NEOTECTONICS OF THE ROUBIDEAU CREEK FAULT, UNCOMPAHGRE PLATEAU, COLORADO; A PRELIMINARY ASSESSMENT

The Roubideau Creek fault on the Uncompahgre Plateau west of Montrose has been classified as a late Quaternary fault by some authors and as an old fault exhumed by landsliding by other authors. I examined the fresh, 25 m-high south-facing scarp in Roubideau Canyon which lies at the center of the controversy, and conclude that it is a fault-line scarp exhumed by Holocene landsliding, as suggested by West (1997). However, west of Roubideau Canyon the surface of the Plateau is displaced 60 m by a broad topographic escarpment underlain by a faulted monocline. Subsequent to the Laramide (?) ductile folding of the Dakota and Morrison Formations here, the monocline has been faulted in a brittle fashion and a 1.5 km-long graben has formed at the base of the escarpment. The southern margin of this graben is marked by steep, young-looking fault scarps 4-13 m high that displace Quaternary colluvium and alluvium. Abrupt decreases in scarp height across younger inset surfaces indicate recurrent faulting. The morphology of these compound scarps suggest several surface-faulting events in the mid to late Quaternary. However, at present Quaternary age control is lacking so slip rates and recurrence estimates are poorly constrained. Preliminary estimates of slip rate range from 0.1-0.2 mm/yr, which is similar to the 0.2 mm/yr estimated by Ake et al. (2002). It is not presently possible to estimate recurrence of characteristic magnitude with confidence, because the fault exhibits the “short, fat fault” syndrome of large scarps coupled with an anomalously short fault length. This discrepancy can only be resolved by a trenching study which could directly determine the number and ages of surface rupturing events. Several potential trench sites were located which are accessible to excavating machinery.