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 transtensional 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).

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; MunhaÂ 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 lineation 1Ls) in this sector of the paths of the Central Unit and the southern part of the CIZ. T ± P Fig. 2. 1a) Geological map of a sector of the Central Unit; 1b) Lower hemisphere stereographic projections of principal foliation 1Sp) and stretching Central Unit; 1c) Cross-section of the Central Unit; and 1d) 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. 2a Ar/Ar in biotite,1Blatrix and Burg,1981); 337 and and c). These faults cut all of the previous structures in 335,Rb/Sr whole rock,1GarcõÂa Casquero et al.,1988); both the Central Unit and the northern border of the 358 ^ 5 and 361 ^ 3,Ar/Ar in hornblende,331 ^ 6 OMZ,thus preventing the original relationships and 339 ^ 7,Ar/Ar in muscovite,1Quesada and Dall- between them from being observed. meyer,1994). The metamorphic evolution of the Central Unit is The lower part of the Central Unit experienced characterized by an initial high-pressure/high- higher pressure and temperature than the upper part, temperature event recorded in the lower part of the with telescoping of the metamorphic isograds. The unit as partially retrogressed eclogite assemblages present visible thickness of the Central Unit is of 3± 1Abalos et al.,1991). Eclogite-facies parageneses 4 km and the difference between peak-pressure in are made up of garnet,omphacitic pyroxene,zoisite, their lower and upper parts is at least of 5 kbar thus rutile and quartz. Omphacitic pyroxene is very scarce, suggesting a thinning of approximately 15 km. In appearing in most cases retrogressed to simplectitic addition,at the contact between the Central Unit intergrowths of Ca-rich pyroxene and albite. The and the CIZ there is a metamorphic gap in pressure high-pressure assemblages were retrogressed ®rst to and temperature: the CIZ,which overlies the Central amphibolite 1hornblendic amphibole and plagioclase) Unit,recorded a maximum pressure of about 5 kbar and then to greenschist 1actinolite amphibole,biotite 1see below),which compared with the approximately and chlorite) facies assemblages during the shearing 10 kbar in the uppermost part of the Central Unit,can of the unit. This high-pressure metamorphism is be taken to imply a gap of about 15 km. On the whole, related to the underthrusting of the Central Unit a vertical thinning of about 30 km 1attenuation in beneath the southern border of the CIZ in an early thickness of the Central Unit and gap-pressure compressional stage whose structural record has between the Central Unit and the southern border of been completely obliterated by the shearing which the CIZ) can be estimated. These data,together with dominates the whole unit. According to geothermo- the above-mentioned kinematics,allow us to conclude barometric calculations,maximum pressure in the that the left-lateral shearing affecting the whole lower part of the Central Unit was at least of Central Unit has,in addition to the obvious and domi- 15 kbar at temperatures between 650 and 7008C nant left-lateral movement,a normal 1extensional) 1Abalos et al.,1991; Azor,1994). The upper part of component. Both the oblique ductile shearing and the unit does not show evidence for high-pressure/ the brittle Matachel Fault located at the top of the high-temperature metamorphism. The assemblages Central Unit,can be grouped under a transtensional recorded here 1garnet±biotite±muscovite±plagio- system with a strong left-lateral component,namely, clase±quartz in metapelites) indicate medium-to-low the Matachel Oblique Extensional System,responsible grade and intermediate-pressure conditions. The for the exhumation of the Central Unit during the Varis- maximum pressure in the upper part is dif®cult to can Orogeny. The subhorizontal stretching lineation in estimate,but the use of geothermobarometers and the Central Unit can not be interpreted as the exact trans- petrogenetic grids yields pressures of no more than port direction,as demonstrated by a number of 10 kbar at temperatures of 500±6008C 1Azor,1994). theoretical studies on three-dimensional