LARAMIDE STRIKE-SLIP DEFORMATION ALONG THE NORTHERN UNCOMPAHGRE PLATEAU, WESTERN COLORADO: THE CACTUS PARK AND GLADE PARK FAULT SYSTEMS

Well-known Laramide-age structures of the northern Uncompahgre Plateau include NW-SE striking monoclines cored by NE-vergent, basement reverse faults. New USGS EDMAP-funded fieldwork reveals a more complex pattern of left-lateral strike-slip faults linked by restraining bends of reverse faults and monoclines. Strike-slip structures include the Cactus Park-Bridgeport and Glade Park faults. The Cactus Park-Bridgeport fault is a WNW-ESE striking, subvertical, oblique-slip structure 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 en echelon, NNE-SSW striking right-lateral, normal-slip faults. These right-slip structures are conjugate with the Cactus Park-Bridgeport fault. The Glade Park fault is another WNW-ESE striking, subvertical, left-lateral strike-slip fault. The sense of dip-slip along this fault changes along strike in a scissor-like manner, from a north-dipping reverse fault (in No Thoroughfare Canyon) to a north-dipping normal fault (in Bangs Canyon). The amount of left-lateral strike-slip displacement along the Cactus Park-Bridgeport and Glade Park faults is small (<1 km). Strain analysis indicates sub-horizontal, WSW-ENE shortening formed the Cactus Park-Bridgeport and Glade Park faults. This strain pattern fits the shortening direction for all monoclines of the northern Uncompahgre Plateau. This strain pattern is consistent with other WNW-ESE striking left-lateral faults found in northern Colorado (e.g., Garmesa Fault). This regional pattern fits a model of NE-directed motion of the Colorado Plateau during the Laramide orogeny. This movement was facilitated by WNW-ESE striking, left-lateral strike-slip faulting along the northern Colorado Plateau and N-S striking, right-lateral strike-slip faulting along the SE margin of the Colorado Plateau in New Mexico. We would contend, however, that the amount of strike-slip displacement along regional left and right-lateral fault systems related to this model is small (<10 km). Many workers have proposed hypotheses regarding late Cenozoic uplift of the Uncompahgre Plateau. We have recognized one location along the Cactus Park-Bridgeport fault that may indicate Quaternary-age, reactivation of this Laramide-age structure.