Dating pseudotachylyte of the Nojima fault using the zircon fission-track method

Abstract

The zircon fission-track analysis of pseudotachylyte answered how old the Nojima fault is, which was activated during the 1995 Kobe earthquake in Japan. The time of seismic fault motions at the crustal interior can be constrained by dating pseudotachylyte formation. We succeeded in separating zircons from the pseudotachylyte layer of the fault as well as in dating them by the fission-track method. Here we show that the layer has a significantly younger age (56 ± 4 Ma) than the initial cooling age of the host rock, suggesting that the fault was already initiated at ∼56 Ma, much older than the previous estimate based on the spatial distribution of sediments around the fault. Copyright 2004 by the American Geophysical Union.