Tertiary Ishizuchi Cauldron, southwestern Japan Arc: Formation by ring fracture subsidence

Abstract

The Tertiary Ishizuchi Cauldron, in the Setouchi volcanic belt of Miocene age in northwestern Shikoku, is 7–8 km in diameter and includes outer and inner ring fractures, inward dipping welded tuffs, central plutons, and ring fault complexes. Andesitic to dacitic welded tuffs within the central subsided block demonstrate repeated explosive activity during evolution of the cauldron. Inward dips of ring fractures surrounding the cauldron indicate that the bounding faults have the form of an upward opening cone. The cone shape is substantiated by peripheral upturning of intracaldera welded tuffs downdropped against the encircling ring faults. The ring fault complexes consist of rhyolitic to andesitic composite dikes, as well as intrusive tuffs and breccias from which some of the adjacent pyroclastic volcanic deposits were probably erupted. Central plutons of porphyritic granodiorite and fine‐grained adamellite were intruded about 14 Ma ago, have walls dipping outward and flat roofs, and may have formed concave‐upward tabular bodies which pass downward into ring dikes and upward into resurgent domes. Many Valles‐type calderas including Ishizuchi Cauldron are located in the area where the direction of the maximum horizontal compression of regional origin is parallel to arc structure in the middle Miocene southwestern Japan arc.