Introduction

The Mitidja basin is a post-nappe Neogene basin in north-central Algeria (Fig. 1a). The basin is characterized by two important geological boundaries, one to the north of the basin and one to the south. The northern boundary lies between the basin and the Sahel anticline of Algiers, which borders the coastline. The southern boundary is a major geological contact between the Mitidja basin to the north and the Tellian Atlas to the south. This contact has been the focus of many multidisciplinary studies, including those concerned with geology, seismology, seismotectonics, and neotectonics. The zone is well marked with several deformed structures and has undergone several moderate to strong seismic events along numerous segments of active reverse and strike-slip faults (Fig. 1a) (Guemache 2010; Harbi et al. 2017; Yelles-Chaouche et al. 2017; Yelles-Chaouche et al. 2018; Maouche and Harbi 2018).

Fig. 1
figure 1

a Geological sketch of the Mitidja basin and the surrounding regions, adapted from the relevant geological map (Flandrin 1952). Yellow circles represent mainschocks of recent seismicity in Algiers province. The black focal mechanism corresponds to the most significant earthquakes in the region (Yelles-Chaouche et al. 2018, modified). b Geological cross section of the Mitidja basin (Glangeaud et al. 1952)

Full size image

The Hammam Melouane region is one of the most representative sites for investigating the structural geology of the southern margin of Mitidja basin. Rocks in this region have undergone several phases of tectonic deformation and can be used to elucidate the formation conditions of the Mitidja basin. Historic studies have investigated the geology of the Hammam Melouane area (Glangeaud 1932; Debrand Passard 1968; Bonneton 1977; Guemache 2010), with more recent work focusing on the seismology and seismotectonics of the region (Harbi et al. 2017; Yelles-Chaouche et al. 2017; Maouche and Harbi 2018).

In this study, our field investigations and structural analyses focus principally on the post-nappe Miocene period. In North Africa, the start of the Miocene signaled the end of the Alpine collision, an event that led to the formation of the Maghrebian belts and the opening of the Mediterranean Basin. The Miocene was an important phase for the genesis and development of Neogene basins in northern Algeria.

The aim of this study is to provide a detailed structural analysis of the cross section of the Hammam Melouane region, particularly investigating fracture networks in the Miocene outcrops in this region, which have not been previously studied. Furthermore, we examine folding and fold-related faulting in the region. We propose and discuss a polyphase tectonic evolution that occurred in two major phases. Lastly, we establish a structural model that summarizes the tectonic relationships between several structural features.

Geological setting

The Mitidja basin

The Mitidja basin is in north-central Algeria (Fig. 1a) and extends between latitudes 36.20°–37.00°N and longitudes 2.00°–3.80°E. The basin is oriented E−W and is 100 km long and 20 km wide. It is bordered to the north by the Sahel anticline; to the east by the region of Grande Kabylie; to the west by the Menaceur syncline and Mount Chenoua; and to the south by the Atlas Mountains of Blida, Bou Maad, and Tablat. The Mitidja basin was generated after the formation of nappes during the early Burdigalian (Wildi 1983) and is hence referenced to the post-nappe tectonic period. Mio-Plio-Quaternary deposits (Fig. 1b; Glangeaud et al. 1952) fill the sedimentary basin. These deposits unconformably overlie Miocene ante-nappes, the crystalline basement and its cover, and Meso-Cenozoic formations of the Tellian Atlas. The formation of the Mitidja basin occurred simultaneously with an episode of calc-alkaline volcanism (Glangeaud 1932; Glangeaud et al. 1952; Hernandez and Lepvrier 1979; Ait Hamou 1987; Abbassene 2016) to the south of the basin (Fig. 1a).

Hammam Melouane cross section

The Hammam Melouane region is located 30 km from the capital Algiers and sits in the transition zone between the post-nappe Mitidja basin and the Blida and Tablat Atlas mountains (Fig. 2a). Sediments of the Tellian Atlas of Blida and Tablat were deposited during the Cretaceous and belonged to the “Schistosity Massifs” (Blès 1971; Kirèche 1993), part of the internal Tellian domain. Morphologically, this transition zone offers a visible contrast between the mountainous area to the south and the flatter Mitidja plains that stretch northwards to the Algerian coastline.

