Antonio Funedda

The Sardinia-Corsica microplate and its role in the Northern Apennine Geodynamics: new insights from the Tertiary intraplate strike-slip tectonics of Sardinia

The behaviour of Sardinia and Corsica within the Alpine-Apennine orogenic events has not been considered in a univocal way; different hypotheses have been proposed, disregarding any eventual effect on the internal structuration of this piece of European crust.Identifying the mechanism and age of the prominent strike-slip tectonics in Sardinia and Corsica allows us to bear new insights on the relationships between the south European crust and Adria plate. Syntectonic Oligocene-Aquitanian deposits fill some intracratonic basins in Sardinia. They developed in correspondence with releasing bends that affect the sinistral strike-slip faults, constraining the time span during which this tectonic regime was active.Thrusts and folds involving the Mesozoic and Lower Cainozoic cover are not ubiquitous in Sardinia, they are mainly localised along deformed corridors in the NE part of the Island where deeply shortened Cainozoic conglomerates were involved in the wrench-thrust faults which, in some case, led the basement to override the Mesozoic cover. The association of these structures to restraining bends is documented, so that they are the coeval transpressive counterparts of the strike-slip basins.Confining most of the Tertiary strike-slip tectonics of Sardinia and Southern Corsica within an Oligocene-Aquitanian time interval involves the following consequences: i) no E-W extension, leading to a N-S trending rift (in present-day coordinates), was active in Sardinia and Corsica during Oligocene-Aquitanian times; ii) the so-called Sardinia Rift is an assemblage of shallow asymmetric basins, trending N150, which developed during the late Burdigalian-Langhian, i.e. contemporary to the onset of the collapse of the North Apennine and Alpine Corsica orogenic wedge and to the opening of the North Tyrrhenian Sea; iii) the Oligocene-Aquitanian strike-slip tectonics in Sardinia is consistent with the deformation of a hinterland involved in collision; this was the collision between Adria and Europe that led to the building of the North Apennines; iv) the collisional event predates the drifting of the Sardinia-Corsica crust and the opening of the Liguro-Provencal basin.

The geological and metallogenic map of the Baccu Locci mine area (Sardinia, Italy)

Abstract Please click here to download the map associated with this article. The study area is in the nappe zone of the Sardinian Variscides in the southeast part of the Island of Sardinia (Italy), and extends between 39°33′14″/9°30′14″ (NW corner) and 39°30′09″/9°35′36″ (SE corner). The area shows a section of the Variscan orogen in Sardinia with three tectonic units that were stacked and folded during the Middle Carboniferous Variscan tectonics. The presented 1:10,000 scale geological map, the cross sections and the 3D models illustrate the complicated tectonic setting of the area, resulting from the polyphasic Variscan collisional evolution as well as from later extensional stages. The geometry resulting from progressive deformation is strongly noncylindrical; it is not balanceable because the polypahsic deformation with different tectonic transport directions and the loss in volume in the different formations occurred under greenschist facies metamorphism. The use of 3D modelling of geological surfaces greatly improved both the map and cross-sections. The Variscan basement of the study area hosts one of the most important mining zones of SE Sardinia (Baccu Locci mine area), which was active until 1961 for the extraction of AsPy, PbS and ZnS. Recent studies also found a noteworthy occurrence of Au. The Baccu Locci mine is assumed to be the eastern part of a mineralized corridor linked with the Variscan shear zone. A metallogenic map of the Baccu Locci mine area at 1:7,500 scale is included in this paper. The primary map associated with this paper actually represents a 4D model (spatial and time dimensions) of ore bodies hosted in a crystalline basement and highlights the overprinting of different paragenetic sequences of mineralizations and their relationships with pre-existing structures.

Tectono-sedimentary evolution of southwest Sardinia in the Paleogene: Pyrenaic or Apenninic Dynamic?

