Between the High Atlas and the Saharan platform, the Anti-Atlas of Morocco offers large exposures of Precambrian rocks beneath the moderately folded Paleozoic series. These inliers allow ...reconstructing a segment of the Pan-African Belt and of its foreland at the northern outskirts of the West African Craton (WAC). From ∼885 Ma to ∼540 Ma, three periods are recognized in the Pan-African cycle. The Tonian–Cryogenian period ends with the obduction of supra-subduction ophiolite and oceanic arc material at ∼640 Ma. The Early Ediacaran period is marked by the development and subsequent closure of a wide marginal basin next to a likely Andean-type arc. The Late Ediacaran period is recorded by subaerial molasse deposits associated with post-collisional high-K calc-alkaline to shoshonitic magmatism. Although a wide consensus has been reached based on the number of new robust datings, several questions still remain pending, which we address taking into account relevant African and European correlations.
The Raman Spectroscopy of Carbonaceous Materials (RSCM) geothermometry approach allows determining the peak temperature recorded by metasediments through their metamorphic history. This technique, ...however, has been calibrated using Meso-Cenozoic metapelitic rocks that underwent a single metamorphic cycle. Until now, the reliability of the RSCM method has never been demonstrated for contexts with superposition of regional and contact metamorphism, such as many Variscan contexts. The present study aims at testing the applicability of the RSCM method to these polyphased metamorphism terrains and at investigating the cumulative molecular transformations of carbonaceous materials related to metamorphic superposition.
To address the above issues, samples were collected in the Variscan Jebilet massif of the Moroccan Meseta. This massif was first affected by a regional, greenschist facies metamorphic event (D1 phase), and then by a higher-T, regional and contact metamorphism that reached the hornfels/amphibolite facies conditions (D2 and D2/D3 phases). Mineralogical, thermobarometric and RSCM methods have been used in this study to determine the peak T recorded by the studied rocks. The results obtained for greenschist facies metapelitic rocks show a good agreement between the mineralogical assemblage Chlorite–Phengite–Felspar–Quartz and the Raman temperatures ranging from 330 to 394±50°C. In the metapelitic rocks that underwent higher metamorphism grades (hornfels/amphibolite facies), four dominant mineral assemblages were observed: (1) Chlorite–Biotite, (2) Cordierite–Biotite, (3) Andalusite–Garnet–Bt, and (4) Andalusite–Cordierite–Biotite. The corresponding Raman temperatures vary respectively between 474±50°C and 628±50°C. The pseudo-sections generated for samples from the hornfels/amphibolite facies confirmed the peak temperatures measured by the RSCM method. Our results do not support clear evidence of potential molecular cumulative effect on CM triggered by overprinted metamorphism. Therefore, the RSCM method is suitable to investigate the peak temperature within a polymetamorphic context. We also note the accuracy of the RSCM geothermometer to delimit the metamorphic area due to hidden intrusions. Concerning the specific case of the Jebilet massif, we emphasize the occurrence of the mineral assemblage Garnet–Staurolite likely developed during the regional metamorphism, which compares with the evolution of the Rehamna massif farther in the north.
•We examine the applicability of the RSCM in polymetamorphic contexts.•We use rocks of different metamorphism grade to compare mineralogical and RSCM data.•No clear molecular cumulative effect on CM structure, evolved under regional and contact metamorphisms, was observed.•The new geodynamic interpretation of Grt–St assemblage shows that the metamorphism of the Jebilet massif needs more studies.
The evolution of the Alpine Tethys margins during the beginning of the African-Eurasian convergence was little studied compared to their evolution during the post-Pangea rifting and oceanic ...expansion,
i.e.
, from the Early Jurassic to the early Late Cretaceous. The present work firstly aims to make up for this shortcoming in the case of the distal European margin of the Alpine Tethys, namely the Briançonnais domain of the Western Alps. We show that this margin was affected by strong post-rifting extension mainly in Late Cretaceous-Paleocene times and propose to make it the type of the (rare) “Late Extension Passive Margins”. Remarkably, this extension shortly preceded Lutetian times, when Briançonnais margin encroached the SE-dipping subduction zone under the Adria microplate. Secondly, we assess the post-rifting evolution of the north-Tethyan paleomargin in the Maghrebides transects,
i.e.
, south-west of the Briançonnais transect along the same European-Iberian paleomargin. For this purpose, we consider the Triassic-Eocene series of the “Dorsale Calcaire” in the Alkapeca Blocks located along southeastern Iberia until the Eocene then transported onto the North African margin. Examination of the literature shows that the Tethyan margin of the Alboran block was strongly affected by normal faulting as early as Late Jurassic-Early Cretaceous times whereas post-rifting extension of the Kabylian blocks mainly occurred in the Late Cretaceous-Paleocene like in the Briançonnais. We propose that post-rifting extension of the Alboran block southern margin resulted from the sinistral movement of Africa relative to Iberia while the later extension of the Kabylian blocks can be related to the further convergence kinematics. Subduction of the Ligurian-Maghrebian slab under the North African margin would have occurred at that time in the southward continuation of the Alpine subduction. The overriding Adria and North African margins did not experience significant compression at that time. During the Eocene, a subduction polarity reversal occurred, which was associated with the relocation of the subduction zone along the Alkapeca block. This was the beginning of the Apenninic subduction, which triggered the back-arc opening of the Mediterranean basins.
