Variscan belt along the edge of the West African craton The Anti

Geological Society, London, Special Publications
The Anti-Atlas chain (Morocco): the southern margin of the
Variscan belt along the edge of the West African craton
Abderrahmane Soulaimani and Martin Burkhard
Geological Society, London, Special Publications 2008; v. 297; p. 433-452
doi:10.1144/SP297.20
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The Anti-Atlas chain (Morocco): the southern margin of the
Variscan belt along the edge of the West African craton
ABDERRAHMANE SOULAIMANI1,2 & MARTIN BURKHARD3,4
1
Université Cadi Ayyad, Faculté des Sciences Semlalia, av. Moulay Abdellah,
BP 2390, Marrakech, Morocco
2
Present address: 18 rue Stephenson, 59000 Lille, France
(e-mail: [email protected])
3
Institut de Géologie, Université de Neuchâtel, rue Émile-Argand 11, CP 2,
2007 Neuchâtel, Switzerland
4
Deceased August 2006
Abstract: Broadly synchronous circum-Atlantic Variscan –Alleghanian orogenic belts developed
during the Late Palaeozoic Gondwana –Laurentia collision. In the northern part of the West
African craton (WAC), the Variscan orogeny produced basement-controlled structures in the
Anti-Atlas, which represents the pericratonic foreland, now located south of the Variscan
domains of Morocco and north of the Mauritanides belt. New structural field observations document the strong involvement of the basement and the inversion and folding of the Palaeozoic sedimentary basins at the edge the WAC. Two contrasting domains differently responding to regional
NW– SE shortening are recognized: (1) a narrow belt along the Atlantic coast characterized by
thin-skinned folding and ESE-vergent thrusting (para-autochthonous Anti-Atlas); (2) a large
area between the WAC sensu stricto and the South Atlas front showing huge basement uplifts
amidst a folded Palaeozoic cover with upright polyharmonic folds (autochthonous Anti-Atlas).
The structural trend of the basement inliers is inherited at least in some case from previous Proterozoic fractures. Compressional reactivations led to basement uplift and concomitant folding of
the Palaeozoic cover. Cover series are horizontally shortened by mostly upright symmetrical
buckle folds of various wavelengths in response to thickness variations between abundant incompetent silt and shale horizons and rare competent carbonate and quartzite beds. Deformation is
greatest near the borders of and between closely spaced basement uplifts. Regionally, deformation
intensity decreases, either abruptly or progressively, towards the SE and it vanishes within the
undeformed Tindouf basin.
The West African craton (WAC), stable since 2 Ga,
constitutes the basement of northwestern Africa
(Rocci et al. 1991). Along the northern margin of
this craton, especially in Morocco, the Reguibate
shield is rimmed by a series of mobile belts with
decreasing ages toward the north (Fig. 1a). The
southernmost of these zones, the Anti-Atlas belt,
is located between the Alpine Atlas chain and the
northern rim of the Palaeozoic Tindouf basin.
Gentil (1918), Lecointre (1926) and many other
workers showed that a Late Palaeozoic orogeny
affected the northern Moroccan Meseta as well as
the Palaeozoic inliers of the Atlas and Rif. The
Anti-Atlas belt exposes thick, unmetamorphosed
and often mildly deformed Palaeozoic rocks,
which unconformably overlie a Precambrian basement included in several NE –SW inliers (Choubert
1952). Piqué & Michard (1989) proposed that these
Moroccan Palaeozoic structures were contemporaneous with the Variscan orogeny in Europe.
The oldest Precambrian rocks exposed in the
Anti-Atlas belt represent splinters derived from
the Eburnean WAC (Aı̈t Malek et al. 1998;
Thomas et al. 2002; Walsh et al. 2002; Gasquet
et al. 2004). However, although comparable rocks
located within the shield remained undisturbed
since about 2 Ga, those of the Anti-Atlas region
display evidence of two superimposed pre-Variscan
orogenic events: (1) a Neoproterozoic compressive
event, related to the Pan-African orogeny (Leblanc
1975; Saquaque et al. 1989); (2) a subsequent
extensional event, which affected the northern
margin of the craton during the Early Palaeozoic,
or as early as the late Neoproterozoic (Piqué et al.
1999; Doblas et al. 2002; Soulaimani et al. 2003).
During this extension, the Anti-Atlas domain
experienced rifting, tilting of basement blocks and
the creation of sedimentary rift basins. This event
is documented by the volcanoclastic series of the
Latest Proterozoic ‘Ouarzazate Supergroup’ to
From: ENNIH , N. & LIÉGEOIS , J.-P. (eds) The Boundaries of the West African Craton. Geological Society, London,
Special Publications, 297, 433–452.
DOI: 10.1144/SP297.20 0305-8719/08/$15.00 # The Geological Society of London 2008.
434
A. SOULAIMANI & M. BURKHARD
Fig. 1. (a) Location of the Anti-Atlas within the main structural domains of NW Africa. (b) Generalized geological map of the Anti-Atlas, simplified from the 1:1 000 000
geological map of Morocco (Maroc Service Géologique 1985); showing the location of the areas studied: 1, western Atlantic Bas Drâa; 2, eastern Bas Drâa and western Bani;
3, Lakhssas Plateau; 4, Irherm–Tata; 5, Bou Azzer–El Graara; 6, Saghro –Ougnate. Inliers: BD, Bas Drâa; If, Ifni; Kr, Kerdous; Ir, Igherm; TA, Tagragra d’Akka;
TT, Tagragra Tata; AM, Agadir Melloul; Ze, Zenaga; Sr, Sirwa; Bz, Bou Azzer– El Graara; Sg, Saghro; Og, Ougnate.
THE ANTI-ATLAS CHAIN, MOROCCO
Earliest Cambrian (terminology after Thomas et al.
2004). Synsedimentary extensional deformations
are also recorded within the Early Cambrian
marine ‘Taroudant Group’ (Buggisch & Siegert
1988; Algouti et al. 2002; Benssaou & Hamoumi
2003). Later, up to 10 km of shallow-marine sediments overlying the synrift successions were deposited during the rest of the Palaeozoic in an
intracratonic setting (Burkhard et al. 2006), at
least in the western –central Anti-Atlas. The architecture of the Precambrian basement inherited
from the Late Proterozoic– Early Cambrian extensional episode plays a significant role in the geometry of subsequent geodynamic events. In
particular, it has a strong influence on the Variscan
structures that developed in the course of the Late
Carboniferous compressive deformations.
The role of the Precambrian basement configuration controlling the mode of Variscan shortening
has been documented in many studies (Michard
1976; Jeannette & Piqué 1981; Soulaimani et al.
