A type sequence across an ancient magma-poor ocean–continent transition: the example of the western Alpine Tethys ophiolites

The ophiolites from the Alpine Tethys are incompatible with the definition of the classical 3-layered Penrose ophiolite sequence, but they also show features that are inconsistent with ultraslow-spreading ridge sequences or transform settings. The existence of pre-rift contacts between subcontinental mantle and continental crust, the association of top-basement detachment faults with continent-derived blocks (extensional allochthons) and tectono-sedimentary breccias overlying subcontinental mantle, and a post-rift sedimentary evolution identical to that of the adjacent distal margin enable to characterize some of the Alpine Tethys ophiolites as remnants of a former Ocean Continent Transition (OCT). Therefore, we propose that at least some of the Alpine Tethys ophiolites are formed by remnants of an ancient Magma-Poor-Ocean Continent Transition, referred to as a MP-OCT sequence. The type sequence consists of the Platta, Tasna and Chenaillet ophiolite units, the former two representing the OCT of the ancient Adriatic and European/Briançonnais conjugate rifted margins, the latter representing a more developed “oceanic” domain. All three units escaped Alpine subduction and preserve pre-Alpine contacts between exhumed basement and a volcano-sedimentary cover sequence. These units preserve the structural, magmatic, hydrothermal and sedimentary record of continental breakup and early seafloor spreading. The observations compare well with those made along the magma-poor Iberia–Newfoundland rifted margins, which are the only example in an OCT where drill holes penetrated into basement. At present, magma-poor rifted margins form up to 50% of all rifted margins worldwide. We argue that MP-OCT sequences are more common in the geological record but were, in part mistaken as either Mid Ocean Ridge or tectonically dismembered Penrose-type ophiolite sections.