The Northwestern (Maghreb) boundary of the Nubia (Africa) Plate

Tectonophysics 429 (2007) 21 – 44 www.elsevier.com/locate/tecto

The Northwestern (Maghreb) boundary of the Nubia (Africa) Plate Alain Mauffret Laboratoire de Tectonique, UMR 7072, Université P. et M Curie, 75256 4 Place Jussieu, Paris 6 Cedex, France Received 27 September 2005; received in revised form 10 July 2006; accepted 13 September 2006 Available online 13 November 2006

Abstract A study of the present compressional deformation of the Northwestern (Maghreb) Nubia (Africa) margin is derived from the analysis of more than 20,000 km of seismic profiles. In the western part the compression is distributed in a large zone with on-land compression in Algeria, mainly strike-slip deformation on the Algerian margin and folds and strike-slip faulting in Eastern Spain. In the middle of the Algerian margin, around Algiers, the evidences of compression become more obvious. In this area a ridge trending N–S that is interpreted as a middle to late Miocene spreading center interacted with the transpressional margin that trends E–W. North of the location of the Boumerdes–Zemmouri earthquake the oceanic crust is deformed by blind thrusts up to 60 km from the coast. These thrusts are south dipping and with the northward dipping thrusts located onshore form a wedge that maybe a positive flower structure at a crustal scale related to the right-lateral transpression of the margin. In the eastern part of the Northwestern (Maghreb) Nubia (Africa) Deformed Belt, off eastern Algeria and Tunisia, the deformation is more intense but limited to the north by the continental slope. Large late Miocene Tortonian folds are cut by the Messinian erosional surface but the present deformation is also evident. It is suggested that the deformation with a double vergence may be followed up to the north of Sicily. After the docking (18 Ma) of the Kabylies to the Africa Plate, the crust has been thinned and the Algerian Basin opened during the middle-late Miocene with an E–W direction. From the late Miocene to the Present the margin has been rethickened by transpression and uplifted. © 2006 Elsevier B.V. All rights reserved. Keywords: North Africa margin; Active tectonic deformation; Nubia

1. Introduction The formation of the Western Mediterranean back-arc Basin is related to the northwards convergence of the Nubia (Africa) Plate relative to the Eurasia Plate since the late Cretaceous (Olivet, 1996). The structure of the backarc basins (Provençal, Valencia Trough, Tyrrhenian) are well known thanks to the acquisition of geophysical data, academic (DSDP and ODP), well data and release of exploratory industrial wells and geological studies carried out in the emerged margins of the basins. However, the E-mail address: [email protected]. 0040-1951/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2006.09.007

core of the deformation, which is located along the North Africa margin, is yet poorly known. The plate boundary between Nubia (Africa) and Eurasia plates is clearly delineated in the Eastern Atlantic Ocean then becomes diffuse in the Alboran Sea and the adjacent areas in Spain and Morocco. Along the North Algeria the earthquakes, with reverse or strike-slip focal mechanisms compatible with NW–SE compression, are localized along a 200 km large stripe from the coast to in-land. The seismotectonics studies were focused on-land where south verging thrusts are predominant. The recent earthquake near Algiers evidenced that the offshore margin was also concerned with north verging thrusts. In order to precise the structure

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Fig. 1. Tectonic sketch of the Mediterranean Region (modified from Barrier et al., 2004).

of the North African margin from the Eastern Alboran Sea to Tunisia, the present study, based on the analysis and synthesis of seismic profiles, has been performed. 2. Geological and geophysical setting of North Africa North Africa can be divided (Fig. 1) into three main structural domains (Frizon de Lamotte et al., 2000): the continental African domain including the Atlas Ranges, the Tellian and Rif Thrust Belt (external Maghrebian Belt) and the internal massifs (internal Rif and Kabylies). It is generally accepted that the internal massifs formed by the internal Rif and Betics, Kabylies, Peloritan and Calabrian Massifs belonged to the Eurasia Plate (Bouillin, 1986)

before the opening of the Western Mediterranean Basin that occurred during the early Miocene (Aquitanian– Burdigalian) by the southeastward rollback of a northwest-dipping subduction zone (Rehault et al., 1984; Gueguen et al., 1998). The sedimentary layers of the Tell, external Rif and the Maghrebian–Appenninic Flysch Units (Bonardi et al., 2003) have been originally deposited in the Tethys Basin located between Africa and Eurasian Plates then southwards transported by thrusting. If a general consensus is obtained for the back-arc opening of the Provençal Basin coeval to the subduction beneath the Sardinia volcanic Arc between the late Aquitanian (21 Ma) and the late Burdigalian (16 Ma; Speranza et al., 2002) it is not the case for the Algerian Basin. A similar history in age and style of formation

Fig. 2. Structural map of the Algerian margin superimposed on a bathymetric map of the Commission Océnographique Internationale (1981). The focal mechanisms of the earthquakes in Algeria, Tunisia and off Sicily have been published in several publications (Hatzfeld, 1978; Dewey, 1990; Buforn et al., 1995; Thio et al., 1999; Bezzeghoud and Buforn, 1999; Ayadi et al., 2002; Pondrelli et al., 2002; Stich et al., 2003; Pondrelli et al., 2004; Braunmiller and Bernardi, 2005) or seismological centers (Harvard; IAG, Grenada; European–Mediterranean Seismological Center, RCMT; SED, Switzerland). Several papers have been consulted for the on-land geology and the seismotectonics (Meghraoui et al., 1986; Meghraoui, 1988; Dlala et al., 1994; Aite and Gélard, 1997; Hamdache, 1998; Boudiaf et al., 2001; Bouhadad, 2001; Ayadi et al., 2002, 2003).

