Riedel-shear control on the development of pennant veins: Field example and analogue modelling

Abstract

The wall rocks of a crustal scale sinistral ductile shear zone in Namibia, the Purros Mylonite Zone, contain two types of asymmetric quartz veins. Bedding surfaces contain sigmoidal quartz veins with limited thickness along their symmetry axes that can be classified as tension gashes. A second type of veins consists of a striated central fault vein separating pennant-type quartz filled terminations. The tips of these “pennant veins” have a different orientation to those of the tension gashes. Analogue experiments were carried out using a sheet of silicone powder suspended on a slab of poly-dimethyl-siloxane (PDMS), both deformed in simple shear. These experiments produced open fractures very similar to the pennant veins that form by intersection of R and R′ Riedel shear fractures. These fractures rotate and slip during progressive deformation, opening pennant shaped gaps. We interpret the natural pennant veins to form by the same mechanism of R and R′ shear fracture initiation, and subsequent rotation and opening. Since this mechanism differs from that of previously described vein types such as wing cracks, tension gashes and swordtail or fishmouth termination veins, which mainly open as tension veins, we consider pennant veins as in new independent class of asymmetric mineral-filled veins.