GEOLOGIC MAP OF THE CENTRAL PORTION OF THE LARAMIDE-AGE REDLANDS MONOCLINE OF THE UNCOMPAHGRE PLATEAU, WESTERN COLORADO

The NW-SE striking Redlands Monocline and associated SW-dipping Redlands Reverse fault are Laramide-age structures found in the Colorado National Monument area of the Uncompahgre Plateau of western Colorado. These structures represent one of the classic Laramide monoclines of the Colorado Plateau. The Redlands Monocline is part of a regional system of NW-SE striking monoclines that are linked to more E-W striking, subvertical, left-lateral strike-slip faults. This Laramide-age fault system forms a pattern of left-lateral strike-slip faults connected by right-stepping restraining bends of NW-SE striking monoclines and reverse faults.

Throughout the Colorado National Monument, the SW-dipping Redlands Reverse Fault forms the core of the Redlands Monocline. This fault places Proterozoic migmatite over Triassic or Jurassic-age sediments and has up to 1,600 ft of structural relief. In map view, dipping strata of the Redlands Monocline in the northwestern and southeastern portions of the Colorado National Monument form a 2,000 ft wide zone. In these areas the strata typically dip <45^o NE. There is a dramatic narrowing of the strain zone towards the central part of the Redlands Monocline. We have produced a large-scale (1:3,500) map of the central part of the Redlands Monocline. Here, the monocline is about 500 ft wide in map view and the strata are subvertical to overturned (dips of 50 to 85^o SW). Sedimentary strata adjacent to the Redlands Reverse fault are also highly brecciated. In places, a 15 ft wide zone of mixed Proterozoic migmatite and Jurassic Wingate Sandstone cohesive breccia occurs along the fault. Where the Triassic Chinle Formation is found along the fault, it has been reduced to fine-grained fault gouge with an exposed thickness of 5 to 20 ft.

We suggest that the overall wide to narrow strain pattern of the Redlands Monocline is the result of multiple, closely-spaced reverse faults in the wide northern and southern areas. These faults merge into a single reverse fault along the narrow central part of the Redlands Monocline.