Issuu on Google+

doi: 10.1111/j.1365-3121.2008.00828.x

Unexpected Jurassic to Neogene vertical movements in ‘stable’ parts of NW Africa revealed by low temperature geochronology B. Ghorbal,1,2 G. Bertotti,3 J. Foeken4 and P. Andriessen1,2 1

Department of Isotope Geochemistry, VU University Amsterdam, De Boelelaan 1085, 1081 HV Amsterdam, NL; 2Netherlands Research Centre for Integrated Solid Earth Science (ISES), De Boelelaan 1085, 1081 HV Amsterdam, NL; 3Department Tectonics, VU University De Boelelaan 1085, 1081 HV Amsterdam, NL; 4Isotope Geoscience Unit, Scottish Universities Environmental Research Centre, East Kilbride G75 0QF, UK

ABSTRACT In Morocco, it is generally considered that post-Hercynian vertical movements were limited to the Atlas system, the passive continental margin and the Rif. Apatite FT and He ages from the Moroccan Meseta (Rehamna and Zaer Massif) document instead two episodes of subsidence and exhumation in Jurassic-Early Cretaceous and during the Late Cretaceous to Neogene. The Meseta subsided to >3 km depth during the Late Triassic to Middle Jurassic and was exhumed to the surface before the Late Cretaceous, during the rift and post-rift stages

Introduction An approximately 500 m thick interval of Lower Cretaceous terrigenous sands was drilled at DSDP sites 370 and 416, offshore Morocco (Fig. 1) (e.g. Price, 1981). The presence of grains of quartz, biotite, zircons and micaschists documents a major episode of erosion in the adjacent domains of NW Africa (Price, 1981). The terrigenous interval has been reported for the entire NW African margin as far S as Guinea (Davison, 2005). A coeval, similar terrigenous succession is also found on the Canadian conjugate margin (Nova Scotia Basin) where it is associated with poorly defined Late Cimmerian contractional deformations (e.g. Jansa and Wiedmann, 1982). Early Cretaceous erosion ⁄ exhumation is enigmatic as it is younger than Atlas and Central Atlantic rifting (Medina, 1995; Le Roy and Pique´, 2001; Ellouz et al., 2003), older than first stages of relief forming in the Rif and Atlas (e.g. Frizon de Lamotte et al., 2000; Chalouan et al., 2001) and is at odds with the notion that NW Africa was roughly stable at that time. In particular, the Moroccan Meseta, the region between the Rif Mountains, the High- and Middle Atlas and the Correspondence: B. Ghorbal, Department of Isotope Geochemistry, FALW, VU Amsterdam, De Boelelaan 1085, 1081 HV Amsterdam, The Netherlands. E-mail: badr.ghorbal@falw.vu.nl

 2008 Blackwell Publishing Ltd

of Central Atlantic opening. Erosion of the exhuming rocks is responsible for a thick package of terrigenous sands found in the Moroccan offshore and elsewhere along the NW Africa margin. About 1 km of subsidence affected the Meseta during the Late Cretaceous to Eocene. During the Neogene, these areas were brought back to the surface in association with bimodal folding with wavelengths of 100–150 km and >500 km.

Terra Nova, 20, 355–363, 2008

Atlantic margin (Fig. 1) is generally recognized as an area of no major post-Hercynian vertical movements (e.g. Michard, 1976; Michard et al., 1989; Guiraud et al., 2005). To constrain exhumation ⁄ erosion patterns in Morocco, we have sampled a >500-km long transect from the Mediterranean to the Anti-Atlas for fission track (AFT) and (U-Th) ⁄ He thermochronology on apatites. These methods constrain cooling histories in the upper crust (temperatures in the 120–45 C range) (Ehlers, 2005). The Moroccan Meseta has provided unexpectedly young ages, which are here documented and interpreted. The geology of the Moroccan Meseta (Fig. 1) is characterized by two WNW–ESE-trending areas of outcropping Palaeozoic rocks, the Rehamna region and the Zaer Massif, separated by an area of Upper Cretaceous to Eocene marine sediments called Settat zone (plateau des phosphates). Palaeozoic rocks are generally Hercynian schists locally intruded and contact-metamorphosed by granodiorites, monzogranite and monzonite to monzodiorite (Michard, 1976; Huon et al., 1987; Michard et al., 1989; Haı¨ meur et al., 2004; Hoepffner et al., 2005). In several localities, as the Jebilet hills, such intrusive and metamorphic rocks are stratigraphically overlain by Triassic sediments documenting their arrival at the surface before Triassic times (Michard, 1976). A pre-Triassic exhumation of

Meseta rocks is compatible with petrological data from intrusive and metamorphic rocks (Mrini et al., 1992; El Mahi et al., 2000 respectively) and is typical of the Hercynian belt. The widespread occurrence of shallow marine to transitional Triassic sediments over large parts of the Meseta (e.g. Michard, 1976) documents coeval gentle subsidence and absence of significant relief. The Meseta basement rocks and their thin and discontinuous Triassic cover are unconformably overlain by Lower Cretaceous red beds in the Rehamna area and by Upper Cretaceous marine sediments in the Zaer Massif. Towards the west, the Meseta passes to Central Atlantic sedimentary basins hosting generally marine and continuous Mesozoic sedimentary successions (e.g. Medina, 1995; Le Roy and Pique´, 2001). We present AFT and (U-Th) ⁄ He (AHe) ages from samples of intrusive and metamorphic rocks from Rehamna and the Zaer Massif (Fig. 2a and Tables 1 and 2). Intrusives are biotitebearing granodiorites cut by monzogranite (Haı¨ meur et al., 2004) and fine-grained monzonite to monzodiorite. Metamorphic rocks are contactmetamorphosed Hercynian schists (Huon et al., 1987). Samples have been processed at the Department of Isotope Geochemistry of the VU University (Amsterdam) and at SUERC (Glasgow) (see the Appendix for full details). 355


Unexpected vertical movements in NW Africa • B. Ghorbal et al.

