Sistemi distensivi in due aree dell'Appennino centrale

Abstract

This paper analyses the main features of extensional fault systems in two sectors of the central Apennines, in the Mt. Giano-Mt. Gabbia area and the Ofena-Tirino River valley. The relationships between extensional fault systems and main compressive structures with reference to inversion tectonic phenomena are emphasized. The structural setting of the areas is a complex thrust belt (with W dipping thrust planes trending NW-SE and N-S), with normal fault systems probably formed in Upper Pliocene times as a consequence of the Tyrrhenian rifting and under conditions of a general uplift of the Apennines chain, the climax of which occurred along a NNW-SSE axis independent of the trend of previous compressive structures. The NW-SE trending fault system gave rise to faulted blocks stepping down towards SW. Some of these normal faults break at the contact with main thrust planes, suggesting a possible reactivation of the thrust planes themselves which invert their movement and produce listric geometries. Other faults, chacterized by high dip angle planes, dislocated the compressive structures and probably reached deep-seated crustal levels. Superficial listric faults are planimetrically short and generally concave towards the hanging wall; high dip angle faults are longer and rectilinear and are not dependent on the structural setting of compressive structures. Differentiated throws show maximum values in the most superficial faults, and lower values in the high dip angle faults reaching deep-seated crustal levels. In many cases, these latter faults displaced the most superficial listric faults, simulating a younger age. Another hypothesis is that both superficial and deep reaching faults originated at the same time, dislocating each other several times during the recent tectonic evolution of the chain. If the crustal depth of fault planes is taken into account as well as seismological data collected on the occasion of recent earthquakes, it seems reasonable to hypothesize a seismogenic character for the deep reaching high dip angle faults.