Heterogeneous strength and fault zone complexity of carbonate-bearing thrusts with possible implications for seismicity

Earth and Planetary Science Letters 408 (2014) 307–318

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Earth and Planetary Science Letters www.elsevier.com/locate/epsl

Heterogeneous strength and fault zone complexity of carbonate-bearing thrusts with possible implications for seismicity Telemaco Tesei a,∗ , Cristiano Collettini b,c , Massimiliano R. Barchi a , Brett M. Carpenter c , Giuseppe Di Stefano c a b c

Dipartimento di Fisica e Geologia, Università degli Studi di Perugia, Piazza Università 1, 06123, Perugia, Italy Dipartimento di Scienze della Terra, Università La Sapienza di Roma, P.le A. Moro 5, 00185 Roma, Italy Istituto Nazionale di Geofisica e Vulcanologia, Via di Vigna Murata 605, 00143, Roma, Italy

a r t i c l e

i n f o

Article history: Received 10 July 2014 Received in revised form 9 October 2014 Accepted 11 October 2014 Available online xxxx Editor: P. Shearer Keywords: friction carbonates earthquakes fault

a b s t r a c t The understanding of fault-slip behaviour in carbonates has an important societal impact due to the widespread occurrence and propagation of earthquakes in these rocks. Fault rock variations in carbonates are systematically controlled by the lithology of the faulted protolith: cataclasis and hydraulic fracturing with evidence of past seismic slip commonly affect fault rocks in competent limestone formations whereas widespread pressure-solution and sliding along clay foliation are observed in marly rocks. We performed a series of friction experiments on carbonatic fault rocks sampled from mature thrusts (>2 km displacement) in the Apennines of Italy. We sheared both intact wafers and powdered fault materials at low (10 MPa) and in situ (53 MPa) normal stress under room-humidity and water-saturated conditions. We used velocity steps (1 to 300 μm/s) and slide–hold–slide (3–1000 s holds) to assess the frictional stability and healing behaviour of these rocks. We observe that cataclastic fault rocks derived from competent limestones are characterized by high friction coefficients coupled with significant post-slip restrengthening and velocity-weakening behaviour. Conversely, intact foliated marly tectonites, sheared under the same conditions, show low friction, null post-slip healing and stable velocity-strengthening behaviour suggesting that these rocks deform aseismically. To extrapolate these opposite mechanical behaviours to the entire fault surface we developed a fault model integrating our mechanical data, field observations and balanced geological cross-sections. The mechanical heterogeneities highlighted in the model provide constraints for the distribution of fault patches with higher seismogenic potential. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Tectonic faults in the upper crust are heterogeneous zones where deformation spanning from the kilometre down to the nanoscale accumulates throughout the entire fault history. Their behaviour has important societal implications that include earthquake hazard assessment and the exploitation of water and hydrocarbon resources. The occurrence of earthquakes results from the episodic release of elastic energy in faults by stick–slip motion and thus is linked to the fault rock properties (e.g. Brace and Byerlee, 1966). However, since the discovery of fault creep along the San Andreas fault (Steinbrugge et al., 1960) and the study of motion in giant thrust sheets (e.g. Price, 1988), it has been clear that faults move

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http://dx.doi.org/10.1016/j.epsl.2014.10.021 0012-821X/© 2014 Elsevier B.V. All rights reserved.

in a range of different slip behaviours that includes the traditional stick–slip model of motion. A whole new spectrum of slip phenomena, ranging from low-frequency earthquakes to tremors, has been recently discovered (e.g. Rubinstein et al., 2010). All of these slip phenomena occur in complex fault zones characterized by unsteady and variable slip behaviour, usually inferred from geodetic data (e.g. Prescott et al., 1981; Perfettini et al., 2010; Jolivet et al., 2013; Thomas, 2014) and characterized by heterogeneous materials and variable states of fluid pressure, inferred from seismic velocity and earthquake distributions (e.g. Nakamura et al., 2008; Saffer and Tobin, 2011; Zhao et al., 2011; Valoroso et al., 2014). Fault zones exhumed to the Earth’s surface show an exceedingly high variability in lithology, structure and deformation mechanisms, suggesting that fault rocks may preserve evidence that can explain the mechanics and the complex slip behaviour of faults (e.g. Sibson, 1977; White, 2001; Cowan et al., 2003; Faulkner et al., 2003; Smith et al., 2011; Kimura et al., 2012 and others). For these reasons, fault rocks have been the target of extensive

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Fig. 1. End-member fault zone types in the thrusts of the Northern Apennines: a) Distributed fault zone characterized by S-CC tectonites developed by pressure-solution ductility in marly limestone (Coscerno thrust) and b) Cataclastic fault zone with sharp principal slip surface (PSS) associated with the brittle deformation of massive and pelagic limestones (Spoleto thrust). (For the reference to colour in this figure, the reader is referred to the web version of this article.)

