Seismic Cycle in Bituminous Dolostones (Monte Camicia Thrust Zone, Central Apennines, Italy)

Seismic ruptures often propagate along fault zones cutting km‐thick sequences of carbonates (e.g., Wenchuan Mw 7.8, 2008, China; L’Aquila Mw 6.1, 2009, Italy). As a consequence, fault rock assemblages may record the seismic cycle under a wide range of loading conditions, temperatures (>1,000°C during co‐seismic slip), and fluid‐rock interactions. The Monte Camicia Thrust Zone in the Italian Central Apennines is exhumed from ∼3 km depth. We studied the seismic cycle recorded by a network of faults cutting bituminous dolostones in the footwall of the Monte Camicia Thrust. These faults accommodate up to several meters of displacement. Slip zones are mm‐ to cm‐thick and bounded by ultra‐polished (“mirror‐like”) surfaces independently of their displacement. At the microscale, deformation is accommodated by cataclasis and pressure‐solution in carbonates, and viscous flow in the foliated bitumen. Some of the faults with displacements >0.10 m have multiple slip zones, separated by “mirror‐like” surfaces, and include clasts of foliated bitumen and fragments of older slip zones sealed by calcite precipitation. We conclude that (a) slip zones record post‐ to inter‐seismic (foliated bitumen) and co‐seismic (fragments of bitumen) deformation in a fluid‐rich environment (calcite precipitation) and (b) mirror‐like surfaces formed during all phases of the seismic cycle.

Here we discuss original field data and microstructural observations of faults cutting the footwall rocks (bituminous dolostones) of the Monte Camicia Thrust located in the Gran Sasso Massif (Central Apennines, Italy).We propose that microstructures of the fault cores (often bounded by MSs) record a polyphasic deformation history that may belong to the different phases of the seismic cycle.The studied (often multiple) slip surfaces and slip zones are associated with pre-and post-seismic phases (cataclasis, smearing and viscous flow of bitumen, pressure-solution), co-seismic slip (bitumen embrittlement and fragmentation) and inter-seismic fault strength recovery.Furthermore, several MSs are also found in faults/fractures accommodating negligible displacements (<1 mm), supporting the hypothesis that these surfaces may be the result of rock fracturing followed by static fluid-rock interaction.In addition to that, the new field data clarify the local relationship between compressive and extensional structures in the area, supporting the interpretation of the reverse kinematics of the Monte Camicia Thrust followed by minor and localized normal fault reactivation (Lucca et al., 2019).
Since late Messinian, the GSTS was subjected to extensional gravitational collapse coaxial with the north-eastward compression, probably due to gravity-induced re-equilibration of the GSTS (Lucca et al., 2019).The GSTS was then dissected by E-W and WNW-ESE trending normal faults with evidence of Quaternary deformation and surface ruptures (since Late Pleistocene-Holocene; Galadini & Galli, 2000; Figure 1a).Stratigraphy and inherited synsedimentary Jurassic normal faults are responsible for spatial heterogeneities in both the compressional and extensional deformation stages (Cardello & Doglioni, 2015) with documented reactivation of Jurassic faults during the Quaternary extension (Centamore et al., 2002).
The study area is a gully located in the Fornaca Valley (Gran Sasso Massif, Figure 1b) between Monte Prena and Monte Camicia along the northern margin of the Campo Imperatore plain (42°25′48.7″N,13°42′00″; Figure 1b).Ghisetti & Vezzani, 1986b;Lucca et al., 2019).Colors refer to the tectonic units (see panel c) defined by Ghisetti and Vezzani (1986b).The black inset shows the location of the study area.(c) Cross-section A-A' (see panel b), stratigraphy and tectonic units.
The bituminous dolostones (i.e., Dolomie Bituminose Fm. auct.)outcrop in the Fornaca Valley has a thickness up to 180 m (Figure 1c), while the true thickness is unknown due to intense tectonization (Adamoli et al., 1990;Bertinelli et al., 2004).The beds are made of thin layers (mm to tens of cm) of gray and black (organic matter-rich) dolomites, alternated to dolostone beds up to 10 m thick organized in coarsening and thickening upwards cycles (Adamoli et al., 1984;Barchi & Bigozzi, 1995).Convolute laminations, synsedimentary micro-faults and mega-breccia bodies (olistoliths up to decametric in size) of dolostones, the latter supplied from the platform edge, are related to sediment compaction, fluid expulsion and gravitational movements (Adamoli et al., 1990;Bertinelli et al., 2004).The depositional environment corresponds to an upper carbonate platform slope to a tidal flat with multiple anoxic episodes (Adamoli et al., 1984).The upper part of the Dolomie Bituminose Fm. contains chert nodules, testifying a deepening of the basin and a possible link to the open sea (Adamoli et al., 1984).The TOC (Total Organic Carbon) content of Dolomie Bituminose Fm. is up to 40% (Katz et al., 2000) and Apatite Fission Track and Vitrinite Reflectance analyses define this as an immature source rock that experienced a maximum temperature of 100°C during burial (Rusciadelli et al., 2005).