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(rollback) of the Provençal and Algerian Basins are suggested by several authors (Vergés and Sabat, 1999; Rosenbaum et al., 2002; Rosenbaum and Lister, 2004). However, it is also proposed (Monié et al., 1992; Aite and Gélard, 1997; Caby et al., 2001) that the extensional collapse of the Kabylide thrust belt and the rifting, that preluded the opening of the Algerian Basin, occurred after (16 Ma) the collision (18 Ma) of the Internal massifs (Kabylies) and Africa. Moreover, the opening of the Algerian Basin may result from the westwards rollback of the Gibraltar Arc (Lonergan and White, 1997; Mauffret et al., 2004; Spakman and Wortel, 2004). The E–W opening may occur between 16 and 8 Ma (Mauffret et al., 2004) although some volcanic activities of the Alboran Arc may persist up to the Messinian (6.6 Ma, Duggen et al., 2004). After 8 Ma (late Tortonian) the compression resumed in Spain, Alboran Sea (Bourgois et al., 1992; Comas et al., 1992; Comas et al., 1999), Morocco and Algeria (Meghraoui et al., 1996; Aite and Gélard, 1997). The Messinian Salinity Crisis may be related to the closure of the communication between the Atlantic Ocean and the Mediterranean Sea around the Gibraltar Arc due to the late Miocene compressional tectonics (Weijermars, 1988; Krijgsman et al., 1999; Duggen et al., 2003; Duggen et al., 2004). In the Algerian Basin the Messinian salt layer is much thinner (less than 1 km) than in the Provençal Basin (more than 1.5 km). Therefore, several windows without salt diapirism allow us to see the acoustic basement. We will show that the Messinian salt is a valuable marker to evaluate the recent compressional tectonics. The limit of the salt dome province is located near the base of the slope (Fig. 2) and correspond to the boundary of the deep basin probably underlain by oceanic crust. However, the salt layer is too thin to generate salt domes off Grande and Petite Kabylie (Fig. 2) where the basement is probably composed of oceanic crust. Between Menorca Rise and Algeria a ridge named Hannibal (Fig. 2) could be a Miocene spreading center (Mauffret et al., 2004). The Pliocene–Quaternary layer is much thinner (0.8 km) in the Algerian Basin than in the Provençal Basin (2 km). At the difference of the north no large rivers like the Ebro and Rhone feed the south basin. The relative motion of Africa and Eurasia is characterized by a 0.5 cm/year convergent oblique motion (Argus et al., 1989). The pole of Africa relative to Eurasia was located near Canary Islands but the GPS data suggest a migration of the pole of Nubia (Africa) towards the south since 3.16 Ma (Calais et al., 2003; McClusky et al., 2003; Nocquet and Calais, 2003). The transpressional tectonics in Algeria is characterized by thrusts and folds often localized (Fig. 2) on the flanks of the former

Miocene basins: Mleta and Habra Basins in the Oran region (Bouhadad, 2001; Ayadi et al., 2002), Cheliff and Mitidja Basins (Meghraoui et al., 1986; Meghraoui, 1988; Ayadi et al., 2003), Soummam Basin (Boudiaf et al., 1999). However, the NW–SE transpressional motion is also responsible of a conjugate system of strike-slip faults: NE–SW for the left-lateral faults (Relizane Fault south of the Cheliff Basin,; Meghraoui et al., 1986) and NW–SE right-lateral faults in the Grande Kabylie region (Boudiaf et al., 1999). In the Eastern Algeria (Hamdache, 1998) and Tunisia (Dlala et al., 1994) the deformation is mainly observed along strike-slip faults (Constantine, Fig. 2) and grabens oriented in a NNW–SSE direction, parallel to the convergent vector. The northern and southern boundaries of the West Algerian Basin are outlined (Fig. 2) by the 2600 m bathymetric contour (Commission Océnographique Internationale, 1981). The slope is moderate off Menorca and Ibiza but steep along the Emile Baudot (Acosta et al., 2001) and Mazarron escarpments. The Algerian Basin is separated from the Alboran Sea by the 2400 m bathymetric contour. The western slope of Algeria is steep (Arzew Escarpment). Several canyons off El Marsa feed a deep sea fan (El Robrini, 1986). The Khar Al Din Bank is an isolated feature off Tipaza connected to the upper margin by the Thenia Escarpment that trends NW–SE. The slope off eastern Algeria is steep and cut by several short canyons. Off la Galite Island (Tunisia) the slope is smooth and oriented NE–SW (Fig. 2). 2.1. Structural description of the Algerian Margin. To obtain a complete view of the Algerian Basin a depth to basement map has been performed (Fig. 3). 23,000 km of seismic profiles have been interpreted with crossing of the lines and correlation of the main reflectors (sea floor, base of Pliocene, top and base of Messinian salt, basement) then digitized and a simple velocity law derived from the analysis of several hundred of RMS velocities (Hsü et al., 1978) has been applied. The values obtained have been decimated along the seismic profiles with a value each 10 shots (500 m) or 5 shots (250 m). The main problem is the poor control in the abyssal plain where the profiles are too distant and disturbed by the salt diapirism. Finally the data have been gridded and the final map is automatically contoured. 2.1.1. Transition between the Alboran Sea and the Algerian Margin (Fig. 2). A plateau, dipping gently northwards, lies between the Morocco and Algerian coasts. This plateau is limited to the north by a steep scarp that is formed by the Yusuf

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Fig. 3. Depth to basement map of the Algerian. Contour interval: 200 m. Basin. Zooms of this map are shown (Figs. 7, 8, 10 and 12).

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Fig. 4. El Marsa deep sea fan. Inset multibeam map of the fan performed during the Bretane transit.