1997). Different trends of the Variscan regional
folds have been interpreted as the result of reactivation of basement faults, in particular by the basement uplifts following the inherited zones of
crustal weakness (Leblanc 1972, 1975; Donzeau
1974; Jeannette & Piqué 1981; Hassenforder
1987; Soulaimani 1998; Belfoul et al. 2001). Similarly, variations in the thickness of the Palaeozoic
cover series between and above the Precambrian
rigid basement uplifts induced the development of
disharmonic folds with different wavelengths, separated by numerous décollement levels (Soulaimani
et al. 1997; Burkhard et al. 2001; Caritg et al. 2004;
Helg et al. 2004).
This paper provides a comprehensive description of the geometry of the Anti-Atlas Variscan
belt, including a kinematic study that has been
hitherto missing from the literature as well as inferences for the geodynamics of this southern branch
of the Variscan belt. The present synopsis is based
on the geometry and finite strain observations of
the Variscan deformation in several key areas of
the Anti-Atlas.
Regional geology of the Anti-Atlas
General structural pattern
The Anti-Atlas chain is a broad anticlinorium some
800 km long and 200 km wide, trending ENE–
WSW (Fig. 1), parallel to the Alpine High Atlas
chain. This situation suggests that Alpine –Atlas
rejuvenation might have contributed significantly
to the present-day elevation (up to 2500 m) of the
Variscan Anti-Atlas belt. Indeed, this recent uplift
is well established in the eastern part of the
435
Anti-Atlas (Ougnat and Tafilalt areas), where the
Cretaceous and Neogene cover series are clearly
tilted northward along the northern border of the
belt (Robert-Charrue 2006). In comparison with
the structural observations in the adjacent High
Atlas, the Alpine uplift of the Anti-Atlas belt
occurred during the Pliocene –Pleistocene (Frizon
de Lamotte et al. 2000), partly owing to a
Neogene thermal uplift (Teixell et al. 2005; Missenard et al. 2006).
Along the main axis of the Anti-Atlas anticlinorium, the most extensively exposed rocks are carbonates of Early to Middle Cambrian age,
wrapping around a series of Precambrian basement
inliers (Bas Drâa, Kerdous, Irherm, Zenaga, Bou
Azzer –El Graara, Saghro, etc.). Because Cambrian
carbonates are more resistant to erosion than the
Proterozoic crystalline basement rocks, these basement ‘uplifts’ often correspond to topographic
depressions. This explains the term ‘boutonnière’
(literally ‘buttonhole’) of the French workers
(Choubert 1952, 1963). Along the southern border
of the Anti-Atlas anticlinorium, Palaeozoic sedimentary series dip generally to the SSE, thus revealing successively younger strata to the south (Fig. 1).
The total thickness of the Palaeozoic stratigraphical
column varies regionally from more than 10 km in
the west (Fig. 2) to less than 5 km in the east.
Palaeozoic cover sequences are disharmonically
folded with deformation intensity decreasing
towards the SE on a given transect. Along strike,
folding style also changes and a general decrease
in folding intensity is apparent from west to east.
In the north, the Alpine orogeny affected the
northern part of the Anti-Atlas, creating thrust
faults and folds, contemporaneous with the High
Atlas structures. These Alpine structures are unconformably overlain by Neogene clastic sediments of
the south-Atlas Souss and Ouarzazate basins.
Tectonostratigraphic units
The Proterozoic crystalline basement of the AntiAtlas and its thick (.10 km) sedimentary cover
have been extensively described (e.g. Choubert
1963; Destombes et al. 1985; Thomas et al. 2004;
Gasquet et al. 2005) and therefore only a synthetic
column will be briefly described below (Fig. 2).
The Proterozoic basement. The Proterozoic rocks of
Morocco are classically subdivided into three major
unconformity-bounded assemblages (Choubert
1963; Leblanc 1975; Charlot 1978; Hassenforder
1987). The lowermost Palaeoproterozoic units
consist of low- to medium-grade schists and intrusive granitoids, some of them now in the form of
orthogneisses, attributed to the Eburnean orogeny
(c. 2 Ga) (Aı̈t Malek et al. 1998; Thomas et al.
436
A. SOULAIMANI & M. BURKHARD
2002; Walsh et al. 2002; Gasquet et al. 2004).
Eburnean basement rocks occur exclusively SW
of the ESE–WNW-trending Anti-Atlas Major
Fault (AAMF) (Choubert 1947). However, they
probably constitute the basement of the Pan-African
rocks to the NE (Ennih & Liégeois 2001; Gasquet
et al. 2005).
Consistently with observations elsewhere in the
West African craton, no Mesoproterozoic event or
rocks are recorded in the Anti-Atlas. Palaeoproterozoic basement rocks are, therefore, unconformably
overlain directly by Neoproterozoic metasedimentary and magmatic units, which were subsequently
affected by the Pan-African orogeny (Clauer
1974; Leblanc 1975; Hassenforder 1987). The
Pan-African event was related to the oblique suturing of the northern rifted margin of the Eburnean
continental mass and the Neoproterozoic Saghro
magmatic arc (Saquaque et al. 1989). An ophiolitic
complex has been described in the central AntiAtlas along the AAMF from the southern side of
the Sirwa Massif (Admou & Juteau 2000; Thomas
et al. 2002) to the Bou Azzer– El Graara inlier
(Leblanc 1975; Saquaque et al. 1992). The polarity
of the Neoproterozoic subduction is still a matter of
debate (Ennih & Liégeois 2001; Hefferan et al.
2000; Gasquet et al. 2005), but the present-day
deep structure supports the hypothesis of a worthdipping subduction zone, at least during the late
Pan-African stage (Soulaimani et al. 2006).
The unconformably overlying syntectonic volcanoclastic molasse deposits, attributed to the top
of the ‘Saghro Group’, have been affected by the
ultimate phases of the Neoproterozoic Pan-African
deformation (Thomas et al. 2002). In the Bou
Azzer –El Graara inlier, the Tiddiline series are
regarded as late Pan-African syntectonic molasse
deposits, folded and cut across by sinistral oblique
north-dipping thrust faults in late Pan-African syntectonic molasse basins (Leblanc & Lancelot
1980; Hefferan et al. 1992).
Fig. 2. Synthetic tectonostratigraphic columns for
the Proterozoic basement and Palaeozoic cover of
the Anti-Atlas.
The Late Precambrian and Palaeozoic cover. The
Latest Neoproterozoic rocks are the sedimentary,
mainly clastic, and volcanic rocks of the ‘Ouarzazate Supergroup’, separated from the crystalline
basement by a major unconformity. They grade
upward after a slight unconformity into the ‘Adoudounian’ carbonates, marls and siltstones, which
record the Early Cambrian marine transgression.
