Granites built by sheeting in a fault stepover (the Sanabria Massifs, Variscan Orogen, NW Spain)

Granites built by sheeting in a fault stepover(the Sanabria Massifs, Variscan Orogen, NW Spain)

N. Vegas, A. Aranguren and J. M. TubõÂaDepartamento de GeodinaÂmica, Facultad de Ciencias, Universidad del PaõÂs Vasco, a.p. 644, E-48080 Bilbao, Spain

Introduction

During the last two decades, struc-tural studies on granites have featureda great variety of emplacement pro-cesses, but reliable information abouttransfer of granitic magma from theirsource to upper crustal levels is stillscarce. Laccoliths are an exception tothis rule because evidence they are fedfrom below by basal feeder dykes isabundant, as reported since the begin-ning of the last century (see Corry,1988). Ascent of felsic magmas bydyking was envisaged as a valid mech-anism to build up large granite plu-tons, alternative to diapirism, fromthe last decade on (Clemens andMawer, 1992; Hutton, 1992; Petfordet al., 1993). This mechanism of mag-ma ascent is now preferably applied togranites related to ductile shear zones(Hutton and Reavy, 1992; Fowler,1994; Tobisch and Cruden, 1995;Aranguren et al., 1997), but also tobatholiths emplaced at shallow levelsin anorogenic settings (Wilson et al.,20001 ). Multiple intrusions throughfeeder channels, as described for theBee Mountain laccolith (Corry, 1988)or suggested for the South MountainBatholith (Benn et al., 1999), are re-quired in order to prevent the suddencrystallization of felsic magmas indykes (Hutton, 1992; Petford et al.,1993).

A di�culty in substantiating dykingin granitic plutons is the lack ofnatural examples reported as yet.Actually, at present erosion levelsfeeding zones are commonly inaccess-ible to direct observation, and magmadeformation in its emplacement sitetends to obliterate map ¯ow patternsthat might indicate feeding throughfractures (Tobisch and Cruden, 1995).These restrictions imply that the loca-tion and strike of root zones in plu-tons, frequently interpreted to befeeding channels, can be deduced onlyby combining gravity and structuralstudies. In regard to syntectonic gran-ites, an additional issue is that thenumber, location and orientation ofthe roots is controlled by the tectonicsetting prevailing during the intrusionprocess (Ameglio et al., 19972 ). Thisfact could partly explain why so dif-ferent structural interpretations theburied feeder zones have received.The solutions proposed includeAndersonian (Aranguren et al., 1996)and non-Andersonian fractures ex-ploiting several structures associatedto strike-slip shear zones: en-eÂ chelontension fractures (Castro, 1986; Amiceet al., 1991; Tobisch and Cruden,1995), Riedel fractures (Arangurenet al., 1997) or the shear zone itself(Hutton, 1992; Ingram and Hutton,19943 ).In this paper, we document the

conspicuous case of two syntectonicplutons constructed by countlessdykes that only occasionally evolvedinto sills. These plutons are con®nedin the releasing bridge of a transpres-

sional dextral shear zone from theIberian Variscan belt, in NorthernSpain (Fig. 1). This particular tectonicsetting was decisive to keep locally thedilation conditions required for therepeated intrusion of dykes to occur.Our work combines ®eld structuraldata from the granites and their coun-try rocks with a detailed study of theanisotropy of magnetic susceptibility(AMS) of the two plutons.

