Unravelling the mysteries of plate tectonics

Plate tectonics is the most unifying and revolutionary theory in Earth sciences. However, 50 years after its formulation fundamental questions about how plate tectonics works still remain unanswered. In particular, the formation of new subduction zones (whereby one tectonic plate starts to sink underneath another one) is poorly understood. Current models suggest that subduction can either initiate spontaneously due to gravitational instability, or be induced by external tectonic forces. In the last decade ophiolites (emerged relics of ancient oceanic lithosphere) have been crucial to study subduction initiation processes. Most ophiolites, in fact, form during subduction initiation above an incipient subduction zone. A recent work in the Oman ophiolite (Guilmette et al., 2018) used the time lag between the formation of the ophiolitic crust and the formation of the metamorphic layer found at its base to discriminate between forced and spontaneous subduction initiation. These metamorphic rocks, known as metamorphic sole, are key to constrain the age of subduction initiation as they form during the initial moment of subduction development. A substantial difference between the ages of ophiolitic crust and metamorphic sole would indicate a forced subduction initiation, while similar ages would suggest a spontaneous subduction initiation.

The primary aim of this project is to reconstruct the style (spontaneous vs. forced) of a past, major subduction initiation event to understand the possible mechanisms and causes behind the inception of a subduction zone. The ideal natural laboratory to study subduction initiation processes is an ophiolite belt running for hundreds of kilometers from the Alps in northern Italy to the Dinarides and Hellenides in the Balkan Peninsula. These ophiolites are relics of oceanic lithosphere formed during, and as a result of a major subduction initiation event occurred within the Neotethys Ocean during the Jurassic. Available geochronological ages from both the crust and metamorphic sole of the Balkan ophiolites are similar, suggesting a spontaneous subduction initiation. However, ages from the ophiolitic crust are sparse and those from the metamorphic sole used Ar39-Ar40 method, which does not date the actual metamorphism but rather the cooling occurring after it. Obtaining new ages from these ophiolites is key to better understand one of the major subduction initiation events of the Earth’s history.