Terra Nova, Vol 20, No. 5, 355–363

.............................................................................................................................................................

Fig. 1 Tectonic map of Morocco and adjacent regions. The box corresponds to the area covered by Fig. 2a. The position of DSDP wells 370 and 416 is indicated. Modified after Hafid (2006).

Results and interpretation Analytical results Apatites of monzonite (samples 4, 5 and 7) and metamorphic rocks (sample 9) are of poor crystallographic quality, have low U or ⁄ and Th- contents (<10 p.p.m.), low density of induced fission tracks and display regular [U] distributions. Monzogranites (samples 1 and 6) and granodiorites (samples 2, 3 and 8) have large and well-shaped apatite crystals, high U and Th contents and U is heterogeneously distributed throughout the crystal. Apatite fission tracks Fission track ages are between 143 and 148 Ma (Fig. 2a and Table 1). 356

Measurement of confined induced fission tracks yielded mean track lengths (MTL) between 13.01 ± 0.25 lm and 13.70 ± 0.11 lm with a narrow length distribution. The MTL distribution and the abundance of long tracks are compatible with rapid cooling through the apatite PAZ and long residence time at shallower depths. Dpar values (diameter of etched spontaneous fission tracks measured parallel to the c axis) are 1.74–2.32 lm. (U-Th) ⁄ He Uncorrected apatite He (AHe) ages (Table 2) range from 169 to 9 Ma and cluster around 140–130 Ma. a-corrected ages (Farley, 2000) are between 252 and 15 Ma concentrating around 160–180 Ma. Some corrected ages are

older than AFT ages on the same or comparable samples. AHe age reproducibility among the different sample aliquots is within error for seven out of nine samples [three of the nine aliquots of sample 1 were processed by conventional mineral separation, while the selFRAG system was used for the other six performed by Dr F. Stuart (2007)] (Table 2). Poor intra-aliquots age replication in samples 4 and 9 is attributed to the presence of undetected mineral ⁄ fluid inclusions or slow cooling (Fitzgerald et al., 2006). We note variability among samples from contiguous outcrops. Samples 4, 5, 7 and 9, for instance, produce ages significantly younger than the rest (Fig. 2a and Table 2). We correlate these differences with lithologies and propose that the excess He observed  2008 Blackwell Publishing Ltd


Terra Nova, Vol 20, No. 5, 355–363

B. Ghorbal et al. • Unexpected vertical movements in NW Africa

............................................................................................................................................................. (a)

(b)

Fig. 2 (a) Geological map of the area between the Rif foredeep and the High Atlas (redrawn and simplified after Hafid, 2006) and apatite FT and (U-Th) ⁄ He ages. For each sample, we indicate the sample name (framed number), fission track ages (with grey background) and uncorrected (U-Th) ⁄ He ages. (b) Time-temperature paths for FT samples with radial plot and MTL distribution. Paths in the dark grey part of the diagram are acceptable with Goodness of Fit (GOF) in the interval 50–80%. Paths in the light grey area have a GOF>80%. The thick solid line is the best path identified by the modelling procedure. The apatite PAZ and the HePRZ for the same mineral are indicated (light yellow and orange bands respectively). Boxes representing the geological constraints adopted are shown and discussed in the text.

in monzogranites and granodiorites results from an implantation effect from U- and Th-rich host rocks  2008 Blackwell Publishing Ltd

(Spencer et al., 2004), and ⁄ or from the modification of the He retention properties of apatite, which becomes

more retentive with increasing number of U-and Th-decays (Green et al., 2006). To test this hypothesis, we 357


Unexpected vertical movements in NW Africa • B. Ghorbal et al.

Terra Nova, Vol 20, No. 5, 355–363

358

All samples pass the v2 test. Error is ±1r, calculated using the Zeta calibration technique (cf. Appendix). Abbreviations are as follows: qS and qI are the spontaneous and induced track density (tracks ⁄ cm2) measured; NS and NI are the number of spontaneous and induced tracks counted; Std and nb the standard deviation and the number of measured confined track and Dpar.

495 507 360 1.74 1.79 2.32 179 229 174 0.15 0.11 0.25 1523 3020 532 12.2 12.3 3.1 12.8 12.7 3.2

1602 3110 557

100 100 100

146.6 143.5 145.9

10.4 9.5 12.5

13.43 13.70 13.01

0.95 0.89 1.37

0.07 0.09 0.10

555 0.06 2.10 145 1.10 0.20 4605 20.6

Sample

Rommani Massif 1 37 Rehamna Massif 5 33 6 39 9 30

21.9

4889

93.2

147.9

5.5

13.48

Dpar (lm) Nb MTL ±1r (lm) qi · 100 000 cm)2 qs · 100 000 cm)2

Ni (no. of tracks) Ns (no. of tracks) No. of grains

Table 1 Apatite fission track analytical data.

