Tectonophysics 461 (2008) 1–8

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Preface

Evolution of the Rheic Ocean

The idea for this special issue grew from three international

meetings of the IGCP (International Geoscience Programme) Project497 — ‘The Rheic Ocean: Origin, Evolution and Correlatives’, hosted inPortsmouth, UK (5–11th July 2005), in Ankara, Turkey (23–30th June2006) and in Évora, Portugal (27th September–4th October 2006).These meetings focused, respectively, on the ‘Devono–CarboniferousEvolution of the Northern Margin of the Rheic Ocean’, the ‘Neoproter-ozoic and Palaeozoic Terranes in Northwest Turkey’ and the ‘Ediacaranto Viséan Crustal Growth Processes in the Ossa–Morena Zone’. Awealth of new data was presented during these meetings, and it wasdecided to prepare a compilation of contributions dealing with Peri-Gondwanan crustal growth in the Alleghanian–Variscan orogenicsystem of northern South America, North America, Western andCentral Europe, and Northwest Africa. The remnants of Rheic Oceanform discontinuous exposures from Mexico in the west to easternEurope in the Dobrogea (Romania) and Turkey. This special issue is notmerely a compilation of the communications presented during thesemeetings. Instead, by inviting additional contributions, the twocoordinated parts are designed to provide a comprehensive view ofour current understanding of Late Neoproterozoic to Early Palaeozoictectonics in relation to Peri-Gondwanan terranes and the develop-ment of the Rheic Ocean.

0040-1951/$ – see front matter © 2008 Elsevier B.V. All rights reserved.doi:10.1016/j.tecto.2008.08.015

Although not totally understood, the initiation of the Palaeozoicrifting that culminated in the openingof theRheic Ocean and correlativebasins between the gigantic continentof Gondwana to the south and thecontinents of Baltica and Avalonia to the north (Fig.1a, b) began in EarlyCambrian time with the break up of a Neoproterozoic continentalmargin (Murphy et al., 2004, 2006a,b). After Neoproterozoic accretion(Nance et al., 2002), the northern Gondwana margin began to befragmented giving rise to the development of marine basins withvoluminous rift-relatedmagmatism. As Baltica and Laurentia convergedduring the latest Ordovician to Silurian, the Rheic Ocean developed as anarrow sea between Avalonia and Gondwana (Fig. 1c). The Rheic Oceancontinued to open at the expense of the Iapetus Ocean and reached itswidest extent during the Silurian (Fig.1b, c). It is now firmly establishedthat a Late Palaeozoic accretion/subduction process was responsible forthe closure and tectonic inversion of thesemarine/oceanic basins due tothe collision of Gondwana and Laurussia (Variscan–Alleghanianorogeny; Matte, 2001) during Carboniferous assembly of the super-continent Pangea (Fig. 1d, e).

Despite this accepted geodynamic scheme, there is no realconsensus about the mode and timing of rifting, oceanization,kinematics, subduction, thickening and collapse-related processesthat occurred along thismargin of Gondwana for a period ofmore than250 million years of Earth history. Until now the available informationrelating to: (1) the extensional structures consequent on crustalthinning, (2) the distribution of anorogenic magmatism and hypothe-tical oceanic crust remnants, (3) the plausible existence of more thanone suture zone and associated high-pressure rocks, (4) the occurrenceof orogenic continental arc-relatedmagmatic complexes, (5) the role oftranscurrent movements in the deformation of the basement anddevelopmentof basins and (6) the reconstruction of subduction relatedthrust tectonics andhigh-pressure units,was not sufficient to allow thedevelopment of well-constrained geodynamic models.

This project contains 27 contributions divided into two parts. Thebulk of both volumes deal with detailed aspects of palaeogeography,palaeontology, geochronology, provenance analysis, geochemistry,deformation and tectonics, magmatism and sedimentation, and areconcerned with the evolution of Peri-Gondwanan correlatives fromEuropean, North and South America areas. The geology is consideredin a general chronological order:

Part I: ‘The foundations and birth of the Rheic Ocean: Avalonian–Cadomianorogenic processes and Early Palaeozoic rifting at thenorthern Gondwana margin’, with 16 papers;Part II: ‘Evolution and closure of the Rheic Ocean: Palaeozoic driftof peri-Gondwanan terranes and Alleghenian–Variscan orogenicprocesses’, with 11 contributions.

Fig. 1. Palaeogeography of Iapetus and Rheic oceans during: (a) ca. 480 Ma (Linnemann et al., 2003; modified from C.R. Scotese: Paleomap web site: www.scotese.com), (b) ca. 440 Ma (Linnemann et al., 2007), (c) ca. 400 Ma (Linnemann et al.,2003;modified fromC.R. Scotese: Paleomapweb site:www.scotese.com), (d) ca. 350Ma (Keppie, 2004) and (e) ca. 280Ma (redrawn from ‘Palaeos: The Trace of Life on Earth’website at http://www.palaeos.com). A—Armorica (Britany, Normandy,Central Massif), B— Barrandian, C— Carolina, EWI — England, Wales, and southern Ireland, F— Florida, I — Iberia, IR— Iran, M —Mexican terranes, NF— New Foundland, NS — Nova Scotia, PA — proto-Alps, RH — Reno-Hercynian, SX — Saxo-Thuringian, TP — Turkish plate, IR — Iran.

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Part I: The Foundations and Birth of the Rheic Ocean: Avalonian–Cadomian Orogenic Processes and Early Palaeozoic Rifting at theNorthern Gondwana Margin edited by M. Francisco Pereira, ErdinBozkurt, Robin Strachan and Cecilio Quesada.

