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

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Apatite ﬁ ssion-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

aLaboratoire Géosciences, Faculté des Sciences, Université Hassan II Aïn Chock, BP. 5366 Maârif, Casablanca, Maroc, Morocco

b10 rue des Jeûneurs, 75002 Paris, France

cUniv Paris Sud, UMR CNRS 8148 IDES, Bâtiment 504, Orsay cedex, F-91405, France

dInstitut de Géologie, Université de Neuchâtel, 11 rue Emile-Argand, 2009 Neuchâtel, Suisse, France

eDépartement 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

a b s t r a c t a r t i c l e i n f o

Article history:

Received 11 April 2008

Received in revised form 15 December 2008 Accepted 2 January 2009

Available online 14 January 2009 Keywords:

Apatiteﬁssion-tracks Thermochronology Vertical movements Morocco Meseta Atlas

This work is based on apatiteﬁssion-track analysis of samples (mostly granites) from the basement of the Cretaceous–Tertiary Phosphate and Ganntour Plateaus, exposed in the Jebilet and Rehamna massifs (Western Meseta, Morocco). This basement belongs to the Carboniferous–Early Permian Variscan Belt, and the earlier marine 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 heating events during the Jurassic and the Eocene, respectively, with cooling during the Permian and Cretaceous intervals. These results are partly accounted for by considering a moderate 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 explore further explanation. We interpret the observed discrepancies by the difference in age and depth of crystallization of the sampled granites in the Variscan Orogen, i.e. 330 Ma, 5–6 km in the Jebiletversus~ 300 Ma, 8–10 km in the Rehamna. The importance of the Late Jurassic–Early Cretaceous uplift and erosion of the entire Meseta and that of its Late Eocene burial are emphasized.

1. Introduction

Thermochronology on apatite have been used recently in Morocco to 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

ﬁ

ssion track (AFT) study in a domain that is de

ﬁ

ned, according to the classical geological criteria (Choubert and Faure-Muret, 1962) as a tabular domain, namely the Moroccan (or Western) Meseta. In fact, this domain forms a relatively stable area bounded by Cenozoic mountain belts to the south, east and north (the High Atlas, Middle Atlas, and Rif belt, respectively), and by the Atlantic Ocean to the west (Fig. 1a). The Western Meseta region includes Paleozoic massifs characterized by Variscan deformation and metamorphism, varied granite intrusions, and late orogenic (Early Permian) continental basins. The Paleozoic basement is directly overlain either by Triassic

–

Liassic series (Tabular Middle Atlas) or by Cretaceous

–

Tertiary plateaus (

“

Plateau des Phosphates

”

and Gann- tour), and surrounded by Neogene basins (Bahira-Tadla, Haouz,

Doukkala, External Rif foredeep). The Cretaceous

–

Tertiary plateaus have been intensely studied for phosphate exploitation (three quarters of the phosphate reserves of the world), oil and deep water resources.

Ultimately, Ghorbal et al. (2008) have presented AFT data from the northern (Zaer) and central (Rehamna) parts of Western Meseta, respectively. Our independent study is mainly based on sampling in the southernmost Meseta massif, i.e. the Jebilet Massif, and on additional sampling in the Rehamna Massif. The Jebilet Massif culminates at 1050 m above sea level (a.s.l.), being bounded to the north by a major Neogene (Atlasic) reverse fault (Ha

ﬁ

d, 2006; Ha

ﬁ

d et 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

”

part of the Meseta, barely affected by the Atlas orogeny.

The striking hiatus of Triassic

–

Liassic deposits over most of Western Meseta is classically interpreted as related to the occurrence, during the Triassic

–

Liassic, of an emergent land between the Atlantic margin and the Atlas basins. Choubert and Faure-Muret (1960

–

62) coined the name of

“

Terre des Almohades

”

for this allegedly emergent domain, which is currently referred to as the West Moroccan Arch (WMA; Ha

ﬁ

d, 2006; El Arabi, 2007). Our AFT data allow us to discuss

Tectonophysics 475 (2009) 29–37

⁎Corresponding author.

