Japan Arc Research Articles

Magma genesis beneath Northeast Japan arc: A new perspective on subduction zone magmatism

It is being accepted that earthquakes in subducting slab are caused by dehydration reactions of hydrous minerals. In the context of this “dehydration embrittlement” hypothesis, we propose a new model to explain key features of subduction zone magmatism on the basis of hydrous phase relations in peridotite and basaltic systems determined by thermodynamic calculations and seismic structures of Northeast Japan arc revealed by latest seismic studies. The model predicts that partial melting of basaltic crust in the subducting slab is an inevitable consequence of subduction of hydrated oceanic lithosphere. Aqueous fluids released from the subducting slab also cause partial melting widely in mantle wedge from just above the subducting slab to just below overlying crust at volcanic front. Hydrous minerals in the mantle wedge are stable only in shallow (< 120 km) areas, and are absent in the layer that is dragged into deep mantle by the subducting slab. The position of volcanic front is not restricted by dehydration reactions in the subducting slab but is controlled by dynamics of mantle wedge flow, which governs the thermal structure and partial melting regime in the mantle wedge.

Contribution of slab-fluid in arc magmas beneath the Japan arcs

Identifying the amount and composition of slab-derived fluid and its spatial variation is key to quantifying fluid processes in subduction zones. Based on the isotopic systematics of arc lavas, we found regional variations over the Japan arcs in terms of the amount and composition of slab-derived fluid added to the melting source region. The average amounts of slab-derived fluid differ among the arcs: 2.6 wt.% for Central Japan, 0.69 wt.% for Ryukyu, 0.17 wt.% for NE Japan, and 0.12 wt.% for both Kurile and Izu–Bonin. These differences may be attributed to the arc setting (oceanic or continental) and the geometry of the slabs. Contribution of sediment involved in the slab-derived fluid is dominant in NE Japan compared to the Izu–Bonin and Central Japan arcs. This could be attributed to mechanical features such as fractures near the subducting plate surface, in addition to the arc setting and the slab geometry. Therefore, the amount and composition of slab-derived fluid are thought to be controlled not only by the thermal conditions, but also by the tectonic and mechanical features around the subduction zone. On top of the variability of slab-derived fluid, the mantle wedge shows the regional variation in terms of proportion of the Pacific-type and Indian-type MORB-source mantle components, which also contributes to the compositional variations of arc magmas.

The Fukuyama volcanic rocks: Submarine composite volcano in the Late Miocene to Early Pliocene Akita–Yamagata back-arc basin, northeast Honshu, Japan

The Fukuyama Volcanic Rocks are composed of pyroxene andesite (FKV-1), hornblende-pyroxene andesite (FKV-2), biotite-hornblende dacite (FKV-3) and volcaniclastic debris-flow deposits and/or turbidites. FKV-1, FKV-2 and FKV-3 are medium-K calc-alkaline rocks depleted in Nd, similar to other back-arc volcanic rocks of the northeast Japan arc and constitute a dome cluster at Fukuyama. Volcaniclastic beds surround the dome cluster and thin and fine upwards. The predominant clast type in the volcaniclastic beds changes upwards from pyroxene andesite, through hornblende–pyroxene andesite, to biotite–hornblende dacite, consistent with the stratigraphic relationships of FKV-1, FKV-2 and FKV-3 lavas. All the siltstones inter-bedded with the volcaniclastic beds and overlying the whole succession contain diatom fossils indicative of the lower part of the Thalassionema schraderi zone (7.8 Ma to 8.5 Ma), compatible with the isotopic ages of FKV-1, FKV-2 and FKV-3. The Fukuyama volcano has a total eruption volume of 60–100 km 3, with a lifetime of the order of 10 5 years, as typically observed for volcanoes in the present back-arc region of northeast Honshu. FKV-1 erupted in deep water and partly disintegrated into hyaloclastite breccias due to direct contact with water. FKV-2 lava repeatedly effused over the FKV-1 lava and produced a volcanic apron of breccias that eventually grew above wave base and was eroded by wave action. The magma of FKV-3 was probably hydrous as it contains biotite and hornblende. The FKV-3 magma could have explosively erupted from a shallow-water dome or vent emergent above the wave base, followed by growth of a degassing lava dome. Repose between eruptions allowed accumulation of silt, and after the Fukuyama eruptions ceased silt entirely mantled the volcano. A small magma supply rate perhaps allowed a relatively long period of quiescence between eruptions of FKV-1, FKV-2 and FKV-3 magmas, resulting in abrasion and reworking of volcanic fragments and accumulation of non-volcanic sediments that constitute part of the volcaniclastic apron. Oil generated in the contemporaneous or underlying sediments, migrated to the upper levels of the volcaniclastic apron and accumulated there under the thick cover of fine-grained sediments that mantle the volcano.