Part I contains contributions dealing with the period of plateconvergence that caused the Cadomian–Avalonian orogeny (c. 635–545 Ma) along the northern Gondwana margin during the Neoproter-ozoic. This was followed by Early Palaeozoic rifting (c. 540–420 Ma)and opening of the Rheic Ocean.

The separation of Avalonian blocks from Gondwana by the birth andopening of the Rheic Ocean related to a complex palaeogeography(Figs. 1 and 2), involving local back-arcs or intra-continental rifts, and isintroduced by von Raumer and Stampfli. Based on a review of thepublished lithostratigraphic data from Europe (Ossa–Morena, CentralIberia, Cantabrian, West Asturio–Leonese, Montagne Noir, Saxo-thur-ingian, Barrandian zones) and North Africa (Anti-Atlas) these authorsdefined four main episodes of subsidence during the early Palaeozoic.The presented curves of subsidence reinforce the evidence that thenorthern Gondwana margin had been, in general, a passive marginduring the Cambrian–Ordovician with rapid sedimentation rates. Thisgeodynamic evolution was the expression of an extensional setting,either related to subduction and rollback of Prototethys or tocontemporaneous intra-continental rifting related to a post-Pan-Africanextension, that culminatedwith the initial openingof theRheic Ocean tothe west (Stampfli et al., 2002; von Raumer et al., 2003).

Before the Early Palaeozoic, the evolution of the northern Gondwanamargin was linked to that of an Avalonian–Cadomian continentalmagmatic arc at the periphery of the West African Craton. Probabilitydensity distribution plots of ages of new detrital zircon U–Pb LA-ICP-MSdata of Ediacaran, Cambrian and Ordovician siliciclastic rocks from theOssa–Morena and Saxo-Thuringian zones, by Linnemann et al. yieldedthe intervals c. 500–490 Ma, c. 530–540 Ma. 590–540 Ma, c. 750–600Ma, c. 950–820Ma, c. 2.2–1.8Ga. and an ‘age gap’ between c.1.7 and1.0 Ga. This signature is a general characteristic of a Cadomian–WestAfrican provenance and distinguishes Cadomia–West Africa fromAmazonia and Baltica (Nance and Murphy, 1994; Fernández-Suárezet al., 2002; Linnemann et al., 2004, 2007; Pereira et al., 2008). Thecomparison of these two zones was used to create a model ofdiachronism for the geodynamic processes along the northern Gond-wana margin: (1) arc-continent collision at c. 570–543 Ma with theformation of back-arc and foreland basins in the Saxo-Thuringian andthe Ossa–Morena zones, (2) onset of intra-continental rifting, crustalmelting and magmatism at c. 540–530 Ma, and (3) oblique incision ofthe oceanic ridge into the continent caused the formation of rift basinswithmassivemagmatismduring the Lower toMiddle Cambrian (c. 530–500 Ma) and, finally 4) caused the opening of the Rheic Ocean in theLower Ordovician (c. 490–485 Ma), from the western Peri-Gondwanaand expanded eastward.

The model of a transition from an active to passive setting in thenorthernmargin of Gondwanawas tested by Sláma et al., who obtainedzircon U–Pb LA-ICP-MS data from pebbles of granitoids and matrixfrom a Neoproterozoic–Early Cambrian conglomerate of the Teplá–Barrandian Unit. Ages of detrital zircons from the metaconglomeratematrix indicate two main populations: 2.5–1.7 Ga and Neoproterozoicto early Cambrian ages between c. 520–660Ma (with significant peaksat c. 600 and 544 Ma), while zircons separated from granitoid pebblesyielded c. 610 Ma. For these authors, the absence of Grenvillian zirconages favours a location of the Teplá–Barrandian Unit close to the WestAfrican craton at the Neoproterozoic–Palaeozoic boundary.

Kryza and Zalasiewicz obtained new U–Pb SHRIMP zircon ages inthe Kaczawa Complex (Sudetes) that support the existence ofEdiacaran sediments (c. 550 Ma) with two main populations of ages(c. 550–750 Ma and c. 2.4–3.4 Ga) indicating provenance fromCadomian and older rocks as described in the Saxo-Thuringian Zone(Linnemann et al., 2004). An early Palaeozoic age for bimodal rift-related volcanism (c. 500–485 Ma) in this part of East Europe shows

that lithospheric extension and ascent of magmas was coeval withmagmatism related to the Rheic Ocean opening elsewhere along theperi-Gondwana regions of West Europe (Pin and Marini, 1993; Abatiet al., 1999; Sánchez-García et al., 2003).

New zircon U–Pb SHRIMP and U–Pb TIMS ages constrain theensialic rift-related magmatism in the Ossa–Morena Zone to theLower–Middle Cambrian (c. 530–505 Ma; Sánchez-García et al. andChichorro et al.) and the early Ordovician (c. 494–488Ma; Solá et al.) inthe Central-Iberian — Ossa–Morena transition zone. The igneousexpression of Cambro-Ordovician rifting is represented by a largevolume of magma that ascended during major lithosphere extensionrelated to asthenosphere upwelling (Sánchez-García et al., 2003;Pereira et al., 2007). These authors characterize the large hetero-geneity of the rift-related magmatic associationwith calc-alkaline andtholeiitic signatures related to crustal melting (inherited ‘orogenic’signature; Pin and Marini, 1993) or generated from mantle-derivedmagmas by various petrogenetic processes. Striking similarities withother correlatives from Europe (Crowley et al., 2000) reinforce theexistence of a regional-scale process of high partial melting of crustand primitive-mantle along the northern Gondwana margin.