strain in trans- The P±T paths of the lower and upper parts of the pession±transtension 1Sanderson and Marchini,1984; Central Unit are both characterized by an isothermal Fossen and Tikoff,1993; Lin et al.,1998; Teyssier and decompression after the peak-pressure conditions Tikoff,1999). These studies conclude that in strike- 1Fig. 2d). This decompression took place during the slip dominated transpression or transtension stretch- shearing of the unit. The radiometric ages available ing lineations are frequently horizontal. indicate Late Silurian 1427 ^ 45,Sm/Nd in garnet, SchaÈfer et al.,1991) or earliest Carboniferous 1340 ^ 13,SHRIMP,Ordo ÂnÄez Casado,1998) for the 3. Southern part of the Central Iberian Zone high-pressure/high-temperature metamorphism,and Early to Middle Carboniferous for the medium and In the southernmost CIZ,the NW±SE alignment of 300 J.F. Simancas et al. / Tectonophysics 332 (2001) 295±308 Los Pedroches Batholith 1Figs. 1b and 3) separates Visean 1SaÂnchez Cela and GabaldoÂn,1977) and two parts with some differences in the uppermost unconformably overlies the preorogenic succession Precambrian stratigraphy,but with an identical 1Fig. 3b). This sequence is overlain by two sequences: Palaeozoic stratigraphy. Such a Late Precambrian the ®rst is a paralic-facies 1shallow platform detritic different palaeogeography could be related with an and carbonatic rocks,Late Visean±Early Namurian in ancient fault at present hidden by the Upper Carboni- age,Wagner et al.,1983),and the second is a limnic- ferous granite intrusion. South of Los Pedroches Bath- facies 1continental detritic rocks,Westphalian B±C in olith,there is an Early to Middle Carboniferous thrust age,Andreis and Wagner,1983). In the para-auto- separating an allochthonous and a para-authocthonous chthonous unit,the Culm-facies sedimentation units 1Figs. 1b and 3) with differences in both the 1,7000 m) is continuous from the Late Tournasian structural history and the Carboniferous stratigraphy. until at least the Early Namurian,and conformably overlies the Devonian rocks. However,the Devonian 3.1. Stratigraphy is often incomplete 1SaÂnchez Cela and GabaldoÂn, 1977; Pardo Alonso and GarcõÂa Alcalde,1984). In the CIZ,to the north of Los Pedroches Batholith 1Fig. 3a),the preorogenic succession starts with the so 3.2. Structure called Schist±Greywacke Complex. It is a thick detritic Vendian-Early Cambrian sequence 1,6000 m) During the Variscan Orogeny,the southern CIZ made up of decimetre-scale alternation of slates and constituted the hanging wall of the OMZ/CIZ suture greywackes with intercalations of volcanic rocks, 1Fig. 3b). The ®rst Variscan structure in the southern- limestones and conglomerates. To the south of Los most CIZ consists of NW±SE trending km-scale Pedroches Batholith,instead of the Schist±Grey- recumbent folds verging to the NE 1Fig. 3). The wacke Complex,the following formations appear, largest overturned limb that can be mapped is which widely outcrop in the OMZ: 1i) the Serie ,15 km-long in the SW±NE direction. Associated Negra,composed of Riphean±Vendian dark schists with these folds,there is a syn-schistose planar-linear and metagreywackes with intercalations of black fabric,often mylonitic,which is related to a ductile quartzites,amphibolites and volcanic rocks,1ii) shearing coeval with the recumbent folding. The unconformably overlying the Serie Negra there is a stretching lineation is parallel to the fold axes,and calk-alkaline volcano sedimentary sequence 1the ductile kinematic criteria indicate a top-to-the-SE Malcocinado Formation,Vendian-Early Cambrian). sense of shearing. The metamorphic conditions during Locally,overlying the Malcocinado Formation, the recumbent folding and shearing were of low- to some Cambrian detritic and carbonate formations medium-grade and ,4±5 kbar 1LoÂpez Munguira et are found. However,in most cases,the Serie Negra al.,1991; MartõÂnez Poyatos,1997). The age of these or the Malcocinado Formations are unconformably structures is Middle to Late Devonian because they overlain by Lower Ordovician strata. The Ordovician affect the Lower Devonian rocks but are unconform- to Devonian succession contains a mixture of quart- ably overlain by the Lower Carboniferous