These latest Neoproterozoic–Early Cambrian
cover sequences unconformably overlie older Precambrian basement structures. The ‘Ouarzazate
Supergroup’ sequences are characterized by dramatic thickness changes across fault-bounded basement blocks (Piqué et al. 1999). They were
deposited in faulted basins developed in an
THE ANTI-ATLAS CHAIN, MOROCCO
intracontinental rifting context during a late to
post-Pan-African extensional event (Azizi Samir
et al. 1990; Thomas et al. 2002; Soulaimani et al.
2003). This event is associated with a major
calc-alkaline late to post-orogenic magmatism
(Boyer et al. 1978; Youbi 1998), which evolved
toward tholeitic and alkaline lavas by the end of
the Proterozoic and during the Early Cambrian
(Jbel (J.) Boho seynite) (Ducrot & Lancelot 1977;
Soulaimani et al. 2004; Álvaro et al. 2006).
The geometry and geodynamic significance of this
rifting episode are still open to discussion. This event
could correspond to a late orogenic extensional collapse of the Pan-African chain. In the most recent
interpretation (Soulaimani & Piqué 2004; Oudra
et al. 2006), the Late Proterozoic extensional event
is associated with an important tectonothermal
reworking of the Precambrian basement, which
would have formed metamorphic domes, similar to
the related structures developed along the rim of the
West African craton during Late Precambrian–Early
Cambrian times (Doblas et al. 2002).
The Adoudounian rocks are conformably overlain by a post-rift sedimentary sequence starting
towards the end of the Early Cambrian and continuing through most of the Palaeozoic. From the
Middle Cambrian to the Middle Devonian, the AntiAtlas domain was a shallow-marine shelf, where
mostly fine-grained detrital sediments eroded from
the West African craton accumulated. A longlasting subsidence resulted in a progressive southward migration of the shoreline over the Reguibate
shield. Transgressive sediments of the Anti-Atlas
domain thus became progressively finer grained
upward, forming a megasequence consisting of
Ordovician sands and silts, Silurian shales and,
except in the eastern Anti-Atlas, Early and Middle
Devonian platform carbonates.
The presence of very fine-grained clastic detritus
within the Late Devonian carbonates might indicate
the end of the passive transgressive regime and the
beginning of the central Anti-Atlas uplift (Hassenforder
1987). The Early Carboniferous strata of the eastern
Anti-Atlas, which include olistostromes, were deposited in an intramontane trough. Along the southern
border of the Anti-Atlas, the Late Viséan, Namurian
and Westphalian sequences of the Ouarkziz, Betana
and Bechar remained undisturbed, although they
were deposited in a regressive mega-sequence.
The Late Mesozoic-Cenozoic cover. Triassic and
Jurassic sediments are limited to the north, along
the South Atlas Fault in the Ouarzazate and Souss
basins, and in the SW, in the Atlantic Tarfaya
basins onshore and offshore (e.g. Le Roy & Piqué
2001). They are related to the Pangaea break-up
and opening of the central Atlantic Ocean and
Atlas rift basins in Late Triassic– Early Jurassic
437
times. This extension affects both the crystalline
basement and the folded Palaeozoic cover of the
inner Anti-Atlas itself, being recorded there by the
intrusion of NE– SW-trending dykes and associated
sills of gabbro and dolerite (Sebai et al. 1991).
During the Late Cretaceous, a shallow sea
invaded the internal domains of the Anti-Atlas,
resulting in the deposition of vast flat-lying strata
(hamada). The deposition of these nearly horizontal
plateaux lasted until the Oligocene, and sealed the
eroded Palaeozoic fold belt.
The Variscan deformation
The western Atlantic Bas Drâa area
In the westernmost part of the Anti-Atlas, south of
the Precambrian Ifni inlier, the western Atlantic
Bas Drâa domain is a narrow, elongate terrane
located between the Bas Drâa inlier to the east
and the Atlantic coast to the west (Fig. 1).
Deformed Cambrian rocks constitute NNE–
SSW-trending ridges (Choubert 1963; Soulaimani
1998; Belfoul et al. 2001). The folds, with wavelengths decreasing eastward, are cut by repeated
east-vergent thrust faults (Fig. 3b). These thrust
units bounding minor units form recurrent parallel
ridges responsible for the ‘valley and ridge’ morphology of this region. At the outcrop scale, the
main structure shown in the Cambrian rocks is a
west-dipping penetrative cleavage. Along the
Atlantic coast, Lower Cambrian strata are involved
in tight recumbent folds, trending NNE –SSW and
clearly overturned eastward as shown by their wellpreserved inverted limbs. Their axial planes are parallel to a pervasive foliation, which is slightly
inclined to the WNW to subhorizontal; for
example, at the so-called ‘Plage Blanche’
(Fig. 3d). The WNW-plunging stretching lineation
is parallel to the dip of the cleavage. Shear bands
related to non-coaxial flow along the cleavage
planes represent the latest Variscan structures.
Regionally the deformation intensity decreases
rapidly eastward, and in the Bou-Jerif area the
Cambrian strata are deformed only by decametrescale folds with a westward steeply dipping rough
cleavage, mainly visible in the pelitic layers.
Farther to the east, in the Goulmime area some
20 km from the Atlantic shoreline, the Middle
Cambrian strata are only slightly tilted from a nearhorizontal orientation and lack any cleavage.
The metamorphic conditions associated with the
deformation nowhere exceed the lower greenschist
facies. The typical east-vergent thrusts and folds
described in this area, which considerably differs
from the other Anti-Atlas areas, can be seen as
typical of the external zone of an orogenic belt.
438
A. SOULAIMANI & M. BURKHARD
Fig. 3. (a) Location of the Atlantic Bas Drâa area in the Anti-Atlas belt. (b) Structural map of the western Atlantic
Bas Drâa domain. (c) Stereograms (Wulff stereonet, lower hemisphere) representing bedding data (S0) and
foliation data (S1). (d) Geological cross-section (section location A– A0 in the map).
The eastern Bas Drâa and western Bani area
East of the Goulmine–Tan Tan meridian line, the
Bas Drâa area (Fig. 4b) exposes Precambrian basement (Bourcart 1937; Choubert & Faure-Muret
1969) and its Palaeozoic cover affected by the
Variscan contractional deformation (Mazéas &
Pouit 1968; Soulaimani et al. 1997). The crystalline
basement remained relatively undeformed throughout the Variscan orogeny, showing only reverse
faults along its margins. In contrast, intense deformation is observed at the base of the sedimentary
cover, especially within the Late Precambrian volcanoclastic rocks of the ‘Ouarzazate Supergroup’. A
locally pervasive ENE –WSW-striking, moderately
to steeply dipping foliation occurs NW of the basement massif. To the SE of the massif, where
way-up criteria can be ascertained, kinematic indicators point to a clear thrust component toward the
SE. A strong stretching lineation is observed
within the deformed ‘Ouarzazate Supergroup’ conglomerates, associated with a pervasive axial
planar cleavage and tight to isoclinal SE-vergent
folds. Detailed mapping of these regional structures
around the inlier (Fig. 4b) demonstrates that the cleavage intensity increases towards the margins of the
inlier, where cleavage is rotated into parallelism
with the boundary of the inlier. This cleavage refraction suggests that the basement acted as a rigid
buttress during the contractional deformation. The
basement was pushed upon the cover units along
its southern border (Fig. 4c; Soulaimani et al. 1997).
