AN INTERACTIVE PROGRAM FOR VISUALIZING AND ESTIMATING STRAIN IN DEFORMED CONGLOMERATES

Deformed pebbles and oolites are commonly used by researchers to estimate strain in rocks, and by instructors to teach fundamental principles of strain. The Rf-Phi method is useful for determining strain from a population of deformed pebbles, although it relies on the assumption that pebbles were deposited with no initial preferred orientation of long axes. We developed a simple yet robust interactive program that allows users to explore visually how deformation of a set of elliptical objects appears on Cartesian and polar Rf-Phi plots. The user creates or loads the ellipses and then deforms them by simple shear, pure shear, or rotation. As the shapes (Rf) and long-axis orientations (Phi) of the pebbles change, the Cartesian and polar Rf-Phi plots are instantaneously updated. Deformation can be specified precisely, incrementally changed using the arrow keys, or simulated by click-and-drag with a mouse. The program provides both the Cartesian Rf-Phi graph (Ramsay and Huber, 1987), and the 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 digitized, the ratio of the strain ellipse and the maximum initial ellipticity can be determined with the equations provided by 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 assumption of no initial preferred orientation of the pebble long-axes over a range of strain values. Users can export information about the pebble shapes and orientations to spreadsheets for rigorous statistical analysis. The platform independent program is written in Java, and the source code and application are freely available for academic purposes.