Structural diagenesis of carbonate fault rocks exhumed from shallow crustal depths: An example from the central-southern Apennines, Italy

Journal of Structural Geology 122 (2019) 58–80

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Structural diagenesis of carbonate fault rocks exhumed from shallow crustal depths: An example from the central-southern Apennines, Italy

T

Francesco Ferraroa,∗, Fabrizio Agostaa, Estibalitz Ukarb, Donato Stefano Griecoa, Francesco Cavalcantec, Claudia Belvisoc, Giacomo Prossera a

Department of Science, University of Basilicata, Italy Bureau of Economic Geology, Jackson School of Geosciences, The University of Texas at Austin, Austin, TX, USA c National Research Council - Institute of Methodologies for Environmental Analysis, Italy b

ARTICLE INFO

ABSTRACT

Keywords: Limestone Dolostone Cataclasis Structural diagenesis Italian peninsula

This contribution focuses on field and laboratory analyses of carbonate fault cores pertaining to high-angle extensional fault zones currently exposed in the central and southern Apennines, Italy. The fault zones studied crosscut Mesozoic platform-related carbonate rocks, strike ca. NW-SE, and dip steeply SW. They formed during the Plio-Quaternary downfaulting of the Apennine fold-and-thrust belt and were exhumed from shallow crustal levels (< 1.5 km). The carbonate fault cores include grain-supported, matrix-supported, and cement-supported fault rocks, fluidized layers of ultracataclasites with injection veins, and main slip surfaces. Our results of microstructural, petrographic, and cathodoluminescence analyses highlight the contrasting diagenetic evolution of calcite- and dolomite-rich fault rocks. Physical compaction was common within the dolomite-rich fault rocks, whereas chemical compaction coupled with dissolution characterized the calcite-rich fault rocks. Furthermore, multiple generations of calcite cements are documented in the fault zones. The first generation consists of a microcrystalline calcite cement, which developed around survivor grains and lined intergranular pores. The second generation is made up of light-luminescent, fibrous calcite crystals, which precipitated within open fractures and around survivor grains. The third generation consists of an euhedral calcite cement that surrounded survivor grains and infilled both open fractures and intergranular pores.

1. Introduction

Biegel et al., 1989; Marone and Scholz, 1989; Blenkinsop, 1991; Sammis and King, 2007; Billi, 2010). During the early stages of cataclasis, IEF produces coarse-to-minute, angular rock fragments in contact with each other. Conversely, during the late stages of cataclasis, chipping causes smoothing of both the edges and corners of survivor grains, which undergo translation and/or rotation under an abrasive wear regime (Heilbronner and Keulen, 2006; Keulen et al., 2007; Storti et al., 2007; Billi, 2010; Mair and Abe, 2011). Within carbonate fault rocks subjected to high slip rates (> 0.1 m/s) and pressurization, localized shear and seismic-related thermal decomposition of carbonate minerals may also occur, forming cortex grains, matrix-rich bands, injections, and flame structures (Sibson, 2003; Han et al., 2007, 2010; Billi, 2010; Di Toro et al., 2011; Smith et al., 2011, 2013a,b; De Paola et al., 2011, 2014; Collettini et al., 2013, 2014; Fondriest et al., 2013; Rowe and Griffith, 2015). Focusing on carbonate fault rocks, the predominance of one or the other of the aforementioned micromechanisms determines the size distribution and shape of survivor grains, which in turn affects both

Faults zones localize shear deformation and are made up of intensely deformed fault cores encompassed within fractured fault damage zones (Sibson, 1977; Caine et al., 1996; Shipton and Cowie, 2003; Crider and Peacock, 2004; Agosta and Aydin, 2006; De Joussineau and Aydin, 2007; Wibberley et al., 2008; Faulkner et al., 2010). Fault cores include main slip surfaces, fault rocks, and syntectonic veins and mineral deposits. There, the primary fabric of the host rock is no longer discernible. In contrast, fault damage zones, which are commonly crosscut by minor faults, consist of fractured and fragmented host rock that preserves the primary fabric. In the brittle regime, cataclasis is often responsible for fault rock formation and development. Cataclasis determines grain size reduction, grain shape evolution, and the production of a powder-like matrix by means of two main micromechanisms that are known in the literature as Intragranular Extensional Fracturing (IEF) and chipping, respectively (Gallagher et al., 1974; Allegré et al., 1982; Hadizadeh and Rutter, 1982; Sammis et al., 1987;

∗

Corresponding author. Department of Science, University of Basilicata, Via dell'Ateneo Lucano 10, 85100, Potenza, Italy. E-mail address: [email protected] (F. Ferraro).

https://doi.org/10.1016/j.jsg.2019.02.008 Received 23 April 2018; Received in revised form 18 January 2019; Accepted 25 February 2019 Available online 28 February 2019 0191-8141/ © 2019 Elsevier Ltd. All rights reserved.

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their values of porosity and permeability (Billi et al., 2003; Storti et al., 2003, 2007; Agosta et al., 2007; Bastesen et al., 2009; Bauer et al., 2016; Haines et al., 2016). Besides their deformation mechanisms (Willemse et al., 1997; Kelly et al., 1998; Mollema and Antonellini, 1999; Salvini et al., 1999; Billi et al., 2003; Graham et al., 2003; Micarelli et al., 2006; Cilona et al., 2012, 2014; Tondi et al., 2012; Delle Piane et al., 2016), other factors, including pore types such as vugs, molds, fractures, and channels (Wang, 1997; Lucia, 1999; Lønøy, 2006), also affect the petrophysical properties of carbonate fault rocks. Due to the metastable mineralogical compositions of the host rocks, pores are prone to significant diagenetic modifications (Gale et al., 2004, 2010; Micarelli et al., 2006; Kim and Sanderson, 2009; Michie et al., 2014; Haines et al., 2015, 2016). Accordingly, it is of major importance to couple structural and deformational approaches (Laubach et al., 2010), with petrographical and mineralogical analyses to tackle the diagenetic evolution of carbonate fault rocks over time. This study is aimed at documenting the structural diagenesis of carbonate fault rocks currently exposed in the field. The goal is to assess the relative role played by the host rock lithology on the diagenetic processes of fault rocks sampled along active normal fault zones of the central and southern Apennines, Italy, which were exhumed from shallow crustal depths (Vezzani et al., 2010; Ferraro et al., 2018). Results are first discussed in light of the existing literature and then summarized into a conceptual model of carbonate fault rock diagenesis during ongoing extensional faulting, uplift, and exhumation. Specifically, results of optical and SEM microscopies, XRD, and cathodoluminescence analyses are integrated together to decipher the main processes responsible for physical compaction, chemical compaction, and cement precipitation. The model proposed by Lander and Laubach (2015) for quartz cements is employed to assess the ratio of cement growth to the fracture opening rate by considering the different cement morphologies. The latter authors documented three main quartz cement morphological types, which consist of massive sealing precipitates, thin rinds or veneers that line open fracture surfaces, and bridge surfaces that span otherwise open fractures, in sandstones. They concluded that rind morphologies form when the fracture opening rate exceeds two times the fastest rate of cement growth, whereas massive cements precipitate when the opening rate is slower than twice the rate of the slowest cement growth. Bridge morphologies form during intermediate opening rates. Accordingly, we apply the same criteria to the study of carbonate fault rocks. Results of this work can be helpful in a wide range of applications, including predicting the fate of fluids injected deep underground (Stephansson et al., 1996; Tsang, 1999; Dockrill and Shipton, 2010), extracting hydrocarbon resources from tight carbonate reservoirs (Knipe, 1993; Philip et al., 2005; Lander et al., 2008; Olson et al., 2009), and managing groundwater fluids in carbonate-hosted aquifers (Andreo et al., 2008; Petrella et al., 2015; Kavouri et al., 2017; Corniello et al., 2018). Knowledge of structural diagenesis might be crucial to better decipher the petrophysical and rock physical properties of carbonate fault rocks (Micarelli et al., 2006; Agosta et al., 2007; Mavko et al., 2009; Delle Piane et al., 2016; Trippetta et al., 2017), and provide hints on the fluid–rock interactions along active normal fault zones (Miller et al., 2004; Chiaraluce, 2012; Malagnini et al., 2012; Walters et al., 2018).

