Zircon Fission-track Thermochronology of the Nojima Fault Zone, Japan
Fission-track (FT) thermochronologic analysis was performed on zircon separates from rocks in and around the Nojima fault, which was activated during the 1995 Kobe earthquake. Samples were collected from the University Group 500-m (1600-ft) (UG 500) borehole, Geological Survey of Japan 750-m (2500-ft) (GSJ 750) borehole, the fault trench at Hirabayashi, and nearby outcrops. Zircon FT ages from the UG 500 borehole record about 2-Ma cooling age in the zircon partial annealing zone (ZPAZ) for samples within about 25-m (80-ft) distance from the fault plane, whereas those of the GSJ 750 borehole record about 30-40-Ma cooling ages in the same fault-zone width. On the basis of one-dimensional heat conduction modeling as well as the consistency between the degree of FT annealing and the degree of deformation and alteration of borehole rocks, these cooling ages in both boreholes are interpreted as consequences of ancient thermal overprints by heat transfer or dispersion via fluids in the fault zone. For the fault trench samples, zircon FTs of the 2 -10-mm (0.08-0.4-in.)-thick pseudotachylyte layer were totally reset (or remained reset) and subsequently cooled at about 56 Ma, which is interpreted as the time of final cooling through ZPAZ immediately after the pseudotachylyte formation. It is therefore suggested that the present Nojima fault system was reactivated in the middle Quaternary from an ancient fault initiated at about 56 Ma at midcrustal depths.
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Japan National Oil Corporation (JNOC) (presently Japan Oil, Gas and Metals National Corporation) launched a multidisciplinary and international project on the Evaluation of Traps and Seals in 1997. The project ended in 2003. This publication resulted from that project and includes JNOC research articles as well as contributions from industry and academia. The 17 papers in this volume cover topics such as a method to estimate the amount of oil/gas accumulation using the concept of equivalent grain size in seal rock, and oil/gas migration to and spill-point geometry of petroleum traps; two case studies of fault seal assessment applied to normal faults in Tertiary clastic reservoirs in offshore Sarawak and offshore Gulf of Thailand; and physical analog studies of the development of extensional faults. This publication also contains a valuable bibliography of nearly 1000 additional articles and books published on fault traps, fault seal processes, and fault-related fluid flow in sedimentary basins, for use as a reference tool to delve into publications preceding this volume.