Monte Camicia Thrust
At the bottom of the Fornaca valley two faults of the GSTS are outcropping, the Vado di Ferruccio Thrust ("ϕ7" in Ghisetti, 1987;"Upper Thrust" in D'Agostino et al., 1998) and the Monte Camicia Thrust ("ϕ1/T1" to "ϕ2/ T2" in Ghisetti, 1987;Ghisetti & Vezzani, 1986b;Leah et al., 2018;Pace et al., 2014), bounding a tectonic sliver of Upper Triassic bituminous dolostones (Figures 1b and 1c).The Vado di Ferruccio Thrust dips 11°-50° to the S-SW (Leah et al., 2018) and places the bituminous dolostones of the hanging wall on Corniola limestones of the footwall.The Monte Camicia Thrust dips at an average of 33° to the south and places Cretaceous rudist calcarenites of the hangingwall onto Upper Triassic bituminous dolostones of the footwall (Figures 1c, 2, and 3a;Lucca et al., 2019).Because of the younger-on-older contact typical of normal faults, various hypotheses were proposed regarding the kinematics of the Monte Camicia Thrust: 1. Out-of-sequence thrust fault.This hypothesis is supported by the presence of Riedel-type secondary faults, fault-related veins and stylolites networks whose attitude and sense of shear are consistent with the transport of the thrust sheet from the southern backlimb onto the northern forelimb of the Gran Sasso anticline (Ghisetti andVezzani, 1991, 1997;Vezzani et al., 2010); 2. Synsedimentary Mesozoic steeply dipping normal fault rotated to lower dip angle during the Neogene thrust-related folding.This hypothesis is supported by structural data (i.e., drag folds, synthetic R-type Riedel shear planes, cut-off angle relations), stratigraphic constraints and balanced geological cross sections (Calamita et al., 2002(Calamita et al., , 2004;;Pace et al., 2014;Tozer et al., 2002); 3. Low angle normal fault which reactivates late Messinian-middle Pliocene thrusts.This hypothesis is supported by structural analysis and field mapping together with simple geometrical and flexural elastic models (D'Agostino et al., 1998); 4. Out-of-sequence thrust reactivated as a normal fault (this hypothesis reconciles the hypothesis 1 and 3).
The Monte Camicia Thrust formed as an out-of-sequence thrust in a subaerial environment and then was reactivated in extension, due to footwall collapse coeval with the development of the Campo Imperatore (subaerial) extensional fault system during Quaternary (Lucca et al., 2019).This hypothesis is based on structural plus isotopic data, and further supported by evidence presented in this study.
The outcrops chosen for this study are located in an N-S oriented gully about 150 m long in the Fornaca Valley (Figures 1b and 1c).Here WNW-ESE and NW-SE trending normal faults with MSs cut the bituminous dolostones and are related to the Quaternary extension that dissected the GSTS.

Fieldwork and Sample Preparation
Field-geology surveys at 1:1,000 were conducted using orthorectified aerial photographs (spatial resolution 0.20 m/pix, courtesy of the Regione Abruzzo: opendata.regione.abruzzo.it)as mapping bases.Structural data of faults, fractures, folds and bedding were collected in four locations and digitized using QGIS© 3.10 and Stereonet 11 (Allmendinger et al., 2012;Cardozo & Allmendinger, 2013).Fault separation (d) and attitude of outcrop surfaces were also measured for not dip-slip faults to compute the displacement (D) using the following equation:  10.1029/2023GC011063 7 of 21 where β is the angle between fault lineation and the intersection fault surface-outcrop surface, and γ is the angle between the fault lineation and the intersection bedding-fault surface (Figure 2b).
Around 60 samples of fault and host rocks were collected both in the footwall and the hangingwall of the Monte Camicia Thrust.Nine samples were consolidated and prepared in cm-in-size rock chips or polished "thin" sections (3 mm and 30 μm thick respectively) cut perpendicular to the fault surface and parallel to striations for optical and Scanning Electron Microscope (SEM) microstructural observations.For the SEM observations, the samples were coated with a ca. 10 nm thick carbon film.Sample preparation was conducted at the Department of Geosciences of the University of Padua.

Microstructural Analyses
Transmitted-light optical microscopy was used to recognize specific microstructural features at the thin section scale while detailed microstructural analyses were then performed with two Field-Emission Scanning Electron Microscopes: • JEOL JSM-6500F FE-SEM installed at the High Pressure-High Temperature Laboratory of the Istituto Nazionale di Geofisica e Vulcanologia in Rome (INGV); • TESCAN SOLARIS dual beam FIB-FE-SEM installed at the Department of Geosciences at the University of Padua.
Operational conditions of the electron beams were 10-15 keV with ∼6-10 mm as working distance for the analysis scan mode; 5-7 keV and 3-4 mm as working distance for the ultra-high resolution scan mode in University of Padua.We performed Back-Scattered Electron across the principal slip zones and Secondary Electron (SE) imaging of the MSs.

