Post-Variscan tectonics in eastern Anti-Atlas (Morocco

Post-Variscan tectonics in eastern Anti-Atlas (Morocco)

Marco G. Malusa,1,2 Riccardo Polino,2 Andrea Cerrina Feroni,2 Alessandro Ellero,2 Giuseppe Ottria,2

Lahssen Baidder3 and Giovanni Musumeci2,4

1Dipartimento di Scienze Geologiche e Geotecnologie, University of Milano-Bicocca, Milan, Italy; 2Institute of Geosciences and Earth

Resources – Consiglio Nazionale delle Ricerche, Pisa-Torino, Italy; 3Department of Geology, University Hassan II, BP 5366 Maarif

Casablanca, Maroc; 4Department of Earth Sciences, University of Pisa, I-56126 Pisa, Italy

Introduction

The Anti-Atlas of Morocco, located atthe northern rim of the West AfricanCraton, represents the Phanerozoicforeland of two intracontinental oro-genic belts: the Late Palaeozoic NorthAfrican Variscides and the CenozoicAtlas belt (Frizon de Lamotte et al.,2000; Hoepffner et al., 2006). In thisregion, the uneven uplift of the Pre-cambrian basement and the deforma-tion of the Palaeozoic cover aremainly referred to the Variscan orog-eny (e.g. Helg et al., 2004), but thelack of Meso-Cenozoic sequences inlarge areas makes the recognition ofpost-Variscan tectonics difficult. As aconsequence, the effects exerted by theAtlasic orogeny and the Mesozoictranstensional tectonics coeval withthe Atlantic rifting are poorly under-stood and probably underestimated.The aim of this study was to illus-

trate the impact of post-Variscan tec-tonics on the northern boundary ofWest African Craton, on the basis ofnew stratigraphic and structural data(see also El Boukhari and Musumeci,2007; El Boukhari and Ottria, 2007;Malusa and Schiavo, 2007) integratedby the first fission-track (FT) datasetfrom the Anti-Atlas region.

Regional geological setting

The West African Craton (Fig. 1)comprises a basement consolidatedduring the Pan-African orogeny anda thick Palaeozoic cover (Choubert,1963; Destombes, 1976; Ennih andLiegeois, 2001). The Variscan orog-eny led to the suturing of the cratonwith the Meseta block that wasaffected by penetrative deformationand metamorphism. Conversely, theAnti-Atlas acted as the southernVariscan foreland (Hoepffner et al.,2006) and experienced mild deforma-tion. During the Triassic rifting ofPangea, two en-echelon rift zonesdeveloped north of the craton be-tween the left-lateral South-AtlasFault (Fig. 1) and the northernmostNewfoundland Fault (Laville andPique, 1991): the Atlantic rift, alongthe present-day continental margin,and the Atlasic rift in the Atlasregion. In the Early–Middle Jurassic,the Atlantic rift widened and gaveway to the Atlantic oceanic crust,while the Atlasic rift aborted and wasdissected into smaller basins con-trolled by a left-lateral transtensionalregime (Laville et al., 2004). Duringthe Cenozoic, the Atlasic domain wasinverted and uplifted to form theAtlas belt (Giese and Jacobshagen,1992). The Atlasic deformation wasmainly confined north of the South-Atlas Fault; the Anti-Atlas acted infact as the southern Atlasic forelandand it is commonly considered tohave undergone negligible Meso-Cenozoic deformation (e.g. Frizon

de Lamotte et al., 2000; Helg et al.,2004), even if minor faults crosscut-ting Late Cretaceous and Cenozoicsediments are described in the Saharadomain east of the study area (e.g.Fabre, 1976).

