Geological Society, London, Special Publications

doi: 10.1144/SP405.7p249-264.

2014, v.405;Geological Society, London, Special Publications Jordi Carreras and Elena Druguet segmentFraming the tectonic regime of the NE Iberian Variscan

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Framing the tectonic regime of the NE Iberian Variscan segment

JORDI CARRERAS* & ELENA DRUGUET

Departament de Geologia, Universitat Autonoma de Barcelona, 08193 Bellaterra,

Barcelona, Spain

*Corresponding author (e-mail: jordi.carreras@uab.cat)

Abstract: Variscan massifs of NE Iberia occur along the Pyrenees, Catalan Coastal Ranges,Iberian Range and Minorca. Despite the effects of the Alpine cycle, which involve localizedreworking, tilting, translation of basement units and blocks drifting, the Variscan evolution canbe reconstructed. Geological features evidence significant differences across the zones in thenorth, with high-grade metamorphic rocks, structural domes and syntectonic plutons more internalthan the zones in the south, where unmetamorphosed and low-grade rocks are present along withundeformed late- to post-tectonic plutons. This setting contradicts existing schemas where this partof the Variscan belt is located in an external foreland. Variscan structures evidence bulk transpres-sion gradually evolving from a NNW–SSE-directed crustal shortening to NW–SE wrench-domi-nated tectonics. The low-pressure–high-temperature (LP/HT) metamorphic peak and magmatismcoincide with the latest stages of the NNW–SSE event. There is no field evidence based on pen-etrative structures for a widespread late orogenic extensional collapse.

In NE Iberia the Variscan basement forms scatteredoutcrops along three Alpine chains: the Pyrenees,the Catalan Coastal Ranges and the Iberian Chain.These three domains bound the Mesozoic–Ceno-zoic cover that fills the Ebro Basin (Fig. 1).

The setting of these massifs in the broaderVariscan belt context becomes problematic. Thisis because most of the existing schemas (e.g.Ziegler 1988; Matte 2001; Cavazza et al. 2004;Edel 2012) locate these massifs in an externalposition of the Variscan belt (Fig. 2) while the geo-logical features of most of these outcrops, especiallythose located on the NE side, do not correspondto an external position. An alternative setting pro-posed by Garcia-Sansegundo et al. (2011) consid-ers that the Variscan segment of the Pyrenees isdivided into two domains, a northerly located Varis-can hinterland (correlated with the West-Asturian–Leonese Zone, WALZ) and a southerly located fore-land (correlated with the Cantabrian Zone, CZ).This correlation is not, however, sustained by anyanalogy concerning the tectonic, plutonic and meta-morphic features of the compared domains.

The fact that the three referred domains corre-spond to Alpine chains leads to the question of theeffects of Alpine tectonics on Variscan structures.This effect is variable, roughly ranging from signifi-cant in some domains of the Pyrenees to slight orabsent along the Catalan Coastal Ranges. How-ever, the effect of the Alpine deformation is dis-tinguishable and does not impede discriminationbetween Variscan and Alpine features and the inter-pretation of the geological evolution of the Variscancycle (Carreras et al. 1997). In contrast, the dis-continuity between these scattered outcrops and

also between other present or presumably formerlyneighbouring areas (Iberian Massif, MontaigneNoire, French Central Massif, Sardinia, Maures)hampers correlation of this Variscan segment. Inaddition, post-Variscan evolution involves trans-lations along faults (e.g. the North Pyrenean faultand the rotation of the Corso–Sardo block) thathave modified the relative position of different out-cropping segments.

The aim of this contribution is to integrateinformation from different Variscan massifs in NEIberia into a model for the Variscan tectonic evol-ution of the NE Iberia segment and to frame thetectonic history of this segment within the contextof the wider Variscan belt.

Restoring the effects of post-Variscan events

A first step for any interpretation of the Variscanmassifs is the complicated reconstruction of theVariscan basement in Triassic time through restor-ation of the effects of Alpine tectonics. ConcerningNE Iberia, three main restoration steps associatedwith crustal block motions are involved: (1) rota-tion of the Corso-Sardo block relative to the sta-ble Western European plate during the Neogenerifting event; (2) collision of Iberia with the Eura-sian plate during the Palaeogene; and (3) rotationof Iberia with respect to the stable Eurasian plate,involving a sinistral translation along the North Pyr-enean Fault (NPF) during Cretaceous time (Fig. 3).These three main post-Variscan tectonic events andtheir restoration are discussed separately in thefollowing sections.