Monte Camicia Thrust Zone
In the southernmost part of the studied gully, the Monte Camicia Thrust (Figures 1c, 2a, and 3a) strikes on average E-W to WNW-ESE and is characterized by a sharp principal fault surface that dips between 40° and 50° to the south (Stop 1 in Figure 2a) as measured by Lucca et al. (2019) in the south-eastern side of the gully (see structural site CF15 in Lucca et al., 2019).
The hangingwall of the Monte Camicia Thrust (Figures 2a and 3a) consists of several tens of meters thick shattered rudist calcarenites (pertaining to the Calciruditi a Rudiste Fm.) with fractures spaced less than 1-2 cm, and cut by minor faults striking E-W and NNE-SSW and dipping 40°-80° mainly toward ESE-SSE (i.e., parallel to oblique to the Monte Camicia Thrust) and few others toward N and NW, in proximity to the principal fault surface (Stops 1 and 3 in Figure 2a).Faults with normal kinematics dip 40°-90° toward the S (i.e., sub-parallel to the Monte Camicia Thrust) with transtensional sinistral shear (Stop 1 in Figure 2a).On top of the principal fault surface (Stop 1 in Figure 2a), joints dip mainly toward the S, while their attitude is more scattered moving to the west (Stop 3 in Figure 2a).
The footwall block of the Monte Camicia Thrust includes dm-to m-thick strata of folded and faulted bituminous dolostones (pertaining to the Dolomie Bituminose Fm.) (Figure 3a).A gray color gradation across individual strata reflects the percentage of organic matter, which decreases from the base (dark gray/black) to the top (light gray; Figure 3d) of the stratum, probably due to variation of oxygen content in the euxinic basin during sedimentation.The strata dip toward N and NE; on average, their dip angle gradually decreases from 65° to 88° near the main fault surface to ∼5°-10° at 50-60 m north of the Monte Camicia Thrust (Stop 2 in Figure 2a; Figures 3b and 3c).Similar sub-vertical attitudes of the strata were measured to the west of the study area (see stop 4 in Figure 1a and stereoplot in Figure 2a).Along the east side of the gully the heavily faulted dolomitic beds form a m-scale open asymmetric fold (interlimb angle >100°) with axes plunging WNW (Figure 4a); in the same area, minor asymmetric folds (Stop 2 in Figure 2a; Figures 4b and 4c) are characterized by longer limbs dipping at high angle (>70°) and the shorter ones dip at relatively low angle (20°-40°) both toward the north in a Z-type fashion (Ramsay & Huber, 1987), with axes gently plunging 5°-10° toward WNW (i.e., sub-parallel to the strike of the Monte Camicia Thrust).The polarity of the beds, inferred from the distribution of organic matter, together with the attitude of the strata and the presence of asymmetric folds suggest that the bituminous dolos tones were dragged to form a decametric-scale synform fold with the axis striking ca.E-W in the footwall of the Monte Camicia Thrust (Figure 2a).The footwall of the Monte Camicia Thrust is cut by a dense network of minor faults mostly striking WSW-ENE and dipping 85°-50° toward S and SE, and accommodating normal dip-slip to transtensional sinistral shear displacement (Figure 2a).Few transpressive faults and back-thrusts dipping 30° and 50°-70° toward NNE and SSE respectively are also present (Stop 2 in Figure 2a).Low to high angle normal faults crosscut each other (Figures 4a and 4e); these cross-cutting relations and the separation of the strata along the faults allowed us to estimate fault displacements up to 11 m.The most relevant structural features described above are shown in the geological map of the southernmost part of the Fornaca valley and in the N-S cross section (Figure 2a).

Field Observations
In the bituminous dolostones, minor faults have cataclastic (i.e., cohesive) slip zones whose thickness, generally increases with the displacement, from invisible to the naked eye to few cm.The slip zones are composed of dolostone fragments and, to a less extent, bitumen immersed in a calcite-and/or bitumen-rich matrix.The slip zones are usually bounded by MSs independently of the fault displacement (Figures 4d, 4f, and 4g).MSs are also found at the interface between the folded and unfolded strata (Figure 4b).The MSs reflect incident light, have a porcelain-like appearance, and range from light brown color (when they cut pure dolomitic beds) to dark gray or blackish (when they cut bitumen rich layers).The blackish color of the MS can be attributed to the smearing of bitumen, which contains "truncated" dolostone clasts (zoom of sample 9 in Figure 5).Fault striae are not always recognizable in all the samples despite the mirror-like appearance (Figure 5).About 60 samples of MSs and related slip zones were collected from normal faults with different dip, dip azimuth and cumulative displacement for microstructural analyses.

Microstructural Observations
Two host-rock and nine representative fault rock samples were selected for microstructural analysis from faults with displacement ranging from <1 mm to 11.2 m (Figure 5).

Host Rock
Far from the faults, the host bituminous dolostones consist of a foliated rock (parallel to the bedding) made of an alternation of cm-thick layers with higher content of dolomitic grain aggregates (tens of μm in size), and mm-thick layers with higher content in organic matter (Figure 6a).The latter layers include primary organic matter (i.e., "macerals," recognized at the optical microscope) belonging to the groups of Kerogen type I and II (Mastalerz et al., 2018), pre-oil solid bitumen, framboidal pyrite, clay minerals and euhedral calcite crystals (Figure 6).Pre-oil solid bitumen appears as a dark uniform material that fills both pores and inter-to intra-fractures in the dolomite aggregates (Mastalerz et al., 2018; Figures 6b and 6c).Macerals (alginite) and clay minerals have a preferential shape orientation and sometimes wrap dolomite or calcite clasts with a porphyroclastic-like shape (Figure 6b).

