New 40Ar–39Ar Dating of Lower Cretaceous Basalts at the Southern Front of the Central High Atlas, Morocco: Insights on Late Me

International Journal of Earth Sciences (2018) 107:2491–2515 https://doi.org/10.1007/s00531-018-1609-7

ORIGINAL PAPER

New 40Ar–39Ar dating of Lower Cretaceous basalts at the southern front of the Central High Atlas, Morocco: insights on late Mesozoic tectonics, sedimentation and magmatism

G. Moratti1 · M. Benvenuti1,2 · A. P. Santo1,2 · M. A. Laurenzi3 · E. Braschi1 · S. Tommasini2

Received: 1 August 2017 / Accepted: 28 March 2018 / Published online: 9 April 2018 © Springer-Verlag GmbH Germany, part of Springer Nature 2018

Abstract This study is based upon a stratigraphic and structural revision of a Middle Jurassic–Upper Cretaceous mostly continental succession exposed between Boumalne Dades and Tinghir (Southern Morocco), and aims at reconstructing the relation among sedimentary, tectonic and magmatic processes that affected a portion of the Central High Atlas domains. Basalts interbedded in the continental deposits have been sampled in the two studied sites for petrographic, geochemical and radiogenic isotope analyses. The results of this study provide: (1) a robust support to the local stratigraphic revision and to a regional lithostratigraphic correlation based on new 40Ar–39Ar ages (ca. 120 Ma) of the intervening basalts; (2) clues for reconstructing the relation between magma emplacement in a structural setting characterized by syn-depositional crustal shortening pre-dating the convergent tectonic inversion of the Atlasic rifted basins; (3) a new and intriguing scenario indicating that the Middle Jurassic–Lower Cretaceous basalts of the Central High Atlas could represent the first signal of the present-day Canary Islands mantle plume impinging, flattening, and delaminating the base of the Moroccan continental lithosphere since the Jurassic, and successively dragged passively by the Africa plate motion to NE. The tectono-sedimentary and magmatic events discussed in this paper are preliminarily extended from their local scale into a peculiar geodynamic setting of a continental plate margin flanked by the opening and spreading Central Atlantic and NW Tethys oceans. It is suggested that during the late Mesozoic this setting created an unprecedented condition of intraplate stress for concurrent crustal shortening, related mountain uplift, and thinning of continental lithosphere.

Keywords Central High Atlas · Late Mesozoic basalts · 40Ar–39Ar dating · Geochemistry · Sr–Nd radiogenic isotopes · Tectono-sedimentary evolution

Introduction

The Atlas System, developed at the northern margin of the African plate (Piqué et al. 2002), is a well-studied case of structural inversion occurred in a tectono-sedimentary evolution from continental rifting to convergence of Europe- Electronic supplementary material The online version of this Africa plates. This dynamic setting was affected by regional article (https://doi.org/10.1007/s00531-018-1609-7) contains supplementary material, which is available to authorized users. magmatic activity related to the evolution of the Central Atlantic Magmatic Province (CAMP). The CAMP, one of * G. Moratti the largest igneous provinces on Earth, originated from [email protected] mantle upwelling impinging at the base of the continental 1 CNR, Istituto di Geoscienze e Georisorse-Sede Secondaria lithosphere of the supercontinent Pangea at ca. 200 Ma: 7 2 Firenze, Via La Pira 4, 50121 Florence, Italy its products cover an area of some 10 km and outcrop in 2 Dipartimento di Scienze della Terra, Università degli Studi di North and South America, Africa, and Europe (McHone Firenze, Via La Pira 4, 50121 Florence, Italy 2003; Martins et al. 2008; Marzoli et al. 2011; Merle et al. 3 CNR, Istituto di Geoscienze e Georisorse, Via Moruzzi 1, 2011; Blackburn et al. 2013). This determined continental 56124 Pisa, Italy breakup and sea floor spreading with the formation of the

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Atlantic Ocean basin (e.g., Marzoli et al. 1999, 2004; Hames inversion (Benvenuti et al. 2017). To support this hypothesis, et al. 2000; McHone 2003; Knight et al. 2004; Verati et al. two basalt outocrops, interbedded within the continental 2007; Frizon de Lamotte et al. 2008; Callegaro et al. 2014; deposits at the southern front of the CHA facing the eastern Whalen et al. 2015). Ouarzazate Basin, have been dated (40Ar–39Ar) and analysed Following the CAMP magmatism, transitional to alkaline for their geochemical and radiogenic isotope (Sr, Nd) sig- magmas emplaced all around the Central and South Atlan- nature. The results, along with stratigraphic and structural tic Oceans, forming the so-called Cretaceous Peri-Atlantic field analyses, are discussed for their implications at local Alkaline Pulse (PAAP) (Matton and Jébrak 2009). In the and regional scales. Central High Atlas (CHA), these transitional to alkaline magmas emplaced within thick poly-deformed sedimentary continental successions since the Middle Jurassic (Choubert Geological outline and Faure-Muret 1960–1962; Frizon de Lamotte et al. 2008; Ghorbal et al. 2008; Benvenuti et al. 2017). Previous studies Stratigraphic and structural setting on the Middle Jurassic–Early Cretaceous magmatic stage (Hailwood and Mitchell 1971; Westphal et al. 1979; Har- The tectono-sedimentary events that were involved in the mand and Laville 1983; Beraâouz et al. 1994; Amrhar et al. evolution of the Moroccan High and Middle Atlas have been 1997; Lachmi et al. 2001; Zayane et al. 2002; Charrière et al. subdivided into three main stages (Frizon de Lamotte et al. 2005; Bensalah et al. 2013) provided chronologic calibra- 2008 for a review): (1) the Late Permian–Early Jurassic con- tion and petrographic-geochemical characteristics of basalts tinental rifting stage, characterized by multiple rift activation occurring in the northern front of the CHA. The results of within a generalized process of supercontinent fragmentation these studies have been discussed in the context of a long followed by the Central Atlantic and North-Western Tethys post-rift phase characterized by tectonic quiescence punc- oceans opening; (2) the Middle Jurassic–Cretaceous post-rift tuated by magmatic activity (e.g., Frizon de Lamotte et al. stage during which the Atlantic domain of the Moroccan 2008). Nevertheless, low-temperature thermochronology Atlas underwent a classic passive margin evolution. Mean- studies carried out in the last decade in the High Atlas region while, the Tethyan Atlasic domains experienced a long phase (Ghorbal et al. 2008; Missenard et al. 2008; Teixell et al. of tectonic quiescence and eustatically controlled deposi- 2009; Ruiz et al. 2011; Bertotti and Gouiza 2012; Oukassou tional patterns; (3) the Cenozoic convergent stage, when et al. 2013; Gouiza et al. 2017; Sehrt et al. 2018), attest to Africa–Europe plate convergence, following to the North- major exhumation events that took place during the post- Western Tethys closure, propagated compressive stresses in rift phase, suggesting regional tectonic processes. The wide the Atlasic domains, and caused a progressive full tectonic distribution of continental middle-late Jurassic–Cretaceous inversion of the pre-existing rifted basins (Mattauer et al. successions along with the occurrence of syn-depositional 1977; Giese and Jacobshagen 1992; Laville and Piqué 1992; deformation had long been attributed to compressional/ Beauchamp et al. 1999). During the latest Paleozoic–Early transpressional events prior to the Cenozoic Africa–Eurasia Triassic, these rifted Atlasic basins were filled by a thick convergence (Choubert and Faure-Muret 1960–1962; Laville continental succession followed by mixed clastic-evaporite et al. 1977; Mattauer et al. 1977; Studer and du Dresnay deposits, shallow marine limestones and prominent basalt 1980; Jenny et al. 1981; Schaer 1987; Laville 2002). This flows emplaced at the Late Triassic–Early Jurassic transition Jurassic–Cretaceous deformation, however, was considered (El Arabi 2007; Frizon de Lamotte et al. 2008). In the Cen- of only local importance, and the effective High Atlas build- tral and Eastern sectors of the High Atlas, the Middle Juras- up was largely related to the Cenozoic Africa–Eurasia colli- sic–Cretaceous post-rift phase was marked by long periods sion [late Mesozoic “structural inversion” vs Cenozoic “oro- of continental deposition alternated to shorter transgressive genic inversion” of Laville and Piqué (1992)]. Successively, episodes (Choubert and Faure-Muret 1960–1962; Frizon de the High Atlas Chain has been related to the convergent tec- Lamotte et al. 2008; Fig. 1). These continental successions, tonics between the African and the Eurasian plates, and the widely exposed in the axial zone and in the northern front of documented early compressional tectonics was neglected the CHA (Couches Rouges Auctt.; Jenny et al. 1981; Had- (Fraissinet et al. 1988; Jacobshagen et al. 1988; Görler et al. doumi et al. 2002, 2008, 2010; Andreu et al. 2003; Charrière 1988; El Harfi et al. 1996, 2001; Beauchamp et al. 1999; et al. 2005; Mojon et al. 2009), include: (1) upper Bathonian Gomez et al. 2000, 2002; Teson and Teixell 2008; Frizon fluvial reddish fine-grained conglomerates, mostly sourced de Lamotte et al. 2008). from Lias and Dogger limestones, sandstones and mudstones Recent field studies on Middle Jurassic–Upper Creta- (Guettouia Formation); (2) Bathonian–Callovian/Hauteriv- ceous continental deposits and intervening basalts exposed ian lacustrine reddish mudstones with subordinate marls and along the southern front of the CHA suggested a strict rela- evaporites (Iouaridene Formation); (3) Barremian–Early tion with crustal shortening, predating the Cenozoic tectonic Aptian fluvial reddish sandstones and mudstones (Jbel

