Mechanics of Thick-Skinned Variscan

+ 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).

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

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]).

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Fig. 2. Geological map and interpretative cross section of the Miranda do Douro gneissic complex (adapted from Castro et al. [9]). Fig. 2. Carte et coupe géologique interprétative du complexe gneissique de Miranda do Douro (adaptée de Castro et al. [9]). 122 134 123 include relicts of 605 Æ 13 Ma zircons, suggesting that early extensional detachment, generated near the 135 124 the orthogneisses derived from partial melting of Cambro-Ordovician (‘‘Sardic Phase’’, sensu latu) with 136 125 Cadomian source rocks. The banded gneisses show a top-to-the-west normal sense of shear, responsible for 137 126 Cadomian foliation cut by the intrusive contact with the the tectonic denudation. It is later reactivated as a D1 138 127 orthogneisses and with a slight obliquity due to the D1 décollement with top-to-the-E reverse sense of shear, 139 128 intense variscan deformation that reorientsUNCORRECTED the Cado- overprinting almost PROOF completely the previous detach- 140 129 mian foliation towards the D1 Variscan foliation. ment fabrics. This shear zone is related to the high 141 130 Thinning of the Cambrian cover above the Miranda rheologic contrast between the low-grade Palaeozoic 142 131 do Douro gneissic complex indicates tectonic denuda- cover and the high-grade Cadomian basement. This 143 132 tion during deposition of the slate/greywacke complex, situation is exceptional, contrasting with the general 144 133 throughout inﬁlling of the CIZ intracratonic rift [40]. picture in the internal zones of IBVA where there is no 145 134 Therefore, the Cércio shear zone is interpreted as an detachment between the cadomian basement and it’s 146

A. Ribeiro et al. / C. R. Geoscience xxx (2009) xxx–xxx 5 146 173 147 Palaeozoic cover; this anomalous situation is due to pre- absence of intense ductile deformation along the contact 174 148 Ordovician basement high at the boundary between the suggests a possible mantled gneiss dome of Cadomian 175 149 NE West-Asturian Leonese (WALZ) and the SW age by correlation with the Miranda do Douro Complex, 176 150 Central-Iberian troughs. but this interpretation is not yet confirmed by 177 151 Synchronicity between the Miranda do Douro geochronological data. 178 152 orthogneisses and the Ollo de Sapo volcanic complex 153 (Fig. 2, [48]) favours a common origin for both rocks 2.3. Foz do Douro Complex (Finisterra Plate) 154 groups. The former represents in situ partial melting of 179 155 Cadomian gneisses, whereas the latter corresponds to The Foz do Douro metamorphic complex located 180 156 volcanic (shallow) rocks that were emplaced near the near Porto (Fig. 4, [31]) outcrops in the northern limit 181 157 Cambrian–Ordovician boundary. Mafic dykes, intrud- of the Finisterra Plate. It includes (augen-, biotite- 182 158 ing both the Seixo-Pombal high-grade gneissic complex rich, and leucocratic) orthogneisses, amphibolites and 183 159 and the gneissic protholiths of the Cércio blastomylo- metasediments. Available U–Pb geochronological data 184 160 nites, suggest that high-heat flux associated with initial (605 Æ 17 and 567 Æ 6Ma[31]) indicate a Cadomian 185 161 astenospheric upwelling during CIZ Lower Palaeozoic age for the calc–alkaline (synorogenic) igneous proto- 186 162 rifting was instrumental to trigger Cadomian basement liths, whereas Rb–Sr whole rock dating on biotite 187 163 partial melting. gneisses yielded an age of 575 Æ 5Ma[31]. The Foz do 188 Douro metamorphic complex forms a steep belt strongly 189 164 2.2. Rio Águeda mantled gneiss dome (CIZ, Iberian affected by Variscan deformation. It is bounded on the E 190 Terrane) side by the Porto–Tomar–Ferreira do Alentejo shear zone 191 165 (PTFASZ), a north–south (N–S) dextral transform fault 192 166 The deeper structural levels of CIZ to the south and that separates Finisterra and Iberian Plates, [38] and, on 193 167 east of the Miranda do Douro Complex are represented the W side, by thrusting over the lower-grade units of the 194 168 by a series of migmatitic domes both in Spain (as the Espinho Domain Upper Proterozoic sequence (which 195 169 Tormes and Martinamor Domes [48]) and Portugal (Rio show affinities with OMZ [17,41]). The Cadomian 196 170 Águeda Dome [15]). The Rio Águeda Dome (Fig. 3)is metamorphism affecting the Foz do Douro complex 197 171 composed of high-grade migmatitic gneisses in contact records a much higher pressure regime than that related to 198 172 with the Cambrian/Upper Proterozoic low-grade the Variscan metamorphism and granitic melts produc- 199 173 (mono-)metamorphic slate/greywacke complex. The tion/emplacement in the Espinho Domain [17]. 200

