A 100 m{sup 2} muon detector (MD) was successfully constructed under the existing Tibet air shower (AS) array in the late fall of 2007. The sensitivity of the Tibet AS array to cosmic gamma rays can ...be improved by selecting muon-poor events with the MD. Our MC simulation of the MD response reasonably agrees with the experimental data in terms of the charge distribution for one-muon events and the background rejection power. Using the data collected by the Tibet AS array and the 100 m{sup 2} MD taken from 2008 March to 2010 February, we search for continuous gamma-ray emission from the Crab Nebula above ∼100 TeV. No significant excess is found, and the most stringent upper limit is obtained above 140 TeV.
In this study, multi-system thermochronology, i.e., fission-track (FT), K-Ar and U-Pb methods are used to identify the cooling and denudation history of the Tsuruga body of Kojaku granite, southwest ...Japan. Apatite FT age of 51.8 ± 6.5 Ma, zircon FT age of 70.4 ± 2.0 Ma, biotite K-Ar ages of 66.7–62.0 Ma, and zircon U-Pb age of 68.5 ± 0.7 Ma were obtained for granitic samples, whereas plagioclase K-Ar ages of 19.1–18.8 Ma and whole-rock K-Ar age of 19.0 ± 2.9 Ma were inferred for the basaltic dyke intruding into the granite. The zircon FT lengths are not significantly shorter than their initial lengths, implying rapid cooling at the zircon FT partial annealing zone (PAZ). On the other hand, the apatite FT length distribution shows a typical pattern for granitic pluton without reheating, indicating a slow cooling history at the apatite FT PAZ. Based on the results of these thermochronometric analyses, inverse thermal calculations using the FT data, and simple thermal conduction modeling of the granitic body, the cooling and denudation histories of the Tsuruga body are reconstructed: (1) the Tsuruga body intruded at ca. 68 Ma, late Cretaceous, at a depth of several kilometers, (2) rapidly cooled to below the zircon FT PAZ by heat conduction within a few million years or less, and (3) slowly cooled due to peneplanation during the past 50–60 million years. On the other hand, the whole-rock Rb-Sr age previously reported for the Kojaku body is younger than when the cooling curve of the Tsuruga body obtained by this study intersects with the closure temperature of the whole-rock Rb-Sr system. This may imply a time lag between the formation ages of these bodies, but more thermochronometric studies are required to draw a definitive conclusion. The K-Ar ages of the basaltic dyke are interpreted as its formation age, indicating that dyke intrusion was associated with the Green Tuff movement.
We measured the uranium-lead (U-Pb) and fission-track (FT) ages of single zircon grains from three samples (TKB3, TKB6, and TKB7) of tephras that are intercalated with Neogene to Quaternary deposits ...in the Horonobe area, Hokkaido, Japan, to understand their eruption ages. The weighted mean U-Pb ages and pooled FT ages of the youngest zircon grains are 1.43±0.18 Ma (2σ) and 1.14±0.40 Ma (1σ) for TKB3, and 7.55±0.14 Ma (2σ) and 6.62±0.43 Ma (1σ) for TKB6, respectively. Their U-Pb and FT ages overlap within their respective 2σ uncertainties, which suggests that their ages correspond to the eruption ages. Specifically, the TKB6 tephra age is consistent with the diatom fossil ages from the overlying and underlying silt deposits. The FT ages for most of the zircons in TKB7 are reset to their U-Pb ages or younger. The pooled FT age for all of the analyzed zircons (1.52±0.19 Ma; 1σ) and weighted mean U-Pb age for zircon grains within the youngest age range (1.34±0.16 Ma; 2σ) are coincident within their respective 2σ uncertainties. Therefore, their ages could also show the eruption age. TKB3 and TKB7 are from the Quaternary Sarabetsu Formation, and their U-Pb and FT ages correspond to the stratigraphy. Although TKB6 lies at the boundary between the Yuchi and Koetoi formations, its U-Pb and FT ages are significantly older than the reported depositional age of the Yuchi Formation. The Neogene to Quaternary depositional age of this horizon tends to be older from west to east in the Horonobe area. The older age of TKB6, which is located in the easternmost part of the study area, is consistent with this stratigraphic trend.
ABSTRACT A 100 m2 muon detector (MD) was successfully constructed under the existing Tibet air shower (AS) array in the late fall of 2007. The sensitivity of the Tibet AS array to cosmic gamma rays ...can be improved by selecting muon-poor events with the MD. Our MC simulation of the MD response reasonably agrees with the experimental data in terms of the charge distribution for one-muon events and the background rejection power. Using the data collected by the Tibet AS array and the 100 m2 MD taken from 2008 March to 2010 February, we search for continuous gamma-ray emission from the Crab Nebula above ∼100 TeV. No significant excess is found, and the most stringent upper limit is obtained above 140 TeV.
A 6000-year history of relative sea-level change on the western Nobi plain, at the footwall side of the Yoro fault, was reconstructed from three drilling cores based on analyses of sedimentary ...facies, grain-size distributions and electrical conductivity (EC), as well as 20 14C age determination. The Yoro fault comprises the northern part of the Yoro fault system. These cores show a prograding delta sequence. Two intervals characterized by fine deposits, mainly composed of fine sand, and high EC value are identified in sandy delta front deposits showing a general coarsening upward sequence. In synchronous with these changes in the delta front, a rapid rise of EC value is recognized in the abandoned channel fill deposits on the delta plain. These data suggest that temporal sea-level rise events occurred during the progradation process of the delta system. A probable cause of these events is coseismic subsidence. These subsidence events are dated at around 5600-4700 cal BP and 4600-3800 cal BP, respectively. The above-mentioned subsidence events broadly correspond with previously known faulting events at the Kuwana fault to the south of the Yoro fault. Evidence for the faulting of the Yoro and Kuwana faults associated with two historical earthquakes (AD 745 and AD 1586) accompanied by coseismic subsidence and synchronicity of the subsidence events identified in this study and faulting events at the Kuwana faults suggest that subsidence events identified in this study are caused by faulting at the Yoro fault. These results are consistent with the notion that the Yoro and Kuwana faults comprise a behavioral segment in the Yoro fault system.
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