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End Cretaceous to recent polyphased compressive tectonics along the “Môle Constantinois” and foreland (NE Algeria)
Journal of African Earth Sciences 45 (2006) 123–136 www.elsevier.com/locate/jafrearsci
End Cretaceous to recent polyphased compressive tectonics along the ‘‘Moˆle Constantinois’’ and foreland (NE Algeria) Ramdane Marmi a
a,*
, Rene´ Guiraud
b
Laboratoire Ge´ologie et Environnement, De´partement des Sciences de la Terre, Universite´ de Constantine, Constantine 25000, Algeria b Laboratoire Dynamique de la Lithosphe`re, Case 060, Universite´ Montpellier II, 34095 Montpellier, France Received 1 April 2005; received in revised form 1 December 2005; accepted 11 January 2006 Available online 24 April 2006
Abstract The ‘‘Moˆle Constantinois’’ units and their forelands are part of the Alpine Belt of northeastern Algeria. From the North to the South they correspond to the following three tectonic domains: (i) the ‘‘Moˆle Constantinois’’ thrust unit; (ii) the Pre-Atlas domain corresponding to the folded and/or sliced belt and the Pre-Atlas corridor slightly affected by tectonics; and southwards, (iii) the folded and faulted autochthonous foreland represented mainly by the Aure`s massif. This Belt is the result of a series of short-lived compressional deformations that we summarize in this paper and which occurred successively during the end Cretaceous, the Early-Late Eocene, around the Aquitanian–Burdigalian transition, during the Tortonian, and during the Early Pleistocene. Structural, stratigraphic and micro-tectonic arguments have been used to describe this polyphased event during a compressional tectonic history throughout the different domains. These tectonic events can also be identified in the neighbouring areas of the Maghrebides Alpine Belt and correspond to the major stages of inversion of the African Tethyan margin. However, this tectonics is not homogeneous throughout the entire Maghrebides Belt, as paleo-stress fields and age of major deformations may vary from a region to another. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Alpine Belt; Algeria; ‘‘Moˆle Constantinois’’; Polyphased compression; End Cretaceous; Cenozoic
1. Introduction
2. Geological setting
The ‘‘Moˆle Constantinois’’ (MC) is located in the external zone of the Eastern Algeria Alpine Belt (Fig. 1). The geodynamic history of the thrust units was summarized by Vila (1980) and the foreland was studied by Guiraud (1973, 1990). More recently, some studies have been undertaken by Chadi (1991), Coiffait (1992), Aris (1994), Addoum (1995), Marmi (1995) and Aris et al. (1998). Taking into consideration these works and our new stratigraphical and structural observations, we give here a precise analysis and chronology of the short-lived compressional deformations affecting the MC and its foreland during the latest Cretaceous and the Cenozoic.
From a structural point of view the MC is an allochthonous domain mainly characterized by thrust sheets verging southwards in relation with Cenozoic compressional tectonic events. Further South, in the Aure`s massif, the autochthonous domain and cross-cutted by major NW– SE trending dextral strike-slip faults. Located between the two precedent domains, the Pre-Atlasic (PA) domain (Pre-Atlas Belt and Pre-Atlas corridor) is characterized by thrust faults and narrow folds particularly observed along the Sellaoua trough. These geological structures are in relation with several compressional tectonic phases (Fig. 2). The MC includes many allochthonous units amongst which the principal, in a lower position, corresponds to a carbonate unit defined by Guiraud (1973) and represented by marine platform carbonates, extending
*
Corresponding author. Tel.: +213 31 90 45 95; fax: +213 31 90 45 95/ 38 52. E-mail address: [email protected] (R. Marmi). 1464-343X/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.jafrearsci.2006.01.009
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Fig. 1. Schematic structural map of Alpine northwestern Africa. Modified after Frizon de Lamotte et al. (2000). GK, Grande Kabylie; MC, Moˆle Constantinois; PK, Petite Kabylie. The frame corresponds to boundaries of Fig. 2. Thick line A shows the location of Fig. 4.
Fig. 2. Structural elements of the Moˆle Constantinois and neighbouring areas (after Guiraud, 1973 and Wildi, 1983; modified). 1, Moˆle Constantinois; 2, Pre-Atlas domain (Pre-Atlas Belt and Pre-Atlas corridor); 3, Atlasic domain; 4, thrusting; 5, South-Atlasic flexure; 6, fault; 7, anticline; 8, syncline. The frame (a) corresponds to Fig. 10; the frame (b) corresponds to Fig. 12. BT: Bou Taleb; G: Djebel Guetiane.
from Jurassic to Turonian (Fig. 3a). These series are unconformably overlain by marl-carbonate marine formations of Late Senonian to Middle Miocene age, in which unconformity surfaces and/or sedimentary lacuna were observed. On top of the previous formations lie the ‘‘Tellian’’ thrust sheets s.l., which are themselves covered by the Numidian nappe (Vila, 1980; Fig. 4). The
Late Miocene, Pliocene and Quaternary continental detrital deposits constitute a cover unconformably overlying the older formations. It has been observed that some terms of the MC series show lateral variation of facies and thickness in relation to syn-sedimentary tectonics, which reflect a reactivation of old deep basement faults.
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Fig. 3. Lithostratigraphic columns: (a) lithostratigraphic column of the ‘‘Moˆle Constantinois’’; (b) lithostratigraphic column of Aure`s mountains.
Fig. 4. Synthetic cross-section N–S of the ‘‘Moˆle Constantinois’’ and its foreland (modified after Vila, 1980; Frizon de Lamotte et al., 2000; Pique´ et al., 2002). 1, Paleozoic; 2, Triassic; 3, Jurassic; 4, Early Cretaceous; 5, Late Cretaceous; 6, Paleogene; 7, Neogene. SAF: South-Alasic flexure; SP: Saharan platform.
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Along the southeastern front of the MC the Pre-Atlas belt, corresponding to the ‘‘Chebka Sellaoua’’, mainly involves a succession of marls of Cretaceous to Miocene ages. The NE–SW trending Chebka Sellaoua unit is characterized by thrust faults, very narrow folds and imbricate fan giving a particular structural pattern to this belt. Southwards, along the para-autochthonous Pre-Atlasic corridor, deformations were relatively less severe and generated folds thrusting to the South. The autochthonous Aure`s Atlas domain is represented by a thick series of marls and carbonates of Jurassic age. Upwards, the facies change into sandstones from Neocomian to Early Aptian. From the Albian times, marl and limestone sedimentation returned again and continued until the Lutetian (Fig. 3b). These series are interrupted by some discontinuities like hard-ground surfaces, lacuna and unconformities in relation with tectonic instability affecting the area. The Late Eocene to Quaternary cover, represented by marine or continental detrital deposits, unconformably overlies older formations, and includes very important discontinuities. 3. Tectonic events In this paper, we shall give stratigraphical and structural arguments related to tectonic events affecting the MC and it neighbouring areas. From North to South we find the following structural domains: the ‘‘Moˆle Constantinois’’, the Pre-Atlas Belt and corridor (Pre-Atlas domain) and the Atlas domain (autochthonous). Field observations involve micro-tectonic measurements carried out on striated micro-faults within Cretaceous and Neogene formations. This work aims to assess compressional shortening and determine the chronology of the different tectonic events. The data analysis is carried out in order to determine the orientation of the principal stress axes r1, r2 and r3, and the ratio, R of principal stress differences (R = r2 r3/r1 r3, 0 6 R 6 1). The conditions of application and the limits of the method have been described and discussed in detail by Carey (1979), Etchecopar (1984), Etchecopar and Mattauer (1988) and Angelier (1984, 1989). In the following section, we will be describing the general tectonic phases from the older to the more recent, using the geological time scale of Gradstein and Ogg (1996). 3.1. Maastrichtian compressional deformations (a) On the northern boundaries of the ‘‘Moˆle Constantinois’’, Lahonde`re and Magne´ (1983) have described a marly series of Early Maastrichtian age, topped by latest Maastrichtian–Paleocene deposits at Djebel Debar (NW of Guelma locality). In the same area Aris et al. (1998) have showed the existence of folds and reverse faults affecting Early Maastrichtian marly-limestone deposits and unconformably covered by sediments of Maastrichtian–
Paleocene age. On the South of the Chettabah massif, folds and a faulted anticline have been observed to affect Campanian–Early Maastrichtian marly-limestone, which are unconformably overlaid by Late Maastrichtian–Paleocene formations (Fig. 5). These folds, which extend over several hundreds meters’ in length, are NE–SW trending and associated to E–W reverse faults (Fig. 6). In neighbouring localities, SW Constantine, Van de Fliert (1955) noticed that Early to ‘‘Middle’’ Maastrichtian limestones are surmounted by Paleocene marly series. (b) In the Pre-Atlas domain, South of the Ain Beida locality, Vila (1977) showed that folded and faulted ‘‘Middle’’ Maastrichtian formations unconformably lie over the Campanian (Fig. 7). The former formations are also affected by reverse faults of dominantly ENE–WSW direction. Microtectonic data measured at Dj. Azem (S1) and Ain Beida (S10) stations (Fig. 8) reveal a compressive phase (u1) with NW–SE shortening direction (N130°–N135°E). (c) In the Atlas domain, the only evidence of latest Cretaceous deformation was described by Guiraud (1973) in the El Kantara syncline, located southwestward of Batna where a lacuna at the top of the Maastrichtian is sealed by a probable Paleocene formation (Fig. 9). All the previous stratigraphical and structural evidence militates in favour of a ‘‘Middle’’ to Late Maastrichtian tectonic event throughout the study area. 3.2. Late Eocene compressional event (a) In the ‘‘Moˆle Constantinois’’, in the Chelghoum Laid areas (ex Chateaudun du Rhumel) West of Constantine, Van de Fliert (1955) and Durozoy (1960) described folds involving Early-Middle Eocene and Cretaceous formations. They are sometimes transversally cross-cutted by NW–SE faults. In the Dj. Medelsou syncline, Van de Fliert (1955) showed the existence of Late Eocene lacuna, with Oligocene marly-sandstone series unconformably overlying Late Lutetian rocks (Fig. 10). This author’s observation was confirmed by Coiffait et al. (1983) in the same Djebel, where they dated Priabonian deposits unconformably overlying Late Lutetian strata. Near the Ain Smara locality (W of Constantine, Fig. 6) Van de Fliert describes overthrusts, a faulted anticline and a klippe involving at its base a thin level of Triassic sediments above Senonian marly series (example El Golea, Chettabah, Fig. 5). The major trend of these structural elements is NE–SW to E–W and could have been related to a Late Eocene tectonic phase. Along the northern MC, Raoult (1969) has described a tangential tectonic phase, dated around the Middle-Late Eocene boundary, which generates overlaps extending towards the South. In the ‘‘Constantinois central’’ Aris et al. (1998) described an E–W compressive phase and attributed it to the Eocene phase. However, at a regional scale this phase is well known and associated to a NW–SE shortening direction.
