Measurement of fold axes in drill core

Abstract

Measuring the orientation of fold axes is a routine component of field-based structural analysis. Determining the real-space orientations of small-scale folds in drill cores, however, remains a time consuming and often cumbersome task unless the fold axis lies on an exposed plane. We present a method for accurately determining the orientation of fold axes that do not lie on exposed planes in drill core, provided the core is oriented or can be reoriented using the assumed orientation of one of the fabrics within it. Two angles that uniquely specify the orientation of the fold axis with respect to the core are measured. One is the angle between the fold axis and the core axis. The other, measured in a plane perpendicular to core axis, couples the orientation of the fold axis to either a core orientation mark or another fabric of known orientation. The real-space orientation of the fold axis can either be calculated from the two measured angles and the plunge and azimuth of the drill hole or determined using a simple stereonet construction. Measurements are made with respect to the two points of intersection between the fold axis and the outer surface of the core. While this means the technique is best suited for use with a whole core, it has the advantage that fold axes need not lie on exposed planes in the core.