This paper presents the geophysical mapping of the western Sardinia continental shelf (Mediterranean Sea, Italy). The focus is to highlight the various submerged coastal systems associated with the last eustatic cycle, consisting of beach barriers, submerged depositional terraces, submerged dunes and the current infralittoral prograding wedges. The study was carried out by interpreting MBES data, SSS photomosaics, reflection seismic data and aerial orthophotos, validated by sediment sampling. All data were processed to produce a 1:100,000 scale map showing a wide sector of the western Sardinian shelf and summarising the morphological and sedimentological features. The formation and evolution of the submerged coastal systems have been related to the available models of the last sea-level rise from the LGM to the present day, denoting a pulse pattern at sea-level change mainly triggered by northern hemisphere meltwater pulses. Furthermore, the map shows the general distribution of sediments, marine phanerogam meadows and rocky outcrops.

The Corsica-Sardinia block (CSB) is set in the middle of western Mediterranean between two highly stretched lithospheric domains, the Balearic and Tyrrhenian basins, that opened because of the progressive eastward migration of the Apennine front and roll-back of the Neotethys slab. The main tectonic features recorded in the CSB are Oligocene-Miocene strike-slip faults with either NE-SW orientation in Corsica and northern Sardinia and NW-SE orientation in southern Sardinia. Several evidence indicate multiple reactivation of these trans-crustal structures over time. The oldest stage of reactivation is testified by voluminous Pliocene-Quaternary anorogenic volcanic activity localized along the strike-slip faults in northern and central Sardinia. Farther to the south, strike-slip faults reactivated as normal faults during the Quaternary accommodating the deposition of more than 1000 m of continental deposits in the Campidano basin. Finally, in several sites strike-slip faults reactivated as normal or oblique faults offsetting upper Pleistocene to Holocene coastal deposits.In spite of these evidence of recent deformation, the CSB is characterized by vertical aseismic movements in the order of few mm per year and weak seismicity, with earthquakes occurring usually at depth not higher than 10 km and along the main Cenozoic faults. These structures also control the topography of the Corsica and Sardinia islands, supporting a rugged morphology with peaks close to 2000 m in Sardinia and 3000 m in Corsica, deep river incisions and other geomorphic features typical of relief rejuvenation. To investigate the cause of this cryptic neotectonic activity we run a set of Finite Differences numerical models that simulate the CSB as thin elastic plate overlying an inviscid asthenosphere. The structure of the model lithosphere is based on available geological and geophysical dataset and is divided into six compositionally homogeneous layers: air or water, sedimentary or volcanic cover, crystalline middle crust, lower crust, lithospheric mantle, and asthenosphere. In the experiments, we change the density and heat production rate of the crustal layers to fit the measured Bouguer gravity anomaly and surface heat flow.The best fit experiment shows that the CSB crust consists of a relatively low-density lower crust composed by felsic granulites moderately enriched in heat-producing elements, and a standard-density middle crust composed of highly productive granites or migmatites. The results suggest that neotectonic activity can be related to regional uplift driven by a mass deficit localized in the lower crust. In our opinion, the Cenozoic faults accommodate differential vertical displacements of fault-bounded blocks, occasionally triggering low-magnitude earthquakes in the upper crust. This interpretation account also for the peculiar geomorphic features of Sardinia and Corsica, where the landscape is continuously rejuvenated according to the uplift movements.