To the SE of the Bas Drâa inlier, disharmonic
folds in the Lower Cambrian limestones suggest
strain heterogeneity across the ductile deformed
strata. Overlying Cambrian –Ordovician Bani
sandstones are deformed by kilometre-scale open
and cylindrical folds, slightly overturned southeastward and associated with an incipient cleavage in
their hinges. To the SE, the J. Rich folds developed
within the Devonian sandstones and carbonates are
characterized by a decametre- to kilometre-scale
wavelength with subhorizontal fold axes. Folds
generally appear asymmetrical with their southern
limbs in a subvertical to overturned position.
Oblique folds, with deviations as much as
20 –308E from the regional N708E fold axis trend
of the J. Rich, display an en-echelon orientation.
THE ANTI-ATLAS CHAIN, MOROCCO
439
Fig. 4. (a) Location of the eastern Bas Drâa and western Bani area in the Anti-Atlas belt. The stereograms (Wulff
stereonet, lower hemisphere) represent bedding data (S0) and foliation data (S1). (b) Structural map of the eastern
Bas Drâa and western Bani area and (c) geological cross-section (section location B–B0 in the map).
These en-echelon folds are attributed to ENE–
WSW-striking dextral fractures. The asymmetry
of these folds has been interpreted as the result of
a right-lateral N708E-striking Variscan shear zone
along the southern border of the Anti-Atlas, parallel
to the South Atlas Fault (Michard 1976; Jeannette &
Piqué 1981). However, the geometry of the J. Rich
folds, and the gentle lateral plunge of their axes, is
regarded as incompatible with such a mega-shear
zone interpretation, let alone with the ‘Alpine
wrench folding’ with vertical axes such as proposed
by Weijermars (1993).
South of the Oued Drâa, the Upper Devonian
shales are the southernmost formations of the AntiAtlas affected by metre-size folds. The J. Tazout
and J. Ouarkziz Carboniferous strata are slightly
tilted toward the south, at the northern border of
the Tindouf basin. The J. Ouarkziz limestones are
imprinted only by tectonic stylolites, indicating a
very small amount of horizontal shortening (Helg
et al. 2004).
The Lakhsass Plateau area
North of the Bas Drâa domain, about 100 km south
of Agadir, the Lakhsass Plateau (Fig. 1) is located
between the Ifni and Kerdous inliers. The plateau
area corresponds to a large synclinorium of Lower
Cambrian carbonate rocks showing an anticlinal
structure (J. Inter horst) in its core (Fig. 5b). In
the central part of the plateau, gravimetric and magnetic data suggest the presence of an uplifted basement block (J. Inter) beneath the deformed
limestones. This basement high is interpreted as a
horst produced by the Late Proterozoic extensional
episode, and subsequently inverted during the
Variscan compression (Soulaimani 1998).
The Variscan compressive deformation in the
Lakhsass Plateau area is very heterogeneous, with
the greatest intensity towards the centre (Fig. 5c).
In the cover rocks above the basement horst, and
particularly along its flanks, the prominent fabric
comprises a pervasive north–south-striking axial
planar cleavage associated with tight to isoclinal
upright folds. A subvertical stretching lineation is
observed along ductile shear planes east of the
J. Inter, suggesting a significant component of
dip-slip motion. Deformation decreases laterally
from the central horst area towards the margins of
the synclinorium, where the Upper Proterozoic
and Lower Palaeozoic strata are gently tilted
towards the axis of the plateau. Everywhere,
440
A. SOULAIMANI & M. BURKHARD
Fig. 5. (a) Location of the Lakhssas Plateau area in the Anti-Atlas belt. (b) Structural map of the Lakhssas Plateau area
and (c) geological cross-section (section location C– C0 in the map). The Stereograms (Wulff stereonet, lower
hemisphere) represent bedding data (S0) and foliation data (S1).
bedding strikes north– south and dips either to the
east or west, defining folds with hinges plunging
gently to the north or south.
We conclude that Lakhssas Plateau has been
affected by a major Variscan deformation with a
nearly horizontal regional compressive maximum
stress. During this event, pre-existing basement
fractures were reactivated, leading mainly to the
uplift of the J. Inter horst, and of the Kerdous and
Ifni inliers. These reactivations and the induced
compressional structures in the overlying cover
probably operated in a transpressional zone, as
suggested by the NW– SE direction of shortening
at the regional scale.
THE ANTI-ATLAS CHAIN, MOROCCO
The Irherm – Tata area
Northeast of the Kerdous inlier (Fig. 1), the
Irherm –Tata area is a zone with wide synclines
cored by Middle Cambrian rocks (Fouanou,
Issafene, Talat N’Issi and Tagmout), and separated
from each other by narrow anticlines cored by
Precambrian rocks (Aı̈t Abdellah, Alma, Igherm
and Tata) (Fig. 6b). Within the northern and
southern parts of the area, the structural pattern of
the Variscan folds seems erratic and, according to
one of us (M.B.), at least two successive folding
episodes are recognizable (Caritg et al. 2004;
Helg et al. 2004; Burkhard et al. 2006). However,
this is not supported by field observations of
refolded axes or intersecting cleavage planes.
Moreover, there is a regional pattern of fold
trends that would instead suggest a mosaic of
basement blocks, as follows.
(1) To the south, in the Tata region, the east –west
J. Bani Quartzite ridge (‘Tata Fault’: Hassenforder
1987; Faik et al. 2001; Caritg et al. 2004; Helg
et al. 2004) separates two areas: a southern area
where the NE–SW-trending J. Rich folds are
abruptly juxtaposed against the J. Bani quartzite
ridge, and a northern area where south-vergent
folds strike east– west to NW– SE. The abrupt
change of the general strike from NE –SW to
east –west was interpreted as a response to dextral
shear along the Tata Fault (Hassenforder 1987;
Faik et al. 2001), although transcurrent displacement along this zone is not apparent at the outcrop
scale. The north– south to N208E directions of the
Variscan foliations and folds observed at the
western prolongation of the Tata Fault, close to the
Agouliz inlier, suggest that the latter massif was
less reactivated, with transcurrent motion during
the Variscan compression. It is important to note
that the Tata Fault, like many other east –west
faults within the Anti-Atlas, still controls the
recent Atlas uplift, which was estimated here to
reach some 600 m (Choubert 1952).
