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The Dolomites geologically belong to the south-verging eastern Southern Alps of Italy. They record the effects of multiple deformation episodes from the Permian onward and thus offer the possibility to study the role of earlier inherited structures upon the Cretaceous-Neogene Alpine deformation history. In the Passo Rolle area, Permian extension was followed by Late Triassic-Early Jurassic extensional tectonics during which the NNW-SSE striking Passo Rolle Fault (PRF) contributed to the development of horst and graben structures. The Alpine orogenesis in the Passo Rolle area was responsible for the build-up of the Pale di San Martino mountain range during ~ NNWSSE compression. During this stage, the NNW-verging Cimon della Pala backthrust formed concomitant with the activation of minor thrusts and strike-slip faults, and the reactivation of Late Triassic-Early Jurassic extensional faults. The genetic relationships between the broad suite of tectonic structures in the Passo Rolle area have never been investigated in detail. To this end we performed mesostructural analysis of key representative exposures to add geometric and kinematic constraints on the tectonic structures associated with the different tectonic phases registered in the Passo Rolle area. We focused on collecting fault-slip data that we used for paleostress analysis aiming at reconstructing the local paleostress evolution through time. We show that (i) the PRF accommodated significant Alpine transpression and (ii) the Permian-to-Jurassic tectono-stratigraphic framework promoted the localization of Alpine deformation to the east of the PRF, where a thick carbonate-siliciclastic multilayer deformed through first-order folds and thrusts. To the west of the PRF, where the Permian ignimbrite formed instead a relatively rigid and thus less deformable block, only minor thrusts and strike-slip faults formed, commonly exploiting suitably oriented inherited structures.

Long-lived, mature faults can be architecturally complex. The in-depth unravelling of their complexity, including understanding the dynamic and multiscalar evolution of their mechanical properties, requires detailed multidisciplinary studies. Indeed, an integrated approach is the only viable solution to the deconvolution of the at times very complex internal architecture of brittle fault zones, which represents a phenomenal archive of faulting history and conditions through time and in space. Fault zone architectures are commonly characterized by the spatial juxtaposition of “brittle structural facies” (BSFs), which progressively form and continuously evolve during faulting. The mutual spatial and temporal relationships of BSFs impact directly on the bulk static and dynamic permeability structure of fault zones. The permeability structure, in turn, plays a significant role on the distribution of georesources and on seismogenesis in the brittle upper crust, where both natural and induced seismicity are often associated with fluid migration and overpressure. Detailed models of the complexity of fault zones and of their static and dynamic permeability structure are thus necessary to refine our understanding of fluid pathways and of the mechanisms leading to fluid compartmentalization and possible overpressure in the crust. We present a multidisciplinary workflow to decipher exhumed complex fault zones by integrating detailed structural analyses with geochronological dating of the deformation events recorded by the constituent BSFs and systematic in-situ permeability measurements to connect deformation structures to deformation age and hydraulic properties. The absolute dating of BSFs constrains how fault hydraulic properties change not only through space but also in time (during seismic or orogenic cycles, for example) as the fault architecture (and permeability structure) progressively develop. As a case study, we apply this workflow to the Zuccale Fault in the northern Apennines (Italy), a major low-angle fault that formed and was repeatedly reactivated during a time interval spanning at least the last 22 Myr. A stark spatial heterogeneity of its present-day permeability (up to four orders of magnitude) emerges as a key structural and hydraulic feature, even for tightly juxtaposed BSFs. Results show how complex fault architectures steer the 3D hydraulic structure of the brittle upper crust with direct implications on styles and modes of fluid ingress and flow.

