Fault architecture and deformation mechanisms in exhumedanalogues of seismogenic carbonate-bearing thrusts

Telemaco Tesei a,*, Cristiano Collettini a,b,1, Cecilia Viti c, Massimiliano R. Barchi aaDipartimento di Scienze della Terra, Università degli Studi di Perugia, Piazza Università 1, 06123 Perugia, Italyb Istituto Nazionale di Geofisica e Vulcanologia, Via di Vigna Murata 605, 00143 Roma, ItalycDipartimento di Scienze della Terra, Università degli Studi di Siena, Via Laterina 8, 53100 Siena, Italy

a r t i c l e i n f o

Article history:Received 2 April 2013Received in revised form18 July 2013Accepted 22 July 2013Available online xxx

Keywords:CarbonatesFault complexitySeismic behaviourMicrostructures

a b s t r a c t

Faults in carbonates are well known sources of upper crustal seismicity throughout the world. In theouter sector of the Northern Apennines, ancient carbonate-bearing thrusts are exposed at the surface andrepresent analogues of structures generating seismicity at depth. We describe the geometry, internalstructure and deformation mechanisms of three large-displacement thrusts from the km scale to themicroscale. Fault architecture and deformation mechanisms are all influenced by the lithology of faultedrocks. Where thrusts cut across bedded or marly limestones, fault zones are thick (tens of metres) anddisplay foliated rocks (S-CC0 tectonites and/or YPR cataclasites) characterized by intense pressure-solution deformation. In massive limestones, faulting occurs in localized, narrow zones that exhibitabundant brittle deformation. A general model for a heterogeneous, carbonate-bearing thrust is pro-posed and discussed. Fault structure, affected by stratigraphic heterogeneity and inherited structures,influences the location of geometrical asperities and fault strain rates. The presence of clay minerals andthe strain rate experienced by fault rocks modulate the shifting from cataclasis-dominated towardspressure-solution-dominated deformation. Resulting structural heterogeneity of these faults may mirrortheir mechanical and seismic behaviour: we suggest that seismic asperities are located at the boundariesof massive limestones in narrow zones of localized slip whereas weak shear zones constitute slowlyslipping portions of the fault, reflecting other types of “aseismic” behaviour.

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1. Introduction

Faults hosted in carbonatic sedimentary sequences are frequentstructures in a variety of geodynamic settings, including accre-tionary prisms of some subduction zones, fold-thrust belts andpassive margins (e.g. Bally et al., 1966; Marshak and Engelder, 1985;Alvarez, 1990; Holl and Anastasio, 1995; Willemse et al., 1997;Stern, 2002). Carbonate-bearing faults are important targets forscientific investigation because these lithologies frequently hostore mineralization (e.g. Guilbert and Park, 1986; Anderson andMacqueen, 1982; Gökce and Bozkaya, 2007) and important hy-drocarbon reserves (e.g. Archie, 1952; Borkhataria et al., 2005;Ehrenberg and Nadeau, 2005). Also, in Italy and other parts of theworld, shallow earthquakes nucleate within or propagate through

thick carbonatic sequences in all tectonic regimes (e.g. Chiaraluceet al., 2003; Miller et al., 2004; Di Bucci and Mazzoli, 2003;Bernard et al., 2006; Burchfiel et al., 2008; Mirabella et al., 2008;Ventura and Di Giovambattista, 2013).

Past studies have focused on the deformation of carbonates (e.g.Rutter, 1983; Hadizadeh, 1994; Kennedy and Logan, 1998; Billi, 2010;Smith et al., 2011) and depictedmodels of carbonate-bearing maturefaults in which cataclasis is the main deformation mechanism andstrain increases towards fault cores composed of localized slip sur-faces and pulverized rock (e.g. Hadizadeh, 1994; Agosta and Aydin,2006; Billi and Di Toro, 2008). However, in fault-related carbonaterocks significant influence in deformation is exerted by othermechanisms, such as pressure-solution (e.g. Alvarez et al., 1978;Rutter, 1983; Gratier and Gamond, 1990; Collettini et al., 2009).Some fault zone structures are not consistent with a localized faultcore, but display thick bodies of variously foliated rocks (e.g.Koopman, 1983; Lavecchia, 1985; Ghisetti, 1987; Marshak andEngelder, 1985; Bussolotto et al., 2007; Calamita et al., 2012).

The aim of this paper is to understand and unravel differentfaulting styles and deformation mechanisms of mature, thrust

* Corresponding author. Dipartimento di Scienze della Terra, Università degliStudi di Perugia, Piazza Università 1, 06123 Perugia, Italy. Tel.: þ39 3333728136.

E-mail addresses: telemaco.tesei@ingv.it, teletezzo@msn.com (T. Tesei).1 Present address: Dipartimento di Scienze della Terra, Università La Sapienza di

Roma, P.le A. Moro 5, 00185 Roma, Italy.

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faults cutting through carbonatic multilayers with strong rheo-logical contrasts. In particular, we focus on three faults thatrepresent exhumed analogues of active thrusts of the Apenninesnow buried below the Po Plain foredeep deposits in NorthernItaly (Mmax ¼ 6.0, e.g. Selvaggi et al., 2001; Lavecchia et al., 2003;Ventura and Di Giovambattista, 2013).

1.1. Adopted terminology

In the present study, we adopt a mechanics based terminologythat can be tied to simple geometrical observations in the outcropand in microstructures. This terminology allows making inferenceson failure modes and mechanisms of observed geological bodies.

From a macroscopic point of view (i.e. outcrop-to-hand samplescale), the terms brittle failure and ductile flow reflect a difference inthe failure mode of the material. Brittle vs. ductile behaviourfundamentally depends on the space- and time-scale of observa-tion (e.g. Rutter, 1986) but in general, brittle failure is associatedwith localization of deformation in a small portion of the consid-ered physical system. In experiments, brittle failure usually in-volves an appreciable stress drop. On the other hand, ductile flow isassociated with distributed deformation within a shear zone andoccurs without a significant stress drop (e.g. Paterson, 1958; Rutter,1986; Sibson, 1986; Scholz, 2002; Paterson and Wong, 2005 andothers). Within this framework, we refer to the brittleeductiletransition as the shift from a localized to de-localized deformationin the architecture of fault zones.