Sedimentary processes caused by felsic caldera-forming volcanism in the Late Miocene to Early Pliocene intra-arc Aizu basin, NE Japan arc

Six large Late Miocene to Quaternary calderas, > 10 km in diameter, cluster together with several medium to small calderas and stratovolcanoes in a 60 × 30 km area of the Aizu volcanic field, southern NE Japan arc. These caldera volcanoes were built on a WNW–ESE trending highland coincident with a local uplifted swell since Late Miocene. The flare-up of felsic volcanism occurred synchronously along the NE Japan arc. Pyroclastic flow sheets from the calderas spread over the surrounding intra-arc basins and are interstratified with various sediments. Geochronological data indicates that the large-caldera eruptions have occurred six times since 8 Ma, at intervals of 1 to 2 million years. Late Miocene to Early Pliocene extra-caldera successions in the basin consist of nine sedimentary facies associations: (1) primary pyroclastics, (2) lahars, (3) gravelly fluvial channels, (4) sandy fluvial channels, (5) floodplains, (6) tidal flats, (7) delta fronts, (8) pro-delta slopes, and (9) pro-delta turbidites. The distribution of facies associations show westward prograding of volcaniclastic aprons, made up of braid delta, braidplain, pyroclastic flow sheet, and incised braided river deposits. The extra-caldera successions record: 1) an increase in felsic volcanism with an associated high rate of volcaniclastic sediment supply at about 10 Ma, prior to catastrophic caldera-forming eruptions; and 2) progradation of volcaniclastic aprons toward the back-arc side in response to the succeeding caldera-forming eruptions and sea-level changes, until about 3 Ma.

Principal Component Analysis as a Tool for Materials Characterization of the Plate Boundary — Seismic Activity Application in the Plate Boundary Zone of the Northeastern Japan Arc.

We have analyzed microseismic activity in the boundary zone of the Northeastern Japan Arc by using principal component analysis for discrete units by 25 × 20 of the boundary zone with 10 km thick over the plate boundary. We found two types of reduced parameter trends in the region, concluding that the area has two types of evolutionary tracks; one of isolated activity and the other of strong correlated north-southactivity.

西南日本弧東端における前期白亜紀火成活動：朝日山地，西朝日複合塩基性岩体の多様性

西南日本弧東端における前期白亜紀火成活動：朝日山地，西朝日複合塩基性岩体の多様性

始新統石狩層群の夾炭河川～内湾シーケンスと石狩～三陸沖過充填陸棚型前弧の堆積盆テクトニックセッティング

始新統石狩層群の夾炭河川～内湾シーケンスと石狩～三陸沖過充填陸棚型前弧の堆積盆テクトニックセッティング

Combining 2D-basin and structural modelling to constrain heat transport along the Muroto Transect, Nankai Trough, Japan

