South Atlas Fault Research Articles

Relocation of the 8 September 2023 High Atlas, Morocco, Earthquake Aftershock Sequence

ABSTRACT The earthquake that occurred on 8 September 2023, with a magnitude of 6.8, was the most destructive earthquake event in Morocco in the past decade. This earthquake took place in the Al Haouz region, located in the western part of the High Atlas Mountain range. To better understand what caused and triggered this earthquake, the earthquake catalogs including P and S arrival times were collected from the Moroccan seismic network and combined with regional data from the International Seismological Centre. The mainshock and aftershocks were relocated by using iLoc, a state-of-the-art single-event location algorithm, and then by the multiple event location double-difference algorithm, hypoDD. The improved earthquake relocations using iLoc and the double-difference methods provide sharper lineation of seismicity and agree well with tomographic images of the earthquake zone. The seismicity distribution and the focal mechanism of the mainshock indicate that the earthquake sequence has occurred along the South Atlas fault system.

Fold-thrust belt in the southern front of the central High Atlas (Morocco): Analysis and implications for the tectonic inversion of the atlas system

Thrust folds have developed significantly in the meso-Cenozoic rock units of the Errachidia-Boudnib basin. The study areas correspond to the north part of the Pre-African Trough, which is located between the Anti-Atlas and the Atlas systems. It is characterized by faulted anticlines that are indicative of a foreland orogenic context.The Atlas system's structural evolution underwent several stages, starting with a Triassic extensional event followed by a period of fault reactivation during the Atlas compression. This research focuses on examining the thrust and detachment folds associated with both reverse and strike-slip components. For this purpose, we performed detailed geological mapping and interpretation of folding and fault slip data.As a result of shortening, field observations reveal that the study area exhibited well-developed thrusting geometry. Preexisting blind faults and multiple decollement levels within favorable formations such as Cenomanian evaporitic marls, Cenomanian-Turonian marls rich in organic matter, and Senonian argillites influence the folding patterns, which are not uniform. The findings demonstrate two key points. First, the most significant folds have formed along major blind thrusts. Subsequently, between these major faults, detachment folds were developed within the Jurassic-Cretaceous strata. Currently, due to erosion, certain thrusts have become visible, including the Ta'bbast thrust fault, Ait Atman (from the first stage), and the Timazguit fault zone (from the second stage). The South Atlas fault largely remains a blind fault. Additionally, the study emphasizes the presence of strike-slip components and en-echelon folding, indicating a transpressional regime during the uplift of the Atlas system.

Structural analysis, tectonic fracturing modeling, and kinematic evolution along the South Atlas Fault at the northern border of the Tinghir-Errachidia-Boudenib basin (Pre-African Trough, Morocco)

The structural evolution of the High Atlas orogenic belt is characterized by the formation of the thrust propagation folds. The exploration and understanding of the southern central-eastern High Atlas orogenic wedge system are still in their infancy. The significance of the growth of the Tinghir-Errachidia-Boudenib foreland basin (TEB) is underestimated due to limited constraints on surface structural styles and kinematics analysis, as well as insufficient surface data. This question is addressed by using several multi-sources and multi-scale data including a geological map constrained from fieldwork and kinematic analysis, combined with subsurface magnetic data. The integration and analysis of these data provided a comprehensive understanding tectonic fracturing model and reconstruct the kinematic evolution of the northern border of the TEB basin. Analysis of surface and subsurface data identify E−, ENE-to WNW- trending Jbel Tisdar and Jbel Rhouir anticlines interpreted as a fault-propagation fold. Magnetic investigations show a positive anomaly superposed on the hinge of the Jbel Tisdar and the Jbel Rhouir anticlines indicating a Triassic and/or Paleozoic uplifted magnetic materials. Along the southern Atlas margin, brittle tectonic is represented by E−to ENE-reverse faults with a small dextral strike-slip component and NNE-to NE-trending strike-slip faults with a reverse component and associated with NW- trending ones. Fracturing mechanism fault-propagation fold highlights three fracture sets: (i) syn-folding E−trending axial fractures; (ii) syn-folding NW- trending transverse fractures and (iii) late-folding NE-trending fractures. Fault kinematics analyses provide a transpressional tectonic regime, responsible for the structuring (folding, faulting, fracturing) of the Jbel Tisdar and the Jbel Rhouir fault-propagation fold, developed in response to Alpine compressional inversion of the central-eastern High Atlas belt.

