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Late to Post-Variscan tectonics in the Sardinia Einstein Telescope candidate site (Italy): insights from Structural Survey and Electrical Resistivity Tomography
No full textThe geological characterization preceding the construction of a large-scale underground structure, such as the Einstein Telescope (ET), is a mandatory step of civil engineering studies. The ET Italian candidate site is located in the Variscan basement of Sardinia because of its geodynamic quietness, very low seismicity and anthropogenic seismic noise. The ET layout is currently projected as an underground triangular infrastructure (10 km long sides), whose vertices are located between the villages of Lula, Bitti and Onanì, and is confined within an area not crossed by main regional faults.
The geological features of the Palaeozoic metamorphic rocks are the result of ductile deformation with folds and related planar and linear anisotropies. This polydeformed metamorphic basement was intruded by several granitic bodies and by mafic to acidic dykes, mostly of early Permian age. A brittle to ductile fault network affects the metamorphic-plutonic ensemble. Within such a structural frame it is difficult to accurately predict lithologies at depth by means of the geological survey alone. Thus, we started an integrated multidisciplinary cartographic, structural and Electrical Resistivity Tomography (ERT) study. The results as far obtained are useful to highlight the structural elements at depth, particularly lithological contacts and fault zones, which are relevant for the prediction of mechanic behaviour of the rocks along the tunnels tracks as well as the groundwater occurrence.
We have merged the lithologic information from published maps (also by comparing satellite images) and new data collected in the field. Newly traced morphostructural lineaments mark the distribution of fault zones from the areas that surround the boreholes drilled at two vertices. The analysis of satellite images has thus allowed to define the segmentation of principal faults, whose length is limited at surface to a few kilometers (2.5 km max).
Field structural results provided evidence of at least two ductile phases (D2+3) almost completely transposing the original bedding and the oldest schistosity (S0+1). Faults are mainly NNW-, and WSW-striking and are associated with either more altered bedrock and/or cataclastic bands. The WSW-striking faults are often conjugated with E- to NE-striking faults. Fault zones can be associated with thick quartz veins, or thin chlorite fibers. Locally, they are crossed by intense fracture arrays, pseudotachilites and gouge that can be as thick as a meter.
Near the vertices that were site of drilling (ca. 250 m total depth), ERT tomography was carried out, providing a complex internal resistivity stratification, that consists of up to three levels with variable distribution and thickness. As supported by field observation, we have interpreted the more conductive electrolayer as regolith and alluvial units, while the most resistive electro-layers correspond with the less-altered granitoids. The sudden trend of the isoresistivity line, that also recognized from satellite images or field evidence, was related to saturated fractured zones.
Thus, matching the vertical information provided by the 2D ERT results and the geological information from the study area, we provide a more accurate estimate of satured fault geometry at depth. Beside the ET vertices, similar approach can be adopted in predicting zone of hazards during the tunnel drillings
Exploring the Variscan syn- to post-orogenic tectonics at the Einstein Telescope site of Sardinia (Italy).
No full textThe Variscan tectonics of north-eastern Sardinia is defined by syn-orogenic collision-related middle to high-T metamorphic terrains and by post-orogenic intrusions. The aim of this study lies in the understanding of the relationships between faults and intrusions in an area that has been candidate for hosting the Einstein Telescope (ET), a European third-generation underground interferometric detector of gravitational waves.
Free of regional faults, the ET layout is currently projected as a triangular infrastructure (10 km long sides), whose location has been proposed because of its present-day geodynamic quietness, very low seismicity and anthropogenic seismic noise. Despite previous maps underestimated the presence of faults in the area, new fieldwork has mapped them in higher detail.
The syn-orogenic deformation of the Palaeozoic metamorphic rocks consists of distinguished fold and cleavage generations with at least two ductile phases (D2+3) almost completely transposing the original bedding and the oldest schistosity (S0+1), that is still visible in the south were the thermometamorphic grade is lower. The later brittle fault network affects the metamorphic-plutonic ensemble with faults that mostly run parallel to the orientation of both dykes and plutonic contacts. Fault zones are generally NNW-, and WSW-striking and are associated with either more altered bedrock and/or cataclastic bands that are locally affected by late hydrothermal circulation with thick quartz veins, thin chlorite fibers or pseudotachilites and gouge that can be as thick as a meter each. In the surroundings of the boreholes drilled at two ET vertices, multiscale morphostructural analysis and 2D Electrical resistivity tomographies was carried out. The distribution of fault zone-related morphostructures shows maximum length up to 2.5 kilometres. At depth (ca. 250 m), the
tomographies show also a complex internal resistivity stratification, that consists of up to three electrolayers with variable distribution and thickness. This is related to the occurrence of faults in the bedrock corresponding to the larger morphostructural lineaments.