structural and experimental investigation (e.g. Byerlee and Summers, 1976; Sibson, 1977; Byerlee, 1978; Logan et al., 1979; Berthé et al., 1979; Chester and Chester, 1998; Rutter et al., 1986; Sammis et al., 1986; Marone et al., 1998; Collettini et al., 2009; Ikari et al., 2011 and many others). The lithological variability and the geometry of rock bodies, within and around the fault, may ultimately control mechanical processes active within the fault zone, for example by controlling preferential seismic rupture propagation (Ben-Zion and Shi, 2005), strain rates (e.g. Cloos and Shreve, 1988; Fagereng and Sibson, 2010), fluid pressure, friction and the trigger of different deformation mechanisms (e.g. Rice, 2006; Collettini et al., 2011; Kimura et al., 2012). However, fault models that provide a link between the complexities of fault structures and earthquake potential are not common, especially for carbonates. Here, we aim to contribute to this topic by presenting the results of friction experiments on carbonatic fault rocks, derived from mature faults (displacement of several km). These faults, exposed in the Northern Apennines, represent the exhumed analogues of structures presently active at depth and display a variety of fault architecture and rock deformation mechanisms (Tesei et al., 2013). We then discuss the intimate link between mechanical results and existing field and microstructural observations and provide a lithology-dependent model for the mechanical behaviour and seismic potential of carbonate-bearing thrust faults. 2. Geological background In the Northern Apennines of Italy (Umbria–Marche region), a thick carbonatic sequence constitutes the backbone of the outcropping thrust belt. The stratigraphic succession of the region is representative of the lithological variability of marine carbonate rocks, including massive platform limestone at the base of the sequence overlain by finely bedded pelagic limestone intercalated with marly formations (e.g. Barchi et al., 2012). The mechanical heterogeneity of the multilayer and the strong competence contrasts control the geometry of the thrusts, inducing disharmonic tectonic stacking and flat–ramp–flat geometries. The compressional tectonics in the Apennines migrated with time from west to east (Barchi et al., 1998; Carminati et al., 2012). The result of this migration is that active compressional structures are located beneath the Po Plain and in the Adriatic foreland (e.g. Doglioni, 1993; Montone et al., 2004; Govoni et al., 2014) whereas ancient and exhumed thrust faults are exposed in the axial zone of the northern Apennines, e.g. Umbria–Marche region. Major thrusts

exposed in the Umbria–Marche Apennines are mature faults, usually tens of kilometres long that accumulated several kilometres of displacement and have been exhumed from a minimum depth of 2 km that corresponds to the thickness of the faulted sedimentary sequence (Tesei et al., 2013 and references therein). However, even lacking convincing geological evidence, we cannot exclude exhumation of these rocks from larger depth because of multiple thrust stacking and/or fault growth below foredeep deposits (now eroded). In the Umbria–Marche Apennines, different types of fault zones develop mostly as a response to lithological contrasts (Fig. 1) along the same fault trajectory: ductile fault zones develop predominantly in marls and finely bedded pelagic limestone (Alvarez et al., 1978; Koopman, 1983) whereas brittle zones develop in competent limestones (e.g. Tesei et al., 2013). Within ductile fault zones (Fig. 1a), thick, up to 200 m, bodies of foliated S-CC tectonites (Berthé et al., 1979) develop around principal slip planes and locally disrupt their continuity. Conversely, brittle zones (Fig. 1b) are characterized by sharp principal slip planes and cataclastic fault rocks (Sibson, 1977) that usually develop in more competent lithologies. Deformation in pure limestone lithologies occurs along strongly localized and polished slip planes (e.g. Smith et al., 2011; Fondriest et al., 2013) that present microstructures indicating thermal decomposition (Collettini et al., 2013; Bullock et al., 2014). This evidence suggests that strong portions of the fault might be prone to seismic faulting. On the other hand, pressure solution in combination with ductile deformation along the foliation (mostly interlayer sliding, dislocations and nanofaults in smectite crystals; Viti et al., 2014) occur within S-CC tectonites derived from marls or bedded marly limestones, suggesting a dominantly aseismic mode of deformation. The competition of cataclastic and diffusive processes is common in carbonatic fault rocks derived from lithologies having intermediate competence, such as bedded pelagic limestone, and is evidenced by the occurrence of foliated breccias/cataclasites. One of the factors that control the switch between cataclastic and diffusive deformation mechanisms in carbonatic fault rocks is the abundance of clay minerals that enhance pressure-solution of calcite (e.g. Gratier et al., 2013). 3. Experimental methods Since the observed fault rocks of the Apennines thrusts are represented by 3 end-members (cataclasites, S-CC tectonites and

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Fig. 2. Experimental materials and representative friction curves. a) Powdered sample (simulated cataclasite). b) Intact S-CC tectonite wafer sheared along the natural sense of shear. c) Simulated fault plane constituted by a slab of polished massive limestone (right) and fault gouge derived from cataclasites (left). d) Representative experimental curves showing the evolution of friction with slip in a simulated cataclasite (red) and intact S-CC wafers (blue). Velocity step and slide–hold–slide sequences start after friction reaches a steady-state value. Inset: double direct shear configuration, in which three steel blocks are compressed and then shear is imposed through the movement of the central block. (For interpretation of the references to colour in this figure, the reader is referred to the web version of this article.)

principal fault planes, e.g. Fig. 1), we performed friction experiments to reproduce these fault rocks in the lab and to tie together the observed fault complexity with fault mechanical properties. We collected cataclasites derived from pelagic limestone and from massive platform limestone cropping out in the Spoleto thrust (Barchi and Brozzetti, 1991). S-CC tectonites, derived from either marls or pelagic limestone, have been sampled at the Coscerno and Fiastrone thrusts (Barchi and Lemmi, 1996; Lavecchia, 1979). Cataclastic rocks are composed almost entirely of calcite, with minor clay amounts (Viti et al., 2014), whereas the tectonites have a significant clay fraction, qualitatively ranging from 20 to 50%, as estimated in thin section. Clay mineralogy is mostly represented by smectite (Viti et al., 2014). To obtain experimental samples, each type of fault rock was crushed in a disk mill and sieved to obtain powders with a grain size <150 μm, in order to simulate the natural cataclasites (Fig. 2a). However, the foliated fabric of some fault rocks is known to dramatically influence their frictional behaviour (Collettini et al., 2009), therefore we tested solid intact wafers of S-CC tectonites (“wafers”, Fig. 2b). To obtain intact wafers, we selected finely foliated, marl-derived, S-CC tectonites from the Coscerno thrust. Samples were carefully cut using a precision rotary blade to produce wafers of 5 × 5 cm2 nominal area and approximately 1–1.2 cm thick in order to capture the natural heterogeneity of the rock. Wafers were cut with shearing surfaces parallel to the naturally-occurring C planes and were oriented in the in situ sense of shear during the experiments, following the method described in Collettini et al. (2009). In addition, two experiments were carried out to simulate slip on a sharp, smooth fault plane, as in the example of extreme localization documented in the Spoleto thrust (Fig. 1b). In this arrangement, slabs of oolitic limestone were polished with #1000 grit sandpaper to simulate roughness of the natural fault plane, and then juxtaposed to powders derived from pelagic calcareous mudstone (Fig. 2c). We performed friction experiments in a servo-controlled biaxial deformation apparatus in the High Pressure–High Temperature Laboratory of the INGV in Rome, Italy (Collettini et al., 2014a). Applied force were measured via stainless steel submersible load cells with 0.03 kN resolution and load points displacements were measured via LVDT sensors with ±0.1 μm resolution.

