Orientations and magnitudes of paleostress in the Great Valley Province of northern Virginia

Abstract

Calcite c-axes and e-twin plane orientations were measured in both matrix cements and younger fracture fills from late Cambrian to Middle Ordovician age limestone samples taken from the NW and SE limbs of the Massanutten Syncline, located within the North Mountain thrust sheet. Paleostress magnitude estimates using the Rowe and Rutter (1990) twin density technique indicate a differential stress of 240±31 MPa for samples collected from both limbs of the syncline.

Three distinct patterns of paleostress orientations (compression directions) have been detected in the samples; each pattern is observed on both the NW and SE limbs of the syncline. The first pattern, exhibited by calcite grains cementing late fractures, is characterized by a maximum of compression axes oriented sub-perpendicular to bedding possibly indicating either thrust sheet loading or stress refraction associated with folding.

Samples in which calcite grains from both fracture fills and earlier matrix cements were measured are characterized by a bimodal distribution of compression axes—the first point maximum being oriented sub-perpendicular to bedding, the second maximum placing compression directions at low to moderate angles to bedding. Restoration of bedding to horizontal results in this second set of compression axes plunging to either the SE or NW, sub-parallel to the regional thrust transport direction.

The third pattern, originating from early cements, places compression directions plunging to the NE-SW at angles which are sub-parallel to bedding. These compression directions do not seem to correlate with major tectonic movements or thrust sheet loading and may reflect stresses associated with either movement over lateral ramps or oblique thrusting.