Japan Arc Research Articles

Strain buildup and release in the Northeast Japan arc on a geologic time scale with implications for gigantic subduction earthquakes

Strain buildup and release in the Northeast Japan arc on a geologic time scale with implications for gigantic subduction earthquakes

Sr and Nd isotopic compositions of mafic xenoliths and volcanic rocks from the Oga Peninsula, Northeast Japan Arc: Genetic relationship between lower crust and arc magmas

Whole-rock and constituent hornblende and plagioclase geochemical and isotopic compositions of 52 mafic xenoliths from the Ichinomegata maar in the Oga Peninsula, located on the backarc side of Northeastern Japan, were investigated to further understand the nature of lower crustal materials beneath the Oga Peninsula. The inter-rock variations in isotopic compositions (87Sr/86Sr ratios of 0.703245–0.705246 and 143Nd/144Nd ratios of 0.512910–0.512608) correlate negatively with 87Rb/86Sr and 147Sm/144Nd ratios. A continuous and arcuate trend in a Sr–Nd isotope diagram suggests a two-component mixing curve is present; at lower 87Sr/86Sr and higher 143Nd/144Nd ratios, this trend extends towards and partly overlaps Quaternary volcanic rock compositions from the Toga, Ichinomegata (xenolith-hosting pumice), and Kampu volcanoes on the Oga Peninsula (herein, Oga volcanic rocks). This overlapping suggests a common control on the isotopic variations within both xenoliths and volcanic rocks. This common control is most likely to be the metasomatism of intact original lower crustal material by parental magmas of the Oga volcanic rocks, herein termed the Oga parental magma, in addition to the contamination of the Oga parental magma by the crustal material after contact between the two. This metasomatism also caused isotopic re-homogenization of these constituent minerals, meaning hornblende and plagioclase within individual xenoliths have the same Sr–Nd isotopic compositions; i.e., they show no intra-rock variations, suggesting thermal re-setting.However, inter-rock variations imply that full metasomatism and destruction of the original isotopic and geochemical characteristics of the lower crust did not occur. These inter-rock variations are consistently present as changes in the geochemistry of constituent minerals, with K2O, Rb, Sr, Sm, and Nd concentrations varying in hornblende, and anorthite contents (An %) varying in plagioclase. The original lower crustal material, as one end-member of a mixing curve, is assumed to have a 87Sr/86Sr ratio of 0.705250 and a 143Nd/144Nd ratio of 0.512570, whereas the Oga parental magma, forming the other end-member on the mixing curve, has a 87Sr/86Sr ratio of 0.702958 and a 143Nd/144Nd ratio of 0.512933, as represented by Oga volcanic rocks with the lowest 87Sr/86Sr and highest 143Nd/144Nd ratios (stage 3 lavas from Kampu volcano near Ichinomegata maar). The isotopic compositions of other Oga volcanic rocks vary from the most contaminated Toga rhyolitic pumices (87Sr/86Sr of 0.703723–0.703885 and 143Nd/144Nd of 0.512906–0.512847) through to the Ichinomegata host pumice (87Sr/86Sr of 0.703398 and 143Nd/144Nd of 0.512895). This research indicates that both the Ichinomegata mafic xenoliths and the Oga volcanic rocks have undergone isotopic changes, creating an overlap in compositions caused by metasomatism and contamination, respectively. This relationship between lower crustal and volcanic rocks can be also be applied to trench-side volcanic rocks. If the Ichinomegata lower crustal material extends to the trench side of the NE Japan Arc, the island-arc tholeiites typically found within the volcanic front probably also contain a similar, if not identical, lower crustal component.

