Arcuate pyroclastic conduits, ring faults, and coherent floor at Kumano caldera, southwest Honshu, Japan

At the 41×23 km Miocene Kumano caldera in southwestern Japan, a large arcuate pyroclastic breccia unit is interpreted as the dissected conduit for voluminous explosive eruptions. The pyroclastic breccia and associated granite porphyry occur along the southern margin of the caldera. These rocks, collectively as much as 22 km long and 800 m wide, intrude the arcuate fault bounding the southern caldera margin. The breccia is interpreted as the vent facies of ash-flow tuff adjacent to the caldera, based on similar lithologies, phenocryst modes of juvenile clasts, and matrix character. Cataclasites along the major faults include both country rocks and pyroclastic breccia, yet some cataclasite blocks are contained within the pyroclastic breccia. These geometric and textural relations suggest that the faulting took place during caldera-forming eruption. Prevolcanic sedimentary rocks enclosed by the arcuate faults are interpreted as the dissected coherent floor of the caldera, which were subsided several hundreds to a thousand meters. No large-scale piecemeal disruption of the caldera floor is evident. Orientations of striations with the cataclasites and altitudes of the base of the tuff suggest the asymmetric trap-door subsidence of this caldera. The overall caldera geometry is the nested trap-door piston-cylinder subsidence, each 21×15 and 25×23 km in diameter, in contrast to previous interpretation of a funnel shape.