GEOLOGY OF THE NORTH MOUNTAIN FAULT ZONE OF VIRGINIA—CONSISTENT DEVELOPMENT IN A CHAOTIC REGIME

Detailed geologic mapping along the North Mountain fault zone in northwestern Virginia shows that this major thrust fault developed from a non-coaxial planar fault plane in relation to an overturned anticlinal fold. Evidence of fault development comes from upright versus overturned strata in horses within the ¼ to 1 mile-wide fault zone.

The North Mountain fault zone is a major thrust feature with reportedly tens of miles of horizontal displacement that extends from central Virginia to south-central Pennsylvania and contains Cambrian and Ordovician rocks in the hanging wall and Silurian and Devonian rocks in the footwall. For this study, the fault zone was mapped in parts of seven 7.5-minute quadrangles from Rockingham County, Va northeastward to Berkeley County, WV, a linear distance of 75 mi. Generally, the Cambrian Elbrook Formation is thrust northwestward over Silurian and Devonian clastic rocks. The fault zone is as much as 1-mile wide and contains rocks from Ordovician to Devonian age in a series of horses. Rocks on the east side of the zone are almost exclusively upright, whereas rocks on the west side of the zone are almost exclusively overturned. This change from upright to overturned units occurs as a decollement in the shaly upper part of the Middle Ordovician Edinburg Formation and the lowermost calcareous shale of the Martinsburg Formation. Near Middletown, Frederick County, VA, the fault zone is 0.5-mi wide and contains nine horses of various Ordovician and Silurian units with as many as five horses across strike. Horses of Lower and Middle Ordovician carbonate rocks were derived from the upright limb of an anticline on the southeast side of the fault zone. Horses of Upper Ordovician and Silurian rocks were derived from the overturned limb of the anticline. This consistent change from upright to overturned strata in horses suggests that the fault zone developed on the overturned southeast limb of a footwall syncline on the northwest and the adjacent upright limb of a faulted-out anticline to the southeast. The strata in the eastern part of the fault zone were derived from the upright limb, and the overturned strata in the west part of the fault zone were derived from the overturned limb. These relationships suggest that the fault zone developed after much of the folding had taken place.