The origin of the Betic orogen, southern Spain

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  • Tectonophysics, 27 (1975) 39-56 @ Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

    THE ORIGIN OF THE BETIC OROGEN, SOUTHERN SPAIN

    W. KAMPSCHUUR and H.E. RONDEEL

    Department of Structural Geology, Geological Institute, University of Amsterdam, Amster- dam (The Netherlands)

    (Submitted June 18, 1974; revised version accepted December 24, 1974)

    ABSTRACT

    Kampschuur, W. and Rondeel, H.E., 1975. The origin of the Betic orogen, southern Spain. Tectonophysics, 27: 39-56.

    Data are presented on the polyphase deformation and plurifacial metamorphism in the eastern part of the Betic Zone of southern Spain. An older period of polyphase deforma- tion with roughly NW-SE fold axes is succeeded by a younger period with NE-SW to E-W axes which coincide with the present grain of the belt. Metamorphism during the older period occurred in two episodes, each consisting of a kinematic stage succeeded by a static stage. These episodes seem to be present in all of the tectonic complexes recog- nized in the area.

    Dating of the periods of deformation by means of stratigraphical methods suggests a Neogene or possibly older age for the younger period and a post-Early Jurassic-pre- Neogene age for the older period. Various considerations, e.g. concerning plate-tectonic origin, suggest a Mesozoic age for the latter.

    The fold orientations are thought to be in disagreement with the normally assumed northward direction of tectonic transport in the Betic Zone. Movements in this direction are restricted to the younger period of deformation, whereas NE-SW to E-W movements are deduced to have been of major importance during the older period of deformation which relates to the overthrust structures in the Betic Zone. It is here supposed that the original palaeogeographic zones were roughly oriented in a NW-SE direction.

    Between the younger and older periods, the Betic Zone is supposed to have ap- proached the Subbetic Zone along megashears. The latter zone has not been influenced by the older period of deformation.

    INTRODUCTION

    The alpine fold-belt of southern Spain - the Betic Cordilleras - can be subdivided in three zones (Fig. 1):

    (1) The Prebetic Zone which consists of the autochthonous and parau- tochthonous, non-metamorphic sedimentary cover of the hercynian massif of the Spanish Meseta and its subsurface continuation.

    (2) The Subbetic Zone which is composed of parautochthonous to alloch- thonous, non-metamorphic sediments, originally deposited in a realm to the south of that of the Prebetic Zone.

  • Fig. 1. Map showing the tectonic zones of the Betic Cordilleras, southern Spain.

    (3) The Betic Zone, now to the south of the Subbetic Zone, which is built up of a large number of over-thrust tectonic units. These differ in degree of alpine metamorphism.

    The Prebetic and the Subbetic Zone are together normally referred to as the external zone of the Betic Cordilleras. The Betic Zone is the internal zone.

    Various tectonic units of the Betic Zone continue across the Strait of Gibraltar, to form the internal part of the Rif (Didon et al., 1973). The flysch deposits of the Campo de Gibraltar continue into the external, alloch- thonous Moroccan flysches. In more external parts, the allochthonous to parautochthonous alpine units of the Rif mountains occur (see Fig. 3). The hercynian Moroccan Meseta with its folded Mesozoic cover .-. the Atlas - is the foreland of the alpine, north Moroccan chain (Choubert and Faure- Muret, 1971).

    The tectonic units of the Betic Zone can be grouped into several com- plexes which regionally overlie one another. Tectonic level and (interrelated) degree of (synkinematic) alpine metamorphism largely determine the group- ing in use (Egeler et al., 1971). In the eastern part of the Betic Zone, the superposition is Nevado-Filabride complex, Ballabona-Cucharon complex, Alpujarride complex, MaIaguide complex in ascending order (Egeler and Simon, 1969a). The lowermost complex is of medium-grade metamorphism; the uppermost and Malaguide complex has only been slightly influenced by alpine metamorphism. The others have intermediate metamorphic grades. A nappe character of the complexes and of the units therein, is normally ac- cepted; the character of the lowermost tectonic unit - the Nevado-Lubrin unit - is in debate (see Fallot et al., 1960).

