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Distribution of linear mesostructures in oblique folded surfaces: Unravellingsuperposed Ordovician and Variscan folds in the Pyrenees

Josep Maria Casas*, Pilar Queralt, Joana Mencos, Oscar GratacósDepartament de Geodinàmica i Geofísica, Universitat de Barcelona, Institut de recerca GEOMODELS, Martí Franquès s/n, 08028 Barcelona, Spain

a r t i c l e i n f o

Article history:Received 27 January 2012Received in revised form16 July 2012Accepted 23 August 2012Available online 1 September 2012

Keywords:Ordovician foldsPyreneesSuperposed foldingVariscan mesostructures

a b s t r a c t

Detailed mapping and structural analysis yielded new data on the geometry of Ordovician folds in thePyrenees. These folds caused the Variscan mainphase folds to develop over non-horizontal beddingsurfaces in the pre-Upper Ordovician rocks. As a result, the Variscan mainphase fold axes and inter-section lineations in these rocks present a wide range of orientations. Bedding surfaces exhibit a complexpattern in map view making it necessary to reconstruct the structure in 3D. Computer aided recon-struction tools such as FoldPlot proved useful for visualizing and reconstructing the geometry of suchrefolded structures in 3D. In the absence of map-scale key horizons, evidence of superposed folds isprovided by detailed mapping of mesostructures. This requires a good and continuous outcrop over anextended area since one single fold system can account for a complex distribution of the subsequentintersection lineation. As a result, a cartographic study and a structural analysis are warranted becausethe direction of the Ordovician folds cannot be established only by the orientation of the L2mesostructures.

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1. Introduction

Superposed folds caused by polyphase deformation or succes-sive deformational events are common in many deformed areas. Insuch areas, fold interference gives rise to 3D refold structures andoutcrops with 2D complex patterns that are easily interpretablewhen stratigraphic or structural markers are deformed (Ramsay,1962). When monotonous successions are involved, it is not easyto elucidate the number and geometry of superposed fold systems.This is the case of the pre-Upper Ordovician sequence, which cropsout extensively along the backbone of the Pyrenees (Fig. 1). ThePyrenees is an Alpine range formed by the collision between Iberiaand Europe from the late Cretaceous to the Miocene (Muñoz,1992a). This range includes a Variscan basement formed by lateNeoproterozoic to Carboniferous sedimentary and igneous rocksdeformed by the Variscan orogeny during Carboniferous times(Guitard, 1970; Zwart, 1979). The pre-Upper Ordovician sequenceunder study is mainly constituted by a monotonous rhythmicalternation of thin layers of sandstones and argillites affected by

a polyphase deformation. As in the rest of the pre-Variscansequence, it is assumed that (1) deformation mesostructuresrecognized in these rocks resulted mainly from Variscan deforma-tion and that (2) the Alpine deformation did not lead to a consid-erable penetrative deformation (Muñoz, 1992a,b).

Recently, the presence of pre-Variscan folds, Mid Ordovician inage, has been proposed in the Pyrenees (Casas, 2010). These foldsaffect the pre-Upper Ordovician metasediments and are inter-preted to be fossilized by an angular unconformity of UpperOrdovician age. In this paper, we focus on 1) the geometry andorientation of the Ordovician folds, 2) their influence on theorientation of Variscan mesostructures and 3) the interferencerefold structures resulting from the superposition of the Variscanfolds. Datawere collected in LaMolina, on the southern slope of theCanigó massif, by detailed mapping and classic structural analysis.In order to construct accurate sections and adopt a better approachto the geometry of the superposed structures, data were processedwith 3D software. 3D interference refold structures were recon-structed using the package FoldPlot (Moore and Johnson, 2001).Our work seeks to provide new insight into superposed foldingevents in zones where no map-scale markers are available. More-over, the results can help us to understand the orientation of themain deformational mesostructures on the pre-Upper Ordovicianlow-grade metamorphic rocks in other areas of the Pyrenees.

* Corresponding author. Tel.: þ34 93 402 13 88; fax: þ34 93 402 13 40.E-mail addresses: casas@ub.edu (J.M. Casas), pilar.queralt@ub.edu (P. Queralt),

jmencos@ub.edu (J. Mencos), ogratacos@ub.edu (O. Gratacós).

