Quantitative assessment of low-temperature deformation mechanisms in a folded quartz arenite, Valley and Ridge Province, West Virginia

Abstract

Analysis of the grain-scale deformation mechanisms in folded Tuscarora Sandstone from the western margin of the Appalachian foreland near Keyser, WV, reveals that deformation was accomplished by a combination of pressure solution, microfracturing, and crystal plastic mechanisms. The finite strain was assessed using the normalized Fry method and showed a predominant apparent flattening strain regime resulting from diagenetic compaction. Partitioning of the strain between operative deformation mechanisms revealed that layer-normal shortening (25%) and layer-parallel shortening (10%) by pressure solution were the greatest contributors to the finite strain, followed by microfracturing and crystal plastic strains respectively. Although the crystal plastic contribution to the finite strain is small (1.0% layer-normal shortening), strain partitioning allowed detection of layer-parallel extension, consistent with the structural position of the samples on the outer arc of a fold. The variation in inferred stress directions throughout the progressive deformation indicates that local stratigraphic and structural control on the stress field is important in the development of certain microstructures. Only the earliest stages of deformation record far-field Alleghanian compression, whereas later stages reflect local stress fields controlled by bedding orientation and position on developing folds. Similarities in the microstructure evolution between the western and eastern margins of the Appalachian foreland indicate that the entire belt underwent a similar deformation history.