    The Ballabona-Cucharon complex consists exclusively of Permo-Triassic

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    and Triassic sediments and some metabasites. The other complexes contain in addition rock sequences of presumed pre-Permo-T~assic age that are sup- posed to show the effects of pre-alpine erogenic events (e.g. Egeler and Simon, 1969a; see, however, Kampschuur et al., 1973).

    The present stack of tectonic units is the result of at least two phases of overthrusting; the older one having caused an initial empilement of the com- plexes (nappes) and the younger disrupting the pile (Egeler and Simon, 1969b). The direction of thrusting is not known. Our lack of knowledge in this respect - noto~ously denied by Van Bemmelen (1973) - can best be illustrated by a quotation from a publication by Egeler and Simon (1969a, p. 67, literally translated): The opinions diverge on the direction in which the large-scale overthrust movements took place because of uncertainty about the original palaeogeographic arrangement of the complexes. Note that the reverse, i.e. the uncertainty of palaeogeographic arrangements because of ignorance of the direction of over-thrusting, was never discussed. Another illustration of our lack of acquaintance with the direction of tectonic trans- port is given in the publication by Andrieux et al(l971, p. 192, literally translated) who state: The major structures in the E-W part of the Betic Cordilleras are very clearly and energetically overturned to the north; . ... whereas (p. 193, literally translated): ... the direction of overturning of the major phases is not evident since they have been energetically refolded by later phases.

    Concerning reconstruction of the Betic realm of sedimentation, two fun- damentally different possibilities with respect to the paiaeogeo~aphic ar- rangement of the rock sequences now forming the complexes are normally considered. In north to south palaeogeographic order these are:

    (1) Nevado-Filabride complex, Ballabona-Cucharon complex, Alpujamide complex, Malaguide complex.

    The present superposition is in this case ascribed to overthrusting directed between the north and the northwest (e.g. Brouwer, 1926; Blumenthal, 1935; FaIlot, 1948; Hoeppener et al., 1963; Egeler and Simon, 1969b; Fontbote, 1970).

    (2) MaIaguide complex to Nevada-F~ab~de complex. In this case south- to southeastward directed overthrusting (see MacGiI-

    lavry, 1964) or north- to northwestward underthrusting (e.g. Durand-Delga, 1966) resulted in the present tectonic pile.

    In a study on polyphase structures in the eastern Betic Zone (Kampschuur et al., 1973), we concluded that very similar schemes of alpine deformation affected rock sequences belonging to the different tectonic complexes. Since then many new data relating to the polyphase alpine defo~ation in the eastern Betic Zone have become known. They are the result of post-graduate studies by S.I. During, J.A. Verburg, F.M. Voermans and E.H.M. Wolff and of studies for the Magna-project of the Spanish government by W. Kamp- schuur, R.L.M. Vissers and F.M. Voermans. These data confirm the conclu- sion reached earlier of a uniformity in the deformation schemes of tectonic units belonging to separate tectonic complexes in the eastern Betic Zone.

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    In our opinion, these results are in disagreement with the palaeogeo- graphic reconstructions previously considered, since they reflect neither thrusting directed between N and NW nor between S and SE. After presenta- tion of these data, their consequences will be considered for the megatec- tonic plate model of the westernmost part of the Mediterranean area.

    DEFORMATIONAL EVENTS

    Table I summarizes the succession of structural events for the areas in the eastern Betic Zone which are indicated in Fig. 2. The columns in this table have been correlated using the best fit of the locally established deformation sequences. In this correlation it has been assumed that regional metamor- phism affects large domains roughly contemporaneously and that the results of folding phases accompanied by penetrative deformation are recognizable over considerable areas. Attention is drawn to the special character of the

    LOCATION OF AREAS with respect to the TECTONIC COMPLEXES OF THE BETIC ZONE

    -

    Fig. 2. Map showing the distribution of tectonic complexes in the southeastern part of the Betic Zone. The areas from which data are presented in Table 1 are numbered.

  • I

    I 4

    43

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    deformation scheme in which phases of folding succeed thrust phases. It brought us to the use of D, , Dz etc. for distinct periods or intervals of any duration (deformation phases) during which any or all of the following struc- tures were formed: folds, foliations, lineations and thrust faults.

