Jurassic-To-Present Thermal History of the Central High Atlas (Morocco) Assessed by Low-Temperature Thermochronology

doi: 10.1111/j.1365-3121.2006.00715.x Jurassic-to-present thermal history of the central High Atlas (Morocco) assessed by low-temperature thermochronology Luis Barbero,1 Antonio Teixell,2 Marıá-Luisa Arboleya,2 Pedro del Rıó,1 Peter W. Reiners3 and Blaid Bougadir4 1Departamento de Geologıá, Universidad de Ca´diz, 11510 Puerto Real, Ca´diz, Spain; 2Departament de Geologia, Universitat Auto`noma de Barcelona, 08193 Bellaterra, Spain; 3Department of Geology and Geophysics, Yale University, New Haven, CT, USA; 4Faculte´ des Science et Techniques, Universite´ Cadi Ayyad, Marrakech, Morocco ABSTRACT Apatite fission track (AFT) and (U–Th)/He data from the High AFT data and constrained by (U–Th)/He suggest that these ages Atlas have been obtained for the first time to constrain the are included in a slow cooling trend from intrusion age to tectono-thermal evolution of the central part of the chain. c. 50 Ma ago that we attribute to post-rift thermal relaxation. Results from Palaeozoic basement massifs indicate long resi- This is followed by a stability period of c. 30 Ma and then by a dence at low temperatures, consistently with their original final exhumational cooling until present exposure. Eocene location out of the deepest Mesozoic rift troughs and indicating intrusives yield AFT ages similar to those of Rb–Sr and K–Ar minor exhumation. The best rocks for extracting the Alpine suggesting rapid emplacement in the uppermost crust. history of the Atlas Mountains are Jurassic intrusives, which yield AFT ages centred on c. 80 Ma; thermal models based on Terra Nova, 19, 58–64, 2007 et al., 2003; Arboleya et al., 2004). sediments; their ages range between Introduction The Atlas experienced moderate cru- the late Eocene and the Quaternary The Atlas Mountains represent the stal shortening and exhumation, de- (Fraissinet et al., 1988; Go¨ rler et al., most significant relief in North Africa. spite their high elevation (see 1988; El Harfi et al., 1996; Frizon de They extend from Morocco towards structural style in Fig. 2). Accord- Lamotte et al., 2000; Morel et al., Algeria and Tunisia along 2000 km, ingly, crustal thickening is modest 2000). attaining elevations up to 4000 m. (Wigger et al., 1992; Ayarza et al., Apatite fission track (AFT) analysis Two branches can be distinguished in 2005), and the Atlas uplift has been combined with (U–Th)/He chronol- Morocco: the NE-trending Middle partly attributed to mantle causes ogy can be a powerful tool to unravel Atlas and the ENE-trending High (Teixell et al., 2003, 2005; Zeyen et al., exhumation and uplift histories of Atlas (Fig. 1), which attains the high- 2005). orogens (Gallagher et al., 1998). To est altitudes. Traditionally, the Atlas The timing of the uplift in the shed more light on the history of the Mountains have been considered in- Moroccan Atlas Mountains is still shortening and exhumation of the tracontinental chains because they are poorly constrained and constitutes a central High Atlas Mountains, we located far from collisional zones and matter of debate. Gomez et al. (2000, have performed, for the first time, because of the lack of fold nappe 2002) and Laville (2002) recently FT and (U–Th)/He dating in apatite structures, regional metamorphism, argued on the chronology of folding crystals recovered from Palaeozoic to granitoid intrusions and other features and uplift. Laville (2002), following Jurassic supracrustal igneous com- typical of interplate orogens. Mattauer et al. (1977) and Laville and plexes and sedimentary rocks of this Several studies have focused on Pique´(1992), argued that total shor- mountain chain. various aspects of the Atlas structure tening includes a: (a) pre-Cretaceous component resulting in folding, clea- and evolution, from rift or transcur- Summary of geological framework rent sedimentary troughs during the vage and erosion of Jurassic igneous Mesozoic, to compressional belts of and sedimentary rocks; and (b) late The Atlas Mountains derive from tectonic inversion in Cenozoic to pre- Cretaceous to Miocene component earlier intracontinental basins, filled sent times (Choubert and Faure-Mu- responsible for rapid uplift of the with thick sedimentary deposits from ret, 1962; Mattauer et al., 1977; High Atlas and for basin development the Triassic to the late Cretaceous and Laville, 1988; Frizon de Lamotte to the south of the chain. On the other affected by episodic magmatic events et al., 2000; Piqueét al., 2000; Teixell hand, Gomez et al. (2000, 2002) con- (see summaries in Choubert and sidered that the shortening of the Faure-Muret, 1962 and Piqueét al., Moroccan Atlas is the entire result of 2000). Pre-Mesozoic rocks, cropping Correspondence: Dr Luis Barbero, Depar- Cenozoic plate convergence, with no out in the margins of the High and tamento de Geologıá, Facultad Ciencias del significant pre-Cretaceous shortening. Middle Atlas and in sparse internal Mar y Ambientales, Universidad de Ca´diz, Campus de Puerto Real, 11510 Puerto