UNCORRECTED PROOF

Fig. 3. Geological map and interpretative cross section of the Rio Águeda anatectic dome (adapted from Dias et al. [14]). Fig. 3. Carte et coupe géologique interprétative du dôme anatectique de Rio Águeda (adaptée de Dias et al. [14]).

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Fig. 4. Geological map and interpretative cross sections of the Foz do Douro metamorphic complex (adapted from Chaminé et al. [11]). Fig. 4. Carte et coupes géologiques interprétativesUNCORRECTED du complexe métamorphique de Foz do Douro (adaptéePROOF de Chaminé et al. [11]). 200 204 201 2.4. Neoproterozoic sequences in Finisterra Plate PTFASZ (between Porto and Tomar), as well as on the 205 and NE domain of OMZ (Iberian Terrane) NE domain of OMZ (which is bounded to the southwest 206 202 by the Tomar–Badajoz–Córdoba shear zone [TBCSZ]). 207 203 This section summarizes the main results of recent Consideration of the above mentioned domains is based 208 204 work on the Neoproterozoic sequences to the W of the on the fact that TBCSZ is probably a Cadomian suture, 209

A. Ribeiro et al. / C. R. Geoscience xxx (2009) xxx–xxx 7 209 259 210 as suggested by the occurrence of ophiolitic and HP belt, comprising migmatitic ortho- and paragneisses, as 260 211 assemblage relicts [35,38]. Significantly, the Neopro- well as ophiolitic relicts and retrograded (to amphibolite 261 212 terozoic sequences of the Finisterra Plate and of the facies) eclogite lenses [44]. These rocks preserve 262 213 OMZ NE domain, being both part of the Iberian Cadomian igneous/metamorphic ages [42] and were 263 214 Terrane, are easily correlated; those sequences con- affected by Early-Variscan partial melting (particularly, 264 215 stituted the sources for conglomerates and other along the steep axial zone between Crato [west of 265 216 sediments deposited in the upper part of the slate/ Portalegre] and SE Azuaga) related to Lower Palaeo- 266 217 greywacke complex in CIZ [40], and were geographi- zoic extensional events; these features indicate that the 267 218 cally close at the beginning of the Variscan cycle high-grade rocks preserved inside the TBCSZ represent 268 219 (before the main plate displacement along the PTFA reworked Cadomian basement, according to a general- 269 220 transform). ised cross section [38] (Fig. 1). 270 221 The upper unit of the Neoproterozoic sequences Recent fieldwork indicates that the TBCSZ has a 271 222 (‘‘Série Negra’’) is characterized by sequences of black northwestern tip in the Abrantes region (Fig. 5). Indeed, 272 223 phyllites with cherty layers and interlayered bimodal the northeast-verging northeastern branch of TBCSZ is 273 224 volcanics that indicate a possible back-arc extensional connected to its southeast-verging southwestern branch 274 225 setting [18]. These sequences were strongly deformed, by means of a macrosheath fold whose nose points to 275 226 displaying D1 recumbent folds with N–S axes; in cherty NW; this is due to the buttress effect of the PTFASZ (see 276 227 layers, however, it can be seen that D1 structures refold above), an interplate transform that blocks the TBCSZ 277 228 early recumbent folds with E–W axes, which are propagation towards northwest, as implied by north- 278 229 interpreted as a preserved feature developed during westwards increasing 40Ar/39Ar cooling ages [35]. 