Early Permian 908 clockwise rotation of the Maures – Este ´rel– Corsica – Sardinia block confirmed by new palaeomagnetic data and followed by a Triassic 608 clockwise rotation J.-B. EDEL 1 *, L. CASINI 2 , G. OGGIANO 2 , P. ROSSI 3 & K. SCHULMANN 1,4 1 Ecole et Observatoire des Sciences de la Terre, Universite ´ de Strasbourg, UMR 7516 of CNRS, 1 rue Blessig, 67084 Strasbourg, Cedex, France 2 Universita ` di Sassari, DiSBEG, via Piandanna n84, 07100 Sassari, Italy 3 BRGM, 3 avenue Claude-Guillemin, BP 36009, 45060 Orle ´ans, Cedex 2, France 4 Center for Lithospheric Research, Czech Geological Survey, Kla ´rov 3, 118 21, Prague 1, Czech Republic *Corresponding author (e-mail: [email protected]) Abstract: Palaeomagnetic investigations of the Corso-Sardinian block and Maures – Este ´rel show that there has been a change in their magnetic orientation during the Late Carboniferous– Early Permian period (305–280 Ma). This trend is interpreted in terms of a large-scale 908 clockwise rotation of the southern branch of the Variscan belt that matches the successive change in shorten- ing directions revealed by structural geology. The evidence is based on existing structural studies of the fabrics of syntectonically emplaced granitoids partly based on the anisotropy of magnetic sus- ceptibility, combined with a large database of isotopic ages. The chronological match between the palaeomagnetic and tectonic datasets is interpreted here as a result of large-scale dextral wrench movements in the lithosphere between the Gondwana and Laurussia supercontinents. This wrench deformation is regarded as a sequel to the dextral rotation of the northern branch of the Var- iscan belt during 330–315 Ma which terminated in frontal collision with Avalonia. The continu- ation of movement in the southern Variscan realm was due to shearing along the southern margin of the Avalonian block. An additional clockwise rotation is inferred to have taken place during the Triassic period. The age of this motion remains to be determined. Supplementary material: Palaeomagnetic and geochronological data from the Maures–Este ´rel, Corsica–Sardinia block presented in Figure 7 and discussed in the text are available at http:// www.geolsoc.org.uk/SUP18742 The northern Variscides reached their present con- figuration with respect to Avalonia in the Late Car- boniferous after a long tectonic history involving a succession of compressional and extensional move- ments associated with strike-slip faulting and block rotations (Matte 1991; Franke 2000; Edel et al. 2013). Large clockwise rotations associated with dextral wrench faults succeeded the anticlockwise rotations of Armorican Massif, Vosges and Bohe- mian Massif during 335 – 330 Ma up to the collision with Avalonia around 315 Ma. Afterwards, the Var- iscides continued to rotate clockwise but together with Laurussia up to the Middle Permian. In con- trast, after the collision, the southern Variscides i.e. Iberia, Maures – Este ´rel, Corsica–Sardinia and the Alpine blocks which were still located in a large-scale wrench zone generated by sinistral dis- placement of Gondwana supercontinent, underwent additional rotations relative to the northern Varis- cides (Arthaud & Matte 1977; Edel et al. 1981; Bard 1997). As a result, intense magmatism took place in this sheared corridor, associated with important deformation during the Late Carbonifer- ous – Permian (e.g. Casini et al. 2012). During the same period, oroclinal bending manifested by antic- lockwise rotations of the southern branch of the Cantabrian – Asturian arc affected the western ter- mination of the southern Variscides (e.g. Pastor- Gala ´n et al. 2011; Weil et al. 2013). Since wrenching is generally associated with block rotations (Jackson & McKenzie 1988), palaeomagnetism is the appropriate tool for deci- phering this Late Variscan tectonic history (Edel et al. 2003, 2013). Palaeomagnetic investigations were possible thanks to the occurrence of numerous outcrops of Late Carboniferous volcanic units and associated dykes in Maures, Este ´rel, Corsica and Sardinia (MECS) and the southern Alps (Corte- sogno et al. 1998). The palaeomagnetic investi- gations showed contrasting directions of remanent From:Schulmann, K., Martı ´nez Catala ´n, J. R., Lardeaux, J. M., Janous ˇek, V. & Oggiano, G. (eds) The Variscan Orogeny: Extent, Timescale and the Formation of the European Crust. Geological Society, London, Special Publications, 405, http://dx.doi.org/10.1144/SP405.10 # The Geological Society of London 2014. Publishing disclaimer: www.geolsoc.org.uk/pub_ethics
Transcript
Early Permian 9088888 clockwise rotation of the Maures–Esterel–
Corsica–Sardinia block confirmed by new palaeomagnetic data and
followed by a Triassic 6088888 clockwise rotation
J.-B. EDEL1*, L. CASINI2, G. OGGIANO2, P. ROSSI3 & K. SCHULMANN1,4
1Ecole et Observatoire des Sciences de la Terre, Universite de Strasbourg,
UMR 7516 of CNRS, 1 rue Blessig, 67084 Strasbourg, Cedex, France2Universita di Sassari, DiSBEG, via Piandanna n84, 07100 Sassari, Italy
3BRGM, 3 avenue Claude-Guillemin, BP 36009, 45060 Orleans, Cedex 2, France4Center for Lithospheric Research, Czech Geological Survey,
Abstract: Palaeomagnetic investigations of the Corso-Sardinian block and Maures–Esterel showthat there has been a change in their magnetic orientation during the Late Carboniferous–EarlyPermian period (305–280 Ma). This trend is interpreted in terms of a large-scale 908 clockwiserotation of the southern branch of the Variscan belt that matches the successive change in shorten-ing directions revealed by structural geology. The evidence is based on existing structural studies ofthe fabrics of syntectonically emplaced granitoids partly based on the anisotropy of magnetic sus-ceptibility, combined with a large database of isotopic ages. The chronological match between thepalaeomagnetic and tectonic datasets is interpreted here as a result of large-scale dextral wrenchmovements in the lithosphere between the Gondwana and Laurussia supercontinents. Thiswrench deformation is regarded as a sequel to the dextral rotation of the northern branch of the Var-iscan belt during 330–315 Ma which terminated in frontal collision with Avalonia. The continu-ation of movement in the southern Variscan realm was due to shearing along the southern margin ofthe Avalonian block. An additional clockwise rotation is inferred to have taken place during theTriassic period. The age of this motion remains to be determined.
Supplementary material: Palaeomagnetic and geochronological data from the Maures–Esterel,Corsica–Sardinia block presented in Figure 7 and discussed in the text are available at http://www.geolsoc.org.uk/SUP18742
The northern Variscides reached their present con-figuration with respect to Avalonia in the Late Car-boniferous after a long tectonic history involving asuccession of compressional and extensional move-ments associated with strike-slip faulting and blockrotations (Matte 1991; Franke 2000; Edel et al.2013). Large clockwise rotations associated withdextral wrench faults succeeded the anticlockwiserotations of Armorican Massif, Vosges and Bohe-mian Massif during 335–330 Ma up to the collisionwith Avalonia around 315 Ma. Afterwards, the Var-iscides continued to rotate clockwise but togetherwith Laurussia up to the Middle Permian. In con-trast, after the collision, the southern Variscidesi.e. Iberia, Maures–Esterel, Corsica–Sardinia andthe Alpine blocks which were still located in alarge-scale wrench zone generated by sinistral dis-placement of Gondwana supercontinent, underwentadditional rotations relative to the northern Varis-cides (Arthaud & Matte 1977; Edel et al. 1981;
Bard 1997). As a result, intense magmatism tookplace in this sheared corridor, associated withimportant deformation during the Late Carbonifer-ous–Permian (e.g. Casini et al. 2012). During thesame period, oroclinal bending manifested by antic-lockwise rotations of the southern branch of theCantabrian–Asturian arc affected the western ter-mination of the southern Variscides (e.g. Pastor-Galan et al. 2011; Weil et al. 2013).
Since wrenching is generally associated withblock rotations (Jackson & McKenzie 1988),palaeomagnetism is the appropriate tool for deci-phering this Late Variscan tectonic history (Edelet al. 2003, 2013). Palaeomagnetic investigationswere possible thanks to the occurrence of numerousoutcrops of Late Carboniferous volcanic units andassociated dykes in Maures, Esterel, Corsica andSardinia (MECS) and the southern Alps (Corte-sogno et al. 1998). The palaeomagnetic investi-gations showed contrasting directions of remanent
From: Schulmann, K., Martınez Catalan, J. R., Lardeaux, J. M., Janousek, V. & Oggiano, G. (eds)The Variscan Orogeny: Extent, Timescale and the Formation of the European Crust.Geological Society, London, Special Publications, 405, http://dx.doi.org/10.1144/SP405.10# The Geological Society of London 2014. Publishing disclaimer: www.geolsoc.org.uk/pub_ethics
magnetization in the different areas of the MECS(Fig. 1). This was already postulated in an earlyreconstruction that was based on palaeomagneticdata and on the similarity of geological units foundin Maures and in northern Sardinia (Westphal et al.1976). The directions measured in Late Carbonif-erous volcanics from northern Corsica (Westphal1976) and in ignimbrites from northern Sardinia(Zijderveld 1975; Westphal et al. 1976) led West-phal et al. (1976) to postulate a post-Permianrotation of Sardinia relative to Corsica. This inter-pretation was contested by Vigliotti et al. (1990),
who found that the palaeomagnetic directions inthe Late Variscan dykes of southern Corsica andnorthern Sardinia are very similar. Edel et al.(1981) obtained consistent directions in variousvolcanic units in central and southeastern Sardinia,but these deviate significantly from the directionsmeasured in the northern part of the island. Twointerpretations were therefore proposed: (1) theCorsica–Sardinia Block (CSB) has been affectedby relative block rotations, sealed by younger gran-ites; and (2) the directions recorded in the mag-matic rocks were not acquired at the same time,
Fig. 1. Late Triassic reconstruction of Maures–Esterel–Corsica–Sardinia based on palaeomagnetic directions inPermo-Triassic sandstones (B directions); bathymetry; and processed Bouguer anomaly maps (data from BureauGravimetrique International). Geological correlations between Maures and Sardinia (Westphal et al. 1976) revised afterEdel (2000). GAPL, Grimaud–Asinara–Posada Line.