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Fig. 5. Geological cross-section of Late Cretaceous, South of Chettabah massif. 1, Barremian–Aptian; 2, Campanian–Early Maastrichtian; 3, Late Maastrichtian–Paleocene.
Fig. 6. Structural sketch of Constantine neighbouring. 1, neretic; 2, overlapping; 3, fault; 4, reverse fault; 5, anticline; 6, syncline.
(b) In the Pre-Atlas domain, in the Sellaoua area, overlaps of Cretaceous to Middle Eocene marly-limestones
along NE–SW reverse faults are observed (Fig. 2). NE– SW trending folds and SE verging thrusts are also be observed. In the Hodna Mountains, Guiraud (1973, 1990) describes N040°–N060°E folds, extending over a few tens of kilometers, resulting from the earliest Late Eocene Atlas phase. The stations at measured micro-faults in Barremian– Aptian formations at the Djendli (Dj. Bou Arif, S4), Dj. Guellif (S8) and Dj. Azem (S1) areas allow us to confirm the presence of a tectonic event (u2) with a maximum compression direction of N160°–165°E (Fig. 8). Considering field data, this event can be associated with the earliest Late Eocene phase, as it does not affect the Neogene deposits. (c) In the autochthonous Atlas domain, in the Aure`s massif, Laffitte (1939) attributed the general emersion to a post-Lutetian age, whereas Guiraud (1990) associated a very important deformation to the beginning of the Late Eocene. According to Guiraud (1973, 1990) the so-called Atlas event is responsible for the large symmetrical folds trending globally along N060°E. It also entails a conjugate strike-slip fault swarm synchronous to folding, with the dextral set oriented NW–SE and the senestral set oriented NE–SW (Fig. 11). A Pre-Late Eocene phase was well dated by Coiffait et al. (1984) and Coiffait (1992) in the southern Nementcha mountains, in the Bir El Ater area. The latter author also observed folded Lutetian deposits covered by Priabonian deposits with a strong unconformity (Fig. 12). All these geological structures are related to compressive deformations showing major shortening directions trending N–S to NNW–SSE (Guiraud et al., 1987; Addoum, 1995).
Fig. 7. Geological cross-section of Late Cretaceous in South Ain Beida (after Vila, 1977). 1, Campanian; 2, Early Maastrichtian; 3, marine Miocene.
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Fig. 8. Paleostress tensors map defined in Cretaceous and Miocene formations. 1, Triassic; 2, Jurassic; 3, Cretaceous; 4, Paleogene; 5, Miocene–Pliocene. M.St: Microtectonic stations data; PT: Paleostress tensors; TT: Tilted tensor.
Fig. 9. Geological cross-section of the western Aure`s Mountains (after Guiraud, 1990). 1, Early Cretaceous; 2, Canomanian; 3, Turonian; 4, Senonian; 5, Early Eocene; 6, Middle Eocene; 7, Miocene–Early Pliocene; 8, Pliocene-Quaternary.
A detailed analysis of micro-faults realised by Ble`s (1969) in the Tebessa area (SE Constantine) confirmed these directions. In conclusion, the Late Eocene compressional event is clearly expressed in the MC and particularly its forelands. This phase generated remarkable NE–SW
trending folds, NW–SE dextral strike-slip faults, and NE–SW senestral faults, both with important displacements. Micro-faults analysis confirmed macrostructural arguments and precise further the shortening direction of this major compressional phase (N150°– 160°E).
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Fig. 10. Geological cross-section of Dj. Medelsou syncline, South of Tadjenanet (after Van de Fliert, 1955). 1, Early Eocene; 2, Lutetian; 3, Priabonian; 4, Oligocene; 5, Pliocene.
Fig. 11. Structural sketch of Hodna and Aure`s Massif (after Guiraud, 1977, completed). 1, overlapping front; 2, fault; 3, reverse fault; 4, anticline; 5, syncline; 6, flexure.
Fig. 12. Geological cross-section in SW of Bir El Ater, Nementcha (after Coiffait, 1992). 1, Ypresian limestones; 2, Lutetian clays and gypsum; 3, Priabonian sandstones.
3.3. Early Miocene compressional event (a) Along the southern ‘‘Moˆle Constantinois’’ boundaries, Lahonde`re et al. (1979) consider that the first tectonic
event affecting Numidian deposits is Burdigalian s.l. in age, and they describe the sealing of Early Miocene structures by little basins of latest Burdigalian age. In the Mila basin (N Constantine) red conglomerate formations dated as
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latest Burdigalian–Langhian (Coiffait, 1992) sometimes unconformably overly the Early Miocene (Fig. 13). Vila (1971) showed the existence of an Early Burdigalian lacuna in the study area. Always in Constantine area, Guiraud (1990) suspected an Early Burdigalian age for the overlapping of Tellian series on the carbonate unit of the MC. All the field arguments show a tectonic activity during the Early Miocene. Results of micro-fault analysis realised at stations of Dj. Bou Arif (S4), Dj. Toumbaı¨t (S2), Aı¨n Kercha (S7), Chemora (S5) and Dj. Hanout Seghir (S6), (Fig. 8) show a compressional episode (u3) with a shortening oriented NE–SW (N040°–050°E). In ‘‘Petite Kabylie’’, northern MC, Raoult (1969) recognized an important tectonic event during the Early Miocene. (b) In the Pre-Atlas domain, NW of Ain Fakroun, we observed a clear unconformity of marine Miocene sandstones above continental Early Miocene (Fig. 14). The deformation is related to a tectonic phase occurring about the Aquitanian–Burdigalian boundary. Lessard (1957) attribute the unconformity of the Late Burdigalian over the Aquitanian to a tectonic event located at the Aquitanian–Burdigalian boundary. In the Sellaoua area, Vila et al. (1995) mentioned an unconformity of Middle to Late Miocene over the Eocene–Oligocene. (c) In the autochthonous domain, between the Metlili massif (W Aure`s) and the western Belezma mountains (Fig. 11), Guiraud (1990) described an unconformity characterized by red series of Miocene1 (Aquitanian) covered by marine or lagoonal formations of Miocene2 (Langhian). This author observed the same type of unconformity within
similar formations located at SE of Dj. Melah (El Outaya, Fig. 15). The shortening direction of this compressional event, probably oriented NE–SW, is different from preceding ones. 3.4. Late Miocene compressional event (a) In the ‘‘Moˆle Constantinois’’, continental to lacustrine deposits of Late Miocene–Early Pliocene age, corresponding to the Mila-Constantine Neogene Basin, cover Early Tortonian through a slight angular unconformity. According to Vila (1980) the latest noticeable unconformity is situated under the Late Tortonian levels. He also indicates that traces of tangential tectonics are fingerprinted in Early to Middle-Late Miocene marine formations of the Alpine Belt. The Late Miocene compression has generated asymmetric folds, oriented N060°–080°E, and reverse faults NNE–SSW (Aris et al., 1998). South of El Eulma (ex Saint Arnaud) Pliocene carbonates are laid unconformably on middle Tortonian formations (Coiffait, 1992). This author has also suspected a tectonic event occurring during the Late Tortonian at Dj. Rherour (SE of Tadjenanet, Fig. 6). Durozoy (1960) has observed some rejuvenation of landscape which can be dated Late Miocene; it is characterized by the occurrence of conglomeratic beds within lacustrine deposits of latest Miocene age. Vila et al. (1995) think that piggy-back basins within the ‘‘Constantinois’’ have taken place during Tortonian over-thrust. However, in the North of the study area, Raoult (1969) indicated an intra-Tortonian compressional phase inducing over-thrusts towards the southeast.
