Strain analysis of deformed granitic rocks (Helikian), Muskoka District, Ontario

Abstract

Three-dimensional analysis of irrotational, longitudinal, finite strain was carried out on samples from a crescentic sheet which intruded and was deformed with a host gneiss unit of probable Helikian age. Analytical methods were compared using deformed feldspar grains representing four ideal degrees of strain intensity observed in the porphyritic sheet. The polar plot and R f / φ, R s methods proved most reliable and sensitive. Data derived from fabric and strain analysis at 38 sites in the units suggest a two-stage deformational sequence. The first stage produced recumbent, isoclinal, similar (class 2) folds with northwest-trending hinge surface traces. This fold form was modified during the second stage to produce an overall type 2 fold interference pattern. The second stage produced upright, open buckle folds as well as the resultant strain fabric currently observed. Strain analysis confirms the general fold geometries of the model, and also documents competency contrasts between the matrix and feldspar grains with increased strain intensity and magnitude. Deformation of feldspar grains in the sheet involved modification of a fabric of low strain magnitude ( ϵ s = 0.3) and a k value near unity to magnitudes of ϵ s = 2.6 and k = 0.6. Matrix strain intensities and magnitudes are consistently higher than those of the feldspar markers in the sheet. This variation is related to competency differences between the matrix and the feldspar grains. Fabric anisotropy accounts for the strain gradient observed between the sheet and gneiss.