Timing of Fracture Formation and Duration of Mineralization at the Hishikari Deposit, Southern Kyushu, Japan     

Ryota Sekine    Eiji Izawa  Koichiro Watanabe 《Resource Geology》2002,52(4):395-404

Abstract. The Hishikari epithermal gold deposit consists of the Honko-Sanjin zone and Yamada zone. On the basis of observed crosscutting relationships of veins, vein system in the Hishikari deposit was classified into the early and late veins. They differ from orientation, width, control by unconformity, the amount of displacement and structural features, but have similarity in gold grade and Au/Ag ratios. K-Ar ages are presented for the classified veins to clarify timing of fracturing and duration of mineralization.
Age determination revealed that the early veins in the Honko-Sanjin zone range from 0.86 to 1.11 Ma and that the late veins range from 0.73 to 0. 84 Ma. The economically most important mineralization in the Hishikari deposit occurred around 0.90 Ma in a very short period (about 0.05 million years).
The distribution of mineralization age in the whole Hishikari deposit, occurrences of slickensides and ground water temperatures imply that mineralization lasted longer in the southern part of the deposit. As compared the early veins with late veins, structural changes including width, strike, dip and vein characteristics occurred at around 0.85 Ma.  相似文献   

Dynamic branching, arresting of rupture and the seismic wave radiation in self-chosen crack path modelling     

Nobuki Kame  Teruo Yamashita 《Geophysical Journal International》2003,155(3):1042-1050

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Mean slope-angle frequency distribution and size frequency distribution of landslide masses in Higashikubiki area, Japan   总被引：1，自引：0，他引：1  

Junko Iwahashi  Shiaki Watanabe  Takahiko Furuya 《Geomorphology》2003,50(4):349-364

Landslide mass size frequency distributions and mean slope-angle frequency distributions were calculated for slump, slide, and creep type landslides in the Higashikubiki area. Mean slope-angle frequency distributions closely approximated Weibull distributions. Size frequency distributions show power-law dependencies. Both can be explained by modeling landslides as linked uniform blocks in tensile force. Power coefficients for size frequency distributions were 2.01–2.32 (approximation to power functions) or 2.10–2.24 (approximation to Pareto distributions).  相似文献   

Oxygen isotope systematics in Diploastrea heliopora: new coral archive of tropical paleoclimate     

Tsuyoshi Watanabe  Michael K. Gagan  Heather Scott-Gagan  Wahyoe S. Hantoro 《Geochimica et cosmochimica acta》2003,67(7):1349-1358

Diploastrea heliopora forms dense, robust, dome-shaped coral colonies throughout the reef ecosystems of the tropical Pacific and Indian Oceans. This slow-growing (2 to 6 mm/yr) coral has the potential to yield continuous paleoclimate records spanning up to 1000 yr for the warmest waters on Earth, the Indo-Pacific Warm Pool, and has a long fossil history as a single recognizable species. Despite the potential of Diploastrea to be an important new paleoclimate archive, little is known about the systematics of geochemical tracers incorporated into its skeleton. To fill this knowledge gap, we compared skeletal δ¹⁸O signatures in live Diploastrea and Porites colonies from Southwest Lagoon, near Amédée Lighthouse, New Caledonia (at the southern latitudinal limit for Diploastrea) and Alor, Indonesia (in the core area of the Indo-Pacific Warm Pool). We designed a new microsampling technique to minimize smoothing and distortion of the isotopic records due to the complex calyx architecture and slow growth of Diploastrea. High-resolution isotope profiles from the septal portion of the Diploastrea corallite are attenuated, whereas those restricted to the central columella are similar in quality to those extracted from the well-established Porites coral archive. The δ¹⁸O-temperature relationship for the columellar portion of Diploastrea (−0.18‰/°C) is in good agreement with that derived for a nearby Porites (−0.19‰/°C; Quinn et al., 1996), on the basis of comparison with an in situ sea surface temperature record from Amédée Lighthouse. There is a measurable difference of 0.3 ± 0.1‰ between the kinetic/biological disequilibrium offsets from seawater δ¹⁸O composition for Diploastrea and Porites. Despite this offset in mean δ¹⁸O, Diploastrea accurately records the regional differences in mean temperature and salinity between New Caledonia and the Indo-Pacific Warm Pool. Additional tests show that Diploastrea records El Niño-Southern Oscillation (ENSO) interannual variability in sea surface temperature and salinity across the southwestern Pacific, indicating that it should yield dependable paleo-ENSO records. Based on these results, we propose that D. heliopora has the potential to provide an important new coral archive of tropical paleoclimate.  相似文献   