sediments. zites,sandstones and slates deposited in a siliciclastic The second Variscan structure developed in the platform. southern CIZ is the brittle thrust that separates the The synorogenic sedimentation was not homoge- allochthonous unit 1previously deformed by the recum- neous in the southern CIZ,showing differences on bent folding and ductile shearing) and the para- both sides of a thrust that separates two tectonic autochthonous unit 1which until now had not been units 1i.e. allochthonous and para-autochthonous affected by any penetrative deformation) 1Figs. 1b units; Fig. 1b). In both units there is a thick marine and 3). This thrust strikes NW±SE and kinematic Culm-facies sequence made up of decimetre-scale criteria developed along the fault zone 1breccias s.l.) alternation of slates and greywackes with intercala- indicate a top-to-the-NE sense of movement. Thrust tions of conglomerates,limestones and abundant propagation exerted some in¯uence on the Carboni- volcanic rocks. In the allochthonous unit,the age of ferous sedimentation on the hanging wall 1MartõÂnez the Culm-facies sequence is Late Tournasian to Late Poyatos et al.,1998). J.F. Simancas et al. / Tectonophysics 332 (2001) 295±308 301 1the displacements along high-angle late Variscan faults have been 0 Fig. 3. 1a) Geological map of the southern part of the CIZ; and 1b) Simpli®ed composite cross-section 1±1 restored). 302 J.F. Simancas et al. / Tectonophysics 332 (2001) 295±308 In Late Westphalian times,all the previous struc- olith 1Serie Negra and Malcocinado Formations). The tures were affected by large NW±SE trending upright Cambrian sequence begins with conglomerates, folds. In the pre-Carboniferous rocks of the allochtho- arkoses and slates followed by platform limestones nous unit,this deformation constituted a crenulation 1LinÄaÂn and Quesada,1990). These sedimentary condi- phase,while in the Carboniferous rocks of the tions remained until the Early-Middle Cambrian, allochthonous unit and in the whole para-autochtho- when the preorogenic succession became mainly nous unit the upright folds were the ®rst penetrative detritic with volcanic and plutonic activity associated deformation. These folds are associated with a slaty with continental rifting. In the Silurian,pelagic black cleavage developed in the pelitic rocks. The meta- shales and siliceous rocks were deposited. The young- morphic conditions during this folding event were of est rocks of the preorogenic succession are Early P 1±2 kbar and T # 3008C 1MartõÂnez Poyatos, Devonian in age 1Oliveira et al.,1992),being uncon- 1997). formably covered by synorogenic ¯ysch sediments of North of Los Pedroches Batholith,the above- Early Carboniferous age 1van den Boogaard and described upright folds have a moderate SW-vergent VaÂzquez,1981). It is remarkable that the pre-Ordovi- attitude,being affected by NW±SE trending open cian sequence is very similar on both sides of the upright folds 1Fig. 3) without any associated cleavage Central Unit,whereas the Ordovician,Silurian and development. Finally,systems of NW±SE trending Lower Devonian successions of the OMZ differ to a high-angle faults generated,mainly concentrated in considerable extent,regarding lithologies and faunal the southwesternmost part of the CIZ,near the Central content,from those of the CIZ 1Robardet and GutieÂr- Unit. Movements along the main fault systems were rez Marco,1990). reverse 1NE-directed) and/or left-lateral. 4.2. Structure 4. Northeastern part of the Ossa-Morena Zone The structure of the northeastern part of the OMZ 1Fig. 4) results basically from the superposition of two The OMZ is located to the SW of the Central Unit. penetrative deformation phases,both developed under The original contact between both units cannot be low-grade metamorphic conditions 1chlorite and directly observed,since it has been hidden by a late biotite zones). The map 1Fig. 4a) shows how the subvertical left-lateral fault system 1Azuaga and axial traces of the ®rst-phase folds are unconformably Higuera de Llerena faults; Apalategui and Higueras, covered by the Lower Carboniferous rocks,which,in 1980; Azor,1994) 1Fig. 4). Nevertheless,the principal turn,are folded by the second folding phase. The ®rst