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

doi:10.1016/j.tecto.2009.01.007

Contents lists available atScienceDirect

Tectonophysics

j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t e c to

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this classical description, and to suggest that, in fact, the WMA subsided before being eroded during the Late Jurassic

–

Early Cretaceous.

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

belonging to the Jebilet and Rehamna massifs, respectively. Discussing these discrepancies represents one of the most signi

ﬁ

cant topics of the present study. Our study points to the importance of considering not only the evolution of the subsidence/uplift history of a tabular area, but also the structure and evolution of its basement, as the depth and

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 basement massifs and surrounding areas, afterHollard et al. (1985)andHoepffner 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.

Fig. 2.(a) Topographic model of the Moroccan Meseta (GETOPO data) and surrounding areas, with location of the studied granites. Bold line: approximate trace of cross-section (b).

Asterisk: location of the photosFig. 5. (b) Geological cross-section of the Moroccan Meseta south of the Central Massif, located on (a) and more exactly inFig. 1b. Notice the strong vertical exaggeration. The shape of the granite intrusions is diagrammatic.

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

2. Geological setting

The basement of the Western (Moroccan) Meseta belongs to the southern branch of the Variscan Orogen, within which a number of granite stocks emplaced during the Visean to Early Permian interval (Hoepffner et al., 2006; Michard et al., 2008). In particular, the Jebilet granites are dated at 330 Ma (U

–

Pb zircon; Essai

ﬁ

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

ﬁ

rst during the Late Carboniferous

–

Early Permian (Hmich et al., 2006; Saber et al., 2007). Then, the rifting associated with the Pangaea break-down occurred on both sides of the future Western Meseta, i.e. in the Central

Atlantic (Favre et al., 1991; Medina, 1995; Ha

ﬁ

d, 2006) and Atlas (Tethyan) Gulf (Zizi, 2002; El Arabi et al., 2006a,b), respectively. Rifting occurred during the Late Permian

–

Late Triassic, ending temporarily at 200 Ma with the emplacement of the basalts of the Central Atlantic Magmatic 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 basement units, from north to south, the Central Massif and Coastal Meseta, the Rehamna and the Jebilet (Fig. 1b). Triassic and Liassic cover sequences are preserved only at the fringe of Western Meseta, either exposed as outcrops to the northeast and north (e.g. Tabular Middle Atlas, Prerif Ridges; Fig. 1a; Hollard et al., 1985) or documented in the subsurface to the southeast (Tadla, Bahira), southwest (Essaouira

Fig. 3.Sampling maps and AFT results (mean ages). (a) Central Jebilet; geologic contours afterHuvelin (1977). (b) Central Rehamna; geologic contours afterMichard (1982). SeeFig.

1b for location.

Table 1

Apatiteﬁssion-track analyses of Jebilet and Rehamna samples.

Sample Lithology n ρs Ns ρi Ni ρd Nd P(χ²) FT age N L Dpar

× 10⁵t/cm² × 10⁶t/cm² × 10⁵t/cm² % Ma ± 1σ µm ± 1SD µm

Jebilet massif

JGO1 Granodiorite 26 7.76 5496 3.76 2224 5.41 16,464 99.9 193 ± 5 111 11.90 ± 2.12 1.48 ± 0.12

JOG3 Granodiorite 29 6.21 4029 2.91 1891 5.41 16,464 99.1 199 ± 6 – – 1.43 ± 0.19

JGO4 Granodiorite 26 5.55 4881 2.72 2394 5.41 16,464 100 190 ± 5 100 11.07 ± 2.27 1.28 ± 0.12

JTB2 Granophyre 29 6.52 4229 3.28 2126 5.41 16,464 99.3 186 ± 5 80 11.37 ± 2.45 1.32 ± 0.10

