral strike-slip fault system connected by restraining bends of reverse faults and monoclines. One of these strike-slip structures is the >10-mile long Cactus Park-Bridgeport fault. This structure is a subvertical, oblique-slip fault with a predominance of left-lateral strike-slip and lesser amounts of reverse and normal dip-slip. In some locations the Cactus Park-Bridgeport fault is composed of an en echelon series of closely spaced (<3 to 5'), NNE-SSW striking right-lateral, normal-slip faults. These right-slip structures are conjugate with the main Cactus Park-Bridgeport fault and are considered to be R' structures. Field observations and kinematic analysis of fault slip data along the Cactus Park-Bridgeport fault indicate 50 to 200' of dip-slip and a minimum of 200 to 400' of left-lateral strike-slip displacement. Thick breccia zones (150' thick) and the presence of subhorizontal slickenlines suggest greater amounts of left-lateral strike-slip. Strain analysis indicates sub-horizontal, WSW-ENE shortening formed the Cactus Park-Bridgeport fault. This strain pattern fits the shortening direction for all monoclines and reverse faults of the northern Uncompahgre Plateau.

Many workers have proposed hypotheses regarding significant amounts of late Cenozoic uplift of the Uncompahgre Plateau. Was this uplift associated with normal-slip reactivation of Laramide-age structures such as the Cactus Park-Bridgeport fault? We have recognized one location along the Cactus Park-Bridgeport fault that may indicate Quaternary-age, down-to-north, normal-slip reactivation. Additionally, a recent, small magnitude seismic event (M<1.4) may have occurred along the Cactus Park-Bridgeport fault (Mesa State College seismic network, D. Wolney, pers. comm., 2004).