The Variscan belts of North-West Africa: An African legacy to the Wilson Cycle concept

The concept of cyclic closure and opening of oceans along the same crustal scar was introduced by J.T. Wilson (1966) based on the example of the Atlantic Ocean and its continental borders. At that time, the only Variscan orogen cited south of Europe along the Atlantic coast of Africa was the Mauritanides of Mauritania. Here we report on the recent achievements in the Mauritanides from Mauritania to Morocco, and on the Moroccan Meseta orogen, which also records the Variscan orogeny. The Southern and Central Mauritanides are a poly-orogenic, Pan-African and Variscan orogen characterized by a thin-skinned tectonic style that mirrors the structure of the southern Appalachians, but with a different tectonic history. The Northern Mauritanides crops out in the Moroccan Oulad Dlim massif, northwest of the Reguibat Rise. This part of the belt compares with the southern part, but additionally exhibits in the west a Silurian-Devonian sector that shows possible affinities with Gondwana-derived Appalachian terranes. The Western Meseta is only affected by Variscan events, which were mild in the westernmost Meseta Coastal Block, while the Eastern Meseta was also affected by Eo-Variscan events. The along-strike change from the Mauritanides to the Meseta orogen is interpreted as a transition from a head-on collision south of the South Meseta transform fault (SMF, precursor of the South Atlas Fault, SAF) to a dextral, transpressional collision north of the SMF. South of the SAF, the Anti-Atlas and the Dhlou-Zemmour expose the foreland foldbelts of the Meseta and northernmost Mauritanides. The Coastal Block was likely displaced from the south-westernmost Anti-Atlas during the Early Carboniferous.The Wilson Cycle concept mostly applied in that the Atlantic Ocean opened where the prior Rheic Ocean had closed. Possible exceptions are the Sehoul Block north of Western Meseta and the Silurian-Devonian Sector of the Oulad Dlim massif, which may have separated from NW-Africa and re-amalgamated to it during the Variscan orogeny. Likewise, a NW-African fragment from the Anti-Atlas may have stranded in offshore Massachusetts in eastern North America. An early Ediacaran-Cambrian Wilson cycle nested in the classic cycle introduced by Wilson (1966) occurred along the margin of NW-Africa, where Cadomian terranes rifted off Africa, and some were transferred to Europe and some accreted back to NW-Africa. This early cycle likely controlled the localization of the subsequent Rheic rift, and that of the Atlantic rift along the Mauritanides after the Variscan collision.

Discussion on ‘From Pan-African transpression to Cadomian transtension at the West African margin: new U–Pb zircon ages from the Eastern Saghro Inlier (Anti-Atlas, Morocco)’ by Errami et al . 2020 ( SP 503, 209–233)

