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Understanding the factors controlling fracture frequency distribution can greatly improve the assessment of fluid circulation in fault damage zones, with evident implications for fault mechanics, hydrogeology and hydrocarbon exploration. This is particularly important for relay zones that are usually characterized by strong damage and structural complexity. We investigated the fracture frequency within an outcrop adjacent to the front fault segment of a relay ramp, hosted within peritidal carbonates that forms part of the Tre Monti fault (Central Italy). We analysed the distribution of fracture frequency in the outcrop through (1) scanlines measured in the field, (2) oriented rock samples, and (3) scan-areas performed on a virtual outcrop model. Fracture frequency increases with distance from the front segment of the relay ramp. Moreover, supratidal and intertidal carbonate facies exhibit higher fracture frequency than subtidal limestones. This trend of increased fracture frequency has two main explanations. (1) The number of subsidiary faults and their associated damage zones increases moving away from the front segment. (2) the supratidal and intertidal carbonate facies content increases toward the centre of the relay ramp. Our results indicate that the fracture frequency pattern is very complex in relay ramps hosted in shallow-water limestones and that its prediction necessitates a good control on structures and sedimentary facies distribution.
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Lithological and structural control on fracture frequency distribution within a carbonate-
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hosted relay ramp
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Marco Mercuria, Eugenio Carminatia, Maria Chiara Tartarelloa, Marco Brandanoa, Paolo Mazzantia,b,
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Alessandro Brunettib, Ken J. W. McCaffreyc, and Cristiano Collettinia
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Affiliation addresses:
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a Dipartimento di Scienze della Terra, Sapienza Università di Roma, Piazzale Aldo Moro 5, 00185,
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Rome, Italy
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b NHAZCA S.r.l., spin-off company Sapienza Università di Roma, Via Vittorio Bachelet 12, 00185
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Rome, Italy
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c Earth Sciences Department, Durham University, South Road, Durham, DH1 3LE, UK
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E-mail addresses: marco.mercuri@uniroma1.it (M. Mercuri), eugenio.carminati@uniroma1.it
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(E.Carminati), mariachiara.tartarello@uniroma1.it (M. C. Tartarello), marco.brandano@uniroma1.it
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(M. Brandano), paolo.mazzanti@uniroma1.it (P. Mazzanti), alessandro.brunetti@nhazca.com (A.
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Brunetti), k.j.w.mccaffrey@durham.ac.uk (K. J. W. McCaffrey), cristiano.collettini@uniroma1.it (C.
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Collettini).
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*corresponding author: tel. +393342844933; e-mail: marco.mercuri@uniroma1.it; postal address:
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Dipartimento di Scienze della Terra, Sapienza Università di Roma, Piazzale Aldo Moro 5, 00185
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Rome, Italy
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keywords: fractures; virtual outcrop; FracPaQ; carbonate facies; relay ramp
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Abstract
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Understanding the factors controlling fracture frequency distribution can greatly improve the
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assessment of fluid circulation in fault damage zones, with evident implications for fault mechanics,
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hydrogeology and hydrocarbon exploration. This is particularly important for relay zones that are
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usually characterized by strong damage and structural complexity. We investigated the fracture
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frequency within an outcrop adjacent to the front fault segment of a relay ramp, hosted within peritidal
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carbonates that forms part of the Tre Monti fault (Central Italy). We analysed the distribution of
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fracture frequency in the outcrop through (1) scanlines measured in the field, (2) oriented rock
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samples, and (3) scan-areas performed on a virtual outcrop model. Fracture frequency increases with
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distance from the front segment of the relay ramp. Moreover, supratidal and intertidal carbonate facies
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exhibit higher fracture frequency than subtidal limestones. This trend of increased fracture frequency
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has two main explanations. (1) The number of subsidiary faults and their associated damage zones
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increases moving away from the front segment. (2) the supratidal and intertidal carbonate facies
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content increases toward the centre of the relay ramp. Our results indicate that the fracture frequency
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pattern is very complex in relay ramps hosted in shallow-water limestones and that its prediction
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necessitates a good control on structures and sedimentary facies distribution.
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1. Introduction
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Fractures in the damage zone (Chester and Logan, 1986; Chester et al., 1993) constitute the main
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pathway for fluids within faults hosted in low-porosity rocks (Caine et al., 1996; Aydin, 2000;
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Gudmundsson et al., 2001; Bense et al., 2013; Bigi et al., 2013). Fracture frequency and the variation
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of geometrical and topological properties of fracturing in space are an important control on
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permeability, and hence on fluid flow and fault mechanics. For example, these variations in these
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attributes may control traps and leakage points within hydrocarbon reservoirs affected by the presence
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of faults and promote or prevent local fluid overpressures. A poorly connected fracture system might
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lead to the development of high fluid pressures, which can in turn influence the evolution of the stress
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state (Sibson, 1994) with profound implications for earthquake triggering (e.g., Nur and Booker,
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1972; Miller et al., 2004). Conversely, a well-connected fracture system prevents the development of
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fluid overpressures and this leads to the maintenance of a strong but critically stressed crust (Townend
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and Zoback, 2000). Furthermore, fracture distribution can have a direct effect on fault mechanics: a
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change of elastic properties of host rock promoted by fracturing may lead to a stress field rotation
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within damage zone, allowing reactivation of unfavourably orientated faults (Faulkner et al., 2006).
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Characterization of fracture distribution and its controlling factors is therefore fundamental to better
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understand fluid circulation and mechanics of fault zones, with obvious consequences for
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hydrogeology and hydrocarbon exploration. Assessing fracture distribution is particularly relevant
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for relay ramps (and generally, for zones of faults interaction) as they are commonly characterized
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by stronger damage than isolated fault segments (Kim et al., 2004; Peacock et al., 2017) and by high
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structural complexity (Kattenhorn et al., 2000; Peacock et al., 2000; Peacock and Parfitt, 2002; Fossen
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et al., 2005; Ciftci & Bozkurt, 2007; Bastesen and Rotevatn, 2012; Peacock et al., 2017), with
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important consequences for fluid flow (Sibson, 1996; Rotevatn et al., 2007; Fossen and Rotevatn,
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2016 and references therein).
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Here we integrate classical and modern structural geology techniques to investigate the fracture
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frequency distribution and its controlling factors within a well-exposed portion of a carbonate-hosted
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relay ramp damage zone that is part of the Tre Monti fault, a normal fault in the Central Apennines
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of Italy. We observe that lithology (carbonate facies) and the distribution of secondary faults
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accompanying relay ramp development play an important role in the fracture density.
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1.1. Factors controlling fracture distribution within fault zones.
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Many field and laboratory studies have been carried out to investigate factors controlling fracture
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distribution within fault zones. A first factor is represented by distance from the main fault: both
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microfracture and fracture density generally increase moving toward fault core (Brock and Engelder,
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1977; Wilson et al., 2003; Faulkner et al., 2006; Mitchell and Faulkner, 2009). However, fracture
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intensity does not scale with displacement accommodated by the main fault (Anders and Wiltschko,
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1994; Shipton and Cowie, 2003). This has been attributed to the existence of a critical value of
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deformation intensity marking the transition from a strain hardening to a strain softening behaviour
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induced by the development of slip surfaces (Shipton and Cowie, 2003). Instead, higher
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displacements accommodated by faults lead to an increase in the damage zone thickness until a
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critical width is reached (Shipton and Cowie, 2001, 2003; Mitchell and Faulkner, 2009; Savage and
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Brodsky, 2011). This can be attributed to a continuous development of subsidiary faults producing
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their own damage zone (Shipton and Cowie, 2003). Other fault-related factors that influence
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distribution and the geometrical/topological properties of fractures are related to the stress field. For
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example, the asymmetric pattern of the stress field occurring during the long-term propagation of a
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fault (Berg and Skar, 2005), and rupture directivity during earthquakes (Dor et al., 2006a, 2006b;
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Mitchell et al., 2011) may produce an asymmetric damage distribution between hangingwall and
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footwall, whilst development of local stresses may promote a deflection of fractures (Gudmundsson
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et al., 2010).
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Lithology plays another important role in fracture frequency distribution. A stratigraphic or tectonic
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juxtaposition of different lithologies leads to contrasts in mechanical properties (e.g., brittleness;
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Peacock and Xing, 1994) causing a mechanical layering that influences deformation pattern (Tavani
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et al., 2008), and fracture spacing, propagation and arrest (Odling et al., 1999; McGinnis et al., 2017).
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In general, fractures tend to form in more brittle layers and they often arrest at interfaces where
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mechanical contrasts are present (e.g., bedding). For carbonate lithologies, even a variation in
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carbonate facies at metric to decametric scale can affect fracturing (Wennberg et al., 2006; De Paola
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et al., 2008; Larsen et al., 2010a, 2010b; Michie et al., 2014; Rustichelli et al., 2016; Volatili et al.,
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2019). For example, Rustichelli et al. (2016) observed higher fracture intensity, trace length and
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connectivity in platform compared to ramp carbonates, whilst Larsen and co-authors (2010a, b) found
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that fractures forming in the subtidal facies tend to arrest in proximity to intertidal laminated
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limestones. Finally, thickness of sedimentary beds can influence fracturing: a widely observed
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relationship is that, for strata-bound fractures, fracture intensity is inversely proportional to bed
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thickness (Ladeira and Price, 1981; Pollard and Aydin, 1988; Huang and Angelier, 1989; Narr and
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Suppe, 1991; Wu and Pollard, 1995; Bai and Pollard, 2000).
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1.2. The structure-from-motion algorithm to build virtual outcrops
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In this study we integrate classical field techniques (i.e., scanlines; Wu and Pollard, 1995) and a
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virtual outcrop (Bellian et al., 2005; McCaffrey et al., 2005a, 2005b) to investigate fracture frequency
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distribution and its controlling factors in a relay ramp system formed in carbonate host rocks.
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In the last decade, virtual outcrops have been extensively used in structural geology (Bemis et al.,
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2014; Telling et al., 2017 for a review), and in particular for studies dealing with fractures (Olariu et
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al., 2008; Vasuki et al., 2014; Pless et al., 2015; Casini et al., 2016; Seers and Hodgetts, 2016;
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Corradetti et al., 2017; Bonali et al., 2019 and many others). The employment of virtual outcrops in
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geology has increased our ability and efficiency to collect data, allowing the collection of high-
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precision georeferenced datasets, also from inaccessible portions of the outcrop (Bellian et al., 2005;
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McCaffrey et al., 2005a, 2005b). An increasingly adopted methodology to build virtual outcrops is
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represented by the structure-from-motion technique (Westoby et al., 2012; Bemis et al., 2014;
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Colomina and Molina, 2014; Tavani et al., 2014; Vasuki et al., 2014; Bistacchi et al., 2015; Bonali et
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al., 2019), because it has a higher efficiency to cost ratio than other techniques such as laser scanning
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(LiDAR) (Wilkinson et al., 2016; Cawood et al., 2017). The structure-from-motion algorithm exploits
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a series of overlapping photos taken from various positions by a person or a drone (UAV, Unmanned
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Aerial Vehicle) to build a 3D model of the scene (Bemis et al., 2014). The model can be sized and
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georeferenced using the knowledge of the geographic position of some objects (i.e., ground control
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points) in the scene (Bemis et al., 2014). For this study, the employment of a virtual outcrop allowed
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us to accurately map the fracture distribution in our study outcrop.
