Data used in fault dating at three localities in the northern Shimanto belt of Japan: the Goshikinohama Fault, the roof thrust of the Okitsu melange, and the Nobeoka Thrust. Samples were separated into different size fractions, percentage of illite polytypes were determined by modeling XRD patterns, and K-Ar dates were obtained for each size fraction as a function of illite polytype percentages. Geographic coordinates for sample localities are included