New data on low-grade metamorphism in the Brianconnais domain of the Prealps, western Switzerland

Numerous measurements by XRD of the Scherrer width at half-peak height for the 001 reflection of illite and of the illite/smectite ratio in mixed-layer minerals show evidence of an increase in the degree of metamorphism from diagenesis in the plastic zone of the Median Prealps to very low/low-grade metamorphism in the trailing part of the rigid zone, along a NW-SE cross-section. Detailed work on illite/smectite (I/S) mixed-layer minerals from the Median Prealps, using XRD patterns (air-dried and glycolated), showed that both the illite content and the size of coherent diffracting domains increase during deep diagenesis and very low-grade metamorphism. The identification of regional trends was possible using these techniques and suggest that the main metamorphic event probably affected the Median Prealpine nappes during transport from their Penninic origin, the process responsible being progressive burial by thrust stacking, probably during the late Eocene.