northern, central, and southern sectors, respectively; they are bounded by lithospheric discontinuities oriented at a high angle with respect to the main NW–SE trend of the belt (Locardi, 1988; Ghisetti and Vezzani, 1999; Vai and Martini, 2001; Patacca and Scandone, 2007; Vezzani et al., 2010). The central Apennines are characterized by N- to ENEverging thrust faults and by high rates of uplift; differently, E- to ENEverging thrusting and duplex geometries are typical of the southern Apennines (Vezzani et al., 2010, and references therein). In both cases, although multiple episodes of out-of-sequence thrusting were documented, the ∼ 4–5 km-thick Lazio-Abruzzi and Campania-Lucania carbonate platforms and transitional units were juxtaposed onto synorogenic deposits as a consequence of predominant piggyback thrusting (Parotto and Praturlon, 1975; Damiani et al., 1991; Vezzani et al., 1998; Patacca and Scandone, 2007; Cosentino et al., 2010; Vezzani et al., 2010; Carminati et al., 2013). In contrast, synorogenic deposits consist of Middle Miocene-to-Pliocene hemipelagic marls, deep-marine siliciclastic sandstones, and intercalated clayey layers (Patacca and Scandone, 2007; Cosentino et al., 2010; Santantonio and Carminati, 2011). Since the Late Miocene, while contractional deformation was still active along the eastern belt, the inner portion of the Apennines experienced extension and exhumation (Ghisetti and Vezzani, 1999, and references therein; Ghisetti et al., 2001). From west to east, the following structural domains are nowadays documented inland: (i) the thinned peri-Tyrrhenian inner belt composed of a 25–30 km-thick crust; (ii) the strongly shortened peri-Adriatic outer belt made up of ∼ 35 kmthick crust currently affected by extensional deformation; and (iii) the Adriatic foredeep, which is currently affected by contractional deformation. The extensional regime that characterizes the inner belt (Cavinato and De Celles, 1999; Morewood and Roberts, 2000; Cavinato et al., 2002) is due to the formation of the Tyrrhenian backarc basin (e.g., Doglioni, 1991) and/or gravitational collapse of the orogen (Ghisetti and Vezzani, 1999). This work focuses on large, NW–SEstriking and SW-dipping fault zones responsible for active tectonics and intense seismicity (i.e., Mw = 7.0 Val D'Agri, 1857; Mw = 7.0 Avezzano 1915; Mw = 5.7 Sulmona 1933; Mw = 5.6 Mercure, 1998), as is widely documented in the literature (Boschi et al., 1997; Galadini and Galli, 2000, 2003; Pondrelli et al., 2004; Fracassi and Valensise, 2007; Rovida et al., 2011). 2.2. Normal fault zones The faults studied underwent a considerable amount of regional uplift and exhumation during the Quaternary. In the central Apennines, both the Morrone and Maiella tectonic units display the highest uplift and exhumation rates, with a total of ∼5 km of uplift and ∼3 mm/yr of exhumation rate documented since the Late Pliocene (Ghisetti and Vezzani, 1999, and references therein). Differently, the whole portion of the southern Apennines underwent a significant uplift during the Quaternary, with average rates of 0.2–1.3 mm/yr (Westaway, 1993; Schiattarella et al., 2003). Specifically, uplift rates of about 0.6–0.7 mm/yr, with peaks up to ∼1.2–1.3 mm/yr, characterized both the Agri Valley and Pollino Ridge areas (Schiattarella et al., 2017), whereas two main pulses of exhumation occurred in the Monte Alpi area during the late Pliocene–early Pleistocene and post-early Pleistocene, respectively, with rates of ∼0.4 mm/yr and 4 mm/yr (Corrado et al., 2002; Mazzoli et al., 2006). All five normal faults, herein labeled the (a) Marsicovetere Fault, (b) Venere-Gioia dei Marsi Fault, (c) Madonna del Soccorso Fault, (d) Vetrice Fault, and (e) Roccacasale Fault, are characterized by quite similar dimensional properties, crosscut Mesozoic carbonates that formed in a similar paleogeographic realm (Vezzani et al., 2010, and references therein), and developed during Pliocene–Quaternary downfaulting of the Apennine belt. Limestone-hosted normal faults crop out along the Fucino and Agri Valley basins (Fig. 1a and b), whereas dolostone-hosted normal faults are exposed along the Mercure Basin and in the vicinity of

2. Geological setting 2.1. Central and southern Apennines The Apennines fold-and-thrust belt is part of a circumMediterranean orogeny in which the structural and stratigraphic evidence of Oligocene-to-Pliocene collisional tectonics and subsequent downfaulting are preserved (Royden et al., 1987; Patacca et al., 1990; Doglioni, 1991; Cavazza et al., 2004; Dilek, 2006). The Apennines belt is often subdivided into three main sectors, which are named the 59

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Fig. 1. Structural map of central and southern Italy (modified from Vezzani et al., 2010) in which the locations of the five normal fault zones studied are shown: a) Marsicovetere Fault (modified from Bucci et al., 2012), b) Venere-Gioia dei Marsi Fault (modified from Agosta and Aydin, 2006); c) Madonna del Soccorso Fault (modified from Cavalcante et al., 2009); d) Vetrice Fault (modified from Giano et al., 2018); e) Roccacasale Fault (modified from Vezzani et al., 1998). 60

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Fig. 2. Plane-light microphotographs showing microstructures of carbonate host rocks. Thin sections were cut orthogonal to bedding. (a), (b) and (c) Marsicovetere host rock composed of greyish limestones, which include bed-parallel stylolites crosscut by high-angle-to-bedding calcite veins. Granular calcite cements infilled pores. (d), (e) and (f) Venere-Gioia dei Marsi host rock made up of peloidal limestones with abundant bed-parallel stylolites. Multiple generations of calcite veins crosscut the stylolites. Pores are completely filled with equant calcite cements. (g), (h) and (i) Madonna del Soccorso host rock made up of greyish dolostones, which include multiple generations of calcite veins and stylolites. (j), (k) and (l) Vetrice host rock made up of Triassic dolostones, which include multiple sets of calcite veins affected by pressure solution. (m), (n) and (o) Roccacasale host rock composed of peloidal limestone and yellowish-greyish dolostones, which are crosscut by bed-parallel and high-angle-to-bedding calcite veins.

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Fig. 3. Cross-sectional view of the main structural domains documented along the 100s of meter-throw carbonate fault zones studied (modified after Agosta and Aydin, 2006). The insets show a schematic cross section of both 10s of meter-throw and 100s of meter-throw carbonate fault cores including the lateral distribution of main fault rock textures.

the Vietri di Potenza village (Fig. 1c and d). The mixed limestone-dolostone normal fault flanks the eastern edge of the Sulmona Basin (Fig. 1e). The Marsicovetere Fault is a ∼5 km-long, 10s of m-throw fault zone that bounds the northern margin of the Agri Valley (Fig. 1a) and juxtaposes Mesozoic limestones against Quaternary breccias (Di Niro et al., 1992; Giano et al., 2000). Specifically, the studied portion of the fault zone crosscuts Jurassic–Lower Cretaceous, light-grey limestones made up of oolitic and lithoclastic grainstones to floatstones, debrites, and calcirudites (Bucci et al., 2012). The limestone host rocks are crosscut by bed-parallel stylolites and high-angle-to-bedding sets of calcite veins (Fig. 2a–c). The Marsicovetere Fault shows Quaternary vertical and horizontal slip rates of 0.7–1.3 mm/yr and 0.6 mm/yr, respectively (Papanikolaou and Roberts, 2007).

The Venere-Gioia dei Marsi Fault is a ∼10 km-long, 100s of mthrow fault zone that bounds the southeastern edge of the Fucino Basin (Fig. 1b), which juxtaposes Mesozoic limestones against Quaternary fluviolacustrine sediments (Bosi et al., 1995; Cavinato et al., 2002; Agosta and Aydin, 2006). This fault zone forms the southern segment of a larger, seismically active, composite source characterized by average slip rates of 0.4–1.0 mm/yr (DISS Working Group, 2015). The portion of the fault zone studied crosscuts well-bedded Lower Cretaceous limestones made up of peloidal mudstones to wackestones grading to calcirudites (Vezzani et al., 2010), which include bed-parallel stylolites and both high-angle- and low-angle-to-bedding calcite veins (Fig. 2d–f). The Madonna del Soccorso Fault is a ∼1.5 km-long, 10s of m-throw fault zone (Fig. 1c) that borders the eastern side of the Mercure Basin (Vezzani, 1967; Cavalcante et al., 2009; Giaccio et al., 2014; Robustelli 62

