Underplating of mélange evidenced by the depositional ages: U–Pb dating of zircons from the Shimanto accretionary complex, southwest Japan

Abstract   Growth of an accretionary prism is effected by frontal accretion and deep subsurface underplating at the base of the prism. A systematic oceanward and structurally downward younging of underplated sequences is expected as the prism thickens and grows. To test this hypothesis and explore the processes of underplating, the U–Pb ages of zircon grains contained in underplated mélange sequences or packages of the Late Cretaceous and early Paleogene accretionary complex of the Shimanto Belt, southwest Japan, were determined using LA–ICP–MS laser technology. The results document systematic but intermittent younging ages within a single underplated mélange package. This finding suggests that underplating took place episodically during a period of several million years and that between episodes of underplating, a large amount of sediment was subducted to depths much greater than where underplating was occurring.     

Origin of the early Cenozoic belt boundary thrust and Izanagi–Pacific ridge subduction in the western Pacific margin

The belt boundary thrust within the Cretaceous–Neogene accretionary complex of the Shimanto Belt, southwestern Japan, extends for more than ~ 1 000 km along the Japanese islands. A common understanding of the origin of the thrust is that it is an out of sequence thrust as a result of continuous accretion since the late Cretaceous and there is a kinematic reason for its maintaining a critically tapered wedge. The timing of the accretion gap and thrusting, however, coincides with the collision of the Paleocene–early Eocene Izanagi–Pacific spreading ridges with the trench along the western Pacific margin, which has been recently re‐hypothesized as younger than the previous assumption with respect to the Kula‐Pacific ridge subduction during the late Cretaceous. The ridge subduction hypothesis provides a consistent explanation for the cessation of magmatic activity along the continental margin and the presence of an unconformity in the forearc basin. This is not only the case in southwestern Japan, but also along the more northern Asian margin in Hokkaido, Sakhalin, and Sikhote‐Alin. This Paleocene–early Eocene ridge subduction hypothesis is also consistent with recently acquired tomographic images beneath the Asian continent. The timing of the Izanagi–Pacific ridge subduction along the western Pacific margin allows for a revision of the classic hypothesis of a great reorganization of the Pacific Plate motion between ~ 47 Ma and 42 Ma, illustrated by the bend in the Hawaii–Emperor chain, because of the change in subduction torque balance and the Oligocene–Miocene back arc spreading after the ridge subduction in the western Pacific margin.     

A water flow model of the active crater lake at Aso volcano,Japan: fluctuations of magmatic gas and groundwater fluxes from the underlying hydrothermal system

The first crater of Nakadake, peak of Aso volcano, Japan, contains a hot water lake that shows interesting variations in water level and temperature. These variations were discovered by precise, continuous observations of the lake independent of precipitation. We developed a numerical model of a hot crater lake and compared with observational data for the period from July 2006 to January 2009. The numerical model revealed seasonal changes in mass flux (75–132 kg/s) and enthalpy (1,840–3,030 kJ/kg) for the fluid supplied to the lake. The relation between the enthalpy and mass flux indicates that the bottom input fluid is a mixture of high- and low-temperature fluids. Assuming a mixture of high-temperature steam at 800°C and liquid water at 100°C, we evaluated the liquid and steam fluxes. The liquid water flux shows a seasonal increase lagging behind the rainy season by 2 months, suggesting that the liquid water is predominantly groundwater. The fluctuation pattern in the flux of the high-temperature steam shows a relation with the amplitude of volcanic tremor, suggesting that heating of the hydrothermal system drives the tremor. Consequently, precise observations of a hot crater lake represent a potential method of monitoring volcanic hydrothermal systems in the shallow parts of the volcanoes.     

3-D Modelling of Plate Interfaces and Numerical Simulation of Long-term Crustal Deformation in and around Japan

— We developed a 3-D simulation model for long-term crustal deformation due to steady plate subduction in and around Japan by incorporating viscoelastic slip-response functions into a realistic 3-D plate interface model, constructed on the basis of the topography of ocean floors and hypocenter distributions of earthquakes. The lithosphere-asthenosphere system is modelled by an elastic surface layer overlying a Maxwellian viscoelastic half-space. Kinematic interaction at plate interfaces is rationally represented by the increase of tangential displacement discontinuity (fault slip) across the interfaces. With this model, giving the steady slip rates at plate interfaces calculated from NUVEL-1A, we simulated long-term crustal deformation due to steady plate subduction in and around Japan. The simulated crustal deformation pattern is characterized by steep uplift at island arcs, sharp subsidence at ocean trenches and gentle uplift at outer rises. The numerical results show the strong dependence of the deformation pattern on the 3-D geometry of plate interfaces.     