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P-Q Fluvio-lacustrine clastics and marine carbonates (Paleogene-Quaternary) T-J C Fluvial clastic and shallow marine carbonates NAF C LM N and evaporites of the study area (late Mesozoic) C P-Q Ait Tafelt, Ouaouizarth, Ait Attab-Ben Cherrou Fms (north), 0 Km 10 C Ifezouane, Aoufous, Akrabou, Iflit Fms (south) (Aptian-“Senonian”) J3 J2 Fluvial clastics and shallow marine carbonates and evaporites Naour Iouaridene and Jbel Sidal Fms (Callovian?-Barremian) J3 T-J T-J J-C Fluvio-lacustrine clastics J-C T-J C Tizi ‘n Isli J3 Jb Jb Upper Mesozoic basaltic lava flows, dykes, 170 subvolcanic intrusive complexes Beni Mellal Guettouia and Tilogguite Fms (Bathonian) n J2 T-J T-J J3 136 Fluvial and coastal clastics P-Q 123 145 151.3 Bin el Ouidane and Tanant Fms. (late Aalenian- adla Plai Jb J2 157-163 J2 Bajocian) Marine carbonates T T-J NAF Taguelft Jb Wazzant, Amzerai, Tafraout, Azilal Fms Jb J3 J1 (late Pliensbachian-early Aalenian) C J3 Jb Fluvial clastics T-J b J-C C Jb J2 J1 Jb Imilchil b Aganane Fm (Trias-early Jurassic) Jb Ait Attab Ouaouizeght Jb J3 Jb T-J Continental clastics, volcanics b 173 (b), marine carbonates Jb C 110 J-C 122 J-C J2 B Precambrian-Paleozoic 128 basement J2 Tilouguite Jb Jb undifferentiated fault J-C Jb J1 T-J Jb Jb main thrust J2 J2 Ouzoud J3 (triangles on hangingwall) J1 32°N synclinal axis J1 b T-J Jb Jb Haouz Plain J2 Jb Jb b P-Q J2 b J3 J1 J1 T-J Demnat J-C Iouaridene Jb J3 J2 T-J J2 Mserir J1 J1 J1 J-C J1 J2 T-J b b 7°W J2 a J-C ean Se J1 Mediterran B J1 B b J1 T-J J3 36°N Atlantic Ocean C C b Study area SAF Rabat AtlasOranese HA24 J3 Meseta T-J Moroccan B Fig. 3 Tinghir Meseta Middle 34°N B LM B NAF b HA1 Eastern High Atlas and Fig. 2 P-Q Marrakech al SAF B Centr 32°N A J3 J3 Errachidia B gh T-J P-Q Boumalne Dades W. Hi M OB Atlas T-J W Skoura Culmination C Ouarzazate eld 30°N n Shi SAF Atlas frican 200 Km Toundoute Pana P-Q P-Q Anti T-J9°WC 7°W 5°W 3°W Ouarzazate Basi 6°W

Fig. 1 Schematic geological map of the Central High Atlas (modi- 40Ar–39Ar step-heating. White boxes report ages of upper Mesozoic fied from Carte Géologique du Maroc 1985) with location of the magmatic bodies after Hailwood and Mitchell (1971), Westphal et al. study area and Figs. 2, 3. The main synclines filled with Juras- (1979), and Armando (1999) recalculated, when necessary, with the sic–Cretaceous continental red beds (J1, J2, J3, J-C, C) in the chain decay constants of 40K reported in Steiger and Jäger (1977). Inset: are indicated. Stars represent the sampling sites of magmatic rocks. Structural domains in Morocco. NAF North Atlas Fault, SAF South HA1 (Db) and HA24 (JIb) are the two basalts newly dated through Atlas Fault, WMA Western Moroccan Arch

Sidal Formation). Two basalt lava flow units (B1 and B2, (Ettachfini and Andreu 2004; Ettachfini et al. 2005). Upper Haddoumi et al. 2010; Bensalah et al. 2013; Jb in Fig. 1) Cretaceous continental deposits, referred to as Sénonien, at the top of the Guettouia Formation and at the base of document a further regression followed by a last regional the Jbel Sidal Formation, also include diffuse sub-intrusive marine ingression testified by Maastrichtian–Middle Eocene products (Westphal et al. 1979; Harmand and Laville 1983; mostly carbonate deposits, on the southern front of the CHA, Haddoumi et al. 2010). included in the Sub-Atlas Group (Herbig and Trappe 1994). The continental deposition recorded by the Couches Finally, the convergent stage was marked by continental Rouges (red beds) was interrupted by a marine transgression deposition represented on the southern front and foreland during the Aptian, recorded in the northern CHA front by basins of the CHA by the upper Eocene–Present Imerhane marls and limestones (Ait Attab Formation). Albian–Cenom- Group (El Harfi et al.2001 ). anian fluvial-coastal deposits (Ifezouane–Aoufous forma- The present structuration of the CHA consists of a tions along the CHA southern front; Ettachfini and Andreu series of thrust-related folds elongated in a NNE–SSW to 2004; Cavin et al. 2010) testify a return to continental con- E–W direction, retracing the Middle Jurassic–Lower Cre- ditions all over the CHA. A renewed generalized transgres- taceous basins where the Couches Rouges were deposited sion occurred during the late Cenomanian–Turonian with (e.g., Mattauer et al. 1977; Fig. 1). Large synclines alter- the deposition of limestones and marls (Akrabou Formation) nate with narrow anticlinal ridges often intruded by Middle

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Jurassic–Lower Cretaceous subvolcanic complexes, or by complex, with multiple reverse faults and folds affecting the diapirs of evaporite-bearing Triassic rocks. Timing and evo- sedimentary successions accumulated since the Triassic. lution of these structures have been related to different tec- The local geology has been reported in previously pub- tonic scenarios: (1) early compressive/transpressive tectonic lished geological maps (Bourcart et al. 1942; Hindermeyer phases of Middle/Late Jurassic–Cretaceous age (Laville et al. 1977; Milhi 1997) and consists of largely continental et al. 2004 and references therein), (2) extensive diapirism upper Mesozoic–Cenozoic deposits related to the post-rift affecting the CHA in Early–Middle Jurassic (Michard et al. and convergent stages (Fig. 4). A stratigraphic revision of 2011; Saura et al. 2014). Cenozoic compressional tectonics, the continental red beds, previously ascribed to the Late related to the Africa–Europe convergent stage, successively Cretaceous, was stimulated by the mapping of the 1:50,000 affected the entire chain, superimposing and enhancing Boumalne and Imitir sheets (Geological Map of Morocco, previous structures and/or inverting ancient normal/tran- Massironi and Moratti 2007; Schiavo and Taj Eddine 2007). stensional faults (e.g., Frizon de Lamotte et al. 2008, and Further fieldwork in the Dadès River Valley (Ait Youl-Ait references therein). This is evident in both the northern and Ibriren area) allowed reconsidering the stratigraphic archi- southern borders of the chain, which are marked by regional tecture of red beds and carbonates to the Middle Juras- faults—the North Atlas Fault and the South Atlas Fault— sic–Late Cretaceous (Fig. 4, Benvenuti et al. 2017). considered to be the ancient rift master faults inverted during the convergent stage (Giese and Jacobshagen 1992). Both faults underwent a full inversion marked by the overthrust- Geological, petrographic, ing of the Mesozoic CHA successions onto the Haouz-Tadla- and geochronological results Missour, and the Ouarzazate and Goulmima–Errachidia foreland basins, to the north and south, respectively (Morel Stratigraphy and depositional setting of the Middle et al. 1993; Massironi et al. 2007; Fig. 1). Jurassic–Upper cretaceous succession

Mesozoic magmatism The sedimentary succession unconformably bracketed between the Lower Jurassic and the Paleogene carbon- The Atlas domains were affected by two intense mag- ates, is subdivided into five units (units 1–5) on the basis matic events, during the Late Triassic–Early Jurassic of angular unconformities and differences in sedimentary (200–195 Ma), and the Middle–Late Jurassic to Early Cre- facies and architecture. Limited outcrops of basalts within taceous (from ~ 170 to ~ 110 Ma) (e.g., Westphal et al. 1979; this succession are found in the Dadès Valley and in the Martins et al. 2008, 2010; Marzoli et al. 2004; Verati et al. Jbel Istifane area, and have been referred in previous stud- 2007; Frizon de Lamotte et al. 2009; Bensalah et al. 2013; ies to the CAMP event (Hindermeyer et al. 1977) and to the Whalen et al. 2015). Cenomanian (Milhi 1997), respectively. A description of this The first event was related to the CAMP with emplace- succession, based upon our own observations and previous ment of basaltic lava flows (b in Fig. 1), sills and dykes. The work (Benvenuti et al. 2017), is given below starting from second event was likely related to the Peri-Atlantic Alka- the stratigraphic bottom (Fig. 4). line Pulse (PAAP) (Matton and Jébrak 2009). It consists Unit 1 unconformably overlies the uppermost Liassic of basaltic lava flows and subvolcanic intrusive complexes Tafraout Formation and consists at the bottom of polygenic (gabbro-doleritic dikes and sills; Jb in Fig. 1) interbedded conglomerates (sub-unit 1a) with sub-rounded clasts, up to in the continental successions. This High Atlas magmatic 20 cm in diameter, in an abundant sandy-silty matrix. Thick- activity displays an overall transitional to moderately alka- ness varies from a few meters in the Jbel Istifane area up to line geochemical signature with enrichment of incompatible 200 m in the Dadès Valley. Clast composition is dominated trace elements typical of continental intraplate magmatism by volcanics and metasedimentary rocks derived from the (e.g., Frizon de Lamotte et al. 2008; Bensalah et al. 2013). pre-Mesozoic basement with minor calcareous clasts derived from the Lower Jurassic limestones. In the Dadès Valley, The study area massive dark reddish mudstones alternating with conglomerates become dominant upward. These deposits form the The study area located between Boumalne Dades and Ting- infill of the wide (Dadès) and narrow (Jbel Istifane) SSE- hir, at the southern front of the CHA, includes two specific trending paleovalleys drained by torrential streams incised in outcrop sites (Fig. 1). The first site is in the Dadès River an incipient relief to the north (the southern front of a proto- Valley, near Ait Ibriren village (Fig. 2). The second site is CHA; Benvenuti et al. 2017). Pebbly sandstones alternating located few km W of Tinghir, dominated by the Jbel Istifane with reddish-whitish massive mudstones become prevalent relief (Fig. 3). Similar to the other sectors along the northern toward the top (sub-unit 1b) and unconformably overlie margin of the Ouarzazate Basin, the study area is structurally sub-unit 1a. This is the record of a second valley incising