From: Schulmann, K., Martınez Catalan, J. R., Lardeaux, J. M., Janousek, V. & Oggiano, G. (eds) 2014.The Variscan Orogeny: Extent, Timescale and the Formation of the European Crust. Geological Society, London,Special Publications, 405, 249–264. First published online February 25, 2014, http://dx.doi.org/10.1144/SP405.7# The Geological Society of London 2014. Publishing disclaimer: www.geolsoc.org.uk/pub_ethics

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Neogene extensional tectonics

The most recent event to be restored is the Neogenerifting of Western Europe, involving an anticlock-wise rotation and a SE drift of Corsica and Sardinia(Westphal et al. 1973; Cohen 1980; Olivet et al.1996; Speranza et al. 2002; Gattacceca et al.2007; Oudet et al. 2010; Turco et al. 2012), whichbehaved as a rigid microplate (Alvarez 1972)called the Corso–Sardo block. This rifting alsocaused the separation of the Balearic Islands fromthe European–Iberian continental margin, formingindependent small blocks (Pares et al. 1992).

This Neogene extensional tectonics of WesternEurope is also responsible for the segmentationof Variscan domains in NE Iberia. The NE–SWelongate Valles–Penedes and Cerdanya grabenparticipated in this segmentation in the CatalanCoastal Ranges and Eastern Pyrenean Axial Zone,respectively (Fig. 1).

Palaeogene collision of the Iberian

and Eurasian plates

The Pyrenees, the Catalan Coastal Ranges and theIberian Chain formed during the Palaeogene as aresult of convergence between the European plateand the Iberian micro-plate (Pyrenees) and intra-plate deformations (Catalan Coastal Ranges andIberian Chain). The Variscan basement and a Meso-zoic–Palaeogene cover are involved in the threechains. The Variscan basement is significantlyaffected by these Alpine deformations in the Pyre-nees, although extensive reworking can be dis-carded. The main effects of Alpine compressionaltectonics consist of involvement of Variscan base-ment in thrust sheets, as observed along the SouthPyrenean Zone (Verges et al. 1992), and tilting ofthe original structures due to antiformal stackpiling. The effects of Alpine tectonics on the

Fig. 1. Map of NE Iberia and south France showing Variscan massifs and other Variscan basement outcrops (depictedin dark colour). NPF, North Pyrenean Fault which separates Iberia and the European plate; VP, Valles–PenedesNeogene graben; C, Cerdanya Neogene graben.

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Pyrenean Axial Zone remain a matter of debatebecause of the difficulty of identifying the tracesof thrusts over the Variscan structures. Some inter-pretations, drawn on the basis of seismic data andsections restoration (ECORS Pyrenees Team 1988;Roure et al. 1989; Munoz 1992), consider the exist-ence of Alpine thrusts involving large displace-ments along the length of the Pyrenean AxialZone. Some of these major thrusts are also drawnalong major mylonite belts (Soula et al. 1986a, b)that to other authors are Variscan in age (Carreras2001; Denele et al. 2008). Arguments against therestricted effects of Alpine tectonics along theVariscan of the Pyrenean Axial Zone point to thespeculative interpretations of these Alpine AxialZone thrusts. The Merens Fault is a major Variscanmylonite zone (Zwart 1958; Saillant 1982; Deneleet al. 2008; Mezger et al. 2012), developed acrosscrystalline basement rocks (mainly orthogneisses),that vanishes westwards when entering the low-grade rocks of the Pallaresa dome (Carreras &Cires 1986, fig. 6). This structure has been con-sidered by some authors (e.g. Williams & Fischer1984; Casas et al. 1989, 2012) as a major Alpinestructure extending along the Pyrenees. There isno evidence for major Alpine thrusting along otherlithological boundaries such as the southern limbof the Llavorsı Syncline (Casas & Poblet 1989).The untouched character of most of the Variscangranitoid batholiths (Zwart 1979; Gleizes et al.1997) strengthens the argument that Alpine thrustsare absent from vast areas of the Pyrenean Axial

