Evolution of rift-related cover-basement decoupling revealed by brecciation processes in the eastern Pyrenees

Breccias associated with tectonic, fluid and sedimentary evolution of rifted margins can provide information on a variety of processes reflecting the modes of extension. In this paper, we analyze the numerous breccias exposed in the Agly Massif that was part of the European rifted margin now inverted in the eastern Pyrenees. Using a combination of petrologic and sedimentologic analyses, field-based structural study, and multivariate analysis of clast shape and diversity, binding lithology and size, and breccia fabrics we distinguish 5 types of breccias reflecting depositional, tectonic, and salt-related processes. The integration of these processes in the tectonic history of the eastern Pyrenees confirms the attribution of these breccias to the Cretaceous rifting. We emphasize the major role played by the evaporitic Triassic particularly during the first stages of rifting as a major ductile decoupling level at the basement/cover interface. Salt tectonics and shearing assisted by the circulation of fluids are reflected by hydrofracturing at the base of the Mesozoic cover. As this weak mechanical layer is later extracted as extension increases, a brittle detachment system developed along the cover-basement interface to exhume of deep crust and mantle. The relationships between brecciation and Cretaceous extension in the Pyrenees argue for a hydrid mode of rifting during the formation of hyper-extended margins.