JTB3 Granophyre 24 1.81 2482 8.47 1163 5.41 16,464 100 199 ± 7 – – 1.31 ± 0.10

JTB4 Granophyre 26 2.05 2702 9.42 1243 5.41 16,464 99.8 203 ± 7 – – 1.27 ± 0.13

Rehamna massif

RH1 Schist 11 1.90 633 1.80 598 8.52 17,645 99.9 148 ± 9 – – 1.75 ± 0.14

RH4B Schist 10 0.19 168 0.18 154 8.52 17,645 99.8 153 ± 18 – – 1.83 ± 0.15

RH8 Leucogranite 15 3.88 649 3.62 605 8.52 17,645 99.6 150 ± 9 100 12.43 ± 1.96 1.30 ± 0.09

RH9A Leucogranite 15 4.00 997 3.55 885 8.52 17,645 100 153 ± 8 100 11.91 ± 1.94 1.49 ± 0.13

SeeFig. 3for sample location.

n,NsandNi, respectively number of crystals dated, total number of spontaneous and induced tracks counted; s and i, respectively spontaneous and induced track density in apatite grains and their detectors (JGO-JTB: kapton; RH: muscovite);d, means induced track density in the detectors associated to NIST neutron glass monitors 962 (JGO-JTB) and CN5 (RH). P(2) is the probability of obtaining a 2 value for n-1 degrees of freedom. As all samples passed the test at a 95% conﬁdence level with P(2)N5%, ages were calculated using pooled statistics (Green, 1981). Zeta value of F.Z. El Haimer, analyst, 350 17 (JGO-JTB) and O. Saddiqi, analyst, 333 8 (RH) (1). Conﬁned tracks: L and l s.d. are respectively the mean value and standard deviation, (N) is the number of tracks measured.

O. Saddiqi et al. / Tectonophysics 475 (2009) 29–37 31

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Basin; Ha

ﬁ

d, 2006) and west (Doukkala, Abda; Echarfaoui et al., 2002a,b). Elsewhere, the Western Meseta basement is onlapped by younger deposits, Late Jurassic

–

Early Cretaceous in the Mouissat hills west 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 are summarized in the cross-section inFig. 2. The latter

ﬁ

gure also makes clear that the southernmost part of the Meseta has been involved in the Atlas shortening, with the Jebilet Massif thrust over the Bahira Triassic

–

Neogene deposits through a southward-dipping reverse fault. In contrast, further to the north, the Meseta basement and overlying Cretaceous

–

Eocene tabular sequence are only affected by much weaker deformation.

3. Sampling and experimental procedure

Eight samples out of ten (Fig. 3; Table 1) have been collected from granites, among which three from the east Central Jebilet (Ouled Ouaslam granite laccolith; Boummane and Olivier, 2007), three from the west Central Jebilet (Tabouchent granophyric granite; Huvelin, 1977; Essai

ﬁ

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

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 the external detector technique using kapton foils (Jebilet samples) or muscovite (Rehamna samples). Tracks were etched in apatite with 1 M HNO

₃

solution at 20 °C for 45 s, in kapton using a boiling solution of potassium hypochlorite for 8 min, and in muscovite in 40% HF for 45 min. For each sample, a single age population is observed. Dpar has been measured for each sample (

ﬁ

ve measurements per grain).

Horizontal con

ﬁ

ned track length (TINTS) measurements have been performed on

ﬁ

ve samples using a digitising tablet (Table 1).

4. Results

Our AFT analyses on the Jebilet granites show AFT ages grouped between 202.6 ± 7.1 and 185.7 ± 5.1 Ma (Table 1). The Mean Track Length (MTL) varies from 11.90 to 11.07 m with standard deviations in the range 2.4

–

2.1 m. These results are consistent with those obtained by Mansour (1991) using the population method, with AFT ages ranging 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

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

Fig. 5.The Cretaceous transgression at the northern border of the Rehamna massif, as seen in the Oum-er-Rbia valley (seeFig. 2a for location). (a) Overview of the Lower Cretaceous (LC)—Cenomanian–Turonian (C–T) escarpment (about 100 m high) above the Paleozoic basement (Cb: Middle Cambrian). (b) The major unconformity at the bottom of the Lower Cretaceous red beds is marked by coarse conglomerates with poorly sorted, barely rounded pebbles of Ordovician quartzites likely sourced in the Central Rehamna (3 m high roadcut). S1: Variscan cleavage. (For interpretation of the references to colour in thisﬁgure legend, the reader is referred to the web version of this article.)

