Fold interference pattern in thick-skinned tectonics; a case study from the External Variscan Belt of Eastern Anti-Atlas...
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DOI: 10.1016/j.jafrearsci.2016.04.003
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Fold interference pattern in thick-skinned tectonics; a case study from the External Variscan Belt of Eastern Anti-Atlas, Morocco
L. Baidder, A. Michard, A. Soulaimani, A. Fekkak, A. Eddebbi, E.-C. Rjimati, Y. Raddi
PII: S1464-343X(16)30119-4
DOI: 10.1016/j.jafrearsci.2016.04.003 Reference: AES 2539
To appear in: Journal of African Earth Sciences Received Date: 12 November 2015
Revised Date: 1 April 2016 Accepted Date: 2 April 2016
Please cite this article as: Baidder, L., Michard, A., Soulaimani, A., Fekkak, A., Eddebbi, A., Rjimati, E.-C., Raddi, Y., Fold interference pattern in thick-skinned tectonics; a case study from the External Variscan Belt of Eastern Anti-Atlas, Morocco, Journal of African Earth Sciences (2016), doi: 10.1016/
j.jafrearsci.2016.04.003.
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Fold interference pattern in thick-skinned tectonics; a case study
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from the External Variscan Belt of Eastern Anti-Atlas, Morocco
2
L. Baiddera, A. Michardb, *, A. Soulaimanic, A. Fekkakd, A. Eddebbi c, 3
E.-C. Rjimatie, Y. Raddie 4
5
a Hassan II University, Faculty of Sciences Aïn Chock, Geosciences Laboratory, BP 5366 Maârif, Casablanca, Morocco
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b Pr. Em. University of Paris-Sud, 10, rue des Jeûneurs, 75002 Paris, France
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c Department of Geology, Faculty of Sciences-Semlalia, Cadi Ayyad University, P.O. Box 2390, Marrakech, Morocco
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d Chouaïb Doukkali University, Faculty of Sciences, Earth Sciences Department, B.P. 20, 24000 El Jadida, Morocco
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e Direction de la Géologie, Ministère de l'Energie et des Mines, B.P. 6208, Rabat Instituts Haut Agdal, Rabat, Morocco
10 11
Abstract 12
Conflicting views are expressed in literature concerning fold interference patterns in thick- 13
skinned tectonic context (e.g. Central Anti-Atlas and Rocky Mountains-Colorado areas). Such 14
patterns are referred to superimposed events with distinct orientation of compression or to the 15
inversion of paleofaults with distinct strike during a single compressional event. The present 16
work presents a case study where both types of control on fold interference are likely to be 17
combined. The studied folds occur in the Tafilalt-Maider area of eastern Anti-Atlas, i.e. in the 18
E-trending foreland fold belt of the Meseta Variscan Orogen in the area where it connects 19
with the SE-trending, intracontinental Ougarta Variscan belt. Detail mapping documents 20
unusual fold geometries such as sigmoidal and croissant- or boomerang-shaped folds 21
associated with a complex major fault pattern. The folded rock material corresponds to a 6-8 22
km-thick Cambrian-Serpukhovian sedimentary pile that includes alternating competent and 23
incompetent formations. The basement of the Paleozoic succession is made up of 24
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rhomboedric tilted blocks that formed during the Cambrian rifting of north-western 25
Gondwana and the Devonian dislocation of the Sahara platform. The latter event is 26
responsible for an array of paleofaults bounding the Maider and South Tafilalt Devonian- 27
Early Carboniferous basins with respect to the adjoining high axis. The Variscan Orogeny 28
began during the Bashkirian-Westphalian with a N-S direction of shortening that converted 29
the NW-trending Ougnat-Ouzina paleogeographic high into a mega dextral shear zone. Folds 30
developed on top of a moving mosaic of basement blocks, being oriented en echelon along the 31
inverted paleofaults or above intensely sheared fault zones. However, a dominantly NE-SW 32
compression responsible for the building of the Ougarta belt also affected the studied area, 33
presumably during the latest Carboniferous-Early Permian. The resulting fold interference 34
pattern and peculiar geometries would exemplify a dual control of deformation by both the 35
variably oriented basement paleofaults and the evolution of the regional shortening direction 36
with time.
37
Keywords: Thick-skinned tectonics, Superimposed folding, Inversion tectonics, Variscan Belt, 38
Anti-Atlas, Ougarta.
39
40
1. Introduction
41
The fold geometry resulting from the superposition of folds of similar type has been the 42
object of numerous classical studies. John Ramsay (1962) first described the interference 43
patterns produced by two successive foldings with different relative orientation of their shear 44
and flattening directions. However, he basically considered small scale natural examples of 45
such superimposed folds. In contrast, Jean Goguel (1937, 1939) and Marcel Lemoine (1972) 46
described examples of superimposed folding at map scale in south-eastern France where the 47
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Alpine folds (Miocene) superimpose the Pyrenean-Provençal ones (Late Eocene). In their 48
works, the superposition of two folding events resulted in fold tightening (when shortening 49
directions were similar) and in the formation of large, arcuate fold systems. Folding occurred 50
there in a Mesozoic-Cenozoic sedimentary sequence detached above a thick Triassic evaporite 51
basin (Le Pichon et al., 2010; Andreani et al., 2010).
52
Investigating the fold relationships with basement faults in the very different context of 53
the Laramide Rocky Mountains, Mitra and Mount (1998) showed that the orientation of the 54
basement-cored folds is directly controlled by that of the reverse fault underneath. Looking at 55
the same region, Marshak (2000) insisted on the following corollary: the initial rifting pattern 56
of the basement and the concept of fault inversion (Cooper and Williams, 1989; Turner and 57
Williams, 2004) are critical for the interpretation of thick-skinned tectonics. As soon as the 58
basement has been affected by two distinct sets of paleofaults during its early rifting 59
evolution, then their inversion will result in two distinct, but possibly coeval directions of fold 60
axis in the frame of a single regional compression.
61
2.
The Anti-Atlas Paleozoic fold belt (Fig. 1) is a typical example of dominantly thick 62skinned belt (Burkhard et al., 2006) that extends at the south-western front of the 63
Alleghanian-Variscan (Hercynian) orogen in southern Morocco (Soulaimani and 64
Burkhard, 2008; Michard et al., 2010). Its western-central part (Akka-Tata area) offers 65
excellent examples of interference between two sets of flexural-slip folds with differently 66
oriented axes associated with faulted basement inliers. This interference pattern received 67
contradictory interpretations. According to Faik et al. (2001), it would result from one 68
single compressional event (i.e. the main, NW-oriented Variscan compression) acting on a 69
formerly rifted basement with two sets of faults with different strike. The authors argue 70
that the observed, E-trending folds would have formed prior to the interfering NE-trending 71
ones. In contrast, Caritg et al. (2004) argue that the dome-and-basin structures of the Tata 72
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area are typical for the class 1 or 1-2 interferences as defined by Ramsay and Huber 73
(1987), involving a first generation of SW-NE open folds superimposed by a second 74
generation with similar style and wavelengths trending in an E-W direction. Albeit they 75
clearly recognize the control of folding by the inversion of basement paleofaults with 76
different strike, Caritg et al. (2004) as well as Helg et al (2004) conclude that a rotation of 77
the compressional stress occurred in the area during the Variscan orogeny. In the present 78
paper, we present another case study from the Tafilalt-Maider area of easternmost Anti- 79
Atlas, which is the very specific area where the ENE-trending Anti-Atlas belt connects 80
with the NW-trending Ougarta belt (Fig. 1A). Although both these belts result from the 81
Variscan orogeny, it was suggested that Ougarta would have formed basically during the 82
Permian contrary to the Anti-Atlas that would have mainly formed during the Bashkirian- 83
Moscovian (Menchikoff, 1952; Fabre, 1971, 1976, 2005; Haddoum et al., 2001; Michard 84
et al., 2008, 2010). Therefore, the area where these belts connect is potentially fitted for 85
studying interference patterns of two sets of folds with different strike and age. In fact, the 86
studied area exposes surprising structures such as the croissant- or boomerang-shaped 87
Tijekht anticline, a surprising structure when seen in satellite view via Google earth. We 88
propose in the following their interpretation in terms of thick-skinned inversion tectonics 89
with superimposed folding events.
