Kilometre-scale palaeoescarpments as evidence for Cretaceous synsedimentary tectonics in the External Briançonnais Domain (Ligurian Alps, Italy)

Abstract

In the central part of the External Ligurian Briançonnais in the Ligurian Alps (NW Italy), a large and diverse set of stratigraphic, sedimentologic and petrographic data provides evidence for the occurrence of Cretaceous kilometre-scale palaeoescarpments. These palaeoescarpments consist of irregular erosional surfaces more or less deeply incising a stratigraphic succession that ranges from Upper Jurassic limestones down to Permian volcano-sedimentary rocks; at present they are partly exposed and partly covered by Upper Cretaceous–Upper Eocene sediments, and patchily encrusted by authigenic minerals. Palaeoescarpments resulted from the remodelling of palaeofault planes by gravity-driven rock fall processes, whose products are represented by breccias and metre- to tens of metres-sized blocks of Upper Jurassic limestones either lying directly over the surfaces or embedded within onlapping sediments. Palaeofault-related tensional stresses are also documented by the occurrence of subvertical sedimentary neptunian dykes and tabular breccia bodies, interpreted as fault rocks and locally bearing evidence of the uprise of hot, overpressured fluids. Fault activity started during the Aptian, with most of the displacement—which locally reached several hundred metres—accomplished during the Late Cretaceous, resulting in deep morpho-structural depressions that, by Eocene times, were not yet levelled out. Two systems of kilometre-long palaeoescarpments are recognized and mapped, suggesting the existence of a Cretaceous kilometre-sized fault-bounded basin limited to the north and south by two main transcurrent zones, presently striking E–W and internally partitioned by N–S oriented east-dipping normal faults. This type of setting could be consistent with the Western Tethys tectonic context, in which the Ligurian Briançonnais Domain, close to the Early–Late Cretaceous boundary, was located at the easternmost end of a transcurrent belt connecting the Bay of Biscay to the Valais and Ligurian–Piedmont oceans.