Late Paleozoic contractional and extensional deformation at Edna Mountain, Nevada

Abstract

New mapping, structural analysis, and biostratigraphy of the late Paleozoic sequence at Edna Mountain, Nevada, document several distinct deformational events between the Devonian–Mississippian Antler orogeny and the Permo-Triassic Sonoma orogeny. Twofold sets (F 1 , mid-Pennsylvanian, and F 3 , mid-Permian) and the Iron Point fault are late Paleozoic. These record both shortening and extension in late Paleozoic time, and provide further evidence that western North America was not relatively quiescent between the Antler and Sonoma orogenies. These structures are overprinted by folding thought to be associated with emplacement of the Golconda allochthon (F 4 ), indicating that the rocks at Edna Mountain were in approximately their present location and orientation by that time. Unconformities within the overlap section record periods of uplift and erosion related to late Paleozoic tectonism. They are recognized based on angular truncations of the underlying units, basal conglomerates containing fragments of the underlying units, and local erosional features including channels and karst. Biostratigraphic age control makes it possible to bracket the ages of the unconformities, and to correlate some of them with known regional unconformities. The detailed deformation record at Edna Mountain demonstrates the strength of combined biostratigraphic, stratigraphic, and structural analysis of the assemblage for deciphering previously unrecognized late Paleozoic deformation. The Iron Point fault is a mid-Pennsylvanian, down-to-the-east, low-angle normal fault. Previously mapped as the Iron Point thrust (Erickson and Marsh, 1974a, 1974b, 1974c), this fault places younger rocks over older, truncates folds in both hanging wall and footwall, and cuts downsection to the east in the hanging wall. Reconstruction of the original fault orientation by removing younger deformation events results in a gently east- or northeast-dipping surface. Motion along the Iron Point fault may have been a response to crustal thickening during the development of west-southwest–verging F 1 folds, the most penetrative deformation at Edna Mountain. To our knowledge, the Iron Point fault is kinematically unique for this region and time period. Its presence suggests that the rocks at Edna Mountain were near the locus of mid-Pennsylvanian deformation and that this deformation was probably related to events at the western boundary of the continent.