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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 the range 12.43

–

11.91 m (less by ~ 1 m than the MTL given by Ghorbal et al.) with standard deviation 1.96

–

1.94 m. Dpar values are homogeneous within samples and are between 1.27 and 1.83. There is no signi

ﬁ

cant variation between the Dpar values for the two studied massifs.

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 total annealing at 280 Ma as i) the studied granites are dated at 330 Ma in the Jebilet (Essai

ﬁ

et al., 2003; Boummane and Olivier, 2007) and ~300 Ma in the Rehamna (Baudin et al., 2003) and ii) the lack of granite pebbles in the Autunian deposits occurring at the rim of both massifs (Fig. 1b) testi

ﬁ

es that the granites were still below the surface at 280 Ma. Then 280 Ma represents a reasonable proxy for the initiation of the low temperature cooling history.

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

(3) The presence of the samples close to the surface at the end of the 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 the samples towards the surface. This exhumation is coeval with the collapse and erosion of the Variscan chain. Distance to the surface appears however different for the two massifs: the Jebilet granite was at least partly at the surface (cf. onlap of Triassic sediments) whereas the Rehamna massif was still at depth.

(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 Platform and Prerif domain (Charrière, 1990; Zizi, 2002).

(3) A renewed exhumation bringing the Rehamna granites and schists as well as the Jebilet granite up to the surface before the transgression of the Cenomanian

–

Turonian, and probably as early as the Barremian

–

Aptian, i.e. at 120

–

100 Ma (Gigout, 1954; Bolleli et al., 1959; Baudin et al., 2003; Frizon de Lamotte et al., 2008).

A moderate burial during the Late Cretaceous

–

Eocene until 35

–

40 Ma, which corresponds to the last marine sedimentation in the Atlantic gulf where the phosphate series deposited over the Meseta and Atlas domains (Boujo, 1976; Charrière, 1990; Herbig and Trappe, 1994; Zouhri et al., 2008).

The

T–t

paths show two humps of the acceptable

ﬁ

t domain, whatever the sampled granite will be, whereas good

ﬁ

ts correspond to slightly higher

T

in the Rehamna with respect to the Jebilet. The thermal amplitude de

ﬁ

ned by the good

ﬁ

t is small, as maximum temperature remains always within the APAZ domain, with

Tb

100 °C during the earliest heating episode (at 180

–

170 Ma), and

Tb

80 °C during 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: Ha

ﬁ

d, 2006; El Arabi, 2007). Contrastingly, we might infer from the heating which affected the Jebilet and Rehamna granites up to 80

bTb

100 °C (Fig. 4) before their Late Jurassic

–

Early Cretaceous cooling that the southern Meseta basement subsided signi

ﬁ

cantly during the Triassic

–

Middle Jurassic, a conclusion also reached independently by Ghorbal et al. (2008). In the case of the Jebilet granites, which were cropping out at 260

–

250 Ma (see above), heating would have attained 60

–

80 °C. With a conservative geothermal gradient of 30 °C/km, this would 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 likely steeper during the 200 Ma

–

185 Ma interval, due to the huge CAMP magmatism (Knight et al., 2004; Verati et al., 2007). In particular, the widespread barite veins of western Jebilet yield evidence of pervasive hydrothermal activity during the Triassic

–

Middle Jurassic Atlantic opening (Valenza et al., 2000). Likewise, based on K

–

Ar analysis of the

b

0.2

–

0.4 µm micas in the Cambrian metapelites and Triassic argillites, Huon et al. (1993) documented the occurrence of a Triassic

–

Liassic thermal event (195 ± 4 Ma, locally 184 ± 4 Ma) in western Meseta.