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In the majority of our experiments, we used an unconfined “double-direct” shear configuration (e.g. Dieterich, 1972) in which two identical layers of sample are sandwiched between a threeblock assembly and simultaneously sheared (Fig. 2 inset). Sliding area is maintained constant throughout the experiment (25 cm2 ) with an approximate initial sample thickness of 0.8 cm (except for intact wafers). In the two experiments where we sheared powder against a solid slab (simulated fault planes), the assembly was sheared in a single-direct experimental configuration (e.g. Carpenter et al., 2012). In this configuration, the sliding area is also maintained constant throughout the experiment (16 cm2 ) During each experiment, normal stress (σn ) is maintained constant at 10 MPa or 53 MPa, i.e. the estimated in situ normal stress inferred from the minimum exhumation of these faults, corresponding to about 2 km. Tests on simulated smooth planes were performed at 10 MPa normal stress only, due to limited strength of the unconfined limestone slabs in the single-direct configuration. Initially, normal stress is increased to the target value, shearing is then initiated by imposing a load point velocity of 10 μm/s to the central block until a steady state shear stress is attained, usually after ∼10 mm of displacement (Fig. 2d). Friction is calculated from the Coulomb criterion as the ratio between the measured shear and normal stresses. After this initial shearing, we impose a series of computer-controlled changes in sliding velocity to test frictional stability and healing behaviour of our materials, which we address within the framework of Rate-State friction (e.g. Marone, 1998). We impose velocity step sequences of 1 to 300 μm/s, with a constant displacement of 1 mm at each step. We subsequently modelled the resulting friction curves with the Dieterich, timedependent, version of the rate-state friction equation (Dieterich, 1979) and calculated the friction rate parameter, (a − b). This parameter is defined as:

(a − b) = μss / ln( V ) Positive values of (a − b) indicate an increase in strength with accelerating slip, which results in stable sliding behaviour, whereas negative values indicate the potential for the material to host slip instabilities due to decreasing frictional strength with increasing velocity. Frictional healing behaviour was studied with slide–hold–slide sequences during which sliding at the background velocity of 10 μm/s is halted for a given period of time (3, 10, 30, 100, 300 and 1000 s) and then resumed. Peak friction measured after each hold represents the static friction that has to be overcome to continue to slide and the difference of peak friction with steady-state dynamic friction (μ) represents the frictional healing μ. Frictional healing rates (β ) have been calculated for all materials as:

β = μ/ log10 (t ). Where t is the hold time in seconds. The evaluation of frictional healing is crucial to assess the ability of a material to accumulate elastic strain energy in the immediate aftermath of an earthquake and thus allow fault locking and stress drop in the next slip event. The total shear displacement achieved during our experiments ranged from 22 to 35 mm. Experiments were carried out under both room-dry and saturated conditions. Saturation of the samples was accomplished by wrapping the experimental assembly in an impermeable plastic membrane filled with hard water (366.4 mg/l HCO3 and 91.3 mg/l Ca in solution) at the beginning of the experiment. As fluid introduction into the assembly typically induces enhanced compaction of the experimental material that usually lasted for 30–40 min. We only began shearing at the end of such fluid-induced compaction. In addition, for the low permeability S-CC tectonite samples, the rocks were kept under conditions of 100% relative humidity for

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Fig. 3. a) Friction results (measured at 10 μm/s sliding velocity) for saturated experiments. Samples derived from cataclasites and S-CC tectonites are shown in warm cold colours respectively. All powdered materials and the fault plane analogue show friction close to 0.6. Intact wafers show low friction coefficient. Post-experimental microstructures: b) deformation in powdered material (i120) is characterized by cataclasis and grain size reduction (GSR). c) in powders, cataclastic deformation is organized along Y and R 1 shear bands (i020). d) detail of a cataclastic R 1 shear band with angular clasts <1 μm size. e) Intact S-CC wafer (i026) shows shearing accommodated by sliding along the pre-existing and interconnected clay-rich network (brown material) around sigmoidal calcite veins. f) and g) Details of the iso-oriented clay lamellae in contact with calcite. (For interpretation of the references to colour in this figure, the reader is referred to the web version of this article.)

