The origin of intraplate deformation in the Atlas system of western and central Algeria: from Jurassic rifting to Cenozoic–Quaternary inversion

Tectonophysics 357 (2002) 207 – 226 www.elsevier.com/locate/tecto

The origin of intraplate deformation in the Atlas system of western and central Algeria: from Jurassic rifting to Cenozoic–Quaternary inversion Rabah Brace`ne a,b, Dominique Frizon de Lamotte a,* a

De´partement des Sciences de la Terre (CNRS, U.M.R. 7072), Universite´ de Cergy-Pontoise, 95 031 Cergy-Pontoise Cedex, France b SONATRACH Explo, Avenue du Premier Novembre, Boumerde`s, Algeria Received 6 May 2000; accepted 10 August 2001

Abstract Analysis of petroleum exploration data supplemented by paleostress data enabled discussion of the origin of the deformation in the western and central Saharan Atlas (Algeria). This intraplate area has recorded the breakup of Pangea (Late Triassic), the opening of the Maghrebian Tethys (since the Dogger) and subsequently its closure (Oligocene to Present). However, the two periods of strong coupling between Europe and Africa (late Lutetian and Pleistocene), which correspond to rapid uplifts of the Atlas system and important deformations, are not collision-related. They can be correlated to the beginning and the end of the development of the western Mediterranean Sea. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Northern Algeria; Intraplate deformation; Rifting; Inversion

1. Introduction The Atlas system of North Africa is composed by the domain situated between the Sahara platform to the south and the Tell –Rif system to the north (Fig. 1A). This system comprises fold-thrust belts (the Atlas Mountains) and two more or less rigid cores (the Moroccan Central Massif and the Algerian High Plateaux, respectively) corresponding to areas where the Mesozoic cover is thinner or absent. Since the Triassic up to present, the Atlas system has undergone episodes of both subsidence and uplift,

*

Corresponding author.

which, at regional scale, can be correlated to plate kinematics. In particular, there is general agreement that the Atlas domain is developed along zones of crustal weakness inherited from rifting episodes (Aı¨t Ouali, 1991). This rifting is associated with the opening of the Tethyan and Central Atlantic oceans during Late Triassic to Liassic times (Stets and Wurster, 1977; Mattauer et al., 1977; Laville and Petit, 1984; Winterer and Hinz, 1984; Andrieux et al., 1989; Aı¨t Ouali, 1991; Stets, 1992). On the other hand, the inversion of the Atlas basins, occurring during Cenozoic and Lower Quaternary times, is classically related to the convergence between Europe and Africa (Mattauer et al., 1977; Giese and Jacobshagen, 1992; Guiraud and Bosworth, 1997). However, the relationships between

0040-1951/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 1 9 5 1 ( 0 2 ) 0 0 3 6 9 - 4

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Fig. 1. (A) Schematic map of North Africa showing the main structural domains and locating the study area. (B) Structural map of Northern Algeria locating the seismic cross-sections presented in the paper.

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this convergence, which is continuous and quite regular since the late Cretaceous (Le Pichon et al., 1988), and the distinct periods of the Atlas construction are not straightforward (Frizon de Lamotte et al., 2000).

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This paper aims to give an overview of the structure and the development of a segment of the Atlas system since the Triassic. Access to the subsurface database (seismic profiles and wells) of SONA-

Fig. 2. Three cross-sections (TWT, seconds) through the studied area (see location on Fig. 1B), (A) In Western Algeria crossing the Telagh trough, the Meseta block, the Pre-Atlas, the Saharan Atlas and the South Atlas Front. (B) In Central Algeria crossing the Tell foredeep, the foreland (Pre-Atlas), the Saharan Atlas and the South Atlas Front. (C) In Eastern Algeria from the Tertiary Hodna basin to the Sahara platform.

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TRACH Petroleum provides us an opportunity to specify the timing of this evolution from rifting to inversion and to address the general question of the

origin of intraplate deformation. Subsidiary problems such as the role of salt mobility, paleostress reconstruction and control exercised by basement faults on

Fig. 3. (A) Seismic section (TWT, seconds) and interpretation through the Hodna Tertiary basin showing the following unconformities: Palaeogene/Upper Cretaceous; Oligocene/Eocene; Miocene/Paleogene (see location on Fig. 1B). (B) Composite line drawing (TWT, seconds) in the Hodna basin showing the southern migration of Miocene depocentres (see location on Fig. 1B).

