NEOTECTONICS OF UNCOMPAHGRE PLATEAU, COLORADO; A PRELIMINARY ASSESSMENT

Mapping performed for the US Bureau of Reclamation in Sept. 2006 revealed a NW-trending zone of Quaternary faulting in the central Uncompahgre Plateau at least 32 km long, which coincides with parts of previously-mapped faults Q20, Q22, and Q23 (Widmann et al., 1998). The youngest fault scarps identified are 1-12 m high, have steep scarp faces (slightly below the angle of repose), and in some cases truncate Quaternary landforms such as stream valleys. These late Quaternary fault scarps all lie at the toes of ca. 60-m high, NE-facing escarpments and form the margins of prominent grabens at the toes of the escarpments. Of the three prominent escarpments that have such grabens (Roubideau Creek fault, fault Q20/2270; Roubideau Ranch fault, fault Q22/2272; Old Paradox Road fault, fault Q23/2273), the first is clearly a monocline, and the others probably so. Additionally, older scarps disrupt the smooth plain of the Dakota dipslope. These scarps are at least partly erosional, but appear to be younger than the removal of the Mancos Shale from atop the Dakota-Burro Canyon sandstones on the plateau dipslope. One such area of scarps is the Donley-Garrison faults, a group of 7 faults that separates the Roubideau Ranch fault from the Old Paradox Road fault. Mapping identified many late Quaternary and Quaternary (undivided) scarps, but the age of the scarps, and whether they are fault scarps, fold scarps, or some combination of the two, remains untested. Previous studies concluded that only the grabens and isolated antislope scarps were of Quaternary age, and the much higher 60 m-high monoclines were pre-Quaternary. In contrast, if the entire monocline also formed in Quaternary time, the long-term Quaternary slip rate on these normal faults would be much faster than previously supposed.