Geological setting and stratigraphical
90
outline
91
The Anti-Atlas and Ougarta Paleozoic fold belts extend on the northern and north-eastern 92
border of the West African Craton (WAC; Fig. 1A; Hollard et al., 1985; Ennih and Liégeois, 93
2008), whose western border is made up by the Mauritanides (Sougy, 1962; Villeneuve et al., 94
2006; Michard et al., 2010). Their basement crops out in numerous faulted antiforms or inliers 95
(“boutonnières”), which constitute as many opportunities to observe the Neoproterozoic Pan- 96
African Belt which formed between ca. 700-640 Ma (Caby, 2003; Gasquet et al., 2008; Blein 97
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et al., 2014; Triantafyllou et al., 2015, and references therein). The Anti-Atlas inliers (Fig. 1B) 98
south of the Anti-Atlas Major Fault (AAMF; Choubert, 1947) expose Paleoproterozoic 99
terranes overlain by deformed and metamorphic deposits from the former WAC platform, 100
recently dated from the Mesoproterozoic-Tonian (Ikenne et al., 2016). Along the AAMF 101
itself, metaophiolites and oceanic arc units from the Pan-African suture zone dated at ca. 760- 102
700 Ma crop out in the Siroua and Bou Azzer inliers. Northeast of the AAMF, namely in the 103
Saghro and Ougnat massifs, only the youngest, 630-610 Ma-old (Liégeois et al., 2006; Abati 104
et al., 2010, 2012) and lowermost-grade metamorphic units crop out beneath the 105
unconformable late Ediacaran volcanic and volcaniclastic formations of the Ouarzazate 106
Group. The latter group surrounds all the Anti-Atlas inliers, although with a strongly uneven 107
thickness (Soulaimani et al., 2014), and accumulated between 575-550 Ma, being coeval with 108
numerous HKCA granitoid intrusions (Gasquet et al., 2008; Blein et al., 2014).
109
The overlying Paleozoic sequence (Michard et al., 2008, and references therein) begins 110
with the lowermost Cambrian in the Western Anti-Atlas, but not before the late Early 111
Cambrian sensu Destombes et al. (1985) or early Middle Cambrian, sensu Geyer & Landing 112
(1995) in most of the Eastern Anti-Atlas (Fig. 2A), and not before the Middle Cambrian in the 113
northern flank of the Ougnat Massif (Destombes & Hollard, 1986). This results from the 114
activity of synsedimentary ENE-trending normal faults, also responsible for alkaline basalt 115
outpours during the Early and Middle Cambrian (Raddi et al., 2007; Soulaimani et al., 2014).
116
The Ordovician-Silurian period is characterized by the rather monotonous sandy to 117
argillaceous deposits of the Saharan platform where the main perturbations occurred during 118
the end-Ordovician glacial events (Destombes et al., 1985; Clerc et al., 2013; Ghienne et al., 119
2014, and references therein). The Middle-Upper Devonian deposits show dramatic thickness 120
and facies variations (Fig. 2A, B) illustrating the coeval disintegration of the northern Saharan 121
platform (Wendt, 1985; Baidder et al., 2008; Ouanaimi & Lazreq, 2008). Two subsiding 122
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basins formed at that time south of the Saghro-Ougnat and Erfoud high, namely the Maider 123
and the South Tafilalt basins, separated by a paleogeographic high labeled the Ougnat-Ouzina 124
Axis (Fig. 3). This occurred through synsedimentary normal faulting (Baidder et al., 2008) 125
arguably due to back-arc extension in the foreland of the Rheic subduction (Michard et al., 126
2010) or to the effect of the Rheic subduction slab-pull assuming it occurred along the 127
northwestern flank of the ocean (Gutiérrez-Alonso et al., 2008; Frizon de Lamotte et al., 128
2013).
129
The youngest terms of the folded sequence are late Visean in age (Destombes & Hollard, 130
1986). Post-folding, molasse-type subaerial deposits, Bashkirian and Pennsylvanianin age 131
(from ca. 320 to 300 Ma; Fabre, 1976, 2005; Cavaroc et al., 1976) are preserved in the 132
Tindouf cratonic basin south of the Anti-Atlas. In contrast, marine deposits accumulated up to 133
the late Moscovian (ca. 305 Ma) in the Bechar-Abadla Basin to the east of the Ougarta belt.
134
The Abadla basin received subaerial, red beds deposits during the late Westphalian-Autunian 135
(Fabre, 1976, 2005; Bouabdallah et al., 1998), suggesting diachronic folding of the western 136
Anti-Atlas and eastern Anti-Atlas-Ougarta belts.
137
The Anti-Atlas Paleozoic fold belt is intruded by numerous dykes and sills of the Central 138
Atlantic Magmatic Province, dated by place at 200-195 Ma (Hailwood & Mitchell, 1971;
139
Hollard, 1973; Sebai et al., 1991; Derder et al., 2001; Youbi et al., 2003; Chabou et al., 2007;
140
Verati et al., 2007), but no surface outpours or coeval sediments have been preserved except 141
at the northern fringe of the central Anti-Atlas on top of the Abadla Basin (Fabre, 2005). The 142
Paleozoic fold belt is surrounded by the unconformable, weakly faulted and tilted Cretaceous- 143
Neogene deposits of the Saharan plateaus or hamadas (Draa and Guir Hamada, Kem Kem 144
plateaus; Zouhri et al., 2008) to the south and east, and by those of the discontinuous, shallow 145
sub-Atlas basins (Souss and Ouarzazate basins; Frizon de Lamotte et al., 2008) to the north.
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3. Methods
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The present work is mainly based on our detail mapping of the area (L.B., A.F.) covering 148
the Al Atrous, Irara, Marzouga, Mfis and Taouz sheets of the Geological map of Morocco 149
1:50,000, on the preparation (A.M.) of the corresponding explanatory notices (Benharref et 150
al., 2014a-c; Alvaro et al., 2014a, b), and on several common field trips. The methods used 151
besides of mapping are structural observations and measurements (bedding and fault planes, 152
axes of minor folds, etc.) and analysis of satellite imagery, which is particularly informative in 153
these arid regions. The thermal conditions that prevailed during deformation are defined 154
through the observation of the structural features at the outcrop scale (bedding, joint systems, 155
and locally spaced cleavage) or at the optical microscope scale (thin sections), and illite 156
cristallinity measurements from the literature (Ruiz et al., 2008).
