Mechanism of caldera collapse and resurgence: Observations from the northern part of the Kumano Acidic Rocks, Kii peninsula, southwest Japan

The northern part of the Kumano Acidic Rocks on the Kii peninsula, southwest Japan, formed during Miocene felsic igneous activity that involved caldera-forming eruptions and subsequent intrusion of a massive sheetlike body of granite porphyry into intracaldera deposits. Detailed mapping has revealed that the northern part of the Kumano Acidic Rocks has two separate collapse structures that show the structural margin of a caldera. One is an arcuate fault zone characterized by several normal faults with gentle dips. Pyroclastic deposits characterized by the dominance of crystal-rich welded ash vary in thickness across the fault zone. The ash beds contain numerous boulders on the inner side of the collapse structure, representing caldera-collapse breccias sourced from the fault scarp during the caldera-forming eruption. The other is a 1–3 km wide ring dike of granite porphyry that encloses the prevolcanic sedimentary rocks and the pyroclastic deposits, and is interpreted as a magma conduit along a subsurface circular fracture. The caldera collapse area enclosed by these two structures is elliptical and about 20 × 30 km in size. Our observations suggest a coherent, piston-type collapse for the northern part of the caldera, while asymmetric subsidence along the fault zone resulted in trapdoor or piecemeal subsidence in the southern part. The ring dike is connected to the sheetlike body of granite porphyry. We suggest that this sheetlike intrusion is a laccolith and may have caused resurgent doming of the caldera floor.