Arc and forearc rifting in the Tyrrhenian subduction system

The evolution of backarc and forearc basins is usually treated separately, as the volcanic arc represents a clear barrier between them. We analyse their spatial and temporal relationships in the Tyrrhenian subduction system, using seismic profiles and numerical modelling. Our results highlight that the Marsili volcano, commonly interpreted as the spreading centre of the Marsili backarc basin, was instead a part of an old (Pliocene) volcanic arc associated with the development of the Vavilov backarc basin (4.3-4.1 to 2.6 Ma). The old volcanic arc was successively affected by arc rifting. This process caused the shift of the Marsili volcano eastwards and the formation of an oceanic backarc basin (~ 1.8 Ma) located between the Marsili volcano and the old remnant arc, which remained fixed. The eastern side of the Marsili basin, previously considered as the other half of the oceanic backarc basin, is instead a part of the forearc domain floored by serpentinised mantle. As slab rollback continued, volcanism migrated towards the trench and a new volcanic arc (Aeolian Island) formed at ~1 Ma in the forearc domain. The formation of the new volcanic arc represents the onset of the forearc-rifting that could lead to the opening of a new backarc basin between the old and young volcanic arc, resulting in the decrease of the initial forearc region extension.

The example of the Tyrrhenian Sea illustrates how the evolution of forearc and backarc domains is intimately interconnected. Fluids, released from the downgoing plates, control lithospheric hydration and mantle serpentinisation as well as asthenospheric mantle melting. Fluids and melts induce weakening of the volcanic arc region and drive the arc-rifting that led to the backarc basin formation. Later, the slab rollback causes the trench-ward migration of volcanism that led to the forearc- rifting under the control of fluids released from the downgoing plate.