The detectability of hypothetical "preslip" of the great Tonanaki and Nankai earthquakes is investigated using the integrated groundwater observatories currently being established by the Geological Survey of Japan, AIST. The detectability is calculated in terms of three characteristic parameters of the preslip, i.e., time constant (slip duration), spatial extent, and direction of the preslip. If it is assumed that the crustal deformation caused by the hypothetical preslip follows the elastic dislocation theory, and the noise level of the line strain observation of the network is 2×10^-8, it is expected to detect preslip with a moment magnitude of greater than 6.5 in the wide area of the network when the characteristic time constant (duration of preslip) and the spatial extent of preslip are about 1 day and 12 km×10 km, respectively. The detectability gets worse when either of the quantities is larger.

View full abstract

    This paper reports geochemical characteristics of granitic rocks and enrichment of rare earth elements (REE) in their weathered crusts in central and southern Laos in order to assess a REE resource potential of ion-adsorption type mineralization. The granitic rocks in the studied area consist mainly of biotite ± hornblende granodiorite and biotite granite with low to moderate total REE contents with the range of 36 - 339 ppm. The granitic rocks are enriched in light REE (LREE) with the depletion of heavy REE (HREE) compared with HREE-enriched granites in southern China and southwest Japan.This difference in the enrichment of HREE is probably derived from poor differentiation of magma for granite. Weathered crusts of the granitic rocks are generally well developed and rich in kaolin and illite.The weathered crusts can be divided into the A, B and C horizons from top to bottom and the B horizon is enriched in REE relative to the parent rocks whereas the A and C horizons are generally depleted or less enriched in REE. Weathered crusts with relatively high REE contents are locally identifi ed in the Attapu and Xaisomboun districts. Geochemical data and results of sequential leaching suggest that the enrichment of REE is attributed to the occurrence of ion-exchangeable clay minerals and REE phosphates in the weathered crusts and that HREE are selectively adsorbed on the clay minerals than LREE by weathering.

View full abstract

    Pollen data from Pleistocene sediments in the Fukasaku A-1 drill core, obtained from Saitama City, Japan, provide a basis on which a pollen biostratigraphy for subsurface geology of the central Kanto plain is constructed. In this study, 131 sediment samples were collected at ca. 2 m intervals from the Fukasaku A-1 core, which consists mainly of marine and fluvial sediments intercalated with two major Pleistocene marker tephra. The pollen assemblages from the Fukasaku A-1 core distinguish 28 local pollen assemblage zones in informal nomenclature by a distinctive assemblage of taxa, indicating local environmental conditions as a rudimentary biostratigraphic classifi cation. An abundance horizon of Cyclobalanopsis pollen and the uppermost documented occurrence of Quercus pollen are useful marker horizons for Middle Pleistocene pollen biostratigraphic correlation in not only central and south Kanto but also in other sedimentary basins. On the other hand, the Fs-Pol-22 zone in the Fukasaku A-1 core is currently a barren interval in informal nomenclature, because of the signifi cantly low occurrence of tree and shrub pollen. Therefore, the pollen biostratigraphic character of this zone remains unresolved.

View full abstract

    Tin skarn ores from the Pin Yok mine, southern Thailand, were studied for Sn and REEbearing minerals, because these elements are generally rich in "tin granites" and are supposed to be enriched in the surrounding rocks, especially of skarn-type ore deposits. The related granite of possibly Triassic age contains no magnetite (i.e., ilmenite-series granites) but biotite and some secondary muscovite, and is high in silica (75.0 % SiO₂) and potassium (5.2% K₂O) , with moderate contents of Sn (12 ppm) and REE+Y (232 ppm). Accessory REE+Y-bearing minerals observed under the microscope, are allanite, monazite, zircon and apatite in this granite.
    The Sn skarn ores are composed of clinopyroxene (40~90 vol.%), grossular garnet (2~20 vol.%), stilpnomelane (2~20 vol.%), ilvaite (~5 vol.%), calcite (~10 vol.%), and quartz (~10 vol.%). Arsenic is locally abundant up to 4,490 ppm As, as possibly arsenopyrite. Tin contents of the skarn ores go up to 7,070 ppm Sn. Cassiterite but no malayaite, in very fi ne grain size, has been identifi ed by microscopic observation and qualitative analyses by EPMA. REE+Y contents are high in two samples of 327 ppm (77110903-6) and 260 ppm (77110309-5). Yet, no REE and Y-bearing minerals have been identified, except for some grains of allanite. Other skarn ores were found to be very low in REE+Y contents.

View full abstract