Geofluids play an important role to seismic faulting both at hypocentral depth during earthquake nucleation and at shallower crustal levels during seismic rupture propagation. Pre- to co-seismic anomalies of crustal fluid circulation have thus far been identified by hydrogeochemical and seismological (Vp/Vs) monitoring and are generally interpreted as potential precursors, triggers, and/or facilitators of large magnitude earthquakes. Structural and geochemical studies on syn-tectonic mineralizations have highlighted different patterns of fluid ingress, circulation and fluid-rock interaction during the seismic cycle of thrusts and normal faults. Understanding fault rock-fluid relationships in exhumed faults is useful for revealing the role of fluids also in still ongoing seismic cycles from different tectonic settings. We present a review of published studies and original data on the chemical-physical characteristics of syn-tectonic mineralizations formed by fluids that assisted fault-related deformation in the Apennines (Italy). We use our data to build a general model of fluid circulation during thrusting and normal faulting during the seismic cycle, and define a multi-technique workflow to identify tectonic-related physical/chemical equilibria/disequilibria in fluid-rock systems. The proposed workflow relies upon multiscale structural analysis, stable C, O, and clumped isotope analysis, radiometric dating and burial-thermal modeling. The chosen study area is the Central Apennines fold-and-thrust belt, where post-orogenic extensional deformation, characterized by numerous Mw ≥ 6.5 earthquakes, currently overprints Neogene-Quaternary folds and thrusts. We show that thrusting mostly develops in closed fluid systems where fluid and host rock are close to chemical equilibrium. Subsequent normal faulting is characterized by a dominant upward and/or downward open fluid circulation system with an overall range of δ18O of paleofluids from -10‰ to +13.5‰ (V-SMOW). Isotopic and thermal fluid-rock disequilibria are particularly evident during pre-/ and co-seismic extensional deformation with mineralizations that are up to 30° C warmer and 16° C colder than the host rock and are associated with the ingress of exotic (meteoric or deep crustal) fluids.

Constraining the timing of tectonic events is of prime importance for the in-depth understanding of the complex evolution of orogenic deformation, particularly in the case of fold-and-thrust belts. In this work, we combined U-Pb dating of tectonic carbonates and K-Ar dating of fault gouges of selected key outcrops along two main thrusts of the Paleogene-Neogene Eastern Southern Alps (ESA), Italy. The ESA are the south-verging fold-and-thrust retrobelt of the Alpine orogen, offering spectacular exposures to study the details of past tectonic processes. However, despite a few published papers regarding the deformation mechanisms of a major thrust in the ESA (the Belluno Thrust), modern, multiscale structural and radiometric studies of fault zones in the ESA are missing, such that detailed reconstructions of the local and regional tectonic evolution through space and time remain only loosely constrained. We focused on the (i) Valsugana Thrust, which is a first-order thrust separating the Dolomites s.s. to the north from the Venetian Pre-Alps to the south, and (ii) the more external Belluno Thrust. We coupled U-Pb dating of tectonic carbonates and X-ray diffraction and K-Ar dating of clay minerals in fault gouges with structural analysis and microtextural characterization. We show that the Valsugana Thrust represents an inherited pre-Alpine structure that (i) registered far field deformation during the Early Cretaceous (K-Ar gouge age of 140 ± 32 Ma), (ii) strongly influenced the geometry and kinematics associated with deformation structures during the Alpine orogenesis and (iii) recorded multiple reactivations in Late Cretaceous (K-Ar age of 79.2 ± 8.4 Ma and 76.2 ± 1.4 Ma), late Miocene (U-Pb age of 9.1 ± 0.8 Ma), and Miocene-Pliocene (U-Pb age of 5.3 ± 1.6 Ma) times. Radiometric constraints from the Valsugana Thrust attest to remarkable out-of-sequence compressional movements in the inner ESA after the orogenic wave had progressed farther south to the more external Belluno Thrust, whose activity is constrained to the Oligocene by a 30.6 ± 5.8 Ma K-Ar gouge age and a 23 ± 14 Ma U-Pb syn-tectonic vein age.

