Harold Robert Leah, Michele Fondriest, Alessio Lucca, Fabrizio Storti, Fabrizio Balsamo, Giulio Di Toro

Recent high resolution hypocentral localisation along active fault systems in the Central Apennines illuminates the activation of seismogenic volumes dipping at low angle (<30°) in extensional settings overprinting contractional deformations affecting the continental crust of the Adria microplate. Individuation of the geological structures and of the fault processes associated with these seismic patterns will contribute to the interpretation of seismic sequence evolution, and seismic hazard studies.

Here we report field and microstructural evidence of seismogenic extensional faults localized within pre-existing thrust fault zones. The Vado di Ferruccio Thrust Fault (VFTF) is a narrow fault zone (<2.5 m thick fault core) in the Central Apennines of Italy, accommodating ∼1 km of shortening during Miocene-Pliocene and exhumed from <3.5 km depth. In the thrust zone, exposures throughout the Fornaca Tectonic Window show Late Triassic bituminous dolostones thrust over Middle Jurassic interlayered carbonates upon a SSW-dipping fault. Isoclinal folds are dragged and sheared by thrust-parallel reverse faults in the footwall block whereas NW-striking faults occur within the hanging wall. Fault core observations are consistent with stable pressure solution-mediated aseismic sliding towards N024° during thrusting, with cyclic veining and faulting. Later extension has been accommodated at the regional scale by major normal faults cutting through the VFTF, while veins and pressure-solution seams crosscut the microstructures associated with thrusting and record the extensional stress regime within the thrust fault core. Lenses of shattered rocks (up to 10 s m thick), cut by a dense network of small displacement (<1.2 m) mirror-like normal faults, are reported in the hangingwall of the VFTF. These minor faults, related to a sharp principal slipping surface on the upper margin of the VFTF fault core, are interpreted as fossil evidence of microseismicity compartmentalized within the hanging wall of the VFTF. Synthetic and antithetic normal faults within the VFTF hangingwall damage zone are geometrically and kinematically similar to small earthquake ruptures (Mw < 2) in the hangingwall of low angle structures such as the thrust flats illuminated during the 2009 Mw 6.1 LAquila seismic sequence.