Therefore, assuming a geothermal gradient of 40 °C/km, burial of the WMA could have been limited to 1.5

–

2 km, less than the

N

3 km value proposed by Ghorbal et al. (2008).

The 1.5

–

2 km burial here restored compares with the thickness of the Triassic

–

Liassic sequences preserved, respectively, i) west and northwest of the Jebilet Massif, beneath the Upper Jurassic

–

Lower Cretaceous unconformable sequences, i.e. 2

–

2.5 km in the Essaouira 1 well (Ha

ﬁ

d, 2006), and 1.5

–

2 km in the seismic pro

ﬁ

les from the Doukkala-Abda Basin (Echarfaoui et al., 2002a,b); ii) north and northeast 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 Arabi et al., 2001, 2004). The contemporaneous deposits in the Atlas basins are thicker (~2

–

3.5 km), and subsidence continued there during the Dogger, so as the thickness of the sequences predating the Late Jurassic

–

Early Cretaceous regression attain ~3.5 km in the Middle Atlas (Charrière, 1990) and along the northern border of the Central

Fig. 6.EstimatedP–Tconditions of crystallization of the Jebilet and Rehamna schists and

granites plotted on the petrogenetic grid, based on the mineral associations described by Essaiﬁet al. (2001)andHoepffner et al. (1982), respectively. Dashed: high-temperature P–T–tpaths of the studied samples. Horizontally ruled: andalusite–sillimanite transition (Pattison, 1992). Vertically ruled: cordierite–garnet transition (Holdaway and Lee (1977). Staurolite–cordierite experimental curve afterRichardson (1968). Peraluminous granite melt afterWillye (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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High Atlas (Ellouz et al., 2003), and ~ 8 km in the axis of the Central- Eastern High Atlas (Studer, 1987, Warme, 1988). In contrast, the relatively moderate Triassic

–

Liassic subsidence suggested by our AFT data for the southern part of the Western Meseta compares favourably with that of the High Moulouya

–

Missour Basin in Eastern (Oran) Meseta, which varies from 1 to 2 km (Beauchamp et al., 1996; Ellouz et al., 2003). To conclude, the Western (Moroccan) Meseta would have been a submarine high in the Triassic

–

Liassic paleogeography of Morocco, comparable to the Eastern (Oran) Meseta, instead of being an emergent land as postulated up to now. The partitioned Liassic high formed 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

–

Early Cretaceous uplift and erosion.

Another implication of the reported AFT results (Fig. 4) is the importance of the Late Jurassic

–

Early Cretaceous uplift which affected the Jebilet and Rehamna Massifs, and likely the entire WMA, resulting in the very active erosion (Fig. 5) of the Triassic

–

Jurassic cover and underlying basement. This phase of uplift (responsible for the second hump of the

T–t

paths) is coeval with the emersion of most of the Atlas domain, 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 geodynamic meaning 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, thermal modelling indicates that the south-western Meseta underwent a phase of heating up to ~ 60°

–

80 °C, modelled from Early Cretaceous till 35

–

50 Ma. This would represent (assuming a gradient of 30 °C/km) a burial of 1.5

–

2 km, which compares with the 1 km value proposed by Ghorbal et al. (2008). In fact, in the Phosphate Plateau, the preserved Cretaceous

–

Eocene sequence is only 200

–

400 m thick as observed in industrial wells (Bolleli et al., 1959; Anonymous, 1986), whereas it attains 1000 m in the Tadla and Bahira further south (Ha

ﬁ

d, 2006;

