Étude géologique du socle ante-mésozoique au nord du Massif Armoricain: limites et structures de la Domnonée

Abstract

The study of samples taken on the bottom of the sea north of the Massif Armoricain, and the geological, magnetic, gravimetric and seismic data collected on land and at sea, confirms that this region is characterized by: (1) a series of horsts where fragments of Pentevrian and Cadomian orogenies outcrops; (2) gullies, or synclines, of Palaeozoic terrain. The separation of the horsts is the result of tectonic shearing which developed during the Ordovician, Carboniferous and Triassic ages. The Cadomian chain, which constituted the fundamental structure of the region, was broken up very early and is no longer recognizable. In the west, traces of this chain seem to have lasted until the Llanvirnian. In the east, the uplands, which were probably less harsh, were levelled before the start of the lower Cambrian age. A general upwarp of the eastern part between the middle Cambrian and the middle Ordovician could correspond to a reaction to the Caledonian orogeny. The northern part of the Alassif Armoricain had little part in the Hercynian paroxysm, however, this period witnessed an important tangential tectonism, the horsts riding over the edges of the synclines. The western formations show more marked structures than the eastern formations. The area which has been studied largely corresponds to the Domnonean domain, a puzzle of horsts and grabens. The Domnonea is itself a large horst, limited to the north by thick Palaeozoic sequences and to the south by the central Armorican zone and the Mancellian domain.