The blueschist–eclogite transition in the Alpine chain: P–T paths and the role of slow-spreading extensional structures in the evolution of HP–LT mountain belts

High-pressure metamorphic rocks exhumed in mountain belts provide a unique window on deep processes at subduction zones, such as the progressive transformation of blueschist into eclogite, which has important geochemical and geophysical implications, along with information on their exhumation mechanism. We provide a detailed characterization of the field and metamorphic relationships between blueschist- and eclogite-facies terranes of Alpine Corsica (France), where both primary, pre-subduction structures and Alpine high-pressure assemblages are very well preserved. We then compare our data with available observations from the Western Alps. Altogether, these data show systematic metamorphic patterns across the blueschist–eclogite boundary: temperature increases progressively without any gap across the contact, whereas a significant pressure jump (ca. 0.4 GPa) is observed. Lithostratigraphy in the two units suggests that they belong to two different types of oceanic (or transitional) crust, structures of which may have controlled their different mechanisms of decollement, accretion and exhumation. Lastly, the comparison of the exhumed terranes in Alpine belts with structures of modern analogues in present-day oceans, such as large detachment faults and oceanic core-complexes, stresses the importance of inherited extensional structures for subduction, exhumation and orogenic processes.