ESR dating of fault gouge: The effect of grain size

ESR signals of quartz grains in fault gouge have been partially or completely reset by movements on the fault, which allows the dating of the last fault movement. In order to better understand the zeroing process, we have studied the variation in ESR signal, AD, and age in various grain sizes, since it can be expected that larger grains will be less affected by stress than smaller grains. We propose that a plateau of constant, lower age for the smallest grains indicates total resetting; this is confirmed in gouge from a fault in California. Only such samples should be used for dating.