Enéchelon sigmoidal vein arrays hosted by faults

The morphology of enéchelon sigmoidal veins generally supports an extension fracture origin for host fractures. Nevertheless, a shear fracture origin has been proposed in some cases, mainly on geometric arguments, often without morphological support. This study describes a system of sigmoidal enéchelon quartz veins in a deformed greywacke which displays morphological evidence that the veins are hosted by faults. The veins occur in the hinge zone of an upright horizontal antiform and their configuration is consistent with both faulting and vein opening occurring during folding. The formation of close-spaced vertical dextral faults transformed thick massive greywacke beds into a discontinuous multilayer structure of rock bridges between faults. As the length to thickness ratio of the multilayer structure increased, the deformation mechanism changed to one of rotation and bending of rock bridges along sinistral shear displacement zones resulting in sigmoidal openings between rock bridges. Morphological features which indicate that the host fractures are faults include: (1) continuity of host fractures well past the tips of the sigmoidal veins and shear displacement of markers by unopened host fractures, (2) pinnate extension veins emanating from host fractures particularly near terminations and (3) shear quartz fibres on host fractures. Basic kinematic analysis shows that shear displacement (with opposite shear sense to the array) must occur before sigmoidal forms develop as the curvature produces contractional jogs which are incompatible with vein opening. Recognition of these features may assist in determining the role of shear displacement on host fractures of sigmoidal enéchelon vein arrays.