Quaternary Arc Volcanism and Its Bearing on Tectonics

Abstract

Quaternary arc volcanism in circum-Pacific arcs, especially the Japanese Islands, is reviewed and discussed from a tectonic viewpoint. The summary is as follows; (1) There is no correlation between the chemical and petrological characteristics of volcanic rocks and the depths of deep seismic zone. (2) The geographic position of “volcanic front” defined by SUGIMURA (1960) does not coincide with any contour of the hypocentral depth of deep seismic zone. (3) It is difficult to consider the inclined low velocity layer in the sinking slab as a partially molten part, on the basis of the fact that the upper one of the deep seismic planes coincides with the layer, for example in Northeast Japan. (4) There is a good correlation between the type of crystallization sequence in volcanic-front volcanoes determined by SAKUYAMA (1983) and the age of oceanic plate at trench in the circum-Pacific arcs. This suggests that the degree of partial melting in mantle wedge may be subject to a magnitude of slab pull. (5) There is a good correlation between the tectonic stress field and the variation of strontium isotope ratios for volcanic rocks. It is suggested that the degree of mixing between magma and crustal material may be subject to regional tectonic stress. (6) It is inferred that geographic position of volcanic front, degree of partial melting in mantle wedge, and tectonic stress field in the crust may be essentially subject to a secondary convection induced by a downgoing of slab.