Tectonophysics 475 (2009) 29–37

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Apatite fission-track analyses on basement granites from south-western Meseta,Morocco: Paleogeographic implications and interpretation of AFT age discrepancies

O. Saddiqi a,⁎, F.-Z. El Haimer a, A. Michard b, J. Barbarand c, G.M.H. Ruiz d, E.M. Mansour a,P. Leturmy e, D. Frizon de Lamotte e

a Laboratoire Géosciences, Faculté des Sciences, Université Hassan II Aïn Chock, BP. 5366 Maârif, Casablanca, Maroc, Moroccob 10 rue des Jeûneurs, 75002 Paris, Francec Univ Paris Sud, UMR CNRS 8148 IDES, Bâtiment 504, Orsay cedex, F-91405, Franced Institut de Géologie, Université de Neuchâtel, 11 rue Emile-Argand, 2009 Neuchâtel, Suisse, Francee Département des Sciences de la Terre et de l'Environnement (CNRS, UMR 7072), Université de Cergy-Pontoise, 5 mail Gay Lussac, Neuville/Oise 95 031 Cergy-Pontoise cedex, France

⁎ Corresponding author.E-mail address: omarsaddiqi@yahoo.fr (O. Saddiqi).

0040-1951/$ – see front matter © 2009 Elsevier B.V. Aldoi:10.1016/j.tecto.2009.01.007

a b s t r a c t

a r t i c l e i n f o

Article history:

This work is based on apat Received 11 April 2008Received in revised form 15 December 2008Accepted 2 January 2009Available online 14 January 2009

Keywords:Apatite fission-tracksThermochronologyVertical movementsMoroccoMesetaAtlas

ite fission-track analysis of samples (mostly granites) from the basement of theCretaceous–Tertiary Phosphate and Ganntour Plateaus, exposed in the Jebilet and Rehamna massifs (WesternMeseta, Morocco). This basement belongs to the Carboniferous–Early Permian Variscan Belt, and the earliermarine onlap is Late Triassic in age. However, the AFT ages are post-Triassic and different in the Jebilet (186–203 Ma) and Rehamna (148–153 Ma). Track length modelling support the occurrence of moderate heatingevents during the Jurassic and the Eocene, respectively, with cooling during the Permian and Cretaceousintervals. These results are partly accounted for by considering amoderate subsidence during the Late Triassic–Liassic, which is a noticeable change in the regional paleogeographic concept of “West Moroccan Arch”.However, the discrepancies between the AFT results from the studied massifs make necessary to explorefurther explanation. We interpret the observed discrepancies by the difference in age and depth ofcrystallization of the sampled granites in the Variscan Orogen, i.e. 330Ma, 5–6 km in the Jebilet versus ~300Ma,8–10 km in the Rehamna. The importance of the Late Jurassic–Early Cretaceous uplift and erosion of the entireMeseta and that of its Late Eocene burial are emphasized.

© 2009 Elsevier B.V. All rights reserved.

1. Introduction

Thermochronology on apatite have been used recently in Moroccoto discuss the vertical movements of the basement of the High Atlas(Barbero et al., 2007) and Anti-Atlas mountain belts (Malusà et al.,2007). In this work, we present an apatite fission track (AFT) study ina domain that is defined, according to the classical geological criteria(Choubert and Faure-Muret, 1962) as a tabular domain, namely theMoroccan (or Western) Meseta. In fact, this domain forms a relativelystable area bounded by Cenozoic mountain belts to the south, east andnorth (the High Atlas, Middle Atlas, and Rif belt, respectively), and bythe Atlantic Ocean to the west (Fig. 1a). The Western Meseta regionincludes Paleozoic massifs characterized by Variscan deformation andmetamorphism, varied granite intrusions, and late orogenic (EarlyPermian) continental basins. The Paleozoic basement is directlyoverlain either by Triassic–Liassic series (Tabular Middle Atlas) or byCretaceous–Tertiary plateaus (“Plateau des Phosphates” and Gann-tour), and surrounded by Neogene basins (Bahira-Tadla, Haouz,

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Doukkala, External Rif foredeep). The Cretaceous–Tertiary plateaushave been intensely studied for phosphate exploitation (threequarters of the phosphate reserves of the world), oil and deep waterresources.