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Fig. 2 Geologic-structural map and geological cross-section of the Dadès River Valley, eastern Ouarzazate Basin (adapted from Massironi and Moratti 2007 and Benvenuti et al. 2017) (see Fig. 1 for location)

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Fig. 3 Geologic-structural map and geological cross-sections of the Jbel Istifane area, eastern Ouarzazate Basin (see Fig. 1 for location) the previous one, and affected by a relative base-level rise to medium sandstones in lenticular beds up to 2 m thick at with a progressive deactivation of channel belts replaced by the base passing upwards into prevailing reddish mudstones mud-dominated floodplains. In the Dadès Valley, lacustrine recorded only in the Dadès Valley. Sub-unit 1c is interpreted mudstones at the top of sub-unit 1b bear a fossil flora (Dadès to record low-sinuosity river channels transverse to the chain flora), ascribed to the Early Cretaceous (Hauterivian–Bar- and flowing towards the SSE (Benvenuti et al.2017 ). remian interval; Benvenuti et al. 2017). Unit 1 is topped Unit 2 consists of meter-thick pebbly sandstone tabular by sub-unit 1c consisting of planar cross-laminated coarse beds with minor mudstone beds. The pebbly sandstones are

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southern CHA Dadès River Valley Jbel Istifane southern CHA between Boumalne Dades and Tinghir (from literature) (Benvenuti et al. 2017; this study) (this study) (Hyndermeyer et al. 1977) (Mihli, 1997)

70 sub-unit 5b

75 n

Unit 5 e

i Cs Cr1 n

o Iflit F sub-unit 5a

80 n

e S

85 U. Cretaceous nit 4 sub-unit 4a 90 Turonian Akrabou F sub-unit 3d sub-unit 3d Ct Cr3

95 sub-unit 3c sub-unit 3c Cenoman. Aoufous F Cr2

Unit 3U sub-unit 3b sub-unit 3b Cim 100 Cr1 Ifezouane F sub-unit 3a sub-unit 3a1-2 β' (JIb)

105

s Albian

110 northern CHA (from literature) 115

Lower Cretaceou Db JIb 120 Aptian Ait Tafelt F

125 Barremian Jbel Sidal F Unit 2 Unit 2 B2 130 s sub-unit 1c sub-unit 1c

Dadès 135 Flora basalt

140 main unconformity Iouaridene F 145 Unit 1 150 sub-unit 1b sub-unit 1b 155 Callovian? to Barremian

160 Upper Jurassic to Lower Cretaceou B1 c 165 Bathonian Guettouia F sub-unit 1a sub-unit 1a

170 Bajocian Middle Jurassi Aalenian 175

180 c Toarcian J1-J2 J1-J2 J1-J2 J1-J2 J1-J2

185 Ma Pliensbach. Lower Jurassi βτ (Db)

Fig. 4 Stratigraphic comparison among Lower Jurassic–Upper Cre- and 5), and the composite stratigraphic succession of the entire CHA taceous sediments of the Dadès River Valley and Jbel Istifane areas as defined in literature (column 1). Pre-Guettoia formations are indi- with the five Units defined in this study (columns 2 and 3), previous cated with the annotation “J1–J2”, as in Fig. 1. Empty spaces along local composite sections relative to the southern CHA/eastern Ouar- the different columns outline stratigraphic hiatuses. It is worth noting zazate Basin (Hindermeyer et al. 1977) and Milhi (1997) (columns 4 the different stratigraphic location of Db and JIb basalts

1 3 2498 International Journal of Earth Sciences (2018) 107:2491–2515 characterized by horizontal and trough cross lamination, a rhythmic bedding. Poorly exposed and strongly deformed suggesting 3D dune-scale bedforms migrating from NNW pebbly sandstones and reddish mudstones in the Dadès to SSE. In the Dadès Valley this unit is subdivided into three Valley, continuous sandstone bedsets alternating with red- (2a–c) fining upward sub-units. In both areas, unit 2 is inter- dish mudstones in the Jbel Istifane area (sub-unit 3c), rest preted to record a SSE-trending fluvial valley entrenched through an erosive surface on the previous deposits. The in unit 1 and characterized by wide and shallow braided sandstone is arranged in meter-thick planar cross-laminated channels. beds pointing to a dominant flow to the E in the lowermost Unit 3 consists of five sub-units. The lowermost (sub- beds and to the W in the overlying beds. The lamination unit 3a1) is made of reddish conglomerates alternating with has a sigmoidal geometry indicating the influence of tidal massive sandstones and mudstones, incising the Dadès and currents. The deposits record a system of E–W trending flu- Jbel Istifane basalts. The conglomerates consist of sub- vial channels affected in their late development by flow-tide rounded pebble-size clasts in an abundant sandy to muddy entering from the East. The topmost portion of unit 3 (sub- matrix, arranged in dm- to m-thick lenticular beds. Clasts are unit 3d) is made of whitish limestones consisting of three mainly derived from Paleozoic and Precambrian basement stacked lithofacies: (a) basal massive bioclastic calcarenites rocks and the underlying basalts. In the Dadès Valley, the including mollusc shells, echinoids and vertebrate bone frag- conglomerates and sandstones grade upward into massive ments (in the Jbel Istifane the vertical basal surface shows reddish mudstones (Benvenuti et al. 2017). These deposits the marks of megaripples); (b) middle thinly bedded cal- are interpreted as the basal infill of a fluvial valley nested cisiltites with diffuse chert nodules; (c) upper calcarenites in in units 1–2, transverse to a relief to the north and flowing dm- to m-thick inclined beds dipping to the WNW. The latter toward an alluvial plain to the south. In the Jbel Istifane area, sub-unit records the development of a carbonate ramp in a pale reddish, well-stratified, coarse to medium sandstones major transgressive–regressive cycle starting with intertidal form a laterally continuous stratigraphic horizon (sub-unit to subtidal condition of a proximal ramp (basal), deepening 3a 2), truncating sub-unit 3a 1 deposits. The sandstones show into a distal ramp (middle), returning to a proximal ramp a well-developed planar cross lamination indicating dune characterized by westward progradation (upper). migration towards the ESE within braided channels. The Unit 4 consists of dolomudstones of dm- to m-thick overlaying sub-unit 3b consists of reddish-greenish mud- tabular beds, interbedded with reddish-whitish mudstones stones, dolomudstones and anhydritic gypsums arranged in (sub-unit 4a) exclusively recognized in the Jbel Istifane area

Fig. 5 Attitude of Db and JIb basalts. a Jbel Istifane area: the weath- unconformities. Unit 3 outcrops at the core of the syncline. Thick dot- ered zone between JIb basalt and unit 3, unconformable onto JIb ted line = limit of the basaltic bodies; continuous white line = base of basalt. b Dadès River valley: synclinal–anticlinal pair along the unit/sub-unit; dotted white line = bedding attitude (dip direction and Dadès River. Db basalt outcrops at the core of the anticline made dip of the beds are indicated) of unit 2 sediments, arranged according to syntectonic progressive

1 3 International Journal of Earth Sciences (2018) 107:2491–2515 2499 where it overlies sub-unit 3d. These deposits, characterized laminated in dm- to m-thick tabular beds. Mudstones are by disharmonic folding along the southern limb of the Jbel massive and crosscut at places by satin-spar gypsum veins. Istifane anticline, are overlain by a chaotic mixture of anhy- The upper sub-unit 5b consists, in its lower to middle part, dritic gypsum and reddish mudstones (sub-unit 4b).The low- of reddish fine-grained conglomerates dispersed in a sandy ermost portion of unit 4 (sub-unit 4a), not preserved or not matrix alternating with plane-bedded coarse to medium accumulated in the Dadès Valley, records a coastal sabkha sandstones. An overall fining-upward trend culminates in that marked the beginning of a new regressive event culmi- few meters thick pink-whitish mudstones and nodular marls nating into the mixed evaporitic-terrigenous deposition in a being in turn angularly overlain by the Paleogene Sub-Atlas playa setting (sub-unit 4b). Group. Unit 5 indicates an alluvial environment represented Unit 5 occurs in both sites of the study area (although by a terminal fan spreading on a muddy plain (sub-unit 5a), illustrated only in the Dadès Valley map, Fig. 2), and con- followed by an incised fluvial valley (sub-unit 5b) up to a sists of two sub-units. The lower sub-unit 5a is made of alter- coastal plain (Benvenuti et al. 2017). nating coarse to medium sandstones and subordinate red- The igneous rocks found in the study area are both dish mudstones. Sandstones are massive and locally planar hypabyssal intrusions and lava flows. The Jbel Istifane

