Faults play a key role in the hydrological, hydrogeological and geochemical cycle, where volatiles from the hydrosphere are transferred to wall and fault rocks. Generally a fragmented rock mass is more porous and less cohesive than the protolith, allowing enhanced fluid-rock interaction. Faulting may lower kinetic barriers to low-temperature (<100°C) mineral reactions as well. The consequences of these changes are manifested in the microfabric, as well as in geochemical properties of fault rocks.
The microstructures of fault rocks give valuable information on the structural setting and deformation mechanism. Microstructural studies are important tools to get information on their deformational behaviour and rheology, and on deformation - recrystallization mechanisms. These mechanisms change continuously when a rock mass passes through different lithospheric levels during exhumation. In contrast to the main minerals (particularly quartz and feldspar) forming upper crustal rocks, calcite mainly deforms by twinning, cataclasis, and/or grain-size-sensitive processes such as pressure solution, solution transfer and grain-boundary sliding at temperatures below 250°C. Dislocation creep and dynamic recrystallization, typically resulting in porosity reduction, are subordinate deformation mechanisms in calcite carbonates at upper crust conditions. The dominating mechanism is a function of environmental factors (fluids, grain size, pressure, temperature, differential stress). In carbonates affected by layer-parallel shear fault zone development starts with the generation of fracture zones with an internal lamellae structure of R - orientation. These evolve from transfer zones between layer-parallel shears. The consequent synthetic rotation of lamellae results in the development of pervasive kink zones with an axial plane sub-parallel to the shear zone boundary. Kinking results from the longitudinal constraint of lamellae associated by impeded shear zone widening. Rupturing along kink bands and breaking-up to smaller fragments with rotational and translational displacement may mark the transition to the formation of cataclastic fault rocks. Subsequent shear is assumed to be localized along these brecciated zones, including the evolution of a fault core. The aspect ratios of host rock fragments decrease continuously towards the fault core. These processes proceed repeatedly. Earlier- formed breccias get cemented and subsequently undergo continuous shear deformation - a cyclic process of healing and re-fracturing termed "fault-rock-recycling". Thus, pressure solution and precipitation from fluids play a fundamental role in the evolution of these fault zones. Stable isotope compositions (δ13C, δ18O) indicate a continuous homogenisation of the isotope signatures from the lamellae dominated damage zone towards the fault core cataclasites.