An important paper has recently been published in Geological Society, London, Special Publications by Errami et al . (2020). On the basis of new U–Pb zircon radiometric data on magmatic and sedimentary rocks in the Saghro Massif, the authors revised the history of magmatic and geodynamic evolution of the Pan-African belts of the Moroccan Anti-Atlas along the northern edge of the West African Craton. The authors focused on the two main accretionary stages widely recognized in the Anti-Atlas (Ennih and Liegeois 2008; Gasquet et al. 2008): (1) a Paleoproterozoic episode, known as the Eburnean Orogeny, characterized by the extensive emplacement of granitoids at c. 2.1 − 2.0 Ga; (2) Neoproterozoic (800–450 Ma) events corresponding to the Pan-African Orogeny. However, in their lithostratigraphic framework of Proterozoic units of the Anti-Atlas, the authors do not account for the magmatic event recently recognized in several inliers in the Anti-Atlas (Fig. 1): Ighrem (Ikenne et al. 2017), Kerdous (Youbi et al. 2019) and Zenaga (Ait Lahna et al. 2020). Sills intrusive into the mixed carbonate–siliciclastic series of the Taghddout and Lkest groups, previously considered as Cryogenian, have been dated at between 1.7 and 1.6 Ga (Fig. 2). These new ages are c . 1 Ga older than previously thought, and call into question the stratigraphic position of the Tagdoute and Lkest groups in the southwestern domain of the Anti-Atlas. These series are now interpreted to be deposited in an intracratonic basin that developed after the assembly of the Nuna–Columbia supercontinent and before the break-up of the Rodinia supercontinent. The previous Cryogenian age for these deposits and their geodynamic setting thus need to be revised. Fig. 1. Geological map of the Anti-Atlas with the locations of the dated sills in the Ighrem, Kerdous and Zenaga inliers shown. SAF, South Atlas Fault; WAC, West African Craton; AVFZ, Ameln Valley Fault Zone; TTFZ, …

Joint modeling of potential-field data and geodynamic interpretation for northeast Algeria

The crustal structure beneath the northeastern continental part of Algeria is still largely unknown. Here we use potential field data processing, modeling and interpretation in order to investigate the deep sedimentary layers of this area and the transition with the basement. Indeed the gravimetric and magnetic field maps unveil the main geological domains of North Algeria: the AlCaPeKa domain, the Tellian Nappes, the Atlas and the South-Atlasic Fault (SAF). Our modeling approach combines the prediction of gravimetric and magnetic field signals along 4 N–S profiles that cross those domains and using constraints from seismic lines. The resulting cross-sections reveal the roots of the main E-W tectonic detachments of this area: the northern Suture Zone, the Tell Front, and the SAF. It seems that they are all initiated in the lower crust, nearby the continental/oceanic crustal transition for the Suture Zone, and several tenths of kilometers south from this transition for the Tell Front and the SAF detachments. Using these detachments, the lower parts of the crust are sometimes uplifted to only 5 km deep in the crust, while the upper crust is sometimes thickened to reach 15 km of thickness. Large magnetization and density contrasts are observed right beneath the Suture Zone.

Contribution of integrated geophysical data in the structural characterization of the El Hamma graben zone (Southern Tunisia): Example of a pull-apart basin along the South Atlasic Fault Corridor

The El Hamma-Gabes Region in southeastern Tunisia belongs to the crustal shear zone of the South Atlasic Fault Corridor, which corresponds to the Meso-Cenozoic North African margin. So far, most geological works on this region are on the stratigraphic order. However, there are few tectonic and structural studies carried on the style and mechanism of structural deformation. The present work analyzes the subsurface structure of the El Hamma grabens area based on gravity and seismic data, calibrated by stratigraphic and tectonic elements. The El Hamma-Gabes Zone is characterized by a horst and graben system associated with N130˚-150° trending parallel extensive/transtensive faults. Two main grabens are recognized: the El Hamma graben in the west and the Chenchou graben in the east. They are separated by the Jebel Hallouga horst. These structures relate to two N100˚-120° trending principal shear corridors (PSC) arranged in right echelon, being the Oum Ali-Fejej Shear corridor towards the northwest and the Matmata-Medenine Shear corridor towards the southeast.Stratigraphic and structural data collected from several outcrops were correlated with the tectonic events and paleostress reconstruction. New data shows that the PSCs marked a dextral transtensive kinematic framework during the N80° directed Upper Cretaceous extension. The extensional movements are the N150˚-180° Paleogene extension and associated transpression during the N130° Neogene shortening. The dextral sliding generated and maintained the opening of the El Hamma-Gabes graben system, creating a “pull-apart” basin that opened in an extensive ramp between the two PSCs. The overall extension of this ramp is estimated to 15% of the initial width.