Zone. Although we exclude a large Alpine rework-ing, the regional Alpine crustal shortening has,however, modified the relative position of Europawith respect to Iberia. Pre-Alpine restorationimplies moving these blocks away from each other.Some geologists evaluate crustal shortening-relateddisplacement in the range of 130–150 km (Roure &Choukroune 1998; Beaumont et al. 2000). Theseamounts, based on cross-section balancing, mightbe overestimated due to the overestimation of theeffect of Alpine thrusting on the Axial Zone. Insummary, the expected effects of the Alpine com-pression on the Variscan of the Pyrenees wouldbe: (1) a horizontal translation (and related rota-tions) of basement rock units with no widespreadpenetrative deformations; (2) tilting of the Variscanunits, especially noticeable along the southernpart of the Pyrenean Axial Zone; and (3) faultingwith development of brittle to brittle–ductilefault rocks. On the Catalan Coastal Ranges andthe southeastern part of the Iberian Chain, theAlpine overprint is very reduced except for localtilting and the development of some faults. Weconclude that the Variscan structures along the NEIberian segment are preserved over large areas.The Variscan stratigraphic, structural, metamorphicand magmatic features can therefore be identified.

Mesozoic rotation of Iberia

During Mesozoic time and prior to the Iberia–Europe collision, Iberia rotated anticlockwise and

Fig. 2. Two interpretations of the configuration of the Variscan belt of Western Europe: (a) after Matte (2001); and(b) after Edel (2012). Note the external location (square) of NE Iberian Variscan outcrops in both interpretations.

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drifted eastwards while the Bay of Biscay formed.This movement was localized along the North Pyr-enean fault and caused the separation of Galicia inIberia from Brittany in Europe. This displacementimplies that, during Triassic time, the North Pyre-nean massifs and the Pyrenean Axial Zone laidlaterally apart. Proposed restorations differ mainlyon the amount of sinistral displacement along theNorth Pyrenean fault, ranging from ,100 km upto 400 km (Matte 1986; Olivet et al. 1996; Lefort& Agarwal 1999; Gong et al. 2008; Vissers &Meijer 2012a, b), but also differ on the amount ofrotation from 208 up to 408 and on the initial separ-ation (e.g. Vissers & Meijer 2012a, b). Modelsemphasizing rotation over translation imply largeramounts of later crustal shortening to close theoceanic gap. On the basis of correlation of structuresbetween the Axial Zone and North Pyreneanmassifs, a sinistral displacement ranging between150 and 200 km seems reasonable (Carreras &Cires 1986). Models including extensive oceaniccrust creation between Iberia and Europe (e.g.Srivastava & Verhoef 1992) are less consistentwith the geological features of the Pyrenees, includ-ing the absence of a large oceanic suture.

A tentative restoration is depicted in Figure 3,which includes the effects on Neogene, Alpineand eo-Alpine tectonics based on mean values ofinvolved crustal displacement for each event.

Geology of the Variscan basement

in NE Iberia

Pre-Variscan rocks

In the NE Iberian terrains, the Stephano-Permianand/or Permo-Triassic sequences overlay uncon-formably (Variscan unconformity) the deformedNeoproterozoic and Palaeozoic pre-orogenic se-quences. Classically, these pre-Variscan serieswere divided into two units. The Upper Unitcomprises the first chronostratigraphically well-characterized rocks (Upper Ordovician) to thelower Stephanian (Colmenero et al. 2002). TheLower Unit has been referred to as Cambro-Ordovician (Guitard 1970; Zwart 1986), but recentdating by Castineiras et al. (2008) of porphyriesemplaced into this lower series indicates that theoldest rocks in the Eastern Pyrenees are Ediacaran

Fig. 3. Probable Triassic positions of Variscan massifs of SW Europe and main structural trends. Based on geologicalmaps of BRGM (France), IGME (Iberia) and Servizio Geologico d’Italia.

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(late Neoproterozoic). Upper and lower series areseparated by the Caradoc unconformity (Santanach1972; Casas & Fernandez 2007). A lower uncon-formity between the Cambrian sequences andthe Neoproterozoic sequences (Fig. 4) has beenreported in the Eastern Pyrenees (Cap de Creus)by Carreras & Druguet (2013).

