Quaternary capable folds and seismic hazard in Lombardia (Northern Italy): the Castenedolo structure near Brescia

F. Livio(*), A.M. Michetti(*), G. Sileo(*), C. Carcano(**), K. Mueller(***), S. Rogledi(**), L. Serva(****), E. Vittori(****) & A. Berlusconi(*)
(*) Università dell'Insubria, Dipartimento di Scienze Chimiche e Ambientali, Via Valleggio, 11 - 22100 Como, Italy. (**) ENI Exploration & Production, San Donato Milanese (MI), Italy. (***) University of Colorado at Boulder, Department of Geological Sciences, 2200 Colorado Blvd, Boulder, Colorado, USA. (****) ISPRA, Dipartimento Difesa del Suolo/Servizio Geologico d'Italia, Via V. Brancati, 48 - 00144 Roma, Italy

We identify evidence of late Quaternary compressive tectonics in the Northern sector of the Central Po Plain through a systematic revision of the literature, new field mapping, and a new study of seismic reflection data obtained by ENI E&P. In particular, the reinterpretation of ca. 18.000 km of seismic profiles clearly shows a belt of segmented, 10 to 20 km long, fault propagation folds, controlled by the Plio-Quaternary growth of several out-of-sequence thrusts. As an example of this active structural style, in this paper we focus on a buried fold located just south of the Castenedolo Hill, a few km SE of Brescia. Although the Castenedolo anticline has long ago been described as a young compressional structure (e.g., DESIO, 1965), no detailed structural analysis of this feature has been performed until now. We calculated the uplift rates of this fold through the analysis of its syntectonic sedimentary record as imaged by the extremely high quality ENI E&P subsurface data available in the area. The evolution of this anticline was a discontinuous process characterized by several tectonic uplift pulses (with rates of ca. 0.1 mm/yr) of different duration, separated by periods of variable extent in which no fold growth occurred. The Quaternary growth history of this anticline and the presence of faulted and folded late Pleistocene to Holocene deposits at nearby sites (Ciliverghe and Monte Netto) demonstrate that the significant seismicity of this area (e.g., the December 25, 1222, Io = IX MCS Brescia earthquake,MAGRI & MOLIN, 1986; GUIDOBONI, 1986) must be related to active compressional structures within the Brescia piedmont belt. Our regional investigations show that the structural and paleoseismic setting illustrated near Castenedolo is typical of the whole Lombardia domain of the Southern Alps. This implies that the currently accepted seismotectonic model for this region, and related seismic hazard assessment, should be thoroughly and carefully re-evaluated.

Active tectonics, Southern Alps, fault propagation folds, capable faults.