J.-B. EDEL ET AL.
but were recorded during incremental stages of alarger global rotation of the whole block. This latterexplanation was also preferred in a later review ofthe Late Variscan palaeomagnetic data (Edel 2000).However, the difference in directions between thevolcanic rocks of northern Corsica and the dykesof southern Sardinia, which were assumed to be ofthe same age, led the author to place the boundaryof the domains of relative rotation in centralCorsica, where strike-slip faults are present.
The large angular rotation of up to 908 infer-red by the palaeomagnetic measurements made insoutheastern Sardinia (Edel et al. 1981; Emmeret al. 2005) raises two questions: (1) did the wholeCorsica–Sardinia block undergo rotation; and (2)was the sense of rotation anticlockwise, or clock-wise as postulated by Edel (2000), based on geo-logical correlations with the northern Variscides?Because the palaeomagnetic measurements insoutheastern Sardinia were made on rocks of theLate Carboniferous magmatic episode, with agesin the range 310–295 Ma (Edel et al. 1981), addi-tional measurements of the volcanic and plutonicrocks emplaced during the same time-span in north-western Corsica and northeastern Sardinia are pre-sented below. The palaeomagnetic directions ofolder magmatic rocks belonging to the Early Car-boniferous Mg–K magmatic suite of northwesternCorsica, as well as different generations of dykes,were also measured in order to clarify the Late Car-boniferous–Permian evolution of the MECS. Theseinvestigations have enabled us to provide a betterinterpretation of the overall pattern of palaeomag-netic measurements in the context of the geody-namic framework of the Late Variscan, so that anew reconstruction of Late Variscan palaeogeogra-phy can be proposed.
Geological setting
One of the first palaeomagnetic studies of the MECSsuggested that the metamorphic and tectonic zoningof Maures and northern Sardinia was continuous(Westphal et al. 1976). This reconstruction isbased chiefly on palaeomagnetic measurements ofthe Permian volcanic rocks of Esterel, northernCorsica and Gallura. The shared geological evol-ution of Maures and Sardinia has been confirmedrepeatedly by the close similarity in the P–T– tpaths of metamorphic rocks and the ages of theprotholiths occurring in the Corsica–Sardiniablock (Palmeri et al. 2004; Giacomini et al. 2005;Gaggero et al. 2007) and in the Maures–EsterelMassif (Ricci & Sabatini 1978; Corsini & Rolland2009). The second argument for this is the presenceof a non-metamorphosed lower Palaeozoic succes-sion lying on Pan-African basement in both NW
Corsica and the Eastern Maures Massif, which istaken as evidence of their affinity to the southernVariscan branch (Rossi et al. 2009). This prop-osition is also constrained by: (1) palaeomagneticmeasurements of Permo-Triassic sandstones fromthe Maures, the Nurra and Catalonia areas (Fig. 1;Table 1), and (2) the bathymetry and processing ofthe Bouguer anomalies compiled by the BureauGravimetrique International (BGI), which permitthe definition of continental blocks as shown inFigure 1.
Magmatism
Three main magmatic episodes have been recog-nized within the CSB (Orsini 1976, 1979; Ghezzo& Orsini 1982; Rossi & Cocherie 1991; Ferre &Leake 2001). (1) The early magmatic sequence ofnorthwestern Corsica is manifested by high-Mg–Kcalc-alkaline plutons, designated as U1, emplacedover a short time span (,5–10 Ma) at c. 340 Ma(Paquette et al. 2003). (2) A later magmatic unit(U2) is characterized by granitoids with lowerMgO content emplaced between 320 and 280 Ma,making up the largest part of the Corsica–Sardiniablock (del Moro et al. 1975; Paquette et al. 2003;Oggiano et al. 2005, 2007; Casini et al. 2012).According to Paquette et al. (2003), the climax at305 Ma of the calc-alkaline granitoids correspondsclosely to the onset of the major period of LateVariscan strike-slip faulting (Arthaud & Matte1977; Bard 1997). (3) The final magmatic suiteU3 was partly coeval with the latest U2 leuco-monzogranites. It consists of tholeiitic complexesemplaced from 304 + 2 Ma (Paquette et al. 2003)to 279.9 + 2.5 Ma (Cocherie et al. 2005) and subor-dinate alkaline granitoids formed at 287.8 + 1.8 Mato 281 + 3–283 + 2 Ma (Van Tellingen et al.1988; Cocherie et al. 2005; Renna et al. 2007).Emplacement at 285–280 Ma of layered mafic–ultramafic complexes at middle and lower crustallevels is interpreted to reflect (1) the delaminationof the lower part of the thickened lithosphericmantle resulting from the Variscan collision, oralternatively (2) incipient lithospheric break-upassociated with transcurrent faulting (Rossi et al.1992; Paquette et al. 2003). In any case, the latterauthor considers that the 280 Ma magmatism prob-ably points to the switch to a new geodynamiccontext, characterized by an active extensional tec-tonic regime. Despite this long-lasting magmaticevolution (Paquette et al. 2003; Cocherie et al.2005) the growth of the CSB was interpreted as anepisodic rather than a continuous process. U1 mag-matism is 30 myr older than that of U2, and theboundaries within some U2 plutons can be sharpand marked by periods of apparent magmatic quies-cence. On a smaller scale, a similar history of
P–TR ROTATIONS OF CORSICA–SARDINIA
Table 1. Palaeomagnetic results from northern Corsica
Site Locality Rock type Direction Max Tub n Dec. Inc. k a95 (%) Lat. Long. Lat.rotn
Max Tub: maximum unblocking temperature; n: number of samples. The directions after tectonic corrections are in italic. The adopted mean results are in bold.a95%: confidence circle; Lat./Long.: latitude/longitude of the VGP; Lat./long. rotn: latitude/longitude of the VGP rotated (pole of rotation Corsica–Sardinia/Maures–Esterel: 42.78N/8.88E/2708).
J.-B.
ED
EL
ET
AL
.
episodic growth has been proposed recently for theU2 suite in Sardinia (Casini et al. 2012). Parts ofthe U2 and U3 intrusions were emplaced withintheir own volcanic apparatus (Vellutini 1977;Rossi et al. 1992, 1993).
The late and post-Variscan dykes of differentfelsic/mafic compositions, which show an alka-line signature from the Early Triassic, cut throughthe metamorphic basement of the MECS and theplutons (Baldelli et al. 1987; Gaggero et al. 2007).Most of the dykes in northern Sardinia wereemplaced along the direction N10–408, that is,almost perpendicular to the average trend of pre-300 Ma plutons. This trend is roughly parallel tothe alignment of the younger magmatic complexes(300–280 Ma).
Metamorphism and tectonics
The Corsica–Sardinia segment of the southernrealm of the Variscan orogenic belt has been re-stored by Rossi et al. (2009). An unmetamorphosedPalaeozoic succession lying on Pan-African base-ment is exposed in the Argentella area of northernCorsica. The contact of these rocks with the Varis-can Internal Zone is obscured by Mg–K U1 intru-sions so that the collisional scenario proposed byRossi et al. (2009) cannot be confirmed. Carmignaniet al. (1994) have subdivided the Variscan colli-sion in this area into three different structural zones:
(1) The external zone, which covers the south-western part of the island of Sardinia, con-sists of an unmetamorphosed fold-and-thrustbelt (Funedda 2009) consisting of a sedimen-tary succession ranging in age from upperVendian to Lower Carboniferous;
(2) The nappe zone is affected by greenschistfacies metamorphism and consists of a thickcontinental arc-related volcanic suite (Gag-gero et al. 2012) of Middle Ordovician age(Oggiano et al. 2010) embedded within athick Palaeozoic metasedimentary succes-sion. Both the external and the nappe zonesshow evidence of an angular unconformitybetween the Early and Late Ordovician depos-its that is ascribed to the Sardic Phase.
(3) The inner zone is characterized by medium- tohigh-grade metamorphic rocks intruded byLate Variscan granites. It consists of two dif-ferent metamorphic complexes.
There is a polymetamorphic high-grade complexwith anatexites and metatexites which occupiesthe northernmost part of Sardinia and extends intoCorsica. This complex contains orthogneisses andminor metabasites of Ordovician age. Locally,relics of high- to intermediate-pressure granulite
assemblages of early Carboniferous age are wellpreserved in the metabasic rocks (Ghezzo et al.1979; Di Pisa et al. 1992; Cortesogno et al. 2004;Giacomini et al. 2006; Cruciani et al. 2012). Thesecond metamorphic complex is characterized bymicaschists and paragneisses containing kyanite +staurolite + garnet assemblages (Franceschelliet al. 1982), with intercalations of quartzite andmetabasalt lenses showing a N-MORB (normal-type mid-ocean ridge basalt) geochemical signa-ture (Cappelli et al. 1992).
A Late Variscan retrograde dextral strike-slipshear zone along the Posada Valley (Elter et al.1990; Padovano et al. 2012) marks the contactbetween the high-grade complex in the north andmedium-grade complex in the south (Elter 1987).This and other dextral shear zones have beendated as Late Carboniferous (Carosi et al. 2012).However, thrusting of the high-grade metamor-phic complex over the medium-grade rocks hasbeen inferred in several places (Cappelli et al. 1992;Di Pisa et al. 1993; Di Vincenzo et al. 2004; Casiniet al. 2010).
Thermobarometric estimates together withmicrostructural observations suggest that the twocomplexes followed different P–T– t trajectories(Di Pisa et al. 1993) after having shared the sameeclogitic metamorphism. During the early Carbon-iferous, the high-grade complex underwent an inter-mediate-pressure granulitic event at temperaturesof 730–830 8C and pressures of 0.8–1.1 kbar (Cor-tesogno et al. 2004; Cruciani et al. 2012). Locally,the collisional prograde Barrovian metamorphiczoning is obscured by high-temperature–low-pres-sure (HT/LP) re-equilibration dated at 300 Ma,which affects both metamorphic complexes (DelMoro et al. 1991). This late HP/LP metamor-phic evolution is associated with the emplace-ment of the late Carboniferous granitoids and isbelieved to be coeval with a period of extensionaltectonics, which reactivated earlier syncollisionalthrusts as low-angle ductile shear zones (Casini &Oggiano 2008).