Geological setting

The Ribadelago and Sotillo plutons(NW Spain) were emplaced into theOllo de Sapo Domain of the Central-Iberian Zone (CIZ), a major tectonicunit of the Iberian Variscan belt wheregranites spread over large areas anddextral shear zones are common(Fig. 1B). The Sotillo pluton is boun-ded to the south (Fig. 2) by Ordovi-cian black schists and to the north byglandular gneisses, the so-called Ollode Sapo formation, of controversialorigin and age, Late Precambrian toOrdovician (MartõÂnez-GarcõÂa, 1973).The Ribadelago pluton is entirelybounded by the Ollo de Sapo forma-tion (Fig. 2). In the Sanabria region,part of the Ollo de Sapo formation isa�ected by a thermal dome (Fig. 2),since its metamorphic conditions varywithin a few kilometres betweengreenschist facies to the north-eastof the Ribadelago pluton, and mig-matization in southernmost areas(MartõÂnez-GarcõÂa, 1981).The Ribadelago and Sotillo plutons

are characterized by well-de®ned

ABSTRACT

The Sanabria region (Central Iberian Zone, Variscan belt ofSpain) shows an asymmetric thermal dome marked by migma-tites accompanied by the Ribadelago and the Sotillo plutons.These small plutons display pronounced mineralogical varia-tions. Biotite granodiorites and tonalites prevail, and granitesand gabbros are common. Both plutons are con®ned in thereleasing stepover of a transpressional shear zone that strikes120°E and dips 70°SW. Most of the igneous rocks form sheetsparallel to the shear zone. Magnetic foliations and lineations in

the igneous rocks are parallel, respectively, to the shear bandsand stretching lineations observed in the shear zone. Theformation of these igneous sheets at high angle to the mainaxis of the regional ®eld stress is explained by a combination ofthe fault-valve behaviour of the shear zone with the power ofmelt overpressure to open and ascend through previouslyformed planar structures, like S- or C-planes.

Terra Nova, 13, 180±187, 2001

Correspondence: J. M. TubõÂa, Departa-

mento de GeodinaÂ mica, Facultad de Cien-

cias, Universidad del PaõÂs Vasco, a.p. 644,

E-48080 Bilbao, Spain. Fax: 34 94 4648500;

E-mail: [email protected]

180 Ó 2001 Blackwell Science Ltd

lithological variations, whose distri-bution is heterogeneous. Fine-grained,biotite- and hornblende-bearinggranodiorites and tonalites are thedominant rocks, but pyroxene-bearinggabbros, diorites and porphyriticgranites are common. These rocksform sheeted intrusions closely inter-layered at the outcrop scale (Fig. 3A),

with layer thickness varying between afew centimetres to less than 50 m andaverage thickness of 4 m. The mainvariation to this heterogeneous pat-tern is found at the eastern part of theSotillo pluton, where coarse-grained,porphyritic two-mica granite withK-feldspar megacrysts is the dominanttype. The contacts between sheets vary

from gradational to sharp, and ®ne-grained margins are frequent. Ma®cschlierens, from disruption of gabbro-ic enclaves in the granodiorites, andgranitic pipes in sill-like gabbros areabundant, pointing to the input ofma®c magma during the crystal-lization of the granitic sheets. In thegabbros, magmatic epidote is

Fig. 1 Geological map of the northern part of the Iberian Massif showing the great extent of Variscan granitoids within theCentral-Iberian Zone (modi®ed from MartõÂnez CatalaÂ n et al., 1997). The inset (Fig. 2) shows the location of the Sanabria region.

Terra Nova, Vol 13, No. 3, 180±187 N. Vegas et al. · Granites built by sheeting.............................................................................................................................................................

Ó 2001 Blackwell Science Ltd 181

observed forming corroded, subhedralcrystals with allanite cores. Numerousdiatexite sheets and raft trains ofxenoliths from the country rocks areintermingled with the igneous rocks inboth plutons (Fig. 2).