In conclusion, these results highlight the interaction between the syn- and the post-orogenic features that seem related to the inherited Variscan crust structure, which has a present-day implication in the groundwater flow. Evidence shows that the post-orogenic Variscan structures guided the post-orogenic dyke and vein injection. Further, the main fault zones were site of later hydrothermal circulation, possibly reactivated during the Oligocene-Aquitanian tectonics. Further studies should constrain the contribute of the eventually current differential uplift into reactivating the inherited Variscan structures to exclude the presence of neotectonics in the area
Quaternary fault-controlled volcanic vents and crustal thinning: new insights from the magma-rich Tyrrhenian passive margin (Italy)
No full textFrom thrusting to back-arc extension: seismic structure and field evidence of the Apennine Tyrrhenian margin (Central Italy)
Full text linkThe Apennine Tyrrhenian margin records the evolutionary steps of the back-arc basin developed at the rear of a E-ward migrating fold-and-thrust belt. As well-documented in literature, the counterclockwise rotation of the Apennines is related to the southward increase of the roll back-related subduction of the Adria slab. This led first to the progressive incorporation of thrust sheets within the Apennine prism in the upper plate and later to its subsequent back-arc extension that is contemporaneous with the continuate inarching of the Apennine front towards the Adriatic and Ionian seas. Uncertainties arise on the structural style and timing in the internal Apennines between the orogenic and post-orogenic stages, that are respectively represented by thrust-sheet implacement, and crustal thinning
First comparison of subsidence/uplift rates between Copernicus European Ground Motion Service data and long-term MIS 5.5 geological record in Mediterranean regions
No full textThe European Ground Motion Service (EGMS), a component of the Copernicus Land Monitoring Service, offers a valuable tool for investigating vertical ground motion in coastal regions that are subject to different natural and anthropogenic processes. To conduct effective coastal assessments, it is essential to consider the multiscale interactions of these processes. This review presents a methodology for comparing EGMS Ortho (Level 3) data, based on Sentinel-1, and calibrated with measurements from the Global Navigation Satellite System (GNSS) and long-term rates based on the markers of the Marine Isotope Stage (MIS 5.5; Late Pleistocene). This study examines the Italian and Greek coasts, encompassing a range of geodynamic settings. In stable crustal segments, such as Sardinia, the EGMS Ortho (Level 3) data roughly align with both GNSS and post-MIS 5.5 rates. However, long-term based observed variations are considerably below the current 1 mm/yr EGMS precision. Conversely, in foreland basins, EGMS maps show higher values than GNSS data, which reflect regional interactions between ongoing tectonics and compaction. Local discrepancies between EGMS and GNSS in industrialized areas are attributable to industrial activities, as evidenced by the EGMS subsidence rates (8 mm/yr) observed in Ravenna, which exceed regional long-term estimates (0.8 mm/yr). In seismically active regions, such as Calabria and Sicily (e.g. the Messina Strait), the data reveals complex short- and long-term interactions linked to the seismic cycle. Conversely, in volcanic areas, like Campi Flegrei, Mt. Etna, Santorini and Nisyros, the EGMS data proves most valuable for highlighting complex volcano-tectonic movements. The findings of this study can be extended to other areas worldwide
The stratigraphic architecture of Macco (middle Pliocene) in the Tarquinia Basin as controlling factor in the Monterozzi Necropoli placement.
No full textThe Middle Pliocene shallow water deposits cropping out in Tuscany and Latium (Central Italy) are
represented by a lithostratigraphic unit, known as “Macco” (Fazzini et al., 1972). In the Tarquinia basin, the
Macco consists of poorly lithified bioclastic calcarenites intercalated with hybrid sandstones and sandstone
often with lateral heteropic transitions. The skeletal assemblage is dominated by free living branches of
coralline algae, molluscs and foraminifers deposited in a carbonate ramp environment. The terrigenous
fraction is mainly composed of monocrystalline grains of quartz and subordinate detrital micas and feldspars,
sedimentary lithoclasts (arenites, siltites, sparse limestones). Glaucony grains and opaques are also present.
