Analysis of the internal structure of a carbonate damage zone: Implications for the mechanisms of fault breccia formation and fluid flow
Introduction
Knowledge of the internal structure of brittle fault zones has been gathered from both field studies (e.g., Anderson, 1951, Tchalenko, 1970, Sibson, 1986, Chester and Logan, 1987, Chester et al., 1993, Caine et al., 1996, Billi et al., 2003, Faulkner et al., 2003, Wibberley and Shimamoto, 2003), and laboratory experiments (e.g., Riedel, 1929, Reches, 1978, Reches, 1983, Logan et al., 1979, Logan et al., 1992, Reches and Dietrich, 1983, Chester and Logan, 1990, Sagy et al., 2001, Katz et al., 2003). Fluid infiltration into faults, and the subsequent fluid–rock interaction, influence the fault mechanical behaviour (Hubbert and Rubey, 1959, Janssen et al., 1998, Kurz et al., 2008). Characterization of the internal structure of fault zones is an essential pre-requisite to understanding and predicting their mechanical, hydraulic and seismic properties (e.g., Faulkner et al., 2003, Woodcock et al., 2007). Generally, the following structural elements may be discriminated across brittle fault zones (following Chester and Logan, 1986, Caine et al., 1996, Caine and Foster, 1999, Billi et al., 2003, Faulkner et al., 2003):
- (1)
The host rock, or protolith, consists of the rock mass bounding the fault-related structures.
- (2)
The damage zone is characterized by secondary faults of small displacement, veins, and networks of shear and extensional fractures generally related to the processes of fault growth. Generally, the transition from the host rock to the damage zone is quite gradual.
- (3)
The fault core is where shear displacement is localized. The core is associated with the development of fault rocks by bulk crushing, particle rotation, abrasion and grain size diminution that obliterate the original host rock fabric (e.g., Billi et al., 2003, Billi et al., 2003, Storti et al., 2003, Billi and Storti, 2004, Billi, 2005, Billi, 2007).
- (4)
Following the definition of Vermilye and Scholz (1998), the process zone comprises those features that result directly from propagation of the fault tip. As the damage zone, the process zone is characterized by secondary faults of minor displacement, veins, and networks of shear and extensional fractures. It may overlap with the damage zone as well as with parts of the fault core.
The damage zone and the fault core may also be seen as representing the evolutionary steps of fault development. As the fault core evolves continuously within the damage zone (e.g., Billi et al., 2003, Faulkner et al., 2003), the spatial zoning from the protolith to the core, including the development of fault rocks, corresponds to these evolutionary steps (e.g., Micarelli et al., 2006).
Section snippets
Objectives
In this study we discuss the structural evolution of carbonate fault rocks along a major fault zone in the Eastern Alps, and its implications for fluid flow and the permeability evolution within and along the fault zone. As fault zones highly affect the hydrogeological properties of the rock mass, we will discuss the role of fluids for fault zone evolution, and the interaction of these fluid phases with adjacent rocks. Information on these processes can be obtained from stable isotope
Methods of structural analysis
Samples were taken from the damage zone towards the fault core in order to infer the evolution of structures as displacement increases. Samples were saw-cut into serial sections parallel to the local direction of shear, and perpendicular to the shear zone boundary (SZB). The saw-cut sample sections were stained with black ink and subsequently polished in order to highlight the traces of the fracture network, voids and pores.
We describe the brittle structures by considering their orientation
Geological and tectonic setting
We selected well exposed sites along the Talhof fault (Gmeindl, 1999), which is a 15 km long segment of the Salzach-Ennstal-Mariazell-Puchberg (SEMP) fault system in the Eastern Alps (Fig. 1) (Ratschbacher et al., 1989, Ratschbacher et al., 1991, Decker et al., 1993, Decker and Peresson, 1996, Wang and Neubauer, 1998, Frisch et al., 2000). The WSW–ENE oriented SEMP fault extends for 400 km along the Eastern Alps. The maximum left-lateral displacement along the SEMP is 70 km (Linzer et al., 2002).
Site description
The sites described in this study provide a nearly complete exposure of the Talhof fault segment, i.e. from the protolith to the fault core. Special emphasis will be given on a site exposing the intersection of the Talhof and Giessgraben fault zones (Fig. 3). At the Talhof–Giessgraben fault intersection (Fig. 2), Lower Triassic (Anisian) fine-grained layered marbles are separated from quartzites of Permian to Triassic protolith age, both belonging to the Lower Austro-Alpine unit (Fig. 1).
Evolution of joints and shear fractures at the damage zone–fault core transition
Rocks of the damage zone adjacent to domain 1 are characterized by fractures at high (50–120°) and low (10–30°) angles anticlockwise from to the shear zone boundary. In this study, these fractures are named as high- and low-angle fractures, respectively. The relative timing of fracture formation can be determined by abutting and crosscutting relationships.
Low-angle fractures at angles between 20 and 30°, measured in an anticlockwise sense to the SZB, are sheared fractures. Structures at angles
Fault rocks along the damage zone–fault core boundary
The fault rocks (coarse-grained cataclasite, fault breccia) are characterized by an irregular grain size distribution (Fig. 9, Fig. 10). Sample SHWK2 is entirely made up of a medium-grained cemented cataclasite. Sample Talhof5 is characterized by two distinct domains (Fig. 9). The portions immediately adjacent to the fault plane consist of medium-grained carbonate cataclasites, while those near the damage zone grade into a grain-supported fault breccia. Remnants of former joint-bounded slices
Oxygen and carbon stable isotope compositions
Powder samples were obtained by microdrilling of the rock specimens described above. These samples were analysed using stable isotope geochemistry in order to determine the isotopic composition of (1) host rock-derived slices within the damage zone (Fig. 4, Fig. 5, Fig. 6, Fig. 7, Fig. 8, Fig. 9), of (2) cataclasite fragments and of (3) fault rock cements (Fig. 9). The analytical techniques are described in the Appendix. Carbon and oxygen isotopic ratios are plotted in Fig. 11 and listed in
Implications for the formation of carbonate fault rocks
The structural evolution presented in this study (Fig. 12) is that fault zone development within layered carbonates starts with shearing of the primary composite foliation. This includes cross jointing between layer-parallel shears (Fig. 12a, b). These joints form a penetrative fabric in terms of a fracture cleavage, which is in contrast to some previous examples (e.g., Wilcox et al., 1973, Paterson, 1978, Olson and Pollard, 1991, Davis et al., 1999, Mandl, 2005). Example: (1) high-angle
Conclusions
- (1)
In layered rocks, formation of cataclastic shear zones may start with layer parallel shear and the development of cross-joints at high angles to the shear zone boundary (SZB). This results in the formation of joint-bounded, closely spaced slices, forming a penetrative fabric (fracture cleavage) at the scale of the shear zone.
- (2)
The formation of slices bounded by en-echelon joints at high angle to the SZB was probably related to the high ratio of effective normal stresses acting orthogonal to the
Acknowledgements
This study has been carried out during a research project (P 17697-N10) granted by the Austrian Science Fund (FWF). The formal reviews by David Peacock and Fabrizio Storti contributed a lot to the improvement of the first manuscript version. Fabrizio Agosta and Emanuele Tondi are thanked for their thorough editorial comments.
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