The study of fault damage zones is key to the understanding of fault-related fluid flow in the upper crust with many applications, including groundwater and hydrocarbon exploration, and underground storage of CO2 and H. Many studies reveal that a relationship exists between fault damage zone width and net fault displacement. Despite this positive relationship, several factors such as the tectonic setting, the depth of deformation, the deformation mechanisms, and the evolving mechanical properties of fault rocks affect damage zone characteristics (e.g., width, asymmetry, fracture attitude, deformation intensity). Furthermore, recent studies show that the overall along-strike fault geometry may play a pivotal role in controlling damage zone characteristics. In particular, areas such as tip regions, linkage sectors, relay ramps and step-overs can be characterised by fault damage zone parameters markedly different from sectors away from these structural complexities. In this contribution, we present new structural data of fault damage zone parameters acquired along the 8 km long extensional Kornos-Aghios Ioannis Fault (KAIF) on Lemnos Island, North Aegean Sea, Greece. The KAIF deforms lower Miocene effusive and hypabyssal magmatic rocks and middle Eocene to lower Miocene turbidites. Deformed rock volumes along the KAIF are locally strongly altered by hydrothermal fluids (e.g., hydrothermal silicification). We provide a detailed characterization of the KAIF in terms of 2D fault geometry (mapped at 1:1000 scale) and kinematics and we present a characterization of fault damage zone parameters, including frequency and attitude of subsidiary fault-related fractures, in different fault sectors. The acquired data allowed us to define the boundaries of fault damage zones in the different sectors and to discuss the differences in terms of fracture attributes in linking- and tip-damage zones compared to damage zones away from these structural complexities. Our results show that fault damage zones in linkage and tip sectors are wider and that fault-related fractures are more clustered around several subsidiary faults with centimetre- to metre-offsets. Also, secondary fractures in linkage and tip sectors are less systematically oriented, thus increasing fracture network connectivity and, consequently, facilitating fluid mobility in structurally complex fault sectors.

Supplemental Text: Methods. Figure S1: (a) View of the ca. 300 m long tunnel. (b) Entrance of the tunnel, where the NE-verging thrust surface which juxtaposes the Scaglia Bianca limestones onto the Scaglia Rossa limestones (intermediate horses) is exposed. (c) NW-plunging fault-bend fold along the tectonic contact between the structurally highest and the underlying horses (structural site no. 4; see Figs. 2 and 3 for location). The contact is represented by SW-dipping, NE-verging thrusts which juxtapose the Maiolica cherty limestones onto the Marne a Fucoidi marly limestones. (d) Tectonic contact between the structurally highest and the underlying horses (structural site no. 4; see Figs. 2 and 3 for location). The contact is represented by three SW-dipping, NE-verging thrusts which juxtapose the Maiolica cherty limestones onto the Marne a Fucoidi marly limestones. The Maiolica lithotypes form metric lenses characterized by an internal structure imbricated toward NE. The Marne a Fucoidi pelagites exhibit highly (marly) and poorly (calcareous) foliated domains. Note a NW-plunging fault-bend fold above the lower thrust. (e, f) Detail of the metric lensoidal lithons of Maiolica characterized by an internal structure imbricated toward NE and embedded within highly-foliated Marne a Fucoidi pelagites. (g) Highly-foliated Marne a Fucoidi pelagites. (h) Metric imbricated sigmoidal lithons of Maiolica limestones characterized by and internal structure imbricated toward NE. Figure S2: X-ray diffraction patterns for randomly oriented mounts for the different grain-size fractions of sample GI12 with reflections of illite polytypes. Table S1: Summary of in situ permeability measurements both within and outside the deformation zone. Table S2: K-Ar data for the gouge samples of the Sibillini Mts. Thrust.

