Active stress along the NE external margin of the Apennines: the Ferrara are, northern Italy

We have analysed borehole breakout data from 12 deep wells in order to constrain the direction of the minimum and maximum horizontal stress in a part of the Po Plain, northern Italy, characterised by a similar to N-S prevailing compressional stress regime, and in order to shed light on the regional state of stress and on the correlation between the active stress field and the orientation of tectonic structures. The results have been compared with seismological data relating to 1988-1995 crustal seismicity (2.5<M-d<4.8) and to the 1983 Parma (M-s=5.0) and the 1996 Reggio Emilia (M-s=5.1) events. Plio-Pleistocene mesostructural data are also described in order to better define the present-day stress field and to understand the active tectonic processes in particular stress provinces. The borehole breakout analysis, in accordance with the seismicity and mesostructural data, shows the presence of a predominant compression area, characterised by approximately N-S maximum horizontal stress, along the outer thrust of the Ferrara are. Particularly, the breakout analysis indicates a minimum horizontal stress, N81W +/- 22 degrees relative to a total of eleven analysed wells, with 3746 m cumulative total length of breakout zones. Among these, nine wells are located in the same tectonic structure, consisting of an are of asymmetric folds overthrust towards the NE. The breakout results for these wells are quite similar in terms of minimum horizontal stress direction (similar to E-W oriented). The other two wells are located in the outside sector of the are and one of them shows a different minimum horizontal stress direction, probably distinctive of another tectonic unit. On the basis of these new reliable stress indicators, the active compressive front in this area is located along the termination of the external northern Apenninic are. (C) 1999 Elsevier Science Ltd. All rights reserved.