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2. Geological setting
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2.1. The central Apennines tectonic framework
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The central Apennines are an active NE to ENE verging fold-and-thrust belt that started to form in
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the late-Oligocene in response to the westward directed subduction of the Adria plate beneath the
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European plate (Doglioni, 1991; Carminati et al., 2010). Thrusting scraped-off and piled up the
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sedimentary sequence overlying the continental basement of Adria, including a shallow- to deep-
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water Upper Triassic to Middle Miocene carbonate succession (Cosentino et al., 2010 and references
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therein). Since the Early Pliocene, NE-SW oriented extension started to act in the Central Apennines
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to the west of the compressive front, in response to the opening of the Tyhrrenian back-arc basin
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(Doglioni, 1991). The compressive-extensional couple has continuously migrated to the northeast
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(Cavinato and De Celles, 1999). Extension is currently active in the Central Apennines (D’Agostino
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et al., 2001a; Devoti et al., 2010) and is accommodated by normal faults striking mainly NW-SE,
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although rare SW-NE trending fault, such as the Tre Monti fault are present (Fig. 1a). These faults
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cut through both the pre-orogenic carbonates and the syn-orogenic flysch deposits (Fig. 1a), and their
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activity is manifest in the numerous earthquakes that have affected Italy in the recent past, such as
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the L’Aquila 2009 (Chiaraluce, 2012 and references therein), and the 2016-17 central Italy seismic
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sequences (Chiaraluce et al., 2017; Scognamiglio et al., 2018). The exhumation associated with the
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uplift that accompanies the extensional tectonic regime (D’Agostino et al., 2001b; Devoti et al., 2010)
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has exposed formerly buried active normal faults that now usually constitute the borders of the
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intermountain basins. The Tre Monti fault forms the north-west borders of the Fucino intermontane
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basin (Fig. 1a). In the Fucino basin, thrusting occurred from the Late Miocene to Early Pliocene,
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whilst the extensional tectonics started in the Late Pliocene and is still ongoing, as testified by the
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1915 Avezzano earthquake (e.g., Galadini and Galli, 1999).
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2.2. The Tre Monti fault
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Tre Monti fault has been exhumed from a depth < 3 km (Smeraglia et al., 2016) and crops out as a
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series of right-stepping, SE dipping fault scarps for a length of ~ 7 km (Fig. 1b). The fault
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accommodates a throw that varies from ~ 0.7 km in the SW to ~ 2 km towards the NE (Smeraglia et
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al., 2016). The fault scarps juxtapose Early Cretaceous to Miocene carbonates in the footwall with
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Pliocene to Holocene continental deposits in the hangingwall (Fig. 1a, b). The predominance of dip
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slip slickenlines on the main fault scarps (Fig. 1b; See also Morewood and Roberts, 2000; Smeraglia
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et al., 2016; Mercuri et al., 2020) and paleoseismological investigations (Benedetti et al., 2013; Cowie
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et al., 2017) indicate that the Tre Monti fault has been active as a normal fault since the Pliocene,
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probably acting as a release fault (sensu Destro, 1995) for the San Potito – Celano fault (SPCF; Fig.
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1a). Finally, the Tre Monti fault has experienced past earthquakes, as suggested by microstructural
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studies of the fault core (Smith et al., 2011; Smeraglia et al., 2016, 2017).
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A key outcrop for the Tre Monti fault zone structure is provided by an abandoned quarry located ~ 2
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km WSW of Celano village (42°04’35’’N 13°30’00’’E; see also Fig. 1a, b). The quarry is located
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within a portion of a relay zone delimitated by two right-stepping segments on the main fault (zoom
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in Fig. 1b) and has been named “La Forchetta quarry” in previous studies (Smeraglia et al., 2016,
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2017, Mercuri et al., 2020). The quarry extends for ~ 200 m in a NE-SW direction and for ~ 100 m
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in the NW-SE direction (inset of Fig. 1b). The south-eastern limit of the quarry is marked by the front
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segment of the relay ramp (Fig. 1b-c), which dips (~55°) to the southeast (156° mean dip azimuth)
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(Smeraglia et al., 2016, Mercuri et al. 2020; see also the stereoplot in Fig. 1c). The slickenlines on
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the front segment indicate a right-transtensional to right-lateral kinematics (mean rake 155°; see
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stereoplot in Fig. 1c). The kinematics observed here may be due to a stress field rotation promoted
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by the interaction of the segments that border the relay zone (Mercuri et al., 2020).
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The fault damage zone is exposed in almost 360° perspective on the quarry walls (Fig. 1c) and is
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hosted by Lower Cretaceous limestones pertaining to the Calcari Ciclotemici a Gasteropodi e ooliti
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Formation (Centamore et al., 2006). They were deposited at the transition between tidal flat and
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lagoon carbonate platform environments (Fig. 2a) and are organized in metric-scale peritidal cycles
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(Fig. 2b), reflecting the variation of accommodation space (c.f., Osleger 1991; D’Argenio et al.,
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1997). The supratidal facies comprises light-gray to havana-brown poorly sorted grainstones with
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radial ooids and pisoids (Fig. 2e, h). The intertidal facies is defined by laminated, white coloured
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microbial bindstones with birdseyes and fenestrae (Fig. 2f, i). Finally, the subtidal facies is mainly
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composed of white packstones with peloids and oncoids (Fig. 2g, j), although some sporadic
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floatstones with gastropods and some oncoidal rudstones are present.
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The bedding organization is strongly controlled by the relative abundance of the carbonate facies
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mentioned above. Where the supratidal and the intertidal are the most abundant facies, the limestones
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are arranged in cm- to dm- scale tabular beds (Fig. 2c). Conversely, a predominance of the subtidal
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facies has beds that are more than 1 m thick (Fig. 2d).
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3. Methods
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In this section, we present the methodology employed to extract fracture properties from scanlines
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(section 3.1), samples (section 3.2), and the virtual outcrop (section 3.3).
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3.1. Scan-lines
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We performed 26 scan-line surveys (Priest and Hudson, 1981; see the example in Fig. 3) in the quarry
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area (see Section S1.1 for their location). Length, position, and orientation of the scan-lines were
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chosen in order to maximise their length and to maintain a sub-horizontal direction in irregular
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outcrops. The effective length and the orientation of each scanline is reported in Table S4.1. The
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effective length of the scan-line surveys was calculated by subtracting portions of outcrop hidden by
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vegetation from their total length. For each scanline survey we collected trace lengths and orientations
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of all the fractures (mostly joints, minor shear fractures, and rare veins) intersecting the measuring
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tape. For trace length analysis we considered only fractures having both the terminations visible
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(~94% of all collected fractures). Fracture orientation was investigated by producing contour plots
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with the software Stereonet (Allmendinger et al., 2012; Cardozo & Allmendinger, 2013). The
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contours account for the inhomogeneous sampling of fractures along a scanline depending on their
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orientation (Terzaghi, 1965). The Terzaghi correction (Terzaghi, 1965) was applied by inserting the
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trend and plunge of each scanline in the Stereonet software (Allmendinger et al., 2012; Cardozo &
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Allmendinger, 2013). When data from scanlines with similar orientations (coming from the same
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sector of the quarry) were plotted in a single stereoplot, we applied the Terzaghi correction by using
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the mean direction of the scanlines (e.g., Figure 6). We calculated the mean fracture spacing by
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dividing the effective length of the scanline for (N-1), where N is the number of fractures intercepted
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by the scan-line. The linear fracture frequency, or P10 (Sanderson and Nixon, 2015), was calculated
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as the reciprocal of the mean spacing (Fig. 3). Finally, we assigned a carbonate facies to each scanline
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through a visual inspection in the field (Table S4.1). Due to the nature of the quarry this was limited
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to the intertidal facies and supratidal facies only.
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3.2. Samples
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27 oriented hand-samples (Fig. 4a) were collected, mostly in the same locations as the scanlines
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(section S1.2). Oriented samples were cut along vertical sections striking ~ 155° N (i.e., parallel to
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the front fault segment dip), polished, and scanned at a 1200 dpi resolution. Fracture traces were
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digitized using a commercial vector graphic software (Fig. 4b). For each sample, we evaluated the
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fracture spacing, the linear and areal fracture frequency (P10 and P20 respectively; Sanderson and
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Nixon, 2015), and fracture intensity (P21; Sanderson and Nixon, 2015). The spacing and the linear
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fracture frequency (P10), were calculated by tracing a series of sub-parallel scanlines on each sample
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(Fig. 4c), and following the same procedure adopted for the “regular” scan-lines (section 3.1). The
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others fracture properties were extracted using the FracPaQ (v. 2.4) Matlab tool (Fig. 4d; Healy et al.,
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2017). This software takes a .svg file containing the polylines of fracture traces as input, and,
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according to the parameters inserted by the user, calculates the fracturing properties mentioned above.
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We refer the reader to the paper of (Healy et al., 2017) for a complete description of the algorithms
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used by the FracPaQ software. For each sample, we inserted the appropriate pixel/m ratio, in order to
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obtain the outputs in unit length (Healy et al., 2017). Furthermore, a carbonate facies was assigned to
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each sample by visual inspection. The lists of fracture parameters obtained are summarised in section
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S4.2.
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3.3. Fracture analysis on the virtual outcrop
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The photos used for the structure-from-motion algorithm were captured by an Unmanned Aerial
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Vehicle survey performed with an Aeromax X4 quadcopter equipped with a Sony Alpha 5000 camera
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(Fig. 5a). We collected 650 photos with an overlap of ~ 70% between adjacent pictures. The workflow
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we adopted to build the 3D model is very similar to that described by other authors (e.g., Tavani et
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al., 2014; Bistacchi et al., 2015; Bonali et al., 2019): photos were aligned through a semi-automatic
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identification of common points in adjacent pictures in order to create a point cloud. The point cloud
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is subsequently used to build a mesh and, finally, a textured mesh, that is the virtual outcrop (Fig.
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5b). The virtual outcrop was scaled and georeferenced with respect to a previous terrestrial laser-
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scanner survey (Mercuri et al. 2020). We constructed 6 ortho-mosaics (such as the one represented
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in Fig. 5c), with a resolution of 1 pixel per 1 cm, from the virtual outcrop, one for each quarry wall
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(labelled with capital letters in the inset in Fig. 6a). We subdivided each ortho-mosaic into several
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squares with 5 m side length, to form virtual scan-areas (Fig. 5c, d). The dimension of virtual scan-
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areas was established in order to have the side length bigger than most of the fracture trace lengths
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observed in scanlines (Fig. S2.1a). The location of all the virtual scan-areas is shown in Section S1.3.
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All the processing for the virtual outcrop and ortho-mosaic were executed within the 3DFlow Zephyr
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Aerial software. Each scan-area was manually interpreted in Adobe Illustrator® by drawing
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polylines, representing the traces of fractures, minor faults, and bedding (Fig. 5e), and polygons to
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map the supratidal and the intertidal facies (Fig. 5f). The supratidal and intertidal facies were
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recognized by the visible cm to dm thick beds. The fracture analysis was performed in FracPaQ, using
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the same parameters as described in the previous section, to evaluate the areal fracture frequency
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(P20), fracture intensity (P21), and trace length. We also evaluated the minimum content of supratidal
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and intertidal facies in each scan-area by calculating their area in pixel2 and dividing it by 250,000
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px2 (the scan-area). The fracture analysis results for each scan-area are reported in Section S4.3.
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Finally, we captured 420 aerial photos using a Phantom 4 Pro quadcopter. The photos were processed
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using the same procedure described above to produce an aerial orthophoto of the quarry. This
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orthophoto was used as base map to check the position of all the georeferenced data we collected.
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4. Results
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Fractures in the quarry are mainly joints and shear fractures. Calcite-filled veins are quite rare and, if
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present, can be appreciated only at the hand sample scale. Fractures are accompanied by at least 80
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minor faults with various orientations and kinematics (Fig. 6; see Mercuri et al., 2020 for further
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details). In the present study we distinguish the minor faults from the shear fractures by the presence
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of a fault core. Fractures exhibit a centimetre- to a meter-scale trace-length, with modal values
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between 10 and 50 cm (Section S2.1). The mean trace length calculated for each scanline is quite
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homogeneous throughout the whole quarry and generally smaller than 0.25 m (section S2.2). Virtual
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scan-areas suggest that the mean trace length is heterogeneous, with longer fractures located in the
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northern (trace lengths > 0.58 m) and in the western (0.46 m < trace length < 0.58 m) sectors of the
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quarry (S2.3). Most of the fractures are sub-vertical and E-W striking, while two minor clusters
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indicate the occurrence of sub-vertical fractures striking approximately NE-SW and NNW-SSE (Fig.