157
4. Structure of the Southern Tafilalt-Maider area
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4.1. General structure and fault pattern
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The South Tafilalt-Maider area comprises five structural domains (Fig. 3). The 160
northernmost one corresponds to the Ougnat Massif-Erfoud Anticlinorium structural axis that 161
forms the eastern continuation of the Saghro Massif. This domain was a paleogeographic high 162
during the Cambrian and again during the Devonian-Carboniferous (see above section). In 163
particular, the Erfoud anticlinorium expose condensed Devonian formations typical for a 164
pelagic high (Hollard, 1967, 1974; Wendt, 1985, 1988; Wendt and Belka, 1991; Baidder et 165
al., 2008). The Ougnat-Erfoud domain is characterized by dominantly E-W structures. The 166
Bouadil area south of the Ougnat Massif shows a mosaic of tilted basement blocks associated 167
with dominantly NE- and SE-trending folds in the Paleozoic cover series (Raddi et al., 2007).
168
In the Erfoud anticlinorium the Paleozoic succession also overlies Precambrian rocks in the 169
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north (Gour Brikat and Ras el Kahla tiny massifs; Fig. 1B). The southern boundary of the 170
Ougnat-Erfoud structural domain is made up by globally E-trending faults such as the North 171
and South Mecissi Faults and the Erfoud Fault.
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The southernmost domain of the studied area is labeled hereafter the Kem-Kem Domain.
173
The Paleozoic terranes there are widely hidden beneath the Hamada Cretaceous-Neogene 174
formations that have been preserved due to a set of normal faults along the northern boundary 175
of the domain. However, the structural pattern is well-defined by large SSE-trending folds 176
such as the Ouzina and Aroudane anticlines. The Kem Kem Domain looks like the direct 177
continuation of the Ougarta Belt (Fig. 1A; see Discussion section). This domain ends abruptly 178
in the north when crosscut by the Oumjerane-Taouz Fault (OJTF). The latter is a complex 179
fault zone extending over 250 km up to Zagora in the Central Anti-Atlas (Fig. 1B; (Baidder et 180
al., 2008)) where it would connect with the Anti-Atlas Major Fault.
181
The structural domains between the Ougnat-Erfoud and Kem Kem domains are threefold:
182
two basinal domains, i.e. the Maider and South Tafilalt basins, respectively, and the 183
intervening Ougnat-Ouzina Axis. The broadly quadrangular, synformal Maider Basin 184
contains poorly deformed, relatively thick (~3000 m) Devonian-Lower Carboniferous 185
deposits (Figs. 1B, 3). The northern border of the basin corresponds to the North and South 186
Mecissi faults (NMF, SMF), and its southern border to the Oumjerane-Taouz Fault (OJTF). In 187
the east the basin is bounded by the East Maider Fault (EMF) that connects with the Mecissi 188
faults around the J. Signit through a system of curved faults, including the East Signit Fault 189
(ESF). In contrast, the western border of the downwarped basin consists of a simple flexure 190
zone. Scattered Middle Devonian reef mounds underline the borders of the adjoining platform 191
areas, i.e. the Kem Kem Domain and the Ougnat-Ouzina Axis (Kaufmann, 1998).
192
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The South-Tafilalt Basin compares with the Maider basin, but it was more severely 193
deformed than its western counterpart. The northern and southern borders of the South 194
Tafilalt Basin are the same fault zones as that of the Maider Basin, whereas its western limit is 195
a system of NW-trending faults, including the Oued Ziz Fault (OZF), mostly hidden beneath 196
the Oued Ziz alluvium, and its northern branch that forms the Taklimt Fault Zone (TFZ). The 197
Middle-Upper Devonian series crop out in two anticlines in the southeastern corner of the 198
basin, namely the Mfis and Znaigui anticlines. The folded Lower Carboniferous beds occupy 199
the wide Marzouga synclinorium. The famous Emsian-Givetian Hamar Laghdad mud mounds 200
(Montenat et al., 1996; Mounji et al., 1998; Aitken et al., 2002; Cavalazzi et al., 2007; Franchi 201
et al., 2014, 2015) are located on the northern slope of the South Tafilalt basin. The Lower 202
Carboniferous series continue eastward beneath the Guir Hamada, then beneath the Upper 203
Carboniferous-Permian deposits of the Bechar-Abadla area before outcropping again in the 204
Saoura valley (Fabre, 1976, 2005). In other words, the subsiding basin is widely open to the 205
east.
206
The Ougnat-Ouzina Axis (“anticlinorium de Taouz” in Destombes, 2006a, b) is a large 207
NNW-trending strip that exposes dominantly Cambrian and Ordovician terranes with 208
subordinate Silurian and Devonian terranes preserved in narrow synclines. This uplifted axis 209
separates the Maider Basin in the west from the South-Tafilalt Basin in the east (Fig. 3). The 210
thick basinal infill contrasts with the thin coeval deposits of the intervening domain (Fig. 2), 211
which derives from a paleogeographic high (Korn et al., 2000; Lubeseder et al., 2009). So, the 212
EMF and OZF faults that bound the Ougnat-Ouzina Axis derive from the former paleofaults 213
on both sides of the Devonian high. A number of secondary faults subdivide the domain into 214
smaller units described in the following sub-sections.
215
4.2. The Kem Kem, Ougarta-type domain
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The only outcrops of Precambrian and Early Cambrian beds in the whole Tafilalt area 217
occur in the core of the Tazoult n’Ouzina anticlinal vault (Fig. 4A, B). This very open 218
anticlinal structure is abruptly bounded northward by the OJTF whereas its axis plunges 219
gently southeastward and forms (after a small dextral offset beneath the Oued Ziz alluvium) 220
the core of the SE-trending, open Ouzina anticline. The Early Cambrian sandstones overlie 221
directly the Precambrian brittle basement, so as the deepest potential décollement level 222
corresponds to the Middle Cambrian Schistes à Paradoxides. However, the main décollement 223
occurs in the Lower Ordovician (Fezouata and Tachilla Fms.; Fig. 2) between the Cambrian 224
Tabanit sandstones and the quartzites and pelites of the Middle and Upper Ordovician 225
formations.
226
About 20 km further in the east, the Aroudane-J. Zorg anticline is quite similar in 227
geometry and direction to the Tazoult n’Ouzina-Ouzina anticline. However, the axial 228
culmination of the eastern fold is well-preserved (Cambrian massif of J. Zorg; Fig. 3) as the 229
OJTF cuts the fold north of it, then offering a natural cross-section of the Ordovician envelope 230
(J. Aroudane; Fig. 4C). Going again some 25 km to the east, another SE-trending anticline can 231
be seen in the Silurian-Devonian formations at the foot of the Hamada (Oued Nebech area).
232
Therefore, the Kem Kem structural domain is characterized by SSE- to SE-trending, quasi 233
cylindrical open folds with 20-25 km wavelength, hardly detached from their Precambrian 234
basement. This is typically the structure of the Ougarta belt (Donzeau, 1972, 1983; Zazoun, 235
2001; Haddoum et al., 2001; Haddoum, 2009), whose northernmost folds are visible on the 236
south border of the Kem Kem and Daoura Cretaceous-Neogene plateaus (Guir Hamada s.l.), 237
i.e. at about 80 km south-southeast of Taouz (Fig. 1A).
