Unravelling polyphase brittle tectonics through multi-software fault-slip analysis: The case of the Voltri Unit, Western Alps (Italy)

We have analyzed the brittle structures in a key area of the Alps–Apennines transition zone (NW-Italy): here two orogens have interfered with each other since Oligocene times, producing a complex structural evolution and a heterogeneous fault population. Only relative chronologies can be reconstructed as stratigraphic constraints are lacking. We have performed the inversion of fault-slip data with two softwares for paleostress calculations, combined with field observations at selected structural stations and photo-interpretation. The resulting incompatible stress tensors can be grouped in this sequence: i) a strike-slip tensor, with NNW–SSE trending σ1 (Event A); ii) a strike-slip tensor, with NE–SW trending σ1 (Event B) and iii) an extensional/transtensional tensor, with NW–SE or NE–SW trending σ3 (Event C). We correlated our results with structures of known age from adjacent areas. Event A is possibly Rupelian–early Chattian, linked to far-field incipient rifting in the future Ligurian-Provençal basin. Event B fits Oligo–Miocene shortening: the faults may belong to a sinistral strike-slip zone that accommodated the oblique component of deformation during the rotation of the Corsica–Sardinia block. Event C is attributed to a Pliocene/Quaternary? (neotectonic) event. Therefore combining different inversion procedures with a detailed structural analysis has successfully unravelled the polyphase brittle tectonics.