The role of continental margins in the final stages of arc formation: Constraints from teleseismic tomography of the Gibraltar and Calabrian Arc (Western Mediterranean)

The deep seismicity and lateral distribution of seismic velocity in the CentralWesternMediterranean, point to the
existence under the Alboran and Tyrrhenian Seas of two lithospheric slabs reaching the mantle transition zone.
Gibraltar and Calabrian narrow arcs correspond to the slabs. Similarities in the tectonic and mantle structure of
the two areas have been explained by a common subduction and roll-back mechanism, in which the two arcs
are symmetrical end members.
We present a new 3-D tomographic model at mantle scale for the Calabrian Arc and compare it with a recently
published model for the Gibraltar Arc by Monna et al. (2013a). The two models, calculated with inversion of
teleseismic phase arrivals, have a scale and parametrization that allow for a direct comparison. The inclusion in
both inversions of ocean bottom seismometer broadband data improves the resolution of the areas underlying
the seafloor networks. This additional information is used to resolve the deep structure and constrain the reconstruction
of the Central Western Mediterranean geodynamic evolution. The Gibraltar tomography model suggests
that the slab is separated from the Atlantic oceanic domain by a portion of African continental margin,
whereas the Calabrian model displays a continuous oceanic slab that is connected, via a narrow passage
(~350 km), to the Ionian basin oceanic domain. Starting from the comparison of the two models we propose
the following interpretation: within the Mediterranean geodynamic regime (dominated by slab rollback) the geometry
of the African continental margin, located on the lower plate, represents a critical control on the evolution
of subduction. As buoyant continental lithosphere entered the subduction zones, slab pull caused tears in the
subducted lithosphere. This tectonic response,which occurred in the final stages of arc evolution and was strongly
controlled by the paleogeography of the subducted plates, explains the observed differences between the
Gibraltar and Calabrian Arcs.