3-D Simulation of Earthquake Generation Cycles and Evolution of Fault Constitutitve Properties

--The earthquake generation cycle consists of tectonic loading, quasi-static rupture nucleation, dynamic rupture propagation and stop, and subsequent stress redistribution and fault restrengthening. From a macroscopic point of view, the entire process of earthquake generation cycles should be consistently described by a coupled nonlinear system of a slip-response function, a fault constitutive law and a driving force. On the basis of such a general idea, we constructed a realistic 3-D simulation model for earthquake generation cycles at a transcurrent plate boundary by combining the viscoelastic slip-response function derived for a two-layered elastic-viscoelastic structure model, the slip- and time-dependent fault constitutive law that has an inherent mechanism of fault restrengthening, and the steady relative plate motion as a driving force into a single closed system. With this model we numerically simulated the earthquake generation cycles repeated in a seismogenic region on a plate interface, and examined space-time changes in shear stress, slip deficits and fault constitutive properties during one complete cycle in detail. The occurrence of unstable dynamic slip brings about decrease both in fault strength and shear stress to a constant residual level. After the arrest of dynamic slip, the breakdown strength drop j†_p of fault is restored rapidly and the process of stress accumulation resumes in the seismogenic region. On the other hand, the restoration of the critical weakening displacement D_c proceeds gradually with time through the interseismic period. The restoration of D_c can be regarded as the macroscopic manifestation of the microscopic recovery process of fractal fault surface structure. Through numerical simulation with a multi-segmented fault model, we examined the effects of viscoelastic fault-to-fault interaction. The effect of transient viscoelastic stress transfer through the asthenosphere is significant as well as the direct effect of elastic stress transfer, and it possibly explains the time lag of the sequential occurrence of large events along a plate boundary.     

Gaseous formic and acetic acids in the atmosphere of Yokohama,Japan

Gaseous formic acid (HCOOH_g) and acetic acid (CH₃COOH_g) were measured every 30 minutes during a 10 hour daylight period in August, and a 24 hour period in October, 1990 in the urban atmosphere of Yokohama, Japan. An aqueous nebulizer sampler and ion-chromatography exclusion (ICE) were used for the measurements. In the August experiment (0800–1800 local time) the mean HCOOH_g concentration was found to be 7.3±2.5 ppbv. The mean CH₃COOH_g concentration was 3.8±1.2 ppbv. In the 24 hour experiment in October, concentrations of both acids were lower between 0800–1800 than during the same time-period in August (mean HCOOH_g=4.4±2.7 ppbv, mean CH₃COOH_g=1.4±0.5 ppbv). In October, concentrations of both acids were higher in daylight hours than at night; sporadic high HCOOH_g concentrations were observed. In both experiments the ratio HCOOH_g/CH₃COOH_g of individual samples was usually 2.0 (mean ratio of 2.0 in August, 3.1 in October).     

Three-dimensional analysis of groundwater flow in neogene rocks at the Rokkasho low-level radioactive waste disposal center, Japan

As a preliminary step for predicting groundwater flow in a plateau 30–50 m above sea level, a model for three-dimensional analysis of groundwater flow was formulated and its validity was verified. The plateau consists of Neogene sedimentary rocks and a Quaternary deposit. Most of the groundwater originates in precipitation, with the groundwater table lying in the Quaternary deposit. Steady-state analysis was conducted by using the finite element method. The results of pore-water pressure measurement and water examination were useful in verifying the validity of the model. In constructing the model, reducing the hydraulic conductivity according to the depth on the basis of the results of the actual measurement was important.     

Citrate sorption and biodegradation in acid soils with implications for aluminum rhizotoxicity

Citrate and other organic acids play an important role in the rhizosphere and pedogenic processes. Although secreting citrate from roots in response to Al and heavy metal stress has been recognized as a central mechanism for plants to avoid toxicity, the efficiency of root citrate on metal detoxification is still contradictory in acid soil with abundant oxide minerals that serve as a potential sorption site for citrate. The objective of this study was to investigate sorption and biodegradation of citrate in subtropical acid soils with different mineralogical properties. A batch experiment was conducted to assess the possible fates (adsorption and biodegradation) of citrate in the three acid soils (Cecil, Creedmoor and Norfolk) under microbial-active and inactive conditions. Citrate adsorption isotherms for all soils were adequately described by the Freundlich equation with the R² value being over 0.90. The Cecil soil had the highest affinity for citrate adsorption among the soils with 99% adsorption observed throughout the citrate concentration range, which was due primarily to the abundant Al and Fe oxides. Citrate sorption to the mineral phase significantly reduced its biodegradation by 56%, 65% and 99% for the Creedmoor, Norfolk and Cecil soils, respectively. The results suggest the efficiency of rhizosphere processes for Al detoxification by root-secreted citrate would be significantly reduced in acid soil with abundant Al and Fe oxides.