Outline
Comptes Rendus

Transpressional deformation in northwestern Sardinia (Italy): insights on the tectonic evolution of the Variscan Belt
Comptes Rendus. Géoscience, Volume 334 (2002) no. 4, pp. 287-294.

Abstracts

Structural investigations in northwestern Sardinia highlight the occurrence of a regional D2 transpressional deformation related both to NNE–SSW direction of compression and to a NW–SE shear displacement. The deformation is continuous and heterogeneous, showing a northward strain increase, indicated by progressively tighter folds and occurrence of F2 sheath folds. D2 transpression is characterised by the presence of a crustal-scale shear deformation overprinting previous D1 structures, related to nappe stacking and top-to-the-south and southwest thrusting. The L2 prominent stretching lineation points to an orogen-parallel extension and to a change in the tectonic transport from D1 to D2. It is attributed to the position of Sardinia close to the northeastern part of the Cantabrian indenter during the progressive evolution of the Ibero-Armorican Arc. D1 phase developed during initial frontal collision, whereas D2 deformation characterised the progressive effect of horizontal displacement during the increasing curvature of the arc.

L'analyse structurale a mis en évidence dans le Nord-Ouest de la Sardaigne une déformation D2 transpressive, continue et hétérogène, qui augmente d'intensité vers le nord. Elle se superpose aux structures précédentes D1, liées à la mise en place des nappes vers le sud et le sud-ouest. La linéation d'allongement L2 met en évidence une extension parallèle à la chaı̂ne et un changement net, de la phase D1 à la phase D2, de la direction du transport tectonique. La phase D1 s'est développée durant la collision frontale initiale, alors que la déformation D2 est due à l'augmentation progressive de la composante en décrochement pendant le développement de l'arc Ibéro-Armoricain.

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DOI: 10.1016/S1631-0713(02)01740-6
Keywords: tectonics, transpression, Variscan Belt, folds, stretching lineations, Sardinia, Tectonics
Mot clés : tectonique, transpression, chaı̂ne Varisque, plis, linéations d'allongement, Sardaigne, Tectonique

Rodolfo Carosi 1; Giacomo Oggiano 2

1 Dipartimento di Scienze della Terra and CSGSDA, CNR, via S. Maria 53, 56126 Pisa, Italy
2 Istituto di Scienze Geologico-Mineralogiche, Corso Angioi 10, 07100 Sassari, Italy
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     author = {Rodolfo Carosi and Giacomo Oggiano},
     title = {Transpressional deformation in northwestern {Sardinia} {(Italy):} insights on the tectonic evolution of the {Variscan} {Belt}},
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Rodolfo Carosi; Giacomo Oggiano. Transpressional deformation in northwestern Sardinia (Italy): insights on the tectonic evolution of the Variscan Belt. Comptes Rendus. Géoscience, Volume 334 (2002) no. 4, pp. 287-294. doi : 10.1016/S1631-0713(02)01740-6. https://comptes-rendus.academie-sciences.fr/geoscience/articles/10.1016/S1631-0713(02)01740-6/

Version originale du texte intégral

Version abrégée

1 Cadre géologique

Le socle cristallin de Sardaigne appartient à la chaı̂ne Varisque sud-européenne et il est constitué par des roches magmatiques et sédimentaires du Carbonifère et par des métasédiments allant du Cambrien au Carbonifère inférieur, dont le degré métamorphique augmente du sud-ouest vers le nord-est (Figs. 1 et 2) [2, avec bibliographie]. Le socle varisque de la Nurra est constitué par des phyllades, quartzites, métaconglomérats, schistes noirs, métavolcanites, micaschistes, paragneiss, appartenant aux nappes internes, transportées vers le sud-ouest pendant la collision du Carbonifère [3]. Dans la zone étudiée, située dans la région septentrionale de la Nurra, le métamorphisme prograde varie de la zone à biotite, puis grenat, jusqu'à la zone à oligoclase dans le Nord (Figs. 2 et 3) [3,9]. Ici, affleurent essentiellement des phyllites, micaschistes et paragneiss, d'âge Ordovicien à Carbonifère [3]. Au nord de la suture de Posada–Asinara (PAL, [1]), dans l'ı̂le d'Asinara, affleurent des migmatites et des gneiss migmatitiques de haut degré, avec des lentilles d'amphibolites.