P (v2) %

Pooled age (Ma)

±1r (Ma)

MTL (lm)

Std (lm)

Std (lm)

Nb Dpar

............................................................................................................................................................. derived a theoretical concentration of 4 He ([4He]th) using [U] and [Th] ICP-MS values and AFT age (as time reference) and adopting the U-Th decay equation (Farley, 2002), assuming zero initial He content and we compared it with the 4He measured by crystal outgassing ([4He]me) for each sample (Fig. 3; Table 3). Monzonites and greenschists produce comparable values of [4He]me and [4He]th. On the contrary, monzogranites and granodiorites have [4He]me > [4He]th. For these samples, the correction factor will overestimate the [4He] ejected or diffused from the crystal thereby producing too old ages (e.g. Belton et al., 2004). Thermal modelling Time-temperature paths are obtained by modelling apatite fission track and (U-Th) ⁄ He data integrated with independent geological constraints using the HeFTy model, which generates random t–T paths using a Monte Carlo algorithm (Ketcham, 2005). Inputs include track density and length, Dpar values, U and Th contents, uncorrected AHe ages and aliquots size. As measured Dpar values point to low chlorine compositions similar to Durango apatite (Barbarand et al., 2003), we used the annealing model of Ketcham et al. (1999). Predicted t–T paths are considered reliable in the 120–45 C temperature window (Ketcham, 2005). Relevant geological information used to constrain the t–T paths is expressed as boxes in Figs 2b and 4. Box A marks the Late Hercynian arrival of the sampled rocks close to the surface (see introductory chapter). Box B between 250 and 210 Ma includes possible ages for the onset of regional subsidence associated with Central Atlantic rifting (e.g. Medina, 1995; Le Roy and Pique´, 2001). Box C marks the position of the samples close to the surface at 100 Ma, when the Cenomanian transgression caused deposition of shallow marine sediments over the Meseta. As basement rocks in the Rehamna region are covered by poorly dated (Upper Jurassic to Lower Cretaceous) continental red

beds, we have adopted for this region a slightly older box (C’). Box D corresponds to the end of marine sedimentation in the Settat area (and elsewhere) and is used here to constrain the end of subsidence. The box is basically an age constraint.

Post-Palaeozoic evolution of the Moroccan Meseta and implications for NW Africa Time–temperature paths (Figs 2b and 4), homogeneous over the study area, document unexpected stages of subsidence and exhumation affecting the entire Meseta in the Jurassic to Early Cretaceous and, although of more limited magnitude, in the Late Cretaceous to Tertiary. Triassic-Middle Jurassic subsidence The Rehamna and Zaer regions experienced substantial Middle Triassic to Dogger heating. Assuming a gradient of 30 C km)1, the reconstructed heating translates in a subsidence of 3–4 km. The occurrence and magnitude of Late Triassic to Early Jurassic heating and, therefore, subsidence, are robust conclusions not significantly affected by modifications of the dimension and position of boxes A and B. At the regional scale, the Late Triassic to Early Jurassic is the main stage of Central Atlantic rifting and was associated with major subsidence in the High and Middle Atlas and the Atlantic rift (e.g. Lancelot and Winterer, 1981). Our discovery that the Meseta experienced significant coeval subsidence indicates that downward movement was not limited only to the Atlantic basins and the Atlas system, but it affected also the intervening Meseta. The age we propose for the end of subsidence (c. 170 Ma) is roughly comparable with the age of crustal separation in the Central Atlantic along the Morocco – Nova Scotia transect (e.g. Pique´ and Laville, 1996) (see Sahabi et al., 2004 for an alternative interpretation). Late Jurassic-Early Cretaceous exhumation Shortly after having reached their largest depth, the dated rocks began

 2008 Blackwell Publishing Ltd


Rock type

 2008 Blackwell Publishing Ltd

606

560

490

204

444

466

479

509

444

3327¢04¢¢ ⁄ 0636¢19¢¢

3310¢25¢¢ ⁄ 061036¢¢

3308¢09¢¢ ⁄ 0609¢¢43¢¢

3238¢54¢¢ ⁄ 0747¢41¢¢

3223¢09¢¢ ⁄ 0759¢12¢¢

3217¢22¢¢ ⁄ 0803¢19¢¢

3216¢33¢¢ ⁄ 0800¢21¢¢

3221¢22¢¢ ⁄ 0756¢07¢¢

3223¢09¢¢ ⁄ 0759¢12¢¢

Coordinates

9.63E)07 1.57E)06 1.96E)06 1.08E)05 9.65E)06 2.08E)06 7.56E)06 5.25E)06 4.05E)06 4.75E)06 1.89E)06 2.47E)06 4.76E)06 4.75E)06 6.37E)06 4.48E)06 5.44E)06 2.35E)06 2.72E)06 6.57E)06 3.77E)06

8.17E)06 1.07E)05 1.16E)05 3.05E)06 5.38E)06 5.90E)06 3.32E)06 3.58E)06 3.00E)06

Mass (g)

33 43 35 70 80 48 57 51 44 50 36 37 47 50 65 54 52 37 43 62 50

51 70 63 44 62 63 46 48 47

Grain radius (lm)

90 83 156 220 150 90 231 200 205 189 143 180 210 189 150 151 203 169 149 170 150

311 218 289 158 140 148 155 155 136

Grain length (lm)

2.4E+08 1.1E+09 1.7E+09 1.3E+10 6.0E+09 3.9E+09 6.4E+10 1.2E+11 5.6E+10 6.6E+10 1.9E+10 1.5E+09 3.1E+09 4.5E+09 4.6E+09 7.9E+10 4.8E+10 1.6E+10 4.7E+09 6.3E+08 3.8E+10

7.9E+10 1.1E+11 1.2E+11 2.3E+09 1.4E+09 1.4E+09 2.7E+11 7.6E+10 6.5E+10

Helium (no. of atoms)

9.0E)12 4.2E)11 6.4E)11 4.8E)10 2.2E)10 1.4E)10 2.4E)09 4.3E)09 2.1E)09 2.5E)09 7.0E)10 5.4E)11 1.1E)10 1.7E)10 1.7E)10 2.9E)09 1.8E)09 6.0E)10 1.7E)10 2.3E)11 1.4E)09

2.9E)09 4.2E)09 4.6E)09 8.4E)11 5.2E)11 5.1E)11 1.0E)08 2.8E)09 2.4E)09

Helium (ccSTP)

0.17 0.48 0.58 0.80 0.41 1.23 5.65 14.77 9.20 9.27 6.60 0.39 0.43 0.63 0.48 11.70 5.86 4.53 1.15 0.06 6.76

6.41 7.01 7.09 0.49 0.17 0.15 54.83 14.19 14.44

4He (p.p.m.)