R. Brace`ne, D. Frizon de Lamotte / Tectonophysics 357 (2002) 207–226 Fig. 4. Composite line drawing and seismic cross-section (TWT, seconds) in central Algeria crossing the Tell front, the Tell foredeep and the african foreland (see location on Fig. 1B). 1—Triassic, 2—top of the Liassic, 3—Middle Jurassic, 4—Upper Jurassic, 5—top of the Jurassic, 6—Early Cretaceous, 7—Aptian, 8—Upper Cretaceous, 9—Miocene, 10— Quaternary.

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Fig. 5. Composite line drawing and seismic cross-section (TWT, seconds) west of the Hodna basin illustrating the erosion surface at the base of the Tell foreland basin.

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the development of cover structures during inversion will be also addressed. We will focus on the central and western Algerian regions (Fig. 1A and B) where the structures are particularly well exposed and the wells controlled by seismic data. After a short geological presentation of the studied area, we will examine the Tell – Atlas transition zone and the Saharan Atlas system from the northern flank of the High Plateaux to the South Atlas Front region. A geodynamic scenario will be proposed in the discussion.

2. Geological setting Along its southern border, the Tell system is bounded by an E – W trending foreland basin filled by Miocene deposits (Fig. 1B). This basin lies unconformably on the Atlas system, which had been previously structured along SW –NE trends (see below). As a consequence, this foreland basin rests on different structural domains. The general geometry of the region

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is illustrated by three parallel (in two-way time) crosssections (Fig. 2A,B and C) built from seismic profiles. In the west, from north to south (Fig. 2A), we can distinguish: the ‘‘Telagh trough’’, the ‘‘Meseta block’’, the ‘‘Pre-Atlas’’ and the Saharan Atlas domains. From a geographic and topographic point of view, the three first domains belong to the High Plateaux region, which is characterised by a relatively moderate altitude (about 1000 m). Furthermore, quite continuous Tertiary (Miocene –Pliocene) continental deposits cover their Mesozoic series. The Telagh trough is a zone where the Tell margin is not buried below the foreland basin but exposed in the field. It is relatively well preserved from the last compressive events (Elmi et al., 1998). In this structural framework, the Meseta constitutes the true rigid block of the system. It is characterised by a hiatus of the Triassic evaporites, a thin Mesozoic cover and by the lack of Tertiary deformation. This block presents also strong magnetic anomalies confirming that the basement (Paleozoic intruded by volcanic rocks) is closed to the surface. South of the Meseta block, the Pre-Atlasic domain represents the northern margin of

Fig. 6. Map of the main normal faults active during the Liassic and the assigned extension direction.

214 R. Brace`ne, D. Frizon de Lamotte / Tectonophysics 357 (2002) 207–226 Fig. 7. Seismic cross-section (TWT, seconds) and interpretation through the western ‘‘Pre-Atlas’’ illustrating the tilted block geometry which is developed during Liassic, the thickness variation of the Jurassic sequences, the early mobility of the Triassic evaporites. The early salt ridges have been cut out during inversion periods (see location on Fig. 1B).

R. Brace`ne, D. Frizon de Lamotte / Tectonophysics 357 (2002) 207–226 Fig. 8. Seismic cross-section (TWT, seconds) and interpretation through the western Pre-Atlas, illustrating the geometry of the transition between the High Plateaux domain and the Atlas basin (see location on Fig. 1B). 215

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Table 1 Paleostress during the Late Lutetian (see Fig. 9A)