Fig. 6 Lower hemisphere equal angle stereonets illustrating U1 1 1A 1B 2 2A 2B structural data collected at 23 sites of measurement. Continu- ous cyclographic lines indicate faults; black circles and dotted cyclographic lines indicate fold axes and axial planes, 3 3A 4 5 6 respectively. The convergent arrows indicate the σ1 direction computed by the right dihedron inversion method (Angelier and Mechler 1977) in the case of fault-slip data, or evaluated U2 7 7A 8 9 U3 10 from fold data. In case of different groups of mesostructures at the same site, A and B have been added to the site number. Data are listed in ESM Table 1 11 11A U4 12 13 13A U5 14

U1 15 15A U3 16 17 18

19 19A 19B 20 20A

21 21A 22 U4 23 NNE-SSW shortening direction

NW-SE shortening direction

~ E-W shortening direction

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Fig. 7 Features and structures affecting units 1–5:a –c Dadès River ted on the Schmidt net (lower hemisphere). Fold axes data allow Valley; d–f) Jbel Istifane area. SAIG = Cenozoic SubAtlas and Imer- evaluating a NNE-SSW-trending direction of shortening (see also hane groups; continuous white line = base of unit/sub-unit; dashed ESM Table 1). To the north, the thrust of J1a onto J3 and U1-3 at white line = bedding attitude; red line = thrust or reverse fault. a Pano- Jbel Istifane; e folded calcite veins in a vertical bed of sub-unit 3d. ramic view to the west along the Dadès River showing the angular The inset reports the attitude of five fold axes plotted on the Schmidt unconformities between units 3, 4 and 5 and the fan of growth strata net (lower hemisphere), that allow to evaluate a NNE–SSW-trending of sub-units 5a–5b, as well as the folding of Unit 4 multilayer. Loca- direction of shortening (see also ESM Table 1); f panoramic view to tion of the photographs reported in b, c is shown. b Reverse fault the west showing units 1–2, altogether outlining an anticline-syncline affecting sub-unit 5a;c Fault-related fold in unit 4a. d Panoramic narrower than the one described by the unconformable unit 3 deposits view to the west of the folded and verticalized sub-unit 4d. The inset above reports the attitude of the fold axial plane and of six fold axes plot-

1 3 International Journal of Earth Sciences (2018) 107:2491–2515 2501 dike (JId) consists of a few meters thick deeply weathered shortening affects units 3 and 4 along the Dadès River. This basalt intruded in the Liassic Tafraout Formation (Fig. 3). fault displaces the E–W backthrust that affects sub-unit 3d The Jbel Istifane basalt (JIb) is located at the core of an and is sealed by unit 5 deposits (Fig. 2). Large-scale folds anticline and passes upward to an up to 8 m thick sand- are related to NE–SW to E–W trending thrusts and back- sized basalt-derived debris, suggesting weathering of a lava thrusts with north-dipping axial planes (cross-section A–B flow (Fig. 5a). Here, the JIb lava flow rests unconformably in Fig. 2). Inversion of fault-slip data on minor faults affect- over Units 1–2 (Fig. 3) supporting a subaerial emplacement ing all five units was carried out at 14 sites of measurement over a surface deeply incised in the older units. Based upon (Fig. 6; ESM Table 1). The structural analysis of both faults field mapping, the Dadès basalt (Db) forms a lens-shaped and folds allowed to recognize three different shortening body (Fig. 2) although no weathered zone is found on its directions: NNE–SSW, E–W and NW–SE (Fig. 6; ESM top, preventing a clear indication of subaerial vs hypabyssal Table 1). At the mesoscopic scale superposing relationships emplacement. between faults were found only at site 11 (unit 3; Figs. 2, 6; ESM Table 1): faults related to the NW–SE compression Structural features overprint those coherent with the NNE–SSW shortening. These deformation features and the presence of progressive In both areas, the Liassic carbonates, arranged in complex unconformities in the sediments, with the typical basinward structural stacks affected by opposing thrust and backthrust fanning and wedging of units 1–5, record the continuing systems and related folds, are unconformably overlain by syndepositional vertical movements produced by the large- units 1–5 (see contacts between J3 and unit 1 in cross-sec- scale south-verging thrusts. tions A–B of Figs. 2, 3). These are affected by E-W to ENE- In the Jbel Istifane area, the main structure consists of a WSW-oriented folds, sometimes refolded along an about poly-deformed 4 Km long arcuated anticline-syncline pair E–W-trending shortening direction (Figs. 2, 3). (i.e., the Jbel Istifane Anticline and the Tamersgerouine Syn- In the Dadès Valley area (Benvenuti et al. 2017), units cline), altogether elongated in the E–W to NW–SE direction 1–5 are deformed into a series of thrust-related folds such as (Fig. 3). Both folds are overthrust to the NW by the Aguerd the km-scale Ait Ibriren Syncline. The core of the syncline n’ Syad thrust system, a stack made of J1a and unit 3 (Fig. 3, is made of Unit 1 deposits and is overthrust from both the cross-section C–D). The core of the Jbel Istifane Anticline north and the south by the Ait Arbi Thrust Zone and the is affected by a series of anticlines and synclines related Ait Ibriren Backthrust (Fig. 2). The syncline shows very to top-to-the-south thrusts and top-to-the-north backthrusts steep to overturned limbs, an axis gently plunging to the (Fig. 3, cross-section A–B). The attitude of these structures West and a steep north-dipping axial plane (cross-section indicates that they formed under a NNE–SSW trending A–B in Fig. 2). Parasitic fold axes show a double plunge compression and were successively refolded according to that indicates a successive refolding under an almost E-W a NW–SE shortening direction (Fig. 3). These structures shortening [sites 1A, 2A, 3A; Figs. 2, 6; Electronic Supple- superpose J1a onto J3 and units 1–2 deposits in a complex mentary Material (ESM) Table 1]. Few NNE–SSW minor pop-up structure. Backthrusts possibly deform the early fore- folds superimposed to the major syncline are also coher- thrusts (cross-section A–B in Fig. 3), as often evidenced in ent with the E–W compression (site 2B; Figs. 2, 6; ESM natural and analogue modelling studies (e.g., Serra 1977; Table 1). Unit 2 deposits are angularly unconformable onto Bonini 2007). To the NW, the central pop-up, which delimits the Liassic rocks and unit 1. They outcrop along the Dadès the highest topographic zone of this area, overthrusts the River and are folded in an E–W-trending synclinal–anticlinal already structured Jbel Istifane Anticline and Tamersger- pair. In the northern limb of the syncline, sediments of unit ouine Syncline (Fig. 3), pointing to a post-Cretaceous reac- 2 outline a wedge of strata growth in the younging direc- tivation. The southern limb of the Jbel Istifane Anticline tion, from sub-unit 2a to sub-unit 2c (cross-section A–B in shows an apparently simple geometry: the Liassic J1a and Figs. 2, 5b). Sub-unit 3a shows an angular unconformity J1b formations at the core of the anticline are superposed with unit 2 and covers the Db basalt that forms the core of toward the south onto the upright succession made by J3 the anticline (Figs. 2, 5b). Units 1–2 altogether draw a com- and units 1–3 (cross-section A–B in Fig. 3). All units show posite syntectonic progressive unconformity (sensu Anadon bedding attitudes varying from steeply inclined to vertical et al. 1986), with the older angular unconformities tectoni- and overturned (cross-section A–B in Figs. 3, 7d). Minor cally deformed. This documents a shifting to the south of the parasitic folds developed under a NNE-SSW compression, depocenter. Progressive unconformities also affect units 3–5, affect the bedding (sites 15A, 19A, 20A, 21; Figs. 3, 6; ESM that are unconformable one onto the other. All these units Table 1).Verticalization of this southern limb of the Jbel are also deformed by folds and minor reverse faults related Istifane Anticline led, at places, to steeply plunging axes to blind thrusts (sites 10–14; Figs. 2, 6, 7a–c; ESM Table 1). and axial planes (sites 20A and 21; Figs. 6, 7d, e; ESM A dextral transtensional fault coherent with an almost E–W Table 1). The JIb lava unconformably covers units 1–2. Unit

