Mohr Construction in the Analysis of Large Geologic Strain

Abstract

A semigraphical method of analysis of large strain based on Nadai9s strain components and utilizing a Mohr construction is outlined for problems of interest in structural geology. Finite homogeneous strain theory is applicable to measurement and analysis of strains from geologic features small enough to be included within regions of homogeneous strain. Use of this theory and of the strain ellipsoid and its properties implies nothing about isotropy or homogeneity of the rocks or about the stress-strain relation during deformation, and therefore its application is valid over a much wider range of phenomena than most geologists realize. Strain can be measured, and under favorable conditions the principal strain axes can be determined, by using not only the familiar ooids, pebbles, and crinoid columnals but also a large group of fossil remains and impressions and primary sedimentary structures. The most useful features are single linear elements in which extension can be measured; the next most useful are groups of three directions of known original relative orientation, and the least useful are originally perpendicular line elements. Under ideal and unusual conditions volume change can be measured. Examples of strain analysis are worked out in detail to illustrate both the versatility and limitations of the method.