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_Journal of Structural Geology 32 (2010) 1334–1348_ Contents lists available at ScienceDirect Journal of Structural Geology journal homepage: www.elsevier.com/locate/jsg Fault zone structure and fluid–rock interaction of a high angle normal fault in Carrara marble (NW Tuscany, Italy) G. Molli a,*, G. Cortecci b, L. Vaselli b, G. Ottria b, A. Cortopassi c, E. Dinelli d, M. Mussi b, M. Barbieri a Dipartimento di Scienze della Terra, Università di Pisa, Via S. Maria 53, I-56126 Pisa, Italy b CNR Istituto di Geoscienze e Georisorse Pisa, Italy c Azienda U.S.L. n°141 Massa-Carrara, Unità operativa Ingegneria Mineraria, Italy d Centro di Ricerca Interdipartimentale per le Scienze Ambientali, Università di Bologna, Ravenna, Italy e Dipartimento di Scienze della Terra, Università ‘‘La Sapienza’’, Roma, Italy Article info Article history: Received 20 March 2008 Received in revised form 21 April 2009 Accepted 27 April 2009 Available online 21 May 2009 Keywords: Fault zone architecture Carrara marble Microstructures Fault rocks Fluid–rock interactions Alpi Apuane Abstract We studied the geometry, intensity of deformation and fluid–rock interaction of a high angle normal fault within Carrara marble in the Alpi Apuane NW Tuscany, Italy. The fault is comprised of a core bounded by two major, non-parallel slip surfaces. The fault core, marked by crush breccia and cataclasites, asymmetrically grades to the host protolith through a damage zone, which is well developed only in the footwall block. On the contrary, the transition from the fault core to the hangingwall protolith is sharply defined by the upper main slip surface. Faulting was associated with fluid–rock interaction, as evidenced by kinematically related veins observable in the damage zone and fluid channelling within the fault core, where an orange–brownish cataclasite matrix can be observed. A chemical and isotopic study of veins and different structural elements of the fault zone (protolith, damage zone and fault core), including a mathematical model, was performed to document type, role, and activity of fluid–rock interactions during deformation. The results of our studies suggested that deformation pattern was mainly controlled by processes associated with a linking-damage zone at a fault tip, development of a fault core, localization and channelling of fluids within the fault zone. Syn-kinematic microstructural modification of calcite microfabric possibly played a role in confining fluid percolation. © 2009 Elsevier Ltd. All rights reserved. 1. Introduction Fault growth processes commonly produce a fault zone architecture comprising a fault core, bounded by slip surfaces, including comminuted and fragmented rock material and a volume of distributed deformation surrounding the core denoted as damage zone (e.g., Chester and Logan, 1986; Knipe and Lloyd, 1994; McGrath and Davison, 1995; Caine et al., 1996; Cello et al., 2001a,b; Storti et al., 2003; Billi et al., 2003; Kim et al., 2004; Agosta and Aydin, 2006). The fault core represents the part of a fault zone in which most of the displacement was accommodated (Caine et al., 1996), whereas the damage zone, which includes fault-related subsidiary structures can be associated with distributed deformation developed during the different stages (pre-faulting, fault propagation, displacement and linkage) of fault zone growth (e.g., Chester and Logan, 1986; Peacock, 2002; Faulkner et al., 2003). * Corresponding author. Tel.: +39 050 2215749. E-mail address: gmolli@dst.unipi.it (G. Molli). 0191-8141 $ – see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.jsg.2009.04.021 Kim et al. (2004) divided the fault damage zone into three types, namely: tip-, linking- and wall-damage zones based on position within and around the fault zone itself. Although physical models of fault growth address the complexity of fault propagation (e.g., Vermilye and Scholz, 1998; Peacock, 2002 and references), actual three-dimensional damage zone geometry should be considered to understand the slip mode and evolutionary stages of a given zone. Inclined faults commonly show an asymmetric strain pattern around the fault core which justifies the proposed separation of the damage zone into distinct footwall- and hangingwall-damage domains (Berg and Skar, 2005). The asymmetric deformation pattern has been related (e.g., Mandl, 2000; Flodin and Aydin, 2004; Berg and Skar, 2005 and references therein) to: i) geometric controls (e.g., irregularity of fault trace); ii) differences in stress conditions during faulting; iii) differences in rock properties across the fault; iv) growth process of the fault zone. Fault zones may have important control on subsurface fluid flow (McCaig, 1988; Sibson, 1992, 1996; Antonellini and Aydin, 1994), acting as localized conduits, barriers