A Late Cretaceous-Paleogene cauldron cluster: the Nôhi Rhyolite,central Japan

The Nôhi Rhyolite now occupies an area of about 5000 km² in central Japan. It is the largest cluster of volcanic piles of Late Cretaceous to Paleogene age on the Inner Side of SW Japan. It unconformably overlies Paleozoic to Jurassic rocks and is intruded by Paleogene granitic intrusions in its southern and northern parts. The Nôhi Rhyolite consists chiefly of rhyolitic to rhyodacitic welded-tuff sheets with subordinate amounts of volcaniclastic sedimentary rocks. Each welded-tuff sheet is usually 200 to 700 m thick, extends laterally 20 to 60 km, and is composed of nearly homogeneous densely welded tuff. Six volcanic sequences (I–VI) are recognized in the southern and central parts of the Nôhi Rhyolite. Each sequence generally comprises successive accumulations of welded-tuff sheets and volcaniclastic sedimentary layers underlying the sheets. Sequence I, III and V volcanism was derived from zoned magma bodies and was accompanied by granodiorite porphyries. Sequence II, IV and probably VI volcanism was derived from homogeneous magma bodies and was not accompanied by granodiorite porphyries. The estimated total volume is of the order of 300–700 km³ for Sequences I, II and IV, 1600 km³ for Sequence III, and 50 km³ or less for Sequences V and VI. The volume of Sequence III is one of the largest volumes of ashflow tuffs so far recognized. One or more polygonal cauldrons, each 15–40 km across and 400–1000 m deep, formed during each of Sequence I through IV. These cauldrons were generally formed by two successive depressions. The first depression in each sequence is of the Motojuku-type, characterized by collapse prior to the principal eruption. The second depression was formed during the principal eruption. The cauldron of Sequence V has an unusual shape and was probably formed during or after the eruption. The development of a cauldron in Sequence VI is uncertain. The Nôhi Rhyolite is elongate in a NW direction and contains a cluster of seven or eight cauldrons. These cauldrons formed in a tension field trending N and NE, and their volcanism migrated northward. The main regional tectonic framework controlling the cauldrons resulted from uplifting of the basement, which is ascribed to the ascent of a large volume of magma in Late Cretaceous to early Paleogene time.