A MAJOR GEODYNAMIC CHANGE REVEALED BY QUATERNARY STRESS PATTERNS IN THE SOUTHERN APENNINES (ITALY)

In southern Italy, analysis of fault slip data sets, in particular from Quaternary formations, provides evidence for a recent change of the stress field. During the Early Pleistocene, the horizontal maximum stress axes were ENE-WSW trending. The deformation was ENE-WSW compression near the front of the chain, and NNW-SSE extension close to the back-arc basin. Some time after the Early Pleistocene, the direction of the largest horizontal stress axes changed to northwest-southeast. Only extensional deformation (sigma2, NW-SE trending; sigma3, NE-SW trending) is observed for this phase and focal mechanisms indicate that it is still active. This NE-SW extension invaded areas previously affected by compression or NNW-SSE extension and coincides with major uplift of Pleistocene marine sediments in the chain and the foredeep up to 700 m above sea level. This change in the stress regime corresponds to the end of accretion processes that had prevailed since the Middle-Late Miocene. As a consequence of this discovery of a recent regional stress and deformational style change, the present-day normal and strike-slip faulting earthquake focal mechanisms in the Southern Apennines should not be considered representative of Tyrrhenian Sea opening and Apennines accretion.