279 230 Cadomian times [34]. A staurolite-garnet bearing Thus, it is suggested that TBCSZ acted as an intraplate 280 231 micaschists/gneissic unit occurs beneath the ‘‘Série transform during the Variscan cycle, partially obliterat- 281 232 Negra’’ in the Tomar region (part of the southern ing an earlier Cadomian suture that controlled Lower 282 233 segment of the Finisterra Plate); the sharp contacts Palaeozoic intracratonic rift. 283 234 among different Cadomian basement rocks, displaying The deep structure and kinematics of the TBCSZ 284 235 variable metamorphic grade, suggest a possible during the Variscan cycle1 constitutes a good example Q2285 236 discontinuity during the Cadomian Orogeny, which of vertical coupling/decoupling across the lithosphere, 286 237 implies that a polyphase evolution may have had took illustrating the concept of attachment [46,47] or 287 238 place. accommodation zones [22]. Along most of its trace, 288 239 The northeast domain of OMZ preserves critical the axial zone (or Central Unit) of TBCSZ is a steep 289 240 exposures of Cadomian rocks covered by the Cambrian structure with strike–slip sinistral kinematical regime, 290 241 basal metaconglomerate (containing pebbles of the separating two branches with opposing vergences of a 291 242 underlying rocks) in angular unconformity. The upper transpressive flower structure (Fig. 5). The fabrics 292 243 series (‘‘Série Negra’’) are similar to the Brioverian generated in the two branches are of the same age 293 244 (Upper Proterozoic) of Armorica and Finisterra, and because they are both developed previous to a Culm 294 245 rest on top of staurolite-garnet bearing micaschists [34]. synform in the core of the flower [44] and give the same 295 246 Augen gneisses and a gneiss–migmatitic complex isotopic cooling ages [35]. The NW TBCSZ tip sector at 296 247 represent the lower series. Below this complex, there Abrantes and the corresponding flat-lying SE sector of 297 248 is a thrust slice of mafic granulites whose age and Hinojosa del Valle, Hornachos, represent attachment or 298 249 petrologic significance remains uncertain [34,39]. The accommodation zones, with top-to-northwest sense of Q3299 250 development of a Cadomian tectonometamorphic cycle shear. In these zones, the upper brittle part of the 300 251 is proved both by the presence of an axial plane lithosphere, strike–slip faulting evolves to a subhor- 301 252 cleavage that stops against the Cambrian basal izontal ductile shear (to northwest) in the lower part of 302 253 unconformity, as well as by K–Ar dating of regional the lithosphere, consistent with the dominant sinistral 303 254 biotite [6]. strike–slip regime. The IBERSEIS seismic profile data 304 UNCORRECTED[44] favour the concept PROOF of accommodation at the 305 255 2.5. TBCSZ and southwest domain of OMZ (Iberian Terrane) 256 1 A. Ribeiro, J. Romão, R. Dias, A. Mateus, E. Pereira, Deep 257 The major, sinistral TBCSZ corresponds to a structure of strike–slip deformation belts: from kinematics to mecha- 258 transpressive flower structure located near the OMZ– nical coupling/decoupling across the lithosphere (two examples from 259 CIZ boundary [38]. The TBCSZ axial zone is a steep SW Iberian Variscides) (in prep.).

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Fig. 5. Kinematics and deep structure of the Tomar–Badajoz–Córdova shear zone (TBCSZ), from southeast to northwest. A. Hinojosa del Valle–Hornachos sector, accommodation zone. B. Badajoz–Portalegre sector, ﬂower structure with transpressive left-lateral regime. C. Abrantes sector (NW tip of TBCSZ) and interference domain with Porto–Tomar–Ferreira do Zêzere shear zone (PTFASZ). Fig. 5. Cinématique et structure profonde de la zone de cisaillement Tomar–Badajoz–Córdova (TBCSZ), du sud-est au nord-est. A. Secteur de Hinojosa del Valle–Hornachos, zone d’accommodation. B. Secteur de Badajoz–Portalegre, structure en ﬂeur en regime transpressif sénestre. C. Secteur de Abrantes (extremité nord-ouest de TBCSZ) et domaine d’interférence avec la zone de cisaillement Porto–Tomar–Ferreira do Zêzere (PTFASZ).

