The minimum strain ellipsoid using deformed quartz veins

Abstract

A method is described for constructing strain ellipsoids for rock domains which have deformed homogeneously. It is applicable to gneisses as well as to deformed rocks of lower metamorphic grade. It depends on the qualitative observation of deformed linear or planar elements in competent sub-fabrics of known orientation. Plotted on equal area stereographic projections these observations allow the definition of a field which is related to the orientation and shape of the relevant strain ellipsoid. The strain ellipsoid so constructed describes the mean strain of the rock mass or masses from which the observations were collected. Practical limitations will normally only allow the description of part of the deformation suffered by the sub-fabric's host rock i.e., they usually allow only the construction of a minimum strain ellipsoid. The frequency and original orientation of the elements (e.g., segregation veins, igneous sheet intrusions, etc.) making up the sub-fabric is the greatest limitation to the method. The method has great potential for regional and structural deformation studies, and also for studying the deformation of the sub-fabrics themselves. This latter will be illustrated here with some comments on the ductile deformation of quartz vein sub-fabrics used to construct strain ellipsoids for some slates and kyanite bearing gneisses.