Gravity modellings of the Cadomian active margin of northern Brittany

The geometrical relationships existing between the Saint-Brieuc, Saint-Male and Guingamp units, which form the Cadomian active margin of northern Brit tany, have been determined by gravity analyses and 2.5D modelling, correlat ed with other geophysical (magnetic, seismic, etc.) and geological observat ions. Automatic structural analyses were performed before modelling, to ide ntify lateral gravity discontinuities. Averaged density measurements were u sed for 2.5D gravity modelling along six profiles across the arcuate geolog ical and gravity shape of the Cadomian basement. This study allowed the thi rd dimension to be obtained through correlating each profile with its neigh bours. We propose that orthogneiss formations equivalent to the Port-Morvan orthogneiss, constitute the Saint-Brieuc upper-crustal unit, below the Lan vollon Formation. 2.5D gravity modelling outlines downward geological diffe rences in the western and eastern parts of the Ploufragan Complex, as previ ously indicated by surface mapping. These results stress the necessity for distinguishing between the Guingamp and Saint-Male units. (C) 2001 Elsevier Science B.V. All rights reserved.