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)
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.
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
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.
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.
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.
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):
(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.
(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.
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
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...