Imaging a lithospheric detachment at the continent–ocean crustal transition off Morocco

Earth and Planetary Science Letters 241 (2006) 686 – 698 www.elsevier.com/locate/epsl

Imaging a lithospheric detachment at the continent–ocean crustal transition off Morocco Agne`s Maillard, J. Malod *, Emmanuelle Thie´bot, Frauke Klingelhoefer, Jean-Pierre Re´hault Brest, France Received 4 February 2005; received in revised form 18 October 2005; accepted 7 November 2005 Available online 15 December 2005 Editor: V. Courtillot

Abstract The SISMAR seismic survey on the Moroccan Atlantic Margin recorded deep penetration images of the continent–ocean boundary. This paper focuses on the 3D observation of a landward dipping reflector, overlaid by a layered unit. The deep part of the reflector plunges towards the continent beneath thinned continental crust whereas its upper part finishes at the top of the basement in a transition zone where likely volcanics dipping reflectors are visible and where a magnetic anomaly named S1 marks the continent–ocean boundary location. As the landward dipping reflector crosses the Moho, it is interpreted as a lithospheric detachment with associated volcanic material originating from the asthenospheric mantle. Observations are used (i) to build a 2D model to explain the mechanisms of the lithospheric breaking off between Moroccan margin and its conjugate Canadian margin, the landward dipping reflector allowing mantle exhumation; (ii) to propose an ENE–WNW trend for the initial rifting extension in this part of Atlantic during late Triassic, direction compatible with that of the southern Grand Banks transform margin. D 2005 Elsevier B.V. All rights reserved. Keywords: deep seismic profiling; Moroccan Atlantic margin; lithospheric detachment; mantle exhumation

1. Introduction Models proposed to explain continental margin formation are based mainly either on pure shear mechanisms or on simple shear, or more often on a mixing of these both types. In the simple shear case, detachment faults are proposed to occur at 2 levels: – crustal with blocks tilted above low angle faults generally observed on seismic profiles as bSQ horizons, – lithospheric to explain mantle exhumation and extreme thinning of the crust in the deep margin [1–3]. A deep penetration seismic survey, carried out during the SISMAR cruise (Fig. 1), allowed us to image * Corresponding author. E-mail address: [email protected] (J. Malod). 0012-821X/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2005.11.013

large faults at a crustal or upper lithospheric scale. The aim of this paper is to report the seismic reflection images of a prominent landward dipping reflector, emerging at the continent–ocean boundary (COB), in order to discuss the nature and location of the transition zone, the role of volcanism and more specifically the processes of rupture of this passive margin. Finally, we propose a model of the conjugate margins at the time of the lithospheric rupture. 2. The NW Moroccan and Nova Scotia conjugate margins The Atlantic margin off NW Morocco is one of the oldest margins on earth [4–6]. It is conjugate to the Canadian Atlantic margin off Nova Scotia. The oceanic

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crust adjacent to the Moroccan margin was generated during the Jurassic magnetic quiet period [7] and rifting on land is known to be of Triassic–Liassic age [8]. The final stage of rifting in this part of the Atlantic may have been associated with a large episode of magmatism [9,10], which is widespread in a very large magmatic province (Central Atlantic Magmatic Province: CAMP). Dykes and basaltic lava flows (tholeiites), outcropping in numerous locations around the Central Atlantic, are dated from a short range of time around 198–200 Ma [10,11]. The Central Atlantic North American deep margin and the continent to ocean transition zone are characterized by seaward dipping reflectors (SDR) interpreted as volcanic strata [12–14]. These SDR’s are clearly related to a prominent magnetic anomaly, the East Coast Magnetic Anomaly (ECMA) [15,16]. As this magnetic anomaly vanishes progressively northwards, the question is raised to know if a transition occurs from a volcanic margin to a non-volcanic one [17]. To know the age of this deep margin magmatism would be very important to date the onset of sea floor spreading and then calculate the initial rate of oceanic accretion. Most of the authors assume that the SDR’s are related to the CAMP on the base of magnetic or seismo-stratigraphic observations [18]. However, there is still no direct evidence available [12] and an age younger than 200 Ma cannot be ruled out. Later magmatic events are known in Morocco during the mid Jurassic and the early Cretaceous but are localized in the Atlasic domain [19]. A magnetic anomaly named S1 (Fig. 1, Roeser, 1982) located on the deep Moroccan margin may represent a conjugate of the ECMA. As on the Canadian side, the COB is thought to lie along the S1 magnetic anomaly [20,21] whose origin is debated [4,20]. Evaporites are present on both margins in grabens or half-grabens on the shelf and on the deep margin delimiting a diapiric province. On the Canadian side, the salt has been cored in shelf basins (Argo formation) and a late Triassic to early Liassic age is given [22]. On the Moroccan margin, a salt diapiric structure was also drilled (hole DSDP 546, Fig. 1) [5] in a half graben of the continental slope. From geochemical studies of the Br, the marine origin of the salt is confirmed [23], but the salt is not dated. Actually, the age of the salt in the deep offshore is not constrained by any direct evidence. In summary, though some points have been already discussed [21], several questions remain unsolved on the Moroccan margin: Is it a volcanic margin, with typical SDR’s or not? What is the location, nature of

