Characterisation of Mesozoic–Cenozoic deformations and palaeostress fields in the Central Constantinois, northeast Algeria

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Tectonophysics 290 (1998) 59–85

Characterisation of Mesozoic–Cenozoic deformations and palaeostress fields in the Central Constantinois, northeast Algeria Yassine Aris a , Philippe Emmanuel Coiffait a , Michel Guiraud b,* a

Laboratoire de Ge´ologie Applique´e au Ge´nie Civil, Universite´ Henri Poincare´ de Nancy I, B.P. 239, 54506, Vandoeuvre-Le`s-Nancy, France b Centre des Sciences de la Terre, UMR 5561: Pale ´ ontologie Analytique et Ge´ologie Se´dimentaire, Universite´ de Bourgogne, 6 Bd. Gabriel, 21000, Dijon, France Received 24 September 1996; accepted 26 November 1997

Abstract Tectonic analysis in conjunction with the microtectonic study of Mesozoic–Cenozoic series of the Central Constantinois of Algeria are used to reconstruct the sequence of tectonic phases since Cretaceous times. The retrotectonic method used to marshal the microtectonic data makes it possible to distinguish deformations related to Mesozoic tectonic phases from those associated with Cenozoic pre- and post-thrust sheet phases. A N120ºE extensional and a N180ºE compressional phase are highlighted in Albian–Cenomanian and latest Maastrichtian times, respectively. The Cenozoic era is marked by a series of three compressional phases oriented N90º–120ºE in the Late Eocene, and N20º–30ºE and N170ºE in the Late Miocene. The first Late Miocene compressional phase related to the emplacement of Numidian flysch thrust sheets was followed by N–S compression affecting the overriding thrust sheets and their substratum. During latest Miocene–Pliocene and Quaternary times the tectonic pattern changed with NE–SW regional extension giving way in Quaternary and present-day times to a N130º–150ºE compressional episode. The characteristics of the palaeostress fields and the regional structures are specified for the different phases defined.  1998 Elsevier Science B.V. All rights reserved. Keywords: brittle microtectonics; palaeostress tensor; tectonic phase; palaeostress field; Mesozoic; Cenozoic; Algeria

1. Introduction The Central Constantinois domain of northeast Algeria is connected to the external zones of the Alpine belt range (Fig. 1). The southwest area of the domain features a Late Jurassic–Early Cretaceous to Cenomanian carbonate substratum which is evidence of a differentiated neritic carbonate platform within the basins of the North African Tethyan margin (Coiffait, 1992; Aris, 1994; Figs. 2 and 3). Many workers consider the platform to be autochthonous (DuŁ Corresponding

author. E-mail: [email protected]

rand-Delga, 1955; Durozoy, 1960; Voute, 1967; Durand-Delga, 1969; Lahonde`re, 1987; Coiffait, 1992). This idea was challenged by Guiraud (1973) and Vila (1980) who emphasised the allochthonous nature of the unit. The Constantinois carbonate platform is overlain as a regional angular unconformity by the platform cover which grades upward from Campanian to Maastrichtian marls and calcareous marls (late Senonian) to Ypresian marls and carbonates (Early Eocene) and Oligocene marls (Lahonde`re, 1987; Chadi, 1991; Coiffait, 1992; Aris, 1994). In the north and northeast of the study area a set of overriding thrust sheets composed of Oligocene–

0040-1951/98/$19.00  1998 Elsevier Science B.V. All rights reserved. PII S 0 0 4 0 - 1 9 5 1 ( 9 8 ) 0 0 0 1 2 - 2

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Fig. 1. Location of the study area in the Alpine belt of Algeria.

early Burdigalian flysch is unconformably overlain by Pliocene to Quaternary continental formations (Constantine and Guelma zones: Fig. 2; fig. 3 of Vila, 1980; Coiffait, 1992). To the south of a line between Mila and Guelma, the most recent marine autochthonous deposits are of late Burdigalian– Langhian age in the Mila basin (Coiffait, 1992) and of Langhian age in the Sellaoua basin (Voute, 1967; Vila, 1980). The allochthonous series are composed of three units from base to top: (1) Cretaceous to Eocene Tellian thrust sheets, (2) Cretaceous–Palaeogene flysch thrust sheets (Ge´lard, 1969; Bouillin et al., 1970), and (3) Oligocene to early Burdigalian Numidian flysch thrust sheets (Bouillin and Raoult, 1971; Raoult, 1974; Hoyez, 1989). The various tectonic studies of the Central Constantinois concentrate mainly on characterising the Cenozoic tectonic phases and associated regional structures. The emplacement of overriding thrust sheets is thus related to a late Burdigalian phase. This tangential tectonic episode is also thought to account for the Constantinois platform overriding the

northern series of the foreland (Guiraud, 1973, 1975; Vila, 1978, 1980). In the Central Constantinois the major Miocene phase was preceded by late Lutetian compression which has been identified in the Foreland (Laffitte, 1939; Guiraud, 1973; Kazi Tani, 1986) and in the internal zones (Raoult, 1974; Mahdjoub, 1992). The various Cenozoic tectonic phases reactivated ancient NE–SW, NW–SE and E–W basement-induced faults (Voute, 1967; Wildi, 1984). However, despite these studies the Mesozoic– Cenozoic tectonic phases and their associated palaeostress states and regional structures were not precisely documented in the Central Constantinois. This paper brings together microtectonic data correlated with tectonic data effectively tracing the successive stages of basin structuring in the Central Constantinois. Using a tectonic field survey and quantitative microtectonic analysis by fault striation processing we set out to specify for each tectonic stage: (1) the age of the deformation and the associated regional palaeostress state; and (2) the nature of the regional structures that were generated or reactivated.

