Microstructure and origin of faults in siliceous mudstone at the Horonobe Underground Research Laboratory site, Japan

Fault-related microstructures and fault orientations in siliceous mudstone were studied at the Horonobe Underground Research Laboratory site in Japan. Thin dark bands define a weak foliation that exhibits compactional cataclastic fabrics in thin section. These fabrics were observed both along the faults and beyond the fault tips, in outcrop and in drill core. Most of the bands are so thin that it was not possible to measure their thickness in thin section. However, scanning electron microscope images of fault surfaces revealed evidence of chemical compaction within the bands. Millimeter-scale displacements occur along similar dark bands in diatomaceous mudstone, which overlies the siliceous mudstone. In all boreholes, the dominant orientation of unfolded faults is WNW–ESE strike and a near-vertical dip. These observations suggest that the faults formed along compactional shear bands that in turn nucleated under ductile or brittle–ductile conditions. The development of compactional shear bands preceded fault formation, occurring just prior to folding, in response to E–W compression related to the eastward migration of the Amurian plate.