BS in Materials Engineering
Materials Engineering Department
Scott Johnston, Jean Lee
This study analyzes crystal preferred orientation (CPO) patterns within constrictional quartz-rich gneisses. Quartz deformation and CPO patterns are an area of interest because quartz is one of the most prevalent minerals in earth's crust. Understanding the typical way that quartz crystals reorient under different states of strain can provide geologists with an additional tool for understanding paleo-strain. Temperature and strain geometry are two dominant factors that control the orientation of quartz crystals within a rock. Temperature determines which slip planes are active and slip planes typically reorient so that they are perpendicular to maximum stress. Previous workers have suggested that the respective orientations and degrees to which quartz c- and a-axes cluster to form girdles or points can be used to determine if a rock deformed in constrictional, flattening, or plane strain states (Barth et al. 2010). This study serves to test this hypothesis. Two samples with high and low temperature quartz fabrics were obtained for this study. Both samples showed clear constrictional geometries in the field. Further measurement of the quartz grains verified that the ratio of S1/S2 and S2/S3 for both samples yielded points on the Flinn diagram within the constrictional field. Electron backscatter diffraction maps of the quartz crystal preferred orientations were inconsistent with the predictions for constrictional fabrics. The results of the Waterman Hills sample are similar to the pattern for prism slip and rhomb slip with a component of non-coaxial shear, and suggest that quartz CPOs are sensitive to very small differences in the orientation of principal strain. The Ontario, Canada sample was higher temperature and had few individual measured grains. This sample appeared to have a mix of prism and prism slip. Our results suggest that the degree to which quartz axes cluster to form girdles or points is not a reliable tool for interpreting strain geometry for constrictional fabrics.