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the COB and its relationship with the observed S1 magnetic anomaly? We try to answer theses questions with the new data presented in this paper. 3. New data and processing In addition to the existing BGR data (Meteor Cruise 67) [5], the Sismar cruise, carried out by the University of Brest and Ifremer in 2001, acquired 24 deep penetration seismic lines and 48 OBS refraction records. We study a transect of the margin off El Jadida (Fig. 1), a place where additional data were already available including seismic reflection profiles and DSDP well logs [4,5]. There, ship tracks were set up to realize a transect across the whole margin. Four seismic lines are crossing the S1 magnetic anomaly and the main OBS refraction profile (SIS04) was continued on land by means of seismic land stations (Fig. 1) [24]. Seismic reflection was acquired using a 12 air guns array (4800 ci) tuned in single bubble mode (i.e. airguns signals are tuned with respect to the first bubble), giving deep penetration but poor resolution. The profiles have been processed to get a minimum phase record and then migrated [25]. Further, profile SIS04 has been pre-stack migrated in order to get a depth section comparable to the OBS results [24]. 4. Where is the COB off Morocco? As many authors do, we will consider that the ECMA marks the COB on the North American side. Symmetrically, on the Moroccan side, the COB could be located along the S1 magnetic anomaly [7,20,21]. The shape of the ECMA is quite similar to the one of the S1 magnetic anomaly underlining their correspondence on each side of the Atlantic and allowing to make a plate reconstruction at a time just after their emplacement [21] (Fig. 2). To discuss further this assertion, we define the COB as the line of maximum extent of continental crustal material, and then we use several criteria: – The upper crustal seismic signature: On the continent side, the upper continental crust has been sampled in hole DSDP 544 (Fig. 1) drilled at the top of a large tilted block [5]. The seismic reflection facies of this crust shows on profile SIS04 a diffracting pattern above a strong low angle reflector interpreted as a detachment fault of bSQ reflector type [1] (Fig. 3). Oceanwards, on the same profile, this fault is cut by a later normal fault and crustal elements of the same seismic facies

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Fig. 1. Data used in this paper. The long dashed lines are SISMAR cruise seismic reflection profiles. Black lines indicate sections shown in Figs. 4–6. Black triangles are SISMAR OBS and dark grey circle on land are SISMAR seismic land stations. Thin dashed lines are profiles from Meteor 67 [4]. Open circle indicates the location of the S1 magnetic anomaly separating the area with salt diapirism (small grey polygons) from the transitional crust shaded in grey. DSDP holes are indicated. In the oceanic crust, magnetic lineation from the Jurassic magnetic quiet zone is outlined.

are observed with a reduced thickness. Velocities given by the OBS records (5.25 to 6 km/s) are in agreement with upper crustal continental material [24]. These velocities at the top of the basement are increasing progressively toward the vicinity of the S1 magnetic anomaly (Fig. 4). At the location of the S1 magnetic anomaly and as observed earlier [4, 7] (Fig. 1) the seismic facies of the basement appears different showing a more diffracting

surface and, at least locally, possible seaward dipping reflectors as on the profile SIS09 (Fig. 3). According to the seismic refraction results [24], the transition to a more typical oceanic crust occurs further oceanwards at about 50–70 km from S1 (Fig. 3). – The salt diapiric structure repartition in the deep basin: Evaporite occurrences are clearly linked to the half-grabens formed between tilted crustal

