Structural analysis of the valley and ridge extension of the Parsons Lineament

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Virginia Polytechnic Institute and State University

The Parsons Lineament (a major Appalachian Cross-strike Structural Discontinuity, CSD) extends from the Plateau of West Virginia into the Valley and Ridge Province in northern Rockingham County, Virginia. Regional and second—order folds change in strike from typical Central Appalachian trends (N30—35°E) to near due north within the CSD, regional anticlines plunge out, and major thrusts terminate within thisi zone. ln the Rockingham County portion of the Broadtop Synclinorium, the CSD is marked by an increase in intensity (ratio of surface area to volume) of systematic joints and second—order, sinistral strike-slip faults. Two sets of regional joints occur : J1, a pervasive east—west— trending, near vertical joint set, and J2 which varies with structural position. Peaks in J1 intensity coincide with known strike-slip faults and pronounced photolineaments. J2 is associated with closure on the Bergton anticline. Drilled depths to the Oréskany horizon, age relations and intensity of systematic joints suggest deformation associated with a northeast—facing lateral ramp is responsible for both the Parsons lineament and closure on the Bergton natural gas field.

Within the North Mountain thrust sheet, the Parsons CSD consists of a linear zone of anomalously-trending, second-order folds and shears near the nose of the Linville anticline. Blind thrusts responsible for second-order folding are exposed in a quarry face and strain values (from pressure fringes adjacent to pyrite framboids in Martinsburg shales) suggest that the second-order folds adjacent to the Linville anticline are the result of movement on the Saumsville fault in the subsurface. Age relations of S1 and S2 cleavages and V2 calcite veins indicate an east-to-west order of thrusting for the Pulaski-Staunton and Saumsville faults. The linear disturbed zone results from displacement transfer at the termination of the Saumsville fault and, together with the structural anomalies in the Broadtop Synclinorium, constitutes the Valley and Ridge extension of the Parsons CSD.