3. The transition zone between the Tell and Atlas systems

Site

Number of site

Age of rocks

Method

Type of event

j1

Authors

A B C D E F G H I J K L M

1 1 1 1 1 1 1 1 1 1 1 1 1

Eocene Eocene Turonian Jurassic Jurassic Turonian Turonian Eocene T Eocene Jurassic Liassic Liassic Liassic

a a a a a a a a a a a a a

RF, S, S0 RF, S RF, S, S0 RF, S, S0 RF, S, S0 RF, S0 RF RF and S RF, S0, T RF, S0 RF, S, T RF, S, T RF, S, T

140 135 138 140 145 140 140 135 138 130 140 120 135

6 3 6 6 6 6 6 6 3 3 6 6 3

RF: reverse fault. S: stylolites. S0: bedding. T: tension gash. SF: strike-slip fault. a: geometric analysis. b: Carey method. c: from paleostress maps.

the Atlas trough. It is characterised by a moderate deformation with some tight SW – NE trending anticlines separated by wide synclines. To the south, the Saharan Atlas is built on the site of the Mesozoic subsiding Atlas basin. It looks like a ‘‘classical’’ foreland fold-thrust belt. However, an important point emphasised by Frizon de Lamotte et al. (2000) is that the Atlas domain has been affected by an early (Late Lutetian) compressive event (Laffitte 1939; Kieken, 1974, 1975; Ghandriche, 1991; Frizon de Lamotte et al., 2000). This event predates the development of the Tell foreland basin located to the North. The southern border of the Atlas system corresponds to the South Atlas Front, which is the transition between the Sahara platform and the Atlas. It is underlined by an alignment of anticlines built on blind ramps climbing from the basement/cover interface (Brace`ne et al., 1998). Eastward, in the Hodna basin, the cross-sections B and C (Fig. 2) highlight the thickening of Cretaceous deposits. In this domain, the Meseta block no longer exists (Fig. 2C). Folded Cretaceous to Eocene rocks are buried below a thick partly marine Miocene series with clear onlaps progressing southward (Fig. 3A and B). Outside of the Hodna basin, thin Paleogene series exist in some places situated along the South Atlas Front and the Miocene – Pliocene deposits are of continental origin.

This domain, located south of the Tell front, is presently overlapped by the Tell nappes issued from the Tell margin and emplaced during the Langhian to Tortonian. It has been successively the southern passive margin of the Tethys and later the Tell foreland basin. (Fig. 2B). These two successive stages are well illustrated by some seismic lines (Figs. 4 and 5). 3.1. From rifting to passive margin At the scale of the Western Mediterranean region, it is now well known that rifting occurred in two successive stages of late Triassic and early to Middle Jurassic ages (Favre and Stampfli, 1992). The late Triassic period is characterised by diffuse extensional tectonics leading to the development of some tilted blocks filled by clastic continental sediments. Deposition of evaporite layers and interbedded basaltic flows that occurred at the end of Triassic times sealed this early event. Marine sedimentation began in the Rhaetian and continued in a shallow-water environment up to the Middle Jurassic (Bassoulet, 1973; Delfaud, 1986). It is worth noting a fault-controlled increase of thickness of the Liassic carbonates and the coeval development of salt ridges and salt pillow, leading to the present-day pinch-and-swell geometry of the salt layers (Fig. 4). Normal faults and salt ridges are both trending ENE – WSW, suggesting a NNW – SSE extension direction (Fig. 6). Post-rift thermal subsidence lasted from Middle – Late Jurassic up to the Late Cretaceous. Over the previous tilted blocks, the Middle – Late Jurassic sedi-

Table 2 Paleostress during the Miocene extension (see Fig. 9B) Site Number Age Method Type j1 of site of rocks of event

Authors

A

1

4

B C

1 1

Lower b NF Miocene Miocene a and c NF Miocene a and c NF

032

NNE – SSW 1 NNE – SSW 1

RF: reverse fault. S: stylolites. S0: bedding. T: tension gash. SF: strike-slip fault. a: geometric analysis. b: Carey method. c: from paleostress maps.

R. Brace`ne, D. Frizon de Lamotte / Tectonophysics 357 (2002) 207–226 Table 3 Paleostress during the Tortonian (see Fig. 9C) Site

Number of site ref.