Fig. 2
figure 2

a Satellite image showing the study area and the locations of different analyzed sites (the yellow line shows the location of the studied cross section and the red circle represent volcanic outcrop). b Cross section of the Hammam Melouane region

Full size image

The geology of the Hammam Melouane region consists mainly of post-nappe Miocene deposits that unconformably overlie Ante-nappes formations. The whole Miocene series is folded and overlapped (Fig. 2b) by the Tellian Atlas formations from the Cretaceous (Senonian).

From the microfauna abundance, Bonneton (1977) assigned an early Miocene age (late Burdigalian) to the limestone deposits and a middle Miocene age (Langhian−Serravallian) to the marls, sandstones, and sand deposits. Importantly, we note a lack of Tortonian deposits. In Fig. 3, we distinguish from bottom to top: (1) a clayey limestone basement with schistosity formed during the Cretaceous (Senonian); (2) a red conglomerate from the late Burdigalian, unconformably deposited on the Cretaceous rocks; (3) massive limestones from the late Burdigalian, straighted vertically with varying thicknesses; (4) a series of marls of Langhian−Serravallian age; (5) sandstone, sands, and marls with intercalated conglomerates, representing the continuation of the Langhian−Serravallian series; (6) a conglomerate series comprising claystone with limestone pebbles and calcareous sandstone from the Pliocene (Astian); and (7) Quaternary alluvium. We also note the presence of volcanic rocks, shown by the notation “VR” in Fig. 3, interstratified with the early Miocene marine formations.

Fig. 3
figure 3

Lithostratigraphic column of the Hammam Melouane area (modified from Bonneton 1977). 1 Cretaceous (Senonian) folded and metamorphosed limestone basement. 2 Late Burdigalian conglomerate. 3 Late Burdigalian limestones. 4 Langhian−Serravallian marls. 5 Continuation of the Langhian−Serravallian series (sandstone, sands, and marls with intercalated conglomeratic layers). 6 Pliocene conglomerate. 7 Quaternary alluvium. VR, volcanic rocks

Full size image

Hammam Melouane volcanism

The volcanic rocks in the study area (shown by the red area in Fig. 2a) are part of the tertiary volcanic deposits that border the Mitidja basin along its southern edge and extend to the Menaceur syncline (Fig. 1a). These rocks comprise tuffs, rhyolite, dacite, and andesite. The rocks are calc-alkaline (Glangeaud et al. 1952; Hernandez and Lepvrier 1979; Ait Hamou 1987; Abbassene 2016) and are closely related to the opening of the Mitidja basin. The volcanic rocks are interbedded within marine formations from the late Burdigalian and Langhian−Serravallian. The marine sediments have enabled dating of the volcanic units from the late Burdigalian to the Tortonian (16–9 Ma), which is in accordance with the radiometric age (Bellon and Letouzey 1977). In the Hammam Melouane area, volcanic rocks crop out on the western bank of the El Harrach River. These dacites and tuffs have previously been studied by Glangeaud (1932) and Bonneton (1977).

Methodology and material

We conducted detailed field investigations across sites in the Hammam Melouane region (Fig. 2a), from the southern boundary of the Mitidja basin to the Tellian Atlas belt. We used a regional map (Flandrin 1952) and aerial and satellite photos to identify the locations of Miocene formations. The Miocene strata are highly deformed and exhibit fractures (faults and joints) and folds. We conducted a comprehensive suite of structural measurements, including strikes and dips of faults, striae, and fold axes. We measured the kinematics and geometry of faults and joints using a compass clinometer and recorded the location of each measurement in UTM coordinates using a GPS. These data were measured at four sites across the Hammam Melouane region.

Crosscutting relationships of fault planes, joints, and fold-related faults reveal polyphase tectonic deformation during the Miocene, for which we distinguish two major phases. We identify an extensive phase involving normal faults, synsedimentary structures, and tension gashes, as well as a compressive phase characterized by folds and reverse, thrust, and strike-slip faults. We used the Win-Tensor program (Delvaux 1993; Delvaux and Sperner 2003) to calculate the paleostress tensors of each phase using the Right Dihedron method. For this analysis, we selected only those faults affecting Miocene formations that had well-defined kinematic indicators (e.g., striae, calcite steps, crag-and-tail markings). The Win-Tensor results showed a quality ranking of E owing to the small number of data points and the wide dispersion of these data. The results of the stress tensor inversion indicate a NW−SE direction for the extensive stress regime and a NNW−SSE direction for the compressive stress regime. We summarize the results of mapping, field measurements, and analysis and interpretation of structural data collected from the Hammam Melouane area in a structural model.

