K-Ar age and geochemistry of the SW Japan Paleogene cauldron cluster: Implications for Eocene-Oligocene thermo-tectonic reactivation

► The Sanin cauldrons formed during Middle Eocene to Early Oligocene time (43-30 Ma). ► Geochemical data show trends of depletion from the Late Cretaceous to the Paleogene. ► Isotopic shift is explained as mobilization of lithospheric mantle by asthenosphere. ► Rifting may have been triggered by upwelling of hot asthenosphere. ► Upwelling of asthenosphere was caused by rollback of subducted Pacific slab.