Figure 1

Tectonic sketch-map of the Variscan Belt in Sardinia and position of the study area. 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); 11: traces of the cross-sections in Fig. 2.

Carte tectonique simplifiée de la chaı̂ne Varisque de Sardaigne et localisation de la zone étudiée. 1 : Dépôts de couverture post-varisque ; 2 : batholite varisque ; 3 : complexe métamorphique de haut degré ; 4 : nappes internes (métamorphisme de degré moyen à faible) ; 5 : nappes internes (métamorphisme de faible degré) ; 6 : nappes externes ; 7 : zone externe ; 8 : chevauchements ; 9 : failles ; 10 : accident de Posada–Asinara (PAL) ; 11 : localisation des coupes de la Fig. 2.

Figure 2

Geological cross-sections across the Variscan basement in northwestern Sardinia. Section 12: Asinara Island; section 34: Nurra Peninsula (redrawn from Carmignani et al. [3]). Isograds of the Barrovian metamorphism are from Carmignani et al. [3]. 1: Amphibolite; 2: migmatite; 3: Cala d'Oliva orthogneiss; 4: mylonitic micaschist; 5: Cala Reale ortogneiss; 6: andalusite bearing micaschist and paragneiss, quartzites; 7: micaschist, garnet + albite/oligoclase paragneiss, quartzites; 8: paragneiss, micaschist, subordinated quartzite and phyllites; 9: phyllite and metaconglomerate; 10: graphitic phyllite, metabasalt and metagabbro (black) (Silurian–Devonian); ‘oolitic ironstone’, phyllites, metagreywacke, porphyroids (Ordovician); 11: phyllite and metaconglomerate; 12: metasandstone, phyllite and quartzites (Carboniferous?); 13: Variscan granitoids.

Coupes géologiques dans le socle Varisque du Nord-Ouest de la Sardaigne. Coupe 12 : ı̂le d'Asinara ; coupe 34 : presqu'ı̂le de la Nurra (d'après Carmignani et al. [3]). Isogrades du métamorphisme barrovien d'après Carmignani et al. [3]. 1 : amphibolites ; 2 : migmatites ; 3 : gneiss du Cala d'Oliva ; 4 : micaschistes mylonitiques ; 5 : gneiss du Cala Reale ; 6 : paragneiss, micaschistes à andalousite, quartzites ; 7 : micaschistes, paragneiss à grenat + albite/oligoclase, quartzites ; 8 : paragneiss, micaschistes, quartzites subordonnées et phyllites ; 9 : phyllites, métaconglomérats ; 10 : shales noirs, métabasites (noir) (Siluro-Dévonien) ; oolitic ironstone, phyllites, métagrès, métarhyolites (Ordovicien) ; 11 : phyllites et métaconglomérats ; 12 : métagrès, phyllites et quartzites (Carbonifère ?) ; 13 : granitoı̈des varisques.

Figure 3

Geological map of the northwestern Sardinia basement in the study area. 1: Post-Variscan covers; 2: micaschists, garnet + albite/oligoclase paragneiss, quartzites; 3: paragneiss, micaschists, subordinated quartzites and phyllites; 4: phyllites, black phyllites, metavolcanites, metabasites, metasandstones and quartzites; 5: trend and plunge of A2 folds axes; 6: strike and dip of the S2 foliation; 7: trend and plunge of L2 stretching lineation; 8: isograds of the Barrovian metamorphism (after Carmignani et al. [3]).

