REINTERPRETATION OF THE IRON POINT “THRUST” AT EDNA MOUNTAIN, NEVADA, AS A PENNSYLVANIAN LOW-ANGLE NORMAL FAULT

Our structural study of upper Paleozoic rocks at Edna Mountain, Nevada, focused on three anomalous characteristics recognized by the original mappers (Erickson and Marsh, 1974 a,b): (1) a low-angle, younger-over-older fault known as the “Iron Point thrust”; (2) both east- and west-vergent folding; and (3) deformation between the Antler and Sonoma orogenies.

The “Iron Point thrust” places folded Pennsylvanian rocks in the hanging wall over Cambrian-Ordovician Preble Formation in the footwall. The present fault surface is sub-horizontal. Folding in the upper plate is asymmetric to overturned, WSW-vergent, and truncated by the fault. The fault surface is seldom exposed, and kinematic indicators are not preserved.

Several lines of evidence suggest that the Iron Point fault is extensional: (1) The fault places younger rocks over older. (2) The fault appears to cut down-section toward the east in the upper plate. (3) The original orientation of the fault was reconstructed in several places, by progressively removing subsequent deformation. The resulting surface dips gently to moderately NE to ESE, with an average orientation of 336/25 NE. These suggest an east-dipping, normal fault origin for the Iron Point fault.

The age of the Iron Point fault is mid-Pennsylvanian, and is tightly constrained by stratigraphic relationships. The folded Highway Limestone in the upper plate is Lower Pennsylvanian. Both the folding and the Iron Point fault are unconformably overlain by the Upper Pennsylvanian Antler Peak Limestone. Therefore, WSW-vergent folding, closely followed by down-to-the-NE normal faulting, occurred in mid-Pennsylvanian time. NW-verging folding and imbricate thrust faulting also occurred during this time period 140 km to the east at Carlin Canyon in the Adobe Range (Trexler et al. 2004).