Fault rocks such as gouge, breccias and cataclasiteseultra-cataclasites (e.g. Sibson, 1977) with random-fabric and well-localized deformation along Principal Slip Zones (PSZ) representbrittle failure modes whereas foliated rocks with distributeddeformation represent ductile failure modes. The foliated rocks aredistinguished in two broad groups depending on the dominantdeformation process (e.g. McClay, 1977):

(1) YPR (or foliated) cataclasites (after Logan et al., 1979) charac-terized by Y and Riedel shear surfaces mainly deformed bycataclasis and hydrofracturing;

(2) S-C or S-CC0 tectonites (e.g. Ramsay and Graham, 1970; Berthéet al., 1979; Bos and Spiers, 2001), form as consequence ofpressure-solution and frictional sliding.

Although shifts in deformation mechanisms are not abrupt innature, but rather transitional, the main process is recognizableamong the competing mechanisms.

In both cases, YPR and S-CC0 surfaces define rock sigmoidsaffected byminor internal deformation. These surfaces have similarkinematic significance: P and S surfaces are perpendicular to themaximum flattening of the strain ellipsoid, Y and C are shear par-allel surfaces, and R1 and C0 are late, synthetic shear surfaces obli-que with respect to the macroscopic shear sense (Platt, 1984;Scholz, 2002).

2. Geological framework

The Northern Apennines are an arcuate fold and thrust beltoriginated by collision of a formerly European continental block(Corsica-Sardinia) and the Adria microplate with African affinity(e.g. Reutter et al., 1980). Apennines development is embeddedwithin the EuropeeAfrica convergence that initiated in theCretaceous (Dewey et al., 1989). The belt has an E-NE vergence andits development generated a series of foredeep and piggy-backbasins rejuvenating from W to E. The age of these synorogenicdeposits constrains the timing of deformation from more internalOligocene-Miocene domains (Tuscan domain) to the Pliocene-present day Adriatic foredeep (Merla, 1951; Boccaletti et al.,1990; Barchi et al., 2012). Present-day compression is active inthe Adriatic foredeep that extends from the Po-Plain and con-tinues along the Eastern Italian Coast and into the AdriaticOffshore (e.g. Pieri and Groppi, 1981; Doglioni, 1993; Chiarabbaet al., 2005). Since the Miocene, extension roughly coaxial withcompression has been active in dissecting previous compressionalfeatures (Elter et al., 1975).

The study area is part of the external relief of the NorthernApennines known as the Umbria-Marche Apennines; it is locatedbetween the Tuscan Domain to the W and the Adriatic Foothills tothe E (Fig. 1a). The Umbria-Marche Apennines consist of a thick pileof Meso-Cenozoic passive margin-related carbonates that lie overTriassic rift-related deposits and Hercynian crystalline basementand below the foredeep turbidite cover.

The carbonaticmultilayer constitutes the bulk of the outcroppingorogen (e.g. Lavecchia et al., 1988) and presents strong rheologicalcontrasts because of the alternation of limestone and marly

Fig. 1. (a) Digital elevation model of Umbria-Marche Apennines prepared with GeoMapApp (http://www.geomapapp.org) displaying traces of main thrusts and the location ofstudied faults. (b) Schematic stratigraphic column of the sedimentary sequence cropping out in the study area. A distinction between competent and incompetent formations hasbeen highlighted.

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formations (synthesized in Fig.1b). At themacroscale, the base of thesequence (the Calcare Massiccio Fm, constituted of massive lime-stone) behaves generally as a rigid body. The marly, less competent,levels (Marne a Fucoidi and Scaglia Variegata/Cinerea Fms marls),embedded within pelagic limestone, act as decolleménts of thetectonic pile (e.g. Barchi et al., 2012). During the Jurassic, synsedi-mentary normal faults dismembered the carbonatic platforms instructural heights and troughs leading to significant variations of thesuperimposed carbonatic multilayer (Fig. 1b, e g. Colacicchi et al.,1970; Alvarez, 1990; De Paola et al., 2007).

Subsequently, folding and thrusting in the Umbria-MarcheApennines occurred between the Late Miocene and Early Plio-cene. Box folding preceded the development of flat-ramp-flat andshort cut trajectories of thrusts, which subsequently propagatedthrough the whole carbonatic sequence commonly decapitatingprevious anticlines (e.g. Lavecchia et al., 1988; Tavarnelli, 1997).Thrust development and imbrication in a multicompetent sedi-mentary cover gave rise to disharmonic stacking and off-sequencethrusting (e.g. Ghisetti and Vezzani, 1997; Massoli et al., 2006).Therefore, stratigraphic heterogeneity and structural complexity

inherited by pre-thrusting deformation (e.g. Butler et al., 2006),likely affected both along-strike and along-dip fault architectureand deformation mechanisms within each thrust fault.

2.1. Investigation methods

In order to address complexity of thrusts in multicompetentcarbonate sequences, we studied fault architecture and defor-mation mechanisms of three major faults (Fig. 1a) exhumed fromsimilar PeT conditions (less than 3 km depth and temperaturesbelow 100 #C). They are representative end-members of generalgeometry and slip behaviour of shallow Apennines thrust faults.In particular we want to extensively document well-establishedprocesses occurring in mature faults; therefore we leave detailsof the top-to-the-East kinematics of these thrusts and the com-parison with small-displacement structures to the availableApennines literature (e.g. Lavecchia, 1985; Ghisetti and Vezzani,1997; Agosta and Aydin, 2006; Calamita et al., 2012; Petracchiniet al., 2012 among many others). The Coscerno fault representsan example of thrust propagation within incompetent lithologies.

Fig. 2. Coscerno thrust: (a) Geologic cross section. (b) Overview of the shear zone developed in marly limestone and in marls. The two protoliths are mixed within the fault zone. (c)S-CC0 tectonites developed in marly limestone. Sigmoids bounded by pressure-solution seams are locally refolded. (d) Highly foliated S-CC0 tectonites developed in marls. Cal-carenite blocks behave as rigid bodies within the shear zone.

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The Spoleto thrust represents faulting between competent for-mations (massive limestone over bedded limestone) and theFiastrone thrust is the example of a fault zone cutting differentstructural levels of the multilayer. The Coscerno, Spoleto andFiastrone thrusts are mature faults, whose displacement is on theorder of a several kms (Barchi and Brozzetti, 1991; Grandinettiet al., 2000; Mazzoli et al., 2005). Shortening accommodated bythese structures has been evaluated with restoration of geologicalcross sections and thus can be considered as a conservativeestimate.

In the following sections we present these fault zones from thekm scale to the microscopic scale. For each fault, we constructedgeological cross sections (1:10,000) through the studied areas toevaluate exhumation, stratigraphic superposition and overall ge-ometry of the fault structures. Then, we describe the fault archi-tecture and rocks at the outcrop scale. Finally, we describerepresentative fault rock microstructures with particular focus ondeformation mechanisms using polished cuts on hand samples andthin sections observed with both classical petrographical micro-scope and scanning electron microscope (SEM) backscatteredelectron (BSE) microscopy.

