Reconstructing the pre–Variscan puzzle of Cambro–Ordovician basement rocks in the western Mediterranean region of Gondwana

In today’s western Mediterranean region, Variscan and Alpine thrusts and shear zones combine to hamper a correct identification and palinspastic reconstruction of Cambro-Ordovician sequences. However, gap-related stratigraphic, climatically sensitive facies associations, sedimentary, volcanosedimentary, biogeographic, biodiversity and detrital zircon data mainly made available during the last two decades allow envisaging a new palaeogeographic scenario by linking proximal-to-distal transects across the western and eastern branches of the Ibero-Armorican Arc. Variscan parautochthonous and autochthonous domains are represented palaeogeographically by, from SW to NE: (i) the Central Iberian, West Asturian-Leonese and Cantabrian zones of the Iberian Massif and their laterally correlative Central Armorican Domain, fringed marginally by the Ossa-Morena and North Armorican thinned outer margin of Gondwana, reminiscent of the rift axis during the Cambrian; and (ii) the southeastern Pyrenees, Occitan and SW Sardinia domains, fringed marginally by the slope-to-basinal South Armorican, Thiviers-Payzac, Albigeois and northeastern Pyrenees domains. These proximal-to distal transects of West Gondwana record a diachronous SW-to-NE migration of evaporites, phosphorites and maximum peak of trilobite diversity, related to the counter-clockwise migration of the Gondwana supercontinent, supported by a gradual modification of detrital zircon provenance. Both branches of the Ibero-Armorican Arc also display a diachronous migration of Cambro-Ordovician rift-to-drift conditions associated with distinct igneous manifestations (volcanosedimentary and plutonic). This migration is related to the development of the Furongian (Toledanian) to Mid-Late Ordovician (Sardic) geodynamic events, in response to gap-related thermal doming, subaerial denudation and magmatic activity evolving from calc-alkaline to tholeiitic affinity.