Results

Analysis of deformation in the study area

Miocene outcrops in the Hammam Melouane region are an important record of tectonic deformation and include several folds (Bonneton 1977) and faults. Joints and faults at different scales record brittle deformation of the Miocene rocks. In this study, we provide detailed results for folding structures. In particular, we analyze the distribution of folds and their relationship to brittle deformation through analysis of several fault systems.

To highlight these structural elements, we selected four sites for investigation (Fig. 2a). Site I: the reoriented northern limb of the Rocher des Pigeons syncline; site II: the Rocher des Pigeons syncline; site III: the Cretaceous−Miocene contact area; and site IV: the Tiamamine River syncline.

Folds in the study area

Kilometer-scale folds occur throughout the Miocene series. A succession of synclines and anticlines characterize this transitional zone over a transverse distance of about 3 km (Fig. 2b). From north to south, we identify the following structures (Fig. 4): an anticline (F1) that deforms the Eocene formations; a syncline (F2) that deforms the late Burdigalian limestones on the western bank of the El Harrach River; the Tiamamine River syncline (F3) in the Langhian−Serravallian formations; an anticline (F4) with an Oligo-Miocene core; and the “Rocher des Pigeons” syncline (F5) with fold limbs of Burdigalian limestones and a Langhian–Serravallian marl core.

Fig. 4
figure 4

Locations of mapped synclines (red) and anticlines (yellow) in the study area

Full size image

Analysis of fold wavelengths showed an increasing intensity of deformation from north to south. The F1 anticline and F2 syncline dip gently to the WSW and are slightly deformed. The F4 anticline and the F3 and F5 synclines are intensively deformed. The F5 fold is particularly deformed and shows a pinched shape marked by a subvertical axis and steeply dipping sediment beds.

In the field, synclines are well preserved, whereas anticlines have undergone significant erosion that has removed their ancient cores. We explore folds F3 and F5 in detail, as they are well preserved and are representative of folds across the Hammam Melouane region. The identification of the other folds (F1 and F4 anticlines, F2 syncline) helps determine the logical succession of the different folds of the region.

Rocher des Pigeons syncline

The Rocher des Pigeons syncline (RPS) (Figs. 4(F5) and 5a) trends WNW−ESE and dips 70°ESE, and its plunging axis is located on the western bank of the El Harrach River. The near-vertical fold limbs are late Burdigalian limestone, and the fold core consists of gray Langhian−Serravallian marls rich in microfauna (Bonneton 1977). On the western bank of the El Harrach River (hinge zone; Fig. 5b), the fold is pinched. Limestone layers of the northern limb trend 80°N and dip 70°SSE, whereas the southern limb dips 80°NW. On the right bank of the El Harrach River, limestone layers show a reverse dip direction. The southern limb trends 60°−80°N and dips 35°−75°SSE, whereas the northern limb trends 40°N and dips 70°SE. These direction and dip measurements indicate a 40° northward rotation from the western to eastern bank of the El Harrach River. This rotation is a result of a sinistral strike-slip fault, which broadly aligns with the El Harrach River and trends at approximately 30°−40°N. Furthermore, the RPS structure is affected by numerous fractures, well marked in the limestone formations, as well as several faults in the fold limbs. Normal, reverse, strike-slip (both dextral and sinistral), and composite faults are observed at different scales.

