Subvolcanic structure of the central dike swarm associated with the ring complexes in the Shitara district, central Japan

Abstract

The Subvolcanic structure of the central dike swarm associated with the Miocene Otoge ring complex and the Shitara igneous complex, central Japan, has been reconstructed. The central dike swarm was supplied from several aligned magma reservoirs. Flow lineations observed at the margin of the dikes converge towards a region that is regarded as a magma reservoir about 1–2 km below present sea level. The minimum diameter of the magma reservoir corresponds to the width of the central dike swarm, estimated to be about 3–4 km. The inferred magma reservoir of the Otoge ring complex, may have a zoned structure, as suggested by the flow lineations of dikes and the arrangement of cone sheets. Felsic magma occupied the upper part, about 1–2 km below present sea level, and basic magma the lower part, deeper than 2 km. The centre of the Shitara igneous complex is interpreted to be composed of several other shallow magma reservoirs. The distribution pattern in plan view of the central dike swarm is summarized from the frequency of dikes (defined by the number of dikes per kilometre in the direction normal to the trend of the dike swarm) and the variations of the different properties of individual dikes along the dike swarm. It has a plane of symmetry normal to the dike swarm above the magma reservoir. The patterns critical to a general understanding of the dike formation are: 1. A region of low dike frequency is present above the magma reservoir and a radial dike pattern occurs around the magma reservoir. 2. From both sides of the magma reservoir, the axes of high dike frequency extend symmetrically along the central zone of the dike swarm. 3. The number as well as the individual and total thickness of felsic dikes increases towards the magma reservoir. 4. The number of basic dikes increases towards both sides of the magma reservoir, while the individual thicknesses of basic dikes increase with distance from the magma reservoir.