The Nankai Trough located southeast of Shikoku Island, Japan, exhibits a zone of exceptionally high heat flow. In the central part of the Nankai Trough the fossil spreading centre of the Shikoku Basin is subducted beneath the southwest Japan arc. We have modelled the temperature and maturation history along the Muroto Transect reaching from the tip of the thrust zone out into nearly undeformed Quaternary and Tertiary sediments seawards of Nankai Trough. We used two balanced cross-sections defining the sections before and after overthrusting as input for 2D-basin modelling. We can show that rapid burial and overthrusting during the Quaternary in combination with a heat flow history following the cooling curve of a 15 Ma old oceanic plate is not sufficient to explain the measured maturity of organic material in the sediments. Several heat flow scenarios derived from theoretical concepts [Yamano, M., Kinoshita, M., Goto, S., Matsubayashi, O., 2003. Extremely high heat flow anomaly in the middle part of the Nankai Trough. Physics and Chemistry of the Earth, Parts A/B/C 28, 487–497.] and previous modelling approaches [e.g. Brown, K.M., Saffer, D.M., Bekins, B.A., 2001. Smectite diagenesis, pore water freshening, and fluid flow at the toe of the Nankai wedge. Earth and Planetary Science Letters 194, 97–109; Spinelli, G.A., Underwood, M.B., 2005. Modeling thermal history of subducting crust in Nankai Trough: constraints from in situ sediment temperature and diagenetic reaction progress. Geophysical Research Letters 32(L09301): doi:10.1029/2005GL022793; Steurer, J., Underwood, M.B., 2003. Clay mineralogy of mudstones from the Nankai Trough reference sites 1173 and 1177 and frontal accretionary prism site 1174. In: H. Mikada et al. (Eds.), pp. 1–37. Available from: <http://www-odp.tamu.edu/publications/190196SR/VOLUME/CHAPTERS/211.PDF>] were tested. The best match between observed maturity levels, temperature and heat flow measurements is reached for a heat flow history which initially assumes the cooling of a 15 Ma old oceanic lithosphere but is reheated to 170–180 mW/m 2 during the phase of rapid burial in the Quaternary. This can be achieved either by assuming the onset of hydrothermal circulation in the cooling crust or by reheating caused by off-axis volcanism at about 6 Ma [Yamano, M., Kinoshita, M., Goto, S., Matsubayashi, O., 2003. Extremely high heat flow anomaly in the middle part of the Nankai Trough. Physics and Chemistry of the Earth, Parts A/B/C 28, 487–497.].

訂正：東北日本弧，稲庭岳に産するソレアイト系列岩の岩石学的研究 ─鮮新世に噴出した高温タイプのソレアイトマグマ─

岩石鉱物科学, 35, 78-96 の Fig. 1 図中の経度140度を141度に訂正します。

Mapping the mantle wedge and interplate thrust zone of the northeast Japan arc

We detected sP depth phases from three-component, short-period seismograms from the forearc earthquakes under the Pacific Ocean recorded by the dense seismic network on the Japan Islands. Then we use the sP depth-phase data together with P and S wave arrival times to locate the suboceanic earthquakes accurately. A large number of P and S wave arrival times from the relocated suboceanic events and earthquakes under the land area are used to determine the 3-D P and S wave velocity and Poisson's ratio structures under the entire NE Japan arc from the Japan Trench to the Japan Sea. Our results exhibit strong lateral heterogeneity under the forearc region and a clear correlation between the structural heterogeneity and distribution of large earthquakes (M 7.0–8.2) occurred in the interplate thrust zone. Prominent low-velocity and high Poisson's ratio zones are visible in the mantle wedge under the volcanic front and back-arc region, which show nicely the images of arc magma caused by the slab dehydration and corner flow in the mantle wedge.

Mid-crustal horizontal shear zone in the forearc region of the mid-Cretaceous SW Japan arc, inferred from strain analysis of rocks within the Ryoke metamorphic belt