foliation in both the Central Unit and the northeastern deformation phase,which only affects the pre-Carboni- part of the OMZ,once the late folds are restored,dips ferous rocks,gave rise to km-scale SW-vergent recum- towards the NE,which indicates that the Central Unit bent folds followed by SW-directed thrusts. This structurally overlies the OMZ. Moreover,there is also deformation is the most penetrative one,giving way to a metamorphic contrast between both units,since the a well-developed slaty cleavage associated to folds and maximum pressure in the northeastern part of the a mylonitic foliation in the proximity of thrusts. Subse- OMZ is of about 4 kbar 1Azor et al.,1994). This quently to the main deformation,low-angle normal metamorphic contrast,together with the restoration faults with hanging wall movement towards the north of the principal foliation,demonstrate that the original seem to have controlled the deposition of the Lower contact must have involved a thrust component of the Carboniferous sediments. The main deformation phase Central Unit over the OMZ. developed between the Early Devonian,which is the age 4.1. Stratigraphy of the youngest preorogenic metasediments,and the Early Carboniferous,which is the age of the synoro- The lowest parts of the preorogenic succession in genic sediments unaffected by it. the OMZ are Upper Precambrian to lowermost The second deformation phase is characterized by Cambrian formations,which are the same as those NW±SE trending upright folds,with variably plun- described for the CIZ south of Los Pedroches Bath- ging axes which are at a low angle to those of the J.F. Simancas et al. / Tectonophysics 332 (2001) 295±308 303

Fig. 4. 1a) Structural map of a transect across the northern half of the OMZ; and 1b) Composite cross-section 1see location in 1a)). 304 J.F. Simancas et al. / Tectonophysics 332 (2001) 295±308 main deformation phase. A subvertical crenulation of CIZ 1Fig. 5); 1b) the NE dip of the mylonitic folia- cleavage developed during this deformation. The tion in the Central Unit 1i.e. the Central Unit is located superposition of the two fold generations gave rise over the OMZ and under the CIZ); and 1c) the pressure to downward-facing anticlines,the double plunge of gap between the OMZ 1low-pressure metamorphism) which can be observed on the map 1Fig. 4a). and the Central Unit 1high-pressure metamorphism). The plurikilometric size of the recumbent folds Due to the underthrusting,the lower crust of the 1characteristic of collisional settings) together with northeastern part of the OMZ reached a range of their geometry,provides evidence for an important depths consistent with eclogite-facies metamorphism shortening of the OMZ upper crust 1Fig. 4b). The 1Fig. 6a). As will be discussed later,a slice of these structures where probably generated by the under- eclogitic rocks would ascend to the surface. In the thrusting of the OMZ lower crust beneath the CIZ. footwall,deformation propagated to the SW; in the The detachment seems to be placed at the base of upper crust of the OMZ,large-scale SW-vergent the Serie Negra Formation,since older rocks do not recumbent folds and thrusts were formed,implying crop out in this region. The structure demonstrates a a decoupling between upper and lower crust and,as shortening of approximately 125 km for the northeast- a result,an important underplating of the OMZ lower ern half of the OMZ. Assuming an equal shortening crust 1Fig. 6b). In the hanging wall,local but signi®- for the southwestern half 1some observations suggest cant back-folding and associated back-shearing took a similar structural style),the length of the underthrust place,forming a pop-up structure 1Fig. 6a and b). We lower crust of the OMZ can be estimated to be at least can hypothesize that the main thrust putting the CIZ of 250 km. onto the OMZ could have slightly modi®ed its trajec- tory in the deep crust,so that a slice of eclogitic rocks became attached to the hanging wall early in the colli- 5. Tectonic evolution of the OMZ/CIZ boundary sional history 1®rst step of the eclogite exhumation). The tectonic scenario during this stage could have The geometry of the transect studied is presented in been a deformation belt to the SW 