Extension of a lower plate passive margin coeval with subduction of the adjacent slab: The Western Alps and Maghrebides cases
Authors: Farah Aboubaker , Saddiqi Omar , Chabou Moulley Charaf , Michard André 
Published: February 2023

The Piemonte-Ligurian-Maghrebian Tethys or &#8220;Alpine Tethys&#8221; developed at the western tip of the Tethys Ocean between Eurasia and Gondwana. The evolution of the Alpine Tethys during the post-Pangea rifting and oceanic expansion from the Early Jurassic to the Early Cretaceous has been well documented compared to its evolution from the onset of the African-Eurasian convergence during the Late Cretaceous upward. In this contribution, based on our studies and the literature, we try to decipher the evolution of this ocean through the study of its inverted margins during Late Cretaceous-Paleocene times. In the Western Alps, the Brian&#231;onnais domain, which constituted the distal European magma-poor passive margin of the Alpine Tethys, was affected by a systemic extension in the Late Cretaceous-Paleocene. This late extension, poorly described so far, operated only a few million years before the Brian&#231;onnais encroached the SE-dipping subduction zone under the Adria microplate. In the Maghrebides transects from the Rif belt (Northern Morocco) to the Peloritani Mountains of Sicily (e.g., Bouillin, 1986; Bouillin et al., 1986) the Alkapeca (Alboran-Kabylias-Peloritan-Calabre) terranes were part of south-eastern Iberia until the Early Jurassic opening of the narrow Betic Ocean (Puga et al., 2011) or OCT domain (Jabaloy S&#225;nchez et al., 2019). The Alkapeca blocks preserve in their &#8220;Dorsale calcaire&#8221; units remnants of the northern margin of the Alpine Tethys and then are southwestern equivalents of the Brian&#231;onnais domain, except they were fragmented and carried onto the African and south-eastern Iberia margins during the Tertiary opening of the back-arc Mediterranean basins. We observe that the Dorsale calcaire units testify to extensional deformation like the Brian&#231;onnais during the Late Cretaceous-Paleocene, i.e., when Africa-Eurasia-Iberia convergence was active and then subduction of the intervening Tethyan slab must have occurred somewhere. We propose here for the first time that the Late Cretaceous-Paleocene subduction of the Ligurian-Maghrebian slab occurred under the North African margin in the southward continuation of the Alpine subduction. Contrary to some early claims, the North African margin did not experience significant compression during the Late Cretaceous-Paleocene, which compares with the Adria margin case during the same period. During the Eocene, a Subduction Polarity Reversal occurred, which was associated with the relocation of the subduction zone along the Alkapeca blocks. This was the beginning of the Apenninic subduction, which triggered the back-arc opening of the Mediterranean basins and corresponds to the back-thrusting tectonic phase in the Western Alps.References &#160;Bouillin J-P, 1986. Le &#171;&#160;bassin maghr&#233;bin&#160;&#187;&#160;: une ancienne limite entre l&#8217;Europe et l&#8217;Afrique &#224; l&#8217;ouest des Alpes. Bull. Soc. G&#233;ol. Fr. (8) 2&#160;:547-558.Bouillin JP, et al., 1986. Betic-Rifian and Tyrrhenian Arcs : Distinctive Features, Genesis and Development Stages. Developments Geotect. 21:281-304.Puga E, et al., 2011. Petrology, geochemistry and U-Pb geochronology of the Betic Ophiolites: Inferences for Pangaea break-up and birth of the Westernmost Tethys Ocean. Lithos 124:265-272.Jabaloy S&#225;nchez A, et al., 2019. Lithological successions of the Internal Zones and Flysch Trough Units of the Betic Chain. In : Quesada C and Oliveira JT (eds.), The Geology of Iberia: A Geodynamic Approach. Region. Geol. Rev. (Springer Nature Publ.)