Transient fluvial incision as an indicator of active faulting and Plio-Quaternary uplift of the Moroccan High Atlas

One of the challenges facing Earth Scientists is to determine the extent to which geomorphic features can be used to extract tectonic signals from landscapes. Here, we quantitatively analyse the long profiles of rivers that drain southwards across the South Atlas Fault (SAF), a thrust fault that forms the southern margin of the High Atlas Mountains in Morocco, to derive new data on the Late Cenozoic activity of this fault system. River long profiles were extracted for 32 major rivers flowing southwards into the Ouarzazate Basin. Of these, eleven exhibit concave-up river profiles with a mean concavity of 0.64 and normalized steepness indices in the range 47.5–219.0m0.9. By contrast, 21 rivers exhibit at least one knickpoint upstream of the thrust front. Knickpoint height varies from 100 to 1300m, with calculated incision at the range bounding fault ranging from 80 to 900m, despite the drainage areas upstream of the knickpoint ranging over several orders of magnitude. In map view, knickpoint locations generally plot along sub-parallel lines and there are no obvious relationships with lithological units for knickpoints exhibiting slope-break morphology. Channel reaches below slope-break knickpoints have higher mean concavities (0.76) than above the knickpoint indicative. This observation combined with a lithological or river-capture origin for the knickpoints having been ruled out suggests that an increase in uplift rate along a planar fault zone during the Plio-Quaternary caused the initiation of the transient response (i.e., knickpoint formation) to a change in base-level observed in the river profiles. This uplift event can be correlated to the convective removal of the lithospheric root to the Atlas Mountains resulting in the anomalously high topography at the present day.

Reactivated lithospheric-scale discontinuities localize dynamic uplift of the Moroccan Atlas Mountains: COMMENT

The Atlas Mountains of Morocco have received considerable attention in the past decade regarding the lithospheric structure and the uplift mechanisms of this structurally simple mountain belt. Miller and Becker (2014) have recently added to the discussion with new results from receiver functions and shear-wave splitting analysis. These results confirm the mantle upwelling hypothesis for the Atlas uplift that was postulated 10 years ago and we now see highlighted in several forums. The relationship between modest tectonic shortening and high topography in the Atlas was first addressed by Teixell et al. (2003). They concluded that crustal thickness is insufficient to account for the observed elevations, and proposed a mantle-driven (upwelling) component of uplift, supported by low seismic velocities in the Atlas mantle reported already by Seber et al. (1996). Subsequent efforts focused on modeling of gravity, the geoid, and topography that indicated a prominent lithospheric thinning, placing the lithosphere-asthenosphere boundary (LAB) at 70–90 km beneath the Atlas Mountains (e.g., Teixell et al., 2005; Zeyen et al., 2005; Missenard et al., 2006; Fullea et al., 2010). New contributions on the basis of active and passive seismology, partly under the Picasso project frame, are on their way (e.g., Bezada et al., 2013; Carbonell et al. 2013; Palomeras et al., 2013; Miller and Becker, 2014). Miller and Becker present a receiver functions profile that also shows a thinned crust and lithosphere beneath the Atlas Mountains. However, the thinned domain is bounded by abrupt steps coinciding with the location of the South Atlas thrust front; the authors propose offsets of 9 km of the Moho and 26 km of the LAB across the boundary thrust fault at Errachidia (Miller and Becker, 2014, their figure 2), arguing for the important role of lithosphericscale discontinuities in the Atlas mountain building. As for the crust, this is intriguing because early refraction studies by Wigger et al. (1992) and a more dense wide-angle survey by Carbonell et al. (2013) failed to detect a crustal step in this position, but especially because there is ample evidence that the South Atlas fault is a low-angle thrust zone (Saint Bezar et al. 1988; Teixell et al., 2003). Moreover, Miller and Becker imply that the same fault discontinuity goes down to offset the LAB, which means that the South Atlas thrust projects vertically to a depth of 90 km. This has doubtful geological sense even assuming that the present-day thrusts derive from the inversion of earlier extensional faults: those would also have a dip, and at a depth of 90 km they would be off the vertical projection of the fault surface exposure. Miller and Becker recognize that previous authors document shallowly dipping thrust faults from outcrop and seismic reflection studies, but they do not further discuss this issue. We are left questioning whether there is an enigmatic geological feature in the Atlas that escapes our understanding, or an issue of seismic data resolution. Miller and Becker compare the detected position of the LAB with a modeled position on the basis of the residual topography and on the assumption of isostasy, resulting in a large discrepancy, as the modeled LAB is even shallower than the actual base of the crust. This indicates a strong contribution of dynamic topography due to active mantle upwelling. We do not question the validity of dynamic topography for the Atlas, but we miss a discussion of the earlier potential field models which, on the basis of isostatic equilibrium, fitted the residual topography and the geoid with a greater lithospheric thickness, which would be in accord with the new receiver function results (see references above). Finally, we wish to conclude with a formal comment on the referencing to earlier work in the Atlas. Miller and Becker report a small orogenic shortening (15%–25%) inconsistent with Airy isostatic support and cite four papers, but none of them resulting in these small values of shortening nor do they address relationships between shortening and topography. Fullea et al. (2010) are not the primary reference for the distribution of Quaternary basalts, Frizon de Lamotte et al. (2009) do not provide cross sections to demonstrate shallowly dipping thrust faults, Sebrier et al. (2006) do not provide regional tomography models, and Beauchamp et al. (1999) do not suggest a mantle thermal anomaly as the source of volcanism in the Middle Atlas. Although these citations by Miller and Becker do not alter the main point of their paper, we wish to raise a word of caution for the persistence of incorrect citations.