Ha

ﬁ

d 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 thick

deposits prior to the Late Eocene

–

Oligocene Atlas phase (Frizon de

Lamotte et al., 2008). Interestingly, gypsiferous marls up to 400 m

thick are preserved above the Lutetian limestones in the Timhadite

syncline of Middle Atlas, i.e. in the westernmost part of the

Cretaceous

–

Tertiary marine gulf (Charrière, 1990; Herbig and Trappe,

1994), beneath the unconformable, Oligocene (?) continental deposits

(J. Hayane conglomerates: Martin, 1981; Charrière, 1990). We assume

that similar deposits accumulated up to greater thickness in the

central and western part of the Cretaceous

–

Eocene gulf. This implies

that the uplift and subsequent erosion of the WMA was important

during the Atlas orogeny, consistent with the last part of the

T–t

paths

of both the Rehamna and Jebilet Massifs (Fig. 4), and although only the

Fig. 7.Interpretation of the AFT results obtained on the basements massifs of the southern Moroccan Meseta (diagrammatic cross-sections along the same trace asFig. 2b). (a) At ~250 Ma (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 the maximum of the Triassic–Liassic subsidence, i.e. at 180–170 Ma (latest Liassic–early Dogger), heating is comparable in both massifs, suggesting a southward thickening of the sedimentary burial.—(c) Late Eocene (40–35 Ma) heating event. The Late Jurassic–Early Cretaceous uplift and erosion (not shown) have completed the denudation of the Rehamna granites, whereas remnants of Triassic–Jurassic sequences are preserved southward. The Meseta basement is buried beneath the Upper Cretaceous–Eocene series, the thickness of which slightly increases south–westward.—(d) During the Atlas Orogeny, the Meseta Domain itself has been deformed, particularly close to the High Atlas (not shown, south of the Tadla and Haouz Basins). The basement massifs are exhumed and cooled below the apatite PAZ temperature. The vertical movement is related to a major reverse fault in the Jebilet, and to a very large wave-length crustal fold in the Rehamna.

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latter is bounded by a major reverse fault (Fig. 3). However, crustal shortening, which really began by the Late Eocene (Frizon de Lamotte et al., 2009-this issue), only represents a rather little part of the WMA recent uplift; the most signi

ﬁ

cant part results from the regional lithosphere thinning 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 the sampled granites

The Jebilet and Rehamna granites yield heterogeneous AFT ages, i.e. 186

–

203 Ma and 148

–

153 Ma, respectively, although they followed only slightly distinct

T–t

paths (Fig. 4). We argue in the following that this surprising AFT age discrepancy can be explained by the differences in the age and depth of emplacement of the studied granites.

The Jebilet granites emplaced as shallow stocks or laccoliths at

~ 330 Ma, prior to and during the main folding event, in still weakly deformed turbidite formations of late Early Carboniferous age (Essai

ﬁ

et al., 2001, 2003; Boumanne and Olivier, 2007), i.e. in the upper structural level at probably less than ~ 7 km depth. This estimation is supported by the petrology of the country-rock schists, characterized by 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 270 Ma (Mrini et al., 1992), and emplaced probably at 300

–

290 Ma, as the subsequent array of microdiorite dykes was locally dated at 285 ± 6 Ma (U/Pb zircon;

Baudin et al., 2003). This late-orogenic batholith, hardly older than the Early Permian rhyolitic

–

dacitic volcanism, emplaced at the very bottom 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 the Oulmes granite in the Central Massif; Tahiri et al., 2007).

The eastern part of the batholith and the adjoining apexes such as the studied Ras-el-Abiod leucogranite emplaced at similar depth in the high grade unit of the Western Meseta Shear Zone (WMSZ; Hoepffner et al.,1982; Lagarde and Michard, 1987; Michard et al., 2008). The latter unit, characterized by the widespread development of staurotide and kyanite, equilibrated

ﬁ

rst at about 15 km depth (Fig. 6) during the Early Namurian (ca. 330 Ma), which corresponds to the main Variscan folding and metamorphic event in the entire WMSZ, including the Central Jebilet (Fig. 1b). When the Sebt Brikiine granite and associated apexes emplaced (i.e. at ~ 300 Ma), the high-grade schists were already exhumed to the ~10 km depth of the granite apex due to extensional collapse (Aghzer and Arenas, 1995; Baudin et al., 2003) and erosion.