Since the early Paleozoic, numerous metallogenic events produced in the Sardinian massif a singular concentration of mineral deposits of various kinds. Among them, the Variscan metallogenic peak represents a late Paleozoic phase of diffuse ore formation linked to the tectonomagmatic evolution of the Variscan chain. Two main classes of ores may primarily be attributed to this peak: (1) mesothermal orogenic-type As-Au ± W ± Sb ores, only found in E Sardinia, and (2) intrusion-related Sn-W-Mo-F and base metals-bearing ores found in the whole Sardinian Batholith, but mainly occurring in central–south Sardinia. Both deposit classes formed diachronously during the Variscan post-compressional extension. The orogenic-type ores are related to regional-scale flows of mineralizing fluids, and the intrusion-related ores occur around fertile intrusions of different granite suites. Metallogenic reconstructions suggest almost entirely crustal processes of mineralization without a significant contribution from the mantle. We summarized these processes with a holistic approach and conceptualized the Sardinian Variscan Mineral System (SVMS), a crustal-scale physical system of ore mineralization in the Sardinian basement. The SVMS required suitable metal sources in the crust and diffuse crustal reworking triggered by heat that allowed (a) the redistribution of the original metal budget of the crust in magmas by partial melting and (b) the production of metal-bearing fluids by metamorphic dehydration. Heat transfer in the Sardinian Variscan crust involved shear heating in lithospheric shear zones and the role of mantle uplift as a thermal engine in an extensional tectonic setting. Lithospheric shear zones acted as effective pathways in focusing fluid flow through a large-scale plumbing system into regional-scale structural traps for ores. Pre-Variscan metal sources of metallogenic relevance may have been (1) the magmatic arc and magmatic arc-derived materials of Ordovician age, extensively documented in E Sardinia crust, and (2) an inferred Precambrian crystalline basement lying under the Phanerozoic crustal section, whose presence has been assumed from geophysical data and from petrological and geochemical characteristics of granite suites. At shallower crustal levels, important contributions of metals may have come from pre-Variscan ore sources, such as the Pb-Zn MVT Cambrian ores of SW Sardinia or the REE-bearing Upper Ordovician paleoplacers of E Sardinia.

Keywords: ore geology, structural geology, prospection, exploration The requirement for Critical Raw Materials (CRMs) of the last years brought the need for systematic and large-scale prospecting in fast and efficient ways. An example is the ISPRA – UniCa project developing the Metallogenic Map of Sardinia, Italy, drawing upon classifying CRMs’ occurrences. In Sardinia, recent studies highlighted the discrete structural controls displayed by two types of CRMs-bearing ore deposits: a) mixed sulphides (Cu, Pb, Zn, Fe) skarn deposits with variable contents of W-Sn-Bi, Ni-Co, and F minerals, set along shear zones associated with the several thrusts marking the emplacement of the Variscan Nappes with a low-angled, top-to-the SW transport direction; b) F-Ba (±Pb±Zn) low-temperature hydrothermal veins hosting REEs and subordinate Ni-Co-Bi sulfides-sulfarsenides filling normal, high-angled, N-S and E-W striking faults with strike-slip components, crosscutting any Variscan structure. We present two case studies in southwestern Sardinia prospected by comparative modelling: the Rosas Shear Zone (RSZ) and the Santa Lucia-Su Zurfuru (SLZ) mining areas. In the RSZ, the skarn’s protolith is the Cambrian limestones embedded within the Ordovician succession by a system of anastomosing NW-SE Variscan thrusts, enhancing the circulation of the mineralizing fluids and the structurally passive skarn’s emplacement. N-S striking F-Ba veins cut the thrusts. In the SLZ, the protolith and host rocks of skarn are analogous to the RSZ: km-sized Cambrian limestone bodies are metasomatised at their peripheries and show a distinct lithological zonation: limestone, marble, and chlorite-skarn. Preliminary field surveys indicate that the Cambrian limestones outcrop at the cores of NW-SE striking anticlines, whose limbs have been reactivated at the stratigraphic contact by later high-angled faults, which hosts the F-Ba (Pb-Zn) hydrothermal veins. Decimetre-thick veinlets striking N-S also crosscut the limestone bodies. The vein system displays a generally good continuity, and is distinctly zoned: a) the northern zone is barite-rich and characterized by breccia textures, with several generations of fluorite cementing brecciated marble/skarn, rare galena and sphalerite; b) the central zone is dominated by banded fluorite and galena; here, the main system is intercepted by a two kilometres E-W quartz vein that ends up in a barite deposit at the contact with the limestones; c) the southern zone is fluorite-rich: it cuts across the skarn ore at depth. The comparison between RSZ and SLZ helps us to investigate the metallogenic processes that led to skarn and F-Ba hydrothermal systems emplacements, providing new guidelines for regional-scale mineral explorations and potential evaluation. The RSZ skarn model gave us a key to understanding the structural setting of SLZ. The SLZ veins model suggests some mineralogical potential left in the area and guides us in prospecting the same type of deposit by verifying and comparing zonation, continuity, and geometries. Globally, skarn and hydrothermal veins are common and great bearers of CRMs, and the structural-mineralogical comparative modelling we leveraged is an extremely efficient, quick, and low-cost way for prospecting and exploring, meeting all the requisites for modern economic needs.