Carte géologique du socle du Nord-Ouest de la Sardaigne dans la région étudiée. 1 : Couvertures post-varisques ; 2 : micaschistes, paragneiss à grenat + albite/oligoclase, quartzites ; 3 : paragneiss, micaschistes, quartzites subordonnées et phyllites ; 4 : phyllites, phyllites noires, roches volcaniques métamorphiques, métabasites, métagrès et quartzites ; 5 : direction et plongement des axes de plis A2 ; 6 : direction et plongement de la foliation S2 ; 7 : direction et plongement des linéations d'extension L2 ; 8 : isogrades du métamorphisme barrovien (d'après Carmignani et al. [3]).

Dans le socle de bas et moyen degré métamorphique s'observent trois phases de déformations compressives [3]. La phase D1 est documentée, soit par des plis déversés vers le sud-ouest avec une foliation de plan axial syn-métamorphique, soit par des zones de cisaillement avec direction de transport vers le sud. Une déformation hétérogène D2 se superpose aux structures de la phase D1, avec une intensité croissante vers le nord (Figs. 2 et 3). Enfin, le socle est intéressé par deux systèmes de plis tardifs, développés en conditions tectoniques opposées.

2 Analyse structurale de la déformation D2

La phase D2 est caractérisée par des plis F2 à déversement nord, avec des axes est–ouest et NW–SE, avec une foliation de plan axial plongeant vers le sud (∼40–60°) (Figs. 2, 3, 4a et 4b).

Figure 4

A. Isoclinal F2 fold in paragneisses near Punta Falcone. B. Type 3 fold interference pattern between F1 and F2 folds; micaschists and paragneiss near Punta Falcone. C. Syn-D2 albite porphyroblast (Ab) showing asymmetric pressure shadow; porphyroblastic paragneisses in the northern Nurra; CPL, Fow is 5 mm. D. Rotated syn-D2 garnet in micaschists of northern Nurra; dextral sense of shear. CPL, Fow is 6.8 mm. Mylonitic foliation in C and D strikes roughly west–east and dips 40–60° to the south. Dextral sense of shear corresponds to a top-to-the-northwest sense of shear in the field.

A. Plis isoclinaux F2 dans le paragneiss près de Punta Falcone. B. Plis superposés de type 3 entre les plis F1 et F2 ; micaschistes et paragneiss près de Punta Falcone. C. Porphyroblastes d'albite (Ab) syn-D2 montrant des ombres de pressions asymétriques ; paragneiss porphyroblastiques dans la Nurra septentrionale ; LPC, le champ de vision est de 5 mm. D. Grenats hélicitiques syn-D2 dans les micaschistes de la Nurra septentrionale ; direction de cisaillement vers la droite. LPC, le champ de vision est de 6,8 mm. La foliation mylonitique dans C et D montre une direction environ est–ouest et plonge de 40–60° vers le sud. La direction droite de cisaillement correspond sur le terrain à une direction de transport tectonique vers le nord-ouest.

Les angles d'ouverture des plis F2 varient de 60–70° au sud à moins de 10° au nord. Même la géométrie des plis varie de la classe 1C [16], au sud de l'Isola dei Porri, à la classe 3 [16] au nord de Stintino. La déformation finie, mesurée sur les profils des plis selon la méthode de Lisle [11], varie entre 1,5 au sud auprès de Punta Rumasinu à 4,7 près de P. Falcone au nord. La foliation de plan axial est constituée par un clivage de crénulation S2, qui devient de plus en plus pénétratif vers le nord, où il transpose la foliation S1. La foliation S2 se développe depuis un clivage de crénulation espacé au sud jusqu'à une schistosité, au nord, soulignée par quartz, muscovites, chlorites, oxydes et porphyroblastes de grenat et plagioclases [9]. Les relations microstructurales indiquent que la croissance des porphyroblastes de grenats et de plagioclases est pré et syn-D2 [9]. La linéation d'allongement L2, subhorizontale, a une orientation approximative est–ouest (Fig. 3) ; elle est marquée par des alignements de muscovites, quartz et biotites, ainsi que par des porphyroclastes allongés et fracturés. Les linéations L2 sont parallèles aux axes des plis contemporains F2 (Fig. 3) et, à partir de l'Isola dei Porri, se développent des plis en fourreau d'échelle moyenne. Le sens de cisaillement, déduit de coupes parallèles à la linéation L2 et perpendiculaires à la foliation S2, indique un transport du toit vers l'ouest et le nord-ouest (Figs. 4c et 4d).