14 16 5 2 1 3 28 100 70 58 37 4 3 5 2 22 39 10 6 2 2

15 16 14 3 2 2 77 22 22

232Th (p.p.m.)

6 4 2 3 1 5 11 27 16 14 10 1 1 1 1 37 9 20 5 1 29

21 20 21 1 2 1 129 46 41

238U (p.p.m.)

2.41 3.89 2.13 0.95 1.07 0.61 2.44 3.75 4.24 4.08 3.55 3.07 3.32 3.89 1.30 0.60 4.36 0.50 1.16 2.17 0.09

0.74 0.81 0.70 2.37 1.43 1.43 0.60 0.49 0.54

Th ⁄ U

8.7 27.4 73.7 117.7 118.9 90.5 143.9 133.9 127.9 151.5 157.0 85.3 108.7 111.6 122.2 127.1 146.6 92.8 81.7 21.3 105.1

120.2 131.7 134.3 123.3 38.5 37.0 169.5 126.1 142.3 ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

± ± ± ± ± ± ± ± ± 1.2 2.1 5.3 8.2 8.3 6.3 10.5 10.1 9.8 22.2 22.7 6.4 8.0 8.4 8.5 8.8 11.2 12.4 5.8 3.0 7.3

8.5 9.2 9 9.0 7.8 5.2 11.7 8.7 9.9

Uncorr. age (Ma)

0.56 0.67 0.58 0.79 0.81 0.69 0.75 0.72 0.69 0.71 0.62 0.64 0.71 0.71 0.77 0.73 0.72 0.65 0.67 0.76 0.72

0.75 0.79 0.78 0.68 0.74 0.75 0.70 0.71 0.69

Ft

15.5 40.4 127.5 148.8 146.3 130.8 191.4 186.0 185.8 212.9 253.4 133.4 153.8 156.7 159.6 174.2 203.2 142.6 121.2 28.0 146.8

160.9 166.8 171 181.8 51.8 50.6 242.9 178.5 205.3

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

± ± ± ± ± ± ± ± ±

2.8 3.4 10.6 10.4 10.6 9.2 14.7 14.3 14.9 31.4 38.0 13.2 11.7 12.1 11.5 14.3 16.7 20.9 9.1 4.0 10.5

11.7 11.7 11.5 13.7 10.5 7.0 17.5 12.9 14.9

Corr. age (Ma)

1.7 1.5 0.7 0.6 0.3 1.2 3.4 9.4 6.1 5.2 3.6 0.4 0.3 0.5 0.3 7.9 3.4 4.2 1.2 0.3 5.7

4.4 4.4 4.4 0.3 0.4 0.3 26.8 9.4 8.4

He theoret (p.p.m.)

Uncorrected and a-corrected AHe ages are shown. All aliquots pass successfully the reheating test of He extraction. Error is ±1r, calculated by adding errors from the analytical procedure, the crystal size and on the variability of the Durango standard measures (cf. Appendix). The He extracted is reported in number of atom, ccp and in p.p.m. U, Uranium content; Th, Thorium content; FT, factor of correction (Farley, 2000).

Zaer Massif 1.1 Monzogranite 1.3 1.5 2.3 Granodiorite 2.1 2.2 3.1 Granodiorite 3.3 3.4 Rehamna Massif 4.3 Monzonite 4.5 4.4 5.1 Monzonite 5.4 5.5 6.1 Monzogranite 6.2 6.3 6.4 6.5 7.1 Monzonite 7.2 7.3 8.1 Granodiorite 8.2 8.4 8.3 9.1 Green schist 9.2 9.4

Sample

Alt (m)

Table 2 Apatite (U-Th) ⁄ He (AHe) analytical data.

Terra Nova, Vol 20, No. 5, 355–363

.............................................................................................................................................................

B. Ghorbal et al. • Unexpected vertical movements in NW Africa

359


Unexpected vertical movements in NW Africa • B. Ghorbal et al.

Terra Nova, Vol 20, No. 5, 355–363

.............................................................................................................................................................

NE–SW-trending folding resulting from middle or possibly Late Jurassic shortening (Echarfaoui et al., 2002). Authors of fundamental regional studies such as Faure-Muret and Choubert (1971) and Michard (1976) refer to Late Jurassic to Early Cretaceous deformation without, however, presenting further details. Late Cretaceous-Eocene

Fig. 3 Plot of predicted He and measured He showing the behaviours of different lithologies. See Table 2 for detailed values.

Table 3 AHe ages obtained with different separation techniques.

