Comment and Reply on "Neogene extensional collapse in the western Mediterranean (Betic-Rif Alpine orogenic belt): Implications for the genesis of the Gibraltar Arc and magmatic activity"
Dominique Frizon de Lamotte, Jean-Claude Guezou, Jean Andrieux, Marie-Anne Albertini, Michel Coulon, Andr Poisson, Laboratoire de Gologie Structurale (CNRS URA 1369), Universit Paris-Sud, Bataillon 504, 91405 Orsay, France
Doblas and Oyarzun (1989a) have proposed an extensional collapse model in order to explain the Gibraltar Arc. In their hypothesis, the uplift of Beni Boussera-Ronda peridotites, the culmination of metamorphic windows (Nevado-Filabride complex), and the initiation of the Alboran Sea are interpreted as the side effects of a single, Serravallian to present, extensional tectonic event. This "exotic" model is in conflict with field data.
The Ronda and Beni Boussera peridotites have been classically inter-preted as uplifted mantle material (Kornprobst, 1971-1974). However, they are currently included in crustal material (Lundeen, 1978; Reuber et al., 1982; Tubia and Cuevas, 1987) and thrust onto the Subbetic nappe. The uplift of the peridotites predates the overthrusting (and not the reverse, as assumed by Doblas and Oyarzun) and thus cannot be linked to the subsequent extensional regime responsible for the opening of the Alboran Sea.
Various authors have reported Neogene extensional structures, in the Alboran Sea, in the Inner Zones of the Betic orogen (Balanya and Garcia-Duenas, 1986; Garcia-Duenas et al., 1988), and in the overlying basins (Montenat et al., 1987). These tectonic studies all show that the mean stretching direction is northeast-southwest or east-west, and not north-south as assumed by Doblas and Oyarzun's model. On the other hand, these extensional structures are coeval with compressional structures in Neogene basins (Montenat et al., 1987) and in metamorphic windows (Guzou and Frizon de Lamotte, 1988). Doblas and Oyarzun failed to show any kinematic criteria to support their interpretation of the Betic-Rif floor thrusts as "breakaway faults of detachment systems" (1989a, p. 432). On the contrary, all data published so far are consistent with an interpreta-tion in terms of fronts of fold and thrust belts (see cross sections in Garcia-Hernandez et al., 1980; Leblanc, 1986; Morley, 1987; Frizon de Lamotte, 1987).
Doblas and Oyarzun's model also interprets the metamorphic win-dows of the Betic Zone (Nevado-Filabride complex) as metamorphic cores. This hypothesis disregards two crucial data: first, the presence of weakly metamorphosed terranes (Nevada schists) in the core of the struc-ture immediately beneath units affected by much higher grade assemblages (Garcia-Duenas et al., 1988), and second, the trend of stretching lineations which, in the Nevado-Filabride complex, is nearly east-west (Campos et al., 1986), and not north-south as suggested by Doblas and Oyarzun (1989, Fig. 3B). The kinematic indicators associated with this lineation agree with a bulk east to west tectonic transport (Garcia-Duenas et al., 1988; Guzou and Frizon de Lamotte, 1988). In this context, the meta-morphic windows are best interpreted as tectonic culminations related to the hanging-wall stacking above a major floor thrust (Guzou and Frizon de Lamotte, 1988). However, in the Western Alpujarride complex (a higher group of nappes including the massive peridotites), the direction of the stretching lineation shows some discrepancy with the general kine-matic framework: it trends northeast-southwest in the Ronda massif, north-south in the Ceuta massif, and northwest-southeast in the Beni Bous-sera massif (Tubia and Cuevas, 1987). A possible cause of this variation is the rotation of an early-formed north-south lineation during the westward transport of the nappes. Such a model implies an increase of thrust velocity from the lateral boundaries to the axis of the arc and requires that the peridotites form completely allochthonous bodies, and not kinds of diapirs
as indicated in Doblas and Oyarzun's Figure 2. In spite of the assertion of Doblas and Oyarzun, this is not in conflict with recent gravimetrical evidence (Belot, 1985).
A major problem in the Betic-Rif orocline is how to link extension in the Alboran Sea with coeval thrusting in adjacent mountain belts. An explanation in terms of back-arc spreading behind an east-west-migrating subduction zone remains, in spite of its atypical character, one of the more realistic models for the Gibraltar Arc, the Alboran Sea, and the associated magmatic activity. The uplift of Beni Boussera-Ronda peridotites is a different problem pertaining to an older event (e.g., opening of the Tethyan Oceanf?]).