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et al., 2014). The portion of the fault studied juxtaposes Quaternary carbonate breccia against Triassic dolostones (Cavalcante et al., 2009), and it was active during the late Pliocene (Papanikolaou and Roberts, 2007), early Pleistocene (Schiattarella et al., 1994), and/or early–middle Pleistocene times (Monaco, 1993; Marra, 1998) with slip rates of ∼0.9 mm/yr (Serpelloni et al., 2002). The Triassic rock consists of greyish, well-bedded dolostones with widespread thin stromatolitic levels and rare oncoidal levels grading upward to dolomitic limestones with large megalodonts (Patacca and Scandone, 2007; Cavalcante et al., 2009). The dolostone host rock is crosscut by stylolites, which preferentially localize within multiple sets of calcite veins (Fig. 2g–i). The Vetrice Fault is a ∼5 km-long, 10s–100s of m-throw fault zone (Fig. 1d) that juxtaposes Lower–Upper Miocene clastic rocks of the Monte Serio Fm. against Triassic dolostones (Castellano and Sgrosso, 1996; Giano et al., 2018). The Vetrice Fault formed in the Vietri di Potenza underlap zone (DISS Working Group, 2015) and was active in the last 18 kyr with slip rates of 0.3–0.6 mm/yr (Giano et al., 2018), similar to those documented for the nearby San Gregorio Magno Fault (D'Addezio et al., 1991; Galli et al., 2014). The portion of the Vetrice Fault studied crosscuts whitish-to-greyish, highly fractured dolostones with stromatolitic levels (Patacca and Scandone, 2007). There, stylolites localize within multiple sets of calcite veins (Fig. 2j–l). The Roccacasale Fault is a 10 km-long, 100s of m-throw fault zone (Fig. 1e) that bounds the eastern edge of the Sulmona Basin and juxtaposes Mesozoic limestones and dolostones against Quaternary fluvial, lacustrine, and alluvial deposits (Vezzani et al., 1998). This fault zone forms the southern element of a composite seismic source characterized by a cumulative throw up to ∼400 m (Cavinato and Miccadei, 1995; Galadini and Messina, 2004) and average slip rates of 0.1–1.0 mm/yr (DISS Working Group, 2015). In particular, the Roccacasale Fault is characterized by Quaternary slip rates of 0.4 ± 0.07 mm/yr (Vittori et al., 1995; Miccadei et al., 1998; Galadini and Galli, 2000; Gori et al., 2007, 2011). The portion of the fault zone studied crosscuts Upper Cretaceous greyish dolostones and yellowish limestones, which are made up of peloidal packstones and grainstones grading to bioclasticrich mudstones and wackestones (Vezzani et al., 2010). At the microscale, abundant isopachous, equant, and drusy calcite cements and multiple sets of calcite veins are visible (Fig. 2m to 2o).

than 75–80% in volume (Ferraro et al., 2018). Fg-type fault rocks are interpreted as being due to chipping, localized shear, and coseismic thermal decomposition of carbonates (Sibson, 2003; Han et al., 2007, 2010; Smith et al., 2011; De Paola et al., 2011, 2014; Rowe and Griffith, 2015). According to the large-scale structural setting of seismic normal fault zones of central and southern Italy, the 10s of m-throw fault zones are interpreted as 2nd order fault strands of the seismically active fault zones forming the 100s of m-throw, 1st order fault strands (Boschi et al., 1997; Galadini and Galli, 2003; Pondrelli et al., 2004; Agosta and Aydin, 2006; Fracassi and Valensise, 2007; Rovida et al., 2011; Storti et al., 2013; Pischiutta et al., 2017). Although the aforementioned texture types are distributed throughout the entire fault cores studied, their relative thickness varies between 1st and 2nd order fault strands. The latter ones include up to ∼40 cm-thick fault cores made up mainly of Gs-type and Ms-type fault rocks with a thickness ratio ∼3:1 (Fig. 3). The Fg-type fault rock, if present, forms isolated and discontinuous patches along the main slip surfaces. In contrast, 1st order fault strands are made up of up to 1 m-thick fault cores, which include mainly Mstype and Gs-type fault rocks with a thickness ratio ∼2:1 (Fig. 3). There, Fg-type fault rocks are both vertically and laterally persistent along the main slip surfaces at the hanging wall-footwall contacts. 4. Methods A total of 56 hand specimens were sampled from the carbonate fault cores studied. Sampling was conducted along transects at semiregular intervals, moving away from the main slip surfaces into the carbonate fault footwalls. The collected specimens are representative of the fault textures documented by Ferraro et al. (2018). Each sample was collected after removing 10–15 cm of weathered outcrop material and cut both parallel and perpendicular to the main slip direction. A subset of 36 rock samples was selected for mineralogical and petrographical analyses. This subset includes five host rock samples, one for each fault zone, four fault rock samples from the Marsicovetere Fault, eight from the Roccacasale Fault, six from the Venere-Gioia dei Marsi Fault, seven from the Madonna del Soccorso Fault, and six from the Vetrice Fault. The whole-rock mineralogy of all samples was determined by X-ray powder diffraction analysis (XRD) performed on randomly oriented powders following the sideloading method (Środoń et al., 2001). XRD analyses were done using a Rigaku Miniflex powder diffractometer equipped with a sample spinner with Cu-Kα radiation operated at 30 kV and 15 mA at the National Council Research of Italy (CNR-IMAA). Data collection was carried out in the 2θ range 2–63° with a step size of 0.02° and 5 s/step speed. Quantitative analysis was performed following the RIR method (Chung, 1974) using corundum (20%) as the internal standard. Peak areas for each mineral were measured using the WINFIT computer program (Krumm, 1994). Data reported in the present manuscript represent an average of the intensity (counts per second, cps) measured for powder samples either from inner or outer fault core domains or from the surrounding carbonate host rock. Optical microscopy analysis was conducted under transmitted light on 51 thin sections (16 calcite-rich, 18 dolomite-rich, and 17 mixed dolomite/calcite-rich fault rock thin sections) using a Nikon Eclipse E600 optical microscope equipped with a Nikon E4500 camera. The oriented thin sections obtained from the hand specimens were prepared to a standard size (2.48 cm × 4.8 cm) and thickness (30 μm), and impregnated with blue-dyed epoxy resin. On the basis of the optical microscopy analysis, nine representative thin sections of both outer and inner fault cores were further investigated by means of Scanning Electron Microscopy (SEM) to better assess both texture and mineralogy of microcrystalline cements. SEM analyses were conducted at the Science Department of the University of Basilicata, Italy, and acquired at 20 kV using a Philips XL30 ESEM equipped with backscattered (BSE) and energy-dispersive spectroscopy (EDS) detectors. After optical and electron microscopy analyses, seven thin sections

3. Carbonate fault rock textures Despite the different nature of the protoliths, inherited structural fabrics, and overall amount of fault throw, all fault cores studied exhibit a similar distribution of cataclastic textures (Fig. 3). The most common fault rocks include grain-supported (Gs-type) and/or matrix-supported (Ms-type) textures (Ferraro et al., 2018). Gs-type fault rocks consist of cm-to mm-sized, angular, poorly sorted grains, which are mostly in contact with each other. The grains are surrounded by a carbonate matrix < 50–55% in volume and show D0 values — the Box-counting fractal dimension related to grain size distribution (Mandelbrot, 1985; Falconer, 2003) — down to ∼ 1.8 (Ferraro et al., 2018). This type of texture localizes mainly in the outer portion of the fault cores, and it is interpreted as being due to pervasive IEF and incipient chipping (Allegré et al., 1982; Sammis et al., 1987; Blenkinsop, 1991; Sammis and King, 2007; Storti et al., 2007; Billi, 2010). In contrast, Ms-type fault rocks consist of mm-sized survivor grains with a low-to-moderate angularity, a moderate-to-high sphericity, and D0 values between ∼1.8 and ∼1.6; the survivor grains are embedded within a carbonate matrix up to 75–80% in volume (Ferraro et al., 2018). This type of texture is localized mainly within the inner portion of fault cores, and it is interpreted as being due to predominant chipping and minor IEF (Heilbronner and Keulen, 2006; Keulen et al., 2007; Storti et al., 2007; Billi, 2010; Mair and Abe, 2011). More comminuted fault rocks (Fgtype) are present around the main slip surfaces; they consist of a few well-rounded, well-sorted survivor grains with D0 values < 1.6, which are embedded in a carbonate matrix and cement that constitute more 63

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of limestone-hosted and dolostone/limestone-hosted fault zones were selected for SEM-based cathodoluminescence (CL) analysis. SEM-CL images provide information about textures that are not discernible using conventional transmitted light microscopy (Milliken and Laubach, 2000). Furthermore, SEM-CL is preferred over optical-CL because of higher spatial resolution and magnification, and the ability to correlate different SEM emissions. The polished thin sections were carbon coated (20 nm) and then imaged with a Gatan MonoCL4 cathodoluminescence system attached to a Zeiss Sigma field emission (FE)SEM at the Bureau of Economic Geology, The University of Texas at Austin, USA. Panchromatic images of carbonate fault rocks were acquired at 5 kV, 120 μm aperture, high current, and 130 ms dwell time, generating sample currents on the order of 3.5 nA and routine SEM-CL images with spatial resolution as low as a few nanometers per pixel. At these operating conditions, smearing caused by phosphorescence in carbonates is rarely a problem (Ukar and Laubach, 2016). Automated SEM-CL mosaics were acquired and stitched using Digiscan and Adobe Photoshop. In the present work, panchromatic SEM-CL images are described in terms of relative luminescence in a grayscale (light-, medium-, dark-, or nonluminescence). For color information, panchromatic SEM-CL images are calibrated with CL emission spectra to obtain information about the true luminescence color (wavelength of emission within the visible light spectrum).