Sample

Rock type

Coordinates

Rommani Massif Conventional mineral separation 1.1 Monzogranite 3327¢04¢¢ ⁄ 0636¢19¢¢ 1.3 1.5 SelFRAG mineral preparation 1.6 Monzogranite 3327¢04¢¢ ⁄ 0636¢19¢¢ 1.7 1.8 1.9 1.10 1.11

Grain radius (lm)

Th ⁄ U

Uncorr. age (Ma)

606

51 70 63

0.74 0.81 0.70

120.2 ± 8.5 131.7 ± 9.2 134.3 ± 9

606

78 72 70 70 70 70

0.85 1.40 1.02 1.08 0.81 0.68

Alt (m)

128 133 129 137 141 138

± ± ± ± ± ±

7 7 10 6 8 7

(U-Th) ⁄ He data from sample 1 (monzogranites of the Zaer Massif). Note how crystals separated with the selFRAG apparatus and those separated with conventional methods produce indistinguishable ages. The two sets of aliquots were measured using the same He extraction technique (e.g. Foeken et al., 2006).

moving upwards and reached the surface before the Cenomanian when they were unconformably covered by marine sediments (Hollard et al., 1985) (box C). Rocks in the Rehamna region are overlain by poorly dated Lower Cretaceous clastics and could, therefore, have been exhumed slightly earlier (box C’) (Michard et al., 1989 and geological maps). The area experiencing exhumation was limited to the W by the basins associated with the Central Atlantic rifted margin. To the E, Jurassic exhumation has been proposed by Barbero et al. (2007) for samples from the Meseta east of the Middle Atlas – High Atlas junction. Towards the S, exhumation might have affected also the Anti-Atlas from where Malusa` et al. (2007) have produced Mesozoic Fission Track ages and t–T paths pointing 360

to very gentle Mesozoic cooling. More geological and thermochronological work is needed to better define the area experiencing exhumation. Understanding the tectonics of Late Jurassic to Early Cretaceous exhumation is challenging as upward vertical movements took place during the post-rift stage of the Atlas system and Atlantic rifted margin. In Morocco, there is widespread evidence of Late Jurassic to Early Cretaceous angular unconformities and deformations, but they are generally attributed to transtension, for instance, in the Middle Atlas and High Atlas (e.g. Stets, 1992a,b; Charriere, 1996 respectively) or to diapirism (for instance in the Western High Atlas e.g. Hafid, 2000, 2006 and references therein). However, in the Abda basin (western Morocco), seismic data document

Marine Upper Cretaceous sediments overlying the Meseta basement provide a younger limit for its arrival at the surface and document the onset of a subsidence period able to create the necessary accommodation space. We tentatively estimate at 1 km (with a gradient of 30 C km)1) the maximum depth reached by Rehamna and Zaer samples. We use the Late Eocene end of marine sedimentation (33 Ma) in the same area to date the end of subsidence (box D in Figs 2b and 4). In a modelling perspective, Late Cretaceous subsidence is needed to fit the AFT ages and track length distribution, and is compatible with the young AHe ages obtained from some aliquots (Fig. 2a). The first implication of the documented heating stage is that Late Cretaceous to Eocene subsidence was not only limited to the Settat area where up to 500 m of sediments are documented, but it affected also the adjacent Rehmana region and Zaer Massif suggesting that subsidence involved a larger area possibly stretching for 100 s of km from the northern side of the Atlas belt to the future Rif. We note that the subsidence estimated for the Rehamna and Zaer samples is somewhat larger than the one deduced from the thickness of Upper Cretaceous to Eocene sediments in the intervening Settat domain. Oligocene to present Having reached temperatures of 60– 80 C in the Late Cretaceous-Paleogene, rocks of the Meseta were exhumed to the surface cooling to ambient temperatures (Fig. 4). The t–T model suggests that rocks of the Zaer Massif began moving upwards before those of the Rehamna area. Exhumation was associated with the formation of two WNW–ESE-trending regional anticlines, Zaer Massif and Rehamna, separated by the Settat  2008 Blackwell Publishing Ltd


B. Ghorbal et al. • Unexpected vertical movements in NW Africa

Terra Nova, Vol 20, No. 5, 355–363

.............................................................................................................................................................

Fig. 4 Best fit time-temperature paths for the Moroccan Meseta samples. Boxes A–D represent geological constraints (see text for discussion). Subsidence curves in the Atlantic margin (external Essaouira Basin), the High Atlas and the Middle Atlas are shown for comparison (simplified after Ellouz et al., 2003). Magnitude of subsidence should be read on the vertical axis on the right hand side of the diagram. At the bottom of the figure, we have indicated the syn-rift and the post-rift stages as estimated by most authors (e.g. Pique´ and Laville, 1996). Note that Sahabi et al. (2004) propose a slightly older age for the rift-drift transition. The two stippled bands indicate the apatite HePRZ and the apatite PAZ (upper and lower band respectively).

ments deposited of large parts of the Atlantic margin (Price, 1981; Davison, 2005). Late Cretaceous to Tertiary subsidence affected a large area between the Rif and the Atlas. From the late Eocene, folding with wavelengths of 100–150 km caused the formation of the Rehamna and Zaer anticlines separated by the Settat syncline and was coeval with larger wavelength folding, which brought the entire area above sea level. Folding preceded and was coeval with localized shortening at the margins of the Meseta leading to the formation of the Atlas and Rif foldand-thrust belts.

Acknowledgements We thank A. Teixell, D. Frizon de Lamotte and M. Hafid for stimulating discussions on Moroccan geology. F. Stuart (SUERC) is acknowledged for the help and assistance. We also thank R. van Elsas, T. Vogel and M. Hooyen for their technical supervision. G. Ruiz and two anonymous reviewers are thanked for their thorough revisions. We appreciate the supervision and help of A. Borcherds and the Journal Editor.