Fault Slip Zones and Associated Mirror Surfaces
All the selected normal fault samples include MSs, independent of their displacement.Usually, the slip zones cut intensely deformed dolostones, microbreccia and proto-cataclasites (Figures 7a-7c and 7f) or less deformed dolostones (Figures 7d and 7e) sometimes affected by bitumen-rich stylolites with the orientation of the foliation  perpendicular to the maximum shortening direction (tectonically induced foliation; Figure 7c).Small (<1 cm) displacement faults include a single slip zone composed of patches of continuous <20 μm-thick layer of smeared bitumen that contains ultra-fine dolostone grains (from hundreds of nm to few μm in size; Figures 7a, 7b, 7d, and 7e).The elongated grains are aligned and oriented with their long axis parallel to the fault surface (Figure 7b).At higher displacement (123 cm) the slip zone consists of <200 μm-thick dolostone ultracataclasite with lobate-cuspate grain boundaries immersed in a bitumen-rich matrix and topped by a 1 to 15-20 μm-thick layer of smeared bitumen (Figure 7f).
The MSs are thus defined by these thin layers of smeared bitumen and the exposed surfaces of (a) ultra-fine dolostone grains dispersed in the bitumen layers (Figures 7a, 7b, 7d, 7e, and 8), (b) dolostone grains composed by a thin layer of aggregate nanograins (Figure 8b), or (c) dolostone grains showing rough grain boundaries when in contact with bitumen (Figure 7d).
Some of the samples collected from faults that accommodated slip ≥12 cm present fragments of multiple slip zones.The thickness of the composite slip zone broadly increases with fault displacement.Evidence of fluid infiltration associated with calcite precipitation is also widespread (Figure 9).For instance, the fault core of a normal fault accommodating 12 cm of displacement (sample 5 in Figure 5) is composed of at least two slip zones (Figure 9a).The upper slip zone (probably the youngest) is ca. 100 μm thick, and it is made of angular to sub-rounded dolostone clasts (<50 μm in size) and few bitumen-rich clasts as well as clasts of previous dolostone cataclasites cemented by calcite (5-10 μm in size crystals; Figures 9a-9d).This upper slip zone is limited to the top by a MS and to the bottom by a lower slip zone locally developed on ∼20 μm bitumen layer (Figure 9a).The sharp boundary between the two slip zones is defined by flat grain boundaries of dolomite and calcite; the bitumen-rich layer contains ultra-fine grains of dolostone with elongated clasts aligned with the slip zone direction (Figures 9a and 9b), similar to the ones found in small displacement faults (Figure 7b).This layer is fragmented across the analyzed thin section and some fragments are embedded in the upper slip zone.Beneath the bitumen-rich layer of the lower slip zone, it is possible to observe an ultracataclasite made of dolostone grains immersed in a calcite-rich matrix (Figures 9a and 9b).The number of slip zones increases with displacement (sample 1 in Figure 5; 3.8 m of displacement), as attested in Figure 9e.Fragments of ultracataclasite made of indented grains of dolostone (from 10 to 1 μm in size) and bitumen, bounded by a sharp slip surface (Figures 9e  and 9f) are embedded in a sparry calcite cement (Figure 9f).

Compressional Versus Extensional Structures
The study area is characterized by the presence of structures related to a Late Miocene-Middle Pliocene compressional phase (i.e., syncline and asymmetric folds, verticalized bedding, reverse faults) cut and dissected by mainly NE-SW and E-W striking normal Middle Pliocene to Holocene faults.
The tilting and verticalization of the bituminous dolostone beds approaching the Monte Camicia Thrust (Figures 2  and 3) coupled with the presence of Z-type asymmetric parasitic folds along the east side of the studied gully (Stop 2 in Figures 2a and 4a-4c) can be interpreted as part of the limb of a decametric-in-scale synform fold, with the hinge buried beneath the valley and with a fold axis striking ca.E-W (Figure 2a).This interpretation is supported by the stratigraphic polarity of the bitumen-rich dolostone strata (Figure 3d).Therefore, the Monte Camicia Thrust would be responsible for the dragging, folding and verticalization of the strata of the bituminous dolostones during the Miocene-Pliocene thrusting.Our interpretation supports the hypothesis that the Monte Camicia Thrust formed as an out-of-sequence thrust during the Pliocene (Ghisetti & Vezzani, 1986a, 1986b, 1991;Lucca et al., 2019) and placed the Jurassic-Cretaceous rudist calcarenites onto the Triassic bituminous dolostones.The younger-on-older geometry of this thrust can be explained by its propagation through an already folded stratigraphic sequence due to the activity of the lower and older Vado di Ferruccio Thrust (Ghisetti & Vezzani, 1991).The Vado di Ferruccio Thrust itself has been interpreted as an original out-of-sequence during the complex spatio-temporal shortening history driven by the GSTS (Ghisetti & Vezzani, 1991;Lucca et al., 2019).
In the study area (Figure 2a), most of the normal faults strike NE-SW and E-W and accommodate only a few cm of displacement with dip-slip to transtensional kinematic (Figures 2a and 4c).These normal faults dip mainly at >55° toward the south and are associated with ultra-cataclastic slip zones bounded by mirror-like slip surfaces with smeared bitumen (Figures 4 and 5).Many normal faults cut the compressional structure described above and other crosscut each other (Figure 4).Based on the field evidence, we propose that while few and minor normal faults formed during the Monte Camicia Thrust activity, most formed during the subsequent extensional phase.These transtensional faults (Figure 2a) are probably related to the normal faulting, proposed by Ghisetti andVezzani (1986a, 1986b), which cut with the same trend the hangingwall block of the Monte Camicia Thrust in the study area (Stop 1 and 3 in Figure 2a).Following Leah et al. (2018), in the geological cross-section (Figure 2a) we propose that normal faults dipping >60° toward S-SW and located in the northernmost part of the valley cut the Vado di Ferruccio Thrust, that bounds the thrust sheet of bituminous dolostones and truncates the Corniola limestones in its footwall (see Figures 2a and 2e in Leah et al., 2018).Furthermore, the dip angle of the southern and minor faults decreases toward the south so that these faults terminate gently on the thrust (Figure 2a).This interpretation implies the reactivation of the Vado di Ferruccio Thrust as a low-angle normal fault (see evidence in Leah et al., 2018).