The switch in kinematic regimes is recorded bothin the metamorphic rocks and in the syntectonicgranitoids. However, the granitoids provide morereliable information because the orientation offabric elements can be chronologically constrainedby using existing U–Pb zircon ages. The oldestplutons were emplaced at c. 340 Ma (Mg–K U1granites, Paquette et al. 2003) and show north–south-trending subvertical foliation, which car-ries a subhorizontal magmatic lineation (Fig. 2a).Lower Carboniferous fabrics in the metamorphicunits are also characterized by a north–southmineral stretching lineation and foliation consis-tent with that in the U1 magmatic unit. In con-trast, the magmatic fabrics in all the U2 plutons
P–TR ROTATIONS OF CORSICA–SARDINIA
emplaced between 320 and 300 Ma (Casini et al.2012) are characterized by NW–SE-striking mag-netic lineations and generally steep NW–SE foli-ations (Paquette et al. 2003; Gattacceca et al.2004). This pattern corresponds to the preferredorientation of feldspars, suggesting that the fab-rics are of magmatic origin. The stretching linea-tion developed in synmetamorphic shear zones ofsimilar age is almost parallel to both the shape ofthe U2 plutons and their emplacement fabrics, indi-cating a consistent NW–SE direction of tectonictransport (Carosi et al. 2012). A major change inthe orientation of structural elements occurred
over a short time span between c. 295 and 285 Ma(Oggiano et al. 2005; Casini et al. 2012). The U2plutons younger than 295 Ma were emplaced alonga NE–SW direction as confirmed by the shape ofthe plutons and also by the preferred orientation ofK-feldspar megacrysts in coarse-grained megacrys-tic granitoids, which was systematically measured(Cocherie et al. 2005). In addition, the magmaticlineations of Early Permian U2 granitoids corre-spond to the direction of tectonic transport intop-to-the-SW thrust zones developed close to theforeland in southern Sardinia (in present coordi-nates; Sarrabus Phase, Conti et al. 2001).
Fig. 2. (a) Geological map of NW Corsica and location of sampling sites. Dashed lines represent magmatic lineations ingranites. (b) Directions of maximum (K1) and minimum (K3) magnetic susceptibility are plotted on equal-areaprojections. Mean directions (large symbols) are shown with confidence circle. Magnetic anisotropy ellipsoid Tv. anisotropy degree P′ (Jelinek 1981). In (b2) small symbols represent samples of Cg2 close to the dyke Cd3.
J.-B. EDEL ET AL.
Palaeomagnetism
Sampling
In Corsica, sampling was carried out at five sites ingranites and granodiorites of the Mg–K magmaticsuite (Cg1, 2, 3, 4, 5), four sites in the Osani andesite(Os1, 2, 3, 4), one site in the Ota gabbro (Cg7) and infour dykes intruding into the Mg–K granitoids(Cd1, 2, 3, 4) (Table 1; Fig. 2a). In north Sardinia,the samples were taken from three sites in theTrinita d’Agultu granodiorite (Sg4, 5, 6), three sitesin the Isola Rossa diorite (Sg1, 2, 3), three sites inthe La Ettica quartzdiorite (Sg8, 9, 10) and onesite in the Arzachena granite (Sg7) (Table 2; Fig.3a). In the Barrabisa granodiorite and its surround-ing metatexite-diatexite country rocks, 34 regularlydistributed, oriented blocks were collected (Bbin Table 2; location: open dots in inset of Figure3a). Metamorphic rocks, mainly amphibolites, weresampled from one site close to the Posada line andtwo sites on Asinara island (Sm3, 1, 2). In addition,one lamprophyre, two dolerites, two trachyandesiticdykes and one camptonite dyke were also sampled(Sd4, 2, 3, 1, 5, 6 respectively).
Magnetic susceptibility and anisotropy
of magnetic susceptibility
North Corsica. The magnetic susceptibility andthe magnetic anisotropy were measured usingmodified Digico instruments. The susceptibilitiesof the Mg–K granitoids (U1 unit) range from 200to 7000 × 1026 (SI) with outliers exceeding20000 × 1026 in the close vicinity of dyke Cd3.The degree of anisotropy (P′) scatters around avalue of 1.06 and the parameter T (the shape of mag-netic anisotropy ellipsoid) varies from prolate tooblate. The large majority of the granodiorite sam-ples exhibit a north–south foliation and a SSE-striking lineation consistent with the orientationsof the feldspars (Fig. 2b1a, 2b2). In contrast, thehighly magnetic samples of Cg2 close to the dykeCd3 show an oblate fabric parallel to the ENE–WSW-striking dyke (Fig. 2b1b).
The susceptibility of the Osani andesite scattersaround 200 × 1026 in three sites. Site Os2 showssignificantly higher values in the range 1500–2000 × 1026 (SI). The P′ factor is low, rangingfrom 1.005 to 1.05, and the T parameter indicatesa dominantly plano-linear fabric. The magneticfoliation dips 558 to the SW (Fig. 2b3). The dip ofthe flattened clasts varies around 408 and the colum-nar joints plunge 40–508 to the NE, nearly perpen-dicular to the mesoscopic foliation. We interpretthe foliation of the volcanic fabric to be the resultof flow in the lava. The steeper dip of the magneticfoliation may be the result of imbrication.
North Sardinia. A wide range of susceptibilitiesfrom 200 to 32000 × 1026 (SI) characterizes theIsola Rossa and Trinita d’Agultu magmatic bodies(Sg1–6). The highest values are found in the cen-tral part of the Isola Rossa diorite. The most highlymagnetic samples show high anisotropy factors P′
ranging from 1.14 to 1.29 and a dominating pro-late fabric with a lineation dipping to the SE. Theanisotropy factors P′ of the other samples are below1.14. The shape ellipsoids of the pluton are plano-linear to oblate with a foliation mainly subverticalor dipping steeply to the NE (Sg4) (Fig. 3b2).
The Barrabisa granodiorite and migmatitesshow a wide range of susceptibilities ranging from50.10 × 1026 to 5000 × 1026 (SI) with a maximumin the population around 125 × 1026. The low ani-sotropy factor P′ in the range 1.02–1.18 for themajority of granites confirms the magmatic originof the measured fabric (Fig. 3b1; Bouchez 1997).Higher values, indicating deformation, were mea-sured in metatexites from the northern and south-western boundaries of the Barrabisa pluton (Casiniet al. 2012). The magnetic foliation strikes predom-inantly east–west and dips moderately to the south.The magnetic lineation plunges slightly to the ESE,parallel to the elongation of the pluton. In contrast,metatexites and orthogneisses show a steep north–south fabric, consistent with the orientation of pre-320 Ma quartz and feldspar fabrics (Casini et al.2012). The quartz diorite of La Ettica (Sg8–10)displays a large range of susceptibilities, variabledegrees of anisotropy and a predominant oblatemagnetic fabric. Foliation is dipping to the SE(Fig. 3b3), parallel to a N308 fault which boundsthe pluton in the east. The strike of the lineation isvariable but the dominant ESE strike is similar tothat of the older plutons.
Remanent magnetizations
The remanence was measured using a JR6A SpinnerMagnetometer produced by the company AGICO.As we observed in most basement rocks of the Var-iscan belt, thermal demagnetization proved to bemore efficient than alternative field (AF) to separatethe components of the NRM. Stepwise thermaldemagnetization was therefore carried out on 212samples from Corsica and 295 samples from Sardi-nia. Susceptibility measurements of test sampleswere made after each heating step in order todetect possible mineralogical transformations. TheOsani andesite, the Mg–K granodiorites, the Otagabbro and the Isola Rossa diorite show almost uni-vectorial behaviour during demagnetization up to580–590 8C (Fig. 4). This unblocking temperatureindicates that magnetite is the probable carrier ofthe remanence. In some of the samples from sitesOs3, Os4, Sg4 and Sg5, the increasing susceptibility
P–TR ROTATIONS OF CORSICA–SARDINIA
Table 2. Palaeomagnetic results from northern Sardinia
Site Locality Rock type Direction n Max Tub (8C) Dec. Inc. k a95 (%) Lat. Long. Lat.rotn
The directions after tectonic corrections are in italic. The adopted mean results are in bold. See Table 1 for abbreviations.
P–
TR
RO
TA
TIO
NS
OF
CO
RS
ICA
–S
AR
DIN
IA
above 400–500 8C shows that magnetite was cre-ated. This has a greater or lesser effect on the demag-netization curves (Os4.421, Fig. 4a7): on orthogonalplots the curves miss the origin and diverge above500 8C. In this case, the last step before an increasein susceptibility took place was considered to bethe direction of characteristic magnetization. Thisdirection is generally consistent with the direc-tions of the samples, which are not affected bysuch mineralogical transformations. In some othercases where the curves miss the origin, the presenceof a high-temperature mineral is probable. Forinstance, in the reddish altered Mg–K granite Cg5(Fig. 4a12), a high-temperature component carriedby hematite is likely present. With an unblocking
temperature above 670 8C, hematite is also thecarrier of the main magnetization in the hydrother-malized red migmatite Sg11 (Fig. 4b5), located atthe southern rim of the Barrabisa pluton. In contrast,at the same locality the grey migmatite is charac-terized by a low-temperature component with adirection opposite to the previous (Sg11.412,Sg11.331, Fig. 4b5). Characteristic magnetizationwith unblocking temperatures of 330–350 8C, indi-cating pyrrhotite, titanomagnetite, titanohematite ormaghemite as possible carriers of the remanence(Sg6, 7, 9 and Bb) (Fig. 4b7, b9) were also evi-denced at several granitic sites of NE Sardinia(Sg5, 6, 7, 9 and Bb). Because pyrite and titanomag-netite are observed in thin sections, pyrrhotite or
Fig. 3. (a) Geological map of north Sardinia and location of sampling sites. Dashed lines represent magmatic lineationsin granites. (b) Directions of maximum (K1) and minimum (K3) magnetic susceptibility are plotted on equal-areaprojections. Mean directions (large symbols) are shown with confidence circle. Magnetic anisotropy ellipsoid shape Tv. anisotropy degree P′.
