Apatite fission-track dating of fault-related rocks along the Nojima and Kusumoto faults and its tectonic significance

The tectonic history of the Nojima fault, a similar to10 kmlong active fault whose movement caused the disastrous 1995 Kobe earthquake, was studied by the apatite fission-track (AFT) dating method. AFT ages of 63-83 Ma from a thermally undisturbed uplifted block indicate relatively minor uplift since similar to65 Ma with an estimated total exhumation of similar to3.6 km assuming a geothermal gradient of 25degreesC/km. AFT ages of 44-54 Ma and the nature of the track length distribution from fault-related rocks indicate that fault-related hydrothermal minerals, such as laumontite and chlorite, and foliated cataclasite were formed by ancient fault activity that occurred before similar to44 Ma. It is inferred that the Quaternary fault activity of the Nojima fault is a relatively major phase in the entire tectonic history of the Nojima fault.