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6). We do not observe any systematic cross-cutting relationship between the different fracture sets.
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Although the entire quarry is characterized by high fracture frequency values, both scanlines and
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virtual scan-area show similar fracture frequency distribution patterns (Fig. 6). The portions of the
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quarry located immediately at the footwall of the front segment of the relay ramp are characterized
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by relatively low fracture frequency values (Fig. 6). On the SW side of the quarry (sectors E and F;
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see Fig. 6) the linear fracture frequency (P10) is lower than 25 m-1, reaching a value of 10 m-1 close
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to front segment (for the scanline SL13; see S1.2 and S4.1), whilst the areal fracture frequency values
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(P20) are lower than 27 m-2. The whole NE side of the quarry is characterized by relatively low
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fracture frequency values (Sector A in Fig. 6); in this sector the linear fracture frequency is generally
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lower than 28 m-1, although it locally reaches values higher than 38 m-1 near the front segment (for
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the scanline SL12; see S1.2 and S4.1). High linear fracture frequency values (P10 ³ 39 m-1) are also
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located far from the front segment (for scanlines SL21 and 22; see S1.2 and S4.1). The areal fracture
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frequency is always smaller than 34 m-2 in the NE sector of the quarry. The portions of the quarry
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located far from the front segment of the relay ramp (sectors B, C, D; see Fig. 6) are characterized by
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the highest fracture frequencies. In detail the sectors B and D show areal fracture frequencies reaching
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values larger than 48 m-2, up to 60 m-2 (Fig. 6, S4.3). Furthermore, the northern sector shows the
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highest concentration of minor faults, that are often associated with foliated breccias (Fig. 6). Breccias
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are characterized by anastomosing foliations, consisting of closely spaced undulated, striated slip
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surfaces, which are roughly parallel to the associated subsidiary faults (Fig. 7; see also Smeraglia et
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al., 2016). At hand-sample scale, the clasts are characterized by chaotic to crackle breccia textures
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(Woodcock and Mort, 2008; Smeraglia et al., 2016). The scan-area derived fracture intensity (P21)
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distribution mimics the distribution mentioned above (section S3.2).
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In Figure 8 we show the variation in fracture frequency with distance from the principal fault in the
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quarry (i.e., the front segment of the relay ramp). Despite the high variability in fracture frequency
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for each fixed distance from the front segment, we recognize a general trend of fracture frequency
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increase moving away from the front segment (Fig. 8). The linear fracture frequency measured from
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scanlines increases from a median value of 23 m-1 at distances < 60 m from the front segment to 32
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m-1 at distances > 60 m (Fig. 8). Analogously, the areal fracture frequency measured from virtual
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scan-areas increases with distance from the front fault segment from a median value of 18 m-2
303
(distances < 60 m) to 29 m-2 (distances > 60 m) (Fig. 8). Conversely, we do not observe any particular
304
relationship between fracture frequency/intensity distribution and distance from the front segment
305
from data retrieved from the oriented samples (section S3.1.4).
306
We observe that supratidal and intertidal carbonates are more fractured than subtidal carbonates both
307
in scanlines and oriented samples (Fig. 9a,b). The median of the linear fracture frequency retrieved
308
from scanlines measured in supratidal and intertidal facies (28 m-1) is ~40% larger than that measured
309
in subtidal facies (20 m-1) (Fig. 9a). Intertidal and supratidal oriented samples show median areal
310
fracture frequencies (P20) that are respectively 170% (5.4 ×104 m-2) and 100% (4.0×104 m-2) higher
311
than the subtidal samples (2.0×104 m-2) (Fig. 9b). The relationship between fracture frequency and
312
carbonate facies is clearer in virtual scan-areas (Fig. 9c), where the areal fracture frequency increases
313
with the supratidal and intertidal content (Fig. 9c). In detail, fracture frequency ranges between 10 m-
314
2 and 30 m-2 for supratidal and intertidal content <50%, whilst it reaches ~ 60 m-2 where the percentage
315
is ~ 80 %.
316
317
5. Discussion
318
5.1 Classical field techniques vs. virtual outcrop models
319
Our data show a consistent fracture distribution in the fault damage zone in both data retrieved from
320
the scanlines and from the virtual scan areas (Figs. 6, 8). The strong similarity of results produced by
321
classical field techniques such as scanlines (Priest and Hudson, 1981; Wu and Pollard, 1995) and by
322
the virtual scan areas, further demonstrates the high potential of virtual outcrops in structural geology
323
(e.g., McCaffrey et al., 2005a, 2005b; Tavani et al., 2014; Bistacchi et al., 2015; Cawood et al., 2017).
324
However, we do observe a small difference between the fracture trace length distribution computed
325
from scanlines and virtual scan areas (S2.1). This small discrepancy can be only partially attributed
326
to the employment of a virtual outcrop. We believe that such a difference is due to two main biases.
327
Firstly, scanlines are subjected to higher censoring effects (e.g., Priest and Hudson, 1981 among
328
others) than virtual scan-areas. In fact, due to the vertical cliffs of the quarry, the sampling of vertical
329
fractures longer than ~ 2 m - 3 m was impossible during most of the scanlines, whilst all the 5 m ´ 5
330
m virtual scan-areas allowed the collection of trace lengths smaller than 5 m. Secondly, scanlines
331
allowed the collection of very small (< 10 cm) fractures that were quite impossible to identify in
332
virtual scan-areas. The biases mentioned above produce a censoring of long fractures and
333
oversampling of small fractures during scanlines, and this is evident when the histograms of trace
334
lengths measured through the two methods are compared (S2.1).
335
The main advantage of using a virtual outcrop is the ability to collect fracture data on inaccessible or
336
dangerous portions of the quarry. In this way we exploited most of the quarry wall surfaces for data
337
collection (section S1.3), whilst only the base of the cliffs was analysed with scanlines for safety
338
reasons (section S1.1). Since we manually interpreted the fractures, the employment of a virtual
339
outcrop has not provided a consistent advantage in a matter of time efficiency. In fact, in addition to
340
the generation of virtual outcrop model (photo acquisition and processing), which took about a week
341
of work, the interpretation of each scan-area took approximately 2 hours, whilst the time needed for
342
the data collection along a scanline in the field was ~2-3 hours. Despite the time requirements, the
343
manual interpretation of fractures enabled us to preserve the interpretation ability of the user. In
344
addition, the virtual scan-areas method enabled us to use the FracPaQ software (Healy et al., 2017)
345
on the virtual outcrop models (Vinci et al., 2018; Giuffrida et al., 2019), which means that once the
346
interpretation is complete it is easy to extract in a very short time (few minutes) a large number of
347
fracture parameters. We believe that an important improvement in time-efficiency for the fracture
348
analysis from virtual outcrops would be provided by the development of algorithms and workflows
349
for the semi-automatic identification of fractures (e.g. Vasuki et al., 2014).
350
351
5.2 Fracture density distribution
352
The employment of scanlines allowed us to collect more than 1800 fracture attitudes (stereoplot in
353
Fig. 6) that were used as a control on the fracture frequency distribution obtained from the virtual
354
scan-areas. Fractures are mostly subvertical and strike in an E-W direction (± 20°). The pole to such
355
an orientation is coherent with the orientation of the T axis obtained by inverting the kinematic
356
indicators on the front segment of the relay ramp (stereoplot in Fig. 1c). The other fracture sets
357
striking NE-SW and NNW-SSE (Fig. 6) are likely to be related to the evolution of the fault structure.
358
In particular, the NE-SW striking set is coherent with the orientation of the T axis obtained by
359
inverting the kinematic indicators collected on the entire Tre Monti fault (stereoplot in Fig. 1b). The
360
NNW-SSE striking fracture set can be interpreted as related to the development of the relay ramp:
361
bending of strata around an axis orthogonal to the main fault segments may lead to ENE-WSW
362
extension, consistent with the NNW-SSE striking joint set. The data on the widespread population of
363
fractures, i.e. those E-W striking, may be influenced by the direction of the scanlines, most of them
364
performed on NW-SE oriented quarry walls (Fig. 6; see also Fig. S1 and Table S4). However, E-W
365
striking fractures constitute the main set independently on the quarry wall orientation (Figure 6), and
366
therefore, if present, the bias induced by scanlines orientation is limited.
367
Although many studies have demonstrated that the fracture frequency in fault damage zones increases
368
moving toward the main fault segment (Brock and Engelder, 1977; Wilson et al., 2003; Faulkner et
369
al., 2006; Mitchell and Faulkner, 2009; Savage & Brodsky, 2011), in our case study both scanlines
370
and virtual scan-areas show that fracture frequency increases with distance away from the most
371
important fault in the outcrop, represented by the front segment of the relay ramp (i.e., moving from
372
SE to NW; Figs. 6, 8). The observed trend is not due to a geometric bias. Since the most abundant set
373
is E-W oriented, this has the biggest impact on the fracture density calculation and we would expect
374
the highest fracture density in the quarry sectors that have an orientation close to N-S (e.g., sectors
375
A, C, and E; see Fig. 6). Conversely, we observe the highest fracture frequency in sectors B, C and
376
D (Fig. 6). Therefore, if any geometric bias affects the absolute values of fracture frequency, it would
377
lead to an underestimation of the rate of fracture frequency increase with distance from the front
378
segment of the relay ramp.
379
We interpret this unusual trend of fracture frequency to be the result of two main factors. The first
380
control is structural and related to the higher density of minor faults away from the front segment of
381
the relay ramp (i.e., in the north-western sector of the quarry; Figs. 6, 10, 11a). In this scenario, due
382
to the direct relationship between the number of fractures and faults, relatively higher fracture
383
frequencies reflect the development of fractures pertaining to the damage zones of the subsidiary
384
faults (e.g., Shipton and Cowie, 2003). The second important control on fracture distribution is played
385
by lithology and, in particular, by the carbonate facies. Approaching the centre of the relay zone we
386
document an increase in supratidal/intertidal facies (Fig. 11) that are characterized by a higher
387
fracture frequency (Fig. 9).
388
The role of different carbonate facies in fracture density is further testified by fracture frequency
389
measured on oriented samples showing that supratidal and intertidal rock samples are more fractured
390
than the subtidal samples (Fig. 9b). Other authors have shown that carbonate facies can control
391
fracture spacing in shallow-water limestones because of different Dunham’s textures (Wennberg et
392
al., 2006; Larsen et al., 2010b) or different mechanical properties (e.g., Giorgetti et al., 2016;
393
Rustichelli et al., 2016). In particular, Wennberg et al. (2006) show that carbonate facies can be even
394
more important than the mechanical layer thickness if the interbeds are strong (e.g., absence of a well-
395
developed bedding). In our case study, the effect of carbonate facies on fracture frequency is related
396
to the supratidal/intertidal facies being characterized by thinner bedding (cm- to dm- scale)
397
facilitating a larger fracture frequency (Ladeira and Price, 1981; Pollard and Aydin, 1988; Huang and
398
Angelier, 1989; Narr and Suppe, 1991; Wu and Pollard, 1995; Bai and Pollard, 2000).
399
Independently of the cause, the alternation of subtidal and intertidal/supratidal lithofacies at the
400
outcrop scale is responsible for the formation of a mechanical stratigraphy, with strongly fractured
401
intervals confined in the supratidal/intertidal facies beds (Fig. 12a,b; see also Fig. 5e-f). The relative
402
content of supratidal/intertidal facies plays an important role also in the deformation style developed
403
in the northern sector of the study outcrop, which is characterized by the presence of foliated breccias
404
(Fig. 11a). We suggest that during the fault activity, the high fracturing within the supratidal/intertidal
405
facies increased permeability, favouring the influx of fluids into these portions of the relay zone.
