Isamu Hattori

Rock Fragmentation and Particle Size in Crushed Zones by Faulting

The size distribution of fragments in the crushed zone adjacent to a fault, and that of grains in a zone of very fine‐grained particles (the fault gouge), was determined for two different faults in central Japan. One fault has a wide (ca. 100 m) zone of rock fragmentation between the fault gouge zone and the basement rock formation. Dimensions of the 50 largest rock fragments were measured in the field at locations within a continuous outcrop of this fault. At some places, bulk samples were also collected, and the particle, fragment, and grain size distributions were determined in the laboratory. The relationship between the logarithmic mean of the sizes of the largest 50 fragments and their distances from the fault suggests that size reduction of fragments is due to bulk crushing away from the fault and to surface grinding near the fault. The other fault has no crushed zone of basement rocks but does have a 5‐m‐wide zone of fault gouge. Ten samples of ca. 200 g each and three bulk samples of ca. 2 kg each were obtained from this faults fault gouge, and the logarithmic means of the sizes of the largest 50 fragments in each of these 13 samples were determined. It was found that the dominant process in this fault gouge zone is not fragmentation but, rather, the polishing and grinding of fragments. This analysis indicates that the dominant fragmentation processes at places distant to, intermediate to, and within the fault gouge zone are bulk crushing, surface grinding, and surface polishing, respectively.

From Chalcedonic Chert to Quartz Chert: Diagenesis of Chert Hosted in a Miocene Volcanic-Sedimentary Succession, Central Japan

ABSTRACT Chert beds are intercalated in a Miocene sedimentary succession of the Niu Mountains, central Japan, which was deposited in very shallow water. The provenance of sediments was a volcanic suite from eruptions during back-arc spreading in the Sea of Japan. The cherts accumulated in a small lake underlain by lithic sandstone, and lithification was rapid and before deep burial. Much of the silica appears to be derived from extensive devitrification and weathering of volcanic ash and sediments nearby. X-ray diffraction analyses and microscopic observations reveal that the cherts consist of various types of chalcedony, microcrystalline quartz, and megaquartz of very low crystallinity indices. Transformation of chalcedony to megaquartz was common in the cherts. Scanning electron microscopic analysis shows that a chainlike alignment of micron-size silica spheres forms fibers of chalcedony. Morphological change and coalescence of the chalcedony fibers result in formation of crystalline quartz with numerous crocks and defects. The apparent crystalline quartz in the cherts has inner porous domains and small holes or pipes at the crystal surfaces. Rotation, movement, and rearrangement of the spheres seem to be as important as age effects and solubility of silica minerals.