    The following observations can be made (see Table I):

    Deformation

    (1) The oldest phase of deformation is synme~o~hi~. It produced a schistosity (slaty cleavage in low-grade rocks) accomp~ying folds which are normally isociinal or very tight. The folding is assumed to have acted during the initial empilement of tectonic units (Kampschuur et al., 1973). Only in the Sierra de Carrascoy (area 6 of Fig. 2) has the vergence direction been established for the structures generated during this phase (Kampschuur, 1972). There, it is to the southwest*.

    (2) Two periods of alpine deformation can be distinguished, viz. an older and a younger one. They have different fold orien~tions (Kamps~huur et al., 1973). The orientations of the fold axes of the older period of alpine de- formation, here defined as D, to D5, are confined between S and E. Folding was not coaxial, but at any locality the angles between the fold axes gener- ated during succesive phases within this period are now relatively small.

    Folding about axes trending between S and E and hence thought to be correlatable with the older period of alpine deformation, has also been recog- nized in the western part of the Betic Zone, viz. in the area studied by Wes- terhof (in prep~ation) in the Sierra Blanca.

    (3) The axial orientations of the folds formed during the younger period of alpine deformation (post-D,) are everywhere at a large angle to those of the previous period. Axial planes invariably show steep inclinations. Fold axes are roughly oriented NE-SW to E-W, coinciding with the present grain of the Betic Zone.

    (4) Important thrust movements only occurred in the early phases of the deformation history (D1 -D4 ), The translations involved are thought to have been directed at right angles to the fold axes which formed during these phases.

    Metamorphism

    (5) The oldest phase of deformation took place whiIe the rocks of the Nevado-Filabride complex were being metamorphosed under conditions which produced mineral assemblages now classified as glaucoph~ic green- schist facies (see Kampschuur, 1975) or as glaucophane schist facies (De Roever

    * Directions are indicated with respect to present geographical coordinates. A possible influence due to post-deformational rotation of the Iberian Peninsula or parts of it, has not been taken into account. This applies throughout the paper.

  • 45

    and Nijhuis, 1964). At the same time, greenschist-facies conditions prevailed in the Permo-Triassic and Triassic rock sequences of the overlying Ballabona- Cucharon and Alpujarride complexes.

    (6) A subsequent metamorphic stage had a static nature. It took place between successive fold phases D1 and D3, and is of retrograde character. Linked with the preceding kinematic stage it forms the first episode of meta- morphism.

    (7) A second episode of metamorphism also comprised a synkinematic and a subsequent, static (intermicrofolding) stage. The synkinematic stage was active during D3, producing mineral associations of the greenschist facies in the Nevado-Filabride complex and in the Ballabona-Cucharbn complex. The degree of metamorphism is apparently lower in high complexes.

    Langenberg (1972) showed that mineral assemblages of the greenschist- amphibolite transition facies formed in the Nevado-Filabride rocks during D3, but also subsequently. Kampschuur (1975) explained these mineral associations by a syn-D, stage of metamorphism in the greenschist facies and a subsequent static stage.

    (8) The stage of static metamorphism in this second episode is of more than local extent and its effects can be found in the three lower tectonic complexes. Kampschuur (1975) considers it to be of prograde character, grading into the greenschist-amphibolite transition facies in Nevado-Fila- bride rocks.

    The subdivision in metamorphic episodes used here has also been used by Egeler (in preparation) in his comparison of the metamorphism of the inter- nal zones of the Betic Cordilleras and the Alps. He stressed the independance of the two episodes.

    Using the information on deformation and metamorphism as presented sub 1-8, it is now possible to discuss, in some detail, the two major thrust phases proposed by Egeler and Simon (196913). It may be remembered that these two phases were held responsible for the superposition of the tectonic complexes and for discontinuities in the degree of alpine regional meta- morphism at contacts of major tectonic elements. The thrust phases were restricted to the older period of alpine deformation.

    The first phase was responsible for the empilement of complexes by crus- taI shortening in a NE-SW to E-W direction. Isoclinal folding (D, ) accom- panied the empilement. Structures generated are strongly penetrative. In the lowermost units deformation took place under high pressure (be it glauco- phanic greenschist or glaucophane schist facies). Towards the higher com- plexes pressure conditions are lower and the degree of metamorphism de- creases. In the Malaguide complex deformation took place only under very weak metamorphic conditions.

