GENERATION AND EVOLUTION OF FAULT-PARALLEL VEINS IN A FOLD-THRUST BELT

Lineated veins are commonly found on fault surfaces. In the Hudson Valley Fold-Thrust Belt, many examples of these lineated, fault-parallel veins are found in Paleozoic limestones. In this study, evidence from hand samples, thin sections, CL, and ESEM analyses indicate that the veins include blocky segments, elongate crystal segments, and fibrous segments, and the lineations on the surface are grooves carved by the asperities during movement of the overriding wall. Veins occur on both ramps and flats, as well as on the bedding planes accommodating flexural slip during folding. It is usually assumed that these lineations are fibers consisting of elongate crystals that form as slip occurs along the fault. However, these veins can become up to 2 cm thick and have a blocky texture with euhedral crystals up to 5 mm in size. The question then becomes whether the lineations are a thin layer of fibers grown on the surface, or are grooves carved into blocky veins. Formation of these veins begins within cracks that are perpendicular to the fault surface created during slip. As the fault moves and the spaces open, the veins grow parallel to the direction of movement of the overriding wall. The morphology of the veins depends on the size and rate of the opening of these cracks. Asperities in the overriding wall carve grooves into the sealed portions of the veins as slip continues along the fault. This process continues until the veins grow long enough to overlap each other and cover the grooves on the underlying veins. These individual wedges can be separated by μm thick screens of wall rock, or may grow into each other to become a much thicker wedge if no wall rock screen is present. In some locations, the lineations continue onto the unveined portions of the fault surface. In all of the cases in this study, either the screens of wall rock or the overlapping veins will preserve the grooves carved into the lower veins by the overriding wall.