Seismic Cycle Recorded in Bituminous Dolostones
The microstructural analysis conducted on the slip zones of normal faults reveals the activation of several deformation mechanisms.These mechanisms can be associated with the different phases of the seismic cycle (pre-seismic, co-seismic, post-seismic, and inter-seismic; Scholz, 2019), as illustrated in the conceptual model of Figure 10.In bitumen-rich slip zones a foliated fabric may be representative of the viscous behavior of bitumen that acts as a weak phase (Rutter et al., 2013) respect to the surrounding dolostone clasts.Elongated dolostone clasts within the bitumen-rich slip zones are oriented with the long axis parallel to the MSs (Figures 7b and 9b).This shape preferred orientation could be the result of fracturing of the dolostone grains in the wall rocks during the initial stages of slip, their progressive capture by the slip zone, and the orientation of the "rigid" particles during viscous flow (e.g., Cladouhos, 1999) in tandem with bitumen smearing and squeezing (inter-seismic or pre-seismic, Phase 1 in Figure 10).Viscous shear structures in bitumen-rich layers are observed in all the studied normal faults, independent of the accommodated displacement.Flow structures and porphyroclastic-like dolomitic aggregates in bitumen-rich layers are also found parallel to the bedding in the host rock away from normal faults.These viscous deformations potentially occurred at low strain rates throughout the burial and tectonic history (i.e., thrusting) of the bituminous dolostones (Figure 6).The occurrence of viscous flow localized in the proximity of MSs may indicate a "slow" (aseismic) deformation during inter-seismic and/or pre-seismic loading (Phase 1 in Figure 10).
Conversely, the slip zones of faults accommodating larger displacement include tabular fragments of previous slip zones with sharp edges probably corresponding to older MSs (based on roughness and similarities with the exposed ones) embedded in a new cataclastic layer or cemented by calcite precipitation (Figures 9a, 9c, 9e, and 9f).Following Lucca et al. (2019), this sparry calcite cement is precipitated from percolating carbonate-rich fluids meteoric in origin and infiltrated in the vadose zone during the extensional reactivation of the Monte Camicia Thrust.In these younger slip zones, the presence of fragments of bitumen-rich slip zones, which should record viscous flow deformation (Figures 9a and 9b; Cheung & Cebon, 1997;Trippetta & Geremia, 2019), suggests an abrupt co-seismic rupture and fast slip that could be the geological record of a co-seismic phase (Phase 2, Figure 10), followed by fluid infiltration.Fragments of bitumen can be found only in this slip zone, while in all the other fault samples and in the host rock, organic matter deforms by viscous flow.Fault strength recovery by calcite precipitation causes the hardening of the slip zone once the fault motion is halted during the post-seismic phase (Phase 3, Figure 10).Following this interpretation, whenever the slip zones of large faults are bounded by two MSs and include fragments of former bitumen-rich slip zones and/or fragments of other MSs and cataclastic calcite (as in Figure 9a and 9c), they would register multiple events of seismic rupture.Subsequently, ruptured faults sealed by calcite would undergo precipitation-hardening of the slip zone (e.g., Callahan et al., 2020;Di Toro & Pennacchioni, 2005;Tarasewicz et al., 2005;Woodcock et al., 2007).Hence, the slip associated with further seismic ruptures would be localized out of plane in the principal slip zone and reworks the existing cemented fault rocks (new co-seismic Phase 5 in Figure 10).As observed by Leah et al. (2018), the total length of normal and transtensive faults in the Fornaca Valley is difficult to assess but it is possible that some are tens to hundreds of meters-long with cm or even m of slip.Therefore, they could have hosted small earthquake ruptures (M w < 2) compatible with multiple events recorded in our microstructures.Distributed seismicity along minor normal faults in a rock volume at shallow crustal depths is also consistent with the structure of normal seismic faults in the same region, as illuminated by small (down to M w = 0.7) aftershocks after the l'Aquila M6.1 earthquake (Valoroso et al., 2014).
The percolation of calcite-enriched, meteoric fluids through seismically active faults has been extensively documented in the Northern and Central Apennines (Agosta & Kirschner, 2003;Smeraglia et al., 2021) and is also consistent with our observations of cemented slip zones of normal faults (Figure 9).Dolostone clasts in the ultracataclasites of large faults have lobate-cuspate grain boundaries typical of stylolites, further evidence of the activation of viscous pressure-solution processes after cataclastic fragmentation (Figures 7d-7f and 9f).The occurrence of bitumen-rich stylolites in the wall rocks with the orientation of the foliation (i.e., perpendicular to the maximum shortening direction) consistent with the normal sense of shear (top to the right in Figure 7c) is also in agreement with the activation of pressure-solution processes (Figure 7c).Transgranular fracturing during the co-seismic phase and the subsequent increase in the permeability of the slip zone can accelerate the kinetics of pressure-solution in the post-seismic phase by opening pathways for diffusion mass transfer until these fractures are closed during fault recovery (Gratier et al., 1999;Renard et al., 2000; Phase 4 in Figure 10).Furthermore, the presence of organic matter in the cataclastic matrix (Figure 7d) (i.e., an insoluble phase) increases the dissolution rate along the mutual contacts between dolomite and calcite and bitumen with contrasting solubility and by the preservation of water films (Gratier & Gueydan, 2007;Gratier et al., 2013).This body of evidence suggests that the pressure-solution process is widespread in these faults and acted at different dissolution rates both before and after the fragmentation of old slip zones and the formation of new ones by co-seismic fracturing in a fluid-rich environment.