We studied the Zuccale Fault (ZF) on Elba, part of the Northern Apennines, to unravel the complex deformation history that is responsible for the remarkable architectural complexity of the fault. The ZF is characterized by a patchwork of at least six distinct, now tightly juxtaposed brittle structural facies (BSF), i.e. volumes of deformed rock characterized by a given fault rock type, texture, colour, composition, and age of formation. ZF fault rocks vary from massive cataclasite to foliated ultracataclasite, from clay-rich gouge to highly sheared talc phyllonite. Understanding the current spatial juxtaposition of these BSFs requires tight constraints on their age of formation during the ZF lifespan to integrate current fault geometries and characteristics over the time dimension of faulting. We present new K-Ar gouge dates obtained from three samples from two different BSFs. Two top-to-the-east foliated gouge and talc phyllonite samples document faulting in the Aquitanian (ca. 22 Ma), constraining east-vergent shearing along the ZF already in the earliest Miocene. A third sample constrains later faulting along the exclusively brittle, flat-lying principal slip surface to < ca. 5 Ma. The new structural and geochronological results reveal an unexpectedly long faulting history spanning a ca. 20 Myr time interval in the framework of the evolution of the Northern Apennines. The current fault architecture is highly heterogeneous as it formed at very different times under different conditions during this prolonged history. We propose that the ZF started as an Aquitanian thrust that then became selectively reactivated by early Pliocene out-of-sequence thrusting during the progressive structuring of the Northern Apennine wedge. These results require the critical analysis of existing geodynamic models and call for alternative scenarios of continuous convergence between the late Oligocene and the early Pliocene with a major intervening phase of extension in the middle Miocene allowing for the isostatic re-equilibration of the Northern Apennine wedge. Extension started again in the Pliocene and is still active in the innermost portion of the Northern Apennines. In general terms, long-lived, mature faults can be very architecturally complex. Their unravelling, including understanding the dynamic evolution of their mechanical properties, requires a multidisciplinary approach combining detailed structural analyses with dating the deformation events recorded by the complex internal architecture, which is a phenomenal archive of faulting and faulting conditions through time and space., Solid Earth, 13 (8), ISSN:1869-9510, ISSN:1869-9529

Bedding-perpendicular joints striking parallel (longitudinal) and perpendicular (transverse) to both the axis of the hosting anticline and the trend of the foredeep-belt system are widely recognized in fold-and-thrust belts. Their occurrence has been commonly attributed to folding-related processes, such as syn-folding outer-arc extension, although they can also be consistent with a pre-folding foredeep-related fracturing stage. Here we report the pre-folding fracture pattern affecting the Pietrasecca Anticline, in the central Apennines (Italy), resolved by a detailed field structural analysis. Field observations, scan-lines and interpretation of virtual outcrops were used to study the intensity, distribution and the orientations of fracture pattern along the anticline. The fracture pattern of the Pietrasecca Anticline consists of longitudinal and transverse joints, oriented approximately perpendicular to bedding, and of a pre-folding longitudinal pressure-solution cleavage set, which is oblique to bedding regardless of the bedding dip. Cross-cutting relationships show that joints predated the development of the pressure-solution cleavage. Furthermore, joint intensity does not relate to the structural position along the anticline. Taken together, these observations suggest that jointing occurred in a foredeep environment before the Pietrasecca Anticline growth. Our work further demonstrates that joints striking parallel and orthogonal to the main fold axis do not necessarily represent syn-folding deformation structures.

The inner Apennines (Italy) are characterized by scattered outcrops of continent‐derived orogenic metamorphic units exposed along the Tyrrhenian coast from northern to southern Apennines. At least since the 1970s, some peculiar rocks exposed on Zannone Island (central Italy) have been described as the only Paleozoic‐Triassic metamorphic complex linking those exposed in the northern—with those in the southern Apennines. Assessing the protolith nature, thermobaric conditions, and structural features of what is accepted to be the metamorphic unit of Zannone is, therefore, crucial to elucidate the early paleotectonic evolution of the Apennines‐Tyrrhenian orogenic system. To that end, we interpreted seismic reflection profiles offshore Zannone, we carried out mesoscale and microscale structural investigations of representative outcrops on the island, performed X‐ray diffraction analysis, and K‐Ar and U‐Pb geochronology of representative clay gouge and syntectonic carbonate veins. Results show that the metamorphic rocks of Zannone can actually be reinterpreted as belonging to nonmetamorphic siliciclastic turbidites, likely deposited in foredeep settings, and coeval to the Oligocene‐Miocene Macigno Fm. of the northern Apennines. The turbiditic sequence was overthrust by Triassic dolostone in the early Miocene (~22 Ma), weakly deformed at