References

Fig. 5 Schematic section from the Atlas Mountains to the Mediterranean showing present day alternation of highs and lows. Note the strong vertical exaggeration. Topography is obtained from satellite data.

zone syncline (Fig. 5b). Indeed, the northern and southern terminations of the marine sediments of the Settat zone are erosional and no onlap relations are observed on the metamorphics of the substratum. Oligocene-Miocene NE–SW compression has been proposed by Ait Brahim et al. (2002). The young age of vertical movements is underlined by the higher elevation of the anticlines with respect to the synclines despite the larger erosion they experienced. The folding wavelength is in the order of 100– 150 km. Formation of the Rehamna and Zaer anticlines and of the intervening syncline was accompanied by upward vertical movements at a much larger wavelength responsible for bringing into emersion the entire domain of the plateau des phosphates. Such bimodal folding has been pre-

 2008 Blackwell Publishing Ltd

dicted for lithospheres of intermediate age where the upper crust and upper mantle buckle independently (Cloetingh et al.,1999).

Conclusions Combining apatite Fission Tracks and (U-Th) ⁄ He thermochronology with regional geology, we have discovered vertical movements of timing and magnitude hitherto unknown in the Moroccan Meseta, a part of NW Africa previously considered stable. Subsidence coeval to Atlantic rifting affected an area much larger than previously expected stretching from the Atlantic margin to the Atlas. During the post-rift, exhumation affected a several hundreds km long area elongated in NNE–SSW direction and produced terrigenous sedi-

Ait Brahim, L., Chotin, P., Hinaj, S., Abdelouafi, A., El Adraoui, A., Nakcha, C., Dhont, D., Charroud, M., Sossey Aloaoui, F., Amrhar, M., Bouaza, A., Tabyaoui, H. and Chaouni, A., 2002. Paleostress evolution in the Moroccan African margin from Triassic to Present. Tectonophysics, 357, 187–205. Barbarand, J., Carter, A., Wood, I. and Hurford, T., 2003. Compositional and structural control of fission-track annealing in apatite. Chem. Geol., 198, 107–137. Barbero, L., Teixell, A., Arboleya, M.-L., Del Rio, P., Reiners, P.W. and Bougadir, B., 2007. Jurassic-to-present thermal history of the central High Atlas (Morocco) assessed by low-temperature thermochronology. Terra Nova, 19, 58–64. Belton, D.X., Kohn, B.P. and Gleadow, A.J.W. 2004. Quantifying excess helium: some of the issues and assumptions in combined (U–Th) ⁄ He and fission track analysis. In: 10th International Fission track Dating Conference (P.A.M. Andriessen, ed), p. 18. Vrije Universiteit, Amsterdam. Chalouan, A., Michard, A., Feinberg, H., Montigny, R. and Saddiqi, O., 2001. The Rif mountain building: a new tectonic scenario. Bull. Soc. Ge´ol. de Fr., 172, 603–616.

361


Unexpected vertical movements in NW Africa • B. Ghorbal et al.

Terra Nova, Vol 20, No. 5, 355–363

............................................................................................................................................................. Charriere, A., 1996. Contexte Pale´oge´ographique et pale´otectonique de la formation des basins cre´tace´s du Moyen Atlas (Maroc) a` la lumie`re des donne´es stratigraphiques re´centes. Bull. Soc. Ge´ol. Fr., 167, 617–626. Cloetingh, S., Burov, E. and Poliakov, A., 1999. Lithospheric folding: primary response to compression? (from central Asia to Paris Basin). Tectonics, 18, 1064–1083. Davison, I., 2005. Central Atlantic margin basins of North West Africa: geology and hydrocarbon potential (Morocco to Guinea). J. Afri. Earth Sci., 43, 254–274. Echarfoui, H., Hafid, M., Aı¨ t Salem, A. and Aı¨ t Fora, A., 2002. Analyse sismostratigraphique du Bassin d’Abda (Maroc occidental), exemple de structures inverses pendant le rifting atlantique. C. R. Geosci., 334, 371–377. Ehlers, T.A., 2005. Crustal thermal processes and the interpretation of thermochronometer data. In: Reviews in Mineralogy and Geochemistry (P.W. Reiners and T.A. Ehlers, eds), pp. 315– 350. Vol. 58. Mineralogical Society of America, Washington, DC. El Mahi, B., Hoepffner, Ch., Zahroui, M. and Boushaba, A., 2000. L’e´volution tectono-me´tamorphique de la zone hercynienne des Rehmana centraux (Maroc). Bull. Inst. Sc. Rabat, 22, 41–57. Ellouz, N., Patriat, M., Gaulier, J.-M., Bouatmani, R. and Sabounji, S., 2003. From rifting to Alpine inversion. Mesozoic and Cenozoic subsidence history of some Moroccan basins. Sed. Geol., 156, 185–212. Farley, K.A., 2000. Helium diffusion from apatite: general behaviour as illustrated by Durango fluorapatite. J. Geophys. Res., 105, 2903–2914. Farley, K.A., 2002. (U–Th) ⁄ He dating: techniques, calibrations, and applications. In: Noble Gases in Geochemistry and Cosmochemistry (D. Porcelli, C.J. Ballentine and R. Wieler, eds), pp. 819– 844. Reviews in Mineralogy and Petrology, Mineralogical Society of America, Washington, DC. Faure-Muret, A. and Choubert, G. 1971. Le Maroc: Domaine rifain et atlasique. In: Tectonics of Africa (L.C. King, ed.), pp. 17–46. UNESCO, Paris. Fitzgerald, P.G., Baldwin, S.L., Webb, L.E. and O’Sullivan, P.P., 2006. Interpretation of (U-Th) ⁄ He single grain ages from slowly cooled crustal terranes: a case study from the Transantarctic Mountains of southern Victoria Land. Chem. Geol., 225, 91–120. Foeken, J.P.T., Stuart, F.M., Dobson, K.J., Persano, C. and Vilbert, D., 2006. A diode laser system for heating minerals