J.-B. EDEL ET AL.
E. Carboniferous MGK granitoids
30
NupCg3.321
W 50N
S
up
down
Cg4.132E 1
N
S
up
down
Cg6.132E
1
J/J0
0 300 600°C
1
J/J0
0 300 600°C
1
J/J0
0 300 600°C
(a) NW Corsica
(b) NE Sardinia
10
N
E
S
up
down
Sg4.312granodiorite
540°
580°
520°
Barrabisa granodiorite -migmatite
Isola Rossa diorite
1
J/J0
0 300 600°C
Trinita granodiorite
W100
NupCg1.321
1
J/J0
0 300 600°C
C' C'
C
C
W
Sg11.412grey migmatite
0.5 E
Sg11.331red migmatitehydrothermalalteration
N
S
up
down 670°
1
J/J0
0 300 600°C
1J/J0
0 300 600°C
C'
C TJ
E600°
2
down S
Bb12.41granodiorite
540°1
J/J0
0 300 600°C
C
400
Sdown
Sg1.233granodiorite
E450°
1
J/J0
0 300 600°C
C
500 E
Sdown
Sg3.232granodiorite
585°
1
J/J0
0 300 600°C
CC
Osani andesite
40
N
S
up
down
Os1.41150
N
S
up
down
Os2.231
1
J/J0
0 300 600°C
1
J/J0
0 300 600°C
CC
590°590°
S
0.2
Nup
Os4.421
B
585°
0 300 600°C
1J/J0
K/K00.2
E
1
S
Sg10.122quartz-diorite
305°
355°
560°
580°
N
E
B
D
1
J/J0
0 300 600°C
down
Pta Ettica quartz-diorite
1
J/J0
0 300 600°C
40
N
W
S
upSg8.122quartz-diorite
D'
4000
S
up
Cg7.221E
1
J/J0
0 300 600°CA
Ota gabbro
5
S
up
Cd4.221U3 dyke
E
1
J/J0
0 300 600°CA
A'
N
5
S
down
Cg5.212remagnetized MGK granodiorite
E
1
J/J0
0 300 600°C
down
Cd3.432dyke
E
S
1
J/J0
0 300 600°C
B
Cd1.621dyke
1
J/J0
0 300 600°C
5
S
upE
TJ
B
Nup
B
S
Bb30.31granodiorite
E
Nup
1
J/J0
0 300 600°C
A'
N
10
Sd4.331camptonite
E
1
J/J0
0 300 600°C
B
S
200
Sd3.212dolerite
E
Nup
1
J/J0
0 300 600°C
dykes
dykes
Sd2.212lamprophyrealtered
TJ'
N
0.5
405°
525°
W
up
1
J/J0
0 300 600°C
a1)a2) a3) a4)
a5) a6)a7) a8)
a9) a10) a11) a12)
b1) b2) b3)b4)
b5) )7b)6b
b8) b9)b10) b11)
350°
450°
Fig. 4. Examples of orthogonal projections of thermal demagnetization process and associated J/J0 curves.Magnetizations are in units of A m21 (×1026). The directions are labelled A/A′, B/B′, C/C′, D/D′ and TJ/TJ′.
P–TR ROTATIONS OF CORSICA–SARDINIA
titanomagnetite are the most likely carriers of mag-netizations. The samples from site Sg1 on the north-ern boundary of the Isola Rossa diorite disintegrateabove 450 8C, but the directions up to this tempera-ture as well as the end points are consistent withthose of samples from the neighboring sites (Fig.4b3). Multivectorial behaviour of the demagnetiza-tion due to the coexistence of both intermediate-and low-temperature minerals was rare. SampleSg10.122 (Fig. 4b9) is one exception.
The dykes show magnetic characteristics similarto those of the granitoids. Single components likelycarried by magnetite are found in dykes Cd4(Fig. 4a9), Sd3 (Fig. 4b10) and Sd4 (Fig. 4b4). Atthe other sites, a low-temperature magnetization inthe range 330–400 8C is removed (Fig. 4a10, b11).At site Cd3, a secondary magnetization with ran-dom distribution of the directions is much harderand only disappears at 530 8C (Fig. 4a11).
Directions of remanent magnetizations
In stereonets the mean directions computed afterthermal demagnetizing (Kirschvink 1980; Kentet al. 1983) show several clusters which, for thepurpose of discussion, are labelled A, B, C, D andTJ (Fig. 5). All groups show opposite A′, B′, C′,D′ and TJ′ directions. The validity of a componentis supported by a good definition in stereoplots, agood cluster within a site and/or a unit and mainlyby the coexistence of normal and reversed polar-ities. In magmatic and particularly in plutonic rocksseveral field reversals can be recorded during thegenerally long cooling and associated hydrothermalhistory. The occurrence of normal and reversedpolarities implies that at least one component is aremagnetization. Opposite magnetizations can beobserved at a same site, for instance the dyke Sd3(Table 2) in which a B′ component with an unblock-ing temperature of 500–5308 coexists with a weakopposite B component that is removed at c. 5808C. Opposite directions occur in the Mg–K grano-diorite of Capo Cavallo where sites Cg2 and Cg4display a C direction and site Cg3 the opposite C′
direction (Fig. 4a2, a3). In these two latter sites,the very similar J/J0 curves suggest that the carriersare the same.
Easterly and westerly C–D directions. The demag-netization procedure reveals a population of direc-tions with shallow inclinations and declinationsranging from ESE to ENE, designated the C and Ddirections, respectively (Figs 4 & 5). The oppositedirections are also present and are labelled C′ andD′. In NW Corsica, Westphal (1976) observeddirections with shallow inclinations and declina-tions ranging from south to east over the southeast-ern quadrant in the Osani andesites. The present
investigation confirms the existence of such direc-tions, but here the easterly C directions are foundat all four sites and, in particular, at the site Os1 onthe Col de la Croix (Table 1; Fig. 4a5, a6) where therocks are freshest. In sites Os1–3 the demagnetizingprocedure shows a univectorial behaviour of mag-netization with a C direction and an unblockingtemperature of c. 580 8C which implies magnetiteas carrier. At site Os4 the directions are scatteredin the southeastern quadrant. C, B and A directionscoexist in the site. The susceptibility and the rema-nence of the samples carrying B and A directionsand of a few samples with C directions increaseabove 450 8C and the direction becomes erratic,revealing mineralogical transformations (Fig. 4a7).The C component is therefore considered as theoldest, and the A and B components are likely over-prints. The validity of the B directions intermediateto C and A is unclear.
The Osani andesites show bedding criteria thatcan be used for tectonic correction. AMS fabric, flat-tened clasts and columnar joints indicate a dip to theSE. Assuming that in the pyroclastic flow the flat-tened clasts are subhorizontal at a certain distancefrom the feeding point, the mesoscopic foliation wasused for the tectonic corrections (Table 1; Fig. 5a1).
The oldest units in which C directions occurare the Mg–K granodiorites (Cg1–4, 6) (Table 1;Fig. 4a1–4). Sites Cg4 and Cg3, separated by c.1 km, show C and opposite C′ directions, respect-ively (Fig. 4a2, 3). Opposite C′ directions are alsofound in mafic inclusions from the Mg–K grano-diorite Cg1 (Fig. 4a1). These C and C′ directionsare remarkably consistent with the C directions ofthe Osani andesite and suggest that magnetizationin both rock types took place during the same periodand in the same palaeogeographical context. Thefreshest granodiorite shows an apparent unblock-ing temperature in the range 250–3308 and totalremoving of the magnetization at 580–590 8C(Fig. 4a1–3). The directions in both temperatureranges remain similar. We suspect the coexistenceof titanomagnetite and magnetite as carriers ofthe remanence.
ESE to east directions with shallow inclinationswere also obtained from the Isola Rossa diorite,the Trinita d’Agultu granodiorite and the Barrabisagranodiorite and migmatites (Table 2; Fig. 4b1–3,5, 6). As in Corsica, most C components from north-ern Sardinia show unblocking temperatures ofc. 580–590 8C, probably due to magnetite (Fig.4b1, 2, 6). This is in accord with the high suscepti-bilities. Low unblocking temperatures in the range330–400 8C, indicating minerals such as titano-magnetite, titanohematite, pyrrhotite or maghemiteas possible carriers, occur in a few sites. In generalthe susceptibility remains relatively stable or in-creases with temperature, so that maghemite is the
J.-B. EDEL ET AL.
Fig. 5. Stereographic projection of the characteristic directions of (a1) NW Corsica and (a2) north Sardinia. Thedirections are represented with their a95% confidence circle. A line joins the in situ C or D direction without confidencecircle and the tilt-corrected direction with confidence circle. In (b1) and (b2), the north to western directions have beenreversed and highlighted in red in order to illustrate the reversal test. The unclear polarity of the C and D directionsinvolves two possible distributions of south directions (c1) and (c2).
P–TR ROTATIONS OF CORSICA–SARDINIA
least probable. The low-unblocking-temperature C′
component observed at the southern rim of the Bar-rabisa migmatite (Sg11.412, Fig. 4b5) was acquired(1) during uplift of the pluton, (2) during reheatingand emplacement of the younger Arzachenagranite in the south or (3) reheating during intrusionof the adjacent dyke Sd1. At the same locality, thered hydrothermalized migmatite shows a C com-ponent carried by hematite (Sg11.331, Fig. 4b5).
The mean C directions from the tectonizednorthern and southern boundaries (Sg1, Sg4) areslightly different from the mean C direction of thecore of the small Isola Rossa pluton (Sg2, Sg3),whereas the central part of the larger Trinita plu-ton only shows late B low-temperature overprintslikely acquired during the late stages of upliftand/or the intrusion of the younger dykes (Sg5,7). The sheared southwestern boundary (Sg4) hasa mesoscopic and magnetic foliation dipping by558 to the NNE, while in the central part of theIsola Rossa diorite (Sg2, Sg3) the foliation issteeper and dips by 808 to the SSW. The magnetiza-tions of the tectonized northern and southern bound-aries are assumed to be the youngest. After tiltingthe core of the Isola Rossa pluton by 458 to theSSE, the foliation of Sg2 and Sg3 becomes parallelto Sg4 and the mean C directions of Sg2 and Sg3converge on the C directions of the uncorrectedsites Sg1 and Sg4. Such a correction assumes thatthe foliation of the diorite was at first dipping tothe NNE like the southern rim of the Trinitapluton, and was steepened by the ongoing transpres-sion. This hypothetical correction of Sg2 and Sg3is supported by the consistency with the other Cdirections of the batholite. In short, pre- and post-tilting C magnetizations coexist in Isola Rossa–Trinita granitoids.
