A área de Rosmaninhal (Beira Baixa, Portugal Central) foi cartografada em detalhe. A litoestratigrafia do Grupo das Beiras (parte do Complexo 
Xisto-Grauváquico - CXG, com idade do Câmbrico ao Proterozóico superior) foi estabelecida, o que permitiu cartografar dobras e cavalgamentos de idade 
sarda, entre o Ordovícico e Câmbrico, às quais são sobrepostas dobras variscas com clivagem de plano axial, produzindo estruturas de interferência.
Propomos que a deformação sarda é devida à inversão dum aulacógeno em regime de transpressão dextra do Proterozóico superior ao Câmbrico no terreno

Ribeiro, António

Romão, José

Repositório do LNEG

Comun. Seru. Geol. Portugal, 1992, t. 78, fase. 2, pp. 87 - 95 Comunicações dosServiços Geológicosde PortugalThrust tectonics of sardic age in the Rosmaninhal area(Beira Baixa, Central Portugal)JOSÉ ROMÃO * & ANTÓNIO RIBEIRo ‘~Key-words: Centro-Iberian Zone; Beiras Group; lithostratigraphy; sardic phase; sardic thrusts; slickensides; geodynamic significance; aulacogene; dextral transpressive inversion.Abstract: The Rosmaninhal area (Beira Baixa, Central Portugal) was mapped in detail. The lithostratigraphy of the Beiras Group (part of the«Xisto-Grauváquico» Complex — CXG, Cambrian to upper Proterozoic in age) was established and allowed the mapping of folds and thrusts ofSardic age (Cambrian to Ordovician), on which are superposed variscan folds and axial plane cleavage, producing interference patterns. It is proposed that the sardic deformation is due to dextral transpressive inversion of an aulacogene of upper Proterozoic to Cambrian age inside the Ibero-Armorican Terrane.Palavras-chave: Zona Centro-Ibérica; Grupo das Beiras; litoestratigrafia; fase sarda; cavalgamentos sardos; estrias; significado geodinâmico; aulacógeno; inversão transpresssiva dextra.Resumo: A área de Rosmaninhal (Beira Baixa, Portugal Central) foi cartografada em detalhe. A litoestratigrafia do Grupo das Beiras (partedo Complexo Xisto-Grauváquico — CXG, com idade do Câmbrico ao Proterozóico superior) foi estabelecida, o que permitiu cartografar dobras ecavalgamentos de idade sarda, entre o Ordovícico e Câmbrico, às quais são sobrepostas dobras variscas com clivagem de plano axial, produzindoestruturas de interferência. Propomos que a deformação sarda é devida à inversão dum aulacógeno em regime de transpressão dextra do Proterozóicosuperior ao Câmbrico no terreno Ibero-Armoricano.INTRODUCTIONThe present geological mapping study is part ofResearch Project 1.2 of the Geological Survey ofPortugal «The Stratigraphy and Structural Study ofthe Xisto-Grauváquico Complex-XGC»; it covers theregion of the Centro-Iberian Zone located near its SWlimit (LOTZE, 1945), N of the Tejo River and W of theErges River, being included in the so-called «BeirasGroup» (SILVA et ai., 1988).The main geologic units present in the area are:metasediments of the Beiras Group, included in the«Douro-Beiras Supergroup» of Cambrian and UpperProterozoic age; quartzite, arenite and siate(Armorican Quartzite Formation) of LowerOrdovician age; continental sediments of Cenozoicage and late-syntectonic variscan granitoid plutons.Acid and basic dykes of pre-variscan and variscan age(fig. 1) are also present.The main structural features show clearly a sardicdeformation phase and superposed variscan orogenicphases. The sardic phase generated upright folds withaxial trends towards 45°±15° and dominant vergence toSE and, less frequently, to NW. The first variscanphase (F1) is caracterizated by WNW-ESE trendingfolds with associated penetrative vertical cleavagesubparalell to the axial planes of the folds.Superposition of sardic and variscan folds (P1. 