305 334 306 middle-lower crust interface rather than attachment; the the prevailing Upper Proterozoic autochthonous 335 307 main reﬂective horizon controls a large-scale, sill-like sequences (mostly known as Série Negra Group 336 308 intrusive body according to some authors [7], postdating [SNG] [33]) were only affected by mild (greenschist 337 309 the main displacement in TBCSZ. The presence of an facies) Variscan metamorphism. Hence, some ‘‘pre- 338 310 accommodation zone within the deeper part of TBCSZ Variscan’’ (calc–alkaline) magmatic bodies and med- 339 311 favours a model of predominant strike–slip (interplate ium-/high-grade metamorphic assemblages still pre- 340 312 or intraplate) regime with (partial) vertical coupling serve Lower Cambrian/Upper Proterozoic ages ( 540– 341 313 across the lithosphere. Accordingly, the preserved 600 Ma [12,32,43]), indicating that large-scale Cado- 342 314 suture rocks (eclogitic and ophiolitic remnants; referred mian tectonics must have been operative in the OMZ. 343 315 above) should have been inherited from a previous At Serpa-Briches, Alvito-Viana do Alentejo and 344 316 orogenic cycle, recording an earlier Cadomian subduc- Escoural (Alentejo, S Portugal), SNG lithological units 345 317 tion/obduction geodynamic process. From this per- (garnet-biotite schists/felsic gneisses–migmatites/ 346 318 spective, the kinematical regime during the Variscan amphibolites) are tectonically imbricated along large- 347 319 cycle is consistent with the overall evidence supporting scale, west-to-southwest-verging anticline, D1 Variscan 348 320 a polycyclic evolution (Cadomian overprinted by structures [33]. Locally, at the core of these structures, 349 321 Variscan) for the TBCSZ main structure. microstructural analysis demonstrates that the earliest 350 322 Towards the southwest of TBCSZ, demonstrable Variscan fabric was superimposed on an older 351 323 high-grade Variscan metamorphism is restricted to a (compressive) shearing foliation; this is in accordance 352 324 prominent ( 340 Ma) high temperature/low pressure with geochronological data and provides further support 353 325 (HT/LP) band along the southern border of the OMZ to the hypothesis of polycyclic geodynamic evolution 354 326 [4,8,13,36] and to ( 370 Ma eclogite) allochthonous for the authocthonous units on the southwestern domain 355 327 klipen [21,26] within the western domain of OMZ of the OMZ. 356 328 (Alentejo, South Portugal). Apart fromUNCORRECTED these main PROOF 329 occurrences, only a few small medium-/high-grade 3. Geodynamic evolution and mechanics of 357 330 (low-P) metamorphic areas (e.g., Valuengo, Mones- overprinting of Cadomian basement by the 358 331 terio; SW Spain) are found in the central OMZ, whose Variscan cycle 332 origin has been ascribed to thermal doming during 359 333 Lower Palaeozoic (500–450 Ma) continental rifting Geological data summarized in the previous section 360 334 tectonics [16,38]. Thus, over large areas of the OMZ, can be rationalized in terms of the Variscan and 361