Neogene exhumation of the Marrakech High Atlas (Morocco) recorded by apatite fission‐track analysis

AbstractApatite fission‐track (AFT) data have been obtained along a traverse across the Marrakech High Atlas to constrain its tectono‐thermal evolution. AFT ages vary between 212 ± 15 Ma and 20 ± 4 Ma. An Early Miocene AFT age accompanied by long mean track length from the central part of the chain has been interpreted as the timing of the main inversion of this region with the creation of relief because of the shortening induced by the interplay between the African and Eurasian plates. Thermal modelling of samples collected south of the South Atlas Fault Zone indicates a Middle‐Late Miocene or even later cooling that has been attributed to the component of the uplift of the chain related to the thermal anomaly present beneath the Atlas Mountains.

The magmatic evolution at the Moroccan outboard of the West African craton between the Late Neoproterozoic and the Early Palaeozoic

Abstract The Late Neoproterozoic Ouarzazate Group crops out on the north margin of the West African craton (WAC). In this group an important post-collisional magmatism is characterized by a great diversity in plutonic and volcanic rock types of the high-K calc-alkaline series. This series evolved mainly by crystal fractionation and by an important crustal contamination from an anomalous mantle source. The Early Cambrian magmatism began at the same time on both sides of the Anti-Atlas Major Fault, the southwestern side (Kerdous region) and northeastern side (Ouarzazate-Agdz region), interbedded in the Early Cambrian Basal Series and spread later to the Western High Atlas of the Morocco northern WAC outboard areas. This magmatism changes from a continental tholeiitic series (HPT and LPT) at the beginning to an alkaline series at the top (Adoudou and ‘Lie de vin’ formations). Fractional crystallization and pelagic or crustal contamination were the most important processes in the magma differentiation. The geochemical inversion from calc-alkaline to tholeiitic magmatism between the Late Neoproterozoic and the Early Cambrian is documented, as is the major extension of the tholeiitic activity on both sides of the South Atlas Fault. This geochemical variation indicates a transition of the tectonic regime from compressive to extensional. The late local Jbel Boho alkaline magmatism indicates the sink of the source and the mitigation or closure of the extensional cycle at this time.