406
Fluids reacted with the fine grains within the fractured rocks promoting fluid-assisted dissolution and
407
precipitation mass transfer processes (i.e., pressure-solution; Rutter, 1983; Gratier et al., 1999;
408
Collettini et al., 2019). In addition, small amounts of clay minerals present in the supratidal facies
409
(Strasser et al 1999; Fig. 12c) may further enhance pressure-solution (Gratier et al., 1999; Renard et
410
al., 2001).
411
Since the quarry intercepts only a portion of the relay ramp (see Fig. 10), no constraints allow us to
412
evaluate whether the fracture intensity distribution was prevalently structurally or lithologically
413
controlled. We provide two end-member scenario depending on the main controlling factor on
414
fracture distribution. In a first more conservative scenario, it is assumed that the distribution of
415
carbonate facies is homogeneous and facies variations control fracture frequency only at metric to
416
decametric scale (Fig. 13a). As a consequence, the increase of fracture frequency away from the front
417
segment of the relay ramp is related to tectonic factors, such as the presence of an incipient breaching
418
zone between the front and rear segment of the relay ramp that is not directly observable in map view
419
because it is hidden by the presence of Pleistocene breccias (Fig. 13a; see also Fig. 1b). A clue for
420
the presence of a breaching zone may be represented by the numerous subsidiary faults in the northern
421
sector of the quarry. In this case, the increase in fracture frequency with distance from the front
422
segment would be explained by the abandoned quarry intercepting the damage caused by the incipient
423
breaching zone (Fig. 13a). In a second scenario, the distribution of carbonate facies is assumed as
424
heterogeneous (Fig. 13b). As a consequence, the damage is heterogeneously distributed, and
425
relatively higher fracture frequency is expected to follow the primary distribution of supratidal
426
carbonate facies (Fig. 13b). According to this hypothesis, the increase of fracture frequency moving
427
away from the fault segment of the relay ramp would be explained by the presence in the northern
428
sector of the study outcrop, of a stratigraphic interval characterized by a high supratidal facies content
429
(Fig. 13b).
430
Our results indicate that fracture frequency pattern is very complex in relay ramps hosted in shallow-
431
water limestones and that its prediction necessitates a good control on structures and sedimentary
432
facies distribution. We suggest that both of these factors should be considered during fluid flow
433
modelling within relay ramps hosted in shallow water limestones.
434
435
6. Conclusions
436
We evaluated the fracture distribution and its controlling factors within a relay ramp damage zone
437
hosted in shallow water limestones. Combining classical (i.e., scanlines) and modern (i.e., virtual
438
scan-areas) techniques, we have shown that fracture frequency increases moving toward the centre
439
of the relay zone. Two main factors can explain this trend:
440
1) The number of subsidiary faults and their associated damage zones accommodating the
441
development of the relay ramp increases moving toward the centre of the relay zone.
442
2) The supratidal and intertidal carbonate facies abundance increases toward the centre of the relay
443
zone. All the employed techniques show that supratidal and intertidal carbonate facies are
444
characterized by higher fracture frequencies than the subtidal carbonates.
445
To conclude, our results highlight that fracture distribution patterns with respect to the main faults
446
are not easily predictable within a relay ramp, because they can be modulated by subsidiary faults
447
formation and slip during the relay ramp development. Moreover, carbonate facies may play a non-
448
negligible role in fracture distribution within fault zones hosted in shallow water carbonates. Our
449
results therefore provide important suggestions for factors controlling fracture distribution and fluid
450
flow within relay ramps hosted by shallow water limestones.
451
452
Acknowledgements
453
We thank Dr. S. Mittempergher and an anonymous reviewer for their constructive comments which
454
helped to improve the manuscript. We thank Billy Andrews, Sabina Bigi, Carolina Giorgetti, Marco
455
Scuderi, Luca Smeraglia, Telemaco Tesei and Fabio Trippetta for fruitful discussions, Damiano Steri
456
for his help during the aero-photogrammetry survey, and Domenico (Mimmo) Mannetta for his help
457
during rock samples cutting and polishing and for high-quality thin sections preparation. We
458
acknowledge 3DFlow for providing the Education License of Zephyr Aerial. MM also thanks Manuel
459
Curzi, Roberta Ruggieri and Lavinia Squadrilli for their help in the fieldwork and Marta Della Seta
460
for her help with the QGIS software. This research was supported by the Sapienza University of
461
Rome Earth Sciences Department Ph.D. funds and Sapienza Progetti di Ateneo 2017 and 2019 to EC.
462
463
References
464
Allmendinger, R.W., Cardozo, N., Fisher, D.M., 2012. Structural Geology Algorithms: Vectors and
465
Tensors. Cambridge University Press.
466
467
Anders, M.H., Wiltschko, D.V., 1994. Microfracturing, paleostress and the growth of faults. Journal
468
of Structural Geology 16, 795–815.
469
470
Aydin, A., 2000. Fractures, faults, and hydrocarbon entrapment, migration and flow. 17, 797–814.
471
https://doi.org/10.1016/s0264-8172(00)00020-9
472
473
Bai, T., Pollard, D.D., 2000. Closely spaced fractures in layered rocks: initiation mechanism and
474
propagation kinematics. Journal of Structural Geology 22, 1409–1425.
475
https://doi.org/10.1016/S0191-8141(00)00062-6
476
477
Bastesen, E., & Rotevatn, A. (2012). Evolution and structural style of relay zones in layered
478
limestone–shale sequences: insights from the Hammam Faraun Fault Block, Suez rift, Egypt.
479
Journal of the Geological Society, 169(4), 477-488.
480
481
Bellian, J.A., Kerans, C., Jennette, D.C., 2005. Digital Outcrop Models: Applications of Terrestrial
482
Scanning Lidar Technology in Stratigraphic Modeling. Journal of Sedimentary Research 75, 166–
483
176. https://doi.org/10.2110/jsr.2005.013
484
485
Bemis, S.P., Micklethwaite, S., Turner, D., James, M.R., Akciz, S., Thiele, S.T., Bangash, H., 2014.
486
Ground-based and UAV-Based photogrammetry: A multi-scale, high-resolution mapping tool for
487
structural geology and paleoseismology. Journal of Structural Geology 69, 163–178.
488
https://doi.org/10.1016/j.jsg.2014.10.007
489
490
Benedetti, L., Manighetti, I., Gaudemer, Y., Finkel, R., Malavieille, J., Pou, K., Arnold, M.,
491
Aumaître, G., Bourlès, D., Keddadouche, K., 2013. Earthquake synchrony and clustering on Fucino
492
faults (Central Italy) as revealed from in situ 36Cl exposure dating. Journal of Geophysical
493
Research: Solid Earth 118, 4948–4974. https://doi.org/10.1002/jgrb.50299
494
495
Bense, V.F., Gleeson, T., Loveless, S.E., Bour, O., Scibek, J., 2013. Fault zone hydrogeology.
496
Earth-Science Reviews 127, 171–192. https://doi.org/10.1016/j.earscirev.2013.09.008
497
498
Berg, S.S., Skar, T., 2005. Controls on damage zone asymmetry of a normal fault zone: outcrop
499
analyses of a segment of the Moab fault, SE Utah. Journal of Structural Geology 27, 1803–1822.
500
https://doi.org/10.1016/j.jsg.2005.04.012
501
502
Bigi, S., Battaglia, M., Alemanni, A., Lombardi, S., Campana, A., Borisova, E., Loizzo, M., 2013.
503
CO2 flow through a fractured rock volume: Insights from field data, 3D fractures representation and
504
fluid flow modeling. International Journal of Greenhouse Gas Control 18, 183–199.
505
https://doi.org/10.1016/j.ijggc.2013.07.011
506
507
Bistacchi, A., Balsamo, F., Storti, F., Mozafari, M., Swennen, R., Solum, J., Tueckmantel, C.,
508
Taberner, C., 2015. Photogrammetric digital outcrop reconstruction, visualization with textured
509
surfaces, and three-dimensional structural analysis and modeling: Innovative methodologies applied
510
to fault-related dolomitization (Vajont Limestone, Southern Alps, Italy). Geosphere 11, 2031–2048.
511
https://doi.org/10.1130/GES01005.1
512
513
Bonali, F.L., Tibaldi, A., Marchese, F., Fallati, L., Russo, E., Corselli, C., Savini, A., 2019. UAV-
514
based surveying in volcano-tectonics: An example from the Iceland rift. Journal of Structural
515
Geology 121, 46–64. https://doi.org/10.1016/j.jsg.2019.02.004
516
517
Brock, W.G., Engelder, T., 1977. Deformation associated with the movement of the Muddy
518
Mountain overthrust in the Buffington window, southeastern Nevada. GSA Bulletin 88, 1667–1677.
519
https://doi.org/10.1130/0016-7606(1977)88<1667:DAWTMO>2.0.CO;2
520
521
Caine, J.S., Evans, J.P., Forster, C.B., 1996. Fault zone architecture and permeability structure.
522
Geology 24, 1025–1028.
523
524
Cardozo, N., Allmendinger, R.W., 2013. Spherical projections with OSXStereonet. Computers &
525
Geosciences 51, 193–205. https://doi.org/10.1016/j.cageo.2012.07.021
526
527
Carminati, E., Lustrino, M., Cuffaro, M., Doglioni, C., 2010. Tectonics, magmatism and
528
geodynamics of Italy: What we know and what we imagine. Journal of the Virtual Explorer 36.
529
https://doi.org/10.3809/jvirtex.2010.00226
530
531
Casini, G., Hunt, D., Monsen, E., Bounaim, A., 2016. Fracture characterization and modeling from
532
virtual outcrops. AAPG Bulletin 100, 41–61. https://doi.org/10.1306/09141514228
533
534
Cavinato, G.P., De Celles, P.G., 1999. Extensional basins in the tectonically bimodal central
535
Apennines fold-thrust belt, Italy: response to corner flow above a subducting slab in retrograde
536
motion. Geology 27, 955–958. https://doi.org/10.1130/0091-
537
7613(1999)027<0955:EBITTB>2.3.CO;2
538
539
Cawood, A.J., Bond, C.E., Howell, J.A., Butler, R.W., Totake, Y., 2017. LiDAR, UAV or compass-
540
clinometer? Accuracy, coverage and the effects on structural models. Journal of Structural Geology
541
98, 67–82. https://doi.org/10.1016/j.jsg.2017.04.004
542
543
Centamore, E., Crescenti, U., Dramis, F., 2006. Note Illustrative della Carta Geologica d’Italia alla
544
scala 1:50.000. Foglio 368, Avezzano.
545
546
Chester, F.M., Evans, J.P., Biegel, R.L., 1993. Internal structure and weakening mechanisms of the
547
San Andreas fault. Journal of Geophysical Research 98, 771–786.
548
https://doi.org/10.1029/92JB01866
549
550
Chester, F., Logan, J., 1986. Implications for mechanical properties of brittle faults from
551
observations of the Punchbowl fault zone, California. Pure and Applied Geophysics 124, 79–106.
552
https://doi.org/10.1007/BF00875720
553
554
Chiaraluce, L., 2012. Unravelling the complexity of Apenninic extensional fault systems: A review
555
of the 2009 L’Aquila earthquake (Central Apennines, Italy). Journal of Structural Geology 42, 2–
556
18. https://doi.org/10.1016/j.jsg.2012.06.007
557
558
Chiaraluce, L., Stefano, D.R., Tinti, E., Scognamiglio, L., Michele, M., Casarotti, E., Cattaneo, M.,
559
Gori, D.P., Chiarabba, C., Monachesi, G., Lombardi, A., Valoroso, L., Latorre, D., Marzorati, S.,
560
2017. The 2016 Central Italy Seismic Sequence: A First Look at the Mainshocks, Aftershocks, and
561
Source Models. 88, 757–771. https://doi.org/10.1785/0220160221
562
563
Çiftci, B., Bozkurt, E., 2007. Anomalous stress field and active breaching at relay ramps: a field
564
example from Gediz Graben, SW Turkey. Geological Magazine 144, 687–699.