Structural constraints to plutonemplacement

The structure of the Ollo de SapoDomain is ascribed to three mainphases of deformation (Aller and Bast-ida, 1996). The oldest deformation(D1) led to recumbent, east-vergingfolds, which evolved into east-directedthrusts during D2. Finally, coaxialfolds with a penetrative, steep-dippingcrenulation cleavage formed duringD3. Within the study area, the struc-tural evolution of the country rocks

during D3 reveals that a progressivedeformation led to strain partitioningbetween large zones having foldedcomposite foliations, in the sense ofTobisch and Paterson (1988), andnarrow zones forming strike-slip shearbands. The shear bands, which strikeN120°E and dip 50° to 70° SW,become more intense and abundanttoward both the northern contact ofthe Ribadelago granite and thesouthern contact of the Sotillo granite,ultimately de®ning S-C tectoniteswithin a few metres against the granitecontacts. These S-C tectonites carry apronounced lineation trending N120°to 130°E and plunging 20°E, andde®ned by elongate, bluish-quartzaggregates in the Ollo de Sapo gneissesand stretched andalusite porphyro-blasts in the Ordovician hornfels.

Fig. 2 Geological map of the Sanabria region, with location of two dextral shear zone terminations. Note that the migmatite dome(dark grey) and the Ribadelago and Sotillo plutons are located inside the releasing stepover de®ned by the linked shear zones. Theinset illustrates a releasing stepover (shaded area) related to a dextral shear zone; the development of folds parallel to the shearzone rather than en-eÂ chelon folds, along with the attitudes of Riedel fractures are consistent with a transpressional shear zone.

Fig. 3 Field structures and microstruc-tures in XZ (parallel to lineation andperpendicular to foliation) sections.A: steeply dipping sheets of igneousrocks in the Ribadelago pluton. B: gran-ite dyke intruding the Ollo de Sapocountry gneiss. This dyke evolvesupwards to a sill. C: magmatic foliationde®ned by the preferred orientation ofbiotite (Bt) and plagioclase (Plg) crystalsin a tonalite of the Ribadelago pluton.The grains of quartz (Q) show equiaxialshapes, and are devoid of solid-statedeformation. D: conjugate sets of lateaplite dykes oblique to the foliation (F)of the Sotillo granodiorite. E: quartzaggregate with mosaic subgrainsshowing subboundaries at right angles,attesting for high-temperature deforma-tion conditions (Mainprice et al., 1986).

Granites built by sheeting · N. Vegas et al. Terra Nova, Vol 13, No. 3, 180±187.............................................................................................................................................................




Kinematic criteria provide a dominantdextral sense of shear. Folds withtrends parallel to the shear zone andsteep-dipping axes and cleavage-tran-sected hinges, typical features in trans-pressively deformed areas (Woodcockand Schubert, 1994), are commonalong these shear bands.Figure 2 shows that these dextral

shear zones compose a linked fault-system. The Ribadelago and Sotilloplutons are precisely emplaced in thereleasing stepover of the fault-system.The metamorphic dome is also boun-ded by the two segments of the shearzone (Fig. 2), suggesting that it has anasymmetric shape consistent with theNE-ward vergence of adjacent thrusts(MartõÂnez-GarcõÂa and Quiroga, 1993).The contacts between the igneous

and the host rocks are sharp.Countless sheets that strike 120°Eand dip » 70°SW are the most prom-inent structures of the plutonic rocks(Figs 2 and 3A). These sheeted-bodiesare here interpreted as dykes injectedparallel to the shear zones, based ontheir steep dips, occurrence of chilledmargins and their obliquity to thefoliation of the country rock xeno-liths. By contrast, close to the Riba-delago pluton roof, corresponding toits western and topographically highersector, sills are very common anddykes evolving into sills are observedlocally (Fig. 3B). Many dykes show amagmatic foliation parallel to thedyke walls, de®ned by the preferredorientation of strain-free, euhedralcrystals of biotite, hornblende orplagioclase (Fig. 3C). In both plutons,a strain gradient is evidenced by theprogressive replacement of the mag-matic foliation by S-C structures to-ward the southern shear zone for theSotillo massif and toward the nor-thern segment of the shear zone forthe Ribadelago pluton (Fig. 2). It isworth noting that dykes totally devoidof solid-state deformation are closelyinterlayered with deformed sheets,strongly suggesting that multipleintrusions took place during transientmagmatic pulses.Late aplites and pegmatites form

