Tectonophysics 603 (2013) 179–188 Contents lists available at ScienceDirect Tectonophysics journal homepage: www.elsevier.com/locate/tecto Insights into extensional events in the Betic Cordilleras, southern Spain: New ﬁssion-track and U–Pb SHRIMP analyses José Julián Esteban a,⁎, José María Tubía a, Julia Cuevas a, Diane Seward b, Alexander Larionov c, Sergey Sergeev c, Francisco Navarro-Vilá d a Departamento de Geodinámica, Facultad de Ciencia y Tecnología, Universidad del País Vasco UPV/EHU, Apartado 644, 48080 Bilbao, Spain School of Geography, Environment and Earth Sciences, Victoria University, PO Box 600, Wellington, New Zealand Centre of Isotopic Research, VSEGEI, 199106 St. Petersburg, Russia d Departamento de Geología, Facultad de Ciencias, Universidad de Salamanca, Plaza de la Merced s/n, 37007 Salamanca, Spain b c a r t i c l e i n f o Article history: Received 21 December 2012 Received in revised form 17 May 2013 Accepted 23 May 2013 Available online 3 June 2013 Keywords: Fission-track analysis U–Pb SHRIMP Extensional tectonics Malaguide Complex Los Reales nappe Betic Cordilleras a b s t r a c t A combination of the U–Pb SHRIMP and zircon ﬁssion-track analyses has been used to constrain the age and evolution of the extensional tectonics in the Betic Cordilleras, southern Spain. The studied rocks are dolerite dykes intruded in the Malaguide Complex and in the underlying Los Reales nappe and their country rocks. The dykes are grouped in two nearly perpendicular systems, characterized by N–S strikes in the Los Reales nappe and E–W strikes in the Malaguide Complex. Two periods of extension have been identiﬁed. The age of the older event, ranging from ~ 33 to 25 Ma, is constrained by the intrusion of the dykes (33 Ma) and the joint cooling of the Los Reales–Malaguide domain from 25 Ma onwards. This event is associated with the structural emplacement of the Malaguide Complex over the Los Reales nappe, along a brittle–ductile extensional detachment and is tentatively ascribed to the tectonic collapse of the orogenic belt. A minimum time span of 10 m.y. is established between the onset of the extensional collapse and the widespread Aquitanian HT-LP metamorphism. In the present geographic frame, the kinematics of this event is characterized by a change from an E–W extension at lower levels (Los Reales nappe) to a N–S extension in upper levels (Malaguide Complex). A younger extensional event of Miocene age (~18 Ma) has been deduced based on the ﬁssion-track analysis within the “calizas alabeadas” member of the Malaguide Complex. © 2013 Elsevier B.V. All rights reserved. 1. Introduction The Betic Cordilleras in Spain and the Rif in Morocco form the western termination of the Mediterranean Alpine Orogen. The Betic Cordilleras represent a good example of a collisional orogen disaggregated by extensional collapse in a continuous convergent setting between the Iberia and Africa plates. In this regard, it is worth noting that in the Internal Zones of the Betic Cordilleras, which are composed by the Nevado–Filabride, the Alpujarride and the Malaguide complexes from bottom to top, the structures related to crustal thickening are overprinted by late extensional tectonics. The main tectonic contacts such as that between the Nevado–Filabride and Alpujarride complexes (Álvarez et al., 1989; García-Dueñas et al., 1986, 1992; Jabaloy et al., 1992) or the Alpujarride and Malaguide complexes (Aldaya et al., 1991; Lonergan and Platt, 1995; Tubía et al., 1991, 1993) are nowadays interpreted as extensional detachments. A few of them have been dated by ﬁssion-track analysis (e.g. Johnson et al., 1997), but little is still known about their precise age of movement, as the Aquitanian–Burdigalian age of most of these ⁎ Corresponding author. E-mail address: [email protected] (J.J. Esteban). 0040-1951/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tecto.2013.05.027 detachments, has been established based on the palaeontological ages from unconformably overlying sedimentary units (e.g. Aguado et al., 1990; Durand-Delga et al., 1993; Martín-Algarra and Estévez, 1984). However, these palaeontological ages are clearly inconsistent with the youngest isotopic data of the Alpujarride Complex (e.g. Esteban et al., 2004), as these nappes should still be at depths where the temperatures were higher than ≫300 °C at Aquitanian–Burdigalian times. This work deals with the Alpujarride–Malaguide boundary (AMB) located in the western part of the Betic Cordilleras (Fig. 1), where the Malaguide Complex appears over the highest tectonic unit of the Alpujarride Complex, the Los Reales nappe (Navarro-Vilá and Tubía, 1983). The AMB, identiﬁed as a brittle–ductile extensional detachment (Tubía et al., 1993), postdates the thrust stacking as evidenced by the duplication of the some of the stratigraphic units throughout the complexes. The structural and kinematic analysis of the boundary (Tubía et al., 1993) conﬁrms that the Los Reales nappe and the Malaguide Complex comprise a single crustal tectonic domain formed during thinning of previously thickened crust, in direct relationship with the exhumation of the Ronda peridotites. In this regard, Tubía et al. (1993) in the Betic Cordilleras and Negro et al. (2006) in the Rif already suggested that after the emplacement of the complexes this crustal domain underwent the same Alpine thermal evolution. 