Antonio Luca Funedda

Change of nappe transport direction during the Variscan collisional evolution of central-southern Sardinia (Italy)

Abstract The collisional Variscan (Early Carboniferous) evolution of the greenschist facies metamorphic basement of Sardinia is characterized by three deformation phases and two 90° reorientations of shortening direction. During the first phase of deformation N–S shortening occurred, with south-facing isoclinal folding, south-directed nappe emplacement and mylonitization in central and southeastern Sardinia, and large-scale E–W striking upright folds in southwestern Sardinia. Emplacement of the Barbagia, Meana Sardo, Gerrei and Riu Gruppa units took place during this event. This deformation phase was followed by E–W shortening, with W-directed emplacement of the Sarrabus and Arburese units along the Villasalto thrust and upright to E-dipping large-scale N–S striking upright folds in southwestern Sardinia. During the third deformation event, the shortening direction again rotated 90°, and N–S shortening produced kilometer-scale antiforms and synforms, the most prominent features in the metamorphic basement of Sardinia. The limbs of the kilometer-scale antiforms and synforms are vertically shortened during gravitational collapse, with contemporaneous faulting and conjugate shear zone development. Presence of large-scale E–W oriented antiforms was therefore critical for the formation of late Variscan extensional features, such as E–W striking, low-angle normal faulting.

Rifting and Arc-Related Early Paleozoic Volcanism along the North Gondwana Margin: Geochemical and Geological Evidence from Sardinia (Italy)

Three series of volcanic rocks accumulated during the Cambrian to Silurian in the metasediment-dominated Variscan basement of Sardinia. They provide a record of the changing geodynamic setting of the North Gondwana margin between Upper Cambrian and earliest Silurian. A continuous Upper Cambrian–Lower Ordovician succession of felsic submarine and subaerial rocks, dominantly transitional alkaline in character (ca. 492–480 Ma), is present throughout the Variscan nappes. Trace element data, together with Nd isotope data that point to a depleted mantle source, indicate an ensialic environment. A Middle Ordovician (ca. 465 Ma) calc-alkaline bimodal suite, restricted to the external Variscan nappes, overlies the Sardic Unconformity. Negative εNdi values (−3.03 to −5.75) indicate that the suite is a product of arc volcanism from a variably enriched mantle. A Late Ordovician–Early Silurian (ca. 440 Ma) volcano-sedimentary cycle consists of an alkalic mafic suite in a post-Caradocian transgressive sequence. Feeder dykes cut the pre-Sardic sequence. The alkali basalts are enriched in Nb-Ta and have Zr/Nb ratios in the range 4.20–30.90 (typical of a rift environment) and positive εNdi values that indicate a depleted mantle source. Trachyandesite lavas have trace element contents characteristic of within-plate basalt differentiates, with evidence of minor crustal contamination.

Mylonite development in the Hercynian basement of Sardinia (Italy)

Abstract Plastic deformation predominates over large areas in the Hercynian basement of Sardinia during Lower Carboniferous continental shortening. Mylonitization associated with ubiquitous top-to-south-southwest thrusting decreases from internal (central Sardinia) to foreland areas (southern Sardinia). In the Barbagia unit, mylonites completely obliterate all the previous structures; in the Meana Sardo unit, plastic deformation strongly overprints early folds; and in the Gerrei unit, mylonites are found only below the Meana Sardo thrust. A thick shear zone, the Baccu Locci mylonite zone, develops between the Gerrei and Riu Gruppa units. Inferences from deformation mechanisms in quartz mylonites suggest a temperature during deformation below 400 °C along thrusts throughout the study area. Overprinting relationships between foliations, deformation of annealed microstructures and porphyroblasts with inclusion patterns indicate that crystal growth occurred after early thrusting events and before the main nappe emplacement phase. Microstructural investigations in the Baccu Locci mylonites record two changes in the main deformation mechanism operating during progressive mylonitization. In the early stages of deformation, most of the strain is accommodated by dislocation creep in a fine-grained quartz matrix. At higher strain, dynamic recrystallization fully affects larger quartz crystals, producing pure quartz layers where plastic deformation and strain localize. Ongoing deformation together with syntectonic breakdown of feldspar producing mica leads to mineral changes, grain size reduction and reaction softening with strain localization in the fine matrix again. Dynamically deformed quartz is boudinaged in the later stages of mylonitization. Significant fluid infiltration during deformation can account for some of the large ore bodies hosted in the Baccu Locci mylonites.

3D hydrogeological modelling supported by geochemical mapping as an innovative approach for management of aquifers applied to the Nurra district (Sardinia, Italy)

A hydrogeological 3D modelling, supported by hydrogeochemical mapping and integrated interpretation of available data, was performed. The proposed methodology was applied to the strategic aquifer of the Nurra district (NW Sardinia, Italy). The finding of this work highlighted that structural history of the Nurra district exerts a relevant control on the hydrogeology and hydrochemistry of groundwater. The local connection of Triassic and Jurassic aquifers was proposed. The knowledge-base system will provide a suitable and effective tool for understanding and monitoring pollutants transfer dynamics into groundwater to better manage water resources and mitigate desertification processes.

Geological modelling for hydrogeological purposes in Oum Zessar area (SE Tunisia)

Lucchi, Renata G. ... et. al.-- 87° Congresso della Societa Geologica Italiana e 90° Congresso della Societa Italiana di Mineralogia e Petrologia, The Future of the Italian Geosciences - The Italian Geosciences of the Future, 10-12 September 2014, Milan, Italy.-- 1 pageThe Montellina Spring (370 m a.s.l.) represents an example of groundwater resource in mountain region. It is a significant source of drinking water located in the right side of the Dora Baltea Valley (Northwestern Italy), SW of Quincinetto town. This spring shows a morphological location along a ridge, 400 m from the Renanchio Torrent in the lower sector of the slope. The spring was investigated using various methodologies as geological survey, supported by photo interpretation, structural reconstruction, NaCl and fluorescent tracer tests, discharge measurements. This multidisciplinary approach, necessary due to the complex geological setting, is required for the importance of the Montellina Spring. It is interesting in the hydrogeological context of Western Alps for its high discharge, relatively constant over time (average 150 l/s), and for its location outside a fluvial incision and suspended about 40 m above the Dora Baltea valley floor (Lasagna et al. 2013). According to the geological setting, the hydrogeological reconstruction of the area suggests that the large amount of groundwater in the basin is essentially favoured by a highly fractured bedrock, covered by wide and thick bodies of glacial and gravitational sediments. The emergence of the water along the slope, in the Montellina Spring, is essentially due to a change of permeability between the deep bedrock and the shallow bedrock and/or surficial sediments. The deep bedrock, showing closed fractures and/or fractures filled by glacial deposits, is slightly permeable. The shallow bedrock, strongly loosened as result of gravitational phenomena, and the local gravitational sediments are, on the contrary, highly permeable. The concentration of water at the spring is due to several reasons. a) The spring is immediately downward a detachment niche, dipping towards the spring, that essentially drains the water connected to the change of permeability in the bedrock. b) It is along an important fracture, that carries a part of the losses of the Renanchio Torrent. c) Finally, it is favored by the visible and buried morphology. Although it is located along a ridge, the spring occurs in a small depression between a moraine and a landslide body. It also can be favored by the likely concave trend of buried base of the landslide. At last, tracer tests of the Renanchio Torrent water with fluorescent tracer are performed, with a continuous monitoring in the Montellina Spring. The surveys permit to verify and quantify the spring and torrent hydrogeological relationship, suggesting that only a small fraction of stream losses feeds the spring.