238
4.3. Cambrian-cored folds of the Ougnat-Ouzina Axis
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The Tawjit n’Tibirene unit belongs to the group of tilted blocks described by Raddi et al.
240
(2007) south of the Ougnat Massif. This unit is a large, almost monoclinal Cambrian slab that 241
dips gently southeastward and forms the northeastern “root” of the Ougnat-Ouzina Axis.
242
Three Cambrian-cored folded units occur in the Ougnat-Ouzina Axis itself, from north to 243
south the J. Taklimt, J. Renneg and J. Tijekht units (Fig. 3). The surface of the Cambrian 244
exposures increases from the Taklimt to the Tijekht units, which suggests a southward 245
shallowing of the basement in the Ougnat-Ouzina Axis.
246
The Taklimt unit in the northwest part of the Ougnat-Ouzina Axis is a good example of 247
asymmetric, sub-cylindrical fold developed in correspondence with a deep fault zone that 248
connects further in the SE with the Oued Ziz Fault (Figs. 5A, 3). This NW-trending fold is 249
well designed by the First Bani quartzites that display box fold geometry next to its 250
southeastern pericline. The southwestern limb of the fold is steeply dipping (Fig. 5B) in 251
contrast with the northeastern. Taking into account the low temperature conditions of folding 252
(see below, Discussion) and the brittle behavior of the basement, a dense set of faults must be 253
hypothesized beneath the Middle Cambrian anticline within the basement and the overlying 254
Lower Cambrian sandstones (Fig. 5C). Another branch of the Taklimt fault zone (TFZ) occurs 255
along the NE limb of the fold, which separates the Taklimt unit from a foundered block 256
transitional between the Ougnat-Ouzina Axis and the South Tafilalt Basin. A group of E-W 257
open folds (including the Amelane and Mech Irdane synclines) are seen on this transitional 258
block and reveals a dextral throw along the TFZ, coeval with what can be regarded as the 259
main folding phase (“D1”; see sect. 5). Remarkably, the Taklimt fold is affected by a 260
transverse fold whose axis plunges southward (Fig. 5A). This folding event suggests a minor, 261
and probably late compressional event (“D2”) almost normal to the main one.
262
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The Renneg anticline is located along the opposite margin of the Ougnat-Ouzina Axis 263
(Fig. 3). This structure is widely overlain by sandy deposits, which hampers its detail analysis.
264
However, the bean-shaped horizontal section of its 8 km-long Cambrian core reveals the 265
curvature of its axis. The direction of the Renneg axis is about N120E at its eastern pericline, 266
but tends to parallel the EMF at its western pericline, suggesting a dextral throw along the 267
fault.
268
The croissant- or boomerang-like J. Tijekht anticline is the southernmost, and most 269
surprising Cambrian structure of the Ougnat-Ouzina Axis (Fig. 6A). This structure is 270
bounded in the east and south by two sinistral fault zones connected to the OJTF, i.e. the 271
ENE-trending Tizi n’Ressas fault (TRF) and the latitudinal South Tijekht and Oumjerane- 272
Taouz faults (STF and OJTF), respectively (Fig. 3). The deepest outcropping beds of the 273
Tijekht anticline belong to the Schistes à Paradoxides Fm and their competent carapace is 274
made up of Tabanit sandstones (Destombes & Hollard, 1986), the dip of which remains quite 275
shallow everywhere (Fig. 6A, B). The broadly semicircular crest is in fact composed by two 276
distinct parts separated by a reverse fault associated with two NNE-trending folds, suggesting 277
a late, approximately E-W compression (cf. J. Taklimt “D2”). The eastern corner of the 278
Tijekht croissant broadly parallels the two transverse folds and can be associated with the 279
same compressional event. Remarkably, the system of fractures that affects both the eastern 280
and western corners of the croissant is a homogeneous N35-N70 system of steeply dipping 281
open faults mostly mineralized in barite (Fig. 6A). Thus these fractures record an ultimate 282
tectonic event (“D3”) with a broadly NE-directed horizontal compression. In addition to the 283
main system of fractures, the conical periclines are truncated by transverse normal faults.
284
West of the TRF and north of the STF-OJTF boundary faults, the croissant-shaped 285
Cambrian core is surrounded and overlain by Ordovician formations whose geometry is much 286
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simpler. Above the thick, lower Ordovician pelites (Fezouata and Tachilla Fms.) that drape 287
the Tabanit irregular vault, the competent formations of the First Bani, Ktaoua and Second 288
Bani are organized in two sets of relatively tight folds slightly fanned with respect to the NW- 289
SE direction (Figs. 3, 6A&). Their mean direction is precisely that of the west corner of the 290
Cambrian massif. This implies that the main folding event responsible for the structure of the 291
Tijekht unit corresponds to a NE-trending compression, also found in the J. Taklimt (“D1”
292
event) and J. Renneg (eastern part). In contrast, the east corner of the Tijekht Cambrian massif 293
would result from the transverse “D2” event evidenced in the J. Taklimt (see Discussion 294
section).
295
4.4. Ordovician-cored folds of the Ougnat-Ouzina Axis
296
In this section we consider three antiformal units (Fig. 3), from north to south: i) the Bou 297
Mayz anticline immediately south of the Taklimt and Amelane-Mech Irdane units studied 298
above; ii) the large Shayb Arras anticline and its second order folds, and iii) the J. Tadaout 299
system in the southeastern most part of the Ougnat-Ouzina Axis, immediately north of the 300
OJTF. These units are cored by Ordovician terranes and separated from each other by narrow 301
Devonian-Carboniferous synclines, namely the Ottara and Amessoui synclines north and 302
south of the Shayb Arras anticline, respectively. As the Cambrian does not crop out in these 303
anticlines we infer they are built over low basement blocks with respect to the Tijekht and 304
Renneg Cambrian-cored structures. In other words, the basement is higher in the west of the 305
Ougnat-Ouzina Axis than in the east, which is in fact inherited directly from the Devonian 306
paleogeography (Korn et al., 2000; Lubeseder et al., 2009).
307
The Bou Mayz anticline globally consists of an E-trending, 20 km-long cylindrical fold 308
made up of Upper Ordovician formations and bounded southward by the Ottara syncline (Fig.
309
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3). However, the western pericline of the fold is particularly interesting as it displays a clear 310
interference pattern (Fig. 7A). The main fold is crosscut here by transverse folds trending 311
NNE-SSW with crest lines strongly curved in their vertical axial plane. These secondary folds 312
are reminiscent of the “D2” fold observed in the J. Taklimt at a short distance further north.
313
The crest of the Bou Mayz anticline describes a dextral bayonet at about half its length, in 314
the area where the Oued Rheris crosses the fold. Some 6 km further in the east, the eastern 315
pericline is twisted dextrally to a SE direction close to the Oued Ziz dextral strike-slip fault 316
(OZF). Most of the NE-trending fractures here are mineralized in barite, which compares with 317
the fracture systems of the Tijekht (see above) and Shayb Arras anticlines.