1OMZ) and a the composite cross-section of Fig. 5,in which the plateau to the NE 1CIZ). In this regard the lacunes vergence change and the main structures on both in the Middle Devonian to earliest Carboniferous of sides of the OMZ/CIZ contact can be observed. the southern CIZ can be related to the uplift due to the Three main stages in the tectonic evolution can be underplating of the OMZ lower crust. distinguished: 1i) a continental underthrusting 1®rst The kinematic picture during this stage still remains transpressional stage),1ii) an oblique extension 1trans- unclear,as the main thrust is at present hidden by late tensional stage),and 1iii) a late shortening episode faults 1Fig. 5). Nevertheless,it will be seen in the ending with left-lateral faulting. A sketch of the envi- remainder of this paper that a left-lateral component saged evolution is shown in Fig. 6,but note that there of movement is present throughout the subsequent are very important lateral movements that can not be evolution. Thus,we tentatively believe that a left- shown on the ®gure. The evolution previous to the lateral component was also a part of the main thrust collision has too many uncertainties to be discussed kinematics. Under this view,it must be noted that the in the present paper,but the well-documented Early opposite kinematics 1top to the SE) of the back-shear- Palaeozoic rifting in the OMZ 1LinÄaÂn and Quesada, ing in the southern border of CIZ 1Fig. 6a and b) is 1990) should have been concentrated in what will perfectly consistent with this proposal. become later the OMZ/CIZ collisional boundary. A radiometric age of 427 ^ 45 Ma 1Sm/Nd in 5.1. Continentalunderthrusting garnets) for the retrograde eclogites cropping out in the Central Unit was determined by SchaÈfer et al. The underthrusting of the OMZ beneath the CIZ is 11991). Recently,Ordo ÂnÄez Casado 11998) has supported by the following evidence: 1a) the SW obtained a much younger age for the same retrograde vergence of the km-scale recumbent folds and thrusts eclogites: 340 ^ 13 Ma 1SHRIMP),which can be in the OMZ,clearly dominant as compared with the interpreted to be close to the age of the thermal peak. extent of the NE vergent folds in the southern border As the PT path is one of isothermal decompression J.F. Simancas et al. / Tectonophysics 332 (2001) 295±308 305 ry important. Fig. 5. Simpli®ed composite cross-section of the region studied. For location see Fig. 1b. Note that lateral displacements 1out of the section) are ve 306 J.F. Simancas et al. / Tectonophysics 332 (2001) 295±308

Fig. 6. Evolutionary model for the Ossa-Morena/Central Iberian contact. 1a) and 1b) Continental underthrusting of the OMZ beneath the CIZ; 1c) Oblique extension 1Transtensional stage); and 1d) Final architecture after the late shortening episode 1but late reverse and strike-slip faults are not shown). Note that the lateral displacements,very important in this oblique collisional boundary,cannot be adequately shown in cross- section. See text for further explanations. Only 1d) is rigourously to scale 1vertical scale horizontal scale).

1Azor,1994),high-temperatures must have been relics of eclogites in the OMZ/CIZ contact suggest the sustained some time after the peak-pressure,i.e. the possibility of rooting those allochtonous high-pres- peak-pressure must be a bit older than the determined sure rocks in this contact. age. In any case,this new age would imply an extre- mely rapid exhumation rate,considering that the sedi- 5.2. Oblique extension (Transtensional stage) ments related to the extension responsible for the exhumation of the retrograde eclogites are Early A new tectonic scenario starts in the Early Carbo- Carboniferous in age. In view of the uncertainty,a niferous. Following the main thrust closely,the NE- Late Devonian age for the eclogites appears to be dipping Matachel Oblique Extensional System 1Fig. more plausible. 