1, photo 1) originated type 1 and II interference patterns(RAMSAY & HUBER, 1987).The second variscan phase (F2) produced a flatter~2 crenulation cleavage with NNW-SSE trend and~<pitch» to SE.The late variscan phase is expressed by conjugateNNE-SSW to ENE-WSW, sinistral and NNW-SSE toNW-SW dextral systems. These systems are compatible with a N-S stress-field in the Upper Carboniferous(RIBEIRO et ai., 1979). Subvertical «kink-bands» with* Serviços Geológicos de Portugal. Rua da Academia das Ciências,19, 2°, 1200 Lisboa** Departamento de Geologia, Faculdade de Ciências, Universidadede Lisboa, 1200 Lisboa88N-S to NNW-SSW trend and fractures with ESE-WNW to E-W trend, filled by gabbro and microgabbro are also observed. This brittle event was producedby W-E maximum compressive stress, probably in theLower Permian.LITHOSTRATIGRAFy OF THE BEIRASGROUPThe «Xisto-Grauváquico» Complex, also called«Douro-Beiras Supergroup» of pre-Ordovician age, issubdivided into the Douro Group (SousA, 1982) andthe Beiras Group (SILVA et ai., 1988 and ROMÃ0, inpress). Both groups are composed of flyschoid sequences which are exposed in large areas of Northern(Douro Group) and Central (Beiras Group) Portugal.The Beiras Group in the Rosmaninhal-Moradal sector has been recently subdivided from the bottom tothe top into the following units (R0MÃ0, 1990):— Malpica do Tejo Formation: Thin (centimetricto milimetric) bands of laminated pelite and siltstonewith occasional metric sized interbeds of greywacke,in the lower member, and thick sequences of greywacke with pelite and siltstone intercalations in theupper member. The lower member has a thickness ofat least 150-200 metres (base not seen) and the upperone a thickness in excess of 1000 metres.— Rosmaninhal Formation: Microconglomerates,conglomerates and metric layers of greywacke intercalated in a very thick pelitic sequence. This unit has athickness in excess of 1000 meters.Almaceda Formation: A thick sequence madeup predominantly of greywacke with subordinate pelite and bands of siltstone. This unit is lithologicallysimilar to the upper member of the Malpica do TejoFormatjon. The thickness of this unit is unknown,because its top is not exposed in the mapped area, butit should be in excess of 1000 meters.Recently, SEQUEIRA (1991) has found acritarchs inthe upper part of the Beiras Group near the Cabeço dasPopas region, just north of our study area, which mdi-cate a Middle to Upper Vendian age (UpperProterozoic).The Armorjcan Quartzite Formation (LowerOrdovician) is composed of quartzites with minorintercalations of pelite; it rests unconformably on theBeiras Group sediments. Between the ArmoricanQuartzite Formation and the Beiras Group another unitcrops out. II is locally composed of conglomerate, arenite, quartzite and volcanjcs, and has been given diferent names: Serra Gorda Formation (SEQUEIRA, 1991),Envendos Formatjon (RIBEIRO et ai., 1991), Vale doGrou Formation (SILvA ei’ ai., 1988) and «SérieIntercalar» (CONDE, 1971). This unit is conformable toslightly unconformable with respect to the overlyingArmorican Quartzite and is strongly unconformable onthe Beiras Group.REGIONAL STRUCTUREIn the study area, the Beiras Group has a generalstructure with WNW-ESE trend and the ArmoricanQuartzite Formation shows NW-SE trend; thus there isan unconformity between both units (fig. 2). Themetasediments of the Beiras Group are deformed bysardic and variscan phases and the ArmoricanQuartzite only by variscan phases.The Malhadio-Serrinha das Corgas Antiform andthe Rosmaninhal Synform represent first ordermacroscopic structures. B oth structures were generated by the interaction of the sardic upright folds with afirst order variscan anticline-syncline pair, with development of axial plane cleavage (si). This interferenceoriginates oscillation in the strike and dip of the limbsof the major structures.In the north limb of the Malhadio-Serrinha dasCorgas Antiform the normal succession is found, i. e.,upper member of the Malpica do Tejo Formation overIam by the Rosmaninhal Formation, as we walktowards the NNE. The south limb of the antiform isreversed, dipping to the NNE except near theArmorican Quartzite (rio Aravil area) where the succession is normal, dipping to SSW. This antiform verges to the NNW and its core becomes steeply plungingwhen it crosses a sardic syncline, with subhorizontalaxis in the NE-SW direction, in the Pônsul river sector,6 km W of Ladoeiro.Adjacent to the 5 limb of the Malhadio-Serrinhadas Corgas Antiform is the N limb of the Monforte daBeira-Rosmaninhal Synform defining everywhere areverse limb, dipping to the 5. Thus, the Monforte daBeira-Rosmaninhal Synform shows, from E to W, bothlimbs reversed or only the 5 limb reversed.1GEOLOGICAL MAP OF THE ÁREA BETWEEN THE PÔNSULAND ERGES RIVERS, BEIRA BAIXA (CENTRAL PORTUGAL)Recent depositsArmorican Quartaite FormationBEIRAS GROUP~ Rosmaninhal Formationconglomerates and microconglomerates ()~ Malpica do Tejo Formationo o upper member microconglornerates (1MalDica do Tejo FormationLower memberERUPTIVE ROCKSCastelo Branco granites and granodiorites+++ Zebreira granodiorites.~. Segura granites______ Basic dykesIntermediate to acrd dykesContact— — Iníerred contact—J ~ Andaluzite isograd— Fault— — Inferreci fautt~ ThrustInlorred thrustSCALE 1 100 000• . • LADOEIRO •• • •• •• MONFORTE A BEIRA~ri.‘t’NLEGENDz.J~.-rC0)s PNO 3kmFig. 1 — Geological map of the arca between lhe Pônsul and Erges rivers, Beira Baixa (Cenlral Porlugal)./‘ i•~1,,4•N N~ r~:..°°~.---./ \...~SEGURA~:oooaos~:i:~:oo~AS~ ./- -Sardic axial traceLEGENOArmorican Quartzite FormationRosmaninhal Formation000000. ConglomeratesMalpica do Tejo FormationJ—X .—~_x — S y n cli n eVariscanaxial trace [_x__!_x_Anticline— — Anti c li n eSardicaxial tracejSyncline1+•-~NormalBedding ReverseFig. 2— Schematic map of the regional structures (scale 1/200 000).a — Identification óf Malhadio-Serrinha das Corgas Antiform and Monforte da Beira-Rosmaninhal Synform and its relation with the Armorican Quartzite syncline of Monforte da Beira.b — Axial zone of the Malhadio-Serrinha das Corgas Antiform./.,x• .X•77$Variscan axial trac7-ThrustO 6km91THE VALE DA MORENA AND BOIZANATHRUSTSThree exposures of the upper member of theMalpica do Tejo Formation, contrasting by the predominance of the greywacke component with the surrounding sequences of mainly pelitic composition,occur in the SE comer of the study area; we namedthem the Fervedouro, Vale da Morena and Boizanasubdomains (fig. 1).The Fervedouro structure is a brachyanticline generated by the interference of a sardic anticline of longwavelength, vergent to WNW, with shorter wavelength variscan folds (R0MÃ0, in press).The Vale da Morena structure has a NW limb younging to NW; it corresponds to the envelope of a sardicanticline cut by higher order variscan folds vergent tothe 5W. The SE limb shows that the younging to NWRosmaninhal Formation is dipping below the olderupper member of the Malpica do Tejo Formation, alsoyounging to NW. This is due to the presence of theVale da Morena thrust (fig. 3). This thrust is itselfaffected by the variscan folds and cleavage, causinghighly variable dips in the thrust plane.The Boizana structure is similar to the Vale daMorena one, with the Boizana thrust carrying theMalpica do Tejo upper member over the RosmaninhalFormation situated to the SW. Both thrust planesduplicate the stratigraphic sequence from NW to SE.Thrust indicators were defined in the Vale da Morenathrust plane by quartz slickensides at the millimeter tocentimeter scale oriented N60-75W, 55-65 SE (P1. 