The tyrrhenian-apennines system: structural setting and seismotectogenesis

Abstract

The available geological, geophysical and seismological data of the Tyrrhenian-Apennines system are briefly reviewed and synthesized in a number of sketch maps. Integrating and comparing these data, a large-scale simple-shear kinematic model is proposed for the Tyrrhenian-Apennines system that is here interpreted as an oblique asymmetric passive continental margin developed during the progressive counterclockwise rotation of the Adriatic foreland, from the Late Miocene to the present. In the frame of this working hypothesis, the thinning and stretching of the Tyrrhenian crust were accommodated by progressive eastward motion of extensional allochthonous slices, bounded by low-angle west-dipping oblique shear zones, rotating above a common detachment that penetrates into the mantle lithosphere. At any given time, the most external eastward moving slice is bounded by a transtensional shear zone at the top and by a transpressional one at the bottom. The displacement within the hanging wall of the transtensional shear zone is accommodated by high-angle normal faults and en échelon grabens; the displacement within the hanging wall of the transpressional shear zone is instead accommodated by co-axial en échelon sets of folds, thrusts and high-angle strike-slip faults. Thus, the development of the Late Miocene-Pliocene-Pleistocene contractional strain field of the Apennines of Italy might not be a consequence of complex subduction processes, but subordinate to the deformation processes that lead to the progressive opening of the Tyrrhenian Sea. The seismicity of the crustal Apennines of Italy is analyzed in the frame of the above history of progressive deformation, and a seismotectogenetic model is outlined.