Origin of Late Oligocene to Middle Miocene Adakitic Andesites, High Magnesian Andesites and Basalts from the Back-arc Margin of the SW and NE Japan Arcs

Late Oligocene to Middle Miocene adakitic andesites are found in the southern part of Okushiri Island, the northern Noto Peninsula and in the Toyama region in the present-day back-arc margin of the SW and NE Japan arcs. On Okushiri Island, adakitic andesite is accompanied by moderately alkaline basalt, whereas on the Noto Peninsula, adakitic andesite has been erupted along with high magnesian andesite (HMA), bronzite andesite and tholeiitic basalt. Adakitic andesites from all three locations are characterized by high Sr/Y and low Y, and have higher MgO contents than adakitic melts generated by experimental melting of metabasalt and amphibolite. They also have higher Ni and Cr contents than either Archaean tonalite–trondhjemite–granodiorite (TTG) suites or Early Cretaceous adakitic granites, which have been attributed to partial melting of subducted oceanic crust. The Noto Peninsula adakitic andesite has Sr and Nd isotopic compositions identical to normal mid-ocean ridge basalt (N-MORB), whereas the Okushiri Island and Toyama adakitic andesites are more isotopically primitive than N-MORB. The Noto Peninsula primary adakitic melt was derived from subducted oceanic N-MORB crust, whereas the Okushiri Island and Toyama primary adakites are interpreted as melts of subducted N-MORB and sediment that have subsequently interacted with the overlying mantle wedge peridotite. To explain the comagmatism of adakite, HMA and basalt, the following model is proposed. A hydrated adakitic diapir ascends from the subducting slab and is heated because it enters the overlying hot mantle wedge. The subsequent establishment of thermal and H2O gradients in the adakitic diapir and surrounding mantle wedge peridotite results in concurrent generation of adakitic andesite magma in the inner adakitic diapir region (low temperature and high H2O content), HMA and bronzite andesite magmas in the intermediate peridotite region (intermediate temperature and H2O content), and tholeiitic basalt magma in the outer peridotite region (high temperature and lower H2O content). Comagmatic adakite and mildly alkaline basalt are found in cooler and wetter adakitic diapirs and hotter and drier peridotite regions respectively. The most likely tectono-magmatic situation for the genesis of adakitic magmas in this example of a cool subduction zone involves upwelling of hot asthenosphere into the subcontinental lithosphere beneath the back-arc side of the NE Japan arc and northern end of the SW Japan arc, during the period spanning the pre-Japan Sea opening to syn-opening stages. The unusually high temperature conditions established in the mantle wedge owing to upwelling of hot asthenosphere caused partial melting of the relatively cool subducting Pacific plate, resulting in the generation of adakitic magmas.

Relocating the 2011 Tohoku-oki earthquakes (M 6.0–9.0)

The great Tohoku-oki earthquake (Mw 9.0) on 11 March 2011 was accompanied by nearly 100 large foreshocks and aftershocks (M≥6.0) in the Northeast Japan forearc. In this study we relocated 77 of these large events using three different velocity models. An updated three-dimensional velocity model significantly reduced the travel-time residuals in the earthquake relocation. We found that the large thrust earthquakes are generally located in or around high-velocity patches, suggesting that the generation of the large thrust earthquakes is controlled by structural heterogeneities in the megathrust zone. The high-velocity patches may represent asperities in the megathrust zone where the subducting Pacific plate and the overriding Okhotsk plate may be coupled strongly. Many non-thrust earthquakes occurred in the crust of the overriding Okhotsk plate, indicating that the stress field there has been changed by the Tohoku-oki mainshock. The present results may improve our understanding of the subduction dynamics and seismotectonics of the Northeast Japan arc.

A3-07 東北日本弧の第四紀マグマのH2O/K2O比の水平変化(火山の岩石学2,口頭発表)

A3-07 東北日本弧の第四紀マグマのH2O/K2O比の水平変化(火山の岩石学2,口頭発表)

Primary Magmas at the Volcanic Front of the NE Japan Arc: Coeval Eruption of Crustal Low-K Tholeiitic and Mantle-derived Medium-K Calc-Alkaline Basalts at Azuma Volcano