The magmatism of the Acatlán Complex in Mexico was dated byKeppie et al. at Ordovician (c. 461–442 Ma; U–Pb TIMS and U–Pb zirconages). These within-plate tholeiites define a period of magmatism thatmay have started earlier at c. 478 Ma, probably related to rifting in thesouthernmargin of the Rheic Ocean. This magmatismwas contempora-neouswith the separation of peri-Gondwanan terranes, such asAvaloniaand Carolinia, from Gondwana during opening of the Rheic Ocean(Keppie, 2004; Nance et al., 2007), while the Iapetus Ocean was beingsubducted throughout the Ordovician.

Evidence for Devonian rift-related alkaline mafic magmatism in theCentral-Iberian Zone, traditionally regarded as Lower Silurian, issupported by new zircon U–Pb TIMS data (c. 394 Ma) by Gutiérrez-Alonso et al. This study discusses a ridge subduction model that resultedin geodynamic coupling of the northern flank (Acadian ‘orogeny’;Murphy and Keppie, 2005) and the southern flank (extension as amanifestation of slab pull along the northern flank) of the Rheic Ocean.

The variety of sources of the early Ordovician rift-related magmatismof Northwestern Iberia (Galicia, Central-Iberian and Cantabrian zones),including contaminated juvenile mantle, subcontinental lithosphereenriched mantle and crust are discussed byMurphy et al. (a) in the lightof Sm–Nd isotopic data. They suggest that arc activitywas coeval with theopeningof theRheicOceanduring the earlyOrdovician (Martínez-Catalánet al., 2007) asoccurredalongotherpartsof theGondwanapassivemarginlike Oaxaquia (Murphy and Dostal, 2007). The Sm–Nd isotopic resultspresented byMurphy et al. (b)were used to discuss the same problem inrelation to theNeoproterozoic–MiddleDevonian felsic andmaficmagmasof the Antigonish Highlands. Magmatism developed during the rift anddrift of Avalonia from theGondwananmargin during the formation of theRheic Ocean in the Ordovician, its accretion to Baltica and to Laurentia inthe Silurian, and post-accretionary strike-slip movement of Avalon alongthe Laurentian margin (Keppie et al., 2003). This suggests that Avaloniancrustal basement and its enrichedmantle were both formed between 0.8and 1.1 Ga in a Panthalassa-type ocean that surrounded Rodinia (Meertand Torsvik, 2003) andwere accreted to the northernGondwananmarginat about 650 Ma, prior to the oldest magmatic event in the AntigonishHighlands. López-Guijarro et al. also present Sm–Nd isotopic dataindicating a remarkable geochemical and isotopic similarity of theNeoproterozoic to early Devonian sediments and magmatic rocks forthe Ossa–Morena and Central-Iberian zones. For these authors, with theexception of juvenile inputs during the Cambrian–Early Ordovician and,the local Silurian rifting events in the Central-Iberian Zone, the Palaeozoicrocks indicate a dominant contribution from old crustal componentsduring a passive margin stage (Quesada, 2006).

Alvaro et al. show evidence for Cadomian and/or earliest Cambriantectonic instability in the IberianChains. Deformed andmetamorphosedNeoproterozoic sediments are overlain by an early Cambrian erosive

Fig. 2. Schematic paleogeographic reconstruction of the transition from an active (Ediacaran) to passive (Cambrian–Ordovician) setting in the northern margin of Gondwana leadingto the birth of the Rheic Ocean (Adapted from Linnemann et al., this volume).

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unconformity also identified in other areas of the Iberian Massif(Quesada, 1991; Eguiluz et al., 2000; Rodríguez-Alonso et al., 2004;Pereira et al., 2006). The Neoproterozoic–Cambrian transition is markedby strong palaeogeographic transformations related to a sharp relativesea-level fall, the onset of alluvial plain sequences and increasedsubsidence rates and sedimentation controlled by rift-related synsedi-mentary faults. In contrast, the Cambrian rifting event has not beenidentified in the Pyrenees. New zircon U–Pb SHRIMP data obtained byCastiñeiras et al. frommetaigneous rocks from the Canigó, Roc de Frausaand Cap de Creus massifs (Eastern Pyrenees) confirm the presence ofEdiacaran–earlyCambrian (c. 580–540Ma)andearlyOrdovician (c. 475–460Ma)magmatism. Inherited ages yielded the intervals c. 530–560Ma,c. 800–660Ma,1.1–0.9 Ga,1.8–2.5Ga and c. 3.1 Ga. For these authors, theobtained ages are in agreement with the probable development of anactive continental margin related to the closure of the Iapetus or Proto-Tethys Ocean, followed by a back-arc rifting episode prior to theseparation from the northern Gondwana margin (Neubauer, 2002;Stampfli et al., 2002; von Raumer et al., 2002).

Bozkurt et al. investigate in a Proterozoic inlier of the istanbul Block(Sünice Massif), the geochemistry of mafic and ultramafic rocks(island arc tholeiites and transitional to calc-alkaline basalts) whichunderlie Ediacaran calc-alkaline volcanics intruded by granitoids(c. 576–565 Ma; Ustaömer and Rogers, 1999; Ustaömer et al., 2005).These authors test a palaeogeographic model that views the İstanbulBlock as a detached part of Avalonia (easternmost extremity) thatdocked with Laurussia during the Silurian (Winchester et al., 2006).Okay et al. describe the Neoproterozoic record in the Pontides. Theyprovide new zircon U–Pb LA-ICP-MS data from Ediacaran granitoids(c. 570 Ma) and Ordovician granitoids (c. 460 Ma) included in theboundary between the İstanbul and Sakarya terranes. This studydiscusses the transition from an active to passive setting along theGondwana margin, with the Neoproterozoic granitoids representingthe Pan-African plutonism (correlatable with the İstanbul terrane) andthe Ordovician granitoids, on the other hand, are probably related tothe drifting of the İstanbul terrane away from Gondwana during theopening of the Rheic Ocean.