perpendicular to the main slip surfaces and are partially filled with sharply pointed calcite crystals and, in some cases, isolated cement pillars spanning across individual fractures (Figs. 4h and 5d). Results of SEM-CL analyses show the presence of a light-luminescent, fibrous calcite cement characterized by face-normal growth when gap sizes (distance between adjacent grains or fracture walls) are small (Fig. 5e and f). Furthermore, these results also exhibit a light-luminescent, zoned calcite cement with euhedral terminations, which overgrows the light-luminescent, fibrous cement and lines small fractures (Fig. 5e and f). 5.1.2. Venere-Gioia dei Marsi fault zone The fault rocks studied are part of a laterally continuous, up to 1 mthick carbonate fault core flanked by light-grey, well-bedded, fractured, fragmented, and heavily pulverized (sensu Agosta and Aydin, 2006) Cretaceous limestone rocks on the footwall side (Fig. 6a). Results of XRD analyses performed on powder samples obtained from one host rock, three outer fault core, and three inner fault core hand specimens indicate that calcite is the most common mineral and that both quartz and siderite occur only in traces (Fig. 6b). The amount of calcite is ∼93.8% in the host rock, ∼94.2% in the outer fault core, and ∼94.1% in the inner fault core. Siderite is characterized by subhedral-to-euhedral terminations and coats both carbonate grains and calcite cement (Fig. 7b). The outer fault core includes whitish, cohesive, Ms-type fault rocks and rare pockets of Gs-type fault rocks (Agosta and Aydin, 2006, and references therein; Ferraro et al., 2018). The Gs-type fault rocks include a condensed fabric in which both grain interpenetration and relicts of microcrystalline cements within individual survivor grains are documented (Fig. 6c and d). SEM-CL analyses show that survivor grains have micron-sized rinds and/or veneers of a dark-luminescent calcite cement and that pore space is filled with a zoned calcite cement with euhedral terminations (Fig. 7a and b). We note that well-cemented areas are adjacent to poorly cemented ones and that this distribution is irrespective of grain size (Fig. 7c). Differently, the inner fault core contains Ms-type fault rocks, yellowish-to-pinkish cemented fault rocks (Fg-type), and main slip surfaces (Agosta and Kirschner, 2003; Agosta and Aydin, 2006; Ferraro et al., 2018). Locally, the most prominent slip surfaces crosscut narrow zones of slope debris and incohesive fault breccias. Ghosts of a condensed fabric in which microstylolites crosscut large survivor grains, calcite veins, and relicts of microcrystalline cements are documented (Fig. 6e and f). The Fg-type fault rock texture is made up of a microcrystalline calcite cement, which completely fills the pores, and one main set of calcite veins and open fractures that mainly subparallel the main slip surfaces (Fig. 6g). Open fractures and pores are often partially filled with sharply pointed calcite crystals with euhedral terminations (Fig. 6h). SEM-CL images show that some of the survivor grains in Mstype fault rocks are characterized by irregular edges and include concentric rinds of a few micron-thick, dark-luminescent calcite cement overgrown by zoned calcite with euhedral terminations (Fig. 7e and f). The innermost zone of the euhedral crystals is dark-luminescent, similar to the cement rinds that surround survivor grains (Fig. 7f).

5. Results 5.1. Calcite-rich fault rocks 5.1.1. Marsicovetere fault zone Fault rocks crop out within a laterally discontinuous, up to 20 cmthick fault core flanked by a footwall damage zone made up of fractured and slightly faulted, dark-to-light-grey Cretaceous limestone rocks (Fig. 4a). The outer fault core includes Gs-type fault rocks encompassing lenses of Ms-type fault rocks, whereas the inner fault core is made up of main slip surfaces (MSS), Ms-type fault rocks, and thin, discontinuous, yellowish/brownish cemented fault rocks (Ferraro et al., 2018). Results of XRD analyses performed on powder samples obtained from one host rock, two outer fault core, and two inner fault core hand specimens show that calcite is the main mineralogical phase and that both quartz and siderite are present in trace (Fig. 4b). In fact, the average amount of calcite is ∼94.6% in the host rock, ∼94.5% in the outer fault core, and ∼94.3% in the inner fault core. Results of SEM analyses highlight a clay-rich (illite) matrix (Fig. 5a) and confirm the occurrence of minor detrital quartz (Fig. 5b). Clay minerals and detrital quartz likely derived from either the Miocene clastic deposits (cf. Bucci et al., 2012), as documented for other carbonate fault zones in central Italy (Smeraglia et al., 2016a), and/or from the clay-rich Sicilide Units topping the Mesozoic carbonates (Bucci et al., 2012). In the outer fault core, a condensed fabric (sensu Logan and Semeniuk, 1976) made up of angular survivor grains in contact with each other is documented (Fig. 4c). There, grain interpenetrations at contact points and a few, short, randomly oriented microstylolites are common (Fig. 4d). Results of SEM-CL analyses are consistent with small amounts of light-luminescent, locally fibrous calcite cements surrounding the survivor grains (Fig. 5a). These results also show the presence of zoned calcite crystals with euhedral terminations, which overgrow light-luminescent cements and infill the largest pores (Fig. 5c). The Ms-type fault rocks pertaining to the inner fault core include a condensed fabric and widespread calcite cements (Fig. 4e and f). Within the condensed fabric, microstylolites crosscut both reworked survivor grains, which include calcite cements at their edges, and the finegrained carbonate matrix. Grain interpenetration is uncommon. The cemented fault rocks include microcrystalline calcite cements, which line pores and surround the survivor grains (Fig. 4g). Cemented fault rocks are crosscut by high-angle fractures that strike either parallel or

5.2. Dolomite-rich fault rocks 5.2.1. Madonna del Soccorso fault zone The fault rocks studied are part of a laterally continuous, up to 40 cm-thick carbonate fault core flanked on the footwall side by darkto-light-grey, well-bedded, fractured, and slightly faulted Triassic dolostones (Fig. 8a). The outer fault core consists of whitish-to-greyish, cohesive, Gs-type fault rocks that include discontinuous lenses of Mstype fault rocks, whereas the inner fault core is made up of Ms-type fault rocks and main slip surfaces (Ferraro et al., 2018). Locally, patchy and isolated, cm-thick, whitish cemented fault rocks coat the main slip surfaces. Results of XRD analyses performed on powder samples 64

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Fig. 4. (a) Field photograph of the Marsicovetere fault scarp. (b) XRD patterns showing the average of the intensity (counts per second, cps) measured for powder samples collected from both fault core (inner and outer) and host rock. (c) and (d) Thin sections obtained from hand specimens collected from the outer fault core. Plane-light microphotographs showing condensed fabric and grain interpenetrations among survivor grains (pink arrows) in Gs-type fault rocks. Red arrows indicate the position of the Main Slip Surface (MSS). (e) and (f) Thin sections obtained from hand specimens collected from the inner fault core. Plane-light microphotographs showing a condensed fabric and microstylolites subparallel to the main slip surface in Ms-type fault rock. (g) Inner fault core. Cross-polarized microphotograph of pores completely filled with microcrystalline calcite cements. (h) Inner fault core. Cross-polarized microphotograph of euhedral calcite cement that lines, and in cases spans across, fractures forming isolated cement pillars. Fractures partially cemented by calcite strike perpendicular to the main slip surfaces. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

obtained from one host rock, three outer fault core, and four inner fault core hand specimens indicate that the host rock consists of almost pure dolomite (∼99.7%), whereas higher amounts of calcite are present in both outer (∼5.4%) and inner fault cores (∼10%) (Fig. 8b). A condensed fabric is documented within both the inner and outer fault cores (Fig. 8c, e, and 9b). There, grain reorganization, pore collapse, and minor grain crushing are documented within Ms-type fault rocks (Fig. 8f), whereas grain crushing is predominant in the Gs-type fault rocks (Fig. 8d). The inner fault core is crosscut by open fractures, which strike either orthogonal to or parallel to the main slip surfaces; locally, small amounts of calcite cement span across fracture walls (Fig. 8g and h) and partially fill micropores of the dolomitic matrix (Fig. 9a–d).

rocks, which form a ∼1 m-thick volume flanked by dark-grey, wellbedded, fractured, and slightly faulted Triassic dolostones on the footwall (Fig. 10a). The outer fault core is made up of greyish-to-whitish, Gs-type fault rocks that contain pockets of Ms-type fault rocks, whereas the inner fault core includes Ms-type fault rocks, Fg-type fault rocks, and main slip surfaces (Ferraro et al., 2018; Giano et al., 2018). Results of XRD analyses performed on powder samples obtained from one host rock, three outer fault core, and three inner fault core hand specimens indicate that the host rock is composed of ∼98.7% dolomite and traces of albite and calcite. Calcite is present in both outer and inner fault cores, and constitutes between ∼0.4% and 1% of the bulk volume of the rock, respectively (Fig. 10b). A condensed fabric is documented within both inner and outer fault cores. Grain crushing is common in the condensed fabric of Gs-type fault rocks (Fig. 10c and d), whereas pore collapse, grain reorganization, and minor grain fracturing are dominant in the condensed fabric