A typical sequence cannot be presentedbecause of significant changes across the Variscanbasement. In general, most complete sequencescrop out on the NE Pyrenees while rocks croppingout towards the west and south mainly belong tothe Upper series (with the exception of outcropson the Iberian Chain, which mainly consist ofLower Palaeozoic sequences; Linan et al. 2002). Acharacteristic feature of these sequences is the pres-ence of interlayered igneous pre-Variscan rocks,both intrusive and extrusive and ranging frombasic to acid compositions (Navidad & Carreras1995; Liesa et al. 2010; Navidad et al. 2010; Casti-neiras et al. 2011; Martınez et al. 2011). Two mag-matic episodes have been recognized: a morerestricted Cadomian event and a widespread UpperOrdovician event (Castineiras et al. 2008). Thelatest event is responsible for the emplacement oflarge sheets of granitoid which were transformedinto gneisses during the Variscan Orogeny. Thesegranitoids had been considered as Cadomian base-ment rocks by Guitard (1970); this interpretation hasbeen discarded in favour of the previous widely

accepted hypothesis that the gneissic units werederived from Ordovician granitoid intrusions(Zwart 1965).

A sketch of the representative sedimentarysequences and interlayered pre-Variscan intrusivescropping out on the Eastern Pyrenees massifs is pre-sented in Figure 4.

This sequence differs substantially from thoseobserved in Minorque (Colmenero et al. 2002) andthe southern Catalan Coastal Ranges (Anadonet al. 1983; Saez & Anadon 1989; Maestro-Maideuet al. 1998), where Carboniferous Culm sequencesdominate, and from those of the Iberian Chain,where well-characterized Cambrian sequences cropout (Linan et al. 2002).

Variscan tectono-plutono-metamorphic

evolution

The NE Iberia segment of the Variscan belt showsmanifest differences between the northern andsouthern domains. While the northern part consistsof a tectono-plutono-metamorphic belt, deformationand metamorphism are less intense in the southerndomain although granitoids may be present. Thecharacteristics described here therefore correspondmainly to the Pyrenean domain and northern partof the Catalan Coastal Ranges. Here, the Variscanevent is characterized by polyphase tectonics invol-ving the widespread development of penetrative

Fig. 4. Layout of the pre-Variscan series and inter-layered igneous rocks. This is a comprehensive sketch broadlycorresponding to the Central and Eastern Pyrenees where series are more complete. U1, Cambrian unconformity;U2, Upper Ordovician unconformity; U3, Post-Variscan non-conformity. 1, Ediacaran granitoids and porphyres;2, Ediacaran(?) basic subvolcanic and volcanic rocks; 3a, Ordovician granitoids and porphyres; 3b, Ordovicianacidic volcanics.

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foliations. Progressive deformation is associatedwith polyphase metamorphism and syn- to late-kinematic magmatic activity.

Deformation. Variscan polyphase tectonics displaysdiverse structural styles which correspond to spatialand time zonations (Carreras & Capella 1994).Early tectonic phases are responsible for thrustingin shallow levels and low-dipping penetrative foli-ations in deeper levels. The main Variscan tectonicphase (namely D2, Gleizes et al. 1998; Druguet2001) corresponds to a transpressional event thatis characterized by folding and development ofpenetrative foliations (S2) with largely variableattitudes. At deeper tectonic levels, D2 fabrics rangefrom crenulation (Fig. 5a) to transposition foli-ations and were developed close to coeval with thethermal peak of metamorphism and granitoidemplacement. At shallow levels, axial planar slatycleavages or penetrative crenulation foliations(Fig. 5b) are the most representative structuresassociated with the main D2 phase. Late tectonicphases (mainly D3) progressively developed underretrograde metamorphic conditions, giving rise tolocalized strike-slip ductile shear zones in highlycrystalline rocks (granitoids, orthogneisses orhigh-grade schists; Fig. 5c) and to folds, crenulationfoliations and kink-folding in lower-grade meta-sedimentary rocks (Fig. 5d), compatible with a con-tinuing transpressional deformation (Carreras &Capella 1994; Carreras 2001).

