Collisional type orogens are characterized by crustal thickening due to overthrusting and nappe development. The following tectonic evolution of the orogen and its overthickened crust is controlled by several factors, such as the progressive cessation of the driving forces, the change of the kinematics of the involved tectonic plates or extensional tectonics. As the tectonic compressive forces tend to diminish, the overthickened crust at high-temperature tends to collapse inducing the development of widespread extensional structures superimposed over compressional ones. However, large-scale plate kinematics often plays a first-order role in determining the fate of an orogen. For example extensional tectonics in the Caledonides affecting the belt soon after the development of compressive structures is attributed to a change from converging to diverging movement induced by far plate tectonic movement (Rey et al., 1997).
In the last twenty years an increasing number of evidence led to the recognition of extensional structures in many orogens (Dewey, 1988; Platt, 1986) such as the Variscan belt (Eisbacher et al., 1989; Van Den Driessche & Brun, 1992; Malavieille et al., 1990; Rey et al., 1992; Costa & Rey, 1995; Gardien et al., 1997). Metamorphic core complexes have been recognized in several parts of the Variscan belt and extensional tectonics is regarded as an efficient mechanism of exhumation of metamorphic rocks in the later tectonic history (Malavieille et al., 1990; Carmignani et al., 1993, 1994; Rey et al., 1997; Gardien et al., 1997).
Compression and/or transpression are often active after the thickening stage and could play an important role in the tectonic evolution and in the exhumation (Matte et al., 1998; Carosi & Palmeri, 2002; Carosi et al., 2004). The continuing compression and the change in the direction of convergence may originate transpressional deformation that deeply affects the thermal evolution of orogens (Thompson et al., 1997a, b).
However, there are increasing examples of metamorphic terrains in which transcurrent or transpressional shear zones overprint a previous D1 deformation responsible of crustal stacking (Carosi & Palmeri, 2002; Little et al., 2002; Vassallo & Wilson, 2002; Goscombe et al., 2003; Konopásek et al. 2005)
This paper is based on structural investigations in three key sectors of the Variscan basement of northern Sardinia, located respectively in the NW (Nurra and Asinara Island transect), in the centre (SW Gallura and Anglona) and in the NE (Baronie transect) of the island (Figs. 1 and 2). The aim of the paper is to show the syn- and post-thickening mechanisms which controlled the tectonic-metamorphic evolution and the exhumation of this part of the belt. The thickening-related structures are well documented all over the island but the following tectonic history remains unclear.
Figure 1. Tectonic sketch-map of the Variscan belt
Tectonic sketch-map of the Variscan belt in Sardinia and position of the study areas. 1: Post-Variscan cover deposits; 2: Variscan batholith; 3: High Grade Metamorphic Complex; 4: Internal nappes (low- to medium-grade metamorphism); 5: Internal nappes (low-grade metamorphism); 6: External nappes; 7: External Zone; 8: Thrusts (a: main thrusts; b: minor thrusts); 9: Faults; 10: Posada-Asinara Line (PAL).
Figure 2. Geological structural map of the Variscan basement in northern Sardinia
Modified after Carmignani et al., (1986), Carmignani & Rossi, (2001), and Carmignani et al., (2001). The main structural elements of the D1, D2 and D3 deformation phases have been reported.
A general change in the direction of tectonic transport from perpendicular to parallel to the belt during collisional and post collisional tectonics is described and a transpressional tectonic model is proposed as the main mechanism of exhumation of large sectors of the belt before the onset of the later extensional collapse at higher structural levels (Carosi & Oggiano, 2002; Carosi & Palmeri, 2002).