Initiation of subduction in the Alps: Continent or ocean?

The triggering event for the modern evolution of the European Alps was the onset of subduction in the Middle Jurassic. However, there is some debate as to how this onset occurred: as (1) oceanic subduction; (2) the succession of an earlier (Tethys) and a later (Penninic) oceanic subduction; or even (3) intracontinental subduction. Here we show that it is plausible that the continental lithosphere in the center of the Adriatic plate was gravitationally unstable, causing the onset of subduction inside the continent. This gravitational instability was provided by thermal thickening of the mantle lithosphere in the wake of the Permian extensional event, and onset of subduction was facilitated by a strong rheological contrast inside the Adriatic plate.