The strain geometry of a Cretaceous low- P/high- T metamorphosed accretionary complex in SW Japan is one of the key geological constraints in understanding the Cretaceous tectonics of the forearc region at the eastern margin of Eurasia. We studied the strain geometry of upper greenschist to lower amphibolite facies metamorphic rocks from the mid-Cretaceous low- P/high- T Ryoke metamorphic belt, which during the mid-Cretaceous were located at mid-crustal depths close to the volcanic front. Strain analysis focused on deformed radiolarian fossils in metachert and sandstone/chert clasts within metapelite. The maximum stretching direction ( X-direction) of the strain ellipsoid of the schistosity-forming deformation at the peak metamorphism is oriented E-W, and the XY-plane of the strain ellipsoid developed parallel to schistosity. The metachert and metapelite exhibit plane strain to general flattening strain that probably largely resulted from the schistosity-forming deformation. Based on the results of the previous strain analyses and this study, it reveals that the schistosity-forming deformation occurred under bedding-normal shortening, resulting in general flattening strain throughout the Ryoke metamorphic belt. The X-direction of the strain ellipsoid of the schistosity-forming deformation is oriented parallel to the length of the Ryoke metamorphic belt throughout the SW Japan arc. The schistosity formed parallel to bedding planes that developed horizontally in non-metamorphosed accretionary complexes of SW Japan, indicating horizontal XY-plane of the strain ellipsoid. These observations indicate the occurrence of horizontal shear deformation within the middle crust (∼15 km depth) at the time of the low- P/high- T metamorphism. The deep-seated horizontal shear zone and the shallower incipient Median Tectonic Line (MTL) fault system could result in a crustal-scale detachment fault in the forearc region of the mid-Cretaceous SW Japan arc. After the formation of the detachment, the left-lateral pull-apart forearc basins, Onogawa-Izumi Group distributed on the north side of the MTL and the southern marginal mylonite zone might be formed by the sinistral movement along the detachment fault.

Simultaneous high P‐T measurements of ultrasonic compressional and shear wave velocities in Ichino‐megata mafic xenoliths: Their bearings on seismic velocity perturbations in lower crust of northeast Japan arc

Simultaneous measurements of ultrasonic compressional (Vp) and shear wave velocities (Vs) were conducted under pressures and temperatures representative of the lower crust on five mafic xenoliths derived from the northeast Japan arc crust. For comparison with the observed seismic heterogeneity of the lower crust of the northeast Japan arc, the velocity deviations (dVp and dVs) of the xenoliths were calculated using the average reference Vp (6.61 ± 0.1 km/s) and Vs (3.76 ± 0.1 km/s). The dVp and dVs values of the xenoliths at 700°C and 0.8 GPa are 0.8% and −1.2% for hornblende gabbro, 4.8% and 4.0% for hornblende‐pyroxene gabbro, 0.9% and −1.5% for amphibolite, 2.5% and 1.7% for pyroxene amphibolite, and 0.9% and −5.6% for hornblendite, respectively. We compared the velocity perturbations in the lower crust determined by seismic tomography with dVp and dVs of the xenoliths using “Vp−Vs−Vp/Vs deviation diagrams.” On the basis of our measurements, we suggest that (1) the seismically high‐Vp and Vs regions beneath the Tobishima Basin consist of hornblende‐pyroxene gabbro, (2) hornblende gabbro is a predominant rock type beneath the Dewa Hills and Ou Backbone Range, (3) the low‐velocity anomalies beneath the active volcano areas may be caused by the existence of partial melts of hornblende gabbro, and (4) the low‐Vp and high‐Vs regions beneath the Kitakami Mountains consist of quartz‐plagioclase‐bearing rocks. Our data demonstrate that the seismic heterogeneity in the lower crust of the northeast Japan arc reflects variations in rock composition and temperature that are related to the regional geological history.

New Insights into Andesite Genesis: the Role of Mantle-derived Calc-alkalic and Crust-derived Tholeiitic Melts in Magma Differentiation beneath Zao Volcano, NE Japan

