WEAKENING MECHANISMS ALONG LOW-ANGLE NORMAL FAULTS

Traditional models - with vertical sigma 1 and friction in the range 0.6<µs<0.85 - predict that normal faults should ‘lock up’ once they rotate down to dips of less than 30-40°. This seems consistent with the observed dip-range of most moderate and large normal fault seismogenic ruptures identified using positively discriminated focal mechanisms. However, microseismically active low-angle normal faults (LANF) are now 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. These ancient examples give geological insights into fault zone processes that occur at depth in order to facilitate movement along the currently active LANF. 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 (pressure solution) as the dominant grain-scale deformation process in the pre-existing cataclasites leading to shear localisation and the formation of a narrow foliated fault core dominated by fine-grained hydrous mineral phases. These rocks exhibit deformation textures closely comparable 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. Based on the experimental analogue data, we propose a slip model for LANF in the Apennines in which aseismic frictional-viscous creep occurs on a weak (µs <0.2), slow-moving (slip rate < 1 mm/yr) fault, interdispersed with small seismic ruptures due to cyclic hydrofracturing events. Our findings are potentially applicable to many other examples of weak faults and LANF in a wide variety of tectonic settings.