Based on new field survey in the area and on the comparison with previous works, a new geological map of
Tarquinia area has been produced. Multiscale morphologic and structural analysis shows that morphostructural
lineaments are fault-controlled mostly by E-W and NW-SE striking fault and fracture zones, locally interested
by calcite-bearing veins. Widespread evidence of transtensive syn-sedimentary tectonics is documented by
thickness and facies variations across faults, slumps and fault-related unconformable contacts.
In the Tarquinia area, this lithostratigraphic unit hosts the Etruscan necropolis of Monterozzi, UNESCO site
since 2004. The necropolis consists of more than 6000 burials of different typology (room tombs, pit tombs and
hole tombs) spanning a chronological period from 7th to 2nd century BC (Cataldi 1993; 2001). The aim of this
work is to evaluate how the stratigraphic architecture of Macco may have influenced the distribution of burials
Initiation and development of the Pennine Basal Thrust (Swiss Alps): a structural and geochronological study of an exhumed megathrust
No full textThe Pennine Basal Thrust (PBT) is an exhumed megathrust developed during continental collision from late Eocene to Miocene. To trace its evolution, five samples, with indications for up to three microstructurally diachronous white-mica generations, were investigated by laser in-situ and step-heating 40Ar–39Ar dating. Three deformation-related crystallization ages can be distinguished: (1) D1, characterized in the PBT hanging wall by an S1 foliation defined by white mica + chloritoid, began at or before ∼38.0 Ma; (2) D2 formed a pervasive S2 cleavage and synchronous white-mica rich veins dated at ∼27 Ma; (3) D3 produced an S3 crenulation cleavage and chlorite + white-mica veins dated at ∼23 Ma. Older ages of ∼96 Ma (footwall) and ∼115 Ma (hanging wall) are interpreted as minimum ages for the detrital component. Finally, discrete faulting produced fault gouge, with an illite K–Ar age of ∼19 Ma. A simplified back-restored reconstruction provides a tectonic context for the dated structures. In this framework, D1 occurred during middle to late Eocene tectonic accretion. After late Eocene initiation of continental collision, D2 reflects Oligocene top-to-NW shearing, with both in- and out-of sequence thrusting. D3 then developed from 23 to 19 Ma during the progressive deactivation of the PBT
Carbonate deformation through the brittle-ductile transition: The case of the SW Helvetic nappes, Switzerland
Full text linkCarbonate deformation through the brittle-ductile transition (BDT) remains incompletely documented in the field. We therefore investigate the exhumation of the SW-Helvetic nappe stack using a new multi-method approach that integrates optical observations with a revised nomenclature, thermochronology, stable isotopes, and clumped isotope thermometry, aiming to constrain the time-temperature history of BDT deformation processes in carbonates. Single grain (U-Th)/He zircon and apatite fission track ages establish new burial/exhumation trajectories from different nappes, allowing us to infer the thermal history of the Rawil Depression. This prominent doubly-plunging hinge zone between the Mont Blanc and Aar Crystalline Massifs underwent postnappe faulting, now constrained to the Tortonian-Early Pliocene, reflected in differential exhumation rates of -0.2 km/Myr between the most depressed area and its easternmost side. Calculating and modelling the rockbuffered clumped isotope temperatures (ranging from -250 degrees C to 55 degrees C), we indirectly date the BDT processes, exemplified by the Rezli Fault. On this structure, possible shear heating during mylonitisation is indirectly dated at 18-15.5 Ma. Progressive embrittlement started around 11-9 Ma at temperatures of about 150-110 degrees C. Lower clumped isotope temperatures correspond to recent brittle faulting between 9 and 5 Ma. Our results on the regional evolution can be applied to analogous seismogenic carbonate-rich crustal sections
The Volsci Volcanic Field (central Italy): Anatomy of a tectonically controlled, carbonate-seated, volcanic activity