The resulting structural pattern consists of south-verging structures describing a large arc in mapview (see Fig. 3) and including open antiformscored by high-grade rocks or gneisses (the largegneiss domes of the Pyrenean Axial Zone) andpinched synforms of non-metamorphic to low-gradePalaeozoic sequences. This domal structure, bestdeveloped in the Central and Eastern Pyrenees andthe North Pyrenean massifs (Fig. 6), likely reflectsthe superimposition of the D2 and D3 foldingevents. A broad spatial structural zonation can bedrawn from these Pyrenean massifs towards thewest and south (Western Pyrenees, South CatalanCoastal Ranges and Iberian Chain) where slate-beltand fold–thrust-belt tectonics prevailed (e.g. Juli-vert & Duran 1990).

Metamorphism. The Variscan thermal regime in thePyrenees is a classic example of low-pressure–high-temperature (LP/HT) metamorphism. Mineralassemblages developed around the metamorphicpeak, coeval with the main D2 tectonic phase,reflecting this LP/HT prograde metamorphism.However, medium-pressure conditions might havebeen reached at both early and late orogenicstages. At early stages this is indicated by the spora-dic presence of relics of staurolite in cordierite or

andalusite (Sebastian et al. 1990; Guitard et al.1996; Druguet 2001), garnet + staurolite (Mezger2005) or kyanite + staurolite (Azambre & Guitard2001). At the latest phases this is evidenced bykyanite pseudomorphs of andalusite localizedalong some pinched synforms (Autran et al. 1970).

A widespread feature in the Pyrenees is thedistribution of prograde metamorphic isogrades,where large areas of low- to very-low-grade metase-diments surround amphibolite to granulite faciesmetamorphic and migmatite cores. Most of thesezones are concentric and dome-shaped, thus cal-led thermal domes. A high thermal gradient ofbetween 65 and 85 8C km21 is inferred fromregional studies for the Variscan massifs in the Pyr-enees between the beginning of the amphibolitefacies and the beginning of anatexis (Zwart 1979;Guitard et al. 1996). For the Cap de Creus massif,Druguet (2001) describes the interplay betweencoeval transpressional deformation, metamorphismand magmatism as the main cause for such astrong metamorphic gradient, accentuated by thelate D3 folding and shearing event.

Some of the domes have an orthogneissiccore, such as the Aston, St Barthelemy, Canigo,Roc de Frausa and Albera massifs, which mighthave acted as channels inducing and controllingthe metamorphic gradient (Fonteilles & Guitard1968, 1977). However, other massifs are cored bymigmatitic schists, such as in the Trois Seigneursand Cap de Creus massifs. In these cases, the mig-matite core is usually associated with small funnel-shaped intrusions of mantle-derived magmas(Druguet 2001).

The Variscan metamorphism in the western andsouthern areas of NE Iberia are restricted to narrowaureoles of contact metamorphism around plutonsintruded into Upper Palaeozoic rocks (Sebastianet al. 1990).

Magmatism. Batholiths and stocks are widespreadall along the NE Iberia Variscan segment. Calc-alkaline granite to granodiorite compositions pre-vail in most plutons that are sheet-shaped andemplaced into intermediate and shallow levels(Fig. 5e). The composition of some intrusionsextends from gabbros to leucogranites, gabbros,diorites and tonalites being the earliest and deepest-seated intrusions while leucogranites are the young-est and occupy the apical parts of the intrusions.In medium- to high-grade metamorphic areas, mig-matites and associated granitoids form subverticalpipe or funnel-shaped domains (usually complexlydeformed), commonly referred to as migmatitecomplexes. There, swarms of anatectic peralumi-nous leucogranites and pegmatites abound (referredto as perianatectic leucogranites by Autran et al.1970). They are different from the leucogranitic

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Fig. 5. Structures from Variscan rocks of NE Iberia. (a) D2 folds with associated S2 crenulation cleavage affectingsillimanite schists. [UTM 524979, 4685646] Puig Culip, Cap de Creus. Width of view: c. 40 cm. (b) Tight foldsaffecting Upper Ordovician–Lower Silurian sequences of slates and quartzites. The regional foliation consists of a S2