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2. Geological setting

2.1. Stratigraphic setting

As a result of exhumation due to the Alpine tectonics, rocksranging in age from the late Neoproterozoic to the pre-VariscanCarboniferous crop out extensively in the central part of the Pyr-enees (Fig. 1). The Canigó massif, in the Eastern Pyrenees, exhibitsone of the most complete pre-Variscan successions. In this massifthe pre-Upper Ordovician sequence constitutes a thick (3000 m)succession of metasedimentary rocks that has classically beendivided into two parts (Fig. 2). Its lower part (“Série de Canaveilles”,Cavet, 1957; Canaveilles Group, Laumonier, 1988) is a 1500 m thick,azoic, lithologically heterogeneous sequence made up of meta-pelites and metagreywackes interbedded with numerous layers ofmarbles, calcesilicates and laminar orthogneissic bodies derivedfrom Ordovician granites (Cocherie et al., 2005; Castiñeiras et al.,2008). It includes syn-sedimentary bimodal metavolcanic rocks(Ayora and Casas, 1986; Navidad and Carreras, 2002) that haveyielded late Neoproterozoiceearliest Cambrian ages for this lowersuccession (SHRIMP UePb in zircon; 581 � 10 Ma, Cocherie et al.,2005; w540 Ma, Castiñeiras et al., 2008) (Fig. 2). In contrast, theupper part is a rather monotonous, 1500 m thick, succession ofrhythmic alternation of sandstones, siltstones and argillites(“schistes de Jujols”, Cavet, 1957; Jujols Group, Laumonier, 1988).Layers vary in thickness from 1 mm to several cm and range incolour from grey to light green or light brown. Quartzitic sandstonelayers up to 1 m in thickness may be present especially at the top ofthe series. Recent data based on Acritarchs indicate that theuppermost part of this series under the Upper Ordovician rocks hasa late Cambrian (Furongian)eearly Ordovician (Tremadocian) age(Casas and Palacios, 2012).

A well-dated Upper Ordovician succession (Cavet, 1957;Hartevelt, 1970) lies unconformably over the former series(Santanach, 1972a; Laumonier and Guitard, 1986; García-Sansegundo and Alonso, 1989; Den Brok, 1989; Kriegsman et al.,1989; Poblet, 1991; García-Sansegundo et al., 2004; Casas andFernandez, 2007) (Fig. 2). Although it is difficult to evaluate thehiatus represented by this unconformity, it may be assumed thatconsiderable erosion took place before the Upper Ordoviciandeposition given that the middle Ordovician rocks are lacking and

the Upper Ordovician rocks overlie different terms of the pre-UpperOrdovician succession, from late Neoproterozoic to early Ordovi-cian, in the Central and Eastern Pyrenees (Santanach, 1972a;Laumonier and Guitard, 1986; Cirés et al., 1994; Vergés et al., 1994).The Upper Ordovician rocks constitute a fining upwards sequencewith marked thickness variations between 100 and 1000 m inwhich Hartevelt (1970) defined five main siliciclastic formations(Fig. 2). In the study area, Upper Ordovician rocks are representedby the Rabassa conglomerates and by the Cava sandstones andshales (Figs. 3 and 4). The Estana limestones are absent and theAnsovell shales and Bar quartzite crop out further South. TheRabassa Conglomerate Formation is made up of red-purple,unfossiliferous conglomerates and microconglomerates that formdiscontinuous lenses that range in thickness from a few to 200 m.Clasts of slates, quartzites and quartz veins are subrounded to wellrounded and can attain 50 cm in diameter. Hartevelt (1970)attributed the Rabassa conglomerates to Sandbian. The Rabassaconglomerates are conformably overlain by the Cava Formationthat ranges in thickness from 100 to 800 m. Microconglomeratesand feldspatic sandstones predominate in the lower part, overlainby shales, siltstones and fine-grained sandstones, typically green orpurple in colour in the upper part. Brachiopods and bryozoans arelocally abundant. Gil-Peña et al. (2004) assigned a Katian age to thisformation.

