A graphical representation of coaxial plane strains and volume changes

Abstract

Coaxial strain history, although not representing a common situation in rocks, can provide the opportunity to quantify very important parameters such as the amount of volume change occurring during compaction and tectonic deformation. In the case of coaxial superposition of compaction and tectonic plane strain, a co-ordinate system is defined with the x-y plane lying parallel to bedding and the z axis normal to it, and with the x-z plane corresponding to the plane in which tectonic strain occurs. Bulk strain data are represented on a simple deformation plot with axes 1 + e x , 1 + e x (where e x and e x are the finite extensions along the x and z axes of the co-ordinate system, respectively). With respect to the conventional Flinn diagram, the use of this plot for representing coaxial deformation paths brings several advantages, in that: (1) the magnitudes of the finite strains and their orientation with respect to the chosen co-ordinate system are represented; (2) the volume changes occurring during both compaction and tectonic strain are easily visualized for all the stages of progressive deformation; and (3) the transitions from the oblate to the prolate shape (and vice versa) of the finite strain ellipsoid during progressive deformation are shown continuously, without the artificial zig-zag pattern of the Flinn diagram.