Strain paths of three small folds from the Appalachian Valley and Ridge, Maryland

Structural analysis indicates that, for a given set of conditions, subtle differences in layer configuration and rheology can result in major differences in fold kinematics. We studied three hand-sample scale folds from the Maryland Valley and Ridge province in an effort to understand the accommodation of folding on meso- and microscopic scales, and to constrain the deformation histories of the folds sampled. Two single-layer folds and one multilayer fold were taken from two nearby outcrops of the Wills Mountain Anticline, and are therefore interpreted to have developed under virtually identical pressure and temperature conditions. Although all three folds are dominated by carbonate material, structural fabrics are unique for each fold, indicating rheological contributions to and local structural control of fabric development. Both single layer folds have asymmetric vein distributions that are consistent with strain directions expected from asymmetric flexural flow. Slickenlines (on one sample) and cleavage in an adjacent layer (in the other sample) support this interpretation. The first of these folds appears to have undergone late-stage hinge tightening, as evidenced by the development of crosscutting bed-normal stylolites. In contrast, veins, stylolites, and the intracrystalline deformation in the multilayer fold are suggestive of (symmetric) tangential longitudinal strain followed by heterogeneous sub-horizontal flattening. The three folds are interpreted to be buckle folds, with differing mechanisms accommodating strain within the competent layers.