+ Models CRAS2A 2812 1–13 1 C. R. Geoscience xxx (2009) xxx–xxx http://france.elsevier.com/direct/CRAS2A/ 2 Tectonics 43 Mechanics of thick-skinned Variscan overprinting of 5 Cadomian basement (Iberian Variscides) 6 a a, a a 7 António Ribeiro , José Munhá *, António Mateus , Paulo Fonseca , b c d b 8 Eurico Pereira , Fernando Noronha , José Romão , José Rodrigues , Paulo Castro b, Carlos Meireles b, Narciso Ferreira b 9 10 a Department Geologia and CEGUL, Faculdade de Ciências, Universidade de Lisboa, Ed. C6, Piso 3, Campo Grande, 11 1749-016 Lisboa, Portugal 12 b Department de Geologia, Laboratório Nacional de Energia e Geologia, 4466-956 S. Mamede Infesta, Portugal 13 c Department Geologia and CEGUP, Faculdade de Ciências, Universidade do Porto, Rua do Campo Alegre 687, 14 4169-007 Porto, Portugal 15 d Department Geologia, Laboratório Nacional de Energia e Geologia, Apartado 7586, 16 2721-866 Alfragide, Portugal 17 Received 19 February 2008; accepted after revision 25 November 2008 18 211920 Written on invitation of the Editorial Board 22 Abstract 23 24 Remnants of the Cadomian basement can be found in the Iberian Variscides (IBVA) in several key sectors of its autochthonous 25 units (composed of Neoproterozoic to Lower Palaeozoic metasedimentary sequences) and within the Continental Allochthonous 26 Terrane (CAT). Comprehensive characterization of these critical exposures shows that the prevailing features are related to major 27 geological events dated within the age range of 620–540 Ma. Indeed, near the Cambrian–Ordovician boundary, the IBVA Internal 28 Zones experienced pervasive basement thinning and cover thickening, reflecting diffusive displacement of intracratonic rifting that 29 continued until Lower Devonian times. In the thick-skinned Internal Zones, Helvetic/Penninic style nappes were generated, 30 whereas flower upright axial structures developed along transpressive, intraplate shear zones. These features contrast with those 31 preserved in the thin-skinned IBVA External Zones, dominated by décollements above (un-)deformed Palaeozoic and Cadomian 32 basement. The inferred attenuation of rheological contrast between Cadomian basement and Palaeozoic cover can be explained by 33 inherited fabrics due to thermal softening operated during the Cambrian–Lower Devonian extensional regime. Deeper décollements 34 (and subsequent strain partitioning) are also expected to develop at the upper-lower crust (and at the Moho?) transition, as imaged by 35 the available seismic profiling and MT surveys. The whole data implies a significant discontinuity between Cadomian and Variscan 36 Cycles that should have constrained subsequent lithospheric evolution. To cite this article: A. Ribeiro et al., C. R. Geoscience xxx 37 (2009). 403839 # 2008 Académie des sciences. Published by Elsevier Masson SAS. All rights reserved. 41 Résumé 42 43 Mécanique de la réactivation tectonique profonde du socle Cadomien au cycle Varisque en Ibérie. Dans les Variscides 44 Ibériques (VAIB), on trouve des reliques de socleUNCORRECTED Cadomien dans plusieurs secteurs-clé d’unitésPROOF autochtones (composés par des * Corresponding author. E-mail address: [email protected] (J. Munhá). 1631-0713/$ – see front matter # 2008 Académie des sciences. Published by Elsevier Masson SAS. All rights reserved. doi:10.1016/j.crte.2008.12.003 Please cite this article in press as: A. Ribeiro, et al., Mechanics of thick-skinned Variscan overprinting of Cadomian basement (Iberian Variscides), C. R. Geoscience (2009), doi:10.1016/j.crte.2008.12.003 + Models CRAS2A 2812 1–13 2 A. Ribeiro et al. / C. R. Geoscience xxx (2009) xxx–xxx 44 45 séquences métasédimentaires du Néoprotérozoïque et du Paléozoïque inférieur) et dans la formation du allochtone. La 46 caractérisation d’ensemble de ces affleurements critiques montre que leurs aspects dominants sont en rapport avec des 47 événements géologiques majeurs datés de l’intervalle d’âge 620–540 Ma. En fait, près de la limite Cambrien-Ordovicien, les 48 zones internes de VAIB ont subi un amincissement du socle et un épaississement tectonique de la couverture qui expriment un 49 déplacement diffusif de rifting intracratonique jusqu’au Dévonien inférieur. Dans les zones internes à tectonique profond il y a 50 Q1 mise en place de nappes de style helvétique/pennique, tandis que dans les zones de cisaillement transpressive intraplaque se 51 développe des structures en fleur à zone axiale redressée. Ces aspects contrastent avec ceux qui sont préservés dans les zones 52 externes, pelliculaires des VAIB, dominées par des décollements au-dessus d’un tégument Paléozoïque et du socle Cadomien 53 sous-jacent. L’atténuation impliquée par les contrastes rhéologiques entre la couverture PaléozoïqueetlesocleCadomien 54 peut être expliquée par la présence des fabriques héritées par amollissement thermique qui a opéré pendant le régime 55 extensionnel depuis le Cambrien jusqu’au Dévonien inférieur. Des décollements plus profonds (et le compartimentage dû à la 56 déformation) doivent aussi se développer au niveau de la transition croute supérieure–inférieure (et du Moho ?), d’après les 57 images disponibles de profils séismiques et levers magnétotelluriques. Toutes ces donnés impliquent une discontinuité 58 significative entre les cycles Cadominen et Varisque qui ont dû contraindre l´évolution lithosphérique ultérieure. Pour citer 59 cet article : A. Ribeiro et al., C. R. Geoscience xxx (2009). 