Fig. 13. Geological cross-section of Mila Miocene. 1, Early Miocene; 2, Late Burdigalian–Langhian; 3, Messinian; 4, Pliocene-Quaternary.
Fig. 14. Geological cross-section of Miocene, NE Ain Fakroun (SW of Sellaoua). 1, Cenomanian; 2, Early Miocene; 3, Burdigalian–Langhian.
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In ‘‘Grande Kabylie’’, Aite (1995) mentioned a Tortonian compressional phase with N160°E shortening inducing N050°–N080°E folds. (b) Along the Pre-Atlas domain, in Taxas syncline (Sellaoua), Vila (1980) described a klippe laying over marine Middle Miocene (Langhian–Serravallian, Fig. 16). Towards the South, on northern flank of Dj. Bou Arif we have observed marine Miocene deposits (Langhian–Serravallian) with overturned conglomeratic beds dipping 050–060° to the South. Moreover, in the oriental part of this Djebel, Serravallian marly-calcareous formations are folded along an axis oriented N080°E. In the Hodna Mountains, Guiraud (1990) showed the existence of an angular unconformity between the
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Miocene3 and Miocene4 formations, the latest presumably corresponding to late Tortonian–Messinian series. (c) In the autochthonous domain, in the ‘‘Ravin Bleu’’ anticline and the Batna syncline, Vila (1980) established a geological cross-section (Fig. 17) showing Middle Miocene occurring under Liassic dolomite, through overthrust reverse fault. In the Dj. Chelia (Aure`s Massif), Ghandriche (1991) and Frizon de Lamotte et al. (2000) described an Early-Middle Miocene formation wedged under Barremian by means of overthrust reverse fault (Fig. 18). Probably these structures are related to an intra-Tortonian compressional phase. Generally, Laffitte (1939) thinks that Aure`s massif is then affected by a new uplift accompanied by folding.
Fig. 15. Geological cross-section of Miocene on SE of Dj. El Melah (El Outaya), after Guiraud (1990). 1, Triassic; 2, Aquitanian–Burdigalian; 3, Burdigalian; 4, Langhian–Tortonian.
Fig. 16. Geological cross-section of Taxas syncline (after Vila, 1980). 1, Cenomanian; 2, Campanian; 3, Early Maastrichtian; 4, Paleocene; 5, Ypresian; 6, Lutetian; 7, Miocene; 8, Pliocene-Quaternary.
Fig. 17. Geological cross-section of ‘‘Ravin bleu’’, Batna Mountains (after Vila, 1980). 1, Triassic; 2, Liassic; 3, Dogger-Malm; 4, Neocomian; 5, Barremian; 6, Miocene.
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Fig. 18. Geological cross-section of Dj. Chelia (Aure`s), after Ghandriche (1991) and Frizon de Lamotte et al. (2000). 1, Neocomian; 2, Barremian; 3, Aptian–Albian; 4, Cenomanian; 5, Turonian; 6, Senonian–Eocene; 7, Miocene.
These folds are of moderate extension with an E–W dominant orientation, thus they are different from those attributed to the Atlasic phase. On the northeastern of the Aure`s massif, in the Touffana basin, marine detrital series dated Serravallian-earliest Tortonian (Vila, 1977) are folded into an anticline with E–W general direction (Fig. 19). Moreover, it has been observed in this basin that clay or gypseous Messinian deposits lay unconformably over Serravallian-earliest Tortonian. In the Oum El Bouaghi area (Fig. 8), carried out microtectonic data on striated micro-faults in different stations within Mio-Pliocene conglomerates, at Dj. Guellif (S11 and S9), give an orientation of the principal stress axis of N140°–145°E (u4). This later phase (u4) characterizes a compressional episode. The intra-Tortonian compressional phase is more expressed in the external zones of eastern Algeria Alpine Belt and it is characterized by imbricate fan structures. This phase has induced thrust sheets in the MC, reverse faults and over thrusts in its foreland. 3.5. Early Quaternary compressional event The structures related to the Early Quaternary tectonic event are largely expressed in the Pliocene formations of the study area, particularly around Constantine (Fig. 6). (a) In the northern part of the ‘‘Moˆle Constantinois’’ the E–W trending M’Cid Aicha-Debar transverse fault is well known and mentioned by many authors such as Durand-
Delga (1969), Raoult (1974) and Bouillin (1977). The reactivation of this dextral strike-slip fault into a reverse fault, during ‘‘post-nappe’’ tectonic activity, affects Miocene– Pliocene deposits (Coiffait, 1992). In the ‘‘Constantinois Central’’ this author indicates a compressional tectonic episode (N–S to N135°E) affecting earliest Pliocene sediments. In Ain El Bey Shelf (South of Constantine) this deformation generates folds globally oriented N70–80°E (Fig. 6) and extending on several hundred meters. On the southern boundary of the Khroub basin (southeastern of the Khroub locality) Ravin (1957) and Durozoy (1960) describe Late Tortonian limestone layers (probably Messinian) and Messinian sands and clay beds turned up and strongly tilted, often overturned, showing the intensity of post-Miocene deformation (Fig. 20). We have also observed small folds E–W oriented, involving Pliocene lacustrine limestones. South of Ain Smara, the Pliocene beds are bended in contact with the E–W strike-slip fault, indicating its reactivation. Always in the same zone, Durozoy (1960) describes an unconformity of a probable Moulouyan (Early Quaternary) limestone over red marls and Pliocene lacustrine limestones. (b) In the Pre-Atlas belt, at the western boundary of the Dj. Oum Chkrid (South of Ain Fakroun) we have observed wide anticlines within the Pliocene lacustrine deposits, globally oriented N110°E, extending over few hundred meters. These folds are often crossed by senestral decametric strike-slip faults, trending NNE–SSW. These structural elements are in agreement with a compressional
Fig. 19. Geological cross-section of folded Middle-Late Miocene in the Touffana basin (NW of Khenchela). 1, Langhian–Serravallian; 2, Messinian; 3, Quaternary.
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Fig. 20. Geological cross-section of Miocene–Pliocene of Sidi Lakhdar (South of Constantine), after Ravin (1957). 1, Langhian–Serravallian; 2, Early Tortonian; 3, Messinian; 4, Pliocene.
deformation, characterized by a sub-meridian trending principal stress axis. In the same zone, sub-parallel dextral strike-slip faults cross-cut marine Cretaceous formations and continental Pliocene. West of Dj. Guellif, red Miocene–Pliocene conglomerates are strongly tilted; they contain sheared pebbles with senestral and dextral micro-faults, respectively oriented N040–050°E and N110–120°E. These microstructures are related to submeridian shortening (Marmi, 1995). In the same place, a reverse fault oriented N080–100°E and slightly deeping to the North, allows Triassic formation to appear over Miocene–Pliocene outcrops, indicating recent tectonic activity (Fig. 21). In the Bou Taleb and Guetiane massifs (Hodna Mountains, Fig. 11), Guiraud (1977) described many structural elements such as folds, reverse faults oriented E–W, dextral strike-slip faults N010–050 °W and senestral strike-slip faults N010–020°E. These structures are the result of a compressional shortening oriented globally N–S, they contribute to the making of the actual landscape. Around the same area, Guiraud (1990) observed folded Villafranchian deposits often cut by strike-slip faults.
(c) In the Pre-Atlas corridor some folds and flexures, E– W oriented and affecting marginal-marine latest Miocene and continental formations of the Pliocene, are described by Lessard (1957) in north Aure`s boundary (Sebkha zone). Guiraud (1990) showed the existence of folds trending E–W to ENE–WSW affecting Miocene–Pliocene and Cretaceous bedrock in the Timgad Basin. Many NW–SE dextral strike-slip faults displace Bou Arif-Fedjoudj belt, originally oriented ENE–WSW (Fig. 11). Similarly, near E–W post-Miocene dextral fractures tear off the flanks of Dj. Bou Arif anticline. The micro-tectonic study carried out on striated microfaults stations near Oum El Bouaghi (S9), Dj. Guellif (S8, S11), Dj. Bou Arif (S3) and Dj. Azem (S1) (Fig. 8) shows a later compressional episode (u5) with major principal stress axis of N–S to N010°E direction. (d) In the Atlas domain (East of Biskra) along the Saharan flexure, which corresponds to the Aure`s meridional boundary, Guiraud (1973) and then Aı¨ssaoui (1984) described Villafranchian formations affected by E–W oriented reverse faults (Fig. 22) and E–W to NW–SE dextral strike-slip faults, generating drag folds in many places.
Fig. 21. Geological cross-section West of Dj. Guellif (SW of Oum El Bouaghi). 1, Triassic; 2, Aptian; 3, Miocene–Pliocene; 4, Quaternary.