2.2. Tectonic setting

Santanach (1972a) in the study area and García-Sansegundoet al. (2004) in the Garona dome attribute the Upper Ordovicianunconformity to basement tilting and to subsequent erosionbecause of a late Ordovician faulting episode. In the Lys-Caillaouasmassif, Den Brok (1989) and Kriegsman et al. (1989) propose theexistence of a pre-Variscan deformational event and Clariana andGarcía-Sansegundo (2009) describe a pre-Upper Ordoviciandeformation episode that gave rise to a slaty cleavage not associ-ated with folds in the eastern part of the Pallaresa dome. Moreover,a pre-Upper Ordovician folding episode has been proposed as theorigin of the unconformity on the southern slope of the Canigómassif (Casas, 2010).

Subsequently, late Visean to Serpukhovian Variscan deforma-tion (Cygan et al., 1981; Delvolvé et al., 1993, 1998) affected the

Fig. 1. Simplified geological map of the central part of the Pyrenees with the location of study area (Fig. 3).

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whole succession accompanied by prograde high temperatureelowpressure metamorphism (Guitard, 1970; Zwart, 1979). Syn to lateorogenic (MoscovianeKasimovian, Romer and Soler, 1995) granit-oids intruded mainly into the upper levels of the succession,producing local contact metamorphism (Autran et al., 1970). TheVariscan deformation exhibits a different signature in the infra- andin the supra-Silurian rocks. A pervasive cleavage is the maindeformational Variscan structure in the pre-Silurian rocks. Thiscleavage is subvertical or dips regularly to the North in the low-grade metasediments in adjacent areas (North of the Canigómassif, Rabassa dome, Massana anticline, and south of the Pallaresadome, Hartevelt, 1970; Laumonier and Guitard, 1978; Speksnijder,1986; Poblet, 1991; Capellà, 1995), whereas in the Orri dome itexhibits an antiformal attitude (Speksnijder, 1986; Poblet, 1991).The intersection lineations and minor fold axes related to thiscleavage exhibit a wide range of orientations and in order toaccount for this dispersion, some authors have invoked the occur-rence of early pre-mainphase Variscan folding (Guitard, 1967:

Laumonier and Guitard, 1978; Speksnijder, 1986; Poblet, 1991;Capellà, 1995 among others). By contrast, stretching lineationrelated to the main cleavage displays a rather regular attitude,subvertical or with steep plunges to the NW in the Massana anti-cline, the Rabassa dome and in the Pallaresa dome (Santanach,1972b; Poblet, 1991; Capellà, 1997; Casas et al., 1998). Moreover,X-axes of the finite strain ellipsoid measured using Rf/F method incoarse-grained deposits, plunge steeply to the NW in the Pallaresadome, the Massana anticline and the Orri dome (Capellà, 1997),always forming a high angle to the fold axes (Corstanje et al., 1989;Carreras and Capellà, 1994).

South-directed Variscan thrust sheets are the most prominentstructures in the Silurian, Devonian and Carboniferous successions,near the study area, in the Cerdanya and in the Segre unit (Mattaueret al., 1967; Hartevelt, 1970; Llac, 1979; Domingo et al., 1988; Casaset al., 1989 among others). Thrust sheets involve mainly Silurianand Devonian rocks although some climb up into the Carboniferous“Culm” sediments. Their basal detachment is located at the base ofthe Silurian black shales. In the Segre unit, thrusts cut fold-relatedcleavage, and in turn thrusts and the lower detachment are foldedby south verging cleavage-related folds. Thus, thrust and folddevelopment could be interpreted as broadly synchronous (Poblet,1991). The structure of the Silurian and Devonian rocks locatedmore to the west in the Llavorsí syncline and in the southernmostoutcrops of the Pyrenees differs because of the presence of severalfold systems and thrusts (Mey,1967; Casas and Poblet, 1989; Poblet,1991). In these areas, pre-mainphase Variscan folds have also beendescribed in the Devonian rocks (Mey, 1967).

EeW upright broad folds related to the Alpine deformationdeformed the entire edifice. These folds caused a bulk antiformaldisposition of the entire zone and are responsible for the antiformaldisposition of the Variscan cleavage (Muñoz, 1992b).

