Plio-Quaternary changes of the normal fault architecture in the Central Apennines (Italy)

Abstract

The definition of the active fault geometry and kinematics in young evolving orogens may be difficult owing to changes in the structural architecture which may occur with a frequence of few hundred thousand years. Cases from the central Apennines well illustrate this problem. The Avezzano-Bussi and Vallelonga-Salto Valley fault systems (65 and 85 km long, respectively) show clear evidence of Pliocene-early Pleistocene activity and have been responsible for the formation of intermontane basins. Available geological data, however, indicate that only minor segments (the Tre Monti and Trasacco faults, both 7 km long) of the mentioned faults have to be considered active during the late Pleistocene-Holocene, as faults accommodating minor deformations inside an intermontane basin. The LˈAquila fault system underwent significant geometrical and kinematic modifications during the Quaternary, with the reactivation of minor portions of parallel normal faults to draw a new system of en-echelon normal-oblique left-lateral faults. The Laga Mts. fault experienced an along-fault activity migration. The portion of the fault which was active earlier during the Quaternary shows a significant decrease or end of the activity while a portion previously not active displays impressive evidence of late Pleistocene-Holocene displacements. Structural changes in the intermontane basins bounded by the Colfiorito fault system also indicate that the intensity of the tectonic activity decreased during the Quaternary. Not defining the structural evolution in the above mentioned cases would imply wrong conclusions for both the fault geometry and kinematics which may be delivered for seismotectonics and seismic hazard assessment. This typically leads to overestimate the fault length and the expected magnitude or to the increase in the number of seismogenic sources affecting an area. Finally, the definition of the structural evolution permits to select between different geometrical options in terms of active faulting framework (e.g. a system of parallel normal faults vs. a system of en-echelon normal oblique faults as in the case of the LˈAquila fault system) related to different geometries at depth (detachment normal fault vs. high-angle oblique fault).