Two-stage collision: Exploring the birth of Pangea in the Variscan terranes

The Variscan suture exposed in NW Iberia contains a stack of terranes including two allochthonous units with continental affinity and Gondwanan provenance (Upper and Basal Units), separated by an ophiolite belt where the most common units show protolith ages at c. 395 Ma. Recent Lu–Hf zircon data obtained from these ophiolites indicate interaction between the gabbroic magmas and old continental crust. Hence, the ophiolites could not have originated in a deep ocean basin associated with a mature mid-ocean-ridge or intraoceanic subduction. The tectonothermal evolution of the continental terranes bounding the suture zone records two consecutive events of deep subduction. The Upper Units record an initial high-P/ultra-high-P metamorphic event that occurred before 400–390 Ma, while the Basal Units were affected by a second high-P/low-to-intermediate-T metamorphic event dated at c. 370 Ma. Continental subduction affected the most external margin of Gondwana and developed in a setting of dextral convergence with Laurussia. Development of the two high-P events alternated with the opening of an ephemeral oceanic basin, probably of pull-apart type, in Early Devonian times. This ephemeral oceanic domain is suggested as the setting for the protoliths of the most common ophiolites involved in the Variscan suture. Current ideas for the assembly of Pangea advocate a single collisional event between Gondwana and Laurussia in the Carboniferous. However, the new evidence from the allochthonous terranes of the Variscan belt suggests a more complex scenario for the assembly of the supercontinent, with an interaction between the colliding continental margins that started earlier and lasted longer than previously considered. Based on modern analogs of continental interaction, the development of complex collisions, as here suggested for Gondwana and Laurussia during the assembly of Pangea, could have been the norm rather than the exception throughout Earth history.