Fig. 22. Geological cross-section of Dj. Rheliss, Guiraud (1990). 1, Campanian–Maastrichtian; 2, Messinian; 3, Early Pliocene; 4, ‘‘Middle’’ Pliocene; 5, Late Pliocene.
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Macro- and micro-structural analysis confirm that a regional compressional tectonic event, characterized by a sub-meridian shortening, took place during Early Quaternary. This recent deformation is very well recorded in the ‘‘Moˆle Constantinois’’ as well as in its foreland. This stress regime continues from the beginning of the Quaternary until today, but with low intensity as illustrated by the seismic activity in the region, particularly Constantine earthquake in 1985 (Bounif et al., 1987). 4. Regional geodynamics and conclusions Clues of the tectonic events which we described for the ‘‘Moˆle Constantinois’’ and forelands have been reported along adjacent domains of the Northwestern African Alpine Belt. We shall give a brief review of these polyphased deformations, hereafter to draw a hierarchy between them, and discuss their links with the regional geodynamics. We first mention here that we do not in this paper faced with the Santonian event which entailed the oldest alpine compressional deformations along the African Alpine plate margin (Guiraud and Bosworth, 1997), as it presents very minor echoes in our study area. Permanent tectonic instability characterized the Maastrichtian times (Guiraud and Bosworth, 1997). Local folding and reverse faulting then affected the ‘‘Moˆle Constantinois’’ and the Pre-Atlas domain. In central and eastern Tunisia, Zouari et al. (2004) have mentioned compressional strike-slip tectonics of ‘‘Middle’’ Maastrichtian– Paleocene age, with sub-meridian shortening. Along the southern Moroccan High Atlas, southwards thrusting was registered (Laville et al., 1977). So, diachronous and local compressional deformations, exhibiting NW–SE to N–S shortenings (Table 1), occurred during Maastrichtian times along the Maghrebian Alpine Belt. Regionally, they characterize a minor event which was witness to dextral transpression along the northwestern African plate margin, in response to differential opening of the Central, South and North Atlantic oceans (Guiraud and Bosworth, 1997; Guiraud et al., 2005). A more severe tectonic event occurred in the earliest Late Eocene (37 Ma), particularly identified around the ‘‘Moˆle Constantinois’’. This event entailed strong
Table 1 Synoptic table of compressional phases recognized in the ‘‘Moˆle Constantinois’’ and neighbouring regions Tectonic phases
Shortening direction (NE Algeria)
Chronology of compressional events
/1 /2 /3 /4 /5
N130°–135°E N155°–160°E N40°–50°E N140°E NNW–SSE to N–S
Maastrichtian (68–65 Ma) Earliest Late Eocene (37 Ma) Aquitanian–Burdigalian (21–18 Ma) Tortonian (8.5 Ma) Early Quaternary (1.5–1.3 Ma)
folding and strike-slip faulting along the Saharan Atlas and Pre-Atlas domains, as well as the Tunisian Atlas and Moroccan High and Middle Atlas (‘‘Atlas event’’) (Guiraud et al., 1987, 2005; Dlala and Rebai, 1994; Frizon de Lamotte et al., 2000; Pique´ et al., 2002; Mahboubi et al., 2003). Throughout these domains it resulted in NNW–SSE to NW–SE shortening (Table 1). It also affected the internal Tellian–Rifian domain (Kabylies– Alboran) that then underwent thrusting and slight metamorphism (Raoult, 1969; Guiraud et al., 1987, 2005), but saved up the external Tell. Identified along the entire northern African–Arabian plate margin, as well as along the large intra-plate fault zones (Bamby and Guiraud, in press), this compressional event corresponds to a major tectonic stage in the development of the Maghrebian Alpine Belt. Like the late Santonian and Maastrichtian events, it resulted from changes in the rates and directions of opening of the Central, South and North Atlantic oceans (Guiraud et al., 2005). These differential openings generated compressions and/or dextral transpressions in the western Mediterranean Alpine domain and probably the initiation of the subduction of the Maghrebide Tethys underneath the Iberian-Balearic margin (Frizon de Lamotte et al., 2000). During the latest Aquitanean–Early Burdigalian times (21–18 Ma) the ‘‘Moˆle Constantinois’’ and surroundings experienced compressional deformations whose intensity strongly decreased from northern to southern domains. The internal domain of the Tellian–Rifian Belt then underwent thrusting and metamorphism (Raoult, 1974; Guiraud et al., 2005) while southern forelands only registered minor folding. The micro-tectonic studies which we realised show that the major shortening was then NE–SW oriented (Table 1). Similar results were obtained along the Moroccan and Tunisian Atlas (Letouzey and Tre´molie`res, 1980), as well as in several intra-plate domains of Northwestern Africa (Guiraud and Bellion, 1995). This shortening direction is very different from preceding and following ones. It results of a sharp change in plate motions, as Africa then moved northeastwards (Guiraud and Bosworth, 1997). This change, synchronous with the opening of the Western Algerian–Alboran ocean basin (Mauffret et al., 1992), generated compressional deformations (Guiraud et al., 2005). During Tortonian times (9–8 Ma) the ‘‘Moˆle Constantinois’’ rapidly shifted southwards, pressing the Pre-Atlas domain, which then was strongly sliced and folded (Guiraud, 1973, 1990; Vila, 1980). Similar southwards thrusting occurred all along the external zones of the Tellian–Rifian Belt (Vila, 1980; Frizon de Lamotte et al., 2000; Guiraud et al., 2005), as especially evidenced in Tunisia (Rouvier, 1977) and Oranie (Gardia, 1975). These displacements entailed slight folding of the Atlas domain s.l., from Morocco to Tunisia (Dlala and Rebai, 1994; Ghribi et al., 2004; Guiraud et al., 2005). The shortening was NNW–SSE (Table 1). This event resulted from a new change in the plate motions as Africa then moved
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northwestwards, entailing dextral transpression along its northern margin (Guiraud and Bosworth, 1997, 1999). At last, Early Quaternary (1.5–1.3 Ma) compressional event affected the ‘‘Moˆle Constantinois’’ and foreland domains, giving E–W folds and reverse faults and conjugate strike-slip faults, witnessing for N–S to NNW–SSE shortening (Table 1). Identical deformations occurred along the Rif–Tell and Atlas Mountains (Philip and Thomas, 1977; Amari and Bedir, 1989; Dlala and Rebai, 1994; Addoum, 1995) that then grew up (Frizon de Lamotte et al., 2000; Guiraud et al., 2005). Most of these structures will develop, with decreasing intensity, up to recent times as evidenced by neotectonic analyses and seismicity (Girardin et al., 1977; Philip and Thomas, 1977; Boudiaf et al., 1998, 1999). Along the southern edges of the ‘‘Grande Kabylie’’, Boudiaf et al. (1999) documented a large thrust developing during the Pleistocene times. During this stage, the approximately northward motion of Africa, initiated in the Senonian times, continued. For Frizon de Lamotte et al. (2000) the Quaternary phase corresponds to the cessation of the Maghrebian Tethys subduction, while the fluxuration of the plate margin developed. In conclusion, for many aspects, the End Cretaceous to recent polyphase compressive tectonics of the ‘‘Moˆle Constantinois’’ and foreland exemplies the history of the Maghrebian Alpine Belt and, on a broader scale, of the Northwestern African plate domain. Acknowledgements Suggested improvements by Fred Beekman, Pat Eriksson and an anonymous reviewer were greatly appreciated. We are grateful to Dr. A. Boumezbeur for his help in the translation of the original manuscript of this paper. References Addoum, B., 1995. L’Atlas Saharien sud-oriental: Cine´matique des plischevauchements et reconstitution du bassin du Sud-Est Constantinois (Confins alge´ro-tunisiens). Thesis Sci., University Paris Sud Orsay, p. 198. 4 pl. h.t. Aite, M.O., 1995. Pale´ocontraintes post-collision identifie´es dans le Ne´oge`ne de Grande Kabylie (Alge´rie). C.R. Acad. Sci. Paris, t. 320 (Se´rie IIa), 433–438. Aı¨ssaoui, D., 1984. Les structures lie´es a` l’accident sud-atlasique entre Biskra et le Djebel Mannda (Alge´rie). Evolution ge´ome´trique et cine´matique. The`se 3°cycle, Strasbourg, p. 154. Amari, A., Bedir, M., 1989. Les bassins quaternaires du Sahel central de la Tunisie. Gene`se et e´volution des Sebkhas en contexte de´crochant compressif et distensif. Ge´odynamique 4 (1), 49–65. Angelier, J., 1984. Tectonic analysis of fault slips data sets. J. Geophys. Res. 89 (B7), 5835–5848. Angelier, J., 1989. From orientation to magnitudes in paleostress determinations using fault slip data. J. Struct. Geol. 11 (1/2), 37–50. ´ tude tectonique et microtectonique des se´ries jurassiques a` Aris, Y., 1994. E plio-quaternaires du Constantinois (Alge´rie nord-orientale): Caracte´risation des diffe´rentes phases de de´formation. Thesis, University Nancy I, France, 215 pp.