Deep-rooted “thick skinned” model for the High Atlas Mountains (Morocco). Implications for the seismic Eurasia-Africa plate boundary region

Previous crustal models of the High Atlas suppose the existence of a mid-crustal detachment where all the surface thrusts merged and below which the lower crust was continuous. However, both seismic refraction data and gravity modeling detected a jump in crustal thickness between the High Atlas and the northern plains. Here we show that this rapid and vertical jump in the depth of Moho discontinuity suggests that a thrust fault may penetrate the lower crust and offset the Moho (deep-rooted “thick skinned” model). The distribution of Neogene and Quaternary volcanisms along and at the northern part of the High Atlas lineament can be related to the beginning of a partial continental subduction of the West African plate to the north underneath Moroccan microplate. Allowing from the complex problem of the plate boundary in the western zone of the Mediterranean, we propose to interpret the South-Atlasic fault zone as the actual northwestern boundary of the stable part of the African plate rather than the Azores-Gibraltar fault currently used.

Age of the Wirgane granodiorite intrusions (Western High-Atlas, Morocco): New U–Pb constraints

U–Pb geochronological data, obtained on single zircon grains using the ion microprobe Cameca IMS 1270 (CRPG-CNRS, Nancy), indicate a Neoproterozoic age (625 ± 5 Ma) for the two intrusions of Wirgane granodiorite. This age is in variance with the previously published conclusions where the intrusions were supposed Variscan. Consequently, the Wirgane intrusions are interpreted as remnants of the Neoproterozoic basement of the Variscan belt. They are similar to the Anti-Atlas calc-alkaline intrusions of the collisional to post-collisional stages of the Pan-African orogeny. At that time, the Wirgane region was an area of continental growth to the north of the major South Atlasic Fault.

Deep-rooted “thick skinned” model for the High Atlas Mountains (Morocco). Implications for the structural inheritance of the southern Tethys passive margin

A re-interpretation of the deep structure of the High Atlas is presented through integration of geophysical and geological data, highlighting the architectural significance of the southern Tethys palaeomargin. Previous crustal models suggest the occurrence of a flat intra-crustal detachment at a depth of −20 km, a zone where surface thrusts merge and below which the lower High Atlas crust appears continuous. However, within this study seismic refraction data, electrical resistivity surveys and gravity modelling all appear to detect a jump in crustal thickness between the High Atlas and the northern plains. We interpret these data as penetration by thrusts within the “South-Atlasic fault” zone through the lower crust to offset the Moho in accordance with a “deep-rooted thick skinned tectonic model”. The “South-Atlasic fault” zone corresponds to a series of crustal-scale structures inherited from a series earlier orogenic events (Pan-African, Hercynian and Atlasic) and thus is sufficiently weak to create separate crustal blocks in the northern part of the stable African plate. This structural configuration was achieved during Cenozoic plate collision between Africa and Europe culminating in tectonic inversion of Mesozoic extensional basins, linked to the development of Tethys and opening of the Atlantic. The inherited structures typically comprise normal faults and tilted blocks affecting the upper crust, and crustal-scale detachments affecting the lower crust.

Active tectonics in the Moroccan High Atlas

Review of seismological and structural data coupled with new data on topographical, geomorphology, and Quaternary geology allows delineating the major active faults of the High Atlas. These are the North and South border faults of which fault segmentations correspond to Mw max ranging between 6.1 and 6.4. Detail active tectonics analyses were performed on the South Atlas Fault Zone in the Souss and Ouarzazate basins, where deformed Quaternary levels permit to estimate slip rates on individual faults in the order of 0.1 mm yr −1. Such low slip rates imply that large observational time-window is needed to analyze active deformation in low-seismicity regions. However, the complex 3D geometry of reverse or thrust faults may cause difficulty to relate surface observations with the deeper faults that have the potential to nucleate big earthquakes. Further studies are necessary to interpret the Anti Atlas seismicity. To cite this article: M. Sebrier et al., C. R. Geoscience 338 (2006).