With regard to the main Cenozoic massifs (Fig. 1), were previously affec- Real, Ca´diz, Spain. Tel.: +34 956 016279; deformation and uplift, several ted by the Hercynian orogeny. fax:+34 956 016797;e-mail:luis.barbero@ authors have proposed different During much of the Mesozoic, the uca.es phases on the basis of syntectonic High Atlas experienced extension and 58 Ó 2007 Blackwell Publishing Ltd Terra Nova, Vol 19, No. 1, 58–64 L. Barbero et al. • Jurassic-to-present thermal history of the Atlas ............................................................................................................................................................. Fig. 1 Geological map of the Central High Atlas (Teixell et al., 2003) showing the location of samples. SM, Skoura massif; OB, Ouarzazate basin; AK, Aı¨ t Kandoula basin; TZ, Tamazerht massif; TS, Tassent massif; TI, Tizi n’Isly syncline. Inset shows the location of the Atlas Mountains in the North African foreland (HA, High Atlas; MA, Middle Atlas). Fig. 2 Cross-section through the High Atlas in the Imilchil region indicating the structural location of the Jurassic intrusive, the Jurassic red bed samples, and the projected position of sample IM-16 (section after Teixell et al., 2003). See location in Fig. 1. rifting. The first rifting period oc- coinciding with the present day High whose manifestations crop out along curred during the Triassic as evi- and Middle Atlas. Up to 5000 m of the axis of the central High Atlas denced by thick red beds and by a marls, calciturbites and reefal lime- (Figs 1 and 2). Three magmatic for- basaltic magmatic episode. A post-rift stones were accumulated from Toar- mations can be distinguished (Laville Liassic platform was drowned and cian to Bajonian times in central High and Harmand, 1982; Beraaˆ ouz et al., disrupted during the late Liassic– Atlas. Red beds indicating generalized 1994): (a) epizonal plutonic complexes Dogger by a renewed rifting episode filling and regression became wide- emplaced along the main fault zones (Warme, 1988), and subsiding basins spread during the Bathonian. Crustal at pressures <1.5 kbar (Lhachmi started to differentiate, essentially thinning led to alkaline magmatism, et al., 2001), mainly composed of Ó 2007 Blackwell Publishing Ltd 59 Jurassic-to-present thermal history of the Atlas • L. Barbero et al. Terra Nova, Vol 19, No. 1, 58–64 ............................................................................................................................................................. cumulate rocks and syenites-monzo- with associated carbonatites), clus- He is retained. For the FT system, the diorites; (b) olivine dolerite dykes, tered into two age groups: 40–45 and uppermost limit of the partial anneal- which feed sills intercalated within 15–0.5 Ma (Tisserant et al., 1976; ing zone for a Durango-type compo- mid-Jurassic deposits; and (c) plateau Harmand and Cantagrel, 1984; Berra- sition apatite (0.4 wt % Cl) cooled at a ) basalts produced by fissural volcan- hma and Hernandez, 1985; El Azzouzi rate of 5 °CMa 1,isc. 108 °C (Bran- ism. K–Ar radiometric ages of the et al., 1999). This magmatism has don et al., 1998). Mesozoic intrusions of the High Atlas been attributed to a mantle upwelling Sample description, FT and (U– are compiled in Laville and Harmand occurring synchronically with the At- Th)/He data are presented in Tables 1 (1982) and range from 119 to 173 Ma. las compression (Teixell et al., 2005). and 2. Samples from Palaeozoic Mattauer et al. (1977) and Laville and blocks to the north and south of the Pique´(1992) proposed that in the late High Atlas (Fig. 1) show AFT ages of Methods and results Jurassic there was a major phase of 270 and 143 Ma; average track length transpressional folding and erosion. Samples for thermochronological ana- in the former is 11.59 lm, being The Cretaceous is represented by a lysis were collected from diverse rock slightly negatively skewed. The Dpar basal red bed formation followed by a types in the central High Atlas values (Table 2) vary from 1.61 to platform limestone of Cenomanian– (Fig. 1). These include granitic and 1.80 lm. AFT ages in the red bed Turonian age; both units either cover porphyritic intrusions within Palaeo- sample could be resolved, on the basis the earlier Jurassic deposits or overlap zoic basement (IM-16 and DMN-1), of binomial peak-fit procedures (Bran- the basement to the N and S of the late Jurassic to early Cretaceous detri- don, 1992, 1996), into two peaks, at High Atlas (Figs 1 and 2), being tal red beds (IM-2) Jurassic syenites 165 and 295 Ma. AFT ages of Jurassic interpreted as post-rift deposits (Teix- (IM-4–IM-6, TAS-1 and TAS-3, syenites from the Imilchil area range ell et al., 2003). The uppermost Cre- AMG-1), and Eocene syenites (IM-9 from 73 to 92 Ma, most showing very taceous consists of fine-grained red and IM-10). FT and (U–Th)/He ana- low dispersion (chi-squared test beds which according to some authors lyses in apatite crystals were carried passed; Galbraith, 1981).

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