Two distinctive differentiation trends, tholeiitic and calc-alkalic, are recognized in Zao volcano, which is located immediately behind the volcanic front of the NE Japan arc. The genetic relationship between these two magma series is critical for a better understanding of andesite genesis, because they often coexist in close spatial and temporal proximity in arc volcanoes. Petrographic features indicative of ‘disequilibrium’ , such as reversely zoned pyroxene phenocrysts, the wide and bimodal compositional distribution in Ca/(CaþNa) of plagioclase phenocrysts, honeycomb textures and dusty zones that these plagioclase phenocrysts often exhibit, and the presence of olivine^pyroxene pairs with different Mg/Fe, are observed exclusively in calc-alkalic rocks. In tholeiitic rocks the Sr isotopic ratios of plagioclase phenocrysts, determined by both micromilling combined with thermal ionization mass spectrometry, and laser-ablation inductively coupled plasma mass spectrometry techniques, are constant at 0 7042^0 7044. On the other hand, those in calc-alkalic rocks (0 7033^0 7042) show more complex characteristics, which can be best understood if at least three end-member components, a calc-alkalic basaltic melt, a tholeiitic basaltic melt and a tholeiitic felsic melt, contribute to the production of mixed calc-alkalic magmas.The Sr/Sr and trace element compositions of the leastdifferentiated basalt magmas, which are inferred from the composition of the calcic plagioclase [Ca/(CaþNa)40 9], suggest that two types of basaltic magma, calc-alkalic and tholeiitic, exist beneath the volcano. The tholeiitic basalt magma has a higher Sr/Sr than the calc-alkalic magma (0 7042 vs 0 7038) and a characteristic trace element signature consistent with the presence of plagioclase and amphibole as melting residues.This suggests that the tholeiitic magmas are produced via anatexis of amphibolitic crust caused by underplating and/or intrusion of mantle-derived calcalkalic basalt magmas into the sub-Zao crust. The mantle-derived calc-alkalic basalt magma mixes with crust-derived tholeiitic melts to form calc-alkalic andesite magmas.The hypothesis proposed here requires revision (or even abandonment) of the general consensus that calc-alkalic magmas have greater contributions of a crustal component than tholeiitic magmas.

Characterization of low-temperature pyroclastic surges that occurred in the northeastern Japan arc during the late 19th century

Three eruption events occurring in the central part of the northeastern Japan arc were investigated and compared: Adatara AD1900, Zao AD1895, and Bandai AD1888. Producing low-temperature (LT) pyroclastic surges, these events are characterized by steam eruptions ejecting no juvenile material. These eruptions' well-preserved eruptive deposits and facies facilitated granulometric analyses of the beds, which revealed the transport and deposition mechanisms of LT surges. Combining these results with those of investigations of documents reporting the events, we correlated each eruption to the relevant individual bed and reconstructed the LT surge development sequence. Important findings related to the transport and deposition modes are the following. (1) Bed sets consisting of thin, laminated ash and its overlying thick massive tuff were recognized in the Adatara 1900 proximal deposits. The bed set was probably produced by a strong wind that discharged and propagated quickly from the vent (leading wind) and a gravitationally segregated, highly concentrated flow originated from the eruption column, within a discrete eruption episode. A similar combination might have occurred during the first surge of the Bandai 1888 event. (2) Comparison of the proximal and distal facies for the largest eruption of Adatara 1900 event indicates that the initial turbulence of the eruption cloud decreased rapidly, transforming into a density-stratified surge with a highly concentrated part near the base. Similar surges occurred in the climatic stage of Zao 1895. (3) Bandai 1888 ejecta indicate massive beds deposited preferentially at topographic lows. Co-occurring planar beds showed no topographic affection, as indicated by the topographic blocking of a stratified surge. The observed facies–massive tuffs, crudely stratified tuffs, and thin bedded tuffs–are compatible with those for high-temperature surges. At Bandai, absence of dune bedded tuffs and commonly poorer sorting in the LT surge deposits might be attributable to poor thermally induced turbulence of eruption columns. Condensation of vapor in the surges might have contributed to the poor sorting. The estimated explosion energies were 6 × 10 13 J for Adatara AD1900, 6.5 × 10 10 J for Zao AD1895, and 6.5 × 10 15 J for Bandai AD1888, implying that the three events were hydrothermal eruptions with distinctive eruptive mechanisms. Regarding eruption sources, the Adatara 1900 event was caused solely by thermal energy of the hydrothermal fluid, although magma intrusion likely triggered evolution of hydrothermal systems at Zao in 1895. Steam eruptions in the Bandai 1888 event occurred simultaneously with sudden exposure of the hydrothermal system, whose triggers require no internal energy.