180 J.J. Esteban et al. / Tectonophysics 603 (2013) 179–188 Fig. 1. Geological map of the western part of the Betic Cordilleras (modiﬁed from Navarro-Vilá et al., 2007), showing the location of sample sites for ﬁssion-track and U–Pb SHRIMP analysis. Square marks Fig. 2 location. However, although its structure and kinematics are well known, its age is still a matter of question. In order to clarify the age of the extensional episode of the detachment located between the Los Reales nappe and Malaguide Complex, zircons from two dolerite dykes were dated by U–Pb SHRIMP and nine samples from different structural levels (schists, phyllites and greywackes) by zircon ﬁssion track (ZFT) analysis. The new ZFT data are compared with other published data, in order to understand the extensional collapse of the Betic Cordilleras. In this regard, it should be noted that ZFT analysis emerges as a useful and suitable tool for resolving the timing of extensional events in this orogen. 2. Geological setting The Betic Cordilleras, in southern Spain, formed in response to the convergence of the African and Iberian plates from Late Cretaceous to Tertiary times. The External Zones of the chain, known as the Prebetic and Subbetic, mainly comprise deformed sedimentary rocks of Mesozoic and Tertiary age, deposited on the southern palaeomargin of Iberia. The Internal Zones, also called Betic Zone, are composed mainly of metamorphic rocks with original Palaeozoic and Triassic ages distributed in the three tectonic complexes mentioned above. The contacts between these complexes have been regarded as large-scale thrusts, but nowadays it is agreed that they have been reactivated as large-scale detachments, during the extensional process related to the orogenic collapse of the chain (e.g. García-Dueñas et al., 1986, 1992; Lonergan and Platt, 1995) and subsequent to the crustal thickening induced by orogenic compression. Although the division into three Complexes for the Internal Zone of the Betic Cordilleras is generally accepted, recent works have already pointed out the possibility for a new reorganization (Navarro-Vilá et al., 2007; Platt, 2007). The Malaguide Complex, deﬁned in the Montes de Malaga region by its extensive outcrops (Figs. 1 and 2), consists of a dominant Palaeozoic series unconformably overlain by Permo-Triassic redbeds and a condensed rock sequence (Azéma, 1961; Mäkel, 1985), of Jurassic dolomites and limestones, Cretaceous marls and Tertiary sediments. The lowermost part of the Palaeozoic series is composed of ﬁne-grained bluish-grey schists that have experienced the static growth of garnet and andalusite postdating the main foliation deﬁned by biotite, chlorite and quartz (Tubía, 1988; Tubía et al., 1993). The thermal structure of the Malaguide Complex indicates a temperature variation from ~500 °C at its base to b 330 °C at the top (Negro et al., 2006), consistent with the aforementioned garnet–andalusite recrystallization. Such thermal imprint has been ascribed to the widespread LP-HT Alpine metamorphism of the Betic Cordilleras (Negro et al., 2006; Tubía et al., 1993). In this regard, K–Ar ages of white micas from the lowest most parts of the Ghomaride Complex (equivalent to the Malaguide in the Rif, Morocco) yield fully reset ages of ca. 25 Ma (Chalouan and Michard, 1990). Furthermore, Platt et al. (2003) have reported a still younger, 18.6 ± 1.6 Ma, Ar–Ar age on muscovite from the lowest part of the Malaguide Complex. Upwards, the schists are gradually replaced by grey phyllites with sparse beds of strained quartz–pebble conglomerates and dark Silurian limestones (Agard et al., 1958; Geel, 1973). The phyllites grade into slates and greywackes that alternate with extremely folded limestones referred to as the ‘calizas alabeadas’ (de Orueta, 1917) of Devonian age (Kockel, 1963). Over the ‘calizas alabeadas’, the Lower Carboniferous greywacke–slate member is located with several beds of polymictic conglomerates, the Marbella conglomerate, interlayered with slates and greywackes in the upper levels. The Marbella conglomerate, post-Early Namurian in age (Herbig, 1983), is unconformably covered by a Mesozoic and Cenozoic condensed sequence up to Chattian–Aquitanian (Jutson, 1980; Mäkel, 1985). The Palaeozoic basement, metamorphosed to greenschist facies conditions during the Variscan, and the Permo-Triassic redbeds can be correlated with the Piar Group and Saladilla Formation, in an eastern section of the Malaguide Complex (Geel, 1973). In the Montes de Malaga region, the Benamocarra Unit represents the Alpujarride Complex to the east (Figs. 1 and 2). The latter lacks ﬁssiontrack data and some members are missed compared to its western tectonic equivalents. Thus we focused our study to the west, where the Malaguide Complex overlies the Los Reales nappe (Tubía et al., 1993). The Los Reales nappe is a composite thrust sheet that includes, in ascending order, a portion of subcontinental mantle – the Ronda peridotites – and a crustal sequence of felsic granulites (kinzigites), gneissic migmatites, and medium-to-low grade schists. The crustal sequence preserves evidence of a complex polymetamorphic history with preAlpine HP-HT (Sánchez-Rodríguez, 1998) and Alpine LP-HT metamorphic conditions (Argles et al., 1999; Zeck et al., 1992). The AMB corresponds to a brittle–ductile extensional detachment developed on ductile andalusite-bearing schists, which locally form S–C structures. However, breccia and fault gouges, with thickness ranging between 15 cm and several metres, predominate along this brittle–ductile detachment. In some