Ultimately, Ghorbal et al. (2008) have presented AFT data from thenorthern (Zaer) and central (Rehamna) parts of Western Meseta,respectively. Our independent study is mainly based on sampling inthe southernmost Meseta massif, i.e. the Jebilet Massif, and onadditional sampling in the Rehamna Massif. The Jebilet Massifculminates at 1050 m above sea level (a.s.l.), being bounded to thenorth by a major Neogene (Atlasic) reverse fault (Hafid, 2006; Hafidet al., 2006; Frizon de Lamotte et al., 2008). The Rehamna Massif,which culminates at ca. 600 m a.s.l., belongs to the most “stable” partof the Meseta, barely affected by the Atlas orogeny.

The striking hiatus of Triassic–Liassic deposits over most ofWestern Meseta is classically interpreted as related to the occurrence,during the Triassic–Liassic, of an emergent land between the Atlanticmargin and the Atlas basins. Choubert and Faure-Muret (1960–62)coined the name of “Terre des Almohades” for this allegedly emergentdomain, which is currently referred to as the West Moroccan Arch(WMA; Hafid, 2006; El Arabi, 2007). Our AFT data allow us to discuss

Fig. 1. Location maps. (a) Structural provinces of northern Morocco. PR: Prerif Ridges; SB: Selloum Basin; TMA: Tabular Middle Atlas. — (b) Schematic map of the studied basementmassifs and surrounding areas, after Hollard et al. (1985) and Hoepffner et al. (2006). Dashed lines (DK 25, etc.): Seismic profiles (Hafid, 2006; Hafid et al., 2008); Kh: Khouribga;WMSZ: Western Meseta Shear Zone, Y: Youssoufia.

30 O. Saddiqi et al. / Tectonophysics 475 (2009) 29–37

this classical description, and to suggest that, in fact, the WMAsubsided before being eroded during the Late Jurassic–EarlyCretaceous.

On the other hand, we recognized puzzling discrepancies betweenthe AFT ages from two different groups of granite intrusions,

Fig. 2. (a) Topographic model of the Moroccan Meseta (GETOPO data) and surrounding areaAsterisk: location of the photos Fig. 5. (b) Geological cross-section of theMoroccanMeseta soutexaggeration. The shape of the granite intrusions is diagrammatic.

belonging to the Jebilet and Rehamnamassifs, respectively. Discussingthese discrepancies represents one of themost significant topics of thepresent study. Our study points to the importance of considering notonly the evolution of the subsidence/uplift history of a tabular area,but also the structure and evolution of its basement, as the depth and

s, with location of the studied granites. Bold line: approximate trace of cross-section (b).h of the CentralMassif, located on (a) andmore exactly in Fig.1b. Notice the strong vertical

Fig. 3. Sampling maps and AFT results (mean ages). (a) Central Jebilet; geologic contours after Huvelin (1977). (b) Central Rehamna; geologic contours after Michard (1982). See Fig.1b for location.

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age of emplacement of a given basement granite control its distance tosurface after a given erosional event.

2. Geological setting

The basement of the Western (Moroccan) Meseta belongs to thesouthern branch of the Variscan Orogen, within which a number ofgranite stocks emplaced during the Visean to Early Permian interval(Hoepffner et al., 2006; Michard et al., 2008). In particular, the Jebiletgranites are dated at 330 Ma (U–Pb zircon; Essaifi et al., 2003;Boummane and Olivier, 2007), and those of the Rehamna at ~300 Ma(Baudin et al., 2003). The belt collapsed and was eroded first duringthe Late Carboniferous–Early Permian (Hmich et al., 2006; Saber et al.,2007). Then, the rifting associated with the Pangaea break-downoccurred on both sides of the futureWesternMeseta, i.e. in the Central

Table 1Apatite fission-track analyses of Jebilet and Rehamna samples.

Sample Lithology n ρs Ns ρi Ni ρd

×105 t/cm2 ×106 t/cm2 ×

Jebilet massifJGO1 Granodiorite 26 7.76 5496 3.76 2224 5.JOG3 Granodiorite 29 6.21 4029 2.91 1891 5.JGO4 Granodiorite 26 5.55 4881 2.72 2394 5.JTB2 Granophyre 29 6.52 4229 3.28 2126 5.JTB3 Granophyre 24 1.81 2482 8.47 1163 5.JTB4 Granophyre 26 2.05 2702 9.42 1243 5.