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End Cretaceous to recent polyphased compressive tectonics along the ‘‘Moˆle Constantinois’’ and foreland (NE Algeria) Ramdane Marmi a
a,*
, Rene´ Guiraud
b
Laboratoire Ge´ologie et Environnement, De´partement des Sciences de la Terre, Universite´ de Constantine, Constantine 25000, Algeria b Laboratoire Dynamique de la Lithosphe`re, Case 060, Universite´ Montpellier II, 34095 Montpellier, France Received 1 April 2005; received in revised form 1 December 2005; accepted 11 January 2006 Available online 24 April 2006
Abstract The ‘‘Moˆle Constantinois’’ units and their forelands are part of the Alpine Belt of northeastern Algeria. From the North to the South they correspond to the following three tectonic domains: (i) the ‘‘Moˆle Constantinois’’ thrust unit; (ii) the Pre-Atlas domain corresponding to the folded and/or sliced belt and the Pre-Atlas corridor slightly affected by tectonics; and southwards, (iii) the folded and faulted autochthonous foreland represented mainly by the Aure`s massif. This Belt is the result of a series of short-lived compressional deformations that we summarize in this paper and which occurred successively during the end Cretaceous, the Early-Late Eocene, around the Aquitanian–Burdigalian transition, during the Tortonian, and during the Early Pleistocene. Structural, stratigraphic and micro-tectonic arguments have been used to describe this polyphased event during a compressional tectonic history throughout the different domains. These tectonic events can also be identified in the neighbouring areas of the Maghrebides Alpine Belt and correspond to the major stages of inversion of the African Tethyan margin. However, this tectonics is not homogeneous throughout the entire Maghrebides Belt, as paleo-stress fields and age of major deformations may vary from a region to another. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Alpine Belt; Algeria; ‘‘Moˆle Constantinois’’; Polyphased compression; End Cretaceous; Cenozoic
1. Introduction
2. Geological setting
The ‘‘Moˆle Constantinois’’ (MC) is located in the external zone of the Eastern Algeria Alpine Belt (Fig. 1). The geodynamic history of the thrust units was summarized by Vila (1980) and the foreland was studied by Guiraud (1973, 1990). More recently, some studies have been undertaken by Chadi (1991), Coiffait (1992), Aris (1994), Addoum (1995), Marmi (1995) and Aris et al. (1998). Taking into consideration these works and our new stratigraphical and structural observations, we give here a precise analysis and chronology of the short-lived compressional deformations affecting the MC and its foreland during the latest Cretaceous and the Cenozoic.
From a structural point of view the MC is an allochthonous domain mainly characterized by thrust sheets verging southwards in relation with Cenozoic compressional tectonic events. Further South, in the Aure`s massif, the autochthonous domain and cross-cutted by major NW– SE trending dextral strike-slip faults. Located between the two precedent domains, the Pre-Atlasic (PA) domain (Pre-Atlas Belt and Pre-Atlas corridor) is characterized by thrust faults and narrow folds particularly observed along the Sellaoua trough. These geological structures are in relation with several compressional tectonic phases (Fig. 2). The MC includes many allochthonous units amongst which the principal, in a lower position, corresponds to a carbonate unit defined by Guiraud (1973) and represented by marine platform carbonates, extending
*
Corresponding author. Tel.: +213 31 90 45 95; fax: +213 31 90 45 95/ 38 52. E-mail address: [email protected] (R. Marmi). 1464-343X/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.jafrearsci.2006.01.009
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Fig. 1. Schematic structural map of Alpine northwestern Africa. Modified after Frizon de Lamotte et al. (2000). GK, Grande Kabylie; MC, Moˆle Constantinois; PK, Petite Kabylie. The frame corresponds to boundaries of Fig. 2. Thick line A shows the location of Fig. 4.
Fig. 2. Structural elements of the Moˆle Constantinois and neighbouring areas (after Guiraud, 1973 and Wildi, 1983; modified). 1, Moˆle Constantinois; 2, Pre-Atlas domain (Pre-Atlas Belt and Pre-Atlas corridor); 3, Atlasic domain; 4, thrusting; 5, South-Atlasic flexure; 6, fault; 7, anticline; 8, syncline. The frame (a) corresponds to Fig. 10; the frame (b) corresponds to Fig. 12. BT: Bou Taleb; G: Djebel Guetiane.
from Jurassic to Turonian (Fig. 3a). These series are unconformably overlain by marl-carbonate marine formations of Late Senonian to Middle Miocene age, in which unconformity surfaces and/or sedimentary lacuna were observed. On top of the previous formations lie the ‘‘Tellian’’ thrust sheets s.l., which are themselves covered by the Numidian nappe (Vila, 1980; Fig. 4). The
Late Miocene, Pliocene and Quaternary continental detrital deposits constitute a cover unconformably overlying the older formations. It has been observed that some terms of the MC series show lateral variation of facies and thickness in relation to syn-sedimentary tectonics, which reflect a reactivation of old deep basement faults.
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Fig. 3. Lithostratigraphic columns: (a) lithostratigraphic column of the ‘‘Moˆle Constantinois’’; (b) lithostratigraphic column of Aure`s mountains.
Fig. 4. Synthetic cross-section N–S of the ‘‘Moˆle Constantinois’’ and its foreland (modified after Vila, 1980; Frizon de Lamotte et al., 2000; Pique´ et al., 2002). 1, Paleozoic; 2, Triassic; 3, Jurassic; 4, Early Cretaceous; 5, Late Cretaceous; 6, Paleogene; 7, Neogene. SAF: South-Alasic flexure; SP: Saharan platform.
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Along the southeastern front of the MC the Pre-Atlas belt, corresponding to the ‘‘Chebka Sellaoua’’, mainly involves a succession of marls of Cretaceous to Miocene ages. The NE–SW trending Chebka Sellaoua unit is characterized by thrust faults, very narrow folds and imbricate fan giving a particular structural pattern to this belt. Southwards, along the para-autochthonous Pre-Atlasic corridor, deformations were relatively less severe and generated folds thrusting to the South. The autochthonous Aure`s Atlas domain is represented by a thick series of marls and carbonates of Jurassic age. Upwards, the facies change into sandstones from Neocomian to Early Aptian. From the Albian times, marl and limestone sedimentation returned again and continued until the Lutetian (Fig. 3b). These series are interrupted by some discontinuities like hard-ground surfaces, lacuna and unconformities in relation with tectonic instability affecting the area. The Late Eocene to Quaternary cover, represented by marine or continental detrital deposits, unconformably overlies older formations, and includes very important discontinuities. 3. Tectonic events In this paper, we shall give stratigraphical and structural arguments related to tectonic events affecting the MC and it neighbouring areas. From North to South we find the following structural domains: the ‘‘Moˆle Constantinois’’, the Pre-Atlas Belt and corridor (Pre-Atlas domain) and the Atlas domain (autochthonous). Field observations involve micro-tectonic measurements carried out on striated micro-faults within Cretaceous and Neogene formations. This work aims to assess compressional shortening and determine the chronology of the different tectonic events. The data analysis is carried out in order to determine the orientation of the principal stress axes r1, r2 and r3, and the ratio, R of principal stress differences (R = r2 r3/r1 r3, 0 6 R 6 1). The conditions of application and the limits of the method have been described and discussed in detail by Carey (1979), Etchecopar (1984), Etchecopar and Mattauer (1988) and Angelier (1984, 1989). In the following section, we will be describing the general tectonic phases from the older to the more recent, using the geological time scale of Gradstein and Ogg (1996). 3.1. Maastrichtian compressional deformations (a) On the northern boundaries of the ‘‘Moˆle Constantinois’’, Lahonde`re and Magne´ (1983) have described a marly series of Early Maastrichtian age, topped by latest Maastrichtian–Paleocene deposits at Djebel Debar (NW of Guelma locality). In the same area Aris et al. (1998) have showed the existence of folds and reverse faults affecting Early Maastrichtian marly-limestone deposits and unconformably covered by sediments of Maastrichtian–
Paleocene age. On the South of the Chettabah massif, folds and a faulted anticline have been observed to affect Campanian–Early Maastrichtian marly-limestone, which are unconformably overlaid by Late Maastrichtian–Paleocene formations (Fig. 5). These folds, which extend over several hundreds meters’ in length, are NE–SW trending and associated to E–W reverse faults (Fig. 6). In neighbouring localities, SW Constantine, Van de Fliert (1955) noticed that Early to ‘‘Middle’’ Maastrichtian limestones are surmounted by Paleocene marly series. (b) In the Pre-Atlas domain, South of the Ain Beida locality, Vila (1977) showed that folded and faulted ‘‘Middle’’ Maastrichtian formations unconformably lie over the Campanian (Fig. 7). The former formations are also affected by reverse faults of dominantly ENE–WSW direction. Microtectonic data measured at Dj. Azem (S1) and Ain Beida (S10) stations (Fig. 8) reveal a compressive phase (u1) with NW–SE shortening direction (N130°–N135°E). (c) In the Atlas domain, the only evidence of latest Cretaceous deformation was described by Guiraud (1973) in the El Kantara syncline, located southwestward of Batna where a lacuna at the top of the Maastrichtian is sealed by a probable Paleocene formation (Fig. 9). All the previous stratigraphical and structural evidence militates in favour of a ‘‘Middle’’ to Late Maastrichtian tectonic event throughout the study area. 3.2. Late Eocene compressional event (a) In the ‘‘Moˆle Constantinois’’, in the Chelghoum Laid areas (ex Chateaudun du Rhumel) West of Constantine, Van de Fliert (1955) and Durozoy (1960) described folds involving Early-Middle Eocene and Cretaceous formations. They are sometimes transversally cross-cutted by NW–SE faults. In the Dj. Medelsou syncline, Van de Fliert (1955) showed the existence of Late Eocene lacuna, with Oligocene marly-sandstone series unconformably overlying Late Lutetian rocks (Fig. 10). This author’s observation was confirmed by Coiffait et al. (1983) in the same Djebel, where they dated Priabonian deposits unconformably overlying Late Lutetian strata. Near the Ain Smara locality (W of Constantine, Fig. 6) Van de Fliert describes overthrusts, a faulted anticline and a klippe involving at its base a thin level of Triassic sediments above Senonian marly series (example El Golea, Chettabah, Fig. 5). The major trend of these structural elements is NE–SW to E–W and could have been related to a Late Eocene tectonic phase. Along the northern MC, Raoult (1969) has described a tangential tectonic phase, dated around the Middle-Late Eocene boundary, which generates overlaps extending towards the South. In the ‘‘Constantinois central’’ Aris et al. (1998) described an E–W compressive phase and attributed it to the Eocene phase. However, at a regional scale this phase is well known and associated to a NW–SE shortening direction.
