The analysis and interpretation of minor folds developed in the Moine rocks of Monar, scotland

Abstract

Abstract A series of minor folds developed in the Moine rocks of Monar, Scotland, have been studied in detail using various geometrical methods of analysis and are interpreted as having formed by a process involving both buckling and uniform “shortening”. The rocks, highly metamorphosed sediments, are well-banded psammitic to pelitic granulites with interlayered quartzo-feldspathic pegmatitic veins. Studies of thickness variation with apparent dip and isogon patterns within the folded layers show that the overall fold geometry is close to that of a similar fold. Together with the results of harmonic analysis these studies also indicate that the fold geometries of both layers and single surfaces show considerable variation, and that there are slight but systematic differences in fold shape that can be related to differences in composition of the layers. These differences are related to differences in competence of the layer material. Most competent are the quartzo-feldspathic pegmatitic veins that often form ptygmatic structures with an average layer geometry least removed from that of a parallel fold. Competence decreases with increasing biotite content in the granulitic layers and is least for the pelitic layers whose mean geometry is furthest removed from that of a parallel fold. Where competence contrast is least across single folded surfaces their shapes are near sinusoidal, and with increasing competence contrast they become progressively non-sinusoidal and more variable. Interpretation of the fold geometry in terms of folding processes has been made by analogy with theory and the results of experiments. All geometric features are consistent with a process of fold development involving both buckling and uniform shortening (flattening) normal to the axial surfaces of the folds and to the schistosity. During deformation the rock mass behaved as a complex multilayer in which the more competent layers controlled fold initiation and development by buckling. Some of the pegmatitic veins buckled to form ptygmatic folds for which there is an approximately linear relationship between wavelength and thickness. These folds have developed as if the veins were single competent layers embedded in a uniform incompetent medium, and their geometry shows the effects of a modifying flattening deformation. Some folds also show features to indicate that buckling and flattening have proceeded simultaneously. Where pegmatitic veins are absent in the layered granulites and competence contrast is low, effects of buckling are slight and almost homogeneous strain has produced nearly perfect similar folds with sinusoidal form. The strongest effects of buckling and least effects of flattening are seen in the ptygmatic folds. Applying single-layer buckling theory to the ptygmatic folds in the pegmatitic veins suggests that the maximum viscosity (competence) ratio, between the quartzo-feldspathic layers and the pelitic granulites, that existed at the time of fold development was about 10 1 , and that the total strain in the profile plane of the folds is given by a value for √ λ 1 λ 2 of 11 1 . Studies of early linear structures deformed around the folds show that the fold axes are nearly parallel to the maximum extension axis of the finite bulk strain ellipsoid.