It has long been thought that the low-K tholeiitic (TH) and medium-K calc-alkaline (CA) lavas in the NE Japan arc were produced by fractional crystallization from mantle-derived basalt magmas, and that the latter formed from mixing of mafic and felsic magmas, both derived from a common primary TH basalt through fractionation. An alternative view was recently proposed on the basis of Sr isotope microanalysis of plagioclase phenocrysts from the Zao volcano, suggesting that (1) the low-K TH basaltic andesites formed by melting of lower crustal amphibolite and that (2) the medium-K CA basalts to andesites formed by mixing of mantle-derived basalt and crustal TH melts. To investigate further the origin of the ‘primary’ low-K TH and medium-K CA basalts, we investigated basalts and andesites from Azuma volcano. Azuma is a Quaternary eruption center at the volcanic front in the NE Japan arc that has erupted two types of basalt: (1) radiogenic-Sr (€87Sr/€86Sr = 0·7058–0·7062) low-K TH basalt lavas without evidence of magma mixing and assimilation; (2) unradiogenic-Sr (€87Sr/€86Sr = 0·7039–0·7041) medium-K CA basalt lavas with subtle evidence for magma mixing. Associated intermediate lavas are voluminous and are all (3) mildly radiogenic-Sr (€87Sr/€86Sr = 0·7044–0·7055) medium-K andesites, all of which have CA affinities with evidence for rigorous magma mixing but no crustal assimilation. The low-K TH basalt has an isotopic composition similar to that of crustal granitoids beneath Azuma and has a composition indicating that it potentially formed from a high-degree lower crustal amphibolite melt. The medium-K CA basalt has a basaltic groundmass with Mg-rich olivine (Fo89) and calcic plagioclase phenocrysts (An90) and the most unradiogenic Sr (€87Sr/€86Sr = 0·7037–0·7038), suggesting that it originated from a primary mantle melt. Major and trace element microanalysis of the basaltic groundmass indicates that the primary magma composition is close to high-K. We conclude that the mantle-derived basalt at Azuma is the result of a high- to medium-K magma that was later mixed with a low-K TH basalt melt from the amphibolitic lower crust to form medium-K CA basalts and andesites. This supports the view of a lower crustal origin of the low-K TH basalts and simultaneously requires a reappraisal of the origin of the across-arc variation in K contents of the mantle-derived primary arc basalts, as the high- to medium-K CA basalt is geochemically fairly similar to the high-K rear-arc basalt in the NE Japan arc.

Fluid upwelling beneath arc volcanoes above the subducting Philippine Sea Plate: Evidence from regional electrical resistivity structure

[1] At subduction zones, aqueous fluids released from the subducting slab play an important role in island arc volcanism, triggering partial melting and accretion magmatism. We used the Network-magnetotelluric (MT) method to obtain the deep (>100 km), large-scale electrical resistivity structure beneath Kyushu, Japan, with the aim of identifying regions of deep fluids and magma that feeds subduction zone volcanoes in the region. Network-MT observations, in which dipoles of about 10–30 km in length were used to measure the electric potential difference, were performed over all of Kyushu, in the Southwest Japan Arc, where the Philippine Sea Plate (PSP) is subducting beneath the Eurasian Plate at a high angle. Using the Network-MT data, we obtained the large-scale electrical resistivity structure along four profiles across each of four Quaternary active volcanoes at the volcanic front of the Kyushu subduction zone. The resistivity models have two features in common: a conductor beneath each volcano, whose base extends to the backarc side, and a resistor along the hinge line of the subducting PSP in the forearc. The conductor indicates regions of fluid released from the slab or partial melting of the mantle, representing a magma source for subduction zone volcanoes in this region.