No full textThe Quaternary Volsci Volcanic Field (VVF) represents one of the products of the west-directed subduction of the Adriatic slab that drove the development of the Apennine mountain belt in central Italy. Here, we present new results on the eruptive history and the diatreme processes of exemplar tectonically controlled carbonate-seated maar-diatreme volcanoes. The VVF is defined by phreatomagmatic surge deposits, rich in accidental carbonate lithics, and subordinate Strombolian scoria fall deposits and lava flows, locally sourced from some tens of monogenetic eruptive centers, mostly consisting of small volume (0.01-0.1 km3) tuff rings and scoria cones. In light of new 40Ar/39Ar geochronological data and compositional characterization of juvenile eruptive products, we refine the history of VVF activity and envisage the implications on the pre-eruptive magma system and the continental subduction processes involved. Leucite-bearing, high-K (HKS) magmas mostly fed the early phase of activity (∼761-539 ka); primitive, plagioclase-bearing (KS) magmas appeared during the climactic phase (∼424-349 ka), partially overlapping with HKS ones, and then prevailed during the late phase of activity (∼300-231 ka). As the volcanic centers cluster along high-angle faults, we investigate the relationships between faulting and explosive magma-water interaction, as well as the distribution pattern of the eruptive centers. New field data allowed to retrieve the fold-and-thrust belt structure associated with the eruptive centers. Analysis of componentry, grain-size, degrees of whiteness and roundness of carbonate lithic inclusions, along with their micropaleontological features, has allowed to establish volcano tectonic correlations. In our interpretation, the clustering of eruptive centers is controlled by tectonic features. Specifically, a first order control is tentatively related to crustal laceration and deep magma injection along a ENE-trending Quaternary lateral tear in the slab and to Mesozoic rift-related normal faults. A second-order control is provided by orogenic structures (mainly thrust and extensional faults). In particular, magma-water explosive interaction occurred at multiple levels (< 2.3 km depth), depending on the structural setting of the Albian-Cenomanian aquifer-bearing carbonates, which are intersected by high-angle faults. The progressive comminution, rounding and whitening of entrained carbonate lithics allow us to trace multistage diatreme processes. Finally, our findings bear implications on volcanic hazard assessment in the densely populated (> 0.4 million people) areas of the Volsci Range and adjoining Pontina Plain and Middle Latin Valley
Groundwater mixing assessment in the Pontina Plain (Central Italy): preliminary results of the CARG survey performed for the Terracina geological map
No full textAssessing water resource quality, and ground-and-surface water interactions related to land use and
climate changes is essential to groundwater assessment. In Central Italy, springs fed by carbonate aquifers are commonly the major contributors of drinking and irrigation water supply. In the Pontina Plain, they encompass mixing processes that we aim to understand. In the frame of the new geological map survey of Terracina, the groundwater flowpaths will be retrieved from a hydrogeochemical monitoring of about 20 springs. In particular, we aim at defining the hydrogeochemical processes that control the groundwater chemistry and to determine the suitability of springs and groundwater for irrigation and drinking purposes based on the water quality indexes.
Additionally, in order to investigate possible temporal variations in hydrogeochemical composition, historical
data were collected too as they may be related also with ongoing tectonics. Chemical-physical parameters (pH, Electrical Conductivity, and Temperature) were measured on field, and groundwater samples to analyze major, trace and isotope content (2H and 18O of H2O) were sampled. The Piper diagram revealed that groundwater shows a hydrogeochemical evolution from the Calcium-Bicarbonate facies to the Sodium-Chloride water type.
Geochemical modeling and saturation index computation of springs and wells confirm an interaction with
carbonate rocks. Most of the springs and well water samples are saturated with respect to calcite and dolomite, however the existence of mixing process with deep fluids and/or sea water cannot be excluded in some cases as supported by saturation indices of halite and anhydrite. The analysis of time series allowed the identification of trends in groundwater ions concentrations. Near the sea, these variations can be attributable to the increase in the salinization process in the coastal aquifer, while far from that it could be related to the upwell of deep fluids across major faults. Most water samples are suitable for irrigation and drinking purposes, except for the samples influenced by seawater and prolonged water-rock interaction. This study highlights the need of a detailed hydrogeological characterization for strategic groundwater management policy and planning, to avoid the decline of groundwater quality due to a decrease in recharge rates induced by climate changes
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