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Fault. A very deep basin (more than 4 km deep, Fig. 3) has been investigated by the deep sea (923 m) Habibas well (Kheidri et al., 2000; Cope, 2003). The metamorphic basement (4496 m deep) is covered by 843 m of Tortonian–Serravallian, 325 m of Tortonian, 1271 m of Messinian marine at the base with some gypsum and halite on the top and 571 m of Pliocene–Quaternary. The enormous thickness of the Messinian (1271 m) seem to be exaggerated and could be attributed to a wrong micro-paleontological determination because the Messinian has been sometimes in the past confused with the Tortonian (Montenat et al., 1975). The main tectonic feature of this region is the right lateral Yusuf Fault and its pull-apart (Mauffret et al., 1987; Watts et al., 1993; Alvarez-Marron, 1999). An extensional focal mechanism (1) with a component of strike-slip is related to this fault (Hatzfeld, 1978). 2.1.2. El Marsa deep sea fan (Figs. 4–6) The WNW–ESE Yusuf Fault can be traced eastwards up to the coast of Algeria and Habibas Island (El Robrini, 1986), then, off Oran, the slope has a NE–SW trend along the steep Arzew Escarpment. The Arzew

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well was drilled (Burollet et al., 1978) on a seismically defined Miocene–Pliocene nose that we can observe in the depth to basement map (Figs. 3 and 6). The upper Tortonian (818 to 1022 m) is made of grey marls with intercalations of pyroclastic material. The lower Messinian (671–818 m) is composed of grey marls and clays with some gypsum intercalations. The massive gypsum Messinian bed has drilled between 542 and 671 m and is overlain by 542 m of Pliocene–Quaternary marls and limestones. The Cheliff Basin, were lies a thick (up to 300 m) gypsum layer (Perrodon, 1957) was connected with the Mediterranean Sea in the area of the Arzew well during the Messinian before the Pliocene compressional tectonic uplift of this region (Meghraoui et al., 1986). A small deep-sea fan, labelled El Marsa, is fed by several canyons (El Robrini et al., 1985; El Robrini, 1986). A turbidity current broke several telephonic cables in 1954 during the earthquake (Heezen and Ewing, 1955) of the former town named Orléanville (then El-Asnam and presently Ech Chelif). This turbidity current started probably in the Khadra canyon then turned downslope toward the north (Fig. 4). A multibeam map of the lower part of the deep-sea fan has been

Fig. 5. 3.5 kHz. Profile crossing the El Marsa deep sea fan. Location of the profile shown in Fig. 4. Observe the active faults.

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acquired during the Bretane transit of the Atalante Oceanographic Vessel (Fig. 4, inset). The convex shape of the fan is outlined by the 2700 m bathymetric contour (Fig. 4, inset). The contrast between the steep Arzew Escarpment west of the Khadra canyon and the gentle slope east of the valley is prominent. Close to the boundary with the abyssal plain, the fan is pierced by several salt Messinian diapirs and cut by very recent faults (Figs. 4 and 5) as shown by a 3.5 kHz profile recorded during the Bretane transit (Fig. 5). The vertical exaggeration is high (X10) and it is difficult to determine the motion of the faults that could be reverse or related to the downslope movement of a slide (Fig. 5, A,B). The same high situation as the shallow deep sea fan relative to the abyssal plain is observed at crustal scale. Beneath the deep sea fan the depth to basement map (Fig. 6) shows a high limited by the 6000 m depth contour. The seismic profile Géomède 3 (Fig. 6), that crosses the high, shows that the Messinian salt is absent on the high where a Messinian erosional surface cut the Miocene strata. A part of the salt may glide downslope to contribute to the formation of the salt dome observed in Fig. 6 but the presence of the Messinian erosional surface demonstrates that the high was already formed prior to the Messinian. However, in the left side of the profile (22–23 h) the upper evaporitic Messinian layer and the overlying Pliocene are also deformed in a broad anticline. A shallow unconformity in the sedimentary section suggests that the deformation ended during the Quaternary. Therefore, the Arzew Escarpment may be a NE–SW left-lateral fault with a component of reverse motion. The trend of this fault is compatible with a compressional deformation and several onshore faults with the same orientation are reverse (Mascara area) or with a strike-slip component (Fig. 4; Meghraoui et al., 1986). The seismic control of the conjugate right-lateral strike-slip fault is poor. However, the depth to basement map (Figs. 3 and 6) shows the eastern end of the El Marsa structural high. An aftershock, a day later (10/09/1954, 10 Fig. 4) of the 09/09/1954 earthquake that destroyed the former town named Orléanville (El-Asnam = Ech Chelif), occurred offshore and its focal mechanism indicates a strike-slip motion off Ténès (Dewey, 1990). The recent faults shown on the 3.5 kHz profile (Fig. 5) are also placed on the western flank of the El Marsa deep sea fan. The depth to basement, The focal mechanism and the faults in

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the recent sediments suggest that the deep-sea fan is controlled by a right-lateral fault in its eastern part. We named this feature Salah El Din Fault. The focal mechanism 5 (Hamdache, 1998). suggests a right-lateral strike-slip motion in this region affected by the Arzew Escarpment. 2.1.3. Khayr Al Din Bank (Figs. 4 and 7) Eastwards of the Ténès Meridian (Fig. 4) the slope is divided into two escarpment separated by a terrace. The northern escarpment, oriented ENE–WSW, is located between the abyssal plain and the Khayr Al Din Bank that culminates at 2000 m as shown by the Bretane transit (Fig. 7F). A seismic profile (Polymède 2 1–2/5/ 1972; Fig. 7A,B,C) crossing the bank suggests a tilted block. The Messinian salt is restricted to the abyssal plain (Fig. 7). The line drawing (Fig. 7B) suggests that the tectonics that formed the bank occurred after the Messinian because the Messinian upper evaporites show the same thickness in the abyssal plain and beneath the bank and the terrace. In the depth section (Fig. 7C), with no vertical exaggeration, the extensional nature of the fault that limits the block is not evident and a deep seated anticline could be also invoked. 2.1.4. Chenoua–Algiers Massif (Fig. 8) The bathymetric contours that bounds the Khayr Al Din Bank to the north and the bathymetric lines located along the coast merge to form a steep slope north of Tipaza and Algiers (Figs. 2, 4 and 7). The well Alger 1 is located in the Tipaza Bay between the Chenoua and Algiers massifs (Fig. 8). It was drilled on the top of a presently active anticline (Burollet et al., 1978; El Robrini, 1986). The Pliocene and the Quaternary are thin (0–165 m). A marl (165–341 m) rich in pelagic foraminifera with some gypsum is Messinian in age. This layer overlies a Tortonian marl (with some fine intercalations of sandstones and some gypsum (341–635 m). From 635 to 1062 m lies a marl with limestone intercalations that is Serravallian in age. The lower layers (1062–1183 m) are made of Upper Langhian to Serravallian marls with calcareous intercalations. The base of the well is composed (1183–1195 m) of volcanic sedimentary formation (tuffs and trachy-andesitic elements). The depth to basement map (Fig. 8A) shows a high that is connected to the Khayr Al Din Bank. The focal mechanism of the Tipaza earthquake (12) corresponds to a

Fig. 6. The El Marsa deep sea fan is located above a high in the basement as shown by the depth to basement map (D) and the seismic profile Géomède 3 (A, B, C). The Messinian erosional surface on the top of the high suggests that the uplift occurred before the Messinian. However, a very recent deformation is also evidenced by the unconformities in the upper sedimentary layers. The study of the aftershocks after the El Asnam earthquake suggests to Yielding et al. (1989) a rethickening of the margin (inset E lower right side). This hypothesis is adopted with the addition of a thrusting dipping to the south and a double vergence of a tectonic wedge.

