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The Posada–Asinara Line is a crustal-scale transpressive shear zone affecting the Variscan basement in northern Sardinia during Late Carboniferous time. We investigated a structural transect of the Posada–Asinara Line (Baronie) with the aid of geological mapping and structural analysis. N-verging F2 isoclinal folds with associated mylonitic foliation (S2) are the main deformation features developed during the Posada–Asinara Line activity (D2). The mineral assemblages and microstructures suggest that the Posada–Asinara Line was affected by a retrograde metamorphic path. This is also confirmed by quartz microstructures, where subgrain rotation recrystallization superimposes on grain boundary migration recrystallization. Crystallographic preferred orientation data, obtained using electron backscatter diffraction, allowed analysis of quartz slip systems and estimation of the deformation temperature, vorticity of flow and rheological parameters (flow stress and strain rate) during the Posada–Asinara Line activity. Quartz deformation temperatures of 400 ± 50 °C have been estimated along a transect perpendicular to the Posada–Asinara Line, in agreement with the syn-kinematic post-metamorphic peak mineral assemblages and the late microstructures of quartz. The D2 phase can be subdivided in two events: an early D2early phase, related to the metamorphic peak and low kinematic vorticity (pure shear dominated), and a late D2late phase characterized by a lower metamorphic grade and an increased kinematic vorticity (simple shear dominated). Palaeopiezometry and strain rate estimates associated with the D2late deformation event showed an intensity gradient increasing towards the core of the shear zone. The D2early deformation developed under peak temperature conditions, while the D2late event was active at shallower structural levels. [ABSTRACT FROM AUTHOR]

The Punta Bianca Unit (NW Italy) is a continent-derived metamorphic unit that experienced underthrusting and later exhumation in the hinterland sector of the Northern Apennines fold-and-thrust belt. We present a novel 1:5000 scale geological map that aims to illustrate the polyphase tectonic evolution of the Punta Bianca Unit and its relationships with non-metamorphic units. The geologic data presented in the map are coupled with the structural analysis of the main tectonic elements, lithostratigraphic, finite strain and microstructural data that allow to highlight the tectonic history of the study area. In particular, we recognized that the Punta Bianca Unit underwent an early phase of underplating (D1), followed by syn-orogenic exhumation to shallow crustal levels and coupling with the overlying Tuscan Nappe (D2), and nappe stacking/refolding (D3). Low-angle semibrittle (D4) and high-angle brittle (D5) faulting affected both the Punta Bianca Unit and the Tuscan Nappe during the latest stages of deformation. [ABSTRACT FROM AUTHOR]

The Ferriere-Mollieres Shear Zone (FMSZ) is a regional shear zone cross-cutting the Argentera External Crystalline Massif (Western Alps). It shows a NW-SE striking dextral shear zone separating two Variscan migmatitic complexes: the Tinèe to the SW and the Gesso-Stura-Vesubiè to the NE. Geological-structural mapping at 1:10,000 scale focused on the characterization of mylonitic deformation. A deformation gradient has been observed towards the core of the shear zone marked by the occurrence of ultramylonites and rare phyllonitic layers. Protomylonites passing to unsheared migmatites occur in the outer zones. Low-angle shear zones with a top-to-the S and SW sense of shear cross-cut the previous mylonitic foliation. The FMSZ is a Variscan transpressive shear zone activated during the Late Carboniferous under amphibolite-facies metamorphic condition. The shear zone has been partially reactivated under greenschist-facies metamorphic conditions during Alpine Orogenesis. [ABSTRACT FROM AUTHOR]

Terra Nova, 24, 42-51, 2012 Abstract The post-collisional tectonics in the inner zone of the Variscan belt of Corsica-Sardinia recorded a dextral transpression developed with an orogen-parallel tectonic transport. Recent works documented the presence of nearly parallel dextral and sinistral crustal-scale shear zones responsible for the exhumation of the metamorphic complexes. To constrain, for the first time, the timing of both shear zones and the timing of the transpressional shearing, zircons and monazites were collected for U-Th-Pb ages. Results indicate that the shear zones have been active at c. 320 Ma in a transpressional setting, widespread in the Southern European Variscan belt. [ABSTRACT FROM AUTHOR]

Structural analysis performed in the southern sector of the Apuan Alps Metamorphic Complex (AAMC) and on the overlaying Tuscan Nappe (TN) pointed out a structural evolution much more complex than that outlined till now. The comparison between the structural evolutions of the two tectonic units could provide new important constraints on the tectonic evolution of the whole belt. The two tectonic units recorded different tectonic evolution during the first stages of compression, while they shared the same deformation history later after the Tuscan Nappe overlapped the AAMC. The coupled tectonic units have been then deformed by two systems of folds, in a compressive tectonic regime. Extensional tectonics affected the units later, at upper crustal levels. To cite this article: R. Carosi et al., C. R. Geoscience 334 (2002) 339–346. [Copyright &y& Elsevier]

Abstract: Quartz c-axis fabrics, vorticity of flow and deformation temperatures were analyzed in samples collected in the central sector of the Variscan inner zone in Sardinia (Italy). In order to constrain the kinematics of exhumation of medium- and high-grade metamorphic rocks during D2 post-collisional deformation, we investigated two shear belts with opposing shear senses. These shear belts are developed parallel to the trend of the inner zone which marks the boundary between two metamorphic complexes. Our data suggest that the sinistral shear belt initiated during the early stage of D2 deformation in a simple-shear-dominated regime, whereas the dextral shear belt initiated later in a pure-shear-dominated regime. We document a general non-coaxial flow regime with contemporaneous pure and simple shear components. Geometrical relationships reveal the operation of oblique extrusion during a compressive regime where the coaxial component acted sub-horizontal and orthogonal to the trend of the axial zone. Changes in the kinematics, vorticity of flow and strain geometry that occurred during D2 deformation may be related to post-collisional orogen-parallel displacement caught up in the Variscan orogeny. [Copyright &y& Elsevier]