318
The Shayb Arras anticline is remarkable in two respects, i) its overall axis is sigmoidal, 319
and ii) its core of Middle Ordovician formations displays a brachyanticlinal shape contrasting 320
with the elongated shape of the Upper Ordovician-Devonian envelope (Fig. 8A). The 321
sigmoidal shape of the whole structure is particularly clear in the Silurian-Devonian eastern 322
pericline. There, the curvature of the axis occurs through a complex pattern of strike-slip, 323
normal or reverse faults suggesting brittle deformation of a previously more rectilinear 324
cylindrical fold (Fig. 8A, D). This forced curvature is consistent with the dextral throw along 325
the OZF, already documented further in the north (J. Taklimt and Bou Mayz region; Fig. 3).
326
The western curvature is less visible and occurs along a sinistral strike-slip fault that follows 327
the southwest border of the Mech Agraou plateau, surrounds the Amessoui western pericline 328
and then follows eastward the southern border of the syncline, thus being labeled Amessoui- 329
Mech Agrou fault (AMF; Fig. 8E). The AMF basically appears at the surface as a corridor of 330
en echelon folds or asymmetric shear folds in the Devonian formations, but likely corresponds 331
to a basement fault at depth.
332
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The central brachyanticline of the Shayb Arras structure is defined by the First Bani 333
quartzites that show a box-type profile formed through buckling above the incompetent 334
Lower Ordovician pelites. The geometry of the Upper Ordovician competent formations 335
(Tiouririne sandstones, Second Bani) broadly mimics that of the First Bani, but second order 336
flexural folds develop in these beds outside the core area (Fig. 8A). Second order kink folds 337
are also observed in the Middle Devonian limestones of the northeastern limb of the major 338
fold (Fig. 8B). Layer-parallel shear is documented by minor folds at varied places (Fig. 8C).
339
The entire anticline is crosscut by a set of vertical faults directed NE to ENE and frequently 340
mineralized in barite. The walls of these veins bear conspicuous horizontal striations with 341
sinistral kinematic indicators (Fig. 8F). All these observations document a flexural-slip 342
mechanism of folding that evolved toward a more brittle style of deformation. The shortening 343
direction would have rotated from N-S to NE-SW in the meanwhile.
344
Similar to the Amessoui syncline, the Ottara syncline is also bordered by a sinistral fault 345
corridor broadly parallel to its axis. At the western pericline (Fig. 9), the fault is located south 346
of the fold and curves northwest-ward as the syncline axis. Several transverse faults are also 347
observed in the pericline, that likely result from the complete or partial inversion of the 348
normal paleofaults that were active during the Middle-Upper Devonian (Lubeseder et al., 349
2010).
350
The Tadaout massif is located in the trapezoidal block bounded by the OZF and OJTF 351
main faults in the east and south, and the AMF and TRF subsidiary faults in the north and 352
west, respectively (Fig. 3). In other words, the Tadaout massif occupies the very southeast 353
corner of the Ougnat-Ouzina Axis, and this probably accounts for the great structural 354
complexity of this kind of “Gordian knot” (Fig. 10). The rock material involved in the massif 355
spans from the Lower Ordovician Fezouata pelites to the Lower Carboniferous in the faulted 356
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J. Mraier syncline, which suggests a downwarped basement with respect to the adjoining J.
357
Tijekht massif and Kem Kem domain. The internal structure of the Tadaout massif can be 358
untangled by distinguishing two sets of superimposed folds, broadly E-W and N-S, 359
respectively. The major and earlier folds appear to be the E-W directed, better said N100 in 360
the west to N120 in the east, suggesting a dextral twist comparable with that observed in the 361
Shayb Arras unit. In contrast, the N30 to N160-trending folds appear to postdate the E-W 362
ones as they are linked to transverse faults crosscutting the latter folds. These faults are the 363
Tadaout Central fault (TCF) in the middle of the massif and the Bou Hmid and Tizi n’Ressas 364
faults (BMF and TRF) in the west. The BMF fault is rectilinear and parallel to the TRF in the 365
southwest, but it curves in the north around the J. Bou Hmid monocline, which is the substrate 366
of the Mraier syncline, and finally ends against the TCF (Fig. 10). Thus this very peculiar 367
fault seems to detach the Mraier-Bou Hmid unit from the south part of the Tadaout massif and 368
carry it further in the north like a drawer between two N20-striking ramp faults. Along the 369
west border of the tectonic drawer and at its front, the Silurian-Lower Devonian limestones 370
show numerous minor folds recording the displacement of the Mraier-Bou Hmid unit, which 371
however was probably limited to less than 1 km. The N-S compression of the Tadaout block 372
against the Cambrian-Ordovician Kem Kem domain in the south is attested by the occurrence 373
of hectometric lenses of verticalized beds and the coexistence of dextral and sinistral minor 374
structures in the OJTF (Fig. 11).
375
4.5. Structure of the Maider and South Tafilalt basins
376
In this section we briefly consider the folds that affect the Devonian-Lower 377
Carboniferous formations of the two basinal areas on both sides of the Ougnat-Ouzina Axis, 378
i.e. the Maider and South-Tafilalt Basin (Fig. 3). The overall outline of these basins has been 379
defined above (sect. 4.1).
380
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The quadrangular Maider Basin is poorly deformed, except along its borders. The 381
Ordovician-Lower Devonian series crop out as E- to ENE-trending anticlines in the south 382
(Msiouda). Submeridian fold axes are observed in the NE corner of the quadrangle, where 383
they would record E-W compression against the Ougnat-Ouzina Axis. A NNW-plunging 384
minor fold affects the Lower and Middle Devonian formations of the east border of the basin, 385
consistent with a dextral throw along this inverted paleofault zone. Lastly, a poorly marked 386
anticline occurs in the southeastern part of the basin, resulting in a wide exposure area of the 387
lowest Upper Devonian series there. The remarkable sinusoidal shape of the Fezzou Lower 388
Carboniferous syncline cannot be easily accounted for by fold interference, and would rather 389
result from the adaptation of the sedimentary infill to the inversion of the surrounding or 390
underlying paleofaults during a moderate NW-SE to NE-SW compression. In particular, the 391
NE-striking, NW-dipping Fezzou paleofault documented by the Devonian stratigraphy 392
(Baidder et al., 2008) would have controlled the NE trend of the syncline axis east of Fezzou 393
village.
394
The South-Tafilalt Basin is much more deformed than its western homologous. The most 395
complex structures appear in the Znaigui and Mfis anticlines in the southwest corner of the 396
basin bounded by the OJTF and OZF fault zones (Fig. 13A). Both anticlines show exposures 397
of Middle-Upper Devonian competent formations. They are crosscut by ENE-trending 398
sinistral faults. The Mfis anticline displays a brachyanticlinal geometry, which could 399
corresponds to the interference of a submeridian “D2” fold superimposed on an latitudinal 400
“D1” fold. The core of the fault is crosscut by a complex set of open faults mineralized in 401
barite (Fig. 13B), and intruded by several dolerite bodies of probable Triassic-Liassic age.
