Incremental strain analysis using two generations of syntectonic coaxial fibres: an example from the Monte Marguareis Briançonnais Cover nappe (Ligurian Alps, Italy)

Abstract

Composite calcite–quartz fibres, developed in strain fringes on pyrites in the Monte Marguareis cover nappe (Ligurian Briançonnais, Northern Italy), are utilised for an incremental strain analysis. Classical methods require non-coaxial fibres to reconstruct incremental strain histories. As an innovation this contribution shows that two generations of coaxial fibres grew independently on different pyrite sites during the two major deformation phases (D1 and D2) which affected the Monte Marguareis structural unit. Rare curved fibres are shown by meso- and microstructural analyses to have grown during D1 and been deformed by D2 folds. This peculiarity provides a way to estimate, quantitatively, the strain associated with these deformation phases without the uncertainties related to classical methods. D1 strain shows a marked strain partitioning among different lithologies and within the same lithology, the measured elongations ranging between 1.84 and 8.45. Measurements of D2 incremental elongation are available only for one lithology and indicate that D2 stretching is approximately half that of D1 and its contribution to the finite strain ellipsoid cannot be neglected. This finding is consistent with qualitative inferences on strain partitioning between D1 and D2 phases. Measured D2 elongations display higher homogeneity throughout the area.Both L1 fibre orientation (generally NE–SW) and D1 fold asymmetry suggest D1 deformation occurred in a ductile shear zone possibly induced by the south-westward motion of nappes, consistently with regional tectonics. The L2 lineation orientation (approximately NE–SW) and the D2 folds asymmetry suggest that the D2 phase is likely to be the product of a regionally recognised backthrusting event.