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Fig. 2. Plate reconstruction between Morocco and Nova Scotia (Canada) at the time of juxtaposition of the magnetic anomalies S1 and ECMA [21]. The line of breakup is shown as a stippled–dashed line. To the north, a line indicates the Southern Grand Banks transform margin. The location of the conjugate sections SMART line 01 (squares) and the composite profile build on SIS04 and SIS10 are underlined by a thick dashed line. Thin continuous lines represent faults on each margin.

blocks. The salt was deposited in these structurally controlled depressions of both the deep and shallow margin (Fig. 3). This supports the idea of a single evaporitic basin during the late Triassic–early Liassic. In the studied area, there is no evidence of significant oceanward salt migration. This allows us to use the diapiric province extent as a minimum extent of the continental crust. Off El Jadida, it appears clearly that all the diapirs are located east of the S1 magnetic anomaly (Fig. 1). – The magnetic anomaly pattern: Looking towards ocean, the first typical oceanic magnetic linea-

tions are those of the Mesozoic sequence (M25 to M0). Between this sequence and the S1 magnetic anomaly lies the Jurassic magnetic quiet zone with weak lineations (Fig. 1) interpreted tentatively as resulting of magnetic inversion M28 to M40 [7]. These lineations are slightly oblique to S1 in the northern area [7,26]. This suggests a propagation of the spreading from the south to the north in agreement with subsidence studies from the Moroccan platform [8,27]. S1 is difficult to identify north of 33.38 N and vanishes north of 34.58 N (Fig. 2). Likewise, the

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Fig. 3. Profiles SIS05, 04, 10, 09 aligned with respect to the S1 magnetic anomaly axis (location on Fig. 1). The LDR is well defined on profile SIS10. Dipping reflectors are imaged on profile SIS09. Note the significant Tertiary tectonic inversion on profiles SIS04 and 05.

ECMA diminishes in intensity and disappears just north of 448 N on the Nova Scotia margin (north of 338 N on the Meseta fixed plate reconstruction, Fig. 2) [17,28].

In summary, our new data confirm that the S1 magnetic anomaly is closely located at the COB. In the following we will address the deep structure of this transition zone to try to answer some questions as the

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Fig. 4. Seismic profile SIS04, pre-stack depth migrated, superimposed on the refraction velocity model from OBS’s and land stations data. The LDR is not really defined on profile SIS04. The location where the subsidence was the strongest, close to anomaly S1 is the place where the LDR should emerge by comparison with profile SIS10.

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nature of the crust at this transition zone or the origin of the S1 magnetic anomaly. 5. Imaging the COB: evidences for a large landward dipping reflector Four Sismar profiles are crossing the S1 magnetic anomaly (Figs. 1–3). They show different images at the COB. The most interesting is the profile SIS10 which exhibits a strong reflectors zone within the basement, dipping apparently towards the continent (Fig. 3 and 5). This zone is precisely made of several reflectors interpreted as layered material with a thickness of up to half a second double time, which constitutes the upper part of the basement on the ocean side west of S1. We will name LDR (Landward Dipping Reflector) the reflector at the base and LU the layered unit above it. At this location, the crust below the LDR exhibits almost flat reflections and no obvious Moho is apparent. The LDR plunges to the NE below the thinned continental crust. The place, where the LDR and its associated LU are reaching the top of the basement, is located slightly east of S1 axis (Fig. 5). Here, the basement’s surface is depressed and forms a transitional basin filled by the thick almost horizontal sediments of the deep margin on lapping the LU oceanwards and the diffractive conti-

