Thermal constriction and slab tearing at the origin of a superinflated spreading ridge: Marsili volcano (Tyrrhenian Sea)



[1] Recently acquired swath bathymetry of the Marsili basin has offered an unprecedented opportunity to study the processes of back arc ocean basin development in the Tyrrhenian Sea. In particular, the detailed morphology of Marsili seamount, a large, strongly elongated volcano located in an axial position within the <2 Ma ocean crust floored Marsili basin, is a key to understanding the mechanisms governing lithosphere formation in this young basin. The basin is near circular in shape with diameter on the order of 120 km and is positioned in the southern Tyrrhenian Sea, above the steeply dipping Ionian oceanic slab of Mesozoic age. It is bounded southward by the Aeolian volcanic arc and the Calabrian accretionary wedge, surface evidence of the northwesterly directed subduction. The most outstanding feature of the basin is the elongated, 3000-m-high Marsili volcano which reveals distinctive morphology strikingly akin to the high-order segmentation and volcanic landforms described in mid-ocean slow spreading ridges. On the basis of its distinctive morphology and incremental growth relationship we propose that Marsili volcano represents a superinflated spreading ridge resulting from a distinct thermal pulse of increased melt production occurring within the young and immature Marsili basin. Surrounding cooler continental lithosphere thermally constricts ridge propagation and crust production in Marsili basin to the finite scale of Marsili volcano. Increased melt production to feed the superinflated Marsili ridge is generated by deep, lateral asthenospheric mantle flow produced at the edges of tears that bound the subducting ocean crust of the Ionian plate. Slow spreading plate separation, outpaced by the increase in magma generation, results in vertical accretion to produce the superinflated ridge. The existence of dip-directed tears delimiting the narrow Ionian slab is supported by the geological evolution of the surrounding foreland and Apennine/Maghrebid mountain belt during early/middle Pleistocene, i.e., the time of formation of the Marsili volcano. Present-day structure and volcanism furnish direct and indirect surface evidence of the presence and location of the slab tears.