slaty cleavage parallel to the axial planes of folds in layering (Ss/S1). [UTM 423307, 4584586] Collserola, CatalanCoastal Ranges. Width of view: c. 5 m. (c) Dextral D3 shear zone affecting high-grade schists and pegmatites. [UTM525311, 4686005] Cullero, Cap de Creus. Width of view: c. 5 m. (d) Late kink folds affecting low-grade metasedimentsbearing a S2 – 3 foliation. [UTM 354197, 4716656] Pui Tabaca, Pallaresa Dome, Pyrenean axial zone. Width of view:c. 40 cm. (e) Intrusive contacts induced by magma stopping of granodiorites (Grd) in hornfelses (Hf), followed by a laterintrusion of leucogranites (Lgr) into both. [UTM 517178, 4639609] Cala Pedrosa–Costa Brava, Catalan CoastalRanges. Width of view: c. 15 m. (f) Ptygmatically folded peri-anatectic pegmatites, syntectonically emplaced during theD2 transpressional deformation episode. [UTM 52534, 4685961] Sa Llambrusquera, Cap de Creus. Width of view:c. 35 cm. All coordinates correspond to UTM ETRS 89 system.

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Fig. 6. Sketch showing main large structures along the Central and Eastern Pyrenean Axial Zone. Structural domes (whether cored by gneisses or by metasediments) result as aninterference pattern of D2 and D3 structures. Wide domes contrast with pinched synforms. Sheet-shaped plutons are located on relatively shallow levels and are affected by D3 event.

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differentiates, which are characteristic of the apicalparts of the large plutons.

Field and anisotropy of magnetic susceptibility(AMS) studies of granitoid plutons in the Pyreneanand North Pyrenean domains have demonstratedthat their emplacement was coeval with the mainVariscan D2 transpressive event (Leblanc et al.1996; Evans et al. 1997; Gleizes et al. 1997;Aurejac et al. 2004; Carreras et al. 2004; Vilaet al. 2005). Migmatites and crustal anatecticgranitoids (Fig. 5f) are also syntectonic in a trans-pressional environment (Druguet & Hutton 1998;Druguet 2001).

The structural zonation that we have describedfrom the more internal (NE) to the external (westand SW) zones can be correlated well with themain metamorphic and magmatic trends. The degreeto which plutonic rocks are deformed increases fromsouth to north. The plutons from the central andeastern Pyrenean Axial Zone and from the NorthPyrenean massifs are ductilely deformed by vari-ous amounts depending on the emplacement level.In contrast, plutons located along the CatalanCoastal Ranges correspond to shallow intrusionswith little or no apparent sign of deformation.

Up-to-date chronological studies on magmaticrocks have not assisted greatly in constraining thetiming of the Variscan event in NE Iberia. Magma-tism apparently extends in age over 50 Ma. Theoldest rocks correspond to deformed granitoidsfrom the Eastern and North Pyrenean zones (317 +3 Ma for granites from the Aglı massif, Olivieret al. 2004), while the youngest correspond toPermo-Stephanian late-orogenic calc-alkaline vol-canism in the Pyrenees (Cabanis & Le Fur-Balouet1989; Colmenero et al. 2002) and to Permian unde-formed granitoids from the Catalan Coastal Ranges(288 Ma for the Costa Brava granitoids, Ferres1998; 291–285 Ma for gabbroic rocks and grani-toids from the Montnegre massif; Sole et al. 2002)and from the Western Pyrenees (267 Ma for theAya pluton; Denele et al. 2012).

Model for NE Iberia

Despite the reduced segment of the Variscan beltcropping out on NE Iberia and the discontinuity ofcropping out massifs, a general geological zonationcan be proposed (Fig. 7) with Internal zones in theNE and External zones (foreland) towards thesouth. An intermediate zone formed by shallow low-grade rocks but abundant granitoids can be distin-guished. While the massifs located in the IberianChain bear analogies with the WALZ, the Variscanmassifs of the Southern Catalan Coastal Rangeswhere magmatic rocks are abundant cannot becorrelated with the WALZ or CZ. It is unlikely

that such a NE–SW zonation was produced by aNE-directed increase in post-Variscan exhumationand erosion. The present Moho structure of theeastern part of NE Iberia derived from seismic pro-files (Rivero et al. 2002) shows similar Mohodepths for the Eastern Pyrenees and the CatalanCoastal Ranges.