5.2.2. Vetrice fault zone The fault core studied includes both Gs- and Ms-type, cohesive fault 65

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Fig. 5. SEM images of outer and inner fault rocks from the Marsicovetere fault zone. (a) Panchromatic SEM-CL image of light-luminescent, fibrous calcite cement surrounding rounded limestone survivor grains within the outer core. Intergranular clay mineral cements are locally present. (b) X-ray EDS element map for Ca, K, and Si showing the distribution of clay material (red and yellow) and limestone grains (dark blue); porosity is imaged in black. (c) Panchromatic SEM-CL image of a fracture in the outer core containing calcite cement crystals with euhedral terminations and residual fracture porosity. (d) BSE image of a partially filled fracture. In some cases, calcite cement spans across the fracture forming isolated cement pillars. (e) and (f) Panchromatic SEM-CL images of light-luminescent, fibrous cement spanning across narrow intergranular pores (gap sizes) in the inner core. Note that where pores are wider, calcite cements have euhedral terminations. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

of Ms-type fault rocks (Fig. 10e–h). Results of SEM analyses confirm the previous assessment and show that the orientation of the numerous intra- and intergranular fractures is highly variable (Fig. 11a–d).

specimens indicate that dolomite is more abundant than calcite. The relative proportion is 69.5% and 27.5% in the host rock, 89.8% and 8.6% in the inner fault core, and 54.8% and 42.4% in the outer fault core, respectively. Traces of albite and siderite are present (Fig. 12b). A condensed fabric in which grain crushing is common characterizes the Gs-type fault rocks (Fig. 12c and d). SEM-CL images show that some dolostone survivor grains have irregular edges and embayments, probably indicating that dissolution occurred prior to precipitation of intergranular calcite cement (Fig. 13c). This calcite cement is patchily distributed, zoned, encompasses both dolostone and limestone survivor grains, and completely fills micron-sized, intergranular pore spaces (Fig. 13c). Unlike in all previously described fault zones, calcite cement crystals with euhedral terminations and residual intergranular macropores are rare. Within Ms-type fault rocks, both grain interpenetrations and microstylolites are common in the fine-grained calcite matrix adjacent to the largest dolostone survivor grains (Fig. 12e). SEM-CL images show the presence of both microcrystalline calcite and euhedral calcite. The

5.3. Calcite/dolomite-rich fault rocks 5.3.1. Roccacasale fault zone The fault rocks studied are part of a laterally continuous, up to 1 mthick carbonate fault core flanked by light-grey, well-bedded, fractured, fragmented, and pulverized (sensu Agosta and Aydin, 2006) Cretaceous limestone rocks on the footwall side (Fig. 12a). The outer fault core contains yellowish, cohesive, Ms-type fault rocks that include pockets of cohesive, Gs-type fault rocks. In contrast, the inner fault core is made up of yellowish and pinkish, cohesive, cemented fault rocks and main slip surfaces (Agosta and Kirschner, 2003; Ferraro et al., 2018). Locally, cmthick, sheared slope debris and discontinuous, incohesive fault breccias are present. XRD analyses performed on powder samples obtained from one host rock, four outer fault core, and four inner fault core hand 66

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Fig. 6. (a) Cross-sectional view of the Venere-Gioia dei Marsi fault scarp. (b) XRD patterns showing the average of the intensity (counts per second, cps) measured for powder samples derived both from the fault core domain (inner and outer) and from the surrounding carbonate host rock. (c) and (d) Thin sections obtained from hand specimens collected from the outer fault core. Gs-type fault rocks showing evidence of condensed fabric and grain interpenetrations coupled with dissolution at the contact point between neighboring survivor grains. Red arrows indicate the position of the Main Slip Surface (MSS). (e) and (f) Thin sections obtained from hand specimens collected from the inner fault core. Condensed fabric and microstylolites in the Ms-type fault rock. (g) and (h) Inner fault core. Pores are partially or completely occluded by microcrystalline cements in matrix-supported fault rocks.

former cement type fills pores, whereas calcite with euhedral terminations fills fractures, and it is present within some of the reworked survivor grains (Fig. 12f and g). Within the inner fault core, some dolostone survivor grains have irregular edges overgrown by intergranular calcite cement (Fig. 13d). Alternations of brownish and yellowish, cm-thick bands, which subparallel the main slip surfaces, are observed. These bands are characterized by sharp and curved irregular edges, indicating dissolution, and resemble the fluidization textures similar to sedimentary flame structures (Figs. 12h, 13a and 13b). Within the bands, 10s of μ-sized, angular dolostone survivor grains embedded in a very fine calcite matrix made up of micron-sized, microcrystalline, reworked calcite cements and limestone survivor grains are documented (Fig. 13a, b, and 13e). There, carbonate cements are virtually absent. In fact, SEM images revealed that micropores present within the narrow bands are completely filled with a very fine-grained calcite matrix (Fig. 13e).

6. Discussion This work provides a comprehensive documentation of the diagenetic phases present in carbonate fault rock samples collected from extensional fault zones currently exposed in central and southern Italy, which formed at similar shallow crustal depths during the PlioQuaternary downfaulting of the Apennine belt. First, we discuss the data in light of the existing literature to assess the main diagenetic processes that took place during concomitant deformation, uplift, and exhumation. Then, we summarize the main results into a conceptual model of structural diagenesis of carbonate fault rocks, which invokes five main stages that took place during fault rock exhumation. 6.1. Diagenesis of carbonate fault rocks exhumed from shallow crustal depths Compaction of granular and cohesive materials during burial occurs 67

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Fig. 7. SEM images of outer and inner fault rocks from the Venere-Gioia dei Marsi fault zone. (a) Panchromatic SEM-CL image of dark-luminescent cement rinds around larger survivor limestone grains as well as very fine matrix grains within the outer core. (b) Panchromatic SEM-CL image of potential calcite cement with euhedral terminations and concentric zones in SEM-CL growing into fracture pore space in an outer core sample. Dark-luminescent euhedral crystals within the cataclasite are siderite. (c) BSE image of angular calcite survivor grains surrounded by an incipient porous matrix. (d) Calcite cement with euhedral terminations surrounding limestone survivor grains. (e) Panchromatic SEM-CL image of limestone survivor grains with irregular edges and a concentric rind of dark-luminescent calcite cement. Larger survivor grains are embedded in a very fine grained calcite matrix. (f) Panchromatic SEM-CL image of inner core limestone survivor grains with irregular edges, dark-luminescent rinds, and overgrowing zoned cements with euhedral terminations (dogtooth) that partially infill porosity in between survivor grains.

at increasing pressures and temperatures, producing the concomitant decrease of both bulk volume and porosity (Meyers and Hill, 1983; Rutter, 1983; Scholle and Halley, 1985). During compaction, physical processes are controlled mainly by effective stress, hence they are largely time-independent under the low-temperature conditions prevailing in sedimentary basins, whereas chemical processes are time-dependent because they are regulated by fluid–rock interactions involving mineral dissolution and/or precipitation (Rutter, 1983; Bjørlykke et al., 1989; Passchier and Trouw, 1996). Fracture is sensitive to linked chemical-

mechanical processes like stress corrosion (Atkinson, 1982), and such processes might be more prevalent in chemically reactive limestones. Since dolostones are usually more chemically and mechanically stable than limestones (Glover, 1968; Bathurst, 1971; Hugman and Friedman, 1979), both physical and chemical processes will be discussed separately in the following text. The modalities of cement precipitation will be assessed by considering the host mineralogy. Within the fault rocks studied, the volumes of calcite cement overgrowths vary tremendously for a pattern of 68

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Fig. 8. (a) Cross-sectional view of the Madonna del Soccorso fault core. (b) XRD patterns showing the average of the intensity (counts per second, cps) measured for powder samples from the fault core domain (inner and outer) and from the surrounding carbonate host rock. (c) and (d) Thin sections obtained from hand specimens collected from the outer fault core. Condensed fabric in which survivor grains are mostly in contact with each other. Note the development of intragranular fractures at the contact point among neighboring survivor grains. Red arrows indicate the position of the Main Slip Surface (MSS). (e) and (f) Thin sections obtained from hand specimens collected from the inner fault core. Evidence of condensed fabric and grain reorganization by means of pore collapse. (g) and (h) Inner fault core. Open fractures partially filled with a small amount of calcite cement spanning across fracture walls.

