Thermal state and deep earthquakes in the Southern Tyrrhenian

Abstract

The geodynamic processes in the Southern Tyrrhenian are approached by comparing the thermal structure with the seismic activity. Subsidence data are used to determine the boundaries of the oceanic domains in the abyssal area. The regional pattern of the surface heat flux reflects lateral variations of structural properties of the lithosphere. In the oceanic domain, a mean heat flux of 160 mW m −2 is observed, to which a convective component of about 100 mW m −2 should be added if an ocean-plate cooling model is applicable. For a pure shear stretching model, the heat-flux values of the continental margins indicate a thinning factor of 3.5–4.0. The maximum frequency of seismic events occurs in the 200–350-km depth interval, and the energy released is maximum between 250 and 300 km. Most earthquakes are confined within a 30–50-km-thick zone. The slab dip angle is about 65° in the 100–400-km depth range characterised by predominant down-dip compression. Then both the slab and the maximum compression axis slightly deflect down to 500 km depth where the deepest event occurs. Under the hypothesis that the seismic activity is confined within portions of downgoing lithosphere which have a temperature below a depth-dependent critical value, calculations for a 60–70-km-thick slab sinking at a velocity of 6×10 −2 m yr −1 yield a thermal structure compatible with the distribution of the deepest seismic activity. Such a thermomechanical picture and the correlation between the slab dip angle and the total tectonic subsidence of the oceanic domain are typical features of a young marginal basin and are related to the drag stress in the vicinity of the downgoing lithosphere.