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Outcrop view of fault rocks from different BSFs. Panels (a, b) show BSF 2. (a) Upper yellowish cohesive cataclasite with gently W-dipping foliation capped by the subhorizontal ZF principal slip surface. (b) Detail of foliated cataclasite with oblique foliation indicating top-to-the-east shearing. (c) Compositionally banded phyllonite of BSF 3. Note the shear bands constraining top-to-the-east shearing along the subhorizontal foliation. (d) Outcrop view of the complex spatial arrangement of four different BSFs at the western termination of the E-W section at Punta Zuccale. Note the eastern lateral closure of BSF 3, which forms a metric lens in the complex architecture of the ZF (see Fig. 2). The width of the photograph corresponds to ca. 8-10 m. (e) BSF 4 greenish cataclasite with millimetric angular to subrounded quartz clasts deriving from underlying Triassic quartzites in a very fine-grained matrix made up of ultracataclasite and gouges. (f) Detailed view of a foliated gouge level within BSF 4.
Source publication
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 cha...
Contexts in source publication
Context 1
... foliated cataclasite occurs in the northernmost exposed ZF, where it is sandwiched between BSF 1 above and BSF 5 below ( Fig. 3a), and it tapers off progressively towards the south (Fig. 2a, b). BSF 2 fault rocks appear as a cohesive foliated whitish-yellowish cataclasite in the immediate footwall of the ZF striated PSS (Fig. 5a, b). The PSS and subparallel striated planes dip gently to the west (Viola et al., 2018, their Fig. 3). W-plunging slickenlines are associated with slickensides and other kinematic indicators (i.e. S/Ctype shear bands in the foliated cataclasite) and are consistent with a top-to-the-east sense of shear. The cataclasite is made of ...
Context 2
... the coarsest clasts mainly localized in up to decimetre-thick breccia pockets and/or coarse-grained cataclasite layers. Millimetric clay-rich layers locally define a coarsely spaced subhorizontal or gently dipping foliation. Dark green to rust brown foliated to non-foliated gouge occurs as millimetre-to centimetre-thick discontinuous layers ( Fig. 5f) with sharp contacts to the contiguous breccia and cataclasite (Fig. 4g). In the gouge, the clasts are represented by rounded quartz grains and/or older breccia clasts. The clasts define a weak shape-preferred orientation (SPO) that is generally parallel to the gently dipping foliation of the clayrich ...
Citations
... Based on cartographic evidence, the horizontal throw of the Zuccale Fault is estimated to be about 6 km, partially gained after the emplacement of the Porto Azzurro monzogranite (Pertusati et al., 1993). The internal structure of the Zuccale Fault zone, which is up to 5 m thick, has been described in several papers (e.g., Pertusati et al., 1993;Collettini and Barchi, 2004;Collettini and Holdsworth, 2004;Smith et al., 2011;Viola et al., 2022): the shear zone shows S-C and C-C′ structures, clearly indicating a top-to-the-East shear sense (Collettini and Holdsworth, 2004), and the main slip surface is characterised by grooves and mechanical striations with Fe-oxides and/or Fe-hydroxides (Pertusati et al., 1993;Collettini and Barchi, 2004;Liotta et al., 2015). This evidence suggests that the Zuccale Fault evolved from ductile to brittle deformation (Pertusati et al., 1993;Collettini and Holdsworth, 2004;Collettini and Barchi, 2004) when the permeable cataclastic level played the role of conduit and trap for geothermal fluids . ...
... In particular, there is clear evidence of compressive structures controlling the basement reactivation and shortening of Miocene and Pliocene Basins in southern Tuscany and also in Corsica (e.g., Finetti et al., 2001;Bonini and Sani, 2002;Cerrina Feroni et al., 2006;Musumeci et al., 2008;Sani et al., 2009;Benvenuti et al., 2014;Bonini et al., 2014). Moreover, field evidence suggests that the emplacement of Miocene and Pliocene plutons in the upper crust in the Tuscan Archipelago and western Tuscany occurred within an overall compression regime (Musumeci et al., 2005;Mazzarini et al., 2011;Musumeci et al., 2015;Papeschi et al., 2017Papeschi et al., , 2021Viola et al., 2018Viola et al., , 2022. ...