402
The Marzouga synclinorium in the central part of the basin displays WNW-ESE directed fold 403
axis, broadly parallel with those of the Erfoud anticlinorium in the Widane Chebbi area. Some 404
hectometric second order folds appear in the major hinges (e.g. Hassi Merdani area), and the 405
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dip of bedding may reach about 60° in the highest stratigraphic levels within the deeper part 406
of the synclinorium, east of Hamou Rhanem. This attests for the importance of the 407
submeridian shortening of the sedimentary basin between the northern (Erfoud) and southern 408
(Nebech) uplifted blocks. The weak inflexion of the fold axes toward a NW direction next to 409
the OZF is consistent with a dextral movement along this fault zone.
410
5. Discussion
411
5.1. Folds interferences and involvement of the faulted basement
412
The structural data presented above are gathered together in a synthetic map (Fig. 14). At 413
first glance, this map illustrates the varied, interfering directions of fold axes and their close 414
relationships with the regional fault array:
415
- in the southernmost Kem Kem domain, folds and faults are regularly oriented NNW- 416
SSE, which is the main Ougarta direction (Menchikoff, 1952; Donzeau, 1972, 1974, 417
1983; Zazoun, 2001; Haddoum et al., 2001); in the central and northern domains, i.e.
418
the Ougnat-Ouzina Axis, the South Tafilalt Basin, and the south border of the Ougnat 419
Massifand Erfoud Anticlinorium, fold axes are dominantly directed WNW-ESE to E- 420
W, which is the direction of the Hercynian structures in the eastern Anti-Atlas and 421
adjoining Meseta units (Fig. 1B); the main faults strike either E-W to ENE or NW-SE, 422
which are the dominant fault directions in the eastern Anti-Atlas and Ougarta belts, 423
respectively;
424
- most fold axes are sigmoidal, and the curvature of their periclines give evidence of 425
strike-slip displacements along several fault zones; dextral movements are particularly 426
documented along the EMF and OZF faults that bound the Ougnat-Ouzina Axis, as 427
well as along the TFZ branch of the latter; sinistral displacements are documented 428
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along dominantly E-W to ENE-striking faults or strike-slip corridors such as the STF, 429
AMF, SOF structures (from south to north within the Ougnat-Ouzina Axis);
430
- clear fold interferences occur in the Taklimt and Bou Mayz anticlines in the north of 431
the Ougnat-Ouzina Axis, with the NW- to W-trending main folds “D1” deformed by 432
NNE- to N-S trending minor folds “D2”; the relative chronology of these folds is 433
discussed in the following section;
434
- folds at the southern part of the Ougnat-Ouzina Axis, next to the major Oumjerane- 435
Taouz fault (OJTF) show strong axis curvature and internal faulting; the Tijekht 436
anticline offers a croissant or boomerang shape in map view, and the adjoining 437
Tadaout anticlinal massif looks like a Gordian knot of intersecting folds and faults.
438
So, folding interferences and folds peculiar geometries appear to be controlled by an 439
array of intersecting faults in the basement allowing relative displacements of basement 440
blocks to occur. These basement faults correspond to inverted synsedimentary faults, mostly 441
Devonian paleofaults (Baidder et al., 2008) as exemplified in particular by the EMF, OZF and 442
OJTF faults (Fig. 15; see also sect. 2 and 4). Hence, we deal with a thick-skinned inversion 443
tectonics as observed immediately in the north around the Ougnat inlier (Raddi et al., 2007) 444
and further in the west in the central Anti-Atlas (Faik et al., 2001; Burkhard et al., 2006). The 445
synthetic profile here proposed (Fig. 16) makes visible the involvement of the brittle 446
basement in the first order folds of the cover. Folding of the sedimentary cover above the 447
moving mosaic of basement blocks was permitted because of the occurrence of ductile, pelitic 448
or argillaceous formations (Schistes à Paradoxides, Fezouata-Tachilla pelites, Silurian shales 449
and upper Emsian marls). The Lower Cambrian sandstones remained globally stuck onto the 450
basement, in the absence of thick “lie-de-vin” pelites and layered limestones at the bottom of 451
the sequence, which contrasts with the Western Anti-Atlas setting (Helg et al., 2004;
452
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Burkhard et al., 2006). Folds are very open in the Cambrian Tabanit sandstones, whereas they 453
become tighter in the overlying Ordovician and Devonian formations.
454
In this profile, the dip of the strata has been extrapolated at depth admitting a flexural-slip 455
mechanism of folding, which is well-documented by the field observations at every 456
stratigraphic level and consistent with the low-temperature conditions of the Variscan 457
deformation (see next section). The brittle behavior of the basement in such conditions is 458
illustrated in the Ougnat Massif (Raddi et al., 2007) and Erfoud anticlinorium in the north, as 459
well as in the Ediacaran outcrops beneath the Tazoult n’Ouzina vault in the south. The Dip of 460
the faults at depth remains speculative. Unpublished seismic profiles acquired by the Office 461
National de Recherche et d’Exploitation Pétrolière (ONAREP, now renamed ONHYM, 462
Office National des Hydrocarbures et des Mines) in the Erfoud-Rissani basin have been 463
tentatively interpreted in the last couple of years (Baidder, 2007; Toto et al., 2008; Robert- 464
Charrue and Burkhard, 2008), but resulted contradictory due to the poor quality of these 465
ancient 2D-seismic lines. Here the style of faulting is inspired from the Laramide examples 466
(Mitra and Mount, 1998) on the one hand (Tijekht and Shayb Arras anticlines, Erfoud 467
anticlinorium), and on the other hand from the flower geometry (Harding, 1985) where 468
vertical throw is minimum (e.g. Mech Agrou-Amessoui strike-slip fault).
469
Fold orientation is mostly oblique to the major basement faults (Fig. 14), suggesting the 470
regional stress was oblique to these faults during at least part of the Variscan orogeny.
471
However, along some of the inverted paleofaults the development of multiple shear planes 472
may result in pseudo continuous deformation of the basement at the vertical of a fold in the 473
cover. This was described in the case of the J. Angad anticline in the Bou Adil area south of 474
the Ougnat Massif (Raddi et al., 2007), and seems also appropriate to the J. Taklimt case in 475
the Ougnat-Ouzina Axis (Fig. 5).
476
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477
5.2. Low temperature conditions of strain
478
In order to ascertain the above interpretation of the regional tectonic style, it is 479
appropriate specifying the physical conditions that prevailed in the rock material during 480
folding. This is particularly true to understand the formation of the Tijekht and Tadaout 481
croissant-shaped folds, seldom described in the literature. Besides of crescent folds associated 482
occasionally with diapirism (Jackson et al., 1990), crescent fold pattern caused by flattening 483
and flexural flow folding have been described in the southernmost Altaids where the 484
sediments were unconsolidated and enriched in fluids during their deformation (Tian, 2013), 485
which is clearly not the case of the Cambrian and Ordovician strata of the Tafilalt area.
