The measurement of strain and displacement in orogenic belts

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Abstract
Summary: Many of the premises that have been used in evaluating the crustal shortening across orogenic belts are open to question. Past estimates of displacement have often depended upon the validity of some preconceived tectonic hypothesis and its mechanical feasibility. Displacements describe the changes in position of particles in space, and can be defined in three dimensions by a set of three equations. Displacement leads to three main effects: (1) body translation, (2) body rotation, (3) strain. The inter-relationships and properties of these features are discussed to determine which features the geologist can measure in naturally deformed rocks. Absolute values for the amount of displacement are impossible to determine. Displacement differences between different localities are related to crustal shortening and under special circumstances may be computed with precision. Techniques for determining displacement difference employ two methods, (1) removal of the effects of strain through the deformed zone, and (2) resection of once continuous linear features in the foreland and hinterland. These have been applied successfully in parts of deformed zones but their application to a complete orogenic belt is for the future.