565
https://doi.org/10.1017/S0016756807003500
566
567
Collettini, C., Tesei, T., Scuderi, M.M., Carpenter, B.M., Viti, C., 2019. Beyond Byerlee friction,
568
weak faults and implications for slip behavior. Earth and Planetary Science Letters 519, 245–263.
569
https://doi.org/10.1016/j.epsl.2019.05.011
570
571
Colomina, I., Molina, P., 2014. Unmanned aerial systems for photogrammetry and remote sensing:
572
A review. ISPRS Journal of Photogrammetry and Remote Sensing 92, 79–97.
573
https://doi.org/10.1016/j.isprsjprs.2014.02.013
574
575
Corradetti, A., Tavani, S., Parente, M., Iannace, A., Vinci, F., Pirmez, C., Torrieri, S., Giorgioni,
576
M., Pignalosa, A., Mazzoli, S., 2017. Distribution and arrest of vertical through-going joints in a
577
seismic-scale carbonate platform exposure (Sorrento peninsula, Italy): insights from integrating
578
field survey and digital outcrop model. Journal of Structural Geology.
579
https://doi.org/10.1016/j.jsg.2017.09.009
580
581
Cosentino, D., Cipollari, P., Marsili, P., Scrocca, D., 2010. Geology of the central Apennines: a
582
regional review. 36. https://doi.org/10.3809/jvirtex.2010.00223
583
584
Cowie, P., Phillips, R., Roberts, G., McCaffrey, K., Zijerveld, L., Gregory, L., Walker, F.J.,
585
Wedmore, L., Dunai, T., Binnie, S., Freeman, S., Wilcken, K., Shanks, R., Huismans, R.,
586
Papanikolaou, I., Michetti, A., Wilkinson, M., 2017. Orogen-scale uplift in the central Italian
587
Apennines drives episodic behaviour of earthquake faults. Scientific Reports 7, 44858.
588
https://doi.org/10.1038/srep44858
589
590
D’Agostino, N., Giuliani, R., Mattone, M., Bonci, L., 2001a. Active crustal extension in the Central
591
Apennines (Italy) inferred from GPS measurements in the interval 1994–1999. Geophysical
592
Research Letters 28, 2121–2124. https://doi.org/10.1029/2000GL012462
593
594
D’Agostino, N., Jackson, J., Dramis, F., Funiciello, R., 2001b. Interactions between mantle
595
upwelling, drainage evolution and active normal faulting: an example from the central
596
Apennines (Italy). Geophysical Journal International 147, 475–497. https://doi.org/10.1046/j.1365-
597
246X.2001.00539.x
598
599
D'Argenio, B., Ferreri, V., Amodio, S., & Pelosi, N. (1997). Hierarchy of high-frequency orbital
600
cycles in Cretaceous carbonate platform strata. Sedimentary Geology, 113(3-4), 169-193.
601
602
De Paola, N., Collettini, C., Faulkner, D. R., & Trippetta, F. (2008). Fault zone architecture and
603
deformation processes within evaporitic rocks in the upper crust. Tectonics, 27(4).
604
605
Destro, N., 1995. Release fault: A variety of cross fault in linked extensional fault systems, in the
606
Sergipe-Alagoas Basin, NE Brazil. Journal of Structural Geology 17, 615–629.
607
608
Devoti, R., Pietrantonio, G., Pisani, A., Riguzzi, F., Serpelloni, E., 2010. Present day kinematics of
609
Italy. 36. https://doi.org/10.3809/jvirtex.2010.00237
610
611
Doglioni, C., 1991. A proposal for the kinematic modelling of W dipping subductions possible
612
applications to the Tyrrhenian Apennines system. Terra Nova 3, 423–434.
613
614
Dor, O., Ben-Zion, Y., Rockwell, T., Brune, J., 2006a. Pulverized rocks in the Mojave section of
615
the San Andreas Fault Zone. Earth and Planetary Science Letters 245, 642–654.
616
https://doi.org/10.1016/j.epsl.2006.03.034
617
618
Dor, O., Rockwell, T., Ben-Zion, Y., 2006b. Geological Observations of Damage Asymmetry in the
619
Structure of the San Jacinto, San Andreas and Punchbowl Faults in Southern California: A Possible
620
Indicator for Preferred Rupture Propagation Direction. Pure and Applied Geophysics 163, 301–349.
621
https://doi.org/10.1007/s00024-005-0023-9
622
623
Faulkner, D., Mitchell, T., Healy, D., Heap, M., 2006. Slip on “weak” faults by the rotation of
624
regional stress in the fracture damage zone. Nature 444, nature05353.
625
https://doi.org/10.1038/nature05353
626
627
Fossen, H., Johansen, T. E. S., Hesthammer, J., & Rotevatn, A. (2005). Fault interaction in porous
628
sandstone and implications for reservoir management; examples from southern Utah. AAPG
629
bulletin, 89(12), 1593-1606.
630
631
Fossen, H., Rotevatn, A., 2016. Fault linkage and relay structures in extensional settings—A
632
review. Earth-Science Reviews 154, 14–28. https://doi.org/10.1016/j.earscirev.2015.11.014
633
634
Galadini, F., Galli, P., 1999. The Holocene paleoearthquakes on the 1915 Avezzano earthquake
635
faults (central Italy): implications for active tectonics in the central Apennines. Tectonophysics 308,
636
143–170. https://doi.org/10.1016/S0040-1951(99)00091-8
637
638
Giorgetti, C., Collettini, C., Scuderi, M., Barchi, M.R., Tesei, T., 2016. Fault geometry and
639
mechanics of marly carbonate multilayers: An integrated field and laboratory study from the
640
Northern Apennines, Italy. 93. https://doi.org/10.1016/j.jsg.2016.10.001
641
642
Giuffrida, A., Agosta, F., Rustichelli, A., Panza, E., La Bruna, V., Eriksson, M., ... & Giorgioni, M.
643
(2019). Fracture stratigraphy and DFN modelling of tight carbonates, the case study of the Lower
644
Cretaceous carbonates exposed at the Monte Alpi (Basilicata, Italy). Marine and Petroleum
645
Geology, 104045.
646
647
Gratier, J.-P., Renard, F., Labaume, P., 1999. How pressure solution creep and fracturing processes
648
interact in the upper crust to make it behave in both a brittle and viscous manner. Journal of
649
Structural Geology 21, 1–9. https://doi.org/10.1016/s0191-8141(99)00035-8
650
651
Gudmundsson, A., Berg, S.S., Lyslo, K.B., Skurtveit, E., 2001. Fracture networks and fluid
652
transport in active fault zones. Journal of Structural Geology 23, 343–353.
653
https://doi.org/10.1016/S0191-8141(00)00100-0
654
655
Gudmundsson, A., Simmenes, T.H., Larsen, B., Philipp, S.L., 2010. Effects of internal structure and
656
local stresses on fracture propagation, deflection, and arrest in fault zones. Journal of Structural
657
Geology 32, 1643–1655. https://doi.org/10.1016/j.jsg.2009.08.013
658
659
Healy, D., Rizzo, R.E., Cornwell, D.G., Farrell, N., Watkins, H., Timms, N.E., Gomez-Rivas, E.,
660
Smith, M., 2017. FracPaQ: A MATLABTM toolbox for the quantification of fracture patterns.
661
Journal of Structural Geology 95, 1–16. https://doi.org/10.1016/j.jsg.2016.12.003
662
663
Huang, Q., Angelier, J., 1989. Fracture spacing and its relation to bed thickness. 126, 355–362.
664
https://doi.org/10.1017/S0016756800006555
665
666
Kattenhorn, S., Aydin, A., Pollard, D., 2000. Joints at high angles to normal fault strike: an
667
explanation using 3-D numerical models of fault-perturbed stress fields. Journal of Structural
668
Geology 22, 1–23. https://doi.org/10.1016/S0191-8141(99)00130-3
669
670
Kim, Y.-S., Peacock, D., Sanderson, D., 2004. Fault damage zones. Journal of Structural Geology
671
26, 503–517. https://doi.org/10.1016/j.jsg.2003.08.002
672
673
Ladeira, F.L., Price, N.J., 1981. Relationship between fracture spacing and bed thickness. Journal of
674
Structural Geology 3, 179-183.
675
676
Larsen, B, Grunnaleite, I., Gudmundsson, A., 2010a. How fracture systems affect permeability
677
development in shallow-water carbonate rocks: An example from the Gargano Peninsula, Italy.
678
Journal of Structural Geology 32, 1212–1230. https://doi.org/10.1016/j.jsg.2009.05.009
679
680
Larsen, B, Gudmundsson, A., Grunnaleite, I., Sælen, G., Talbot, M.R., Buckley, S.J., 2010b.
681
Effects of sedimentary interfaces on fracture pattern, linkage, and cluster formation in peritidal
682
carbonate rocks. Marine and Petroleum Geology 27, 1531–1550.
683
https://doi.org/10.1016/j.marpetgeo.2010.03.011
684
685
Marrett, R., Allmendinger, R.W., 1990. Kinematic analysis of fault slip data. Journal of Structural
686
Geology. 973-986.
687
688
McCaffrey, K., Holdsworth, R., Imber, J., Clegg, P., Paola, N., Jones, R., Hobbs, R., Holliman, N.,
689
Trinks, I., 2005a. Putting the geology back into Earth models. Eos, Transactions American
690
Geophysical Union 86, 461–466. https://doi.org/10.1029/2005EO460001
691
692
McCaffrey, K.J.W., Jones, R.R., Holdsworth, R.E., Wilson, R.W., Clegg, P., Imber, J., Holliman,
693
N., Trinks, I., 2005b. Unlocking the spatial dimension: digital technologies and the future of
694
geoscience fieldwork. Journal of the Geological Society 162, 927–938.
695
https://doi.org/10.1144/0016-764905-017
696
697
McGinnis, R.N., Ferrill, D.A., Morris, A.P., Smart, K.J., Lehrmann, D., 2017. Mechanical
698
stratigraphic controls on natural fracture spacing and penetration. Journal of Structural Geology 95,
699
160–170.
700
701
Mercuri, M., McCaffrey, K. J., Smeraglia, L., Mazzanti, P., Collettini, C., & Carminati, E. (2020).
702
Complex geometry and kinematics of subsidiary faults within a carbonate-hosted relay
703
ramp. Journal of Structural Geology, 130, 103915.
704
705
Michie, E., Haines, T., Healy, D., Neilson, J., Timms, N., Wibberley, C., 2014. Influence of
706
carbonate facies on fault zone architecture. Journal of Structural Geology 65, 82–99.
707
https://doi.org/10.1016/j.jsg.2014.04.007
708
709
Miller, S.A., Collettini, C., Chiaraluce, L., Cocco, M., Barchi, M.R., Kaus, B.J., 2004. Aftershocks
710
driven by a high-pressure CO2 source at depth. 427, 724–727. https://doi.org/10.1038/nature02251
711
712
Mitchell, T.M., Ben-Zion, Shimamoto, 2011. Pulverized fault rocks and damage asymmetry along
713
the Arima-Takatsuki Tectonic Line, Japan. Earth and Planetary Science Letters 308, 284–297.
714
https://doi.org/10.1016/j.epsl.2011.04.023
715
716
Mitchell, T.M., Faulkner, D.R., 2009. The nature and origin of off-fault damage surrounding strike-
717
slip fault zones with a wide range of displacements: A field study from the Atacama fault system,
718
northern Chile. Journal of Structural Geology 31, 802–816.
719
https://doi.org/10.1016/j.jsg.2009.05.002
720
721
Morewood, N.C., Roberts, G.P., 2000. The geometry, kinematics and rates of deformation within an
722
en échelon normal fault segment boundary, central Italy. Journal of Structural Geology 22, 1027–
723
1047. https://doi.org/10.1016/S0191-8141(00)00030-4
724
725
Narr, W., Suppe, J., 1991. Joint spacing in sedimentary rocks. Journal of Structural Geology 13,
726
1037–1048. https://doi.org/10.1016/0191-8141(91)90055-N
727
728
Nur, A., Booker, J.R., 1972. Aftershocks Caused by Pore Fluid Flow? Science 175, 885–887.