cases, the Malaguide Complex is involved in major rollover anticlines near the AMB (Tubía et al., J.J. Esteban et al. / Tectonophysics 603 (2013) 179–188 181 Fig. 2. Geological map and location of dated samples within the Montes de Malaga region (map modiﬁed from Cuevas et al., 2001). Sample of Duggen et al. (2004) is located outside the map. 1993). Kinematic criteria point to downward motions of the hanging wall rocks, but according to structural and kinematic observations compiled in the Montes de Malaga region (Fig. 1) the detachments and high-angle normal fault associated with the AMB can be grouped in two generations linked to the development of extensional domes: the ﬁrst one dipping to the east and with top-to-the-east shearing and a second cross-cutting set, dipping to the west in the present-day reference frame (Cuevas et al., 2001). 3. Dolerite dyke swarm in the AMB The Malaguide Complex, Los Reales nappe and the Benamocarra unit are cut by a dolerite dyke swarm. These dykes are thought to represent the remnants of the earliest magmatic episode related to Alpine extensional collapse of the Betic Cordilleras (Torres-Roldán et al., 1986), resulting in the exhumation of deep metamorphic rocks and the lithospheric mantle during a continuing N–S to NW–SW convergence between the Iberian and African plates (Dewey et al., 1989; Platt and Vissers, 1989). The swarm is formed by two perpendicular sets of subvertical dykes. They intruded within the Malaguide Complex with a mean E–W strike whereas those emplaced in the Los Reales nappe and Benamocarra unit show a N10 °E strike. The dyke thickness varies from 10 cm to 4 m and they crosscut the main structure of the country rocks. The fresh dykes are greyish-green in colour and have ophitic texture, in thin section, deﬁned mainly by plagioclase and pyroxene. Other minor minerals like chlorite, epidote and pyrrhotite with minute inclusions of magnetite, transformed to hematite in altered samples, are present. In the study area, the dolerite dykes are conﬁned to the lower part of the Malaguide Complex (from garnet- and andalusite-bearing schists to the Devonian ‘calizas alabeadas’) and in the crustal sequence of the Los Reales nappe from migmatites to schists (Tubía, 1988). Similar dolerite dykes have been described within the Malaguide Complex of the easternmost part of the Betic Cordilleras, even within the Middle-Upper Triassic pelites of the Saladilla Formation (Fernández-Fernández et al., 2007). The dolerite dykes represent a signiﬁcant volume of outcrop in the Montes de Malaga region (Fig. 3a). They maintain nearly vertical dips, except close to the AMB, showing that at the regional scale no signiﬁcant tilting has occurred after dyke emplacement. This ﬁeld observation contrasts with the inference of a 34° rotation of about a horizontal axis trending 025°, proposed by Platzman et al. (2000) aiming to explain the shallow inclination of the high-temperature component of magnetization revealed by palaeomagnetic analyses of the dykes. Nevertheless, the local nature of such tilting is clear, since otherwise the vertical N–S striking dykes should dip less than 60°. Indeed, all the palaeomagnetic samples analysed by Platzman et al. (2000) come from the Malaguide Complex and are located in the western limb of a late, gentle and upright antiform with a N-striking axial surface. The andalusite-bearing schists of the Malaguide Complex crop out in the core of such antiform (Fig. 2). Towards the lower part of the Malaguide Complex, most dykes are cut by late normal faults (Fig. 3b) that exhibit a near NS-strike and gentle dips, less than 30°, to the E or W. The number of normal faults and their displacement increases notably towards the AMB. Some dykes display asymmetric folds with ﬂat-lying axial surfaces parallel to the regional cleavage (Fig. 3c). The folds denote a vertical shortening consistent with the extensional tectonics. Almond-shaped fragments of dolerite with asymmetric tails, resembling σ-porphyroclasts, are isolated within the fault zone (Fig. 3d). Taking into account that cooled dolerite is much more competent than schist, the dykes had to be still hot enough for these quasi-plastic structures to develop. These observations have two implications: (1) the AMB was already activated as an extensional detachment by the time of the dolerite intrusion, and (2) interpreting the dykes as Andersonian intrusions, an overall N–S extension arises, in the present-day geographic frame, from the consistent E–W strike of the dolerite dyke swarm in the Malaguide Complex. This point will be discussed later because, as we already mentioned, structural indicators from the fault zones point rather to E-directed extension (Tubía et al., 1993). In the Los Reales nappe the overall number of dykes is far lower than in the Malaguide Complex. Not only the orientation of the dykes but also that of the main foliation in the country rocks differ in these two tectonic domains. Both dyke systems intruded at high-angle with respect to the penetrative foliation of the country rocks, which changes from a dominant N–S strike in cleavage of the Malaguide Complex to an E–W strike in the schistosity of the Los Reales nappe. With increasing distance to the AMB, the dolerite dykes of the Los Reales nappe become 182 J.J. Esteban et al. / Tectonophysics 603 (2013) 179–188 Fig. 3. Field structures of the dolerite dyke swarm within the Malaguide Complex showing the strain increasing from the lower phyllite