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

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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-compactional 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 Pliocene and Late Pliocene–Early Pleistocene. The Early–middle Pliocene sediments show a weak but well defined magnetic lineation parallel to the main fold and thrust axes throughout the region, analogously to the Messinian sediments located in a more internal (western) position of the Apenninic chain. Since the Late Pliocene, the AMS data are not regionally coherent and indicate a reduced and locally variable tectonic influence on the magnetic fabrics. The AMS data have been integrated with the available geophysical information 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 the 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 the stress field through the area in the last 5 Ma. In the northern sector the tectonic regime shows a maximum compressional axis trending NE–SW, that is constant at least since the Early Pliocene. In the southern sector the stress field has drastically changed: a compressional stress with a maximum compressional axis trending E–W was active during the Pliocene and was likely preserved until Early Pleistocene; then the stress field changed, presently showing a NE–SW minimum horizontal stress that mirrors the trend of the active extension observed in the southern Apennines. Between these two sectors a transitional area is identified at about 43°N, that we associate with a main NNE–SSW lithospheric discontinuity pointed out by the available geological and geophysical data.