This paper presents a finite-differences 3D numerical model that simulates the gravity and thermal structure of the Corsica-Sardinia Block (CSB), an apparently stable lithospheric domain characterized by cryptic tectonic activity. In the experiments, we change the density and heat production rate of the model crust within a range of geologically realistic values to fit the measured Bouguer gravity anomaly and surface heat flow pattern. The discrepancy between the observed geophysical structure and the outcomes of numerical modelling are discussed in relation to the composition of the CSB crust and finally recast in the geodynamic framework of the western Mediterranean region.

<p>The Einstein Telescope (ET) is a proposed underground infrastructure to host a third-generation, gravitational-wave observatory. There are currently two candidate sites to host it: one of this is located in Sardinia region (Italy), in a favourable geological context, the other one in the Meuse-Rhine Euregion. Site-characterization studies are under way towards the site selection, which is expected for 2024. The scope work of this research is to evaluate the surface deformation of this site by integration of remote sensing techniques with geological and geophysical data. In this framework the PSI (Persistent Scattered Interferometry) technique with SAR data is the proposed approach for the analysis of a long time-series imagery. Although recent crustal movements in the study area are supposed to be very small (≃ - 0.5 mm/years from 2014 as measured by EUREF Permanent Network https://epnd.sgo-penc.hu), ESA Sentinel-1 data from Copernicus program, represents an effective tool to update this knowledge and monitor the phenomenon. A first analysis has been performed in the study area using the Snap2Stamps methodology. During the first assessment of this research, this methodology has been tested to a dataset of 94 images from Sentinel-1. The radar data (SLC, Single Looking complex) acquired from January 2021 to July 2022 for both descending and ascending orbits on an area of 250 sq km has been managed. The applied methodology requires a long-time for the processing in order to derive vertical velocities, and we considered the opportunity to use a cloud service. So, we exploited the possibility offered by SNAPPING service provided by Terradue (https://www.terradue.com/portal/), a cloud on-demand computing service for Sentinel-1 Multi-Temporal DInSAR processing, based on integrated SNAP and StaMPS chain. In this service, the dataset can be improved, considering a longer time of acquisition, exploiting the complete Sentinel revisiting time, starting from 2014.</p><p>The first results of this analysis have been calibrated with the existing GNSS measures provided by EUREF using the data of the Nuoro station. The ground vertical displacement calculations, composing data from both acquisition orbits, confirm the existing evaluations and extend the current information to the whole study area. Moreover, it will be possible to consider also future acquisition with a continuous monitoring process.</p><p>These results can be considered an important value for the proposed Italian site and the ET infrastructure realization.</p><p> </p>

In this study, we present a new Matlab-derived software, MYflow, developed to perform rheological modelling of high-strain rocks and mylonites. The software handles both monomineralic and compositionally heterogeneous rocks made of various proportions of the most common minerals such as quartz, feldspar, calcite, olivine, plagioclase, micas, pyroxene, amphibole and garnet. The rheology of composite mylonites is evaluated using a suite of mixing models combined with two complementary mechanical constraints derived from the assumption of either uniform stress or strain-rate. Various compositions can be used to run either 0th dimensional rheological models corresponding to classical strength profiles, or 2D maps showing the grain-scale spatial variability of stress and strain rate as a function of composition, grain size and effective deformation mechanism. The applicability of the code, along with its main functionalities, is demonstrated using a model of composite mylonite that reproduces the typical microstructure of rocks deformed in high-strain zones. The software is further benchmarked by modelling the grain-scale distribution of effective deformation mechanism, stress, and strain-rate of three natural mylonite developed under different pressure, temperature, and strain-rate conditions. The outcomes of our modelling approach are compared to the results obtained from classical paleopiezometry studies and evaluated in relation to the processes that yield to partitioning of stress and strain rate in shear zones. Finally, we discuss the significance of mean stress and strain rate in mylonites addressing the applicability of recrystallized grain-size paleopiezometry.

