STRUCTURE OF THE LARAMIDE-AGE REDLANDS MONOCLINE, COLORADO NATIONAL MONUMENT, WESTERN COLORADO

The Laramide-age, NW-SE striking Redlands monocline and reverse fault are the dominant structures of the Colorado National Monument. The Redlands reverse fault is a steeply SW-dipping structure (60 to 85 degrees SW) that places Proterozoic basement rocks over NE-titling Triassic and Jurassic-age strata of the Redlands Monocline. Strata along the monocline structure typically dip 80 to 30 degrees NE and there is up to 1,600 ft of vertical structural displacement along the Redlands Fault. The Redlands fault and monocline are not simple structures. For example, the width of the Redlands monocline changes significantly along strike. The monoclinal width is measured perpendicular to strike of the dipping strata in the center of the monocline, from the lower to the upper monoclinal hinge (or exposed reverse fault if the upper hinge is eroded away). Along the central part of the Redlands monocline and reverse fault structure (Gold Star Canyon area), the width of the monocline is very narrow (250 ft) and the strata are overturned (dips of 50 to 85 degrees SW). The overturned Jurassic-age strata in the monocline have been thinned to 20 – 30% of their original thickness.

The width of the monocline broadens southward in the area of the East Entrance to Colorado National Monument. Here, the width of the monocline widens to 2,000 ft, the Jurassic-age strata in the monocline are not significantly thinned, and monoclinal dips are more gentle (dips of 18 to 37 degrees NE). In this area the Redlands Fault also steps NE-ward along a S-dipping lateral ramp. This ramp is well-exposed in a small, unnamed canyon just to the NW of the East Entrance. The lateral ramp has reverse-slip displacement, dips 63 degrees S and results in a 350 ft of horizontal offset of the Redlands Fault. It is likely that the Redlands Fault splits from a single fault along the narrow portion of the Redlands Monocline in the Gold Star Canyon area to multiple, subparallel splays in the East Entrance area where the monocline broadens. Also in the East Entrance area, a wide expanse of strata up to 1 mile SW of the Redlands fault has a gentle NE dip of between 3 – 9 degrees NE, suggesting the presence of additional reverse faults SW of the Redlands fault. Typically, strata up to 1 mile SW of the Redlands fault only dip 2 – 3 degrees NE.