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Fig. 5. Geological cross-section of Late Cretaceous, South of Chettabah massif. 1, Barremian–Aptian; 2, Campanian–Early Maastrichtian; 3, Late Maastrichtian–Paleocene.
Fig. 6. Structural sketch of Constantine neighbouring. 1, neretic; 2, overlapping; 3, fault; 4, reverse fault; 5, anticline; 6, syncline.
(b) In the Pre-Atlas domain, in the Sellaoua area, overlaps of Cretaceous to Middle Eocene marly-limestones
along NE–SW reverse faults are observed (Fig. 2). NE– SW trending folds and SE verging thrusts are also be observed. In the Hodna Mountains, Guiraud (1973, 1990) describes N040°–N060°E folds, extending over a few tens of kilometers, resulting from the earliest Late Eocene Atlas phase. The stations at measured micro-faults in Barremian– Aptian formations at the Djendli (Dj. Bou Arif, S4), Dj. Guellif (S8) and Dj. Azem (S1) areas allow us to confirm the presence of a tectonic event (u2) with a maximum compression direction of N160°–165°E (Fig. 8). Considering field data, this event can be associated with the earliest Late Eocene phase, as it does not affect the Neogene deposits. (c) In the autochthonous Atlas domain, in the Aure`s massif, Laffitte (1939) attributed the general emersion to a post-Lutetian age, whereas Guiraud (1990) associated a very important deformation to the beginning of the Late Eocene. According to Guiraud (1973, 1990) the so-called Atlas event is responsible for the large symmetrical folds trending globally along N060°E. It also entails a conjugate strike-slip fault swarm synchronous to folding, with the dextral set oriented NW–SE and the senestral set oriented NE–SW (Fig. 11). A Pre-Late Eocene phase was well dated by Coiffait et al. (1984) and Coiffait (1992) in the southern Nementcha mountains, in the Bir El Ater area. The latter author also observed folded Lutetian deposits covered by Priabonian deposits with a strong unconformity (Fig. 12). All these geological structures are related to compressive deformations showing major shortening directions trending N–S to NNW–SSE (Guiraud et al., 1987; Addoum, 1995).
Fig. 7. Geological cross-section of Late Cretaceous in South Ain Beida (after Vila, 1977). 1, Campanian; 2, Early Maastrichtian; 3, marine Miocene.
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Fig. 8. Paleostress tensors map defined in Cretaceous and Miocene formations. 1, Triassic; 2, Jurassic; 3, Cretaceous; 4, Paleogene; 5, Miocene–Pliocene. M.St: Microtectonic stations data; PT: Paleostress tensors; TT: Tilted tensor.
Fig. 9. Geological cross-section of the western Aure`s Mountains (after Guiraud, 1990). 1, Early Cretaceous; 2, Canomanian; 3, Turonian; 4, Senonian; 5, Early Eocene; 6, Middle Eocene; 7, Miocene–Early Pliocene; 8, Pliocene-Quaternary.
A detailed analysis of micro-faults realised by Ble`s (1969) in the Tebessa area (SE Constantine) confirmed these directions. In conclusion, the Late Eocene compressional event is clearly expressed in the MC and particularly its forelands. This phase generated remarkable NE–SW
trending folds, NW–SE dextral strike-slip faults, and NE–SW senestral faults, both with important displacements. Micro-faults analysis confirmed macrostructural arguments and precise further the shortening direction of this major compressional phase (N150°– 160°E).
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Fig. 10. Geological cross-section of Dj. Medelsou syncline, South of Tadjenanet (after Van de Fliert, 1955). 1, Early Eocene; 2, Lutetian; 3, Priabonian; 4, Oligocene; 5, Pliocene.
Fig. 11. Structural sketch of Hodna and Aure`s Massif (after Guiraud, 1977, completed). 1, overlapping front; 2, fault; 3, reverse fault; 4, anticline; 5, syncline; 6, flexure.
Fig. 12. Geological cross-section in SW of Bir El Ater, Nementcha (after Coiffait, 1992). 1, Ypresian limestones; 2, Lutetian clays and gypsum; 3, Priabonian sandstones.
3.3. Early Miocene compressional event (a) Along the southern ‘‘Moˆle Constantinois’’ boundaries, Lahonde`re et al. (1979) consider that the first tectonic
event affecting Numidian deposits is Burdigalian s.l. in age, and they describe the sealing of Early Miocene structures by little basins of latest Burdigalian age. In the Mila basin (N Constantine) red conglomerate formations dated as
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latest Burdigalian–Langhian (Coiffait, 1992) sometimes unconformably overly the Early Miocene (Fig. 13). Vila (1971) showed the existence of an Early Burdigalian lacuna in the study area. Always in Constantine area, Guiraud (1990) suspected an Early Burdigalian age for the overlapping of Tellian series on the carbonate unit of the MC. All the field arguments show a tectonic activity during the Early Miocene. Results of micro-fault analysis realised at stations of Dj. Bou Arif (S4), Dj. Toumbaı¨t (S2), Aı¨n Kercha (S7), Chemora (S5) and Dj. Hanout Seghir (S6), (Fig. 8) show a compressional episode (u3) with a shortening oriented NE–SW (N040°–050°E). In ‘‘Petite Kabylie’’, northern MC, Raoult (1969) recognized an important tectonic event during the Early Miocene. (b) In the Pre-Atlas domain, NW of Ain Fakroun, we observed a clear unconformity of marine Miocene sandstones above continental Early Miocene (Fig. 14). The deformation is related to a tectonic phase occurring about the Aquitanian–Burdigalian boundary. Lessard (1957) attribute the unconformity of the Late Burdigalian over the Aquitanian to a tectonic event located at the Aquitanian–Burdigalian boundary. In the Sellaoua area, Vila et al. (1995) mentioned an unconformity of Middle to Late Miocene over the Eocene–Oligocene. (c) In the autochthonous domain, between the Metlili massif (W Aure`s) and the western Belezma mountains (Fig. 11), Guiraud (1990) described an unconformity characterized by red series of Miocene1 (Aquitanian) covered by marine or lagoonal formations of Miocene2 (Langhian). This author observed the same type of unconformity within
similar formations located at SE of Dj. Melah (El Outaya, Fig. 15). The shortening direction of this compressional event, probably oriented NE–SW, is different from preceding ones. 3.4. Late Miocene compressional event (a) In the ‘‘Moˆle Constantinois’’, continental to lacustrine deposits of Late Miocene–Early Pliocene age, corresponding to the Mila-Constantine Neogene Basin, cover Early Tortonian through a slight angular unconformity. According to Vila (1980) the latest noticeable unconformity is situated under the Late Tortonian levels. He also indicates that traces of tangential tectonics are fingerprinted in Early to Middle-Late Miocene marine formations of the Alpine Belt. The Late Miocene compression has generated asymmetric folds, oriented N060°–080°E, and reverse faults NNE–SSW (Aris et al., 1998). South of El Eulma (ex Saint Arnaud) Pliocene carbonates are laid unconformably on middle Tortonian formations (Coiffait, 1992). This author has also suspected a tectonic event occurring during the Late Tortonian at Dj. Rherour (SE of Tadjenanet, Fig. 6). Durozoy (1960) has observed some rejuvenation of landscape which can be dated Late Miocene; it is characterized by the occurrence of conglomeratic beds within lacustrine deposits of latest Miocene age. Vila et al. (1995) think that piggy-back basins within the ‘‘Constantinois’’ have taken place during Tortonian over-thrust. However, in the North of the study area, Raoult (1969) indicated an intra-Tortonian compressional phase inducing over-thrusts towards the southeast.