362

for (U-Th) ⁄ He chronometry. Geochem. Geophys. Geosyst., 7, 1–9. Frizon de Lamotte, D., Saint Be´zar, B., Brace´ne, R. and Mercier, E., 2000. The two main steps of Atlas building and geodynamics of the western Mediterranean. Tectonics, 19, 740–761. Green, P.F., Crowhurst, P.V., Duddy, I.R., Jaspen, P. and Holford, S.P., 2006. Conflicting (U–Th) ⁄ He and fission track ages in apatite: enhanced He retention, not anomalous annealing behaviour. Earth Planet. Sci. Lett., 250, 407–427. Guiraud, R., Bosworth, W., Thierry, J. and Delplanque, A., 2005. Phanerozoic geological evolution of Northern and Central Africa: an overview. J. Afr. Earth Sci., 43, 83–143. Hafid, M., 2000. Triassic-Early Liassic extensional systems and their Tertiary inversion, Essaouira Basin (Morocco). Mar. Petrol. Geol., 17, 409–430. Hafid, M., 2006. Styles structuraux du Haut Atlas de Cap Tefelney et de la parte septentrionale du Haut Atlas Occidental: Tectonique salife`re et relations entre l’Atlas et l’Atlantique. Notes et Me´m. Serv. Ge´ol. Maroc, 465, 174 pp. Haı¨ meur, J., El Amrani El Hassani, Iz.-E. and Chabane, A., 2004. Petrology and geochemistry of the Zae¨r calc-alkaline granitoids (central Morocco): petrogenic model. Bull. Inst. Sci., Rabat, Earth Sc. section, 26, 27–48. Hoepffner, C., Soulaimani, A. and Pique´, A., 2005. The Moroccan Hercynides. J. Afr. Earth Sci., 43, 144–165. Hollard, H., Choubert, G., Bronner, G., Marchand, J. and Sougy, J., 1985. Carte ge´ologique du Maroc, scale 1:1000000. Notes et Me´m. Serv. Ge´ol. Maroc, Royaume du Maroc, 260. Huon, S., Pique´, A. and Clauer, N., 1987. Etude de l’orogenese hercynienne au Maroc par la datation K-Ar de l’e´volution me´tamorphique de schistes ardoisiers. Sci. Geol., Bull., 40, 273–284. Hurford, A.J. and Green, P.F., 1983. The zeta age calibration of fission-track dating. Isot. Geosci., 1, 285–317. Jansa, L.F. and Wiedmann, J., 1982. Mesozoic-Cenozoic development of Eastern North American and northwest African continental margins: A Comparison. In: Geology of the Northwest African Continental Margin (U. von Rad, K. Hinz, M. Sarnthein and E. Seibold, eds), pp. 213–272. Springer Verlag, Berlin-Heidelberg, New York. Ketcham, R.A., 2005. Forward and inverse modeling of low-temperature thermochronology data. Rev. Mineral. Geochem., 58, 275–314. Ketcham, R.A., Donelick, R.A. and Carlson, W.D., 1999. Variability of

apatite fission track annealing kinetics: III. Extrapolation to geological time scales. Am. Mineral., 84, 1235–1255. Lancelot, Y. and Winterer, E.L., 1981. Evolution of the Moroccan oceanic basin and adjacent continental margin – a synthesis. Initial Rep. Deep Sea Drill. Project, 50, 801–821. Le Roy, P. and Pique´, A., 2001. TriassicLiassic Western Moroccan synrift basins in relation to the Central Atlantic opening. Mar. Geol., 172, 359–381. Malusa`, M., Polino, R., Cerrina Feroni, A., Ellero, A., Otaria, G., Baidder, L. and Musumeci, G., 2007. Post-Variscan tectonics in eastern Anti-Atlas (Marocco). Terra Nova, 19, 481–489. Medina, F., 1995. Syn- and post-rift evolution of the El Jadida-Agadir basin (Morocco): constraints for the rifting models of the central Atlantic. Can. J. Earth Sci., 32, 1273–1291. Michard, A., 1976. Ele´ments de Ge´ologie Marocaine. Notes et Me´m. Serv. Ge´ol. Maroc, Royaume du Maroc, 252, 409.pp. Michard, A., Cailleux, Y. and Hoepffner, C., 1989. L’oroge´nese me´se´tien du Maroc: structure, deformation hercynienne et de´placements. Me´m. Serv. Ge´ol. Maroc, 335, 313–327. Mrini, Z., Rafi, A., Duthou, J.L. and Vidal, P.H., 1992. Chronologie Rb-Sr des granitoı¨ des hercyniens du Maroc et conse´quences. Bull. Soc. Ge´ol. Fr., 163, 281–291. Pique´, A. and Laville, E., 1996. The Central Atlantic rifting: reactivation of Paleozoic structures? J. Geodynam., 21, 235–255. Price, I., 1981. Provenance of the JurassicCretaceous flysch, Deep Sea Drilling Project sites 370 and 416. Initial Rep. Deep Sea Drill. Project, 50, 751–757. Sahabi, M., Askanian, D. and Olivet, J.-L., 2004. Un nouveau point de depart pour l’histoire de l’Atlantique central. C.R. Geosci., 336, 1041–1052. Spencer, A.S., Kohn, B.P., Gleadow, A.J.W., Norman, M., Belton, D.X. and Carter, T.J. 2004. The importance of residing in a good neighbourhood: rechecking the rules of the game for apatite (U-Th) ⁄ He thermochronology. In: 10th International Fission track Dating Conference (P.A.M. Andriessen, ed.), pp. 20. Vrije Universiteit, Amsterdam. Stets, J., 1992a. Mid-Jurassic events in the Western High Atlas (Morocco). Geol. Rund., 81 ⁄ 1, 69–84. Stets, J., 1992b. Mid-Jurassic events in the Western High Atlas (Morocco). Geol. Rund., 81, 69–84. Received 5 December 2007; revised version accepted 16 June 2008