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3 is angularly unconformable onto JIb and units 1–2, as evi- magmas (Fig. 8a). Chondrite-normalized Rare Earth Ele- dent in correspondence of the minor anticlines refolding the ment (REE) pattern (Fig. 8b) is similar for all the sam- north-western flank of the Jbel Istifane Anticline (cross sec- ples, with overall REE fractionation [(La/Yb)n = 5.3–8.3], tion C–D in Figs. 3, 7f). Unit 4a deposits are in turn angu- along with light [(La/Sm)n = 2.0–2.6] and heavy [(Tb/ larly unconformable with an angle of 30°–40° onto the verti- Yb)n = 1.8–2.3] REE fractionation and no Eu anomaly. The cal unit 3d (see both cross-sections in Fig. 3). In the hinge of dike sample HA 22 has the lowest REE content and a slightly the anticline, to the SW, two refolding events are particularly positive Eu anomaly, consistent with cumulus of olivine and evident at map scale. The first refolding is related to the plagioclase as observed in thin section (e.g., see partition NW–SE shortening, which is the same leading the Aguerd coefficients in Rollinson 1993). The REE patterns of other n’ Syad thrust system onto the main structure (sites 19B and volcanic and hypabyssal samples collected at the northern 23; Fig. 3, cross-section CD, Fig. 6; ESM Table 1). The sec- front of the CHA (Bensalah et al. 2013) are reported as a ond refolding is related to an about E–W-trending shortening grey shaded area for comparison and mimic the pattern of direction (site 18; Figs. 3, 6; ESM Table 1). The core and our samples (Fig. 8b). Primitive mantle-normalized dia- hinge of the anticline are also affected by syn-sedimentary grams (Sun and McDonough 1989) of incompatible trace NE–SW left-lateral faults. Across these faults, from west elements (not shown) exhibit a pattern intermediate between to east, the thickness of sub-unit 1b is strongly reduced and Ocean Island and Enriched Mid-Ocean Ridge Basalts, unit 2 disappears. Both faults also dislocate unit 3 (and pos- although having positive K and Pb anomalies. Radiogenic sibly unit 4) deposits (Fig. 3). Structural analysis of minor Sr and Nd isotope compositions of representative CHA structures at nine sites of measurements confirmed the exist- basalt samples define a negative correlation and exhibit a ence of the three different shortening directions (NNE–SSW, E–W, NW–SE), that are the same found in the Dadès Valley (Figs. 3, 6; ESM Table 1).

Petrography and geochemistry of basalts

Two basalt samples, namely, HA1 and HA24 have been collected from Db and JIb outcrops, respectively (Figs. 1, 2, 3). For comparative purposes, basalts related to the second period of magmatism (170–110 Ma), have been sampled in the northern front and the axial zone of CHA (black stars in Fig. 1). The overall set of samples represents lava flows, hypabyssal sills, and dikes with porphyritic to ophitic and inter- granular textures. The main phenocryst paragenesis consists of plagioclase + olivine + clinopyroxene; apatite and Fe–Ti oxides are present as accessory mineral phases; serpentine, calcite, chlorite, iddingsite, and sericite can occur as secondary minerals in weathered samples. Mineral chemistry (ESM Table 2) along with whole rock (major and trace elements, and Sr and Nd isotopes, ESM Table 3) analyses were carried out following standard analytical procedures (see ESM Methods) on selected samples devoid of weathering (based upon secondary mineral occurrence in thin sections). Plagioclase is the most abundant mineral phase, and consists of zoned (An 69–46) and twinned phenocryst and microphenocryst. Olivine occurs as subhedral to euhedral Fig. 8 a Total alkali vs silica classification diagram (Le Maitre 2002) phenocryst, and displays normal zoning (Fo 80–70). Clinopy- of the CHA volcanic rocks. The dashed blue line represents the divi- roxene occurs as subhedral phenocryst and microphenocryst sion between alkaline and subalkaline volcanic rock associations with augitic composition (Wo45–42–En46–39–Fs11–20). (Irvine and Baragar 1971). b Chondrite-normalized rare earth ele- The studied CHA volcanic and hypabyssal rocks display ment (REE) patterns of the CHA volcanic rocks. The shaded grey area represents literature data of CHA volcanic rocks. Normalizing a restricted compositional range from basalt to trachyba- values from Boynton (1984). Literature data source from Bensalah salt, and are transitional between alkaline and subalkaline et al. (2013)

1 3 International Journal of Earth Sciences (2018) 107:2491–2515 2503

(a) 140 0.90 (b) 0.004 HA 1 g.m. 130 0.75 119.74 ± 0.51 Ma 0.003 [MSWD = 1.84] 120 0.60 40 36 Ar/ Arin= 286.6 ± 45.3 Ar ] 40 110 0.45 / 0.002 Ar K/Ca 36

e [ Ma 100 0.30

Ag HA 1 g.m. 0.001 90 Plateau age = 119.71 ± 0.57 Ma 0.15 MSWD = 1.59 80 0.00 0.000 020406080100 0.00 0.02 0.04 0.06 0.08 0.10 39 Cumulative Ar Released [ % ] 39Ar/40Ar

(d) 0.004 (c) 140 0.90 HA 24 g.m. HA 24 g.m. 119.24 ± 0.62 Ma Plateau age = 119.14 ± 0.67 Ma 130 0.75 [MSWD = 0.85] MSWD = 0.91 0.003 40 36 Ar/ Arin= 280.0 ± 12.2 120 0.60 Ar 40 ] / 110 0.45 0.002 Ar 36 K/Ca

e [ Ma 100 0.30

Ag 0.001 90 0.15

80 0.00 0.000 020406080100 0.00 0.01 0.02 0.03 0.04 Cumulative 39Ar Released [ % ] 39Ar/40Ar

Fig. 9 40Ar–39Ar step-heating data of analyzed samples. a, c Age lines with arrows indicate steps used in the plateau age calculation. b, spectra (black line and left axis) and K/Ca spectra (red line and right d Isochron plots, with solid symbols representing data points used for axis) of groundmasses, with boxes heights at the 2σ analytical level; age calculation, and empty symbols unused ones

87 86 143 144 40 36 range of Sr/ Sri from 0.7034 to 0.7041 and Nd/ Ndi due to the slightly sub-atmospheric initial Ar/ Ar ratio from 0.51258 to 0.51271 (ESM Table 3). All the samples (280.0 ± 12.2) compared to 295.5 (Nier 1950). in this study and Bensalah et al. (2013) have no significant HA 1 age spectrum is more complex, with a main plateau geochemical and radiogenic isotope difference suggesting a (63.63% of 39Ar release) at 119.71 ± 0.32 Ma (2σ analytical, common mantle source origin. MSWD = 1.59), followed by a mini-plateau (24.94% of 39Ar release) at 115.11 ± 0.58 Ma (2σ analytical, MSWD = 1.44) 40 39 Ar– Ar dating of Db and JIb basalts at high laser powers (i.e., temperature) and low K/Ca ratios. This age spectrum shape might be indicative of 39Ar recoil, 40Ar–39Ar step-heating analyses of the Db (HA 1) and JIb due to the energy associated to the nuclear reaction that (HA 24) basalts are reported in Fig. 9 and summarized in formed 39Ar (Turner and Cadogan 1974). In fine-grained ESM Table 4. HA 24 age spectrum is fairly flat, as evidenced crystals of the groundmass 39Ar atoms may have been dis- by an integrated age (calculated from the sum of all radio- placed from their original position to adjacent ones with 40 39 40 39 genic Ar and K-derived Ar, and equivalent to K/Ar age) different thermal characteristic, thus causing a Arrad/ Ar identical within error to plateau age. Only the first three bias during the step-heating, hence in the age. However, the steps, amounting to less than 3% of the total release of 39Ar, main central plateau is regular, and should be unaffected are younger, likely due to some alteration. Thirteen steps, by recoil-induced redistribution of 39Ar, maintaining its equal to 97.36% of 39Ar release, identify a plateau age of validity (see Fleck et al. 2014, for an updated discussion). 119.14 ± 0.29 Ma (2σ analytical, MSWD = 0.91); the same The eight steps of the main plateau identify an isochron data points yield an isochron age of 119.24 ± 0.16 Ma (2σ age of 119.74 ± 0.23 Ma (2σ analytical, MSWD = 1.84), analytical, MSWD = 0.85). The small difference between the with an initial 40Ar/36Ar ratio of 286.6 ± 45.3. It is worth plateau age and isochron age, however within the error, is to note that both samples have high radiogenic 40Ar yields,