729
https://doi.org/10.1126/science.175.4024.885
730
731
Odling, N.E., Gillespie, P., Bourgine, B., Castaing, C., Chiles, J.P., Christensen, N.P., Fillion, E.,
732
Genter, A., Olsen, C., Thrane, L. and Trice, R., 1999. Variations in fracture system geometry and
733
their implications for fluid flow in fractures hydrocarbon reservoirs. Petroleum Geoscience, 5, 373-
734
384.
735
736
Olariu, M.I., Ferguson, J.F., Aiken, C., Xu, X., 2008. Outcrop fracture characterization using
737
terrestrial laser scanners: Deep-water Jackfork sandstone at Big Rock Quarry, Arkansas. Geosphere
738
4, 247–259. https://doi.org/10.1130/GES00139.1
739
740
Osleger, D. (1991). Subtidal carbonate cycles: Implications for allocyclic vs. autocyclic controls.
741
Geology, 19(9), 917-920.
742
743
Peacock, D.C.P., Dimmen, V., Rotevatn, A., Sanderson, D.J., 2017. A broader classification of
744
damage zones. Journal of Structural Geology 102, 179–192.
745
https://doi.org/10.1016/j.jsg.2017.08.004
746
747
Peacock, D.C.P., Parfitt, E.A., 2002. Active relay ramps and normal fault propagation on Kilauea
748
Volcano, Hawaii. Journal of Structural Geology 24, 729–742. https://doi.org/10.1016/S0191-
749
8141(01)00109-2
750
751
Peacock, D.C.P., Price, S.P., Whitham, A.G., Pickles, C.S., 2000. The World’s biggest relay ramp:
752
Hold With Hope, NE Greenland. Journal of Structural Geology 22, 843–850.
753
https://doi.org/10.1016/S0191-8141(00)00012-2
754
755
Peacock, D.C.P., Xing, Z., 1994. Field examples and numerical modelling of oversteps and bends
756
along normal faults in cross-section. Tectonophysics 234, 147–167. https://doi.org/10.1016/0040-
757
1951(94)90209-7
758
759
Pless, J., McCaffrey, K., Jones, R., Holdsworth, R., Conway, A., Krabbendam, M., 2015. 3D
760
characterization of fracture systems using Terrestrial Laser Scanning: an example from the
761
Lewisian basement of NW Scotland. Geological Society, London, Special Publications 421, 125–
762
141. https://doi.org/10.1144/SP421.14
763
764
Pollard, D.D., Aydin, A.A., 1988. Progress in understanding jointing over the past century.
765
Geological Society of America Bulletin 100, 1181–1204.
766
767
Priest, S.D., Hudson, J.A., 1981. Estimation of discontinuity spacing and trace length using scanline
768
surveys. Int. J. Rock Mech. Min. Sci. & Geomech. Abstr 18, 183–197.
769
770
Renard, F., Dysthe, D., Feder, J., Bjørlykke, K. and Jamtveit, B, 2001. Enhanced pressure solution
771
creep rates induced by clay particles: Experimental evidence in salt aggregates.
772
https://doi.org/10.1029/2000GL012394
773
774
Rotevatn, A., Fossen, H., Hesthammer, J., Aas, T., Howell, J., 2007. Are relay ramps conduits for
775
fluid flow? Structural analysis of a relay ramp in Arches National Park, Utah. Geological Society,
776
London, Special Publications 270, 55–71. https://doi.org/10.1144/GSL.SP.2007.270.01.04
777
778
Rustichelli, A., Torrieri, S., Tondi, E., Laurita, S., Strauss, C., Agosta, F., Balsamo, F., 2016.
779
Fracture characteristics in Cretaceous platform and overlying ramp carbonates: An outcrop study
780
from Maiella Mountain (central Italy). Marine and Petroleum Geology 76, 68–87.
781
https://doi.org/10.1016/j.marpetgeo.2016.05.020
782
783
Rutter, E.H., 1983. Pressure solution in nature, theory and experiment. Journal of the Geological
784
Society 140, 725–740. https://doi.org/10.1144/gsjgs.140.5.0725
785
786
Sanderson, D., Nixon, C. (2015). The use of topology in fracture network characterization Journal
787
of Structural Geology 72, 55-66. https://dx.doi.org/10.1016/j.jsg.2015.01.005
788
789
Savage, H., Brodsky, E., 2011. Collateral damage: Evolution with displacement of fracture
790
distribution and secondary fault strands in fault damage zones. Journal of Geophysical Research:
791
Solid Earth (1978–2012) 116. https://doi.org/10.1029/2010JB007665
792
793
Scognamiglio, L., Tinti, E., Casarotti, E., Pucci, S., Villani, F., Cocco, M., Magnoni, F., Michelini,
794
A., Dreger, D., 2018. Complex Fault Geometry and Rupture Dynamics of the MW 6.5, 30 October
795
2016, Central Italy Earthquake. Journal of Geophysical Research: Solid Earth 123, 2943–2964.
796
https://doi.org/10.1002/2018JB015603
797
798
Seers, T.D., Hodgetts, D., 2016. Extraction of three-dimensional fracture trace maps from calibrated
799
image sequences. Geosphere 12, 1323–1340. https://doi.org/10.1130/GES01276.1
800
801
Shipton, Z.K., Cowie, P.A., 2003. A conceptual model for the origin of fault damage zone
802
structures in high-porosity sandstone. 25, 333–344.
803
804
Shipton, Z.K., Cowie, P.A., 2001. Damage zone and slip-surface evolution over μm to km scales in
805
high-porosity Navajo sandstone, Utah. Journal of Structural Geology 23, 1825–1844.
806
807
Sibson, R.H., 1994. Crustal stress, faulting and fluid flow. Geological Society, London, Special
808
Publications 78, 69–84. https://doi.org/10.1144/GSL.SP.1994.078.01.07
809
810
Sibson, R.H., 1996. Structural permeability of fluid-driven fault-fracture meshes. Journal of
811
Structural Geology 18, 1031–1042. https://doi.org/10.1016/0191-8141(96)00032-6
812
813
Smeraglia, L., Berra, F., Billi, A., Boschi, C., Carminati, E., Doglioni, C., 2016. Origin and role of
814
fluids involved in the seismic cycle of extensional faults in carbonate rocks. Earth and Planetary
815
Science Letters 450, 292–305. https://doi.org/10.1016/j.epsl.2016.06.042
816
817
Smeraglia, L., Billi, A., Carminati, E., Cavallo, A., Toro, G., Spagnuolo, E., Zorzi, F., 2017. Ultra-
818
thin clay layers facilitate seismic slip in carbonate faults. 7, 974. https://doi.org/10.1038/s41598-
819
017-00717-4
820
821
Smith, S.A., Billi, A., Toro, G., Spiess, R., 2011. Principal Slip Zones in Limestone:
822
Microstructural Characterization and Implications for the Seismic Cycle (Tre Monti Fault, Central
823
Apennines, Italy). Pure and Applied Geophysics 168, 2365–2393. https://doi.org/10.1007/s00024-
824
011-0267-5
825
826
Strasser, A., Pittet, B., Hillgartner, H. and Pasquier, J.-B. (1999) Depositional sequences in shallow
827
carbonate dominated sedimentary systems: concepts for a highresolution analysis. Sed. Geol., 128,
828
201–221.
829
830
Tavani, S., Granado, P., Corradetti, A., Girundo, M., Iannace, A., Arbués, P., Muñoz, J.A., Mazzoli,
831
S., 2014. Building a virtual outcrop, extracting geological information from it, and sharing the
832
results in Google Earth via OpenPlot and Photoscan: An example from the Khaviz Anticline (Iran).
833
Computers & Geosciences 63, 44–53. https://doi.org/10.1016/j.cageo.2013.10.013
834
835
Tavani, S., Storti, F., Salvini, F., Toscano, C., 2008. Stratigraphic versus structural control on the
836
deformation pattern associated with the evolution of the Mt. Catria anticline, Italy. Journal of
837
Structural Geology 30, 664–681. https://doi.org/10.1016/j.jsg.2008.01.011
838
839
Telling, J., Lyda, A., Hartzell, P., Reviews, G.C., 2017. Review of earth science research using
840
terrestrial laser scanning.
841
842
Terzaghi, R.D., 1965. Sources of error in joint surveys. Géotechnique 15 (3), 287–304.
843
http://dx.doi.org/10.1680/geot.1965.15.3.287
844
845
Townend, J., Zoback, M.D., 2000. How faulting keeps the crust strong. 28, 399–402.
846
https://doi.org/10.1130/0091-7613(2000)028
847
848
Vasuki, Y., Holden, E.-J., Kovesi, P., Micklethwaite, S., 2014. Semi-automatic mapping of
849
geological Structures using UAV-based photogrammetric data: An image analysis approach.
850
Computers & Geosciences 69, 22–32. https://doi.org/10.1016/j.cageo.2014.04.012
851
852
Vinci, F., Tavani, S., Iannace, A., Parente, M., Pirmez, C., Torrieri, S., Giorgioni, M., Pignalosa, A.,
853
Mazzoli, S., 2018. Extracting and quantifying fracture patterns from a reservoir-scale Virtual
854
Outcrop Model. EGU General Assembly Conference Abstracts (Vol. 20, p. 4797).
855
856
Volatili, T., Zambrano, M., Cilona, A., Huisman, H., Rustichelli, A., Giorgioni, M., Vittori, S.,
857
Tondi, E., 2019. From fracture analysis to flow simulations in fractured carbonates: The case study
858
of the Roman Valley Quarry (Majella Mountain, Italy). Marine and Petroleum Geology 100, 95–
859
110. https://doi.org/10.1016/j.marpetgeo.2018.10.040
860
861
Wennberg, O., Svånå, T., Azizzadeh, M., Aqrawi, A., Brockbank, P., Lyslo, K., Ogilvie, S., 2006.
862
Fracture intensity vs. mechanical stratigraphy in platform top carbonates: the Aquitanian of the
863
Asmari Formation, Khaviz Anticline, Zagros, SW Iran. Petroleum Geoscience 12, 235–246.
864
https://doi.org/10.1144/1354-079305-675
865
866
Westoby, M.J., Brasington, J., Glasser, N.F., Hambrey, M.J., Reynolds, J.M., 2012. ‘Structure-
867
from-Motion’ photogrammetry: A low-cost, effective tool for geoscience applications.
868
Geomorphology 179, 300–314. https://doi.org/10.1016/j.geomorph.2012.08.021
869
870
Wilkinson, M.W., Jones, R.R., Woods, C.E., Gilment, S.R., McCaffrey, K.J.W., Kokkalas, S.,
871
Long, J.J., 2016. A comparison of terrestrial laser scanning and structure-from-motion
872
photogrammetry as methods for digital outcrop acquisition. Geosphere 12, 1865–1880.
873
https://doi.org/10.1130/GES01342.1
874
875
Wilson, J., Chester, J., Chester, F., 2003. Microfracture analysis of fault growth and wear processes,
876
Punchbowl Fault, San Andreas system, California. Journal of Structural Geology 25, 1855–1873.
877
https://doi.org/10.1016/S0191-8141(03)00036-1
878
879
Woodcock, N.H., Mort, K., 2008. Classification of fault breccias and related fault rocks. Geological
880
Magazine 145, 435–440. https://doi.org/10.1017/s0016756808004883
881
882
Wu, H., Pollard, D.D., 1995. An experimental study of the relationship between joint spacing and
883
layer thickness. Journal of Structural Geology 17, 887–905. https://doi.org/10.1016/0191-
884
8141(94)00099-L
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Figure captions
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Figure 1 – Geological setting of the analysed outcrop. (A) Simplified geological map of the Fucino
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basin in the central Apennines, Italy (the black arrow in the upper right inset indicates the location).