nental basement on the other side. The LDR is clearly a major feature of the transition zone. At depth, it reaches the Moho level, but its relationship to the Moho cannot be clearly defined on profile SIS10 (Fig. 3). Profile SIS09 allows us to correlate the LDR of profile SIS10 with a prominent deep reflector on profile SIS08 (Fig. 3). Oceanwards, the profile SIS09 shows also some westward dipping diffractive reflectors which may be interpreted as seaward dipping reflectors of limited extent (SDR’s) [4]. The relationship between these reflectors and the LDR is difficult to specify from one single profile. Continuity seems to link these dipping reflectors with the layered unit LU associated with the LDR. However, the LU is less well imaged on profile SIS08 than on profile SIS10 (Fig. 6). On profile SIS08 the LDR seems to cross the Moho and to penetrate into the mantle below the continental crust (Fig. 6). Above it, several reflectors within the overlying crust connect to the Moho and outline large lenses of material whose nature will be discussed later. Profile SIS04 shows a basement topography very similar to the one observed in profile SIS10. However, only some weak landward dipping reflections may suggests the occurrence of a LDR (Fig. 3). A basin filled by old sediments occurs at the transition between thinned continental crust and transitional crust. Later, this sedimentary basin has been inverted and recent sediments were deposited more eastward (Fig. 3). The transition zone, which was initially the most subsiding area, forms now a broad faulted anticline. This tectonic inversion affects Tertiary sediments likely of Miocene age as determined by correlation with the DSDP hole 416 [29]. Northwards, the profile SIS05 is rather similar to the profile SIS04 but tectonic inversion is more pronounced and there is no evidence for a LDR (Fig. 3). The LDR observed on profiles SIS08, 09 and 10 can be drawn in space (Fig. 6). Taking into account the weak reflections seen on profile SIS04, we extrapolate to profile SIS05, but the LDR is actually well defined only to the south (Figs. 6 and 7). It dips with a slope of about 68 to 78 northeastwards, which is not the expected direction if the margin extensional motion was along the known NW–SE oceanic spreading directions [6,21,30]. 6. Interpretation of the LDR

Fig. 5. Close up of the COB on profile SIS10. Note the LDR and the LU above it. The COB is characterized by the deepest sedimentary basin filled by post-detachment sediments. The occurrence of salt diapirism indicates that upper continental crust is very close.

LDR’s have been observed within the basement at the COB in other margins. This is the case in the Tagus Abyssal Plain [31], off the Grand Banks [32]. On the outer Vo¨ring basin a landward dipping reflector has been recognized beneath the prominent lava flows dipping toward the ocean [33]. However in this region, the

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Fig. 6. Perspective view of the LDR surface on profiles SIS08, 09 and 10 (Location on Fig. 1). Note the occurrence of crustal lenses above LDR on profile SIS08.

reflector seems linked to intra-crustal reflectors topping a high velocity body interpreted as eclogitic crust. In all of these cases only limited interpretation of the relationship with the Moho has been proposed. In this paper, we interpret the observed LDR as a large fault, which seems to cut the Moho at depth and then could be a lithospheric detachment [2]. According to seismic facies, very thinned continental crust is clearly recognized above it. In addition, the occurrence on profile SIS10 of a diapiric structure close to the emergence zone of the LDR is likely the signature of the continental domain. Thus, the emergence of the LDR coincides with the COB location. Because it is clearly associated with the COB and forms the top of

basement on the oceanic side, we rule out the possibility of an old detachment exhumed during the Jurassic break-up. The thick sediments filling the transitional basin have been clearly deposited after the activity of the large detachment fault, which may have played later than the Triassic–early Liassic rifting. In this hypothesis, the salt diapir located nearby landwards above this depression may originate from older syn-rift sequence attached to the continental basement. Other questions raised by our data are the nature 1) of the material below the LDR and 2) of the material above it: the layered unit LU and the lenses. Considering the LDR as a lithospheric detachment cutting the Moho, then we expect that the lower compartment is

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Fig. 7. LDR’s isobaths in km. The surface is extrapolated towards profile SIS05 where the LDR is not observed. The LDR dips east–northeastwards. Arrows indicate the possible direction of motion during the detachment play parallel to the south Grand Banks transform margin (see Fig. 2). Thin black lines are the seismic lines of Sismar cruise. The S1 magnetic anomaly and salt diapirs are indicated.