 2008 Blackwell Publishing Ltd


B. Ghorbal et al. • Unexpected vertical movements in NW Africa

Terra Nova, Vol 20, No. 5, 355–363

............................................................................................................................................................. Appendix Apatite fission track method Apatite concentrates were embedded in araldite, polished, etched during 20 s at 21 C in 1N HNO3 to reveal spontaneous tracks and irradiated against a mica detector CN5 with thermal neutrons at the low flux reactor of the ECN at Petten (NL). Detectors were etched in 48% HF at room temperature for about 30 min. Neutron flux was monitored by standard glasses CN5 (Corning Glass Works). Fish canyon tuff and Durango apatites were used as age standards to determine the f factor (e.g. Hurford and Green, 1983). Density, length and Dpar tracks were measured with a TrackScan system (Autoscan Systems PTY, LPTD, P.O. Box 112, Ormond, Victoria, Australia). (U-Th) ⁄ He method Apatite crystals were selected and measured (for Ft correction) under an optical microscope at 160· magnification following standard procedures (Farley, 2002). Selected grains

ing 5% HNO3 on a hotplate for 48 h. Each sample batch was prepared with a series of procedural blanks (including Pt foil blanks for apatite) and spiked to check the purity and calibration of the reagents and spikes. Spiked solutions were analysed as 1 mL of 1–5 ppb U-Th solutions by isotope dilution on an X series II ICP-MS (Thermo Electron Co, Winsford, UK) with a Teflon microflow nebulizer and double-pass spray chamber. Procedural U and Th blanks by this method are 1 ± 0.5 pg and 2 ± 1 pg, respectively and are <10% of the samples’ U and Th concentrations. Routine in-run precisions and long-term reproducibilities of standard 232Th ⁄ 229Th and 238U ⁄ 233U are 0.1–0.4%, and estimated to be 1–2% (2r). Replicate analyses of Durango apatite during the period of analyses yielded a mean age of 32.7 ± 1.2 Ma (r) (Table 4). On the basis of the reproducibility of the Durango standard measures, we estimate an analytical uncertainty of 6–7% (2r), which is added to a conservative crystal size uncertainty of 3–4% for apatite age determinations in this study.

are (near) euhedral and lack visible inclusions or fractures. Apatite Helium extraction was conducted at the SUERC (U-Th) ⁄ He facility following procedures in Foeken et al. (2006). Apatites were packed in Pt foil tubes (99.9% pure). Gases liberated by heating the Pt-foil capsules are purified using a hot SAES TiZr getter and two liquid nitrogen-cooled charcoal traps. 4He abundances along with 3 He, H (mass 2) and CH4 (mass 16), are determined by an electron multiplier in a Hiden HAl3F quadrupole mass spectrometer operated in static mode. Absolute He concentrations are calculated from peak height comparison against a calibrated standard. Re-extracts (Farley, 2002) were run following each analysis to ensure complete He degassing. All samples passed the re-extract test. U and Th analyses were conducted at the ICP-MS facility at the VU in Amsterdam. Outgassed crystals that passed the re-extract test plus Pt tubes were retrieved, transferred into 15 mL Teflon beakers and spiked with a calibrated 229Th and 233U solution (233U ⁄ 229Th = 0.96). Total dissolution of apatites was obtained by add-

Table 4 VU Durango apatite (U-Th) ⁄ He analytical data measured with the samples. Sample VU VU VU VU VU VU VU VU VU VU VU

Dur Dur Dur Dur Dur Dur Dur Dur Dur Dur Dur

1 2 4 5 6 8 11 12 13 14 15

4He (cc STP)

4He blank correction

238U (ng)

238U blank correction (%)

232Th (ng)

232Th blank correction (%)

232Th ⁄ 238U

Uncorrected age (Ma)

1.13E)09 2.33E)09 2.92E)09 3.62E)09 5.83E)09 6.13E)09 8.90E)10 1.87E)09 1.43E)09 3.40E)09 1.60E)09

1.63E)03 7.85E)04 6.11E)04 8.44E)04 3.12E)04 3.44E)04 3.77E)03 2.99E)03 1.27E)03 8.42E)04 1.83E)03

4.17E)02 9.74E)02 1.32E)01 1.62E)01 2.65E)01 2.56E)01 4.48E)02 8.59E)02 6.37E)02 1.32E)01 6.33E)02

5.6 2.5 1.8 1.5 0.9 1.0 5.2 2.9 3.7 1.8 3.8

1.02 2.08 2.64 2.89 5.52 5.51 0.77 1.55 1.22 3.16 1.42

0.59 0.29 0.23 0.21 0.11 0.11 0.78 0.39 0.50 0.19 0.43

25.2 21.9 20.5 18.3 21.4 22.1 17.7 19.2 19.6 24.4 23.0

32.7 32.5 31.7 35.2 30.5 32.4 32.2 34.1 33.5 31.9 33.0

± ± ± ± ± ± ± ± ± ± ±

0.7 0.6 0.6 0.5 0.6 0.6 0.5 0.5 0.5 0.7 0.6

± ± ± ± ± ± ± ± ± ± ±

1.4 1.4 1.4 1.4 1.3 1.3 1.7 1.4 1.4 1.4 1.4

Error is ±1r, calculated by adding calibration gas and blank error (cf. Appendix). The Th ⁄ U ratio is between 17.7 and 25.2.

 2008 Blackwell Publishing Ltd

363



movements in stable parts of Africa