東北日本脊梁地域に分布する中新世後期〜鮮新世のカルデラ群について  [in Japanese] Mio-Pliocene calderas in the Backbone Region in northeast Japan  [in Japanese]

A number of latest Miccene to Pliocene (7 to 2 Ma.) calderas are distributed in a narrow terrane of the Backbone Region in Northeast Japan. They were formed under subaerial circumstances where most of the terrane had already emerged from the sea in late Miocene. The stratigraphic successions of the caldera sediments are very characteristic. The basal part consists of fairly thick pumice flow deposits which are partly welded. The eruption of them may have essentially caused for changing the area at and around the eruption center to be subsided into a depression. The depression was mostly filled by lacustrine tuffaceous beds and subordinately by intra-caldera volcaniclastics. Slump-, talus- and sliding deposits are common, especially near the foot of the depression wall. These sediments were covered by lava and/or pumice flow deposits, and were intruded by felsic to intennediate volcanic rocks at the latest stage. The formation of depressions may have been strongly controlled by regional tectonics. Therefore these calderas belong to "volcanotectonic depressron (VTD)". A unique rock alteration, caldera type alteration, appeared in and around these calderas. It is characterized by zeolitization and production of high-quality mordenite. Various kinds of hydrothermal alterations were locally superimposed upon this alteration. A model for development of the Sanzugawa caldera is presented as a typical example, based on the descriptions on its stratigraphy, geological structure, and rock alteration.