Crustal structure of the Ionian margin of Sicily: Etna volcano in the frame of regional evolution

Abstract

Crustal imaging could be achieved on normal-incidence reflection profiles offshore eastern Sicily by using industrial-grade reflection seismic with improved marine sources. Thick recent sediments, a reflective pile including the Mesozoic deposits, a transparent upper crust, and a band of low frequency reflections attributed to the lower crust, are imaged in the seismic sections. The structure of the crust and its thickness show features inherited from the Mesozoic evolution as a passive margin, by which Ionian basin crust was formed around the Hyblean continental promontory of Africa to constitute the southern plate in the later convergence with Europe. The seismic images are also marked by the lithospheric deformation due to the Neogene overriding of the northern part of this paleomargin by the Calabro-Peloritan block of European continental crust. This transpressive motion may have been guided along the northern part of this paleomargin where the seismic profiles evidence a hinge line between the northward upslope of the Moho of that old passive margin and its downslope to the present slab under the Tyrrhenian Sea. Etna volcano is located at the intersection of this mantle upwarp by a zone of active sea-bottom normal-faults, which cut across the formerly constructed compressional belt. The onset of its volcanic activity is roughly coeval with that of the cessation of interplate thrusting and could hence be related to a change of the coupling of the Ionian slab. This slab is now probably disconnected from the overriding plate and rolled back in front of the expanding hot Tyrrhenian asthenospheric dome with the mobilisation of a viscous mantle material at depth. An active lithospheric fault is here imaged which cuts over more than 100 km into the Ionian basin. The fault runs from the Tyrrhenian margin in the SSE direction of Etna updip of the southwestern lateral edge of this slab, leaving north-eastward the extruded Calabrian block and on its south-westward edge the uplifted crust and mantle structures of Etna. Along it, the crust, including the Mesozoic and deeper layers, has sagged vertically in the segment in front of the slab, with a finite throw across the fault increasing from the basin towards Etna.