A balanced foreland–hinterland deformation model for the Southern Variscan belt of Sardinia, Italy LEONARDO CASINI 1 * , ANTONIO FUNEDDA 2 and GIACOMO OGGIANO 1 1 Department of Botanics, Ecology and Geology, University of Sassari, Sassari, Italy 2 Department of Earth Science, University of Cagliari, Cagliari, Italy Deformation related to orogenic collision has been investigated along a complete crustal section exposed throughout the Sardinian Variscides, Italy. The dynamics of grain-scale deformation processes and palaeo-differential stress have been examined within three major shear zones (Rosas Shear Zone —RSZ, Baccu Locci Shear Zone—BLSZ and Giuncana Badesi Shear Zone—GBSZ) developed at progressively deeper structural levels from the foreland to the inner zone during the D1 phase of shortening associated with Barrovian-type metamorphism. Microstructural analysis reveals that the overall strain in quartz-feldspathic and calc-mylonites results from a combination of and, possibly, competition between several deformation mechanisms. At the sub-greenschist level (RSZ), operating mechanisms are pressure-solution associated with grain boundary sliding. Dislocation creep is the dominant process at the greenschist level (BLSZ), although pressure-solution is still effective in zones of strain concentration. In the lower crust (GBSZ), deformation occurred in the range of 600–6208C and 0.7–0.9 GPa by a combination of dislocation creep and high-T diffusional processes. Differential stresses inferred from quartz grain-size and calcite twin piezometers decrease with increasing depth, as predicted by most common rheological models. Despite this general trend, the stress profile across each shear zone suggests a pronounced stress gradient towards the zone of maximum deformation, leading to an increase of strain-rate up to two orders of magnitude. The results of this regional study demonstrate that both stress and strain within orogenic wedges are localized rather than distributed, allowing the crust to deform coherently at different structural levels. Copyright # 2010 John Wiley & Sons, Ltd. Received 7 April 2009; accepted 4 November 2009 KEY WORDS shear zones; stress and strain; palaeopiezometry; mylonites; Variscides; Sardinia 1. INTRODUCTION Orogenic systems merge with plate boundaries and require large strains to be accommodated within seams that are 30– 120 km thick and several hundred, or even thousands, of kilometres wide (Ziegler, 1989; Hill et al., 1995). At a first order approximation, the strength of the continental litho- sphere controls the way in which strain is achieved at increasing depths into these huge lithospheric wedges. It is now widely accepted that the continental crust is strong within the upper 12–15 km (Brace and Kohlstedt, 1980; Kohlstedt et al., 1995; Ranalli, 1995, 1997), which means that the upper crust is assumed to be in equilibrium failure with the state of stress controlled by its frictional strength. Three decades of research indicate that most crustal rock types have a relatively uniform coefficient of friction between 0.6– 0.85, which should hinder fault propagation at depth (Byerlee, 1978). Even so, the direct extrapolation of these data to natural faults is questionable, since many seismically active fault zones are weaker than would be expected from laboratory friction experiments (e.g. Brune et al., 1969; Kanamori and Anderson, 1975; Rice, 1992; Williams et al., 2004). In fact, there is positive feedback between most faults (active and fossil) and zones of reduced strength as a result of tectonic crushing, inherited geometrical discontinuities, pore pressure gradients, shear heating or compositional differences (Butler, 1989; Massoli et al., 2006; Haines, 2008). In other words, deformation in the uppermost part of the lithosphere is not homogeneously distributed, but probably localizes along weakened fault zones which pass next to relatively undeformed and strong domains. These assumptions have been successfully demonstrated by experimental modelling of inverted foreland basins in which deformation is localized into thrust systems (Dahlen, 1990; Vanbrabant et al., 1999). Current interpretations of most fold-and-thrust belt zones accept that tens or even a few hundred kilometres of shortening are accommodated within the upper 10–20 km of the orogenic wedge (Burkhard, 1990a,b; Becker, 2000; McQuarrie and Davis, 2002). However, deep seismic tomography indicates that orogenic GEOLOGICAL JOURNAL Geol. J. 45: 634–649 (2010) Published online 18 May 2010 in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/gj.1208 * Correspondence to: L. Casini, Department of Botanics, Ecology and Geology, University of Sassari, Corso Angioy 10, Sassari 07100, Italy. E-mail: casini@uniss.it Copyright # 2010 John Wiley & Sons, Ltd.