ANALYSIS OF GRAIN-SCALE DEFORMATION USING A THIN SECTION GIS

Quartz-rich rocks deformed at low temperatures are characterized by a heterogeneous distribution of microstructures that reflects the sequential or simultaneous operation of several disparate grain-scale deformation mechanisms. At small scales, extrinsic factors such as pressure and temperature are not expected to vary; therefore, intrinsic factors such as grainsize or cement may be responsible for the heterogeneity. To investigate this possibility, a thin section GIS was built from a quartz arenite deformed at <250°C to first characterize the spatial variability of microstructures and then look at relationships between them and different intrinsic parameters.

An automated method was developed to digitize grain boundaries from a series of digital photomicrographs. Once the grain boundary base map for the GIS was created, the associated database was populated with the type of microstructures and their orientations as well as intrinsic factors such as grain shape, grain size, and cement. Using scripts written for ArcView, statistical tests were done to characterize the spatial distribution of microstructures and intrinsic parameters and to look for relationships between them. Scripts were also developed to quantify various fabric descriptors including pressure solution and finite strain.

Chi-squared tests showed the spatial distribution of most microstructures to be non-random, non-uniform, and non-clustered. Exceptions were microveins and pressure solution surfaces which have a clustering distribution. Semivariograms for all microstructures have similar forms and small ranges indicating that the occurrence of all microstructures is controlled by the same factors and that microstructures become independent of one another after a distance of only 2-3 grain diameters. For each microstructure except microveins, directional semivariograms have similar ranges for all azimuths showing that the probability of occurrence is isotropic. Correlations between the occurrence of microstructures and intrinsic variables indicates that grain size was an important factor in controlling the distribution of microstructures.