3. Structure

In the study area, Santanach (1972a) deduced the existence ofthe Upper Ordovician unconformity on the basis of the differentattitude of the bedding surfaces in the low metamorphic gradeUpper Ordovician and Cambro-Ordovician metasediments. Thisauthor attributed this difference to a pre-Upper Ordovician tiltingrelated to fault development that affected only the Cambro-Ordovician rocks. However, a closer examination of the datasuggests that the bedding disposition of the Cambro-Ordovicianseries cannot be explained by bedding rotation related to tilting(Casas, 2010). Detailed geological mapping (1/5000) and structuralanalysis of the outcrops on both sides of the unconformity wereperformed to provide further insight into this subject.

To better visualize and reconstruct the fold geometry, datawere integrated into a$3D environment, which also facilitates theconstruction of geological cross-sections. Each section was dividedinto several cylindrical domains and for each domain the appro-priate vector projection was chosen. This procedure is used toavoid errors derived from the projection of complex structuresusing an average vector projection. This is a time-consumingprocedure and in-house developed routines for CAD-type toolswere used to project data and calculate apparent dip values(Fernández, 2004).

In the rhythmic Cambro-Ordovician rocks, the most visiblestructures are the bedding surfaces that present a marked disper-sion (Fig. 5a). This dispersion results from the superposition ofthree main deformational episodes D1, D2 and D3. D1 mainly gaverise to map-scale structures whereas D2 and D3 mainly developedoutcrop-scale structures and penetrative foliations (Fig. 3). Bycontrast, in the Upper Ordovician rocks, bedding is regularlyoriented and only the D2 and D3 folding phases can be recognized

Fig. 2. Synthetic stratigraphic column of the pre-Silurian rocks of the Canigó massifwith the location of the dated volcanic rocks: (a) Cocherie et al. (2005); (b) Castiñeiraset al. (2008). Data from Guitard (1970), Hartevelt (1970), Santanach (1972b) and Ayoraand Casas (1986).

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(Figs. 3, 4 and 5b). Minor variations in strike affecting both seriesare due to kilometric scale folds that are probably Alpine in age. Inthe description that follows, L1, L2 and L3, refer to different fold axes,and L0e2, L0e3 to bedding/cleavage intersection lineations.

3.1. The D1 folding event

In the study area, the presence of several antiforms and syn-forms that affect the Cambro-Ordovician rhythmites can bededuced. These folds are responsible for the NWeSE trending andthe NE and SW dipping of the bedding and are, in turn, deformed bythe D2 and D3 structures (Figs. 3 and 4). In profile, these D1 folds areopen to close, have a hectometric wavelength and amplitude, withupright or steeply inclined axial surfaces although its dip variesbecause of subsequent deformation, mainly D3. D1 folds aregenerally upright and occasionally asymmetric, facing towards theNE since the NE limbs dip more strongly than the SW ones (Fig. 4).D1 axial directions are difficult to establish, due to later deforma-tions. However, on the basis of bedding attitude at map scale, it canbe proposed that L1 are subhorizontal with trends ranging from130�e150� to 170�. There is no evidence of an axial planar cleavagedevelopment related to the D1 fold formation. Several axial tracesare curved owing to subsequent deformation, as can be seen in theeastern half of the study area, where D1 folds are deformed bya large D2 antiform (Fig. 3). Some axial traces tend to convergewhereas others endwithout converging and are relayed by other D1axial traces.

Some decametric folds cut by the S2 cleavage can be recognizedin the Cambro-Ordovician rhythmites (Fig. 6). They trend NeS withsteep limbs and like the hectometric D1 folds are not related tocleavage development. Although the relationship between thesemetric NeS and the hectometric NWeSE D1 folds is not wellestablished, we propose that these folds represent decametric sizeD1 folds.