B-type olivine fabrics developed in the fore-arc side of the mantle wedge along a subducting slab

B-type olivine fabrics are pervasive within highly depleted dunites of the small-sized Imono peridotite body located within the subduction-type Sanbagawa metamorphic belt of the southwest Japan arc. The dunites contain various microstructures, ranging from porphyroclastic to fine-grained intensely sheared textures. The Mg/(Mg + Fe) atomic ratios (Fo number) of olivine within these dunites are consistently around 0.9, as are the Cr/(Cr + Al) atomic ratios (Cr number) of chromian spinel, suggesting their evolution from a highly depleted magma (boninite). These data provide strong thermal constraints on the formation of the highly depleted dunites, as their formation requires hot, hydrous, shallow mantle (> 1250 °C at < 30 km depth) in the mantle wedge. Because the Sanbagawa metamorphic belt finally entrained these peridotites during progressive retrogression, B-type olivine fabrics probably developed in the fore-arc side of the subduction zone, above or along the subducting slab, possibly in association with dehydration fluids derived from the slab. The previously documented small magnitude of S-wave splitting can be explained by the seismic properties of B-type peridotites within an anisotropic layer of approximately several kilometers in thickness, oriented by flow parallel to the subducting slab, under maximum temperatures of 880–1030 °C depending on the flow stress. These findings indicate that such a B-type layer could constitute a dominant source of seismic S- and P-wave anisotropy in mantle wedge regions.

Stress due to the interseismic back slip and its relation with the focal mechanisms of earthquakes occurring in the Kuril and northeastern Japan arcs

AbstractAssuming a uniform and inhomogeneous distribution of back slip, based on a realistic configuration of the plate boundary that was newly determined from the distribution of earthquakes, we have investigated if the stress due to the interseismic back slip can explain the focal mechanisms of earthquakes occurring in the Kuril and northeastern Japan arcs. The inhomogeneous back-slip model was taken from the distribution of slip deficit estimated from GPS data collected in recent years. We examined the change in the Coulomb Failure Function (ΔCFF) for several target faults to evaluate the fitness of the models to the observed focal mechanisms. The results of ΔCFF are clearly different in the two back-slip models near the down-dip end of the locked zone. Despite this difference, the models can generally explain the earthquake focal mechanisms in the overriding plate as well as those on the megathrust plate boundary. However, they cannot explain the focal mechanisms of intermediate-depth earthquakes in the double-planed deep seismic zone. Neither can they fully explain the focal mechanisms with the pressure axis parallel to the trench that are observed at the junction of the Kuril and northeastern Japan arcs. Other tectonic processes, such as the transcurrent movement of the forearc sliver along the Kuril arc, may be involved in producing the stress anomaly.

Postcollisional exhumation history of the Tanzawa Tonalite Complex, inferred from (U‐Th)/He thermochronology and fission track analysis

The Tanzawa Mountains in the South Fossa Magna region, in the middle of the Japan Arc, were formed due to collision of the Izu‐Bonin Arc against the Japan Arc during the last several million years. The Tanzawa Tonalite Complex is exposed in the Tanzawa Mountains and is inferred to represent middle crust of the Izu‐Bonin Arc. We performed new thermochronometry, consisting of (U‐Th)/He dating, fission track dating, and fission track length analysis for the complex to investigate the exhumation history of the Tanzawa Tonalite Complex caused by arc‐arc collision. Typical (U‐Th)/He ages were 2.0 ± 0.2 Ma on apatite, 3.3 ± 0.2 Ma on zircon, and 4.5 to 6.9 Ma on zircon using the fission track method. The mean fission track length was 9.7 ± 0.6 μm (2 SE). The cooling rate of the tonalite has been calculated to be in the range of 20°–60°C Ma−1 from ∼7 to 3.3 Ma, ∼80°C Ma−1 from 3.3 to 2.0 Ma, and ∼30°C Ma−1 from 2.0 to 0 Ma assuming a surface temperature of 10°C. The exhumation rate has been calculated to be 0.5 to 1.5 mm a−1 from ∼7 to 3.3 Ma, ∼2 mm a−1 from 3.3 to 2.0 Ma, and ∼0.8 mm a−1 from 2.0 to 0 Ma with an assumed geothermal gradient of 40°C km−1. The exhumation rate has not changed significantly despite the simultaneous tectonic changes around the Tanzawa Mountains after ∼7 Ma.