A multi-disciplinary approach for the hydrogeological assessment and characterization of groundwaters in a coastal area with high anthropogenic pressure and ongoing seawater intrusion phenomena is presented. Such phenomena are increasingly widespread in coastal areas all over the world and could seriously threaten groundwater resources and socio-economic development of territories. The coastal plain of Muravera, in south-eastern Sardinia (Italy), has been studied since the sixties because of important seawater intrusion phenomena. Over the years, many research and studies, including geological, geophysical and geochemical, have been carried out, but dynamics and processes controlling the groundwater flow system were not fully understood. To define a three-dimensional (3D) hydrogeological conceptual model, all the available existing data were integrated within a 3D GIS environment along with those collected during new field surveys, including piezometric, hydrochemical and multi-isotope data, namely deuterium and oxygen isotopic composition of water (δ2H and δ18O), tritium(3H), strontium (86Sr/87Sr), and boron (δ11B). Stratigraphic logs and geophysical, interpreted according to a geological–depositional model based on sequential stratigraphy, allowed to constrain the geometry of the groundwater system, resulting in a complex multilayer aquifer, mostly phreatic and locally confined. Results from bulk chemistry and isotopes provided information regarding recharge sources, flow paths and residence times of groundwaters. Four main flow paths, including lateral recharge from bedrock, surface water infiltration from the Flumendosa river and Rio Flumini Uri, and the occurrence of young mixing processes between fresh and sea waters were recognized. Moreover, a major contribution of meteoric water to groundwater recharge has been documented. The proposed approach improves the understanding of the aquifer system under investigation and reduces uncertainties about main groundwater dynamics. Moreover, results of the conceptualization become new input information and data required in the development of a groundwater flow numerical model. The latter represents a useful tool for an efficient management of groundwater resources aimed at improving the quality and availability of water resources by local government.

Seawater intrusion is a global phenomenon occurring in many coastal aquifers. The excessive and uncontrolled withdrawal of groundwater and/or reduction in recharge to aquifers decrease the freshwater hydraulic head and can result in the saline front advancing inland toward abstraction boreholes. Sea-level rise due to the climate change can exacerbate these effects. Old saline groundwater related to eustatic effects resulting from climate change during the last post-glacial period can also occur in coastal aquifers. According to the Ghyben-Herzberg principle, the depth of fresh-saline groundwater interface is mainly controlled by density and, in turn, by salinity. However, aquifer geometries and intrinsic heterogeneity of the geological medium, can affect the fresh-saline groundwater interface position and the response times to the forcing that control the salinization processes. Therefore, the knowledge of the response dynamics of the aquifer conditioning the position of the interface are essential to design countermeasures to compensate the salinization processes. Results of the monitoring of electric conductivity, temperature, pH and Eh log profile at about 30 m deep boreholes in the highly anthropized coastal plain of Muravera, in south-eastern Sardinia (Italy), are presented. Since the early fifties, in the plain area the natural hydrodynamic equilibrium between groundwater, surface-water, and seawater has been deeply modified by the construction of dams across the Flumendosa river, embankments, and the development of agriculture, tourism, and aquaculture activities along the coast. Moreover, abandoned branches of the river have been salinized by a fishpond that created a direct opening to the sea. According to a geological–depositional model based on sequential stratigraphy, the geometry of the aquifers in the Muravera coastal plain has been defined integrating stratigraphic, geophysical, geochemical, and isotopic data. A complex multilayer aquifer, mostly phreatic and locally confined, has been recognized. Results of the monitoring campaigns showed that the position of the fresh-saline groundwater interface along the plain cannot be explained by the Ghyben-Herzberg model. In the north area of the Muravera Plain, where the semi-confined condition of the aquifer occurs, the position of the interface doesn’t change significantly. Moreover, the lowering of pH as conductivity increases suggests high residence time of saline groundwater in the aquifer and interaction with marshes sediments. In the central sector of the plain, in unconfined conditions, the deepening of the interface as the piezometric head increases occurs and has been related to the higher transmissivity of the aquifer and a recharge rate coming from the Flumendosa river during extreme rainfall events. The multilayered aquifer geometry and the relationship between surface waters and groundwater have been recognized as responsible for the recharge rate of the aquifer and for the relative position of the freshwater–saltwater interface.

