A NEW APPRECIATION OF OUT-OF-SEQUENCE THRUSTS AND EXTENSIONAL FAULTING IN PAGE VALLEY, VIRGINIA, BASED ON RECENT 1:24,000-SCALE MAPPING

West-directed thrusting and associated folding are well documented in the northern Valley and Ridge of western Virginia. East-directed backthrusts and late faults that cross-cut and truncate the dominant NE-SW Alleghanian fabric of the region are less recognized. Recent geologic mapping in Page Valley, Virginia suggests that these late faults significantly impacted the tectonic development of the region. Our detailed 1:24,000-scale mapping extends from the Massanutten ridges to just west of the Blue Ridge, and includes the Elkton West and Stanley 7.5-minute quadrangles, and the western part of the Tenth Legion quadrangle. Previous work in this area recognized an extensive system of NW-verging folds and minor thrusts that imbricate and shorten the carbonate rocks of Page Valley and the clastic ridges of the Massanutten synclinorium. Minor east-directed thrusts in Siluro-Devonian rocks in the core of the synclinorium were also noted. Significant geologic features documented by our recent work include: 1) Additional east-directed backthrusts that occur in Cambro-Ordivician rocks on the east flank of the synclinorium; 2) An extensive array of NW-striking normal faults, predominantly northeast side down, that crosscut and offset the characteristic NE-striking fabric of the Valley and Ridge; and most significantly 3) A reinterpreted Stanley fault that is a system of west-directed thrust splays that sole into a master detachment along the base of the Martinsburg Fm. Previous work in the Elkton area by P.B. King (1950) and others showed the Stanley fault as a north to east striking thrust (or reverse) fault that surfaces within, and truncates, Cambrian and Ordovician carbonate units. We now recognize this as a minor splay of a master detachment surface that surfaces farther west as the boundary between Edinburg limestones and Martinsburg shales. Our hypothesis is that the mechanically weak shales facilitated most of the west-directed thrust movement. Unresolved issues include the location of the Stanley fault in the southern part of our field area, and the relationship between this thrust system and the main Blue Ridge thrust that previous mappers have inferred along the western base of the Shenandoah Blue Ridge.