    The second phase of thrusting disturbed this initial pile. It is correlatable with the series of deformation phases (Dz -DS ) with a crust&shortening direction roughly coinciding with that of the previous event. D2 and D4

  • I &

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    caused imbrications (and layer-parallel thrusting) due to which metamorphic isogrades are disrupted at major thrust contacts. D3 and D, are fold phases during which strongly penetrative structures were generated. The second episode of metamorphism occurred syn- and postkinematically with D3. It is reported to have a decreasing influence on higher tectonic complexes, imply- ing that the thrusting during Dz cannot have very strongly disturbed the order of the pre-existing pile.

    These events were followed by the younger period of alpine deformation with a roughly NW-SE and N-S crustal shortening direction. Open to closed folds were formed and upthrusting occurred.

    Wrench faulting in a late stage of the alpine history locally disturbed the general trend of the Betic Zone (Fig. 3) which became fixed during the younger period of deformation.

    AGES OF THE PERIODS OF DEFORMATION

    The oldest deposits which are certainly unconformable upon the Betic thrust masses in the eastern part of the Betic Zone are Early Miocene (Rondeel and Simon, 1973), Serravallian (Kampschuur, 1972) or Early to early-Middle Miocene (Geel, 1973). Paquet (1967) considered that Middle to lower-Upper Eocene deposits transgressed onto Lutetian rocks of different MaIaguide units, but this interpretation was rejected by Hermes and Kuhry (1969).

    The folding and thrusting recognized in the transgressive sequence seal- ing the Betic thrust masses leads to the conclusion that during and after the Miocene, N-S to NW-SE directed shortening occurred. Rondeel (1965) reported Miocene and younger folding about ENE-WSW to E-W oriented axes in the Vera and Sorbas basins (see also Valk, 1967). In the area of the Lorca basin and Sierra de Tercia (Geel, in preparation) and in the extreme eastern part of the Betic Zone - north and east of the Sierra de Carrascoy - Neogene deposits have folds with NE-SW to E-W oriented axes (Kamp- schuur, 1972; Montenat, 1973).

    Folding about NW-SE to N-S axes has not been found in Neogene rocks in the eastern Betic Zone.

    In the external zone of the Betic Cordilleras folding is normally about NE-SW to E-W oriented axes. There is general agreement concerning the Neogene (mainly Miocene) age of the essential erogenic movements that affected this domain, as illustrated recently by the publications of Gee1 (1973), Linares and Rodriquez (1973), and Paquet (1967). Hoedemaeker (1973) proposed that folds in the Subbetic came into existence during the Eocene at the locus of major basement hinges.

    Most of the tectonic units in the eastern Betic Zone consist exclusively of pre-Jurassic rocks. Only in the Malaguide complex there are also Jurassic, Cretaceous and Palaeogene sediments. Therefore, it is the only complex in which the ages of the deformation periods might be more precisely defined by st,ratigraphic methods.

  • Post-Triassic rocks assigned to the Malaguide complex are common in the corridor of Velez Rubio which is a zone of intensive tectonic imbrication at the boundary between the Betic and Subbetic Zones. There it is uncertain which Tertiary formation forms the upper part of the Malaguide succession and which is post-nappe. In any case, Early to early-Middle Miocene deposits postdate the final emplacement of the Malaguide units and it is the Oligo- Miocene rocks whose position is debated (Geel, 1973; MacGillavry, 1964).

    Malaguide rocks also occur in isolated patches to the south of the Velez- Rubio corridor. In the Sierra Cabrera it has been ascertained that Lower Jurassic rocks were affected by thrusts of D2 and D4 age (IGME, in prepara- tion).

    It thus appears that the younger period of deformation, characterized by the NE-SW to E-W axes is of Neogene age, or possibly older, and that the older period of alpine deformation is pre-Neogene and post-Early Jurassic. Precise dating is not possible with the information currently available but more general considerations might further delimit the ages of these periods.

    The lack of post-Triassic sediments from the lower tectonic complexes has been explained in various ways. For example, complete erosion of a relative- ly thin sequence of post-Triassic sediments prior to nappe formation or a prolonged period of non-deposition during the younger Mesozoic and the early Tertiary. Both these hypotheses assume the alpine erogenic movements to have taken place during the Tertiary. A third hypothesis is that of a Meso- zoic tectonic transgression (Late Triassic for Banting, 1933; and Jurassic for Van Bemmelen, 1927, Hoeppener et al., 1963). This would not only explain the absence of post-Triassic sediments from most tectonic units, but also the great similarity between the post-Triassic rock sequence of the highest unit - the Malaguide complex - and the Subbetic (De Booy and Egeler, 1961; MacGillavry, 1964; Hoeppener et al., 1963, 1964). Moreover, this might explain the occurrence of Lower Cretaceous to Palaeogene flysch deposits in the Campo de Gibraltar and Morocco (Didon et al., 1973).

