Remarks on the Quaternary tectonics of the Insubria Region (Lombardia, NW Italy, and Ticino, SE Switzerland)

In the foothills of the Southern Alps between Lake Garda and Lake Maggiore, the relations between the recent tectonic evolution and the accompanying seismic potential have been overlooked. Published interpretations of structures consider shortening to cease at the end of the Messinian; this is in contrast with geomorphic and stratigraphic evidence for Plio-Pleistocene reverse faulting and folding presented in this paper. Evidence for Quaternary shortening near Lake Garda includes Quaternary folds with 200 m of structural relief recorded by Lower-Middle Pleistocene deposits. These crop out on top of the Castenedolo and Ciliverghe Hills, within the epicentral area of the historical December 25, 1222 Brescia earthquake (epicentral intensity IX MCS, Me 6.2; GUIDOBONI, 2002). Shortening further west may also be occurring, but there is no record of local damaging historical earthquakes along the foothills of the Alps. This motivated us to review available data and undertake new reconnaissance field work, in order to better understand the Quaternary tectonic evolution of the Southern Alps near Lake Como in the Insubria region. We focused our studies on two major structures located east and west of Como, respectively, the Gonfolite backthrust and the Albese con Cassano anticline. The Gonfolite backthrust, first recognized by BERNOULLI et alii (1989) and reported as late Miocene in age, is accompanied by geomorphic features consistent with Quaternary compressional structures. Our new field mapping suggests that this thrust offsets deposits of Pliocene and possibly younger age. Neotectonic evidence for the recent evolution of Albese con Cassano anticline was first presented by OROMBELLI (1976). Our investigations confirm Orombelli's observations, and suggest that the Albese con Cassano is an actively growing anticline that accumulated ca. 200 m of post-Middle Pleistocene vertical displacement. In addition, paleoliquefaction features in Mid-Pleistocene proglacial deposits at this site suggest that growth of this fold has been accompanied by strong local earthquakes. Evidence of Quaternary tectonics in the Insubria region is consistent with that observed in the well-known seismic area near Lake Garda. Therefore, the seismic potential of this area should be carefully re-evaluated based on the examination of available geological data and the results of new ad hoc paleoseismological analyses. Current assessment of seismic hazard, based only on the historical seismic catalog, supposedly heavily underestimates the earthquake potential in one of the most vulnerable areas of the whole Europe.