A refined classification of 3D shape preferred orientation patterns in fault gouges: Insights from the Yangsan and Ulsan Fault Zones, South Korea

Shape preferred orientation (SPO) analysis of rigid clasts in fault rocks is a fundamental tool for understanding grain-scale deformation process. However, the occurrence of anomalous patterns in 3D-SPO data has challenged traditional interpretations that rely solely on clast rotation systems observed in cross-sections. Here, we present a novel classification system for 3D-SPO patterns that accounts for the critical influence of clast morphology. Through integrated analysis of X-ray diffraction patterns, μ-CT imaging, 3D-SPO analysis, and electron microscopy data for three samples from the Yangsan and Ulsan fault zones (southeastern Korean Peninsula), we demonstrate that clast final shape and fabric under shear are primarily controlled by clast morphology, possibly combined with initial orientation. We classified all samples into four types based on clast morphology and identified six distinct SPO patterns associated with these classifications: top-spinning (type-S) and oblate-rolling (type-OR) for oblate type clasts, rotating (type-R) and prolate-rolling (type-PR) for prolate type clasts, and two distinct rotating patterns (types H and V) for bladed type clasts. Natural fault gouge typically exhibits a composite of these patterns, with the dominant pattern reflecting the predominant clast shape population. This refined understanding of the relationship between clast shape and SPO patterns significantly improves the reliability of SPO analysis and enhances our ability to reconstruct fault deformation process.