Rehamna massifRH1 Schist 11 1.90 633 1.80 598 8.RH4B Schist 10 0.19 168 0.18 154 8.RH8 Leucogranite 15 3.88 649 3.62 605 8.RH9A Leucogranite 15 4.00 997 3.55 885 8.

See Fig. 3 for sample location.n,Ns andNi, respectively number of crystals dated, total number of spontaneous and induced trand their detectors (JGO-JTB: kapton; RH:muscovite); d, means induced track density in the deprobability of obtaining a 2 value for n-1 degrees of freedom. As all samples passed the test at a1981). Zetavalueof F.Z. ElHaimer, analyst, 35017 (JGO-JTB)andO. Saddiqi, analyst, 3338 (RH) (1the number of tracks measured.

Atlantic (Favre et al., 1991; Medina, 1995; Hafid, 2006) and Atlas(Tethyan)Gulf (Zizi, 2002; El Arabi et al., 2006a,b), respectively. Riftingoccurred during the Late Permian–Late Triassic, ending temporarily at200 Ma with the emplacement of the basalts of the Central AtlanticMagmatic Province (CAMP: Knight et al., 2004; Verati et al., 2007).During the Liassic–early Dogger, rifting resumed in the Middle Atlas(Charrière, 1990) and High Atlas Basins (Studer, 1987; Warme, 1988),resulting in the accumulation of 3 to 8 km-thick deposits, respectively.In contrast, the Western Meseta exhibits large exposure of basementunits, from north to south, the Central Massif and Coastal Meseta,the Rehamna and the Jebilet (Fig. 1b). Triassic and Liassic coversequences are preserved only at the fringe of Western Meseta, eitherexposed as outcrops to the northeast and north (e.g. Tabular MiddleAtlas, Prerif Ridges; Fig. 1a; Hollard et al., 1985) or documented inthe subsurface to the southeast (Tadla, Bahira), southwest (Essaouira

Nd P (χ2) FT age N L Dpar

105 t/cm2 % Ma±1σ µm±1SD µm

41 16,464 99.9 193±5 111 11.90±2.12 1.48±0.1241 16,464 99.1 199±6 – – 1.43±0.1941 16,464 100 190±5 100 11.07±2.27 1.28±0.1241 16,464 99.3 186±5 80 11.37±2.45 1.32±0.1041 16,464 100 199±7 – – 1.31±0.1041 16,464 99.8 203±7 – – 1.27±0.13

52 17,645 99.9 148±9 – – 1.75±0.1452 17,645 99.8 153±18 – – 1.83±0.1552 17,645 99.6 150±9 100 12.43±1.96 1.30±0.0952 17,645 100 153±8 100 11.91±1.94 1.49±0.13

acks counted; s and i, respectively spontaneous and induced track density in apatite grainstectors associated to NIST neutron glass monitors 962 (JGO-JTB) and CN5 (RH). P(2) is the95% confidence level with P(2) N5%, ages were calculated using pooled statistics (Green,). Confined tracks: L and l s.d. are respectively themeanvalueand standarddeviation, (N) is

Fig. 4. Modelling experiments (AFTSolve) for the Central Jebilet granite (a, b) and Central Rehamna granite and schists samples (c, d).

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Basin; Hafid, 2006) and west (Doukkala, Abda; Echarfaoui et al.,2002a,b). Elsewhere, the Western Meseta basement is onlapped byyounger deposits, Late Jurassic–Early Cretaceous in the Mouissat hillswest of the Jebilet (Hollard et al., 1985), and Cretaceous (mainly post-Aptian) in the Rehamna and southern Central Massif (Gigout, 1954;Bolleli et al., 1959; Baudin et al., 2003).

In addition to the map in Fig. 1b, these geological constraints aresummarized in the cross-section in Fig. 2. The latterfigure alsomakes clearthat the southernmost part of the Meseta has been involved in the Atlasshortening,with the JebiletMassif thrustover theBahiraTriassic–Neogenedeposits through a southward-dipping reverse fault. In contrast, further tothe north, theMeseta basement and overlying Cretaceous–Eocene tabularsequence are only affected by much weaker deformation.