Stratigraphy

In the eastern Anti-Atlas, the Pre-cambrian basement is exposed in theSaghro and Ougnat inliers (Figs 1and 2), and it is locally topped byLate Neoproterozoic volcanoclasticrocks preserved in small, often fault-bound, extensional basins (Fig. 3A).These rocks are unconformably over-lain by Cambro-Ordovician terrige-nous clastics and minor carbonatesediments deposited in an epicontinen-tal sea (Buggisch and Siegert, 1988;Landing et al., 2006) (Fig. 3A,B).To the north, the Cambro-Ordovi-cian sequence is directly overthru-sted by a stack of allochthonous unitsascribed to the Variscan belt(Fig. 3C) (Michard et al., 1982),which involves most of the LowerCarboniferous terrigenous clasticsexposed between Tinerhir and Tinej-dad as well as older terms of thePalaeozoic sequences. The post-Ordovician autochthonous cover isthus preserved only to the south-east(Fig. 1) and consists of Silurianblack-shales followed by Siluro-Devonian carbonates.Our field data show that the Pal-

aeozoic succession, whose thicknessexceeds 10 km in the central andwestern Anti-Atlas (e.g. Faik et al.,

ABSTRACT

New field data integrated by fission-track (FT) analysis unravelan innovative scenario for the post-Variscan evolution of theeastern Anti-Atlas. This area, unaffected by Meso-Cenozoictectonics according to most workers, is crosscut by crustalfaults bearing evidence of a polyphase deformation history.Apatite FT ages, ranging between 284 and 88 Ma, point to fastNeogene exhumation and unravel contrasting cooling pathsacross major faults. Results show that the study area was buried

beneath 2 km of allochthonous Variscan units, now eroded. Theeastern Anti-Atlas acted as the southern shoulder of the Atlasicrift in the Mesozoic, and underwent a dextral transpressionalstructuring of Neogene age followed by sub-meridian shorten-ing. The southern front of Atlasic deformation is thereforelocated inside the Anti-Atlas region, and it is still active.

Terra Nova, 19, 481–489, 2007

Correspondence: Marco Giovanni Malusa,

Dipartimento di Scienze Geologiche e

Geotecnologie, Universita di Milano-

Bicocca, Piazza delle Scienza 4, 20126

Milan, Italy. Tel.: +39 02 6448 2071; fax:

+39 02 6448 2073; e-mail: marco.malusa@

unimib.it

� 2007 Blackwell Publishing Ltd 481

doi: 10.1111/j.1365-3121.2007.00775.x

2001), is just a few kilometres thick ineastern Anti-Atlas and it is increas-ingly thinner from south to north(Fig. 2). The overall thickness of theCambro-Ordovician cover is just1050–1200 m north of the Saghro(section 1), it increases up to 2200–2700 m between the Saghro and theOugnat (section 2), and reaches2800–3300 m south of the Ougnat(section 3) where the Cambro-Ordo-vician succession is also followed bythin Siluro-Devonian deposits.The Palaeozoic sequences and the

Variscan units are intruded by NE–SW tholeiitic dykes dated at 196.9 ±1.8 Ma (40Ar ⁄ 39Ar) (Sebai et al.,1991), and are unconformably overlainby flows of olivine nephelinites datedat 4.8–2.9 Ma (K–Ar) (Berrahmaet al., 1993).

The network of major faults

On a regional scale, south of the flat-lying Variscan thrusts described byMichard et al. (1982), the study area isdeformed by a basement-cored E–Wanticline of Variscan age crosscut bythree major crustal structures: the Iskn�Izekelli Fault, the Tizi n�BoujouFault and the Bou Larhzazil-TinififtFault (Fig. 1).The Isk n�Izekelli Fault is a few

kilometres thick deformation zonedeveloped on the northern side of theSaghro inlier. It consists of NNW-dipping intermediate to high-anglefaults that crosscut the sedimentarycover and bound tectonic slivers ofPrecambrian basement (W–W¢ inFig. 4). It bears the evidence of trans-pressional tectonics associated with

the upthrow of the northern blocks(Fig. 3D), as attested by hectometre-sized drag folds. Faulting led to acumulative throw along cross-sectionW–W¢ of ca. 700–800 m, beside aright-lateral separation of severalkilometres along strike (Fig. 1). TheTizi n�Boujou Fault is a polyphaseENE–WSW deformation zone thatconsists of linked fault segments dip-ping at high-angle to the NNW (X–X¢in Fig. 4). Its western segment juxta-poses the Middle Cambrian sedi-ments, on the southern side, to theOrdovician shales and sandstones onthe northern side (Fig. 3E). The east-ern segment bounds to the north theOugnat inlier, and juxtaposes thePrecambrian basement to the Ordovi-cian cover (Fig. 3F). These relation-ships are consistent with normal