Formation of Mirror-Like Surfaces
A prominent structural feature in the area is the abundance of exposed MSs, independent of the displacement accommodated by the associated fault/fracture (Figures 4d-4g and 5).In all the studied slip zones, the MSs sharply "truncate" the dolomitic grains (samples 7, 8, 9 in Figure 5; Figures 7a, 7e, and 9c-9e).MSs are most prominent and continuous in large displacement faults; however, some MSs are found in faults/fractures often with wavy or bent surfaces (perpendicular and parallel to slip direction) that accommodate less than 1 mm of displacement, or along the bedding interface of folded strata (i.e., flexural slip surfaces Figure 4b; sample 6 in Figure 5).Continuous, sharp mirror-like fault surfaces truncating clasts were obtained in dry experiments performed on pure dolostones gouges sheared at seismic slip rates under low normal stresses (up to 27 MPa) by Fondriest et al. (2013).In addition, the shattered rudist calcarenites located in the hangingwall of the Monte Camicia Thrust are similar to in situ shattered carbonates found in damage zones of other faults with clear evidence of seismogenic history (Demurtas et al., 2016;Fondriest et al., 2015Fondriest et al., , 2020)).This evidence suggests that the studied faults accommodated seismic slip at some stage during their activity (see Section 6.2).
Moreover, several microstructures outline the MSs can be interpreted as the result of pressure-solution processes (e.g., indentation and flattening of dolostone grains, Figure 9d) or viscous flow (smeared bitumen, Figure 8a), which is typical of "aseismic" creep deformation (Gratier et al., 2011(Gratier et al., , 2013)).As a matter of fact, MSs outlined by nanoparticles have also been produced in experiments performed at sub-seismic slip rates (≈0.1-10μm/s; see Section 1).Proposed mechanisms for the formation of seismic and aseismic MSs are (a) localized dynamic recrystallization due to frictional heating during seismic deformation conditions (Smith et al., 2013), (b) formation of ) and the development of a shape preferred orientation of the dolostone clasts (e.g., Figure 7b).These processes also contribute to the formation of mirror-like fault surfaces (Verberne et al., 2019).( 2) Co-seismic phase: co-seismic rupture in a fluid-rich environment causes fragmentation of both dolostone and bitumen and fluid percolation/infiltration (e.g., Figure 9a).Co-seismic slip may also contribute to the formation of mirror-like fault surfaces (Fondriest et al., 2013).( 3) Post-seismic phase: calcite precipitation from percolating fluids and pressure-solution deformation causes fault strength recovery and hardening of the slip zones and of the damaged fault rocks (e.g., Figure 9d).( 4) Inter-seismic phase (and/or pre-seismic phase): dissolution of dolostone grains and viscous flow of bitumen in the slip zone.(5) Co-seismic phase: seismic ruptures propagate along sealed and hardened faults.Previously formed slip zones are cut by the newly formed slip zones (e.g., Figure 9e).
nanograins by strain-induced plasticity followed by brittle fracturing of crystals and abrasion at very small scales during aseismic, sub-seismic and seismic deformation (Siman-Tov et al., 2013;Tesei et al., 2017;Verberne et al., 2014).Pozzi et al. (2018) suggest that MSs are not frictional slip surfaces, but mark the broken boundaries between strong, sintered wall rocks and a weak and viscously deforming principal slip zone.
Based on the field and microstructural evidence listed above, we propose an alternative mechanism for MSs formation at these shallow crustal depths (1-4 km) and low temperature (<100°C, see Section 2.2) conditions in the presence of fluids.These conditions are quite common in carbonate-built mountain belts, especially in the Apennines, where polished slip surfaces are widespread (Demurtas et al., 2016;Fondriest et al., 2012Fondriest et al., , 2015;;Jackson & McKenzie, 1999;Leah et al., 2018;Ohl et al., 2020;Siman-Tov et al., 2013).The proposed mechanism includes (a) propagation of mode II transgranular cracks through the intact bare rock forming two rough fracture surfaces filled by gouge and nanoparticles that are in contact through highly stressed asperities (Figure 11, stage 1) and, (b) ingression of a fluid phase (carbonated water) that triggers the reduction of fracture roughness (at the micro and nanoscale) by the dissolution or by cold sintering of nanoparticles and asperities (Figure 11, stage 2).This "static" flattening of rough surfaces and slip zones may operate along faults and fractures during the inter-seismic period (Figure 11, stage 3).The proposed mechanism would help explain the observation of Ms along faults with negligible slip displacements.
The high bitumen-content, the low temperature of the fluid (<80°C), and abundance of micro-cracks (e.g., Figure 7a) would increase the rate and effectiveness of the dissolution of dolomite (Gratier & Gueydan, 2007).The above mechanism possibly operates during the inter-seismic/post-seismic phases at shallow crustal depths and could reduce the surface roughness also in the case of slip zones of large displacement faults that potentially experienced seismic rupture (e.g., Figure 9).This series of processes could result in the formation, during fault quasi-stationary phases, of extensive patches of ultra-polished and even striae-free MSs (Figure 5).According to this model, the MSs can represent the ultra-flattened or dissolved surface of a major rock discontinuity, such as a fracture, a fault, or flexural slip surface and may not always be associated with seismic slip displacement.Nevertheless, the studied mirrors are developed on a very peculiar rock type; smearing of bitumen (which occur not only in the slip zone, but also in the host rock, probably during burial) can smooth out the irregularities and fill in the grooves, contributing to the formation of smooth rock surfaces at aseismic slip rates without significant amount of displacement.