Fig. 5
figure 5

a Satellite image and b interpretative sketch of the Rocher des Pigeons syncline

Full size image

Tiamamine River syncline

The Tiamamine River syncline (TRS) is located in the north of the study area (Fig. 4(F3) and Fig. 6a). The axis of this fold is oriented NE−SW and dips approximately 60°NE. The perisynclinal closure, located on the western bank of the Tiamamine River, consists of 350 m high of yellowish sandstone cliffs. On the eastern fold limb flank, the sandstone formation dips 45° towards the NNW (Fig. 6b). We distinguish poorly consolidated red, yellow, and white sands in the fold limbs. On the eastern bank of the Tiamamine River and on the western limb of the syncline, yellowish conglomerate beds overlie the sands and dip 45° towards the ESE. These formations are interstratified with gray clay marls, which form the core of the fold. This lithology dates to the Langhian−Serravallian of the middle Miocene (Bonneton 1977). On the eastern bank of the Tiamamine River, the western limb of the syncline is approximately 1000 m long, and its northern end coincides with the southern boundary of the Mitidja basin. Field measurements of the western limb show a change in direction, whereby the fold axis trend rotates towards the north, from 190°N through to 220°N. This limb evolution is terminated by a deep valley through which a river flows. The changes in direction of the fold limb are controlled by a dextral fault trending 120°N (Fig. 6b(F3)). This fold shows other decametric faults, for example, a sinistral strike-slip fault trending 20°N (Fig. 6b(F1)) and a dextral strike-slip fault trending 120°N (Fig. 6b(F2)).

Fig. 6
figure 6

a A satellite image and b an interpretative sketch of the Tiamamine River syncline

Full size image

Analysis of fracturing

A multidirectional network of fractures and faults, affecting Miocene formations, shows brittle deformation of the Hammam Melouane region. This brittle deformation is closely related to the fold series described above, forming characteristic fold-related faulting structures. To examine the relationship between folding and faulting, we performed structural analysis at different selected sites (Fig. 2a) to define the different parameters of fracturing (e.g., direction, dip, nature) and establish chronological links with post-nappe Miocene sedimentation.

Site I

Site I corresponds to the reoriented northern limb of the RPS (Fig. 7a). The late Burdigalian limestones show a characteristic brittle deformation with brecciation (Fig. 7b) linked to a complex fracture network (Fig. 7, stereographic projection). The fracture network consists of various joints, reverse faults, and normal faults (Fig. 7a). The faults tilted to the south are reverse faults, but those tilted northwards show a normal sense of movement with striated planes and calcite step markers (Fig. 7c). This site shows clear evidence of extensional tectonic deformation in the late Burdigalian limestones, supported by well-marked normal faults. Secondary deformation followed the generation of these normal faults to produce the northern vergence of the reverse faults.

Fig. 7
figure 7

Site I features. a Burdigalian limestone showing a multidirectional fracture network. b Prismatic structures in the Burdigalian limestone. c A striated normal fault plane. The rose diagram shows the principal directions of fractures and faults with the stratification orientation in red

Full size image

Site II

Site II corresponds to the southern continuation of site I and contains a sinistral strike-slip fault that trends 40°N. The El Harrach River fault is responsible for the reorientation of the RPS structure (Fig. 5b) and is visible in the limestone formations. On the eastern bank of the El Harrach River, the late Burdigalian limestones of the southern fold limb trend 80°N, dip 75°SSE, and are strongly affected by several fault systems. A road cutting (Fig. 8a) exposes a longitudinal section of these limestones. We identify a reverse fault (Fig. 8b) oriented 60°N with 70°SE dip, and a normal fault with slickensides (Fig. 8c) trending 250°N with a dip of 50°NW and a pitch of 70°NW.

Fig. 8
figure 8

Site II features. a Photograph showing the intersection of two different fault planes. b Reverse fault F2. c Normal fault with striation F1

Full size image

On the western bank of the El Harrach River, limestone beds trend 80°−95°N and dip 85°−90°NW. We also identified the following fracture networks in these limestones:

  1. (1)

    A centimeter-scale listric fault network imparting a micro-graben structure (Fig. 9a).

  2. (2)

    Karstic features, formed by water circulation through the El Harrach River fault and other fracture networks, causing dissolution of the limestone formations. Limestone dissolution due to sulfate-laden water leads to yellow−ocher rocks formation, which are characterized by irregular-shaped cavities termed “cargneules” (Fig. 9b).

  3. (3)

    Tension gashes trending 40°N, with a calcite fiber filling. The opening direction is parallel to the mineral fiber growth, giving a NW−SE orientation, as shown by the stress ellipsoid in Fig. 9c.