A. Ribeiro et al. / C. R. Geoscience xxx (2009) xxx–xxx 9 361 413 362 Cadomian geodynamic evolution, proceeding back- This allowed development of spectacular Helvetic style 414 363 wards in time. A pervasive and distributed extensional nappes within crustal domains subjected to strong 415 364 tectonics event took place across the Iberian Terrane tangential tectonics (e.g., Mondoñedo [WALZ] and 416 365 (comprising the WALZ, CIZ and OMZ), particularly Juromenha–Monasterio [OMZ] nappes [15,38,48]) and 417 366 during Upper Cambrian and Lower Ordovician times promoted basement shearing/folding within transpres- 418 367 with high strain rate that generates mylonite fabrics in sive domains along the axis of flower structures (e.g., 419 368 the localized extensional detachments. This tecto- TBCSZ, including ‘‘relicts’’ of a Cadomian suture which 420 369 nothermal event extended until Lower Devonian and controlled the aborted rift during Lower Palaeozoic times 421 370 was initially triggered by high heat flux recorded by [15,38,48]). 422 371 widespread bimodal magmatism and LP–HT meta- CAT of NW Iberia [38] derives from the thinned 423 372 morphism, reflecting mantle-derived magma under- continental margin of Armorica (it may be seen as the 424 373 plating (astenospheric upwelling) and related partial Variscan equivalent of the Austro-Alpine Nappes) and 425 374 melting of Cadomian basement (igneous plutons/ were affected by extensional tectonics (with detachment 426 375 laccoliths intruded into near the Cambrian–Ordovician to west, in present geographical coordinates) coeval 427 376 interface). During this time span, the Cadomian with mafic–ultramafic underplating below the Cado- 428 377 basement was thinned by extensional detachments mian crust; these features are related to the initial stages 429 378 (acting both at midcrustal levels and within the of Rheic ocean opening, splitting Armorica from 430 379 Palaeozoic cover), explaining the presence of several Avalonia near the Cambrian–Ordovician boundary. 431 380 discontinuities that are recognised at the cover–base- The resulting tectonometamorphic regime constrained 432 381 ment interface [38]. From Ordovician towards Lower both the Cadomian basement décollement from 433 382 Devonian times, extension should have preceded at Palaeozoic cover and the Penninic style crystalline 434 383 lower and steadier strain rates [38], when compared to nappe emplacement [37] that may still be recognized in 435 384 the higher strain rate near the Cambro-Ordovician the Cabo Ortegal Massif [23]. Different levels of 436 385 boundary that corresponds to a pick in the stretching– Cadomian lithosphere (including, underplated mafic– 437 386 extension history. ultramafic igneous bodies ( 500 Ma) folded and 438 387 In the course of the Variscan Orogeny, the imbricated with Cadomian subcontinental upper mantle 439 388 mechanical behaviour of basement-cover interfaces is (Fig. 6A) may still be observed within basement nappes 440 389 twofold. Tectonic and seismic data [44] for the thin- of CAT (despite the strong Variscan tectonometa- 441 390 skinned thrust belts in external zones (Cantabria [CZ] in morphic overprinting). 442 391 the Iberian Terrane, and SPT), support décollement of Inside the Autochthonous of the Iberian Terrane 443 392 Palaeozoic sequences from the underlying Cadomian Internal Zones, and in the Finisterra Plate, direct 444 393 basement that may have attached a tegument of the exposure of Cadomian basement is restricted to 445 394 basal Palaeozoic succession. In contrast, basement– scattered domains (see above) and its extrapolation to 446 395 cover interface décollements are absent from the thick- depth remains conjectural (even assisted with seismic 447 396 skinned belts in internal zones (WALZ, CIZ and OMZ, profiling). In some domains (Miranda do Douro in CIZ 448 397 in the Iberian Terrane). Field mapping differentiates and Foz do Douro in Finisterra Plate), the lowermost 449 398 between a set of Upper Cambrian–Lower Ordovician exposed structural level is eventually incorporated in 450 399 magmatic complexes (including granitoid plutons that fold basement nappes (Peninic style), corresponding to 451 400 intrude metasediments of those same ages and coeval their root zones. It is inferred that, within these domains, 452 401 felsic/mafic volcanism emplaced on top of them [24]), the rheology contrast between Cadomian basement and 453 402 from another set of antiform basement cores (with no Palaeozoic cover was attenuated and locally annihilated 454 403 signs of localized basal décollement) that display a more by thermal softening of both lithologies during the 455 404 complex deformation history than their Palaeozoic Cambrian to Lower Devonian high heat flow exten- 456 405 cover; within the latter, mafic dykes (displaying sional regime. This long-term thermal regime inhibited 457 406 monocycle, Variscan, tectonothermal evolution, such development of a generalized décollement at the 458 407 as that of the Palaeozoic cover) cut throughUNCORRECTED previously basement-cover interface, PROOF although allowing restricted 459 408 sheared basement gneisses that display (clear) poly- decoupling in domains where the previously acquired 460 409 cycle tectonic/metamorphic history [38]. high rheology contrast was preserved. Inhibition of 461 410 In the thick-skinned domains of Iberian Terrane, early décollement tectonics may have been (locally) extended 462 411 overprinting of Cadomian basement (folding and ductile towards Neovariscan times (from Upper Devonian to 463 412 shearing) took place during Eovariscan times, under a Westphalian [38]), due to (LP–HT) orogenic thermal 464 413 dominant medium-/low-pressure tectonothermal regime. doming and coupled partial melting, as recorded by Rio 465