L'analyse de la vorticité Wm [10,15], conduite sur des porphyroblastes de plagioclases hélicitiques, appartenant à 14 échantillons de paragneiss, confirme que la déformation D2 est caractérisée autant par une composante de déformation pure que par un cisaillement simple.

3 Déformations tardives

La déformation S2 est affectée par deux systèmes de plis tardifs développés dans des conditions tectoniques opposées. Les plis F3, de type ouvert, avec des plans axiaux sub-verticaux, sont encore liés à une compression sensiblement nord–sud. Au contraire, les plis F4 ont des plans axiaux sub-horizontaux et sont asymétriques, avec une vergence vers le sud. Ils sont quelquefois associés à des zones de cisaillements fragiles et fragiles–ductiles, qui accommodent la déformation extensive après la phase d'épaississement.

4 Discussion et conclusion

Le socle de la Nurra a été affecté par deux déformations pénétratives principales pendant la convergence varisque (Fig. 4a) [3]. La phase D1 voit le développement de plis à vergence sud, de chevauchements et de charriages des unités tectoniques vers le sud ou le sud-ouest dans les nappes internes et dans les nappes externes [3]. La déformation D2 est hétérogène et accompagnée par des plis à vergence nord, qui deviennent, en allant du sud au nord, de plus en plus fermés, jusqu'à atteindre une géométrie isoclinale (Fig. 4b) [3,17]. Avec l'accroissement de la déformation D2 s'observe un net parallélisme entre les axes des plis A2 et les linéations d'allongement L2, avec aussi le développement de plis en fourreau dans les zones à plus haute déformation. En même temps, la foliation S2 devient toujours plus pénétrative, jusqu'à transposer les structures D1. L'analyse de la vorticité cinématique, conduite sur des porphyroclastes hélicitiques, confirme les effets simultanés d'un cisaillement et d'une déformation pure qui, en accord avec le fort pendage de la foliation S2 et la linéation d'allongement L2 sub-horizontale, est caractéristique d'une « transpression ». Les données structurales indiquent la présence d'une zone de cisaillement crustale oblique, avec une déformation partagée en plis F2, comme réponse au raccourcissement approximativement nord–sud et en cisaillement dextre parallèle à la chaı̂ne comme réponse à la composante de strike-slip.

La présence d'une telle zone de cisaillement dans le Nord-Ouest de la Sardaigne suggère que le métamorphisme barrovien (pré- à syn-D2), qui augmente vers le nord, ne traduit pas une situation stable pendant la collision, mais qu'il est plutôt le résultat de la déformation D2 sur les précédents isogrades. Les données thermo-barométriques de Franceschelli et al. [9] ont mis en évidence le fait que la déformation D2 s'est développée avec une diminution de pression et une augmentation de la température initiale. Ces données indiquent le rôle important joué par la déformation transpressive D2 dans l'exhumation du socle. Des caractéristiques structurales très semblables pour les déformations D1 et D2 ont été reconnues également dans le socle de bas et moyen degré affleurant dans le Nord-Est de la Sardaigne [4]. Les données thermo-barométriques y indiquent aussi une diminution de pression pour la phase D2 [7,8]. Le changement de cinématique, de perpendiculaire à la chaı̂ne durant la phase D1 à parallèle à celle-ci durant la phase transpressive D2, doit être lié à la position de la Sardaigne pendant le développement de l'arc Ibéro-Armoricain [13]. La courbure progressive de l'arc, due à l'indentation du bloc Cantabrique [6,14], a causé le passage progressif d'une convergence frontale à une convergence oblique, avec comme conséquence l'augmentation du cisaillement parallèle à la chaı̂ne [4,5], qui a accompagné une grande partie de la ré-exhumation du socle.