Fig. 13. Geological cross-section of Mila Miocene. 1, Early Miocene; 2, Late Burdigalian–Langhian; 3, Messinian; 4, Pliocene-Quaternary.
Fig. 14. Geological cross-section of Miocene, NE Ain Fakroun (SW of Sellaoua). 1, Cenomanian; 2, Early Miocene; 3, Burdigalian–Langhian.
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In ‘‘Grande Kabylie’’, Aite (1995) mentioned a Tortonian compressional phase with N160°E shortening inducing N050°–N080°E folds. (b) Along the Pre-Atlas domain, in Taxas syncline (Sellaoua), Vila (1980) described a klippe laying over marine Middle Miocene (Langhian–Serravallian, Fig. 16). Towards the South, on northern flank of Dj. Bou Arif we have observed marine Miocene deposits (Langhian–Serravallian) with overturned conglomeratic beds dipping 050–060° to the South. Moreover, in the oriental part of this Djebel, Serravallian marly-calcareous formations are folded along an axis oriented N080°E. In the Hodna Mountains, Guiraud (1990) showed the existence of an angular unconformity between the
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Miocene3 and Miocene4 formations, the latest presumably corresponding to late Tortonian–Messinian series. (c) In the autochthonous domain, in the ‘‘Ravin Bleu’’ anticline and the Batna syncline, Vila (1980) established a geological cross-section (Fig. 17) showing Middle Miocene occurring under Liassic dolomite, through overthrust reverse fault. In the Dj. Chelia (Aure`s Massif), Ghandriche (1991) and Frizon de Lamotte et al. (2000) described an Early-Middle Miocene formation wedged under Barremian by means of overthrust reverse fault (Fig. 18). Probably these structures are related to an intra-Tortonian compressional phase. Generally, Laffitte (1939) thinks that Aure`s massif is then affected by a new uplift accompanied by folding.
Fig. 15. Geological cross-section of Miocene on SE of Dj. El Melah (El Outaya), after Guiraud (1990). 1, Triassic; 2, Aquitanian–Burdigalian; 3, Burdigalian; 4, Langhian–Tortonian.
Fig. 16. Geological cross-section of Taxas syncline (after Vila, 1980). 1, Cenomanian; 2, Campanian; 3, Early Maastrichtian; 4, Paleocene; 5, Ypresian; 6, Lutetian; 7, Miocene; 8, Pliocene-Quaternary.
Fig. 17. Geological cross-section of ‘‘Ravin bleu’’, Batna Mountains (after Vila, 1980). 1, Triassic; 2, Liassic; 3, Dogger-Malm; 4, Neocomian; 5, Barremian; 6, Miocene.
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Fig. 18. Geological cross-section of Dj. Chelia (Aure`s), after Ghandriche (1991) and Frizon de Lamotte et al. (2000). 1, Neocomian; 2, Barremian; 3, Aptian–Albian; 4, Cenomanian; 5, Turonian; 6, Senonian–Eocene; 7, Miocene.
These folds are of moderate extension with an E–W dominant orientation, thus they are different from those attributed to the Atlasic phase. On the northeastern of the Aure`s massif, in the Touffana basin, marine detrital series dated Serravallian-earliest Tortonian (Vila, 1977) are folded into an anticline with E–W general direction (Fig. 19). Moreover, it has been observed in this basin that clay or gypseous Messinian deposits lay unconformably over Serravallian-earliest Tortonian. In the Oum El Bouaghi area (Fig. 8), carried out microtectonic data on striated micro-faults in different stations within Mio-Pliocene conglomerates, at Dj. Guellif (S11 and S9), give an orientation of the principal stress axis of N140°–145°E (u4). This later phase (u4) characterizes a compressional episode. The intra-Tortonian compressional phase is more expressed in the external zones of eastern Algeria Alpine Belt and it is characterized by imbricate fan structures. This phase has induced thrust sheets in the MC, reverse faults and over thrusts in its foreland. 3.5. Early Quaternary compressional event The structures related to the Early Quaternary tectonic event are largely expressed in the Pliocene formations of the study area, particularly around Constantine (Fig. 6). (a) In the northern part of the ‘‘Moˆle Constantinois’’ the E–W trending M’Cid Aicha-Debar transverse fault is well known and mentioned by many authors such as Durand-
Delga (1969), Raoult (1974) and Bouillin (1977). The reactivation of this dextral strike-slip fault into a reverse fault, during ‘‘post-nappe’’ tectonic activity, affects Miocene– Pliocene deposits (Coiffait, 1992). In the ‘‘Constantinois Central’’ this author indicates a compressional tectonic episode (N–S to N135°E) affecting earliest Pliocene sediments. In Ain El Bey Shelf (South of Constantine) this deformation generates folds globally oriented N70–80°E (Fig. 6) and extending on several hundred meters. On the southern boundary of the Khroub basin (southeastern of the Khroub locality) Ravin (1957) and Durozoy (1960) describe Late Tortonian limestone layers (probably Messinian) and Messinian sands and clay beds turned up and strongly tilted, often overturned, showing the intensity of post-Miocene deformation (Fig. 20). We have also observed small folds E–W oriented, involving Pliocene lacustrine limestones. South of Ain Smara, the Pliocene beds are bended in contact with the E–W strike-slip fault, indicating its reactivation. Always in the same zone, Durozoy (1960) describes an unconformity of a probable Moulouyan (Early Quaternary) limestone over red marls and Pliocene lacustrine limestones. (b) In the Pre-Atlas belt, at the western boundary of the Dj. Oum Chkrid (South of Ain Fakroun) we have observed wide anticlines within the Pliocene lacustrine deposits, globally oriented N110°E, extending over few hundred meters. These folds are often crossed by senestral decametric strike-slip faults, trending NNE–SSW. These structural elements are in agreement with a compressional
Fig. 19. Geological cross-section of folded Middle-Late Miocene in the Touffana basin (NW of Khenchela). 1, Langhian–Serravallian; 2, Messinian; 3, Quaternary.
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Fig. 20. Geological cross-section of Miocene–Pliocene of Sidi Lakhdar (South of Constantine), after Ravin (1957). 1, Langhian–Serravallian; 2, Early Tortonian; 3, Messinian; 4, Pliocene.
deformation, characterized by a sub-meridian trending principal stress axis. In the same zone, sub-parallel dextral strike-slip faults cross-cut marine Cretaceous formations and continental Pliocene. West of Dj. Guellif, red Miocene–Pliocene conglomerates are strongly tilted; they contain sheared pebbles with senestral and dextral micro-faults, respectively oriented N040–050°E and N110–120°E. These microstructures are related to submeridian shortening (Marmi, 1995). In the same place, a reverse fault oriented N080–100°E and slightly deeping to the North, allows Triassic formation to appear over Miocene–Pliocene outcrops, indicating recent tectonic activity (Fig. 21). In the Bou Taleb and Guetiane massifs (Hodna Mountains, Fig. 11), Guiraud (1977) described many structural elements such as folds, reverse faults oriented E–W, dextral strike-slip faults N010–050 °W and senestral strike-slip faults N010–020°E. These structures are the result of a compressional shortening oriented globally N–S, they contribute to the making of the actual landscape. Around the same area, Guiraud (1990) observed folded Villafranchian deposits often cut by strike-slip faults.
(c) In the Pre-Atlas corridor some folds and flexures, E– W oriented and affecting marginal-marine latest Miocene and continental formations of the Pliocene, are described by Lessard (1957) in north Aure`s boundary (Sebkha zone). Guiraud (1990) showed the existence of folds trending E–W to ENE–WSW affecting Miocene–Pliocene and Cretaceous bedrock in the Timgad Basin. Many NW–SE dextral strike-slip faults displace Bou Arif-Fedjoudj belt, originally oriented ENE–WSW (Fig. 11). Similarly, near E–W post-Miocene dextral fractures tear off the flanks of Dj. Bou Arif anticline. The micro-tectonic study carried out on striated microfaults stations near Oum El Bouaghi (S9), Dj. Guellif (S8, S11), Dj. Bou Arif (S3) and Dj. Azem (S1) (Fig. 8) shows a later compressional episode (u5) with major principal stress axis of N–S to N010°E direction. (d) In the Atlas domain (East of Biskra) along the Saharan flexure, which corresponds to the Aure`s meridional boundary, Guiraud (1973) and then Aı¨ssaoui (1984) described Villafranchian formations affected by E–W oriented reverse faults (Fig. 22) and E–W to NW–SE dextral strike-slip faults, generating drag folds in many places.
Fig. 21. Geological cross-section West of Dj. Guellif (SW of Oum El Bouaghi). 1, Triassic; 2, Aptian; 3, Miocene–Pliocene; 4, Quaternary.