Part II: Evolution and Closure of the Rheic Ocean: Palaeozoic Drift ofPeri-Gondwanan Terranes and Alleghenian–Variscan Orogenic Processesedited by Erdin Bozkurt, M. Francisco Pereira, Robin Strachan andCecilio Quesada.

The second part (Part II) contains 11 contributions that cover thePalaeozoic drift of Peri-Gondwanan terranes across the Rheic Ocean,the tectonic inversion of basins, and mountain building processes.

The first paper is concerned with Maritime Canada which recordsthe accretion of peri-Gondwanan terranes (e.g., Avalonia, Meguma) toLaurentia during the closure of the Iapetus Ocean, followed bysubduction along the northern flank of the Rheic Ocean prior to theLate Paleozoic collision between Laurentia and Gondwana (Murphyet al., 1999). A recent study of detrital zircons from coeval UpperOrdovician to Lower Devonian sedimentary successions in theAvalonia and Meguma terranes of Nova Scotia (Murphy et al., 1999)concluded that the two terranes were fellow travellers throughout thePalaeozoic and collided with Laurentia as a single terrane in the EarlySilurian. Murphy and Collins present 40Ar–39Ar data from muscovitesfrom the same successions. Detrital muscovites are interpreted to havebeen derived from neighbouring terranes during the accretion ofAvalonia to Baltica and Laurentia. However, many muscovite ages aresignificantly younger than the depositional age of the strata, andreflect distributed fluid flow coeval with dextral shear along theAvalon–Meguma terrane boundary and closure of the Rheic Ocean.

Keppie et al. synthesizes recent studies in the Acatlan Complex(Mexico). The following sequence of events is recorded: (i) Ordovicianrifting on the southern margin of the Rheic Ocean, (ii) passive driftingwith Amazonia during the Silurian, (iii) Devonian–Permian subductionbeneath southern Mexico producing an arc complex that was partiallyremoved by subduction erosion, subjected to high-pressure meta-

morphismandMississippian extrusion into the upper plate, followed byre-establishment of a Permian arc. In the Oaxaquia terrane, the 920–1300 Ma basement is unconformably overlain by a ∼200 m uppermostCambrian–lowest Ordovician shelf sequence containing Gondwananfauna (Tiñu Formation), unconformably overlain by 650 m of shallowmarine-continental Carboniferous sedimentary rocks containing aMidcontinent (USA) fauna. In the Mixteca terrane, the low-gradePalaeozoic sequence is composed of: (i) a ?Cambrian–Ordovician clasticsequence intruded by ca. 480–440 Ma bimodal, rift-related igneousrocks; and (ii) a latest Devonian–Permian shallow marine sequence(N906m) consisting ofmetapsammites,metapelites and tholeiiticmaficvolcanic rocks. The Ordovician granitoids contain concordant inheritedzircons that range in age from ca. 900 to 1300 Ma, indicating a source inthe Oaxacan Complex. Concordant ages of detrital zircons in both thelow- and high-grade Cambro–Ordovician metasedimentary rocksindicate a provenance in local Ordovician plutons and/or ca. 1 GaOaxacan basement, and distal northwestern Gondwana sourceswith anunique source in the 900–750 Ma Goiás magmatic arc within theBrasiliano orogen. These data combined with the rift-related nature ofthe Cambro–Ordovician rocks are most consistent with an origin alongthe southern margin of the Rheic Ocean. The high-pressure Cambro–Orodivician rift-shelf rocks are inferred to have originated in the forearcregion of the upper plate that was removed, alongwith undated arc andoceanic rocks, by subduction erosion, carried down the subductionchannel, and then extruded into the upper plate during the Mississip-pian in the middle of the Mixteca terrane. During the Permian, arc-related intrusions in both the Mixteca and Oaxaquia terranes wereaccompanied by dextral transtensional deformation and deposition ofclastic rocks containing Permian detrital zircons and carbonates inperiarc, pull-apart basins. These Permo–Carboniferous events areinferred to have taken place on the western margin of Pangea.

The paper byRamos-Arias et al.documents the penetrative structuresproduced during Mississippian exhumation of high-pressure rocks inthe northern part of the Acatlan Complex. These structures grade frompolyphase deformation under blueschist-eclogite facies through poly-phase and single phase deformation under greenschist facies to nodeformation. Kinematic indicators show that these structures formedbylistric normal movements, which were synchronous with deposition oflatest Devonian to Lower Permian rocks. Possible models for the originsof these structures include a core complex, extrusion, and gravitationalcollapse. That these structures formed on thewesternmargin of Pangeais suggested by the appearance of Mid-Continent (USA) Mississippianfauna in the adjacent Oaxaquia terrane, i.e. after the closure of thewestern end of the Rheic Ocean.

Morales-Gámez et al. record U–Pb data that bracket the deposition of:(i) twoOrdovician clastic units intruded by a bimodal suite of dykes; (ii) aCarboniferous unit intruded by mafic dykes; and (iii) a Lower Permianclastic-carbonate unit. The Ordovician units are inferred to have beendeposited in a rift during the formation of the Rheic Ocean. TheCarboniferous unit is synchronous with exhumation of high pressurerocks elsewhere in the Acatlan Complex, and the Lower Permian unitformed during arc magmatism on the western margin of Pangea. Thepreponderance of 510–630Maand900–1250Maages in both thedetritaland inherited ages in some of the dated samples suggests a source in theadjacent and subjacent Maya and Oaxacan Complex, respectively.

