The metamorphic complexes of Anglona and SW Gallura (Northern Sardinia, Italy): New constraints on the post-collisional evolution of the Variscan belt in Sardinia

The Variscan belt in Sardinia can be divided in three tectonometamorphic units that show an increasing both in the metamorphic grade and in the intensity of deformation moving from South toward North. A wide dextral shear zone (Posada Asinara line: PAL, CAPPELLI et alii, 1992) with a WNW-ESE trend divided the high(HGMC) and the medium-grade metamorphic (MGMC) complexes. This study is focused on a transect of the inner zone of the belt in Northern Sardinia through the Anglona and South-West Gallura regions where, structural and kinematic analyses at mesoscopic and microscopic scale highlight a similar tectonic evolution for all the studied complexes. After the continental collisional (D1 deformational phase), the metamorphic complexes were affected by a polyphase post-collisional tectonics characterized by three deformation phases (D2, D3 and D4) followed by a later extensional tectonic phase associated with collapse folds. The D2 phase, developed after the Barrovian metamorphism related to the collisional phase, is the main deformation phase in all the complexes. In SW Gallura, the S2 foliation shows an heterogeneous shear deformation, with a welldeveloped mylonitic fabric especially near the phyllonitic high strain zone. CPO and vorticity analyses, performed along a transect perpendicular to the main trend of high strain zone, testify a dominant non-coaxial flow with a major simple shear component. Two systems of shear zones has been recognized in this area. The stretching lineation shows a sub-horizontal plunge and its trend is generally parallel to the main trend of the belt and to F2 and F3 fold axes.