Ce contexte géodynamique peut expliquer l'évolution différente du socle de Sardaigne par rapport aux autres secteurs de la chaı̂ne Varisque européenne, où l'on décrit des phénomènes extensifs plus importants [12].

1 Geological setting

The crystalline basement of Sardinia is a segment of the Southern European Variscan Belt, made up of Carboniferous magmatic and sedimentary rocks and metasedimentary Cambrian to Lower Carboniferous rocks, with metamorphic grade increasing from very low-grade in the southwest side of the island up to medium- and high-grade in the northern sector [3,7] (Figs. 1 and 2). Polyphase deformation and poly-metamorphism occurred during the collisional and post-collisional stages [3, and quoted references]. The study area, located in the northern part of the Nurra Peninsula (Figs. 1 and 3), represents a transition from low- to medium-grade basement [3]. The area is located roughly, 30 km south of the Posada–Asinara suture zone (PAL [1], Fig. 1). Metamorphic rocks outcropping in the area are phyllites, micaschists and porphyroblastic paragneisses, generally referred to as being of Ordovician–Carboniferous age [3]. This metamorphic pile is intruded by Carboniferous granitoids. North of the PAL, in the Asinara Island, high-grade rocks (Sill + Ms and Sill + Kfs zones), consisting of migmatites and migmatitic gneisses, with lenses and bodies of amphibolites, granulites and retrogressed eclogites are found (Fig. 1).

The increase of metamorphism from the chlorite zone to the sillimanite + K-feldspar zone is regarded as a typical collision-related prograde Barrovian metamorphism [3,8,9].

The Variscan basement in the Nurra Peninsula is constituted by phyllites, quartzites, metaconglomerates, black shales, metavolcanics, micaschists, paragneiss and porphyroblastic paragneisses belonging to the Internal Nappes transported toward the southwest during the Carboniferous collisional event [2]. In the study area, prograde metamorphism varies from biotite zone, through garnet, up to oligoclase zone to the north (Figs. 2 and 3) [3].

Structural analysis indicates the existence of three phases of compressive deformation in the low- to medium-grade rocks of northern Nurra [3]. The D1 deformation phase is characterised by overturned folds in central and southern Nurra, facing to the southwest and syn- to late-D1 ductile to brittle shear zones, with a top-to-south and southwest sense of shear [17]. A low-grade S1 penetrative axial plane foliation developed during D1 deformation. A heterogeneous D2 deformation overprinted D1 structures with northward increasing strain (Figs. 2 and 3). Two later folding phases affect all the previous penetrative structures.

2 Structural analysis of D2 deformation

2.1 Folds

Prominent F2 folds characterise the D2 deformation phase, with east–west- to NW–SE-trending axes and moderately to strongly south- and southwest-dipping axial planes (Figs. 2 and 3). F2 folds verge toward the north in Nurra and southern Asinara Island, showing an opposite vergence with respect to the D1 phase. The interference pattern between F1 and F2 folds varies but the type 3 [16] is more frequent (Fig. 4a).

In southern Nurra, F2 folds are open and associated to a S2 spaced crenulation cleavage, whereas going northward folds became more and more tight until S2 foliation progressively transposes the S1 foliation. Interlimb angles range from nearly 60–70° south of Isola dei Porri to less than 10° in northernmost area (Fig. 4b). F2 folds have thickened hinges and stretched limbs, but we observed a progressive change of the geometry of the F2 folds from a prevailing class 1C [11] south of Isola dei Porri to class 3 north of Stintino. Sheath folds are quite common and mainly developed in northern Nurra starting from the Isola dei Porri area.