1 3 2504 International Journal of Earth Sciences (2018) 107:2491–2515 of the order of 96%. Ages recommended for these samples unit 1 is considered an equivalent of the Guettouia (1a) are the isochron ages: 119.24 ± 0.16 (± 2σ analytical)/0.62 and Iouaridene (1b–c) formations bracketed between the (± 2σ J, i.e., with uncertainty in J value) Ma for HA24, and Bathonian and the early Barremian (Haddoumi et al. 2010 119.74 ± 0.23/0.51 Ma for HA1, which are identical within and references therein). Unit 2 is made of fluvial deposits error. resting unconformably over the previous deposits and is These two 40Ar–39Ar ages start to fill a gap in the chro- correlated to the Barremian Jbel Sidal Formation ascribed nology of CHA Mesozoic magmatism, given that while the to riverine settings (Haddoumi et al. 2010 and references chronology of CAMP rocks has been thoroughly studied in therein; Figs. 2, 3, 4). The mid-Aptian age of the over- the last years (e.g., Knight et al. 2004; Marzoli et al. 2004; laying Db–JIb basalts supports this correlation and indi- Verati et al. 2007), other scattered CHA magmatic rocks cates that the magmatic activity was coeval with a marine rely only on ages dating back to the 1970s (Hailwood and ingression recorded apparently only on the northern flank Mitchell 1971; Smith and Pozzobon 1979; Westphal et al. of the CHA by the Ait Tafelt Formation (Souhel 1987). 1979), and to stratigraphic correlations, often poorly defined. Units 3–5 record the Cenomanian–Maastrichtian inter- Only two other 40Ar–39Ar datings (151 and 145 Ma, Fig. 1) val as indicated by previous studies (Hindermeyer et al. have been performed in another location of the CHA, the 1977; Milhi 1997) (Fig. 4). The lithological and deposi- Jbel Hayim massif (Armando 1999). Literature data on tional features of sub-unit 3a suggest a correspondence CHA magmatism (without CAMP samples), when neces- to the Cenomanian Ifezouane Formation, exposed on the sary recalculated with Steiger and Jäger (1977) decay con- southern side of the eastern Ouarzazate Basin (Massironi stants of 40K, are reported in Fig. 1. Data range from 170 to and Moratti 2007; Schiavo and Taj Eddine 2007), and rep- 110 Ma, but it is difficult to assess whether this is the actual resenting a large drainage system confined between the time lapse because chronological data are almost exclusively CHA and the Anti-Atlas range. This drainage consisted K/Ar ages, which are model ages unable to correctly detect of localized transverse gravel-bed streams sourcing from disturbances in the isotopic system (gain or loss of father a relief to the NNW (3a1) and merging into a wider plain and daughter isotopes). crossed by sandy braided channels (3a2) opening to the ESE. Sub-units 3b-c are equivalent to the Aoufous Forma- tion (Cavin et al. 2010), pointing to alternating terrigenous Discussion supply and chemical precipitation in a coastal sabkha (3b), replaced by an ESE-directed fluvial system, tidally influ- Tectono‑sedimentary implications of the dated enced in its later development (3c). Such evidence indi- basalts cates a transgressive signal during the late Cenomanian that culminated into a full marine ingression attested by A first important result of this study is represented by the the overlying sub-unit 3d. The latter is biostratigraphically new 40Ar–39Ar dating of the Db and JIb basalts, which constrained to the Turonian (Benvenuti et al. 2017), thus had been previously attributed to the Late Triassic–Early equivalent to the Akrabou Formation (Cavin et al. 2010). Jurassic CAMP in the Dadès Valley (Hindermeyer et al. Finally, units 4–5, marking an overall Late Cretaceous 1977) and to the Cenomanian in the Jbel Istifane area regression, coincide with the regionally known Sénonien, (Milhi 1997). The Db and JIb basalts yield, in fact, the previously mapped as an undifferentiated complex (Hin- same middle Aptian age corresponding to the final stage of dermeyer et al. 1977). the second period of Mesozoic magmatism (i.e., 120 Ma, Apart from the stratigraphic implications, the Db–JIb Fig. 9), and show a similar age to dated basalts from the basalts represent important elements to reconstruct the mode northern CHA front (Hailwood and Mitchell 1971; West- of syn-depositional deformation of the encasing continental phal et al. 1979; Armando 1999). The new 40Ar–39Ar dat- deposits. ing provides robust support to the proposed stratigraphic The Dadès Valley had already been investigated in the revision of the upper Mesozoic successions strongly frame of structural studies on the Ouarzazate Basin and the deformed at the southern front of the CHA and their southern front of the CHA (e.g., El Harfi et al. 2001; Teson correlation to the regional lithostratigraphic framework and Teixell 2008). In those previous studies, however, no (Benvenuti et al. 2017). These mostly continental depos- revision was made on the Mesozoic portion of the strati- its, previously ascribed to the Cenomanian–Maastrichtian graphic succession. Teson and Teixell (2008), in particular, interval (Hindermeyer et al. 1977; Milhi 1997), first do described a conformable Triassic-basal Paleogene succes- not form a continuous succession, and, second, include a sion and referred it to a “predeformational” stage in the portion (units 1–2) older than this interval. On the basis of structural development of the CHA. In their analysis, only the fluvio-lacustrine depositional setting and the prelimi- the upper Eocene-Quaternary “syndeformational” succes- nary calibration of the Dadès flora to the Early Cretaceous, sion was related to the collisional Atlasic orogeny (Teson

1 3 International Journal of Earth Sciences (2018) 107:2491–2515 2505 and Teixell 2008). Other authors hypothesized an early com- related to the movement of the Aguerd n’ Siad thrust zone, pression/transpression both in the axial portion of the CHA is responsible for major refolding of their axes, as well and at its southern front (Laville et al. 1977; Mattauer et al. as the development of folds with NE–SW trending axis 1977; Laville 1980, 1988; Studer and du Dresnay 1980), (Fig. 3, cross-section C–D). The E–W shortening direction although they referred this deformation to local tectonic develops local minor folds, with an about N–S axis, that events, not able to develop an “orogenic” inversion (Laville are well expressed in the southwestern portion of the area. and Piqué 1992; Piqué et al. 2002). The results of our study Here, the N–S folds affect the external hinge of the Jbel definitely highlight the relevance of an Early Cretaceous, if Istifane Anticline, already refolded by the NE–SW trend- not Middle-Late Jurassic, tectonic inversion affecting the ing folds, thus representing a later contractional episode. southern front and possibly the entire CHA. The JIb basalt lies unconformable onto both unit 1 and unit In the Dadès Valley the relative timing of the main thrust 2 deposits, thus pointing to an emplacement by lava flows faults activation is given by the syn-depositional tectonic over a surface deeply incised in these older units, affected influence on the (1) development of the unconformity- by syn-sedimentary strike slip faults (Fig. 3). These latter bounded units 1–5, (2) southward shift of their depocen- also dislocate unit 3 (and possibly unit 4) deposits, thus tres, (3) angular and progressive unconformities within pointing to a successive, late to post-Cretaceous, reacti- each unit. The structural analysis indicates that this pattern vation under an about N–S shortening direction that is resulted from a two-step almost coaxial tectonic history: a consistent with the geometry of the major structures. This late Mesozoic syn-depositional thrusting and folding affect- suggests that the NW–SE compressional tectonics related ing units 1–5 under an almost NNE–SSW shortening direc- to the Africa–Europe convergence overprints the earlier tion, was overprinted by the late Eocene–Present inversion tectonic events also in this area. tectonics tied to the NNW–NW-directed Africa–Europe Syntectonic deposition and the magmatic episode convergence. An E–W to ENE–WSW-trending shortening occurred before deposition of unit 3, separated from unit 2 direction locally affected units 1–4 with structures sealed by an angular unconformity (see cross-sections in Figs. 2, 3, by unit 5 deposits. We, therefore, postulate a relative chro- 7f). The architecture of units 3–5 (Fig. 7a) and the structural nology among these three tectonic events with the almost data point to a continuous uplift of the CHA under a com- E–W-trending shortening direction intervening between the pressive stress sub-perpendicular to the proto-chain. NNE–SSW and the NW–SE ones. In the Dadès Valley, the geometrical and petrographic characteristics of the Db basalt The geochemical message from the basalts suggest differential cooling in a hypabyssal setting, so that a syn-kinematic emplacement as a laccolith at the core of the The geochemistry of the Db–JIb basalts (120 Ma) and growing anticline formed by the coeval unit 2 deposits can those of the northern front of the CHA (170–110 Ma) be envisaged. The occurrence of sand-sized basalt debris share common geochemical and radiogenic isotope charac- in the overlaying beds of sub-unit 3a points to a successive teristics (Fig. 8, ESM Table 3) and are referred hereafter as rapid exhumation and denudation of the laccolith. Consid- Middle Jurassic–Lower Cretaceous (MJLC-CHA) basalts. ering the syntectonic deformations affecting unit 3 deposits In our interpretation, these basalts can help to bridge the and the angular unconformity between units 2 and 3, the temporal and mantle source gap from the CAMP (200 Ma) exhumation of the Db basalt was most probably driven by to the Neogene–Quaternary magmatism occurring in the tectonics. The emplacement of the laccolith may have been Atlas domains of Morocco and eventually culminating favoured by the opening of dilatation zones at the core of in the present-day plume activity of the Canary Islands the anticline, where the melt was possibly conveyed by blind (Sleep 1990; Malamud and Turcotte 1999; Carracedo thrusts (see cross-section in Fig. 2). These latter represent et al. 2001). The Neogene-Quaternary basalts, in particu- preferential traps for the rising magma that accumulated as a lar, have been related to a geodynamic model invoking passive intrusion in the low-pressure area developed within mantle plume material funnelled along a subcontinental the thrust-related anticline, as suggested by field examples lithospheric corridor running from the Canary Islands to in active thrust systems and by analogue models (Musumeci the Alboran Sea and marked by a trail of intraplate vol- et al. 2005; Galland et al. 2007; Tibaldi 2008; Montanari canoes activated in the last 15 Ma (Duggen et al. 2009). et al. 2010). The data presented in this study provide preliminary argu- The Jbel Istifane structural setting also provides evi- ments suggesting an alternative scenario that can explain dence of polyphase deformation. The thrust-related Jbel the complex geodynamic setting and tectono-sedimentary Istifane Anticline and Tamersgerouine Syncline developed evolution of this region. in late Mesozoic under a NNE–SSW shortening direction The CAMP basalts are widely distributed in the Moroc- and were successively refolded under two different pale- can Atlasic domains (e.g., Whalen et al. 2015), whilst the ostress directions. The NW–SE trending compression, Mesozoic and Neogene–Quaternary basalts occur mostly

