Homogeneous Deformation of the Gneisses of Vesterland, South-West Greenland.

Abstract

Structural development of the migmatitic gneisses of the Vesterland area is described first by reference to successive fold episodes (F₁-F₃) and then by reference to the estimated pattern of three dimensional homogeneous strain. Mylonite zones, up to 2km in width, are located along at tenuated steep limbs of relatively open F₃ folds. In acid gneisses F₂ folds are usually isoclinal, with axial planes and axes parallel to those of F₁ folds. In basic gneisses F₂ folds are of relatively open style and both axes and axial planes differ from those of F₁ folds. The macroscopic penetrative fabric of the gneisses is of the L-S type in which three mutually perpendicular axes can be recognised on a variety of scales, the axes corresponding to the principal axes of the strain ellipsoid. The strain ellipsoid corresponding to deformation in the acid gneisses, as estimated from the disposition of discordant acid veins, is of prolate type, with deformation path (k) approximately 17, and axial ratios 1 :3 :100. The relationship between homogeneous deformation and varying styles and attitudes of F₁ and F₂ folds is discussed.