The development of quartz-dolomite hydrothermal vein complexes synchronous with slip along the Zuccale low-angle normal fault, Italy: evidence for overpressure, weakening and paleoseismicity

Slip along upper-crustal fault zones severely misoriented with respect to the maximum principal stress, including low angle normal faults, remains an enduring mechanical paradox in crustal tectonics. The Zuccale low-angle normal fault in Italy cross-cuts a lithologically heterogeneous sequence of wall rocks and underwent kilometre-scale displacements broadly synchronously with emplacement of upper crustal igneous bodies. Microstructural analyses of fault rocks formed at <6km depth reveal systematic changes in fault rock mineralogy and texture related to progressive exhumation during regional extension. Recent attention has focussed on weakening mechanisms associated with the development of phyllonitic fault rocks derived from the interaction of dolomitic host rocks with silica-rich fluids. These fault rocks facilitated low friction creep by a combination of localised slip along interconnected networks of talc and the more general operation of grain-scale dissolution-precipitation processes. The present contribution focuses on the less well understood role played by the development of fluid overpressure and its possible relationship to mineralisation and palaeoseismic events in the core of the Zuccale fault. Complex intergrown vein networks of low temperature quartz and dolomite (and later calcite) occur throughout much of the fault core suggesting intermittent supra-lithostatic fluid pressures. The vein networks are internally deformed, with widespread textural evidence for episodes of ductile creep under high fluid pressures (with folding and shearing of dolomite-quartz vein - hydrothermal fluid 'mushes') interdispersed with more rapid shearing events along discrete brittle detachments. These localised slip events are associated with the precipitation (and subsequent cataclastic deformation) of jet black, ultra-fine grained (1-5 micron), zoned authigenic quartz layers up to 3 cm thick that may have initiated as gels. These observations point to a close association between fluid overpressure development, interdispersed episodes of creep and seismogenic slip, and changing styles of mineral precipitation along low angle normal faults and may provide further evidence of long term weakening.