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SPCF: San Potito – Celano Fault. (B) Simplified geological map of the Tre Monti fault area and
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zoom of the studied area. (C) Panoramic view of the study outcrop. Stereoplots with Linked
891
Bingham solution in (B) and (C) show the overall kinematics of the Tre Monti fault and of the front
892
fault segment in the relay ramp respectively. P: pressure axis; T: tension axis. Blue and red dots are
893
P and T axis calculated individually for each slickenside data. Kinematic inversions have been
894
performed using FaultKin (Marrett and Allmendinger, 1990; Allmendinger et al., 2012).
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Figure 2 -Lithological characterization of the damage zone host rock. (A) Cartoon representing the
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hypothesized depositional environment of the limestones in the quarry: the transition between a
898
tidal flat and a lagoon carbonate platform environment. The subtidal facies content increases
899
moving toward the lagoon environment. (B) Representation of an ideal peritidal cycle with the
900
associated carbonate facies. (C) Example of an outcrop where supratidal and intertidal facies,
901
characterized by centimetric to decimetric thick beds, predominate. (D) Outcrop characterized by
902
the predominance of subtidal facies and characterized by > 1 m thick beds. (E-J): scans (E-G) and
903
optical micrographs at plane polarized light (H-I) of samples pertaining to the supratidal (E, H),
904
intertidal (F, I), and subtidal (G, J) carbonate facies.
905
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Figure 3 – Scanlines. Example of a scanline survey (SL13, see Section S1 for the location) and
907
linear fracture frequency calculation. L: scanline length; N: number of fractures intercepted by the
908
scanline; P10: linear fracture frequency.
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Figure 4 – Fracture analysis on the oriented samples. (A) Collected rock sample with marked
911
orientation. (B) Fracture traces digitized on a high-resolution scan of the sample (dark blue lines).
912
(C) The linear fracture frequency has been calculated by counting the fracture traces sampled by
913
sub-horizontal scanlines (yellow lines). (D) Other fracturing parameters such as areal fracture
914
frequency and fracture intensity have been calculated by using the FracPaQ software (Healy et al.,
915
2017).
916
917
Figure 5 – Fracture analysis on the virtual outcrop. (A) Unmanned Aerial Vehicle survey in the
918
study outcrop. (B) Virtual outcrop model of the quarry obtained by a structure-from-motion
919
processing. (C) Example of an orthorectified panels with 1 mm per pixel resolution extracted from
920
the virtual outcrop model. A1-12 indicate the label of the virtual scan-areas (D) Example of a
921
virtual scan-area (A3). (E, F) The orthorectified squares were interpreted by drawing fractures
922
(yellow lines in panel E), bedding (green lines in panel E), and supratidal/intertidal carbonate facies
923
(F). The fracture analysis was performed using FracPaQ software (Healy et al., 2017).
924
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Figure 6 – Fracture frequency and geometry. (A) Space distribution of the linear (diamonds, P10)
926
and areal (circles, P20) fracture frequencies respectively measured along scanlines and obtained
927
from virtual scan-areas. The minor faults traces are retrieved from Mercuri et al. (2020). The
928
stereoplots (Schmidt’s net, low hemisphere) show the density contour of the poles to fractures in
929
different sectors of the quarry (see inset in the upper left). The label of the scanlines used as input
930
are reported in brackets (B) Density contour plot of the poles to the fractures collected along the
931
scanlines.
932
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Figure 7 – Foliated breccias. Photo (A) and interpretation (B) of an exposure of foliated breccias. B:
934
bedding, F: foliation, J: joints, SS: slip surfaces. The location of the photo is reported in Figure 6.
935
936
Figure 8 – Evolution of the linear and areal fracture frequency with distance from the front segment
937
of the relay ramp respectively measured through scanlines (blue) and virtual scan-areas (red).
938
939
Figure 9 – Relationship between fracture frequency and carbonate facies. (A) Box plot showing
940
fracture frequency for different carbonate facies in scanlines (B) Box plot showing fracture
941
frequency for different carbonate facies in oriented samples (C) Fracture frequency vs. supratidal
942
and intertidal facies content in virtual scan-areas.
943
944
Figure 10 – Structural control on the fracture frequency. The fracture frequency increases with
945
density of subsidiary faults that increases moving from SE to NW in the quarry, i.e., approaching
946
the centre of the relay ramp.
947
948
Figure 11 – Facies distribution in the quarry. (A) Map of the quarry showing the percentage of
949
supratidal and intertidal carbonate facies measured in the virtual-scan areas. The supratidal and
950
intertidal content is higher in the north-western sector of the quarry. High supratidal/intertidal facies
951
contents are often accompanied by the development of foliated breccias. (B) Supratidal and
952
intertidal carbonate facies content with distance from front segment (i.e. moving toward NW).
953
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Figure 12 – Damage evolution versus supratidal and intertidal facies content. (A) The alternation of
955
supratidal/intertidal and subtidal carbonate facies promotes a mechanical stratigraphy. The higher
956
fracture intensity observed in the supratidal and intertidal facies can be related to smaller thickness
957
of the beds (cm- to dm-thick, whilst the subtidal facies is characterized by m-thick beds) and to the
958
development of compartmentalized fractures. The supratidal portions can contain small amount of
959
clay minerals. (B) The average fracture intensity increases with increasing supratidal/intertidal
960
content for a fixed sampling area (C) Foliated breccias can eventually develop in portions of the
961
quarry dominated by the supratidal facies.
962
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Figure 13 – Hypotheses for the role of carbonate facies on fracture intensity distribution. (A)
964
Carbonate facies define a mechanical stratigraphy at metre scale, with highly damaged supratidal
965
intervals but has no effect on fracture intensity distribution at hundreds of meters scale. (B)
966
Supratidal facies distribution guides the intensity of subsidiary faults and fractures at hundreds of
967
meters scale. Pleistocene continental breccias (see Fig. 1b) cropping out in the relay zone are not
968
represented in the cartoon.
969
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... As displacement accumulates on normal faults, it has been shown that their segmentation can variably influence the spatial distribution of fractures and fault rock (Aarland and Skjerven, 1998;Rotevatn et al., 2007;Childs et al., 2009;Michie, 2015;Michie and Haines, 2016). For example, it has been demonstrated that shear strain can be greatest in relay zones between adjacent fault segments (Fossen et al., 2005;Rotevatn et al., 2007;Childs et al., 2009;Nixon et al., 2018Nixon et al., , 2020Mercuri et al., 2020). Furthermore, with increasing displacement these relay zones breach (e.g., Peacock and Sanderson, 1991;1994;Childs et al., 1995;2009;Crider and Pollard, 1998;Peacock, 2002;Soliva and Benedicto, 2004), and can eventually form fault-bound lenses of fault rock (Childs et al., 2009). ...
... The triangle corresponds to the average pole to bedding, which is gently dipping to the NNW. Demurtas et al., 2016;Mercuri et al., 2020), supporting the embryonic nature of the studied fault zone. In this regard, occurrence of similar fractures in adjacent unfaulted carbonate cliffs (e.g., Corradetti et al., 2018), indicates that part of the studied fractures has formed during extension but before incipient faulting, and should be regarded as background fractures. ...
Article
Fault zones can often display a complex internal structure associated with antithetic faults, branch and tip points, bed rotations, bed-parallel slip surfaces, and subordinate synthetic faults. We explore how these structural complexities may affect the development of fault-related fractures as displacement accumulates. We analysed in detail an incipient fault zone within well-bedded, shallow-water carbonates of the southern Apennines thrust belt (Italy). The fault zone crops out with quasi-complete exposure at a reservoir scale on an inaccessible sub-vertical cliff face, and fault and fracture mapping were carried out on a 3D virtual outcrop model of the exposure built for this study using photogrammetry. Comparing the structure of the fault zone and the density of 9444 mapped fractures allowed us to unravel their spatial relationships. Our results show that the areas of denser fractures coincide with: (i) rock volumes bounded by antithetic faults developed within the fault zone, (ii) branch points between these antithetic faults and fault zone-bounding fault segments, (iii) fault zone-bounding fault segments associated with significant displacement gradients, and (iv) relay zones between subordinate synthetic faults. These findings may aid locating sub-seismic resolution volumes of dense fracturing and associated enhanced permeability within faulted reservoirs.
... Detection, mapping, and analysis fracture network can often be hampered when outcrops are inaccessible (e.g., high and steep rock cliff), large (>units/tens km2) and in remote areas. In this frame, the Unmanned Aerial Vehicle and Digital Outcrop Models (DOM), often called Virtual Outcrop Model (VOM), can be extremely useful to properly characterize the fault-related fracture network (e.g., Mercuri et al., 2020;Ceccato et al., 2021). In recent years, DOMs have become predominate in geosciences (e.g., Powers et al., 1996;Pringle et al., 2004;Bellian et al., 2005;Sturzenegger and Stead, 2009;Humair et al., 2013;Corradetti et al., 2018;Inama et al., 2020;Camanni et al., 2021). ...
Article
The Mt. Vettore area is located in the Central Apennines (Italy), a region characterized by intense seismic activity that has recorded multiple moderate-to-high magnitude seismic sequences. The seismic activity is due to the presence of normal fault systems, among which is the Mt. Vettore Fault System (VFS), which was last activated during the 2016-17 Central Italy seismic sequence.. Moreover, the region has experienced three major tectonic phases over geological history, thus it is important to unravel their contribution to the current fracture network. Based on the integration of field observation with Unmanned Aerial Vehicle - Digital Photogrammetry data, we aim to analyze the fracture network on eight different outcrops located at different structural positions with respect to VFZ. Results show that the Late Miocene−Early Pliocene compressional phase deeply affected the present-day fracture pattern, which is especially related to the evolution of the Mt. Sibillini regional thrust and its related anticline. The present-day Quaternary extensional phase, and the associated normal faults, mostly reactivate some of the pre-existing fracture sets.
... Fracture density is related to nucleating fracture networks according to fracture linkage configuration (Myers & Aydin, 2004;Agosta & Aydin, 2006;De Joussineau & Aydin, 2007;Antonellini et al. 2008;Agosta et al. 2010), and to rock elastic properties (Gross et al. 1995;Agosta et al. 2015;Rustichelli et al. 2016). However, fracture intensity is associated with well-connected fracture networks, which often localize within fault damage zones (De Joussineau & Aydin, 2007;Aydin et al. 2010;Demurtas et al. 2016;Giuffrida et al. 2019;Mercuri et al. 2020;Camanni et al. 2021). At the Viggiano Mountain, neither P20 nor P21 varies proportionally with the bed thickness (Fig. 14a, d, e). ...
Article
The Viggiano Mt. platform carbonates form a layered succession cross-cut by a dense array of pressure solution seams, and five sets of fractures and veins, which together form a sub-seismic structural network associated with polyphasic tectonic evolution. To assess the influence exerted by depositional and diagenetic heterogeneities on fracture geometry, distribution and multiscale properties, we present the results of stratigraphic, petrographic, mineralogical and mesoscale structural analyses conducted at the Viggiano Mountain, southern Italy. Based on rock textures and fossil associations, we documented that the Sinemurian–Pleinsbachian carbonates were deposited in a low-energy open lagoon, the Toarcian carbonates in a ramp setting rimmed by sand shoals, and the Cenomanian carbonates in a medium- to high-energy, lagoonal–tidal setting. Fracture-density (P20) and intensity (P21) values computed after circular scanline measurements show similar trends in both Sinemurian–Pleinsbachian and Toarcian carbonates, consistent with the bed and bed-package heterogeneities acting as efficient mechanical interfaces during incipient faulting. On the other hand, P20 and P21 do not show very similar variations throughout the Cenomanian carbonates due to pronounced bed amalgamation. Throughout the study area, the aforementioned parameters do not vary in proportion to the bed thickness, and show higher values within the coarse-grained carbonate beds. This conclusion is confirmed by results of linear scanline measurements, which focus on the P10 properties of the most common diffuse fracture set. The original results reported in this work are consistent with burial-related, physical–chemical compaction and cementation processes affecting the fracture stratigraphy of the Mesozoic platform carbonates.