3. Sampling and experimental procedure

Eight samples out of ten (Fig. 3; Table 1) have been collected fromgranites, among which three from the east Central Jebilet (OuledOuaslam granite laccolith; Boummane and Olivier, 2007), three fromthe west Central Jebilet (Tabouchent granophyric granite; Huvelin,1977; Essaifi et al., 2001, 2003), and two from the Central Rehamna(Ras-El-Abiod leucogranite; Hoepffner et al., 1982; Baudin et al.,2003). Additionally, two samples were collected in the Paleozoic

Fig. 5. The Cretaceous transgression at the northern border of the Rehamna massif, as seen in(LC) — Cenomanian–Turonian (C–T) escarpment (about 100 m high) above the Paleozoic baCretaceous red beds is marked by coarse conglomerates with poorly sorted, barely roundroadcut). S1: Variscan cleavage. (For interpretation of the references to colour in this figure

schists from the Central Rehamna. All samples were collected at 500–600 m of elevation.

Apatite grains were separated using conventional heavy liquids/magnetic separation procedures. Samples were dated with theexternal detector technique using kapton foils (Jebilet samples) ormuscovite (Rehamna samples). Tracks were etched in apatite with1 M HNO3 solution at 20 °C for 45 s, in kapton using a boiling solutionof potassium hypochlorite for 8 min, and in muscovite in 40% HF for45 min. For each sample, a single age population is observed. Dpar hasbeen measured for each sample (five measurements per grain).Horizontal confined track length (TINTS) measurements have beenperformed on five samples using a digitising tablet (Table 1).

4. Results

Our AFT analyses on the Jebilet granites show AFT ages groupedbetween 202.6±7.1 and 185.7±5.1 Ma (Table 1). The Mean TrackLength (MTL) varies from 11.90 to 11.07mwith standard deviations inthe range 2.4–2.1 m. These results are consistent with those obtainedby Mansour (1991) using the population method, with AFT agesranging from 218±23 Ma to 170±15 Ma (mean age 186 Ma).

In the Rehamna samples the AFT ages of the granite and country-rock schists are grouped between 148.4±9.3 and 157.8±8.4 Ma

the Oum-er-Rbia valley (see Fig. 2a for location). (a) Overview of the Lower Cretaceoussement (Cb: Middle Cambrian). (b) The major unconformity at the bottom of the Lowered pebbles of Ordovician quartzites likely sourced in the Central Rehamna (3 m highlegend, the reader is referred to the web version of this article.)

Fig. 6. Estimated P–T conditions of crystallization of the Jebilet and Rehamna schists andgranites plotted on the petrogenetic grid, based on themineral associations described byEssaifi et al. (2001) and Hoepffner et al. (1982), respectively. Dashed: high-temperatureP–T–t paths of the studied samples. Horizontally ruled: andalusite–sillimanite transition(Pattison, 1992). Vertically ruled: cordierite–garnet transition (Holdaway and Lee(1977). Staurolite–cordierite experimental curve after Richardson (1968). Peraluminousgranite melt after Willye (1977). And: andalusite; As: aluminium silicate; Chl: chlorite;Cld: chloritoid; Crd: cordierite; Fe–Ctd: iron-rich chloritoid; Grt: garnet; Kfs: K-felspar;Ky: kyanite; Ms: muscovite; Qtz: quartz; Sil: sillimanite; St: staurolite.

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(Table 1), which is identical to the ages obtained by Ghorbal et al.(2008) within the error bar. The MTL in the granite samples are in therange 12.43–11.91 m (less by ~1 m than the MTL given by Ghorbalet al.) with standard deviation 1.96–1.94 m. Dpar values arehomogeneous within samples and are between 1.27 and 1.83. Thereis no significant variation between the Dpar values for the two studiedmassifs.

Data have been modelled using the Ketcham et al. (1999)annealing model and AFTSolve (Ketcham, 2005). Geological con-strains used in this modelling are:

(1) The presence of both massifs close to the temperature of totalannealing at 280 Ma as i) the studied granites are dated at330 Ma in the Jebilet (Essaifi et al., 2003; Boummane andOlivier, 2007) and ~300 Ma in the Rehamna (Baudin et al.,2003) and ii) the lack of granite pebbles in the Autuniandeposits occurring at the rim of both massifs (Fig. 1b) testifiesthat the granites were still below the surface at 280 Ma. Then280 Ma represents a reasonable proxy for the initiation of thelow temperature cooling history.

