Sedimentation and deformation in a wrench tectonic context. The Lower Palaeozoic Plougastel Formation: the key to a better understanding of Variscan geodynamics in the Central Armorican domain, Brittany, France.

In many deformed, low-grade middle- to upper-crustal terrains, rocks commonly bear the evidence of an often complex pre-lithification deformation history. Once lithified and affected by tectonometamorphic events, recognising the early 'soft-sediment' deformation structures may become evermore difficult, even to the point that such structures end up being erroneously interpreted as part of a sequential fabric that results from multiple tectonometamorphic events. The Lower Palaeozoic metasediments of the Plougastel Formation in Central Armorica, exposed in western Brittany (France), are an ideal case to deal with this research question. The current project will use the Plougastel Formation as a tool (1) identify particular pre-lithification structures, (2) define criteria to distinguish 'soft-sediment' deformation features, (3) reconstruct the pre-lithification history, and (4) assess correctly the Variscan deformation in middle- and upper-crustal levels in Central Armorica. To achieve these objectives three tracks of research will be pursued: (1) a field-based sedimentological and structural analysis in different target areas in western Brittany; (2) a laboratory-based analysis of the diagenetic and metamorphic conditions; and (3) an analysis of the rock magnetic properties to further refine the applicability of magnetic fabrics in geological research.

Keywords:Siliciclastics, Variscan, Soft-sediment deformation, Armorica, Low-grade metamorphism, Lower Palaeozoic

Disciplines:Geophysics, Geology