Distribution of fault rocks in the fracture zone of the Nojima Fault at a depth of 1140 m: Observations from the Hirabayashi NIED drill core

Abstract Characteristics of deformation and alteration of the 1140 m deep fracture zone of the Nojima Fault are described based on mesoscopic (to the naked eye) and microscopic (by both optical and scanning electron microscopes) observations of the Hirabayashi National Research Institute for Earth Science and Disaster Prevention (NIED) drill core. Three types of fault rocks; that is, fault breccia, fault gouge and cataclasite, appear in the central part of the fault zone and two types of weakly deformed and/or altered rocks; that is, weakly deformed and altered granodiorite and altered granodiorite, are located in the outside of the central part of the fault zone (damaged zone). Cataclasite appears occasionally in the damaged zone. Six distinct, thin foliated fault gouge zones, which dip to the south-east, appear clearly in the very central part of the fracture zone. Slickenlines plunging to the north-east are observed on the surface of the newest gouge. Based on the observations of XZ thin sections, these slickenlines and the newest gouge have the same kinematics as the 1995 Hyogo-ken Nanbu earthquake (Kobe earthquake), which was dextral-reverse slip. Scanning electron microscopy observations of the freeze-dried fault gouge show that a large amount of void space is maintained locally, which might play an important role as a path for fluid migration and the existence of either heterogeneity of pore fluid pressure or strain localization.     

A Paleogene magmatic overprint on Cretaceous seamounts of the western Pacific

The West Pacific Seamount Province (WPSP) represents a series of short-lived Cretaceous hotspot tracks. However, no intraplate volcanoes in advance of petit-spot volcanism erupted near a trench have been identified after the formation of the WPSP on the western Pacific Plate. This study reports new ages for Paleogene volcanic edifices within the northern WPSP, specifically the Ogasawara Plateau and related ridges, and Minamitorishima Island. These Paleogene ages are the first reported for basaltic rocks on western Pacific seamounts, in an area that has previously only yielded Cretaceous ages. The newly found Paleogene volcanisms overprint the Early–middle Cretaceous volcanic edifices, because the seamount or paleo-island material-covered reefal limestone caps on these edifices are uniformly older than the Paleogene volcanism identified in this study. This study outlines several possible causative factors for the Paleogene volcanism overprinting onto existing Cretaceous seamounts, including volcanism related to lithospheric stress, or a younger hotspot track within the northern part of the WPSP that records magmatism from ~60 Ma.     

Origin of Neoproterozoic ophiolitic peridotites in south Eastern Desert, Egypt, constrained from primary mantle mineral chemistry

The ophiolitic peridotites in the Wadi Arais area, south Eastern Desert of Egypt, represent a part of Neoproterozoic ophiolites of the Arabian-Nubian Shield (ANS). We found relics of fresh dunites enveloped by serpentinites that show abundances of bastite after orthopyroxene, reflecting harzburgite protoliths. The bulk-rock chemistry confirmed the harzburgites as the main protoliths. The primary mantle minerals such as orthopyroxene, olivine and chromian spinel in Arais serpentinites are still preserved. The orthopyroxene has high Mg# [=Mg/(Mg + Fe²⁺)], ~0.923 on average. It shows intra-grain chemical homogeneity and contains, on average, 2.28 wt.% A1₂O₃, 0.88 wt.% Cr₂O₃ and 0.53 wt.% CaO, similar to primary orthopyroxenes in modern forearc peridotites. The olivine in harzburgites has lower Fo (93?94.5) than that in dunites (Fo_94.3?Fo_95.9). The Arais olivine is similar in NiO (0.47 wt.% on average) and MnO (0.08 wt.% on average) contents to the mantle olivine in primary peridotites. This olivine is high in Fo content, similar to Mg-rich olivines in ANS ophiolitic harzburgites, because of its residual origin. The chromian spinel, found in harzburgites, shows wide ranges of Cr#s [=Cr/(Cr + Al)], 0.46?0.81 and Mg#s, 0.34?0.67. The chromian spinel in dunites shows an intra-grain chemical homogeneity with high Cr#s (0.82?0.86). The chromian spinels in Arais peridotites are low in TiO₂, 0.05 wt.% and Y_Fe [= Fe³⁺/(Cr + Al + Fe³⁺)], ~0.06 on average. They are similar in chemistry to spinels in forearc peridotites. Their compositions associated with olivine’s Fo suggest that the harzburgites are refractory residues after high-degree partial melting (mainly ~25?30 % partial melting) and dunites are more depleted, similar to highly refractory peridotites recovered from forearcs. This is in accordance with the partial melting (>20 % melt) obtained by the whole-rock Al₂O₃ composition. The Arais peridotites have been possibly formed in a sub-arc setting (mantle wedge), where high degrees of partial melting were available during subduction and closing of the Mozambique Ocean, and emplaced in a forearc basin. Their equilibrium temperature based on olivine?spinel thermometry ranges from 650 to 780 °C, and their oxygen fugacity is high (Δlog ?O₂?=?2.3 to 2.8), which is characteristic of mantle-wedge peridotites. The Arais peridotites are affected by secondary processes forming microinclusions inside the dunitic olivine, abundances of carbonates and talc flakes in serpentinites. These microinclusions have been formed by reaction between trapped fluids and host olivine in a closed system. Lizardite and chrysotile, based on Raman analyses, are the main serpentine minerals with lesser antigorite, indicating that serpentines were possibly formed under retrograde metamorphism during exhumation and near the surface at low T (<400 °C).     

