196-2 Evaluating the use of Automated Image Processing Techniques for Finite Strain Analysis

Session: Strain and Displacement: Patterns, Gradients, Partitioning, and Reconstructions (Posters)

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Traditionally, finite strain analysis involves the mathematical and graphical description of the shapes and orientations of strain markers in 2D thin sections and/or outcrop exposures. This process is time-consuming, requiring manual identification and tracing of markers. Image processing techniques such as edge fabric ellipse (EFE) analysis have been suggested as a means of expediting this process, where the contrast of neighboring pixels is used to generate a gradient matrix that directly correlates to a 2D shape matrix, giving the aspect ratio (R) and orientation (ϕ) of the 2D strain ellipse. Here we test EFE analysis on two collections of thin sections: strained quartzite from the Blue Ridge Mountains, VA and granitoid mylonite from the Buckskin Mountains, AZ, as well as 12 outcrop photographs of the Purgatory Conglomerate in Middletown, RI. We compare all EFE results to shape matrix eigenvector (SME) analyses performed on the same sections/photographs. EFE derived values for ϕ in 2D sections, and the orientation of the maximum elongation axes in 3D compilations of oriented sections, are within 15° of that calculated with SME analysis for samples with SME R values >1.5 in 2D or natural strains >0.3 in 3D. Similarly, in the 3D compilation of Purgatory Conglomerate photographs, SME and EFE analyses yielded only a ~3° difference in the orientation of the maximum elongation axis. EFE analysis was also found to preserve strain symmetry in most samples of moderate-to-high strain. For the Purgatory Conglomerate photographs, SME and EFE analyses yielded Lode’s values of -0.893 and -0.824, respectively. Although EFE analysis consistently under-predicts strain magnitude, a moderate linear relationship can be seen between SME and EFE R values in 2D (EFER=0.18*SMER+0.91, R2=0.59), and natural strain values (ε) in 3D (EFEε=0.33*SMEε+0.024; R2=0.80), suggesting that relative strain magnitude between analyzed sections is preserved. More data are required to establish a stronger correlation between the two methods. Due to higher variability in strain magnitude compared to traditional methods such as SME analysis, EFE analysis is not recommended as a tool for detailed finite strain analysis. However, EFE may prove an effective tool for the approximation of strain shape and orientation and for evaluating relative strain magnitude. 

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-5542

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