... The absolute isotopic ages of the thrusts exposed in the Calamita peninsula are 6.14 Ma and 4.9 Ma (K-Ar on authigenic illite, Viola et al., 2018), suggesting that the activity of the T6 thrust can be extended to the Messinian. In this context, we also possibly suggest that the main thrust T4 may have been reactivated in the Pliocene, as the Zuccale Fault (post 4.9 Ma; Viola et al., 2018Viola et al., , 2022, namely an out-of-sequence thrust during the activity of T6 or an easternmost thrust such as that postulated for the emplacement of the Gavorrano granite ( Fig. 1) at about 4.5 Ma (Musumeci et al., 2005). Such an outermost thrust that probably spreads onshore may justify the outcrop of the Monte dell'Uccellina and the Monte Argentario promontory, where the deepest metamorphic terms of the Apennine stack are now exposed Fig. 1). ...
... In this plausible scenario, we suggest that the areas outside the preformed orogenic wedge (e.g., the Messinian basins of southern Tuscany) would represent inherited intermontane depressions and not rift and/or retro-arc areas as commonly assumed. Furthermore, we cannot exclude the propagation of late Messinianearly Pliocene out-of-sequence thrusts and back-thrusts (as documented in the nappe stack of eastern Elba island, Musumeci et al., 2015;Viola et al., 2018Viola et al., , 2022. ...
... It was initially interpreted as a LANF (low angle normal fault) accommodating late Miocene and Pliocene extension (e.g., 34 ) and this interpretation is still valid to many researchers. Subsequently, it has been suggested that the ZF represents the flat segment of an Aquitanian thrust reactivated during early Pliocene out-of-sequence thrusting (e.g., 33,35 ). Regardless of this debate and the regional impact of the ZF upon the local tectonic evolution, the ZF stands out as a remarkable and complex fault accommodating a kilometric displacement (Fig. 2b). ...
... Structural analysis and in-situ outcrop permeability measurements were carried out on the main structural elements of the ZF and BF (principal slip surface-PSS, brittle structural facies-BSF, and, where possible, undeformed host rock). In-situ outcrop permeability measurements from the ZF followed the description of fault architecture and BSFs by Ref. 35 , who provide the most recent characterization of the ZF as a patchwork of at least six BSFs formed at different times during long-term fault activity. Measurements of in-situ outcrop permeability along the BF were conducted according to our own identification of BSFs done by expanding upon the available fault characterization by Refs. ...
... We studied the ZF at its best-known exposure, Punta Zuccale on Elba (Figs. 2b and 3), where continuous outcrops allow the detailed analysis of the fault internal architecture. We adopted the structural characterization of the ZF by Ref. 35 , which, from bottom to top of the Punta Zuccale section, distinguishes six BSFs (Fig. 3). We focused specifically on the following structural elements (Fig. 3), which are the most representative of the internal architecture of this mature fault: The Boccheggiano Fault: structural framework and BSFs. ...
The permeability of fault zones 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 permeability structure
of fault zones are thus necessary to refine our understanding of natural fluid pathways and of the
mechanisms leading to fluid compartmentalization and possible overpressure in the crust. Fault
zones commonly contain complex internal architectures defined by the spatial juxtaposition of
“brittle structural facies” (BSF), which progressively and continuously form and evolve during faulting
and deformation. We present the first systematic in-situ outcrop permeability measurements from
a range of BSFs from two architecturally complex fault zones in the Northern Apennines (Italy). A
stark spatial heterogeneity of the present-day permeability (up to four orders of magnitude) even
for tightly juxtaposed BSFs belonging to the same fault emerges as a key structural and hydraulic
feature. Insights from this study allow us to better understand how complex fault architectures steer
the 3D hydraulic structure of the brittle upper crust. Fault hydraulic properties, which may change
through space but also in time during an orogenesis and/or individual seismic cycles, in turn steer the
development of overpressured volumes, where fluid-induced seismogenesis may localize.
... In other words, we speculate upon the role of inherited orogenic structures (i.e., the now inactive thrusts) on present-day seismicity (i.e., post-orogenic extensional earthquakes). Since the studied thrust zone is clay-rich, we also performed K-Ar dating of synkinematic and authigenic clay minerals to better constrain the age of thrusting (e.g., Van der Pluijm et al., 2001;Pană and Van der Pluijm, 2015;Torgersen et al., 2015;Viola et al., 2022). We discuss K-Ar ages in the framework of the Apennines evolutionary history to better constrain structural inheritance as a major factor influencing the ongoing geological evolution of the belt. ...
