Structural characterization and K–Ar illite dating of reactivated, complex and heterogeneous fault zones: lessons from the Zuccale Fault, Northern Apennines

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 t

Evolution of segments of the central Apennine wedge through structural, geochemical, and geochronological constraints in fault zones

The timing of the progressive eastward migration of fluid-assisted orogenic (compressional) and post-orogenic (extensional) deformations in the Apennines is essentially constrained by stratigraphic data. Therefore, the reconstruction of tectonic evolution of the Apennine belt suffers from the paucity of high-resolution temporal constraints. Finding a more reliable methods for dating fault activity is therefore crucial to constrain the evolution of orogenic belts. Tectonic activity along faults is assisted by geofluids which are responsible for syn-tectonic mineralizations in different structures. Hence, such mineralizations offer the opportunity to unravel the fluid-assisted tectonic-thermal history of faults through time by coupling multiscale structural investigations, geochemical and radiometric analyses. The aim of the thesis is to constrain the compressional and post-compressional tectonic evolution of three segments of the central-northern Apennines and to frame them within the tectonic evolution of the central-northern Apennine wedge. To reach this aim, I studied the fluid-assisted deformation histories of three key area in the Apennines, where compressional deformation was followed by later extensional tectonics, through a multidisciplinary approach including geological- structural field observations, multiscale structural analyses, P-T tracers, and geochemical and radiometric analyses on syn-tectonic mineralizations. The studied areas are representative of an ideal transect oriented W-E across the Apennine belt and arranged from its inner portion (Tyrrhenian coast, to the west) to its axial sector (toward east and where extensional tectonics is actually active), passing through an intermediate sector of the belt. In detail, from W to the E, I focused on a thrust fault cut by later extensional faults exposed on Zannone Island (located on the western margin of the continental shelf in the central Italy), on an extensionally-inverted thrust fault exposed at Mt. Tancia (~50 km to the NE of Rome), and on compressional structures cut by later extensional structures exposed along the seismically active Mt. Gorzano normal Fault

Supporting Information and data for Curzi et al., 2024 - Tectonics - Tectonic evolution of the Eastern Southern Alps (Italy): a reappraisal from new structural data and geochronological constraints

This material includes the description of the investigation methods and the U-Pb data.</p

U-Pb data

Data of the tectonic carbonates dated through U-Pb dating</p

Middle Triassic subsidence in the Dolomites (Italy): control of tectonics.

In the Dolomites region, Middle Triassic tectonics was associated with fast subsidence rates and a Late Ladinian magmatic event that induced epicrustal intrusions (Monzoni, Predazzo, Cima Pape) and a significant shoshonitic-basaltic volcanism. Contrasting scenarios (from transpressional/transtensional tectonics to subduction backarc extension) were proposed to explain these tectonics and magmatism (Lustrino et al., 2019 and reference therein). In order to contribute to unravel of this issue, we built subsidence curves and maps for different Triassic periods, based on cross sections and well data. In particular, we analysed the thicknesses of four Triassic carbonate systems (represented by Contrin Fm., Sciliar Fm., Cassian Dolomite and Heiligkreuz Fm.) in 21 areas spread on the central-eastern Southern Alps. Subsidence calculations, covering a time span of about 12 Ma (from Illyrian to Tuvalian), provided us the opportunity to quantify local and regional subsidence rates, and to constrain and reconstruct the time-space evolution of subsidence at a regional scale. From preliminary results, we can observe that: 1) a depocenter of subsidence was persistent in the eastern Dolomites from late Anisian to middle-late Carnian; 2) subsidence migrated toward east and north-east in Julian time; 3) the time-space evolution and migration of the subsidence was controlled by the principal Triassic (strike-slip and extensional) tectonic lineaments; 4) subsidence rates were faster both in the eastern Dolomites and in areas characterized by Ladinian magmatism (central Dolomites), reinforcing the idea of a control of tectonics (i.e. crustal thinning). Based on our observations, we speculate the occurrence, during Middle Triassic time, of a wide (crustal scale) strike-slip system, associated with magmatism, in which the Dolomites represented a pull apart basin. Within this scenario, in the Dolomites, N70°-90° strike slip tectonics generated transtensional-transpressional faults (associated with local extra subsidence and uplift, respectively)

Alpine transpression in the Passo Rolle area (Dolomites, Italy): new structural and paleostress constraints

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

Structural characterization and K-Ar illite dating of reactivated, complex and heterogeneous fault zones: lessons from the Zuccale Fault, Northern Apennines