Comparative morphotectonics in the Himalayan foreland and the forearc of Southwest Japan

The Southwest Japan, which is developed along a subduction boundary, is not a typical island arc. It has rich granitoid and crystalline, poor active volcano, thick upper crust and so on. These geophysical characteristics are in common with a collision mountain range such as the Himalayas. Geodetic surveying and seismology in SW Japan are ahead with those in the Himalayas. On the other hand, the Himalayan front that is a subaerial plate boundary has advantage for geologic and geomorphic studies. Therefore, comparative studies in the two boundaries aid to clarify their active tectonics by recovering the deficiency of each other. Reflecting crustal thickness and shortening rate, the Himalayas is about twice larger in relief energy and far shorter in topographic wavelength than the SW Japan arc. In a seismotectonic sense of the boundary fault, the arc is divided into proto-thrust zone, imbricate thrust zone, multi-decollement zone, and earthquake thrust zone (Kagami et al. 1983). Morphostructural similarity in the Himalayas and the SW Japan is obvious as close to the plate boundaries. The proto-thrust zone is just on the plate boundary and is filling up with thick sediments. These characteristics are in common with the Ganges foredeep. Geostructure and morphology on the imbricate thrust zone is similar to those of the Sub-Himalaya. Both areas are deformation fronts with relatively low relief, and consist of Neogene-Quarternary deposits, which are sliced by imbricate thrusts in piggyback sequence. In a sense of lithostrigraphy, the area above the multi-decollement zone can not correlate to the Lesser Himalaya directly. However, nappe structure and its related crustal processes in the Lesser Himalaya resemble the development of the multi-decollement. In addition, its geomorphic pattern composed of the Mahabharat Range and the midland depression also corresponds to a series of outer ridges (Tosa-bae etc.) and forearc basins (Muroto trough etc.) on the decollement zone. The area above the earthquake thrust zone forms steep slope from the Muroto Trough to the Shikoku Island. Relief energy of the slope is the largest in the SW Japan arc. Though there is rare similarity in lithostratigraphy, south face of the Higher Himalaya may equivalent to the slope on the earthquake thrust zone by its remarkable topographic gap. The above mentioned meso-scale morphotectonic features, which are over 10 km in topographic wavelength, are essentially formed by stable slip of flat-lying boundary faults. A dislocation model (Kimura and Komatsubara 2000) can reproduce it. In this model, ridges and basins and of the Himalayan foreland are mainly simulated by slip on thrust ramps branched off from the decollement. Also, slip on the decollement may be considered. Its topographic profile is in accord with the interseismic (post-seismic) crustal movement. On the other hand, remarkable uplift by the 1905 Kangra earthquake, which is considered to be a huge inter-plate earthquake, was observed along topographic depressions in the sub-Himalaya (Yeats and Lillie 1991). This is simulated chiefly by rupture on the decollement beneath the Lesser to Higher Himalayas. These facts show that coseismic crustal movement is less effective to form morphotectonic sequence of the Himalayas. Similar trend is known in the SW Japan Arc. For example, seismic uplift by the 1946 Nankai earthquake (M 8.0) was only observed on the south margin of the Shikoku Island (Ohmori 1978) that is topographic depression in front of the pre-Neogene prism. Regardless, seismic slip by inter-plate earthquake occurs beneath a frontal margin of a pre-Neogene accretionary wedge, which forms as a steep topographic gap. It means that implications of the next inter-plate earthquake are hidden in the Lesser Himalaya to the southern slope of the Higher Himalaya. Intensive research of seismology and geodesy should be focused to these zones. In addition, the areas above the earthquake thrust zone and their backarc side, there are minor active faults, which are causes of intra-plate earthquakes. Though the earthquakes are relatively small and their recurrent interval are usually over 103 years, they must be care as secondary important seismic hazards. Their behavior can be clarified by geomorphic and trenching survey.