A B C D E F G

1 1 1 1 1 1 1

Age of rocks

Method

Type of event

j1

Authors

b b b a a a a

RF RF SF RF and S RF and S SF SF

322 335 146 150 155 030 000

5 4 4 3 3 1 1

RF: reverse fault. S: stylolites. S0: bedding. T: tension gash. SF: strike-slip fault. a: geometric analysis. b: Carey method. c: from paleostress maps.

ments (Fig. 4) display a typical ‘‘steer’s head’’ geometry (White and Mc Kenzie, 1988). In contrast, thickness and facies of Cretaceous strata change slowly in the western and Central Saharan Atlas. During Early Cretaceous, a thick ‘‘flysch-type’’ sedimentation took place on the Tethys margin. The detrital material came from the western Saharan Craton through the ‘‘Monts des Ksours’’ delta, situated in the present-day Saharan Atlas (Delfaud et al., 1974; Vila, 1980). A general marine transgression starting in the late Albian led to the development of a carbonate platform extending almost everywhere in Africa during the Cenomanian and the Early Turonian. Subsidence was still active

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during the Senonian in the Tell domain with pelagic or hemipelagic facies covering the whole Tell domain (Wildi, 1983). During the Senonian, occurrence of conglomerates and some uncomformities have been described in some places in the Tell (Babors) (see Leikine, 1971; Obert, 1981) and in the Hodna basin (see Bertraneu, 1955;Guiraud, 1973; and review in Guiraud and Bosworth, 1997). Some authors have linked these conglomerates and unconformities to inversions occurring during the Upper Cretaceous (Guiraud and Bosworth, 1997). According to Kireche (1993), we consider that the conglomerates do not seal compressive structures, but more likely, underline apex of tilted blocks developed during an extensional event and subsequently eroded. Furthermore, in the Aure`s basin of eastern Algeria (Fig. 1B), Upper Cretaceous extensional events have been already described by Bureau (1986). In the Central Saharan Atlas (Djelfa syncline, for instance) where the Upper Cretaceous series are continuous, there are neither conglomerates nor unconformity exposed in the field. Towards the SW in the western Saharan Atlas (Fig. 1B), the latest Cretaceous levels are Turonian in age (Galmier, 1970; Bassoulet, 1973). During the whole post-Rift period, lasting from Middle Jurassic to late Lutetian, there is no evidence of compressional event in the studied area. In

Table 4 Paleostress during the Pleistocene (see Fig. 9C) Site

Number of site

Age of rocks

Method

Type of event

j1

Authors

A B C D E F G H I J K L M N

1 1 1 1 1 1 1 1 3 1 1 1 1 1

Villafranchian Villafranchian Villafranchian Villafranchian Miocene Miocene Mio – Pliocene Quaternary Pliocene Pliocene Pliocene Pliocene Pliocene Pliocene

a a b b b b a a a a a a a a

RF and S RF RF RF RF RF RF ? RF SF SF, S, T, RF SF, S, T, RF RF RF

000 180 180 001 000 357 000 NNW – SSE 150 169 163 170 005 N000

6 5 5 5 4 4 3 7 2 1 1 1 3 3

RF: reverse fault. S: stylolites. S0: bedding. T: tension gash. SF: strike-slip fault. a: geometric analysis. b: Carey method. c: from paleostress maps.

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particular, neither transpressive nor transtensive events described in the eastern High Atlas in Morocco (Laville, 1985; Pique´ et al., 1998) nor those in the Saharan Atlas (Aı¨t Ouali, 1991) are detected. Similarly, the Eo-Cretaceous extensional tectonics described in the eastern regions (Burollet and Ellouz, 1986) or in the Tell (Wildi, 1983; KaziTani, 1986) seems to be absent in the studied area (Fig. 1A). Marine marly and calcareous sedimentation is continuous until the Late Lutetian. The major facies change occurred during the Oligocene with the development of series of flysch (Wildi, 1983). The detrital material came from Africa likely as a response to the tectonic event, which took place in the Atlas system during the late Lutetian. 3.2. Foreland basin stage The Tell system foreland basin has been formed as a flexural response to the emplacement of the Tell external nappes from Middle Miocene to Tortonian times (Wildi, 1983; Courme-Rault, 1985). More precisely, in central Algeria, the latest levels overthrusted by the Tell nappes are late Langhian (OGN well, SONATRACH, unpublished data) (Fig. 1B). As shown in the Figs. 4 and 5, the geometry of the basin is quite classical with southward migrating onlaps. An important point is that the basin developed on an erosion surface. The base of the basin overlies unconformably the Mesozoic (Figs. 4 and 5). Along the seismic section (Figs. 5), the Miocene deposits rest directly on the Campanian in DEG well (Draa El Ghozlane), on the Turonian in OGS well (Oued Gue´te´rini South) and on the Albian in OGN well (Oued Gue´te´rini North) (Fig. 1B). For this last well, this implies more than 3000 m of eroded sedimentary cover below the unconformity (Fig. 5). Erosion appears mainly as a response to the pre-Neogene uplift of the Tell foreland due to the late Lutetian compressive event which affected the Atlas system (Laffitte, 1939; Guiraud, 1975; Ghandriche, 1991;