Fig. 9
figure 9

Site II photographs of microstructures observed in Burdigalian limestones. a A micro-graben. b Cargneules. c Tension gashes

Full size image

Decametric strike-slip faulting is also recorded in the Burdigalian limestones and intensively deforms the limbs and hinge of the RPS. One of these strike-slip faults (Fig. 10a) trends 90°N and dips 80°S. This fault surface shows striations at 45°SW with calcite step markers (Fig. 10b). The movement indicators on the surface suggest a component of both dip-slip and strike-slip faulting (oblique-slip faults), implying dextral transtension motion.

Fig. 10
figure 10

Site II features. a Photograph of a decametric fault surface in the late Burdigalian limestones. b Tectoglyphs and sense of displacement of the missing bloc indicated by the black arrow

Full size image

Site III

The outcrops at site III expose rocks deposited between the Cretaceous and the post-nappe Miocene. There is also a reverse fault with a northern vergence. The fault surface is not visible at the site, but it covers a large zone (about 300 m) where Cretaceous schist and late Burdigalian red conglomerate are highly deformed. Escarpments, reverse faults, and overlapping show the intensity of the deformation. Escarpments oriented E−W to NE−SW (Fig. 11a) clearly mark the landscape and indicate a northern vergence for the overlapping. The Cretaceous formations overlap the post-nappe Miocene deposits. The late Burdigalian red conglomerate beds are overturned to the north (Fig. 11b), trending 110°N and dipping 70°SW. The schist shows syn-schistosity folding with fold hinges associated with fracturing, suggesting intense frictional motion.

Fig. 11
figure 11

Site III features. a Satellite image showing escarpments of the overlapping faults (white arrows). b Miocene conglomerate beds displaced northwards

Full size image

Site IV

The TRS structure crops out in site IV, located on the edge of the Mitidja basin. This syncline has been affected by brittle tectonic deformation, expressed by deformation of the fold limbs, associated with decametric strike-slip faults and the overlapping of Cretaceous formations in the eastern part of the syncline. The strike-slip faults occur in the Langhian−Serravallian sandstone, with one set of dextral faults trending WNW−ESE and a set of sinistral faults trending NNE−SSW. The F1 fault surface (Fig. 12a) is oriented 20°N with a dip of 80°ESE, showing horizontal striations with calcite steps (Fig. 12b) and crag-and-tail markings (Fig. 12c) suggesting sinistral strike-slip movement. The F2 faults trending 120°N deform the TRS structure by dextral motion highlighted by torsion of the fold limbs (Fig. 6b). The same fault orientation (F2, 120°N) affects the Tiamamine River in the north (Fig. 6(F3)), where dextral motion is revealed by direction changes of the western fold limb. We measured the following strike directions from south (Tiamamine River) to north: 190°N, 200°N, and 220°N.

Fig. 12
figure 12

Site IV features. a Fault surface in the Langhian−Serravallian sandstone. b Calcite steps markers. c Crag-and-tail markings showing sinistral motion (the red arrow indicates the sense of displacement)

Full size image

Polyphase tectonics

The geological structures in the Hammam Melouane region are complex, as reflected by the range of post-nappe deformation structures (folds, joints, and faults). Our structural model (Fig. 13) summarizes the relationships between these deformation structures. It shows that folds and faults are closely related, giving a fold-related faulting complex. The strike-slip faults are responsible for the shape of the deformed Miocene strata. In addition, the Cretaceous rocks of the Tellian Atlas overlap Miocene formations through ENE−WSW to NE−SW-oriented thrusting faults. Detailed structural analysis and the crosscutting relationships between folds and faults reveals polyphase tectonic deformation during the Miocene. We identify two major tectonic phases responsible for deformation of the Miocene formations.

