Tectonic evolution of the Betic–Rif arc: New constraints from 40Ar/39Ar dating on white micas in the Temsamane units (External Rif, northern Morocco)

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  • ewer

    . Vd, 75- Ma

    Mi

    Accepted 10 June 2008Available online 26 June 2008

    Keywords:Alboran

    Miocene in the lower Alboran Domain units (Platt et al., 2006). This

    Lithos 106 (2008) 93109

    Contents lists available at ScienceDirect

    Lith

    j ourna l homepage: www.e lThe building of the Rif (northern Morocco) and the Betics(southern Spain), which represent the western termination of theAlpine orogenic system, has been the subject of many debates andseveral hypotheses are proposed to explain the formation of the BeticRif arc including: (1) convective removal of subcontinental lithosphere(Platt & Vissers, 1989; Platt et al., 2003a, Platt et al., 2006), (2)lithospheric delamination (Seber et al., 1996; Calvert et al., 2000;Tuba et al., 2004), (3) slab break off (Blanco and Spakman,1993; Zeck,1996) or (4) roll-back of an east dipping subduction zone (e.g.Lonergan and White, 1997; Frizon de Lamotte et al., 2000; Faccennaet al., 2004; Jolivet et al., 2006). The internal zones (Alboran Domain)

    event is overprinted during the early Miocene by HTLP metamorph-ism related to the crustal extension that follow the shortening, andaffected the whole Alboran Domain units (Balany et al., 1997; Plattet al., 2003a; Negro et al., 2006). This high thermal event reset most ofisotopic systems in the whole area (e.g. Zeck et al., 1992; Platt et al.,2003a) and the beticorifan accretionnarywedge early building stagesremain difcult to constrain. Furthermore, even if several geochrono-logical studies have been carried out to constrain the Alboran Domainmetamorphic evolution, data are lacking in the External Rif.

    Recent studies show that the Temsamaneunits (External Rif), in spiteof their external position in the Rif, underwent medium-pressure low-(Fig. 1) of this orogen recorded high pressureLT) (Gmez-Pugnaire and Fernandez Soler, 1metamorphism generally considered to be E

    Corresponding author. Present address: InstitutNeuchtel, 11 rue Emile Argand, CP158, 2009 Neuchte26 70; fax: +41 32 718 26 01.

    E-mail address: francois.negro@unine.ch (F. Negro).

    0024-4937/$ see front matter 2008 Elsevier B.V. Aldoi:10.1016/j.lithos.2008.06.011(e.g. Platt et al., 2005; Michard et al., 2006), but more recently dated as1. IntroductionBeticsRifArArTemsamaneMoroccothe AfricaEurasia convergence. The Temsamane units, in spite of their external position in the Rif (northernMorocco), underwent medium-pressure low-temperature (MPLT) conditions (ca. 79 kbar; 330430 C).We propose a new tectonic and metamorphic evolution scenario for the Rif (southern) branch of the orogenon the basis of rst 40Ar/39Ar dating on petrologically and structurally characterized white micas. Threegroups of 40Ar/39Ar ages are observed: (1) Chattian or older Si-rich (highest-pressure) mica relics, (2) 1512.5 Ma corresponding to the micas dening the foliation and (3) Messinian or younger late micas andalteration. We propose that the MPLT metamorphic event in the External Rif is Oligocene in age,highlighting a subduction event during this period which could be almost contemporaneous with the burialof HPLT units from the internal Rif (Alboran Domain). The exhumation of these units characterized by anintense EW stretching and by top-to-the-west shear senses, is Middle to Late Miocene in age. We propose acorrelation of tectonic and metamorphic events at the BeticRif arc scale. We argue that the exhumation ofthe external units of the Rif (1) is younger than that of the Alboran Domain (internal) unit of the Rif, andmirrors a different tectonic setting, but (2) strongly resembles to that documented in the lower AlboranDomain units of the Betics. We show that a regional EWextension is recorded on both sides of the BeticRifarc during the Middle Miocene. This extension probably reects back-arc deformation of an eastward dippingsubduction that retreated westward during the Middle to Late Miocene in the Western Mediterranean.

    2008 Elsevier B.V. All rights reserved.Article history:Received 18 February 2008The BeticRif orogen, at the western end of the Mediterranean, is a key region to improve our knowledge onA B S T R A C TA R T I C L E I N F OTectonic evolution of the BeticRif arc: Nwhite micas in the Temsamane units (Ext

    F. Negro a,, J. de Sigoyer a, B. Goff a, O. Saddiqi b, I.Ma Laboratoire de Gologie, UMR 8538 du CNRS, Ecole Normale Suprieure, 24 rue Lhomonb Dpartement de Gologie, Facult des Sciences, Universit Hassan II An Chock BP 5366c Institut fr Geologie, Universitt Bern, Baltzerstrasse 3, 3012 Bern, Switzerlandd Dipartimento di Scienze Geologiche e Geotecnologie, Universit di Milano Bicocca, 20126low temperature (HP987; Goff et al., 1989)oceneOligocene in age

    de Gologie, Universit del, Switzerland. Tel.: +41 32 718

    l rights reserved.constraints from 40Ar/39Ar dating onnal Rif, northern Morocco)

    illa c,d

    005 Paris, Francerif Casablanca, Morocco

    lano, Italy

    os

    sev ie r.com/ locate / l i thostemperature (MPLT) conditions (ca. 79kbar; 330430 C).Weproposethat these metamorphic conditions are due to the subduction ofTemsamane units as it is proposed for the high pressure units of theinternal Rif (Negro et al., 2007). The exhumation of Temsamane units,which is characterized by an intense ~EW stretching and by top-to-the-west shear senses, looks like the one documented in the core of theinternal Betics (Galindo-Zaldvar et al., 1989; Garca-Dueas et al., 1992;Jabaloy et al., 1993; Martnez-Martnez and Azaon, 1997; Martnez-

  • Fig. 1. Geological and tectonic map of the Rif. Modied after Chalouan et al. (2001) and Frizon de Lamotte (1987) for the External Rif.