The Narcao and Cixerri basins in Southwestern Sardinia are east-west trending basins of Oligocene age. Recent geological mapping, combined with structural and stratigraphical analyses, support the proposed hypothesis that these basins were very open growth synclines confined within a structural high, delimited by northwest trending dextral strike slip faults. Previously the basins have been interpreted as fault-bounded grabens. The newer revised interpretation is consistent with the existence of NNW trending dextral strike-slip dynamic, related to a north-south shortening which has generated reverse faulting and tight folds in the underlying, pre-synclinal evolution, Eocene succession. This deformation, along with an interfering sub-orthogonal thrust and fold system which affects the Mesozoic sequence, was traditionally linked to the Pyrenean Orogenesis. The Oligocene–Aquitanian shortening, which resulted in the growth synclines and strike-slip faulting, is consistent with the structural development recognized in north-central Sardinia; there structures related to the collision between continental margins that resulted in the Northern Apennines are well documented. Therefore, the Oligocene tectonics of Southwestern Sardinia also must be related to the collision event between the Southern Europe margin (i.e. a crustal sector corresponding to the future Corsica-Sardinia block) and the Adria Plate, which generated the Northern Apennines. Conversely, the previous E-W shortening- related structures must be related to Pyrenean tectonics.

Geology of the Variscan basement of the Laconi-Asuni area (central Sardinia, Italy): the core of a regional antiform refolding a tectonic nappe stack

The study area extends in central Sardinia between the coordinates 8°56′00″/39°55′00″ (NW corner) and 9°03′00″/39°50′00″ (SE corner). It forms part of the Sardinian Variscides that are characterized here by the western-most aspect of a regional structure known as the Flumendosa Antiform, which runs ENE-trending for more than 50 km along its axis. It represents the envelopment of several, upright km-scale minor folds that refold the different tectonic units of the Variscan Nappe zone characterized by a number of isoclinal folds, with axial plane foliation and thick ductile shear zones. The antiform is in turn deformed by late-orogenic extensional structures, namely asymmetric folds and narrow, low angle ductile shear zones that generally reactivate earlier collisional structures. The 1:12,500 scale map completes the mapping of this important regional structure, meaning that a detailed survey of the entire mega-structure is now available. This enables more detailed structural analyses of poly-deformed areas in low-grade metamorphic conditions to be conducted, given the robust knowledge available of the lithostratigraphic succession and the geometric and kinematic outlines.

Reply to: Discussion on «The Sardinia-Corsica microplate and its role in the Northern Apennine Geodynamics: new insights from the Tertiary intraplate strike-slip tectonics of Sardinia» by Igino Dieni & Francesco Massari

In this reply we stress the role played by the Sardinia-Corsica microplate in the geodynamic evolution of the present-day W-Mediterranean during Tertiary. We answer to each of the seven remarks of the discussion proposed by Dieni & Massari, mainly concerning misleading scientific literature on the age of deposits involved in the strike-slip faults of NE Sardinia. Finally, we summarise and best outline the tectonic features of the whole Sardinia and Corsica microplate, which leads us to discuss the interaction between strike-slip tectonics and the N-Apennine evolution during Oligo-Miocene.

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.

Tectonics, ore bodies, and gamma-ray logging of the Variscan basement, southern Gennargentu massif (central Sardinia, Italy)

ABSTRACT We present a structural geological map (1:14,000 scale) that covers a 100 km2 area of Variscan basement rocks exposed in central Sardinia. The mapped area is located between 39°56′20′′ N 9°04′59′′ E (northwestern corner) and 39°51′47′′ N 9°13′16′′ E (southeastern corner) on the southern slope of the Gennargentu massif, surrounding the mining village of Gadoni. This village was the hub of a mining district in central Sardinia. The region extends between the external and inner nappe zones of the Variscan orogenic wedge of Sardinia. Despite significant mining, the area lacked an up-to-date structural and stratigraphic synthesis comparable to that achieved in the southern Sardinia. This gap in knowledge was due to: (i) more complex structural deformation; (ii) slightly higher grade regional metamorphism including a late Variscan high-temperature overprint; (iii) difficulty in distinguishing terrigenous stratigraphic units that belong to different tectonic units; and (iv) the absence of key stratigraphic marker for resolve complex structures in the uppermost tectonic unit. Integration of field mapping, structural analysis, portable gamma-ray spectroscopy, and zircon U-Pb ages of intrusive rocks has enabled a new geological map and cross-sections. These contributions synthesize the collisional and postcollisional evolution of the region and its relationship with ore genesis.