40Ar-39Ar and K-Ar geochronology for plutonic rocks in the central Abukuma Plateau, northeastern Japan

The Abukuma Plateau is one of the major granitic terranes in the Japan Arc. Older granitic rocks (foliated and intermediate), younger granitic rocks (massive and felsic), and minor gabbroic rocks are complexly distributed in this region. In order to examine the cooling histories of the plutons, we have determined four 40Ar-39Ar hornblende ages and five K-Ar biotite ages of the plutonic rocks in the central Abukuma Plateau. The results are as follows: Utsushiga-take gabbroic body: 103.8 ± 0.5 Ma (hbl) Nagaya body (older): 97.3 ± 0.6 Ma (hbl), 93.4 ± 3.1 Ma (bt) Shikayama body (older): 103.0 ± 0.4 Ma (hbl), 86.1 ± 3.8 Ma (bt) Ishimori body (older): 99.0 ± 0.5 Ma (hbl), 94.2 ± 2.6 Ma (bt) Miharu body (younger): 91.3 ± 2.3 Ma (bt) Hatsumori body (younger): 90.5 ± 2.6 Ma (bt) (hbl, 40Ar-39Ar hornblende age; bt, K-Ar biotite age).On the basis of these analytical results and the petrography of the samples, we made the following observations:(1) The cooling rates of the Nagaya and Ishimori bodies (56 and 46 °C/m.y.) are equivalent to the common cooling rates of the Japanese granitic batholiths. The cooling rate of the Shikayama body (13 °C/m.y.) is slow because its volume is large.(2) The excellent 40Ar-39Ar plateau age of and the scarcity of metamorphic minerals in the Utsushiga-take gabbroic body suggest that the hornblende age is not a rejuvenated age but the original cooling age.(3) We were unable to detect any distinct geochronological boundary between the older and younger granitic rocks in the central Abukuma Plateau, in terms of the K-Ar biotite ages.

海嶺の沈み込みと三波川‐領家変成岩および花崗岩の成因‐広域ダイナミクス，物質循環との関連性‐

The origin of the Sanbagawa-Ryoke metamorphic rocks and associated granitic rocks is discussed with respect to thermal structure and water cycling in subduction zones. First, the relationship between thermal structure and water cycling is summarized based on previous studies of numerical modeling for three different settings along the Japan arcs. This shows that water cycling becomes shallower in a warmer environment, which has been demonstrated or supported by studies on seismic structures and distribution of volcanoes. In a very hot environment associated with a young (<10 Ma) plate, including ridge subduction, arc magmatism is shut down to switch to granitic magmatism and regional metamorphism in the forearc region, as both heat and water are supplied to the region. Numerical modeling studies show that a paired metamorphism can occur associated with ridge subduction in a single forearc domain within a relatively short period (a few tens of million of years), which explains the spatial association of Sanbagawa (high-P type) and Ryoke (high-T type) metamorphic rocks associated with granitic batholiths, their similar metamorphic ages, and a common mode of syn-metamorphic deformation (prolate strain). Therefore, in terms of thermal structure and water circulation, arc magmatism and regional paired metamorphism associated with granitic magmatism are regarded as representing different stages of a series of thermal and fluid processes in subduction zones. At any stage of these processes, even after major dehydration of the subducting slab and the overlying mantle wedge, nominally anhydrous minerals, such as olivine, pyroxene, and garnet, can carry a significant amount of water (up to several 1000 ppm) to the deep mantle. Recent geochemical and statistical studies have revealed that such signatures of deeply subducted fluid and associated elements are indeed identified in oceanic basalts as exits of global material circulation. Based on an understanding of the thermal structure and water circulation in subduction zones, it is argued that the fluid processes and slab configuration (e.g., cold hard slab vs. hot soft slab) exercise great control on large-scale dynamics in subduction zones, such as formation of backarc basins (e.g., Japan Sea), and wet region representing extensive magmatism with abundant volatile components over eastern Asia. Regional metamorphism, arc magmatism, large-scale dynamics in Eastern Asia, and global material circulation are potentially all connected with each other, irrespective of qualification or disqualification of the model or working hypothesis discussed in this paper.