Finite strain was calculated from F2 fold profiles according to the method of Lisle [11]. It assumes initial parallel folds; strain is obtained by measuring the change of thickness in the folded layers and plotting it in polar coordinates. Two-dimensional strain on fold profiles increases from 1.5 up to 4.7 from Punta Rumasinu, to the south, up to Punta Falcone, to the north.

2.2 Foliation

In the northern part of the Nurra Peninsula, S2 foliation becomes pervasive both at the outcrop scale and at the microscopic scale. The S1 foliation was identified only in the thin sections in the low- and medium-grade micaschists. It consists of a relict foliation within D2 microlithons and aligned inclusion trails within garnet and plagioclase, which define a foliation made of quartz, flakes of muscovite, biotite, muscovite, chlorite, sphene and graphite [9].

The S2 foliation is a zonal to discrete crenulation cleavage; it strikes WNW–ESE and strongly to moderately dips toward the southwest (Fig. 3). The S2 schistosity consists of quartz, muscovite, chlorite, oxides and garnet and albite/oligoclase porphyroblasts. Albite porphyroblasts contain S1 inclusion trails that range from rectilinear to curved, suggesting a pre- to syn-D2 growth of albite (Fig. 4c) [8]. Garnet porphyroblasts are often idioblastic and show snowball inclusion trails, suggesting their rotation during syn-D2 growth (Fig. 4d).

2.3 Lineation

The S2 foliation bears a sub-horizontal stretching lineation (L2) marked by the alignment of muscovite, quartz and biotite and by stretched and fractured porphyroclasts of garnet and feldspar in a west–east and WNW–ESE direction (Fig. 3). L2 lineation becomes prominent starting from Cala Coscia di Donna.

L2 stretching lineation is parallel to F2 fold axes (Fig. 3) and sheath folds have been recognised in the northern part of Nurra.

Sense of shear has been detected in sections perpendicular to the S2 schistosity and parallel to the L2 stretching lineation in the northernmost termination of Nurra. Shear bands, rotated syn-D2 garnets and asymmetric tails around syn-D2 albite and oligoclase porphyroclasts point to a top-to west and northwest sense of shear (Figs. 4c and 4d).

2.4 Vorticity analysis

Vorticity analysis was performed on 14 samples of paragneiss from the higher strain zone using porphyroblasts analysis [10,15] to check the characters of the D2 deformation. The degree of non-coaxiality of deformation is indicated by Wm, the ratio between mean angular velocity of lines and stretching rate (0<Wm<1). In pure shear, Wm=0 and in simple shear Wm=1. In a general deformation, rigid particles with an aspect ratio greater than a critical aspect ratio Rc rotate until they reach a stable orientation, while those with an aspect ratio lower than Rc rotate continuously. The value of Rc depends on the degree of non-coaxiality of deformation [15]. Albite porphyroblasts, showing rigid behaviour during the D2 deformation, have been used to calculate Rc and Wm. The values of Wm were scattered from 0.5 to 0.7, with a mean value of 0.6, suggesting that D2 deformation was characterised both by a component of simple shear and by an important component of pure shear.

D2 deformation develops heterogeneously in northern Nurra and it is characterised both by nearly NW–SE-trending folds and stretching lineation. The former testifies a nearly north–south compression, whereas the latter accommodates a component of deformation parallel to the belt.

3 Later folding

S2 foliation is affected by two systems of later folds developed in contrasting tectonic regimes. D3 deformation produces large-scale open folds, with steeply dipping axial planes coaxial to the F2 folds, and is related to continuing north–south horizontal compression in the basement.

Centimetric to metric asymmetric F4 folds, with sub-horizontal to shallowly dipping axial planes and with a mainly top-to-the-south vergence, are widespread all over the basement. Top-down to the south ductile/brittle to brittle shear zones and collapse folds accommodate the extensional collapse of the belt at upper crustal levels, after the thickening stage.