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Fig. 2. Structural map and N–S-trending section AB of the Central Constantinois domain: ADF D Aicha Debar Fault, AKF D Ain Kercha Fault, BRF D Bourma–Regada Fault, OSF D Oum Settas Fault, SF D Sellaoua Fault, 1 D Hercynian basement, 2 D Jurassic and Early Cretaceous to Cenomanian carbonate platform, 3 D Jurassic and Early Cretaceous marine deep-water deposits, 4 D platform cover (late Senonian to Oligocene), 5 D thrust-sheet of the Ultra-Tellian and the Massilian–Mauretanian flyschs (Cretaceous to Eocene), 6 D Numidian flysch thrust-sheet (Oligocene to early Burdigalian), 7 D marine Early Miocene deposits of the external domain (late Burdigalian– Langhian), 8 D continental Pliocene–Quaternary deposits, 9 D fault, 10 D thrust fault, 11 D syncline axis, 12 D anticline axis.

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Fig. 3. Lithological, stratigraphic and tectonic column of the formations of the Central Constantinois domain. Stratigraphic setting of the microtectonic stations: 1 D conglomerate deposits, 2 D calcareous marl, 3 D slumps and slope deposits, 4 D marls.

2. Purpose and methods The findings set out in this paper are based on microtectonic analysis from 51 stations of mi-

crofault populations distributed throughout the autochthonous, allochthonous and post-thrust sheet formations (Figs. 3 and 4). More than 1500 brittle to hydroplastic microfaults were considered with an av-

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Fig. 4. Location of the microtectonic stations: 1 D fault, 2 D microtectonic station reference number.

erage 30 microfaults per station. Criteria used for determining movement are those defined by Petit et al. (1983). Several authors (Angelier and Goguel, 1978; Carey, 1979; Angelier and Manousis, 1980; Etchecopar et al., 1981; Etchecopar and Mattauer, 1988) have proposed quantitative inverse computer-aided methods for interpreting the striations in limited areas of microfaulting (100 to 2500 m2 ). These are based on an inverse problem of finding the deviatoric stress tensor that best accounts for the measured fault slickensides. Rocks in tectonic areas have generally been submitted to successive tectonic phases with different tectonic stress tensors. The major problem is to sort out from the set of N measured microfaults the n1, n2 ( : : : ) slickensides generated by each successive tectonic phase and to calculate the corresponding stress tensors T1, T2, ( : : : ). The computer-aided Etchecopar fault method is used in

this work for its ability to separate and calculate successive deviatoric stress tensors (Etchecopar, 1984; Etchecopar and Mattauer, 1988). The method provides the stereographic projection of the fault planes and the axes of the principal stresses ¦1 , ¦2 and ¦3 , as well as the ratio R D ¦2 ¦3 =¦1 ¦3 of the calculated stress tensor (Fig. 5b). The reliability of the calculation is evaluated by considering the location of the fault plane poles in a Mohr diagram and the histogram of angular differences between calculated and measured striations as well as the angular deviation AD (Fig. 5a and c). Depending on the number of striations per station, an average of two palaeostress tensors was defined for the more recent formations. A significant number of striations were taken into account for the older formations subjected to many large tectonic events meaning that three and exceptionally four palaeostress tensors could be defined. Philip (1987) and Guiraud et al. (1989) presented

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Fig. 6. Bidimensional representation of stress tensors (modified from Guiraud et al., 1989; Ritz, 1991): ¦v D vertical stress R D ¦2 ¦3 =¦1 ¦3 , with 0  R  1); 1–5 D general extensional stress tensors; 6–10 D general strike-slip stress tensors; 11–15 D general compressional stress tensors; 1 D radial extensional stress tensor .R D 0/; 3 D pure extensional stress tensor .R D 0:5/; 5, 10 D uniaxial extensional stress state .R D 1/ related to strike-slip extensional deformation; 6, 11 D uniaxial compressional stress tensor .R D 0/ related to strike-slip compressional deformation; 8 D pure strike-slip stress tensor .R D 0:5/; 13 D pure compressional stress tensor .R D 0:5/; 15 D radial compressional stress tensor .R D 1/.

a general classification of the deviatoric stress tensors combining the nature of the vertical stress (¦1 , ¦2 , ¦3 ) and the value of the ratio R (R D ¦2 ¦3 =¦1 , 0 < 1). This classification is subdivided into nine major deviatoric stress tensors. Three classical types

of deviatoric stress tensors successively of pure extensional, pure strike-slip and pure compressional types (3, 8 and 13: Fig. 6) are characterised. Four other types of deviatoric tensors are specified by an ellipsoidal shape. Types 1 and 15 correspond respec-

Fig. 5. Calculation of the palaeostresses tensors using the Etchecopar computer-aided method (Etchecopar and Mattauer, 1988; Taboada et al., 1991). (a) Location of the poles of the fault planes in a Mohr diagram. (b) Stereographic projection of the fault planes and the principal stress axes (measured and calculated striations are indicated respectively by arrows and by circles). (c) Histogram of angular differences between calculated and measured striations. (d) Microfault planes excluded during the calculation (numbers in a, b, c and d correspond to the reference numbers of the microfault measurements).

