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

Composite calcite-quartz fibres, developed in strain fringes on pyrites in the Monte Marguareis cover nappe (Ligurian Brianconnais, Northern Italy), a re utilised for an incremental strain analysis. Classical methods require n on-coaxial fibres to reconstruct incremental strain histories. As an innova tion this contribution shows that two generations of coaxial fibres grew in dependently on different pyrite sites during the two major deformation phas es (D1 and D2) which affected the Monte Marguareis structural unit. Rare cu rved fibres are shown by meso- and microstructural analyses to have grown d uring D1 and been deformed by D2 folds. This peculiarity provides a way to estimate, quantitatively, the strain associated with these deformation phas es without the uncertainties related to classical methods. D1 strain shows a marked strain partitioning among different lithologies and within the sam e lithology, the measured elongations ranging between 1.84 and 8.45. Measur ements of D2 incremental elongation are available only for one lithology an d indicate that D2 stretching is approximately half that of D1 and its cont ribution to the finite strain ellipsoid cannot be neglected. This finding i s consistent with qualitative inferences on strain partitioning between D1 and D2, phases. Measured D2 elongations display higher homogeneity througho ut the area. Both L1 fibre orientation (generally NE-SW) and D1 fold asymmetry suggest D 1 deformation occurred in a ductile shear zone possibly induced by the sout h-westward motion of nappes, consistently with regional tectonics. The L2 l ineation orientation (approximately NE-SW) and the D2 folds asymmetry sugge st that the D2 phase is likely to be the product of a regionally recognised backthrusting event. (C) 2001 Elsevier Science Ltd. All rights reserved.