    The tectonic transgression concept has been considered untenable by various authors, because the Mesozoic and Palaeogene hiatuses in the Mala- guide and in the Subbetic sequences (see Hoeppener et al., 1963, and his bibliography; Geel, 1973; Garcia-Duefias, 1969) were regarded as being insufficiently important to record nappe movements. In view of the scarcity of data from the Malaguide complex, we would like to comment retrospec- tively on this in two ways. Firstly, the reconstruction of the detailed Mala- guide column was only made in a few areas. Secondly, the reconstructions come from areas with scattered outcrops in very strongly tectonically frag- mented terrain.

    Flysch sedimentation is, however, a good indicator of compressive oro- genie events and continuous, conformable sedimentation contemporaneously with deformation in adjacent areas - as suggested for the Alps (Trilmpy, 1973) - could also be the case in the Betic Cordilleras. We favour a Mesozoic age for the beginning of the older period of deformation in the Betic Zone, in view of the age of the flysch deposits.

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    In the western part of the Betic Zone, Bourgois (1974) proved alpine deformation preceding Early Cretaceous flysch deposition.

    RELATION TO PLATE TECTONICS OF GENERAL AREA

    The evolution of the Betic Cordilferas is clearly in some way related to the rel.ative motions of the European, African and American Plates. In effect the history of the entire western part of the Mediterranean since Triassic times cannot be evaluated without taking into account, the opening of the proto- ALlantic and the resulting interaction of the European and African Plates (see e.g. Dewey et al., 1973). In plate models based on Atlantic sea-floor spreading, the area of t,he Alboran Sea is a tectonic junction. This is the area where the European and African Plates met and where, during early opening of the Atlantic, the Tethyan trench of crustal uptake passed westwards into a megashear zone (Dietz and Holden, 1970). Megashearing continued from the Eocene onwards with accompanying N-S compression. The mega- shear(s) indicated in this area by most authors (e.g. Le Pichon, 1968; Smith, 1971; Dewey et al., 1973) have sinistral movements from the beginning of opening of the Atlantic until about 80 m.y. ago when they accommodated movements in the opposite dire&on (Smith, 1971; Pitman and Talwani, 1972; Dewey et al., 1973). The anti-clockwise rotation of Iberia (1 30) is considered to result from the sinistral inter-plate movements and has been dated as Jurassic- Cretaceous (Choukroune et al., 1973; Van der Voo and Zijderveld, 1971) although an Oligocene age has recently been proposed (Storetvedt, 1973).

    The post-Palaeocene convergence of the African and European Plates could explain the folding about NE-SW to E-W axes produced during the younger period of deformation. Auzende et al. (1973) concluded that in the west.ernmost Mediterranean, parts of the Alboran Plate moved to the south- west during the Neogene. This motion occurred along shears whose traces are small circles on the surface of the globe and comparable to those indicated in Fig. 3. It is said to be the cause of the NE-SW features off the North Afri- can coast, the Alboran Sea included, where Tortonian sediments directly overlie the substratum (see also Nesteroff and Ryan, 1973).

    It is suggested here that the older period of deformation occurred in pre- Eocene times. The reason is that the post-Palaeocene Africa--Europe con- vergence across the separating E-W oriented megashear(s) is unlikely t.o have provoked the NW-SE to N-S oriented fold axes of this older period. Neither can the pre-Eocene dextral movements along the megashear(s) ac- count, for the folding. The older deformation might, however, be assumed to have been causally and therefore geometrically related to the Mesozoic sinis- tral wrench movements. However, its penetrative nature and the contem- porary conditions, as deduced from m~tamorphi~ facies, would not be ex- pected in a wrench tectonics regime (see Wilcox et al., 1973; Moody, 1973). Even less satisfactory is that one cannot account for the translatory move-

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    ments of the nappes. An alternative explanation is that the folding took place between successive megashears as the result of Atlantic sea-floor spreading which also controlled strike-slip movements along the shears. The convergent plate margin at which deformation occurred should not in that case be a continuous feature but a series of consumption zones offset by the successive megashears.