Fig. 1 Geological sketch map of the study area (eastern Anti-Atlas) reporting the major faults detected in the field, the post-Variscan magmatic bodies (in black), fission-track data (see Tables 1 and 2 for details), and the frontal thrusts of the Variscan beltaccording to Michard et al. (1982). Numbered white circles are stratigraphic sections of Fig. 2; W–W¢, X–X¢ and Y–Y¢ are cross-sections of Fig. 4. Acronyms are as follows: 21, chiefly Lower Cambrian cover; 22, Middle Cambrian cover; J, Jurassic tholeiiticdykes; MZ-CZ, Meso-Cenozoic cover; O, Ordovician cover; P, Mio-Pliocene nephelinite flows; PR, Precambrian basement; S-D,Silurian-Devonian cover. Inset: tectonic setting of Morocco.

Post-Variscan tectonics in eastern Anti-Atlas • M. Malusa et al. Terra Nova, Vol 19, No. 6, 481–489

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482 � 2007 Blackwell Publishing Ltd

motion leading to the downthrow ofthe northern blocks, which exceeds800 m north of the Ougnat inlier. Thisextensional tectonics is followed by aremarkable right-lateral transpressionattested by top-to-SE drag folds(Fig. 4).Farther to the south, the Bou Lar-

hzazil-Tinifift Fault is a complexENE–WSW deformation zone con-sisting of linked near-vertical shearplanes developed between the Saghro

inlier and the southern side of theOugnat inlier (Y–Y¢ in Fig. 4). Itinvolves kilometre-sized tectonic slicesof deformed basement and coverrocks (Fig. 3G), and accommodatesa polyphase tectonics responsible forthe relative uplift of the southernblocks. Throws are estimated in ca.1500 m along cross-section Y–Y¢,while a right-lateral separation of thebasement-cover boundary, on theorder of 5 km, can be observed bothin the Ougnat and in the Saghroinliers.

Fission-track data

Fission-track analysis on apatite pro-vides (i) information on past strati-graphic and ⁄or tectonic loads and (ii)chronological constraints on exhuma-tion of rocks in the uppermost crustallevels, where temperatures are below100 ± 20�C (e.g. Gallagher et al.,1998). Samples for FT analysis werecollected along a NW–SE transectacross the major faults of the studyarea (Fig. 1), and were analysedaccording to the external detectormethod (Hurford, 1990).All detrital samples yielded an FT

age younger than the stratigraphic agepointing to post-depositional anneal-ing. Three groups of ages were recog-nized: samples north of the Iskn�Izekelli Fault yielded ages between88 and 95 Ma (s1–s3), while samplescollected between the Isk n�Izekelliand the Bou Larhzazil–Tinifift Faultsyielded ages between 139 and 159 Ma(s4–s8). The oldest ages (>200 Ma)are from samples collected south ofthe Bou Larhzazil–Tinifift Fault (s9–s10) (Tables 1 and 2). Modelling oftrack-length distributions (Laslettet al., 1987; Crowley et al., 1991)points to very slow cooling duringMesozoic–Palaeogene followed byrapid Neogene cooling, which wasfaster north of the Isk n�Izekelli Faultsince Late Miocene–Pliocene (Fig. 5).Grain ages in granites from the