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reverse fault dipping to the NW beneath the Chenoua Massif (Bounif et al., 2003). The large aftershock 10 min after the main shock shows a NE–SW trend and a leftlateral motion (Bounif et al., 2003). 2.1.5. Algiers–Dellys margin (Figs. 8, 9, and 10) The E–W steep slope that limits the Algiers Massif is interrupted toward the east by an escarpment that trends NW–SE and the continental slope is offset to the south. A destructive earthquake occurred the 21/05/2003 in an area named Boumerdes–Zemmouri (Delouis et al., 2004) located between Algiers and Dellys (18, Fig. 8). After the earthquake the multibeam Maradja cruise (Déverchère et al., 2003) has been performed and the structural map presented (Figs. 2, 3 and 8) is inspired from the main results of this survey (Déverchère et al., 2005) with the addition of our seismic profiles (Figs. 8 and 9) and the Bretane transit (Fig. 10). In agreement with the observations of Déverchère et al. (2005) the steps in bathymetry (0.8 km at the foot of the slope to 0.1 km in the abyssal plain, Fig. 9) are underlain by large displacement of the salt layer (up to 1 km, Fig. 8) probably induced by blind thrusts. A 300 m offset can be observed at the base of the Messinian salt layer (4600, 4900 m; Fig. 9) and 500 m at the top (3800, 4300 m; Fig. 9). The reverse fault responsible of the earthquake has a strike of 70° whereas the seismic profiles (Fig. 8) and the multibeam map (Déverchère et al., 2005) show a 65–70° orientation of the steps in the bathymetry. The fault related to the earthquake may emerge at the foot of the slope (Déverchère et al., 2005) at about 15 km of the shoreline (Semmame et al., 2005) although the coastal uplift suggests a possible sea bottom rupture between 5 and 10 km from the coast (Meghraoui et al., 2004). Anyway our seismic profiles were shot before the 21/05/ 2003 earthquake and the steps in bathymetry probably connected to blind thrusts are up to 60 km far from the coastline. The observations of the multichannel seismic profiles (Déverchère et al., 2005) acquired by WesternGeco (Cope, 2003) and this study show that the reverse faults have a high dip (up to 50°, Fig. 8) when they emerge at the sea floor. The depth to the base of Pliocene map (Fig. 8B) shows an image of the compressional structures although the automatic contour is not always precise with the complexity generated by the Messinian salt diapirs. Two 4000 m deep Pliocene troughs are localized in the west (Ténès) and the east (Bejaia) of the

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deformed Algiers Region (Fig. 8B). The base of the Messinian salt is a very important marker to evidence the blind thrusts. The Messinian salt layer is also involved in the deformation. The southern limit of the diapirism is generally parallel to the continental slope and outlined by large salt domes that are more or less oriented E–W (Fig. 10). However, the line drawing of a seismic profile (3, Fig. 10) shows a diapir that is activated along a reverse fault. The transit Bretane shows that this diapir is oriented N–S. Moreover, the limit of the salt dome province is clearly offset along this diapir. Therefore, it is suggested that the N–S diapir has been formed along a reverse fault with an important component of left-lateral strike-slip (Fig. 10). 2.1.6. Dellys–Bejaia margin (Figs. 8 and 11) The continental slope is narrower than to the west and just one step can be identified in the abyssal plain (Figs. 8 and 11). North of the Algerian slope lies the Hannibal Ridge (Fig. 2), now buried beneath 1 km thick late Miocene and Pliocene to Quaternary sedimentary layer. The seismic profile ALE 05 (Fig. 11) shows a basement high (Fig. 8A) that belongs to the Hannibal Ridge. The Messinian salt layer is thin or absent in this region (Figs. 2 and 11). However this thin salt layer can be deformed by underlying blind thrust. The salt structure shown Fig. 11 is not a salt diapir because the salt layer is too thin to generate such a structure but an anticline that deforms the Pliocene–Quaternary series up to the surface where a 100 m step is observed. The base of the Pliocene rises from 3800 m to 2600 m along a convex slope. Between the base of the Messinian salt or its equivalent and the sea floor a 3 km thick chaotic body is composed of dipping reflectors. This chaotic body looks like an accretionary prism that is mainly Messinian in age. However, the 600 m high step in the bathymetry may be related to a blind thrust although the seismic profile is unclear in this area. 2.1.7. Petite Kabylie continental margin (Figs. 12 and 13) The continental slope is offset to the south off Bejaia. The Petite Kabylie Massif is limited to the north by a steep slope that trends E–W. There is no step in the abyssal margin and the deformation front is limited to the foot of the continental slope (Fig. 12). The seismic profile shown Fig. 13 illustrates two anticlines that deform the recent sediments up to the surface. 2 km of

Fig. 7. Seismic profile Polymède 2 (A) crossing the Khayr Al Din Bank. Location of the profile shown Fig. 4 and on the multibeam map (inset F lower right-side) performed during the Bretane transit. Note the thick Messinian upper evaporites on the terrace (line drawing B). The bank on the seismic profile looks like a tilted block. However, the profile drawn without exaggeration (C) suggests an anticline. The Bank presents some similarities with the Murjadjo Mont (D) near Oran (Bouhadad, 2001, see inset E, this figure and Figs. 2 and 4 for location).

