Fluid- and reaction-assisted low-angle normal faulting: evidence from rift-related brittle fault rocks in the Alps (Err Nappe, eastern Switzerland)

Brittle fault rocks associated with a rift-related low-angle detachment system, exposed in the Lower Austroalpine Err Nappe (eastern Switzerland), exhibit systematic changes in mineralogy, fabrics, and structures, which are reflected by the transition from an undeformed granite to a cataclasite and a phyllosilicate-rich gouge across the fault zone. Cross-cutting relationships between the brittle fault rocks and syn-kinematic quartz veins suggest that a major part of the 11 km (min.) of displacement along the fault zone was accommodated in the thin, continuous gouge layer. localization in the fault zone is controlled by fluid-assisted deformation which enhances break-down reactions of feldspar to phyllosilicates. An increasing proportion of phyllosilicates decreases the strength and the permeability of the fault zone and may facilitate inter-crystalline deformation mechanisms such as grain-boundary sliding in phyllosilicate-rich zones. Normal slip of the hanging wall relative to the footwall at low angles of inclination may be facilitated by weakening of the fault zone and transient high pore pressure events.