APPLICATION OF GIS-BASED SOFTWARE FOR QUANTIFYING MICROSTRUCTURES

Microstructural analysis on photomicrographs or rock samples is a commonly used method for studying deformed rocks. One major challenge for quantifying microtextures is the tedious process of digitizing grain boundaries. Consequently, the nature of studying microtextures remains largely descriptive. Here we present a method for quantifying mineral shapes, sizes, and distribution patterns by using remote sensing and GIS-based software. Such an approach can provide insights into fabric formation and grain scale processes associated with deformation and metamorphism.

This method includes three steps: 1) a semi-automated process using ArcGIS ModelBuilder® to trace grain boundaries on images; 2) classification of images using ERDAS Imagine 9.3® software, commonly used for analyzing remotely sensed images; 3) spatial pattern analysis using Fragstats 3.3®, a public domain program for quantifying landscapes. Using this three-part method, we maintained the spatial characteristics of the minerals within a geographical information system, and created a database listing the shape, size, and distribution characteristics of each mineral.

In one application, we quantified microstructures in the Economy Falls mylonite, an orthogneiss from the Cobequid Highlands of Nova Scotia. We quantified the microstructures of two regions from the same thin section including plagioclase and quartz grains and gneissic banding defined by alternate layers of fine-grained mica and quartz aggregates. Spatial distribution data shows how the mineral distribution, banding, and connectivity are heterogeneous within the scale of the thin section.

In another application at a larger scale, we quantified mineral distribution patterns across a small granitic gneiss shear zone from the larger Mountain Shear Zone of northeastern Wisconsin. The shear zone itself is approximately 2 cm wide, and we analyzed fabric from it and adjacent regions, resulting in a 12 cm transect. Spatial distribution data allows us to quantify the degree of fabric formation, in this case determined by connectivity of mafic minerals, and relate it to the approximate shear strain determined from fabric deflection. This type of analysis can only be achieved if one uses such a method in which the spatial relationships between minerals are maintained.