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The Structure of a Major Suture Zone in the SW Iberian Massif: the Ossa-Morena/Central Iberian Contact

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The Structure of a Major Suture Zone in the SW Iberian Massif: the Ossa-Morena/Central Iberian Contact Tectonophysics 332 12001) 295±308 www.elsevier.com/locate/tecto The structure of a major suture zone in the SW Iberian Massif: the Ossa-Morena/Central Iberian contact J.F. Simancas*,D. MartõÂnez Poyatos,I. ExpoÂsito,A. Azor,F. Gonza Âlez Lodeiro Departamento de GeodinaÂmica, Universidad de Granada, Campus de Fuentenueva, Granada E-18002, Spain Abstract We have investigated the stratigraphy,structure and metamorphism of the boundary between the Ossa Morena Zone 1OMZ) and the Central Iberian Zone 1CIZ),two signi®cant continental portions of the Variscan Iberian Massif. The OMZ/CIZ contact is marked by a strongly deformed and metamorphosed NW±SE trending narrow band,namely,the Central Unit,in which partially retrogressed eclogites are included. During the Middle-Late Devonian the CIZ overthrust the OMZ,and in the footwall km-scale recumbent folds and thrusts developed with decoupling and underplating of the lower crust. At the same time,in the hanging wall there took place intense though localized back-folding and back-shearing. In the Early Carboniferous a transten- sional tectonic regime sank the overthrust block resulting in the exhumation of eclogites. These eclogites probably came from the underthrust OMZ lower crust,and they are at present included in the suture zone 1Central Unit) of this continental collision. The extension is responsible for the origin of a basin and bimodal magmatism on the southern border of the CIZ. A late episode of folding and fracturing signi®cantly contributed to the ®nal complex picture of this suture boundary. q 2001 Elsevier Science B.V. All rights reserved. Keywords: collision tectonics; eclogite exhumation; oblique extensional collapse; Variscan suture; SW Iberian Massif 1. Introduction western Iberian Massif,consisting of a pile of allochthonous tectonic units 1Ries and Shackleton, The Iberian Massif is the westernmost outcrop of 1971) some of which show high-pressure metamorph- the Variscan Orogen,and shows an almost complete ism and/or ophiolitic rocks 1Arenas et al.,1986,1997; transect of this orogenic belt. This transect has been Ribeiro et al.,1990) is particularly relevant. It is a divided into a number of zones shown in Fig. 1a. The commonly accepted view that these units must Cantabrian Zone,to the north,and the South Portu- be rooted in an orogenic suture located somewhere guese Zone 1SPZ),to the south,represent the external either to the north or to the west of their present zones of the Variscan belt in the Iberian Massif. outcrop 1Matte,1986; Martõ Ânez CatalaÂnetal., Consequently,the suture1s) of the orogen must be 1997). Because of the curvature of the orogen located somewhere in between. 1Ibero-armorican Arc),this suture must continue The Galicia Tras-Os-Montes Zone in the north- towards the southern part of the Iberian Massif. More precisely,in the southern Iberian Massif two suture contacts have been recognized: the boundary between the SPZ and the Ossa-Morena * Corresponding author. Tel.: 134-958-243353; fax: 134-958- 248527. Zone 1OMZ),and the boundary between the OMZ and E-mailaddress: [email protected] 1J.F. Simancas). the Central Iberian Zone 1CIZ) 1Fig. 1a). 0040-1951/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S0040-1951100)00262-6 296 J.F. Simancas et al. / Tectonophysics 332 (2001) 295±308 Fig. 1. 1a) Geological sketch of the Iberian Massif showing the main zones; and 1b) Simpli®ed geological map of the region studied; location of geological maps in Figs. 2±4 and composite cross-section 1±10 in Fig. 5 are indicated. The SPZ/OMZ contact can be considered to be a lack of high-pressure rocks 1Bard,1977) and the suture drawing on evidence from the existence of constant tectonic vergence to the SW at both sides basic igneous rocks with oceanic af®nity,namely, of the contact 1Crespo-Blanc and Orozco,1991). the Beja-Acebuches amphibolites 1Bard,1977; Small dispersed klippes containing eclogite-facies Dupuy et al.,1979; Munha et al.,1986) and the basalts rocks have been recently discovered in Portugal in included in the Pulo do Lobo Group 1MunhaÂ,1983). the southern part of the OMZ 1asterisk in Fig. 1a). It Additional important features of this contact are the has been argued that these klippes can be related to the J.F. Simancas et al. / Tectonophysics 332 (2001) 295±308 297 SPZ/OMZ contact 1Fonseca et al.,1993; ArauÂjo et al., consist of migmatitic gneisses. As for the orthog- 1994),but such a connection has not yet been ®rmly neisses,there are several types with different textural demonstrated. and compositional features: leucocratic orthogneisses, The OMZ/CIZ boundary 1Fig. 1) is the second biotitic augen-gneisses and