Active faulting, 3-D geological architecture and Plio-Quaternary structural evolution of extensional basins in the central Apennine chain, Italy

Abstract

The general “basin and range” Apennine topographic characteristic is generally attributed to the presently active normal fault systems, whose long-term activity (throughout the Quaternary) is supposed to have been responsible for the creation of morphological/structural highs and lows. By coupling field geological survey and geophysical investigations, we reconstructed the 3-D geological model of an inner tectonic basin of the central Apennines, the Subequana Valley, bounded to the northeast by the southern segment of one of the major active and seismogenic normal faults of the Apennines, known as the Middle Aterno Valley– Subequana Valley fault system. Our analyses revealed that, since the late Pliocene, the basin evolved in a double halfgraben configuration through a polyphase tectonic development. An early phase, Late Pliocene–Early Pleistocene in age, was controlled by the ENE–WSW-striking and SSEdipping Avezzano–Bussi fault, that determined the formation of an early depocentre towards the N–NW. Subsequently, the main fault became the NW–SE-striking faults, which drove the formation during the Quaternary of a new fault-related depocentre towards the NE. By considering the available geological information, a similar structural evolution has likely involved three close tectonic basins aligned along the Avezzano–Bussi fault, namely the Fucino Basin, the Subequana Valley, and the Sulmona Basin, and it has been probably experienced by other tectonic basins of the chain. The present work therefore points out the role of pre-existing transverse tectonic structures, inherited by previous tectonic phases, in accommodating the ongoing tectonic deformation and, consequently, in influencing the structural characteristics of the major active normal faults. This has implications in terms of earthquake fault rupture propagation and segmentation. Lastly, the morpho-tectonic setting of the Apennine chain results from the superposition of deformation events whose geological legacy must be considered in a wider evolutionary perspective. Our results testify that a large-scale “basin and range” geomorphological feature – often adopted for morpho-tectonic and kinematic evaluations in active extensional contexts, as in the Apennines – just led by rangebounding active normal faults may be actually simplistic, as it could not be applied everywhere, owing to peculiar complexities of the local tectonic histories.