Saghro basement (s8) show an extra-Poissonian distribution. This is acommon character in the Anti-Atlasgranitoids (cf. FT data by M.L. Bale-strieri in Dal Piaz et al., 2007) and isprobably because of compositionaleffects and U-zoning caused by slowapatite crystallization (Burchart, 1981;Kral and Burchart, 1983; Dempsteret al., 2003). Conversely, the occa-

sional extra-Poissonian grain-age dis-tribution in fast-cooled apatites fromvolcanoclastic samples (s9) is bestexplained by partial post-depositionalannealing, occurred at temperaturesaround but not higher than100 ± 20�C after deposition. Thepresent-day temperatures measuredin deep wells from the eastern Anti-Atlas are on the order of 100–105�C at3 km depth (Rimi, 2000). Therefore, asensible estimate of maximum burialexperienced by sample s9 is on theorder of 3 km, while burial was defi-nitely higher for the other samples.

Chronology of mesoscaledeformation

Bedding planes in the Palaeozoic suc-cession are deformed by submeridianfolds and NE–SW to E–W foldsclassically referred to the Variscanorogeny. This fabric is crosscut bystructures referred to four deforma-tion phases, distinguished in the fieldon the basis of deformation style,superposition relationships and anal-ysis of fault rocks. Deformation phaseD1 is marked by thick horizons ofcohesive cataclasites, best developedalong the Bou Larhzazil–Tinifift Faultand the Tizi n�Boujou Fault. Thearchitecture of lower-order fault seg-ments, the widespread fault fibres andS-C structures (Fig. 6A), and thearrays of en-echelon veins are consis-tent with left-lateral motion and thedownthrow of the northern blocks(Fig. 4). D1 mesostructures point toincremental NW–SE extension andNE–SW shortening consistent with aleft-lateral transtension on a regionalscale (Fig. 7). During deformationphase D2, the right-lateral reversere-activation of pre-existing faults isattested by top-to-SE drag folds, bestdeveloped along the Isk n�Izekelli andTizi n� Boujou Faults (Figs 4 and 6B),and by fault fibres, en-echelon veins(Fig. 6C) and S-C structures (Fig. 6A)consistent with a NW–SE incrementalshortening. D2 deformation inside theslightly deformed fault blocks isexpressed by conjugate arrays ofen-echelon veins (Fig. 6D), pressure-solution cleavage, and flexural slipalong bedding planes consistent withan E–W incremental shortening.These data are supportive of aNW–SE regional shortening duringphase D2 (Fig. 4). Finally, a late

Fig. 2 Stratigraphic sections of theCambro-Ordovician cover sequence ofthe eastern Anti-Atlas (location inFig. 1), showing the decreasing thick-ness of the main lithostratigraphic unitsfrom south to north. Above the Pre-cambrian basement and the Neoprote-rozoic volcanoclastic rocks (a), thelowermost Palaeozoic cover sequence21 (Lower–Middle Cambrian) startswith conglomerates and sandstones (b),followed by shales and carbonates (c),and by sandstones classically referred toas �Gres Terminaux� (d). The onset ofsequence 22 (Middle Cambrian) ismarked by a discontinuous level of�Breche a Micmacca� (e), followed byshales (f) and sandstones, the so-called�Gres du Tabanit� (g). After a LateCambrian hiatus of likely eustatic ori-gin, clastic sedimentation is resumed inthe Ordovician (O) with shales andsandstones (h to k), the latter formingtwo competent layers classically referredto as �1st Bani� (i) and �2nd Bani� (k). Thecover sequence of section 1, where Mid-dle Cambrian sediments rest directly onthe Precambrian basement, is locallydetached and probably allochthonous(Cerrina Feroni et al., 2007).

Terra Nova, Vol 19, No. 6, 481–489 M. Malusa et al. • Post-Variscan tectonics in eastern Anti-Atlas

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A B

C D

E F

G H


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reactivation of fault planes accordingto a left-lateral reverse kinematics isattested by widespread fibres andstriae along the Isk n�Izekelli and Tizin� Boujou Faults. Superposed slicken-

lines show that the regional-scaleshortening direction varied throughtime from NNE–SSW (D3) to NNW–SSE (D4). Phase D4 also affects theQuaternary deposits south of the

Atlas belt, which are deformed by E–W folds and NNW-dipping thrusts(Fig. 3H).

