Leonardo Casini

Early Permian 90° clockwise rotation of the Maures–Estérel–Corsica–Sardinia block confirmed by new palaeomagnetic data and followed by a Triassic 60° clockwise rotation

Abstract Palaeomagnetic investigations of the Corso-Sardinian block and Maures–Estérel show that there has been a change in their magnetic orientation during the Late Carboniferous–Early Permian period (305–280 Ma). This trend is interpreted in terms of a large-scale 90° clockwise rotation of the southern branch of the Variscan belt that matches the successive change in shortening directions revealed by structural geology. The evidence is based on existing structural studies of the fabrics of syntectonically emplaced granitoids partly based on the anisotropy of magnetic susceptibility, combined with a large database of isotopic ages. The chronological match between the palaeomagnetic and tectonic datasets is interpreted here as a result of large-scale dextral wrench movements in the lithosphere between the Gondwana and Laurussia supercontinents. This wrench deformation is regarded as a sequel to the dextral rotation of the northern branch of the Variscan belt during 330–315 Ma which terminated in frontal collision with Avalonia. The continuation of movement in the southern Variscan realm was due to shearing along the southern margin of the Avalonian block. An additional clockwise rotation is inferred to have taken place during the Triassic period. The age of this motion remains to be determined. Supplementary material: Palaeomagnetic and geochronological data from the Maures–Estérel, Corsica–Sardinia block presented in Figure 7 and discussed in the text are available at http://www.geolsoc.org.uk/SUP18742.

Structural map of Variscan northern Sardinia (Italy)

In this paper, we present a geological structural map (1:100,000 scale, ∼2300 km2 surface area) of the Variscan basement of northern Sardinia. The map integrates field structural analysis, extensive gamma-ray spectrometry, and high-resolution ELA-ICP-MS U/Th-Pb zircon and monazite dating. A set of 10 samples of granitic rocks collected from different plutons were characterized for their crystallization age. This provided an accurate timing of magmatic events related to the development of the Corsica-Sardinia Batholith. The structural map, complemented with geochronological results represents a benchmark for future studies on Variscan geodynamics.

Can weathering improve the toughness of a fractured rock? A case study using the San Giacomo Granite.

The physical–mechanical behaviour of the weathered San Giacomo granite, Sardinia, has been investigated using a combination of physical–mechanical tests and microstructural analysis. Fieldwork and microscopy suggest a pervasive microfracture system developed on a pre-existing magmatic quartz anisotropy. Alteration indices, chemical analyses and mineralogy indicate that the yellow staining is produced by the circulation of oxygen-rich rainwater through microfractures in the rock, resulting in the oxidation of biotites. Apart from the obvious modification of the original grey colouring, the precipitation of Fe-oxides and hydroxides has welded fractures together, leading to the partial recovery of the physical–mechanical properties previously compromised due to microfracturing.RésuméLe comportement physico-mécanique du granite altéré de San Giacomo (Sardaigne) a été étudié à partir de plusieurs essais physico-mécaniques et des analyses microstructurales. Les observations de terrain et au microscope suggèrent qu’un système généralisé de micro-fractures s’est développé à partir d’une anisotropie préexistante des quartz d’origine magmatique. Des indices d’altération, des analyses chimiques et minéralogiques indiquent que la coloration jaune est produite par la circulation d’eaux de pluie, riches en oxygène, au travers des micro-fractures de la roche, avec pour conséquence l’oxydation des biotites. En dehors de la modification évidente de la couleur grise d’origine, les précipitations d’oxydes et d’hydroxydes de fer ont soudé ensemble les fractures, conduisant à une récupération partielle des propriétés physico-mécaniques qui avaient été affaiblies par la micro-fracturation.

Uranium distribution in the Variscan Basement of Northeastern Sardinia

ABSTRACT We present a detailed map of uranium distribution and its uncertainties in the Variscan Basement of Northeastern Sardinia (VBNS) at a scale of 1:100,000. An area of 2100 km2 was investigated by means of 535 data points obtained from laboratory and in situ gamma-ray spectrometry measurements. These data volume corresponds to the highest sampling density of the European Variscides, aimed at studying the genetic processes of the upper crust potentially triggered by an enrichment of radiogenic heat-producing elements. For the first time, the Kriging with Variance of Measurement Error method was used to assign weights to the input data which are based on the degree of confidence associated with the measurements obtained using different gamma-ray spectrometry techniques. A detailed tuning of the model parameters for the adopted Experimental Semi-Variogram led to the identification of a maximum distance of spatial variability coherent to the observed tendency of the experimental data. We demonstrate that the obtained uranium distribution in the VBNS, characterized by several calc-alkaline plutons emplaced within migmatitic massifs and amphibolite-facies metamorphic rocks, is an excellent benchmark for the study of ‘hot’ collisional chains. The uranium map of VBNS, and in particular the Arzachena minor pluton, confirms the emplacement model based on the recognition of the different petrological associations characterizing the Variscan magmatic processes in the Late Paleozoic. Furthermore, the presented model of the uranium content of the geological bedrock is a potential baseline for future mapping of radon-prone areas.

