Viscous remanent magnetization dating for tsunami boulders

Erratic boulders have been reworked intermittently or continuously in geological processes. The age estimation of boulders is an important task for understanding past geological events. In rock magnetism and paleomagnetism, partially overprinted viscous remanent magnetization (VRM) of boulders has been used as a geochronometer for a variety of rocks. Rocks have an assemblage of ferrimagnetic particles that record the geomagnetic field. As the boulder is moved with rotation, subset particles in an assemblage change the direction parallel to current geomagnetic field direction. The magnetization acquired with the realignment of magnetic particles is VRM. The theoretical framework of VRM in single domain grains is based on Néel's thermal relaxation theory. According to the theory, the relaxation time of VRM depends very strongly on temperature, and the exposure to high temperature causes the shortening of relaxation times. Thus, an experimental combination of short relaxation time and high temperature for removing VRM (unblocking temperature) can determine the unknown relaxation time (the age of geological event) at room temperature. Here, we report the application of this time-temperature relation (Pullaiah's nomogram) to determine the VRM age for twenty-six core samples from four coral boulders reworked by paleo tsunami events in Isigaki Island, Japan. The VRM age of several samples were good agreement with the 14C age of coral boulders although the others showed older age, yielding high unblocking temperatures. To explain this age difference, we improve the loss of precision in input ambient temperature because the time resolution changes logarithmically with respect to temperature.