THE INFLUENCE OF PRIMARY SEDIMENTARY FEATURES ON FOLD-HINGE DEVELOPMENT IN QUARTZ ARENITE DEFORMED AT LOW TEMPERATURE

The Wills Mountain anticline is a 300+ km long, northeast trending first-order fold in the Appalachian Valley and Ridge province that formed in response to blind thrusting during the Alleghanian orogeny. At Mays Gap, WV, folding of the Lower Silurian Tuscarora Sandstone was accommodated by the development of multiple angular fold hinges separated by planar fold limbs. Samples of the Tuscarora quartz arenite were collected from the same stratigraphic horizon across a hinge zone to assess the variation of deformation microstructures and primary sedimentary features/textures. Our study investigates how this variation may have influenced the location of the fold hinges. The type and abundance of microstructures vary considerably from samples located in the fold limbs to those in the fold hinge zone. Hinge zone samples show a greater abundance of brittle microstructures, such as transgranular microcracks and wide cataclastic bands (>5 mm), and a greater abundance of crystal plastic-related microstructures, such as deformation bands and kink bands. Microstylolites, a pressure solution-related microstructure, are more abundant in the limbs, whereas other pressure-solution microstructures, such as truncated and indented grains, show no significant variation in abundance with respect to fold location. Interestingly, in addition to documenting a variation in microstructures, we also observed changes in the primary sedimentary features of the rock across the hinge zone. Samples from the fold limbs generally have lower porosity, are finer grained, and contain more clay matrix. Thus, our preliminary results suggest that primary features in the Tuscarora influenced the selection of grain-scale deformation mechanisms during folding. The resulting microstructures may have altered the mechanical properties of the rock, thereby localizing the fold hinges. Moreover, the greater porosity (an thus, more available water) in the hinge zone may have promoted hydrofracturing that resulted in a greater abundance of brittle microstructures compared to the fold limbs.