member (a, b) towards the AMB (c, d). (a) General view of the vertical dyke swarm of dolerite dykes in the A-45 road to Malaga. (b) Fractured dolerite dykes showing downward hanging wall movement. (c) Sheared and folded dyke parallel to the main foliation of the andalusite-bearing schists. (d) Almond shape block of dolerite where the asymmetry provides a top-to the SE shear sense. free of structural evidences on extensional deformation. In contrast, there is a widespread development of extension related structures, S–C tectonites and shear bands in the metamorphic country rocks (Tubía et al., 1993), suggesting that the localized deformation leading to the development of discrete brittle–ductile detachments along the AMB gives way to a distributed deformation at deeper levels. Interestingly though, the dykes in the Los Reales nappe are nearly perpendicular to the trend, N110 °E, of the stretching lineation associated with the extensional structures. 4. U–Pb SIMS SHRIMP dating of the dolerite dyke swarm Zircons were extracted from 20 to 30 kg blocks of dolerite dykes. After routine mineral separation only two (see sampled-sites location in Fig. 1) of the nine sampled dykes yielded zircons. These two dykes are located within the migmatites (tb-06-833) and sillimanite-bearing schists (tb-08-03) of the Los Reales nappe. Selected zircon grains were mounted in epoxy resin together with the TEMORA 1 (Black et al., 2003) and 91500 (Wiedenbeck et al., 1995) reference zircons and analysed on a SHRIMP-II SIMS in the Centre of Isotopic Research (CIR) at VSEGEI (Saint Petersburg). Cathodoluminescence and electron backscattering images were used to reveal the internal structures of zircons and as a guide for analytical spot choice. The results were obtained with a secondary electron multiplier in peak-jumping mode following the procedure described by Larionov et al. (2004). The U–Pb ion microprobe data are presented in Table 1. The results were processed with the SQUID 1.13a (Ludwig, 2005a) and Isoplot/Ex 3.22 (Ludwig, 2005b) software, using the decay constants of Steiger and Jäger (1977). The common lead correction was done on the basis of measured 204Pb/206Pb according to the model of Stacey and Kramers (1975). Separated zircon grains display rounded, may be eroded edges that attest to their inherited, possibly detrital origin. Some of them (Fig. 4) display: a) a very thin and light luminescent external rim or, b) dark/light luminescent irregular areas that truncate their internal structures or protrude into their cores. The light luminescent external rims are thinner than the spot diameter (≈20 μm) and only two analyses were carried out on sufﬁciently wide enough rims to yield reliable 206Pb/238U ages of 980 and 377 Ma. Otherwise, those dark/ light luminescent irregular zircon areas yield 206Pb/238U ages ranging from 1187 to 28 Ma. Four of the youngest analysed zircons show very low Th/U (b 0.05) that is consistent with a metamorphic origin and contrasts with high Th/U values of zircons extracted from basic igneous rocks (e.g. Wingate, 2001). The zircon ages deﬁne a mixing line intersecting the Concordia at 20.9 ± 8.9 Ma (Fig. 5). Although the error is large (~43%), the result is consistent with earlier reported ages for the thermal Alpine metamorphism of the Betic Cordilleras (e.g. Sánchez-Rodríguez and Gebauer, 2000). Zircons with igneous appearance (euhedral, oscillatory zoned, high Th/U ratio) yield 206Pb/238U ages between 1799 and 263.7 Ma, but this cannot be the intrusion age of the dolerite dyke swarm as they intrude into Middle-Late Triassic pelites of the Malaguide Complex (Fernández-Fernández et al., 2007). Only two dark luminescent areas with high Th/U (spots tb-06-833_4.1 and tb-06-833_3.2) provide concordant Palaeogene ages (Fig. 5). The youngest 206Pb/238U age, 33.1 ± 0.7 (1σ) Ma, is the only one to have geological meaning, as similar ages have been obtained using 40Ar/39Ar geochronology for similar dykes (see Figs. 1 and 2 for samples location) by Turner et al. (1999) in the Malaguide Complex (30.2 ± 0.9 (1σ Ma and by Duggen et al. (2004) in the Los Reales nappe (33.6 ± 1.4 (1σ Ma). Regarding the oldest age (39 Ma), although from the statistical point of view it could be considered as a coherent age for the intrusion, no magmatic events older than 33 m.y. have been detected within the Malaguide Complex. Therefore, we suggest that the age of 33.1 ± 0.7 (1σ) Ma could represent the intrusion age of the dolerite dyke swarm whereas the age of 20.9 Ma, would represent the superimposed thermal Alpine metamorphism. These ages agree J.J. Esteban et al. / Tectonophysics 603 (2013) 179–188 183 Table 1 SHRIMP U–Pb–Th isotopic data for analysed zircons recovered from two dolerite dykes of Los Reales nappe. Bold analyses are those used for individual zircon Tera-Wasserburg plots in Fig. 3, whereas the square represents the best age estimation for dyke intrusion. 