Talc Formation Along Continental Low-Angle Normal Faults: Growth, Deformation, and Mechanical Consequences

Fault zones which slip under low-resolved shear stress, i.e. weak faults, represent a mechanical problem. Low-angle normal faults in particular have been the subject of much debate since they have been widely documented in the extending continental crust, yet no moderate to large earthquakes have been positively identified along these structures. Here we present a study of a regional low-angle normal fault, the Zuccale fault on the Island of Elba (Central Italy), which has a displacement of 6-8 km and carries fault rocks exhumed from between 3-8 km depth. The key feature of the fault zone is a mesoscopically ductile fault core which is sandwiched between a hanging-wall and footwall where the deformation is exclusively brittle. A significant amount of deformation was accommodated within a pervasively foliated basal horizon within the fault core, containing calcite-mylonites and talc-phyllonites. Field, optical microscope, XRD, SEM and TEM studies show that the phyllonitic foliation mostly consists of talc lamellae, 50-100 nanometres thick. Talc was formed by fracturing of dolomite and interaction with silica-bearing hydrothermal fluids, and often crystallizes within strain shadow regions around carbonate porphyroclasts, indicating a syn-tectonic origin. With increasing deformation, the amount of dolomite within these fault rocks decreases from 80-90% to zero and the amount of talc increases from 5-10% to 60-70%. At the same time, talc lamellae rotate into parallelism with the margins of the fault core, and ultimately form an interconnected network. TEM analysis shows that the talc lamellae contain numerous interlayer delaminations, and that sliding, translation and/or rotation along 001 foliation planes is common, strongly suggesting that deformation was accommodated by frictional slip. Kinking and interlayer bending of talc are rare. Talc has one of the lowest friction coefficients of any mineral (μ <0.2), and its occurrence within the Zuccale fault as continuous and interconnected layers may have fundamental mechanical consequences. The derivation of talc from dolostones suggests that this weakening process may be applicable to many faults within the continental crust, provided that silica-bearing fluids are available. Finally, the syn-tectonic production of stable sliding minerals such as talc may provide an explanation for the lack of large earthquakes on low-angle normal faults worldwide.