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tively to radial extensional and radial compressional tensors. The similar types 5 and 10 are related to uniaxial extensional tensors associated with strike-slip extensional deformation while the identical types 6 and 11 are related to uniaxial compressional tensors associated with strike-slip compressional deformation. The bidimensional representation of stress tensors shown in Fig. 6 is used in this contribution to show the set of palaeostress tensors related to each specified tectonic phase. In the Central Constantinois, analysis of populations of microfaults measured at the stations indicates 6, 16, 28 and 60 palaeostress tensors in the Miocene–Pliocene–Quaternary formations, the thrust sheets series, the cover and the carbonate platform, respectively. These results are analysed by using the retrotectonic method which consists in associating a subuniform set of palaeostress tensors with each tectonic phase (Fig. 7). Each of the phases defined is associated with a comparatively homoge-

neous palaeostress state on the scale of the study area while considering the size of the stress disruption phenomena in the vicinity of the large faults. Apart from the large faults that disrupt the stresses in direction and magnitude (Philip, 1987; Guiraud, 1991a,b), the main horizontal stress related to a tectonic phase can remain comparatively homogeneous over large areas (Bergerat, 1987; Petit and Mattauer, 1995). A set of tensors T1 defined in sedimentary levels A, B and C is related to a recent tectonic phase '1 post-dating formation A. Before '1 , tectonic phase '2 post-dating formation B and pre-dating formation A is characterised by a set of tensors T2 detected in formations B and C. The relative chronology of recent .'1 / and ancient ('2 –'3 ) tectonic phases as evidenced by tensors T1, T2 and T3 is established from the vertical distribution of families of tensors and the age of the sedimentary formations A, B and C. In this paper the relative ages of the phases of deformation are further

Fig. 7. Principle of separation of tectonic phases using the retrotectonic method: T1, T2, T3 D palaeostress tensors related to brittle microfaulting, respectively observed within formations A, B and C.

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Fig. 8. Palaeostress tensors deduced from analysis of the brittle microfaults deforming the Pliocene–Quaternary deposits: 1 D Pliocene– Quaternary deposits; 2 D older deposits; 3 D anticline axis; 4 D normal fault; '1 D Late Quaternary and present-day NW–SE compressional phase; '2 D latest Miocene–Pliocene–Quaternary NW–SE extensional phase. The symbols used to represent the palaeostress tensors are detailed in the Fig. 6 (N D number of measured microfaults; Nc D number of microfaults compatible with the calculation of the stress tensor; Ni D number of microfaults incompatible with the calculation of the stress tensor).

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characterised by analysis of the geometric relations between the different systems of regional structures and of synsedimentary deformations. 3. Deformations and associated palaeostress states The characteristics of the phases of deformation and of the associated sets of palaeostress tensors were established by successively analysing the four major rock groups of the area (Fig. 3): the Miocene– Pliocene–Quaternary formations, the Oligocene– early Burdigalian series of the Numidian thrust sheets, the late Senonian to Oligocene cover and the Jurassic–Early Cretaceous to Cenomanian carbonate platform (Figs. 2 and 3). The thrust sheets emplaced in the late Burdigalian (Coiffait, 1992) include the ultra-Tellian (Cretaceous–Eocene marl and limestone), the Cretaceous–Palaeogene flysch and the Oligocene–early Burdigalian Numidian thrust sheet. 3.1. Mio–Pliocene and Quaternary continental deposits The Mio–Pliocene and Quaternary continental formations overlie the earlier autochthonous and allochthonous units unconformably. An extensional episode (Fig. 8) is related to the formation of basins with continental fill bounded by N45º–80ºE faults (Mila and Tamlouka Basins) and N100º–120ºE faults (Guelma Basin) which clearly post-date the emplacement of the flysch thrust sheets in the Tamlouka and Guelma zones. The greatest extensional displacement is observed along the Aı¨cha–Debar Fault and associated locally with trachyte and andesite flows (Raoult and Veld, 1971). In the south of the area vertical displacement appears less pronounced in the vicinity of the N45º–60ºE Oum Settas, Djaffa and Sellaoua Faults. A compressional episode is characterised by observation of N60º–70ºE folds in Pliocene–Quaternary deposits in the Constantine and Mila sectors (Fig. 8) and of conjugated sinis-

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tral N30º–60ºE (Oum Settas and El Kantour Faults) to dextral N110ºE (Fortas and Nif en Nser Faults) strike-slip faults. The lacustrine limestone and fluviatile sandstone are affected by normal and reverse microfaults and by sub-vertical tension gashes indicative of the multiphase nature of deformation. Judging from the relative chronologies established from intersecting striations on the microfault surfaces, the compressional episode seems to have been the more recent one. Despite the small number of stations considered, analysis of the brittle microfaults produces uniform results on the regional scale and characterises two stages of deformation. The first extensional episode is marked by N140º–150ºE horizontal stretching (stations 80, 97 and 99, Fig. 8). The compressional phase is related to N130º–150ºE horizontal shortening (stations 80, 97 and 99, Fig. 8). The latter data are consistent with the orientation of the N60º–70ºE folds observed in the Pliocene–Quaternary deposits and with the NNW–SSE orientation of the present-day compressional stress field defined in North Africa from in-situ stress measurements, focal mechanisms and microtectonic analysis of Quaternary deformations (Rebai et al., 1992; Philip and Megraoui, 1983; Philip, 1987). These observations are consistent with Guiraud (1977) and Philip (1987) characterising a NW–SE extensional phase of latest Miocene– Pliocene age and a NW–SE compressional phase of Quaternary to present-day age in eastern Algeria. However, the characterisation of two phases of deformation in the Quaternary sediments (stations 80 and 99, Fig. 8) allows us to suggest a latest Miocene–Pliocene–Quaternary age for the NW–SE extensional phase and a late Quaternary to presentday age for the NW–SE compressional phase. 3.2. The overriding thrust sheets and the Lower Miocene sediments of the foreland The Numidian flysch thrust sheet comprises an Oligocene–early Burdigalian sandstone series, the

Fig. 9. Palaeostress tensors deduced from analysis of the brittle microfaults affecting the Oligocene–early Burdigalian Numidian thrust sheets and the autochthonous Early Miocene marine deposits: 1–2 D recent tectonic phases; 3–4 D late Burdigalian–Langhian N20–30ºE and Late Miocene N170ºE compressional phases; 5 D Oligocene–early Burdigalian Numidian thrust sheets; 6 D autochthonous late Burdigalian –Langhian marine deposits of the foreland (Early Miocene).

