Analysis of the internal structure of a carbonate damage zone: Implications for the mechanisms of fault breccia formation and fluid flow

A segment of the Salzach-Ennstal fault zone (Talhof fault, Eastern Alps) shows evidence for joint nucleation by layer-parallel shear, causing the formation of joint-bounded slices oriented at high angles (65–85°) with respect to the shear zone boundary (SZB). Subsequent slice rotation resulted in joint reactivation as antithetic shears, slice kinking, and breaking-up of the individual slices into smaller fragments. The latter process, due to the longitudinal constraint of slices with impeded shear zone widening, marked the transition to cataclasite formation and fault core evolution during shear localization. Cataclasites were subsequently cemented and underwent continuous shear deformation by re-fracturing. Cement precipitation from fluids therefore played a fundamental role in the evolution of the fault zone, with a cyclic change between an open and a closed permeability system during fault evolution. Stable isotope compositions (δ13C, δ18O) of fault rock cements indicate a continuous equilibration between protolith-derived fragments and cements precipitated from those fluids. This points to limited fluid amounts, only temporally replenished by meteoric water, and a hydraulic gradient that directed fluid flow from the damage zone towards the fault core.