Magnetotelluric measurements in La Malbaie area (Quebec): the anomalous vertical magnetic field

Abstract

Magnetotelluric soundings were recorded in 1980–1981 at Ste-Mathilde near La Malbaie, Quebec for periods of 0.04–200 s. The present study deals only with single-station vertical transfer functions. Those functions show very characteristic features in their responses versus period. Of particular interest is a large in-phase (P) peak corresponding to a quadrature (Q) minimum between 0.1 and 0.2 s. For those periods, induction arrows indicate no obvious correlation between the anomalous vertical magnetic field (Z) and the conductive zone of Ste-Mathilde. Rather, they point toward the Malbaie River to the southwest of the survey zone. A possible model for the source of the vertical magnetic field is a shallow N130°E vertical dyke-type conductor, the top of which is some 1400 m under the Malbaie River valley. An interpretation could well be that it is one of the major graben faults related to the meteoritic impact crater of Charlevoix.