486
In the South Tafilalt area, the P-T-fluid content conditions were different during folding 487
from top to bottom of the Paleozoic sedimentary pile. In the Lower Carboniferous formations, 488
rocks were buried at > 2 km depth (Fig. 2), perhaps ca. 5 km assuming ~3 km-thick eroded 489
deposits, and they were still rich in fluids when the Late CarboniferousEarly Permian folding 490
occurred. Therefore, and in the absence of any coeval magmatism, folding occurred at very 491
low temperature (150°C at a maximum) and pressure. Illite cristallinity measurements 492
confirm that these rocks remained in diagenetic conditions, i.e. at T< 200°C (Ruiz et al., 493
2008). Contrary to Benharref et al. (2014) who suggest that the planar fabric observed in these 494
Carboniferous rocks is a metamorphic foliation, we consider that it corresponds generally to 495
the stratification plane enhanced by compaction and locally deformed around the calcareous 496
or cherty concretions (Fig. 17A, B). However, a true tectonic cleavage (Fig. 17C) is observed 497
south of Taouz in the olistolite-bearing deposits accumulated against the OJT fault during the 498
Tournaisian (Fig. 4C). This tectonic fabric is a vertical, spaced cleavage axial-planar to 499
M AN US CR IP T
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decametric folds and almost parallel to the adjoining fault. The asymmetry of these folds and 500
their steeply dipping axis (N70E, 50-60° ENE) indicate a sinistral throw during compression 501
of the Lower Carboniferous series against the Ordovician quartzites of J. Aroudane (Figs. 4, 502
13). The spaced cleavage developed there by pressure-solution at low temperature as the 503
Tournaisian sediments were still rich in fluids.
504
When folding began, burial of the lowermost Paleozoic beds exceeded that of the Lower 505
Carboniferous by ca. 3 km, which is the mean thickness of the Cambrian-Devonian series in 506
the area (Fig. 2). The expected temperature was likely close to 200°C assuming a 25°C/km 507
geotherm, which is typical for continental basins with thick infill (Allen & Allen, 1990). The 508
illite cristallinity indexes measured by Ruiz et al. (2008) in Devonian, Silurian and Ordovician 509
samples from the area indeed indicate diagenetic to anchizonal evolution, whereas epizonal 510
conditions are not observed here contrary to the western and central Anti-Atlas regions. In 511
other words, T remained close to 200°C in most of the Tafilalt-Maider area. This is consistent 512
with the sedimentary fabric observed in the Middle Cambrian formations (e.g. Tijekht 513
anticline; Fig. 17D).
514
5.3. Superposed folding events or fault control during a single deformation
515
This classical problem (e.g. Marshak, 2000; Carciumaru and Ortega, 2008) was addressed 516
in the western-central Anti-Atlas by Faik et al. (2001) and by Martin Burkhard and his alumni 517
(Caritg et al., 2004; Helg et al., 2004; Burkhard et al., 2006) with divergent conclusions. In 518
agreement with Soulaimani (1998), Faik et al. (2001) suggested that the fold interferences of 519
the Tata area were controlled by the inverted paleofaults orientation without any superposed 520
events of differently oriented regional compression. In contrast, Burkhard’s school favored a 521
combination of paleofault control and superposed compression events, oriented firstly south- 522
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eastward, then southward. We argue that such a combination of tectonic events fits the 523
Taflilat study case, although with different orientations of compression with respect to 524
Western Anti-Atlas.
525
First of all, rotation of the direction of compression is clearly documented in the 526
Variscan belt of the Meseta-Atlas domain, from a dominant WNW-ESE trend during the 527
Bashkirian-Moscovian to a N-S trend during the Late Pennsylvanian-Early Permian (De 528
Koning, 1957; Ferrandini et al., 1987; Aït Brahim and Tahiri, 1996; Saidi et al., 2002; Saber 529
et al., 2007). . Indeed, a similar rotation of regional stress is observed as well in the Variscan 530
belt of Western Europe (Marques et al., 2002; Ribeiro et al., 2007; Gutiérrez-Alonso et al., 531
2015).
532
As the Meseta-Atlas domain was coupled with its metacratonic foreland along the SMF 533
(Fig. 1) from the Bashkirian onward, the Late Pennsylvanian-Early Permian rotation of 534
compression occurred also in the Anti-Atlas, and likely in the Ougarta belt further in the 535
south-east. The regional stress reorientation from NW-SE to N-S was described by Caritg et 536
al. (2004) in western-central Anti-Atlas, as reported above. Rotation of regional stress is also 537
reported in the Tineghir area from N-S to NNW-SSE during the same Late Carboniferous- 538
Early Permian span of time (Soualhine et al., 2003; Cerrina-Feroni et al., 2010). In the Eastern 539
High Atlas Tamlelt massif of the South-Meseta Zone immediately north of the Bechar Basin, 540
Variscan E-W folding and dextral shearing record a similar evolution of compressional trend 541
(Houari and Hoepffner, 2003).
542
In the Ougarta-Ahnet belt, folding would have started shortly after the Stephanian- 543
Autunian (Haddoum et al., 2001), whose subaerial red beds deposits (Abadla lower and upper 544
formations; Fabre, 1976, 2005; Bouabdallah et al., 1998) are tilted along the western and 545
M AN US CR IP T
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eastern sides of the belt (Reggane and Bechar Basin, respectively). However, some structural 546
observations suggest that a Visean deformation event occurred (Blès, 1969), which appears 547
consistent with K-Ar datings of <2µ mica fractions from three Ougarta samples at 378±17, 548
323±9 and 246±7 Ma (Bonhomme et al., 1996). Likewise, whole-rock K-Ar datings of 549
Ediacaran volcanics yielded 310 Ma and 264 Ma ages (Hamdidouche and Aït Ouali, 2009).
550
Thus a protracted Variscan evolution of the Ougarta belt must be considered rather than a 551
single Early Permian event. Lamali et al. (2013) even proposed the occurrence of a 552
Famennian-Tournaisian event, based on the paleomagnetic study of the magmatic complex of 553
the Precambrian-Cambrian inliers. This proposal is contradicted by the perfect continuity 554
between the Devonian, Tournaisian and Visean strata of the fold belt (Menchikoff, 1952;
555
Haddoum et al., 2001; Haddoum, 2009). So, we retain at least provisionally that Ougarta 556
deformation occurred during the Late Carboniferous-Early Permian, being coeval of the Anti- 557
Atlas folding.
558
The rotation of regional stress directions is also documented in Ougarta, and fold 559
interferences are also described there (Collomb and Donzeau, 1974; Haddoum, 2009). The 560
main direction of shortening changes from NE-SW to E-W, which is interpreted either as the 561
result of superposed events (Collomb and Donzeau, 1974) or as a continuous reorientation 562
process (Zazoun, 2001). Anyway, the control of fold trends by NW and E-W striking 563
basement faults inherited from the Pan-African orogeny is generally acknowledged in the 564
literature and explain the dominant NW to NNW trend of the Paleozoic belt. The global 565
model that better accounts for this evolution evokes the impingement of the WAC nucleus 566
against the European Variscan belt in the north and the East Sahara metacraton (Ennih and 567
Liégeois, 2008) in the east, linked to a northward and anticlockwise rotational movement of 568
Africa during the Alleghanian-Variscan collision (Lefort, 1988; Lefort and Bensalmia, 1992).
569
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The interpretation of the Tafilalt-Maider structures must be approached within this 570
general framework as a combination of paleofault control of fold orientation and superposed 571
compressional events with different directions of compression. In particular, this general 572
scenario may account for the most complicated fold structures of the area, i.e. the croissant- 573
shaped Tijekht anticline and the curved and fractured Tadaout anticline (Fig. 18).