Conclusions
In this study, we present new field data regarding the structure and kinematics of the Monte Camicia Thrust (Figures 2-4), and microstructural investigations of slip zones and fault/fracture MSs cutting bituminous dolostones at its footwall (Figures 5-9).Based on the interpretation of the microstructures, the rheology of these faults is controlled by the interplay of cataclasis, fluid flow and viscous flow of bitumen spread over the slip surfaces and within the slip zones.Faults may register aseismic deformation occurring via cataclastic flow aided by the viscous rheology of bitumen and by pressure-solution, which may also aid the formation of MSs (inter-seismic/pre-seismic or inter-seismic/post-seismic phase).However, in larger faults, the presence of fragments of older slip zones, of bitumen-rich clasts and of slip zones cemented by calcite suggests the occurrence of multiple seismic ruptures in the presence of carbonate-rich fluids (co-seismic phase, 1 and 3 in Figure 10).Mirror-like surfaces, extremely common in the area, are found in both large faults and in faults/fractures with negligible displacement: we suggest that they may form by a combination of (a) dissolution of fine dolostone grains formed during crack propagation or fault slip (Figure 11) and (b) bitumen smearing causing flattening of fracture roughness.
As a consequence, the microstructures found in the slip zones and surface mirrors cutting the bituminous dolostones in the Fornaca Valley capture the main phases of the seismic cycle, including rapid co-seismic slip, viscous flow and fault strength recovery during post-seismic and inter-seismic phases (1, 3, and 4 in Figure 10).This research was founded by a INGV PhD grant: "Formation of polished surfaces in natural rocks: experimental and field constraints" (MC), and by the European Research Council Consolidator Grant Project NOFEAR No 614705 (MF, RG, and GDT).MF received funding from NextGenerationEU (REACT project) and from the 2021 STARS Grants@Unipd programme (STIFF project).The authors thank Renato Raineri for fieldwork activities, Leonardo Tauro and Silvia Cattò for thin sections preparation, Stefano Castelli for the high-resolution hand sample photographs, Manuela Nazzari, Giorgio Pennacchioni and Jacopo Nava for SEM support, Elena Spagnuolo and Sveva Corrado for the constructive discussions.The authors thank the editor Claudio Faaccenna, Jean-Pierre Gratier and an anonymous reviewer for their extremely constructive feedback.Open access funding provided by Università degli Studi di Padova within the CARE-CRUI agreement.

Figure 1 .
Figure 1.Geological setting of the Campo Imperatore area (Central Italy).Main thrust faults are in red, normal faults in blue and folds in black.(a) Map showing the main faults of the northern portion of the Central Apennines.The focal mechanisms refer to the mainshocks of the L'Aquila 2009 seismic sequence (modified after Pizzi et al., 2017).(b) Structural map of the Monte Camicia area (modified afterGhisetti & Vezzani, 1986b;Lucca et al., 2019).Colors refer to the tectonic units (see panel c) defined byGhisetti and Vezzani (1986b).The black inset shows the location of the study area.(c) Cross-section A-A' (see panel b), stratigraphy and tectonic units.

Figure 2 .
Figure2.Structural data collected in the surveyed area.(a) Map and geological cross section (A-A').Monte Camicia Thrust and reverse faults are in red; normal faults are in blue.The stereoplots (lower hemisphere projection) summarize the structural data (faults, bedding, joints and folds) collected in four localities (Stop 1-4; numbers in white boxes; location of structural stop n. 4 is in Figure1b) in the hangingwall (HW) and in the footwall (FW).In the stereoplots, the contours of the poles to bedding are in black, poles to faults with uncertain kinematics and planes are in black, poles to normal faults and planes are in blue, poles to reverse faults and planes are in red; joint surfaces are in green and fold limbs are in dark yellow and brown.Dashed lines represent mean fault surfaces (blue for normal faults and black for faults with uncertain kinematics).Arrows represent fault striae and point toward the direction of movement of the hangingwall.Diamonds represent fold axis.The stereoplot with a light red background refers to the structural data of the Monte Camicia Thrust.(b) Sketch showing how fault displacement was computed from fault separation and attitude of fault, outcrop surface, and bedding.

Figure 3 .
Figure 3. Monte Camicia Thrust Zone in the surveyed gully.(a) Monte Camicia Thrust exposure.Folded sub-vertical strata of the Dolomie Bituminose Fm. in the footwall (FW) and highly damaged strata of Calciruditi a Rudiste Fm. in the hangingwall (HW).(b) and (c) Sub-vertical (b) and sub-horizontal (c) strata of the bituminous dolostones in the footwall at about 10 and 50 m from the main fault of the thrust.(d) Undeformed bituminous dolostones showing the color layering due to different content in organic matter and the polarity of the strata.

Figure 4 .
Figure 4. Main structural features in the bituminous dolostones at the footwall of the Monte Camicia Thrust (photomosaic of the east side of the surveyed gully).(a) Low and high angle normal faults cross-cutting both each other and folded strata.(b, c) Z-type asymmetric parasitic folds (longer limbs dipping at high angle) sometimes developed on strata with a mirror-like surface (MS) along the bedding interface (b).(d) Small displacement (<8 cm of slip) normal faults with MSs.(e) High angle normal faults with more than 1 m of slip displacement.(f) MS in (e).(g) MS with smeared bitumen (black).

