Cesaro, M.

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.

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.

In order to obtain a reliable map of the present-day stress field in Italy, needed to better understand the active tectonic processes and to contribute to the assessment of seismic hazard, in 1992 we started to collect and analyze new data from borehole breakouts in deep oil and geothermal wells and focal mechanisms of earthquakes (2.5 < M <5) occurred in Italy between 1988 and 1995. From about 200 deep wells and 300 focal mechanisms analyzed to date, we infer that: the internal (SW) sector of the Northern Apenninic arc is extending with minimum compressional stress (Shmin) oriented ? ENE, while the external front is thrusting over the Adriatic foreland (Shmin ? NW-SE). The entire Southern Apennine is extending in NE direction (from the Tyrrhenian margin to the Apulian foreland) and compression (in the foredeep) is no longer active at the outer (NE) thrust front. Between these two arcs, an abrupt change in the tectonic regime is detected with directions of horizontal stress changing by as much as 90º in the external front, around latitude 430N. Along the Ionian side of the Calabrian arc the stress directions inferred from breakouts and focal mechanisms are scattered with a hint of rotation from N-S Shmin close to the Southern Apennines, to ~ E-W directions in the Messina Strait. In Sicily, a NW-SE direction of SHmax is evident in the Hyblean foreland, parallel to the direction of plate motion between Africa and Europe. A more complex pattern of stress directions is observed in the thrust belt zone, with rotations from the regional trend (NW í directed SHmax) to NE oriented SHmax. A predominant NW direction of SHmax is also detected in mainland Sicily from earthquake focal mechanisms, but no well data are available in this region. In the northern part of Sicily (Aeolian Islands) a ~N-S direction of SHmax is observed.

The aim of this work is to understand the complex pattern of active stress field orientations revealed by borehole breakout analysis with respect to the tectonic structures in a wide region of Northern Italy. The area is located in the central-western part of the Po Plain between the south verging Southern Alps structures and the north verging buried folds and thrusts of the Monferrato and Emilia arcs. Little information concerning the active stress field is available because of the low seismicity level and the thick layer of sediments that covers the entire zone. A detailed borehole breakout analysis has been performed in 36 wells with depths ranging from 2.2 to 7.3 km, whose data have been supplied by Eni. Breakout analysis determined the minimum and maximum horizontal stress directions (Shmin and SHmax). The results show a very complex pattern, pointing out that the stress field is not uniform. In this area the regional stress field seems not «strong» enough, compared to the local one, so most of the wells detect only the local field. This work contributes to clarify the various Shmin orientations observed in this area, pointing out at wide scale, a general compression in NNE-SSW direction in this complex region.