Modern tectonic regime in the Tyrrhenian area: Observations and models

Abstract

The rapid opening, since 10 Ma, of the Tyrrhenian Sea in a context of convergence between Eurasia and Africa remains a puzzling geodynamic problem. One of the most plausible scenarios, proposed by Malinverno & Ryan (1986) attributes extension to southward and eastward migration of the trench, resulting from subduction of the African-Adriatic lithosphere below the European lithosphere. At present, subduction only continues along the Calabrian arc. A compilation of recent strain and stress indicators by Rebaï, Philip & Taboada (1992) indicates that an extensional regime still prevails in the Tyrrhenian Sea, whereas compression dominates north and south of this area. In an earlier paper, Bassi & Sabadini (1994) used a numerical model to demonstrate the necessity of a 'trench suction force', acting along the subduction trench, to produce the present extension in the Tyrrhenian domain. This study, however, did not discuss in detail the tectonic regime in the different areas composing the domain, nor the match between the observations and the model. In this paper, we first review the constraints provided by the observations compiled by Rebaï et al. (1992) and by more recent measurements. We then investigate how much of the observed tectonic regime can be understood in terms of plate-interaction forces, using 'thin-plate' models (Bird 1989). The geometry and heterogeneous lithospheric properties of the models are constrained by observations. The best-constrained boundary conditions are held constant; the remaining are regarded as parameters that we tune in order to fit the observations. Our results indicate that the present-day tectonic regime in the Tyrrhenian area can be explained as a consequence of trench retreat associated with the subduction of Ionian oceanic lithosphere under the Calabrian Arc, combined with a rotation of the Apennines, possibly related to a rotation of the Adriatic microplate. This study supports, therefore, the model of Malinverno & Ryan (1986) and suggests, moreover, that rotation of the Apennines is an ongoing process, which is supported by seismological and geodetic data but remains to be understood in terms of geodynamics. A discrepancy between observations and models is observed in western Sicily, which can be attributed to modelling limitations. The weak extensional activity of the Corsica-Sardinia block, however, is not explained by this model.