Extensive normal faulting during exhumation revealed by the spatial variation of phengite K – A r ages in the S ambagawa metamorphic rocks, central S hikoku, SW J apan

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 – A r ages of pelitic schist of the S ambagawa metamorphic rocks (sensu lato) from the S aruta R iver area, central S hikoku, 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 – A r ages were perhaps caused by truncation of the metamorphic layers by large‐scale normal faulting at D 2 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 D 2 phase, the change of 5 my in phengite K – A r 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 A r ages ( ca 85 to 78 Ma) in the actinolite rock could be reasonably comparable to the phengite K – A r ages of the surrounding non‐faulted pelitic schist, because the K – A r 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).