made of deep material being likely exhumed mantle. In this case, the LU may be interpreted as serpentinized peridotites eventually intruded by volcanics [34] or as magmatic layers. The first hypothesis could well explain why the Moho is not visible on the seismic reflection profile at this place. Refraction results may help for the interpretation. Unfortunately, the profile SIS10 is not long enough to provide us with refraction velocities in this area. However, basement shallow structures observed at the COB on profiles SIS04 and SIS10, and particularly the occurrence of a transitional basin, are very similar. This supports the idea of a comparable tectonic evolution between both profiles, even if the LDR is not clearly identified on profile SIS04. Consequently, we use the refraction results of profile SIS04 to comment tentatively the profile SIS10. The velocities at the top of the basement are increasing oceanwards when approaching the transition zone and the location where westwards dipping reflectors are observed (crossing of profiles SIS10 and SIS09). They reach values of 6 to 6.5 km/s that are fully acceptable for serpentinite with a serpentinization rate of about 50% to 60% [35]. Actually, compressional seismic velocities cannot discriminate between serpentinites and volcanics. Our second hypothesis for the LU is thus the occurrence of magmatic material and lavas, mixed or not with sediments. They may originate from asthenospheric melted material rising from the lower extremity of the LDR and guided along it to the surface of the basement. Profile SIS10 shows that, using a constant

velocity, the stratified reflectors of the LU are getting thicker toward the shallower part of the LDR. This may correspond to increasing accumulation of volcanics toward the surface in correlation with cooling process. Moreover, the westward dipping reflectors observed on profile SIS09 may also represent small volcanic bodies intruded at the top of the basement as interpreted in the Bay of Biscay margin [34]. A close proximity between the LDR emergence and the westward dipping reflectors suggests an original relationship. At greater depth, as observed on profile SIS08, the LDR shows no more LU layering: this may also correspond either to a lowering amount of serpentinization or to the fact that lavas have just traveled along the reflector up to the surface. At depth above the LDR, and connected with it by means of large low angle faults, the lenses observed at the base of the thinned continental crust may be either lower crustal material transported along the LDR like eclogitic bodies or magmatic materials underplated at the base of the crust. Another evidence is the occurrence of the S1 magnetic anomaly at the transition zone. The magnetic pattern along the profile SIS10 shows three anomalies: two small anomalies to the west named A and S1, and a large one named B above the edge of the continental crust (Fig. 8). In order to model the magnetic anomaly across the transition zone, we used the geometry given by the seismic reflection, transformed into depth by means of the refraction velocities from profile SIS04. Magnetic susceptibilities have been put to zero for most

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Fig. 8. Magnetic anomaly model along western part of profile SIS10. Modeled magnetic anomalies are represented in dashed line. See text for explanations.

of the layers including the sedimentary cover, the continental crust and the underlying mantle, in order to test the contribution of the LU. We suppose that if the LU is a magmatic body, it has been emplaced at the end of rifting around 180–200 Ma [21] at 298 N of paleolatitude. At the top of basement and in connection with the LU, we added a body corresponding to the dipping reflectors observed on profile SIS09. Varying the remanent magnetization and the magnetic susceptibility, we succeed to model the S1 anomaly and the anomaly A on the oceanic side. Magnetic parameters values obtained in the model (magnetic susceptibility of 12.10 3 and magnetization of 55 nT for LU) are compatible with igneous rocks. However, the magnetic anomaly B (Fig. 8) cannot be modeled. Despite the fact that crustal lenses seen on profile SIS08 (Fig. 6) are not observed on line SIS10, we put a lens of underplated magmatic material at the base of the crust with the same initial parameters that the LU. A magnetic susceptibility of 47.10 3, again compatible with basic igneous rocks, is found to adjust the model. Being fully aware that too many parameters are not well constrained, this model intends to show that the magmatic hypothesis is quite reasonable and compatible with realistic parameters values, even if we cannot completely rule out the magnetization of serpentinites at the top of the mantle. Finally, we favour the interpretation of the LU as magmatic material injected along the LDR in connec-

tion at depth with underplated volcanics and at surface with the occurrence of some volcanics expressed by local dipping reflectors on the ocean side. 7. Comparison with the conjugate margin To build a 2D-model depth section at time of breakup, we have to consider conjugate segments of the Moroccan and Canadian margins. On the Canadian side, the SMART experiment gives a cross section of the margin based on deep seismic reflection and refraction (Fig. 2). It shows the thinning of the continental crust and oceanwards a transitional crust characterized at depth by a high velocity body (7.2 to 7.8 km/s) interpreted as serpentinized peridotites [36,37]. For the Moroccan side we rely mainly on the profile SIS04 where the deep margin structure is best defined from pre-stack depth migrated reflection data and from associated refraction data. However, considering the plate reconstruction at time of break-up, the profile SIS04 is definitively not the conjugate of the SMART 1 (Fig. 2). Therefore, we build a composite depth profile combining the profile SI04 imaging the thinning of the continental crust and the southwestern end of the profile SIS10 imaging the COB and the transition zone (Fig. 2). This depth profile gives a complete cross section of the margin which is not too far to be a conjugate of the SMART 1 (Figs. 2 and 9). We cut the SISMAR composite profile at the location