abrupt variation that resembles that of quartz in sandstone fractures (Laubach, 2003; Lander and Laubach, 2015) and dolomite in some dolostone fractures (Gale et al., 2010). Accordingly, although precipitation and nucleation kinetics for carbonate minerals remain poorly constrained, in the following discussion we will employ a model similar to that proposed by Lander and Laubach (2015), which considers the occurrence of different cement morphologies as dependent upon the ratio of cement growth to the fracture opening rate. On the basis of the abundance of carbonate host rock and cements in the fault rocks studied, we assume that cement supply was probably not a limiting factor. The results of cathodoluminescence analysis will also be discussed in light of the existing literature, which documents that both Mn2+ and Fe2+ can substitute Ca2+ within the crystal structure of carbonates and that their relative abundance can respectively activate or quench luminescence of crystals (Hiatt and Pufahl, 2014). Both Mn and Fe concentrations in carbonate minerals are affected by redox conditions, so

their relative amounts will be used to decipher the diagenetic environments of mineral precipitation (Meyers, 1974, 1978; Frank et al., 1982; Amieux, 1982; Cander et al., 1988; Cander, 1994; Vahrenkamp and Swart, 1994; Kyser et al., 2002). As oxygen concentrations drop from near-atmospheric levels at the surface, Mn is reduced to Mn2+ and readily incorporated into the diagenetic cements. A further oxygen drop during burial causes Fe reduction, so that both Mn2+ and Fe2+ may substitute Ca2+ in the crystal lattice. As a result, redox change may be reflected in the CL pattern of the crystal. In particular, orange wavelengths are predominantly activated by Mn2+ (Calderòn et al., 1984), whereas Fe2+ appears to be the most important luminescence quencher in carbonate minerals. Therefore, brightly luminescent cement overgrowths may indicate an abundance of Mn2+, whereas dark to nonluminescence may be caused by the presence of Fe2+. Of course, the results of cathodoluminescence analysis need to be taken with care because the relationship between Fe2+ concentrations and 69

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Fig. 9. BSE images of dolomite-rich fault rocks from the Madonna del Soccorso fault zone. Outer fault core microphotographs (a) and (b) show presence of a small amount of calcite cement among dolostone survivor grains. Inner fault core microphotographs (c) and (d) show presence of a small amount of calcite cement widespread within the dolomite matrix. Some survivor grains are characterized by intragranular fractures at the contact point between neighboring grains (blue arrows). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

luminescence is nonlinear (Machel et al., 1991; Habermann et al., 1998; Reed and Milliken, 2003).

carbonate matrix, which prevented contact among neighboring survivor grains, hampering their fracturing (Sammis et al., 1987; Sammis and King, 2007). Consequently, we invoke that physical compaction occurred in mostly the dolomite-rich, Ms-type fault rocks by means of pore collapse and grain reorganization (Fig. 8e and f, 10e and 10f).

6.1.1. Physical compaction Physical compaction takes place by means of different mechanisms such as grain sliding, grain crushing, microcrack propagation, and pore collapse (Coogan and Manus, 1975; Ricken, 1987; Renard et al., 2001; Chuhan et al., 2003; Flügel, 2009). In carbonates, these mechanisms are controlled mainly by grain size, grain sorting, cementation processes, and amount of clay (Flügel, 2009). Since pore collapse occurs mainly during the first increments of increasing confining pressure due to the closure of elongated pores and microfractures (Anselmetti et al., 1997; Baud et al., 2000; Vajdova et al., 2004; Jouniaux et al., 2006; Agosta et al., 2007), the overall physical compaction of carbonate rocks is considered as nonlinear in a stress-strain field (Renner and Rummel, 1996; Baud et al., 2000; Couvreur et al., 2001; Palchik and Hatzor, 2002; Eberli et al., 2003; Vajdova et al., 2004; Jouniaux et al., 2006; Vanorio et al., 2008). Within the carbonate fault rocks studied, physical compaction resulted in the formation of a peculiar texture, the condensed fabric (sensu Logan and Semeniuk, 1976), which is characterized by the rearrangement and closer packing of the grains, causing a reduction of the bulk rock volume. We document a more developed condensed fabric throughout the dolostone-hosted fault cores and in the outer fault core of the mixed dolostone/limestone-hosted fault zone. These results are consistent with grain crushing, fracturing, reorganization, and minor pore collapse in the Gs-type fault rocks (Figs. 8c and d, 9b, 10c and 10d, 11a to 11d, 12c and 12d), in which the rare carbonate matrix preserved its microporosity (Coogan and Manus, 1975; Logan and Semeniuk, 1976). Differently, grain fracturing and crushing are uncommon within the Ms-type fault rocks due to the high amount of fine-grained

6.1.2. Chemical compaction Pressure Solution (PS) is the main mechanism for chemical compaction of carbonate rocks (Weyl, 1959; Bathurst, 1971; Tada and Siever, 1989; Railsback and Andrews, 1995; Amrouch et al., 2010; Tavani et al., 2015). PS is a physical/chemical, water-assisted process controlled by the stress-dependent solubility of minerals, which causes dissolution, diffusion, and reprecipitation of the solute material in lowstress regions (Weyl, 1959; Raj, 1982; Rutter, 1983; Tada and Siever, 1989; Lehner, 1990; Gratier et al., 2013, 2015; Croizé et al., 2013; Toussaint et al., 2018). Precipitation takes place within pores and open fractures, which are usually located close to the dissolution areas (Bathurst, 1971; Rutter, 1983; Carrio-Schaffhauser et al., 1990; Renard and Ortoleva, 1997; Agosta and Kirschner, 2003; Agosta et al., 2009, 2012; Croizé et al., 2013). The slowest processes among dissolution, diffusion, and reprecipitation control the overall rate of deformation (Rutter, 1983; Croizé et al., 2013). PS also plays a part in the development of а condensed fabric (sensu Logan and Semeniuk, 1976) by determining grain interpenetration due to μm-thick, mm-long microstylolites from slightly sutured to curved to planar (Tucker and Wright, 1990). We observe that grain interpenetration is widely documented in all the limestone-hosted fault cores and in the inner fault core of the mixed dolomite/calcite-rich fault zone, whereas it is virtually absent in the dolostone-hosted fault cores. Grain interpenetration is most common within Gs-type fault rocks (Fig. 4c and d, 6c and 6d). Differently, microstylolites crosscutting both grains and the fine-grained 70

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Fig. 10. (a) Cross-sectional view of the Vetrice fault scarp. (b) XRD data show the average of the intensity (counts per second, cps) measured for powder samples derived both from the fault core domain (inner and outer) and from the surrounding carbonate host rock. (c) and (d) Thin sections obtained from hand specimens collected from the outer fault core. Condensed fabric made up of angular and poorly sorted dolostone survivor grains characterized by intragranular fracturing (blue arrows) developed at the contact point among survivor grains, embedded in a small amount of porous dolomite matrix. Red arrows indicate the position of the Main Slip Surface (MSS). (e) and (f) Thin sections obtained from hand specimens collected from the inner fault core. Evidence of condensed fabric and grain reorganization by means of pore collapse. (g) and (h) Inner fault core. Open fractures and small intragranular fractures are shown. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

carbonate matrix are widespread in both Ms-type and Fg-type fault rocks (Fig. 4e and f, 6e and 6f, 12e). Occurrence of the aforementioned dissolution features was likely controlled by the shape, size, and sorting of the survivor grains (Rutter, 1976; Raj, 1982; Niemeijer et al., 2002, 2009; Gratier et al., 2009, 2013). All of them are actually known to influence the kinetics of dissolution reactions and the diffusion rates along grain boundaries. Since the driving force for pressure solution is the chemical potential gradient between the highly stressed grain contact points and the free faces of grains (Weyl, 1959; Raj, 1982; Rutter, 1983; Tada and Siever, 1989; Lehner, 1990; Gundersen et al., 2002), the high number of contact points in Gs-type fault rocks likely caused the pervasiveness of grain interpenetration. On the other hand, in order to explain the localization of microstylolites around smaller grains and within the fine-grained

carbonate matrix, we consider grain size as a major controlling factor (Weyl, 1959; Rutter, 1983; Tada and Siever, 1989; Lehner, 1990) because chemical compaction is controlled by either diffusion rate, proportional to 1/r3 (r: radius of the grain; Rutter, 1976; Tada and Siever, 1986), or by dissolution/precipitation processes, proportional to 1/r (Raj, 1982; Tada and Siever, 1989). 6.1.3. Cementation Cements are chemical precipitates whose composition is controlled by both chemical signature and flow rate of the mineralizing fluids, and by temperature (Flügel, 2009). In the carbonate fault rocks studied, multiple textures, such as fibrous cements, cement rinds, and euhedral cements are documented. Fibrous cements are present at Marsicovetere, where their amount is inversely proportional to the gap size, which 71