Discussion

The interpretation of stratigraphic,structural and thermochronologicaldata presented in this work unravelsan innovative scenario for the LatePalaeozoic to Present evolution of theeastern Anti-Atlas, summarized in thefollowing three points.

The missing Variscan belt

Resetting of apatites at the base of thePalaeozoic succession points to burialbeneath at least 3 km of rocks beforethe Mesozoic (Fig. 5). The thicknessof the Palaeozoic cover is consistentwith these findings south of the Oug-nat inlier (3 in Fig. 2), but it is notable to account for apatite resettingnorth of the Bou Larhzazil–TinififtFault, where the missing sedimentaryload ranges from few hundred metres

Fig. 3 (A) Fault-bound extensional basin in the Precambrian basement (PR) filled by Late Neoproterozoic volcanoclastic rocks (a)(Assif Toulla Unit in Dal Piaz et al., 2007), unconformably overlain by the Lower Cambrian cover sequence (21) (Assif Toulla,eastern border of the Saghro inlier); (B) Lower Cambrian conglomerates and sandstones (b) unconformably overlying thePrecambrian basement (PR) (Assif Anesnim, Ougnat inlier); (C) Tectonic unit with a penetrative axial-plane cleavage of Variscanage bounded by a S-verging thrust (V), overlying a mildly deformed Ordovician cover sequence (O); (D) NNW-dippingintermediate-angle reverse plane belonging to the Isk n�Izekelli Fault that displaces the Middle Cambrian shales (f) and sandstones(g) (Assif n�Ouaourguiouat, northern Saghro); (E) The Tizi n�Boujou Fault juxtaposing Middle Cambrian sediments (22) toOrdovician shales and sandstones (O) that are unconformably overlain by flows of olivine nephelinites (P) (Tachtafacht-Tazoultarea); (F) Eastern segment of the Tizi n�Boujou Fault, which juxtaposes the Precambrian basement (PR) to the Ordovician coversequence (O) (Ibenzerg, northern Ougnat); (G) Tectonic slices of deformed basement (PR) and cover rocks (b-to-e, see Fig. 2)along the Bou Larhzazil-Tinifift Fault (Tarhoulast, southern side of the Ougnat inlier); (H) Quaternary deposits (Q)unconformably overlying Upper Cretaceous sandstones (K2), crosscut by a top-to-S thrust (Assif n�Arg n�Sidi Ali ou Bourk,west of Tinerhir).

Fig. 4 Cross-sections across the major faults of the study area (location in Fig. 1).Their importance on a crustal scale is confirmed by N–S seismic reflection profiles eastof the Ougnat inlier (Robert-Charrue, 2006). The depth of the basement-coverboundary is indicated only when constrained by field data.

Table 1 Sample location and description*.

Sample Latitude Longitude Elevation Lithology Location

s1 – mrc 10 ⁄ 05 31�26¢26¢¢ 5�35¢57¢¢ 1300 Tholeiitic dyke Amane Oukider

s2 – mrc 3 ⁄ 04 31�23¢02¢¢ 5�30¢38¢¢ 1320 Neoproterozoic volcanoclastics Tisk n�Temda

s3 – mrc 3 ⁄ 05 31�22¢32¢¢ 5�28¢24¢¢ 1350 Neoproterozoic volcanoclastics Assif n�Ouaourguiouat

s4 – mrc 1 ⁄ 05 31�21¢27¢¢ 5�25¢08¢¢ 1325 Neoproterozoic volcanoclastics Jebel Taouarda

s5 – mrc 4 ⁄ 04 31�18¢32¢¢ 5�16¢33¢¢ 1190 Gres Terminaux Assif Tiferkhine, near the Tizi n�Afgane