Petrology and Geochemistry of Adakitic Dacites and High-MgO Andesites, and Related Calc-alkaline Dacites from the Miocene Okoppe Volcanic Field, N Hokkaido, Japan

In the Okoppe area of North Hokkaido, Japan, the eruption of adakitic and calc-alkaline dacites was followed by high-Mg andesite (HMA) and calc-alkaline dacite during the Middle Miocene (12–10 Ma). Adakitic dacite is characterized by high Sr/Y and low Y, with Sr and Nd isotopic compositions identical to those of mid-ocean ridge basalt. It has higher MgO contents than adakites generated by experimental melting of metabasalt and amphibolite, and higher Ni and Cr contents than either Archean trondhjemite–tonalite–granodiorite or Early Cretaceous adakitic granites, which are considered to represent partial melts of subducted oceanic crust. This provides compelling evidence that adakitic dacite magma from Okoppe resulted from interaction of a melt derived from subducted oceanic basaltic crust and the overlying mantle wedge peridotite, with little modification to the adakitic melt signature and Sr and Nd isotopic values. The compositional variations in the Toyono adakitic dacite and associated calc-alkaline dacite probably resulted from mixing of the reacted magma and an evolved silicic dacite magma formed by fractional crystallization of the reacted magma. A disequilibrium phenocryst assemblage in the HMA may result from mixing of boninite and silicic andesite that resulted from crustal melting. Calc-alkaline dacites associated with the HMA were derived by fractional crystallization of silicic andesite and assimilation of crust with an enriched Sr isotopic signature. The most likely tectono-magmatic model for the production of adakitic dacite and HMA involves upwelling of hot asthenosphere into the subcontinental lithosphere beneath North Hokkaido and the back-arc side of the NE Japan arc, coincident with the spreading of the Kurile back-arc basin and Japan Sea back-arc basin. This resulted in a high geothermal gradient in the mantle wedge beneath North Hokkaido. The subsequent melting of a limited part of the cool oceanic crust subducting beneath Hokkaido produced adakitic magmas, which interacted with the overlying mantle wedge peridotite. These magmas subsequently reacted with an evolved calc-alkaline melt en route to the surface. Boninitic magma derived from the ascending hot asthenosphere in part reacted with crust-derived silicic andesitic magma, undergoing simultaneous fractional crystallization.

Volcanic CO2 flux from Japan Arcs

Volcanic CO2 flux from Japan Arcs

Quantifying crustal extension and shortening in the back‐arc region of Northeast Japan

The Northeast Japan (NEJ) arc has experienced strong extension in the early to middle Miocene time and contractive deformation since Pliocene time. We determine the amounts of the Miocene extension and Pliocene contraction along four transects across the NEJ arc by area‐balancing restoration using seismic reflection, gravity, and surface geologic data. We find that the Miocene extension is as much as 31–56 km while the Pliocene‐Quaternary contraction is only 10–15 km. The style of the Miocene extension in the back‐arc region of NEJ is highly asymmetric; the zone of concentrated extension is characterized by a breakaway fault on the west, a rollover basement anticline on the east, and abnormally deep (∼10 km) basins and strongly rotated fault blocks in between, suggesting the existence of a large‐scale detachment fault at a mid‐crustal level beneath the extended zone. We reveal that the Pliocene‐Quaternary contractive surface deformation in the back‐arc region of NEJ resulted principally from positive tectonic inversion of the underlying detachment fault. Previous works have suggested that Andean‐type orogens have often experienced negative and positive inversions alternately in their back‐arc regions. Although the former has been well documented, for example, for the Cenozoic North American Cordillera, the latter has rarely been reported because subsequent contraction has obscured the older structures. Since the NEJ arc is still at an incipient stage and hence its older structures are well preserved, our study provides an important example of positive tectonic inversion of Andean‐type orogens.

U-Pb zircon ages of Abukuma granitic rocks in the western Abukuma plateau, northeastern Japan Arc

U-Pb zircon ages of Abukuma granitic rocks in the western Abukuma plateau, northeastern Japan Arc

Three-dimensional structure of so-called double seismic zone of deep earthquakes in Northeast Japan.(<Special issue>Crustal structure and geotectonic history, Northeast Japan Arc)

Three-dimensional structure of so-called double seismic zone of deep earthquakes in Northeast Japan.(<Special issue>Crustal structure and geotectonic history, Northeast Japan Arc)