1 3 2506 International Journal of Earth Sciences (2018) 107:2491–2515 along an ENE–WSW direction pointing to the low buoyancy provided by trace elements (Fig. 10a, c). The European flux plume of the Canary Islands (Sleep 1990; Malamud and and African CAMP basalts have radiogenic Sr and unra- Turcotte 1999; Carracedo et al. 2001). diogenic Nd isotope composition indicating a mantle Overall, the geochemical and radiogenic isotope charac- source metasomatized by recycled sediments. In contrast, teristics of the European and African CAMP basalts sug- the Sr–Nd isotope signature of the CI basalts is typical of gest they derived from a depleted mantle source (low Ta/Yb, OIB-type basalts (e.g., Marcantonio et al. 1995; Nikogo- Fig. 10a), which experienced metasomatic enrichment(s) as sian et al. 2002; Duggen et al. 2009; Day et al. 2010) and testified by their high Th/Yb (Fig. 10a) (e.g., Callegaro et al. points to a low buoyancy flux plume (Sleep1990 ; Mala- 2014; Whalen et al. 2015 and references therein). In con- mud and Turcotte 1999; Carracedo et al. 2001) impinging trast, the geochemical signature of the Canary Islands (CI) on thick, Jurassic age oceanic lithosphere. The NQAMA and the Neogene–Quaternary Anti Atlas and Middle Atlas basalts mimic the radiogenic isotope signature of the CI (NQAMA) basalts point to an enriched mantle source typical basalts. The MJLC-CHA basalts have a Sr–Nd isotope sig- of intraplate basalts (e.g., Marcantonio et al. 1995; Nikogo- nature intermediate between the CI-NQAMA and CAMP sian et al. 2002; Duggen et al. 2009; Day et al. 2010). The basalts. MJLC-CHA basalts form a trait d’union between these two The geochemical and radiogenic isotope signature of contrasting mantle sources (Fig. 10). The HREE fractiona- the MJLC-CHA basalts (Fig. 10) sets constraints on the tion along with radiogenic Sr isotope signature (Fig. 10b) tectonic evolution of this sector of the Central Atlantic- provides robust constraints on both the depth of mantle Moroccan High Atlas continuum and permits to suggest melting and enrichment processes. The CAMP basalts have a geodynamic scenario alternative to that proposed by (Tb/Yb)n < 1.5, suggesting they sampled a shallow man- Duggen et al. (2009). tle volume mainly within the spinel stability field, whilst The geochemical signature of the MJLC-CHA basalts the CI and NQAMA basalts have (Tb/Yb)n > 2 providing indicates sampling of a mantle volume intermediate evidence for an origin from a deeper mantle source within between the Cl-NQAMA mantle source (plume-type man- the garnet stability field. The enriched Sr isotope signature tle) and the CAMP mantle source (lithospheric mantle) 87 86 ( Sr/ Sri > 0.705) of the European and African CAMP (Fig. 10). Additionally, there is a progressive increase basalts is consistent with their high Th/Yb (Fig. 10a) and of mantle melting depth and decrease of mantle melt- supports mantle metasomatism by recycled sediments in an ing degrees from the CAMP basalts (subcontinental old subduction zone setting (e.g., Whalen et al. 2015 and ref- lithospheric mantle, 5–10% melting) to the MJLC-CHA 87 86 erences therein). The CI and NQAMA basalts have Sr/ Sri (subcontinental asthenospheric/lithospheric mantle, 5% clustering around 0.703–0.704, suggesting they sampled a melting), NQAMA (subcontinental asthenospheric mantle, mantle volume devoid of recycled sediments. The MJLC- 1–5% melting) and CI (suboceanic asthenospheric mantle, CHA basalts complement the gap between the CI-NQAMA 1–5% melting) basalts. 87 86 and CAMP mantle sources in both (Tb/Yb)n and Sr/ Sri It is thus tempting to hypothesize that, following the (Fig. 10b). CAMP magmatic event, the present-day Canary Islands The different depths of mantle melting are strongly high- mantle plume impinged and flattened at the base of the lighted in the Yb vs La/Yb diagram (Fig. 10c). The range of lithosphere underneath CHA since at most 170 Ma, pro- Yb and La/Yb of the European and African CAMP basalts ducing the MJLC-CHA basalts along with lithosphere provides evidence for 5–10% polybaric mantle melting delamination (Fig. 11). We are unable to be more precise within the spinel (ca. 40%) and garnet (ca. 60%) stability about the arrival of the plume, although plume-related field, whilst that of the CI and NQAMA basalts suggests magmatism in the CHA (MJLC-CHA, Fig. 11) is con- a polybaric mantle melting process (1–5% melting) occur- fined within the stratigraphic units from Middle Jurassic to ring mainly in the garnet stability field (ca. 80%) and sub- Early Cretaceous. Successively, owing to Central Atlantic ordinately in the spinel stability field (ca. 20%) upon man- sea floor spreading and African plate movement to NE, the tle source rising to shallower depths (Fig. 10c) (see also plume head was passively dragged and delaminating the Hoernle and Schmincke 1993; Day et al. 2010). Again, the continental Morocco lithosphere forming an asymmetric MJLC-CHA basalts bridge the gap between the CAMP and flattened plume head (Fig. 11), similar to the well-known CI-NQAMA basalts indicating a polybaric mantle melt- case of the Hawaii mantle plume (e.g., Ribe and Chris- ing process (some 5% melting) occurring, as much as the tensen 1999). CI-NQAMA, in the garnet (ca. 80%) and spinel (ca. 20%) Contemporaneously to Morocco lithosphere movement, stability field. the plume progressively created direct routes to the surface The Sr and Nd radiogenic isotope signature of the without interacting with the lithospheric mantle and gener- CAMP, CI-NQAMA, and the MJLC-CHA basalts ated the Neogene–Quaternary volcanoes (NQAMA, Fig. 11) (Fig. 10d) is consistent with the geochemical signature occurring in the Anti Atlas and Middle Atlas mountain

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(a) (b) 4 10 European CAMP African CAMP Canarie Islands Middle Atlas 3 Anti Atlas CHA literature CHA this work

1 n ) b Yb /Y b/ Th 2 (T

0.1 S W 1

) ) 0.01 0.01 0.1110 0.702 0.7030.704 0.7050.706 0.707 0.708 Ta/Yb 87Sr/86Sr

6 3% 20% 0.5128 5% 40% Nd 4 0.5126 144 pp m

/ BSE 10% 60% Yb Nd 0.5124 80% 14 3 2 0.5122

10% 5% 3% 1% 0 0.5120 010203040506070 0.702 0.7030.704 0.7050.706 0.707 0.708 La/Yb 87Sr/86Sr

Fig. 10 Selected geochemical and radiogenic isotope diagrams of 10% ol, 20% opx, 30% cpx, 40% grt. Spinel lherzolite: 54% ol, 27% the MJLC-CHA volcanic rocks (this study and Bensalah et al. 2013) opx, 9% cpx, 10% sp, that melts in the proportions 10% ol, 20% opx, illustrating the connection with the CAMP and the CI-NQAMA 40% cpx, 30% sp. Partition coefficients from McKenzie and O’Nions basalts. a Th/Yb vs Ta/Yb showing the difference between basalts (1991). Literature data of CAMP, CI, and NQAMA samples have derived from depleted mantle sources (MORB) and enriched man- been selected with MgO > 4 to minimize the effect of open-system tle sources (OIB) with vectors indicating the influence of subduc- processes. It is worth noting that in all diagrams, with no exception, tion (S) and within-plate (W) components (e.g., Wilson 1989). b the MJLC-CHA basalts form a trait d’union between the CAMP and 87 86 (Tb/Yb)n vs Sr/ Sri. c La/Yb vs Yb with reported mantle melting the CI-NQAMA basalts. See text for further explanation. Literature 143 144 87 86 curves. d Nd/ Ndi vs Sr/ Sri with reported BSE (Bulk Silicate data source: Oversby et al. (1971), Cousens et al. (1990), Bertrand Earth) and DMM (depleted MORB-type mantle) composition. Man- (1991), Araña et al. (1994), Ovchinnikova et al. (1995), Thirlwall tle melting curves in c have been calculated assuming non-modal par- et al. (1997), Neumann et al. (1999), Thomas et al. (1999), Widom tial melting of garnet- (red curve) and spinel- (blue curve) lherzolite et al. (1999), Jourdan et al. (2000), Simonsen et al. (2000), Abratis sources with a primitive mantle composition (Sun and McDonough et al. (2002), Cebria et al. (2003), Lundstrom et al. (2003), Mar- 1989), and are intended to be a schematic example of the influence zoli et al. (2004), Deckart et al. (2005), Dorais et al. (2005), Verati of garnet as opposed to spinel during mantle melting (e.g., Thirlwall et al. (2005), Gurenko et al. (2006, 2009, 2010), Praegel and Holm et al. 1994). Numbers along the red and blue curves refer to partial (2006), Mahmoudi and Bertrand (2007), Martins et al. (2008), Berger melting degrees; dashed grey curves represent mixtures among melts et al. (2009), Duggen et al. (2009), Day et al. (2010), Day and Hil- originating in the garnet and spinel stability fields, with percentages ton (2011), Wiesmaier et al. (2011), Deegan et al. (2012), Bensalah referring to the amount of garnet lherzolite melt. Garnet lherzolite: et al. (2013), Bosch et al. (2014), Callegaro et al. (2014), Turner et al. 54% ol, 27% opx, 9% cpx, 10% grt, that melts in the proportions (2015) ranges (e.g., Duggen et al. 2009; El Azzouzi et al. 2010; Geodynamic implications Berger et al. 2014; Bosch et al. 2014). The plume is currently located underneath the Canary Islands although the asym- The arrival of the plume underneath the CHA (Fig. 11) metric flattened plume head is still producing magmas in explains the occurrence of the anomalously thinned the Atlas mountain ranges (e.g., Duggen et al. 2009; Berger lithosphere beneath the Atlas domains (ca. 60–80 km et al. 2009; Miller et al. 2015). thick) in comparison with typical NW African lithosphere (130–160 km) as suggested by the high heat flow, gravity and geoid anomalies (e.g., Teixell et al. 2005; Missenard et al. 2006; Fullea Urchulutegui et al. 2006;