Thus, exhumation of the metamorphic units during the 330

–

300 Ma interval 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 km depth. The 330 My-aged Jebilet granites have already been exhumed (as the Rehamna schists) by several kilometres from their initial emplacement depth (~ 7 km) up to shallow depth (about 3

–

4 km). The Jebilet 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 or even slightly stronger exhumation (6

–

7 km?) of the Rehamna granites between 300 and 250 Ma, they were still located at about 2

–

3 km depth during the Late Permian, consistent with the modelled

T–t

path (Fig. 4). Remarkably, the Zaer granite of NW Meseta (Fig. 1), which displays the same structural characteristics and age of emplacement as the Rehamna batholith yielded also the same AFT results to Ghorbal et al. (2008).

In conclusion, the discrepancy between the older AFT ages yielded by the Jebilet granites (186

–

202 Ma) with respect to the Rehamna granite and country-rocks (148

–

158 Ma) can be explained by the fact that the former emplaced 30 My earlier and 3

–

4 km shallower than the latter, and then crossed the APAZ earlier than the older and deeper

Rehamna batholith (Fig. 7). One could wonder if some Permian fault could have exhumed the Jebilet granite, leaving the Rehamna granite deeper in the APAZ. This tentative hypothesis is contradicted by several observations: i) Autunian deposits are widespread all around the Rehamna and Jebilet massifs, and the associated, synsedimentary normal/wrench faults crosscut both massifs with a dominant NE trend (Saber et al., 2007); ii) the conspicuous, E-trending North-Jebilet reverse fault (NJF), similar to the other faults of the Atlas System, corresponds to a former Triassic

–

Early Jurassic normal fault inverted during the Tertiary orogenic evolution, with a major reverse throw dated from the Neogene (Ha

ﬁ

d, 2006; Frizon de Lamotte et al., 2008);

at that time, both massifs were located at about the same shallow depth and the reverse movement had few consequences, if any, on the AFT ages.

6. Conclusion

The AFT data presented here concern a major structural zone of Morocco, i.e. the West Moroccan Arch (WMA) which constituted during the Late Permian

–

Middle Triassic the eastern shoulder of the Central Atlantic rift and the north-western shoulder of the Atlas (Tethyan) rift. This zone acted as a relatively stable block of Variscan crust during the Mesozoic

–

Paleogene, being widely covered by the tabular Cretaceous

–

Eocene series of the Phosphate and Ganntour Plateaus.

Our AFT results are based on samples collected in two basement massifs of the southern WMA, namely the Jebilet and Rehamna Massifs. Remarkably, they yielded different ages, 203

–

186 Ma and 148

–

153 Ma, respectively. The

T–t

paths produced are characterized by a two-hump aspect. They demonstrate that the WMA subsided during the Triassic

–

Middle Jurassic before being uplifted and eroded during the Late Jurassic

–

Early Cretaceous. Therefore, the previous concept of a permanently subaerial Western Meseta prior to the Cenomanian

–

Turonian transgression must be abandoned. Our results suggest that, during the Early-Middle Jurassic, the WMA could be compared to the Eastern Meseta

–

Missour Basin submarine high, with less than 2 km thick sedimentary cover.

Regarding the surprising discrepancies (40

–

50 My) between the mean AFT ages from the studied massifs, they can be explained by the difference in age and depth of emplacement of the sampled granites:

the older and shallower granites crossed the APAZ earlier than the younger and deeper ones. In other words, in both massifs, the succession of cooling and heating phases was identical as far as the chronology of erosion and subsidence events is considered, but the temperature reached during the earliest phase of erosion has been different. Thus, the initial structure and evolution of the basement of any young tabular or mountainous domain has to be taken into account in order to interpret the potential differences in the AFT ages observed in the various basement rocks.

Acknowledgments

We are indebted to P. Van der Beek and E. Labrin (Joseph-Fourier Univ., Grenoble) and D. Seward (Univ. of Zurich) for their help in irradiation process. Thanks are due to M. Ha

ﬁ

d (Univ. of Kenitra) for helpful discussions and to A. Charrière (Paul-Sabatier Univ., Toulouse) for enlightening comments. We acknowledge useful discussions with B. 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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