several weeks before the experiments. At the end of each experiment, rock layers were carefully extracted and impregnated with epoxy resin for subsequent microstructural study. A list of all performed tests, type of material and experimental conditions can be found in the Supplementary Material Table 1. 4. Frictional behaviour of fault rocks 4.1. First-order frictional strength and experimental microstructures To determine the evolution of the coefficient of friction with slip, we measured steady-state dynamic friction at the end of the run-in stage and at another 5–8 points during shear at 10 μm/s (Fig. 3a). Fig. 3a shows frictional strength of all fault rocks, sheared under saturated conditions, which are thought to represent more realistic conditions of slip at depth (for room humidity friction results see Supplementary Material Fig. S1). Friction of most of the powdered fault rocks (simulated cataclasites) fall in the range 0.57 < μ < 0.70 (Fig. 3a) and is similar to the average frictional strength of Earth materials (“Byerlee friction”, e.g. Byerlee, 1978). The only exceptions are smectite-rich (up to 40%) powdered S-CC tectonites showing friction near 0.5 (Fig. 3a). The behaviour of the simulated fault planes (powders against limestone slab) closely resemble the behaviour of experiments

with powders, showing high coefficients of friction, close to μ = 0.6. On the other hand, the four experiments performed on intact S-CC wafers sheared in the in situ geometry display low friction μ < 0.35 (Fig. 3a) that dramatically decreases in the presence of water (Fig. 3a vs. Fig. S1). Friction of intact wafers with continuous phyllosilicate-rich fabric is notably lower than friction of the powders of the same rocks, consistent with previous studies (e.g. Collettini et al., 2009). Our results identify a first order difference in frictional strength between the limestone-rich cataclasites and marly S-CC tectonites sheared in their in situ geometry. Such a difference is due to different deformation mechanisms that are active during shearing of the various samples. Fig. 3b–d shows the typical microstructure of a powdered sample after the experiment. Powders are characterized by cataclasis with grain size reduction predominantly localized in bands along R1 and Y orientations (Logan et al., 1979), in which grain size is extremely comminuted displaying angular grains with size 10 μm (Fig. 3d). In simulated fault planes, deformation is mostly accommodated by gouge cataclasis and slip against the oolitic limestone slab (Fig. S2) consistently with the high friction measured in these experiments. Within wafers of S-CC tectonites, we observe fibrous calcite veins and marly pockets with sigmoidal shape, together with bands of localized fracture. Most-experimental iso-orientation of clays (Fig. 3e) and veins that retain their shape (i.e. the original fault rock fabric) suggest that

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experimental sliding mostly occurred along the pre-existing foliation. Experimental inter/intra-layer clay sliding is responsible for the low friction we measure in analogy to what is observed in natural fault rocks (Viti et al., 2014). However, at higher shear strains, shear is accommodated by a combination of cataclasis and sliding along clays resulting from the consumption of weak horizons (Fig. S3). This combination of mechanisms results in the evolution of friction from μ < 0.25 to μ > 0.3 we observe for slip exceeding 10–12 mm. 4.2. Frictional stability analysis We evaluated the velocity dependence of friction to establish the potential of earthquake nucleation in the different fault rocks (Fig. 2), namely: cataclasites, simulated fault plane and marly tectonites (Fig. 4). In general, powders obtained from foliated cataclasites and cataclasites have strikingly different behaviour from S-CC tectonites. Powders derived from cataclasites show variable behaviour from velocity strengthening to velocity-weakening (Fig. 4a and b). At low normal stress (10 MPa, Fig. 4a), powders are dominated by velocity-weakening behaviour in both room humidity and saturated experiments with minimum values (a − b) = −0.0023 (upstep velocity v = 10 μm/s) and (a − b) = −0.0018 (v = 30 μm/s) for the wet and dry gouges respectively. Conversely, the same cataclasite powders sheared against a limestone slab (fault plane analogue) show an overall velocity strengthening behaviour (0.0037 < (a − b) < 0.0086). At a normal stress of 53 MPa (estimated in situ conditions of the studied faults, Fig. 4b), the saturated powders exhibit velocitystrengthening behaviour at slow slip velocities (up to 100 μm/s) and evolve towards velocity-weakening behaviour at higher velocities (from (a − b) = 0.0063 at v = 30 μm/s to (a − b) = −0.0028 at v = 300 μm/s). Powders sheared under room humidity conditions show more negative values (minimum (a − b) = −0.0034 at v = 300 μm/s) at high velocity (Fig. 4b). Conversely, clay-rich lithologies generally show velocitystrengthening behaviour (Fig. 4c) with strongly positive values ranging from 0.0043 < (a − b) < 0.01 for all intact and powdered S-CC tectonites. Two exceptions are: the behaviour of powdered tectonites sheared at σn = 53 MPa and under wet conditions that show velocity neutral behaviour (−0.0006 < (a − b) < 0.002) and the 10 μm/s to 30 μm/s velocity step for dry wafers sheared at σn = 10 MPa with a single (a − b) = 0 value. Our results show that velocity-weakening friction behaviour is favoured in calciterich cataclasites, especially at velocities approaching 100 μm/s (Fig. 4a, b), whereas the clay-rich S-CC tectonites show velocitystrengthening sliding behaviour (Fig. 4c). 4.3. Healing behaviour Fig. 5 shows frictional healing μ as function of the log10 (hold time) for all tested materials at low (Fig. 5a) and in situ (Fig. 5b) normal stress (10 and 53 MPa respectively). At σn = 10 MPa, the rate of frictional healing (Fig. 5a) ranges from β = −0.0006 (null healing, intact marly wafers) to β = 0.027 (wet massive limestone cataclasites). Null healing means that we observe a very small, if any, peak in friction upon reshearing after a hold period (e.g. Fig. 2d, blue curve). Simulated cataclasites (powders) all show healing rates 0.009 < β < 0.027. Saturated powders of calcite-rich cataclasites and the simulated fault plane are the samples with the highest healing rates (β = 0.023 and β = 0.027 respectively). These measurements are comparable to tests on similar fault materials conducted under the same experimental conditions that show high healing rates for calciterich gouges (Carpenter et al., 2011). Wet, powdered S-CC tec-

Fig. 4. Velocity dependence of friction for the different materials tested. a) Powdered cataclasites and simulated fault plane (10 MPa normal stress). b) Powdered cataclasites (53 MPa). c) Powdered and intact S-CC tectonites. Open and filled symbols show dry and saturated experiments respectively.