3.2. The D2 folding event

In the Cambro-Ordovician rhythmites D2 folds are recognizableat millimetric to hectometric scale, although they are especiallyvisible at mesoscopic scale (Fig. 7a) and some folds with a hecto-metric wavelength crop out in the central part of the study area. D2folds are open to tight, asymmetrical, south-west verging and withaxial surfaces dipping strongly, 70�e80�, towards the NE. In theCambro-Ordovician rocks, L2 axes of map-scale folds trend 110�

although the minor L2 axes and L0-2 intersection lineations displaya wide range of orientations and exhibit a wide dispersion. L0e2plunge ranges from subhorizontal to subvertical (Figs. 5c and 8)and no simple distribution pattern may be deduced from the mapanalysis (Fig. 8). If L0e2 are plotted together, they are distributedalong a great circle coincident with the S2 average plane, the highplunge being predominant (Fig. 5c).

In the Upper Ordovician rocks, bedding surfaces exhibita regular disposition oriented NWeSE, dipping moderately orstrongly towards the SW (Fig. 5b). As stated above, the D1 episodewas not recognized and the D2 structures are the older ones inthese rocks (Figs. 3, 4 and 8). Asymmetric decametric D2 foldsindicate that the study area constitutes the southern flank ofa south facing first order D2 fold oriented NWeSE (Fig. 4a). In theseUpper Ordovician rocks the L2 axes and the L0e2 intersection line-ations are grouped forming a maximumwith a moderate plunge tothe NW (15/297�) (Fig. 5d).

S2 cleavage is the predominant deformational mesostructure inthe Cambro-Ordovician and Upper Ordovician rocks in the studyarea. The S2 surfaces are subvertical or dip strongly (70�e80�) to theN or NE and are irregularly developed, not being well expressed insandstone and quartzitic levels. S2 is parallel to the axial surface ofthe D2 folds and shows little fanning in fold profile. S2 ranges fromoblique to perpendicular to bedding but is never parallel asobserved in the geological map (Fig. 3).

Fig. 3. Geological map of the contact between the Upper Ordovician and the Cambro-Ordovician successions in La Molina showing the location of Fig. 4aed and 6 (UTM coor-dinates). The number indicates the dip value (See Fig. 1 for location).

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3.3. The D3 folding event

The wavelength of the D3 folds varies from a few to severalmetres (Fig. 7b). When these folds affect south-dipping D1 foldlimbs, D3 folds are asymmetrical with long gently south-dippinglimbs and short north-dipping overturned, limbs (central partof Fig. 4d). When developed over the north-dipping D1 foldlimbs, they show short steeply south-dipping limbs and longsteeply north-dipping limbs (easternmost part of Fig. 4d). Inprofile, folds are open to tight and south to southwest verging,with axial surfaces dipping gently, 15�e30�, towards the NeNE.D3 folds also caused variations in the dip of the S2 surfaces(Fig. 4).

A locally developed S3 cleavage is associated with D3 folds(Fig. 7c). S3 is parallel to the D3 axial surfaces and shows littlefanning in fold profile. S3 dips gently to the N or NE (Fig. 5e andf). In the Cambro-Ordovician rocks, L3 axes show a markeddispersion and are distributed along a great circle that matchesthe mean S3 surfaces (Fig. 5e). In the Upper Ordovician rocks, L3axes are subhorizontal and fairly grouped, oriented 7/099(Fig. 5f).

3.4. Disposition of the mainphase (D2) linear mesostructures

As shown in the 3D reconstruction (Fig. 9a), D1 folds control thegeometry of the D2 mesoscale folds given that D2 folds developedover initially non horizontal surfaces (Turner and Weiss, 1963, p.130; Mey, 1967, p. 194; Ramsay, 1967, p. 539). In the SW-dippinglimbs of D1 folds, L2 and L0e2 are mainly oriented NWeSE or NeSwith moderate plunges towards the NW or N (Fig. 8a and b). Inthis situation, D2 folds exhibit low axial direction stability (Ramsay,1967) because the angle between D2 fold axial direction and the S0strike is low, less than 30�. As a result, a small variation in the S0strike or D2 axial direction may originate L2 and L0e2 meso-structures with moderate axial plunges towards the SE (Fig. 8c). Incontrast, in the NE-dipping limbs of D1 folds, L2 and L0e2 areoriented NEeSW or EeW with strong plunges towards the NE or E(Fig. 8a and e). In general terms, the NE and N plunges are higherthan the NW and W plunges, confirming that the D1 folds are NEfacing (Figs. 5c and 8a). In the central area a hinge zone of a hec-tometric D2 fold crops out. In this area, the D2 and the D1 axialsurface directions form a high angle and as a consequence the D2axial directions form moderate to high angles, ranging from 50� to

Fig. 4. (a)e(d) Cross-sections through the study area. See location in Fig. 3.