Fig. 13
figure 13

Structural model of the Hammam Melouane region

Full size image

Extension phase

The new structural data from this study from sites I and II reveal an extensional regime during the early Miocene. This regime is well preserved in the late Burdigalian marine formations in the study area. The limestones record synsedimentary structures, normal faults, tension gashes, and thin-section scale grabens, all of which support an extensional tectonic regime. The late Burdigalian limestones of the reoriented northern limb of the RPS (site I) are the most representative of this type of tectonic regime. Structural analysis confirms the normal fault motion, as indicated by smooth fault surfaces, striated fault planes, and calcite step markers (e.g., Figs. 7 a and c). The limestones at site II show additional extension structures. Metric and decametric normal fault motion (Figs. 8c and 10), centimetric grabens, listric faults (Fig. 9a), and tension gashes (Fig. 9c), show clear evidence of an extensional regime. The Langhian−Serravallian sandstone deposits do not show evidence of normal faulting, but we suggest that it may have been masked by the later phase of deformation or removed by erosion.

Using the fault-slip data from normal faults, we calculated the paleostress tensor for the late Burdigalian limestones. Only five fault-slip data were used as inputs for the Win-Tensor program. The small number of data and the insufficient diversity in orientations of fault planes and slip lines (close orientations) give a quality ranking of E. The improved Right Dihedron method (Delvaux and Sperner 2003) gives a mean orientation of the principal stress axes of σ1 at 76/052 (vertical axis), σ2 at 14/231, and σ3 at 00/321. The stress ratio of minimum to maximum stress is estimated at 0.5. These results suggest a purely extensional stress regime trending NW−SE (14).

Fig. 14
figure 14

Stereographic projection results for the normal fault data and the stress inversion

Full size image

Compression phase

Folding and overlapping

The extensional regime was followed by a compressional tectonic regime. This compression affected all of the post-nappe Miocene formations in the Hammam Melouane region, forming a succession of synclines and anticlines, and overlapping with a northern vergence. The folding developed across a deformation zone that was approximately 3 km wide and extended from the Cretaceous metamorphic deposits to the edge of the Mitidja basin. The degree of deformation in the Mitidja basin−Tellian Atlas transition zone progressively increases to the south, resulting in pinched and highly deformed folds (e.g., the RPS). This folding was followed by north-verging overlapping, in which Cretaceous (Senonian) strata overlap the post-nappe Miocene formations. A reverse fault (Figs. 7a and 8b), overturned the upper Burdigalian red conglomerate beds (Fig. 11b) and limestone beds. The E−W escarpments (Fig. 11a) demonstrate this overlapping.

The structural directions provided by the fold axes, reverse faults, overlapping faults, and escarpments indicate a N−S- to NNE−SSW-directed compressive stress. This compression superimposes the late Burdigalian extensional structures and the Langhian-Serravallian deposits.

Strike-slip faults

Strike-slip faulting affects all units in the Miocene sedimentary series and is closely linked to folding and the formation of fold–fault complexes. The fold-related faulting relationship can be examined along the deformed fold limbs of the two synclines (the RPS and TRS). We identify two major categories of strike-slip faults: longitudinal dextral faults and transverse sinistral faults, in the RPS and TRS.

  1. (1)

    The RPS structure: this kilometric fold is pinched, and its limbs have been reoriented and now trend NE−SW. This change of direction is clear on the western bank of the El Harrach River, where the limbs trend 80°N on the western bank but 40°N on the eastern bank. This twisting (Fig. 13) can take place only if the fold is associated with a fault. The associated fault is a sinistral strike-slip fault and follows the direction of the El Harrach River (40°N).

  2. (2)

    The TRS structure: a large deep river crosscuts the northern portion of this fold, which indicates post-fold faulting (Fig. 6(F3)). This fault is oriented 120°N with a dextral strike-slip motion. We deduce this movement from the deformation of the lambs extremities. To the south, the sandstone formations show another fault (Fig. 6(F2)) that has the same direction and motion as the fault in the north. These two faults distort the western flank, giving an “S” shape. In this syncline, the sandstone formations on the western fold lamb record another strike-slip fault with a decametric fault plane. This fault aligns with the river bed. It is oriented 20°N, has a dip of 80°ESE, and has subhorizontal striae with a pitch of 5°. The motion of the fault is deduced from the crag-and-tail markers, indicating sinistral motion.