60 # 2008 Académie des sciences. Publié par Elsevier Masson SAS. Tous droits réservés. 61 Keywords: Iberian variscides; Cadomian basement; Thick-skinned tectonics 62 Mots clés : Variscides ibériques ; Socle Cadomien ; Tectonique profonde 63 64 94 1. Introduction 2. General features of Cadomian basement in 95 65 the IBVA Internal Zones 66 Several lines of evidence indicate the presence of a 96 67 Cadomian basement inside the Iberian Variscides Cadomian basement in the Continental Allochtho- 97 68 (IBVA), possibly, also including relicts of earlier nous Terrane (CAT) of NW Iberia (Cabo Ortegal, 98 69 tectonic cycles (Grenville, Eburnean). Actually, uncon- Ordenes, Bragança and Morais massifs) have been 99 70 formities of Lower Cambrian metasediments on reported in many recent studies and will not be 100 71 deformed/metamorphosed Neoproterozoic sequences, discussed here [38]. Therefore, this work will focus 101 72 as well as geochronological data on metamorphic and on remnants of Cadomian basement in IBVA, namely in 102 73 plutonic rocks in the range 540–620 Ma, clearly point to some key sectors of the autochthonous Central-Iberian 103 74 the existence of a (reactivated) Cadomian basement (CIZ) and Ossa-Morena (OMZ) zones (Fig. 1). 104 75 within IBVA [15,48]. The data are compatible with 76 indirect evidence for a Pannotian cratonic basement 2.1. Miranda do Douro Complex (CIZ, Iberian 105 77 inside the Variscan orogen [18,38]. Indeed, the Lower– Terrane) 78 Middle Cambrian (carbonate platform) to Lower 106 79 Ordovician (siliciclastic platform) stable depositional The Miranda do Douro gneiss–migmatitic complex 107 80 environment, as well as the isotopic data on recycled (Fig. 2, [19]) outcrops in an antiform developed during 108 81 zircons (included in the Lower Palaeozoic metasedi- the third phase of Variscan deformation (D3) located at 109 82 ments), demonstrates the development of the Lower the NE sector of the CIZ. The core of this complex is 110 83 Palaeozoic sequences on those cratonic areas [18,38]. composed of the Seixo-Pombal banded gneisses and 111 84 In this study, new and revised data (field mapping, migmatites, mantled by the polymetamorphic Vale de 112 85 petrology, geochemistry and ongoing geochronology) Mira paragneisses and schists [19]. Cércio blastomy- 113 86 obtained for restricted domains of the Cadomian lonites, shown on Fig. 2 (derived from felsic–gneissic to 114 87 basement found in IBVA Internal Zones will be amphibolitic protoliths [19]), imply a tectonic contact 115 88 considered. Most of the data here reportedUNCORRECTED concern developed during thePROOF first phase of Variscan deforma- 116 89 key sectors located in Portugal, but its meaning is tion (D1; with thrusting top to E), that put the Cambrian/ 117 90 evaluated in the larger context of the IBVA. The Upper Proterozoic monometamorphic slate/greywacke 118 91 regional data will be briefly reviewed following the complex over the SW limb of the gneissic basement. 119 92 IBVA subdivision in terranes and zones (Fig. 1)as The Seixo-Pombal banded gneisses were intruded by 120 93 accounted in Bard [4] with general cross sections the Miranda do Douro orthogneisses (dated at 121 94 [15,38,48]. 526 Æ 10 Ma to 483 Æ 3Ma[5,10]; see Fig. 2) which 122 Please cite this article in press as: A. Ribeiro, et al., Mechanics of thick-skinned Variscan overprinting of Cadomian basement (Iberian Variscides), C. R. Geoscience (2009), doi:10.1016/j.crte.2008.12.003 + Models CRAS2A 2812 1–13 A. Ribeiro et al. / C. R. Geoscience xxx (2009) xxx–xxx 3 UNCORRECTED PROOF Fig. 1. Schematic representation and interpretative cross section of the Iberian Variscides subdivision in terranes and zones and location of the proposed boundaries between high and low heat flow domains during extension (adapted from Vera [48] and Ribeiro et al. [38]). Fig. 1. Représentation schématique et coupe géologique interprétative des Variscides ibériques de la subdivision en formations et zone localisation des limites entre les domaines à haut et bas flux de chaleurs pendant l‘extension (adaptée de Vera [48] et Ribeiro et al. [38]). Please cite