New stratigraphic and structural evidence for Late Pleistocene surface faulting along the Monte Olimpino Backthrust (Lombardia, N Italy)

We summarize some preliminary results achieved during the investigations conducted for the CARG Project, geological sheet n. 75 “Como”, i.e. the analysis of the Quaternary evolution of the “Monte Olimpino Backthrust” and the evaluation of its seismogenic activity. Cross-border field mapping between Ticino (CH) and Lombardia (IT) resulted in the finding of new outcrops (Borgo Vico site, in the north-western sector of the urban area of Como) located along the front of the Monte Olimpino Backthrust, that allowed to recognize evidence for Late Pleistocene reverse surface faulting along this structure. At Borgo Vico, a clastic Tertiary unit, the Villa Olmo Conglomerate, intercalated in the Chiasso Fm. of Early Oligocene age, is thrust over a Late Pleistocene fluvioglacial and glacio-lacustrine sequence (Comerci et alii, 2007). Until now, the Monte Olimpino Backthrust was supposed by most authors to have been active until Tortonian times. Sileo et alii (2007) inferred a Pliocene activity and proposed, based on geomorphic evidence, that fault displacement was still taking place during Pleistocene. However, this is the first time that Pleistocene activity along the Monte Olimpino Backthrust has been documented by unequivocal tectonic offset of late Pleistocene deposits. Paleoseismological analyses are in progress in order to distinguish potential coseismic movement from fault creep during the observed recent displacement.