Résumé
Deciphering between deformations related to subduction and exhumation in HP metamorphic rocks represents a challenge for the understanding of the dynamic processes involved and more particularly, the exhumation mechanisms. An analysis of shear criteria in the Ile de Groix blueschists in the Hercynian Belt of southern Brittany (Western France) is carried out in terms of relative chronology and in relation to lithology (mainly micaschists and glaucophane-bearing metabasites). Structural mapping shows that foliations are flat-lying, bearing stretching lineations oriented in a N150°E ± 30° direction. Associated shear criteria are dominantly top-to-northwest and locally, in the southeast part of the island, both top-to-northwest and -southeast. Both shearing deformations lead to intense finite strains as demonstrated by the occurrence of sheath folds and the general orientation of fold axes parallel to the stretching lineation. Detailed mapping in this area showing opposite senses of shear, demonstrates that only top-to-southeast shear is observed in rocks containing well-shaped lawsonite pseudomorphs. In rocks affected by top-to-northwest shear, the lawsonite pseudomorph shape is destroyed by intense stretching. The top-to-southeast shear is coeval with prograde HP metamorphism and precedes a top-to-northwest shear coeval with a retrograde metamorphic path. In the frame of the hercynian belt of southern Brittany, the southeast directed shear is attributed to thrusting related to a northward dipping subduction and the northwest directed shear to exhumation controlled by a northward dipping extensional detachment.