Miguel Doblas, Roberto Oyarzun, Departamento de Geodinmica and Departamento de Cristalografa y Mineraloga, Facultad de Ciencias Geolgicas, Universidad Complutense, 28040 Madrid, Spain
Frizon de Lamotte et al.'s doubts about our ideas on the Neogene evolution of the westernmost Alpine realm (Doblas and Oyarzun, 1989a) should allow the nonspecialist to appreciate more fully the range of opin-ions on this crucial issue.
Regarding their comments about the Ronda and Beni-Boussera peri-dotites, three main points must be highlighted: (1) Frizon de Lamotte et al. seem to be unfamiliar with both the geological nomenclature and the subdivisions of this realm. They state that the Ronda and Beni-Boussera peridotites were thrust onto the Subbetic nappes; however, the term "Sub-betic" applies only to the Betic Cordilleras (and not to the Rif orogen), and the Ronda peridotites are included within the internal Betic units, and therefore have nothing to do with the Subbetic external domain. (2) Con-cerning the geophysical evidence, which Frizon de Lamotte et al. dispute, a very recent gravimetric survey (Casas and Carb, 1989) clearly reaffirms what we suggested in our paperi.e., the deep-rooted character of the ultramafic bodies (A. Carb, 1989, personal commun.). (3) The overall comments of Frizon de Lamotte et al. are based primarily on the classical assumption that these peridotites form allochthonous thrust-related bodies, a hypothesis that has now proved to be inconsistent with indisputable field structural relations (subvertical mylonitic corridors surround some of the ultramafic complexes), and the evidence provided by the geophysical data (see Doblas and Oyarzun, 1989b).
Regarding Frizon de Lamotte et al.'s strong opposition to an extension-related uplift of the Nevado-Filabride complex; recent papers not only propose extensional schemes for this complex that involve de-tachment systems (Garca Dueas and Martinez Martinez, 1988), but even speak in terms of uplift of lower plate rocks, crustal thinning, and exten-sional collapse of a previous orogenic welt (Piatt and Vissers, 1989).
In relation to Frizon de Lamotte et al.'s assertions about coeval compression and extension in this realm: (1) The existence of localized compressional features within a generalized global extensional scenario is news to no one. (2) The Neogene fold and thrust belt model advocated by Frizon de Lamotte et al. for the Betic-Rif realm is utterly incompatible with both the noticeable crustal thinning from the mainland toward the Alboran Sea, and the presence of two major Neogene extensional troughs (Guadalquivir and Sebou) that trend parallel to what Frizon de Lamotte et al. assume to be sites of compression-related frontal stacking phenomena. (3) Frizon de Lamotte et al. uncritically make the assumption that subhor-izontal deformational planes are compressional "floor thrusts," whereas many of them have proved to be extensional detachment surfaces whose kinematic indicators reveal a generalized top-to-the-south displacement.
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Frizon de Lamotte et al.'s central argument about the relevance
of north-south stretching lineations in the Betic realm is weak because
(1) stretching lineations vary in orientation f rom northeast to northwest, as
observed in the field (Garcia Dueas and Martinez Martinez, 1988; Piatt
and Vissers, 1989); and (2) variations in the orientations of these lineations
are the least w e could expect within a gravitationally collapsing edifice.
Regarding the back-arc spreading basin hypothesis, which Frizon de
Lamotte et al. call "realistic," apart f rom the lack of geologic evidence
supporting such a scheme, we stress that the Alpine evolution of the
Alboran Sea is already overcrowded with theoretical subduction zones,
plates, microplates, and mid-ocean ridges. O n e should bear in mind the
small size of this basin, a factor that prevents the development of a full-
scale "Pacific-type" plate-tectonic environment.
Last, but not least, if our model is to be considered "exotic," it should
be welcome, because it provides an entirely different approach on a subject
in which establishment, orthodoxy, and dogmatism are commonplace.
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Belot, A., 1985, Etude gravimtrique du Rif palozoque: La forme du massif des Beni-Boussera [Thse de Docteur-Ingnieur]: Montpellier, France, Universit de Montpellier, 138 p.
Campos, J., Garcia-Dueas, V., Gonzales-Lodeiro, F., and Orozco, M., 1986, La zona de cizalla del contacto entre el grupo de mantos del Mulhacen y la unidad del Veleta (Sierra Nevada y Sierra de Los Filabres, Andaluca): Geogaceta, v. 1, p. 15-17.
Casas, A., and Carb, A., 1989, Deep structure of the Betic Cordilleras derived from the interpretation of a complete Bouguer anomaly map: Annales Geophysicae, Special Issue, p. 15-16.
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Tubia, J.M., and Cuevas, J., 1987, Structure et cinmatique lies la mise en place des pridotites de Ronda (Cordillres Btiques, Espagne): Geodinmica Acta, v. 1, p. 59-69.
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