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90�, with S0 strikes. As a result, L2 and L0e2 are subvertical or witha steep plunge towards the NW or the NE (Fig. 8d), i.e., despiteshowing high axial direction stability, they undergo alternatechanges in orientation depending on the limbs of the D1 foldswhere they developed (Fig. 8f).

4. Discussion

4.1. The quest for the orientation of the D1 and D2 folds

In order to reconstruct the orientation of the earlier folds, someauthors invoke the map-scale distribution of the pitch angle or

trend of the subsequent intersection lineation (Santanach, 1972a;Speksnijder, 1986). That is, lines joining points with the sameintersection lineation pitch angle or trend value may indicate thedirection of the earlier fold limbs. However, in our case this methodcannot be easily applied because of the great spread of the L0e2. Asstated above, this results from the low stability of L0e2 in some D1limbs but it can also be due to the fact that the L0e2/S0 pitch angledepends not only on the angle between S0/S2 trends but also on theS0 dip. That is, the great spread of the L0e2 pitch angle or the L0e2trend cannot inform us easily about the D1 fold orientation.However, as explained in detail by Mey (1967) in the Devonianrocks of the Pyrenees, the distribution of the subsequent intersec-tion lineation can yield information about the previous foldgeometry. In our case, the spread of the L0e2 suggests that the D1folds are NE facing and that their limbs display a wide range of dipfrom subhorizontal to subvertical. In such a situation the D1 foldorientation should be reconstructed mainly from the beddingorientation displayed at map scale. This reconstruction indicatesthat D1 axial direction ranges from 130� to 150� in the areas whereD2 folds are not well developed (Fig. 3).

Likewise, it follows that the orientation of the subsequentmainphase Variscan folds cannot be easily determined by theorientation of themesoscale folds. This may help to explainwhy thecorrelation of the Variscan deformation phases recognized in thedifferent areas continues to be one of the most baffling problems ofthe basement rocks of the Pyrenees. In the study area, given theorientation of map-scale folds and S2 attitude, the D2 event could becorrelated with the main Variscan episode, D3 of Laumonier andGuitard (1978), Speksnijder (1986), Poblet (1991) and Clarianaand García-Sansegundo (2009), or with the second deformationalepisode of Capellà (1995).

In order to resolve these problems, it is necessary to (1) sepa-rately analyze the data collected below and above the UpperOrdovician unconformity because of the differences in the struc-tures of the Cambro-Ordovician and the Upper Ordovician series,

Fig. 5. Equal area lower hemisphere stereoplots of bedding (S0), D2 related meso-structures (S2 cleavage and L0-2 intersection lineation), and D3 related mesostructures(S3 cleavage and L0-3intersection lineation) from the Cambro-Ordovician (a, c and e)and Upper Ordovician successions (b, d and f) in the La Molina area. In (c) and (d), theL0e2 intersection lineations are distributed along a great circle that is coincident withthe S2 average plane. In (e) and (f) the L0e3 intersection lineations are distributed alonga great circle that is coincident with the S3 average plane (n indicates the number ofmeasurements).

Fig. 6. 3D reconstruction of a decametric D1 synform cut by the S2 cleavage. Pictureillustrates S0 and S2 relationships in the eastern limb of the fold (See Fig. 3 for location).

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(2) determine the orientation of the mainphase Variscan folds bythe bedding attitude at map scale and not by the mesoscale folds,and (3) to use kinematic elements such as the X axis of the strainellipsoid or the stretching lineation as correlation criteria since they

are more regularly oriented than the geometric elements. Insummary, a better understanding of the polyphase deformation inareas characterized by the absence of distinct stratigraphic hori-zons requires detailed structural mapping and structural analysis ofa good and continuous outcrop over an extended area. As pointedout by Gutiérrez-Alonso (1996), extensive measurements of struc-tural elements, which enabled the construction of the bedding andcleavage traces, is the only method to reconstruct the macro-structure and to obtain data to establish the orientation of thestructures in polydeformed areas.