3. The Coscerno thrust

The Coscerno thrust fault (e.g. Barchi and Lemmi,1996) is a NeS-trending W-dipping regional feature with displacement of about4 km (Grandinetti et al., 2000) that crops out for about 25 km. In thestudy area the limestones of the Scaglia group, from the overturnedeastern limb of the anticline, thrust on top of the marls of theScaglia Cinerea/Variegata Fms (Fig. 2a). The Scaglia Cinerea/Varie-gata marls and marly limestone come from both the overturnedlimb of the Coscerno anticline and the normal limb of the syncline.The geological cross section constrains a fault exhumation on theorder of 1.2 km (Fig. 2a).

3.1. Fault zone architecture and fault rocks

In one of the best exposures of the fault, the bulk of the defor-mation is distributed (i.e. ductile) in awide (z20m) shear zone thatdeveloped in the Footwall (FW) block, within the less competentScaglia Variegata and Scaglia Cinerea formations (fault core, Fig. 2b).The shear zone is characterized by intense and pervasive pressure-solution cleavage that led to the development of strongly foliated S-

Fig. 3. (a) S-CC0 sigmoids developed in marly limestone with abundant calcite veins (arrows). (b) Hand sample of tectonites developed in the marls shows very fine foliation. (c)Thin section of undeformed grainstone and blocky calcite vein observed in the inner part of a sigmoids. (d) Thin section of undeformed mudstone inside a sigmoid showing pre-tectonic blocky calcite and stylolite association with incipient overprinting by tectonic fibrous calcite. (e) Developed tectonite associated to the grainstone: stylolites and rod-shapedfibrous calcite develop parallel to the shear sense of the fault. (f) Tectonite developed in marls: thick stylolites with smooth profile bound microscopic sigmoids constituted byfibrous calcite.

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CC0 tectonites (Fig. 2c). In the Hangingwall (HW), the fading of thebedding and a stratigraphic elision mark the transition to the shearzone, accommodating a significant portion of the fault displace-ment. Macroscopically, within the shear zone it is possible torecognize two units roughly corresponding to the Scaglia Variegata(more calcareous) and Scaglia Cinerea (marly) deformed protoliths.The two units are tectonically interdigitated within the fault core.

The calcareous unit (Fig. 2c) is constituted by white, greenishand reddish marly limestone rich in oxidized horizons whereas themarly unit (Fig. 2d) is mainly composed of grey-brownish, veryfinely foliated marls, with occasional intercalation of sigmoidalcalcarenite blocks. Calcarenite blocks represent deformed rem-nants of competent strata within the Scaglia Cinerea Fm. Thesecompetent blocks have been initially boudinaged by Riedel shearswithin the more ductile marly tectonites and are now emplacedcoherently with the transport direction of the thrust towards E-NE.

In the tectonites, S, C and C0 planes contain striated clay horizonsand are often decoratedwithfibrous calcite veins documenting syn-kinematicmineral deposition and fluid pressure fluctuations duringfault activity. The intensity of the deformation has been measuredacross the shear zone: it increases abruptly from 19-33 C-planes/mwithin the calcareous unit to 48e52 C-planes/m in the marly unit.The shear zone is thinned byE-NEdipping normal faults (few tens ofcentimetres of slip), here interpreted as major R1 shears.

3.2. Microstructural analysis

We collected samples of S-CC0 tectonites from the two distinctunits of the shear zone. Fig. 3 shows representative samples of theCoscerno tectonites from the cm-scale to the microscale. Intensityof deformation decreases from the sigmoids boundaries towardstheir inner portions. Samples of the calcareous tectonite usuallypreserve, in their interiors, the original protolith and are fracturedby extensive calcite veins (Fig. 3a). On the other hand, marly tec-tonites show less veining, denser foliation and more tightly foldedsigmoids with the original fabric obliterated by the prolongedpressure-solution (Fig. 3b).

Under the optical microscope, fault rocks from low-strain do-mains are characterized by undeformed fossils, blocky calcite veinsand thin tooth-shaped stylolites, probably inherited by previousdeformation stages (Fig. 3c and d). In high-strain domains, i.e. to-wards the boundaries of sigmoids, fabrics or fossils of the protolithare not recognizable. We observe alternation of calcite veins andpressure-solution seams thatdefineamicroscopic fabric sub-parallelto the shear direction (Fig. 3e and f). Calcites tend to develop into arod-shaped, fibrous habitus although some crack-and-seal textureshave been recognized. Pressure-solution and mechanical twinningrework thefibrous calcite rods.Microscopic foliated fabric andcalcitereworking are more intense in the marly tectonite (Fig. 3f).

SEM investigation confirms the characteristic S-CC0 texture,defined by stylolites and sigmoids, is also present at the microscale.Fig. 4 shows a series of SEM-BSE microstructures from undeformed(Fig. 4a) to progressively more deformed portions (Fig. 4b and c) ofa marly tectonite sigmoid. Stylolites evolve from rough, teeth-shaped and isolated segments in undeformed domains (Fig. 4b)towards smooth, continuous and with multiple thick strands thatbound microscopic sigmoids in the fully developed tectonite(Fig. 4c). Microscopic sigmoids consist of aggregates of originallimestone, commonly with microfossils truncated by pressure-solution, and/or reworked calcite veins (Fig. 4c).

4. The Spoleto thrust

The Spoleto thrust is an extensively studied (Lotti, 1905;Decandia, 1982; Barchi and Brozzetti, 1991), regional-scale thrust

with at least 5e10 km of displacement exhumed from about 1.6 kmdepth (Fig. 5a). A rather flat thrust surface juxtaposes Jurassicmassive limestone (Calcare Massiccio Fm) and its layered cover(Corniola Fm) onto the Cretaceous multilayer (Maiolica, Marne aFucoidi and Scaglia group Fms).

4.1. Fault zone architecture and fault rocks

We studied an outcrop where the contact between massivelimestones (Calcare Massiccio Fm) on top of a cataclasite body

Fig. 4. SEM-BSE images showing microstructural evolution from the interior to theouter portion of a marly tectonite sigmoid. (a) Pocket of marly limestone with calcitegrains (light BSE contrast) and interstitial material trapped in the porosity at grainboundaries. (b) Initially discontinuous and teeth-shaped stylolites (bottom) evolve intosmooth and continuous forms (up) through progressive dissolution of calcite grainsand concentration of insoluble material. (c) S-CC0 fabric at the microscale: calcite veins(uniform light BSE contrast) ad marly limestone pockets (heterogeneous BSE contrast)with sigmoidal shapes bounded by thick pressure-solution seams (dark BSE contrast).

