The Shiratakiyama Formation of the Abu Group, southwest Japan, is part of a dissected caldera within a complex of Cretaceous volcanic and plutonic rocks. The formation contains the products of rhyolitic and andesitic magmas emplaced in a back-arc region. It is important to understand the genetic relationship between the volcaniclastic ejecta and structural constraints on these rocks in order to determine the evolution of caldera volcanism.
The orientations of bedding planes within the Shiratakiyama Formation suggest the occurrence of a buried asymmetric structure within basement rocks. The depth of the basement surface increases toward the center of the caldera in the northern part of the Shiratakiyama Formation, dipping at 40° to 70°, whereas in the southern half of the caldera the surface dips at 20° or less. This asymmetric basement surface is also discordant with the orientation of basement rocks themselves. In addition, the formation is bound by intersecting high-angle normal faults and/or intrusive rocks. These observations suggest the presence of a small (6×4 km) cauldron, here named the Shiratakiyama cauldron.
The Shiratakiyama Formation is divided into two members, here named the Futanoigawa rhyolite ash-flow tuff and the overlying Tenjougatake andesite lava. The formation also contains many associated intrusive rocks, such as porphyrites, felsites, granite porphyry, and intrusive breccias. Thick and voluminous ash-flow tuff is the dominant rock within the cauldron interior. The total volume of ash-flow tuff is ≥ 9.6 km
3, and it is locally intercalated with lacustrine rocks, andesite lavas, and volcaniclastic rocks, which represent cooling units. Caldera-collapse meso-breccias occur in the lower part of the ash-flow tuff sequence.
These findings suggest that the deeper structure of the Shiratakiyama cauldron was formed by asymmetric piecemeal collapse rather than by coherent trapdoor subsidence.
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