4.2. Geometry of the superposed folding

The bedding attitude in the Cambro-Ordovician rocks allows usto recognize some interference patterns at map scale between D2and D1 in the eastern area (Fig. 3). It should be noted that inter-ference patterns at map-scale scale between D3 and D1 were notrecognized and that interference patterns at outcrop scale indi-cating superposition of D2 over D1 and D3 over D1 folds were notrecognized either. This could be due to: 1) the smaller dimensionsof the D3 folds, 2) the absence of metric sized marker horizons inthe rhythmic Cambro-Ordovician rocks and 3) the absence or thescarcity of the D1mesostructures. In order to illustrate the expected3D geometry of the superposed folds, we constructed interferencerefold structures using the package FoldPlot developed by Mooreand Johnson (2001) with software Mathematica (Fig. 9). Weattributed mean values to different folding episodes (L1 ¼ 00/140;L2 ¼ 15/297; L3 ¼ 07/099; SAX1 ¼ 67/230; SAX2 ¼ 74/020; SAX3 ¼ 35/020) on the basis of the D2 and the D3 fold orientation in the UpperOrdovician rocks. The refold structures do not coincide with any ofthe four types illustrated by Ramsay and Hubber (1987). D2 over D1lies in between type 0 and type 3 as the angle between L1 and L2 islow (23�) but the angle between the SAX1 and a2 is moderate (43�)(Fig. 9a). D3 over D1 does not match any of the simple types as boththe angle between L1 and L3 and the angle between the SAX1 and a3are moderate (45�) (Fig. 9b). Finally, D3 over D2 also corresponds toan intermediate type between 0 and 3 as the angle between L1 andL2 is low (18�), and the angle between the SAX1 and a2 is moderate(36�) (Fig. 9c). In order to refine these results we use the model ofGrasemann et al. (2004), which classifies the three-dimensionalrefold structures into six different end members and fifteen inter-mediate types. According to this classification, D2 over D1 matchestheir intermediate 1 4 03 type; D3 over D1 does not match any ofthe intermediate types and has some characteristics in commonwith the 3 4 01 type and D3 over D2 corresponds to the interme-diate 34 03 type. Hence, the three refold structures share some ofthe characteristics of the 01e03 and 3 type. These types of inter-ference do not provide two-dimensional closed shapes, which mayexplain why these interference patterns are not easily detectable atmap or at outcrop scale.

4.3. The age of the D1 deformation

Considering a Sandbian age for the basal Upper Ordovicianconglomerates and a late Cambrian (Furongian)eearly Ordovician(Tremadocian) age for the uppermost part of the Cambro-Ordovician sequence, a late Early Ordovician or a middle Ordovi-cian age for the D1 deformation can be proposed (475e460 Ma)(Fig. 10). The presence of NWeSE to NeS trending hectometric tokilometric Mid Ordovician folds together with an important pre-Upper Ordovician erosion could account for the disposition of theUpper Ordovician rocks over different terms, from Neoproterozoicto Early Ordovician, on the southern slope of the Canigómassif. Thisdeformational episode could be correlated with the pre-UpperOrdovician deformation event described in the Lys-Caillaouas

Fig. 7. (a) D2 folds affecting the Cambro-Ordovician rhythmites. Note the high anglebetween bedding (S0) dipping towards the NW and S2 subvertical cleavage, giving riseto an elevated plunge of the L0e2 intersection lineation and D2 fold axes; (b) D3 metricfold developed in the Cambro-Ordovician rhythmites and (c) Upper Ordovicianmicroconglomerates and sandstones exhibiting S0 subvertical planes and S2 and S3cleavage surfaces.

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Fig. 8. (a) Map of the disposition of the D2 linear mesostructures (L0e2 intersection lineation and L2 fold axis). The arrow indicates the trend and the number the plunge value (UTMcoordinates). (b)e(f) illustrate different attitudes of the L0e2 and the L2 mesostructures. See text for explanation.