Zannone is a very important island, located in the Neogene-Quaternary extensional domain of the Tyrrhenian back-arc basin, as it is the unique spot where the Paleozoic (?) crystalline basement is hypotesized to be exposed in central Apennines. The exposure of such hypothetical basement in the Zannone Island is very problematic as it implies very large normal displacements (> 3 km) along surrounding faults. No such displacements are known along faults close to Zannone Island.In this work, we study the hypothetical Paleozoic crystalline basement exposed in the Zannone Island with the main aim of understanding its geological nature and relationships with the surrounding rocks. We use a multidisciplinary approach including 1) interpretation of seismic reflection profile; 2) field survey; 3) petro-textural observations; 4) microthermometry on fluid inclusions; 5) geochemical analyses of stable and clumped isotopes; 6) Illite crystallinity and mineralogical analyses of clays and host rocks; 7) analyses of minor gaseous species (He, Ne, and Ar concentrations and isotope ratios) in fluid inclusions; 8) U-Pb geochronology of syn-tectonic calcite, and 9) K-Ar dating of syn-kinematic clay minerals.Our results show that the hypothetical Paleozoic (?) crystalline basement exposed on the Zannone Island is, instead, represented by siliciclastic rocks of very low metamorphic grade. This is testified by the quartzarenites nature of the rocks, the presence of chloritoid and by the observed incipient foliation marked by fine-grained white micas and disposed parallel to the bedding. The contact between such siliciclastic rocks and the overlapping Triassic Dolostone is represented by a low-angle thrust cut by sets of high-angle normal faults with associated calcite mineralizations. K-Ar dating on clay minerals in fault gouge reveals that at least one event of authigenesis (i.e. fluid-assisted tectonic activity) occurred in Zannone Island

Fault inversion may lead to significant obliteration of earlier tectonic structures, thus preventing the straightforward interpretation of the complete kinematics and deformation history of faults. We adopt a multidisciplinary approach to: (1) reconstruct the tectonic evolution through space and in time of the extensionally-inverted Mt. Tancia Thrust (Central Apennines, Italy); (2) understand the deformation mechanisms and the transition and reactivation of compressional structures during negative tectonic inversion; and (3) constrain the origin of fluids involved during tectonic processes. To this end, we combined: (1) detailed geological mapping and multiscale structural analysis; (2) illite-smectite paleothermal indicators; (3) C, O, and clumped isotopes on calcite mineralizations and (4) K–Ar dating of authigenic and/or syn-kinematic illite from the Mt. Tancia Thrust fault rocks. We show that shortening occurred between ~9 and ~7 Ma, during possibly multiple events of fluid overpressure and shear rupturing involving fluids entrapped over long term within the host rocks. Post-compressive tectonic inversion occurred at ~3 Ma under fluid pressure fluctuations during shear events with an input of meteoric-derived fluids. Tectonic inversion is spatially confined within the first few metres below the thrust surface in a volume dominated by the partial overprinting, folding, transposition, and re-utilization of the earlier compressional fabric.

The Mt. Gorzano Fault (MGF) is a major seismically active extensional fault of the central Apennines, responsible for the destructive Mw 6.0 Amatrice earthquake in 2016. The MGF developed during post-orogenic extensional tectonics, generating a continental basin in the hanging wall. The age of the onset of the MGF and the relationship among faulting, fluid circulation, and the seismic cycles are unknown. We investigate these issues by studying the footwall damage zone of the MGF (exhumed from ~2-3 km depth), where extensional structures and related mineralizations cut and partially overprint the pre-existing compressional orogenic fabric. Structural and geochemical analyses (REE, O, C and clumped isotopes) combined with U-Pb dating of calcite mineralizations show that extensional deformation along the MGF began at least ~2.5 Myr ago. Its activity has continued to the present-day with a mean slip rate of ~0.9 mm/yr, consistent with other seismically active extensional faults of the central Apennines. During Apennine contractional tectonics, orogenic structures and tectonic overburden hindered the ascent of deep fluids, which, therefore, may have been progressively overpressured. We postulate that such overpressure occasionally or cyclically triggered impulsive deformation (earthquakes). During the subsequent extensional phase (younger ~2.5 Ma), pre- or co-seismic crustal dilation opened pathways for the ascent of deep mineralizing fluids, which impregnated the fault damage zone and may have triggered earthquakes. The crustal reservoir of carbonate-rich fluids nowadays is the source of water for the Acquasanta thermal springs close to the Mt. Gorzano seismic area.