(2) The presence of the samples at lower temperature duringTriassic times as Upper Triassic deposits are recovered on top ofthe Ouled Ouaslam granite and to the West of the Jebilet(Huvelin, 1977; Hollard et al., 1985), in the Bahira boreholesnorth of the Jebilet Fault, and in the outcrops west and north ofthe Rehamna Massif (Figs. 1b and 2b).

(3) The presence of the samples close to the surface at the end ofthe Lower Cretaceous prior to the overall Cenomanian–Turonian transgression which extends across North Africa(Guiraud et al., 2005; Frizon de Lamotte et al., 2008).

Results of the modelling show (Fig. 4):

(1) A rapid exhumation during Permian times which brings thesamples towards the surface. This exhumation is coeval withthe collapse and erosion of the Variscan chain. Distance to thesurface appears however different for the two massifs: theJebilet granite was at least partly at the surface (cf. onlap ofTriassic sediments) whereas the Rehamna massif was still atdepth.

(2) A phase of heating until the Toarcian–Bajocian (180–170 Ma).This episode is coeval with the accumulation of sediments

recorded in the adjoining Atlas Basins, Eastern Meseta Platformand Prerif domain (Charrière, 1990; Zizi, 2002).

(3) A renewed exhumation bringing the Rehamna granites andschists as well as the Jebilet granite up to the surface before thetransgression of the Cenomanian–Turonian, and probably asearly as the Barremian–Aptian, i.e. at 120–100 Ma (Gigout,1954; Bolleli et al., 1959; Baudin et al., 2003; Frizon de Lamotteet al., 2008).

A moderate burial during the Late Cretaceous–Eocene until 35–40 Ma, which corresponds to the last marine sedimentation in theAtlantic gulf where the phosphate series deposited over the Mesetaand Atlas domains (Boujo, 1976; Charrière, 1990; Herbig and Trappe,1994; Zouhri et al., 2008).

The T–t paths show two humps of the acceptable fit domain,whatever the sampled granitewill be, whereas good fits correspond toslightly higher T in the Rehamna with respect to the Jebilet. Thethermal amplitude defined by the good fit is small, as maximumtemperature remains always within the APAZ domain, with Tb100 °Cduring the earliest heating episode (at 180–170 Ma), and Tb80 °Cduring the latest (around 40 Ma).

5. Discussion

5.1. Regional implications

The Moroccan Meseta is currently regarded as a former, Triassic–Liassic subaerial land, raised between the Atlantic and Atlas rift basins:this is the classical “Terre des Almohades” (Choubert and Faure-Muret,1962), now referred to as the West Moroccan Arch (WMA: Hafid,2006; El Arabi, 2007). Contrastingly, we might infer from the heatingwhich affected the Jebilet and Rehamna granites up to 80bTb100 °C(Fig. 4) before their Late Jurassic–Early Cretaceous cooling that thesouthern Meseta basement subsided significantly during the Triassic–Middle Jurassic, a conclusion also reached independently by Ghorbalet al. (2008). In the case of the Jebilet granites, which were croppingout at 260–250 Ma (see above), heating would have attained 60–80 °C. With a conservative geothermal gradient of 30 °C/km, thiswould correspond to 2–2.4 km-thick sedimentary burial at 180–170 Ma, disregarding the shape of the geotherm close to the surface(Dempster and Persano, 2006). However, the geotherm was likelysteeper during the 200 Ma–185 Ma interval, due to the huge CAMPmagmatism (Knight et al., 2004; Verati et al., 2007). In particular, thewidespread barite veins of western Jebilet yield evidence of pervasivehydrothermal activity during the Triassic–Middle Jurassic Atlanticopening (Valenza et al., 2000). Likewise, based on K–Ar analysis of theb0.2–0.4 µmmicas in the Cambrian metapelites and Triassic argillites,Huon et al. (1993) documented the occurrence of a Triassic–Liassicthermal event (195±4 Ma, locally 184±4 Ma) in western Meseta.Therefore, assuming a geothermal gradient of 40 °C/km, burial of theWMA could have been limited to 1.5–2 km, less than the N3 km valueproposed by Ghorbal et al. (2008).