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.ISSN:1869-9510ISSN:1869-952

Architecture and evolution of an extensionally-inverted thrust (Monte Tancia thrust, central Apennines, Italy): geological, structural, geochemical, and K-Ar geochronological constraints

Deformation in the upper crust is heterogeneous and mostly localized along brittle faults. Faults and fault rocks may be weak compared to surrounding host rocks and are likely to accommodate repeated slip episodes due to structural reactivation during commonly fluid-assisted faulting events. Thrust fault reactivation by subsequent normal faulting has been commonly documented in orogenic wedges, where extensional tectonics often follows contraction. Fault inversion may lead to variable overprinting of the early tectonic fabrics, which prevents a straightforward interpretation of the complete fault kinematics and deformation history. For this reason, and due to the seismogenic potential of upper crustal faults, much effort has been invested into a better understanding of the architecture of faults, of their deformation mechanisms as well as kinematic evolution through time. In this study, we adopt a multidisciplinary approach to reconstruct the tectonic evolution of the exhumed Monte Tancia Thrust (MTT), central Italy, consisting of a W-dipping thrust fault later inverted into a normal fault. To this goal, we combined: 1) 1:5000 scale geological mapping of an area of about 15 km2 and detailed structural analysis both in the hanging wall and in the footwall of the MTT; 2) microstructural analysis of the thrust wall and fault rocks; 3) cathodoluminescence microscopy (CL); 4) X-ray diffraction (XRD) analysis of the clay content of the MTT fault rocks and surrounding undeformed blocks to evaluate their thermal evolution and 5) K-Ar dating of authigenic synkinematic illite. Further geochemical analyses such as stable and clumped isotopes of calcite mineralizations are in progress. The MTT is a 200 m thick shear zone characterized by a pervasive S-CC’ fabric associated with contractional top-to-ENE kinematics (S1 foliation). Within the first few metres below the thrust surface, a zone dominated by the partial superimposition of S-CC’ fabric showing an extensional top-to-W-SW movement (S2 foliation) on top of the earlier compressional fabric is prominent. Cathodoluminescence suggests that the compressional and extensional deformation stages were facilitated by fluid ingress in the deformation zone, but that the fluid did not undergo significant changes in chemistry. To evaluate the thermal evolution of the MTT and constrain the timing of the tectonic inversion, XRD and K-Ar datings are respectively now in progress. Based on our observation, we speculate that the S1 foliations mainly developed by repeated events of fluid overpressure leading to multiple crack-and-seal events characterized by several veining episodes, followed by pressure-solution creep. S2 foliation characterized by fibrous calcite developed under fluid pressure fluctuations simultaneous with the fault slip in a normal sense

From Fossil to Active Hydrothermal Outflow in the Back-Arc of the Central Apennines (Zannone Island, Italy)

Post-orogenic back-arc magmatism is accompanied by hydrothermal ore deposits and mineralizations derived from mantle and crustal sources. We investigate Zannone Island (ZI), back-arc Tyrrhenian basin, Italy, to define the source(s) of mineralizing hydrothermal fluids and their relationships with the regional petrological-tectonic setting. On ZI, early Miocene thrusting was overprinted by late Miocene post-orogenic extension and related hydrothermal alteration. Since active submarine hydrothermal outflow is reported close to the island, Zannone provides an ideal site to determine the P-T-X evolution of the long-lived hydrothermal system. We combined field work with microstructural analyses on syn-tectonic quartz veins and carbonate mineralizations, X-ray diffraction analysis, microthermometry and element mapping of fluid inclusions (FIs), C, O, and clumped isotopes, and analyses of noble gases (He-Ne-Ar) and CO2 content in FIs. Our results document the evolution of a fluid system of magmatic origin with increasing mixing of meteoric fluids. Magmatic fluids were responsible for quartz veins precipitation at similar to 125 to 150 MPa and similar to 300 degrees C-350 degrees C. With the onset of extensional faulting, magmatic fluids progressively interacted with carbonate rocks and mixed with meteoric fluids, leading to (a) host rock alteration with associated carbonate and minor ore mineral precipitation, (b) progressive fluid neutralization, (c) cooling of the hydrothermal system (from similar to 320 degrees C to similar to 86 degrees C), and (d) embrittlement and fracturing of the host rocks. Both quartz and carbonate mineralizations show noble gases values lower than those from the adjacent active volcanic areas and submarine hydrothermal systems, indicating that the fossil-to-active hydrothermal history is associated with the emplacement of multiple magmatic intrusions.ISSN:1525-202