Structural analysis of Variscan structures inthe Pyrenees reveals a polyphasic tectonics with atime-clockwise rotation of the main structuraltrends (Carreras & Capella 1994). Older structures(e.g. Mey 1968) have a NNE–SSW trend, whilethe trend of the structures corresponding to themain Variscan event is WSW–ENE and that ofthe late structures is NW–SE (Figs 7 & 8). Themain deformation, here referred to as D2, roughlycoincides with the metamorphic peak, migmatiza-tion and granitoid emplacement (Druguet 2001;Vila et al. 2007), while late deformation (referredto as D3) is retrogressive and associated withfolding in low- to medium-grade rocks and ductileshearing in crystalline rocks (medium- to high-grade schists, orthogneisses and granitoids; Carreraset al. 1980). Changes in structural style occur acrossthis segment of the belt but also between shallow-and deep-seated structural levels (infrastructureand suprastructure, De Sitter & Zwart 1960; Oele1966; Carreras & Capella 1994). Superposition ofD2 and D3 structures in infrastructural domains isresponsible for the development of dome-shapedstructures. These can have orthogneissic (e.g.Aston–Hospitalet, Canigo–Caranca), metasedi-mentary (Bossost, Pallaresa) cores or both (Albera,see Fig. 6). It is worth emphasizing that the dis-tinction between infrastructure and suprastructureis far more complex as initially postulated by DeSitter & Zwart (1960). Infrastructural levels, ordeep-seated domains with medium- to high-graderocks, are not always associated with a flat-lyingattitude of the dominant foliation since S2 mayvary from gently dipping to vertical. In addition,since D2 is highly heterogeneous across and alongthe belt, the S2 foliation dominates in some high-strain domains whereas it is absent or appears as aweak crenulation in others. Such a heterogeneity isalso the case for the D3 mylonite belts which are ubi-quitous in the internal domains and absent from theexternal domains where rocks accommodate D3 byfolding. Another important feature of this segmentof the belt is the variability in tectonic complexity,with complex polyphase structures in the internaldomains and rather simple structures in the mostexternal domains. The emplacement of granitoidsalso differs across the belt with a widespreadpresence of forceful syntectonic plutons in the inter-nal parts, contrasting with the rather post-tectonicplutons along with associated magma-stoppingstructures in the intermediate and external domains.

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Fig. 7. Geological zonation of the NE Iberia Variscan segment. Zones are traced on the basis of similarities on tectonometamorphic evolution but their arrangement is stronglycontrolled by the D3 deformation event. Interference of D2 and D3 trends causes difficulties in delineating a zonation where characteristics are maintained along the zone.

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The obliquity between the trend of the main D2

tectonic phase and the D3 late folding and shearing,both phases involving crustal shortening, is at theorigin of the development of the dome-shaped struc-tures. No widespread extensional structures havebeen recognized; the interpretations according towhich a late extensional collapse would accountfor the formation of domes (Verhoef et al. 1984;Eeckhout 1986; Vissers 1992) are therefore not sup-ported by field structural evidence. In conclusion,our model (Figs 7 & 8) attempts to integrate thefollowing points: (1) a geological zonation inwhich the external zones are located towardsthe SW parts of the belt and the more internaldomains towards the NE; (2) a clockwise rotationwith time of the main structural trends explainingthe formation of the dome-shaped antiformal struc-tures; (3) a progressive change in tectonic regimefrom a crustal shortening transpression during D2

to a wrench-dominated transpression during D3;and (4) in the internal zones, granitoid emplacementis chiefly synkinematic with the main deformation(D2) and metamorphic peak, contrasting with thepost-tectonic character of most granitoids emplacedin the intermediate zones.

Placing NE Iberia in a wider framework

The published schemas on the zoning of the Varis-cides highlight the problematic setting of NEIberian zones usually placed in foreland positionsand correlated with the WALZ or CZ of theIberian massifs. Similar problem apply to other for-merly nearby massifs (Maures, Corsica–Sardinia).This problem concerns the fact that the final geo-logical features are the sum of different evolutionstages, each with a particular lithostratigraphicarrangement and structural trend. The widespreadpresence of orthogneisses is a consequence of abun-dant pre-Variscan magmatism along belts that arepresumably not coincident with the later Variscantectonometamorphic belts. The lithostratigraphiczonation is also highly variable across this part ofthe belt. However, most complexity derives fromthe structural features arising from changing tec-tonic regimes and trends. This causes a cross-hatchof structures where two dominant trends interfere,especially on the eastern part of the Variscansegment of the Pyrenees. It is proposed that aWSW–ENE trend was dominant during the maintectonic deformation event (D2) which was in turn

Fig. 8. Sketch showing the main features of the Variscan tectonic and metamorphic evolution and associatedmagmatism.

