Magnetic anisotropy of Plio-Pleistocene sediments from the Adriatic marginof the northern Apennines (Italy): implications for the time-space evolution of the stress field

The anisotropy of magnetic susceptibility (AMS) of the marine fine-grained Plio-Pleistocene sediments that crop out at the eastern (Adriatic) front of the central-northern Apennines (Italy) indicates a prevalent sedimentary-c ompactional magnetic fabric with variable overprint of the tectonic strain. The degree of anisotropy and the geometry of the AMS ellipsoids suggest a subdivision of the studied sediments in two distinct ages: Early-middle Pli ocene and Late Pliocene-Early Pleistocene. The Early-middle Pliocene sedime nts show a weak but well defined magnetic lineation parallel to the main fo ld and thrust axes throughout the region, analogously to the Messinian sedi ments located in a more internal (western) position of the Apenninic chain. Since the Late Pliocene, the AMS data are not regionally coherent and indi cate a reduced and locally variable tectonic influence on the magnetic fabr ics. The AMS data have been integrated with the available geophysical infor mation of the present-day stress in the region, with the aim to reconstruct the time-spatial evolution of the stress field in the Adriatic margin of t he central-northern Apennines, during the past 5 Ma. The present-day stress field was evaluated using borehole breakout in deep wells (about 50 wells) , focal mechanisms of crustal earthquakes (2.5 < M-d < 5) that occurred in the last 10 years and a review of the historical seismicity. The synthesis of these data suggests significant changes in the time-space evolution of t he stress field through the area in the last 5 Ma. In the northern sector t he tectonic regime shows a maximum compressional axis trending NE-SW, that is constant at least since the Early Pliocene. In the southern sector the s tress field has drastically changed: a compressional stress with a maximum compressional axis trending E-W was active during the Pliocene and was like ly preserved until Early Pleistocene; then the stress field changed, presen tly showing a NE-SW minimum horizontal stress that mirrors the trend of the active extension observed in the southern Apennines. Between these two sec tors a transitional area is identified at about 43 degrees N, that we assoc iate with a main NNE-SSW lithospheric discontinuity pointed out by the avai lable geological and geophysical data. (C) 1999 Elsevier Science B.V. All r ights reserved.