Zircon U–Pb ages of granitic and mafic dikes associated with granitoids and metamorphic rocks of the Ryoke Belt in the eastern Kii Peninsula,southwest Japan

Zircon U–Pb dates for felsic and intermediate to mafic dikes intruding into the Ryoke granitoids and metamorphic rocks at selected outcrops in the Takamiyama area of the eastern Kii Peninsula, southwest Japan, were determined along with their geology and petrography to reveal the history of Cretaceous magmatism. At each outcrop, the felsic and intermediate to mafic dikes exhibit specific structures that are indicative of magma intermingling and have coeval intrusion ages of ca. 81–77 Ma. Our zircon U–Pb data complement previously published data, suggesting that the mafic magmatism continued intermittently from 83 to 76 Ma in the Takamiyama area and that magmatism migrated eastward within the Ryoke Belt. A comparison of intrusion ages between a dike and a host Ryoke granitoid at one outcrop indicates that the host rock experienced ductile deformation at ~88 to ~83 Ma. Judging from the small number of zircons and the concordant date distributions, we didn't recognize the evidence suggesting the partial melting of the host rocks, as Nakajima et al. (Journal of the Geological Society of Japan, 2021, 127, 69–78) reported.     

Source models for the 1993 Hokkaido Nansei-Oki earthquake tsunami

A source model for the 1993 Hokkaido Nansei-Oki tsunami must satisfy certain conditions. Such conditions are presented in this paper, and two methods are used to determine the best source model for this event. A trial-and-error method selects DCRC-17a as the best among 24 different models. This model has three fault planes dipping westward. To reproduce well the tide gauge records at two locations, an inversion analysis is used to modify the dislocation of DCRC-17a.     

Evapotranspiration from citrus trees growing in sandy soil under drip irrigation with saline water

An experiment on evapotranspiration from citrus trees under irrigation with saline water was carried out for 4 months. Two lysimeters planted with a citrus tree in the green house were used. One lysimeter was irrigated with saline water (NaCl and CaCl₂ of 2000 mg/L equivalence,EC = 3.8 dS/m, SAR = 5.9) and the other was irrigated with freshwater using drip irrigation. The applied irrigation water was 1.2 times that of the evapotranspiration on the previous day. Evapotranspiration was calculated as the change in lysimeter weight recorded every 30 minutes. The lysimeters were filled with soil with 95.8% sand. The results of the experiment were as follows. (i) The evapotranspiration from citrus tree was reduced after irrigation with saline water. The evapotranspiration returns to normal after leaching. However it takes months to exhaust the salt from the tree. (ii) To estimate the impact of irrigation with saline water on the evapotranspiration from citrus trees, the reduction coefficient due to salt stress (K_s) was used in this experiment. Evapotranspiration under irrigation with saline water (ET _s ) can be calculated from evapotranspiration under irrigation with freshwater (ET) by the equationET _s =K _s × ET. K_s can be expressed as a function ofEC _sw . (iii) The critical soil-water electrical conductivity (EC _sw ) is 9.5 dS/m, beyond which adverse effects on evapotranspiration begin to appear. IfEC _sw can be controlled at below 9.5 dS/m, saline water can be safely used for irrigation.     

Logic-tree Approach for Probabilistic Tsunami Hazard Analysis and its Applications to the Japanese Coasts

For Probabilistic Tsunami Hazard Analysis (PTHA), we propose a logic-tree approach to construct tsunami hazard curves (relationship between tsunami height and probability of exceedance) and present some examples for Japan for the purpose of quantitative assessments of tsunami risk for important coastal facilities. A hazard curve is obtained by integration over the aleatory uncertainties, and numerous hazard curves are obtained for different branches of logic-tree representing epistemic uncertainty. A PTHA consists of a tsunami source model and coastal tsunami height estimation. We developed the logic-tree models for local tsunami sources around Japan and for distant tsunami sources along the South American subduction zones. Logic-trees were made for tsunami source zones, size and frequency of tsunamigenic earthquakes, fault models, and standard error of estimated tsunami heights. Numerical simulation rather than empirical relation was used for estimating the median tsunami heights. Weights of discrete branches that represent alternative hypotheses and interpretations were determined by the questionnaire survey for tsunami and earthquake experts, whereas those representing the error of estimated value were determined on the basis of historical data. Examples of tsunami hazard curves were illustrated for the coastal sites, and uncertainty in the tsunami hazard was displayed by 5-, 16-, 50-, 84- and 95-percentile and mean hazard curves.     

Whole mantle SH velocity model constrained by waveform inversion based on three-dimensional Born kernels

A whole mantle SH velocity model is obtained by using a unique data set and techniques. Body and surface waveforms including major and multi-orbit phases are used as a data set and are inverted by using 3-D Born kernels. The resultant model, SH18CE, reveals the different natures of the two major upwelling systems: the strong low velocity anomalies beneath Africa extend for more than 1000 km from the core–mantle boundary (CMB), whereas those beneath the Pacific are restricted to 300–400 km from the CMB. The results also show the variable natures of stagnant slabs on the 670 discontinuity around Japan: the depths of the strongest high velocity anomalies within the stagnant slabs are different region by region, which is consistent with the detailed delay time tomography model in this area.     

Reactions to define the biotite isograd in the Ryoke metamorphic belt,Kii Peninsula,Japan

Two types of biotite isograd are defined in the low-grade metamorphism of the Wazuka area, a Ryoke metamorphic terrain in the Kii Peninsula, Japan. The first, BI₁, is defined by the reaction of chlorite+K-feldspar= biotite+muscovite+quartz+H₂O that took place in psammitic rocks, and the second, BI₂, by the continuous reaction between muscovite, chlorite, biotite and quartz in pelitic rocks. The Fe/Mg ratios of the host rocks do not significantly affect the reactions. From the paragenesis of pelitic and psammitic metamorphic rocks, the following mineral zones were established for this low-pressure regional metamorphic terrain: chlorite, transitional, chlorite-biotite, biotite, and sillimanite. The celadonite content of muscovite solid solution in pelitic rocks decreases systematically with the grade of metamorphism from 38% in the chlorite zone to 11% in the biotite zone. Low pressure does not prohibit muscovite from showing the progressive change of composition, if only rocks with appropriate paragenesis are chosen. A qualitative phase diagram of the AKF system relevant to biotite formation suggests that the higher the pressure of metamorphism, the higher the celadonite content of muscovite at BI₁, which is confirmed by comparing the muscovites from the Barrovian and Ryoke metamorphism.     

A study of subsolidus relations of the Skaergaard pyroxenes by analytical electron microscopy

Within augite and pigeonite grains of the Skaergaard ferrogabbro 4430, the Ca-poor phases contain only three mole percent of CaSiO₃, and the Mg-Fe partition coefficients between the Ca-poor and Ca-rich phases are extremely small with 0.46 for augite and 0.51 for pigeonite grains. These values indicate existence of diffusion within each grain (intragranular diffusion) at considerably low temperatures.The compositions are slightly but definitely different between the Ca-rich phases in augite and pigeonite grains as well as between the Ca-poor phases in augite and pigeonite grains. This indicates that the diffusion among the grains (inter-granular diffusion) has not taken place under the subsolidus condition of the Skaergaard intrusion.