C directions dominate in the granodiorite andmigmatite of Barrabisa (Table 2; Fig. 4b5, 6),where both polarities coexist. In northeastern Sardi-nia, NNE extension accommodated by EW-directedlow-angle normal faults caused necking of thelithosphere and 35–458 NNE tilting of the fault-bounded blocks to their present geometry. Giventhe emplacement age of most plutons in the area,it is inferred that extension occurred after thepre-300 Ma plutons were completely solidified. Evi-dence of the tilting of the northern part of the Sardi-nian batholith is provided by the mesoscopicfoliation of subvolcanic granites and is confirmedby Al-in-hornblende thermobarometry showingthat the depths of emplacement decreased slightlytowards the north (Casini et al. 2012). The age oftilting is inferred to be late Carboniferous–EarlyPermian because the NW-striking faults do notdisplace some younger granodiorites dated at286.5 + 4.3 Ma (Gaggero et al. 2007). For thatreason a tectonic correction has to be applied to
the oldest magnetizations of the Barrabisa grano-diorites, namely to the C components.
Considering the three units for which tectoniccriteria are available, namely the Osani andesite(Osac, Table 1), the Isola Rossa pluton (IRTA2,Table 2) and the Barrabisa pluton (BbC, Table 2),the tectonic correction improves the cluster of theC directions around the mean value D ¼ 1058,I ¼ 98, a95 ¼ 68 and increases the k factor of theFisher statistics significantly: the ratio kc (after tec-tonic correction) on kis (in situ) kc/kis ¼ 388/28;and for the virtual geomagnetic pole (VGPs) Kc/Kis ¼ 751/42. After adding the mean in situ direc-tion C of the remagnetized Mg–K granodiorites ofnorthern Corsica, for which no tectonic criteriaare available, and the uncorrected probably post-tectonic IRTA1 direction from Isola Rossa, themean C direction for the 310–300 Ma magnetiza-tions becomes D ¼ 1028, I ¼ 78, k ¼ 182, a95 ¼68 and the mean VGP: 278N, 888E, K ¼ 287,A95 ¼ 4.58.
Easterly directions of remanence are rare inthe dykes from Corsica although the dykes Cd1–3, which are intrusive into the Mg–K granodioriteCg3, are of the same generation (305 + 2 Ma,Paquette et al. 2003) as the andesite (308.1 +2.9 Ma). An ENE D direction is preserved in theCd2 dyke of Calvi (Table 1). The D direction isalso characteristic of dyke Sd1 that intrudes thesouthern margin of the Barrabisa granodiorite. Inaddition to the dominant ESE C directions, ENE Ddirections are also present in the Isola Rossadiorite and the Barrabisa granodiorite (Table 2).The absence of C direction in the dykes suggeststhat the D magnetizations of the granitoids areyounger, contemporaneous with the later intrusionsand probably responsible for partial remagnetiza-tion of the early granitoids. For example, D direc-tions occur in the younger basic complex of LaEttica (286.5 + 4.3 Ma) (Sg8 and Sg10). Two sub-sites of Sg8 (Sg8A and Sg8b, Table 2), separatedby c. 100 m, are characterized by opposite D andD′ directions (Sg8.122, Sg10.122, Fig. 4b8, 9) whichsupport the reliability of these directions. Theclose directions Sg8a and Sg10b show a similardeclination as Sg8b, but the inclinations of Sg8aand Sg10b are positive while the negative incli-nation of Sg8b is in agreement with the other Ddirections of Sardinia (Table 2). The quartzdioritepluton is emplaced along a fault that belongs to aseries of N308-oriented sinistral strike-slip faults,which are particularly visible on the processed aero-magnetic maps of northern Sardinia (F in Fig. 1).When assuming that the ESE-dipping foliationwas at first subvertical, the tilt correction shifts thedirections Sg8a and Sg10b close to the uncorrecteddirection Sg8b. The mean direction (LE in Table2) calculated with the corrected Sg8a and Sg10b
J.-B. EDEL ET AL.
directions and the uncorrected Sg8b direction isconsistent with the D directions from the plutonicand volcanic units of other areas of Sardinia,suggesting that pre- and post-tectonic D magnetiza-tions have been preserved in the La Ettica pluton.
Southerly A directions. The A components showsoutherly declinations and shallow–intermediatenegative inclinations. The opposite northerly direc-tions are labelled A′. The southerly direction insamples from the Ota gabbro already found byWestphal et al. (1976) is confirmed at site Cg7,where the unique magnetization is carried by mag-netite (Fig. 4a8). A similar A direction was obtainedon the two adjacent dykes Cd4 near Curzu whichwere emplaced during the U3 magmatic episode.The S–SSE directions of the Osani andesite(sites Os3 and Os4) and of the contemporaneousdyke Cd3, found in addition to C directions, corre-spond to magnetizations which become erraticabove 350–450 8C and which are due to mineralo-gical transformations and creation of magnetite(Os4.421, Fig. 4a7). An A′ direction with normalpolarity occurs in the reddish Mg–K granodioriteCg5 (Cg5.212, Fig. 4a12), the most altered of theinvestigated sites, while the fresh and dark Cg3 con-tains only C′ components. Components with the Adirection were also obtained from a few sites inthe Barrabisa pluton (Table 2). All these A direc-tions are consistent with the results from the Otagabbro and from dykes and volcanic flows of theU3 magmatic episode (Cd4.221, Fig. 4a9), whichare the youngest manifestations of the Late Variscanmagmatism of Corsica. They are therefore con-sidered to be overprints on the earlier units. An A′
direction with normal polarity opposite to the Adirection and likely primary was measured in thecamptonite dyke from eastern Sardinia (Sd4.331,Fig. 4b4). In dyke Sd2 of Conca Verde a componentwhich may belong to the A′ group is removed in therange 200–450 8C, while the remaining magnetiza-tion shows a B direction.
Southeasterly B directions (shallow inclinations).The B directions are characterized by southeasterlydeclinations and shallow, mostly negative incli-nations. The typical B direction is found in thedyke Sd3 from La Collucia (Sd3.212, Fig. 4b10).This direction (132/215) is consistent with themean directions found by Vigliotti et al. (1990) ina large number of dykes from southern Corsica(135/211) and northeastern Sardinia (133/22).The Conca Verde dyke Sd2 shows an oppositeB′ direction at intermediate temperatures and, fora few samples, a B direction that is removedat c. 580 8C. For most samples carrying low–intermediate-unblocking-temperature B′ and A′
directions, the demagnetizing circles converge
towards a mean B direction, however (Table 2).Similar B directions occur in older granites andgranodiorites from northern Sardinia (Sg5–7,Sg10a, BaB) and in metamorphic units from thePosada zone (Sm3) and Asinara (Sm1–2) (Table2). Because the dykes intrude the granites and meta-morphic rocks, the B directions are considered asremagnetizations acquired during the late intru-sions; at that time the batholith had almost cooleddown, as most of these secondary B componentsshow low unblocking temperatures of 320–4008C. In the dyke Cd1 from northern Corsica, the Bdirection coexists with a younger low-unblocking-temperature TJ component (Table 1).
Southeasterly TJ directions (high negative incli-nations). The TJ directions show declinationssimilar to the B directions but with steeper incli-nations, comparable with Late Triassic–Jurassicdirections found in Mesozoic sediments from Sar-dinia. They occur in dykes (Cd1, Cd2, Sd2; Fig.4a10, b11; Tables 1 & 2) and in an amphibolite(Sm3). The unblocking temperatures are low in therange 300–350 8C (Cd1, Sg11, Sd1) or intermediatein the range 500–580 8C (Cd2, Sm3, Sd4). At sitesCd2, Sd2 and Sm3 the direction has the normalpolarity TJ′.
Timing of the magnetizations
A significant number of the magnetizations can beconsidered as a result of overprints, as testified bythe presence of normal and reversed directions; thediscussion will therefore not be restricted to thecrystallization ages obtained from zircons usingthe U–Pb method. Concerning effusive rocks andsmall plutons, which cooled rapidly, 40Ar/39Arand K–Ar dating of hornblende and biotite canalso provide crystallization ages. On large plutonicassemblages these methods can indicate coolingages. In addition, possible resetting by hydrothermalalteration dated using the K–Ar method on plagio-clase, K feldspar and whole rock will be considered.
C components. The C directions of the Osani ande-site were obtained using the freshest samplesshowing a univectorial behaviour during demagneti-zation. According to the unblocking temperature ofc. 580 8C, the carrier is probably magnetite. Thismagnetization is assumed to be primary and itsage is probably close to the age of the rocks thatis 308.1 + 2.9 Ma (U–Pb age of zircon). The 347-Ma-old Mg–K granodiorites, which carry normaland reversed C and C′ directions, are intruded by aseries of ENE–WSW dykes with an age inferredto be close to that of the mafic border of the compo-site dyke from Capo Cavallo (304 + 2 Ma; Paquetteet al. 2003). Some investigators have proposed that
P–TR ROTATIONS OF CORSICA–SARDINIA
these dykes could have been the feeder dykes for theandesitic magmatism (Fumey-Humbert et al. 1986).It is therefore tempting to consider the C magnetiza-tions of the granitoids as remagnetizations acquiredduring this U2 magmatism and also to attribute aLate Carboniferous age to them. In Sardinia, the Cdirections may be primary in the inner part of theIsola Rossa diorite and the age of the magnetizationsmay be close to the 300.1 + 6.1 Ma U–Pb ageobtained from zircons. The existing U–Pb zirconages of the Barrabisa granitoids from northern Sar-dinia range between 320 + 10 and 311 + 6 Ma(Oggiano et al. 2007; Casini et al. 2012). Theyounger age coincides with the age of emplacementof the U2 magmatic suite in Corsica. The mainremanent magnetization C was acquired prior tothe normal faulting which affected the pre-300 Magranitoids, i.e. prior to 286 Ma during the coolingof the early plutons.