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Fig. 9. Seismic line ALE 04. Location of the profile is shown in Figs. 2, 4 and 8). Interpretation shown (2) in lower part of Fig. 8. Two steps in the topography are probably induced by blind thrusts. The base of the Messinian salt is 300 m offset (4600, 4900 m) by a probable thrust.

shortening can be evaluated from the folding of the late Miocene layers. The Messinian upper evaporites placed between the Messinian salt and the Pliocene–Quaternary are 0.8 km thick. The Messinian erosional surface is cut by a small canyon on the upper margin. The lower anticline is dissymmetrical and looks like a rollover with a thick upper evaporite overlying layer that shows a fanshaped pattern. This configuration suggests that the beginning of the deformation is contemporaneous to the Messinian upper evaporites. However, the tilting of the Pliocene–Quaternary series up to the seafloor indicates that the deformation is presently active. The upslope anticline is more symmetrical and the top has been flattened by erosion. In the abyssal plain the basement and its thick Miocene cover are dipping towards the south (Fig. 13) and an 8 km deep depocentre (Fig. 12) is located north of the deformation front. No similar dip is observed neither in the upper late Miocene (Messinian)

nor in the Pliocene–Quaternary layers except at the foot of the seawards anticline where the dip can be induced by a rollover related to the anticline or/and the sliding of the salt towards the north. The dipping of the Pliocene– Quaternary series towards the south was an argument for an incipient subduction (Auzende et al., 1975). Although the continental slope is often bounded by deep Pliocene–Quaternary troughs (Fig. 8B) and a general dip from north to south is observed, however, the Fig. 13 shows that the dipping to the south can be more pronounced in the Miocene layers than in the superficial sedimentary cover. 2.1.8. Annaba margin (Figs. 12, 14, 15, 16, and 17) Off Annaba, the slope, oriented NE–SW, is smoother than to the west and a large plateau is connected to the Tunisia Margin. The seismic profile ALE 09 (Fig. 14) shows a large anticline where the top of the Messinian

Fig. 8. Depth to basement (A; contour interval: 200 m) and depth to Pliocene (B; contour interval: 100 m) in the Algiers region. The depth sections 1 and 2 shown in the lower part of the figure are located on the map. Most of focal mechanisms are related to compressional thrusts like the 16 (Stich et al., 2003). The earthquake of Boumerdes–Zemmouri (18) is relocated (Bounif et al., 2004). The focal mechanisms of the aftershocks 17, 17bis and 17ter are related to strike-slip faults (Braunmiller and Bernardi, 2005).

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Fig. 10. The line drawing of the seismic profile 3 shows a Messinian salt structure completely dissymmetric, probably a thrust dipping to the east. The Bretane transit shows that this feature is a N–S 100 m. high diapir. The salt limit is offset to the south along this structure.

Fig. 11. Seismic profile ALE O5; Location shown in Figs. 2 and 8. The basement high on the left side of the figure is related to the Hannibal Ridge. Below a 600 m step in the topography a wedge looks like an accretionary prism.

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Fig. 12. Depth to basement of the eastern Algerian margin and Tunisia. Contour interval: 200 m. Observe the focal mechanism 27 that is probably related to the Sardinia thrust.

upper evaporites that is equivalent to the base of the Pliocene, outcrops along the slope. These upper evaporites are thick (300 m) on the upper margin and are uplifted from 3700 m in the abyssal plain to 2300 m on the margin. The acoustic basement and the overlying sedimentary layer are flat beneath the anticline and this disposition suggests that the anticline is related to a

southwards reverse fault. A E–W cross-section of the anticline is illustrated Fig. 15. Eastwards, a 200 m step (Fig. 16) limits southward the abyssal plain. Between 2500 and 1000 m the slope is hummocky. Between 1000 m and the continental shelf (180 m) lies a terrace dipping to the north (Fig. 16). A prominent erosional surface, that outcrops on the continental shelf, cuts the

Fig. 13. Seismic profile ALE 07. Location shown in Figs. 2 and 12. Two anticlines are imaged. Note the thickening of the Miocene (8 km deep depocentre, Fig. 12) whereas the Messinian salt layer and the Pliocene–Quaternary sedimentary layer are almost flat.

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Fig. 14. Seismic profile ALE 09. Location shown in Figs. 2 and 12. The profile ALE 09 crosses the ALE 03 seismic line on the slope. Note the anticline along the slope.

Fig. 15. Seismic profile ALE 03. Location shown in Figs. 2 and 12. The profile ALE 03 crosses the ALE 09 seismic line on the slope. Note the 600 m difference in level.

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Fig. 16. Seismic profile ALE 10. Location shown in Figs. 2 and 12. The profile 10 crosses the ALE 03 seismic line on the upper terrace.

deformed Miocene layers. The seismic profile ALE 03 (Fig. 17) is oriented E–W and crosses the profile ALE 10. These two profiles show late Miocene folds eroded by the Messinian surface. The erosion is up to 1 km (Fig. 17). The main deformation occurred during the late Miocene and the Pliocene Quaternary series do not show large deformation compared to the late Miocene. However, a present deformation is seen at the foot of the slope and on the continental shelf (southern end of ALE 10, Fig. 16) where the Miocene outcrops. The Messinian erosional surface is deformed by a fold and the Pliocene Quaternary is slightly deformed above the fold (Fig. 17). The two profiles indicate that the late Miocene folds are dissymmetrical, the steep limb being facing the northwest (Fig. 17). The folds are oriented NE–SW (65°) parallel to the continental slope (Fig. 12). The folds on-land show (Fig. 12) an almost (55°) similar trend (Dlala et al., 1994; Hamdache, 1998). In Eastern Algeria the focal mechanisms suggest normal faults with a strike-slip component (25) or strike-

slip fault that is left-lateral (Bounif et al., 1987) near Constantine (23). 2.1.9. Western Tunisia continental margin (Fig. 12) Off Western Tunisia the slope that trends NE–SW is moderate and the smooth bathymetry of the large flat plateau off Tunisia is interrupted by several banks (Sentinelle, Estafette, Resgui, Skerki banks, Figs. 2 and 12) and the Galite volcanic Island. The seismic profiles on the northeast continental margin of Tunisia suggest that the banks are presently uplifted and folded by compression (Auzende et al., 1974; Tricart et al., 1994). This margin and its relation with Sicily and Sardinia has been well studied (Auzende et al., 1974; Compagnoni et al., 1989; Nicolich, 1989; Blundell et al., 1992; Pierce and Barton, 1992; Tricart et al., 1994; Catalano et al., 2000; Sulli, 2000; Mascle et al., 2004). A seismic profile (MS 115) located north of the Sentinelle Bank that has been presented several times (Nicolich, 1989; Tricart et al., 1994) shows a pile of reflectors dipping to the

Fig. 17. Seismic profile 03. Location shown in Figs. 2 and 12. The profile ALE 03 crosses the ALE 10 seismic line on the upper terrace. Observe the anticlines and synclines cut by the Messinian erosional surface. 1 km of erosion can be evaluated.

