Structural setting of a transpressive shear zone: insights from geological mapping, quartz petrofabric and kinematic vorticity analysis in NE Sardinia (Italy)

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 degrees 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 D2(early) phase, related to the metamorphic peak and low kinematic vorticity (pure shear dominated), and a late D2(late) phase characterized by a lower metamorphic grade and an increased kinematic vorticity (simple shear dominated). Palaeopiezometry and strain rate estimates associated with the D2(late) deformation event showed an intensity gradient increasing towards the core of the shear zone. The D2(early) deformation developed under peak temperature conditions, while the D2(late) event was active at shallower structural levels.