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Frizon de Lamotte et al., 2000). It is likely that a part of the uplift was related to the progression of the forebulge through a region now occupied by the foreland basin. Furthermore, normal faulting observed just below the unconformity (Figs. 4 and 5) could have been developed along the moving flexural bulge. However, the thickness of the eroded sedimentary cover is too large to result from this single process. The development of the foreland basin occurred during the Middle Miocene, a period, which corresponds to the emplacement of the Tell external nappes (Kieken, 1975; Wildi, 1983). This pile of thrust sheets, composed by Upper Cretaceous up to Miocene sediments, can be considered an accretionary prism (Caire, 1957; Mattauer, 1958; Wildi, 1983). Based on the geometry of the thrust system, the tectonic transport is assumed to be N –S. There is no evidence demonstrating that the pre-Triassic basement is involved in the prism, at least close to the front. The Triassic evaporites acted as a major de´collement level, but many other de´collement planes have been activated within the Cretaceous series (Fig. 4). An important point already emphasised by Vially et al. (1994) is that the tilted block geometry inherited from the Triassic – Liassic rifting is preserved, at depth, below the foreland basin. This implies that the far-field compressional stress regime responsible for the first inversion of the Saharan Atlas during the late Lutetian (see below) has been transmitted along a deep detachment on which the Tell passive margin was transported as a rigid body. Late Miocene or Pliocene beds (Fig. 3) seal the foreland basin stage. During the early Quaternary, deformation is concentrated mainly within the Atlas region and particularly along the South Atlas Front (second Atlas inversion, see below). However, presently, the Tell system is again a region of active compressive setting. In western Algeria, the El Asnam active ramp-related fold (Meghraoui et al., 1986) as well as Quaternary thrust faults close to the front of the Kabylies in central

Fig. 9. Paleostress maps: A—Late Lutetian. B—Miocene extension. C—Tortonian and Pleistocene. Note that 1 to 7 refer to the following authors. (1) Thomas, 1985; (2) Meghraoui, 1986; (3) Brace`ne (unpublished data); (4) Ghandriche, 1991; (5 ) Addoum, 1995; (6) Boudjema, 1987; (7) Boudiaf et al., 1999.

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Algeria (Boudiaf et al., 1999) belong to an ‘‘en e´ chelon’’ fold system, locating the present-day Europe –Africa plate boundary.

4. The Saharan Atlas The Saharan Atlas is part of an intra-continental chain, which formed repeatedly during the Tertiary – Quaternary inversion stages of the intra-cratonic Atlas basin. The opening, the filling up and later inversion of the Atlas basin are linked to the geodynamic events occurring in the adjacent areas: opening of Tethys and Atlantic oceans and closure of the Maghrebian Tethys. However, the timing of the inversion of the Saharan Atlas does not necessarily reflect the timing observed along the Tellian margin. 4.1. From rifting to intracontinental basin The same rifting phases that affect the whole North African domain are recognised in the Saharan Atlas basin. In particular, the Late Triassic rifting event has been recently pointed out in zones where the salt deposits are thinner allowing observation of the basement faults (Yelles-Chaouche et al., 2001). However, the kinematics of the faults situated below the salt remain unknown. As in the Tell margin, the Liassic is characterised by fault-controlled carbonate sedimentation and the development of salt ridges (Figs. 7 and 8). The faults and the salt ridges are both NE – SW to ENE –WSW trending in agreement with a NW –SE to NNW –SSE direction of extension (Fig. 6). The major faults dip south and are situated along the boundary between the Meseta block and the Saharan Atlas. This emphasises the asymmetric development of the western Saharan Atlas basin, which appears as a half-graben (Brace`ne et al., 1998). The normal fault system was particularly active during the Liassic (Domerian and Toarcian) leading to erosion and karst development on the tops of footwall blocks (Aı¨t Ouali, 1991; Elmi et al., 1998). Top laps indicative of a relatively low sea level characterise the Jurassic/Cretaceous boundary (Fig. 4). The discontinuity observed at this level must be interpreted as a sequence boundary and not as an