(2) North and NW of the Tata region, the
kilometre-scale Variscan folds yield a roughly
north–south-striking trend (N160 –308E) with symmetrical (Talat n’Issy and Fouanou) (Fig. 6c) or
asymmetrical (Issafene) synclines. In the lowermost
Cambrian pelitic strata, north–south-trending,
decamete-scale folds containing a vertical axialplane cleavage are well developed. These folds progressively vanish upward, indicating a décollement
level between the Cambrian limestones and the
overlying, mainly sandy strata.
(3) Between the Irherm inlier and the Wawfengha inlier, N70 –1108E anticlinal axes, bedding
planes and decametre-scale fold axes in the Lower
Cambrian rocks suggest a sinistral shear controlled
by regional east –west sinistral faults (Fig. 6b).
441
(4) North of the Wawfengha inlier, the Lower
Cambrian limestones rocks are not folded, but
they are cut by east– west subvertical faults and
tilted to the north, and finally concealed below the
Quaternary alluvium of the Souss plain. Most probably, part of these faults operated during
Atlas deformation.
The Bou Azzer – El Graara area
In the central part of the Anti-Atlas (Fig. 1), the Bou
Azzer –El Graara inlier is an eroded NW–SE Variscan basement high with a box-fold shape that
borders the Anti-Atlas Major Fault (Choubert
1947) (Fig. 7). The main part of the Proterozoic
basement consists of a Pan-African dismembered
ophiolitic sequence and arc fragments (Leblanc
1975; Saquaque et al. 1989). The Pan-African structures are unconformably overlain by a thick Neoproterozoic to Cambrian volcano-sedimentary
cover. At its base, the folded Tiddiline Formation
attributed to the ‘Saghro Group’ (Thomas et al.
2004) is confined to NE –SW-trending late
Pan-African basins (Hefferan et al. 1992). The
overlying ‘Ouarzazate Supergroup’ volcanoclastic
sequence, which ranges in thickness from 0 to
700 m, is the highest exposed stratigraphical unit.
The Ouarzazate sequences form impressive cliffs
around the Proterozoic basement high. These
latest Neoproterozoic sequences are disconformably overlain by the Lower Cambrian carbonates
interlayered with the alkaline syenitic Alougoum
volcanic flows (Álvaro et al. 2006), dated at
534 + 10 Ma (Ducrot & Lancelot 1977) and
529 + 3 Ma (Gasquet et al. 2005).
The Late Palaeozoic compressional event reactivated the basement structures along the borders of
the inlier (Fig. 7c). The basement rocks were not
deformed, except in the vicinity of the previous
fractures, where kink bands and a rough cleavage
developed. In contrast, folds of varying wavelengths are very common in the cover rocks. The
regional anticlines are characterized by box-fold
shapes throughout the Bou Azzer– El Graara area,
and by large open synclines of Cambrian rocks.
Metre- to decametre-scale, upright detachment
folds are common in the lowermost Cambrian limestones. The folds exhibit a dominant NW– SE trend
with subordinate NE –SW structures. These folds
are often conical (Leblanc 1975) with gently plunging axes and steep axial planes inclined towards
the margins of the inlier. Such folds are often
associated with reverse faults, which possibly originated from the inversion of former synsedimentary
normal faults. Upwards, these folds disappear progressively in the overlying sedimentary sequences.
Cleavage is either absent or occurs as a rough
spaced cleavage in the pelitic layers of the
442
A. SOULAIMANI & M. BURKHARD
Fig. 6. (a) Location of the Irherm–Tata area in the Anti-Atlas belt. (b) Structural map of the Irherm–Tata area
and (c) geological cross-section of the Irherm–Tata area (section location D– D0 in the map). The stereograms
(Wulff stereonet, lower hemisphere) represent bedding (S0) and foliation (S1) data in the Talat N’Ouamane
domain (dashed inset).
THE ANTI-ATLAS CHAIN, MOROCCO
443
Fig. 7. (a) Location of the Bou Azzer–El Graara area in the Anti-Atlas belt. (b) Geological and structural
map of the Bou Azzer– El Graara area, modified from Leblanc (1975) and (c) geological cross-section (section
location E– E0 in the map).
Cambrian synclines. Along the northeastern margin
of the inlier, the folds are associated with
kilometre-scale sinistral strike-slip faults and
SW-verging
transpressional
reverse
faults
(J. El Hassel), where the ‘Lower Limestones’
formation is stacked upon the ‘Tikirt Sandstones’,
equivalent to the ‘Lie-de-vin series’ rocks. The
southwestern border of the inlier is defined by a
sharp, steep monocline, exposed on the scale of
tens of metres.
The NW–SE direction of the Bou Azzer – El
Graara antiform continues southeastward as far as
the southern Tafilalt area and the Algerian ‘aulacogenic’ Ougarta chain, where the Variscan tectonic
style has similar geometry and structural features
to that of the eastern Anti-Atlas (Donzeau 1974).
The Saghro – Ougnate area
The last example chosen to illustrate the Variscan
tectonics in the Anti-Atlas belt comes from the
vicinity of the Tinghir oasis, just south of the
High Atlas (Fig. 1). The Cambrian to Carboniferous
strata cropping out along the northern border of the
Saghro and Ougnate Proterozoic massifs (Fig. 8b)
are deformed by several thrust faults that dip
gently northward (Hindermeyer 1954; Choubert
1959). Michard et al. (1982) presented a detailed
north–south cross-section of the region south of
Tinghir (Fig. 8c), where they distinguished
three allochthonous units overriding a southern
autochthonous unit apparently undetached from
the Sahgro substratum. The allochthonous units
display have sedimentary features to those of the
northern margin of the Saghro massif. Lower
Ordovician and Silurian shales constitute two
ductile décollement levels accommodating SSEdirected thrusting. A rough, gently north-dipping,
east –west-trending cleavage is locally axial
planar to south-vergent asymmetrical folds. The
cleavage affects all of the Palaeozoic units but is
preferentially developed within the pelitic horizons.
This cleavage also occurs within the latest
Proterozoic volcanoclastic formation along the
northern border of the Saghro massif. The cleavage
progressively dies out northward and disappears
completely in the J. Tisdafine Carboniferous
flysch units, which are, however, affected by
east –west-trending folds associated with thrusting
toward the south (Soualhine et al. 2003). Southvergent thrusts are also developed in the Neoproterozoic cover along the northern side of the
Ougnate inlier.
To the south, between the Saghro and
Ougnate massifs, an uplifted Cambrian outcrop,
about 15 km wide, is squeezed between the
Saghro and the Ougnate basement blocks.
These Cambrian strata between the two Precambrian massifs are locally overprinted by a north –
south rough cleavage, noticeable in the Cambrian
444
A. SOULAIMANI & M. BURKHARD
Fig. 8. (a) Location of the Saghro– Ougnate area in the Anti-Atlas belt. (b) Structural map of Saghro– Ougnate
area, modified from the 1:500 000 geological map of Ouarzazate (Choubert 1959) and (c) geological crosssection of the northeastern border of the Saghro massif (section location F– F0 in the map), reinterpreted from
Michard et al. 1982).