D components. D components are present in theIsola Rossa, the Barrabisa and the La Ettica grani-toids. Based on the zircon age of 288.3 + 2.5 Maand the 40Ar/39Ar ages of 288 + 0.7 Ma on biotiteand 286.5 + 4.3 Ma on hornblende, the La Etticaquartz diorites are the youngest (Gaggero et al.2007). The single D components in the dyke Sd1that intruded the southern boundary of the Barrabisapluton is assumed to be primary. As the C com-ponents are not present in the dykes and in the lategranitoids, the D components are likely youngerthan the C. They are considered as overprints inthe Isola Rossa and Barrabisa granitoids, and mayhave been acquired during the magmatic event at290–286 Ma.
The previous palaeomagnetic data of theLate Carboniferous–Early Permian volcanic andintrusive units of Seui–Seulo, Baunei, Talana–Villagrande, Monte Ferru–Tertenia and Escala-plano areas in eastern and southeastern Sardinia(Edel et al. 1981) belong to the C–D group of direc-tions. Similar directions were obtained from dykesin central and southeastern Sardinia (Emmer et al.2005). A large group of K–Ar ages (Cozzupoliet al. 1971; Lombardi et al. 1974; Edel et al.1981) show three populations of which the oldestare seven ages for hornblende, biotite and plagio-clase ranging from 315 to 280 Ma with a mean of299 Ma. These ages are consistent with the ages ofthe Osani andesites (north Corsica) and the Trinitad’Agultu granodiorite (NE Sardinia) and are con-sidered as emplacement ages. The main group of20 ages obtained by dating K feldspars and wholerocks ranges between 277 and 257 Ma with amean age of 264 Ma, suggesting a resetting of thechronometer during Middle–Late Permian time.This period coincides with a large gap in the strati-graphical record of NW Sardinia, Provence and
Alps (Ronchi et al. 2011). It includes at least alarge part of the Middle Permian, the entire LatePermian and the earliest Triassic, ranging from c.270 Ma to nearly 248 Ma. The youngest group of11 K–Ar ages obtained by dating whole rocksand feldspars ranges between 240 Ma and 202 Mawith a mean of 225 Ma, and may be due to Triassicalteration.
Unless all rocks with C–D components havebeen completely remagnetized during a late hydro-thermal resetting at c. 264 Ma, which is highly unli-kely when considering the large gap involved inpalaeolatitude with Variscan Europe for that date(Fig. 6a2, b2), the C–D components were acquiredduring the period 311 + 10 to 288 + 0.7 Ma. Bothpolarities C–C′ and D–D′ exist, but the easterly Cand D directions dominate. The reversed Kiamansuperchron lasted from 312 to 262 Ma (Eide &Torsvik 1996). Short normal events are reportedwithin this timespan at 305 and 280 Ma, respect-ively (Alva-Valdivia et al. 2002). As a consequenceit is unclear whether the easterly C and D com-ponents were acquired during the Kiaman reversalor not; the interpretation is not unique (Figs 6 & 7).
The A magnetizations were recorded in the Otagabbro Cg7 and in the dyke Cd4, which bothbelong to the U3 magmatic suite of Corsica. West-phal et al. (1976) found similar A directions in thering complexes of Porto, Scandola and Cintowhich belong to the U3 suite (Ota gabbro, Seninoignimbrites, Tuara dolerites, rhyolites). The Otagabbro shows a cooling age of 280 + 12 Ma deter-mined by the 40Ar/39Ar dating of biotite (Maluski1976), a mean K–Ar age of 282 + 10 Ma forbiotite and hornblende and 230 + 9 Ma for plagio-clases (Edel et al. 1981). In addition, the Porto U3granitic complex which includes the gabbro hasprovided concordant laser ablation-inductively cou-pled plasma-mass spectrometry (LA-ICP-MS) ageson zircon of 281+3 and 283 + 2 Ma (Renna et al.2007). A younger date for the Porto complex of259 + 9 Ma is attributed to a later hydrothermalevent (Renna et al. 2007). A U–Pb zircon age of278 + 2 Ma (unpublished) was obtained for theAsco rhyolite erupted during the U3 magmaticepisode.
The reversed polarity of most A components andthe radiometric ages are in favour of an emplace-ment during the Permian Kiaman reversal or even-tually the Triassic, the magnetic field of whichwas alternatively normal and reversed. This is con-sistent with the reversed polarity of the Corsica U3magmatic rocks. The well-defined A′ componentof the camptonite dyke Sd6 from northeastern Sardi-nia is also considered to be primary. According tothe 227 + 3 Ma 40Ar/39Ar age and 247 + 20–224 + 16 Ma fission track age it is significantlyyounger than the A components of the granitoids
J.-B. EDEL ET AL.
of Corsica (Baldelli et al. 1987). The normalpolarity of this A′ component is also compatiblewith a Triassic age, because the polarity wasreversed from 312 to 262 Ma and normal or reversedafter 262 Ma (Eide & Torsvik 1996). Nevertheless,a reconstruction of Variscan Europe and MECSfor an age of 230 Ma involves a distance in palaeo-latitude of the microplates exceeding 500 km(Fig. 6c2). As a consequence, the age of the campto-nite must be older; it may be close to the oldestfission track age. In summary, the rocks that pre-serve A components were emplaced during thePermo-Triassic in the period c. 285–250 Ma.
In the reconstruction based on the continentalboundaries defined by the bathymetry and thegravity (Fig. 1) and on palaeomagnetic B directionsfrom the Permo-Triassic of Corsica–Sardinia (con-sidered as a rigid block) and Eastern Iberia, the Acomponents correspond to the oldest directionsrecorded in the Plan de la Tour ignimbrite of theMaures Massif (Zijderveld 1975; Edel 2000). Theconsistency of the concerned VGPs is illustratedin Figure 7. The Plan de la Tour ignimbrite isan expression of the magmatic event dated at
305+5and 303.6 + 1.2 Ma(Begassat 1985; Corsiniet al. 2010) but the magnetizations are syn- topost-tectonic (Edel 2000), that is, Early–MiddlePermian in age. This is consistent with the magneti-zation ages in northern Corsica. The unpublisheddirection obtained from the microgranite dykes inthe Catalonian Coastal batholith is subparallel tothe A directions described above. These dykes areintrusive into granites and granodiorites dated at284 + 4 Ma and belong to the same magmaticsuite (Sole et al. 1998).
The typical B directions (declinations around1308E) are found in dykes Sd2 and Sd3 from north-ern Sardinia. These are equivalent to the direc-tions obtained from similar dykes investigated byVigliotti et al. (1990) in southern Corsica and north-ern Sardinia and by Emmer et al. (2005) in northernSardinia. The ages of these dykes at Conca Verde(Sd2) have been determined to be 253.8 + 4.9 Maand 248.4 + 8.8 Ma using the 40Ar/39Ar methodon amphiboles (Gaggero et al. 2007). A rejuvenatedage of 204.7 + 5.3 Ma indicates recrystallizationsduring a sub-greenschist facies metamorphic over-print. The B components and the opposite B′
Fig. 6. Alternative palaeopositions for northern Variscan Europe (Edel & Duringer 1997; Edel 2001; Torsvik et al.2012) and of MECS, according to the various ages available for rocks with A, B, C and D directions and the polarities ofthe directions.
P–TR ROTATIONS OF CORSICA–SARDINIA
components are also present in the different gener-ations of granitoids emplaced during the period320–280 Ma in northern Sardinia (Sg6, Sg7, Sg10,Bb; Fig. 4b7) where they are interpreted mostly aslow-temperature overprints acquired during theemplacement of the dyke swarms, resulting eitherfrom partial thermoremanent remagnetization orfrom crystallization of sulphides during the associ-ated hydrothermal activity. B directions also occurin Permo-Triassic sandstones and in the south
Giusta ignimbrite from Nurra in NW Sardinia(Zijderveld et al. 1970; Edel et al. 1981). The ignim-brite is of late Carboniferous age according to theK–Ar date of 302 + 9 Ma for biotite, but youngerages of 244 + 9 and 207 + 11 Ma (Lombardi et al.1974) suggest a Triassic overprint contemporane-ous with deposition of the overlying sediments.
Close to the B directions are the directions140–1508E of the Gallura ignimbrite (Zijderveldet al. 1970; Westphal et al. 1976) which was
Fig. 7. (a) Poles of published and new results from Late Carboniferous to Triassic units from Maures, Esterel, Corsica,Sardinia and APWP of northern Variscan Europe. The VGPs of Corsica–Sardinia are rotated around the Eulerian pole(42.78N/8.88E/2708) leading to the reconstruction of Figure 1. (a1) Hypothesis 1: the polarity of the C, D, B, Adirections is considered to be reversed. (a2) Hypothesis 2: the polarity of the C′ and D′ directions is normal. The APWPof MECS is part of a great circle that corresponds to a rotation around a pole located in the western Mediterranean (blackstar). (b) The poles of MECS in (a2) are rotated around the Eulerian pole: (08N/1808E/308) in order to overlap theMesozoic parts of the MECS and Variscan Europe APWPs.
J.-B. EDEL ET AL.
emplaced in the late Early Permian based on theU–Pb age of 275.6 + 3.4 Ma for zircon (M. Maino,pers. comm. 2012). The K–Ar ages on feldspars andquartz cluster around a mean of 259 + 10 Ma (Edelet al. 1981).
If Maures–Esterel–Corsica–Sardinia is as-sumed to be a rigid block (Fig. 1), the B directionsbecome parallel to the younger directions ofMaures–Esterel recorded in Late Permian–EarlyTriassic sandstones of the western Maures and theToulon basin (Merabet & Daly 1986; Aubele et al.2012) and in the Esterel rhyolites, which show40Ar/39Ar emplacement ages in the range 278–264 Ma (Zheng et al. 1991–1992).