s6 – mrc 28 ⁄ 04 31�20¢18¢¢ 5�08¢52¢¢ 1245 Gres Terminaux Oued Ifni, near Arhenbou n�Khenbour

s7 – mrc 11 ⁄ 04 31�17¢18¢¢ 5�01¢12¢¢ 1410 Neoproterozoic volcanoclastics Azlou

s8 – mrc 32 ⁄ 04 31�14¢58¢¢ 5�33¢58¢¢ 1720 Bou Gafer granite Tine Iouerkane

s9 – mrc 2 ⁄ 05 31�15¢32¢¢ 5�06¢06¢¢ 1140 Neoproterozoic volcanoclastics Amessoui, east of Tinifift

s10 – mrc 34 ⁄ 04 30�59¢00¢¢ 5�49¢20¢¢ 1250 Gres du Tabanit Tanoute n�Ousdidene

*Most of the samples belong to the lowermost levels of the cover sequence. We observed that the apatite content decreases upsection, thus most of the sediments

younger than the Lower Cambrian are not datable by the fission-track method.


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(2 in Fig. 2) up to at least 2 km (1 inFig. 2). Apatite resetting observednorth of the Isk n�Izekelli Fault canonly be explained by deep burialbeneath ca. 1 km of Palaeozoic sedi-ments plus 2 km of Variscan allochth-onous units now eroded, emplacedalong flat-lying thrusts onto thePalaeozoic cover of the Saghro andOugnat basement. Therefore, theVariscan belt was extended furthersouth than previously assumed, andthe southern Variscan front should belocated above the present-day base-ment inliers.

The Mesozoic rift shoulder

Fission-track data show that exhuma-tion in eastern Anti-Atlas was alreadyeffective after the Variscan orogenyand continued during the Mesozoic,but it was rather slow as attested bytime–temperature paths (Fig. 5).Mesozoic denudation in eastern Anti-Atlas was thus coeval with subsidenceand sedimentation in the Atlas region,that was related to the Triassic riftingof Pangea and to an Early–MiddleJurassic transtensional tectonics(Laville et al., 2004). The emplacementof NE–SW tholeiitic dykes of EarlyJurassic age in the Anti-Atlas wasfavoured by NW–SE local extension,which may be related to the left-lateraltranstensional motion described alongthe Tizi n�Boujou and Bou Larhzazil-

Tinifift Faults (D1). This transtension-al regime led to the relative uplift of thesouthern blocks with kilometre-scalethrows (Fig. 4). Therefore, the easternAnti-Atlas has represented the denud-ing shoulder of the Atlasic transten-sional rift during the Mesozoic, anddelivered clastic sediments into the riftand in the nearby craton. In thisframework, the Cretaceous trans-gression affecting large areas east ofTinejdad (e.g. Ferrandini et al., 1985)was probably confined to the westinside narrow fault-bound basins(Laville et al., 2004).

The southern front of Atlasic orogeny

The Atlasic domain was inverted anduplifted during Miocene to form theAtlas belt (Schmidt, 1992; Barberoet al., 2007), and underwent NW–SEshortening followed by submeridiancontraction attested by deformationof Neogene continental deposits(Morel et al., 2000; Ait Brahim et al.,2002). The Anti-Atlas region clearlyrecords the effects of the Atlasic upliftas shown by its fast Neogene exhu-mation (Fig. 5). The apparent breakin FT ages documented across the Iskn�Izekelli Fault (Fig. 1) cannot beexplained by a progressive northwarderosion of the Variscan units exposedon the former rift shoulder, becausethis would produce a gradual varia-tion in FT ages. Conversely, it points