Grodzicki et al. documents polyphase deformation in two clasticunits, one of which is interbedded with tholeiitic pillow lavas in thewestern part of the Acatlan Complex. Deposition of the latter unit post-dates the Devonian age of the youngest detrital zircon and appears to besynchronous with exhumation of high-pressure rocks and subsequentPermian dextral movements at the western end of the Variscan orogen.Detrital zircons in these two units yielded different age populationpeaks: Ordovician and ca. 900–1250 Ma detrital zircons in the Canoasunit, and 357±35 Ma, 563±22 Ma, 837±28 Ma, and 1156±74 Ma in theCoatlaco unit.Whereas a source for the Palaeozoic zirconsmay be foundin theAcatlánComplex, andprovenance for the ca. 900–1250Mazircons

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is likely the adjacent Oaxacan Complex, the Neoproterozoic zirconsprobably have a more exotic source in Amazonia.

Hinojosa-Prieto et al. describes the Late Palaeozoic–Early Mesozoicstructures in the central part of the Acatlan Complex. These structuresoccur in two Ordovician units, and an Ordovician granitoid suite datedusing U–Pb and 40Ar/39Ar geochronology. Whereas sources for most ofthe detrital zircons may be found in either the Acatlán and Oaxacancomplexes, Laurentia or Gondwana, a source for the Neoproterozoiczircons is most likely in Amazonia. Whereas the Ordovician history ofthe plutons and volcano-sedimentary units coincides with thelifespan of both the Iapetus and Rheic oceans, the Late Palaeozoic–Early Mesozoic deformation better reflects closure of the Rheic Oceanand convergence tectonics on the palaeo-Pacific margin following theamalgamation of Pangea.

Sintubin et al. builds on an extensive structural study of the low-grade, siliciclastic metasediments of Pridolian to Lochkovian age(Plougastel formation), exposed in the Monts d'Arrée (Brittany, France).This study revealed a high-strain, contraction-dominated slate belt, inwhich the initial stages of strain partitioning are preserved as strainheterogeneities consistently showing dextral non-coaxial shear defor-mation. A kinematicmodel suggests overall top-to-the-NWthrustingona weakly dipping décollement surface. This deformation is LateDevonian to Early Carboniferous in age; hence it is considered theexpression of the ‘Bretonian’ tectonometamorphic event. The regionalsurvey, presented by Sintubin et al. now shows that this contraction-dominated, ‘Bretonian’ thrust tectonics affected the entire middle-crustal domain of Central Armorica. Such a regional 'Bretonian' eventimplies a diachronous evolution of the main, cleavage-forming,deformation in Central Armorica, younging to the south. This eventcan, moreover, be associated with the Late Devonian to EarlyCarboniferous top-to-the-NW thrusting and nappe stacking in theLéon Terrane, which is considered to be the result of the docking of theLéonmicrocontinental blockwith thenorthernmargin of Armorica. Thiskinematic and temporal association corroborates the geodynamicmodels placing the Léon Terrane within the suture zone along thenorthern margin of Armorica. Hence, the ‘Bretonian’ thrusting event,identified in both the Léon Terrane and Central Armorica, can beassociated with the collisional geodynamics at the southern margin ofthe Rheic Ocean during its closure.

Pin et al. present ID-TIMS U–Pb zircon ages, along with elementaland Sr–Nd isotope data, on the largely mafic Beja Igneous Complex(BIC), and related rocks ascribed to the Beja–Acebuches OphioliteComplex (BAOC) delineating a major (Rheic?) suture between theOssa–Morena Zone (OMZ) and the South Portuguese Zone (SPZ) of theIberian Variscides. According to these authors, the ca. 350 Maintrusion age obtained for the BIC significantly postdates the LateDevonian collision between the OMZ and SPZ, implying that theemplacement of the BIC was not related to an active subduction ofoceanic lithosphere. Instead, the generation in the mantle, andemplacement in the overlying continental crust of copious volumesof mafic magmas with concomitant LP-HT metamorphism (includingthe classical Aracena Belt example), is interpreted in terms oftranscurrent slab detachment that followed oblique subduction andcollision of the OMZ with the SPZ. Based on trace element and Ndisotope signatures, these authors further emphasize that the variouslydeformed gabbros and ultramafic cumulates previously ascribed tothe BAOC depart from the MORB-like metabasalts of the disruptedophiolite, but strongly resemble those from the BIC, thereby castingdoubts on their oceanic derivation. This implies that the ophioliticcomplex should be much narrower than previously thought, andpossibly restricted to discontinuous strips of oceanic metabasalts.These geochemical data are combined with published results on theadjacent regions to propose a new geodynamic model at odds withmost previous reconstructions, in that it involves a southwestwardsubduction polarity of the oceanic portion of the OMZ plate in theDevonian. In this model, closure of the Rheic (?) ocean occurred

through convergence accommodated by ocean-ward dipping subduc-tion of the OMZ plate, prior to oblique collision in Late Devonian–EarlyCarboniferous times. This was followed by break-off of the subductedoceanic slab of the OMZ plate ca. 350 Ma ago, causing theemplacement of mantle-derived magmas with the development ofLP-HT metamorphism in the suture zone. Following this major event,the subsequent geodynamic evolution is interpreted to have involveda tectonic polarity reversal and intracontinental transpression withSW-ward propagating deformation in the SPZ, from Late Visean toWestphalian times.

