ESR plateau dating of fault gouge

The use of ESR dating to determine the time of last motion on a fault requires that the signals must have been fully zeroed at the time of last motion. Zeroing can be demonstrated by comparing the ages of successively finer grain sizes of quartz; if a plateau of low ages is found for the finest grains, this should represent the age of last motion. Testing this model on various strands of the San Gabriel Fault zone in Southern California, we find low-age plateaus in all samples, and also convergence of ages for two different ESR signals, in most samples. ESR dating of single samples of quartz with larger grains than those of the plateau will result in too large an age of last motion, and may fail to observe convergence between ages given by two or more ESR signals.