tonites, dry intact S-CC wafers and dry calcite powders show healing rates of 0.0074 < β < 0.016. Wet, intact wafers show negligible restrengthening (β < 0.003). Analogously, at high normal stress, experiments on powders show significantly healing rates (0.003 < β < 0.015) higher with respect to intact wafers (β = −0.001, Fig. 5b). The systematic absence of healing in intact tectonites can be attributed to the presence of interconnected clay layers that “saturate” the growth of real contact area and at the same time inhibit compaction and healing via solution transfer between soluble calcite grains (e.g. Niemeijer and Spiers, 2006; Tesei et al., 2012). Furthermore, we observe that healing at higher normal stress is sensibly lower (βmax = 0.015) than healing at low stress (βmax = 0.027), for all materials tested. This observation is in contrast with

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Fig. 5. Frictional restrengthening as function of hold time. Open and filled symbols show dry and saturated experiments respectively. a) Experiments carried out at 10 MPa normal stress. b) Experiments at 53 MPa normal stress. (For interpretation of the references to colour in this figure, the reader is referred to the web version of this article.)

previous studies in which frictional healing is reported to increase with increasing normal stress due to the growth in size and number of microscopic asperities responsible for frictional strength (e.g. Dieterich and Kilgore, 1994). The saturation state of the gouge layer also plays a noticeable role in the restrengthening rates of calcite-rich fault gouges. Pure calcite gouges show significantly higher healing rates under watersaturated conditions at both 10 and 53 MPa normal stress (filled vs. open symbols, Fig. 5a, b). Also, the healing rates of room humidity calcite gouges tend to flatten out after hold times greater than ∼100 s (Fig. 5a, b). It is therefore likely that under nonreactive conditions healing is induced mainly by mechanical loading of the material that is maximized after a few tens of seconds of stationary contact. This healing difference between saturated and room dry gouges is strongly reduced in gouges that have variable amounts of phyllosilicates such as powders from S-CC tectonites (open blue symbols, Fig. 5). This suggests that restrengthening of calcite-rich rocks is strongly affected by their environmental reactivity, due to the presence of either pore fluids or phyllosilicates at the scale of the asperities. 5. Geological fault model In the previous section we reported results from friction experiments designed to investigate the mechanical properties of endmember fault rocks observed in the field. In the following we use

sequential geological sections to extrapolate observations collected in isolated outcrops (e.g. Tesei et al., 2013) to the entire extent of an actual fault. In this way we aim to reconstruct the possible spatial distribution of the fault zone heterogeneities described in Section 2 (Fig. 6). In particular, we constructed sequential, balanced geological cross-sections perpendicular to the strike of the Coscerno thrust (Fig. 6b), a N–S trending fault cropping out for about 25 km with excellent exposures (Barchi and Lemmi, 1996). The fault has top-to-the-east kinematics with displacement increasing towards the centre of the structure. We constructed a series of five W–E oriented cross-sections (Fig. 6a and Fig. S4) with ∼2.5 km spacing from near the northern termination to the centre of the structure, i.e. sections with increasing fault displacement. We used the cross sections to reconstruct the fault isobaths map and constrain the occurrence of different fault rocks along different fault patches (Fig. 6c). In the hanging wall of the Coscerno thrust, the Coscerno anticline is a large box fold with steep-to-overturned limbs that was initially formed by buckling; further growth of the fold was induced by subsequent transport towards the East along a W-dipping thrust with staircase trajectory (i.e. fault-bend-folding; Suppe, 1983). In addition, an E–W oriented transfer fault accommodates the higher displacement towards E of the southern segment of the Coscerno thrust with respect to the northern segment. Fig. 6a shows a typical cross-section passing through the Coscerno faulted anticline (for the other cross-sections see Supplementary Material Fig. S4). The reconstruction of geological cross-sections has been constrained by surface geology, in particular for the hangingwall, whereas footwall geometry has been estimated through area balancing (e.g. Hossack, 1979). Starting from the geological cross sections, we reconstructed the fault isobaths and then highlighted the probable distribution of brittle and ductile fault patches. Three steps have been followed for extrapolation of fault mechanical properties: 1) Division of the outcropping formations in three categories: strong, weak, intermediate. Massive limestones that deform mainly by brittle deformation are considered strong. Marls and marly limestone formations deform mainly by pressuresolution and clay smearing and are considered weak. Bedded, pelagic limestone formations typically show interplay between brittle and ductile deformation and thus can be considered to have intermediate strength. 2) As a consequence, there are six possible combinations of (fault-induced) juxtaposition of litho-mechanical units: Strong/ Strong, Strong/Intermediate, Strong/Weak, Intermediate/Intermediate, Intermediate/Weak, Weak/Weak. 3) We predict the occurrence of different fault zones along each cross-section depending on different litho-mechanical juxtaposition described above and consistent with field observations. Strong/Strong or Strong/Intermediate juxtapositions (e.g. Fig. 1b) are considered to produce brittle fault zones. Strong/Weak and Intermediate/Intermediate juxtapositions are assumed to produce a mixed brittle and ductile fault zone. Intermediate/Weak and Weak/Weak juxtapositions would develop fault patches characterized by ductile fault rocks (e.g. Fig. 1a). Fig. 6c illustrates the possible distribution of brittle and ductile fault patches along the Coscerno thrust. We observe that the different fault patches usually have a N–S elongated shape because they result from the intersection of a planar stratigraphy with the fault trace. The presence of a transfer fault within the thrust also provides an additional geometrical discontinuity within the fault

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Fig. 6. Fault model of the Coscerno thrust: a) Geological balanced cross-section of the Coscerno thrust (Section 2) and assigned fault zone structure (see text for the details) b) Topography and trace of the Coscerno Thrust. c) Thrust isobaths extrapolated from geological cross sections and distribution of brittle vs ductile fault zones constrained from exposed fault portions (Figs. 1–3, 7 and details in Tesei et al., 2013). See text for further explanation. (For the reference to colour in this figure, the reader is referred to the web version of this article.)