two sets of subvertical, oblique dykes(Fig. 3D) striking to the north or thenorth-east. Parallel to the wallsof some late dykes, the previouslyemplaced sheets are o�set by, and thegranite foliation de¯ected into, conju-gate shear zones, which have a dextral

sense for the N±S dykes and sinistralfor the NE±SW ones. These latedykes, because of their close associ-ation with such conjugate shears, areconsidered to have intruded throughRiedel fractures related to the trans-pressional shear zone (Fig. 2).

Magnetic fabrics

The internal structure of these plutonshas been determined by anisotropy ofmagnetic susceptibility (AMS) meas-urements using a KLY-2 instrument(Agico, Brno) operating in a lowmagnetic ®eld (� 4 ´ 10±4 T and920 Hz; sensitivity » 5 ´ 10±8 SI).The anisotropy of susceptibility, aphysical property expressed by a sym-metric second rank tensor and geo-metrically represented as an ellipsoidwith K1 ³ K2 ³ K3 as principal axes,often correlates to the main directionsof the rock fabric ellipsoid as dis-cussed for granites in Bouchez (1997).Orientated drill cores for the AMSstudy have been collected at 74 and 56sites in the Ribadelago and Sotilloplutons, respectively. Sampling comesfrom outcrops where solid-statedeformation is negligible to the nakedeye. The bulk magnetic susceptibility,K � (K1 + K2 + K3)/3, ranges from

8 ´ 10±5 to 52 ´ 10±5 SI (average:24 ´ 10±5 SI) in Ribadelago, and from1 ´ 10±5 to 62 ´ 10±5 SI (average:13 ´ 10±5 SI) in Sotillo. K values varyabruptly from one site to another,re¯ecting the lack of petrographiczoning inherent to the interlayerednature of the felsic and ma®c sheets.It is usually concluded that the low Kvalues re¯ect the magnetic contribu-tion of the Fe-bearing paramagneticminerals (Rochette, 1987; Jover et al.,1989), here biotite and hornblende.This is con®rmed in our case bymeasuring susceptibility magnitudevs. temperature curves from powderedrock samples heated above the Curietemperature of magnetite (Fig. 4).The paramagnetic anisotropy,

expressed as P � 100[(K1 ) D)/(K3 ) D) ) 1], where the ubiquitousdiamagnetic contribution of the rock(D) is subtracted to the maximun andminimum susceptibilities (Hrouda,19864 ), increases from 2.4% to 17%principally toward the shear zones atthe pluton borders. Nine out of 10sites provide P values higher than 7%along with well-de®ned fabrics,mainly of oblate type. Such highanisotropy values usually re¯ectthe overprinting of solid-statedeformation onto former magmatic

Fig. 4 Susceptibility vs. temperature curves of three selected samples, ranging fromthe greatest susceptibility value (RL 69; K � 51 ´ 10±5 SI) to intermediate values (RL58; K � 37 ´ 10±5 SI. RL 3; K � 29 ´ 10±5 SI). The curve of sample RL 69 shows asharp susceptibility increase from 400 °C that re¯ects the heating-induced paramag-netic matrix to magnetite reaction and/or the Hopkinson effect.



structures (see Bouchez, 1997). How-ever, in the corresponding samples ourmicroscopic observations reveal littlehigh-temperature plastic deformationfeatures in quartz grains, such asmosaic subgrains (Fig. 3E). Instead,plagioclase crystals often display sub-magmatic microfractures in®lled byquartz and albite (Fig. 3F) that pro-vide evidence for the preservation of

magmatic fabrics (Bouchez et al.,1992). These features show thatsolid-state deformation began at hightemperature, close to the solidus forgranite melts.The patterns of the magnetic folia-

