The Cretaceous Shiratakiyama cauldron in northwest Yamaguchi Prefecture, Japan: an example of asymmetric subsidence

DOI Web Site Web Site Web Site 4 Citations 20 References Open Access

Bibliographic Information

Other Title
  • 山口県北西部の白亜紀白滝山コールドロン：非対称陥没の例
  • ヤマグチケン ホクセイブ ノ ハクアキ シラタキヤマ コールドロン : ヒタイショウ カンボツ ノ レイ

Search this article

Description

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.<br>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.<br>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 km3, 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.<br>These findings suggest that the deeper structure of the Shiratakiyama cauldron was formed by asymmetric piecemeal collapse rather than by coherent trapdoor subsidence.

Journal

Citations (4)*help

See more

References(20)*help

See more

Keywords

Details 詳細情報について

Report a problem

Back to top