206 Spot name Info Pbc (%) U (ppm) Th (ppm) 232 Th / 238 U 206 (1) (ppm) 206 Pb* Age (Ma) Pb / 238 U (1) Age (Ma) 207 Pb / 206 Pb Dis. % (1) 238 U/ ± 206 Pb* (%) (1) 207 Pb*/ 206 ± Pb* (%) Dolerite Dyke (Los Reales nappe–Santi Petri area) [36°45'2.00'' N −4°38'27.03'' W] tb-08-03_11.1 o 1.0 567.9 15.4 0.03 29.7 376.4 ± 3 402.0 ± 120 6.8 16.6 0.8 0.1 5.4 tb-08-03_9.1 er 0.0 190.2 51.0 0.28 9.9 377.6 ± 7.5 340.0 ± 130 − 16.6 2.1 0.1 5.6 tb-08-03_5.2 er 0.5 411.3 34.0 0.09 24.2 426.4 ± 5.9 432.3 ± 81.8 1.4 14.6 1.4 0.1 3.7 tb-08-03_1.1 c 0.0 572.9 136.0 0.25 34.0 430.5 ± 5.3 467.6 ± 29.6 8.2 14.5 1.3 0.1 1.3 tb-08-03_7.1 c 0.8 463.2 62.3 0.14 27.8 436.0 ± 5.9 388.8 ± 88.1 − 14.3 1.4 0.1 3.9 tb-08-03_4.2 c − 800.9 311.8 0.40 64.5 578.1 ± 12.9 604.4 ± 22.3 4.5 10.7 2.3 0.1 1.0 tb-08-03_4.1 c 0.5 406.7 205.0 0.52 33.4 588.1 ± 7.4 559.9 ± 54.8 − 10.5 1.3 0.1 2.5 6.3 tb-08-03_10.1 o 0.0 10.7 0.3 0.03 1.0 655 ± 19 1637 ± 120 149.9 9.4 3.0 0.1 tb-08-03_8.1 m 0.2 1883.3 88.0 0.05 188.0 708.9 ± 11.2 718.7 ± 20.8 1.4 8.6 1.7 0.1 1.0 tb-08-03_5.1 c 0.4 760.4 60.9 0.08 80.7 750.9 ± 8.9 910.3 ± 25.9 18.5 8.1 1.3 0.1 1.3 tb-08-03_3.1 m 0.4 445.9 84.3 0.20 62.9 980.6 ± 14.8 1060.5 ± 40.3 8.1 6.1 1.6 0.1 2.0 tb-08-03_2.1 er 0.2 941.3 18.8 0.02 136.0 − 5.9 1.6 0.1 1.1 tb-08-03_6.1 c 0.2 276.2 398.0 1.49 40.8 1.6 tb-08-03_6.2 er 0.0 1324.6 74.9 0.06 210.0 tb-08-03_12.1 m 3.7 483.4 60.0 0.13 88.0 1002 ± 14.5 973.8 ± 21.8 1023 ± 13 1006 ± 31.9 − 5.8 1.4 0.1 1094 ± 13.4 1102 ± 11.9 0.8 5.4 1.3 0.1 0.6 1187 ± 20 1932 ± 120 62.7 4.9 1.8 0.1 6.5 6.3 Dolerite Dyke (Los Reales nappe–Sierra Alpujata area) [36°31'44.90'' N –4°45'19.75'' W] tb-06-833_9.1 o 22.1 932.5 6.3 0.01 3.5 28.4 ± 0.6 2828 ± 102.5 99.2 226.8 2.0 0.2 tb-06-833_9.2 o 24.5 896.6 5.7 0.01 3.4 28.8 ± 0.4 3152 ± 60.6 99.3 223.7 1.5 0.2 3.8 tb-06-833_10.1 o 31.1 946.8 35.4 0.04 3.8 30.2 ± 2.5 3307 ± 268.8 99.3 213.1 8.2 0.3 17.1 22.0 tb-06-833.3.2 o 2.2 1154.0 16.5 5.22 0.0 33.1 ± 0.7 − − 194.4 2.2 0.0 tb-06-833_10.2 er 35.8 905.0 33.0 0.04 4.4 36.5 ± 2.4 3580 ± 59.9 99.2 176.3 6.7 0.3 3.9 tb-06-833.4.1 o 2.8 270.9 2.6 1.45 0.0 39.0 ± 1.3 174 ± 870 346.4 164.9 3.2 0.1 37.0 tb-06-833_11.1 er 93.0 353.2 102.2 0.30 5.1 106.4 ± 0.5 tb-06-833.5.1 c 0.7 170.9 38.1 6.17 0.2 263.7 ± 4.7 4887 ± 38 261 ± 170 98.5 60.1 0.5 0.8 2.7 − 24.0 1.8 0.1 7.4 − 17.2 1.6 0.1 5.4 33.4 14.7 1.6 0.1 2.7 443 ± 70 − 12.0 1.5 0.1 3.1 540.4 ± 7.7 556 ± 23 2.9 11.4 1.5 0.1 1.1 1.5 566.4 ± 8.8 527 ± 55 − 10.9 1.6 0.1 2.5 161.00 0.1 600.7 ± 9.5 790 ± 15 31.5 10.2 1.7 0.1 0.7 34.70 0.4 633.6 ± 9.4 617 ± 35 − 9.7 1.6 0.1 1.6 61.8 8.14 0.8 697 ± 12 − 8.8 1.8 0.1 6.8 254.4 41.5 38.80 0.2 1051 ± 15 1983 ± 18 88.7 5.6 1.6 0.1 1.0 976.3 157.3 270.00 0.2 1799 ± 23 2059 ± 6.1 14.5 3.1 1.5 0.1 0.4 tb-06-833.3.1 c 0.8 255.2 95.9 12.90 0.4 364.9 ± 5.8 tb-06-833.8.1 c 0.3 337.9 83.6 19.80 0.3 423.5 ± 6.5 565.0 ± 59 tb-06-833.7.1 o 0.4 418.9 53.8 30.10 0.1 516.3 ± 7.7 tb-06-833.2.2 c 0.1 834.0 115.1 62.70 0.1 tb-06-833.1.1 c 0.2 246.5 353.7 19.50 tb-06-833.7.2 c 0.1 1918.3 164.8 tb-06-833.2.1 c 0.1 390.3 159.0 tb-06-833.4.2 c 0.9 82.3 tb-06-833.6.2 o 0.1 tb-06-833.6.1 c 0.0 296 ± 120 595 ± 150 – er: external rim/ir: internal rim/o: overgrowth/c: core/m: mixed analyses. – Errors are 1-sigma; Pbc and Pb* indicate the common and radiogenic portions. respectively. – (1)Common Pb corrected using measured 204Pb. with previous petrological evidence (Torres-Roldán et al., 1986) that point to thermal Alpine metamorphism after the dyke emplacement. 5. Fission-track analysis In order to date extensional detachments and delimit the thermal effect produced by dolerite dykes on country rocks, samples for ﬁssion-track analysis were collected from sillimanite-bearing schists of the Los Reales nappe up to the ‘calizas alabeadas’ of the Malaguide Complex (see Fig. 2 for sample locations). From each locality three samples were collected: one from the inner part of the dolerite dykes and two at increasing distance away from the contact within the country rocks, with the aim to test if the intrusion of the dykes could have reset, and to what extent, the older ﬁssion track ages of the country rocks. Zircon and apatite grains were extracted according to conventional mineral separation routines at the University of the Basque Country, but zircons in an amount appropriate for ﬁssion-track analysis were only recovered from nine of the fourteen country rocks. For comparative studies and for a better understanding on the relationship of the structural position with the ZFT ages, three chlorite–schist ﬁssion-track ages (Esteban et al., 2007) from the western corner of the Ronda peridotites have been included. The FT method used follows that previously described by Seward (1989) using the zeta approach (Hurford and Green, 1983). All samples were irradiated at the ANSTO facility, Lucas Heights, Australia. All age measurements were made by J.J. Esteban at the ETH (Zurich), using a 184 J.J. Esteban et al. / Tectonophysics 603 (2013) 179–188 Fig. 4. Cathodoluminescense images of studied zircons. Ellipses show the spots' location. The attached numbers yield the spot name and the obtained Zeiss microscope with a magniﬁcation of ×1600 (oil) with ζCN1 values of 102.1 ± 3.0. Ages are reported as central ages (Galbraith, 1981) and compared with those previously published for the Los Reales nappe (Esteban et al., 2004) and the Malaguide Complex (Johnson, 1993; Platt et al., 2003). The twelve samples yield apparent ZFT ages between 18.6 ± 1.8 and 25.8 ± 1.5 Ma (2σ) (Table 2). The ages overlap statistically, and do not have a positive correlation with their altitudinal position (Fig. 6a), which could be due to large-scale, late folding throughout the Alpujarride and Malaguide Complexes. But they do have a correlation with their relative structural positions (Fig. 6b). This correlation is even more evident when other published ﬁssion track ages from the Los Reales nappe are plotted together (Fig. 7). A similar resetting of zircon ﬁssion track ages, decreasing from lower to upper levels, has been described previously in the