erosion surface (Kazi-Tani, 1986). In the central and western Saharan Atlas, facies of early Cretaceous series is continental and displays a more or less isopach pattern. The thickness of Cretaceous series increases along strike towards the Aure`s and northward toward the Tell margin. Concurrently, facies changes becoming marine. Summarily, a carbonate platform is developed during the Late Cretaceous and an eroded surface underlines the Cretaceous– Tertiary boundary (Fig. 3A and B). It has been interpreted as a result of the first uplift of the Saharan Atlas (see review in Guiraud and Bosworth, 1997). Unfortunately, no demonstrative structure can be ascribed to this event (see below). 4.2. Inversion of the Atlas basin In western and central Algeria, it has been established that the Atlas inversion resulted from two distinct compressive events. Folds and thrusts are developed in the hanging wall of a major de´collement level located in the Triassic evaporites characterising a typical thin-skinned style (see review in Frizon de Lamotte et al., 2000). In addition, the tectonic history is recorded at outcrop scale by numerous microstructures allowing the determination of ‘‘paleostress’’ orientations, or at least, shortening (or stretching) directions corresponding to each event. The data presented in this paper are compiled from several works (see Tables 1, 2, 3 and 4). They have been obtained by different methods including (a) geometrical (kinematic) analysis, (b) stress tensor reduction when it was possible, or simply (c) extracted from maps. Thus, the degree of confidence that one can have in each site is not equivalent. However, one can notice the great homogeneity of the results at regional scale (Fig. 9). The first event, called the ‘‘Atlas event’’, occurred at the end of Eocene times (Laffitte, 1939; Guiraud, 1975). It is coeval with an important change in sedimentary supply from carbonates to sandy rocks, which is recorded throughout North Africa. This event is well documented in the Aure`s Mountains and in the Hodna basin where the Oligocene sandstones rest uncomformably on folded Maastrichtian carbonates or even (Hodna) on Lower Lutetian evaporites (Laffitte, 1939; Kieken, 1974, 1975; Ghandriche, 1991; Frizon de Lamotte et al., 2000). Numerous seismic

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Fig. 10. Seismic cross-section (TWT, seconds) and interpretation through an anticline buried below the Hodna basin illustrating the two folding events (prior to Oligocene and after Miocene) known at the regional scale (see location on Fig. 1B).

222 R. Brace`ne, D. Frizon de Lamotte / Tectonophysics 357 (2002) 207–226 Fig. 11. Seismic section (TWT, seconds) crossing the South Atlas Front (see location on Fig. 1B). This section shows the South Atlas front geometry and the Oligocene Miocene foreland basin (Benoud).