The central Apennines are a fold-thrust belt currently affected by post-orogenic ex- tensional seismicity. To constrain the influence that the inherited thrust-related structures exert on the present seismic behavior of the belt, we provide the high-resolution structural and hydraulic characterization of one of the most external exposed thrust fault systems of the central Apennines, the Sibillini Mts. Thrust Front (STF). We integrate structural mapping, multiscale structural analysis, and in situ air permeability on the brittle structural facies of the thrust zone. We also performed K-Ar dating of selected fault rocks to better constrain structural inheritance. The STF is defined by a complex, ∼300-m-thick deformation zone involving Meso-Cenozoic marl and limestone that results from the accommodation of both seis- mic and aseismic slip during shortening. Permeability measurements indicate that the low permeability (10^−2 ÷ 10^−3 D) of the marly rich host rock diminishes within the thrust zone, where the principal slip surfaces and associated S-C structures represent efficient hydraulic barriers (permeability down to ∼3 × 10^−10 D) to sub-vertical fluid flow. Field data and K-Ar dating indicate that the STF began its evolution ca. 7 Ma (early Messinian). We suggest that the studied thrust zone may represent a barrier for the upward migration of deep fluids at the hypocentral depth of present-day extensional earth- quakes. We also speculate on the influence that similar deformation zones may have at depth on the overall regional seismotectonic pattern by causing transient fluid overpressures and, possibly, triggering cyclic extensional earthquakes on normal faults prone to slip while crosscutting the earlier thrust zones (as per a classic fault valve behavior). This mechanism may have controlled the ori- gin of the 2016–2017 central Apennines devastating earthquakes.
... The most documented section of the ZF is the one of Punta Zuccale (Fig. 1a) where the fault consists mainly of 1.5-3 m-thick matrixsupported breccias, as well as foliated and massive cataclasites (Keller and Coward, 1996;Musumeci et al., 2015;Viola et al., 2018) and sulfide-bearing cataclasites (Gundlach-Graham et al., 2018). Based on field relationships, Ar-Ar dating of the Porto Azzurro monzogranite, on Ar-Ar and U-Pb dating of hornfels from the metamorphic aureole and from the footwall block, and on K-Ar dates of fault gouges, several authors (Musumeci et al., 2015;Viola et al., 2022;Viola et al., 2018) proposed that the ZF started as a thrust at c. 22 Ma (late Miocene) but became selectively reactivated by out-of-sequence thrusts at <5 Ma due to the progressive structuring of the Northern Apennine orogenic wedge. The younger age identifies the brittle stage of fault activity during which the ZF displaced the thermal aureole of the Porto Azzurro Pluton after the peak metamorphic conditions. ...
... Open questions regarding this model are the relations between the ages of faulting and of the skarn/epithermal ore at Terranera. The available constraints fix the timing of the ZF activity at <5 Ma (Viola et al., 2022;Viola et al., 2018), i.e., after the out-of-sequence thrusting occurring at 4.9 ± 0.27 Ma. This age is slightly younger than that of Rio Marina (5.4-5.6 Ma, Wu et al., 2019). ...
The Terranera magnetite-hematite-pyrite deposit of the Island of Elba (Italy) is an historical skarn deposit hosted by a fault zone of regional importance (Zuccale Fault) and by its hanging wall rocks.
We combine field observations with petrographic data, electron probe microanalyses (EPMA), XRPD data, fluid inclusion microthermometry, and element imaging by Laser Ablation-Inductively Coupled Plasma-Time of Flight Mass Spectrometry (LA-ICP-TOFMS) to define the ore-forming process at Terranera. We show that in this location the fault is made of four levels of mineralized fault rocks having distinct mineral compositions. In these levels, a mineral association made of diopside, clinozoisite, and other Mg-rich minerals is replaced by magnetite, hematite, pyrite, Mg-hornblende, clinochlore, and other Mg-rich phyllosilicates. This paragenesis is overprinted by goethite and clay minerals. Chlorite-quartz geothermometry and fluid inclusion microthermometry show that ore precipitation occurred at 350-180 °C from fluids of distinct bulk salinities, but goethite and clay mineral overprinting progressed at lower T.
