Late extension of a passive margin coeval with subduction of the adjacent slab: The Western Alps and Maghrebides files

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

Première découverte de l’ichnogenre <i>Selenichnites</i> du Jurassique moyen dans le synclinal de Skoura (Moyen Atlas, Maroc) ; contexte paléoécologique et paléoenvironnemental

Des traces d’invertébrés, attribuées à l’ichnogenre Selenichnites , ont été découvertes dans les couches d’âge Bajocien supérieur–Bathonien inférieur (formation d’Ich Timellaline/Bou Akrabène) du synclinal de Skoura, dans le Moyen Atlas marocain. Conservées sur une dalle gréso-calcaire subhorizontale, ces traces montrent des formes en croissant ou fer à cheval avec, dans le spécimen le mieux conservé, une empreinte arrière selon l’axe médian (trace du telson caudal). Elles sont interprétées comme des traces d’enfouissement (fodinichnia) de Xiphosures (= Limules) cherchant leur nourriture sur le fond sablo-carbonaté ou sous un voile de vase le recouvrant. Les caractères sédimentologiques indiquent des dépôts subtidaux dans une zone peu profonde relativement abritée. Ces paléoenvironnements littoraux précèdent la régression bathonienne du domaine atlasique. Cette découverte permet d’établir la présence de Xiphosuridés sur les rivages sud de la Téthys dès le Jurassique moyen.Mesozoic strata of North Africa yield the first occurrence of the ichnogenus Selenichnites. The trace fossils occur on the top surface of a sandy carbonate deposit in the axis of a Middle Atlas syncline (Skoura Syncline, NE Morocco). The ichnofossil-bearing horizon belongs to the Late Bajocian–Early Bathonian Ich Timellaline/Bou Akrabene Formation. The trace fossils are crescent-shaped and the best preserved exhibits a posterior central axial impression (possible telson tail impression). They are interpreted as feeding burrows (fodinichnia) or hiding depressions of Xiphosurids or Limulids (horseshoe crabs) on a sandy carbonate substrate beneath a veneer of muddy deposits. The sedimentological character suggests a relatively protected shallow water subtidal palaeoenvironment preceding the Bathonian regression of the Atlas domain. This discovery provides the first evidence of xiphosurans or xiphosuran-like organisms inhabiting the southern shores of the Tethys in the Middle Jurassic.</p

The West African Craton and its margins. Foreword

International audienceThe present thematic issue is dedicated to the West African Craton (WAC) and its margins (Fig. 1). The project was launched during the "First West African Craton and Margins International Workshop", which took place in Dakhla (Morocco) on 24-29th April 2017. The aim of this meeting was to confront the results of different teams working on various topics, mainly in the northwestern parts of the WAC and their Pan-African to Present margins. The workshop, organized basically by one of us (O. S.) and the University Hassan II of Casablanca, met with great success. Indeed, the Moroccan part of the WAC and the adjoining Oulad Dlim massif (aka Adrar Souttouf) are under the spotlights. Mapping programs, metallogenic prospection, and academic research backed on petrological and geochronological analyses have been intensely focused on the formerly Spanish Rio de Oro, which became Moroccan in 1975. These recent (and ongoing) studies have greatly improved our understanding of the geological structure and tectonic-stratigraphic evolution of the area

Late extension of a passive margin coeval with subduction of the adjacent slab: TheWestern Alps and Maghrebides file

The evolution of the Alpine Tethys margins during the beginning of the African-Eurasian convergence (Upper Cretaceous) was little studied compared to their evolution during the post-Pangea rifting and oceanic expansion, i.e., from the Early Jurassic to the Early Cretaceous. The aim of the present work is first 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 magma-poor passive margin was affected by a systemic extension in Late Cretaceous-Paleocene times. Remarkably, this extensional tectonics shortly preceded Lutetian times, when Briançonnais margin encroached the SE-dipping subduction zone under the Adria microplate (“Alpine subduction”). Secondly, we aim to assess the Late Cretaceous-Paleocene evolution of the north-Tethyan paleomargin in the Maghrebides transects, i.e., south-west of the Briançonnais transect along the same European-Iberian margin. For this purpose, we consider the Triassic-Eocene series of the "Dorsale Calcaire" in the Alboran, Kabylias and Peloritan terranes that constitute with Calabria the Alkapeca blocks formerly located along the southeastern border of Iberia until the Eocene. Reinterpretation of the literature allows us to assert that the Tethyan margin of these blocks was extending like the Briançonnais during the Late Cretaceous-Paleocene, when Africa-Eurasia-Iberia convergence and then subduction of the intervening Tethyan slab were active. We propose here for the first time that the subduction of the Ligurian-Maghrebian slab occurred under the North African margin at that time in the southward continuation of the Alpine subduction. In the Alboran transect, the Rif-Betic Dorsale Calcaire can be seen as the detached cover of the thinned crust of the Alpujarrides-Sebtides Complex. In the same transect, the oceanic domain may have included a continental allochthon of African origin (Ketama Unit). Contrary to some assertions, the North African margin did not experience significant compression during the Cretaceous. 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 and corresponds to the backthrusting tectonic phase in the Western Alps