New arguments about the origin of the Paris Basin Magnetic Anomaly (PBMA) from the electrical properties of the crust.

The PBMA, by its amplitude and extent, is a major structure of the geology of France, but its origin remains poorly known. Based on the data of two im portant national programs, the ECORS program and the Deep Geology of France (DGF) program, the last interpretations propose that the anomaly is relate d to an oblique oceanic suture formed during the Variscan collision between the North Brittany block and the Brabant block. The study of the electrica l properties of the crust, by the magneto-telluric (MT) method, along the s eismic profile ECORS-'Nord de la France', reveals the existence of a strong subvertical electrical discontinuity in the Tourly region, located between the Seine fault and the Bray fault. This discontinuity, marking the fronti er between two electrically different crusts, probably corresponds to a sub vertical and not oblique Variscan suture zone. As the axis of the PBMA. is located about 50 km to the west of this electrical discontinuity, it has no genetic relation with the Variscan suture. Its origin is older, probably P recadomian. (C) 2000 Academie des sciences / Editions scientifiques et medi cales Elsevier SAS.