State of Stress in Southern Italy from borehole breakout and focal mechanism data

Abstract

We present new wellbore breakout data from oil exploration wells drilled in southern Peninsular Italy, from the Tyrrhenian shelf through the Apenninic mountain belt and foredeep, eastward to the undeformed Adria foreland. From the analysis of 64 wells, we observe Shmin oriented around NE-SW, perpendicular to the trend of the Apennines, both within the belt and in the eastern foredeep. Some variations are observed in the few wells analyzed on the Tyrrhenian shelf, possibly due to local tectonic structures, such as transverse NE-SW oriented extensional basins. The Apulian platform overthrust by the Apenninic mountains at 3-4 km depth, shows a very consistent direction of Shmin oriented N40°-50°E, in agreement with the stress regime inferred from focal mechanisms of strong earthquakes at depths of 10-15 km, which show NE extension, mostly accomplished by normal faulting. The overlying sedimentary and allochthonous units of the Apenninic belt exhibit the same NE Shmin direction, but show a larger scatter. Breakout data demonstrate that Quaternary compressional tectonism has ended, as Shmin is oriented NE also in the undeformed foreland as far as 50 km NE from the seismic belt. In the easternmost sector of the Adria foreland there are suggestions of a nearly isotropic horizontal stress field, as most wells do not show significant ellipticity.