Abstract This review illustrates the most important features of the Ordovician successions of the Sardinian basement. We focus on the stratigraphy and tectonic structures in the tectonic units of the External and Nappe zones of the Variscan basement. The Ordovician successions are characterized by unconformities related to tectonic events ascribed to the Sardic and Sarrabese phases. The different durations of the unconformity-related gaps in the External (17 myr) and Nappe (6 myr) zones, recent work on the trilobite fossil content, and the occurrence of a volcanic arc only in the Nappe Zone (Sarrabus and Gerrei units) highlight significant discrepancies suggesting that these domains did not share the same geodynamic setting and palaeogeographical position during the Ordovician. This implies they were amalgamated only in Variscan times. Whereas for the external and nappe zones the Ordovician features are clear, the high-grade metamorphic Inner Zone, where numerous Ordovician ortho- and para-gneiss occur, more detailed studies are needed to define a complete framework for the Ordovician evolution of Sardinia. The present revision of data for the best-preserved succession of Sardinian tectonic units suggests that at least two distinct terranes, which did not share the same Ordovician evolution, were only amalgamated during the Variscan Orogeny.

The case study presented here deals with the Pb-Zn-Cu skarn ores hosted in the Rosas Shear Zone (RSZ), a highly strained domain located in the external zone of the Sardinian Variscan chain. The RSZ is characterized by several tectonic slices of Cambrian limestones within a strongly folded and foliated Cambrian-Ordovician siliciclastic succession, intruded by late Variscan granites and mafic dykes. Based on geological mapping, structural and microscope analyses, our results show that the skarn ores in the RSZ are an example of passive structurally controlled mineralization. The RSZ was structured close to the brittle–ductile transition and, once exhumed to shallower crustal levels, acted as plumbing system favoring a large-scale granite-related fluid circulation. The paragenesis and the mineralization style of the skarn vary slightly according to the peculiarity of the local structural setting: a tectonic slice adjacent to the mafic dyke; an intensely sheared zone or a discrete thrust surface.

The crystalline basement of the Sardinia block is made up of an almost complete segment of the Variscan belt. Along a SW-NE transect of roughly 200 km, it is possible to observe the structure of the chain from the shallowest to the deepest domains, starting from an anchimetamorphic external zone in the SW of the Island, to a green-schist facies nappe zone in the center and to a medium to high-grade metamorphic inner zone in northern Sardinia. The exceptional exposure of the chain in Sardinia makes it an essential piece for the reconstruction of the pre-Variscan geodynamics and the Paleozoic terranes puzzle. In several reconstruction of the pre-Variscan paleogeography, Sardinia is considered a whole single block that experienced, since Cambrian times, several geodynamic settings as part of the northern Gondwana margin, before being involved in the Variscan Orogeny during lower Carboniferous. As stated by previous Authors, the most relevant pre-Variscan geodynamic events recorded in Sardinia occurred during the Lower-Middle Ordovician, when the Sardinian block was located close to a subduction zone where a volcanic arc developed. According to this interpretation, the external, nappe and inner zones acted as back-arc, arc and fore-arc, respectively, belonging to the same lithospheric block. The main evidences of Ordovician tectonics and volcanic activity are a folding event that affect only the Cambrian-Lower Ordovician successions and an angular unconformity related to the folds sealed by continental and tidal deposits in the external zone and by calc-alkaline volcanic products in the nappe zone. The review of the paleontological, stratigraphic, magmatic and structural data highlights significant discrepancies between the external and nappe zones, suggesting that these domains did not share the same geodynamic setting and, possibly, paleogeographic position during the Ordovician, implying they drew close and amalgamated only in Variscan times. This hypothesis is supported by the different ages of the unconformities, Upper Ordovician in the nappe zone and Middle Ordovician in the external zone, and the extent of the stratigraphic gap, long-lasting in the external zone. Furthermore, the activity of the volcanic arc in the nappe zone is contemporaneous to the continentalization and erosive processes in the external zone, that is totally devoid of magmatism and volcano-sedimentary deposits. The Upper Ordovician succession in the external zone define a rift that evolve to a passive margin, whereas in the nappe zone the onset of a passive margin is marked by a nonconformity above the volcanic arc. Note that also the faunas show remarkable differences between the external and nappe zones. Finally, a different paleogeographic position is suggested by the Hirnantian glaciomarine deposit in the external zone, lacking in the nappe zone. The recognition that the Sardinian block consisted of two distinct terranes before the Variscan Orogeny, entails alternative correlations and an adjustment of the arrangement of the now scattered Variscan terranes. In particular, the external and nappe zones should be located in different positions in order to fit the proper geodynamic setting in the paleogeographic reconstruction at the snapshot time.