Pereira et al. documents the results of a detailed microstructural andP–T analysis of mylonites in pelitic and quartzofeldsathic gneisses fromthe Coimbra–Córdoba shear zone (SW Iberian Massif): ductile deforma-tion evolved from a metamorphic peak at c. 650–790 °C and c. 7.5–9.5 kbar and to retrograde conditions at c. 500–600 °C and c. 3.5/5.5–5.5/7.5 kbar. The horizontal component of exhumation path, calculated formiddle and shallower crustal levels, sum to ca. 57 km to 94 km (for thepressure peak). Assuming this offset acted in the Viséan during a timeinterval of ca. 9Ma, theestimatedexhumationhorizontal slip rate is in theorder of 6.3 to 10.4 mm/yr, which corresponds to an oblique-slipexhumation rate of 6.6 to 10.7 mm/yr (for ductile deformation). TheCoimbra-Córdoba shear zone represents amodel example for document-ing the stages and scales of ductile deformation and metamorphicevolution at deep crustal levels typical of large-scale intra-continentaltranscurrent zones, as those from the Alpine-Himalaya orogenic system.This study reinforces the importance of ten-to-one hundred kilometrealong-strike mass movement of material during the exhumation of deepcrustal rocks as result of transcurrent movements. This conclusion iscritical to improving models of Variscan deformation during theamalgamation of Pangea and coeval closure of the Rheic Ocean.

On the basis of petrography, geochemistry and Sm–Nd systematics,Armendáriz et al. describe the provenance, infill history and evolutionof a Mississippian syn-orogenic basin (Pedroches Basin) developed inthe northern foreland of the Variscan suture in SW Iberia. The suturerecords oblique (sinistral) closure of this part of the Rheic Ocean,which culminated in oblique collision between Laurussia andGondwana. In response to orogenesis, transpressional reactivation ofa major tectonic boundary (Cadomian suture) between the CentralIberian and Ossa–Morena zones, located on the Gondwanan side, gaverise during the Early Carboniferous to: (i) the formation of thePedroches basin in response to transtensional processes at a majorreleasing bend in the reactivated suture, and (ii) subsequent basininversion and internal deformation probably related to transpressionat a restraining bend.

Keller et al. describe the record of the closure of the Rheic Ocean inthe Cantabrian Zone of northern Spain. In the Cantabrian Zone, SilurianthroughMiddle Devonian rocks record sedimentation on the northernpassive margin of Gondwana, which was bounding the Rheic Ocean toits south. With the reversal of plate motion during the Devonian, theRheic Ocean began to close and a subduction zone became established.As convergence continued continental crust became involved into thisprocess. The subsequent large-scale thrusting of continental litho-sphere onto the autochthonous Iberian margin led to the formation ofan initial foreland basin and the corresponding peripheral bulge in thepresent-day Cantabrian Zone. Subsequent erosion of the bulge isdocumented in the Upper Devonian deposits. Uplift of the bulgeproduced large amounts of siliciclastic detritus which were depositedas several prograding wedges in the distal foreland basin. Geometry ofthe corresponding unconformity shows an increasing depth of erosiontowards the centre of the Cantabrian Zone, where uppermostDevonian conglomerates and sandstones rest on Lower Ordovicianstrata. After uplift and erosion of the bulge had ceased, the basindeepened rapidly and persisted in starved conditions through most ofthe Mississippian. With the approach of the thrust units, sufficientsubaerial relief was created to produce increasing amounts of detritusthat filled the foredeep thus achieving overfilled conditions.

7Preface

Acknowledgements

We wish to thank all the authors who have contributed to thisvolume. Special thanks are due to Benito Abalos (Spain), RicardoArenas (Spain), Felix Bellido (Spain), Bernd Buschmann (Germany),Josep M. Casas (Spain), Sébastien Castonguay (Canada), Jose R.Martinez Catalan (Spain), Peter A. Cawood (Australia), Emilio Clavijo(Spain), Ray Coish (United States), Quetin Crowley (United Kingdom),Antonio Flores de Dios (Mexico), Alan Dickin (Canada), Antonio CastroDorado (Spain), Jaroslav Dostal (Canada), Kerstin Drost (South Africa),Luis Alarcon Eguiluz (Spain), Andres Perez Estaun (Spain), CarlosFernandez (Spain), Javier Férnandez-Suárez (Spain), Peter A. Floyd(United Kingdom), Mike Fowler (United Kingdom), Axel Gerdes(Germany), Jose Ignacio Gil-Ibarguchi (Spain), Chris Hepburn (UnitedStates), Ralf Hetzel (Germany), Teresa Jeffries (United Kingdom),Stephen T. Johnston (Canada), J. Duncan Keppie (Mexico), Uwe Kröner(Germany), Ryszard Kryza (Poland), Jean-Paul Liegeois (Belgium), UlfLinnemann (Germany), StanislawMazur (United Kingdom), J. BrendanMurphy (Canada), Richard Damian Nance (United States), Aral İ. Okay(Turkey), David C.P. Peacock (United Kingdom), M. Francisco Pereira(Portugal), Christian Pin (France), Anthony Prave (United Kingdom),Victor Puchkov (Russia), Cecilio Quesada (Spain), Victor A. Ramos(Argentina), M. Dolores Rodriguez-Alonso (Spain), J. Francisco Santos(Portugal), Jose Brandao Silva (Portugal), Sven Sindern (Germany), RobStrachan (United Kingdom), Rosa Tejero (Spain), José María Ugidos(Spain), Daniel Vachard (Francea), Jürgen von Raumer (Switzerland),Bodo Weber (Mexico), Chris White (Canada), Ian A. Williams(Australia), John A. Winchester (United Kingdom), and Gernold Zulauf(Germany), for their constructive reviews of the papers contained inthis issue. We would also like to express our sincere thanks to Editor-in-Chief Jean-Pierre Burg (Switzerland) for his encouragement andhelp throughout the preparation of this special issue. AndrésVillavicencio, Journal Manager, is gratefully acknowledgement forher patience and invaluable help during every stage of this process.This volume is a contribution to the UNESCO-IUGS project, IGCP 497—

The Rheic Ocean: Origin, Evolution and Correlatives. At last, but doesnot mean the least, we would also like to thank Project Leader UlfLinnemann for his initiative, enthusiasm, encouragement and endlessefforts that make this project plausible and running smoothly.