10 A. Ribeiro et al. / C. R. Geoscience xxx (2009) xxx–xxx

Fig. 6. Crustal evolution of different domains (Fig. 1) of Iberian Variscides Internal Zones during the end of the Cadomian cycle (590–540 Ma), Cambrian to Lower Devonian extension regime (540–390 Ma) and during convergent Variscan orogeny (390–290 Ma) and regional metamorphism. A. Continental Allochthonous Terrane (CAT) of NW Iberia with high pressure–low temperature metamorphism. B. Autochthonous domains with high heat flow during extension and high temperature–low pressure metamorphism during Variscan orogeny. C. Autochthonous domains with low heat flow during extension and medium to high temperature–low pressure metamorphism during Variscan orogeny. Fig. 6. Évolution crustale de différents domaines (Fig. 1) des zones internes des Variscides ibériques pendant la fin du cycle Cadomien (590– 540 Ma), en régime extensionnel du Cambrien au Dévonien Inférieur (540–390 Ma) et orogenèse convergente Varisque (390–290 Ma) et métamorphisme régional syntectonique. A. Formation du continental allochthone du NW Ibérique métamorphisme haute pression–basse température. B. Domaines autochthones à flux de chaleur élevé pendant l’extension et métamorphisme haute température–basse pression pendant l’orogenèse Varisque. C. Domaines autochtones à bas flux de chaleur pendant l’extension et à métamorphisme de haute/moyenne température–basse pression pendant l’orogenèse Varisque.

465 476 466 Águeda migmatitic domes (Fig. 3) and elsewhere in the complete Variscan reworking of Cadomian crust 477 467 CIZ [48]. These interpretations are consistent with those irrespective of type and strength of previous hetero- 478 468 advanced for other orogenic belts, namely the well- geneities and anisotropies generated during the Cado- 479 469 exposed and comprehensively studied Caledonian Belt mian cycle and the Cambrian to Lower Devonian 480 470 of NW Scotland [37]. There, basement/coverUNCORRECTED interface preorogenic stages PROOF of the Variscan cycle. The presence 481 471 is folded and sheared (obliterating without discontinuity of these heterogeneities and anisotropies will never- 482 472 the original unconformity), being intersected by later theless concentrate the variscan reworking (Figs. 2–5). 483 473 thrusts. Indeed, thin-skinned models are well supported Below the CAT of NW Iberia, two different styles of 484 474 by both tectonic and seismic data in foreland domains, Variscan thick-skinned tectonic (mechanical) over- 485 475 but become inadequate for thick-skinned tectonics in printing may be considered. In domains characterized 486 476 internal orogenic zones [37]. We conclude that there is a by early Variscan high heat ﬂow, the Palaeozoic cover 487