... Therefore, further energy release (i.e., during earthquakes) and fracture development during fault zone growth could have been transferred into the footwall, generating new fractures. As a general conclusion, lithology may prove to be an effective player in the definition of fracture intensity, as inferred also by Mercuri et al. (2020b). ...
Article
We combined structural data collected in the field and those obtained from a virtual outcrop model constructed from drone imagery, to perform Discrete Fracture Network (DFN) modelling and to characterize the fracture distribution within the damage zone of the low-displacement (∼50 m) carbonate-hosted Pietrasecca Fault (PF) (central Apennines, Italy). Both in the hanging wall and in the footwall damage zones, fractures are vertical and parallel to slightly oblique to the fault strike. Fracture length distributions in the footwall damage zone indicate a high degree of fracture maturity, while in the hanging wall damage zone they indicate a low degree of fracture maturity. Pervasive stylolitization in the hanging wall must have hindered the development of through-going fractures, favoring diffuse fracturing characterized by stylolite-bounded fractures. DFN models suggest that permeabilities are 1-2 orders of magnitude greater in the footwall damage zone than in the hanging wall damage zone. As permeability (10⁻¹² to 10⁻¹⁵ m²) is comparable with those measured in large-displacement (up to 600 m) faults in carbonates, our results show that also damage zones accompanying carbonate faults with ∼50 m of displacement could be fracture corridors for efficient fluid flow within subsurface reservoirs. Therefore, we propose that jumps in subsurface permeabilities occurring in many carbonate fractured reservoirs could be associated with to the occurrence of high permeability fracture zones developed within damage zones of low-displacement faults. As the recent advancement in seismic imaging allow the recognition of faults with displacement in the order of a few tens of meters, reservoir geologists and engineers can apply results of this study to better model the subsurface flow pathways near low displacement faults in carbonate reservoirs.
Article
To quantitatively describe the developmental characteristics of tectonic fractures in shale reservoirs, taking the early Silurian Longmaxi Formation in the Changning area in southern Sichuan, China, as an example, the fracture characteristics are described through fracture observations in field outcrops and in cores. The paleotectonic stress field is reconstructed by a numerical simulation method to predict the distribution of tectonic fractures. According to the evolution and characteristics of the study area, two geological models of the main tectonic stages of fracture development are established: the late Yanshanian to early Himalayan and the middle Himalayan to late Himalayan. The results show that the paleotectonic stress in the study area is mainly compressive stress and that the stress is high in the upper part of the structure. In the fault zone, the stress decreases and releases stress. The grade I fracture development area is mainly located near NE-trending faults, has a fracture coefficient of > 1.132, and is strip-shaped, which may lead to shale gas leakage and greatly impact shale gas productivity.
Conference Paper
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The use of multi-view photogrammetry in building Virtual Outcrop Models (VOMs) represents an increasingly accessible source of high-quality, low-cost geological data. In particular, VOMs are nowadays widely used in the characterization of fractured outcrops, since they allow for the collection of large volumes of fracture data from reservoir-scale outcrops. The extraction and analysis of such a big amount of data can be a time-consuming step of the interpretation workflow and, although several software packages dedicated to structural analysis are currently available, their scope is generally restricted to datasets acquired on flat 2D surfaces oriented roughly perpendicular to fractures. In this study, we present a workflow allowing the extraction of structural data from complex 3D surfaces and their quantification through the integration of free and open-access software. The study outcrop is located in the Lattari Mts. of the Sorrento Peninsula (Southern Apennines, Italy). It consists of a pyramid-shaped peak, approximately 200 m high and 150 m wide, exposing gently-dipping Lower Cretaceous shallow-water limestones and dolostones affected by vertical fractures, up to few tens of meters in height. In terms of stratigraphy and facies it represents an excellent outcrop analogue for Southern Italy reservoirs. Due to the inaccessibility of the peak, we used an Unmanned erial Vehicle (UAV) equipped with a camera to acquire photos necessary for building a photorealistic and georeferenced VOM. Structural data from the VOM were acquired by means of the open source software OpenPlot. Distinct fracture clusters are selected and projected independently on the corresponding fracture-perpendicular plane, minimizing the distortion of the measurements. The obtained 2D fracture pattern is then imported in FracPaQ, a MATLAB toolbox for fracture pattern quantification, to perform spacing analysis. The presented workflow expands the scope of the software, originally developed to perform structural analysis on 2D images, and allows for extracting quantitative information from 3D datasets, improving our ability to quantify structural data in a quick and robust way.
Article
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Damage zones have previously been classified in terms of their positions at fault tips, walls or areas of linkage, with the latter being described in terms of sub-parallel and synchronously active faults. We broaden the idea of linkage to include structures around the intersections of non-parallel and/or non-synchronous faults. These interaction damage zones can be divided into approaching damage zones, where the faults kinematically interact but are not physically connected, and intersection damage zones, where the faults either abut or cross-cut. The damage zone concept is applied to other settings in which strain or displacement variations are taken up by a range of structures, such as at fault bends. It is recommended that a prefix can be added to a wide range of damage zones, to describe the locations in which they formed, e.g., approaching, intersection and fault bend damage zone. Such interpretations are commonly based on limited knowledge of the 3D geometries of the structures, such as from exposure surfaces, and there may be spatial variations. For example, approaching faults and related damage seen in outcrop may be intersecting elsewhere on the fault planes. Dilation in intersection damage zones can represent narrow and localised channels for fluid flow, and such dilation can be influenced by post-faulting stress patterns.<br/
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The patterns of fractures in deformed rocks are rarely uniform or random. Fracture orientations, sizes, and spatial distributions often exhibit some kind of order. In detail, relationships may exist among the different fracture attributes, e.g. small fractures dominated by one orientation, larger fractures by another. These relationships are important because the mechanical (e.g. strength, anisotropy) and transport (e.g. fluids, heat) properties of rock depend on these fracture attributes and patterns. This paper describes FracPaQ, a new open source, cross-platform toolbox to quantify fracture patterns, including distributions in fracture attributes and their spatial variation.
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The application of advanced remote sensing technologies, including terrestrial laser scanning (TLS), to the Earth sciences has increased rapidly in the last two decades, improving the spatial and temporal resolution of data. Terrestrial laser scanning units have evolved into a common tool in studies of spectral and structural geology, seismology, natural hazards, geomorphology, and glaciology. Special consideration of the advantages and limitations of TLS in each of these fields is discussed in depth in the context of important work published in each field. The workflow used in a TLS survey is crucial to the success of the survey, and field-specific Earth science workflows are therefore also discussed. Products based on TLS data, such as triangulated irregular networks (TIN) and digital surface models (DSM), are commonplace tools throughout the Earth sciences and the use of these tools to measure slip distributions, fault geometries, aeolian transport, river bed morphologies and flows, and other research problems is expanded on where appropriate. The review concludes with a discussion of recent trends in TLS instrument development and their potential impact on the use of TLS in the Earth sciences in the future.
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Fine-grained low permeability sedimentary rocks, such as shale and mudrock, have drawn attention as unconventional hydrocarbon reservoirs. Fracturing – both natural and induced – is extremely important for increasing permeability in otherwise low-permeability rock. We analyze natural extension fracture networks within a complete measured outcrop section of the Ernst Member of the Boquillas Formation in Big Bend National Park, west Texas. Results of bed-center, dip-parallel scanline surveys demonstrate nearly identical fracture strikes and slight variation in dip between mudrock, chalk, and limestone beds. Fracture spacing tends to increase proportional to bed thickness in limestone and chalk beds; however, dramatic differences in fracture spacing are observed in mudrock. A direct relationship is observed between fracture spacing/thickness ratio and rock competence. Vertical fracture penetrations measured from the middle of chalk and limestone beds generally extend to and often beyond bed boundaries into the vertically adjacent mudrock beds. In contrast, fractures in the mudrock beds rarely penetrate beyond the bed boundaries into the adjacent carbonate beds. Consequently, natural bed-perpendicular fracture connectivity through the mechanically layered sequence generally is poor. Fracture connectivity strongly influences permeability architecture and fracture prediction should consider thin bed-scale control on fracture heights and the strong lithologic control on fracture spacing.
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Terrestrial laser scanning (TLS) has been used extensively in Earth Science for acquisition of digital outcrop data over the past decade. Structure-from-motion (SfM) photogrammetry has recently emerged as an alternative and competing technology. The real-world performance of these technologies for ground-based digital outcrop acquisition is assessed using outcrops from North East England and the United Arab Emirates. Both TLS and SfM are viable methods, although no single technology is universally best suited to all situations. There are a range of practical considerations and operating conditions where each method has clear advantages. In comparison to TLS, SfM benefits from being lighter, more compact, cheaper, more easily replaced and repaired, with lower power requirements. TLS in comparison to SfM provides intrinsically validated data and more robust data acquisition in a wide range of operating conditions. Data post-processing is also swifter. The SfM data sets were found to contain systematic inaccuracies when compared to their TLS counterparts. These inaccuracies are related to the triangulation approach of the SfM, which is distinct from the time-of-flight principle employed by TLS. An elaborate approach is required for SfM to produce comparable results to TLS under most circumstances.
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Sealing layers are often represented by sedimentary sequences characterized by alternating strong and weak lithologies. When involved in faulting processes, these mechanically heterogeneous multilayers develop complex fault geometries. Here we investigate fault initiation and evolution within a mechanical multilayer by integrating field observations and rock deformation experiments. Faults initiate with a staircase trajectory that partially reflects the mechanical properties of the involved lithologies, as suggested by our deformation experiments. However, some faults initiating at low angles in calcite-rich layers (θi = 5°–20°) and at high angles in clay-rich layers (θi = 45°–86°) indicate the important role of structural inheritance at the onset of faulting. With increasing displacement, faults develop well-organized fault cores characterized by a marly, foliated matrix embedding fragments of limestone. The angles of fault reactivation, which concentrate between 30° and 60°, are consistent with the low friction coefficient measured during our experiments on marls (μs = 0.39), indicating that clay minerals exert a main control on fault mechanics. Moreover, our integrated analysis suggests that fracturing and faulting are the main mechanisms allowing fluid circulation within the low-permeability multilayer, and that its sealing integrity can be compromised only by the activity of larger faults cutting across its entire thickness.
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We examine the potentially-seismic right-lateral transtensional–extensional Tre Monti Fault (central Apennines, Italy) with structural and geochemical methods and develop a conceptual evolutionary model of extensional faulting with fluid involvement in shallow (≤3 km depth) faults in carbonate rocks. In the analysed fault zone, multiscale fault rock structures include injection veins, fluidized ultracataclasite layers, and crackle breccias, suggesting that the fault slipped seismically. We reconstructed the relative chronology of these structures through cross-cutting relationship and cathodoluminescence analyses. We then used C-and O-isotope data from different generations of fault-related mineralizations to show a shift from connate (marine-derived) to meteoric fluid circulation during exhumation from 3 to ≤1 km depths and concurrent fluid cooling from ∼68 to <30 • C. Between ∼3 km and ∼1 km depths, impermeable barriers within the sedimentary sequence created a semi-closed hydrological system, where prevalently connate fluids circulated within the fault zone at temperatures between 60 • and 75 • C. During fault zone exhumation, at depths ≤1 km and temperatures <30 • C, the hydrological circulation became open and meteoric-derived fluids progressively infiltrated and circulated within the fault zone. The role of these fluids during syn-exhumation seismic cycles of the Tre Monti Fault has been substantially passive along the whole fault zone, the fluids being passively redistributed at hydrostatic pressure following co-seismic dilatancy. Only the principal fault has been characterized, locally and transiently, by fluid overpressures. The presence of low-permeability clayey layers in the sedimentary sequence contributed to control the type of fluids infiltrating into the fault zone and possibly their transient overpressures. These results can foster the comprehension of seismic faulting at shallow depths in carbonate rocks of other fold-thrust belts involved in post-collisional seismogenic extensional tectonics.