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 CO 2 -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.

Observations from the Basin and Range Province (western United States) pertinent to the interpretation of regional detachment faults

“…Moresby seamount, Woodlark Basin; Floyd et al 2001); and the significance of fluids in fault zones and more generally in crustal rheology (Axen 1992;Manatschal 1999;Collettini & Barchi 2002;Hayman et al 2003;Collettini & Holdsworth 2004;Evans et al 2004;Scholz & Hanks 2004). Wills & Buck (1997) examined the possible role of stress-field rotation in the development of rooted detachment faults, and concluded that applied stresses are not in general sufficient to create low-angle normal faults that would propagate to the Earth's surface (cf.…”

On the mechanical behaviour of a low-angle normal fault: the Alto Tiberina fault (Northern Apennines, Italy) system case study

“…Chiaraluce et al (2007) interpreted these features as the presence within the fault system of fault zones possessing different rheology and/or frictional properties. It is also worth noting that the microseismicity nucleating on the ATF is not able to explain the amount of deformation associated with the short-and long-term slip rate inferred by geological (Collettini and Holdsworth, 2004) and geodetic studies and data (D'Agostino et al, 2009). These observations together with the lack of large-magnitude (M>7) historical earthquakes that ruptured the whole ATF in the past 1000 years (Rovida et al, 2011;Chiaraluce et al, 2014) suggest the occurrence of aseismic deformation or creeping as proposed by Hreinsdottir and Bennett (2009) by interpreting regional GPS data.…”

“…This theory is supported by the evidence that no moderate-to-large magnitude earthquakes have been documented worldwide nucleating on LANFs using positively discriminated slip planes from the focal mechanisms (Jackson and White, 1989;Collettini and Sibson, 2001). On the contrary, observations of large displacements (Lister and Davis, 1989;John and Foster, 1993;Hayman et al, 2003;Collettini and Holdsworth, 2004;Jolivet et al, 2010;Mirabella et al, 2011) and the well-identified microseismic activity (Chiaraluce et al, 2007;Rietbrock et al, 1996) associated with these structures suggest that the LANFs are tectonically active, accommodating crustal extension and possibly formed at low angle. For these reasons, the LANFs cannot be excluded from the assessment of seismic hazard, although their inclusion still represents a debated issue.…”

Episodic Dissolution, Precipitation, and Slip along the Heart Mountain Detachment, Wyoming

“…It is clear from the rock record that significant strains in the brittle regime are accommodated by pressure solution (e.g., Engelder 1979;Wright and Platt 1982), which is likely an important factor along brittle faults (e.g., Collettini and Holdsworth 2004;Anderson et al 2010). In contrast to other known low-to moderate-temperature deformation processes, both theoretical arguments and experimental data suggest that pressure solution in rocks and minerals has a viscous rheology (e.g., Rutter 1983;Bos and Spiers 2001).…”