We propose that Terranera is a magnesian Fe skarn formed due to the interaction between distinct hydrothermal fluids and a dolomitic protolith, which was preserved within the fault zone. These fluids mixed and cooled during protolith metasomatism, causing ore precipitation due to oxidation and desulfidation. A very similar process was described in a large deposit of Elba (Rio Marina). Argillic alteration was widespread within the fault but met permanently intermediate sulfidation conditions. Trace element composition of hematite shows that Terranera has features that overlap those of skarn and epithermal deposits. In particular, elements that are typical of epithermal deposits (Sb, Ga, Ge, As) occur at mass fractions (50-200 g/g) that are either unreported or not typical of hematite from skarn deposits. These features identify Terranera as formed in an ore environment that was transitional between that of a skarn and of an epithermal deposit. These features are shared by other historical deposits located at Elba and in the massive pyritic ore district of south Tuscany (e.g., Gavorrano, Fenice Capanne). This indicates that a similar environment might have occurred during the Neogene beyond Elba, in a much larger ore district of south Tuscany.
The development of fractures and faults in plutonic rocks is significantly influenced by the primary fabrics and long-lived deformation history. Dike swarms and intersection damage zones are highly fractured and have the potential to form naturally fractured reservoirs of hydrocarbons and fluid pathways to zones of host rock matrix alteration.
The Eastern Southern Alps fold‐and‐thrust belt (ESA) is part of the seismically active S‐verging retro‐wedge of the European Alps. Its temporal tectonic evolution during continental shortening has so far been constrained by few and low‐resolution indirect time constraints. Aiming at better elucidating the ESA spatiotemporal evolution, we gathered new structural and geochronological data from two regional thrust systems: the innermost south verging Valsugana Thrust (VT) and the more external Belluno Thrust System (BTS). Field work allowed us to constrain the geometry and kinematics of those thrusts and related folds and informed our sampling strategy to carry out fault gouge K‐Ar and tectonic carbonate U‐Pb dating from representative samples structurally associated with the VT and BTS. Our results suggest that the VT was active already in the Late Cretaceous (between ∼78 and 76 Ma) in response to far‐field stresses, with repeated reactivation continuing to the Late Miocene (∼6 Ma). The BTS recorded two distinct deformation events during the Oligocene (∼30 Ma) and at the Oligocene‐Miocene boundary (∼23 Ma). The VT was active for ∼72 Myr and partly acted during out‐of‐sequence thrusting. Based on regional correlations, we propose that the ESA share a similar spatiotemporal deformation history with the central Southern Alps farther to the west. We suggest a conceptual regional tectonic model wherein multiple, broadly coeval deformation events occurred in the entire Southern Alps during their long‐lived orogenic deformation in response to generally continuous NW‐SE shortening.
Brittle reactivation of plastic shear zones is frequently observed in geologically old terranes. To better understand such deformation zones, we have studied the >700 Ma long structural history of the Himdalen–Ørje Deformation Zone (HØDZ) in SE Norway by K–Ar and ⁴⁰ Ar– ³⁹ Ar geochronology, and structural characterization. Several generations of mylonites make up the ductile part of HØDZ, the Ørje Shear Zone. A ⁴⁰ Ar– ³⁹ Ar white mica plateau age of 908.6 ± 7.0 Ma constrains the timing of extensional reactivation of the Ørje mylonite. The mylonite is extensively reworked during brittle deformation events by the Himdalen Fault. ⁴⁰ Ar– ³⁹ Ar plateau ages of 375.0 ± 22.7 Ma and 351.7 ± 4.4 Ma from pseudotachylite veins and K–Ar ages of authigenic illite in fault gouge at c. 380 Ma are interpreted to date initial brittle deformation, possibly associated with the Variscan orogeny. Major brittle deformation during the Early–Mid Permian Oslo Rift is documented by a ⁴⁰ Ar– ³⁹ Ar pseudotachylite plateau age of 294.6 ± 5.2 Ma and a K–Ar fault gouge age of c. 270 Ma. The last datable faulting event is constrained by the finest size fraction in three separate gouges at c. 200 Ma. The study demonstrates that multiple geologically significant K–Ar ages can be constrained from fault gouges within the same fault core by combining careful field sampling, structural characterization, detailed mineralogy and illite crystallinity analysis. We suggest that initial localization of brittle strain along plastic shear zones is controlled by mechanical anisotropy of parallel-oriented, throughgoing phyllosilicate-rich foliation planes within the mylonitic fabric.
