Deep structure of the northeastern Japan arc and its implications for crustal deformation and shallow seismic activity

Seismic tomography studies in the northeastern Japan arc have revealed the existence of an inclined sheet-like seismic low-velocity and high-attenuation zone in the mantle wedge at depths shallower than about 150 km. This sheet-like low-velocity, high-attenuation zone is oriented sub-parallel to the subducted slab, and is considered to correspond to the upwelling flow portion of the subduction-induced convection. The low-velocity, high-attenuation zone reaches the Moho immediately beneath the volcanic front (or the Ou Backbone Range) running through the middle of the arc nearly parallel to the trench axis, which suggests that the volcanic front is formed by this hot upwelling flow. Aqueous fluids supplied by the subducted slab are probably transported upward through this upwelling flow to reach shallow levels beneath the Backbone Range where they are expelled from solidified magma and migrate further upward. The existence of aqueous fluids may weaken the surrounding crustal rocks, resulting in local contractive deformation and uplift along the Backbone Range under the compressional stress field of the volcanic arc. A strain-rate distribution map generated from GPS data reveals a notable concentration of east–west contraction along the Backbone Range, consistent with this interpretation. Shallow inland earthquakes are also concentrated in the upper crust of this locally large contraction deformation zone. Based on these observations, a simple model is proposed to explain the deformation pattern of the crust and the characteristic shallow seismic activity beneath the northeastern Japan arc.     

Fluid Inclusion Study and Opaque Mineral Assemblage at the Deep and Shallow Part of the Batu Hijau Porphyry Copper-gold Deposit, Sumbawa, Indonesia

The Batu Hijau deposit is the only porphyry type deposit in production in the Sunda‐Banda arc, Indonesia. This study discusses the reason for the localization of copper grade at the deep part of the deposit based on the observation of opaque mineral assemblage. In addition, the formation condition of quartz veins and opaque minerals is discussed on the basis of the fluid inclusion microthermometry. Samples were selected from drill holes SBD100, SBD168, SBD194, SBD254, and SBD257 to cover the wide vertical range. At the Batu Hijau deposit, quartz veins have been classified mainly into four types called A, B, C and D veins, and the A veins contain mainly bornite, often associated with digenite and chalcocite. In addition, magnetite occurs in A veins. However, at the deep part of the deposit, there are quartz veins associated with magnetite, but few copper sulfides such as bornite and chalcopyrite in quartz veins, as observed in SBD257. Quartz veins at depth in SBD257 have abundant magnetite and pyrite. Pyrite in quartz veins at depth in SBD257 mainly occur at the rim of magnetite grains or interstices between them. In quartz veins in SBD254, there are abundant copper sulfides such as bornite and chalcopyrite in spite of the depth. Bornite and chalcopyrite occur as inclusions in magnetite grains in quartz veins in SBD254. Pyrite which often occurs in low grade zone in quartz veins in SBD254 is also recognized at the rims of copper sulfides. This indicates that pyrite in SBD257 and SBD254 formed later than magnetite. On the other hand, blebs of bornite and chalcopyrite inclusions in magnetite grains, which are recognized in quartz veins in SBD168 at shallow high grade part, suggest that the hydrothermal fluid, from which magnetite was deposited also brought the copper sulfides such as bornite and chalcopyrite to the deep part of the Batu Hijau deposit. Therefore, it is concluded that initially the high grade ore zone extended to depth without localization. However due to the later overprinting hydrothermal activity, copper sulfides and magnetite were replaced or dissolved and pyrite was formed, resulting the low grade zone at the deep part of the deposit. Dissolution temperatures (Td) of halite obtained by from fluid inclusion microthermometry show significant differences between SBD168 and other drill holes. The high Td obtained in SBD168 may indicate larger volume of NaCl crystals in hydrothermal fluid at the time of entrapment of the fluid inclusions and formation of other opaque minerals such as magnetite and copper‐iron sulfides. It suggests that the ratio of vapor to brine is also higher at the shallow part of the deposit. The higher vapor to brine ratio may suggest a higher degree of boiling. Removal of vapor phase separated from brine during boiling increases the concentration of substances dissolved in the brine, and this will result in saturation, as evidenced by the salinity and NaCl saturation. The higher degree of boiling suggested by the higher vapor to brine ratio at shallow part may have increased the copper concentration in the brine that may have lead the saturation, resulted in the deposition of copper‐bearing minerals.     