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Fig. 18. Lithospheric section of the Algerian Basin and Nubia (Africa) Plate (modified from Roca et al., 2004). Location of the TRANSMED section shown (Figs. 2, 3 and 8). The Kabylies and the subduction zone dipping to the north rolled back from the Eurasian margin during the early Miocene and the Kabylies collided with the Nubia (Africa margin). Then, the thickened margin has been thinned by rifting and subsequent opening of the Algerian Basin (Aite and Gélard, 1997) probably related to a westward motion of the Alboran Arc (Mauffret et al., 2004). The compression resumed during the Tortonian and a part of the former thinned margin is uplifted and rethickened. However, the crust is yet thin beneath the Kabylies and Tell (Mickus and Jallouli, 1999). The oceanic crust is involved in the compression with formation of blind thrusts and related steps in the bathymetry (Déverchère et al., 2005). The absence of deep earthquakes (no more than 20 km deep) suggests that a subduction zone (Auzende et al., 1975) is not yet formed but is nascent. Observe the double verging wedge of the Kabylies with southward dipping to the north (margin) and northward dipping to the south (Djurdjura, Boudiaf et al., 1999).

south from the surface up to 3 s TWTT deep that are maybe related to northward thrusting. 3. Discussion In the Alboran Sea, Morocco and Spain the compressional deformation related to the convergence between the Nubia (Africa) and Eurasia plates is diffuse (Buforn et al., 1995; Buforn et al., 2004). The focal mechanisms of the

earthquakes in this area show E–W extension and strikeslip faulting (Stich et al., 2003; Buforn et al., 2004). The Alboran ridge is a major compressional feature with a double vergence and a strike-slip component (Bourgois et al., 1992). The Yusuf right-lateral fault limits the Alboran Region toward the east (Mauffret et al., 1987; Watts et al., 1993; Alvarez-Marron, 1999). The El Marsa deep sea fan is placed on a top of a high probably limited by a conjugate system of strike-slip

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faults. This El Marsa offshore block is in the process to be integrated to the continent. On-land the Cheliff Basin with its thick Messinian evaporitic layer was a gulf of the Mediterranean Sea before its uplift (Meghraoui et al., 1986). However, the Messinian erosional unconformity on the high off El Marsa suggests that this offshore block was already uplifted during the late Miocene. A rethickening of the former extended margin has been proposed (Yielding et al., 1989). In addition a double verging of a compressional wedge in the El Marsa area is suggested (Fig. 6, inset E). This double wedge may be related to a positive flower structure on a top of a rightlateral transpressional boundary between Nubia (Africa) and Eurasia Plate. The evidence of compression at sea are limited on the Western Algerian margin whereas the on-land earthquake focal mechanisms are compressional and the seismotectonics indicates a NW–SE compressional stress (Meghraoui et al., 1986; Yielding et al., 1989; Meghraoui et al., 1996) in the Oran–Mascara–El Asnam region. However, like in the Alboran Sea, the deformation is not restricted to Africa but extends to Spain. Active strike-slip faults (Masana et al., 2004) and thrusts (Alfaro et al., 2002) are well known in Eastern Spain. On the Spanish Margin E–W active folds can be traced (Mauffret, work in preparation) as far to the north as the strait between mainland Spain and Ibiza Island. The compressive stress must be transmitted through the oceanic crust between Africa and Spain. In the Algerian Abyssal Plain abyssal plain the evidence of compression are weak although we cannot exclude the activation by compression of Messinian salt ridges. The compression is probably transmitted by a deep detachment in the lower crust or at the Moho level. East of Ibiza Meridian we cannot evidence any present compressional deformation on the Balearic margin and the deformation is now restricted to the Algerian margin. A comparison between the Khayr Al Din Bank and the Murdjadjo Mont (Fig. 2 and inset D, E, Fig. 7) located southwest of Oran (Bouhadad, 2001) shows a prominent similarity. The Murdjadjo Mont is flanked toward the southeast by a reverse fault that forms the boundary of the Mleta Plain. An apparent normal fault is described on the northern flank of the Mont (inset D Fig. 7). This fault maybe in extrados position on the Murdjadjo anticline or the normal fault maybe the superficial expression of a pop-up structure (inset, D Fig. 7). A similar explanation can be proposed for the Khar Al Din Bank although a sliding of a crustal block along the steep slope is also a possible explanation. However, the ENE–WSW trend of the bank is parallel to the Murdjadjo anticline (Bouhadad, 2001) and the Sahel anticline (Fig. 8) located between