Although much is known about the Ordovician tectonics of the South European Variscides, aspects of their geodynamic evolution and palaeogeographic reconstruction remain uncertain. In Sardinia, Variscan tectonic units include significant vestiges of Ordovician evolution, such as a fold system that affected only the Cambrian–Lower Ordovician successions, and are cut by a regional angular unconformity. A comparison of the stratigraphy and tectonic structures of the successions below and above the Lower Ordovician unconformity and a reinterpretation of biostratigraphic data allow us to identify significant differences between the stacked tectonic units. The unconformity is sealed as follows: (i) in the Sulcis–Iglesiente Unit (Variscan External Zone, SW Sardinia) by Middle–Upper Ordovician continental and tidal deposits; and (ii) in the Sarrabus and Gerrei units (part of the Variscan Nappe Zone, SE Sardinia) by Middle–Upper Ordovician calc–alkaline volcanic rocks. Therefore, at the same time, one tectonic unit was situated close to a rifting setting and the others were involved in a convergent margin. Of note are the different durations associated with the unconformities in the tectonic units (17 Myr in the Sulcis–Iglesiente Unit, 6 Myr in the Sarrabus and Gerrei units) and the occurrence (or absence) of glacio-marine deposits indicating that the units were located at different palaeo-latitudes during the Ordovician. These results suggest that the SW and SE Sardinia blocks did not share the same geodynamic setting during the Ordovician, implying that they were situated in different palaeogeographic positions at this time and subsequently amalgamated during the Variscan Orogeny. Furthermore, stratigraphic and tectonic correlations with neighbouring areas, such as the eastern Pyrenees, imply alternative palaeogeographic reconstructions to those proposed previously for some peri-Mediterranean Variscan terranes.

Fold-and-thrust belts have a high variability of structural styles, whose investigation provides continuous updates of the predictive models that try to better approximate the geometries recognized in the field. The majority of studies are focused on the geometry and development of folds and thrust surfaces and the amount of displacement, taking into account the role played by the involved stratigraphic succession assumed as a layer cake. We present a case study from the external zone of the Variscan fold-and-thrust belt in SW Sardinia, where it was possible to investigate the lateral and vertical variations of the mechanical properties of the involved succession, how they related to previous folding, control thrust geometry, and kinematics. In this case, the superposition of two fold systems acted as a buttress that induced extensive back-thrusting. We found that there is a close connection between the attitude of the bedding and the geometry of back thrust surfaces, shear strength during thrust propagation, and variation in the shortening amount, depending on which part of the folds were cut across. The folding-related mechanical anisotropy also seems to have induced a ductile deformation in the footwall of back-thrusts. Although the case study considers the development of back-thrust, the relations between thrust and not-layer cake geometries could also be applied to fore-thrust development.

The mechanical strength is a fundamental characteristic of rock masses that can be empirically related to a number of properties and to the likelihood of instability phenomena. Direct field acquisition of mechanical information on tall cliffs, however, is challenging, particularly in coastal and alpine environments. Here, we propose a method to evaluate the compressive strength of rock blocks by monitoring their thermal behaviour over a 24-h period by infrared thermography. Using a drone-mounted thermal camera and a Schmidt (rebound) hammer, we surveyed granitoid and aphanitic blocks in a coastal cliff in south-east Sardinia, Italy. We observed a strong correlation between a simple cooling index, evaluated in the hours succeeding the temperature peak, and strength values estimated from rebound hammer test results. We also noticed different heating-cooling patterns in relation to the nature and structure of the rock blocks and to the size of the fractures. Although further validation is warranted in different morpho-lithological settings, we believe the proposed method may prove a valid tool for the characterisation of non-directly accessible rock faces, and may serve as a basis for the formulation, calibration, and validation of thermo-hydro-mechanical constitutive models.