The 1.5–2 km burial here restored compares with the thickness ofthe Triassic–Liassic sequences preserved, respectively, i) west andnorthwest of the Jebilet Massif, beneath the Upper Jurassic–LowerCretaceous unconformable sequences, i.e. 2–2.5 km in the Essaouira 1well (Hafid, 2006), and 1.5–2 km in the seismic profiles from theDoukkala-Abda Basin (Echarfaoui et al., 2002a,b); ii) north andnortheast of the Moroccan Meseta, i.e. 1–1.5 km in the Prerif Ridges(Sani et al., 2007), as well as in the Tabular Middle Atlas (Charrière,1990; Gomez et al., 1996) and the Selloum Basin south of it (El Arabiet al., 2001, 2004). The contemporaneous deposits in the Atlas basinsare thicker (~2–3.5 km), and subsidence continued there during theDogger, so as the thickness of the sequences predating the LateJurassic–Early Cretaceous regression attain ~3.5 km in the MiddleAtlas (Charrière, 1990) and along the northern border of the Central

Fig. 7. Interpretation of the AFT results obtained on the basementsmassifs of the southernMoroccanMeseta (diagrammatic cross-sections along the same trace as Fig. 2b). (a) At ~250Ma(Late Permian), the early and shallow Jebilet granites are totally exhumed, whereas the deeper and younger Rehamna granites are still overlain by ~2 km of Paleozoic rocks.— (b) At themaximumof the Triassic–Liassic subsidence, i.e. at 180–170Ma (latest Liassic–early Dogger), heating is comparable in bothmassifs, suggesting a southward thickening of the sedimentaryburial.— (c) Late Eocene (40–35Ma) heating event. The Late Jurassic–Early Cretaceous uplift and erosion (not shown) have completed the denudation of the Rehamna granites, whereasremnants of Triassic–Jurassic sequences are preserved southward. The Meseta basement is buried beneath the Upper Cretaceous–Eocene series, the thickness of which slightly increasessouth–westward.— (d) During the Atlas Orogeny, theMeseta Domain itself has been deformed, particularly close to the High Atlas (not shown, south of the Tadla and Haouz Basins). Thebasementmassifs are exhumedand cooledbelow the apatite PAZ temperature. Theverticalmovement is related to amajor reverse fault in the Jebilet, and to avery largewave-length crustalfold in the Rehamna.

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High Atlas (Ellouz et al., 2003), and ~8 km in the axis of the Central-Eastern High Atlas (Studer, 1987, Warme, 1988). In contrast, therelatively moderate Triassic–Liassic subsidence suggested by our AFTdata for the southern part of theWesternMeseta compares favourablywith that of the High Moulouya–Missour Basin in Eastern (Oran)Meseta, which varies from 1 to 2 km (Beauchamp et al., 1996; Ellouzet al., 2003). To conclude, theWestern (Moroccan)Mesetawould havebeen a submarine high in the Triassic–Liassic paleogeography ofMorocco, comparable to the Eastern (Oran) Meseta, instead of beingan emergent land as postulated up to now. The partitioned Liassic highformed by the Tabular Middle Atlas and Selloum Basin (El Arabi et al.,2001) could be an image of the WMA, prior to its Late Jurassic–EarlyCretaceous uplift and erosion.

Another implication of the reported AFT results (Fig. 4) is theimportance of the Late Jurassic–Early Cretaceous uplift which affectedthe Jebilet and RehamnaMassifs, and likely the entire WMA, resultingin the very active erosion (Fig. 5) of the Triassic–Jurassic cover andunderlying basement. This phase of uplift (responsible for the secondhump of the T–t paths) is coeval with the emersion of most of the Atlasdomain, which was covered by widespread red beds of Bathonian–Barremian age, partly sourced from the rising WMA (Charrière et al.,1994, 2005; Frizon de Lamotte et al., 2008). Discussing the geodynamicmeaning of this uplift event should be beyond the scope of this paper(see Frizon de Lamotte et al., 2009-this issue).

