Fault zone weakening and character of slip along low-angle normal faults: insights from the Zuccale fault, Elba, Italy

Abstract

A seismically active low-angle normal fault is recognized at depth in the Northern Apennines, Italy, where recent exhumation has also exposed ancient examples at the surface, notably the Zuccale fault on Elba. Field-based and microstructural studies of the Zuccale fault reveal that an initial phase of pervasive cataclasis increased fault zone permeability, promoting influx of CO2-rich hydrous fluids. This triggered low-grade alteration and the onset of stress-induced dissolution–precipitation processes (e.g. pressure solution) as the dominant grain-scale deformation process in the pre-existing cataclasites leading to shear localization and the formation of a narrow foliated fault core dominated by fine-grained hydrous mineral phases. These rocks exhibit ductile deformation textures very similar to those formed during pressure-solution-accommodated ‘frictional–viscous’ creep in experimental fault rock analogues. The presence of multiple hydrofracture sets also points to the local attainment of fluid overpressures following development of the foliated fault core, which significantly enhanced the sealing capacity of the fault zone. A slip model for low-angle normal faults in the Apennines is proposed in which aseismic frictional–viscous creep occurs on a weak, slow-moving (slip rate <1 mm a–1) fault, interspersed with small seismic ruptures caused by cyclic hydrofracturing events. Our findings are potentially applicable to other examples of low-angle normal faults in many tectonic settings.