The TJ and TJ′ directions are consistent withthe results from the Late Triassic and Jurassic sedi-ments of Sardinia (Horner & Lowrie 1981; Kirscheret al. 2011); we therefore interpret them as Jurassicoverprints.
To summarize, the oldest C magnetizations werelikely acquired in the Late Carboniferous between308 and 299 Ma during the early stage of the U2magmatic episode. The radiometric ages of rockscarrying A and B magnetizations overlap so thatsome uncertainty remains regarding their relativechronology. The reversed A components in the U3units emplaced at c. 280 Ma should be primary; anA direction with a normal polarity occurs in an alka-line dyke of Triassic age, however. The B com-ponents in the Upper Permian–Triassic doleriticdykes may be primary. We note that in the wholeMECS, radiometric dating reveals rejuvenatedLate Triassic ages (230–200 Ma) with a maximumat c. 220 Ma, which is consistent with the age ofthe alkali basalts (Gaggero et al. 2007). This radio-metric rejuvenation may be associated with amagnetic overprinting.
Edel (1980) reports different results from twosites in Muschelkalk limestones from Nurra inNW Sardinia, which may reflect uncertainty in theA and B ages and could suggest a possible rotationduring the Middle–Late Triassic. One site shows aclear A direction (D ¼ 1848, I ¼ 278, a95 ¼ 78)and the second has a typical B direction (D ¼1218, I ¼ 28, a95% ¼ 78). Preliminary measure-ments of Triassic dykes intruded into the Catalo-nian Coastal batholith, which lay adjacent to SWSardinia, support this assumption. They show asignificant rotation of the declinations during theLate Triassic magmatic event (Edel et al. unpub-lished data).
Tectonic evolution of the MECS
First conclusions
The new palaeomagnetic results lead to the follow-ing important conclusions.
(1) The occurrence of easterly and opposite wes-terly C–D directions with similar acquisi-tion ages in the range 310–286 Ma innorthern Corsica and in northern and south-eastern Sardinia indicates that this area hasbeen a single continental block since thelatest Carboniferous.
(2) The rotation of the declinations by c. 908 fromC to A in northern Corsica can only be inter-preted in terms of a block rotation, anticlock-wise or clockwise (Fig. 5).
(3) Due to the uncertain polarity of the C and Ddirections and relative timing of the A and Bdirections, the changes in directions can beinterpreted in different ways (Fig. 6). Afterplotting the VGPs of the new and publishedresults, several different paths can be con-sidered depending on the polarity of the Cdirections and the relative ages of the com-ponents (Fig. 7).
Possible scenarios of the evolution of
MECS in Late Variscan times
Relative positions of Maures–Esterel/Corsica–Sardinia. In the reconstruction of Figure 1, basedon bathymetry and Triassic palaeomagnetic datafrom MECS and Catalonia, Corsica and Sardiniaare rotated around the Eulerian pole (42.78N/8.88E/2708) relative to Maures and Esterel. InFigure 7 the poles of the mean directions fromCorsica and Sardinia are rotated accordingly, sothat the Late Permian–Triassic poles from Mauresand Esterel become consistent with the B poles ofthe CSB. In this reconstruction, the continuity ofthe metamorphic zoning and of the Grimaud–Asinara–Posada line is preserved (Fig. 1).
Relative positions of MECS/Calabria. Based onpalaeomagnetic results from the Sila nappes inCalabria (Manzoni 1979), Edel et al. (1981) pro-posed a reconstruction with Calabria attached tosoutheastern Sardinia and a late Carboniferous–Permian direction nearly parallel to the C–D direc-tions from Sardinian volcanics and intrusives(Fig. 8). The mineral 40Ar/39Ar thermochronologi-cal analyses document emplacement and coolingof the Sila intrusives during the period 293–289 Ma, which is consistent with the estimated ageof the D components (Ayuso et al. 1994).
Relative positions of MECS/northern VariscanEurope. The Triassic–Jurassic poles/APWP(apparent polar wander path) of the MECS aredistant from the APWP of northern Europe (Edel& Duringer 1997; Torsvik et al. 2012) (Fig. 7a),which means that Maures–Esterel and consequentlythe whole MECS were only welded with northern
P–TR ROTATIONS OF CORSICA–SARDINIA
Europe after Jurassic time. Fitting the poles ofMECS with the poles of northern Europe needs arotation of the MECS VGPs around the Eulerianpole (08N/1808E/2308) (Fig. 7b).
Considering the earlier evolution of MECS withrespect to the Variscan belt, several possibilitieshave to be discussed.
Sequence D–C–B–AB–A. In the first hypothesis(Fig. 5c1), all directions in the southwestern quad-rant have the same reversed polarity. The sequence
D–C–B–AB–A then results from a succession ofanticlockwise rotations amounting to c. 1108. Thisimplies that the A components are younger thanthe B and AB. Following the anticlockwiserotations, the A–TJ path corresponds to a changein the global motion of the whole CSB and a clock-wise rotation during the Late Triassic. The availableages are against such an interpretation. It was shownabove that the D components (c. 288 Ma) are likelyyounger than the C (311–297 Ma). The A com-ponents which occur in rocks that show ages in the
Fig. 8. Palaeoposition of the Variscides and of MECS in Late Carboniferous times according to palaeomagnetic data ofthe Variscides (e.g. Edel 2001; Torsvik et al. 2012) and of MECS in case of hypothesis 2. Main structural features arefrom Burg et al. (1990, 1994), Casini et al. (2012), Charles et al. (2009), Conti et al. (1999), Echtler & Malavieille(1990), Faure et al. (2009), Pitra et al. (1999), Henk (1993), Verner et al. (2009) and Zeh et al. (2000).
J.-B. EDEL ET AL.
range (285–250 Ma) appear to be older than therocks carrying B components (255–240 Ma).Unless this chronology is contested by new data,the B components will be considered as youngerthan the A.
Sequence D–C–A–AB–B–TJ. In this sequencethe D, C, A, AB, B, TJ directions are assumedto be reversed (Fig. 5c1). This involves an anti-clockwise D–C–A rotation exceeding 908 ofMECS during 288–282 Ma, followed by a clock-wise rotation A–AB–B and a northward driftB–TJ in the Triassic (Fig. 7a1). This scenario isalso contradicted by the chronology of the C andD directions. The reconstructions of Figure 6a1and 6a2 correlate the MECS with the BohemianMassif and support the interpretation in terms oforoclinal bending of the eastern Variscan belt fol-lowed by a large dextral wrenching (Guillot et al.2009). However, such a large-scale oroclinal bend-ing model originally proposed by Matte (2001)correlates the high-grade Gfohl unit of the Molda-nubian orogenic root zone in the Bohemian Massifwith north Sardinia and the Corsica internal zoneand the medium-grade rocks of the Drosendorfunit in the Bohemian Massif with the southern exter-nal part of Sardinia. While correlation between thegranulites and Mg–K magmatism from the Gfohlunit and Corsica internal zone is possible from thepoint of view of ages, P–T conditions and the chem-istry of key igneous and metamorphic lithologies(e.g. Giacomini et al. 2008; Rossi et al. 2009;Lexa et al. 2011), the correlation of the externalpart of Sardinia and the Drosendorf unit is unlikely.The main difference is in Late Proterozoic–Cam-brian protolith ages of the Drosendorf unit and itsMP/HT metamorphism including relics of eclogites(Faryad et al. 2006) compared to complete Palaeo-zoic sequence and low-grade to very-low-grademetamorphism in Sardinia. In addition, the me-dium-grade Drosendorf unit alternates with theGfohl granulites due to large-scale gravity overturns(Schulmann et al. 2005, 2009), while the internalzone rocks are thrust over the external zone in Sar-dinia. Altogether, the correlation of tectonic archi-tecture and geological inventory of the BohemianMassif with Corsica and Sardinia is impossible;the Matte’s (2001) model of oroclinal bendingtherefore cannot be applied to explain the palaeo-magnetic record presented in this work.
Sequence C′ –D′ –A–AB–B–TJ. This hypothesisassumes a normal polarity for the C directions and areversed polarity for the C′ directions (Fig. 5c2) andrespects the available, although weakly constrained,chronology of the components. In this case, the C′ –A–AB–B path has to be interpreted in terms of asuccession of clockwise rotations amounting to c.
1408. The resemblance of this APWP with theAPWP of the whole Variscan belt is noticeable,although the rotations of MECS mark the continu-ation of the Middle–Late Carboniferous rotationsof the Variscan belt (Fig. 7). Such a sequence canbe explained using a model of block rotationrelated to the activity of dextral shear zones andstrike-slip faults which are known to have operatedduring Late Carboniferous–Permian time in westand central Europe such as the Bristol Channel–Bray, Elbe and Sudetic fault systems (Arthaud &Matte 1977; Edel & Weber 1995). In Sardinia,major dextral shear zones also operated during330–300 Ma (Elter et al. 1990; Carosi et al. 2012;Padovano et al. 2012) simultaneously with similarlarge-scale fault zones. The dextral shearing canexplain the clockwise rotation of the southern Var-iscan segments in a similar way as recently pro-posed by Edel et al. (2003, 2013) for Palaeozoicorogenic regions in the Central Europe.
Late Carboniferous palaeoposition
of the MECS
Several geological arguments support the latterhypothesis in favour of a large dextral movementof the MECS at the end of the Carboniferous.
(1) In a late Carboniferous palaeogeographybased on palaeomagnetic measurements fromthe Variscides and Laurussia (Edel 2001;Torsvik et al. 2012), and on the C′ –D′ direc-tions from this study, the orientation of theMECS becomes nearly east–west (Fig. 6a1,b1). The metamorphic zoning runs NE–SW,parallel to that of the eastern branch of theVariscides, and the polarity of both domainsis consistent (Fig. 8). In this reconstruction,the internal zones are located on the northwes-tern flank of the MECS, thus close to the Mol-danubian domain. To the SE, the forelandzone of southern Sardinia coincides with theprolongation of the Montagne Noire (south-eastern Massif Central).