The strike-slip faults crosscut the Miocene formations and folded strata. Structural measurements from these faults allow the paleostress tensor to be calculated. Only three fault slip data points could be collected for input into the Win-Tensor program, resulting in a poor quality rank of E. The improved Right Dihedron method (Delvaux and Sperner 2003) gives a mean orientation of the principal stress axes of σ1 at 17/159, σ2 at 70/307 (vertical axis), and σ3 at 10/066. The stress ratio is estimated at 0.33. These parameters suggest a strike-slip stress regime trending NW−SE (Fig. 15).

Fig. 15
figure 15

Stereographic projection results for the strike-slip fault data and stress inversion

Full size image

Discussion

The Mitidja basin is one of several post-nappe Neogene basins in northern Algeria. Several multidisciplinary studies have discussed the origin and evolution of the Mitidja basin. The Miocene is a key period in the history of the basin, and in this study, we have identified polyphase tectonic deformation for this period. The first phase of deformation identified in the Hammam Melouane area is an extensional phase that occurred from the late Burdigalian to the Langhian−Serravallian. Several authors have described this Miocene extension across northern Algeria. Bonneton (1977) described NNE−SSW and WNW−ESE trending normal faults, as recorded in middle Miocene formations in the Meftah region, which is ~ 20 km east of the Hammam Melouane region. Guemache (2010) inferred an extensional tectonic regime on the basis of synsedimentary normal faults trending NE−SW to ENE−WSW in Oligo-Miocene formations in the northern edge of Mitidja basin. Calc-alkaline magmatism accompanied the subsidence of the Mitidja basin and confirmed an extensional regime localized at the edge of the basin (Bellon and Letouzey 1977; Hernandez and Lepvrier 1979; Ait Hamou 1987). Perrodon (1957) identified NNW−SSE trending extension during the early Miocene, which initiated the formation of the Chelif basin and volcanism, which continued until the late Miocene. Thomas (1985) defined a transtensional regime for the opening phase of the Chelif basin from the Serravallian to Tortonian. Aite (1994) described a NW−SE trending extensional phase in the Tizi-Ouzou basin recorded in early and middle Miocene formations.

At the scale of the Mitidja basin, the localized extension described by Bonneton (1977) does not fully represent the extensive deformation regime identified as affecting Miocene formations. Guemache (2010) determined an extensional regime using theoretical striations measurement from the surfaces of normal faults. At a larger scale, the extension phase during the early Miocene controlled the opening of post-nappe basins, thus shaping the structural geology of northern Algeria.

In our study area, well-marked synsedimentary normal faults, multiple extensional structures, and calc-alkaline volcanism imply an extensional regime trending NW−SE. These structures are recorded in the late Burdigalian marine formations, which constitute the first sedimentary fill in the Mitidja basin. The extensional stress regime could have affected the Langhian−Serravallian sandstones, although we found no evidence of normal fault motion. This evidence could, however, have been eroded or masked by the later phase of deformation. We suggest that the features indicating an extensional regime during late Burdigalian age imply the genesis in extension of the Mitidja basin. Thus, this coastal and southern Mediterranean graben can be considered as an annex structure to the opening process of the Mediterranean Basin.

The second deformation phase identified in the study area is the well-known Tortonian inversion, which followed the late Burdigalian extensional phase. The deformation processes in this phase included folding, thrusting, and strike-slip faulting, which are clearly recorded in several post-nappe basins in northern Algeria. On the southern edge of the Mitidja basin and Blida Atlas, this phase created NE−SW- to E−W-trending thrust faults and folds with a subvertical axis. This phase has been described by Thomas (1985) in the Chelif basin, located to the west of the Mitidja basin, where deposits indicate a transpressional zone with a maximum horizontal stress-oriented NW−SE. In the Tizi-Ouzou basin to the east, Aite (1994) distinguished a compressional regime from the late Serravallian to the Tortonian, indicated by folds trending 70°N and thrusting of the Kabylian basement over the post-nappe sediments. Recanati et al. (2019) used new onshore data and thermal modeling to demonstrate that major exhumation of the Algerian margin occurred during the Tortonian, thus confirming the resumption of compression along the Tellian belt.