日本海溝周辺における太平洋プレート内の地震発生機構

A double-planed shallow seismic zone has been found in the northeastern Japan forearc region. However, the characterizations of the focal mechanisms of earthquakes in this zone, especially the lower plane events, cannot be carried out adequately due to low seismicity and poor station coverage on the focal sphere of the onshore P-wave polarity data. In this study, we determine the focal depth using the sP depth phase and the focal mechanisms using the P-wave initial motions observed by the ocean bottom cabled seismic stations and temporary autonomous ocean bottom seismic networks off Miyagi, as well as the onshore seismic networks. Seven focal mechanism solutions of events were precisely determined. Using the classification based on the dip angles of the T, B and P axes, we classified 21 weakly constrained focal mechanisms of other events having poor station coverage. All the determined solutions were of the thrust-faulting type regardless of the focal depth. The offshore observations helped considerably in constraining the focal mechanisms of these far-offshore earthquakes, especially their rake angles. Although previous studies had reported that the focal mechanisms of the upper and lower plane events show predominantly normal and reverse faulting respectively, our result showed that the thrust events in the upper plane seem to occur on the plate boundary. We found that the focal mechanisms change from normal faulting in the upper plane to reverse faulting in the lower plane at a depth of 15km from the plate boundary, possibly defining the depth of the neutral plane in the northeastern Japan forearc region included in the source area of the 1933 Mw8.4 Sanriku earthquake. The western edge of the normal faulting events along the upper plane is located about 70km inward from the trench axis. These results suggest that the earthquake-generating stress field in the double-planed shallow and deep seismic zone in the northeastern Japan arc can be explained by the bending-unbending model of the subducting Pacific plate.

Mantle wedge structure in the Miyagi Prefecture forearc region, central northeastern Japan arc, and its relation to corner-flow pattern and interplate coupling

[1] We applied three-dimensional (3-D) seismic tomography to both active-source data and local earthquake data recorded at onshore and offshore seismic stations in the Miyagi Prefecture forearc region of central northeastern Japan to obtain a detailed seismic velocity model of the mantle wedge. The velocity model showed a clear boundary that divides the mantle wedge into an oceanward high-velocity toe and a landward low-velocity region that extends trenchward from beneath currently active arc volcanoes. The top of the seaward dipping boundary is about 20–30 km landward from the coastline, and its bottom is approximately beneath the coastline. We consider that this velocity boundary represents the trenchward limit of corner flow and the high-velocity toe represents a stagnant mantle wedge. The high spatial resolution of the tomographic image we produced allowed us to identify a spatial correlation between velocity variations in the mantle wedge immediately above the plate boundary and the spatial extent of the rupture areas of the large interplate earthquakes that occur repeatedly in the region. Our results suggest that variations of the volume fraction of serpentinization in the mantle wedge cause the irregularity of rupture patterns of megathrust earthquake sequences. We also identified a swarm of active microseismicity in the mantle wedge, well above the seismogenic plate boundary. This activity may be related to dehydration of a detached seamount.

Back-arc spreading of the northern Izu–Ogasawara (Bonin) Islands arc clarified by GPS data

We examined GPS data in the northwestern Pacific region, which includes the Izu–Ogasawara (Bonin)–Mariana (IBM) arc and the Japan arc. GPS velocity vectors on the Izu Islands, including Hachijo-jima and Aoga-shima, show systematic eastward movement deviating from that predicted by the rigid rotation of the Philippine Sea plate; this deviation supports the active back-arc spreading model suggested by previous geological studies. The results of a statistical F-test analysis with 99% confidence level showed that the forearc of the Izu Islands arc has an independent motion with respect to the rigid part of the Philippine Sea plate. We developed a kinematic block–fault model to estimate both rigid rotations of crustal blocks and elastic deformation due to locked faults on the block boundaries. The model suggests that the back-arc opening rate along the Izu back-arc rift zone ranges from 2 mm/yr at its southern end to 9 mm/yr near Miyake-jima, its northern end. It also predicts 23–28 mm/yr of relative motion along the Sagami Trough in the direction of ~ N25°W, where the Izu forearc subducts beneath central Japan. The orientation of this motion is supported by slip vectors of recent medium-size earthquakes, repeated slow-slip events, and the 1923 M = 7.9 Kanto earthquake.