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derived from layered red limestones (Scaglia group) is continuouslyexposed for more than 40 m along the transport direction: thisallows detailed study of the fault zone (Fig. 5b). The total width ofthe emerging fault zone is 15e20 m, mainly consisting of FW cat-aclasites although the transition to the undeformed protolith is notexposed. The contact is defined by a sharp Principal Slip Zone (PSZ)that separates HW and FW fault rocks (Fig. 5b). The HW is ratherundeformed and presents a comminuted and indurated layer ofcataclasites a few centimetres thick that can be easily recognizedonly in saw-cuts of hand specimens. Below the PSZ an extensivebody, >15 m thick, of red, coarse-grained foliated cataclasitederived from the FW (Scaglia group) is present. A discontinuous,extremely comminuted, pale-red cataclasite layer and isolatedlenses of HW white limestone are sometimes present between thePSZ and the coarse cataclasite (Fig. 5c).

The PSZ gently dips towards West and has a sinuous traceforming a series of metre-scale compressional stepovers where thePSZ ramps. In front of compressional stepovers, cataclasite of thelower body has a smaller mean clast size (Fig. 5d). Within the HW,metre-scale lenses of limestone have been detected in correspon-dence of PSZ bends (Fig. 5e). These lenses are separated by smoothfracture surfaces with negligible slip, which link with the PSZ. Thus,geometrical asperities favour an adhesive-type of wear (e.g.Swanson, 2005) with the FW block characterized by a soft-asperity

behaviour with progressive accumulation of comminuted cata-clasite and the HW characterized by a hard-asperity behaviour withdetached lenses of limestone at fault bends where stressconcentrates.

It is important to emphasize that such a fault zone structure,with awell developed cataclastic body formed from the layered redlimestones of the Scaglia group, is not ubiquitous along the wholethrust. Other outcrops of the same fault, with the same strati-graphic superposition (Calcare Massiccio onto Scaglia group) arecharacterized by S-CC0 tectonites developed within the Scagliagroup. This implies that the deformation mechanism can changefrom cataclasis-dominated to diffusion-dominated from one loca-tion to another, although common features such as the sharpboundary between HW and FW and asymmetric deformation aremaintained (see Section 6).

4.2. Microstructural analysis

Hand samples of the indurated HW cataclasite, usually a fewcentimetres thick, are constituted by bands of comminuted cat-aclasites of different colours ranging from white to dark grey(Fig. 6a and b). Clasts grain size ranges from sub-centimetric tosub-millimetric and clasts are usually well rounded andembedded in a reddish fine matrix. Within this cataclasite, the

Fig. 5. Spoleto thrust fault overview. (a) Geological cross section. (b) The Principal Slip Zone (PSZ) separates poorly deformed massive limestones and several metres thick cat-aclasites derived from layered limestones in the FW. (c) Extremely comminuted cataclasite with FW affinity in small lenses below the PSZ. (d) Compressional stepover corre-sponding to a thrust ramp. FW cataclasites have reduced clast size in front of the jog. (e) Sigmoidal lens of massive limestone separated from the HW by a small slip surface.

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original HW limestone rich in bioclasts and ooids, is sometimespreserved very close to the PSZ. Fine cataclasite bands are char-acterized by lobated (fluid-like) contacts and are sometimesbounded by pressure-solution seams (Fig. 6b). Close to the PSZ, afine reddish layer and calcite veins abruptly cut cataclasite clasts(Fig. 6bee).

We sampled fresh, unaltered fragments of the PSZ lying betweenthe HW and the FW cataclasites. The PSZ is often constituted bysmooth and polished striated surfaces (Fig. 6c). In thin section weobserve a highly comminuted cataclasite with rounded clasts that issharply truncated by an ultracataclasite layer, interpreted as thePrincipal Slip Zone itself (Fig. 6d). The coarser cataclasite above thePSZ is highlymature due to prolonged shear although in some placesthe original sedimentary texture of the HW is preserved (Fig. 6e).Grains are internally strongly deformed and some of them appear asreworked calcite veins and ultracataclasite aggregates. The PSZ

ultracataclasite is usually 0e400 mm thick and is entirely composedby calcite crystals. The PSZ lies betweenHW-derived and FW-derivedcataclasites (Fig. 6e), demonstrating that some km of displacementhas occurred in this thin zone. The PSZ ultracataclasite layer is closelyassociated with 50e1000 mm thick veins (Fig. 6def) constituted bylarge crystals of blocky twinned calcite that in some locationcompletely replace the ultracataclasite layer. The thick veins are, inturn, locally overprinted (crushed) by a fine ultracataclasite (Fig. 6e).Locally, the PSZ continuity is interrupted by brown patches rich inpoorly crystalline silicate material (Fig. 6f). SEM and TransmissionElectron Microscope analyses of the silicatic material revealed acomplexmineralogical association of partially decomposed euhedralcalcite crystals, vesicles, amorphous material with illite-smectitecomposition and skeletal calcite crystals (Fig. 6g). These micro-structures have been interpreted as the result of rock thermaldecomposition induced by seismic slip (Collettini et al., 2013).

Fig. 6. (a) and (b): Hand samples of fine-grained and indurated HW cataclasite. Calcite rounded clasts are embedded in a reddish fine matrix and different cataclasite generationsshow lobate boundaries, sometimes affected by pressure-solution. (c) Smooth and striated fragment of the Principal Slip Zone. (d) Optical microscope microphotograph of thePrincipal Slip Zone: ultracataclastic layers and sub-horizontal calcite veins truncate the HW cataclasite. (e) Intact HW rocks (bioclasts) preserved above the PSZ. Calcite veins showsyntaxial growth and subsequent reworking (white arrow). The thin ultracataclasite layer is broken in many small strands generated by different cataclastic events (black arrows).(f) Brown silicate material (arrow) in occasional patches along the PSZ. (g) SEM image of the brown material showing calcite skeletal crystals within an amorphous silicate matrix(dark BSE contrast). We interpret these structures as the result of seismic-induced thermal decomposition. (h) Hand sample of the FW coarse cataclasite with poorly rounded clastsand several deformation bands with reduced clast size (arrows). (i) Microstructure of FW cataclasites showing a calcite-filled crack that accommodates a small shear displacement(For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).