The Rosas Shear Zone (RSZ) is a 1 km thick brittle-ductile shear zone that outcrops in the Variscan fold and thrust belt foreland of SW Sardinia, where several important ore deposits were mined in the last century. The RSZ lies in the footwall and strikes parallel to the NE-dipping regional thrust that separates the Variscan foreland from the nappe zone. Two thrusts that developed along the limbs of two km-scale overturned antiforms, with NE-dipping axial plane, bound the RSZ. The folds show a SW-facing direction and a well-developed axial plane cleavage, and affect a lower Cambrian-upper Ordovician stratigraphic succession mainly made, from bottom to top, by a sequence about 200 m thick of dolostones and massive limestone followed by 50 m of marly limestones overlain by about 150 m of sandstones, pelites and siltstones, finally unconformable capped by conglomerates and siltstones, ranging in thickness from a few to 200 m. Differently, within the RSZ the bedding is completely transposed along the cleavage and its internal structure is characterized by anastomosing thrusts that affect the stratigraphic succession defining map-scale slices mainly consisting of dolostones and limestones embedded into the siliciclastic formations. It is noteworthy the occurrence of a NE-dipping, up to 100 m thick gabbro-dyke that postdates the deformation phases and that can be related to the exhumation of the chain during late Carboniferous-Permian times. In the whole area, contact metamorphic and metasomatic processes selectively affected the Cambrian carbonate tectonic slices, originating several skarn-type orebodies. Mineralized rocks display the mineralogical assemblages and textures of Fe-Cu-Zn skarns, with relicts of anhydrous calcic phases related to the prograde metamorphic stage (garnet, clinopyroxene, wollastonite), frequently enclosed in a mass of hydrous silicates (actinolitic amphibole, epidote) and magnetite related to the retrograde metasomatic stage, in turn followed by chlorite, sulfides, quartz and calcite associated to the hydrothermal stage. Metasomatic reactions also involved mafic rocks, producing a mineral association marked by clinopyroxene, amphibole, epidote, prehnite and Ba-rich K-feldspar. Sulfide ores are made of prevailing sphalerite, chalcopyrite and galena, with abundant pyrite and pyrrhotite and minor tetrahedrite and Ag-sulfosalts. Garnets are andraditic/grossularitic, distinctly zoned and optically anisotropic. Field surveys pointed out the tight structural controls on skarn and ore formation. On a local scale, the gabbro emplacement along high- to low-angle NNW-SSE structures bordering the carbonate tectonic slices accentuate the effects of contact metamorphism, and metric to decametric mineralogical zonation (garnet→pyroxene→wollastonite) are recognized. On a larger scale, extensive hydrothermal fluid circulations involved the structures of the RSZ. Infilling of metasomatic fluids in carbonate tectonic slices is fault-controlled and aided by the increase in permeability due to the alteration of prograde silicates. The causative intrusion related to skarn ores belongs to the early Permian (289±1 Ma) ilmenite-series, ferroan granite suite which intrudes the RSZ about 3 km east from the studied area. The Fe-Cu-Zn skarn ores of Rosas are best interpreted as distal, structurally-controlled orebodies, connected to large-scale circulation of granite-related fluids in the km-sized plumbing system represented by the RSZ.