Following the Late Jurassic–Early Cretaceous uplift, thermalmodelling indicates that the south-western Meseta underwent aphase of heating up to ~60°–80 °C, modelled from Early Cretaceous till35–50 Ma. This would represent (assuming a gradient of 30 °C/km) aburial of 1.5–2 km, which compares with the 1 km value proposed byGhorbal et al. (2008). In fact, in the Phosphate Plateau, the preservedCretaceous–Eocene sequence is only 200–400 m thick as observed inindustrial wells (Bolleli et al., 1959; Anonymous, 1986), whereas itattains 1000 m in the Tadla and Bahira further south (Hafid, 2006;Hafid et al., 2006). Hence, in the southern part of the WMA, the“Thersitea slab” (Lutetian) which tops presently the Cretaceous–Tertiary tabular sequence must have been covered by ~1 km thickdeposits prior to the Late Eocene–Oligocene Atlas phase (Frizon deLamotte et al., 2008). Interestingly, gypsiferous marls up to 400 mthick are preserved above the Lutetian limestones in the Timhaditesyncline of Middle Atlas, i.e. in the westernmost part of theCretaceous–Tertiary marine gulf (Charrière, 1990; Herbig and Trappe,1994), beneath the unconformable, Oligocene (?) continental deposits(J. Hayane conglomerates: Martin, 1981; Charrière, 1990). We assumethat similar deposits accumulated up to greater thickness in thecentral and western part of the Cretaceous–Eocene gulf. This impliesthat the uplift and subsequent erosion of the WMA was importantduring the Atlas orogeny, consistent with the last part of the T–t pathsof both the Rehamna and JebiletMassifs (Fig. 4), and although only the

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latter is bounded by a major reverse fault (Fig. 3). However, crustalshortening, which really began by the Late Eocene (Frizon de Lamotte etal., 2009-this issue), only represents a rather little part of theWMA recentuplift; the most significant part results from the regional lithospherethinning also responsible for most of the Atlas and Anti-Atlas uplift(Teixell et al., 2005; Missenard et al., 2006; Babault et al., 2008).

5.2. General inference: role of the age and depth of emplacement of thesampled granites

The Jebilet and Rehamna granites yield heterogeneous AFT ages,i.e. 186–203Ma and 148–153Ma, respectively, although they followedonly slightly distinct T–t paths (Fig. 4). We argue in the following thatthis surprising AFT age discrepancy can be explained by thedifferences in the age and depth of emplacement of the studiedgranites.

The Jebilet granites emplaced as shallow stocks or laccoliths at~330 Ma, prior to and during the main folding event, in still weaklydeformed turbidite formations of late Early Carboniferous age (Essaifiet al., 2001, 2003; Boumanne and Olivier, 2007), i.e. in the upperstructural level at probably less than ~7 km depth. This estimation issupported by the petrology of the country-rock schists, characterizedby widespread crystallization of andalusite (Fig. 6).

Contrastingly, the Sebt Brikiine granite from the Rehamna Massif(Fig. 2b) yielded a Rb–Sr whole-rock age at 270Ma (Mrini et al., 1992),and emplaced probably at 300–290 Ma, as the subsequent array ofmicrodiorite dykes was locally dated at 285±6 Ma (U/Pb zircon;Baudin et al., 2003). This late-orogenic batholith, hardly older than theEarly Permian rhyolitic–dacitic volcanism, emplaced at the verybottom of the folded Paleozoic as shown by the detail mapping(Hoepffner et al., 1982; Baudin et al., 2003), i.e. at about 10 km depth(similar to theOulmes granite in the CentralMassif; Tahiri et al., 2007).The eastern part of the batholith and the adjoining apexes such as thestudied Ras-el-Abiod leucogranite emplaced at similar depth in thehigh grade unit of the Western Meseta Shear Zone (WMSZ; Hoepffneret al.,1982; Lagarde andMichard,1987;Michard et al., 2008). The latterunit, characterized by the widespread development of staurotide andkyanite, equilibratedfirst at about 15 kmdepth (Fig. 6) during the EarlyNamurian (ca. 330 Ma), which corresponds to the main Variscanfolding and metamorphic event in the entire WMSZ, including theCentral Jebilet (Fig. 1b). When the Sebt Brikiine granite and associatedapexes emplaced (i.e. at ~300Ma), the high-grade schistswere alreadyexhumed to the ~10 km depth of the granite apex due to extensionalcollapse (Aghzer and Arenas, 1995; Baudin et al., 2003) and erosion.Thus, exhumation of the metamorphic units during the 330–300 Mainterval can be estimated at about ~5 km in the Rehamna transect.

