The East Tenda Shear Zone is the regional structure that marks the Alpine overthrusting of the Ligurian–Piedmontese ocean onto the Variscan Corsica. We present the first report of a Na-pyroxene (acmite)–rutile-bearing assemblage from a phyllonitic shear zone that occurs within the gneissic lithologies of the East Tenda Shear Zone. Acmite hosts inclusions of Na-amphibole and titanite, and is rimmed by retrogressive biotite. Forward modelling of the shear zone assemblages in the NCKFMASTHO chemical system indicates a cold burial–exhumation path (palaeogeothermal gradient <10 °C km −1 ) and a metamorphic climax with minimum pressure of 1.2 GPa and temperatures of 350–400 °C. U–Pb thermal ionization mass spectrometry analyses on synkinematic rutile yield a 3D array with an age of 48 ± 18 Ma (MSWD 7.3), whereas coexisting acmite–phengite and coatings of oxides or sulphides provide an intercept at 54 ± 8 Ma (MSWD = 48). The scatter of the arrays is considered to reflect secondary disturbances of the system and the age provided by acmite–phengite and the coatings at 54 ± 8 Ma is considered the most reliable estimate for shear zone formation. Implications of these new metamorphic and geochronological data are discussed in the regional framework of the Alpine geology and integrated in the plate-tectonic scenario of the central Mediterranean.

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Newly collected structural data in Eastern Sardinia (Italy) integrated with numerical techniques led to the reconstruction of a 2-D admissible and balanced model revealing the presence of a widespread Cenozoic fold-and-thrust belt. The model was achieved with the FORC software, obtaining a 3-D (2-D + time) numerical reconstruction of the continuous evolution of the structure through time. The Mesozoic carbonate units of Eastern Sardinia and their basement present a fold-and-thrust tectonic setting, with a westward direction of tectonic transport (referred to the present-day coordinates). The tectonic style of the upper levels is thin skinned, with flat sectors prevailing over ramps and younger-on-older thrusts. Three regional tectonic units are present, bounded by two regional thrusts. Strike-slip faults overprint the fold-and-thrust belt and developed during the Sardinia-Corsica Block rotation along the strike of the preexisting fault ramps, not affecting the numerical section balancing. This fold-and-thrust belt represents the southward prosecution of the Alpine Corsica collisional chain and the missing link between the Alpine Chain and the Calabria-Peloritani Block. Relative ages relate its evolution to the meso-Alpine event (Eocene-Oligocene times), prior to the opening of the Tyrrhenian Sea (Tortonian). Results fill a gap of information about the geodynamic evolution of the European margin in Central Mediterranean, between Corsica and the Calabria-Peloritani Block, and imply the presence of remnants of this double-verging belt, missing in the Southern Tyrrhenian basin, within the Southern Apennine chain. The used methodology proved effective for constraining balanced cross sections also for areas lacking exposures of the large-scale structures, as the case of Eastern Sardinia.

The East Tenda Shear Zone (ETSZ) is the major Alpine tectonic boundary marking the overthrusting of the ocean-derived Schistes Lustrés nappe onto the Variscan crystalline basement of Corsica. New structural, mineralogical, and geochemical investigations along a transect ranging from the Variscan basement to the contact with the Schistes Lustrés are used to construct a rheological model for the ETSZ during its polyphase deformation history. The progressive transformation of the isotropic granitoid protolith into gneisses, shear zones, and, locally, phyllonites is described. The textural/mineralogical change suggests a concurrent increasing metasomatism associated with structurally controlled fluid flow. The effect of such textural/mineralogical evolution on the bulk rheology of the ETSZ is estimated. Rheological flow laws are obtained using an averaging procedure based on the integration of single-phase rheological behavior. The flow laws are used to infer strain rates and construct strength envelopes for each structural domain during progressive deformation, which highlight the relative rheological differences of the main textural components of the ETSZ and the variations in their brittle/ductile transition depths. Two competing processes are inferred. On the one hand, the combined effects of the feldspar-to-mica reaction and the development of a strong planar fabric induce weakening and strain localization along shear zones. On the other hand, fluid channeling along these shear zones enhances alkali feldspar neoblastesis, inducing strain hardening. Among the possible consequences of such feedback processes between strain localization and fluid-rock interaction are episodes of rheological transitions from ductile to brittle behavior during the polyphase tectonic evolution of the ETSZ.

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