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Tipaza and Alger (Meghraoui, 1988; Ayadi et al., 2003). Moreover, A reverse fault has been described in the abyssal plain at the foot of the bank (Déverchère et al., 2003). In conclusion, the Khayr Al Din Bank is probably a fold and may result from an inversion in compression of a previous Miocene normal fault. The western side of the Khar Al Din Bank and the internal slope limiting the terrace are probably NE–SW left-lateral faults (Fig. 8). A first offshore location of the 21/05/2003 earthquake, at 10 km deep, was proposed by the European– Mediterranean Seismological Centre; the US Geological Survey and the Centre de Recherches Algérien d'Astronomie et de Géophysique (CRAAG), (Ayadi et al., 2003; Yelles et al., 2004) but a relocation (18, Figs. 2 and 8) just beneath the coastline has been determined (Bounif et al., 2004). The reverse fault responsible of the earthquake is southeast dipping and the dip from the Harvard CMT is 43° (Yelles et al., 2004). If the earthquake is located beneath the coastline (Bounif et al., 2004) a flat (Déverchère et al., 2005) must be introduced to correlate the fault located at the foot of the slope and the reverse fault linked with the earthquake whereas this flat was not necessary with the offshore previous location (Ayadi et al., 2003; Yelles et al., 2004). In the other hand, a 25°+ / − 5° dip is also proposed (Braunmiller and Bernardi, 2005) and a flat is not required (29°, Fig. 8) if we accept this dip although a high dip is observed (Déverchère et al., 2005 and Fig. 8) on the seismic profiles when the fault emerges at the sea floor. In conclusion, the seismotectonics interpretations on the 21/05/2003 earthquake are unclear on the position of the epicentre (onshore or offshore) and the dip of the reverse fault but the seismic profiles show that the deformation was active in a large offshore area prior to the earthquake and is not restricted to the nearshore. The Grande Kabylie and the 2300 m high Djurdjura Mountain are presently thrusting the Soummam Basin (Fig. 8) and the motion of the Bouira right-lateral fault evidence this displacement (Boudiaf et al., 1999). The Thenia Fault that is located along the coastline is probably also an active right-lateral strike-slip (Boudiaf et al., 1999). At sea, a steep escarpment north of Algiers is the probable extension of the Thenia fault. A reverse motion is determined (Stich et al., 2003) for a focal mechanism of an earthquake (16, Fig. 8) located at the foot of the scarp. The strike-slip focal mechanisms of three earthquakes (17, 28/05/2003; 17bis, 29/05/2003, 17ter, 10/01/2004, Fig. 8) has been determined by the SED (Braunmiller and Bernardi, 2005). These authors relate these aftershocks to a left-lateral fault connecting the Boumerdes–Zemmouri thrust to the reverse fault that limits the Mitidja Basin to the south (Bounif et al., 2003; Meghraoui et al., 2004).

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However, the focal mechanism 17bis and 17ter are also compatible with a right-lateral component of the Thenia fault. Moreover, the rupture of the fault related to the Boumerdes–Zemmouri earthquake may stop its westward propagation close to the Thenia fault (Semmame et al., 2005). In conclusion it is proposed that the right-lateral strike-slip Thenia fault is active and transmits the deformation from the Boumerdes–Zemmouri region to the Khar Al Din Bank (Fig. 8). The margin between Algiers and Bejaia presents an unusual tectonic framework. Several steps in the bathymetry can be traced (Déverchère et al., 2005) as far as 60 km north of the Algerian coast. These features are underlined by more pronounced steps (up to 1 km, Fig. 8) of the Messinian salt layer. The Boumerdes–Zemmouri thrust and the seismic profiles suggest (Déverchère et al., 2005) that the steps are induced by blind thrusts that participate to the uplift of the margin. In a first interpretation of the TRANSMED lithospheric transect it was proposed (Roca et al., 2004) that the area affected by the offsets in topography was underlain by an intermediate transitional crust between the oceanic crust and the continental crust. However, except its higher situation, there is no geophysical difference (gravity, magnetism, seismic features) between this area and the abyssal plain where lies the oceanic crust. Therefore, it is suggested (Fig. 18) that the ocean–continent boundary is placed at the foot of the continental slope. After the middle Burdigalian (18 Ma) collision between the Kabylies and the African Plate the margin has been extended (Aite and Gélard, 1997) and the oceanic crust has been formed from the early Langhian (16 Ma) to the late Miocene (Tortonian–Messinian, 8–6.6 Ma; Mauffret et al., 2004). The compression resumed during the Tortonian (Aite and Gélard, 1997). The former thinned continental crust may have been integrated to the steep continental crust by blind thrusting (Fig. 18). The Moho is about 25 km deep beneath the Kabylies and the Tell Mountain (Mickus and Jallouli, 1999) and the previously thin crust is not yet completely rethickened. In the Grande Kabylie Region a tectonic wedge shows again a double verging with offshore thrusts dipping to the south and on-land thrusts dipping to the north particularly beneath the Djurdjura Mountain (Aite and Gélard, 1997; Boudiaf et al., 1999). The North African margin has been considered as an incipient active margin (Auzende et al., 1975) and this suggestion fits with the structural framework. However, the oceanic crust is probably not yet subducted because the earthquakes are always located above 20 km and the weak compression (50 to 60 km of shortening since the Tortonian) may be absorbed in a large deformed zone from the offshore region to the Atlas (Frizon de

Lamotte et al., 2000). Moreover, the oceanic crust is probably also thickened by thrusting (Fig. 18). Between Menorca and Algeria the depth to basement map (Figs. 2 and 3) shows a high named Hannibal Ridge. This ridge is interpreted as a spreading center during a middle to late Miocene E–W opening of the Algerian Basin (Mauffret et al., 2004). This high, probably hot, and the western adjacent zone located off the coast between Algiers and Bejaia, may have resisted to the incipient subduction. This structural situation may explain the compression of the oceanic crust. The Hannibal spreading center was yet active during the Tortonian shortening of the margin like in the Chile Triple Junction (Corgne et al., 2001). A similar compression of the oceanic crust has been observed in the Seine abyssal plain off Gibraltar in particularly in the Coral Patch Ridge area (Medialdea et al., 2004). The oceanic slices cut by the thrusts will be incorporated to the upper plate when the subduction will occur. A peeling off of the uppermost portion of the oceanic crust and underplating of a topographic high is proposed for the formation of a melange in Japan (Ikesawa et al., 2005). This process is probably in activity off Algeria. A large right-lateral strike-slip fault off Bejaia may limits toward the east the area where the oceanic crust is involved in the compression and the Hannibal Ridge. Eastwards the compressional front is located at the foot of the continental slope. Very large anticlines are now evident (Figs. 13–15). On the plateau east of Annaba meridian (Fig. 12) and on the Tunisian margin, Tortonian anticlines are eroded by the Messinian surface (Figs. 16 and 17). These compressional features, with a probable vergence to the south, are parallel to the onland structures of the same age and same vergence (Tricart et al., 1994). The Calabro–Peloritan–Kabylian zone extends from Petite Kabylie through the Galite Island up to the north of Sicily and is bounded southwards by the Drepano Thrust Front (Fig. 2; Tricart et al., 1994; Catalano et al., 2000). The Galite archipelago is formed by Oligocene–early Miocene arkosic turbidites of Kabylies and Peloritan affinity intruded by Langhian to Serravallian (14–12 Ma) calc-alkaline volcanic rocks (Rekhiss, 1996; Mascle et al., 2004). The late Oligocene–early Miocene Numidian nappes have been emplaced during the early Miocene but reactivated by compression during the Tortonian and the Pleistocene (Tricart et al., 1994). The Tunisia margin is separated from the Sardinia margin by the Sardinia Thrust Front (Compagnoni et al., 1989; Tricart et al., 1994; Catalano et al., 2000; Sulli, 2000; Sartori et al., 2001). This thrust may result from the final Langhian (16 Ma) docking (Speranza et al., 2002) of the Sardinia block relative to the Kabylian–Calabro–Peloritan block that