Dynamic nucleation process of shallow earthquake faulting in a fault zone     

Nobuki Kame  Teruo Yamashita 《Geophysical Journal International》1997,128(1):204-216

Two distinct phases are commonly observed at the initial part of seismograms of large shallow earthquakes: low-frequency and low-amplitude waves following the onset of a P wave ( P ₁) are interrupted by the arrival of the second impulsive phase P₂ enriched with high-frequency components. This observation suggests that a large shallow earthquake involves two qualitatively different stages of rupture at its nucleation.
We propose a theoretical model that can naturally explain the above nucleation behaviour. The model is 2-D and the deformation is assumed to be anti-plane. A key clement in our model is the assumption of a zone in which numbers of pre-existing cracks are densely distributed; this cracked zone is a model for the fault zone. Dynamic crack growth nucleated in such a zone is intensely affected by the crack interactions, which exert two conflicting effects: one tends to accelerate the crack growth, and the other tends to decelerate it. The accelerating and decelerating effects are generally ascribable to coplanar and non-coplanar crack interactions, respectively. We rigorously treat the multiple interactions among the cracks, using the boundary integral equation method (BIEM), and assume the critical stress fracture criterion for the analysis of spontaneous crack propagation.
Our analysis shows that a dynamic rupture nucleated in the cracked zone begins to grow slowly due to the relative predominance of non-coplanar interactions. This process radiates the P₁ phase. If the crack continues to grow, coalescence with adjacent coplanar cracks occurs after a short time. Then, coplanar interactions suddenly begin to prevail and crack growth is accelerated; the P₂ phase is emitted in this process. It is interpreted that the two distinct phases appear in the process of the transition from non-coplanar to coplanar interaction predominance.  相似文献   

Heat influx and exhumation of the Shimanto accretionary complex: Miocene fission track ages from the Kii Peninsula, southwest Japan     

Noriko  Hasebe  Hiroaki  Watanabe 《Island Arc》2004,13(4):533-543

Abstract   To determine how local geological events contributed to the evolution of accretionary complexes and eventual exposure of rocks with different structural levels, geochronological mapping was carried out using fission track (FT) analysis at the Kii Peninsula, southwest Japan. At this site, the original zonal structure of Cretaceous accretionary complexes parallel to the subduction zone is disturbed by the northward projection of the Shimanto accretionary complex. Twenty-six zircon FT ages were obtained from an area of ∼12 km in an east–west direction and ∼15 km in a north–south direction, and classified into three groups: (i) ages ∼15 Ma (range ∼10–20 Ma), which are distributed along the northwest–southeast valley; (ii) ages of ∼50 Ma in the northwest of the study area; and (iii) ages older than those in Groups 1 and 2. Based on results from eight zircon FT length distributions, the Miocene ages appear to be the result of spatial variations in heat influx and cooling after the regional exhumation of the area, as recorded by FT ages of ∼50 Ma.  相似文献   

Sequence of faulting and deformation during the 2000 eruptions of the Usu Volcano, Hokkaido, Japan—interpretation and image analyses of aerial photographs     

Hiromitsu Yamagishi  Tsukasa Watanabe  Fumiaki Yamazaki 《Geomorphology》2004,57(3-4):353-365

Significant faulting and deformation of the ground surface has been rarely known during volcanic eruptions. Usu Volcano, Hokkaido, Japan, is a unique example of deformation due to felsic magma intrusion. Usu Volcano has a history of such types of eruptions as phreatic, pumice eruption (Plinian type), pyroclastic flowing and lava doming since 1663. On March 31, 2000, phreatomagmatic to phreatic eruptions took place after 23 years of dormancy in the western piedmont, followed by explosions on the western flank of Usu Volcano. They were associated with significant deformation including faulting and uplift. The eruptions and deformation were continuing up to the end of May 2000.We identified the faulting using total nine sets of aerial photographs taken from before the eruption (March 31, 2000) to more than 1 year (April 27, 2001) after the end of the activity, and traced deformation processes through image processing using aerial photographs. We found that some of the new faults and the associated phreatic eruptions were related to old faults formed during the 1977–1981 eruptive episode.The image processing has revealed that the surface deformation is coincident with the area of faulting forming small grabens and the phreatic explosion vents. However, the faulting and main explosive eruptions did not take place in the highest uplift area, but along the margin. This suggests that the faulting and explosive activities were affected by small feeder channels diverging from the main magma body which caused the highest uplift.  相似文献   