(2) The Mg–K granites of the southern Vosges(Ballons granite; Tabaud 2012), southernBlack Forest (Schaltegger 2000), northernCorsica (Rossi & Cocherie 1991; Cocherieet al. 1994) and external Crystalline Alps(Schaltegger et al. 1991; Debon & Lemmet1999) were emplaced during a short-livedmagmatic episode at c. 340 + 5 Ma. Theseultrapotassic rocks show very distinctive Sr–Nd, trace and REE element compositions sug-gesting that these Palaeozoic massifs weregeographically close together (Tabaud 2012;Fig. 8). In contrast, the Sr–Nd compositionof the Mg–K granites of the Central Vosges
P–TR ROTATIONS OF CORSICA–SARDINIA
is similar to the corresponding Moldanubiangranitoids of the Bohemian Massif.
(3) The NE–SW trend of the Stephanian Plan-de-la-Tour and Reyran basins in the Maures isconsistent with the general strike of the LateCarboniferous basins of ‘stable Europe’(Fig. 8).
(4) Magmatic lineations in the Mg–K gran-ites and the direction of tectonic transportrecorded by metamorphic units in the southernMassif Central and southern Sardinia are allconsistent, indicating a general NW–SE toNNW–SSE shortening direction during theperiod 345–335 Ma (Fig. 8). The structuralfeatures of the CSB, including the shapes ofplutons, the orientation of magmatic fabricsand the trend and kinematics of melt-bearingshear zones, are in agreement with the generaltrend.
(5) In Figure 8, the orientation of the CSB is basedmainly on the C′ and D′ directions, the ages ofwhich range between 305 and 288 Ma. Dur-ing the Late Carboniferous (315–300 Ma),numerous granites were emplaced in thesouthern Massif Central and the CSB. Theaverage east–west to ENE–WSW strike ofthe magmatic lineations and of the stretchinglineations in the host rocks, sinistral strike-slip along the NNE–SSW Sillon Houillerfault and dextral strike-slip along the NW–SE Bavarian faults all consistently indicatea general NNW–SSE shortening (Fig. 8;Ziegler 1990; Henk 1993; Burg et al. 1994;Faure 1995; Conti et al. 1999; Zeh et al.2000; Charles et al. 2009; Verner et al.2009; Casini et al. 2012). The NE–SW mag-matic lineations of the U2 granitoids are paral-lel to the stretching lineation in the MontagneNoire granitic and migmatitic dome whichwas emplaced during the period 325–295 Ma under NW–SE-striking compression(Charles et al. 2009; Pitra et al. 2012). Inthis framework, the rotation of the stretch-ing lineations from east–west to NE–SW inthe Cevennes and Montagne Noire can beexplained by the general clockwise rotationof the Variscides, which lasted from the Mid-dle Carboniferous to the Middle Permian.
Early Permian clockwise rotation
of the MECS
The evolution of the Variscides during the Stepha-nian–Autunian is marked by a 908 rotation of theaxis of principal stress from north–south to east–west in present coordinates (Ziegler 1990; Burget al. 1994). Evidence of north–south extension inthe Massif Central is provided by east–west- to
ENE–WSW-normal faulting in the MontagneNoire and the Morvan (Fig. 9; Charles et al. 2009;Faure et al. 2009; Choulet et al. 2012; Pitra et al.2012). The extensional detachment in the MontagneNoire has been dated precisely at 295 Ma byLA-ICP-MS U–Pb–Th dating of monazite (Pitraet al. 2012). In the southern and western Bohe-mian Massif, the sense of shear on the ENE–WSW strike-slip faults changed from dextral tosinistral in the Early Permian (Mattern 2001). Theonset of the north–south extensional regime coin-cides with the large-scale ignimbritic and rhyoliticmagmatism that affected most of the northern Var-iscides during the period 297–295 Ma (Lippolt &Hess 1989; Boutin et al. 1995; von Seckendorffet al. 2004).
The stretching lineations of the U2b and U3granitoids of the CSB deviate by about 908 fromthe lineations of the U2a granites, also indicating arotation of the shortening direction (Figs 8 & 9).The same conclusion can be drawn for Maures andEsterel where the axis of the Bas-Argens Permianbasin, which separates these two massifs, is orthog-onal to the Stephanian basins of the Plan de la Tourand Reyran.
The clockwise rotation of the MECS started withan apparent 208 C′ –D′ rotation. The time range isconstrained by the 300.1 + 6.1 Ma U–Pb age ofzircons from the Isola Rossa diorite, the youngestpluton that bears a C direction, and the 40Ar/39Arages of 288 + 0.7 Ma for biotite and 286.5 + 4.3Ma for hornblende from La Ettica quartz-diorite,which carries a D component.
The orientation of the CSB at the end of the D–Arotation is north–south (Fig. 9); in a reconstructionbased on the parallelism of the stretching lineat-ions in the MECs and the Variscides, the orientationof the batholith is, however, roughly NW–SE. Thiscan be explained by the difference in timingbetween the acquisitions of the magnetizations andthe magmatic fabric in the granitoids. The K–Arage of 282 + 10 Ma and 40Ar/39Ar age of 280 +12 Ma for biotite from the Porto complex (Maluski1976; Edel et al. 1981), marking the closure of theargon system at c. 300 8C, may correspond withthe acquisition of the A components. These agesare close to the 278 + 2 Ma U–Pb age obtainedfor zircon from the Asco rhyolite, which is a latemanifestation of U3 volcanism. Because this rhyo-lite was erupted at the surface, the zircon age maycorrespond with that of the A magnetization. Thesignificantly older U–Pb age 287.2 + 1.7 Ma ofzircon from the Evisa granite (Cocherie et al.2005) that cuts the Porto complex indicates thatthe magmatic fabric of the alkaline U3 granitoidsis at least 5 Ma older than the A magnetizations.
The available ages show that the D–A 808clockwise rotation occurred over the period 288–
J.-B. EDEL ET AL.
280 Ma, which corresponds to a rotation of about108 per Ma. This rate is very similar to the rateof the anticlockwise rotation of the CSB duringthe Tertiary convergence of Africa and Europe,which corresponds to a rotation of 35–308 from21.0–20.5 Ma to 18–17.5 Ma (Edel et al. 2000;Gattacceca et al. 2007). Based on high-resolutionU–Pb and 40Ar/39Ar dating of Early Permianplutons (Paquette et al. 2003; Gaggero et al. 2007;
Casini et al. 2012), the onset of D–A rotation ismostly coeval with intrusion of mantle-derivedsheeted bodies of gabbroic to quartz-dioritic mag-mas at relatively low pressures (0.1–0.25 GPa) cor-responding to shallow emplacement depths (Rennaet al. 2006; Gaggero et al. 2007; Casini et al.2012). The rotation and associated magmatism aretemporally associated with an important thermalevent in the mantle. The onset of the C–D rotation
Fig. 9. Palaeoposition of the Variscides during the Early Permian rotation of MECS according to the (1) structural and(2) palaeomagnetic data, the magnetizations being younger in magmatic rocks than the fabric. Main structural featuresare from Conti et al. (1999), Mattern (2001), Charles et al. (2009), Faure et al. (2009), Casini et al. (2012), Choulet et al.(2012) and Pitra et al. (2012). Legend as Figure 8, unless otherwise indicated. BA, Bas Argens basin.
P–TR ROTATIONS OF CORSICA–SARDINIA
coincides with the global change from north–southcompression to north–south extension and withthe large-scale ignimbritic and rhyolitic volcanicevent that affected the Variscides at 297–295 Ma.Such a large clockwise rotation can be suitablyexplained in the context of dextral wrenchingbetween Laurussia and Gondwana proposed byArthaud & Matte (1977) and Bard (1997). In sum-mary, the Late Carboniferous–Early Permian evol-ution of the southern branch of the Variscan belt ischaracterized by a succession of tectono-magmaticevents accompanied by massive emplacement ofgranitoids and large-scale block rotations associ-ated with dextral wrench-dominated movements.Similarly to the Cantabrian arc, the succession ofmagmatism reflects progressive upwelling of theasthenosphere during the Early Permian (Weilet al. 2013). This wrench deformation is regardedas a sequel to the dextral rotation of the northernbranch of the Variscan during 330–315 Ma whichterminated by frontal collision with Avalonia. Thecontinuation of movement in the southern Variscanrealm was due to shearing along the southern marginof the Avalonian block.
Triassic clockwise rotation of MECS
The 608 clockwise rotation deduced from thechange in declinations A–B occurred during theTriassic. The final orientation of MECS is illus-trated in Figure 1. Due to the uncertain age of theyoungest A components and the oldest B com-ponents, precise timing of the rotation requiresadditional radiometric dating. Given the actualinformation available, we propose that the rotationtook place c. 250–240 Ma, corresponding with theage of the Muschelkalk marine transgression. Thelast clockwise rotation is therefore a manifestationof the opening Tethyan basin.
Conclusions
The evidence presented in this paper indicates amajor dextral rotation of the southern branch ofthe Variscan belt during 300–280 Ma. These obser-vations are supported by a major switch in shorten-ing directions, which is recorded by fabrics inplutons and associated shear zones. The kinema-tic reversal is in agreement with Ziegler’s (1990)observation of a rotation of the principal axis ofhorizontal compressional from north–south toeast–west at the Stephanian–Autunian boundaryin present coordinates.
Dextral wrenching associated with clockwiserotations started at c. 330 Ma in the northern partof the Variscan belt until collision with thewestern margin of Avalonia at c. 315 Ma (Edel
et al. 2003, 2013). The rotation continued in thesouth at c. 300 Ma from the end of the syn-collisionnorth–south compression until the last emplace-ments of granitoids in MECS. This evolution isconsistent with the model based on structural geol-ogy proposed by Arthaud & Matte (1977). Anadditional dextral rotation, likely associated withthe onset of the Tethys, affected the MECS blockin the Triassic. The age of this later rotation hasto be constrained by new geochronological dataand by additional palaeomagnetic data on Triassicdykes and sediments.
This work was carried out under the terms of UMR 7516of CNRS and the University of Strasbourg and theLK11202 programme of the Ministry of Education of theCzech Republic awarded to KS. Bureau de RecherchesGeologiques et Minieres and University of Sassari con-tributed to the costs of the field trips. We express ourwarm thanks to C. Franke and M. Thiry for giving accessto their palaeomagnetic laboratory at the Ecole desMines de Paris after the retirement of JBE.
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