Structural and metallogenic map of late Variscan Arbus Pluton (SW Sardinia, Italy)

ABSTRACT This paper presents the geological, structural and metallogenic map of the Arbus Pluton, a late Variscan composite intrusion belonging to the Corsica-Sardinia Batholith. The pluton is surrounded and crosscut by a wide variety of vein ore deposits. The Arbus Pluton was emplaced at shallow crustal levels at the end of the Variscan Orogeny, along an E–W trending shear zone located in the low-grade external nappe pile of the Sardinian basement. The architecture of the pluton is roughly concentric with a core of cordierite-bearing leucogranites and an outer shell composed of pyroxene-bearing and hornblende granodiorites. New U/Pb dating on zircons of granodiorite yields an emplacement age of 303.7 ± 1.1 which improves previous Rb/Sr and Ar/Ar dating. The map has been compiled on the basis of new geological/structural surveys and petrographical studies coupled with in situ gamma ray spectrometry. All the data-sets have been processed using a geographical information system.

Stratigraphic, magmatic and structural features of Ordovician tectonics in Sardinia (Italy): a review

The Variscan Palaeozoic successions exposed in Sardinia show clear evidences of tectonic instability during Ordovician times. In the Foreland zone (SW Sardinia), the Cambrian to lower Ordovician strata are folded giving way to an angular unconformity sealed by a Middle-Upper Ordovician syn-tectonic succession, characterized at the base by a thick conglomeratic deposit. This is indicative of a tectonic-driven uplift, followed by intensive erosion. A time-equivalent angular unconformity, sealed by Middle Ordovician volcano-sedimentary complexes, is also recorded in the External nappe zone (central–southern Sardinia). These volcanic products became rare toward NE and approaching the Inner zone, where few remnants of Ordovician magmatic rocks consist of the protoliths of small bodies of calc-alkaline orthogneisses and metabasites. Paleogeographic reconstructions generally assume that Sardinia formed a coherent crustal block since the late Cambrian; therefore, the SW–NE trend of magmatic and sedimentary features observed in the Ordovician successions has been interpreted as sequential snapshots captured along an Ordovician volcanic arc. Stratigraphic and paleontological data, sediment provenance and petro-chronology of magmatic products indicate that the Ordovician successions could have belonged to different, and possibly far paleogeographic domains. These data suggest that the actual zonation of the Palaeozoic basement of Sardinia is a puzzle resulting from extensive crustal reworking and amalgamation in Variscan times. Hence it cannot be directly referred to an Ordovician paleogeography.ResumenLa serie Varisca expuesta en Sardinia muestra claras evidencias de instabilidad tectonica durante el Ordovícico. En el ante-país (SW Cerdeña), las capas del Cambrico y Ordovícico estan deformadas y producen una discordancia angular sellada por una serie sin-tectónica del Ordovícico Medio-Superior que se caracteriza por una potente capa de conglomerados a su base. Esto indica un levantamiento tectonico seguido por intensa erosión. Otra discordancia angular de la misma edad que está sellada por una serie volcano-sedimentaria del Ordovícico Medio se encuentra en la zona de los cabalgamientos externos (la parte central y sur de Sardinia). Estos productos volcanicos son menos frequentes en las zonas mas internas hacia el NE donde los pocos restos magmaticos del Ordovícico estan representados por los protolitos de pequeños cuerpos de ortho-gneiss calc-alkalino y de metabasitas. Las reconstrucciones paleogeográfico normalmente asumen que Sardinia constituía un bloque de corteza coherente desde el Cámbrico superior, por lo tanto, el trend de cuerpos magmaticos y sedimentarios que se observan dentro del la serie del Ordovícico desde el SW hacia el NE se ha interpretado como una secuencia de instantaneas tomadas a lo largo de un arco volcanico Ordovícico. Los datos sedimentarios y paleontológico, la origen de los sedimentos y la cronología petrologica de los productos magmaticos indican que la serie del Ordovícico puede haber pertenecido a dominios paleontológico distintos y posiblemente lejanos. Estos datos sugieren que la Zonación del basamiento Paleozoico en Cerdeña es un puzzle que resulta de una amalgamación y reelaboración extensiva durante el Varisco y que por lo tanto no puede directamente relacionarse con una paleontológico del Ordovícico.

2D-thermo-mechanical modelling of spatial P-T variations in heterogeneous shear zones

We present a suite of two-dimensional thermo-mechanical simulations of a compositionally heterogeneous shear zone characterized by viscous matrix and inclusions. The experiments have been performed using a visco-elasto-plastic 2D finite difference code that reproduces the stress, strain, temperature and pressure variations occurring within the shear zone at lithospheric scale. From here, we extrapolated and discuss the results of a narrow (

THE LATE VARISCAN MONTE LINAS PLUTON (SW SARDINIA): A SOURCE OF GRANOPHILE ELEMENT ORE

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.