Malaguide Complex (Platt et al., 2003) and correlated with a temperature increase linked to the regional thermal event of Miocene age, 20 to 22 Ma, recognized in the Betic Zone (e.g. Monié et al., 1994; Priem et al., 1979; Sosson et al., 1998; Zeck et al., 1992). However, this Miocene regional thermal overprint does not explain the full reset of the zircon ﬁssion-track ages from the upper part of the Malaguide Complex at 24–25 Ma, but will support a delay between the beginning of the extension and the thermal overprint. Therefore, we suggest that: a) Los Reales nappe and Malaguide Complex cooled down as a single and thinned crustal domain from at least latest Oligocene times (~25 Ma), b) the structural emplacement was set before the rocks reached the ZFT closure temperature (260 °C; Foster et al., 1996) and c) no partial annealing of the ZFT data can be attributed to contact metamorphism induced by the intrusion of the dolerite dykes as different ZFT ages are obtained from dyke to dyke. Therefore, we tentatively suggest that the Los Reales–Malaguide Complex was already tectonically stacked before the Late Oligocene (≈pre-25 Ma). When the published FT data of Platt et al. (2003) are located within a detailed geological map of the Malaguide Complex (Fig. 2: samples MM), an age-structural position disruption is observed from the ‘calizas alabeadas’ up to the greywacke–slate member (Fig. 6c). Samples located within the upper structural levels attest to variable partial annealing of Palaeozoic or older detrital ZFT ages, while those samples located at the lowermost levels show a continuous cooling from at least latest Oligocene. Consequently, the Los Reales nappe and the lower part of the Malaguide Complex cooled down at higher rates than the upper of part of the Malaguide Complex. The age disruption is located within the ‘calizas alabeadas’ member (Fig. 7). Platt et al. (2003) presented a plausible explanation for the distinctly identiﬁed thermal histories, i.e. that the Malaguide Complex was the upper part of an orogenic pile that was thinned by Miocene detachments. In order to reconcile the partially reset ﬁssion-track data obtained by Platt et al. (2003), we propose that the Malaguide Complex and the Los Reales nappe comprise a unique crustal domain, which underwent several extensional 206 Pb/238U age in brackets. events from Oligocene times onwards as is evidenced by pre-Miocene (older than 25 Ma) and Miocene (younger than 25 Ma) low-angle normal faults developed at brittle–ductile and brittle conditions, respectively (Fig. 7). 6. Discussion The AMB and its eastern equivalents are widely recognized as extensional detachments, according to structural and metamorphic criteria. The shear zones located along this boundary present structures developed in brittle–ductile conditions (Lonergan and Platt, 1995; Tubía and Navarro-Vilá, 1984; Tubía et al., 1993), which conﬁrm that the Malaguide Complex was tectonically stacked over the Los Reales nappe before the rocks were cooled down to the zircon closure temperature, >260 °C (Foster et al., 1996) or even as high as 360 °C depending on the cooling rate (e.g. Bernet, 2009). Moreover, the different ZFT ages obtained in the vicinity of the dykes and their positive correlation with their structural position also suggest that the country rock zircons were not affected by annealing during the dolerite dyke swarm intrusion. The lack of such ZFT annealing could be due to the fact that: a) the dyke intrusion took place when the country rocks were at temperatures above the zircon ﬁssion-track Fig. 5. Tera-Wasserburg diagram for Cenozoic zircon ages. J.J. Esteban et al. / Tectonophysics 603 (2013) 179–188 185 Table 2 Zircon ﬁssion-track data from the Los Reales nappe (LR) and Malaguide Complex (MC). Data from tb-733, tb-734 and tb-735 have been taken from Esteban et al. (2007). Sample locations are shown in Figs. 1 and 2. Sample Geographic location Lithology Altitude (m) Grains Counted ρd (Nd) (106/cm2) ρs (Ns) (106/cm2) ρi (Ni) (106/cm2) U (ppm) PX2 (%) Var (%) Central Age (1σ) DK-1.2 DK-1.3 DK-2.3 DK-3.3 DK-5.2 DK-5.3 DK-6.1 DK-7.1 DK-7.4 Tb-733 Tb-734 Tb-735 36°35′15.81″ N 4°42′25.45″ W 36°35′15.81″ N 4°42′25.45″ W 36°47′3.65″ N 4°25′54.92″ W 36°47′54.37″ N 4°26′32.21″ W 36°48′26.48″ N 4°26′19.65″ W 36°48′26.48″ N 4°2619.65″ W 36°48′40.90″ N 4°22′9.49″ W 36°48′45.24″ N 4°22′4.71″ W 36°48′45.24″ N 4°22′4.71″ W 36°27′35.86″ N 5°13′33.88″ W 36°27′32.80″ N 5°13′36.21″ W 36°27′31.56″ N 5°13′36.18″ W Schist (LR) Schist (LR) Schist (MC) Phyllite (MC) Schist (MC) Schist (MC) Greywacke (MC) Greywacke (MC) Greywacke (MC) Chloritite (LR) Chloritite (LR) Chloritite (LR) 171 171 118 285 196 196 928 930 930 439 426 425 8 13 9 22 19 19 7 24 13 16 24 17 3.799 3.759 3.678 3.637 3.597 3.516 3.435 3.354 3.989 2.948 2.827 2.786 22.105 (252) 20.000 (396) 28.436 (261) 38.631 (1205) 47.556 (1142) 37.569 (1113) 33.541 (275) 38.397 (1034) 35.075 (2899) 50.243 (2899) 99.942 (2156) 79.011 (3516) 20.526 (234) 19.394 (384) 24.600 (225) 27.763 (866) 37.686 (905) 31.324 (928) 24.149 (198) 27.554 (742) 22.140 (380) 40.659 (2346) 76.625 (1684) 55.798 (2483) 171 179 213 246 340 284 265 285 225 440 854 655 99 86 57 93 95 25 79 3 69 82 15 38 0 0 0 0 0 0 0 0 0 0 0 0 20.9 19.8 21.7 25.8 23.1 21.6 24.3 24.1 24.1 18.6 18.8 20.1 (2042) (2042) (2042) (2042) (2042) (2042) (2042) (2042) (2042) (2042) (2042) (2042) ± ± ± ± ± ± ± ± ± ± ± ± 2.0 1.6 2.1 1.5 1.3 1.3 2.2 1.8 1.8 0.9 1.0 0.9 Zircon ﬁssion-track data: ρd: