VISUALIZING STRAIN AND THE RF-PHI METHOD WITH AN INTERACTIVE COMPUTER PROGRAM

Deformed elliptical objects are commonly used by researchers to estimate strain in rocks, and by instructors to teach fundamental principles of strain. It is relatively easy to visualize how strain will affect a population of initially elliptical objects, and how the development of a preferred-shape fabric is related to the intensity and orientation of the strain ellipse. It is more challenging, however, to imagine how the deformation of elliptical objects will manifest itself in Cartesian or polar Rf-Phi plots. To overcome this limitation we developed a program that allows users to explore visually how manipulating a set of elliptical objects affects the corresponding plot. A user first creates or loads the ellipses and then deforms them by simple shear, pure shear, or rotation. As the ellipses' shapes (Rf) and long-axis orientations (Phi) change in one window, the Rf-Phi plot continuously updates in another. Deformation can be specified precisely, or simulated by click-and-drag with a mouse. The program lets users save snapshots of the deformed elliptical objects and their plots to record graphical experiments and to create animations using other applications. The program provides both the familiar Rf-Phi graph (Ramsay and Huber, 1987), and a polar plot of ln(Rf) vs. 2(Phi), as suggested by Elliott (1970). Elliptical objects can be loaded from a text file or quickly traced from an imported photograph. When a population of naturally deformed ellipses is traced, the ratio of the strain ellipse and the maximum initial ellipticity can be determined in the normal way (Ramsay and Huber, 1987). In addition, graphical experiments to un-strain the population using a range of inverse strain ellipses can be done quickly and easily, making it possible to inspect the shapes and orientations of the ‘retro-deformed' objects. Thus, it is possible to assess the initial preferred orientation of the ellipses' long-axes over a range of strain values. This program is written in Java and so can run on virtually any operating system. Both the source code and the finished application will be freely available for academic purposes.