Quartz microstructures and fabrics in the Island of Groix (Brittany,France)

Quartz microstructures and fabrics in the southeastern part of the island of Groix developed during the last stages of the Palaeozoic synmetamorphic deformation. The zonation of the quartz microstructures in map view suggests an upward positive gradient of strain. The plastic flow plane in quartz found in folds with axes aligned parallel to the stretching lineation, is parallel to the axial planes of the folds. The dominant regional sense of shear, as deduced from quartz fabrics, corresponds to the northward displacement of the upper block. This sense of displacement supports the hypothesis made by previous workers that the synmetamorphic deformation in Groix occurred in a N-directed intra-lithospheric thrust rather than in a N-dipping subduction zone. Quartz c-axis patterns argue for the distinction of two synmetamorphic phases with different transport directions. The transition between these two phases is thought to have occurred progressively during the course of the thrusting.