Magnetotelluric survey on the Alps-Apennines boundary (Tertiary Piemonte Basin - northwestern Italy)

A 60-kilometre-long magnetotelluric profile was carried out across the south-western part of the Po plain from Monferrato through the Asti-Alessandria Plio-Quaternary infillings, the Langhe Tertiary Basin and into the Maritime Alps (Voltri Group). Recent works suggest that in this region the step-like geometry of the top of the basement is due to the northward overthrust of an Alpine basement wedge onto the Apenninic sedimentary nappes and the underlying Padane basement. The spatial distribution of the major tectonic discontinuities was well constrained, at crustal scale, by seismic survey and gravimetric modelling. The aim of the magnetotelluric survey was to detect the electrical behaviour of this structure, thus defining the geometric relationship between the overthrusting resistive basement (Alpine crystalline basement, ophiolites, Mesozoic cover) and a less resistive sedimentary thrust belt (Apenninics). Data were collected with an M.S.P.M. (Medium Short Period Magnetotellurics) instrument, which is a real-time data processing system in the frequency range 100 Hz - 1164 Hz. The 2-D magnetotelluric interpretation shows an evident rise in the resistive basement southeastwards: it confirms the existence of tectonic discontinuities constraining the geometries of the Tertiary Basin.