zone. It is worth noting that our model is more suitable for faults hosted in carbonates, rather than in other lithologies, because the change from brittle to ductile fault portions essentially depends on the efficiency of pressure-solution processes in the fault rock. Calcite exhibits high solubility even under uppermost crust conditions and is greatly enhanced by the presence of clays (e.g. Gratier et al., 2013; Viti et al., 2014). 6. Discussion 6.1. Frictional strength heterogeneity Many authors have highlighted the importance of lithological variations, on the mechanical and hydrological properties of crustal faults (e.g. Chester and Chester, 1998; Faulkner et al., 2003). In particular, it is widely known that the occurrence of phyllosilicates or other weak minerals (such as anhydrite or clays) in a fault zone can promote distributed deformation and provide a potential explanation for fault weakness (e.g. Faulkner et al., 2003, 2010; Moore and Rymer, 2007; De Paola et al., 2008; Collettini et al., 2009). We observe a first-order difference in mechanical behaviour between two main groups of fault rocks: the calcite cataclasites and clay-rich S-CC tectonites. Experimental cataclasites show high strength 0.55 < μ < 0.70, in agreement with previous measurements on calcite gouges (e.g. Ikari et al., 2011; Verbene et al., 2013). High frictional strength is explained by the strong interlocking of calcite grains that causes the formation of force chains, intense comminution and dilatancy processes that sum with grain boundary sliding resistance (e.g. Biegel et al., 1989; Marone, 1998; among many others, Fig. 3b).

In addition, high frictional restrengthening rates of calcite (high

β , Fig. 5) suggest a rapid recovery of strength, and thus fault locking after slip, as in the case of the immediate aftermath of an earthquake. This characteristic allows quick accumulation of elastic energy that is a prerequisite for repeatable earthquake nucleation. This rapid frictional healing is probably followed by further restrengthening induced by cementation that increases the fault cohesion (Muhuri et al., 2003; Tenthorey et al., 2003). Although there is, to our knowledge, a scarcity of data on calcite frictional healing (e.g. Carpenter et al., 2011), we note that the healing rates we measured on cataclasites can be up to three times larger than frictional healing of commonly measured quartz-based materials under similar conditions (β = 0.0086 e.g. Marone, 1998). Additionally, in high-velocity experiments commonly used to simulate seismic slip, dramatic restrengthening of calcite gouges is measured at the end of these tests (from μ ∼ 0.2 to μ ∼ 0.6; e.g. Smith et al., 2012). This might suggest that, after an earthquake, the bulk of fault restrengthening is associated with the dissipation of the thermal anomaly caused by the fast slip. However, this large restrengthening is probably limited to the Principal Slip Surfaces (PSS) where co-seismic slip is thought to localize. Conversely, our data on calcite restrengthening apply to the cataclastic rocks surrounding the PSS that slip before seismic localization (e.g. Katz et al., 2003; Heesakkers et al., 2011) and that constitute the loading medium of the fault in the post-seismic and inter-seismic periods. Our observations on the velocity dependence of friction for calcite (negative (a − b) at v > 100 μm/s) correlate well with previous data showing a transition from velocity-strengthening to velocityweakening behaviour at slow velocities (e.g. Verbene et al., 2013).

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Fig. 7. Integrated fault model of the Coscerno thrust. a) Extrapolation of the model in Fig. 6 to the northern fault tips. Former brittle, ductile and brittle/ductile fault zones have been reinterpreted as seismic, aseismic and mixed mode slip behaviour respectively (see text for explanation). Aseismic creep is favoured toward the northern end of the fault because the thrust experiences lower strain rates and juxtaposes less competent lithomechanical units. b) Field example of fault zone dominated by pressure-solution in weak/intermediate lithologies. c) Brittle, potentially seismogenic fault zone due to Strong/Intermediate contact. d) Friction of ductile zones is low, characterized by null restrengthening (no peak upon reshearing) and velocity-strengthening behaviour (a − b) > 0. Note the differences in the friction scale. e) Friction of cataclastic fault zones is characterized by high strength (note the scale), high peaks upon reshearing (μ) and velocity-weakening behaviour (a − b) < 0. (For interpretation of the references to colour in this figure, the reader is referred to the web version of this article.)

In particular it is worth to mention that the velocity-weakening behaviour of calcite-rich gouges can be greatly exacerbated at elevated temperatures above 80–100 ◦ C, i.e. at greater depth (e.g. Ikari et al., 2013; Verbene et al., 2013). Earthquake nucleation is therefore facilitated in cataclastic, calcite-rich materials because of the elastic energy accumulation, due to high strength and high healing rates, combined with the tendency for unstable slip behaviour (negative (a − b) values). On the contrary, fault zones with S-CC tectonites are not prone to host seismic nucleation. S-CC wafers display low frictional

strength (μ ∼ 0.25) due to the presence of interconnected layers of clay that act as efficient sliding horizons (Fig. 3c). In addition, they are characterized by velocity-strengthening frictional behaviour (positive (a − b) values, Fig. 4c) combined with the lack of frictional healing (β = 0) that appears to be a consistent characteristic of many phyllosilicates (e.g., Beeler, 2007; Tesei et al., 2012). These mechanical characteristics have a strong influence on fault behaviour as they imply the lack, or extreme reduction, of interseismic stress accumulation in these fault portions. In addition we note that the frictional characteristics of clays