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Fig. 11. BSE images of dolomite-rich fault rocks from the Vetrice fault zone. Inner fault core microphotographs (a)–(d) highlight the absence of calcite cements and show the presence of both intragranular and intergranular pores and open fractures.

open pore space and/or within open fractures (Figs. 4g and h, 5c to 5f, 6g and 6h, 7d to 7f). In SEM-CL, these euhedral cements are zoned, as outlined by bright-to dark-luminescent bands (Figs. 5c and 7f). Their precipitation therefore occurred in an open hydraulic system, which allowed for fluctuations of both pore fluid composition and oxygen levels (Hiatt and Pufahl, 2014). For this reason, we infer that cements characterized by crystals with a sharply pointed shape likely precipitated in a vadose diagenetic environment (Longman, 1980; Moore and Druckman, 1981; Adams et al., 1984; Tucker and Wright, 1990; Flügel, 2009). Dark zones may have formed during dry periods characterized by high oxygen levels, whereas bright crystal zones could have formed in wet periods characterized by low oxygen levels. A similar conclusion is proposed for the blocky calcite documented at Roccacasale, where precipitation either occurred during deformation events, when calcite cementation rates exceeded fracture opening or grain separation rates, or during interseismic periods in the presence of fluctuating fluids, when cement precipitation occurred relatively slow. Cements are virtually absent in the dolomite-hosted fault zones due to the high chemical stability of dolomite, which does not allow its dissolution at shallow crustal depths (Bathurst, 1971). However, a small amount of calcite cement was documented along the main slip surfaces of the Madonna del Soccorso fault core and in the surrounding matrixsupported fault rocks (Figs. 8g and h, 9a to 9d). Precipitation of these cements could be ascribed to the juxtaposition of Triassic dolostones against Quaternary carbonate breccia (Cavalcante et al., 2009), which likely affected the chemical composition of the fault fluids.

means that small gaps are more pervasively cemented than the larger ones (Fig. 5e and f). Considering the aforementioned model (cf. Lander and Laubach, 2015), such an inverse correlation is consistent with the precipitation of fibrous cements during deformation events at rates similar to the slowest opening rates (Bons, 2001; Hilgers et al., 2001; Hilgers and Urai, 2002; Gale et al., 2010; Ukar and Laubach, 2016). Furthermore, the orientation of individual calcite crystals most likely tracked the direction of the displacement vector (Fig. 5e), as is widely documented in various lithologies (Durney and Ramsay, 1973; Ramsay, 1980; Cox and Etheridge, 1983; Cox, 1987; Passchier and Trouw, 1996; Bons, 2001; Zhang and Adams, 2002; Hilgers and Urai, 2002; Bons and Bons, 2003; Hilgers et al., 2004; Nollet et al., 2005; Okamoto and Sekine, 2011; Ukar et al., 2017). Since these fibrous cements are lightluminescent, we also infer that they precipitated in a shallow-phreatic environment at not very low oxygen levels to allow more Mn than Fe to substitute Ca in the crystal lattice (Longman, 1980; Moore and Druckman, 1981; Adams et al., 1984; Tucker and Wright, 1990; Flügel, 2009). Micron-sized cement rinds, concentric and isopachous with respect to the irregular edges of single grain boundaries, were documented at Venere-Gioia dei Marsi (Fig. 7a and b, 7e and 7f). We interpret them either as micritic envelopes — very common in carbonate rocks (Tucker and Wright, 1990; Flügel, 2009) — or as dark-luminescent cements that precipitated at very low oxygen levels which allowed Fe to substitute Ca (Hiatt and Pufahl, 2014) in a deep, stagnant phreatic environment (Longman, 1980; Moore and Druckman, 1981; Adams et al., 1984; Tucker and Wright, 1990; Flügel, 2009). Based on its distribution throughout the entire Venere-Gioia dei Marsi fault core (Fig. 5a, e and 5f), this microcrystalline calcite is assessed as a relic of the first generation of precipitated cements. Both cement rinds and fibrous cements of the Venere-Gioia dei Marsi and Marsicovetere fault zones, respectively, were overgrown by calcite crystals with dogtooth shapes, indicating that they grew into an

6.2. Conceptual model of structural diagenesis Based on their deformation mechanisms (Ferraro et al., 2018) and both mineralogical and δ13C and δ18O compositions (Ghisetti et al., 2001; Agosta and Kirschner, 2003; Agosta et al., 2008; Smeraglia et al., 2016b), five main diagenetic stages are assessed for the fault rocks 72

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Fig. 12. (a) Field photograph of the Roccacasale fault scarp. (b) XRD patterns showing the average of the intensity (counts per second, cps) measured for powder samples from the fault core domain (inner and outer) and from the surrounding carbonate host rock. (c) and (d) Thin sections obtained from hand specimens collected from the outer fault core. Condensed fabric made up of angular and poorly sorted dolostone survivor grains characterized by intragranular fracturing (blue arrows), embedded in a small amount of porous dolomite matrix. Red arrows indicate the position of the Main Slip Surface (MSS). (e), (f) and (g) Thin sections obtained from hand specimens collected from the inner fault core. Microstylolites oriented parallel to the main slip surfaces. Microcrystalline and granular to blocky cement filling pores. (h) Inner fault core. Alternations of brownish and yellowish bands, which resemble fluidization textures. These bands have sharp borders and are subparallel to the main slip surfaces. Dissolution preferentially localized at the boundaries between the different bands (pink arrows). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

studied (Fig. 14). Structural diagenesis started at depths ≤1.5 km (Stage A), at the inception of exhumation (Vezzani et al., 2010, and references therein) and before the onset of cataclasis (Ferraro et al., 2018). At those depths, pervasive rock fracturing was likely predominant within the incipient fault zones, forming a fragmented host rock (Fig. 14a) in which calcite cements precipitated within individual fractures from either groundwater-derived (Agosta and Kirschner, 2003; Agosta et al., 2008) and/or marine-derived fault fluids (Smeraglia et al., 2016b). At similar depths (Fig. 14), deformation progressively localized forming incipient carbonate fault cores (Stage B). Structural diagenesis of Gs-type fault rocks determined dissolution-related grain boundaries (Fig. 7a–f, and 13c) and precipitation of isopachous, dark cement rinds (Fig. 7a). Cement precipitation likely occurred in a stagnant phreatic environment in which precipitation rates were considerably slow

(Tucker and Wright, 1990; Flügel, 2009). The narrow, isopachous cement veneers precipitated around survivor grains and pores, and did not completely fill the intergranular spaces (Fig. 7a and b). Within an extending upper continental crust, physical compaction was likely dominant within the dolomite-rich fault rocks (Fig. 8c and d, 9b, 10c and 10d, 11a and 11b) during interseismic periods (Sibson, 2000), whereas chemical compaction due to grain interpenetration occurred mainly in the calcite-rich fault rocks (Fig. 4c and d, 6c and 6d). With increasing strain and exhumation from shallow depths (Fig. 14), chipping became the dominant micromechanism forming Ms-type fault rocks (Stage C). The reworking of both isopachous, dark cement rinds and interpenetrated grains was coeval with precipitation of fibrous, lightluminescent cements (Fig. 5e and f). This precipitation occurred in a shallow phreatic zone (Tucker and Wright, 1990; Flügel, 2009) during deformation events, whereas chemical compaction of calcite-rich fault 73

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Fig. 13. SEM images of outer and inner fault rocks from the Roccacasale Fault. (a) and (b) BSE images of multiple injection features characterized by coseismic thermal decomposition of calcite. Generally, limestone survivor grains are smaller and rounder than the dolostone survivor grains. (c) Panchromatic SEM-CL image of outer core dolomite survivor grains with irregular edges and embayments, indicating dissolution. (d) Panchromatic SEM-CL image of dolostone survivor grains embedded in calcite cement that shows patchy zoning within the inner core. (e) Panchromatic SEM-CL image of Fg-type texture in the inner core composed mainly of very fine calcitic grains (matrix) and a few large calcitic and dolomitic survivor grains. The amount of cement in between grains is negligible.