tions (K1/K2 plane) and lineations(K1 axis), very similar in map viewfor both massifs, are characterized bysteeply dipping and 120°E-striking

foliations and lineations gently plun-ging (120°) to the ESE (Fig. 5). Thisstructural arrangement suggests up-magma ¯ow in a top-to-the north-west sense. This ¯ow behaviour isinterpreted as a dyke to sill evolution,like in Fig. 3B, toward the western-most and topographically higher sec-tor, which is close to the pluton roof.However, in some sites of Ribadelagomoderately dipping (20±45°E), nor-therly striking foliations, and linea-tions having northward trends arerecorded. These peculiar sites suggestthat upward magma ¯ow tookadvantage of en-eÂ chelon tension frac-tures locally.

Proposed emplacement model

Our structural data in the igneousand country rocks call for a coevalemplacement of the Ribadelago andSotillo plutons during the formationof a releasing stepover in a transpres-sional shear zone. It is accepted thatthe input of melts into active shearzones triggers strain localization(Tommasi et al., 1994; Brown andSolar, 1998). In our case, this mayexplain that although the shear zonesare also found in the country rocks,their development is greater in theRibadelago and Sotillo plutons(Fig. 2).As illustrated by the schematic

emplacementmodel for theRibadelagoand Sotillo plutons (Fig. 6), we pro-pose that magma sheets ascended re-peatedly through the segmenteddextralshear zone, in such a way that succes-sive younger magma batches were pro-gressively shifted toward the centralpart of the releasing fault-bridge.Interpreting this sheets of magma as

dykes can be debated (Paterson andMiller, 1998; Brown and Solar, 1999)since they are at high angle to theprincipal compressive axis (r1) of theregional stress ®eld. However, theiremplacement was likely controlled bylocal stress conditions rather than bythe regional stress ®eld. Within theshear zone the occurrence of S-Cstructures provided networks of stee-ply dipping surfaces liable to evolveinto conduits along which melt ascentthrough the crust was easier thanelsewhere. For instance, small butfrequent displacements on irregularC-planes may have promoted dilationeffects responsible for the opening of

Fig. 5 Map and stereoplots of magnetic foliations (A) and magnetic lineations (B) inthe Ribadelago and Sotillo plutons. In the Ribadelago massif, the shaded areascontain anomalous sites with oblique lineations, interpreted as intrusions throughtension fractures.



numerous pipe-like channels. All thispoints to transient reductions of thetectonic compressional stress, whichwould ®nally be overcome by themagma overpressure (Hutton, 1997)to form continuous planar injections,the sheeted magma bodies, by propa-gation and connection of neighbour-ing channels. Robin and Cruden(1994) concluded that transpressioninduces a vertical gradient of tectonicoverpressure in the crust, and accord-ing to Saint Blanquat et al. (1998) suchtectonic overpressure may contributedirectly to magmatic overpressuring.Compared to other examples of

granites associated with transpression-al shear zones (Fowler, 1994; Bennet al., 1997), in the Sanabria regiononly a few dykes evolve into sills atupper levels. Along with the smallthickness of the igneous sheets, thisfeature seems to re¯ect fast crystal-

lization and, hence, short periods formelt conduits. Consequently, we pro-pose that multiple injections tookplace during transient magmatic pul-ses helped by the fault-valve beha-viour of the shear zone.

Acknowledgements

Supported by MEC (Pb 96±1452-Co3-03)and UPV/EHU (UPV 001-310-G18/99)projects. The Junta de Castilla y LeoÂ n gavepermission to work on the Parque Naturaldel Lago de Sanabria. We thank J. L.Bouchez and two anonymous referees forconstructive and critical reviews, whichhelped to improve the manuscript. This isa contribution to the IPGC 453.

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Received 12 September 2000; revised versionaccepted 25 June 2001



Documents

Granites built by sheeting in a fault stepover (the Sanabria Massifs, Variscan Orogen, NW Spain)