Slow mass movement in the Kangchenjunga area, eastern Nepal Himalaya     

Dhananjay  Regmi  Teiji  Watanabe 《Island Arc》2005,14(4):400-409

Abstract   The rates of the accumulated and continuous displacement of solifluction lobes in the Kangchenjunga area, eastern Nepal Himalaya, were determined using glass fiber tubes and a strain probe. Ground temperature, precipitation and soil moisture were monitored at two sites, whose altitude differed by approximately 100 m, to understand the solifluction process. The average movement rate of the glass fiber tubes on a 31° slope at altitudes of 5412–5414 m a.s.l. was approximately 11 mm/year, being almost threefold greater than that observed on a 22° slope at 5322–5325 ma.s.l. There was no significant difference in the depth of displacement at these sites. The continuous displacement measurement near the ground surface at 5414 m showed permanent downslope movement from early July. Such movement may be attributed to additional moisture supply during the monsoon season. The amplitude of the displacement cycle was highest at the ground surface, and decreased to virtually zero at and below 20 cm in depth. Probable factors leading to the relatively slow rates of downslope displacement at the surface and depth at the studied altitudes are the lack of concurrence of the freeze–thaw cycles and the high moisture condition in the soil, and the low moisture retention capacity of the soil because of steep slopes and superficial desiccation. The rate of displacement may be more pronounced at altitudes above 5600 m because of the freeze–thaw cycles during the summer season.  相似文献   

K-Ar Ages of Granitic Magmatism and Related Pegmatite Formation at the Umanotani-Shiroyama Mine, Shimane Prefecture, SW Japan and Their Bearings on Cooling History     

Shouji Kihara    Kenichi Hoshino    Makoto Watanabe    Hirotsugu Nishido  Shunso Ishihara 《Resource Geology》2005,55(2):123-129

Abstract. The Umanotani-Shiroyama pegmatite deposits, the largest producer of K-feldspar and quartz in Japan, are of typical granitic pegmatite. Ilmenite-series biotite granite and granite porphyry, hosting the ore deposits, and biotites separated from these rocks yielded K-Ar ages ranging from 89.0 to 81.4 Ma and 95.2 to 93.7 Ma, respectively. Muscovite and K-feldspar separated from the ore zone yielded K-Ar ages with the range of 96.2 to 93.1 Ma and 87.3 to 80.7 Ma, respectively. Muscovites from quartz-muscovite veins in the ore zone and in the granite porphyry yielded K-Ar ages of 90.4 and 76.3 Ma, respectively. K-feldspar is much younger in age than coexisting muscovite. It is noted that the K-Ar ages of biotite separates and the whole-rock ages are identical to those of muscovite and K-feldspar in the ore zone, respectively. These time relations, as well as field occurrence, indicate that the formation of the pegmatite deposits at the Umanotani-Shiroyama mine is closely related in space and time to a series of granitic magmatism of ilmenite-series nature. Using closure temperatures of the K-Ar system for biotite and K-feldspar (microcline), cooling rate of the pegmatite deposits is estimated to be about 82C/m.y. at the beginning, but slowed down to about 15C/m.y. in the later period.  相似文献   

Sorption behavior of the Pt(II) complex anion on manganese dioxide (δ-MnO2): a model reaction to elucidate the mechanism by which Pt is concentrated into a marine ferromanganese crust     

Mamiko Yamashita Maeno  Hironori Ohashi  Kotaro Yonezu  Akane Miyazaki  Yoshihiro Okaue  Koichiro Watanabe  Tamao Ishida  Makoto Tokunaga  Takushi Yokoyama 《Mineralium Deposita》2016,51(2):211-218

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