1,photo 2).An outcrop of Malpica do Tejo greywacke situatedbetween the Vale da Morena and Boizana structurescorrespond, probably, to a «Klippe» of the Vale daMorena thrust but its geometry is ill defined due tolack of exposure.In these areas the L1(S1’~S0) intersection lineationwas systematically mapped at the 1:16 000 scale(fig. 3). By drawing dip isogons of this lineation it ispossible to reconstruct the geometry of the sardic folds(fig. 4). Looking at the isogon rnap we notice that inthe hangingwall of the Vale da Morena thrust the sardic axes strike NNE and in the footwall they strikeNE, being truncated by the thrust plane. This suggestthat the arnount of thrusting is increasing to SSW.A schernatic block-diagram (fig. 5) of the Vale daMorena thrust was constructed from outcrop data; itshows clearly the truncation of the footwall beds bythe thrust plane and also some backfolding.GEODYNAMIC SIGNIFICANCE OF SARDICTHRUST TECTONICSThe geodynamic significance of sardic thrust tectonics observed in the Rosmaninhal area can be analysed if we put it on the larger context of the Centro-Iberian Zone of the Iberian Terrane (DALLMEYER &MARTINEZ GARCIA, 1990) and in the even larger context of the Ibero-Armorican Arc, IAA (RIBEIRO, DIAS& SILVA, in press).In fact, some zones of the Iberian Terrane show apalaeogeographic evolution rnarkedly similar to thecorresponding zones in Armorica. The more affinezones seem to be the Centro-Iberia (CIZ) and Centro-Armorican (CAZ) Zones that forrn the nucleous ofboth variscan rnassifs. The palaeogeographic evolution of both zones is very similar from Cambrian toDevonian times (ROBARDET, 1976; YOUNG, 1990) andnew data (RABU et ai., 1990) show that Brioveran, forrnely considered to be Upper Proterozoic in Britannybut nevertheless very similar to the «XistoGrauváquico» Cornplex of CIZ, which is Cambrian(REBELO & ROMANO, 1986) at least in the upper part,is also post 540 MY in age. This is very importantbecause the sardic unconforrnity of CIZ, betweenUpper Cambrian and Lower Ordovician is also presentin Armorica, with the sarne age, and has been previously considered part of Cadomian cycle. We cantrace a belt affected by the sardic deformation phase,disappearing to the inner side of the IAA and also,probably, to the outer side of the IAA (fig. 6). What isthe geodynarnic significance of this deformation eventwhich produced thrusting and upright folding at highangle to variscan grain in the CIZ (SILVA & RIBEIRO,1985) and spectacular recumbent and upright foldingin the CAZ, also at high angle to the variscan grain?At the sarne time in other zones on both sides thedeforrnation vanishes cornpletely (West-AsturianLeonese Zone) or partially (Ossa Morena Zone) butthe palaeogeographic trend of the Carnbrian is nearlyparallel to the variscan grain. We propose the following rnodel, based on the comparison with transpressiveLEGEND - Contact— —— —- lnferred contactBeiras Group - “Xisto-grauvaqujco” Complex1 ThrustRosmanjnhal FormatjonUpper Proterozojc ~~ lnferred thrust__________ CambrianLI Lii Malpica do TejoNormal polarity 1L Bedding( (50)Mafjc rocks (Lower Permjan) ,~ Reverse polarity jGabbros and microgabbros Primary cleavage (Si)\ Lineatjon (Li)Fig. 3— Geological map ofthe Vale da Morena and Boizana structures (scale 1/16 000).LEGENDBeiras Group - ~‘Xisto-grauv~quico” ComplexRosmaninhal Formation 1 ThrustUpper ProterozoicCambrian~1 1 1 II ~ Inferred thrustii 1 Malpica do Tejo Formation jAntiform hinge zoneMafic rocks (Lower Permian)Gabbros and microgabbros Synform hinge zoneFig. 4 — Axes of rnesoscopic structures of Sardie age.94inversion of Upper Jurassic age in the High-Atlasaulacogene (SCHAER, 1986).The CIZ-CAZ correspond to an aulacogene insidethe Ibero-Armorican undisrupted Terrane that suffersat 500 MY, a very early dextral transpressive inversion(SILVA & RIBEIRO, 1985) with folding, bimodal magmatisrn but no cleavage — except perhaps near somelocal intrusions — and is later incorporated in the variscan coilisional belt, since 390 MY. We can comparethe difference in age between the early inversion andthe beggining of coilision in the High Atlas (200 andless than 100 MY, respectively) and in Iberia (500 andless than 390, respectively) and see that they are of thesarne order of magnitude. We conclude that the intracratonic sardic trough extended with no discontinuityfrom Iberia to Armorica.Fig. 5 — Schematic block-diagram of the Vale da Morena