L’évolution des marges de la Téthys alpine pendant le début de la convergence Afrique-Eurasie au Crétacé supérieur a été peu étudiée par rapport à leur évolution du Lias au Crétacé inférieur, pendant le rifting post-Pangéen et l’expansion océanique. L’objectif du présent travail est en premier lieu de pallier cette lacune dans le cas du Briançonnais, marge européenne distale de la Téthys alpine. Nous montrons que cette marge a été le siège d’une forte extension au Crétacé supérieur-Paléocène et proposons d’en faire le type des rares « Marges Passives à Extension Tardive ». Fait remarquable, cette extension tardive est contemporaine de la subduction de la croûte téthysienne sous la microplaque Adria, le Briançonnais entrant lui-même dans la zone de subduction dès le Lutétien. En second lieu, nous revisitons l’évolution de la bordure nord-téthysienne sur plusieurs transects des Maghrébides, au sud-ouest du transect Briançonnais mais sur la même marge européenne. Nous considérons la série triasico-éocène de la “Dorsale Calcaire” dans les blocs Alkapeca riftés de la bordure sud-est d’Iberia puis transportés après l’Eocène sur la marge nord-africaine. L’examen de la littérature montre que la marge téthysienne du bloc d’Alboran a été fortement affectée par des failles normales dès la fin du Jurassique-début du Crétacé alors que l’extension post-rifting des blocs kabyles s’est principalement produite au Crétacé supérieur-Paléocène comme dans le Briançonnais. Nous proposons que l’extension post-rifting de la marge méridionale du bloc d’Alboran résulte du mouvement sénestre de l’Afrique par rapport à l’Ibérie, tandis que l’extension des blocs kabyles peut être liée à la cinématique ultérieure de convergence. La subduction de la plaque liguro-maghrébine sous la marge nord-africaine se serait produite à cette époque dans le prolongement sud de la subduction alpine. Les marges des plaques supérieures Adria et Africa n’ont pas subi de compression significative à cette époque. Au cours de l’Éocène, une inversion de la polarité de la subduction s’est produite, associée à la relocalisation de la zone de subduction le long du bloc Alkapeca. C’est le début de la subduction apenninique, responsable de l’ouverture arrière-arc des bassins méditerranéens.
The E–W trending Atlas System of Maghreb consists of weakly shortened, intra-continental fold belts associated with plateau areas (“Mesetas”), extending between the south-westernmost branch of the ...Mediterranean Alpine Belt (Rif-Tell) and the Sahara Platform. Although the Atlas system has been erected contemporaneously from Morocco to Algeria and Tunisia during the Middle Eocene to Recent, it displays a conspicuous longitudinal asymmetry, with i) Paleozoic outcrops restricted to its western part; ii) highest elevation occurring in the west, both in the Atlas System and its foreland (Anti-Atlas); iii) low elevation corridors (e.g. Hodna) and depressed foreland (Tunisian Chotts and Sahel area) in the east. We analyse the origin of these striking contrasts in relation with i) the Variscan heritage; ii) crustal vertical movements during the Mesozoic; iii) crustal shortening during the Cenozoic and finally, iv) the occurrence of a Miocene–Quaternary hot mantle anomaly in the west. The Maghreb lithosphere was affected by the Variscan orogeny, and thus thickened only in its western part. During the Late Permian–Triassic, a paleo-high formed in the west between the Central Atlantic and Alpine Tethys rift systems, giving birth to the emergent/poorly subsident West Moroccan Arch. During the late Middle Jurassic–Early Cretaceous, Morocco and western Algeria were dominantly emergent whereas rifting lasted on in eastern Algeria and Tunisia. We ascribe the uplift of the western regions to thermal doming, consistent with the Late Jurassic and Barremian gabbroic magmatism observed there. After the widespread transgression of the high stand Cenomanian–Turonian seas, the inversion of the Atlas System began during the Senonian as a consequence of the Africa–Eurasia convergence. Erosion affected three ENE-trending uplifted areas of NW Africa, which we consider as lithospheric anticlines related to the incipient Africa–Europe convergence. In contrast, in eastern Algeria and Tunisia a NW-trending rift system developed contemporaneously (Sirt rifting), normal to the general trend of the Atlas System. The general inversion and orogenesis of the Atlas System occurred during two distinct episodes, Middle–Late Eocene–Oligocene and Late Miocene–Pliocene, respectively, whereas during the intervening period, the Africa–Europe convergence was mainly accommodated in the Rif-Tell system. Inversion tectonics and crustal thickening may account for the moderate uplift of the eastern Atlas System, not for the high elevation of the western mountain ranges (Middle Atlas, High Atlas, Anti-Atlas). In line with previous authors, we ascribe part of the recent uplift of the latter regions to the occurrence of a NE-trending, high-temperature mantle anomaly, here labelled the Moroccan Hot Line (MHL), which is also marked by a strip of late Miocene–Quaternary alkaline magmatism and significant seismicity.