density of tracks in the glass dosimeter, Nd: number of tracks counted in the glass dosimeter, ρs: spontaneous tracks density, Ns: number of spontaneous tracks counted, ρi: induced track density, Ni: number of induced tracks counted, U: uranium content in the sample, PX2: probability of obtaining X2-values for ν degreees of freedom, where ν is the numbers of crystal-1, Var: variation. Ages were calculated using dosimeter glasses CN-1 (zircon). Analyser (Esteban) ζ values were ζCN-1: 102.1 ± 3.0. annealing window (>260 °C) and both, dykes and country rocks, were later cooled together or b) if the intrusion took place after the country rocks have passed the zircon ﬁssion-track annealing window (b260 °C), the heat issued from the dykes was not enough to cause partial annealing of the ZTF. The second option can be discarded because the calculated intrusion age, 33 Ma, of the dolerite dykes is older than ZFT cooling ages. Besides, the dykes are cut by the AMB and therefore, they predate this detachment. The extensional collapse of the Betic Cordilleras, explained by processes as different as convective removal of the lithosphere (Platt and Vissers, 1989; Turner et al., 1999), lithospheric delamination (Docherty and Banda, 1995; García-Dueñas et al., 1992), back-arc extension (Jolivet et al., 2008; Lonergan and White, 1997) or slab roll-back (Duggen et al., 2004), is generally proposed to be Late Oligocene to Early Miocene in age (33–19 Ma) based on poorly constrained Ar/Ar data of the oldest Alpine magmatic activity recognized in the Internal Zones of Betic Cordilleras (Duggen et al., 2004; Torres-Roldán et al., 1986; Turner et al., 1999). Thermal models (Comas et al., 1999; Platt and Whitehouse, 1999; Platt et al., 1998, 2003) suggest that the extension should have started during the late Oligocene, 27 Ma (Platt and Whitehouse, 1999) or 25 Ma ago (Platt et al., 2003). However, no direct observations of extensional tectonics at that time have been proposed until the present work. In this regard, we suggest that the extensional collapse of the Betic Cordilleras probably started before the intrusion of the dolerite dyke at 33 Ma and the severe thinning of the crust was mainly accomplished between 33 and 25 Ma. The available geochronological data of the Alpujarride Complex reveal that most of the cooling ages are clustered around 22–20 Ma representing the HT peak. Consequently, a minimum time span of 10 m.y. between the starting of the extensional collapse and the related HT-LP metamorphism can be established. This fact suggests that between 33 and 25 Ma the exhumation of the Alpujarride–Malaguide was mainly accomplished by ductile– brittle thinning. The thinning was coeval with the upwelling of the asthenosphere, which will bring up the isotherms leading to the Miocene thermal peak and the resetting of the geochronometers (from U–Pb to FT) from the lowest part of the Los Reales nappe up to the lower part of the Malaguide Complex. The age-structural position disruption (Fig. 6c), characterized by Palaeozoic and Mesozoic ZFT ages in the hanging wall and Cenozoic ZFT ages in the footwall of the Malaguide Complex, were already interpreted by Platt et al. (2003, 2005) as evidence for extensional Miocene tectonics in the Montes de Malaga region. This explanation agrees with our observations, as the intra-Malaguide Complex extensional detachment must be younger than 25 Ma (Fig. 7). Similar extensional timing around 19 to 17 Ma has been also identiﬁed for the Cerro Tajo fault, a tilted extensional fault in the western Los Reales nappe (Esteban et al., 2004). As the AMB detachment and the Cerro Tajo fault yield a similar extensional timing, we tentatively suggest that both shear zones could be ascribed to the same event. The ZFT ages obtained in this work are quite different from those of Johnson (1993) for a lower grade metamorphic equivalent of the AMB, in the Sierra de Las Estancias, located in the eastern part of the Betic Cordilleras. Johnson (1993) interpreted his FT data as indicators of an extensional motion along the boundary in late Oligoceneearlier Miocene time. For Lonergan and Platt (1995), in the same eastern region, the concordant apatite and ZFT ages led them to suggest that the displacement on the Sierra de Las Estancias detachment took place in early Miocene time (~18 Ma), as it does with the intraMalaguide detachment. In our opinion, a late reactivation of the Sierra de Las Estancias boundary could explain the span of 6 m.y. between the AMB from the west to the eastern regions. Therefore, according to the concordance of the post-25 Ma age with the frequently observed age of 18 Ma for the extensional tectonics within the Los Reales nappe and for the reactivation of the Sierra de Las Estancias boundary, we propose that the Malaguide Complex has recorded at least two stages of crustal thinning: the ﬁrst between 33 and 25 Ma, related to the emplacement of the Malaguide Complex over the Los Reales nappe, and the second of Miocene age (~18 Ma) associated with the development of intra-Malaguide Complex faults. We suggest here that the age of 33 Ma (see above), might point to dyke intrusion during a pre-Miocene extensional event. This interpretation agrees with structural evidence suggesting that the dykes were affected by the extensional deformation, while they were still hot enough to be folded (Fig. 3c). Based on isotope and trace geochemistry, Turner et al. (1999) consider that the Malaguide dykes come from decompression