Therefore, by the eve of the Permian (300 Ma), the “just born”Rehamna granites and their country-rocks were located at 9–10 kmdepth. The 330 My-aged Jebilet granites have already been exhumed(as the Rehamna schists) by several kilometres from their initialemplacement depth (~7 km) up to shallow depth (about 3–4 km). TheJebilet granites were probably entering the APAZ at ~280 Ma (Fig. 4),being prone to reach the surface during the Late Permian (~250 Ma),and then to be overlain by Late Triassic deposits. Assuming a similar oreven slightly stronger exhumation (6–7 km?) of the Rehamna granitesbetween 300 and 250 Ma, they were still located at about 2–3 kmdepth during the Late Permian, consistent with the modelled T–t path(Fig. 4). Remarkably, the Zaer granite of NW Meseta (Fig. 1), whichdisplays the same structural characteristics and age of emplacementas the Rehamna batholith yielded also the same AFT results to Ghorbalet al. (2008).

In conclusion, the discrepancy between the older AFT ages yieldedby the Jebilet granites (186–202 Ma) with respect to the Rehamnagranite and country-rocks (148–158 Ma) can be explained by the factthat the former emplaced 30 My earlier and 3–4 km shallower thanthe latter, and then crossed the APAZ earlier than the older and deeper

Rehamna batholith (Fig. 7). One could wonder if some Permian faultcould have exhumed the Jebilet granite, leaving the Rehamna granitedeeper in the APAZ. This tentative hypothesis is contradicted byseveral observations: i) Autunian deposits are widespread all aroundthe Rehamna and Jebilet massifs, and the associated, synsedimentarynormal/wrench faults crosscut bothmassifs with a dominant NE trend(Saber et al., 2007); ii) the conspicuous, E-trending North-Jebiletreverse fault (NJF), similar to the other faults of the Atlas System,corresponds to a former Triassic–Early Jurassic normal fault invertedduring the Tertiary orogenic evolution, with a major reverse throwdated from the Neogene (Hafid, 2006; Frizon de Lamotte et al., 2008);at that time, both massifs were located at about the same shallowdepth and the reverse movement had few consequences, if any, on theAFT ages.

6. Conclusion

The AFT data presented here concern a major structural zone ofMorocco, i.e. the West Moroccan Arch (WMA) which constitutedduring the Late Permian–Middle Triassic the eastern shoulder of theCentral Atlantic rift and the north-western shoulder of the Atlas(Tethyan) rift. This zone acted as a relatively stable block of Variscancrust during the Mesozoic–Paleogene, being widely covered by thetabular Cretaceous–Eocene series of the Phosphate and GanntourPlateaus.

Our AFT results are based on samples collected in two basementmassifs of the southern WMA, namely the Jebilet and RehamnaMassifs. Remarkably, they yielded different ages, 203–186 Ma and148–153 Ma, respectively. The T–t paths produced are characterizedby a two-hump aspect. They demonstrate that the WMA subsidedduring the Triassic–Middle Jurassic before being uplifted and erodedduring the Late Jurassic–Early Cretaceous. Therefore, the previousconcept of a permanently subaerial Western Meseta prior to theCenomanian–Turonian transgression must be abandoned. Our resultssuggest that, during the Early-Middle Jurassic, the WMA could becompared to the Eastern Meseta–Missour Basin submarine high, withless than 2 km thick sedimentary cover.

Regarding the surprising discrepancies (40–50 My) between themean AFT ages from the studied massifs, they can be explained by thedifference in age and depth of emplacement of the sampled granites:the older and shallower granites crossed the APAZ earlier than theyounger and deeper ones. In other words, in both massifs, thesuccession of cooling and heating phases was identical as far as thechronology of erosion and subsidence events is considered, but thetemperature reached during the earliest phase of erosion has beendifferent. Thus, the initial structure and evolution of the basement ofany young tabular or mountainous domain has to be taken intoaccount in order to interpret the potential differences in the AFT agesobserved in the various basement rocks.

Acknowledgments

We are indebted to P. Van der Beek and E. Labrin (Joseph-FourierUniv., Grenoble) and D. Seward (Univ. of Zurich) for their help inirradiation process. Thanks are due to M. Hafid (Univ. of Kenitra) forhelpful discussions and to A. Charrière (Paul-Sabatier Univ., Toulouse)for enlightening comments. We acknowledge useful discussions withB. Ghorbal (Vrije Univ. Utrecht) during the MAPG-ILP congress,Marrakech 2007. This work has been supported by the French-Moroccan program Volubilis (Ma/05/125 and Ma/01/13).

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