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in foliated tectonites are not likely to change much at higher temperatures (across the Smectite-Illite transition 100 ◦ C < T < 200 ◦ C; Ikari et al., 2011) and thus our results are applicable for the same rocks present at greater depths (i.e. 5–8 km). 6.2. Integrated fault model and comparison with seismicity in the Po Plain Here we integrate fault characterization at the outcrop scale (e.g. Fig. 1), mechanical data of the faulted rocks (e.g. Fig. 4) and a geological fault model of a thrust fault for the Apennines (e.g. Fig. 5), to characterize the complexities, both geometrical and mechanical, of a natural fault (summary in Fig. 7). Localized brittle zones in massive limestones (Fig. 7c) are characterized by fault rocks showing a predominant velocity-weakening behaviour, with significant restrengthening (Fig. 7e). This combination of fault zone structure and frictional properties define high-strength zones that are prone to seismic nucleation. These zones (red areas, Fig. 7a), potentially seismogenic, correspond in general to areas overthrust by massive limestone formations. On the other hand, distributed and ductile zones formed from marly limestones, are represented by foliated and phyllosilicaterich fault rocks (Fig. 7b) that show a velocity-strengthening behaviour with no healing (Fig. 7d). These areas (blue in Fig. 7a) likely represent weak fault portions that are prone to long term aseismic creep. Finally we assigned a mixed-mode slip behaviour to fault patches characterized by the interplay of brittle and ductile deformation (green areas in Fig. 7a). In general, we note that in a thrust zone affecting a lithologically complex multilayer, the number and the dimension of fault portions prone to earthquake nucleation depend on the number and thickness of weak levels. In particular, in carbonates, the continuity of seismogenic fault patches can be interrupted by both thick marly units or by the presence of structural complexities such as the E–W striking, left-lateral strike slip fault (Fig. 7a). A combination of other physical parameters such as fluid pressure (e.g. Noda and Lapusta, 2013), fault roughness (e.g. Candela et al., 2011) or fault segments poorly oriented with respect to the stress field (e.g. Sibson, 2007) can influence the slip behaviour of thrust faults. In particular, the common flat–ramp–flat geometry of thrust systems is characterized by ramps formed in correspondence to strong formations that might represent favourable locations for earthquake nucleation (e.g. Chen et al., 2001). The presented model is conceptually similar to the “asperity” model widely described in seismology (e.g. Kanamori and Stewart, 1978; Lay and Kanamori, 1981) and geology (e.g. Cloos, 1992). However, we considered some complementary aspects, regarding the spatial distribution of the complexities along the fault (from geological reconstructions) and the characterization of the frictional properties of the fault rocks (derived from friction experiments). Our model provides a picture of the possible behaviour of a heterogeneous fault defining the stronger and velocity weakening fault portions as the areas where a seismic rupture can nucleate (red in Fig. 7a). However it is worth considering that earthquakes nucleated in strong portions might dynamically propagate into other fault patches, i.e. the green and blue areas in Fig. 7a. Indeed, during high-velocity experiments, phyllosilicaterich rocks exhibit low “fracture” energy (Faulkner et al., 2011; Behnsen and Faulkner, 2012) and, in carbonates, a number of processes activated by high temperature or high power density (e.g. Boneh et al., 2013; Fondriest et al., 2013) might promote coseismic localization and dynamic weakening in normally creeping and/or velocity-strengthening fault zones, as proposed for the M W = 9.0 Tohoku-Oki earthquake (Noda and Lapusta, 2013)

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The geological evidence of the systematic control of the lithology over the fault structure has been widely observed in the Northern Apennines (Alvarez et al., 1978; Koopman, 1983; Agosta and Aydin, 2006; Bussolotto et al., 2007; De Paola et al., 2008; Billi, 2010; Smith et al., 2011; Tesei et al., 2013; Bullock et al., 2014; Collettini et al., 2014b) and other carbonatic sequences throughout the world (e.g. Wojtal and Mitra, 1986; Prior and Behrmann, 1990; Willemse et al., 1997; Lacroix et al., 2011; Gratier et al., 2013). Therefore, we think that our integrated analysis has broad applicability to faults hosted in carbonatic multilayers worldwide. Our reconstruction of the thrust faults affecting the Umbria– Marche carbonatic multilayer, constructed using geology and frictional properties of the fault rocks, represents a reliable model for the active compressional structures beneath the Po Plain (Northern Italy). In this area, the integration of seismic reflection profiles with seismological data shows that the strongest seismic sequences, such as the M W = 5.4 Reggio Emilia sequence in 1996 (Selvaggi et al., 2001) and the M L = 5.9 Emilia sequence in 2012 (Govoni et al., 2014), nucleate within the same sedimentary sequence exposed in the Umbria–Marche Apennines (Pieri and Groppi, 1981). Seismic profiles crossing the active compressional front highlight compressional structures with the same geometry, similar wavelength and fault dimensions of the fault mapped in the Umbria–Marche Apennines (Massoli et al., 2006; Carminati et al., 2010). In particular, the Emilia sequence of May– June 2012 is characterized by two major events with M L = 5.9 and M L = 5.7 nucleated at depths of 6.3 and 8.6 km respectively (Govoni et al., 2014). Each mainshock was followed by thousands of aftershocks with several of them, with M L ∼ 5, nucleating at depth as shallow as 3.5 km (Govoni et al., 2014). The comparison of hypocentres distribution with existing seismic profiles in the area (Pieri and Groppi, 1981; Carminati et al., 2010) and the high inclination of fault planes from moment tensor solutions (Govoni et al., 2014) suggest that mainshocks nucleated along thrust ramps of the carbonatic multilayer, within lithologies likely corresponding to massive limestones. This interpretation is supported by our integrated model, in which ramps represent favourable locations for earthquake nucleation (Fig. 7a). In addition, aftershocks illuminated a fault zone with dimension similar to the Coscerno thrust (∼20–30 km long), and with seismicity concentrated only within the more competent limestone lithologies. To conclude, the proposed fault zone reconstruction highlights which fault portions might host earthquake nucleation and thus have important implications in the study of seismic sequences occurring in carbonates (e.g. Chiaraluce et al., 2003; Bernard et al., 2006; Burchfiel et al., 2008; Nissen et al., 2014) and might help to understand how the growth of large faults can be the result of a combination of seismic pulses and ductile deformation (e.g. Wojtal and Mitra, 1986; Yue et al., 2005). In particular, accurate geological reconstructions together with laboratory characterization of the fault rocks, can help to improve recent numerical and mechanical models used to understand earthquake nucleation/propagation in forearc accretionary prisms (e.g. Cubas et al., 2013) and foreland thrust belts (e.g. Ikari et al., 2013; Noda and Lapusta, 2013). 7. Conclusions Carbonatic multilayers worldwide are often characterized by a multi-competent stratigraphy (limestone and marls). We performed friction experiments designed to investigate the strength and frictional behaviour of brittle and ductile carbonatic fault rocks commonly found in limestone and marls, respectively. We tested natural fault rocks collected in the Northern Apennines, both as simulated cataclasites and intact wafers. We showed that high friction, rapid post-slip restrengthening, and a tendency towards un-

