Quantifying granular material and deformation: Advantages of combining grain size, shape, and mineral phase recognition analysis

Abstract

Granular material from a fault and a clastic dike in granodiorite at the NW contact of the Hornelen basin has been compared by a new digital image analysis tool to extract size and shape characteristics for individual phases. Particle size distributions measured in both samples are consistent with shear fracturing ( D∼3.0–3.2). However, the shape characteristics of the samples are distinct. Granular material from the dike shows no clear shape–size relationship. In contrast, granular material from the fault shows a systematic shape–size relationship (smaller grains being circular and smoother) suggesting a shift in deformation mechanism from intragranular fracturing to abrasion with decreasing grain size. Similarly, field observations, petrography, and the shape and texture of epidote indicate repeated faulting events. Field and textural observations combined with grain size and shape characteristics indicate that the dike sample has a mixed origin. Granulation in fractures connecting to the dike indicates mechanical deformation, while flow structures, texture, grain shape, and high content of epidote in the dike itself suggest that basinal fluids were present. We show that combined size, shape and phase recognition analyses can reveal quantifiable differences in the granular material associated with a fault and a clastic dike, hence allowing us to interpret the distinct origin of these materials.