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

Abstract

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.