    Fig. 2. Geological map of the Temsamane units (modied after Frizon de Lamotte, 1987) showing locations of dated samples by 40Ar/39Ar (for location see Fig. 1).

    94 F. Negro et al. / Lithos 106 (2008) 93109

  • Martnez et al., 2002; Negro et al., 2007). As the retrogression wasintense, the timing of the early MPLT event is very challenging.

    The aim of this paper is to constrain the timing of this early tectonicandmetamorphic event that affected the Temsamane units in order tobetter assess the dynamics of the BeticRif arc.

    We present the rst 40Ar/39Ar dating on the MPLT Temsamaneunits, together with a detailed petrological study of the samples. Wethen compare our results with the available geochronological data inthe whole BeticRif orogen and discuss the implications for regionalgeodynamics.

    2. Geological setting

    TheRif chain, represents thenorthernpart ofMorocco. It is classicallydivided into three geological domains: the External Rif, the Flysch unitsand the Internal Rif, also known as the Alboran Domain (Fig. 1; Suter,1980). The External Rif nappe stack is divided into the Prerif,Mesorif and

    TheTemsamaneunits (Mesorif) cropout in the easternpart of theRif,betweenAlHoceimaandMelilla (Fig.1). Thismassif is elongatedNESWand is bounded to thewest byNekor sinistral strikeslip fault, and to thesouth by the Rif foreland units and the External Rif nappes (Fig. 2). Themassif can be divided into 7 distinct tectonic units, bounded by north-dipping thrusts which strike parallel to the length of the massif (Fig. 2).The Ras Afraou, Tres Forcas and Khebaba units could possibly have aninternal origin related with Alboran Domain (Fig. 2; Suter 1980,Negro et al., 2007). These three units present similar metamorphicassemblages with chloritephengitequartzchloritoidkaolinite+paragonite (Negro et al., 2007). The only geochronological data availableon the Temsamane units (40Ar/39Ar dating on micas), range between 28and8Ma (Moni et al.,1984), butwerenot realizedon theseunits anddonot account for the MPLT metamorphic conditions.

    3. Sample description and petrography

    Table 1Sample description and petrography of the dated samples

    Sample Lithology Unit Main mineral assemblage Si content in white mica X ray diffraction Remarks

    Thin sections

    TS02-03 Schist Ras Afraou Chloritephengitequartz 3.063.32 Illitekaolinite Intense deformationTS03-08 Schist/Vein Ras Afraou Chloritoidphengitekaolinite 3.033.22 Illitekaoliniteparagonite Vein/schist interfaceTS0326 Schist Ras Afraou Chloritoidphengitekaolinite 3.073.26 IlliteTF02-02 Schist Tres Forcas Chloritoidchloritephengite 3.023.33 Illitekaoliniteparagonite Near contactTF02-08 Schist Tres Forcas Chloritoidchloritephengite 3.013.38 IllitekaoliniteparagoniteTF03-01 Schist Tres Forcas Chloritoidchloritephengitealbite 3.033.09 Illite Near contact

    2-02

    .

    5903269913915955040010

    95F. Negro et al. / Lithos 106 (2008) 93109Intrarif domains. The different units of the External Rif are separated bymajor southward to southwestward-directed thrusts (Fig. 1). Strati-graphic arguments suggested that the main tectonic phase on nappestacking in theExternal Rif isMiddle to LateMiocene inage (i.e. FrizondeLamotte et al., 2004). All these units are non-metamorphic or recordedlow-grade greenschist facies metamorphism (Andrieux 1973; Frizon deLamotte, 1985; Azdimousa et al., 1998) but locally MPLT metamorphicconditions (79 kbar; 330430 C) were recently discovered in theTemsamane units (Mesorif; Negro et al., 2007).

    Table 2Representative electron microprobe analysis of phengite in the dated samples

    Sample TF02-08 TF02-08 TF02-08 TF02-08 TF02-08 TF0

    Unit TF TF TF TF TF TF

    Sep. Sep. Th. Sec. Th. Sec. Th. Sec. Sep

    Relic Foliation Foliation

    SiO2 49.615 47.163 50.432 48.057 46.867 46.6TiO2 0.218 0.039 0.000 0.152 0.135 0.2Al2O3 28.547 38.974 28.712 35.425 36.560 33.7FeO 2.804 0.429 3.211 1.602 1.028 2.0MnO 0.024 0.001 0.000 0.000 0.108 0.0MgO 2.302 0.218 2.283 0.859 0.574 1.0CaO 0.035 0.080 0.003 0.006 0.022 0.0Na2O 0.549 6.797 0.221 0.930 1.159 0.7K2O 9.515 1.616 10.058 9.621 8.822 9.1F 0.000 0.000 0.179 0.257 0.172 0.0Total 93.619 95.325 95.104 96.908 95.449 93.7Stru