Paradoxides-bearing shales (Fig. 8b). To the
south, in the Alnif plain, Ordovician rocks have
a south-dipping monoclinal geometry similar to
that observable along the southern flank of the
central and eastern Anti-Atlas chain. Generally,
similar southern-vergent structures are described
in the Palaeozoic inliers of the High Atlas,
south of the ‘South Atlas Fault’, in the
Skoura – Aı̈t Tamlil (Jenny & Le Marrec 1980)
and in the Bechar basin, where overlying strata
are post-Namurian in age (Ball et al. 1975).
Discussion
Para-autochthonous v. autochthonous
Anti-Atlas
The present study suggests that the Variscan AntiAtlas chain consists of two structural domains,
as follows.
To the west, the narrow western Atlantic Bas
Drâa area was subjected to a penetrative deformation (Mattauer et al. 1972; Soulaimani 1998;
Belfoul et al. 2001). The Cambrian series has
been intensely deformed by east –west- to NW –
SE-trending large recumbent folds. The axial
planes of these folds are gently west-dipping or
even subhorizontal in the westernmost outcrops.
The folded units are composed of thrust sheets or
duplexes. The deformation intensity decreases progressively eastward. The analysis of lineations and
kinematic indicators, the occurrence of large
recumbent folds with horizontal foliations, and the
widespread development of low-angle faults indicate a horizontal displacement toward the ESE. In
this area, there is no evidence for any basement
involvement, at least within the exposed supracrustal levels. This thin-skinned tectonic style
suggests that the westernmost part of the Anti-Atlas
Palaeozoic cover can be regarded as a paraautochthonous (rather than an allochthonous) area
THE ANTI-ATLAS CHAIN, MOROCCO
that is the northern prolongation of the Mauritanides
belt. This is the only part of the Anti-Atlas belt that
is clearly part of the external zones of the Variscan
belt. The remaining areas of the Anti-Atlas further
east should be considered as meta-cratonic rather
than part of the Variscan mobile belt (Burkhard
et al. 2006).
Throughout the remaining western and central
Anti-Atlas chain, NE– SW-trending Precambrian
blocks were deeply buried beneath up to 10 km of
Palaeozoic sediments. This Palaeozoic basin was
subsequently reactivated during the Late Palaeozoic
compressive event (Soulaimani et al. 1997; Helg
et al. 2004; Burkhard et al. 2006). The geometry
of the Variscan basement uplifts or ‘boutonnières’
is controlled by inherited fracture zones of the
underlying Precambrian basement. Variscan
folding within the Palaeozoic cover series is in
turn strongly influenced by the inversion of these
basement blocks. Deformation intensity is generally
greatest at the base of the cover, near the reactivated
basement structures. Folding intensity decreases
southeastward as well as upward. Steep thrust
faults are rarely associated with this compressive
event, they are observed only in some places
along the borders of the basement inliers. Cover
shortening is almost exclusively accommodated
by upright folding rather than by duplex formation
or thrusting (Caritg et al. 2004; Helg et al. 2004).
Thus, the greater part of the Anti-Atlas is clearly
an autochthonous thick-skinned tectonic province
characterized by brittle to semi-brittle reactivation
of the Precambrian basement and clear decoupling
of the deformation mode between the basement
and Palaeozoic cover. This tectonic style mostly
related to vertical uplift of basement blocks is
more typical for a foreland area rather than the Variscan external thrust-and-fold belt.
Basement – cover relationships
The structural pattern of the Anti-Atlas, except in its
westernmost part, is characterized by the presence
of Proterozoic inliers separated by wide flat-floored
synclines cored by Cambrian sedimentary rocks. A
map-scale outcrop pattern combined with structural
data indicates a large-magnitude bend in the structural grain of the Anti-Atlas wrapping around the
West African craton. In the western Anti-Atlas,
the inliers are oriented NE– SW to north–south;
in the central and eastern Anti-Atlas, the structural
trends are east –west (Irherm, Saghro) and NW–
SE (Bou Azzer –El Graara), belonging to the
Ougarta trends. On the whole, the Anti-Atlas and
Ougarta domains form a broad arc north of the
Reguibat shield (Lefort 1988; Haddoumi et al.
2001) (Fig. 9).
445
It is commonly accepted that the variation in the
structural trend of Variscan folds and the shape of
inliers throughout the Anti-Atlas are inherited
from Proterozoic basement trends (Choubert 1947;
Leblanc 1972, 1975; Donzeau 1974; Michard
1976; Jeannette & Piqué 1981; Soulaimani et al.
1997). Many arguments and observations suggest
that the pattern and geometry of the inliers, and
even the orientation of their faulted borders,
often at right angles, are controlled by ancestral Precambrian basement structures. The best example is
that of the Bou Azzer –El Graara boutonnière.
The Late Pan-African orogeny, and the subsequent
rift-related extensional event, controls the Palaeozoic and younger structural trends (Piqué et al.
1999; Soulaimani et al. 2003). However, a precise
synthetic reconstruction of the Late Proterozoic
basement –cover configuration has not yet
been compiled.
The Late Proterozoic–Early Cambrian extensional event provides important constraints on the
subsequent development of Variscan structures.
Although parts of some Precambrian inliers may
be inverted basins filled in with the synrift ‘Ouarzazate Supergroup’ deposits (Faik et al. 2001; Helg
et al. 2004), many others were already single
highs during the Late Proterozoic continental
rifting, as shown by the development of surrounding
collapse fault and by the deposition of the post-rift
Cambrian sequences directly over their Proterozoic
basement. Many inliers in the western Anti-Atlas
can be taken as examples of this configuration
(Bas Drâa, Kerdous, Tagragra Akka, Tagragra
Tata, etc.). Furthermore, the mechanism of the
Late Proterozoic extensional event in the Anti-Atlas
is more complex than a classical tilting of basement
rigid blocks, if we consider the coeval expanding
magmatic and hydrothermal activities affecting
the Precambrian basement. As an example, the
eastern Kerdous inlier could correspond to a diapiric metamorphic dome exhumed during the Late
Precambrian transtensional event (Soulaimani &
Piqué 2004). The other western Anti-Atlas inliers
provide evidence for tectonothermal restructuring
of their Precambrian basement during this extensional event (Oudra et al. 2006).
Although the mechanism of this early extensional event is a matter of debate, it is obvious that
the resulting basement –cover configuration has a
significant role in subsequent reactivations of these
structures during the Variscan orogeny. The mechanism governing the development of basement
uplifts during the Variscan epoch seems to be
more complex than simple classical positive inversion (Williams et al. 1989) of Late Proterozoic –
Early Cambrian extensional grabens. The uplift of
the Precambrian basement blocks as a result of the
Variscan shortening, as shown by the geometric
446
A. SOULAIMANI & M. BURKHARD
Fig. 9. Position of the Anti-Atlas and related areas in the limit Palaeozoic–Mesozoic, modified from Burkhard et al.