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stable slip behaviour characterize cataclastic rocks. Conversely, low friction, negligible restrengthening and a strongly stable slip behaviour characterize intact S-CC tectonites. Fault portions localized in pure carbonates show evidence of past seismic slip, whilst portions developed in marls show distributed deformation and suggest fault weakness. Starting from these experimental observations, we built a model that integrates field and mechanical data in order to visualize the fault zone heterogeneity and the earthquake potential of an exhumed real case study. Our study show that an accurate geological model of the fault can be used to infer the actual dimension and orientation of seismogenic fault portions in a complex thrust zone hosted in a carbonatic multilayer. Acknowledgements This research was carried out within the ERC Starting Grant GLASS project (no 259256). We are grateful to Dr. D. Perugini (University of Perugia), for use of his rock preparation facilities, Prof. R. Rettori (University of Perugia) for use of his optical microscope and A. Cavallo for technical help with the SEM. We also wish to acknowledge the thoughtful and very detailed reviews from Z. Reches and another anonymous reviewer that significantly helped to improve this manuscript. Appendix A. Supplementary material Supplementary material related to this article can be found online at http://dx.doi.org/10.1016/j.epsl.2014.10.021. References Agosta, F., Aydin, A., 2006. Architecture and deformation mechanism of a basinbounding normal fault in Mesozoic platform carbonates, central Italy. J. Struct. Geol. 28, 1445–1467. http://dx.doi.org/10.1016/j.jsg.2006.04.006. Alvarez, W., Engelder, T., Geiser, P.A., 1978. Classification of solution cleavage in pelagic limestones. Geology 6, 263–266. Barchi, M.R., Brozzetti, F., 1991. Il sovrascorrimento di Spoleto: un esempio di inversione tettonica nell’Appennino Umbro–Marchigiano? Studi Geolog. Camerti, Vol. Spec. 1, 337–345. Barchi, M.R., Lemmi, M., 1996. Geologia dell’area del M. Coscerno-M. di Civitella (Umbria sud-orientale). Boll. Soc. Geol. Ital. 115, 601–624. Barchi, M.R., Minelli, G., Pialli, G., 1998. The CROP 03 profile: a synthesis of results on deep structures of the Northern Apennines. Mem. Soc. Geol. Ital. 53, 383–400. Barchi, M.R., Alvarez, W., Shimabukuro, D.H., 2012. The Umbria–Marche Apennines as a Double Orogen: observations and hypotheses. Ital. J. Geosci. 131, 258–271. http://dx.doi.org/10.3301/IJG.2012.17. Beeler, N.M., 2007. Laboratory-observed faulting in intrinsically and apparently weak materials. In: Dixon, T.H., Moore, J.C. (Eds.), The Seismogenic Zone of Subduction Thrust Faults, pp. 370–449. Behnsen, J., Faulkner, D.R., 2012. The effect of mineralogy and effective normal stress on the frictional strength of sheet silicates. J. Struct. Geol. 42, 49–61. http://dx.doi.org/10.1016/j.jsg.2012.06.015. Ben-Zion, Y., Shi, Z., 2005. Dynamic rupture on a material interface with spontaneous generation of plastic strain in the bulk. Earth Planet. Sci. Lett. 236, 286–296. http://dx.doi.org/10.1016/j.epsl.2005.03.025. Bernard, P., Lyon-Caen, H., Briole, P., Deschamps, A., Boudins, F., Makrapoulos, K., Papadimitriou, P., Lemeille, F., Patau, G., Billiris, H., Paradissis, D., Papazissi, K., Castaréde, H., Charade, O., Nercessian, A., Avallone, A., Pacchiani, F., Zahradnik, J., Sacks, S., Linde, A., 2006. Seismicity, deformation and seismic hazard in the western rift of Corinth. New insights from the Corint Rift Laboratory (CRL). Tectonophysics 426, 7–30. http://dx.doi.org/10.1016/j.tecto.2006.02.012. Berthé, D., Choukroune, P., Jegouzo, P., 1979. Orthogneiss, mylonite and non coaxial deformation of granites: the example of the South Armorican Shear Zone. J. Struct. Geol. 1, 31–42. http://dx.doi.org/10.1016/0191-8141(79)90019-1. Biegel, R.L., Sammis, C.G., Dieterich, J.H., 1989. The frictional properties of a simulated gouge having a fractal particle-size distribution. J. Struct. Geol. 11, 827–846. Billi, A., 2010. Microtectonics of low-P low-T carbonate fault rocks. J. Struct. Geol. 32, 1392–1402. http://dx.doi.org/10.1016/j.jsg.2009.05.007.

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