Comparative study of cataclastic rocks from a drill core and outcrops of the Nojima Fault zone on Awaji Island, Japan

Cataclastic rocks found in the Disaster Prevention Research Institute, Kyot o University (DPRI) 500 m drill core and outcrops along the Nojima Fault zo ne on Awaji Island, southwest Japan, were examined at mesoscopic and micros copic scales. The damaged zone of this fault in granitic rocks, observed on the southeast side of the fault, is 50-60 m wide and is composed of fractu red host rocks and cataclastic rocks including cataclasite, fault breccia, and fault gouge. The fault breccia and gouge of small scales are scattered in the damaged zone. Fault core (zone of extremely concentrated shearing de formation along a fault) consists of fault gouge measuring several tens to approximately 150 mm in width, as recognized both in the drill core and at outcrops of the Nojima Fault along which surface ruptures formed during the 1995 Kobe earthquake. Fault breccia, measuring a few meters wide, has deve loped pervasively in the damaged zone, just next to the fault core. Pseudot achylyte has been found interlayered with fault gouge within the fault core only at outcrops at Hirabarashi, not in the DPRI 500 m core. Petrological studies and powder X-ray diffraction analysis show that the pseudotachylyte and fault gouge are composed mainly of fine-grained angular clasts of the host granitic rocks, suggesting the pseudotachylyte is of 'crush origin'. F oliated cataclasite is characterized by the preferred orientation of elonga ted biotite clasts and granular aggregates of quartz and feldspar clasts, a nd by the development of cataclastic shear bands. Unlike cataclastically de formed quartz and feldspar in the cataclasite, biotite in the foliated cata clasite shows combinations of brittle and plastic deformation, such as biot ite 'fish', cleavage steps, bending and kinking. These textures suggest tha t the foliated cataclasite formed at a deeper level than the cataclasite, f ault breccia and gouge, possibly before the Quaternary period during which the Nojima Fault has moved as a dextral strike-slip fault with some reverse movement resulting in the uplifting of Awaji Island. Examination of fault rocks from surface outcrops can yield similar results to those obtained fro m drill cores with regard to the internal structures of a fault zone.