Calc-alkaline Arc I-type Granitoid Associated with S-type Granite in the Pan-African Belt of Eastern Anti-Atlas (Saghro and Ougnat, South Morocco)

The Sidi Flah and Ougnat inliers are located in the eastern Anti-Atlas antiform between the Anti-Atlas Major Fault (AAMF) and South Atlas Fault (SAF). They consist of many granitoid intrusions emplaced into Neoproterozoic metasedimentary rocks and surmounted by upper Neoproterozoic A-type granites. The Sidi Flah (Saghro) and Ougnat granitoids are part of the Neoproterozoic magmatic activity related to northwards subduction of an oceanic plate beneath the Saghro continental margin. They are post-orogenic I- and S-type granitoids related to the ending of the compressional deformation in this Pan-African belt. A petrographic, geochemical and zircon typology study leads us to subdivide these rocks into three magmatic groups: (1) a medium- to high-K calc-alkaline group formed by quartz diorites and amphibole granodiorites is found in both Sidi Flah and Ougnat inliers; (2) a high-K calc-alkaline group is present in Sidi Flah. These two groups have a (deeper and) hybrid mantle-crust origin; (3) a peraluminous group in Ougnat is linked to the post-collisional setting and has a shallow crustal source. On a primitive mantle-normalized trace-element diagram, almost all of these rocks show a significant Nb depletion relative to K and La, which is typical of the calc-alkaline magmatism from the subduction-zone environment. Absence of structural marks of thrusting upon the West African craton (WAC) of this arc system and the ophiolitic suite in Bou-Azzer, and the presence of Imiter muscovite-bearing granite as part of Pan-African belt do not support the localization of northern limit of WAC at the level of SAF.

The Moroccan Anti-Atlas: the West African craton passive margin with limited Pan-African activity. Implications for the northern limit of the craton

Since 1980, several diverging models for the Pan-African fold-belt of the Anti-Atlas mountains have been proposed. Differences mainly result from diverging interpretations of geological, geochemical and geochronological data. However, the fundamental point is the lack of a detailed lithostratigraphic framework of the Neoproterozoic successions, supported by geochronological data and regional correlations. Furthermore, these models did not evolve from multidisciplinary studies which are essential in the case of this belt, whose structure is complex and diversified. We discuss here some points taken as a base by Ennih and Liegeois to conclude that the entire Anti-Atlas is underlain by Eburnian crust and that the northern limit of the West African craton (WAC) and its passive margin is located at the South Atlas Fault (SAF). This led the authors to propose a new model of the Anti-Atlas belt, which also explains the origin of the Khzama-Bouazzer ophiolite. 1. Evidence for a Palaeoproterozoic basement in Saghro?

Triassic stratigraphy and structural evolution of a rift basin: the Eç Çour basin, High atlas of Marrakech, Morocco

Stratigraphic and structural analysis of the Triassic sedimentary basins of the High Atlas of Marrakech, in particular of the Eç Çour basin, indicates that the Proterozoic–Paleozoic block (“Massif Ancien”) formed a paleogeographic high during this period, with the basement block acting as an east–west trending horst, separating two large half-grabens to the north and to the south. The marginal half-grabens, which were filled by red beds essentially of continental origin, were most likely linked via transverse pathways. In the Eç Çour basin, the Triassic sedimentary deposits are subdivided into three formations of which the first two are continental and the third shows marine influences. Structurally, this area is characterized by two main (ENE–WSW and NNE–SSW) fault directions. The ENE–WSW trending faults, which include the major South Atlas fault, have an extended history of displacement during pre-Triassic Hercynian to Late Hercynian and during Triassic times. In contrast, the NNE–SSW-striking faults appear to have only a syndepositional Triassic history with, rarely, a late Hercynian heritage. This structural pattern is the result of a pure extensional regime during which synsedimentary normal faulting caused the assymmetry of the basin. This was reflected by high subsidence rates in the NE that progressively decrease to the NW towards the rising Proterozoic–Paleozoic massif of the High Atlas. During the tertiary Alpine–Atlasic orogeny, the ENE–WSW-striking normal faults were inverted.