Architecture,formation and implication of active structure-controlled intraslope channel system southeast offshore Sendai,Tohoku, Japan

Located off the Pacific coast of central Tohoku (NE Japan), the Ishinomaki slope channel (ISC) provides an excellent opportunity to study a structure-controlled intraslope channel and downslope sedimentation along the active margin. The seismic reflection data across ISC show an extensive basal surface and overlying channel complexes between the basement structures of the Abukuma ridge to the south and Kitakami massif to the north, indicating that the formation of the intraslope basin, channelization of ISC and sedimentation of the downstream channel-lobe transition zone (CLTZ) are very likely to be structure-controlled. The oblique channel stacking pattern, faulting of the seafloor and subsurface Abukuma ridge in the upper and lower domains of ISC, collectively suggest that ISC has migrated northward and is currently under the influence of active compression. Differences in styles of accommodation space between the upper and lower domains of ISC suggest that differential subsidence occurred along the strike-slip tectonic line. Based on the regional strike-slip tectonic line, we propose that a Kitakami-Abukuma ridge existed before the formation of ISC. The strike-slip faulting divided the Kitakami-Abukuma ridge into the Kitakami massif to the north and the Abukuma ridge to the south, and an intervening fault trough as the precursor of the intraslope basin and ISC. As the subduction of the Pacific Plate and associated compressional events continued, the Abukuma ridge was reactivated to narrow the intraslope basin into a confined channel. Located near the epicentre of the devastating 2011 Tohoku earthquake event, the ISC, downstream CLTZ and underlying intraslope basin provide information on active basement structure and the evolving sediment routing system on the tectonically active margin.     

Correction of the effect of relative wind direction on wind speed derived by advanced microwave scanning radiometer

The sea surface wind speed (SSWS) derived by a microwave radiometer can be contaminated by changes of the brightness temperature owing to the angle between the sensor azimuth and the wind direction (Relative Wind Direction effect: RWD effect). We attempt to apply the method proposed by Konda and Shibata (2004) to the SSWS derived by Advanced Microwave Scanning Radiometer (AMSR) on Advanced Earth Observing Satellite II (ADEOS-II), in order to correct for the RWD effect. The improvement of accuracy of the SSWS estimation amounts to roughly 60% of the error caused by the RWD effect. Comparison with in situ observation at the Tropical Atmosphere Ocean (TAO) array shows that the root mean square error of the corrected SSWS is 1.1 ms⁻¹. It is found that the impact of the RWD effect on the estimation of the latent heat flux can amount to about 30 Wm⁻² on average. We applied the method to the SSWS derived by AMSR for Earth Observing System (AMSR-E) and obtained a similar result.     

Peculiar Mg–Ca–Si metasomatism along a shear zone within the mantle wedge: inference from fine-grained xenoliths from Avacha volcano,Kamchatka

We found extremely high-Mg# (=Mg/(Mg + total Fe) atomic ratio) ultramafic rocks in Avacha peridotite suite. All the high-Mg# rocks have higher modal amounts of clinopyroxene than ordinary Avacha peridotite xenoliths, and their lithology is characteristically heterogeneous, varying from clinopyroxenite through olivine websterite to pyroxene-bearing dunite. The Mg# of minerals is up to 0.99, 0.98 and 0.97 in clinopyroxene, orthopyroxene and olivine, respectively, decreasing progressively toward contact with dunitic part, if any. The petrographical feature of pyroxenes in the high-Mg# pyroxenite indicates their metasomatic origin, and high LREE/HREE ratio of the metasomatic clinopyroxene implies that the pyroxenites are the products of reaction between dunitic peridotites and high-Ca, silicate-rich fluids. The lithological variation of the Avacha high-Mg# pyroxenites from clinopyroxenite to olivine websterite resulted from various degrees of fluid-rock reaction coupled with fractional crystallization of the high-Ca fluids, which started by precipitation of high-Mg# clinopyroxene. Such fluids were possibly generated originally at a highly reduced serpentinized peridotite layer above the subducting slab. The fluids can reach the uppermost mantle along a shear zone as a conduit composed of fine-grained peridotite that developed after continent-ward asthenospheric retreats from the mantle wedge beneath the volcanic front. The fluids are incorporated in mantle partial melts when the magmatism is activated by expansion of asthenosphere to mantle wedge beneath the volcanic front.     

Operational Data Assimilation System for the Kuroshio South of Japan: Reanalysis and Validation

We describe an operational ocean data assimilation system for the Kuroshio and its validation using a nine-year reanalysis (historical run from 1993 to 2001) dataset of upper-ocean state estimation in the North Pacific. The horizontal structure of volume transport of the Ryukyu Current System (RCS) is shown from the reanalysis: The RCS is connected to the flow of the subtropical gyre, and its volume transport gradually increases from south-east of Okinawa (5–10 Sv) to the east of Amami-Ohshima Island (20 Sv). Comparing the reanalysis with independent observations on the southeast slope of the Amami-Ohshima Island indicates that the root mean square differences (RMSDs) are 0.076 (0.037) m/s in the period of December 1998 to November 1999 (November 1999 to November 2000) respectively. The reanalysis field has a bias (3.1 Sv) of the volume transport of the RCS and the RMSD (3.5 Sv) which is larger than the observed variability (2.81 Sv). Surface velocity and the Kuroshio axis south of Japan are also examined. Comparison of the reanalysis and ADCP data gave maximum RMSD of 0.749 (0.271) m/s in the strong (weak) current regions, respectively. The annual mean value of the axis error is 19 km in 1998. The RMSD of the error is at most 50 km, in 294 cases in the observation period, which is smaller than the observed root mean square variability of the axis (64 km). This revised version was published online in July 2006 with corrections to the Cover Date.