ANCESTRAL ROCKY MOUNTAINS OROGENY IN NEVADA: THE WEST SIDE STORY?

Pennsylvanian folding and thrust faulting at Carlin Canyon, north-central Nevada, is approximately synchronous with Ancestral Rocky Mountains orogenic activity in central North America. Northwest-vergent folding and northwest-directed thrusting in Carlin Canyon are constrained to Desmoinesian time. Deformation in strata including the Tomera Formation (as young as lower Desmoinesian) is unconformably overlain by rocks of the lower Strathearn Formation (lower Missourian). The unconformity (C6) cuts down-section to the west within the canyon, to strata as old as Mississippian. Deformation below the unconformity includes a NW-verging anticline-syncline pair with an amplitude of > 400 m, and a NW-directed thrust duplex with stratigraphic separation of > 500 m. We do not yet know whether northwest vergence is a characteristic of Pennsylvanian deformation, or a local anomaly.

Deformation is intense at Carlin Canyon and decreases eastward, but the angular unconformity, and correlative disconformity, can be mapped east to the Utah-Nevada border. The deformed rocks cannot be mapped to the west because they are structurally overlain by oceanic strata (Devonian and older) of the Roberts Mountains allochthon. The only bracket on the internal deformation of the allochthon is the Missourian (Late Pennsylvanian) overlap strata, so the age(s) of deformation and emplacement are poorly constrained. The lower part of the Strathearn Formation and the Hogan Formation of northeastern Nevada may be synorogenic units resulting from Pennsylvanian tectonism.

We suggest that mid-Pennsylvanian deformation in central Nevada forms a poorly understood northeast-southwest trending fold and thrust belt. The timing suggests a relationship with the Ancestral Rocky Mountains basins and uplifts to the east, but the deformation style is very different. Continent-continent collision along the Marathon-Ouachita orogenic belt, widely cited as the driving force for the Ancestral Rocky Mountains orogeny, cannot alone explain concurrent deformation in the Great Basin. We conclude that tectonic events along the western margin of the continent must have played a significant role in tectonism in North America during the late Paleozoic.