D1 deformation in the southern portions of the studied transects testifies to a thickening stage characterized by isoclinal folds overturned to the SW and ductile to brittle shear zones with the same sense of shear. D1 phase is responsible of the nappe stacking in central and southern Sardinia (Carmignani & Pertusati, 1979; Carmignani et al., 1982, 1994; Carosi et al., 2004).
F1 folds and a north dipping S1 foliation are the prominent features to the south of the studied areas. Recent studies on top-to-the SW ductile/brittle - shear zones in southern Nurra suggest that they accomplished exhumation of the sheared low grade metamorphic rock causing their decompression at the end of the D1 deformation phase (Montomoli, 2003).
D1 fabric has been progressively overprinted by a D2 deformation with strain increasing to the north. Vorticity analysis indicates that D2 deformation is characterized by both simple shear and pure shear acting contemporaneously in a transpressional regime.
D2 non-coaxial regional deformation in northern Sardinia is related to a crustal-scale oblique shear zone (Fig. 3). Shear sense indicators indicate a top-to northwest sense of shear in the western area whereas a top-to-the SE sense of shear in the eastern zone. In both cases they show a transport direction at a high angle with respect to the SW direction of tectonic transport during D1 (Carosi & Oggiano, 2002; Carosi & Palmeri, 2002). The attitude of the L2 stretching lineation implies both a dip-slip movement and a strike-slip movement.
A change in the attitude of the L2 stretching lineations is clearly observable in Asinara Island. In the southern and central portion of the island, L2 stretching lineation trends N090-N110, parallel to F2 fold axes and plunges a few degrees toward the NW and the SE. The L2 lineation switches from sub-horizontal, near Punta Gruzitta, to down-dip in the northern part of the island, whereas the S2 foliation maintains the same attitude. This geometry plays a key role for constraining the kinematic history of the D2 deformation phase.
This complex deformation pattern can be explained in two ways (Carosi et al., 2003; Carosi et al., 2004; Iacopini, 2005):
D2 is a steady-state non coaxial deformation in which pure shear is partitioned in space and increases to the north. This could cause a rotation of the L2 stretching lineation from sub-horizontal to down-dip on the S2 foliation according to the transpressional model proposed by Tikoff & Teyssier, (1994), and Tikoff & Green, (1997).
The switching of the L2 stretching lineation can be caused by a late-D2 ductile "thrusting" of the High Grade Metamorphic Complex onto the Low to Medium Grade Metamorphic Complex with a top-to-the SW sense of shear.
The growth of Barrovian porphyroblasts took place either post-D1 or during the early stages of the D2 shear deformation (Franceschelli et al, 1982, 1989, 1990; Ricci et al., 2004 with references). Variable retrogressions have been recognized along the S2 foliation, so the early stages of its development during medium-grade conditions represent preserved relics. The sense of rotation of D2 porphyroblasts is consistent with the other D2 kinematic indicators.
The metamorphic peak is pre- and partly sin- D2 (Franceschelli et al. 1982, 1989, 1990) Evidences of D1 deformation is found within albite, oligoclase, staurolite and kyanite porphyroblasts. The composition and zoning of small-size garnets, together with the phengitic white mica included in the albites, indicates that D1 deformation was characterized by both increasing temperature and pressure. Mineralogical associations and composition of the main phases indicate that the D2 phase developed in a regime of increasing temperature and decreasing of pressure during the early stage. The latest D2 stages were characterized by decreasing both pressure and temperature (Carosi & Palmeri, 2002; Di Vincenzo et al., 2004; Ricci et al., 2004).
The increase of the metamorphic grade in northern Sardinia in a short distance is not consistent with Barrovian metamorphic gradients. D2 transpressional deformation was responsible for the observed condensed isograds (Franceschelli et al., 1982, 1989) by their stretching and shearing (Carosi & Palmeri, 2002).
Geometric, kinematic and petrological data suggest that the D2 deformation phase drove the decompression of medium-pressure rocks in a transpressive tectonic setting at 315-320 Ma (Carosi & Palmeri, 2002; Di Vincenzo et al., 2004). The crustal-scale transpression controlled the exhumation of medium pressure rocks up to upper crustal level, before undergoing the post-collisional extensional collapse at nearly 300 Ma (Carmignani et al, 1993).