The seismic structure of the Calabrian subduction system (Central Mediterranean): new insights into crustal construction and mantle hydration from wide-angle seismic data

18 International SEISMIX Symposium, Seismology between the Poles, 17-22 June 2018, Cracow, Poland.-- 1 pageSubduction zones are known to be key in shaping the lithospheric structure of the Earth, and consequently its surface. The dehydration of ultramafic products from the subducted oceanic lithosphere triggers the hydration of the overlying mantle wedge, resulting in the production of hydrous flush melting, and the consequent formation of new crustal material at the volcanic arc. However, the evolution of processes such as crustal construction and mantle hydration are still poorly constrained mainly because of the lack of observations from young subduction settings. In this study, we focus on the Plio-Quaternary Calabrian subduction system, located in the Central Mediterranean. This subduction zone is the result of the same slab rollback process that shaped back-arc basins in the Western Mediterranean and is characterized by the subduction of the Ionian Lithosphere under Calabria. To date, there is limited understanding of its shallow crustal structure and the transition between lithospheric domains (e.g. Arc - back-arc) due to the lack of dedicated active-source experiments. Here, we present the 2D P-wave velocity structure of the entire subduction system from travel-time tomography of wide-angle seismic data. The data were recorded during the CHIANTI amphibious experiment in 2015 along a ~600 kmlong transect that extends from the Tyrrhenian back-arc domain to the fore-arc in the Ionian Sea, across Calabria. In the back-arc system, OBS sections lack PmP-like arrivals and the velocity structure of the model shows a continuous and strong vertical velocity gradient of ~1 s-1. These results together with previous tomographic models, analysis of Vp/Vs and basement rock sampling strongly supports the exhumation of the mantle along ~300 km long section in the back-arc region. The back-arc region is also characterized by large volcanic edifices that were emplaced soon after the exhumation of the mantle. The transition between back-arc and Calabria is marked by a steep Moho geometry that shallows from SE to NW, revealing a dramatic crustal thinning along the N Calabrian margin. The lower crust of the margin has localized Vp of ~7 km/s under the submarine volcanic arc. The vertical velocity structure of this young arc (< 2Ma) reveals a thin and mafic crust, suggesting that crustal growth by magmatic addition is still at an early stage in the Calabrian volcanic arc, and that little crustal felsic-to-mafic differentiation has occurred as opposed to older well-established volcanic arcs in the Pacific. SE Calabria, the model shows a strong horizontal velocity gradient that is interpreted as the backstop of the subduction. In the Ionian, a 3-5 km thick sedimentary wedge thickens towards the NW. The frontal part of the wedge shows sub-vertical low-velocity anomalies indicating the presence of fluid-saturated large thrusts faultsPeer Reviewe