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between the transition zone and the more typical oceanic crust, including the dipping reflectors observed at the junction between SIS10 and SIS09. On the Canadian side we cut the SMART 01 profile at the oceanic border of the deep serpentinized body eliminating the typical oceanic crust. Putting these two profiles face to face (Fig. 9) gives a cross section at the time of onset of oceanic spreading. We use this geometry to compare the conjugate margins and discuss the rupture process. Based on the synthetic model proposed by [3] we interpret the LDR as the fault separating the upper African plate from the lower Canadian one. In this model, the crustal thinning resulted from 2 phases: 1) a widespread crustal extensional stage during the continental rifting; 2) exhumation of lower continental crust and mantle through the play of a large detachment. The Fig. 9 shows the stage immediately before the lithospheric rupture and the further rise of the asthenospheric mantle close to the surface. In this sketch, the observed LDR represents the last active part of the detachment, whose a fossil segment is exhumed and partly overlaid by stretched continental crust on the Canadian side. In our opinion, this model integrates our previous observations. The asymmetry of both margins supports the existence of the lithospheric detachment fault. Further, the model points out the probable role of the rising asthenosphere linked to the play of the large detachment fault, which may have produced magmas feeding possible underplated bodies or volcanics along the LDR (LU).

An interesting point of view is to consider homologous points on both sides on the detachment as indicated by black squares (Fig. 9). This suggests that the overall displacement between the 2 plates during the detachment’s activity was about 300 km, much more than the one that is currently supposed to calculate initial rate of motion. A better understanding of the initial kinematics is now dependant upon our ability to determine a tight timing of the different events. Should our hypothesis be verified, it is likely that the volcanics associated to the LDR and the exhumed mantle could be younger than the CAMP magmatism. This would imply a very different rate of initial motion between North America and Africa as proposed by previous authors. 8. Discussion The proposed model is idealized. Some points remain uncertain. The LDR is not clearly observed in the northern part of the study area. Either the LDR exists but is not imaged because of the later compressional tectonics which is increasing northwards, or it does not exist. The latter possibility may be related to the fact that the S1 magnetic anomaly is disappearing in a region which was located close to the Grand Banks southern margin at the time of the ECMA-S1 plate reconstruction (Fig. 1) [21]. Another explanation could be that the LDR surface was undulating with

Fig. 9. 2D-Model of the conjugate margins of Morocco and Nova Scotia at a pre-rupture stage (see location on Fig. 2). The margin structure is build from seismic reflection and refraction data of Sismar and Smart cruises. Note the asymmetry of the margins at the COB. Black squares are homologous points along the large detachment fault, whose latest active part was the LDR.

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changing direction as illustrated on the extrapolated LDR’s isobaths map (Fig. 7). However, where it is well defined in profiles SIS10, 09 and 08 (Fig. 7), the dipping orientation of the LDR suggests an extension along an ENE–WSW direction. It may represent an earlier extensional phase occurring after the late early–early Liassic rifting phase. This phase may be related to a relative motion of the Moroccan Meseta with respect to the rest of Africa, responsible for the formation of an early deep basin. This motion may have been guided by the transform fault south of the Grand Banks in an ENE–WSW direction, different from the later NW–SE early oceanic spreading direction [6,30] (Figs. 2 and 7). Lastly, looking at the plate reconstruction the ECMA is finishing southwest of the Tafelney ridge (Fig. 2). This also may indicate a transition between 2 different segments of the Moroccan margin that favors the hypothesis of an early relative motion of the Moroccan Meseta with respect to the rest of Africa [21]. 9. Conclusion A LDR corresponding to a lithospheric detachment fault is observed on the Moroccan deep margin off El Jadida. Oceanwards, it shapes the top of the basement and may be associated with either serpentinized mantle or according to our preferred interpretation with volcanic rocks. We interpret the LDR as a large detachment fault that may have guided the arrival to the surface of volcanics, producing locally some seaward dipping reflectors. This volcanism of limited extent and importance characterizes a poorly volcanic margin and is associated with the S1 magnetic anomaly, which definitively marks the COB. Thus, the LDR separates the African upper plate from the Canadian– North American lower plate. It may represent the fault along which the lithospheric deformation was concentrated at the end of rifting before the onset of spreading. Following the dip of the LDR, the initial direction of extension in the segment of the Atlantic located off El Jadida may have been ENE–WSW. This extension implies an initial relative motion of the Moroccan Meseta with respect to North America along the South Grand Bank transform zone. This motion should have changed then to NW–SE at onset of spreading. Acknowledgment We acknowledge K. Hinz and H. Roeser of BGR in Germany for communicating reflection profiles used to prepare the SISMAR Cruise. Thanks to T. Funk for