melting within the asthenosphere following removal of lithospheric mantle. Although the timing of the asthenospheric upwelling has not been addressed in detail, some works propose Early Miocene ages for this geodynamic process (Argles et al., 1999; Platt et al., 2003) or Oligocene-Miocene (Esteban, 2005; Tubía et al., 2004). These works are based mainly on geochronological studies trying to date the emplacement of the Ronda peridotites, which occurred around 22 Ma (Esteban et al., 2011; Priem et al., 1979; Sánchez-Rodríguez and Gebauer, 2000; Zindler et al., 1983). Two nearly perpendicular directions of extension arise if the dyke systems from the Los Reales and the Malaguide Complex are interpreted as Andersonian intrusions. In the present location of the Betic Cordilleras, a N–S extension is deduced from the dominant eastward strike of the dykes emplaced in the Malaguide Complex. In contrast, the N10 °E dykes from the underlying Los Reales nappe are consistent with an 186 J.J. Esteban et al. / Tectonophysics 603 (2013) 179–188 (Tubía et al., 1993) and that an E-directed extension is commonly observed in the low-angle normal faults of the region (Cuevas et al., 2001). Palaeomagnetic studies on these dolerite dykes have obtained clockwise rotations in excess of 110° (Calvo et al., 2001; Platzman et al., 2000). Similar rotations have been found in dykes from the Malaguide Complex and the Los Reales nappe (Calvo et al., 2001), which suggests that the nearly perpendicular orientations of these two dyke systems is a primary, intrusion-related structural feature. The older extensional deformation has been related to the upwelling of the asthenosphere subsequent to lithospheric delamination (Tubía et al., 2004). This interpretation, which is consistent with the mantellic origin of the magma (Turner et al., 1999), allows to explain the presence of dykes in the Malaguide Complex and in the metamorphic cover of Los Reales nappe and its lack in the underlying Alpujarride nappes, since the overthrusting of the Los Reales nappe over the underlying Alpujarride nappes postdate the upwelling of the asthenosphere (Tubía et al., 2004). From the regional point of view, the Betic–Riﬁan belt is subjected to a protracted NNW–SSE convergence between the African and Iberian plates from Cretaceous times (Dewey et al., 1989). Such a tectonic setting would favour a short-lived asthenosphere upwelling and the ﬁnal consecution of transpressional conditions (Tubía et al., 2004). In this context, the second extensional event described here could express gravity-driven tectonics in the toe of the extruding lithospheric wedge. 7. Conclusions - U–Pb SHRIMP data suggest that the intrusion of dolerite swarm dykes, connected with mantle partial melts produced during the upwelling of the asthenosphere in response to lithospheric delamination, could be of Oligocene age (33.1 Ma). - The structural emplacement of Malaguide Complex over the Los Reales nappe along an extensional detachment is constrained to the 33–25 Ma span. - A minimum time span of 10 m.y. can be established between the onset of the extensional collapse and widespread Aquitanian HT-LP metamorphism. - The Malaguide Complex and Los Reales nappe were cooled as an already juxtaposed and thinned tectonic domain at least from Oligocene (~25 Ma) to Miocene (~18 Ma) time. - The presence of a disruption in the ZFT age-structural relationship within the rocks of the Malaguide Complex suggests the existence of a second post-25 Ma extensional tectonic event. The age of the second extensional tectonic event can be estimated at around 18 Ma, based on the widely distribution of this age in the Los Reales nappe and in the Sierra de Las Estancias detachment, in the eastern Betic Cordilleras. Acknowledgements We thank the carefully and constructive reviews of Drs. Argles and Negro and the editorial work of Dr. Liu that have contributed to improve the manuscript. The ﬁnancial support was obtained from the Ministerio de Ciencia e Innovación (CGL2010-14869 and CGL2011-23755) of Spain and from the Basque Government (IT364-10). Fig. 6. Zircon ﬁssion-track ages plotted against (a) altitude and (b) relative structural position of the Los Reales nappe and Malaguide Complex. (c) Zircon ﬁssion-track ages and their relative structural position for the analysed samples (dots) and for those of the upper members of the Malaguide Complex (squares) obtained by Platt et al. (2003). Appendix A. Supplementary data ESE–WNW extension. It is interesting to note that in the study region the stretching lineation associated with the extensional deformation in the metamorphic cover of the Los Reales nappe trends to N110 °E Supplementary data associated with this article can be found in the online version, at http://dx.doi.org/10.1016/j.tecto.2013.05.027. These data include Google maps of the most important areas described in this article. J.J. Esteban et al. / Tectonophysics 603 (2013) 179–188 187 Fig. 7. Synthetic illustration of the Los Reales nappe and Malaguide Complex with the relative structural position and zircon ﬁssion-track ages. Ages in white were the obtained in this work, whereas those in light-grey were those obtained by Esteban et al. (2004) in the Los Reales nappe and by Platt et al. (2003) in the Malaguide Complex. The ﬁgure summarizes the extensional evolution proposed in this work. References Agard, J., Destombes, J., Milliard, Y., Morin, P., 1958. 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