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The coarse FW cataclasite (Fig. 6h) is composed of centimetre-to millimetre-size angular clasts of red and white, fossil-richlimestone and displays less mature cataclastic fabric with respectto the HW cataclasite. Large limestone clasts are not embedded in afiner matrix but usually touch each other. Smaller clasts areconcentrated inmore deformed bandswith anastomosing patterns.In these bands, clasts are bounded by tooth-shaped stylolitesformed by pressure-solution. Internal structure of the limestonefragments shows a complex array of stylolites and calcite veins.Fractures sealed by calcite veins reactivated previous stylolites andaccommodated small displacements as demonstrated by truncatedand displaced microfossils (Fig. 6i).

5. The Fiastrone thrust

This fault is part of the frontal thrust of the Umbria-MarcheApennines (Lavecchia, 1979, 1985). In the Fiastrone river gorge,the Mesozoic succession from the Calcare Massiccio Fm up to theScaglia group is folded and overturned over the marls of the ScagliaCinereaeBisciaro Fms (Fig. 7a). The fault accommodates at least6 km of shortening (Mazzoli et al., 2005) and has a complex tra-jectory with a heterogeneous shear zone, exhumed from about1.5 km depth (Fig. 7a). The extensive exposure allows the obser-vation of the architecture and fault rocks evolution of a shear zonethat propagates through different structural levels.

5.1. Fault zone architecture and fault rocks

The fault zone in the Fiastrone gorge is particularly thick (up to200 m) and crops out for several hundreds of metres along boththe Fiastrone riverbed and the Grotta dei Frati outcrop over theflank of the mountain (Fig. 7a and b). A localized slip zone, towhich we refer as the “roof thrust”, occurs at the contact ofmassive limestone with layered limestone and propagatesthrough the overlying carbonate multilayer with a ramp-flat tra-jectory (Fig. 7a and b). The ramp-flat geometry has probably beeninduced by deflection of the first faulting trajectory during thepropagation of the fault through layers with different rheology. Aseries of metric-to-decametric lenses derived from the HW alignabove the localized slip zone. These lenses are found in corre-spondence with minor ramps in the main thrust in a similarfashion to the Spoleto thrust (Fig. 7b). The roof thrust is consti-tuted by a sharp, striated Principal Slip Zone (PSZ) below therather undeformed HW massive limestones. The HW massivelimestone is only affected by minor calcite veins and macroscopicevidence of cataclasis is lacking.

In the Fiastrone riverbed we observe a shear zone rich in S-CC0

tectonites derived from bedded limestones (Scaglia group). Theshear zone developed in front of the massive limestone indenterwithin the bend of the roof thrust (Fig. 7a and b). The S-CC0 tectonitebody is wedge-shaped and accommodated the smoothing of theinitial ramp-flat trajectory of the roof thrust by progressive shearing.The S-CC0 tectonites (Fig. 7c andd) completely obliterate thebeddingof the overturned anticline. Locally, lenses of massive limestone(Calcare Massiccio Fm) or bedded limestones (Maiolica Fm) occurwithin the S-CC0 tectonites or in contact with the PSZ (Fig. 7c).

Transition to the FW calcareous marls of the Scaglia Variegata-Cinerea Fms is gradual and characterized by a series of minor basalthrusts and by the reduction of the cleavage intensity. Chevronfolds that are coherent with the thrust emplacement direction havebeen observed in the FW (e.g. Lavecchia, 1979).

Where the thrust cuts higher structural levels, it juxtaposes theMaiolica Fm, composed of pure and fine grained limestone beds, overthe Scaglia group characterized by a highermarly fraction (Grotta deiFrati outcrop, Fig. 7a). Fault propagation through the Maiolica Fm

produces disruption of the stratification and the development of afoliated cataclasite characterized by YPR fabric (Fig. 7e).

5.2. Microstructural analysis

Hand observation of the PSZ of roof thrust fault rocks retrievedalong the riverbed shows that the HWrock is formed by two distinctcataclasites, one white and one grey, extensively cut by thick veins(Fig. 8a). Under the optical microscope, the two cataclasites showdifferent features: the light cataclasite occasionally shows originalbioclastic texture of the HW Calcare Massiccio Fm, the dark cata-clasite is composed by shards of ultrafine grained limestone withmicrofossils that can be attributed to the Maiolica Fm from theoverturned limb of the HW anticline (cf. Fig. 7a). The contact be-tween the two formations is very sharp and thus this feature can beconsidered a previous PSZ accommodating significant displace-ment. In thin section the contact appears as a diffuse zone, severalmillimetres thick, of very fine-grained cataclasite (Fig. 8b).

Just below the PSZ, fault rock developed within the Scaglia Fmdeforms only by pressure-solution and frictional sliding producingan S-CC0 tectonite with well defined sigmoids often bounded bycalcite fibrous veins. Within the S-CC0 tectonite, bands of moreintense deformation, with S planes almost parallel to shear C sur-faces (Fig. 8c), alternate with less deformed domains. Moredeformed domains are characterized by: (1) smaller and moreclosely spaced sigmoids with frequent C0-type shear surfaces oftencoated with fibrous calcite veins (Fig. 8c right side) and (2) higherconcentration of clay. Sigmoids in highly strained domains showless fossiliferous content and more frequent stylolites compared toless strained sigmoids. Calcite slickenfibres developed in stylolitejogs and at sigmoid boundaries (Fig. 8d).

YPR cataclasites developed in the Maiolica Fm are found in theGrotta dei Frati outcrop and in lenses embedded in the Fiastroneriverbed shear zone. The YPR cataclasite consists of very finegrained limestones with microfossils, which have been extensivelybroken by veining episodes at the site of previous stylolites (the Pfoliation, Fig. 8e). The internal structure of the cataclasite showsintensive hydrofracturing (Fig. 8f).

6. Discussion

In this section we delineate a general model for a thrust cuttingthrough a carbonatic multilayer on the basis of fault zone obser-vations presented in previous sections and summarized in Table 1.

Fault architecture and fault rocks occurring in thrusts of theUmbria-Marche Apennines show remarkable differences and highspatial variability in spite of the fact that they are all exhumed fromsimilar PeT conditions. This variability arises from both heteroge-neities in the faulted multilayer and/or different strain rates. Fig. 9exemplifies the along-dip and along-strike (km scale) structure of asingle thrust in which competence contrasts (clayley vs. non-clayley formations) are highlighted. Faults described in previoussections represent examples of different parts of the modelledthrust (Fig. 9). Displacement of the thrust sheet increases towardsthe centre of the structure from the tip points (in map view, Fig. 9inset). Within the same fault we observe significant differences inthe overall internal architecture and changes in deformationmechanisms between parts of the fault cutting through marly ho-rizons and parts developed in more competent formations.