rocks (Fig. 4e and f, 6e and 6f, 12e) and physical compaction of dolomiterich fault rocks (Fig. 8e and f, 9c and 9d, 10e and 10f, 11c and 11d) took place during interseismic periods (Sibson, 2000, and references therein). A contrasting diagenetic evolution is envisioned for the mixed dolomite/ calcite-rich fault rocks. In fact, while physical compaction happened in the Gs-type fault rocks (Fig. 12c and d), chemical compaction localized in the Ms-type fault rocks (Fig. 12e). Stage D was characterized by slip localization within the inner fault cores (Fig. 14) by means of chipping, shear fracturing, and thermal decomposition of carbonate minerals (Ferraro et al., 2018). The latter mechanism formed vertically persistent, cm-thick, Fg-type fault rocks in which concurrent physical and chemical processes caused dissolution and milling down of both carbonate survivor grains and preexisting diagenetic phases (Fig. 13e). The small veins and microfractures crosscutting Fg-type fault rocks (Fig. 12h) formed during transients of coseismic dilatancy, which likely redistributed the overpressurized, ultrafine, fluidized granular material (Fig. 13a and b) immediately after coseismic rupture (Sibson, 1985, 1986). Physical compaction of dolomite-rich fault rocks occurred during interseismic periods (Fig. 12f and g), whereas precipitation of euhedral and blocky calcite cements took place in calcite-rich fault rocks either during deformation events or

during interseismic periods (Fig. 4g, h, 5d, 6g, 6h, 7d, 13c, and 13d). Results of CL analyses suggest that calcite precipitation occurred from meteoric-derived fault fluids (Agosta and Kirschner, 2003; Agosta et al., 2008; Smeraglia et al., 2016b), which were characterized by fluctuations in oxygen levels typical of a vadose environment (Longman, 1980; Moore and Druckman, 1981; Flügel, 2009). Stage E is associated with the exposure of the fault rocks studied at the surface (Fig. 14). Brecciation and rock pulverization took place during coseimic slip at low confining pressures (Agosta and Aydin, 2006; Dor et al., 2006; Doan and Gary, 2009; Mitchell et al., 2011; Doan and d'Hour, 2012; Fondriest et al., 2015; Aben et al., 2016; Demurtas et al., 2016). Opening-mode fractures mainly formed parallel to the main slip surfaces (Agosta and Aydin, 2006; Aydin et al., 2010), causing a pronounced dilatancy of the fault zones. Due to weathering, both calcite- and dolomite/calcite-rich fault rocks were affected by selective dissolution (Fig. 12g), which occurred during interseismic periods (Wang, 1997; Lucia, 1999; Renard et al., 2000; Gratier et al., 2003; Lønøy, 2006; Zhang et al., 2010; Smith et al., 2011; Bauer et al., 2016). Perhaps further precipitation of euhedral calcite cements also occurred during interseismic periods (Longman, 1980; Moore and Druckman, 1981; Adams et al., 1984; Flügel, 2009). 74

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Fig. 14. Cartoon showing the diagenetic evolution of the shallow portions of carbonate normal fault zones (see text for details).

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7. Conclusions

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Oblique normal faulting along the northern edge of the Majella anticline, central Italy: inferences on hydrocarbon migration and accumulation. J. Struct. Geol. 32, 1317–1333. Agosta, F., Ruano, P., Rustichelli, A., Tondi, E., Galindo-Zaldívar, J., De Galdeano, C.S., 2012. Inner structure and deformation mechanisms of normal faults in conglomerates and carbonate grainstones (Granada Basin, Betic Cordillera, Spain): inferences on fault permeability. J. Struct. Geol. 45, 4–20. Allegré, C.J., Le Mouel, J.L., Provost, A., 1982. Scaling rules in rock fracture and possible implications for earthquake predictions. Nature 297, 47–49. Amieux, P., 1982. La cathodoluminescence: méthode d’étude sédimentologique des carbonates. Bull. Cent. Rech. Explor.-Prod. Elf-Aquitaine 6, 437–483. Amrouch, K., Robion, P., Callot, J.P., Lacombe, O., Daniel, J.M., Bellahsen, N., Faure, J.L., 2010. Constraints on deformation mechanisms during folding provided by rock physical properties: a case study at Sheep Mountain anticline (Wyoming, USA). Geophys. J. Int. 182, 1105–1123. Andreo, B., Vías, J., Durán, J.J., Jiménez, P., López-Geta, J.A., Carrasco, F., 2008. Methodology for groundwater recharge assessment in carbonate aquifers: application to pilot sites in southern Spain. Hydrogeol. J. 16, 911–925. Anselmetti, F.S., von Salk, G.A., Cunningham, K.J., Eberli, G.P., 1997. Acoustic properties of Neogene carbonates and siliciclastics from the subsurface of the Florida Keys: implications for seismic reflectivity. Mar. Geol. 144, 9–31. Atkinson, B.K., 1982. Subcritical crack propagation in rocks: theory, experimental results and applications. J. Struct. Geol. 4, 41–56. Aydin, A., Antonellini, M., Tondi, E., Agosta, F., 2010. Deformation along the leading edge of the Maiella thrust sheet in central Italy. J. Struct. Geol. 32, 1291–1304. Bastesen, E., Braathen, A., Nøttveit, H., Gabrielsen, R.H., Skar, T., 2009. Extensional fault cores in micritic carbonates, a case study from Gulf of Corinth, Greece. J. Struct. Geol. 31, 403–420. Bathurst, R.G.C., 1971. Carbonate Sediments and Their Diagenesis. Elsevier, AmsterdamOxford-New York, pp. 657. Baud, P., Schubnel, A., Wong, T.F., 2000. Dilatancy, compaction, and failure mode in Solnhofen limestone. J. Geophys. Res. B Solid Earth Planets 105, 19289–19303. Bauer, H., Schröckenfuchs, T.C., Decker, K., 2016. Hydrogeological properties of fault zones in a karstified carbonate aquifer (Northern Calcareous Alps, Austria). Hydrogeol. J. 24, 1147–1170. Biegel, R.L., Sammis, C.G., Dieterich, J.H., 1989. The frictional properties of a simulated gouge having a fractal particle distribution. J. Struct. Geol. 11, 827–846. Billi, A., Salvini, F., Storti, F., 2003. The damage zone-fault core transition in carbonate rocks: implications for fault growth, structure and permeability. J. 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We presented a combined field and laboratory structural investigation of carbonate fault rocks pertaining to NW–SE-striking, highangle, extensional fault zones of the central-southern Apennines, Italy. This work was aimed at deciphering possible differences in the diagenetic evolution of calcite- and dolomite-rich fault rocks. The five fault zones studied are characterized by throws ranging between a few 10s and several 100s of meters and were exhumed from shallow crustal depths (< 1.5 km) during Plio-Quaternary downfaulting of this part of the Apennines. They all crosscut Mesozoic platform carbonates. Despite their different amounts of throw, lithologies, and inherited structural fabric of the protoliths, all carbonate fault cores studied are comprised of cataclastic fault rocks with comparable textures and multiscale dimensional properties of survivor grains. However, the results of this work highlight how their diagenetic evolution was controlled mainly by the nature of the protolith and by the phreatic/vadose hydrogeological environments in which they occurred. Physical compaction was predominant in the dolomite-rich fault rocks and within the outer dolomite/calcite-rich fault core. There, pore collapse, grain reorganization, crushing, and fracturing formed a condensed fabric. In contrast, chemical compaction was dominant in the calcite-rich fault rocks and in the inner fault core of the dolomite/calciterich fault zone. Grain interpenetration and microstylolites cutting across survivor grains and fine-grained carbonate matrix were widespread in Gs-type and Ms-type fault rocks, respectively. Cementation was widespread within the limestone-hosted fault cores and in the inner fault core of the mixed dolomite/calcite-rich fault. On the contrary, it was virtually absent in dolostone-hosted fault zones. In the Marsicovetere fault zone, the first generation of preserved cement consists of light-luminescent, fibrous calcite crystals, which display face-normal growth when gap sizes are small. Microstructural evidence is consistent with the precipitation of the fibrous cements in a shallow-phreatic environment. Differently, the first preserved cements of the Venere-Gioia dei Marsi fault zone consist of thin, isopachous, dark-luminescent calcite rinds, which coat the survivor grains. Microstructural evidence is consistent with precipitation of the isopachous cements in a deep, stagnant phreatic zone characterized by very low oxygen levels. In both cases, these early cements are overgrown by intact, zoned calcite crystals with euhedral terminations. Such a morphology indicates that crystals grew into an open pore space and/or within open fractures in a vadose diagenetic environment. At Roccacasale, the massive, blocky calcite cements likely formed either during deformation events, which is consistent with calcite precipitation rates exceeding fracture opening or grain separation rates, or during the interseismic periods in the presence of fluctuating fluids, when cement precipitation was relatively slow. Acknowledgements The present contribution is part of the first author's PhD research work. The editor, C.W. Passchier, E. Michie, an anonymous reviewer, and S.E. Laubach are warmly acknowledged for the detailed comments and suggestions provided on the manuscript. Alessandro Laurita is acknowledged for the help provided during SEM analysis and Sara Elliott for assistance with CL image processing. William Rader is acknowledged for the careful English grammar and syntax revision. This work was funded by the Italian Ministry of Education, Universities and Research (MIUR), the Reservoir Characterization Project (www. rechproject.com), a consortium of universities and energy companies, and the Fracture Research and Application Consortium (FRAC) Industrial Associates at The University of Texas at Austin. References Aben, F.M., Doan, M.L., Mitchell, T.M., Toussaint, R., Reuschle, T., Fondriest, M., Gratier, J.P., Renard, F., 2016. Dynamic fracturing by successive coseismic loadings leads to

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