Geology and tectonics of the Ryukyu Islands

The Ryukyu Islands are divided morphologically and geologically into three island groups: north, central and south Ryukyus. North-central Ryukyu represents the geological continuation of the Outer Belt of southwest Japan, composed of Mesozoic-Eocene sedimentary sequences, whereas south Ryukyu is characterized by high-pressure metamorphic rocks, Eocene volcanics and limestone, and lower Miocene sediments. The geological and structural contrasts between north-central and south Ryukyus are conspicuous before the late Miocene transgression covered the whole area. The Ryukyu Islands have been established since then.The successive developments of the Goto-Tunghai-Senkaku basins since the Paleogene, of the Okinawa Trough since the Miocene, and of the grabens near the Island group since the Pliocene, signify a southeastward shift of the basins in relation to the activity in the granite diapir zones accompanied by volcanism.     

Effects of non‐uniform traction and specimen height in the direct shear test on stress and deformation in a rock fracture

In the direct shear test (DST), an internal moment is distributed within the rock specimen by non‐coaxial shear loads applied to the specimen, which cause non‐uniform distributions of both the traction on the loading planes and the stress and deformation in the specimen. To examine the validity of the DST for a rock fracture and to clarify the effect of specimen height, both the stress and deformation in a fracture in the DST were analyzed for specimens with three different heights using a three‐dimensional finite element method with quadratic joint elements for a fracture model. The constitutive law of the fracture considers the dependence of the non‐linear behavior of closure on shear displacement and that of shear stiffness on normal stress and was implemented in simulation code to give a conceptional fracture with uniform mechanical properties to extract only the effect of non‐uniform traction on the stress and deformation in the fracture. The results showed that both normal and shear stresses are concentrated near the end edges of the fracture, and these stress concentrations decrease with a decrease in the specimen height according to the magnitude of the moment produced by the non‐coaxial shear loads. Furthermore, although closure is greater near the end edges of the fracture, where normal stress is concentrated, this concentration of closure is not so significant within the range of this study because of the non‐linear behavior of closure, that is, closure does not significantly increase with an increase in normal stress at large normal stresses. Copyright © 2012 John Wiley & Sons, Ltd.     

Migmatite tectonics of the hidaka metamorphic belt, Hokkaido, Japan

The Hidaka metamorphic belt is situated at the junction of the Honshu and Kuril arcs in the axial zone of Hokkaido in northern Japan. Various migmatites, which occupy the core of the metamorphic belt, are classified as lens, sheet, falling star and dome facies on the basis of composition, scale and form as proposed by Harland (1956). Each facies is produced progressively. Movement is first lateral and then upwards at the sheet facies stage, followed by the development of the diapiric falling star and dome facies. Subsequently, the granitic phase starts to form from the lens facies, again within the migmatite sheets, leading to the emplacement of granitic plutons. The movement of the migmatite and granite bodies is controlled by the tangential stress field, as well as by the buoyancy in the gravitational field.