6.1. Lithology vs. brittleeductile transition

Fault architecture is strongly tied to the lithology of the pro-tolith: ductile shear zones with distributed deformation, witheither YPR cataclasites or S-CC0 tectonites, develop within bedded

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limestones and marly formations or in front of ramps in the faulttrajectory (e.g. the Coscerno and Fiastrone shear zones respec-tively, Fig. 9). Localized brittle faulting occurs only along theboundaries of massive limestone (e.g. Spoleto, Fiastrone riverbed;Fig. 9, Table 1).

Large scale fault zone geometry and internal structure arestrongly influenced by the competence contrast between Hang-ingwall and Footwall rocks. In the ductile Coscerno thrust, wherethe faulting occurs between bedded limestones and marls, theboundaries of the fault core are diffuse, smooth and character-ized by progressive fading of the bedding. The shear zone lacksany large geometrical asperities and marly formations are me-chanically interdigitated. Moreover small normal faults that

dislocate relatively undeformed HW blocks and abundant C0

shear planes suggest that the the shear zone is smeared parallelto the tectonic transport direction. We interpret this thinning asthe result of prolonged simple-shear deformation along an effi-cient detachment, i.e. a weak fault, where slip is energeticallyfavoured.

In the Spoleto and Fiastrone faults, where the competencecontrast between formations is higher (i.e. massive limestones overbedded limestones), fault zones tend to widen around geometricalirregularities and damage accumulates in weaker lithologies,consistent with classic fault models (e.g. Sibson, 1986; Chester andChester,1998; Scholz, 2002 and references therein; Ben-Zion, 2008;Mitchell and Faulkner, 2009). Irregularities in the fault zone

Fig. 7. Fiastrone thrust fault zone and fault rocks. (a) Geological cross section. (b) View of the fault zone from the Grotta dei Frati outcrop: anastomosing roof thrust underlain by theshear zone. The roof thrust is rather sharp at the contact with massive limestones and diffuses into the shear zone as it propagates through the layered cover. Lenses of massivelimestone detach from the HW in correspondence of bends of the roof thrust. (c) Competent lens of Maiolica limestones embedded within the shear zone. (d) S-CC0 tectonitesdeveloped from Scaglia group layered limestones in the Fiastrone riverbed. (e) Foliated YPR cataclasites developed from Maiolica Fm layered limestones in the Grotta dei FratiOutcrop (coin for scale).

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architecture with contrasting rheology can be explained by bothinherited geometrical complexity and different responses of thelithology to the imposed stress.

Geometrical complexity (Fig. 9) can be induced by both previousdeformation structures (e.g. folds) and initial flat-ramp-flat trajec-tories of faults through the intact multilayer (e.g. Bally et al., 1966;Butler, 1982; Davison, 1987 among many others) as in the caseof the Fiastrone thrust. Moreover, since we studied outcrops ofmature faults, it is likely that the lithological influence on faultgeometry andmechanics is effective at geologic time-scales. Indeedthe flat-ramp-flat trajectory, inherited from the initial faulting, issmoothed by the progressive development of shear zones (Figs. 7a,b and 9). At the same time, fault “roughness” is preserved because

deformation develops asymmetrically within the weaker forma-tions under the same regional stress.

This brittleeductile transition is therefore highly sensitive to theinitial stratigraphic configuration of competent vs. incompetentformations and to the spacing of the bedding. This observation mayjustify the differences with other carbonate-hosted fault modelsdescribed in the literature from clay-free and poorly bedded se-quences (e.g. Agosta and Aydin, 2006; Billi, 2010).

6.2. Lithology vs. deformation mechanisms

From a failure mechanism point of view, rocks deform by cata-clasis, diffusional mass transfer and intracrystalline plasticity (e.g.

Fig. 8. (a) Rock fragment of the roof thrust from the Fiastrone riverbed showing a sharp, Principal Slip Zone (PSZ, solid line) that truncates a previous PSZ (dashed line) thatjuxtaposes two different limestones both fractured by thick calcite veins. (b) Optical microscope structure of the PSZ: bands of fine-grained cataclasite, mutually crosscutting, withcalcite veins separating HW bioclastic limestone from pelagic FW limestone. (c) Interchanging C and S shear surfaces in hand sample of S-CC0 tectonites from the Fiastrone riverbed.A C0 shear surface cuts previous SeC structures and opens as a shear vein. (d) Microphotograph of fibrous calcite bands that developed by solution transfer at the sigmoid-C0

boundary. (e) Hand sample of YPR cataclasite from the Grotta dei Frati outcrop. P foliation defined by thin calcite veins (white dashed lines) that developed at the site of cleavageweakness. Mutual crosscutting of thick calcite veins (blue) and P veins indicate synchronous deformation. (f) Detail of YPR cataclasite with limestone extensively broken by veinsfilled with blocky calcite (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).

Table 1Summary of macro- and micro-scale observations in the fault zones.

Fault Coscerno Spoleto Fiastrone

Hangingwall lithology Bedded limestones Massive limestone Massive limestone; Bedded limestones (pure)Footwall lithology Marls; Marly limestones Bedded limestones Bedded limestones (some clay)Fault zone architecture Ductile shear zone with diffuse

boundaries (z20 m thick)Localized Principal Slip Zone withgeometric asperities þ Asymmetric cataclasticzone (z15 m)

Localized Principal Slip Zone with geometricasperities above a ductile shear zone(up to 200 m thick)

Fault rocks S-CC0 tectonites Polished PSZ with ultracataclasites þ Cataclasites& YPR cataclasites

PSZ with cataclasites þ S-CC0 tectonites þ YPRcataclasites

Main deformationmechanism

Pressure-solution andfrictional sliding

Cataclasis, hydrofracturing Cataclasis, hydrofracturing (PSZ, YPR cataclasites)Pressure-solution (S-CC0 tectonites)

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McClay, 1977; Sibson, 1977; Rutter, 1986). In the Apennineswe observe a competition between cataclasis, hydrofracturing,pressure-solution and frictional sliding for the accommodation offault slip.

S-CC0 tectonites, generated by pressure-solution and frictionalsliding have been observed in the Scaglia Variegata/Scaglia CinereaFms (marlsþmarly limestone, at the Coscerno outcrop) and Scagliagroup Fms (bedded limestones with marly interbeds, Spoleto,Fiastrone riverbed). On the other hand, fault rocks affected only bycataclasis and hydrofracturing developed in the Calcare MassiccioFm (massive, pure limestone, Spoleto, Fiastrone riverbed). TheScaglia group Fms (Spoleto thrust) and Maiolica Fm (bedded purelimestone, Fiastrone roof thrust) developed distributed shear zoneswith either S-CC0 tectonites or foliated YPR cataclasites (summaryin Table 1). Foliated cataclasites show the interplay of pressure-solution and brittle processes at the microscale. Cataclasis islimited to and distributed throughout the shear zone, whilehydrofracturing reactivates stylolites as Mode I cracks (e.g. Figs. 6iand 8e). The competition of such deformation mechanisms iscommonly observed in the same lithologies at the initial stages offault development: shear tends to nucleate around previous weakpressure-solution seams (e.g. Willemse et al., 1997; Petracchiniet al., 2012).