A. Ribeiro et al. / C. R. Geoscience xxx (2009) xxx–xxx 11 487 544 488 remained attached to Cadomian basement. However, Icartian/Eburnian) cannot be excluded and may be 545 489 subsequent tangential tectonics during the Variscan preserved either in the Cadomian nuclei or in the 546 490 compressive stages may have induced upper/lower crust basement Allochthonous of the Variscan Fold Belt. 547 491 décollement tectonics, similar to the imbricate tectonics Further geochronological studies are clearly needed 548 492 displayed by HP–HT mafic granulites and LP–HT to confirm or reject the records of Cadomian high-grade 549 493 granitic gneisses within CAT [38] (Fig. 6B). Domains events in polymetamorphic rocks preserved in the IBVA 550 494 that sustained a preorogenic lower heat flow developed Internal Zones. Nevertheless, the available data suggest 551 495 extensional detachments during Ordovician–Lower a significant discontinuity between Cadomian and 552 496 Devonian times that were later inverted, given rise to Variscan cycles; this should have constrained the 553 décollements at various levels (Fig. 6C): subsequent lithospheric evolution and, consequently, 554 498499497 the build up of Peri-Atlantic Palaeozoic orogens [25]. 555 500501499 inside the Palaeozoic cover; 502503500 at the cover–basement interface; 4. Concluding statement 504501 within basement levels. 556 505502 Variscan tectonics is well exposed in the Iberian 557 506503 The significance of the previous data and inferences Peninsula and corresponds to a major event in the 558 507 on the mechanics of the Variscan Orogeny is twofold. evolution of the European lithosphere. However, over 559 508 From an evolutionary (time framework) perspective, the large domains of the IBVA,the role of neoformation and 560 509 Variscan cycle is discontinuous relatively to the recycling remains uncertain and clarification of this 561 510 Cadomian cycle on the European lithosphere building issue is of utmost importance to understand the 562 511 process (as discussed below). From a spatial point of geodynamics of the Variscan Cycle. In IBVA, leftovers 563 view, two different tectonic styles can be distinguished: of Cadomian basement is demonstrated by sedimentary 564 513514512 Neoproterozoic and Lower Palaeozoic sequences, as 565 515514 a thin-skinned style with décollement at the cover– well as by major geological events that have been dated 566 516517515 basement interface in the Variscides external zones; within the age range of 620–540 Ma. Indeed, near the 567 518516 a thick-skinned style with décollement at midcrustal Cambrian–Ordovician boundary, a pervasive episode of 568 519517 levels, that was assisted by synorogenic underplating, magma underplating and extensional tectonics (asso- 569 520518 with no generalized décollement at the cover–base- ciated with bimodal magmatism and high heat flux) 570 521519 ment interface in the internal zones. caused basement thinning and tectonic stacking in the 571 522520 IBVA Internal Zones; this reflects contemporaneous 572 523 These different tectonic styles express distinct migration of an intracratonic rifting from the CIZ to the 573 524 mechanical responses to variable boundary thermal SW Variscan suture, between the OMZ and the South 574 525 conditions during the Variscan cycle. Indeed, thermo- Portuguese Terrane. High heat flow regime continued 575 526 chronological modelling of Sm–Nd data from Bragança during Ordovician to Lower Devonian, at least in some 576 527 CAT eclogitic granulites [28] favours the hypothesis domains of the internal zones, as indicated by wide- 577 528 that these rocks have sustained long-term HT conditions spread preorogenic magmatism [15,38,43]. Thin- 578 529 during Lower Palaeozoic times. The observed internal skinned tectonics is expressed by décollements of 579 530 errorchrons represent a natural consequence of the deformed and undeformed Palaeozoic and Cadomian 580 531 inferred HT, slow-cooling thermochronological regime, basement in the IBVA External Zones (Cantabrian and 581 532 being consistent with metamorphic petrology and South Portuguese), whereas in the IBVA Internal Zones 582 533 geological observations [38]; all these features are thick-skinned tectonics (without cover–basement inter- 583 534 inherent to widespread/long-term magmatic activity face décollements) generated Helvetic/Penninic style 584 535 and high heat flux associated with the early Variscan nappes (in tangential tectonic domains) and flower 585 536 continental break-up [43], from Cambrian to Lower upright axial structures along transpressive, intraplate 586 537 Devonian [4]. shear zones (TBCSZ). The mechanical behaviour 587 538 The detailed reconstruction of the CadomianUNCORRECTED cycle reflects attenuation PROOF of rheological contrasts between 588 539 remains very sketchy on the basis of present data. A Cadomian basement and Palaeozoic cover caused by 589 540 spectrum of Cadomian ages (620–540 Ma) can be thermal softening during the Cambrian–Lower Devo- 590 541 interpreted on the context of a polyphasic orogenic nian extensional regime. Deeper décollements (and 591 542 evolution, but it remains to be proved what model fits subsequent strain partitioning) are also inferred to occur 592 543 better the data [30]. Relics of cycles older than the at the upper-lower crust (and at the Moho?) transition, 593 544 Cadomian–Avalonian–Panafrican cycle (Greenville, agreeing with results obtained by seismic [7,44] and MT 594