Seasonal variations of the surface currents in the Tsushima Strait

Seasonal variations of the surface currents in the Tsushima Strait were investigated by analyzing the monthly mean surface currents measured with HF radar. Several new features of the surface currents have been found. One notable feature is the large, complicated seasonal variation in the current structure in the eastern channel of the strait. For example, in the southeastern and northwestern regions of the channel, southwestward countercurrents are found in summer while southeastward acrossshore currents are found in autumn and winter. The wind-driven flow (Ekman flow) as well as surface geostrophic currents are responsible for these complicated variations of the surface currents. To quantify each variation of the flow and current, the wind-driven flow was calculated from the monthly wind (more precisely, the friction velocity) using the monthly speed factor and deflection angle estimated in our previous study, and the surface geostrophic currents were then estimated by subtracting the wind-driven flow from the measured surface currents. It was found that the acrossshore currents are the wind-driven flow, and that the surface geostrophic currents flow almost in the along-shore direction, indicating the validity of the decomposition of the surface velocity into the wind-driven flow and the geostrophic currents using the speed factor and deflection angle. A real-vector empirical orthogonal function (EOF) analysis of the surface geostrophic currents shows a pair of eddies in the lee of Tsushima and Iki Islands as the first mode, which indicates that the southwestward countercurrents in the eastern channel are formed primarily by the incoming Tsushima Warm Current.     

Jurassic plume-origin ophiolites in Japan: accreted fragments of oceanic plateaus

The Mikabu and Sorachi–Yezo belts comprise Jurassic ophiolitic complexes in Japan, where abundant basaltic to picritic rocks occur as lavas and hyaloclastite blocks. In the studied northern Hamamatsu and Dodaira areas of the Mikabu belt, these rocks are divided into two geochemical types, namely depleted (D-) and enriched (E-) types. In addition, highly enriched (HE-) type has been reported from other areas in literature. The D-type picrites contain highly magnesian relic olivine phenocrysts up to Fo_93.5, and their Fo–NiO trend indicates fractional crystallization from a high-MgO primary magma. The MgO content is calculated as high as 25 wt%, indicating mantle melting at unusually high potential temperature (T _p) up to 1,650 °C. The E-type rocks represent the enrichment in Fe and LREE and the depletion in Mg, Al and HREE relative to the D-type rocks. These chemical characteristics are in good accordance with those of melts from garnet pyroxenite melting. Volcanics in the Sorachi–Yezo belts can be divided into the same types as the Mikabu belt, and the D-type picrites with magnesian olivines also show lines of evidence for production from high T _p mantle. Evidence for the high T _p mantle and geochemical similarities with high-Mg picrites and komatiites from oceanic and continental large igneous provinces (LIPs) indicate that the Mikabu and Sorachi–Yezo belts are accreted oceanic LIPs that were formed from hot large mantle plumes in the Late Jurassic Pacific Ocean. The E- and D-type rocks were formed as magmas generated by garnet pyroxenite melting at an early stage of LIP magmatism and by depleted peridotite melting at the later stage, respectively. The Mikabu belt characteristically bears abundant ultramafic cumulates, which could have been formed by crystal accumulation from a primary magma generated from Fe-rich peridotite mantle source, and the HE-type magma were produced by low degrees partial melting of garnet pyroxenite source. They should have been formed later and in lower temperatures than the E- and D-type rocks. The Mikabu and Sorachi Plateaus were formed in a low-latitude region of the Late Jurassic Pacific Ocean possibly near a subduction zone, partially experienced high P/T metamorphism during subduction, and then uplifted in association with (or without, in case of Mikabu) the supra-subduction zone ophiolite. The Mikabu and Sorachi Plateaus may be the Late Jurassic oceanic LIPs that could have been formed in brotherhood with the Shatsky Rise.     

Relative chronology of crust formation on asteroid Vesta: Insights from the geochemistry of diogenites

The eucrites and diogenites are meteorites that probably originate from asteroid 4-Vesta. The upper part of the crust of this body is certainly composed of eucrites which are basaltic or gabbroic rocks. Diogenites are ultramafic cumulates whose relationships with eucritic lithologies are unknown. Here, we show that the orthopyroxenes of some diogenites display very deep negative Eu anomalies (Eu/Eu∗ close to 0.1 or lower). The contamination of the parental magmas of diogenites by melts derived by partial melting of the eucritic crust can satisfactorily explain the range of the Eu anomalies displayed by diogenites. Thus, these anomalies are the first firm indication that parental melts of diogenites have intruded the eucritic crust, and consequently are younger than eucrites.