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profiles through the Hodna basin (Fig. 10) exhibit the same geometry. Along the South Atlas Front, a narrow foreland basin developed in the Benoud area (Fig. 1B). The syntectonic deposits (sandstones of Oligocene –early Miocene age) covered the most external folds. From the seismic profiles (Fig. 11), it appears that onlaps migrated southward toward the Sahara platform and northward toward the fold crest. This suggests a rather rapid sedimentation and, according to Doglioni and Prosser (1997), fold uplift rate slower than regional subsidence. At the scale of the Saharan Atlas system, the Late Lutetian event is recorded by numerous brittle structures (microfaults, joints) consistent with a NW – SE shortening direction (Fig. 9A). It is also (and mainly) responsible for NE – SW trending folds (Fig. 1A). Some of them are located on early salt ridges, which were cut out by thrust faults (Figs. 7 and 8) or evolved as diapirs during or after folding. After this first compressional event (late Lutetian), a phase of relative quiescence is marked by the development of continental molasse that seals the Saharan Atlas folds (Fig. 10) and overflows the South Atlas foredeep (Fig. 11). This molasse (Lower Miocene), where preserved, are relatively isopach. Although it is contemporaneous with the Tell foredeep, it does not show any evidence for syn-depositional deformation. The eastern regions (Aure`s, Fig. 1B), however, exhibit some evidence of diffuse normal faulting (Ghandriche, 1991; Addoum, 1995). Analysis of microfault populations is in agreement with a NE – SW extension, parallel to the one described by Thomas (1985) in the Cheliff basin (Fig. 9B). The second phase of rapid uplift occurred after the phase of quiescence. The latter is responsible of a general tilting of the Miocene molasse observed along the South Atlas Front (Fig. 11). Microfault measurements (Boudjema, 1984; Ghandriche, 1991; Addoum, 1995) as well as the development of a new generation of E –W trending folds are in agreement with a N – S direction of shortening (Fig. 9C). This second uplift took place in two steps: a first one, of Tortonian age, is restricted to the Hodna basin and easternmost areas, the second one, Pleistocene in age, is general at the scale of the Atlas (Frizon de Lamotte et al., 2000) (Fig. 9C).

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5. Discussion, conclusions It is well known that sedimentary basins located on cratonic platforms flanking orogens can record the processes governing an ‘‘orogenic cycle’’: breakup of a continent, opening of an ocean and then its closure (Ziegler et al., 1998). From this point of view, the Saharan Atlas system of Eastern and Central northern Algeria has recorded the breakup of Pangea (Upper Triassic), the opening of the Maghrebian Tethys (since the Dogger) and its subsequent closure (Oligocene to present) (Fig. 12). If we compare the North African margin to other margins of the Alpine system, the Middle Miocene foredeep of the Tell system is developed on an already deformed foreland. The late Lutetian event, which characterises the Saharan Atlas domain, can be correlated to events known in the Iberian microplate (i.e. ‘‘Iberian paleostress field’’ of Mun˜oz-Martin et al. (1998) and in the Western European platform, ‘‘Eocene paleostress field’’ of Letouzey and Tre´molie`res (1980);Bergerat (1985) and Le Pichon et al. (1988)). This period, which corresponds also to a decrease of the convergence rate between Africa and Europe and to a change in the convergence direction (Le Pichon et al., 1988), is consequently a period of strong coupling between the two major plates. According to the model of Ziegler et al. (1998), we interpret this coupling as resulting from the initiation of Maghrebian Tethys subduction along its northern margin (Fig. 12B). In contrast, the Oligocene –Miocene period, during which the subduction of the Maghrebian Tethys was active before ending by the emplacement of the whole Tell accretionary prism (Fig. 12C), was a period of tectonic quiescence indicative of relative uncoupling between Europe and Africa. Since the Late Miocene, the two plates were coupled again leading to the second uplift of the Atlas system and the quite diffuse seismic activity, which today characterises the whole domain (Fig. 12D). In the Western Mediterranean, the collision between Europe and Africa is not yet achieved and therefore, the compressional intraplate deformation in the Atlas is not collision-related. As emphasised by Frizon de Lamotte et al. (2000), the two phases of uplift of the Atlas system (Late Lutetian and Pleisto-

224

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cene) correspond roughly to the initiation and the cessation of Maghrebian Tethys subduction. This constitutes a warning to authors who systematically relate intraplate deformation to collision processes

occurring at plate margins. In the studied area, as a matter of fact, we demonstrate that intraplate deformation predates the orogenic development along the North African margin.

Fig. 12. Conceptual model illustrating the proposed scenario along a transect from the Sahara platform to the Mediterranean sea. The model proposes an initial stage and three steps (Late Lutetian, Middle Miocene, Pleistocene) corresponding to the main stages of the Atlas building.

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