This contribution analyses the structural architecture and tectono-metamorphic evolution of Briançonnais units in the southern French-Italian Western Alps. The studied area extends from a virtually ...non-metamorphic area adjacent to the Helvetic-Dauphinois External Domain in the west to the Monviso-Queyras ocean-derived units in the east, where metamorphism increases up to eclogite-facies. Mapping at the scale 1:10.000 of a mountainous part of the Ubaye-Maira transect was performed, which portrays in detail the Briançonnais units over ~ 100 km
2
. The lithologies include meta-andesite, meta-siliciclastics, marbles, turbiditic calcschists and flysch whose age spans from Late Carboniferous to Eocene. Metamorphism is low-grade greenschist-facies in the west but reaches the blueschist-facies to the east. Structures related to four phases of deformation are identified. The first two generations of structures (D1–D2), related to the original top-to-the-west nappe stacking, are associated with conditions close to the peak of Alpine metamorphism. Previous tectonic surfaces are transposed by the dominant deformation structures (D3), developed under retrograde/decompression conditions. This D3 corresponds to a backfolding and back-thrusting event with a top-to-the-E transport. A fourth phase (D4) developed during late low-grade metamorphic conditions and deforms the previous surfaces by locally developing a crenulation cleavage, followed by brittle tectonics. An updated metamorphic map is presented, backed on published estimates and new thermometric data obtained by Raman Spectroscopy of Carbonaceous Material (RSCM). The T
RSCM
values range from ~ 295 °C to > 350 °C, moving from the most external Briançonnais unit to the internal Queyras “Schistes Lustrés” units. Suspected Upper Cretaceous palaeofaults have been documented, allowing us to group the classic Aiguille de Mary and Ceillac (sub-) units into a single tectonic unit, here referred to as Maurin unit. The palaeogeographical reconstruction of the evolution of the studied transect highlights the importance of the Marinet and Maurin axial units in the feeding of the Late Cretaceous-Eocene breccias deposited on the more internal Acceglio-type units.
Terra Nova, 23, 314–323, 2011
We question for the first time the origin of enigmatic structures, herein termed syncline‐topped anticlinal ridges (STARs). In the Central High Atlas (CHA) of Morocco, ...small synclines of Upper Palaeocene‐? Eocene deposits are only preserved on top of Triassic–Jurassic anticlinal ridges. We explain these peculiar structures through a three‐step evolution: (i) early halokinetic evolution leading to the formation of elongated diapiric ridges over basement faults, ending with magmatic intrusions and enhanced diapiric ascent up to the surface; (ii) Palaeocene‐? Eocene unconformable sedimentation; (iii) Late Eocene–Quaternary shortening phases, which resulted in the erosion of the Palaeocene‐? Early Eocene deposits, except in the breached anticlinal axes. The comparison with the breached valley (‘`makhteshim'’) of the Syrian Arc in the Negev Desert allows us to emphasize the role of the early diapiric evolution of the CHA domain in the genesis of the STAR structure.
Here we describe the structure, the high-pressure, low-temperature (HP-LT) metamorphism and tectonic evolution of the Briançonnais distal margin units from the south Western Alps. The studied area ...extends southwest of the Dora-Maira (U)HP basement units and east-southeast of the classical Briançonnais nappes. A new structural map accompanied by geological profiles shows the thrusting of the oceanic nappes (Monviso and Queyras units) onto the distal Briançonnais units (D1 and D2 late Eocene deformation phases) under blueschist-facies conditions. Subsequent deformation during the Early Oligocene (D3 deformation phase) took place under greenschist-facies conditions and was associated with back-folding and -thrusting in the units overlying the Dora-Maira massif and with exhumation related to normal reactivation of former thrusts within the latter massif. Two large cover units, detached from their former distal Briançonnais basement, are redefined as the Maira-Sampeyre and Val Grana Allochthons (shortly: Maira-Grana Allochthons = MGA) including, (i) the Val Maira-Sampeyre unit involving Lower and Middle Triassic formations, seemingly detached from the Dora-Maira units during the subduction process, and (ii) the Val Grana unit with Middle-Upper Triassic and Early-Middle Jurassic formations, which was probably detached from the Maira-Sampeyre unit and correlates with the “Prepiemonte units” known from the Ligurian Alps to the Swiss Prealps. Three major shear zones involving tectonic mélanges of oceanic and continental rocks at the base of the Val Grana, Maira-Sampeyre and Dronero units testify to an early phase of exhumation within the subduction channel in front of the Adria plate. We present a new metamorphic map based on published and new petrological data, including new thermometric data obtained by Raman spectroscopy of carbonaceous material (RSCM). The T
RSCM
values range from ~ 400 °C to > 500 °C, going from the most external Val Grana unit and overlying Queyras schists to the uppermost Dora-Maira unit. During the Late Triassic, the width of the Briançonnais
s.l.
