Geological and mechanical characteristics of fault breccia in the Philippine Mobile Belt region

Distinguishing between fault breccia and volcanic breccia is critical in tectonically active regions, because their mechanical and hydraulic properties differ substantially and directly affect the stability of large engineering structures. This study aims to identify the origin and characterize the engineering geological properties of breccias at a proposed power plant site within the Philippine Mobile Belt, where active faulting and volcanism exist. To achieve this, an integrated investigation was conducted, including detailed geological mapping, thin-section analysis, electrical resistivity surveys, drilling investigations, and in situ /laboratory tests. The results reveal that breccias are spatially associated with linear geomorphological features and subsurface low-resistivity zones, exhibit jigsaw textures and rotated clasts in drill cores and thin sections, lack diagnostic volcanic textures, and show very low mechanical strength compared with surrounding greenschist host rock. These observations collectively indicate that the breccias are fault-derived and form part of a fault damage zone, rather than being volcanic in origin. From an engineering perspective, clay-rich fault breccia has important implications for slope movement, differential settlement, and groundwater control in large infrastructure projects. The integrated multi-method approach adopted in this study provides a robust framework for distinguishing fault breccia in complex tectonic settings such as the Philippine Mobile Belt.