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_Journal of Structural Geology 32 (2010) 1349–1362_ _Contents lists available at ScienceDirect_ _Journal of Structural Geology_ _journal homepage: www.elsevier.com locate jsg _Analysis of the internal structure of a carbonate damage zone: Implications for the mechanisms of fault breccia formation and fluid flow_ _Stefan Hausegger a, Walter Kurz b,*, Robert Rabitsch a, Eva Kiechl a, Franz-Josef Broscha a Institut für Angewandte Geowissenschaften, Technische Universität Graz, Rechbauerstr. 12, A-8010 Graz, Austria; b Institut für Erdwissenschaften, Universität Graz, Heinrichstr. 12, A-8010 Graz, Austria_ _article info_ _Article history: Received 11 November 2008; Received in revised form 17 April 2009; Accepted 27 April 2009; Available online 7 May 2009_ _Keywords: Fault zone Carbonates High-angle fractures Joint-bounded slices Fault rocks Fluid–rock interaction_ _abstract_ _A segment of the Salzach-Ennstal fault zone (Talhof fault, Eastern Alps) shows evidence for joint nucleation by layer-parallel shear, causing the formation of joint-bounded slices oriented at high angles (65–85°) with respect to the shear zone boundary (SZB). Subsequent slice rotation resulted in joint reactivation as antithetic shears, slice kinking, and breaking-up of the individual slices into smaller fragments. The latter process, due to the longitudinal constraint of slices with impeded shear zone widening, marked the transition to cataclasite formation and fault core evolution during shear localization. Cataclasites were subsequently cemented and underwent continuous shear deformation by refracturing. Cement precipitation from fluids therefore played a fundamental role in the evolution of the fault zone, with a cyclic change between an open and a closed permeability system during fault evolution._ _Stable isotope compositions (d13C, d18O) of fault rock cements indicate a continuous equilibration between protolith-derived fragments and cements precipitated from those fluids. This points to limited fluid amounts, only temporally replenished by meteoric water, and a hydraulic gradient that directed fluid flow from the damage zone towards the fault core._ _© 2009 Elsevier Ltd. All rights reserved._ _1. 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., 2003a; Faulkner et al., 2003; Wibberley and Shimamoto, 2003) and laboratory experiments (e.g., Riedel, 1929; Reches, 1978; 1983; Logan et al., 1979, 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 prerequisite 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 Billi et al., 2003a):_ _(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., 2003a,b; Storti et al., 2003; Billi and Storti, 2004; Billi, 2005, 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._ _1350_ _S. Hausegger et al. Journal of Structural Geology 32 (2010) 1349–1362_ _Fig. 1. Tectonic map of the Eastern Alps displaying the Palaeogene to Neogene fault system (after Linzer et al., 2002). The sites discussed in this contribution are located along the Talhof segment of the Salzach-Ennstal-Mariazell-Puchberg fault (SEMP) (see Fig. 2). TH (encircled) – Talhof fault segment of the Salzach-Ennstal-Mariazell-Puchberg fault system; AF – Ahrtal fault; AnF – Annaberg fault; BL – Brenner line; DHL: Dollach-Heiligenblut line; EL – Engadine line; GoT – Göstling fault; HoF – Hochstuhl fault; InF – Inntal fault; IsF – Iseltal fault; KL – Katschberg line; KLT – Königssee-Lammertal-Traunsee fault; LoF – Loisach fault; LS – Lower Schieferhülle; MoF – Molltal fault; OtT – Oetztal thrust; PF – Peijo fault; PeF – Pernitz fault; PLF – Palten-Liesing fault; PoF – Pols fault; PyF – Pyhrn fault; RTS – Radstadt thrust system; RW – Rechnitz window; SaF – Salzsteig fault; TF – Telfs fault; WeF – Weyer fault; WGF – Windischgarsten fault; Z – Zell pull-apart structure; ZC – Zentralgneiss core. GoT – Göriach basin; PaT – Parschlug basin; SeT – Seegraben basin; FoT – Fohnsdorf basin; ObT – Obdach basin; WiT Wiesenau basin; StT – St. Stefan basin._ _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., 2003a), 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)._ _structures related to subsequent fault zone evolution. We focus mainly on the development of structures in the transition from the fault damage zone to the fault core up to the full development of fault rocks. The aim is to provide new insights into the structural_ _2. 