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base of which corresponds to a characteristic clay unit. To the south, the present-day edge of the thrust sheet forms a N120ºE overriding contact with N60º–70ºE reverse faults (Figs. 2 and 9). The thrust sheet material is deformed by parallel folds with N110º–120ºE axes associated with reverse faults and N60ºE sinistral strike-slip displacement dipping steeply northwards. The Early Miocene marine sediments of the foreland exposed in the southeast of the study area are alternating marl–limestone deposits affected by N100º–120ºE folds and N100º–130ºE reverse faults (Fig. 9). These deposits unconformably overlie intensely structured Cretaceous to Eocene deposits to the north of Chebka d’Aı¨n Kercha and west of Sedrata. In the vicinity of the Sellaoua Fault the Miocene sediments are much deformed by southward-dipping N100º–120ºE to N60º–80ºE folds and by northward-dipping N60º–80ºE reverse faults. The palaeostress tensors defined by microtectonic analysis of microfaults observed in the Oligocene– early Burdigalian Numidian thrust sheet and in the late Burdigalian–Langhian marine deposits of the foreland fall into five uniform sets (Fig. 9). The first two sets are related to N130º–150ºE Quaternary to present-day compression and N140º–150ºE latest Miocene–Pliocene–Quaternary extension already detected in the Pliocene–Quaternary sediments (stations 93, 95, 96, '1 and '2 , Fig. 9). Three new uniform sets of tensors are defined highlighting several phases of Late Miocene deformation post-dating the Oligocene–early Burdigalian deposits composing the thrust sheet material and pre-dating the Pliocene–Quaternary deposits. Two sets of compressional tensors are marked respectively by N–S to N170ºE and N20º–30ºE horizontal shortening ('3 and ϕ4a, Fig. 9). They are uniform on the regional scale and characterise Late Miocene deformation phases responsible for the emplacement and structuring of the thrust sheets. The relative chronologies observed during the analysis of microfaults with several striations seem to indicate that the

ϕ4a N20º–30ºE compression pre-dated the ϕ3 N–S to N170ºE compression. Another set of tensors is of the extensional type with N30º–40ºE lengthening related to extensional deformation of the Numidian thrust sheet front in areas where strata are slightly tilted (stations 65 and 94, '4b , Fig. 9). These extensional tensors can be related to gravitational extensional deformation localised on the thrust sheet edge. The first N20º–30ºE compression is related to the main emplacement of the thrust sheets and is likely to be of late Burdigalian–Langhian age (Coiffait, 1992; Aris, 1994). Thus, we consider the contemporaneous Early Miocene marine deposits of the Sellaoua domain (Fig. 2) to represent marine deposition of the foreland. The later N–S to N170ºE compression is associated with the structuring of the thrust sheets during Late Miocene times and is in good agreement with the Tortonian NW–SE compression defined by Ait et al. (1991) in western Algeria. 3.3. The late Senonian–Oligocene cover The term ‘cover’ is used to differentiate the Late Cretaceous and Oligocene argillaceous and phosphatised limestone from the underlying Jurassic– Cretaceous limestone (Fig. 2). This unit comprises, from base to top, the Maastrichtian–Campanian (late Senonian) marl–limestone beds, the Paleocene black marls, the Early Eocene limestone (Ypresian–early Lutetian) and the late Lutetian–Priabonian (Late Eocene) to Oligocene marls. The late Senonian marls and marl–limestones are deformed by N70ºE parallel folds often associated with N45ºE and N60ºE strike-slip faults. The Eocene limestone and marl is structured by N50º–70ºE recumbent folds associated with strike-slip faults and locally by N–S to N30ºE folds. Microtectonic study of the late Senonian argillaceous limestone and the Ypresian phosphatised limestone reveals several uniform sets of tensors characteristic of several phases of deformation. Four sets

Fig. 10. Palaeostress tensors deduced from the analysis of the brittle microfaults affecting the late Senonian to Early Eocene–Oligocene cover of the carbonate platform (the symbols used to represent the palaeostress tensors are detailed in Fig. 6; N D number of measured microfaults; Nc D number of microfaults compatible with the calculation of the stress tensor; Ni D number of microfaults incompatible with the calculation of the stress tensor).