(2006). (a) General map of the African– North American system in the Early Mesozoic from Sahabi et al. (2004),
with the addition of the main structures of the Late Palaeozoic Alleghanian–Gondwanian collision. (b) Global plate
tectonic reconstruction (in the middle) and cross-sections showing Variscan– Alleghanian structures in the Anti-Atlas
belt modified from Burkhard et al. (2006). 1, West African craton and Anti-Atlas cratonic basement uplift;
2, North American craton; 3, Acadian chain; 4, Avalon terrane; 5, Meguma terrane; 6, Proterozoic basement uplift;
7, allochthonous Mauritanide– western Anti-Atlas terranes; 8, Palaeozoic cover; 8, Post-Palaeozoic cover.
array of the post-rift Early Cambrian Limestone
deposits around the Precambrian inliers and supported by kinematic analyses (e.g. Bas Drâa and
Lakhssas Plateau), is superimposed upon the Late
Precambrian initial horst shape. It is therefore difficult to provide quantitative estimates of the amount
of Variscan shortening and uplift for single basement blocks. At Lakhsass, the difference in
elevation between the top of the uplifted basement
massif (J. Lkest, Kerdous) and the bottom of the
adjacent Lakhssas basin is at least 4 km. On the
scale of the entire Anti-Atlas anticlinorium such
assessments of the vertical movements become
even more complex, as we also have to take into
account a significant Late Miocene Atlas– Alpine
uplift affecting all of the Anti-Atlas belt.
Proterozoic basement blocks subjected to the
Variscan compression behaved rigidly and do not
display any evidence for internal deformation
except in peripheral zones. The rheological difference between the metamorphic and granitic basement rocks and the layered cover sediments may
have played a significant role, as the cover rocks
are more easily deformed than their basement.
The shortening of the basement was primarily
accommodated by the reactivation of ancient fractures, whereas cover rocks are mostly shortened
by poly-harmonic folding without any significant
faulting and thrusting. Penetrative plastic deformation with the development of a foliation is
restricted to the margins of the uplifted basement
(e.g. Bas Drâa) or to limited areas between basement blocks (e.g. Lakhssas Plateau).
Around the Bas Drâa inlier, folded limestones
several hundred metres thick are clearly detached
from the weak infra-Adoudounian basal sandstones.
THE ANTI-ATLAS CHAIN, MOROCCO
Such detachments occur at all levels throughout the
Palaeozoic succession but are typical of the frequent shaly levels within the Ordovician and Silurian rocks, especially in the western Anti-Atlas.
They account for the marked changes in style,
wavelengths and amplitudes of folds within the
Palaeozoic cover (Helg et al. 2004). The uppermost
décollement level is postulated within the Late
Devonian marls to explain the apparent ‘unconformity’ below the J. Ouarkziz Carboniferous strata,
which remained undeformed above the folded
J. Rich Devonian sandstones. This latter décollement can be tentatively interpreted as a
regional-scale back-thrust (NNW thrusting) at the
base of J. Ouarkziz (Burkhard et al. 2001; Helg
et al. 2004), although other researchers have
suggested the occurrence of a diffuse dextral shear
zone (Michard 1976). Given the lack of geophysical
data, these interpretations remain provisional, as
they are not yet corroborated by any field evidence.
Along the southern flank of the western AntiAtlas anticlinorium (J. Bani), some 15–20% of
horizontal shortening was deduced from the restoration of Devonian and Ordovician folds in the Tata
zone (Caritg et al. 2004; Helg et al. 2004). Deformation increases in the western Atlantic Bas Drâa,
where a minimum rate of 45% of shortening is
observed within the units that crop out. Elsewhere
in the central and eastern Anti-Atlas, values of
5–10% at most for the Variscan shortening have
been calculated (Leblanc 1975; Hassenforder
1987), except for the thrust-dominated area of
Tineghir, where some 40% of shortening has been
postulated (Michard et al. 1982).
Because of the lack of unconformably overlying
strata, the age of the folding and synchronous basement uplift remains poorly constrained. It is certainly Variscan (Bonhomme & Hassenforder
1985), as folds and thrusts affect Viséan strata in
the Tineghir area (eastern Saghro; Michard et al.
1982), as well as the south Ougnat domain, and as
folds of the entire Anti-Atlas belt are crosscut by
Late Triassic dolerite dykes. The regional
(western Anti-Atlas) structural contrast between
the folded Early and Middle Palaeozoic strata and
the mostly undisturbed Carboniferous sequences is
not a stratigraphic unconformity. Instead, it represents a gradual or faulted transition from the
weakly deformed domain of the southern AntiAtlas towards the mostly undeformed Carboniferous Ouarkziz sequences at the northern border of
the Tindouf platform (Soulaimani et al. 1997).
Furthermore, many Anti-Atlas rocks have experienced a Variscan lower-greenschist facies
metamorphism at 150–300 8C (Buggisch 1988;
Soulaimani 1998; Burkhard et al. 2001), which is
explained by deep sedimentary burial beneath a
10 km thickness, or more, of Palaeozoic overburden.
447
The southern limit of the Variscan belt
As described above, the thin-skinned tectonic style
observed along the Atlantic coast differs considerably from the thick-skinned tectonics prominent
throughout the Anti-Atlas chain, where the involvement of basement blocks plays a critical role during
the Variscan compressive deformations. At a larger
scale, the Anti-Atlas appears as the nonsymmetrical hinterland to the Alleghanian foreland
fold– thrust belt on the American side of the Appalachian chain (Fig. 9). The east-vergent thrusts of
the western Atlantic Bas Drâa area correspond to
the eastern side of the Mauritanides back-thrust terranes and both can easily be seen as the eastern
Appalachian mountain front. The Anti-Atlas belt,
however, corresponds to the SE metacratonic
foreland to the Variscan belt in NW Africa.
Indeed, as defined by Abdelsalam et al. (2002),
the metacraton corresponds to a craton that has
been at least remobilized during an orogenic
event, but that is still rheologically recognizable.
This is the case for the Anti-Atlas belt and overall
the NW border of the West African craton, which
are visibly affected by major basement uplifts and
inversion tectonics during the Variscan orogeny,
after the Pan-African orogeny and subsequent
extensional evolution (Coward & Ries 2003;
Burkhard et al. 2006).
To the north, the Moroccan Variscan belt is represented by the so-called Meseta domain (Piqué
1989; Piqué & Michard 1989; Hoepffner et al.