Figure 5 .
Figure 5. Summary table and photos of the studied samples.

Figure 6 .
Figure 6.Typical facies of the host rock bituminous dolostones.(a) Bituminous dolostones with characteristic foliation (parallel Nicols, optical microscope image).(b) Organic matter-rich layer.Several macerals (i.e., primary organic matter) are recognizable, including alginite (orange in color elongated algal bodies) and resinite (red oval in shape bodies).Solid bitumen is the black matter filling the voids and boundaries between the clasts.Aggregates of dolomite grains are whitish in color (see panel (a) for location, parallel Nicols, optical microscope image).(c) Dolomite grains (dark gray in color) and sub-idiomorph calcite grains affected by the pressure-dissolution process (see panel (b) for location, SEM-BSE image).

Figure 7 .
Figure 7. Microstructures of slip zones collected from normal faults with displacement from less than 1 mm up to 1.23 m.(a) and (b) Slip zone (secondary fault in sample 5) filled by bitumen (dark gray in color) bounded by truncated dolostone clasts (yellow arrows).Dolostone clasts in the bituminous-rich slip zone are preferentially oriented with the long axis sub-parallel to the wall rocks.(c) Slip and damage zone beneath a mirror surface (sample 3).A minor fault with a mirror-like appearance accommodated 2-3 mm of slip ("MS 2 "; fractures are marked with dashed red lines; thick red arrows show the inferred orientation of the maximum principal stress, σ 1 ).(d) Minor fault decorated by bitumen (sample 3) with about 500 μm of slip.Dissolution of dolostone grains in the wall rock (formation of the styloliticlike wall rock-bitumen contact (white arrow) is aided by the presence of bitumen.(e) Sharp contact between dolostone and bitumen (sample 3).Dolostone clasts are embedded in a bitumen-rich layer visible in (c).(f) Slip zone beneath a mirror surface (sample 9, 1.23 m of slip) with dolostone grains with sutured grain boundaries and indented grains immersed in a bitumen-rich matrix.All SEM-BSE images except for (c) (parallel Nicols optical scan of the thin section).D = displacement.Red arrow = shear sense.

Figure 8 .
Figure 8. Top-view of fault mirror-like surface (MS, sample 5, 12 cm of slip).(a) MS made by smeared bitumen (dark gray) and dolostone clasts.(b) The MS is made of nanoparticles of dolomite glued by bitumen.SEM-SE images.

Figure 9 .
Figure 9. Slip zones and associated mirror surfaces from normal faults cutting bituminous dolostones.These slip zones recorded fluid infiltration (calcite precipitation).Panels (a-d) are from sample 5 (0.12 m of slip).(a) Two slip zones beneath the exposed mirror surface (mirror-like surface (MS)).The upper slip zone is dolomite (Dol; light gray) and calcite (Cc; white) rich and includes an angular fragment of an older bitumen-rich slip zone (dark gray; yellow arrow).The lower slip zone is bitumen-rich.The upper and lower slip zones are separated by an ultra-polished slip surface (see detail in b).(b) Bitumen-rich slip zone (yellow arrow) with dolostone clasts oriented with their long axis sub-parallel to the wall rocks.(c) The upper slip zone includes angular grains of dolostone and fragments of older slip zones cemented by calcite.(d) The contact of calcite with dolostone grains is very flat (see yellow arrow), suggesting the activation of pressure-solution processes.Panels (e) and (f) are from sample 1 (3.83 m of slip), which has multiple slip zones bounded by MSs.(e) The slip zones are fragmented and then cemented by calcite precipitation.(f) Detail of an older slip zone (yellow arrow) with sharp boundaries made of indented grains of dolostone.Grain indentation is due to pressure-solution processes.SEM-BSE images.D = displacement.Red arrow = shear sense.

Figure 10 .
Figure 10.Conceptual model of the seismic cycle in faults cutting bituminous dolostones based on field and microstructural observations discussed in this study.(1)Inter-seismic phase (and/or pre-seismic phase): dissolution of dolostone grains and viscous flow of bitumen in the slip zone.Viscous flow causes clast rotation (see the vorticity arrow in t1) and the development of a shape preferred orientation of the dolostone clasts (e.g., Figure7b).These processes also contribute to the formation of mirror-like fault surfaces(Verberne et al., 2019).(2) Co-seismic phase: co-seismic rupture in a fluid-rich environment causes fragmentation of both dolostone and bitumen and fluid percolation/infiltration (e.g., Figure9a).Co-seismic slip may also contribute to the formation of mirror-like fault surfaces(Fondriest et al., 2013).(3) Post-seismic phase: calcite precipitation from percolating fluids and pressure-solution deformation causes fault strength recovery and hardening of the slip zones and of the damaged fault rocks (e.g., Figure9d).(4) Inter-seismic phase (and/or pre-seismic phase): dissolution of dolostone grains and viscous flow of bitumen in the slip zone.(5) Co-seismic phase: seismic ruptures propagate along sealed and hardened faults.Previously formed slip zones are cut by the newly formed slip zones (e.g., Figure9e).

Figure 11 .
Figure 11.New model of mirror-like surface formation proposed in this work.Propagation of mode II transgranular crack results in gouge and nanoparticle formation that in the presence of a fluid phase (water) can smooth the edges of the crack by pressure-solution.The thick arrow shows the orientation of the maximum principal stress, σ 1.