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Fig. 11. Example of geometrical relationships between N80ºE-trending anticline, NE–SW faults and early Senonian stratigraphic hiatus and unconformities in the Jurassic–Early Cretaceous–Cenomanian carbonate platform of the Mazela Massif and its late Senonian to Early Eocene cover. (A) Geological map of the Mazela anticline (location in Fig. 10): 1 D Aptian; 2 D Albian; 3 D Cenomanian; 4 D Campanian–Maastrichtian; 5 D Early Eocene; 6 D microtectonic station; 7 D slumps; '6 D N–S latest Maastrichtian compression; '7 D N095ºE Albian–Cenomanian extension. (B) NNW–SSE section.

of tensors can be made out, related to the recent compressional and extensional phases '1 –'4 detected by microtectonic analysis of the Pliocene– Quaternary and Oligocene–Early Miocene series ('1 –'4 , Fig. 10). Two new uniform sets stand out ('5 and '6 , Fig. 10). The first is marked by compressional tensors, the azimuth of horizontal shortening of which varies from N80ºE to N100ºE (stations 51, 64 and 76, Fig. 10). These tensors characterise an E–W compressional phase associated with post-Ypresian and pre-Oligocene–Miocene tectonic deformations which were therefore probably of Late Eocene age. Based on the use of the retrotectonic method, tensors '3 –4 associated with the recent tectonic phases are

respectively detected in the Oligocene–early Burdigalian thrust-sheets, the Early Miocene autochtonous marine deposits as well as in the late Senonian–Early Eocene platform cover (Figs. 9 and 10). On the other hand, tensors '5 –6 are only detected by analysing the deformation affecting the late Senonian–Ypresian platform cover. The cover series are locally deformed in the study area by N80ºE to E–W folds (Aris, 1994). The cover in the Central Constantinois, represented by the late Senonian marl–limestone, overlies the Jurassic– Early Cretaceous–Cenomanian platform with a regional angular unconformity associated with a substantial gap in deposition. In the Mazela and Felten Massifs, for example, the marl–limestone strata of

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Fig. 12. Example of latest Maastrichtian positive inversion tectonics along a N45ºE-trending fault in the Jurassic–Early Cretaceous– Cenomanian carbonate platform of the Felten Massif and its late Senonian (Maastrichtian) cover (location in Fig. 10). (A) Geological map: 1 D Aptian; 2 D Cenomanian; 3 D Maastrichtian–Early Eocene; 4 D Plio–Quaternary deposits. (B) NNW–SSE section: '6 D N–S latest Maastrichtian compression; '7 D Albian–Cenomanian extension.

the cover in the vicinity of E–W folds are affected by spectacular synsedimentary deformations including numerous N45ºE to N80ºE slumps, parallel to bedding decollements and hydroplastic microfaults (S in Fig. 11A and B, Fig. 12B). These synsedimentary deformations are related to gravitational decollements of the cover brought about by the E–W folding. These observations tend to show that the E–W folds in the Central Constantinois are probably of late Maastrichtian age. The compressional phase associated with E–W folding is related to the second original set of tensors detected by microtectonic analysis of microfaults ('6 , stations 25 and 79, Fig. 11). These compressional tensors exhibit N–S horizontal shortening. 3.4. Limestone of the Constantinois platform The detailed survey map shows that the coverplatform unit is deformed by N80ºE to E–W folds closely associated with E–W to NE–SW faults. In the Mazela Massif the N45º–60ºE faults affect the

Cretaceous carbonate strata of the platform and are cartographically sealed by late Senonian ferruginous conglomerates at the base of the cover (Fig. 11A). In this sector the very coarse Albian carbonate strata include late Aptian limestone blocks, ranging from centimetres to a metre in size, cropping out over small distances along the N45º–60ºE faults (station 16, Fig. 11B). These coarse alluvial fan sediments grade laterally southwards into fine glauconous limestone. In the Felten Massif the N50ºE faults also closely control the gaps and thickness variations marking the Albian–Cenomanian formations (Fig. 12). All of these characters show that the N45º–60ºE faults are synsedimentary fractures contemporaneous with Albian–Cenomanian sedimentation. The brittle microfaults associated with NE–SW faulting of the Aptian limestone were analysed quantitatively. Analysis shows that the extensional tensor is defined by N90º–100ºE horizontal lengthening related to an Albian–Cenomanian E–W extensional phase which initiated the NE–SW synsedimentary

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Fig. 14. Chronology of the tectonic events defined in the Central Constantinois domain.

Fig. 13. Palaeostress tensors associated with brittle microfaults measured within the deposits of the Central Constantinois carbonate shelf and its lateral equivalents: 1 D '6 latest Maastrichtian N–S compression; 2 D '7 Albian–Cenomanian N090–100ºE extension; 3 D tensor prior to the tilting of the stratification; 4 D Jurassic–Early Cretaceous–Cenomanian formations of the carbonate platform; 5 D Jurassic–Early Cretaceous deep-water deposits. (The symbols used to represent the palaeostress tensors are detailed in Fig. 6; N D number of measured microfaults; Nc D number of microfaults compatible with the calculation of the stress tensor; Ni D number of microfaults incompatible with the calculation of the stress tensor.)

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Fig. 15. Palaeostress field related to the N90–100ºE extension of Albian–Cenomanian age: 1 D carbonate platform; 2 D deep-marine basin; 3 D normal fault; 4 D extensional stress tensor .0:6 > R > 0:25/; 5 D radial extensional stress tensor (R D 0, ¦2 # ¦3 ); 6 D ¦3 trajectory; 7 D probable fault.

