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Metamorphic rocks experience change in the mode of deformation from ductile flow to brittle failure during their exhumation. We investigated the spatial variation of phengite K- Ar ages of pelitic schist of the Sambagawa metamorphic rocks (sensu lato) from the Saruta River area, central Shikoku, to evaluate if those ages are disturbed by faults or not. As a result, we found that these ages change by ca 5 my across the two boundaries between the lower-garnet and albite-biotite, and the albite-biotite and upper-garnet zones. These spatial changes in phengite K- Ar ages were perhaps caused by truncation of the metamorphic layers by large-scale normal faulting at D2 phase under the brittle-ductile transition conditions ( ca 300°C) during exhumation, because an actinolite rock was formed along a fault near the former boundary. Assuming that the horizontal metamorphic layers and a previously estimated exhumation rate of 1 km/my before the D2 phase, the change of 5 my in phengite K- Ar ages is converted to a displacement of about 10 km along the north-dipping, low-angle normal fault documented in the previous study. Phengite 40 Ar-39 Ar ages ( ca 85 to 78 Ma) in the actinolite rock could be reasonably comparable to the phengite K- Ar ages of the surrounding non-faulted pelitic schist, because the K- Ar ages of pelitic schist could have been also reset at temperatures close to the brittle-ductile transition conditions far below the closure temperature for thermal retention of argon in phengite (about 500-600°C). [ABSTRACT FROM AUTHOR]