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The core of the Mediterranean Ridge backstop consists of a pile of Hellenic nappes that migrated outward from the Aegean continent within the adjacent Mediterranean basins during late middle Miocene, about 15 Ma, and the present Mediterranean Ridge has developed since that time by accretion of a new wedge. The arguments we use are: (1) a similarity in thickness, seismic velocity and structure between the backstop and the Hellenic nappes in southern Peloponnesus; (2) the geometry of the westward limit of the backstop that is the one expected from the amount of relative displacement of the nappes on land; (3) an agreement between the estimated ages of the present accretionary wedge and the period of activity of two major faults connecting the backstop to the adjacent Aegean continent. The seaward limit of the backstop is situated about 170 km southwest of western Crete where the displacement of the nappes is maximal and it is close to the margin near the Ionian islands where their displacement is small. We assume that the Hellenic subduction zone migrated southwestward relative to Eurasia during middle Miocene and that the corresponding gravity collapse of the Aegean continent was maximum at this time because the westward extrusion of Anatolia had not yet started. [Copyright &y& Elsevier]