normal faulting ('7 , station 16, Fig. 11B). In the Felten Massif the relative chronology of the two Cretaceous tectonic phases is revealed by detailed analysis of the N45ºE fault (Fig. 12). The fault was activated as a synsedimentary normal fault during Albian–Cenomanian times. During the latest Maastrichtian N–S compression the fault was reactivated as a strike-slip reverse fault during and after the deposition of Maastrichtian sediments (Fig. 12B). On the regional scale, the microtectonic study of the platform carbonates highlights two important sets of E–W extensional and N170ºE to N–S compressional tensors associated respectively with the Albian–Cenomanian N90–100ºE extension and the latest Maastrichtian N–S compression ('7 and '6 , Fig. 13). Although, the Albian–Cenomanian extensional phase '7 is associated with several radial

extensional tensors (R D 0, station 68, Fig. 12B; stations 68, 70, 81, 83 and 87, Fig. 13) and therefore is locally related to the undefined direction of the horizontal stress ¦3 . These results indicate that the two phases of Cretaceous deformation are of regional tectonic significance. The two types of tensors associated with the two tectonic phases are not detected in the more recent formations (thrust sheets and post-thrust sheet formations). 4. Mesozoic–Cenozoic tectonic phases and associated palaeostress fields Quantitative analysis of microfault populations carried out in conjunction with the tectonic study of the various units of the Central Constantinois makes it possible to characterise the nature and succession

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Fig. 16. Palaeostress field related to the NS compression of latest Maastrichtian age: 1 D deep-marine basin; 2 D carbonate platform; 3 D thrust fault; 4 D anticline axis; 5 D strike-slip fault; 6 D ¦1 trajectory; 7 D compressional stress tensor .0:7 > R > 0:5/; 8 D Pure strike-slip stress tensor .R D 0:5/; 9 D uniaxial compressional stress tensor (R D 0, ¦2 # ¦3 ) related to strike-slip compressional deformation.

of the extensional and compressional tectonic phases in this area of northeast Algeria from Mesozoic to Quaternary times (Fig. 14). The palaeostress fields associated with the different tectonic episodes have been reconstructed using the following assumptions. The stress field related to a specific tectonic phase is usually relatively uniform on the regional scale (Bergerat, 1987; Zoback, 1989). However, the direction and R ratio of the stress field may be disturbed locally in the vicinity of major faults (Xiaohan, 1983; Taha, 1986; Guiraud, 1991a,b). In this study the palaeostress trajectories have been calculated from different sets of tensors and defined using the linear interpolation program as finite elements developed by Rebai et al. (1992). To illustrate clearly the changing pattern of structures in the area over time, the palaeostress fields and associated regional structures are presented by considering

the Mesozoic tectonic phases first, followed by the Cenozoic to Quaternary phases. 4.1. Cretaceous phases of deformation: early structuring of the carbonate platform The importance of Cretaceous synsedimentary tectonics is emphasised on the regional scale by variations in thickness and gaps in deposition marking the early mid-Cretaceous and late Senonian deposits. The regional stress field related to the Albian– Cenomanian extension is obtained by automatic interpolation of the pure extensional to radial extensional stress tensors associated with the event ('7 , Figs. 13 and 14). The extensional stress field is characterised by uniform trajectories of the horizontal stress ¦3 which are oriented N90–100ºE and imply normal strike-slip displacement along the NE–SW

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Fig. 17. Palaeostress field related to the N20–30ºE late Burdigalian–Langhian compression: 1 D thrust fault; 2 D anticline axis; 3 D strike-slip fault; 4 D ¦1 trajectory; 5 D pure compressional stress tensor (¦3 vertical, R D 0:5); 6 D pure strike-slip stress tensor (¦2 vertical, R D 0:5); 7 D pure extensional stress tensor (¦1 vertical, R D 0:5); 8 D uniaxial compressional stress tensor (¦1 horizontal, R D 0, ¦2 # ¦3 ) related to strike-slip compressional deformation; 9 D late Burdigalian–Langhian autochtonous marine deposits of the foreland.

Sellaoua, Djaffa, Oum Settas, El Kantour Faults and the NW–SE Nif En Nser Fault (Fig. 15). Normal displacement of the NE–SW synsedimentary faults also controlled the emplacement of the carbonate platform and the lateral transition to deep basin marine facies in the northwest and southeast of the area. These observations fit in with the works of Durozoy (1960) and Voute (1967) which show that the NE–SW faults are palaeogeographic boundaries delimiting the Constantinois platform domain from the deeper neighbouring domains. After a period of Early Cretaceous rifting (Guiraud and Maurin, 1992), the African plate underwent a sudden change in stress regime in the late Santonian (ca. 84 Ma) because of convergence between Africa and Europe (Savostin et al., 1986; Guiraud, 1997; Guiraud and Bosworth, 1997).

Guiraud (1997) shows evidence of compressional or wrench-dominated events during late Santonian and latest Maastrichtian times along the Northern African Tethyan Margin. The Central Constantinois was subjected during the latest Maastrichtian to a compressional N–S phase related to the E–W synsedimentary folds structuring the platform and cover ('6 , Figs. 10–12 and Fig. 14). However, some of the N–S compressional stress tensors characterised by the analysis of the brittle microfaults affecting the Barremian and Aptian deposits of the carbonate platform ('6 , stations 5, 55, 59, 70, 71, 81 and 85, Fig. 13) may also be attributed to the late Santonian N–S compression. The compressional stress field obtained by interpolation of data displays uniform trajectories of horizontal stress ¦1 oriented in a mean N–S direc-

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Fig. 18. Palaeostress field related to the N170ºE Late Miocene compression: 1 D thrust fault; 2 D anticline axis; 3 D strike-slip fault; 4 D ¦1 trajectory; 5 D compressional stress tensor (¦3 vertical, 0:6 > R > 0:2); 6 D strike-slip stress tensor (¦2 vertical, 0:4 > R > 0:1); 7 D pure strike-slip stress tensor (¦2 vertical, R D 0:5); 8 D uniaxial compressional stress tensor (¦1 horizontal, R D 0, ¦2 # ¦3 ) related to strike-slip compressional deformation.