domain can be restored at ~ 100 km, whereas it reached ~ 150 km after the Jurassic rifting. A significant, second rifting event affected the Briançonnais domain during the Late Cretaceous-Paleocene, forming the Longet-Alpet chaotic breccias, which deserve further investigations.
The timing and process of exhumation of the subcontinental peridotites of the Gibraltar Arc (Ronda, Beni Bousera) have been discussed extensively over the last decades. In this work, we contribute to ...this debate through the first mapping, structural and petrological analyses, and SHRIMP U-Th-Pb dating of high-grade marbles that crop out around the Beni Bousera antiform of the Alpujarrides-Sebtides units of northern Rif (Morocco). These marbles, here termed the Beni Bousera marbles (BBMs), instead of being intercalations in the granulitic envelope (kinzigites) of the Beni Bousera peridotites, as previously described, form minor, dismembered units within a ∼30 to 300 m thick mylonitic contact between the kinzigites and the overlying gneisses of the Filali Unit (Filali-Beni Bousera Shear Zone, FBBSZ). They display silicate-rich dolomitic marbles, sandy-conglomeratic calcareous marbles and thinly bedded marble with interleaved biotite-rich schists. An unconformable contact, either of stratigraphic or tectonic origin, with the underlying kinzigites, is observed locally. Pebbles or detrital grains include K-feldspar, quartz, almandine garnet and zircon. Peak mineral assemblages consist of forsterite, Mg-Al-spinel, geikielite (MgTiO3), phlogopite and accessory zirconolite, baddeleyite and srilankite in dolomite marble, as well as K-feldspar, scapolite, diopside, titanite and accessory graphite and zircon in calcite marble. These assemblages characterize peak HT-LP metamorphic conditions close to 700-750°C, ≤4.5 kbar. The FBBSZ includes minor ductile thrusts that determine kinzigite horses or slivers carried NW-ward over the marbles. Within the latter, NNE-trending folds are conspicuous. Brittle, northward-dipping normal faults crosscut the FBBSZ ductile structures. Detrital cores of zircon from the BBMs yield two U-Th-Pb age clusters of ∼270 Ma and ∼340 Ma, whereas their rims yield ∼21 Ma ages. Correlations with comparable settings in other West Mediterranean Alpine belts are discussed. The BBMs compare with the Triassic carbonates deposited over the crustal units of the Alpujarrides-Sebtides. The assumed Triassic protoliths may have been deposited onto the kinzigites or carried as extensional allochthons over a detachment in the Early Jurassic during the incipient formation of the Alboran Domain continental margin. Thus, it is concluded that the Beni Bousera mantle rocks were exhumed to a shallow depth during early rifting events responsible for the birth of the Maghrebian Tethys.
This work concerns the northernmost limit of the West African Craton (WAC) and Variscan WAC-related terranes of NW Africa. Based on newly obtained radiometric age of an oceanic gabbro from the ...“Mesorif Suture Zone” of the External Rif Belt, we propose a revised interpretation of this puzzling lineament. We report on a 190±2 Ma LA–ICP–MS U–Pb zircon age of a trondhjemite vein cross-cutting the Bou Adel gabbro, which is one of the largest oceanic units of the quoted suture zone. We previously interpreted the arcuate MSZ in terms of transported, hyper-extended margin of the Alpine Tethys, based on a K–Ar 166±3 Ma age ascribed to the Bou Adel gabbro in the literature. The new, Early/Middle Liassic age coincides instead with the onset of oceanic floor formation in the Central Atlantic. We hypothesize that the Mesorif suture zone corresponds to the transported trace of the West African Atlantic margin surrounding the northwestern Moroccan Meseta promontory and connecting with the ENE-trending North African Transform North African transform. The latter zone sharply bounded the North Africa margin and connected the Central Atlantic with the Alpine Tethys. We propose that transported elements from the North African transform constitute the “Mesorif Basalt–Breccias” lineament parallel to and more external than the Mesorif suture zone. If correct, this new interpretation provides an opportunity to develop detailed field and laboratory studies of an exhumed segment of the up-to-now conceptual Jurassic North African transform.
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