Fault damage zones dominated by high-angle fractures within layer-parallel brittle shear zones: examples from the eastern Alps

Abstract

Abstract In this study, we focus on the transition from the host rock to the damage zone within brittle shear zones in order to document the structures forming during the initial phases of deformation, i.e., the fractures that formed prior to the formation of fault breccias and cataclasites. Structural analyses of rock samples from sites of the Talhof– and Palten–Liesing faults in the eastern Alps show that in these cases well-known R- and P-fracture patterns do not play a dominant role in the early stages of the generation of brittle fracture zones. In the studied layered marble and foliated impure quartzite samples, the boundary between the host rock and the damage zone is characterized by the formation of closely spaced fractures at high angles (70–90°) to the shear zone boundaries, being parallel to pre-existing layering/foliation planes. These fractures bound and define slender slab-like or columnar rock elements, here being termed lamellae. It is assumed that subsequent rotation of these lamellae in the bookshelf and domino modes associated with impeded dilation across the actual shear zone boundaries leads to kinking, splitting and final granulation of the lamellae to generate breccias of later fault core zones parallel to the shear zone boundary. In some cases, the observed bending and buckling of lamellae indicate additional ductile deformation subsequent to the development of the dominant lamellar structures formed by brittle fracturing.