References

Abati, J., Dunning, G.R., Arenas, R., Diaz Garcia, F., Gonzáles Cuadra, P., Martinez Catalán,J.R., Andonaegui, P., 1999. Early Ordovician orogenic event in Galicia (NW Spain):evidence from U–Pb ages in the uppermost unit of the Ordenes Complex. Earth andPlanetary Science Letters 165, 213–228.

Crowley, Q.G., Floyd, P.A., Winchester, J.A., Franke, W., Holland, J.G., 2000. EarlyPalaeozoic rift-related magmatism in Variscan Europe: fragmentation of theArmorican Terrane Assemblage. Terra Nova 12, 171–180.

Eguiluz, L., Gil-Ibarguchi, J.I., Abalos, B., Apraiz, A., 2000. Superposed Hercynian andCadomian orogenic cycles in the Ossa–Morena zone and related areas of the IberianMassif. Geological Society of America Bulletin 112, 1398–1413.

Fernández-Suárez, J., Gutiérrez-Alonso, G., Jeffries, T.E., 2002. The importance of alongmargin terrane transport in northern Gondwana: insights from detrital zirconparentage in Neoproterozoic rocks from Iberia and Brittany. Earth Planetary ScienceLetters 204, 75–88.

Keppie, J.D., 2004. Terranes of Mexico revisited: a 1.3 billion year odyssey. InternationalGeology Review 46, 765–794.

Keppie, J.D., Nance, R.D., Murphy, J.B., Dostal, J., 2003. Tethyan, Mediterranean, andPacific analogues for the Neoproterozoic–Paleozoic birth of Peri-Gondwananterranes and their transfer to Laurentia and Laurussia. Tectonophysics 365,195–219.

Linnemann, U., Nance, R.D., Kraft, P., Zulauf, G., 2007. Preface — the Evolution of theRheic Ocean: From Avalonia–Cadomian Active Margin to Alleghenian–VariscanCollision. The Geological Society of America Special Paper 423, vii–viii.

Linnemann, U., McNaughton, N.J., Romer, R.L., Gehmlich, M., Drost, K., Tonk, C., 2004.West African provenance for Saxo-Thuringia (Bohemian Massif): did Armorica everleave pre-Pangean Gondwana?: U/Pb–SHRIMP zircon evidence and the Nd-isotopicrecord. International Journal of Earth Sciences 93, 683–705.

Martínez Catalán, J.R., Arenas, R., Díaz García, F., Gómez Barreiro, J., González Cuadra, P.,Abati, J., Castiñeiras, P., Fernández Suárez, J., Sánchez Martínez, S., Andonaegui, P.,González Clavijo, E., Díez Montes, A., Rubio Pascual, F.J., Valle Aguado, B., 2007.Space and time in the tectonic evolution of the northwestern Iberian Massif.Implications for the comprehension of the Variscan belt. In: Hatcher, R.D., Carlson,

M.P., McBride, J.H., Martínez Catalán, J.R. (Eds.), 4-D Framework of ContinentalCrust. Geological Society America, Memoir, p. 200.

Matte, P., 2001. The Variscan collage and orogeny (480–290 Ma) and the tectonicdefinition of the Armorica microplate: a review. Terra Nova 13, 122–128.

Meert, J.G., Torsvik, T.H., 2003. The making and unmaking of a supercontinent: Rodiniarevisited. Tectonophysics 375, 261–288.

Murphy, J.B., Dostal, J., 2007. Continental mafic magmatism of different ages in the sameterrane: constraints on the evolutionof anenrichedmantle source. Geology35, 335–338.

Murphy, J.B., Fernández-Suárez, J., Keppie, J.D., Jeffries, T.E., 2004. Contiguous ratherthan discrete Paleozoic histories for the Avalon and Meguma terranes based ondetrital zircon data. Geology 32, 585–588.

Murphy, J.B., Gutierrez-Alonso, G., Nance, R.D., Fernandez-Suarez, J., Keppie, J.D.,Quesada, C., Strachan, R.A., Dostal, J., 2006a. Origin of the Rheic Ocean: rifting alonga Neoproterozoic suture? Geology 34, 325–328.

Murphy, J.B., Keppie, J.D., 2005. The Acadian orogeny in the northern Apalachians.International Geology Review 47, 591–619.

Murphy, J.B., Keppie, J.D., Nance, R.D., Miller, B.V., Dostal, J., Middleton, M., Fernández-Suárez, J., Jeffries, T.E., 2006b. Geochemistry and U–Pb protolith ages of eclogiticrocks of the Asis Lithodeme, Piaxtla Suite, Acatlán Complex, southern Mexico:tectonothermal activity along the southern margin of the Rheic Ocean. Journal ofthe Geological Society, London 163, 683–695.

Murphy, J.B., van Staal, C.R., Keppie, J.D., 1999. Is the mid to late Paleozoic AcadianOrogeny a plume-modified Laramide-style orogeny? Geology 27, 653–656.

Nance, R.D., Fernández-Suárez, J., Keppie, J.D., Storey, C., Jeffries, T.E., 2007. Provenanceof the Granjeno Schist, Ciudad Victoria, Mexico: detrital zircon U–Pb age constraintsand implications for Paleozoic paleogeography. In: Linnemann, U., Nance, R.D.,Zulauf, G., Kraft, P. (Eds.), The Geology of Peri-Gondwana: The Avalonian–CadomianBelt, Adjoining Cratons and the Rheic Ocean. Special Paper, 23. Geological Society ofAmerica, pp. 437–452.