Fig. 22. Geological cross-section of Dj. Rheliss, Guiraud (1990). 1, Campanian–Maastrichtian; 2, Messinian; 3, Early Pliocene; 4, ‘‘Middle’’ Pliocene; 5, Late Pliocene.
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Macro- and micro-structural analysis confirm that a regional compressional tectonic event, characterized by a sub-meridian shortening, took place during Early Quaternary. This recent deformation is very well recorded in the ‘‘Moˆle Constantinois’’ as well as in its foreland. This stress regime continues from the beginning of the Quaternary until today, but with low intensity as illustrated by the seismic activity in the region, particularly Constantine earthquake in 1985 (Bounif et al., 1987). 4. Regional geodynamics and conclusions Clues of the tectonic events which we described for the ‘‘Moˆle Constantinois’’ and forelands have been reported along adjacent domains of the Northwestern African Alpine Belt. We shall give a brief review of these polyphased deformations, hereafter to draw a hierarchy between them, and discuss their links with the regional geodynamics. We first mention here that we do not in this paper faced with the Santonian event which entailed the oldest alpine compressional deformations along the African Alpine plate margin (Guiraud and Bosworth, 1997), as it presents very minor echoes in our study area. Permanent tectonic instability characterized the Maastrichtian times (Guiraud and Bosworth, 1997). Local folding and reverse faulting then affected the ‘‘Moˆle Constantinois’’ and the Pre-Atlas domain. In central and eastern Tunisia, Zouari et al. (2004) have mentioned compressional strike-slip tectonics of ‘‘Middle’’ Maastrichtian– Paleocene age, with sub-meridian shortening. Along the southern Moroccan High Atlas, southwards thrusting was registered (Laville et al., 1977). So, diachronous and local compressional deformations, exhibiting NW–SE to N–S shortenings (Table 1), occurred during Maastrichtian times along the Maghrebian Alpine Belt. Regionally, they characterize a minor event which was witness to dextral transpression along the northwestern African plate margin, in response to differential opening of the Central, South and North Atlantic oceans (Guiraud and Bosworth, 1997; Guiraud et al., 2005). A more severe tectonic event occurred in the earliest Late Eocene (37 Ma), particularly identified around the ‘‘Moˆle Constantinois’’. This event entailed strong
Table 1 Synoptic table of compressional phases recognized in the ‘‘Moˆle Constantinois’’ and neighbouring regions Tectonic phases
Shortening direction (NE Algeria)
Chronology of compressional events
/1 /2 /3 /4 /5
N130°–135°E N155°–160°E N40°–50°E N140°E NNW–SSE to N–S
Maastrichtian (68–65 Ma) Earliest Late Eocene (37 Ma) Aquitanian–Burdigalian (21–18 Ma) Tortonian (8.5 Ma) Early Quaternary (1.5–1.3 Ma)
folding and strike-slip faulting along the Saharan Atlas and Pre-Atlas domains, as well as the Tunisian Atlas and Moroccan High and Middle Atlas (‘‘Atlas event’’) (Guiraud et al., 1987, 2005; Dlala and Rebai, 1994; Frizon de Lamotte et al., 2000; Pique´ et al., 2002; Mahboubi et al., 2003). Throughout these domains it resulted in NNW–SSE to NW–SE shortening (Table 1). It also affected the internal Tellian–Rifian domain (Kabylies– Alboran) that then underwent thrusting and slight metamorphism (Raoult, 1969; Guiraud et al., 1987, 2005), but saved up the external Tell. Identified along the entire northern African–Arabian plate margin, as well as along the large intra-plate fault zones (Bamby and Guiraud, in press), this compressional event corresponds to a major tectonic stage in the development of the Maghrebian Alpine Belt. Like the late Santonian and Maastrichtian events, it resulted from changes in the rates and directions of opening of the Central, South and North Atlantic oceans (Guiraud et al., 2005). These differential openings generated compressions and/or dextral transpressions in the western Mediterranean Alpine domain and probably the initiation of the subduction of the Maghrebide Tethys underneath the Iberian-Balearic margin (Frizon de Lamotte et al., 2000). During the latest Aquitanean–Early Burdigalian times (21–18 Ma) the ‘‘Moˆle Constantinois’’ and surroundings experienced compressional deformations whose intensity strongly decreased from northern to southern domains. The internal domain of the Tellian–Rifian Belt then underwent thrusting and metamorphism (Raoult, 1974; Guiraud et al., 2005) while southern forelands only registered minor folding. The micro-tectonic studies which we realised show that the major shortening was then NE–SW oriented (Table 1). Similar results were obtained along the Moroccan and Tunisian Atlas (Letouzey and Tre´molie`res, 1980), as well as in several intra-plate domains of Northwestern Africa (Guiraud and Bellion, 1995). This shortening direction is very different from preceding and following ones. It results of a sharp change in plate motions, as Africa then moved northeastwards (Guiraud and Bosworth, 1997). This change, synchronous with the opening of the Western Algerian–Alboran ocean basin (Mauffret et al., 1992), generated compressional deformations (Guiraud et al., 2005). During Tortonian times (9–8 Ma) the ‘‘Moˆle Constantinois’’ rapidly shifted southwards, pressing the Pre-Atlas domain, which then was strongly sliced and folded (Guiraud, 1973, 1990; Vila, 1980). Similar southwards thrusting occurred all along the external zones of the Tellian–Rifian Belt (Vila, 1980; Frizon de Lamotte et al., 2000; Guiraud et al., 2005), as especially evidenced in Tunisia (Rouvier, 1977) and Oranie (Gardia, 1975). These displacements entailed slight folding of the Atlas domain s.l., from Morocco to Tunisia (Dlala and Rebai, 1994; Ghribi et al., 2004; Guiraud et al., 2005). The shortening was NNW–SSE (Table 1). This event resulted from a new change in the plate motions as Africa then moved
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northwestwards, entailing dextral transpression along its northern margin (Guiraud and Bosworth, 1997, 1999). At last, Early Quaternary (1.5–1.3 Ma) compressional event affected the ‘‘Moˆle Constantinois’’ and foreland domains, giving E–W folds and reverse faults and conjugate strike-slip faults, witnessing for N–S to NNW–SSE shortening (Table 1). Identical deformations occurred along the Rif–Tell and Atlas Mountains (Philip and Thomas, 1977; Amari and Bedir, 1989; Dlala and Rebai, 1994; Addoum, 1995) that then grew up (Frizon de Lamotte et al., 2000; Guiraud et al., 2005). Most of these structures will develop, with decreasing intensity, up to recent times as evidenced by neotectonic analyses and seismicity (Girardin et al., 1977; Philip and Thomas, 1977; Boudiaf et al., 1998, 1999). Along the southern edges of the ‘‘Grande Kabylie’’, Boudiaf et al. (1999) documented a large thrust developing during the Pleistocene times. During this stage, the approximately northward motion of Africa, initiated in the Senonian times, continued. For Frizon de Lamotte et al. (2000) the Quaternary phase corresponds to the cessation of the Maghrebian Tethys subduction, while the fluxuration of the plate margin developed. In conclusion, for many aspects, the End Cretaceous to recent polyphase compressive tectonics of the ‘‘Moˆle Constantinois’’ and foreland exemplies the history of the Maghrebian Alpine Belt and, on a broader scale, of the Northwestern African plate domain. Acknowledgements Suggested improvements by Fred Beekman, Pat Eriksson and an anonymous reviewer were greatly appreciated. We are grateful to Dr. A. Boumezbeur for his help in the translation of the original manuscript of this paper. References Addoum, B., 1995. L’Atlas Saharien sud-oriental: Cine´matique des plischevauchements et reconstitution du bassin du Sud-Est Constantinois (Confins alge´ro-tunisiens). Thesis Sci., University Paris Sud Orsay, p. 198. 4 pl. h.t. Aite, M.O., 1995. Pale´ocontraintes post-collision identifie´es dans le Ne´oge`ne de Grande Kabylie (Alge´rie). C.R. Acad. Sci. Paris, t. 320 (Se´rie IIa), 433–438. Aı¨ssaoui, D., 1984. Les structures lie´es a` l’accident sud-atlasique entre Biskra et le Djebel Mannda (Alge´rie). Evolution ge´ome´trique et cine´matique. The`se 3°cycle, Strasbourg, p. 154. Amari, A., Bedir, M., 1989. Les bassins quaternaires du Sahel central de la Tunisie. Gene`se et e´volution des Sebkhas en contexte de´crochant compressif et distensif. Ge´odynamique 4 (1), 49–65. Angelier, J., 1984. Tectonic analysis of fault slips data sets. J. Geophys. Res. 89 (B7), 5835–5848. Angelier, J., 1989. From orientation to magnitudes in paleostress determinations using fault slip data. J. Struct. Geol. 11 (1/2), 37–50. ´ tude tectonique et microtectonique des se´ries jurassiques a` Aris, Y., 1994. E plio-quaternaires du Constantinois (Alge´rie nord-orientale): Caracte´risation des diffe´rentes phases de de´formation. Thesis, University Nancy I, France, 215 pp.
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