    We conclude that a pre-Eocene, Mesozoic age is not unlikely for the older period of deformation in the Betic Zone.

    DISCUSSION

    It is our opinion - implicit in the preceding remarks on crustal-shortening directions - that intensive coaxial folding and penetrative deformation which occur together over large areas and which were caused in the higher parts of the earths crust, are indicative of crustal shortening (tectonic trans- port) roughly orthogonal to the orientation of the fold axes. The term crus- tal shortening here relates to shortening in specific levels of the crust only - the entire crust is not necessarily involved. Tectonic fabrics which have other geometrical relations with the direction of tectonic transport are nor- mally of different character.

    On the basis of these considerations, we conclude for the Betic Zone that during the alpine orogeny an older period of NE-SW to E-W shortening was followed by a period of shortening in a NW-SE to N-S direction. The classi- cal concept of folding in erogenic belts approximately parallel with their palaeogeographic (facies) zones supports the ideas of tectonic transport orthogonal to palaeogeographic zones and of the orthogonal relationship between tectonic transport and orientation of fold axes. That concept leads to the supposition that the palaeogeographic realms in the proto-Betic Zone, now thought to be represented by the different tectonic complexes, were originally stretching from northwest to southeast or from north to south. The speculation of Durand-Delga (1973) over the orientation of these realms and over the direction of overthrusting in the Arc of Gibraltar con- forms with our view.

    In plate-tectonic concepts, compressive fold- or thrust-belts are assumed to have formed at continental plate margins (or island arcs) where plate consumption took place by subduction of oceanic crust or where collisions occurred with other continental plates. The belts originate at these conver- gent plate junctions independently of the relative direction of movement of the plates involved. In fact, the slip vector of an underthrusting or colliding plate is not necessarily orthogonal to the plate margins. In a discussion of the evolution of the alpine system, Dewey et al. (1973, p. 3154) also concluded that the direction of tectonic transport of ensuing nappes would not neces- sarily be parallel to the slip vector of the plates.

    The preference of fold-belts to form at continental margins has been stressed by various authors, e.g. Laubscher (1969), Dickinson (1971), Dewey

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    and Bird (1970), Dietz (1972). Smith (1971) in a discussion on alpine defor- mation and the oceanic areas of the Tethys, Mediterranean and Atlantic noted that (p. 2061) ... the over-all trend of compressional structures has been governed by the supposed continental margin, rather than the displace- ment vectors at the margins, which actually make an angle of less than 45 with it. Choubert and Faure-Muret (1971) seem to express the same general idea about the relation of plate margin and fold trend in a discussion on the st,ructures of the Anti-Atlas (p. 363, literally translated): Given that the Hercynian folding has been provoked by the translation of the African con- tinent from southeast to northwest, it is logical that the main folding is oriented parallel to the rigid front of this continent.

    In our opinion, not only the general trend of fold belts is governed by the orientation of the continental plate margin(s) at convergent plate junctions, but also the direction of tectonic transport which is roughly orthogonal to these margins. Fold axes mimic that orientation. In not too deep levels of the crust they will therefore generally form roughly parallel to these margins.

    An implication of this reasoning is that in the case of the Betic Zone one must propose a NW-SE to N-S trending plate margin to which facies zones are parallel. Plates could have moved in nearly any direction within the restrictions imposed by the model of the Atlantic opening.

    In the literature on the Betics, it is assumed implicitly - in our opinion unjustly - that the direction of tectonic transport is roughly orthogonal to the grain of the belt and that the tectonic complexes stem from palaeogeo- graphic zones which are arranged roughly parallel to it (see e.g. Van Bem- melen, 1973; Dewey et al., 1973; Fontbote, 1970). The grain of the belt has here been proved to be unrelated to the phases during which the principal tectonic transport occurred, viz. the older period of deformation. It is con- trolled by events which occurred during and after the younger period of deformation and, with the exception of curved portions, it runs ENE-WSW. In the eastern part of the belt, the grain is indicated by topographic and structural highs and Neogene basins.

