A balanced foreland–hinterland deformation model for the Southern Variscan belt of Sardinia, Italy

Abstract

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.

Evolution of the Corsica–Sardinia Batholith and late-orogenic shearing of the Variscides

“…7h, n). Although extrapolation of averaged strain rates is subject to several uncertainties related to the effective rheology of shear zones (Handy, 1994), the suggested values are consistent with the strain rates extrapolated from late Variscan shear zones close to the southern edge of the C-SB (Casini et al, 2010). The role of shear heating is further supported by the age and duration of anatexis in model r18, which are in excellent agreement with the U-Pb zircon ages of most early U2 plutons Oggiano et al, 2007;Paquette et al, 2003).…”

“…Most U2 plutons are ribbon-shaped (aspect ratio of 1:3 to 1:10 in map view) and with long axes oriented between N120°and N140° (Fig. 1), nearly parallel to the orogenic zonation and to the transpressional D2 structures (Carmignani et al, 1994;Carosi and Oggiano, 2002;Casini and Funedda, 2014;Casini et al, 2010). Although there is only poor evidence for post-magmatic deformation, U2 plutons have a generally well-developed flat magmatic foliation and a gently plunging lineation almost parallel to the metamorphic fabric of country rocks Gattacceca et al, 2004).…”

“…In these experiments, slab break-off occurs at 345 Ma, and ductile shearing of the lithosphere is imposed between 322 and 305 at an average rate of 10 −13 or 10 −12.9 s −1 . Based on published P-T-t paths (Casini et al, 2010;Faure et al, 2010;Giacomini et al, 2006), exhumation rates are varied over about two orders of magnitude, from 1.30 to 0.01 mm/y. Four different exhumation rate histories are simulated (Table 2): i) continuously increasing (r11), ii) continuously decreasing (r15-r18), iii) increasing in a step-like manner (r12), and iv) decreasing in a step-like manner (r13-r14).…”

Geology of the Variscan basement of the Laconi-Asuni area (central Sardinia, Italy): the core of a regional antiform refolding a tectonic nappe stack

“…sub-grain rotation, deformation band, twinning, etc.) indicate a lowgrade metamorphism, as described in the other area of the Nappe zone (Casini & Funedda, 2014;Casini, Funedda, & Oggiano, 2010;Pavanetto et al, 2012). Kinematic indicators along the ductile shear zone and stretching lineations (mostly mineral lineation of muscovite in metarenites and stretched fragments in metaconglomerates and metavolcanites) on the S 1 foliation all support a general transport direction of the 'top-towards-S'.…”

Structural Controls of Ore Mineralization in a Polydeformed Basement: Field Examples from the Variscan Baccu Locci Shear Zone (SE Sardinia, Italy)

“…The D1 early shortening structures are large kilometer-scale recumbent isoclinal folds facing southward, with well-developed penetrative axial plane foliation-generally a slaty cleavage produced in lower green-schist facies metamorphism-followed by almost contemporary south-southwest thrusts that produced thick mylonitic belts. Development of such shear zones between the different tectonic units, thick up to several hundred meters, is common in the Variscan basement of Sardinia [17][18][19][20][21][22][23][24]. Among them, the Baccu Locci shear zone is one of the most noteworthy [3].…”