THE PIÑON OROGENY, A NEWLY DEFINED PENNSYLVANIAN TECTONIC EVENT RECORDED IN THE BASIN AND RANGE

The Devonian-Mississippian Antler and Permian-Triassic Sonoma orogenies are separated by ~100 m.y. However, rocks containing folds and faults that are biostratigraphically bracketed to mid-Pennsylvanian age crop out in a 300 km wide zone across north-central Nevada. New and published data indicate that such structures occur in six mountain ranges across this zone. The strain gradient recorded by these structures increases westward across the zone. Comparable structures of this age do not occur in southern Nevada, providing a southern limit to deformation. We refer to the tightly constrained tectonic event that formed these structures as the Piñon orogeny, for well-exposed structures at Carlin Canyon, northern Piñon Range. 

The mid-Pennsylvanian Piñon orogeny folds consistently verge northwest, opposite to the typical east-vergence of structures in western North America. Fold shapes range from open and locally asymmetric in the east, to asymmetric and locally overturned in the west. In all exposures, the folds have consistent layer thickness and fold axis orientations, consistent with a tectonic origin. In southern Nevada there are no structures of this age, but the sedimentary record reflects changes in depositional environment, basin depth, or sediment provenance synchronous with the mid-Pennsylvanian structures.

We documented Piñon orogeny structures and related stratigraphy across north-central Nevada, and now can summarize the extent and kinematics of this deformation. Late Paleozoic structures mainly occur in the clastic and carbonate rocks known collectively as the "Antler Overlap Sequence", which contains rocks of Early Pennsylvanian through Late Permian age. The wide time gap between the Antler orogeny itself and the "overlap" is seldom addressed, but Piñon orogeny deformation and erosion may account for some of the time that is unrepresented in the depositional record. The wide age range and lateral variability of the "overlap" strata impeded recognition of the internal structures until recently. The combination of careful structural mapping with biostratigraphic age control reveals the relationships within the Antler Overlap, and allows us to recognize and refine the deformation history.