and/or combined conduit-barrier (e.g., Knipe, 1993; Caine et al., 1996; Kirschner and Kennedy, 2001; Rawling et al., 2001; Cello et al., 2001a,b; Storti et al., 2003; G. Molli et al. Journal of Structural Geology 32 (2010) 1334–1348 Micarelli et al., 2006). Because permeability may be several orders of magnitude different from the host rock (Evans et al., 1997; Caine and Forster, 1999; Agosta et al., 2007), fluids channelized within fault zones strongly influence the rheological behaviour and the dynamics of faulting (e.g., Brodie and Rutter, 1985; Marquer and Burkhard, 1992; Bruhn et al., 1994; Sibson, 1992, 1996; Schultz and Evans, 1998; Faulkner and Rutter, 2001; Kennedy and White, 2001; Labaume et al., 2004; Micklethwaite and Cox, 2004; Mancktelow and Pennacchioni, 2005; Miller et al., 2008). In this contribution we present the results of a structural investigation of a high angle normal fault developed in the Alpi Apuane (NW Tuscany, Italy). The fault studied, hosted within Jurassic marbles (the well known Carrara marbles), gives us the opportunity to compare and infer natural fault zone formation products and processes with those developed in experimental deformation of the same material under controlled physical conditions (P, T, strain rate and fluids) (e.g., Rutter, 1972; Schmid et al., 1980; Fredrich et al., 1989; Burlini and Bruhn, 2005; Han et al., 2007). Moreover, the suitability of the described models for an asymmetric strain distribution within a homogeneous coarse grained carbonate protolith can be examined. 2. Geological setting The Alpi Apuane, in which the studied structure is hosted, exposes the deepest structural level of the northern Apennine nappe stack (Fig. 1). The rock units represent the distal part of the Adria continental margin (Tuscan Domain), lying below the westerly derived oceanic Ligurian and sub-Ligurian accretionary wedge units (Elter, 1975; Carmignani and Kligield, 1990; Carmignani et al., 2000; Molli, 2008). The lithostratigraphic sequence exposed in the Alpi Apuane is made up of a Paleozoic basement unconformably overlain by an Upper Triassic–Oligocene metasedimentary sequence. The Mesozoic cover-rocks include thin Triassic continental to shallow water Verrucano-like deposits, followed by Upper Triassic–Liassic carbonate platform metasediments comprising dolostones (‘‘Grezzoni’’), dolomitic marbles and marbles (the ‘‘Carrara marbles’’), and then by Upper Liassic–Lower Cretaceous cherty metalimestone, cherts and calcschists. Lower Cretaceous to Lower Oligocene sericitic phyllites and calcschists, with marble interlayers, were deposited in deep water during drowning of the former carbonate platform. The Oligocene deposition of turbiditic sandstones (Pseudomacigno formation) closed the sedimentary history of the continental margin. The Tertiary tectonic evolution of the Alpi Apuane includes an early stage of deep underthrusting associated with peak metamorphism (temperature of 350–450°C; pressure of 0.5–0.6 GPa) and isoclinal folding. This was followed by deformation associated with syn-contractional exhumation, during which folding and subhorizontal crenulation cleavage were developed (Molli and Vaselli, 2006; Meccheri et al., 2007; Molli, 2008). The latest stages of geological evolution, associated with the final exhumation and uplift of the Alpi Apuane, were characterized by brittle faulting (low angle and high angle faults) during ‘‘post-orogenic’’ regional extension of the inner part of the Northern Apennine wedge (Carmignani and Kligield, 1990; Patacca et al., 1992; Ottria and Molli, 2000; Molli, 2008). In terms of brittle deformation, the Alpi Apuane represents (Fig. 1) a homogeneous domain of ‘‘low strain’’ surrounded, both to the east and to the west, by main faults (border faults) which separate the Alpi Apuane from the seismically active tectonic depressions of lower Lunigiana Versilia, to the west, and Garfagnana to the east (Carmignani et al., 2000; Molli, 2008). Within the Alpi Apuane, the brittle structures formed during a protracted history of deformation in which an initially mutually interfering system of strike-slip and normal faulting was followed by the development of normal faults (Ottria and Molli, 2000). Available thermochronometric studies (Abbate et al., 1994; Balestrieri et al., 2003), in particular those more recently performed including (U–Th) He and fission-track ages on zircon and apatite (Fellin et al., 2007), allow to constrain the stages of high angle faulting within the last 5 Ma, during which 3–4 km of rock uplift occurred. 3. Mesostructural and microstructural data The analyzed fault, exposed in the central part of the Carrara marble basin, is observed across artificial exposures of an underground quarry in the Fantiscritti marble basin (Fig. 2). 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