Quaternary negative tectonic inversion along the Sibillini Mts. thrust zone: the Arquata del Tronto case history (Central Italy)

Geological and geophysical investigations have been carried out in the surroundings of the villages of Pretare and Piedilama, in the Central Apennines. This area, located at the footwall of the prominent NNE-SSW oriented Sibillini thrust front, was struck by the August 24th 2016 Mw 6 earthquake. The results of the geological field survey combined with geological and geophysical subsurface investigations are here illustrated by a detailed geological map and by two significant cross-sections. The studied area is crossed by the Fosso di Morricone valley that, entrenched within the Messinian pelitic-arenaceous succession of the Laga formation, hosts a Late Pleistocene palaeo-landslide. The study evidences the occurrence of a N 10° oriented west dipping normal fault, here named Pretare-Piedilama fault, that controlled the left side of the Fosso di Morricone valley, producing a vertical displacement of the palaeo-landslide deposits of about 50 m. The geological, geomorphological and geophysical data indicate that the fault was active during the Late Pleistocene and part of the Holocene thus constraining a minimum vertical displacement-rate of 0.4 mm/a in the last 125 ka. The geometry of the deformed Laga formation cut by the fault suggests to relate it to the Late Quaternary negative tectonic inversion of a previous thrust plane located in the footwall of the Sibillini thrust front. The Pretare-Piedilama fault would represent the easternmost tectonic expression of this main potentially active structure. Future researches could be addressed to detail the age and the actual motion-rate of the structure and to define its seismic hazard assessment.