Fig. 9. Interference refold structures (middle image) created by superposing the fold on the right on the fold on the left. We attributed mean values to different folding episodes(L1 ¼ 00/140; L2 ¼ 15/297 and L3 ¼ 07/099, SAX1 ¼ 67/230; SAX2 ¼ 74/020 and SAX3 ¼ 35/020). (a) D2 superposed on D1; (b) D3 superposed on D1; (c) D3 superposed on D2.Constructed with the package FoldPlot (Moore and Johnson, 2001) using software Mathematica.

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massif by Den Brok (1989) and in the medium grade metasedi-ments of the Pallaresa dome reported by Clariana and García-Sansegundo (2009) although in this latter case no related foldshave been described to date.

In other areas of the Pyrenees, pre-mainphase fold systemsaffecting Upper Ordovician and supra-Silurian rocks, and henceVariscan in age, have also been recognized (Mey, 1967; Hartevelt,1970; Cirés et al., 1990; Poblet, 1991; Capellà and Bou, 1997among others). Such fold systems were not recognized in thestudy area, which probably reflects an irregular development of thepre-mainphase Variscan deformation along the basement rocks ofthe Pyrenees.

The described D1 folding event should be correlated with the“Sardic” deformations described in Sardinia (Arthaud, 1963;Carmignani et al., 1986, 2001; Pasci et al., 2008; see discussion inCasas, 2010). In contrast, the Iberian massif provides little evidenceof Ordovician tectonic activity. In the Central Iberian zone, Oen(1970) and Roda (1986) described NEeSW oriented folds prior tothemain Variscan deformation and Silva and Ribeiro (1985), Romãoand Ribeiro (1992) and Romão et al. (2005) described a foldingepisode affecting the pre-Armorican quartzite sequence. Thesedeformations are probably older than those described in Sardiniaand in the Pyrenees since they could have developed between postMid Cambrian and Early Arenig times. Eastward migration of thegeodynamic regime should be invoked in order to explain thediachronism between these pre-Variscan events in the Iberianmassif and Sardinia/Pyrenees. In any case, their geodynamicsignificance is unclear in a context dominated by the northernGondwana break up. Stampfli et al. (2002) and Von Raumer et al.(2002) propose the existence of a short-lived cordillera formedduring the Mid Ordovician in the eastern part of the NorthernGondwana margin. This cordillera resulted from the amalgamationof volcanic arcs and continental ribbons (Stampfli et al., 2002; VonRaumer et al., 2002) or from the collision between the Qilian arcand the Northern Gondwana margin (Stampfli et al., 2011) ina transient Mid Ordovician orogenic pulse.

5. Conclusions

In the low-grade Cambro-Ordovician metasediments of thePyrenees, a Mid Ordovician deformational event gave rise to NWe

SE trending, northeast verging, hectometric scale folds. These foldsled to a wide dispersion of the folding axes of the main Variscanphase in the pre-Upper Ordovician sequence. Thus, an earlier foldsystem accounts for a complex distribution of intersection lineationassociated with a subsequent folding event. A better understandingof the Ordovician deformation could help us to unravel the geom-etry of the main Variscan deformations in the Pyrenees. Sucha structural pattern could be extended to other massifs of the

Pyrenees where pre-mainphase deformations involving monoto-nous lithological series have been described. In such a case, foldsuperposition is not evidenced by interference patterns at outcropscale, and detailed mapping of mesostructures over extended areasmust be performed to elucidate polyphase deformation. The Fold-Plot package proves to be a useful tool for visualizing and recon-structing the geometry of such refold structures in 3D.

Acknowledgements

This work was funded by projects CGL2010-21298 and Con-solider-Ingenio 2010, under CSD2006-00041 “Topoiberia”. Equalarea plots weremadewith GEOrient� 9.2 (R.J. Holcombe, Universityof Queensland), map and cross-sections constructed with the aid ofMICROSTATION� (Bentley) and MOVE (Midland Valley), and the 3Dreconstructions with goCAD� (Paradigm). We are especiallyindebted to R.R. Moore, who facilitated the FoldPlot package, and tothe two referees whose helpful comments have considerablyimproved an earlier version of the manuscript.

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