The Variscan foreland of SW-Sardinia consists of a Cambrian to lower Carboniferous succession polydeformed under very low-grade metamorphism. It is characterized by the following superposed structures: 1) E-W-trending upright folds; 2) N-S-trending inclined folds, penecontemporaneous with 3) W-ward fore-thrusts, and 4) E-ward back-thrusts. A peculiar feature of this sector of the Variscan foreland is the widespread occurrence of back-thrusts, apparently more common than fore-thrusts, unlike the majority of foreland fold-and-thrust belts. Our research focuses on the role played by the folded basement in limiting extensive fore-thrusts development and how fold shape and orientation, along with litho-stratigraphic heterogeneity, influenced the back-thrust geometry. Generally, back-thrusts occur when the shortening can no longer be accommodated by fore-thrusts, usually because of buttressing induced by fore-thrust-related thickening and duplication of the stratigraphic succession. However, in the segment of Variscan foreland outcropping in SW-Sardinia, back-thrusting seems to be activated by a different mechanism. The inherited structural setting is characterized by two perpendicular generation of superposed folds that gave rise to a type 1 interference pattern with pluri-km-scale domes and basins. In particular, in the western sector of the foreland (i.e., the farthest from the nappe zone thrusted over the foreland) domes are made up of about 500 m thick lower Cambrian sandstone and limestones formations that may have acted as a buttress, hindering fore-thrusting propagation and facilitating extensive E-ward back-thrusting. This is corroborated by the large number of back-thrusts that crop out between the buttress and the nappe front. In this area, back-thrusts affect the folded sedimentary succession that is progressively younger and weaker E-ward. As commonly accepted in thrust faults, ramps developed in the competent stratigraphic sequence, here made up of sandstones and limestones, and flats in weak stratigraphic horizons, here consisting of marly limestones and shales. As a result, in the study area the dip of back-thrusts decreases towards the nappes front, where the weaker lithologies have been overthrusted. The back-thrusts’ surface is characterized by discontinuous antiforms and synforms that do not affect the underlaying succession; so, a later deformation phase that folded the back-thrusts can be ruled out. Therefore, the fault plane should have been deformed throughout the back-thrusts growth and development. Interestingly, strictly relationships can be noticed between the fault plane geometry and the inherited structures in the footwall of the back-thrusts. Where the back-thrusts cut across upright limbs perpendicular to the back-thrust strike, the fault plane shows an antiformal shape; where the back-thrusts take place above the pre-existing synforms with the axis plunging towards the back-thrust dip, the fault plane takes the form of the underlying synforms. Instead, back-thrusts are uninfluenced by pre-existing folds where they cut either synforms with the axis that plunges opposite to the dip direction of the fault plane or antiforms, regardless the plunging direction of their axis. To conclude, this research highlights the relevant role of the inherited structural setting on fold-and-thrust belt style and suggests that the strata attitude and the axes plunging directions of pre-existing folds could have a control in the back-thrust geometry.

The temporal evolution of the sedimentary source areas of the Armorican Massif, involving Ediacaran to Upper Ordovician strata, is investigated to gain insight into the palaeogeographic affinities and changes that occurred as a result of Cadomian orogenesis. Until now, palaeogeographic reconstructions based on geodynamic, stratigraphic and paleontological data have shown geological continuity between the Armorican Massif and the Iberian and Bohemian massifs and have allowed researchers to locate the Armorican Massif near the West African Craton and the Trans-Saharan Belt. This study goes beyond the interpretations based on lithostratigraphic correlation, which may be influenced by allocyclic factors (e.g., sea-level change) or fauna assemblages that have a wide provincial distribution, to provide a correct assessment of sediment flux. To determine the palaeogeographic location more accurately, the provenance of the siliciclastic sediments was examined in this study using U-Pb LA-MC-ICP-MS geochronology on detrital zircons coupled with whole-rock Sm-Nd and zircon Lu-Hf isotope analysis. This work was carried out on the sedimentary succession of the Medio Armorican Domain. The oldest studied sedimentary rocks were shown to belong to the Brioverian succession, which contains mainly 519-781 Ma old zircons, likely derived from sources that are still present in the Armorican basement. Successively, the lower Paleozoic succession was deposited in the rift stages of the Rheic Ocean, with contributions from a new source of 827-1120 Ma old zircons. A comparison of the zircon populations showed an increase in negative epsilon(Nd(t)) and epsilon(Hf(t)) values of the sedimentary supply in the post-Cadomian samples. Moreover, it revealed that the Medio and North Armorican domains had different locations during the Lower Ordovician, and that some areas of the Iberian Massif and the Medio Armorican Domain close to the Sahara Metacraton and Arabian-Nubian Shield were contiguous. (c) 2020 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.