On the internal structure and mechanics of large strike-slip fault zones: field observations of the Carboneras fault in southeastern Spain
Introduction
Characterization of the internal structure of fault zones is an essential precursor to understanding and predicting their mechanical, hydraulic and seismic properties. Large-displacement faults generally produce much wider zones of deformation than smaller offset features. This provides much more scope for developing a complex internal geometry, which in turn may lead to significant modifications of the properties of these discontinuities. Indeed, several authors have suggested that transcurrent plate-boundaries such as the San Andreas fault are mechanically weak Lachenbruch and Sass, 1980, Zoback et al., 1987, a characteristic which does not seem to apply to smaller displacement faults (Townend and Zoback, 2000) and which by implication probably do not cut the entire thickness of the continental crust.
Previously, field studies of fault zones and geophysical studies, including modelling of seismic fault zone waves and inferences of the seismic velocity structure, have helped elucidate the internal structure of major strike-slip systems.
For field studies, efforts are hampered by the generally poor exposure that accompanies active deformation on a large fault. Additionally, the types of fault rocks that are produced by brittle deformation, such as friable fault gouges, are easily and quickly degraded by surface weathering effects. For this reason, field studies have concentrated on recently exhumed traces of young but extinct fault zones that are exposed in arid climates Caine et al., 1996, Miller, 1996, Foxford et al., 1998. For large transcurrent fault zones which cut through the entire lithosphere (i.e. plate-bounding strike-slip systems), there are very few places that are suitable for studying the detailed internal structure in the field. Owing to these difficulties, there are remarkably few descriptions of large fault internal geometry from direct observations (but see Chester and Logan, 1986, Chester et al., 1993, Evans et al., 1997, Chester and Chester, 1998, Wibberley and Shimamoto, 2003).
Geophysical techniques have provided an opportunity to study actively deforming large fault zones at depth. The most intensively studied fault zone in the world, the San Andreas fault, has yielded a wealth of geophysical information. Characteristics such as high Vp/Vs ratios have allowed us to speculate on fluid pressures within the fault Johnson and McEvilly, 1995, Thurber et al., 1997. Seismic fault zone wave studies have permitted geophysicists to model fault zone widths Li et al., 1990, Li et al., 1997, although this forward modelling relies somewhat on prior knowledge of, or assumptions about, the fault zone structure (Ben-Zion, 1998). Most recently, improved techniques for the processing of seismic signals have led to high-resolution location of earthquake hypocentres, which has made it possible to map out aspects of fault zone internal structure from microseismicity Nadeau et al., 1994, Nadeau et al., 1995, Rubin et al., 1999, Waldhauser et al., 1999.
The purpose of this paper is to present new observations of the internal structure of a large transcurrent fault exposed in southeastern Spain. The Carboneras fault is inferred to have ∼40 km offset and, as such, must cut through the entire crust and lithosphere. The Carboneras fault differs from other large-displacement strike-slip faults described in the literature because it contains a high percentage of phyllosilicate-rich material. This feature undoubtedly affects the mechanical properties, which in turn determine the internal structure of the fault. The fault has been exhumed from depth during the past 10 Ma and the climate is now semi-arid. For these reasons, the fault has unparalleled exposure and the fault rocks are excellently preserved. The paper first outlines the tectonic and geological setting of the Carboneras fault, then describes detailed field observations of the large-scale internal structure of this fault. Next, the predominant types of fault product, with their macroscopic and microscopic internal structures, are assessed in terms of the likely mechanical behaviour they will exhibit (utilizing the results of previous laboratory studies) and hence the type of seismic slip characteristics it is likely to produce. In the discussion, the fault described is in contrast with previous observations of the internal structure of large fault zones that generally have been derived from phyllosilicate-poor protoliths. Finally, the field observations from the Carboneras area are compared with geophysical observations of the San Andreas fault around Parkfield, CA.
Section snippets
The Carboneras fault zone
The Carboneras fault is part of a large fault system that cuts across the southeastern margin of the Iberian Peninsula (Fig. 1). The fault system, known as the Trans-Alborán Shear Zone (Larouzière et al., 1988), constitutes part of the diffuse plate boundary between Africa and Iberia. The Trans-Alborán Shear Zone runs through the westernmost extent of the Alpine Orogenic belt: the Betic Cordilleras. Since the Alpine Orogeny (Early Tertiary), the mountain belt has been dissected by Miocene-age
Field observations
Detailed mapping, at a scale of approximately 1:3700 (using the compass and pace technique), was conducted along a 5-km stretch of the Carboneras fault immediately to the northeast of the village of El Saltador Alto (see Fig. 1 for regional location of the study area). This allowed the broad scale internal structure of the fault zone to be identified and also the different styles of deformation within the fault zone to be characterized.
Discussion
This discussion considers three areas; first, how does the structure and inferred mechanics of the fault zone presented in this work compare with other, previously described large strike slip faults, and what are the main differences? Second, what are the implications of the fault zone structure for fluid flow properties? Third, how does the Carboneras fault zone compare with indirect fault observations from geophysics, particularly in the light of enhanced techniques developed in the past few
Conclusions
- 1.
The internal structure of the Carboneras fault zone does not fit with the previous conceptual models for large strike-slip fault zones. This is due to the fault having a much wider zone of deformation with a heterogeneous distribution of rock types and strain within. The thickness of the zone of deformation (∼1 km wide) is thought to be due to the mechanics of deformation of phyllosilicate-rich fault gouge. In laboratory experiments, this type of gouge has shown strain hardening/velocity
Acknowledgements
We thank the Natural Environment Research Council for support for this work and the Medio Ambiente of the Junta de Andalucı́a, for permission to conduct studies within the Cabo de Gata–Nı́jar National Park. Constructive comments that helped clarify the paper were provided by the reviewers; Karen Mair and Stephen Karner. We also thank the Tectonic Studies Group of the Geological Society of London for financial assistance with the reproduction of the colour figures.
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