Effects of Variations in Mineralogy and Linear Mica Continuity on Ductile Deformation in Metapelitic Rocks

College of Letters & Science, Czeck, Dyanna, Geosciences, Mentor, Poster Presentation, Schneider, J, School and College, Student

J Schneider, “Effects of Variations in Mineralogy and Linear Mica Continuity on Ductile Deformation in Metapelitic Rocks”
Mentor: Dyanna Czeck, Geosciences
Poster #180

The Southern Iberian Shear Zone (SISZ) in Andalucía, Spain is a ductile shear zone—a deep-seated fault which deforms rocks without fracturing, between tectonic plates that obliquely collided approximately 300 Ma. The Pulo do Lobo (PdL) formation contains quartzites and metapelites, rocks deformed and metamorphosed along the SISZ. The goal of this study is to better understand the role that mineral concentrations and arrangements play in strain localization, a phenomenon in which deformation is spatially concentrated. Minerals and their arrangement are studied microscopically using a conventional petrographic microscope and ImageJ, an image analysis software that is used to isolate grains based on their relative colors. The PdL samples are primarily composed of two minerals—quartz and biotite (a type of mica), which display different colors in plane polarized light. By setting a color threshold within ImageJ, particle analysis can be conducted for the entire sample or along a single line; the former provides bulk composition, while the latter provides LMC (Linear Mica Continuity) data. LMC is the percentage of mica intersected along a linear traverse across a sample, which quantifies the way in which micas are clustered. Eight single line (1 pixel width) LMC measurements are calculated on each sample. The maximum LMC value is used to represent the degree of clustering in each sample. LMC is an important deformation parameter, as measured LMCmax values are related to composition and distance from the shear zone. LMCmax increases with higher mica concentrations and greater distance from the SISZ. Decreases in LMCmax are potentially related to increases in strain accumulation; low LMCmax implies greater strength in the PdL. We hypothesize that the effects of prior rock fabrics and metamorphic changes may result in a nonintuitive relationship between mica connectivity and strain accumulation.