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 compositions (d13C, d18O) of host rock fragments and cements that precipitated from fluids entrapped within the fault zone._ _Structures that formed during the initial phases of fracturing, both along the fault segment described in this study and along other major faults in the Eastern Alps, were described in detail by Brosch and Kurz (2008). In this study we mainly focus on the_ _Fig. 2. Fault map showing the surface intersections and strike directions of the Talhof fault and associated conjugate faults (after Kiechl, 2007)._ _S. Hausegger et al. Journal of Structural Geology 32 (2010) 1349–1362_ _1351_ _evolution of fault zones, the mechanisms of fault rock formation, and the related permeability evolution of fault zones._ _In particular we present new observations on the internal structure of a major strike-slip fault within the Eastern Alps (Fig. 1). This structure is controlled by an anisotropic host rock fabric. The importance of fractures at high-angles to any pre-existing deformation-related anisotropy, being reactivated by layer-parallel shear, will be discussed in detail. The pre-existing anisotropy fabric, together with the repeated activation of discontinuities, has obliterated the original fracture-mechanical rock properties, and modified the angular relationships of fractures generated during faulting (e.g., Paterson, 1978; Rispoli, 1981; Peacock and Sanderson, 1992; Willemse et al., 1997; Mollema and Antonellini, 1999). Moreover, we focus on the role of these structures in the development of brittle shear zones and brecciation. Finally we investigate the fluid–rock interaction with emphasis on dissolution and precipitation mechanisms. Stable isotope analysis of selected samples provides information on the interaction of fractured domains and fault rocks with fluid phases and on the origin of fluids entrapped within the fault zone._ _3. 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 with respect to the shear direction along the shear zone boundary (SZB). Orientation distributions of distinct fracture sets were analysed by using the program package Tectonics FP 1.6.2, a 32-bit Windows? software for structural geology (Reiter and Acs, 1996–2001). The sizes of fragments and grains (maximum and minimum)_ _Fig. 3. Field view and section across the Talhof fault–Giessgraben fault intersection at site ‘‘Stiegerinhuette’’, including the subdivision of fault core domain_ Ключевые слова: fracture zone, wibberley, isotope composition, carbonate, process, evolution, secondary fault, evans, joint-bounded slice, isometric shape, benedicto, tectonic map, uids entrapped, shear zone, main fracture, composition, process zone, woodcock, fluid inltration, isotopic composition, cataclastic, shear displacement, american, journal, transition, szb figs, smaller fragment, stress, sparitic cement, permeability evolution, large range, limestone, science, isotope, journal geophysical, subsequent reactivation, study, boundary, kurz, increasing fragmentation, shear sense, dz, isotope signature, kerrich, minor, austria sheet, uidrock ratio, journal structural, sparitic, hausegger, rzzuschlag, geological society, damage, geology, size, high-angle fracture, sampling site, logan, displayed, acid, cyclic change, ?uid, brittle, tectonics, white, layer-parallel shear, distribution, formation, gamond, internal structure, szb arrows, strike-slip fault, cement, fault rock, elsevier, sinistral sense, table, schmid, faulting, high ratio, cemented, storti, development, billi, fmax decrease, appendix, structural, kirschner, area covered, calcite vein, high angle, fracture, brittle fault, chester, ratio, tectonic, kennedy, fault core, slice fragment, slice-bounding joint, oxygen isotope, fmin, fault zone, joint, shear, additional maximum, main, orientation, analysis, antithetic shearing, evolutionary step, austroalpine nappes, upper crust, core, talhof fault, fragment, host, grain, journal structural geology, isotopic, szb sample, eastern alps, reches, damage zone, structural geology, high, rock mass, cm, shear localization, burkhard, mandl, sample, san, janssen, alps, hausegger journal, fault, mechanical property, uidrock interaction, maximum, calcite, rock, eastern, stable, fragment margin, northern margin, fabric, tectonophysics, szb note, szb, stained sample, cataclasite fragment, host rock, cataclasite, fmax, ?uids, uid, general sense, fracturing, characterized, isotopic signature, shear fracture, slice, mechanism, domain, cataclasites, geophysical, deformation, figs, ?ow, uid phase, high-angle, individual slice, structure, fault-valve behaviour, low-angle orientation, frisch, fracture pattern, normal, sheared fracture, der, greywacke zone, quartz gouge, meteoric water, sampling, micarelli, tauern window, general, riedel, zone–fault, rotation, agosta, mm, sibson, joint-bounded, zone, minor joint, site atlitz, angle clockwise, talhof, site, calcite precipitated, szb stylolites, breccia, carbonate rock, internal, angle, wa, structural evolution, displacement, peacock, high-angle joint, subsequent, caine, fault gouge, pull aparts, reaction time, fmaxfmin ratio, subsequent formation, parallel