Arata Sugimura

Quantitative distribution of late Cenozoic volcanic materials in Japan

The late Cenozoic orogeny in Japan is briefly reviewed. Amounts of volcanic materials in the three periods of the orogeny are estimated at: early Neogene 150 × 103 km3 (mafic 40 %, salic 60 %), middle and late Neogene 20 × 103 km3 (mafic 70 %, salic 30 %), Quaternary 5 × 103 km3 (mafic 80 %, salic 20 %). The largest volume per unit time is in the early Neogene, and the smallest in the middle and late Neogene. Volume per unit area becomes larger towards the southeastern margin or «front» of the volcanic belt. Thermal energy transported by volcanic materials is compared with the terrestrial heat flow in the belt.

Late Cenozoic orogeny in Japan

Abstract Late Cenozoic tectonic maps are serially given to show the amount of vertical displacement since Miocene, trends of fold axes and degree of de-formation of sedimentary rocks in Japan. They illustrate some differences between the east Japan and west Japan arc systems. Lines of evidence show that the former is more active. In the east Japan arc system a series of orogenic events started at the beginning of the Miocene. The trend of the Late Cenozoic orogenic belt is discordant with those of Mesozoic orogenic belts. The younger orogeny seems to be genetically independent of the older ones, although the older structures have given some effects on the younger ones in central Hokkaido and in south Fossa Magna. The east Japan arc system consists of the inner volcanic, outer geanticlinal and outermost subsiding trench belts. The inner and trench belts are regarded as forming a pair of high-temperature and low-temperature tectogenic belts. This pair may correspond to that of the Mesozoic orogenic belts as revealed by the study of metamorphic facies around the Pacific.

Atera Fault and Its Displacement Vectors

The northwest-southeast-trending Atera fault, which is expressed geomorphologically and geologically, cuts many terraces of the Kiso River in central Japan. The authors measured the vertical and horizontal displacement of the fault from the offset of the terrace surfaces and faces (scarps). Seven displacement vectors were calculated in the fault plane. They show that (1) the horizontal displacement is about five times larger than the vertical; (2) the faulting, to date, has been consistently left lateral; (3) the rate of faulting seems to have been almost consistent (about 2–4 m/1000 yrs.). It is noted that the Neo Valley fault is subparallel to the Atera fault, and that its displacement is also left lateral. Data on recent strike-slip fault displacements in central Japan, in part from this specific study, show a regular pattern of geographic distribution of strains.

A possible anisotropy of the upper mantle accounting for deep earthquake faulting

Abstract A preferred orientation of olivine crystals in the upper mantle is postulated to account for the observed radiation patterns of deep earthquakes. It is assumed that there is a flow toward the continent in the oceanic upper mantle and underneath the island arc, this current is going down along a slant seismic plane. Such a current would produce a shearing stress in the same direction as that in the case of a reverse faulting along the slant plane, in which the continent thrusts over the ocean floor. In contrast to this anticipation, however, actual radiation patterns of the deep and intermediate earthquakes of the Izu-Mariana Island Arc region indicate that almost always the focal fault planes meet the slant seismic plane at about 45°. This discrepancy in the direction of fault planes may be accounted for by the postulated preferred orientation of olivine crystals. It is suggested that the preferred orientation may be the result of the regional compressive stress associated with the upper mantle current.

Chemistry of volcanic rocks and seismicity of the earth’s Mantle in the Island arcs

Volcanic belts in island ares are associated in space with epicenter belts of mantle earthquakes. Primary magma of volcanoes on the oceanic side of an island are is rich in silica and poor in alkalies. In contrast, it is poor in silica and rich in alkalies on the continental side. Foci of the mantle carthquakes are close to a plane that dips away from the oceanic side toward the continental side. The spatial correlation between nature of primary magmas and depth of earthquake foci suggests a common origin.Moreover, the hypothesis that the generation of magma and the occurrence of mantle earthquakes are genetically related and that the primary magmas in the island ares are generated at levels about 150 to 250 km deep. is supported by the relationship among dillerent island ares, that is, primary magma of volcanoes fronting on oceanic trenches becomes more silicious from one are to another as the seismicity of mantle earthquake zones becomes greatre.

Radiocarbon dates of elevated shorelines, Indonesia and Malaysia. Part 1

Abstract Four new radiocarbon dates of elevated strandlines in tectonically active areas of eastern Indonesia and East Malaysia indicate average rates of uplift that range between 4.5 and 9 mm annually during the past 24,000 yr. These values are at least three times higher than former estimates from eastern Indonesia. Another radiocarbon date from the south arm of Sulawesi—also tectonically mobile—indicates a rate of uplift of 1.4–2.5 mm per year which corresponds with earlier determinations. This particular case, however, suggests that the sample was probably located close to a north-south axis about which southern Sulawesi was tilted during the Quaternary. In the Langkawi islands, West Malaysia, one of the regionally ubiquitous shorelines at about 2 m above sea level was dated at 2590 ± 100 yr BP which corresponds with Fairbridges (1961) Abrolhos Submergence.

Comparison of recent crustal movements with quaternary movements in japan and its plate-tectonic implication

Abstract Recent crustal movements may be classified into two categories: one being associated with major earthquakes and the other being creep deformations without a direct association with any major earthquake. The spatial distribution of the rate of creep deformation during the last 70 years as detected by the precise re-leveling, shows a similar configuration on a map to the distribution of amount of Quaternary vertical movements, throughout the Japanese Islands with the exception of Hokkaido. Comparison of the rate of recent movement with the total amount of Quaternary movement suggests the following two interpretations, which have the advantage of simplicity compared with other possibilities: 1. (1) The rate of the recent movements is four times larger than the average rate for the Quaternary. 2. (2) If the rate of crustal movements is almost uniform as most of creep deformations can be shown to be, then the Quaternary movements must have started about 0.5 million years ago. Nevertheless, the author suggested in 1967, that the active period of Quaternary tectonic movements might have begun about one million years ago. This suggestion was based upon the measurement of the total amount of movement compared with the rate of movement as detected from the deformation of late Quaternary terraces. These tectonic movements, naturally include the crustal movements associated with major earthquakes. It is highly probable that the movements associated with major earthquakes and the creep deformations not directly associated with major earthquakes became active at the same time. If so, a third interpretation could be advanced: 3. (3) The rate of the recent creep deformations is about twice the average, and the crustal movements in the Japanese Islands commenced their active period about one million years ago. On the basis of a bend in the hot-spot trace along the Hawaiian volcanic chain, the Pacific plate seems to have changed the position of its rotation axis and its angular velocity about one million years ago. The agreement of both of these dates with the increasing rates of activity suggests that the Quaternary tectonic movement in Japan was activated by the change in the pole of rotation of the Pacific plate which took place at about one million years ago and in doing so caused the bend in the eastern end of the Hawaiian island chain.