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 Brianconnais 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 Brianconnais 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 Brianconnais 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 Brianconnais. 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'evolution des marges de la Tethys alpine pendant le debut de la convergence Afrique-Eurasie au Cretace superieur a ete peu etudiee par rapport a leur evolution du Lias au Cretace inferieur, pendant le rifting post-Pangeen et l'expansion oceanique. L'objectif du present travail est en premier lieu de pallier cette lacune dans le cas du Brianconnais, marge europeenne distale de la Tethys alpine. Nous montrons que cette marge a ete le siege d'une forte extension au Cretace superieur-Paleocene et proposons d'en faire le type des rares << Marges Passives a Extension Tardive >>. Fait remarquable, cette extension tardive est contemporaine de la subduction de la croute tethysienne sous la microplaque Adria, le Brianconnais entrant lui-meme dans la zone de subduction des le Lutetien. En second lieu, nous revisitons l'evolution de la bordure nord-tethysienne sur plusieurs transects des Maghrebides, au sud-ouest du transect Brianconnais mais sur la meme marge europeenne. Nous considerons la serie triasico-eocene de la "Dorsale Calcaire" dans les blocs Alkapeca riftes de la bordure sud-est d'Iberia puis transportes apres l'Eocene sur la marge nord-africaine. L'examen de la litterature montre que la marge tethysienne du bloc d'Alboran a ete fortement affectee par des failles normales des la fin du Jurassique-debut du Cretace alors que l'extension post-rifting des blocs kabyles s'est principalement produite au Cretace superieur-Paleocene comme dans le Brianconnais. Nous proposons que l'extension post-rifting de la marge meridionale du bloc d'Alboran resulte du mouvement senestre de l'Afrique par rapport a l'Iberie, tandis que l'extension des blocs kabyles peut etre liee a la cinematique ulterieure de convergence. La subduction de la plaque liguro-maghrebine sous la marge nord-africaine se serait produite a cette epoque dans le prolongement sud de la subduction alpine. Les marges des plaques superieures Adria et Africa n'ont pas subi de compression significative a cette epoque. Au cours de l'eocene, une inversion de la polarite de la subduction s'est produite, associee a la relocalisation de la zone de subduction le long du bloc Alkapeca. C'est le debut de la subduction apenninique, responsable de l'ouverture arriere-arc des bassins mediterraneens.