LARAMIDE STRIKE-SLIP DEFORMATION ALONG THE NORTHERN UNCOMPAHGRE PLATEAU, WESTERN COLORADO: THE BULL CANYON-FLUME CREEK FAULT SYSTEM

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 monocline and basement reverse fault structures (Redlands Fault). Additional NW-SE striking monoclines include the Flume Creek, Black Rocks and Ruby Canyon (new name) Monoclines. New USGS EDMAP-funded research has demonstrated 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 Bull Canyon-Flume Creek fault system is one of these strike-slip structures. This structure is a WNW-ESE striking, subvertical, oblique-slip fault with a predominance of left-lateral strike-slip and lesser amounts of dip-slip. Both the dip direction and the sense of dip-slip change along the strike of this fault from steeply N-dipping normal-slip to steeply S-dipping reverse-slip. Complex patterns of en echelon, small-scale structures associated with the Bull Canyon-Flume Creek fault display combinations of normal or reverse dip-slip and right-lateral or left-lateral strike-slip. The overall fault pattern represents a strain compatible, left-lateral strike-slip, flower structure. Regional-scale monoclines splay off the Bull Canyon-Flume Creek fault. Strain analysis, based on slickenside striation data, indicates that sub-horizontal, WSW-ENE shortening formed the Bull Canyon-Flume Creek fault system and associated monoclines. 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).