to major reverse faulting associatedwith the upthrow of the northernblocks. Time–temperature pathsconstrain this faulting event to theNeogene (Fig. 5), with hangingwalluplift starting in the Late Miocene–Pliocene. This kinematic scenario isconsistent with the right-lateral trans-pressional tectonics (D2), associatedwith NW–SE shortening, describedalong the Tizi n�Boujou and Iskn�Izekelli Faults (Figs 4 and 7), andwith the later deformation phasescharacterized by NNE–SSW (D3) toNNW–SSE (D4) regional shortening.It can be reasonably concluded thatdeformation phases D2-to-D4 wereresponsible for the Neogene transpres-sional re-structuring of the easternAnti-Atlas. Deformation phase D4 isstill active, as attested by displacementof Quaternary deposits on the south-ern side of the Atlas belt (Fig. 7). It isassociated with intraplate volcanismand is probably responsible for therecent Rissani earthquake (Hahouet al., 2003). Therefore, the southernfront of Atlasic orogeny, located sofar along the South-Atlas Fault,should be placed further to the southinside the Anti-Atlas region.

Conclusions

This work describes a polyphasethick-skinned tectonics that affectedthe Anti-Atlas during the break-up

Table 2 Apatite fission-track data from the eastern Anti-Atlas*.

Sample Elev., m Xls

Spontaneous Induced

P(v)2

Dosimeter

AFT age, (Ma) ± 1r U, ppm

Mean track length,

lm ± 1r (n)Rho-s Ns Rho-i Ni Rho-d Nd

s1 – mrc 10 ⁄ 05 1300 9 5.351 235 5.647 248 81.42 5.967 1418 95.4 ± 8.8 19.69 –

s2 – mrc 3 ⁄ 04 1320 18 16.486 1050 31.229 1989 10.84 10.110 3281 90.4 ± 4.4 41.50 9.42 ± 2.74 (100)

s3 – mrc 3 ⁄ 05 1350 20 14.470 1088 16.970 1276 99.32 6.109 1452 88.0 ± 3.9 37.00 9.55 ± 2.97 (100)

s4 – mrc 1 ⁄ 05 1325 20 12.523 1894 8.933 1351 89.05 6.203 1474 146.2 ± 5.7 17.36 9.09 ± 2.89 (100)

s5 – mrc 4 ⁄ 04 1190 5 11.214 113 12.604 127 6.02 10.190 3248 159.1 ± 29.6 15.11 –

s6 – mrc 28 ⁄ 04 1245 20 9.681 762 7.470 588 82.33 6.487 1542 141.4 ± 8.1 18.99 –

s7 – mrc 11 ⁄ 04 1410 10 13.839 271 17.056 334 97.84 10.260 3214 140.0 ± 11.7 25.50 –

s8 – mrc 32 ⁄ 04 1720 20 22.85 4280 17.793 3332 <1 6.392 1519 138.5 ± 6.7 33.96 –

s9 – mrc 2 ⁄ 05 1140 21 19.394 3289 9.700 1648 <1 6.156 1463 216.0 ± 11.4 17.87 –

s10 – mrc 34 ⁄ 04 1250 1 24.125 172 8.977 64 – 6.345 1505 283.6 ± 42 16.02 –

*Samples were dated using the external detector method, they were irradiated in the atomic reactor �TRIGA Mk II� at the Oregon State University (USA) and belong to

two different irradiation packages. Neutron fluences were measured counting neutron-induced tracks in Corning glass dosimeters CN-5, placed both at the top and at

the bottom of the sample holder to have the maximum control on the neutron fluence. Durango and Fish Canyon Tuff apatites were used as standards. Apatite grains

were etched with 5 N HNO3 at 20�C for 20 s. Muscovite detectors were etched in 40% hydrofluoric acid for 45 min. Counting has been carried out by M. Malusa

(mean zeta value 340.05 ± 4.29). Abbreviations are as follows: Elev., sample elevation; Xls, number of individual grains dated; Rho-s, spontaneous track density

(·105 tracks per cm2); Ns, number of spontaneous tracks counted; Rho-i, induced track density in external detector (·105 tracks per cm2); Ni, number of induced

tracks counted; P(v)2, probability of obtaining v2 value for n degrees of freedom (where n = number of crystals ) 1): a probability >5% is indicative of a

homogeneous population; Rho-d, induced track density in external detector adjacent to dosimeter glass (·105 tracks per cm2); Nd, number of tracks counted in

determining Rho-d; AFT age is the sample�s central fission-track age calculated using zeta calibration method (s1–s9); only one grain was dated in sample s10, so this

is just a rough estimate of its potential fission-track age; (n) is the number of measured track-lengths.


