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

Journal of the Geological Society, Nov 2004 by Collettini, C, Holdsworth, R E

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

Low-angle normal faults have been extensively documented in areas of continental extension, but their origin, evolution and existence as seismically active structures is controversial. Traditional 'Anderson-Byerlee' frictional fault models (sliding friction coefficient range 0.6

Many models proposed to explain the presence of low-angle normal faults invoke passive reorientation of initially high-angle structures during either domino-rotation of successive normal fault sets (Proffett 1977) or isostatic adjustments generating footwall flexure and uplift (Wernicke & Axen 1988). Other models allow low-angle movement by invoking dramatic departures from an Andersonian state of stress (e.g. Westaway 1999).

In this paper, we examine the Zuccale fault on Elba, a spectacularly exposed low-angle normal fault exhumed at the surface following the effects of Tertiary back-arc extension in the Apennine chain of Italy. There is good evidence to suggest that the Zuccale fault was active as a low-angle structure and, on the basis of new field and microstructural observations, we propose that fault zone weakening in this case occurred as a result of major changes in fault rock rheology following syntectonic fluid influx at depths in the brittle crust below 3 km. As a modern analogue to the Zuccale fault, we refer to a currently active lowangle normal fault in the Northern Apennine system and use both examples to formulate a new slip model for weak low-angle normal faults. Our findings therefore provide an explanation for how low-angle normal faults may exist as active structures in many other regions of crustal extension.

In the following account, the term 'ductile' is used to refer to deformation in which pervasive mesoscopic flow is thought to have occurred, irrespective of whether this was accommodated microscopically by brittle, diffusion- or dissolution-dominated or crystal-plastic deformation mechanisms (see Rutter 1986). In practice, the presence of such flow in fault rocks is inferred here mainly based on the preservation of a well-defined tectonic fabric (foliation, lincation) with associated asymmetric shear criteria (e.g. Hanmer & Passchier 1991).

Regional setting

The Northern Apennines

In the northern Tyrrhenian-Apennine region, the effects of Tertiary post-orogenic extension in a back-arc setting are superimposed upon and closely follow an eastward-migrating pulse of crustal shortening (e.g. Lavecchia et al. 1994; Jolivet el at. 1998, and references therein). As a result, the locus of active extension and its associated diffuse magmatic activity has migrated eastwards from the Tyrrhenian Sea to the Apennine chain over the last 18 Ma (Fig. 1; Serri et al. 1993; Barchi et al. 1998; Jolivet et al. 1998). Geological and geophysical evidence suggests that significant amounts of extension have been accommodated by low-angle normal faults now exhumed in the Tyrrhenian islands and Tuscany (e.g. Carmignani & Kligfield 1990; Keller et al. 1994; Storti 1995; Jolivet et al. 1998). Active extension continues in the Umbria region, where steeply SW-dipping normal faults have generated historical and instrumental earthquakes, 5.0

The low-angle Altotibcrina fault is not at present exposed at the surface, meaning that it is difficult to assess directly the possible causes of weakening and the character of slip along the fault zone. However, a number of older, large-offset low-angle normal faults in the Apennine system are now exposed at the surface in the Tyrrhenian islands and Tuscany (e.g. Jolivct et al. 1998). These structures provide an opportunity to directly study deformation processes and possible slip behaviour operating along such low-angle faults at depth. The Zuccale fault, first recognized by Trevisan (1950) on Elba and named by Keller & Pialli (1990) is one of the best exposed examples and is discussed in the present paper.