FOOTWALL EXTENSION DURING LOW-ANGLE NORMAL FAULTING: CONTROLS ON FAULT ZONE STRUCTURE AND FAULT ROCK DISTRIBUTION

The Zuccale Low-Angle Normal Fault (LANF) on the Island of Elba forms one of several crustal-scale LANF that appear to have accommodated a majority of post-Miocene extension in the northern Apennines of Italy. Initial brittle deformation along the Zuccale Fault increased fault zone permeability allowing the influx of chemically active fluids. This led to reaction softening and the onset of diffusive mass transfer resulting in the development of a pervasively foliated sequence of weak phyllonites. Continued displacement along the Zuccale Fault resulted in the development of foliated cataclasites, foliated fault gouges, and breccias, leading to a fault core several meters thick. Importantly, this complete fault rock sequence is not preserved everywhere along the Zuccale Fault because the distribution of individual fault rock units was strongly controlled by the interaction between the main detachment and a complex suite of brittle faults developed in its footwall. These footwall faults are all relatively steep (40°-85°) normal faults with maximum displacements of several meters. Detailed observations indicate that slip along the main detachment and the subsidiary footwall structures was broadly synchronous. The footwall normal faults link directly into the base of the main foliated fault core, causing the lower margin of the fault core to be displaced which results in significant thickness variations. Additionally, the distribution of early-formed phyllonites is also directly controlled by slip along the footwall faults. As the phyllonitic fault rocks were developing, linkage between the main detachment and the subsidiary footwall structures, and the resulting development of a branch-line, caused parts of the phyllonite to become isolated in thickened areas of fault core. These thickened areas of fault core were ‘protected' from later phases of low-angle detachment faulting and are preserved as elongate lenses of material of varying size and orientation. Fault rock complexity associated with the development of branch lines seems to occur widely. Depending on the petrophysical properties of the fault rocks involved, the generation of laterally persistent lenses of material like those described above may have important consequences for migration of fluids within fault zones.