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was already accreted (18 Ma) by collision to the African Plate. This thrust should be inactive at the present day. However, the only one earthquake of this area (27, Fig. 2), with a compressional mechanism, is probably related to the Sardinia Thrust Front. The crust beneath the Sardinia Channel is thin and the Moho about 20 km deep (Blundell et al., 1992; Pierce and Barton, 1992). The crust beneath the Tellian–Numidian terranes of Tunisia is also thin. The collision of the Kabylian blocks and Africa and the rotation and the Corsica–Sardinia blocks are coeval to a strong early Miocene compressional tectonics in Eastern Algeria and Tunisia and a probable thickening of the crust (Tricart et al., 1994). After this collision the crust has been thinned by collapse of the former orogen. The late Miocene (12– 10 Ma) thinning (Bouillin et al., 1998; Mascle et al., 2004) related to the opening of the Tyrrhenian Sea is evident (Compagnoni et al., 1989). However, the older (middle to late Miocene) thinning related to the opening of the Algerian basin, that is very well imaged by a CROP seismic profile (Catalano et al., 2000; Sulli, 2000), must also take into account. Moreover, in the Sardinia Channel and the adjacent Tunisia margin is probably located a tear fault between the Algerian slab that is stopped and the narrow Calabrian slab that continues its migration toward the Ionian Sea (Faccenna et al., 2004; Goes et al., 2004). Therefore, this area is very complex. The present deformation is observed onshore Tunisia far to the south of the coast as shown by the field works (Dlala et al., 1994) and earthquakes, mainly related to strike-slip faulting, located in the south (Figs. 2 and 3). The thrusts and folds in Northern Tunisia, in the adjacent shelf between Tunisia and Sicily and north of Sicily, show a southward vergence. The northernmost thrust of the Algerian margin, that is southward verging, may extend to the north of la Galite block. Therefore, a wedge with a double vergence is proposed in the Tunisia Region although the northern boundary with a northern vergence is much narrower than the southern limb that extends to the Atlas. The seismicity and the GPS studies (Battaglia et al., 2004; D'Agostino and Selvaggi, 2004; Pondrelli et al., 2004; Neri et al., 2005) suggest a recent reorganisation (1–0.8 Ma) of the plate boundary that jumps and flips from the southern of Sicily to the north. A right-lateral strike-slip fault, where is located the Etna volcano and the Messina Straits, joins the compressional zone to the Calabrian accretionary prism (Fig. 1). In the cross-section of the Tyrrhenian Sea to Sicily (Torelli et al., 1992), the Kabylo–Calabrian units overthrust the fold and thrust Maghrebian Belt and the vergence is always to the southeast. Nevertheless a northward vergence is proposed for the very recent (1–0.8 Ma) movements (Goes et al.,

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2004; Faccenna et al., 2005) although in the Algerian Basin the north front of the compression is older than 1 million year. A northward vergence has not yet been observed on the seismic profiles north of Sicily (F. Pepe and A. Sulli, oral communications). The absence of seismic expression north of Sicily may be due to the young motion where the thin crust of the Tyrrhenian Sea will begin to subduct beneath Sicily like in the southwestern Mediterranean Sea where the oceanic crust of the Algerian Basin will subduct beneath Nubia (Africa) Plate. 4. Conclusions The study of the Northwestern African margin is instructive because the formation of a subduction zone and the processes that are in activity like the formation of melange can be observed. The low velocity of the convergence between Nubia (Africa) and Eurasia plate allow us a fine observation whereas the incipient evidences of subduction are destroyed in faster convergence or already accreted to the continent. The Hannibal Spreading Center was probably yet active when began the southwards dipping thrusts that preluded a subduction that is not yet fully born. It is proposed that a tectonic wedge that is the superficial expression of the transpression extends along the Maghreb Margin up to the north of Sicily. Acknowledgements I thank the TOTAL oil company and particularly B. Fourcade, to allow me to use and publish the ALE seismic lines. N. Wardell and G. Brancolini from OGS (Oceanografica di Geofisica Sperimentale) are thanked to have provided some seismic lines that have been used to complete the seismic grid. The Spanish Margin has been completed with the seismic profiles kindly provided by M. C. Comas. I thank an anonymous reviewer for his valuable suggestions. and P. Alfaro for his particularly useful and constructive review. References Acosta, J., Munoz, A., Herranz, P., Palomo, C., Ballesteros, M., Vaquero, M., Uchupi, E., 2001. Geodynamics of the Emile Baudot Escarpment and the Balearic Promontory, western Mediterranean. Mar. Pet. Geol. 18, 349–369. Aite, M.O., Gélard, J.P., 1997. Distension néogène post-collisionnelle sur le transect de Grande Kabylie (Algérie). Bull. Soc. Géol. Fr. 168, 423–436. Alfaro, P., Delagado, J., Estevez, A., Soria, J.M., Yébenes, A., 2002. Onshore and offshore compressional tectonics in the eastern Betic Cordillera (SE Spain). Mar. Geol. 186, 337–349.

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