    It has now become evident that the direction of the movements during the older period of deformation strongly deviated from those during the younger period, which resulted in the large-scale folding exhibited by the grain of the belt. It seems that in the Betic Cordilleras, as in the Balkans, Alps and Greater Caucasus (Belov, 1972), the formation of the regional mega-anticli- nal structures occurred mainly during, and as the result of, the concluding stages of erogenic development. Our understanding of the Betics has thus been made difficult because the last phases of alpine deformation involved a northward motion of Africa with respect to Europe. Hsii (1971) asserts this view for the Alps and even the entire Mediterranean.

    The bi-periodical character of alpine deformation in the Betic Zone - older and younger period of deformation - can also be recognized in the western part of the Zone in the history of the peridotites of the Sierra Ber- meja. According to studies of Dar-rot (1974) these ultramafics have been

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    emplaced subhorizontally in southwest direction, after which they were thrusted to the northwest.

    Support for the suggested correlation of early alpine NW-SE to N-S folding and comparably oriented palaeogeographic zones is provided by the distribution of some rock complexes (Fig. 3). These are here listed:

    (1) The Ballabona-Cuchar6n complex is only identified with certainty east of the longitude of Almeria.

    (2) Peridotites are only exposed in the western part of the Betic Zone, even though they occur in units which can be found all over the Zone.

    (3) The dorsale calcaire at the fringe of the Betic Zone in its western part (Didon et al., 1973), does not occur in the eastern half of the belt.

    (4) Flysch deposits are concentrated in the western part of the belt on the convex side of the Arc of Gibraltar (Didon et al., 1973).

    It is reported by Helmers (1973) that during an early period of alpine metamorphism, the lower tectonic units were metamorphosed in an inter- mediate- to high-pressure facies series; the Nevado-Filabrides largely in the albite-epidote amfibolite facies (with HP relics) and the directly overlying part of the Alpujarrides in the greenschist facies. The distribution of these facies is concentric around the dome of the Sierra Nevada-Sierra de 10s Filabres. Proceeding west from the Sierra Nevada, metamorphic conditions within the Alpujarride complex change, possibly progressively, as follows (deduced from Helmers data, 1973). Firstly, an intermediate to low-pressure facies series (amfibolite and greenschist facies) e.g. in the Sierra de Almijara. Secondly, a low-pressure facies series (with HT-HP relics) still further to the west, e.g. in the Sierra Blanca and Sierra de Mijas which are here thought to belong to the Alpujarride complex. Whether or not these differences plead in favour of the early NW-SE to N-S trend of the complexes is a matter of debate.

    It is of further interest that the early alpine direction of fold axes coin- cides with the hercynian fold trend of the Iberian Meseta (Fig. 3). The change in trend within the Iberian Hercynides when approaching the Betic Cordilleras is assumed to relate to strike-slip movements between these areas. It might be that hercynian trends influenced later alpine directions in the Betics just as in many other alpine areas (Alps, Balkans, Greater Caucasus, according to Belov, 1972).

    The Prebetic and Subbetic can be distinguished from the Betic Zone; there the later alpine, NE-SW to E-W direction of fold axes is found exclusively while the early alpine direction has not been reported. The age of the essen- tial erogenic movements is Miocene.

    CONCLUSIONS

    The following tentative scheme is offered for the course of events in the alpine erogenic history of the Betic Cordilleras:

    ( 1) Older period of deformation: Mesozoic (and Early Tertiary?) thrust-

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    ing, folding and metamorphism of a NW-SE to N-S oriented geosynclinal area - the proto-Betic Zone - in a number of consecutive deformation phases which are thought to be related to Atlantic opening and concurrent plate consumption.

    (2) Juxtaposition of this deformed Betic Zone assemblage to the Her- cynides of the Iberian continent and its cover series. The megashears be- tween Africa and Europe which were active during Atlantic opening, are of major importance in this event.

    (3) Younger period of deformation: Neogene thrusting and folding result- ing in the NE-SW to E-W fold trend of the Subbetic and Prebetic realms and of the sedimentary basins in the Betic Zone as a consequence of the convergence of the African and European Plates. Development of mega-anti- clinoria; formation of the Sea of Alboran. These events might also be ex- plained as the result of an uplift (centrifugal sliding) collapse sequence of the erogenic centre (Van Bemmelen, 1973).

    In this concept, the Arc of Gibraltar figures as a gigantic interference pattern.

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