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communicating preliminary results from the SMART experience. This study was supported by CNRS (GDR bMargesQ), University of Bretagne Occidentale (IUEM), IFREMER and the Total oil company. We appreciate the strong support of the Moroccan authorities and the collaboration with the El Jadida and Marrakech Universities. We acknowledge deeply the invaluable help of Mohamed Sahabi and many other people to prepare and achieve the SISMAR cruise. The review by Laurent Gernigon and an anonymous reviewer helped to clarify and improve the paper. References [1] T.J. Reston, C.M. Krawczyk, D. LKlaeschen, The S reflector west of Galicia (Spain): evidence from prestack depth migration for detachment faulting during breakup, J. Geophys. Res. 101 (1996) 8075 – 8091. [2] A. Ho¨lker, G. Manatschal, K. Hollinger, D. Bernoulli, Tectonic nature and seismic response of top-basement detachment faults in magma-poor rifted margins, Tectonics 22 (2003) 1035. [3] R.B. Whitmarsh, G. Manatschal, T.A. Minshull, Evolution of magma-poor continental margins from rifting to seafloor spreading, Nature 413 (2001) 150 – 154. [4] K. Hinz, Reflection Seismic, Gravity and Magnetic Measurements in the Diapiric Province and in the bMagnetic Quiet ZoneQ off Central Morocco, B.G.R., Hanover, 1984. [5] K. Hinz, E.L. Winterer, et al., Initial Reports DSDP, U.S. Government Printing Office, Washington D.C., 1984, 934 pp. [6] K.D. Klitgord, H. Schouten, Plate kinematics of the central Atlantic, in: P.R. Vogt, B.E. Tucholke (Eds.), The Western North Atlantic Region The Geology of the North America, M, Geol. Soc. Amer, Boulder, 1986, pp. 351 – 378. [7] H. Roeser, C. Steiner, B. Schreckenberger, M. Block, Structural development of the Jurassic Magnetic Quiet Zone off Morocco and identification of Middle Jurassic magnetic lineations, J. Geophys. Res. 107 (2002) EPM 1-1 – EPM 1-23. [8] P. Le Roy, A. Pique, Triassic–Liassic Western Moroccan synrift basins in relation to the Central Atlantic opening, Mar. Geol. 172 (2001) 359 – 381. [9] H. Bertrand, J. Dostal, C. Dupuy, Geochemistry of Early Mesozoic tholeiites from Morocco, Earth Planet. Sci. Lett. 58 (1982) 225 – 239. [10] K.B. Knight, S. Nomade, P.R. Renne, A. Marzoli, H. Bertrand, N. Youbi, The Central Atlantic Magmatic Province at the Triassic–Jurassic boundary: paleomagnetic and 40Ar/39Ar evidence from Morocco for brief, episodic volcanism, Earth Planet. Sci. Lett. 228 (2004) 143 – 160. [11] A. Marzoli, P.R. Renne, E.M. Piccirillo, M. Ernesto, G. Bellieni, A. De Min, Extensive 200 Million year old continental flood basalts of the Central Atlantic Magmatic Province, Science 284 (1999) 616 – 618. [12] R.N. Benson, Age estimates of the seaward-dipping volcanic wedge, earliest oceanic crust, and earliest drift-stage sediments along the North Atlantic continental margin, in: W. Hames, J.G. McHone, P. Renne, C. Ruppel (Eds.), The Central Atlantic Magmatic Province, Insights from Fragments of Pangea Geophysical Monograph, vol. 136, American Geophysical Union, Washington D.C., 2003, pp. 61 – 75.

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