tion (Fig. 16). The orientation and nature of the pure compressional, pure strike-slip and uniaxial stress tensors display the reactivation of left-lateral and right-lateral strike-slip of NE–SW and NW–SE faults controlling the distribution of E–W folds on the regional scale. 4.2. Cenozoic compressional phases: emplacement and structuring of the thrust sheets The first Cenozoic tectonic phase detected by microtectonic analysis corresponds to E–W compression probably of Late Eocene age ('5 , Figs. 10 and 14). This phase of deformation is related to smallscale regional structures only, consisting of NW–SE and NE–SW strike slip-faulting (Bourma–Regada Fault). The microtectonic and tectonic analyses linking the Late Miocene deformations result in two N20º–

30ºE and N170ºE compressional phases ('3 and '4 , Fig. 14). The results of the quantitative study of Miocene tectonics further show that the stress tensors associated with the structuring of the Numidian thrust sheet are comparable with those defined in the Miocene marine deposits of the foreland (Fig. 9). The Late Miocene phases therefore affected the Numidian flysch and the Miocene deposits of the foreland. During the late Burdigalian–Langhian, the first N20º–30ºE compression was related to the tangential tectonics of thrust-sheet emplacement (Fig. 17). On the regional scale, the palaeostress field was characterised by pure compressional tensors affecting the Numidian flysch thrust sheet (¦3 vertical: stations 66, 93 and 96, Fig. 17) and by pure compressional, pure strike-slip to uniaxial compressional stress tensors in the foreland, in the vicinity of the Oum Settas and Sellaoua Faults. In the northeast of

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Fig. 19. Palaeostress field related to the N140–150ºE latest Miocene–Plio–Quaternary extension (ADF D Aicha Debar Fault, EKF D El Kantour Fault, OSF D Oum Settas Fault, BRF D Bourma–Regada Fault, SF D Sellaoua Fault); 1 D normal fault; 2 D strike-slip fault; 3 D ¦3 trajectory; 4 D extensional stress tensor (¦1 vertical, 0:7 > R > 0:2); 5 D radial extensional stress tensor (¦1 vertical, R D 0, ¦2 # ¦3 ); 6 D uniaxial extensional stress tensor (¦3 horizontal, R D 1, ¦1 # ¦2 ) related to strike-slip extensional deformation; 7 D Plio–Quaternary trough.

the study area, the emplacement of the thrust sheet in a N200ºE direction was associated between the Constantine and Guelma areas with the formation of a N120ºE thrust sheet front dipping gently NE and the genesis of N120ºE folds. In the southeast, uniaxial compressional stress tensors were related to strike-slip compressional deformation exhibited by NW–SE folds and by reverse and sinistral reactivation of NW–SE and NE–SW faults (stations 53, 88 and 106, Fig. 17). The second Late Miocene N170ºE compressional phase was characterised on the regional scale by a palaeostress field with horizontal stress trajectories ¦1 oriented N160ºE to N180ºE (Fig. 18). N170ºE shortening of the Numidian thrust sheet was associated with N60º–80ºE asymmetrical folds deforming the first generation N120ºE folds and N60º–80º reverse faults affecting the thrust sheet front. In the southwest of the area the N170ºE compression was

related to a large set of N60º–80ºE folds and the reverse displacement of the NE–SW Oum Settas and Sellaoua Faults. 4.3. Latest Miocene–Pliocene–Quaternary extensional and late Quaternary to present-day compressional phases The palaeostress tensors associated with the latest Miocene–Pliocene–Quaternary N140º–150ºE extensional and N130º–150ºE late Quaternary compressional phases were characterised by the microtectonic analysis of microfaults observed in the Pliocene–Quaternary continental formations ('1 and '2 , Fig. 8). Many tensors of the same type and orientation are recorded in the underlying autochthonous and allochthonous units and are therefore related to the two recent phases ('1 and '2 , Figs. 9, 10 and 13). The palaeostress field related to the first latest

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Fig. 20. Palaeostress field related to the N130–150ºE late Quaternary to present-day compression: 1 D thrust fault; 2 D anticline axis; 3 D strike-slip fault; 4 D ¦1 trajectory; 5 D compressional stress tensor (¦3 vertical, 0:6 > R > 0:2); 6 D pure strike-slip stress tensor (¦2 vertical, R D 0:5); 7 D strike-slip stress tensor (¦2 vertical, 0:3 > R > 0:15); 8 D uniaxial compressional stress tensor (¦1 horizontal, R D 0, ¦2 # ¦3 ) related to strike-slip compressional deformation.

Miocene–Pliocene–Quaternary extensional episode is characterised by predominant pure extensional tensors (¦1 vertical, 0:7 > R > 0:2) and by horizontal stress ¦3 trajectories with a mean azimuth of N140º– 150ºE (Fig. 19). Locally uniaxial extensional tensors (¦3 horizontal, R D 1, ¦1 , ¦2 ) and radial extensional tensors (¦1 vertical, R D 0, ¦2 , ¦3 ) have been found both in the post-thrust sheet unit and in the older autochthonous and allochthonous formations. The NW–SE orientation of the horizontal stress ¦3 and the extensional nature of the regional stress field caused the normal displacement of the major N45–70ºE faults and the normal strike-slip activation of the N120ºE faults. These synsedimentary faults bound and separate the variously sized Pliocene– Quaternary extensional basins such as the Mila, Con-

stantine, Tamlouka and Guelma Basins. The N70ºE faults in the southwest of the area are associated with small grabens such as the Tolba and Timetlas Basins. In late Quaternary times the deformation regime changed radically to one of N130º–150ºE regional compression (Fig. 14). Interpolation of all of the compressional to strike-slip N130º–150ºE tensors identified in the Miocene–Pliocene–Quaternary sediments and in the earlier formations shows the regional palaeostress field associated with late Quaternary compression (Fig. 20). The uniform N130º– 150ºE trajectories of the horizontal stress ¦1 show the reverse and reverse left-lateral reactivation of N70ºE and N45ºE faults and provide a coherent explanation for the formation of the N60º–70ºE Quaternary fold system.