Previous studies have not investigated the fault networks in this part of the Mitidja basin. In this study, we have explored these faults and provided detailed structural analysis of the relationships between fault networks and folding in the Miocene formations. In the Hammam Melouane area, analysis of crosscutting and the relationship between faults and fold-related faulting allow reconstruction of the deformation history during the Tortonian. At the beginning of the Tortonian, kilometric folds deformed the Burdigalian and Langhian−Serravallian formations. Thrust faults followed the folding and caused overlapping of the Miocene series by the Cretaceous formations of the Blideen Atlas, with north-verging thrusting. The compressional regime continued and deformed folds during strike-slip faulting tectonic. The RPS is a well-deformed fold with a subvertical axis that was reoriented by a sinistral strike-slip fault, from an E−W direction to NE−SW (shown by deformation of the fold limbs). The TRS shows a sinistral strike-slip fault that trends 20°N and a dextral strike-slip fault that trends 120°N.

Importantly, there is a lack of Tortonian sediments in our study area. We identified an angular unconformity between the Pliocene deposits and the early−middle Miocene series. We did not identify any similarities in deformation features in units deposited during the Miocene and those deposited during the Pliocene. Therefore, we suggest that the compressional tectonic regime (involving folds, thrust faults, and strike-slip fault motion) also occurred during the Tortonian.

The deformation we identify in the Tortonian phase agrees with previous studies. The Tortonian phase was crucial in determining the shape of the Mitidja basin and other post-nappe basins in northern Algeria. The compressional regime initiated during the Tortonian phase was maintained throughout the neotectonic period (late Miocene−Quaternary), and up to the present. Several neotectonic and seismotectonic studies confirm this hypothesis through the recent seismic activity of the Mitidja basin (Harbi et al. 2017; Yelles-Chaouche et al. 2017; Maouche and Harbi 2018; Mohammedi et al. 2019). These studies reveal the complexity of structures within the Mitidja basin, especially in its southern margin, where there have been several earthquakes in historic times.

Conclusion

The Neogene post-nappe basins of northern Algeria were formed after collision between the African and Eurasian plates, which created the Maghrebian belts. The Miocene was an important period for the genesis and evolution of these basins. The Mitidja basin is one such Neogene basin in north-central Algeria. Various seismological, seismotectonic, and neotectonic studies (Harbi et al. 2017; Yelles-Chaouche et al. 2017; Maouche and Harbi 2018; Mohammedi et al. 2019) have demonstrated the active nature of the Mitidja basin, in which several moderate to strong earthquakes have been generated in historic times. The most active region of the Mitidja basin is its southern edge, where the Hammam Melouane study area is located.

Our geological field investigation of Miocene outcrops in the Hammam Melouane area reveals polyphase tectonic deformation. We performed a detailed structural analysis at different scales (centimetric to kilometric) and using the well-marked faults. Two main tectonic phases deformed the Miocene formations and created various fractures, faults, and folds. The first deformation phase was late Burdigalian extension. The deformation of this phase is recorded in the late Burdigalian limestones, in which we identified numerous synsedimentary extension structures, normal faults, centimetric micro-grabens, and tension gashes. The Burdigalian limestones are overlain by Langhian−Serravallian deposits.

The extensional regime was followed by a compressional regime, characterized by the formation of kilometric folds deforming the Burdigalian and Langhian−Serravallian formations. The development of thrust faults followed this folding, resulting in the overlapping of the Miocene series by the Cretaceous formations of the Blideen Atlas, with north-verging thrusting. The compressional regime continues and deforms folds in a strike-slip movement. The Rocher des Pigeons syncline has a subvertical axis that was reoriented by a sinistral strike-slip fault, from an E−W direction to NE−SW (shown by deformation of the fold limbs). The Tiamamine syncline shows a sinistral strike-slip fault trending 20°N and a dextral strike-slip fault trending 120°N. Importantly, there is a lack of Tortonian sediments in the study area. Pliocene deposits unconformably overlie the early−middle Miocene series, and no similarities in deformation structures were observed between the Miocene and Pliocene deposits. Therefore, we suggest that the compressional regime occurred during the Tortonian.

The Hammam Melouane site provides an opportunity for detailed structural analysis within the Miocene, which was an important interval for the genesis and evolution of the Mitidja basin. The polyphase tectonic deformation in the Miocene reflects the wider tectonic environment that controlled the development of Neogene post-nappe basins in northern Algeria. The superposition of the different deformation phases needs further analysis to more fully decipher the nature of these basins and their development.