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Middle Jurassic-Early Cretaceous MJLC-CHA (170-110Ma) SW America NE Oceanic crust Continental crust

Continental lithosphere Plume

Astenosphere

Present day CI NQAMA MJLC-CHA NE SW Mid ocean ridge

Oceanic lithosphere Plume head dragged and delaminating continental listhosphere

Astenosphere

Fig. 11 Schematic cartoon (not to scale) illustrating the proposed to Central Atlantic opening and NW Africa motion to NE, the plume geodynamic evolution of the Central Atlantic-Moroccan High Atlas was progressively dragged SW forming an asymmetric, flattened continuum along a NE-SW transect. a Following the commencement plume head that determined a lithosphere delaminated corridor from of the Central Atlantic sea floor spreading (ca. 180 Ma), a mantle the Central High Atlas to the Canary Islands (Miller et al. 2015), plume impinged at the base of the subcontinental lithosphere of the marked by a trail of intraplate volcanoes (NQAMA) and eventually Atlasic domains causing plume head flattening, lithosphere delamina- culminating in its present-day position underneath the Canary Islands tion, and the MJLC-CHA basalt magmatism (110–170 Ma). b Owing

Missenard and Cadoux 2012; Miller et al. 2015). This 1200 km far apart, implying a minor continental lithosphere thinned delaminated subcontinental lithosphere forms a movement of ca. 1.0 cm/a. This, although being a rough narrow ENE–WSW corridor overlain by the above-men- estimation, is consistent with an interplay between the dif- tioned MJLC-CHA and NQAMA intraplate volcanoes, ferent rates of the Central Atlantic Ocean sea floor spread- and can represent the trail of the dragged plume head ing and the Western Tethys opening (e.g., Schettino and (Fig. 11), consistent with the present-day configuration Turco 2011). The cumulative action of opposing extensional of the subcontinental Moroccan lithosphere as imaged forces may have generated a compressive intraplate stress by S receiver functions from more than 360 broadband that affected the CHA prior to continental collision, as also seismic stations (Miller et al. 2015). The high heat flow found in other passive margins such as Australia and South related to the occurrence of the plume below the CHA America (e.g., Hillis et al. 2008; Navarrete et al. 2016). since the Jurassic strongly supports the hypothesis made Changes in the relative motions among Africa, Iberia and by Benvenuti et al. (2017) on apatite fission tracks (AFT) Europe may also have influenced the regional state of stress annealing in the axial portion of the already uplifted late (Dercourt et al. 1986; Rosenbaum et al. 2002; Stampfli and Mesozoic CHA. The late Mesozoic AFT ages in the oppo- Borel 2002; Ford and Golonka 2003; Schettino and Turco site foothills of the chain (Malusà et al. 2007; Ghorbal 2009, 2011). The opening of the northern Central Atlantic et al. 2008; Saddiqi et al. 2009; Ruiz et al. 2011; Oukas- Ocean in Middle-Late Jurassic was accommodated by left- sou et al. 2013; Gouiza et al. 2017) can approximate the lateral strike slip motion between Africa and Iberia along limits over which the thermal anomaly had no effects, thus the Azores-Gibraltar transform fault (e.g., Dercourt et al. recording the early exhumation of the CHA margins. The 1986), under a maximum horizontal shortening sub-parallel Neogene–Quaternary AFT ages found in the axial part of to slightly inclined with respect to the Atlas trend. The faster the CHA (Missenard et al. 2008; Balestrieri et al. 2009; opening of the Central Atlantic south of this fault (Schet- Ghorbal 2009; Gouiza 2011) document the rapid cooling tino and Turco 2009, 2011) was possibly accommodated by of the chain, driven by the tectonic inversion related to the right-lateral strike slip motion along transform faults, which, Cenozoic convergence between Africa and Eurasia. entering the Atlasic domain, could have caused thrusting The oldest oceanic lithosphere to the east of the island along the E-ENE direction, i.e., under a horizontal short- of Lanzarote is ca. 180 Ma (older than the 160 Ma M25 ening direction sub-perpendicular to the Atlasic chain, as paleomagnetic anomaly, e.g., Collier and Watts 2001) and is already proposed by Dewey et al. (1973) (Fig. 12a). During located ca. 2,600 km far apart from the Mid-Atlantic Ridge. the Early Cretaceous, the opening of the Bay of Biscay led to Accordingly, we can calculate that the half mean ocean sinistral movements along the North Pyrenean Fault and to floor spreading was roughly 1.4 cm/a in this sector of the counterclockwise rotation of Iberia with respect to Europe, Central Atlantic. Conversely, the MJLC-CHA magmatism thus continuing to generate a compressional state of stress (140 Ma on average) and the present-day active volcanoes almost perpendicular to the South Atlas Front (Fig. 12b). of La Palma and El Hierro in the Canary Islands are some Considering the counterclockwise rotation of Africa with

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(a) Conclusion Northern America Central Atlantic NPF The multidisciplinary study performed in this paper, resulted GF Iberia in a composite picture of sedimentary, structural and mag- Central matic processes that acted during the late Mesozoic on a Atlantic sector of the CHA domain, being the local expression of the

Ligurian evolution of the northwestern margin of the Africa plate and Tethys adjacent ocean basins. At the scale of the study areas, the Atlasic Domain main conclusion concerns the stratigraphic revision of the Africa mostly continental succession bracketed between the Lower Jurassic and the lower Eocene marine carbonates. The new 40Ar–39Ar dating of the Db and JIb basalts to the middle (b)America Aptian (120 Ma) provides robust support to this revision and Northern Central Atlantic Eurasia to a correlation to the regional lithostratigraphic framework attempted in our previous study (Benvenuti et al. 2017). The NPF Iberia architecture and syntectonic deposition of the studied suc- Central GF cessions in the Dadès River valley and Jbel Istifane area Atlantic point to a Middle Jurassic–Late Cretaceous pre-convergent Ligurian progressive uplift of the CHA under a compressive stress Tethys Atlasic Domain sub-perpendicular to the proto-chain. Besides contributing to reconstruct the mode of syn-depositional deformation Africa of the encasing continental deposits, the dated basalts provide information on the magmatic processes that affected the CHA domain, particularly its northern side, between the Middle Jurassic and the late Early Cretaceous. Overall, Fig. 12 Schematic plate reconstructions around the Atlasic Domain, the geochemical characteristics of the MJLC-CHA basalts, redrawn after Schettino and Turco (2011) and Dewey et al. (1973). Db JIb trait d’union a Middle-Late Jurassic: the opening of the North-Central Atlantic including the and , form a between the Ocean west of Iberia was accommodated by left-lateral strike slip CAMP basalts, sampling a metasomatized lithospheric man- motion along the Azores-Gibraltar transform fault (GF). This defor- tle, and the CI-NQAMA basalts, sampling the mantle plume mation, along with the faster opening of the Central Atlantic, gener- of the Canary Islands. This geochemical signature provides ated compressive stresses sub-perpendicular and sub-parallel to the Atlasic Domain. b Early Cretaceous: the opening of the Bay of Bis- a hint indicating that the MJLC-CHA basalts could repre- cay led to sinistral movements along the North Pyrenean Fault (NPF) sent the first signal of the present-day Canary Islands mantle and then to anticlockwise rotation of Iberia with respect to Europe plume impinging, flattening, and delaminating the base of (arcuate arrow) with compressional state of stress almost perpendic- the Moroccan continental lithosphere since the Jurassic, and ular to the South Atlas Front. Present-day coastlines (dashed lines) are shown for reference; dashed areas indicate basins; arrows across later on dragged passively by the Africa plate motion to NE. the Mid-Atlantic Ridge represent direction and amount of relative Finally, the opening and spreading of two oceans adjacent motion; strike slip sense of movement is indicated by double arrows; to the north-western margin of the Africa plate may have bold black arrow and white arrow indicate maximum subhorizontal determined during the late Mesozoic a peculiar geodynamic stress respectively sub-perpendicular and sub-parallel to the Atlas Domain scenario accounting for the concurrent crustal shortening and related mountain uplift. respect to Europe (e.g., Dewey et al. 1973, 1989; Savostin Acknowledgements The authors wish to thank Mohamed Gouiza et al. 1986), the direction of the Jurassic–Cretaceous com- and Dominik Letsch for their accurate reviews that greatly helped to improve the manuscript. The authors also wish to acknowledge Pro- pression almost coincides with the direction of the Cenozoic fessors Ahmed and Abdellah Algouti of the Cadi Ayyad University of inversion related to the Africa-Europe convergence. In this Marrakech for their unvaluable support during fieldwork. The research scenario, structural data collected in the study area (Figs. 2, has benefitted from University of Florence (I-Fund programme, Fondi 3, 6; ESM Table 1) record the late Mesozoic N-NNE trend- di Ateneo, 2012–2016), CNR-IGG (Geological Mapping Funds), and PRIN 20158A9CBM grants. ing shortening direction, successively overprinted by the pervasive NNW-SSE post-Cretaceous compression. The ENE shortening direction that affected at times the study area, may be coherent with Jurassic-Cretaceous compressive events recorded on both sides of the Atlantic Ocean (With- jack et al. 1998; Bertotti and Gouiza 2012).

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