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Fig. 21. Summary of the Mesozoic–Cenozoic tectonic phases and their associated regional structures and palaeostress states (Central Constantinois zone, northeastern Algeria).

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5. Conclusions The microtectonic and tectonic studies presented in this work characterise the main Mesozoic– Cenozoic tectonic phases in the Central Constantinois and emphasise the preponderant control exercised over the structuring of the area by antecedent NE–SW and NW–SE faults (Fig. 21). In this area of northeast Algeria the Cretaceous was defined by Albian–Cenomanian extension followed by a latest Maastrichtian compressional phase that is characteristic of the Northern African Tethyan Margin (Guiraud, 1997; Guiraud and Bosworth, 1997). The initial extension was marked by N90– 100ºE extension on the regional scale and the normal displacement of NE–SW synsedimentary faults which closely controlled the distribution of carbonate platform and deep basin facies. During the latest Maastrichtian the extensional tectonism gave way to a compressional phase that is in the carbonate platform strata and in the cover by E–W folding and by the reverse strike-slip movement of NE– SW faults. The synsedimentary nature of the E–W folds is demonstrated by the observation of slumps and erosional truncation associated with increasingly prominent gaps along the anticline hinges (Aris, 1994). In Late Eocene times the E–W compressional phase revealed by microtectonic analysis cannot be related to major regional structures. However, from the relative chronology of the deformation phases and associated palaeostress tensors, the E–W compression can be related to the late Lutetian phase that is also seen in both the foreland (Guiraud, 1973, 1975; Guiraud et al., 1987) and in the internal zones of the Alpine range of Algeria (Lahonde`re, 1987). The emplacement of thrust sheets in the late Burdigalian was associated with N20º–30ºE compression. The flysch thrust sheets were also characterised by a purely compressional deformation associated with SW transport while the foreland was marked by a strike-slip compressional deformation. The second compressional stage, which rapidly followed the major overriding thrust, was linked in the Late Miocene with a N170ºE regional shortening. This latter explains the structuring of the thrust sheets and of the foreland by a set of N60º–80ºE folds and reverse faults. Vila (1978, 1980) emphasised the existence

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of a tangential tectonic phase in the Tortonian that was responsible for the southward overthrusting of the whole Constantinois platform. It seems, however, that the N170ºE shortening was of moderate amplitude on the regional scale (Aris, 1994). The tectonic episodes dating from after the emplacement and structuring of the overriding thrust sheets took place in the latest Miocene–Pliocene– Quaternary. These episodes corresponded essentially to N140º–150ºE latest Miocene–Pliocene– Quaternary extension followed by N130º–150ºE late Quaternary compression, that is representative of the present-day stress field. The opening of the latest Miocene–Pliocene–Quaternary post-thrust-sheet basins is related to NE–SW extensional structuring. Acknowledgements We are grateful for the useful reviews and critical comments made by R. Guiraud and an anonymous reviewer. References Ait, M.O., Ge´lard, J.P., Suzzoni, J.M., Gery, B., 1991. De´formations post–nappes et pale´ocontraintes enregistre´es dans le bassin mioce`ne de Tizi-Ouzou (Grande Kabylie). Bull. Office Natl. Ge´ol. 1 (2), 35–47. Angelier, J., Goguel, J., 1978. Sur une me´thode simple de de´termination des axes principaux des contraintes pour une population de failles. C.R. Acad. Sci. Paris 288, 307–310. Angelier, J., Manousis, S., 1980. Classification automatique et distinction des phases superpose´es en tectonique de faille. C.R. Acad. Sci. Paris 290, 651–654. Aris, Y., 1994. E´tude tectonique et microtectonique des se´ries jurassiques a` quaternaires du Constantinois central (Alge´rie nord-orientale): caracte´risation des diffe´rentes phases de de´formation. Thesis, Nancy, 302 pp. (unpubl.). Bergerat, F., 1987. Pale´o-champs de contraintes dans la plateforme europe´enne au front de l’oroge`ne alpin. Bull. Soc. Ge´ol. Fr. 8, 611–620. Bouillin, J.-P., Raoult, J.F., 1971. Pre´sence sur le socle kabyle du Constantinois d’un olistostrome lie´ au charriage des flyschs; le Numidien peut-il eˆtre un ne´o-autochtone? Bull. Soc. Ge´ol. Fr. 7, 338–362. Bouillin, J.-P., Ge´lard, J.-P., Leikine, M., Raoult, J.F., Raymond, D., Te´fiani, M., Vila, J.M., 1970. De´finition d’un flysch massylien et d’un flysch maure´tanien au sein des flyschs allochtones d’Alge´rie. C.R. Acad. Sci. Paris 270, 2249–2252. Carey, E., 1979. Recherches de directions principales de contraintes associe´es au jeu d’une population de failles. Rev. Geol. Dyn. Geogr. 21, 57–66.

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