Nance, R.D., Murphy, J.B., 1994. Contrasting basement isotopic signatures and thepalinspastic restoration of peripheral orogens: example from the NeoproterozoicAvalonian–Cadomian belt. Geology 22, 617–620.

Nance, R.D., Murphy, J.B., Keppie, J.D., 2002. A Cordilleran model for the evolution ofAvalonia. Tectonophysics 352, 11–31.

Neubauer, F., 2002. Evolution of late Neoproterozoic to early Paleozoic tectonicelements in Central and Southeast European Alpine mountain belts: review andsynthesis. Tectonophysics 352, 87–103.

Pereira, M.F., Chichorro, M., Williams, I.S., Silva, J.B., 2008. Zircon U–Pb geochronology ofparagneisses and biotite granites from the SW Iberian Massif (Portugal): evidencefor a paleogeographic link between the Ossa–Morena Ediacaran basins and theWest African craton. In: Liégeois, J.P., Nasser, E. (Eds.), The Boundaries of the WestAfrican Craton. Geological Society, 297. Special Publications, London, pp. 385–408.

Pereira, M.F., Silva, J.B., Chichorro, M., Moita, P., Santos, J.F., Apraiz, A., Ribeiro, C., 2007.Crustal growth and deformational processes in the Northern Gondwana margin:constraints from the Évora Massif (Ossa–Morena Zone, SW Iberia, Portugal). In:Linnemann, U., Nance, R.D., Kraft, P., Zulauf, G. (Eds.), The Evolution of the RheicOcean: From Avalonian–Cadomian Active Margin to Alleghenian–Variscan Colli-sion. Special Paper, 423. Geological Society of America, pp. 333–358.

Pereira, M.F., Chichorro, M., Linnemann, U., Eguiluz, L., Silva, J.B., 2006. Inherited arcsignature in Ediacaran and Early Cambrian basins of the Ossa–Morena Zone (IberianMassif, Portugal): paleogeographic link with European and North African Cadomiancorrelatives. Precambrian Research 144, 297–315.

Pin, C., Marini, F., 1993. Early Ordovician continental break-up in Variscan Europe: Nd–Sr isotope and trace element evidence from bimodal igneous associations of theSouthern Massif Central, France. Lithos 29, 177–196.

Quesada, C., 1991. Geological constraints on the Paleozoic tectonic evolution oftectonostratigraphic terranes in the Iberian Massif. Tectonophysics 185, 225–245.

Quesada, C., 2006. The Ossa–Morena Zone of the Iberian Massif: a tectonostratigraphicapproach to its evolution. Zeitschrift der Deutschen Gesellschaft für Geowis-senschaften 157, 585–595.

Rodríguez-Alonso, M.D., Peinado, M., López-Plaza, M., Franco, P., Carnicero, A., Gonzalo,J.C., 2004. Neoproterozoic–Cambrian synsedimentary magmatism in the CentralIberian Zone (Spain): geologic, petrologic and geodynamic significance. Interna-tional Journal of Earth Sciences 93, 897–920.

Sánchez-Garcia, T., Bellindo, F., Quesada, C., 2003. Geodynamic setting and geochemicalsignatures of Cambrian–Ordovician rift-related igneous rocks (Ossa–Morena Zone,SW Iberia). Tectonophysics 365, 233–255.

Stampfli, G.M., von Raumer, J., Borel, G., 2002. The Paleozoic evolution of pre-Variscanterranes: fromGondwana to theVariscan collision. In:Martinez Catalan, J.R., Hatcher Jr.,R.D., Arenas, R., DiazGarcia, F. (Eds.), Variscan–AppalachianDynamics: theBuildingof theLate Paleozoic Basement, 364. Geological Society of America Special Paper, pp. 263–280.

Ustaömer, P.A., Mundil, R., Renne, P.R., 2005. U/Pb and Pb/Pb zircon ages for arc-relatedintrusions of the Bolu Massif (W Pontides, NW Turkey): evidence for LatePrecambrian (Cadomian) age. Terra Nova 17, 215–223.

Ustaömer, P.A., Rogers, G., 1999. The BoluMassif: remnant of a pre-Early Ordovician activemargin in the West Pontides, northern Turkey. Geological Magazine 136, 579–592.

von Raumer, J.F., Stampfli, G.M., Borel, G.D., Bussy, F., 2002. The organization of pre-Variscan basement areas at the Gondwana margin. International Journal of EarthSciences 91, 35–52.

von Raumer, J.F., Stampfli, G.M., Bussy, F., 2003. Gondwana-derived microcontinents—theconstituents of the Variscan and Alpine collisional orogens. Tectonophysics 365, 7–22.

Winchester, J.A., Pharaoh, T.C., Verniers, J., Ioane, D., Seghedi, A., 2006. Palaeozoicaccretion of Gondwana-derived terranes to the East European Craton: recognitionof detached terrane fragments dispersed after collision with promontories. In:Stephenson, R.A., Gee, D.G. (Eds.), European Lithosphere Dynamics. Memoirs, 32.Geological Society, London, pp. 323–332.

8 Preface

Erdin BozkurtDepartment of Geological Engineering,

Middle East Technical University, TR-06531 Ankara, TurkeyE-mail address: erdin@metu.edu.tr.

Corresponding author.

M. Francisco PereiraDepartamento de Geociências, Centro de Geofísica de Évora,

Universidade de Évora, Évora, PortugalE-mail address: mpereira@uevora.pt.

Robin StrachanSchool of Earth & Environmental Sciences, University Portsmouth,

Portsmouth, United KingdomE-mail address: Rob.Strachan@port.ac.uk.

Cecilio QuesadaInstituto Geológico y Minero de España, Madrid, Spain

E-mail address: cquesada@igme.es.