Common conceptual models, usually describe changes indeformation mechanism as a function of an increase in depth (e.g.Elliott, 1976; Sibson, 1986; Gratier and Gamond, 1990; Chester,1995; Imber et al., 2001). Since mineralogical reactivity, diffusivemass transfer, and intracrystalline plasticity increase with P, T andfluid regime (e.g. Weyl, 1959; Elliott, 1973; McClay, 1977; Rutter,1983; Wintsch et al., 1995), a shift in the deformation mechanismwith increasing depth/temperature is likely and well documented(e.g. Elliott, 1976; Sibson, 1977; Holl and Anastasio, 1995; White,2001). Indeed, cataclasis is commonly observed as the maindeformation mechanism for limestone hosted faults in the upperkms of the crust (e.g. Agosta and Kirschner, 2003; Benedicto et al.,2008; Billi, 2010). However, an increase of pressure-solutioncleavage with clay content and within fault zones has been

recognized in the Northern Apennines (e.g. Alvarez et al., 1978;Lavecchia, 1985; Ghisetti, 1987; Tavarnelli, 1997; Bussolotto et al.,2007; Brogi and Fabbrini, 2010; Calamita et al., 2012; Meneghiniet al., 2012) and in other parts of the world (e.g. Heald, 1956;Marshak and Engelder, 1985; Groshong, 1988 and referencestherein; Kimura et al., 2012). All these observations are consistentwith theoreticaleexperimental evidence that demonstrateenhanced pressure-solution in the presence of clay minerals (e.g.Weyl, 1959; Renard et al., 1997; Bos et al., 2000; Renard et al., 2001;Niemeijer and Spiers, 2005). This leads to an apparent discrepancybetween different geological observations about the factors influ-encing the transition from fracture-dominated to diffusive transfer-dominated deformation.

Pressure-solution depends on PeT conditions, strain rate ( _ε) andthe presence of fluids but at given conditions lithology might play akey role in the transition (Fig. 9).

We here propose a micromechanical, semi-quantitative expla-nation highlighting the role of lithology in the fracture vs. diffusivetransfer transition. Microstructural analysis allows to unravel howthe S-CC0 fabrics progressively develop with deformation. S-CC0

tectonites from the Coscerno and Fiastrone riverbed outcrops showevidence of dissolution and development of fibrous calcite veinswhich is consistent with continuous diffusive mass transferwithout significant cataclasis of the rock (e.g. Passchier and Trouw,2005; Bons et al., 2012, Figs. 3c, d and 8d).

Microstructural evolution from undeformed to deformed do-mains (Fig. 4) can be considered as a proxy of the temporal evo-lution from the protolith to the fault rock. With progressivepressure-solution, catalysed by the presence of clay, insoluble andautigenic minerals concentrate in pressure-solution seams (e.g.phyllosilicates, apatite and quartz). Frictional sliding is needed toform S-CC0 fabric and accommodate further strain. Frictional slidingis inferred by macroscopic slickensides and slickenfibres that occurin CeC0 and sometimes also in S planes. At the microscale, the onsetof frictional sliding is marked by the formation of micro-sigmoids(Fig. 4c). Frictional sliding is therefore energetically favoured tooccur only when pressure-solution seams reach a critical thickness

Fig. 9. Synoptic scheme of a carbonate-bearing Apennine thrust. Vertical dimensions are exaggerated. The inset shows a simplified map view of the fault trace with displacementincreasing towards the centre of the structure. Along strike, the fault cuts progressively different structural levels where the displacement decreases. Along dip, the flat-ramp-flatgeometry of the fault defines a large scale geometrical complexity accommodated by progressive development of ductile shear zones. Brittle-ductile transition along the fault occursat lithostratigraphical changes and in front of geometrical asperities (see text for a discussion).

T. Tesei et al. / Journal of Structural Geology 55 (2013) 1e15 11

and interlocking asperities are dissolved (e.g. tooth-shaped,immature stylolites).

Under the same (low) PeT and conditions rocks with more claycontent tend to quickly develop pressure-solution fabrics toaccommodate tectonic stresses whereas in more pure limestonesthe kinetics of pressure-solution is hindered and cataclasis andhydraulic fracturing are energetically favoured. Indeed, cataclasis-dominated rocks present veining with blocky or reworked calcitethat testifies dilatancy and overpressure within the fault rock (e.g.in the Spoleto thrust).

For lithologies with intermediate clay content (e.g. the Scagliagroup Fms), both mechanisms concur to the deformation (e.g.Hadizadeh, 1994). Lithological heterogeneities in the protolith, likeparts of the formation with less clay, or fluctuations in the strainrate experienced by the fault rock favour either cataclasis orpressure-solution during the deformation. If we compute thesimple shear strain experienced by the fault zone as ε ¼ d=2Dh,where d is the fault displacement and Dh is the fault zone thickness(e.g. Turcotte and Schubert, 2002), then the trajectory and the faultzone architecture influence the strain rate experienced by the faultrocks (Fig. 10) in a given amount of time (e.g. Fagereng and Sibson,2010). The fault structure, and thus the strain rate, can determine ashift in deformation mechanism even when the same lithologiesare juxtaposed in the fault zone. When the fault displacement isaccommodated along a single surface/narrow zone, the strain rateis higher and cataclasis is the dominant deformation mechanism,e.g. Scaglia group cataclasites in Spoleto. On the other hand, S-CC0

tectonites derived from the Scaglia group in the Fiastrone riverbed,accommodate deformation in a wide zone (low strain rate, Fig. 9)between the roof and the basal thrust (Fig. 7a). These field-basedinferences are consistent with slip and stress concentrationobserved in 3D seismic images and numerical modelling ofbranching faults (e.g. Walsh et al., 2003; Soliva et al., 2008).

