Some Tectonic Aspects of Quaternary Volcanoes in Japan

Abstract

Part 1 of this paper is concerned with various types of deformation of volcanic cones as listed in Table 1. In regard to the crustal displacements before and after eruptions, the local upheaval and fracturing near crater remain as a permanent deformed topography, but the areal upheaval and subsidence before and after eruption are believed to be a reiterative movement caused by increase and decrease of the pressure in magma reservoir. Most of collapse calderas in Japan belong to Krakatoan type and some to Kilauean type, but none of resurgent cauldrons is known. These deformations of volcanic origin are generally circular in their horizontal shape and display larger magnitude and rate in vertical displacement than those of non-magmatic gravitational and tectonic origin.Non-magmatic, superficial, gravitational deformation of volcanic cones such as the settlement of cones and landslides of large scale are emphasized, because of their important role to deform volcanic cones on the weak foundations such as Cenozoic altered volcanics. It is also known that non-magmatic tectonic movements such as the active faulting connected with earthquakes, the active folding of the basement etc., deform volcanic cones.In part 2, firstly, horizontal arrangement of volcanoes is separated into three types of different extent. Two volcanic zones, East Japan and West Japan, are recognized. The two display remarkable cross-sectional asymmetry of both volume of volcanic products and nature of magma from oceanic side to continental side. This asymmetry is believed to be due primarily to the continental-ward inclination of the mantle earthquake zone where parental magma generates. Volcanic center alignments are in parallel with mountain ranges of about 50km in width and are related to the pre-existing faults which were formed during the mountain building. Volcanic vent alignments run in perpendicular or diagonal direction to the volcanic center alignments and are inferred to be related to tension cracks parallel to the late Quaternary local principal horizontal stress direction.Secondly, the vertical distribution of volcanoes of different ages in relation to the configurations of their basements is discussed (Figs. 1, 2, 3, 4). Older volcanoes which were formed during middle and early Quaternary usually rest on the crest of the rised or rising mountain ranges. On the contrary, younger volcanoes which are active during late Quaternary tend to occur on the relatively lower places than older volcanoes.The last chapter is devoted to deal with relationship between the distribution of Quaternary volcanoes of different ages and neotectonics in Japan. Fig. 5 shows a possible working hypothesis on the relationship.