THE EVOLUTION OF THE NORTHERNMOST APENNINE FRONT (PIEDMONT, ITALY): PLIO-PLEISTOCENE SEDIMENTATION AND DEFORMATION IN THE PO BASIN AND MONFERRATO HILLS

Abstract

The area under investigation includes the southern Vercelli plain and the lower Cerrina valley, in the northern central Monferrato hills. The comparison between the deformation of the sediments of the foredeep basin, north of the Monferrato thrust front, and of the Cerrina valley syncline, south of the front, enabled our understanding of the Pliocene and Pleistocene environmental and tectonic evolution to be improved. The study of the Plio-Pleistocene sedimentary successions allowed the phases of subsidence and deformation produced by the compressive tectonic to be dated. The subsidence involved the Po foredeep basin and the Cerrina valley syncline during the same periods, but with different intensity. A first phase of subsidence occurred before the sub-chron Olduvai, while the second phase can be dated between about 1.5 and 0.5 MA ago, but the maximum subsidence rate occurred around 1.07 and 0.99 MA ago. Also the main deformation phases of the Cerrina valley syncline are coeval with the activity of the Lucedio and Cavourrina faults, that correspond to two different buried thrust fronts. The Lucedio fault is linked to the Gaminella-Cerrina valley syncline through the NW-SE trending Crescentino fault (west) and the N-S trending Salera Line (east) acting as ramps, while the ramps linking the Cavourrina fault to the syncline are the NNE-SSW trending Fontanetto Po (west) and the N-S trending Trino (east) deformation zones. The tectonic frame indicates that the syncline corresponds to a trough behind the thrust front. However, the asymmetry of the Cerrina valley in its stretch with a W-E orientation, and the presence of a sequence of terraces only on the northern side, suggest that the hills north of the valley were gently uplifted at least until the upper Pleistocene. It follows that the syncline may have acted, until very recent times, as a boundary between two hilly areas (north and south of the valley) subject to different tectonic evolution. The results of the research suggest that the river, which entered the Cerrina valley during the Early and Middle Pleistocene, was probably fed by the Alpine valleys lying SW of the Aosta valley, or a collector of the streams that drained the Northern Monferrato.