574
The first stage of our qualitative model (Fig. 18A) delineates the synsedimentary normal 575
fault array active during the Middle-Upper Devonian to Early Carboniferous. Four uplifted 576
blocks are distinguished: two of them belong to the Kem Kem domain south of the OJTF 577
whereas the other two belong to the Ougnat-Ouzina Axis. The latter blocks are supposedly 578
crosscut by broadly N-S normal faults that would account for the eastward thickening of the 579
Devonian series and the associated debrites facies (Korn et al., 2000; Lubeseder et al., 2009) 580
and for the subsequent activation of strike-slip faults like the Tizi n’Ressas fault (TNR, Fig.
581
14).
582
The earliest compressive event “D1” corresponds to the N-S shortening of the Meseta- 583
Anti-Atlas system reported above, and dated from the Bashkirian-Westphalian. At that stage 584
(Fig. 18B), the South Tafilalt-Maider area is deformed north of the OJTF and the latter fault is 585
partly inverted as a sinistral strike-slip zone. The Ougnat-Ouzina Axis becomes a mega shear 586
zone between the Oued Ziz (OZF) and East Signit-East Maider paleofaults (ESF, EMF), 587
partially inverted into dextral strike-slip faults. The E-W trending folds born at the beginning 588
of this “D1” event become sigmoidal. The basement of the Shayb Arras and Tijekht-Tadaout 589
anticlines tend to shorten through conjugate strike-slip faulting and intense shearing along the 590
inverted paleofaults.
591
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The Ougarta events are characterized by NE-SW to E-W compression dated as 592
Stephanian-Early Permian. The youngest “D2” event (Fig. 18C) is firstly responsible for the 593
development of NNW-trending, Ourgata-like folds south of the OJTF (e.g. Ouzina and 594
Aroudane-Zorg anticlines, Fig. 4). Second, north of the OJTF this event superimposed a 595
transverse shortening onto the earlier structures. The width of the Ougnat-Ouzina mega shear 596
zone lessens and the pre-existing folds are deformed by the development of transverse, 597
broadly N-trending secondary folds. This is exemplified in the north of the Ougnat-Ouzina 598
Axis by the abrupt kink of the J. Taklimt crest (Fig. 5) and the egg-box pattern at the western 599
pericline of the Bou Mayz anticline (Fig. 7). In the southern part of the Ougnat-Ouzina Axis 600
and adjacent South Tafilalt Basin, the effect of the “D2” shortening is twofold. First, the 601
sigmoidal shape of the Shayb Arras anticline and adjacent synclines is accentuated and 602
second, the geometry of the core of the anticlines is modified. At last, an axial culmination 603
deforms the crest of the earlier fold that becomes a brachyanticline in the core region (e.g.
604
Shayb Arras and Mfis anticlines). More significantly, early and secondary fold axes may 605
complicate the geometry as to form croissant- or boomerang-shaped anticlines, either 606
relatively simple (Tijekht anticline, Fig. 6) or deeply fractured (Tadaout anticline, Fig. 10).
607
The poles to bedding are strongly scattered at the scale of the whole region (Fig. 18D). At the 608
scale of individual anticlines, the distribution of the downdip lines of bedding makes visible 609
the contrast between the Kem-Kem Domain (Figs. 4A, D) with its simple, north-trending 610
folds, and the domains north of the OJTF (Figs. 5, 6, 8, 13), where interference of folding 611
episodes are best exposed.
612
It is worth noting that deformation of the Paleozoic terranes did not stop at this stage 613
“D2”. The occurrence of a widespread system of barite veins oriented rather constantly in the 614
NE-SW quadrant in most of the studied anticlines suggests a “D3” stage of NW-SE extension 615
during which the fractures probably created by the previous “Ougarta event” opened and were 616
M AN US CR IP T
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mineralized. This could be ascribed to the well-known Triassic-early Liassic rifting event 617
(Frizon de Lamotte et al., 2008, and references therein; Berrada et al., 2016). The advection of 618
the mineralizing solution may have been enhanced by the Late Triassic magmatic event by the 619
end of the rifting process as suggested by Kharis et al. (2011) for the Oumjerane veins hosted 620
in the Ordovician quartzites west of the Maider Basin.
621
6. Conclusion
622
Sub-Saharan Morocco offers optimal conditions for studying fold geometry and folds and 623
faults relationships in the frame of a thick-skinned foreland belt, namely the Anti-Atlas 624
Paleozoic belt. The present work focused on the Eastern Anti-Atlas where the E-W trending 625
Anti-Atlas connects with the NW-trending Ougarta. Both belts formed during the Variscan 626
(Alleghanian-Hercynian) Late Carboniferous-Early Permian collision between Laurentia- 627
Avalonia and Gondwana, but they developed in distinct structural setting. The Anti-Atlas 628
formed at the expense of the northern margin of the WAC cratonic domain whereas Ougarta 629
developed at the expense of an elongated trough between the WAC and the East-Sahara 630
metacraton. Deformation was possibly slightly diachronic from west to east as sedimentation 631
changed from marine to subaerial during the Bashkirian-Westphalian transition in the west 632
and not before the late Moscovian in the east. All around the north-eastern border of the 633
WAC, the paleofault pattern was different from west to east, showing NE and E-W strikes in 634
the west and E-W to NW-SE strikes in the east.So, the South Tafilalt-Maider area appears as a 635
good example of inversion tectonics with a dual mechanism of fold interference by both fault 636
control of the basement-cored folds and superposed compressional events with different 637
compression trend. Probably the most curious result of this dual mechanism corresponds to a 638
large croissant- or boomerang-shaped Cambrian anticline easily observed in satellite imagery.
639
M AN US CR IP T
AC CE PT ED
We consider the area as a valuable target for advanced structural research on selected 640
individual folds.
641
642
Acknowledgements 643
We are greatly indebted to one of our reviewers, Dominique Frizon de Lamotte, for his 644
accurate and friendly criticism of the earlier version of this work. The Direction of Geology, 645
Ministry of Energy and Mines, Water and Environment, Rabat (Dr. Belkhedim) afforded us 646
logistic support for our conclusive field trip, April 2015.
647
References 648
Abati, J., Aghzer, A.M., Gerdes, A., Ennih, N., 2012. Detrital zircon ages of Neoproterozoic 649
sequences of the Anti-Atlas belt. Precambrian Research 181, 115-128 650
Abati, J., Aghzer, A.M., Gerdes, A., Ennih, N., 2012. Insights on the crustal evolution of the 651
West African Craton from Hf isotopes in detrital zircons from the Anti-Atlas belt, 652
Precambrian Research 212-213, 263-274.
653
Aitken, S.A., Collom, C.J., Henderson, C.M., Johnston, P.A., 2002. Stratigraphy, 654
paleoecology, and origin of Lower Devonian (Emsian) carbonate mud buildups, Hamar 655
Laghdad, eastern Anti-Atlas, Morocco, Africa. Bull. Can. Petrol. Geol. 50, 217-243.
656
Allen, P.A., Allen, J.R., 1990. Basin Analysis. Blackwell Sci. Publ. Oxford, 451 pp.
657
Alvaro, J.J. et al., 2014a. Carte géologique du Maroc au 1/50 000, feuille Tawz - Mémoire 658
explicatif. Notes et Mém. Serv. Géol. Maroc, n°551 bis.
659