4 Discussion and conclusions

The Nurra basement is affected by two main penetrative deformations during the collision stage of Variscan convergence. The D1 deformation phase is related to south- and southwest-facing folds, thrusts and nappe stacking developed during the thickening stage of the belt in the Internal and External Nappes [2,3]. The D2 deformation is heterogeneous and accompanied by F2 northward vergent folds that become tighter up to isoclinal from south to north [3,17]. At the same time, S2 foliation accommodates increasing shortening until it transposes S1 foliation. D2 strain increases from south to north, as indicated by a progressive decrease of interlimb angles in F2 folds [3,17].

Increasing strain led to the development of a L2 stretching lineation that is parallel to F2 fold axes and strikes WNW–ESE shallowly plunging toward the ESE (Fig. 3). Sheath folds are found in micaschists and paragneisses from Isola dei Porri northward, testifying an increasing strain. Kinematic indicators point to a top-to-the-WNW sense of shear. A high strain oblique shear zone, with an inverse component, accommodates the D2 deformation.

D2 deformation is partitioned into F2 folds as response to nearly north–south shortening and nearly sub-horizontal L2 stretching lineation with shear parallel to the belt, as response to strike-slip component. Vorticity analysis on rotated porphyroclasts confirms the existence of pure shear together with simple shear.

The deformation pattern in northwestern Sardinia suggests that the apparent normal Barrovian metamorphism with northward and down increasing metamorphic grade, from chlorite up to sillimanite, is not a frozen state of the collisional prograde metamorphism but, on the contrary, is the result of shear deformation posterior to, and partly contemporaneous with, the Barrovian metamorphism.

Franceschelli et al. [9] showed that D2 deformation developed during decreasing pressure and initially increasing temperature. This indicates the key role played by the D2 transpressional deformation in the exhumation of the basement.

It is worth noting that a similar strain pattern for D2 deformation has been recognized also in northeastern Sardinia [4]. Our data indicate a complex deformation pattern south of the PAL in northern Sardinia, characterised by the occurrence of a D2 shallowly dipping prominent stretching and mineral lineation nearly parallel to the fold axes. This could be related to oblique convergence progressively developed after the frontal collision during D1.

Taking into account the pre-Permian position of Sardinia near southwestern France and northwestern Spain [13], we can relate the deformation pattern in Sardinia to the evolution of the Ibero-Armorican Arc [4,5].

Tightening of the Ibero-Armorican Arc, caused by the indentation of Cantabrian block [6,14], gave rise to a change from nearly pure north–south compression to oblique convergence. Horizontal displacement in the belt became more and more important during collision either in northern or in central–southern Sardinia [4,5]. This can explain the different fate of northern Sardinia with respect to other sectors of the Variscan Belt, where extensional collapse followed crustal thickening [2,12]. In Sardinia, the inner part of the belt started to collapse at the end of the stage of frontal collision. This may correspond to a stage where underthrusting became difficult and metamorphic rocks began to re-equilibrate at depth. This could be testified by increasing temperature after D1 and partly during D2 [9]. The general dynamic changed to oblique collision and the metamorphic rocks were again subject to compression in the developing transpressional setting and became exhumed at relatively slow rate during D2 deformation.

The occurrence of D2 orogen-parallel displacement prevents the crystalline basement of northern Sardinia from a normal evolution from crustal thickening to a generalized post-collisional extensional collapse.

Acknowledgements

This research was supported by the University of Pisa and funds from the ‘Centro di Studio per la Geologia Strutturale e Dinamica dell'Appennino’, CNR, Pisa. P. Matte is acknowledged for helpful comments on the manuscript. The authors are indebted to E. Guidi, A. Gagliardi and D. Iacopini for help during geological work in Nurra and to D. Iacopini for help with the French text.


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