LARAMIDE-AGE LEFT-LATERAL STRIKE-SLIP DEFORMATION ALONG THE NORTHERN UNCOMPAHGRE PLATEAU, WESTERN COLORADO

Previously documented Laramide structures of the northern Uncompahgre Plateau include NW-SE striking monoclines and reverse faults. For example, the Colorado National Monument displays classic NW-SE striking monoclines and basement reverse faults (Redlands Fault). New detailed, regional mapping indicates a far more complex structural pattern. Laramide structures of the northern Uncompahgre Plateau form a left-lateral strike-slip fault system connected by restraining bends of monoclines and reverse faults. The Cactus Park-Bridgeport, Glade Park, and Bull Canyon-Flume Creek faults represent three of these strike-slip structures. These faults are WNW-ESE striking, subvertical, oblique-slip structures with a predominance of left-lateral strike-slip and lesser amounts of reverse and normal dip-slip. Both the dip direction and the sense of dip-slip change along the strike of these faults from steeply N-dipping normal-slip to steeply S-dipping reverse-slip. Complex patterns of en echelon, small-scale structures associated with these structures display combinations of normal or reverse dip-slip and conjugate right-lateral strike-slip. The overall fault pattern represents a strain compatible, left-lateral strike-slip fault system. Multiple, regional-scale, NW-SE striking monoclines formed as splays from the left-lateral strike-slip faults. Strain analysis, based on slickenside striation data, indicates that sub-horizontal, WSW-ENE shortening formed this strike-slip fault-monocline system. This strain pattern is consistent with other possible WNW-ESE striking left-lateral faults found in northern Colorado (e.g., Garmesa Fault). This regional pattern fits a model of ENE-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. Based on a model of more E-directed motion of the Colorado Plateau, I argue that the amount of strike-slip displacement along regional WNW-ESE striking, left-lateral fault systems is significant and that the amount of slip on complimentary N-S striking right-lateral structures is small (<20 km).