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of Pangea and the Neogene con-vergence between the African andEurasian plates. In the Late Palaeo-zoic, the basement inliers were partlyoverlain by allochthonous unitsascribed to the North AfricanVariscan belt. In the Mesozoic, the

eastern Anti-Atlas acted as thesouthern shoulder of the Atlasic rift,developed in the frame of a trans-tensional left-lateral regime coevalwith the opening of the AtlanticOcean. Finally, this area underwenta dextral transpressional structuring

of Neogene age followed by sub-meridian shortening, associated withremarkable fault reactivation,seismicity and volcanism. On thenorthern rim of the West AfricanCraton, NNW–SSE shortening is stillactive.

Fig. 5 Time–temperature paths modelled using the program HeFTy by R. Ketcham. Both the Laslett et al. (1987) (L) and theCrowley et al. (1991) (C) equations have been employed, the latter providing better predictions of track annealing at geologicaltime scales. After 106 iterations by the Monte Carlo method, both methods provided comparable results. On the left, best-fit path(thick line) and acceptable paths envelope (grey area) according to (C); geological constraints to modelling (squares) are theVariscan tectonic load and the age of nephelinite flows. Neogene to present cooling paths in the hanging wall (s2, s3) and footwall(s4) of the Isk n�Izekelli Fault, according to (L) and (C), are compared in the central inset. On the right, measured (histogram) andmodelled (continuous and dashed lines) track-length distributions. T–t paths across the Isk n�Izekelli Fault attest >3 km post-Variscan burial caused, according to field data, by the Cambro-Ordovician cover plus the overlying allochthonous Variscan units(Fig. 2). The slow Mesozoic cooling indicates that the study area was not site of active Mesozoic sedimentation and relatedtemperature increase due to sedimentary burial; conversely, Mesozoic cooling is ascribed to erosion that, even if characterized bymoderate rates, compensated any potential increase of geothermal gradient during rifting. The fast Neogene cooling is a mirror ofthe Atlasic orogeny, and fits nicely with the T–t paths from the central Atlas (cf. Barbero et al., 2007).


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Acknowledgements

This work was carried out within the frame-work of the CNR Research Project �Trans-pressional evolution of orogenic systems:the South-Atlas Fault�. We thank PieroPertusati for the stimulating discussions inthe field, Max Zattin and the fission-tracklab in Bologna for technical support andquality advice, and the �Ministere de l�En-ergie et des Mines� of Morocco for allow-ance to export samples. The manuscriptbenefited from constructive comments andcareful reviews by Andre Michard, MarcJolivet, and Barbara Ventura.

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A B

C D

Fig. 6 (A) Syn-D1 left-lateral S-C structures deformed by syn-D2 right-lateral shear-planes (c2), along the Bou Larhzazil–TinififtFault (Asrhar Loun, SE of Tizi n�Tiferkine); (B) Syn-D2 top-to-SE drag folds consistent with a right-lateral reverse re-activation ofthe Tizi n� Boujou Fault (Aourarh, NE margin of the Saghro inlier); (C) Syn-D2 array of en-echelon veins consistent with right-lateral motion along the Tizi n�Boujou Fault (northern Ougnat); (D) Syn-D2 deformation inside a slightly deformed fault blockexpressed by an array of en-echelon veins (Assif Tiferkine, W of the Tizi n�Afgane).

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Received 28 February 2007; revised versionaccepted 24 September 2007


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Post-Variscan tectonics in eastern Anti-Atlas (Morocco

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