amphibolic orthogneisses suture-like tectonic contact outcropping in the south- 1Apalategui et al.,1980; Azor et al.,1994). The radio- western Iberian Massif 1Burg et al.,1981; Matte, metric ages of the orthogneisses currently available 1986; Azor et al.,1994). This paper aims to describe mostly range from Middle Cambrian to Middle Ordo- the geological structure that resulted from the Varis- vician 1for a complete review including methods and can tectonic evolution of this suture. The OMZ/CIZ numerical values,see Azor et al.,1995; Ordo ÂnÄez contact is marked by a strongly deformed and meta- Casado,1998). The amphibolites form lens- or morphosed NW±SE trending band,namely,the dyke-shaped bodies parallel to the foliation and are Central Unit 1Azor et al.,1994) 1Fig. 1). As will be intercalated with the orthogneissic lithologies. Some shown below,the deformation propagated extensively of these amphibolites are garnet-bearing. throughout both the footwall and the hanging wall of The structure of the Central Unit suggests a ductile the suture,producing large-scale structures involving shearing and two later generations of folds. The main both the OMZ and the southern part of the CIZ. deformation is a ductile shearing that affects the whole unit,producing intense planar-linear fabric. The mylonitic foliation strikes on an average NW± 2. Central unit SE and dips variably due to the late folding. The stretching lineation is subhorizontal or gently plun- The boundary between the OMZ and the CIZ has ging to the NW or to the SE 1Fig. 2b). Shear criteria been considered to be a major left-lateral subvertical associated with the planar-linear fabric 1S±C struc- ductile shear zone 1the Badajoz-CoÂrdoba Shear Zone, tures,asymmetric tails in feldspars,mica-®sh,etc.) Burg et al.,1981; Matte,1986). Azor et al. 11994) indicate a left-lateral sense of movement when the have rede®ned the Badajoz-CoÂrdoba Shear Zone by foliation is subvertical or steeply dipping,and top- stating precisely its boundaries and restricting it to a to-the-NW when the foliation dips moderately to the narrow band of highly sheared and distinctively meta- NE. The shearing in this unit shows an evolution from morphosed rocks. This band,which has been called the bottom to the top 1i.e. from SW to NE). In the Central Unit 1Figs. 1 and 2),is made up of rocks that lower part,the shearing can be established to have are neither represented in the CIZ to the NE nor in the begun under high-temperature conditions,as shown OMZ to the SW. In the original de®nition of the Bada- by the presence of prismatic sillimanite de®ning the joz-CoÂrdoba Shear Zone,the Upper Precambrian S-planes of S±C structures. By contrast,in the upper rocks of the southernmost part of the CIZ were part,the shearing evolved under low-grade conditions taken to form a part of it. However,these rocks and the S±C structures show a ductile±brittle charac- show a structural and metamorphic evolution which ter. The ®nal evolution of the shearing affecting the differs signi®cantly from that of the Central Unit 1see Central Unit resulted in brittle faulting 1the Matachel below). Fault,Figs. 1b and 2) that separates this unit from the The Central Unit is made up of metasediments, CIZ. The later folds affecting the planar-linear fabric orthogneisses and amphibolites. These rocks form a are as follows: ®rst,a generation of folds with SW- sequence in which the structural top is located to the dipping axial surfaces 1Fig. 2c) and,second,a genera- NE 1Fig. 2c). The lower part of the sequence is domi- tion of NW±SE striking upright folds. These folds are nated by orthogneisses and amphibolites with minor associated with crenulation cleavages that appear metasediment intercalations. By contrast,the upper particularly well-developed in the short limbs of the part is dominated by metasedimentary rocks and folds with SW-dipping axial surfaces. Once the late only a few orthogneissic bodies are intercalated. The folds are removed,the original dip of the mylonitic metasediments in the upper part of the unit consist of foliation can be said to be moderate to the NE,thus garnet-bearing micaschists with minor quartzite inter- indicating that the Central Unit is located under the calations. In the lower part,the metasediments mainly southern border of the CIZ. To the SW,the Central 298 J.F. Simancas et al. / Tectonophysics 332 (2001) 295±308 Fig. 2. 1a) Geological map of a sector of the Central Unit; 1b) Lower hemisphere stereographic projections of principal foliation 1Sp) and stretching lineation 1Ls) in this sector of the Central Unit; 1c) Cross-section of the Central Unit; and 1d) P±T paths of the Central Unit and the southern part of the CIZ. J.F. Simancas et al. / Tectonophysics 332 (2001) 295±308 299 Unit is bordered by a brittle subvertical left-lateral low-grade retrogressive metamorphism 1334 ^ 6, fault system that separates it from the OMZ 1Fig.
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