2005). Within the Meseta, the eastern zones differ
from the western ones by their early metamorphic
evolution (Hoepffner 1987). In the western
Meseta, the deformation is heterogeneous and concentrated within sheared zones (Lagarde et al. 1990;
Essaı̈fi et al. 2001). As in the Anti-Atlas, a contrast
exists in the Meseta between homogeneously and
more strongly deformed zones on the one hand
(eastern Meseta), and heterogeneously deformed
zones, where the deformation is concentrated
along block limits (western Meseta) on the other
hand. It is important to emphasize significant differences between the Anti-Atlas and the Meseta
(Michard 1976; Piqué & Michard 1989; Hoepffner
et al. 2005): (1) generally, the orogenic shortening
is much more important in the Meseta than in the
Anti-Atlas chain; also, the development of a
regional metamorphism, sometimes reaching
medium grade (upper greenschist to amphibolite
facies), and the emplacement of Variscan granitoids, which are never represented in the Anti-Atlas,
are noticeable in the Meseta domain; (2) the dominantly northwestward vergence observed in the
eastern Meseta zones differs from the SE-directed
structures of westernmost ‘Atlantic’ domains of
the Anti-Atlas.
448
A. SOULAIMANI & M. BURKHARD
These fundamental differences between the two
segments of the Variscan belt of Morocco call for a
basic role of the ‘South Atlas Fault’ as a major
Variscan lithospheric fault (Mattauer et al. 1972;
Ouanaimi & Petit 1992). This Palaeozoic
expression of the South Atlas Fault has been
named the Atlas Palaeozoic Transform Fault by
Piqué & Michard (1989). A recent plate tectonic
interpretation (Stampfli & Borel 2002) suggests
that the Moroccan Meseta was rifted and drifted
away from Gondwana at c. 490 Ma. However,
many geological observations indicate that the
Meseta domain belongs to, or at least was located
next to, the West African craton during the Palaeozoic times (Dostal et al. 2005; Hoepffner et al.
2005). The Meseta corresponds to relatively distal
parts of the Africa passive margin, which remained
close to the Anti-Atlas throughout Palaeozoic times.
The present-day position results from the Late
Carboniferous–Early Permian collage of the Meseta
domain against and onto the Anti-Atlas domain.
The traces of this collage are widely exposed in
the Tineghir area as folds and thrusts (Michard
et al. 1982) and as dextral wrench zones in the
Tamlelt area (Houari & Hoepffner 2003). Further
west, it is coincident with the Mesozoic –Cenozoic
Tizi n’Test-Meltsen fault system. The ‘South Atlas
Fault’ zone is a major Late Carboniferous –Early
Permian intracontinental fault devoid of any
ophiolitic slivers. The strong contrasts in Variscan
deformation between the Anti-Atlas and Meseta
domains, both in intensity and dominant vergence,
imply that their mutual boundary operated mostly
as a wrench system.
Toward the south, the southwesternmost AntiAtlas belt correlates with the Zemmour belt, considered as the foreland of the northern Mauritanides
chain (Sougy 1969; Lécorché et al. 1991). In the
western Zemmour belt, Devonian units are folded
and thrust to the east, where Lower Palaeozoic sediments unconformably overlie the Reguibat shield.
Further south, the allochthonous Oulad Dlim area
(Adrar Soutouf) consists of crystalline nappes of
Pan-African age in the west (Le Goff et al. 2001;
Villeneuve 2005; Villeneuve et al. 2006), which
were thrust during the Variscan orogeny and override eastward a Palaeozoic sedimentary series
(Ordovician to Devonian).
Finally, because of its position along the
southern border of the Variscan belt, the Anti-Atlas
chain constitutes an example of cratonic basement
blocks with, at its western side, a cratonward vergence. It fundamentally differs from the northern
limit of the Variscan belt in Europe, where the orogenic boundary is a crustal-scale thrust toward the
North European craton and its Caledonian margin
(Matte 1986). Variscan deformation, analysed
above, is different from that of foreland belts
and associated collisional chains such as the
Appalachian Valley and Ridge province (Hatcher
& Odom 1980; Rodgers 1995) or the Alpine
External Crystalline Massifs (Boyer & Elliot
1982). The involvement of basement blocks
throughout the Anti-Atlas chain best compares
with the Wind River basement uplifts of the
Rocky Mountains, which developed far beyond
the orogenic front of this mountain belt. Rodgers
(1987, 1995) in his comparative anatomy of mountain chains called this style ‘basement uplifts within
cratons marginal to orogenic belts’ in contrast to the
more common ‘basement uplifts within external
parts of orogenic belts’.
Conclusion
The main tectonic features of the Anti-Atlas are
summarized briefly below.
(1) The Anti-Atlas consists of the juxtaposition
of two Variscan domains affected by contrasting
tectonic styles: (1) a western ‘Atlantic’ domain
(para-autochthonous Anti-Atlas) marked by flat
foliations, axial planar to large-scale recumbent
folds, and east-verging thrusts; (2) a vast folded
domain (autochthonous Anti-Atlas) characterized
by upright folds of wavelengths varying from less
than 100 m to more than 10 km with broad Precambrian basement-involved anticlines.
(2) Basement blocks limited by Pan-African
reactivated fractures controlled the Variscan deformation within the cover. The vertical uplift of basement inliers in excess of 4 km is the outstanding
feature of this tectonic inversion.
(3) The deformation intensity within the cover is
strongly influenced by the geometry of the basement uplifts. Deformation is most intense near the
steep borders of the basement blocks or between
closely spaced basement blocks, where a more or
less penetrative cleavage develops. Around the
basement inliers, bedding and the geometry of
folds are strongly influenced by the relative displacements of the underlying rigid block.
(4) Shortening within the Palaeozoic cover is
decoupled from the basement deformation and
accommodated by upright folds of different wavelengths. Strong disharmony is accommodated by a
multitude of detachment levels. Deformation intensity generally decreases toward the south and SE
and completely vanishes within the stable
Tindouf platform.
(5) The Anti-Atlas belt represents a particular
style of deformation not seen elsewhere in the Variscan chain of Northern Africa or Europe. Basement
uplifts and strong tectonic inversion are observed all
along the northern border of the West African metacraton. This metacraton occupies a key position
THE ANTI-ATLAS CHAIN, MOROCCO
between the northern domains of Morocco (Meseta
Block) and the Mauritanides belt to the south.
Together, these orogenic belts define a broadly synchronous circum-Atlantic Variscan –Alleghanian
belt developed during the Late Palaeozoic
Gondwana–Laurentia collision.
Professor Martin Burkhard died on 23 August 2006, when
sampling in the Alps. This paper, to which he significantly
contributed, is dedicated to his memory. Thanks are due to
A. Piqué and A. Michard for fruitful discussions and a
careful review of early versions, and to A. Azor and
S. Mazur for constructive reviews of the manuscript.
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