The Neogene-Quaternary evolution of the Northern Apennines: crustal structure, style of deformation and seismicity.

Massimiliano R. Barchi

Since Lower Miocene, the tectonic evolution of the Northern Apennines has been characterised by the contemporaneous activity and eastward migration of coupled compression (in the foreland) and extension (in the hinterland). Compression and extension are co-axial, i.e. the direction of maximum extension is nearly parallel to the maximum shortening induced by the previous compression.

As a result of this tectonic evolution, the Northern Apennines can be divided into two different crustal domains: a western Tyrrhenian domain, where extensional deformation destroyed the pre-existing compressional belt; and an eastern Adriatic domain where the compressional structures are still preserved.

The upper crust of the Tyrrhenian domain is thinned by a set of east-dipping low-angle normal faults, driving the onset and evolution of the syn-tectonic hinterland basins. The age of the syn-rift deposits testifies the regular eastward migration of the extensional deformation. The shallow structures of the Adriatic domain correspond to the arc-shaped Umbria-Marche fold and thrust belt where the timing of deformation is marked by the onset and evolution of syn-tectonic foreland basins.

The structural style of both compressional and extensional structures is strongly influenced by the mechanical anisotropy in the upper crust stratigraphy. Consequently, both thrusts and normal faults show marked staircase trajectories.

The present-day stress field (seismicity, boreholes break-out) as well as the on-going deformation (GPS data) of the region also reflect the contemporaneous activity of compression in the foreland and extension in the hinterland, supporting a uniformitarian view of the recent tectonic evolution of the Northern Apennines.