6c) caused the thickened crust to extend. This system An important outstanding issue about the OMZ is can be said to be responsible for the intense mylonitic the signi®cance of the recently discovered high-pres- retrogression affecting the Central Unit and,according sure metamorphism in allochthonous rocks outcrop- to the metamorphic pressure gap between the eclogitic ping in the southern half of the OMZ in Portugal rocks and the southern CIZ rocks,has a throw of about 1Fonseca et al.,1993; ArauÂjo et al.,1994). Although 30 km. However,its strike slip must have been much there are no speci®c data to support this hypothesis, longer. This is the way in which the eclogitic rocks the overall vergence to the SW in the OMZ and the now cropping out in the Central Unit were exhumed J.F. Simancas et al. / Tectonophysics 332 (2001) 295±308 307 1Fig. 6c). Other important effects of the oblique exten- contact and has also generated a large synorogenic sion were subsidence and bimodal magmatism 1MartõÂ- basin. Finally,Late Carboniferous upright folding nez Poyatos,1997). Thus,in the southernmost part of and faulting can be said to have shaped the ®nal the CIZ a large NW±SE trending ,90 km wide basin, appearance to the OMZ/CIZ contact,which can thus ®lled in with up to ,7 km of Culm-facies synorogenic be interpreted as a suture of the Variscan Orogen in sediments,formed during the Early Carboniferous. the Iberian Massif. Comparatively minor extension of the same age can also be recognized in the OMZ. In the SPZ 1Fig. 1a), Early Carboniferous extension and a bimodal Acknowledgements magmatic association of tholeitic to alkalic basalts and rhyolites 1MunhaÂ,1983) can be said to be the We are very grateful to Francisco GonzaÂlvez most outstanding feature. Therefore,it appears that GarcõÂa for improving our English text. The manu- this extension has been widespread throughout the script has bene®ted from comments by R. Gayer and southern Iberian Massif. an anonymous reviewer. We also thank K. Schulmann for his editorial assistance. Financial support was 5.3. Late shortening episode given by the CYCIT project PB-96-1452-C03-01. A ®nal tectonic regime is widely documented in the whole southwestern Iberian Massif. In the southern border of the CIZ,the inversion of the Lower Carbo- References niferous basin began in Late Visean times with a top- to-the-NE thrust,then the shortening spread all over Abalos,B.,Gil Ibarguchi,J.I.,Eguiluz,L.,1991. Cadomian subduc- the CIZ by the development of Westphalian upright tion/collision and Variscan transpression in the Badajoz- Â folds 1Fig. 6d). In the OMZ,the same upright folding Cordoba shear belt,southwest Spain. Tectonophysics 199, 51±72. can be found,which can be seen to affect the previous Andreis,R.R.,Wagner,R.H.,1983. Estudio de abanicos aluviales recumbent folds and thrusts. Finally,reverse faults en el norte de la cuenca Westfaliense B de PenÄarroya-BeÂlmez 1directed to the NE in the CIZ and to the SW in the 1CoÂrdoba). In: Lemos de Sousa,M.J. 1Ed.),Contributions to the OMZ) and then left-lateral strike-slip faults were Carboniferous Geology and Paleontology of the Iberian Penin- formed,providing new evidence of lateral displace- sula,Universidade do Porto,vol. 193,pp. 172±227. Apalategui,O.,Higueras,P.,1980. Hoja N 8 855 1Usagre) del Mapa ments. In the cross-section of Fig. 6d,we have omitted GeoloÂgico de EspanÄa a escala 1:50 000. Instituto GeoloÂgico y these late faults,with the exception of the most impor- Minero de EspanÄa. tant one,actual limit between the Central Unit and the ArauÂjo,A.,Fonseca,P.,Leal,N.,Lourenc Ëo,N.,MunhaÂ,J.,Ribeiro, OMZ 1i.e. the Azuaga-Higuera de Llerena left-lateral A.,Rosas,F.,Silva,J.B.,Sousa,J.A.,1994. Field-Guide Europ- Â fault system). robe Meeting,E vora. Arenas,R.,Gil Ibarguchi,J.I.,Gonza Âlez Lodeiro,F.,Klein,E., MartõÂnez CatalaÂn,J.R.,Ortega Girone Âs,E.,de Pablo Macia Â, J.G.,Peinado,M.,1986. Tectonostratigraphic units in the 6. 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Tectonic Subsequently,during the Early Carboniferous,an evolution of the boundary between the Central Iberian and oblique extension in the overthrust block 1CIZ) must Ossa-Morena Zones 1Variscan Belt,SW Spain). Tectonics 13, 45±61. have signi®cantly contributed to the exhumation of Azor,A.,Bea,F.,Gonza Âlez Lodeiro,F.,Simancas,J.F.,1995. eclogitic rocks at present included in the OMZ/CIZ Geochronological constraints on the evolution of a suture: the 308 J.F. Simancas et al. / Tectonophysics 332 (2001) 295±308

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