12 A. Ribeiro et al. / C. R. Geoscience xxx (2009) xxx–xxx 594 644 595 surveys [1,2,27,29,49]. The whole data implies a [8] A. Castro, C. Fernández, H. El-Himdi, M. El-Biad, M. Díaz, J.D. 645 596 significant discontinuity between Cadomian and De la Rosa, F. Stuart, Age constraints to the relationships 646 between magmatism, metamorphism and tectonism in the Ara- 647 597 Variscan Cycles that should have constrained subse- cena metamorphic Belt, southern Spain, Intern. J. Earth Sci. 88 648 598 quent lithospheric evolution across the whole Variscides (1999) 26–37. 649 599 [25,45]. [9] P. Castro, E. Pereira, A. Ribeiro, Dados preliminares da litos- 650 tratigrafia do maciço antigo de Miranda do Douro, Comun. Serv. 651 Uncited references Geol. Portugal 84 (2003) D15–D18. 652 [10] P. Castro, C. Tassinari, E. Pereira, G. Dias, J. Leterrier, Geo- 653 600 cronologia do complexo metamórfico de Miranda do Douro (NE 654 601 Q4 [3,20]. de Trás-os-Montes, Portugal), Implicações geodinâmicas, Ciên- 655 cias da Terra (UNL, Lisboa) V (2003) D29–D30. 656 Acknowledgements [11] H.I. Chaminé, L.C. Gama Pereira, P.E. Fonseca, L.P. Moço, J.P. 657 602 Fernandes, S.P. Rocha, D. Flores, A. Pinto de Jesus, C. Gomes, 658 A.A. Soares de Andrade, A. Araújo, Tectonostratigraphy of 659 603 This paper is dedicated to Philippe Matte, as a Middle and Upper Palaeozoic black-shales from the Porto– 660 604 reconaissence of his major contributions to the current Tomar–Ferreira do Alentejo shear zone (west Portugal): New 661 605 knowledge on geodynamics of the Variscan Fold Belt. perspectives on the Iberian Massif, Geobios. 36 (2003) 649–663. 662 606 [12] R.D. Dallmeyer, C. Quesada, Cadomian vs Variscan evolution of 663 Phillippe Matte was an important mentor of large-scale 40 39 607 international collaboration (e.g., within the scope of the Ossa-Morena zone (SW Iberia): Field and Ar/ Ar mineral 664 age constraints, Tectonophysics 216 (1992) 339–364. 665 608 EUROPROBE, ESF), leading to significant progress on [13] R.D. Dallmeyer, R.D. Tucker, U–Pb zircon age for the Lagoa 666 609 the understanding of Variscan geology, which is augen gneiss, Morais Complex, Portugal: Tectonic implications, 667 610 acknowledged here. The authors would like to extend J. Geol. Soc. 150 (1993) 405–410. 668 611 their acknowlegements to the organisers of the meeting [14] G. Dias, J. Brilha, D. I. Pereira, M. I. C. Alves, P. Pereira, E. 669 670 612 and field trip in homage to Phillipe Matte. Funding from Pereira, N. Ferreira, P. Castro, Z. Pereira, Geologia e Património Geológico dos Parques Naturais de Montesinho e do Douro 671 613 FCT (MCTES, Portugal) was awarded through projects Internacional (NE de Portugal): Caracterização do Património 672 614 MODELIB (POCTI/3569/1999), POCA-PETROLOG Geológico. Unpublished report of the research project PNAT/ 673 615 (UI: 263; POCTI/FEDER–CEGUL) and IBERSUT CTE/15008/99, FCT, Lisboa, Portugal, 2006, 30 p.(plus eight 674 616 (POCI/CTE-GIN/56445/2004). Karel Schullman and attached maps). 675 617 referee’s editorial contributions were crucial to improve [15] R. Dias, A. Araújo, P. Terrinha, J.C. Kullberg (Eds.), Geologia de 676 Portugal no contexto da Ibéria, Univ. Évora, Évora, Portugal, 677 618 the clarity of the original text. 2006, p. 418. 678 [16] I. Expósito, Evolución estructural de la mitad septentrional de la 679 References Zona de Ossa Morena, y sua relación con el limite Zona de Ossa 680 619 Morena/Zona Centroibérica, Ph.D. Thesis, Univ. 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