Discussion on the division of the Nevado-Filábride complex (Betic cordillera) based on tectono-stratigraphic, geochronological and petrochronological data

Abstract

Late Orogenic doming in the eastern Betic Cordilleras: Final exhumation of the Nevado-Filabride complex and its relation to basin génesis. Tectonics, 24, 4, art n°TC4003, 19 p. Brouwer, H.A. (1926.). Zur Geologie der Sierra Nevada. Geologische Rundschau, 17: 118-137. De Jong, K. y BAKKER, H. (1991). The Mulhacen and Alpujarride Complex in the eastern Sierra de los Filabres, SE Spain: Litho-stratigraphy. Geology en Mijnbouw, 70: 93-103. Egeler C.G. y Simon O.J. (1969). Orogenic evolution of the Betic Zone (Betic Cordilleras, Spain), with emphasis on the nappe structures. Geology en Mijnbouw, 48: 296-305. Martínez-Martínez J.M., Soto J.I. y Balanyá J.C. (2002). Orthogonal folding of extensional detachments: Structure and origin of the Sierra Nevada elongated dome (Betics, SE Spain). Tectonics, 21: 1- 22. Puga, E., Díaz de Federico, A. y Nieto, J. M. (2002). Tectono-stratigraphic subdivision and petrological characterisation of the deepest complexes of the Betic Zone: a review. Geodinamica Acta, 15: 23-43. Van Bemmelen R.W. (1927). Bijdrage tot de geologie der Betische Ketens in de province Granada. Thesis E.T.S, Delft, 176 p. Vissers R.L.M. (1977). Data on the tectonic and metamorphic evolution of the central Sierra de los Filabres, SE Spain. Geolologische Rundschau, 66, 1: 81-90. Abstract Area of study The Nevado-Filábride complex is in the lowermost metamorphic complex in the Internal zone of the Betic Cordillera. It outcrops in the sierras Nevada, Filabres, Alhamilla and Cabreara and Almegrera. This complex has been subject of study since more of fifty years (e. g. Van Bemmelen, 1927; Egeler y Simon, 1969; Vissers, 1977; Augier et al., 2005). During this time it has been proposed several divisions. Brouwer, (1926) divided the complex in two big sets: a monotonous succession of mica-schists called the " Crystalline of Sierra Nevada " (Kristallijne Schisten), and the uppermost heterogeneous "Mischungzone". Puga et al., 2002 based on tectono-metamorphic criteria proposed a division in two different independent complexes: the lowermost Veleta and the Mulhacén complex. Martínez-Martínez et al., (2002) provided a division in three structural units, from bottom to top: the Ragua, the Calar-Alto and the Bédar-Macael units. The Ragua unit and the Veleta complex are mostly equivalent. Hypothesis The proposed superposition of tectonic units is based on the different P-T conditions reached during Alpine metamorphism. I.e. the Mulhacén complex of Puga et al. (2002), developed higher P-T conditions than the Veleta complex. However several findings seem not to support the proposed divisions: a) In several sectors of the complex the contact between the tectonic units proposed in the literature is characterized lithostratigraphic continuity, and similarities between structural characteristics of the upper and lower part of the Nevado-Filábride complex (e.g. the coincidence of the fold vergences). Although these coincidences may be due by subsequent later deformations during tectonic superposition of nappes, it could alternatively be the result of an former characteristic. b) The contact between the Veleta and Mulhacén complexes of Puga et al. (2002) is characterized by an alternation of schists and quartzites. c) The errors existing in the previous cartographies focused on the Nevado-Filábride complex include the extension of lithological outcrops as well as the distribution and structure of the materials. Accordingly, it is necessary a revision of the existing cartography. In this Ph.D. Thesis it has been has hypothesized that the Nevado-Filábride sequence is a continuous lithological sequence, instead of constituting a tectonically subdivided metamorphic complex. Objective The Nevado-Filábride succession has been studied in detail, on the basis of its petrological, mineralogical, geochronological, cartographic and structural characteristics. The validity of the existing divisions has been reviewed based on a multidisciplinary approach. Accordingly, in the Ph.D. Thesis is proposed a model which satisfactorily explains the results allowing to decipher the process of formation of the Nevado-Filábride, and its role in the tectonic evolution of the Internal zone of the Betic Cordillera. Methodology a) Cartography of selected areas (1:25000 y 1:10000), identification of structures and sample collection. b) Classical thin section analysis for petrography c) Mineral quantification by X-ray diffraction. d) Mineral identification by scanning electron microscopy e) Electron microprobe analysis of samples and quantitative compositional mapping of garnet porphyroblasts. f) Geochemical characterization of samples by X-ray fluorescence. g) Retrieving of P-T conditions on schists