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associated with the onset of magmatism in thenorthern domains. Under progressive deformation,tectonics evolved towards a wrench-dominatedtranspression generating NW–SE-trending struc-tures (D3). High-grade LP/HT metamorphismcoinciding with the culmination of granitoid empla-cement in the internal zone occurred during a trans-pressional regime (D2) progressively evolving toa wrench-dominated regime. These events arealso contemporaneous with exhumation. In thissegment of the Variscan belt there is no evidencefor a widespread crustal extension related to oro-genic collapse. Local horizontal extension doesnot necessarily imply a bulk regional extensionalregime; it may occur in a regional wrench regimealong with local horizontal shortening. In addition,

although graben formed during Stephano–Permiantime, the associated local extension is limited andcan be related to pull-apart structures (Speksnijder1985) and by no means is associated with thedevelopment of penetrative flat-lying foliations assuggested by Verhoef et al. (1984) and Eeckhout(1986). All observed penetrative and ductile struc-tures are compatible with crustal shortening undertranspression and/or wrench regimes.

Considering the geological evolution of thesereduced and scattered Variscan outcrops of NEIberia in a broader framework, it appears thatearlier crustal shortening can be related to theoblique convergence of Laurussia and Gondwanawhile the later wrench-dominated zones can bethe result of strain partitioning at the belt scale

Fig. 9. Attempt to reconstruct the structural trends of the Variscan belt. (a) Model that emphasizes the structuralcomplexity of the belt caused by a gradual change in tectonic regime from compressional to transpressional and finallyto a wrench-dominated regime. Bulk dextral transpression causes the strain localization along dominant dextralwrench-dominated domains (synthetic) with minor antithetic shear belts (not drawn in the figure). White arrows:vergence of dominant structures. Green arrows: late orogenic shear belts. (b) Interpretative sketch where the mainstructural features of the belt are the result of oblique convergence between Laurussia and Gondwana. The black arrowsindicate displacement vectors while the grey arrows indicate components of crustal shortening and wrenching. This bulkdextral transpression is also responsible for both the dextral shear zones and the antithetic folding of the belt (arcs ororoclines) in a similar way as minor structures develop in shear zones.

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(Fig. 9). Late stages of Variscan evolution produceda crustal-scale strain partitioning with coexistingdomains of wrench-dominated transpression withprevalent dextral shearing with domains ofcompression-dominated transpression. Shearingoccurred along relatively well-defined shear zones(e.g. South Armorican Shear Zone), but also alongwider domains (e.g. Eastern Pyrenees). Antitheticshears also formed, some of them exhibiting acomplex kinematics with reversing-shear sense asthe example of the North Armorican Shear Zone(Derez et al. 2013) or the east–west-trendingshear zones in Cap de Creus (Carreras 2001).

The arcuate shape of the belt could be due tocrustal-scale strain partitioning (Fig. 9b) – in a kine-matically similar way as occurs in shear zones atdifferent scales (e.g. Harris 2003; Carreras et al.2005) – a model which is compatible with pub-lished models of continental-scale oroclines forthe Variscan of Iberia (Martınez Catalan 2011;Shaw et al. 2012). The strain partitioning at a crustalscale also leads to the accentuation and closure ofarcs in some parts of the belt (e.g. Central Iberianarc). In some domains, WSW–ENE structuresbroadly parallel to the trend of the belt are interferedby later NW–SE shearing, causing the develop-ment of a high structural complexity. This complex-ity is accentuated by the presumably irregularshapes of the stable plate boundaries of Gondwanaand Laurussia and by the presence of rigid blocksof irregular shape inside the belt.

In the tentative proposal of Figure 9b, the dextralshear zones of NE Iberia bear similarities with otherdomains across the Variscan belt. This schema isadditionally complicated by the closeness to thetransition from a continental collisional domaintowards an eastern subduction zone along thePalaeotethys active margin.

This work was funded by the Spanish Ministry of Edu-cation and Science, project CGL2010–21751. We aregrateful to K. Schulmann for editorial tasks and toD. Gapais, J. L. Bouchez and Y. Denele for providing con-structive reviews that have improved the manuscript.

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