Finite strain states, quartz textures and the significance of lineation in permo-carboniferous metasediments of the Dora Maira Massif, Val Germanasca, Italy

Abstract

The Permo-Carboniferous metasediments of the Dora Maira Massif (Internal Crystalline Massif of the Western Alps) include metaconglomerates in the lower Val Germanasca. Finite strain analysis using deformed pebbles shows that the associated ellipsoids are predominantly of the constrictive type, with K-values ranging from 0.68 to 12.72 and X Z ratios from 3.30 to 13.38. Long axes have gentle to moderate plunges towards the SW, W or WNW. These have resulted from two major tectono-metamorphic cycles: an early, syn-high-pressure metamorphism deformation, D 1, which produced a bedding-parallel foliation, followed by a period of folding, F 1 , which produced isoclinal, WSW-plunging folds with an associated penetrative schistosity, S 1, and a strong stretching lineation, L 1, oriented parallel with long axes of deformed pebbles in S 1. A later, syngreenschist facies deformation, D 2, produced more open folds, F 2, coaxial with the L 1 lineation and with local development of crenulation cleavage, S 2. Optically determined quartz c-axis diagrams, for the quartzite pebbles, show girdle patterns passing through the Y finite strain axis, but oblique by 21° to 19° to the S 1 schistosity pole, showing that the D 1 deformation was rotational. Consideration of the bedding, S 1 and S 2 trace geometries and the strain ellipses, in YZ-sections of the finite strain ellipsoids, at localities where the S 2 cleavage includes the finite X-direction, suggests that the D 1 deformation was not plane-strain and hence not simple shear. The L 1 lineation cannot be interpreted as an “ a-lineation” and no so-called “tectonic transport direction” can be derived directly from it.