Las Alpujarras, al sur de Granada. Interpretaciones sobre su estructura e influencia de las mismas en las propuestas de evolución tectónica del extremo occidental del Cinturón Alpino Mediterráneo

Abstract

Las Alpujarras, South of Granada between Sierra Nevada and the Mediterranean Sea is a mountainous region which has been the aim of geologists attention since long time ago. During the XIX century the Cordillera as a whole, was considered to be autochthonous and the abnormal boundaries, even the major accidents, were merely considered as faults. Nevertheless the Alpujarras was one of the first regions in the internal parts of the Cordillera, where the existence of nappe structures was proved. In fact, Van Bemmelen (1927) and Westerveld (1929) distinguished three tectonic units in the region, which they interpreted as «nappes», —from top to bottom— the «Guajar nappe», the «Lanjarón-Gádor nappe» and the «Lújar nappe». Nevertheless, according to Banting (1933), «Lújar» and «Gádor» would make part of a huge single unit, the «Lújar-Gádor fold-nappe»” which in its turn would be tectonically overlain by the «Lanjarón» and the «Guajar» nappes (in the sense of Banting). But, according to Blumenthal (1935) only one real nappe is present in the Alpujarras region (his «Gádor nappe»). This Gádor nappe, in the sense of Blumenthal, has a different extension and tectonic position than the Gádor nappe of previous authors. According to Blumenthal (1935) his Gádor nappe overlies an autochtonous or parautochtonous Alpujárride sequence which crops out in Sierra de Lújar and in the tectonic windows of Albuñol and Berja. In the sixties and early seventies new investigations are carried out in the Alpujarras region. As a result of this, detailed geological maps were obtained. A fairly high number of tectonic units or nappes - although not all of them with the same regional extension - were differentiated using as a criterium the superimposition of older rocks over younger ones. In other parts of the Cordillera other different tectonic units of local character were also distinguished. Then the use of the term «Alpujarride complex» expanded. The boundary between the Nevado-Filabride and Alpujarride materials in the region of Sierra Nevada which, since the times of Brouwer and his collaborators, had been generally considered as an important thrust surface, was reinterpreted (Aldaya et al., 1984) as an extensional detachment with a west- southwestward hangingwall transport direction. Later detailed studies carried out in different areas, confirmed the extensional character of the contact. Careful revisions of the boundaries between the tectonic units or nappes of the Alpujarras were also carried out. The study of fault-rock bands and shear sense criteria together with the evaluation of unit and sequence omissions showed that many of the contacts formerly considered as thrusts had, in fact, an extensional character. Thus the term “nappe” for the units defined in the Alpujarras was considered to be misused, as they are actually extensional units separated by low-angle normal faults belonging to the so-called Contraviesa Normal Fault System (Crespo-Blanc et al., 1994) with a north-northwestward transport direction. Recent research in the Alpujarras (Orozco et al., 1998, 2004) which involved a detailed structural analysis of representative areas of the region, has revealed the existence of large recumbent folds involving lithological sequences previously considered to belong to different tectonic units. A new explanation according to which the structure of the Alpujarras is formed by a large recumbent syncline-anticline pair which extends west — east, from Sierra de Lújar to Sierra de Gádor, has been proposed. In several places, the fold is disrupted by low-angle normal faults, and it is overlain by an upper Alpujarride extensional sheet. It was suggested (Orozco et al., 2004) that all these structures arose from the extensional deformation under decreasing temperature conditions of a previously thickened and metamorphosed orogenic crust. This view which, according to Orozco et al (2004), is in accordance with P-T paths and the new Ar — Ar and fission track dating, contradicts former interpretations (e.g. Azañón et al., 1998) according to which the development of large-scale folds would have taken place during a compressional event, which followed a previous extensional one and in its turn would predate the early Miocene extensioonal event, according to a model of «alternating compressional and extensional events» (e.g. Balanyá et al., 1997)