The Moroccan Anti-Atlas: the West African craton passive margin with limited Pan-African activity. Implications for the northern limit of the craton

The Moroccan Anti-Atlas region, located south of the South Atlas Fault, has been viewed traditionally as containing two segments separated by the Anti-Atlas Major Fault. These two segments are said to consist of: (a) 600–700 Ma Pan-African segment located in the northeast; and (b) ∼2 Ga Eburnian segment situated to the southwest. On the basis of observations in the Zenaga and Saghro inliers and of a recent literature review, we suggest that this subdivision is inappropriate in that Eburnian and Pan-African materials occur throughout the Anti-Atlas region: the entire Anti-Atlas is underlain by Eburnian crust, unconformably overlain by a lower Neoproterozoic passive margin; allochthonous Pan-African ocean crustal slices were thrust onto the West African craton (WAC) passive margin sequence ∼685 Ma ago as a result of Pan-African accretion tectonics; high-level high-K calc-alkaline and alkaline granitoids locally intruded the Anti-Atlas sequence as a whole at the end of the Pan-African orogeny at 585–560 Ma; the intervening 100 m.y. interval was marked by quiescence. This succession of events can be related to the behaviour of one single rigid cratonic passive margin during an orogeny and correlated to the Pan-African events that occurred to the east in the Tuareg shield and to the north in Avalonian terranes. This model implies that the actual northern limit of the WAC is located at the South Atlas Fault (SAF) and not at the Anti-Atlas Major Fault (AAMF). We propose that the AAMF corresponds to the southwestern boundary of an aulacogen that formed along the northern margin of the WAC during early Neoproterozoic times. This is consistent with the development of the Gourma aulacogen on the eastern side of the WAC. This model further suggests that the northeastern boundary of the WAC occurs north, and not south, of Ougarta (Algeria). Support for this model is provided by geological and geophysical evidence.

Giant Miocene landslides and the evolution of Fuerteventura, Canary Islands

Fuerteventura is the oldest island in the Canaries archipelago; following the growth of a pre-Miocene seamount, its shield stage began 20.6 Ma ago, and ended in the mid-Miocene. There followed a very extended period of quiescence, then very minor Quaternary post-erosional volcanism. The shield stage produced volcanoes up to 3000 m above sea level which were rapidly eroded by 17.5 Ma. Both Fuerteventura and Lanzarote, the older eastern-most islands, are in a post-erosional stage and have subdued topography with elevations rarely rising above 400 m. The western islands, La Palma and El Hierro, together with Tenerife, are the youngest in the archipelago, and are still in their shield stage which began at most 7.5 Ma ago. They have a rugged topography with peaks rising to several thousand metres. Volcanic activity persists in these islands and in Lanzarote. The shield-stage construction of the volcanically active western islands demonstrates that recent eruptive activity has concentrated in volcanic centres aligned in well-defined rift zones in which major dyke emplacement has taken place, and that volcano flank collapse has occurred with giant landslides initiated on the rifts. In Fuerteventura, most of the shield-stage volcanic rocks have been removed; however, evidence has now been advanced to show that major dyke swarms were emplaced in the Oligocene and Miocene. These belonged to a line of intrusive complexes above which, by the early to mid-Miocene, volcanic peaks had been constructed. Three volcanic centres have been identified and in the central one, a peak as high as the present Mount Teide on Tenerife was rapidly denuded in less than 2 Ma. In the west of the island, this erosion has exposed a window of more than 300 km2 of submarine volcanics and sediments belonging to the pre-shield phase seamount. The principal mechanism of erosion seems to have been the generation of giant landslides analogous to those seen in the younger islands. These appear to have removed some 3500 km3 of lavas and volcaniclastics, stripping the western sectors of the Miocene volcanoes down to the pre-shield phase “Basal Complex”, and transporting the mass of volcanic material into the Atlantic ocean as debris flows. Remnants of the shield volcanoes still exist in the eastern part of the island. Present day topographic variation indicates that the Miocene base level of erosion is still visible in a “Central Depression” which runs through the axis of the island, the eastern hills are composed of the Miocene shield-stage volcanics which still preserve a drainage pattern initiated on these peaks, and the “Basal Complex” in the west has a juvenile landscape suggesting incision on a post-Miocene domal uplift of the volcano core. Structural links between intra-island volcano-tectonic events and the South Atlas Fault zone are not thought to be likely.