based on phase equilibria modelling of garnet and phengite. h) SHRIMP U-Pb dating of detrital zircons, to estimate the maximum deposition age of metasediments. i) LA-ICP-MS U-Th-Pb dating of allanites to estimate the age of metamorphism. Results In the first part of the study, new cartography is proposed for key areas of the complex. The cartography is supported by petrological and mineralogical descriptions of the different schist types existing in the complex. Three lithological formations were identified: the Dark schists Fm in the lower part, the Tahal Fm, and the overlying Schists and marbles Fm. The contact between these formations is stratigraphic, characterized by transitional lithological variations. U-Pb dating (SHRIMP) was carried out on inherited detrital zircons. The eight dated samples pertain to the lower and upper part of the complex. The youngest age populations in the majority of the samples are Carboniferous. According to the results, the most probable maximum age of deposition of the Dark schists Fm is estimated to be 349.1±1.6. The most probable maximum age of deposition for the upper part of the complex (Tahal and Schists and marbles Fms) is 334.6±2.9 Ma. The detrital zircon age patterns in most of the studied samples provided common age populations at 480-615 Ma (Cambrian-Ediacaran), ca. 910-1010 Ma (Tonian-Stenian) and ca. 1800-2000 Ma (Orosirian). These age populations are similar to those found in the Cantabrian zone (Iberian Massif) and the Maláguide complex (Betic Cordillera). Hence the results presented in this thesis agree with recent paleogeographic interpretations that consider the Nevado-Filábride complex as part of the south Iberian paleomargin. Five garnet-bearing schists were investigated to constrain and compare the shape of the P-T paths recorded by the lower and the upper part of the complex. Quantitative compositional mapping of garnet was combined with iterative thermodynamic models for garnet and phengite. The resulting P-T trajectories suggest that the upper and lower parts of the complex exhibit similar clockwise P-T path geometries during Alpine metamorphism, but under slightly different conditions. Garnet nucleation took place at high-pressure and low-temperature conditions, ranging ~16 kbar and ~480ºC in the Dark schists Fm; ~17.5 kbar and ~490ºC in the Tahal Fm; and ~18 kbar and ~520ºC in the Schists and marbles Fm. All the samples record a temperature increase of 70-100°C without any significant change in pressure, followed by a stage of isothermal decompression. The garnet rims reflect a high-temperature and low-pressure stage dated at ~13 Ma using allanite U-Th-Pb geochronology (LA-ICP-MS). It is therefore proposed that the isothermal decompression was triggered by slab break-off of the subducting slab. The isothermal decompression also supports rapid exhumation of the Nevado-Filábride rocks. The Nevado-Filábride complex was eventually accommodated below the Alpujárride complex (Betic Cordillera) during the final step of exhumation, describing linear cooling. Conclusions The stratigraphic contacts between formations suggest a continuous sequence rather than a tectonically divided complex. This is in line with the detrital zircon data showing a progression from older age populations in the lower part of the complex to younger age populations in the upper part. Additionally, the similarity of the P-T trajectories traced by the three formations implies all three were already closely related during the burial and exhumation stages. Bibliography Augier, R., Jolivet, L. y Robin, C. (2005). Late Orogenic doming in the eastern Betic Cordilleras: Final exhumation of the Nevado-Filabride complex and its relation to basin génesis. Tectonics, 24, 4, art n°TC4003, 19 p. Brouwer, H.A. (1926.). Zur Geologie der Sierra Nevada. Geologische Rundschau, 17: 118-137. De Jong, K. y BAKKER, H. (1991). The Mulhacen and Alpujarride Complex in the eastern Sierra de los Filabres, SE Spain: Litho-stratigraphy. Geology en Mijnbouw, 70: 93-103. Egeler C.G. y Simon O.J. (1969). Orogenic evolution of the Betic Zone (Betic Cordilleras, Spain), with emphasis on the nappe structures. Geology en Mijnbouw, 48: 296-305. Martínez-Martínez J.M., Soto J.I. y Balanyá J.C. (2002). Orthogonal folding of extensional detachments: Structure and origin of the Sierra Nevada elongated dome (Betics, SE Spain). Tectonics, 21: 1- 22. Puga, E., Díaz de Federico, A. y Nieto, J. M. (2002). Tectono-stratigraphic subdivision and petrological characterisation of the deepest complexes of the Betic Zone: a review. Geodinamica Acta, 15: 23-43. Van Bemmelen R.W. (1927). Bijdrage tot de geologie der Betische Ketens in de province Granada. Thesis E.T.S, Delft, 176 p. Vissers R.L.M. (1977). Data on the tectonic and metamorphic evolution of the central Sierra de los Filabres, SE Spain. Geolologische Rundschau, 66, 1: 81-90.