QUATERNARY EVOLUTION OF THE PLAIN BETWEEN CASALE MONFERRATO AND VALENZA: IMPLICATIONS FOR THE TECTONICS OF THE MONFERRATO THRUST FRONT (PIEDMONT, NW ITALY)

Abstract

The present paper highlights the Quaternary evolution of the plain which lies on the eastern portion of the Monferrato Thrust Front, in order to identify and provide evidence of recent tectonic deformations. The study has made it possible to detect and date late Middle Pleistocene or early Upper Pleistocene tectonic deformations related to the activity of the front, and to suggest the hypothesis of more recent ones. It can be observed that some parts of the plain, located north of the thrust fronts and the area corresponding to some buried synclines, mainly subsided during the Quaternary. On the other hand the area of the isolated hills and terraces suffered an almost continuous uplift until the late Middle Pleistocene or early Upper Pleistocene. The areas where the uplift prevailed were affected by faults both transversal and parallel to the thrust fronts. The overall interpretation of the tectonic structures lying below the plains and on the hills has enabled us to assume that there are two deformation zones where strike-slip or oblique-slip displacements took place. In the first one (Giarole-Lu Deformation Zone) some left-slip tectonic deformations should exist, while in the second one (Valenza Deformation Zone) some right-slip displacements may be present. The movements along the two deformation zones would result in an advance towards the north of the thrust front near the isolated Pomaro-Montevalenza Hill. In the Eastern Monferrato contractional tectonics related to the Monferrato Thrust Front was more recent than in the Central and Western Monferrato. The subsidence of the northern Alessandria piggyback basin, that lasted until the late Middle Pleistocene or the early Upper Pleistocene, would be a consequence of the shift towards the north of the thrust front. The horizontal and vertical deformation rates obtained by the Alessandria GPS station seem compatible with the hypothesis of a shift to the north and with a subsidence still in progress. The morphology and distribution of the most recent terraces suggest that part of the Po Plain may have been affected by Upper Pleistocene and Holocene tectonic movements. These areas include Casale Monferrato, which underwent a remarkable aseismic uplift during the twentieth century.