STRUCTURAL COMPLEXITIES AND DEFORMATIONAL HISTORY OF THE BLUE RIDGE AND VALLEY AND RIDGE BOUNDARY REGION, PAGE VALLEY, VIRGINIA

Recent field work in the Stanley and Tenth Legion 7.5-minute quadrangles of western Virginia has produced new interpretations of major structures along the boundary of the Blue Ridge and Valley & Ridge tectonic provinces. Dominant features include the Massanutten synclinorium, thrust faults in Page Valley and along the western boundary of the Blue Ridge Mountains, and cross-cutting normal faults. The Massanutten Mountain complex consists of resistant ridges of quartz-rich sandstone that weather proud over less resistant shales, forming high-relief topography that dominates Shenandoah and Page Valleys. These ridges strike northeast for ~160 km and are best represented as a synclinorium with shallowly-plunging folds, inclined dominantly to the northwest. Detailed field mapping has revealed several previously unrecognized southeast-directed backthrusts, which may have helped elevate the Massanutten ridges to their present-day high topography relative to the adjacent valleys.

Just east of the Massanutten synclinorium is a major thrust system that cuts west across the Blue Ridge and southwest through Page Valley: the Stanley Fault of King (1950). The fault positions Late Cambrian carbonates over Early Ordovician carbonates in the region of the Stanley 7.5-minute quadrangle. Previous maps of the Stanley area showed a complex system of low and high angle thrust faults, largely based on carbonate breccias within the Beekmantown Group (Sarros, 1995). Recent work reinterprets the breccias as paleokarst features, as documented by previous workers within the upper Beekmantown Group at, or just below, the Knox unconformity. As a result, the new representation of the Stanley fault shows a less complex system of one or two significant low angle, west-directed thrusts that cross-cut Proterozoic rocks in the Blue Ridge, as well as folded and faulted Cambrian-Ordovician carbonates in Page Valley. Thus, the Stanley Fault may be, in part, a late Alleghenian reactivation of parts of the Blue Ridge boundary thrust, which then trends through Valley and Ridge structures to the southwest. Multiple east-west striking normal faults cross-cut all other features in the area and are likely due to Triassic-aged rifting and the opening of the Atlantic Ocean.