thrust.ACKNOWLEDGEMENTSA. RIBEIRO acknowledges support from TESIS project (JNICT). We are grateful to the Drawing Sectionof the SGP for preparing the figures.Fig. 6 — Tectonic model for the transpressive sardic inversion of theCentro-Iberian Zone aulacogene.REFERENCESCoNDE, L. E. N. (1971) — Existência em Portugal de uma série superior à «Formação Xistosa da Beira» e inferior ao Ordovícico.Est. Geol., 36: 131-138.DALLMEYER, R. D. & MARTINEZ GARCIA, E. (1990) — Introduction tothe Pre-Mesozoic Geology of Iberia. lo: Pre-MesozoicGeology of Iberia. J. Dallmeyer & E. Martinez Garcia (Eds.),Springer- Verlag, pp. 3-4.LOTZE, F. (1945) — Zur Glienderung der Varisziden der IberrischenMeseta. Geotek. Forsth., 6; 78-82 (trad. espanhola por J. M.Rios, in Pubi. Ext. Geol. Espafla, t. V).RABU, D., BRUM, J., CHANTRAINE, J., AIJVERAI, D., BALLEVRE, M.,CHAUVEL, J. J. & LEF0RT, J. (1990) — The North ArmoricanCadomian BeIt (France): anintegrated mapping project. In:Terranes in the Circum-Atlantic Paleozoic Orogens, Internacional Conference ou Paleozoic Orogens in the CentralEurope. Gottingen — Giessen, Aug.-Sept., 1990 (Abstract).RAMSAY, J. & HUBBER, M. (1987) — The tecniques of modem structural geology. Folds and Fractures, London, 2, pp. 678.REBELO, J. A. & ROMANO, M. (1986) — A contribution to the lithostratigraphy and palaeontology of the Lower Paleozoic rocks ofthe Moncorvo region, northeast Portugal. Comun. Serv.Geol. Portugal, Lisboa, 72(1/2): 45-57.RIBEIRO, A., ANTUNES, M., FERREIRA, P., ROCHA, R., SOARES, A.,ZBYSZEWSKI, G., M0ITINH0 DE ALMEIDA, F., CARVALHO, D.& MONTEIRO, H. (1979)— Introduction à la geologie génèraledu Portugal. Sere. Geol. Portugal, Lisboa.RIBEIRO, A., DIAS, R. & SILVA, B. J. (in press) — Genesis of the Ibero-Armorican Arc. (submitted to Geodinamica Acta, Paris).NERIBEIRO, M. A., SOUSA, M.B. & NORONHA, F. (1991) — A Formação(Vulcano-Sedimentar) de Envendos e Rochas ácidas associadas. III Congresso Nacional de Geologia, Abstract.,Coimbra.ROBARDET, M. (1976) — L’originalitè du Segment Hercynien sud-Ibérique au Paléozoique inférieur: Ordovician, Silurien etDévonien dans le north de la Province de Sevilie (Espagne).C. R. Acad. Scí. Paris, D 283: 999-1002.R0MÃO, J. M. (1990) — Geologia da região compreendida entre o rioErg~s e a serra de Moradal. Dissertação para as provas deassistente de investigação. Serv. Geol. Portugal, Lisboa.— (in press) — Litoestratigrafia e tectónica do Grupo das Beiras(Complexo Xisto-Grauváquico - CXG), entre o rio Pônsul eo rio Erges. Boletin Geológico y Minero, Madrid.SCHAER, J. P. (1986) - Evolution and structure of the High Atlas inMarocco. In: The Anatomy of Mountain Ranges. SCHAER, J.P. & RODGERS, J. (Eds.). Princeton University Press, pp. 107--127.SEQUEIRA, A. (1991) — O Grupo das Beiras (Complexo Xisto-Grauváquico) entre Zebreira e Penamacor e a sua relação95com o Ordovícico. III Congresso Nacional de Geologia,Abstract., Coimbra.SILVA, A. F. & RIBEIRO, A. (1985) — Thrust tectonics of Sardic age inthe Alto Douro region (northeastern Portugal). Comun.Sere. Geol. Portugal, 71 (2): 15 1-157.SILVA, F., ROMÃO, J. M., SEQUEIRA, A. & RIBEIRO, L. (1988) —Geotransversal no Grupo das Beiras (CXG) entre os sinclinórios de Mação e Penha Garcia (Centro-Leste de Portugal).X Reunião do Oeste Peninsular. Abstract. Bragança.SOUSA, M. B. (1982) — Litoestratigrafia e estrutura do «ComplexoXisto-Grauváquico» ante-Ordovícico, Grupo do Douro (NEPortugal). Tese de Doutoramento, Universidade de Coimbra.YOUNG, T. (1990) — Ordovician sedimentary facies and faunas ofsouthwest Europe: Palaeogeography and Biogeography,Geol. Soc. Mem., 12: 42 1-430.Manuscript received September 1992

Thrust tectonics of sardic age in the Rosmaninhal area (Beira Baixa, Central Portugal)

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Thrust tectonics of sardic age in the Rosmaninhal area (Beira Baixa, Central Portugal)

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