6.3. Seismic behaviour of complex carbonate-bearing faults

Faults presented in this manuscript can be considered as ancientand exhumed analogues of the seismogenic faults presently activein the outer fronts of Northern Apennines (e.g. the southern part ofthe Po Plain), where seismic reflection profiles show that carbon-ates cropping out in the Umbria-Marche Apennines are presentdown to 15 km depth (Pieri and Groppi, 1981; Massoli et al., 2006).In the same area, compressional earthquakes, such as the M5.9 andM5.8 Emilia earthquakes in 2012 (Ventura and Di Giovambattista,2013), occur within the first 15 km of the crust.

Our structural analysis of fault architecture and deformationmechanisms performed along three large scale and exhumedthrusts suggests that the heterogeneous faulting style could resultin heterogeneous seismic behaviour of various fault portions.

Although convincing evidence of seismic faulting in the geologicrecord, excluding rare pseudotachylytes, has been provenextremely difficult to find (e.g. Cowan, 1999; Niemeijer et al., 2012),theoretical (e.g. Rice and Rudnicki, 1980; Rice, 2006; Marone et al.,2009), geological (e.g. Chester and Chester, 1998; Sibson, 2003) andexperimental (e.g. Paterson, 1958; Marone, 1998; Di Toro et al.,2011) work suggests that seismic slip occurs within suddenlylocalized shear zones and is promoted by thermally-induced dy-namic weakening. Fault rocks retrieved along the Spoleto andpossibly the Fiastrone faults meet these requirements. Within theSpoleto Principal Slip Zone, microstructural evidence (Fig. 6d and e)show sharp truncation of the coarser cataclastic matrix suggestingthat the generation of the ultracataclasite layer can be attributed tothe localization of slip in a few hundreds of mm thick layer. Inaddition, thermally decomposed calcite crystals, vesicles andamorphous silicate matrix have been found on the same PSZ andare evidence of high-temperature (>650 #C) along the fault(Collettini et al., 2013). Since the fault has been exhumed from<3 km depth, corresponding to less than 100 #C, twinning of calciteveins is thin (stress build-up at T < 170 #C, Ferrill et al., 2004) andskeletal calcite remnants suggest fast quenching, therefore weconclude that high-temperature products can be the result of africtional heating pulse generated by propagation of a seismicrupture through a very thin principal slip zone (see Collettini et al.,2013). Multiple generations of reworked ultracataclasites layersand veins point to a repeated localization along the fault. Similarbrittle fault rocks have been also found along the Fiastrone riverbedPSZ (Fig. 8b) although conclusive evidence of fast slip rates islacking.

On the other end of the spectrum, fault rocks that show ductileshear and foliation, such as YPR cataclasites and S-CC0 tectoniteshave been considered aseismic (Chester et al., 1985; Sibson, 1986).Due to their foliated nature and the small displacements that can beaccommodated by crack-and-seal veins and discontinuous slipsurfaces, it is unlikely that they are the product of seismic slip (i.e.slip velocity greater than tens of cm/s typical of regular earth-quakes). Potentially, shearing within these rocks may give rise to awhole spectrum of slow-slip phenomena like low- and ultralow-frequency earthquakes, non-volcanic tremors and silent earth-quakes (Rubinstein et al., 2010) that typically do not radiate highfrequency waves and may be broadly ascribed to as “aseismic”.

We therefore hypothesize that a thrust fault such as the onereconstructed in Fig. 9 may also be segmented also from a seismicpoint of view: parts of the fault are governed by ductile, slowshearing where bedded limestones and marly protoliths are pre-sent, whereas in other patches displacement is accommodated byfault zones localized in competent lithologies that are likely tonucleate earthquakes.

The interplay between mechanically heterogeneous thrustpatches is likely to be highly complex. S-CC0 shear zones, foliatedcataclasites and brittle localized faults probably shear at differentrates under a similar regional stress field. This might lead to localstress concentrations in nearby fault zones, in accordance with thedislocation model of slip along large faults (Price, 1988). Thus,ductile shearing can promote “aseismic” loading of seismic faultpatches that can nucleate earthquakes that, in turn, propagate intoslow deforming patches. Moreover, hydrologic properties ofdifferent fault patches are probably heterogeneous. Foliated faultrocks in the shear zones are characterized by a high content of clayminerals organized in interconnected fabrics that may act as effi-cient barriers for fluids (e.g. Caine et al., 1996; Faulkner et al., 2010and references therein). In our model of heterogeneous thrusts,localized brittle zones would be preferential sites of seal breakageand earthquake nucleation during the cyclic fault-valve behaviour(Sibson,1992). Co-seismic stressing and post-seismic fluid drainage

Fig. 10. Schematic cross section of a thrust fault showing strain partitioning modulatedby the fault zone thickness. Given a fixed displacement (d) in the section, lower strainrates are accomodated by wide shear zones compared to localized fault surfaces.

T. Tesei et al. / Journal of Structural Geology 55 (2013) 1e1512

can in turn promote afterslip and “embrittlement” within ductileportions of the fault, leading to aftershock nucleation.

This model of mechanically heterogeneous faulting is consistentwith observations from faults in different lithologies, exhumationconditions and geodynamic settings, in which seismic faulting islikely accommodated in competent lithologies subjected to highstrain rates, whereas “aseismic” behaviour is mechanically fav-oured in incompetent fault rocks with distributed shear (White,2001; Fagereng and Sibson, 2010; Collettini et al., 2011; Kimuraet al., 2012; Gratier et al., 2013).

7. Conclusions

Apennines thrusts display a large variety of architectures andfault rocks. Deformation within each thrust zone can be accom-modated by both cataclastic and pressure-solution-related pro-cesses. At shallow crustal conditions, the transition from cataclasis-dominated to diffusion-dominated deformation is modulated bythe abundance of clay within the fault rock protolith. As the pres-ence of clay speeds up pressure-solution processes, foliated S-CC0

tectonites develop in marly formations whereas pure massivelimestone are affected by cataclasis and localized faulting. Forma-tions with intermediate clay contents develop either ductile cata-clastic zones or pressure-solution tectonites depending on locallithological heterogeneities and/or fault strain rate.

Lithological heterogeneities across fault propagation trajectoriesinduce a long-term mechanical segmentation of the thrust faultwith “strong” or “weak” patches that accommodate strain atdifferent rates and likely have different hydrological properties.These structural characteristics potentially affect the seismicbehaviour of the fault: cataclastic localized patches likely fail byseismic faulting thus representing seismic asperities whereas S-CC0

and YPR tectonites may continuously creep or present other slow-slip phenomena.

Acknowledgements

This research has been carried out within the ERC Starting GrantGLASS (n# 259256). We are grateful to A. Billi and one anonymousreviewer for their comments that helped to improve this paper. Wealso thank B. Carpenter for insightful discussions on early drafts ofthis manuscript.

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