A3-10 2011年東北地方太平洋沖地震に誘発された東北日本弧火山の地殻変動(巨大地震と火山,口頭発表)

A3-10 2011年東北地方太平洋沖地震に誘発された東北日本弧火山の地殻変動(巨大地震と火山,口頭発表)

P64 東北日本弧の第四紀マグマ含水量の水平変化(ポスターセッション)

P64 東北日本弧の第四紀マグマ含水量の水平変化(ポスターセッション)

Spatial distribution of stress and frictional strength along the interplate boundary in northern Japan and its correlation to the locations of large earthquakes

We investigate spatial distribution of stress along the interplate boundary between Pacific and Okhotsk plates in Tohoku and Hokkaido regions, northern Japan from earthquake focal mechanisms compiled from the National Institute for Earth Science and Disaster Prevention (NIED) of Japan. The interplate boundary in this work is inferred as the surface defining the upper boundary of the double seismic zone (Benioff Zone). To investigate spatial variation in stress field we divide the interplate boundary into 20 km × 20 km subfaults and estimate stress field in each subfault of 10 km thickness. We find clear spatial variation in stress state along the subduction zone of the Pacific plate in northern Japan. Though most of the study area is characterized by trench normal uniaxial compression, in the Hidaka Collision Zone, the signature of collision between the Kurile and Japan arcs is evident in terms of radial compression. Distribution of the angle (ψ) between the maximum principal stress direction and the fault normal shows that the large subduction earthquakes have occurred in the region where this angle ψ varies between 30 and 45°. However in the case of recent M9.1, 2011 Tohoku-Oki earthquake, this correlation is not observed. We further investigate the frictional strength along the interplate boundary. Our results suggest: (1) The fault segments with ψ between 30 and 45 ° are frictionally strong (with friction coefficients between 0.6 and 0.8), (2) The fault segments with ψ between 45 and 62 ° are intermediately strong (with friction coefficients between 0.4 and 0.6), and (3) The fault segments with ψ above 62 ° are weak (with friction coefficients below 0.4).

Geological comparative studies of Japan arc system and Kyushu-Palau arc

Based on the published data of structure geology, geochronology, petrology and isotope geochemistry, the authors of this paper have conducted studies on the tectonic evolution history of Japan arc system and Kyushu-Palau ridge (KPR). The studies show that the initial Japan arc system was resulted from the subduction of ancient Pacific plate beneath Eurasian Plate in Permian. It was part of an Andean-type continental volcanic arc which occurred in the offshore in the east of Asian during late Mesozoic era. The formation of tertiary back-arc basin (Japan Sea) resulted in the fundamental tectonic framework of the present arc system. Since Quaternary the system has been lying at E-W compression tectonic setting due to the eastward subduction of Amur Plate. It is expected that Japan arc system will be juxtaposed with Asian continent, which is similar to the present Taiwan arc system. The origin of Philippine Sea Plate (PSP) is still in debate. Some studies argued that it is a trapped oceanic crust segment, while the others insisted that it is a back-arc basin accompanied with ancient IBM arc. However, it is all agreed that the tectonic evolution of PSP started since 50 Ma, i.e., PSP has drifted from the site around equator at 50 Ma to the present site, and the subduction of PSP along Nankai trough-Ryukyu Trench beneath the Japan arc system during 6–2 Ma led to the formation of the present Ryukyu arc system. Of the PSP, the KPR has been found with the oldest rocks formed at 38 Ma. Combining with its geochemical characteristics of oceanic arc tholeiite, it is suggested that KPR is an intraoceanic volcanic arc, more specifically, a relic arc (i.e., rear arc of the ancient IBM) after rifting of ancient IBM. In addition, Amami-Daito province is of arc tectonic affinity, but has been affected by mantle plume. Therefore, based on their respective tectonic evolution history and geochemical characteristics of rock samples, it is inferred that there is no genetic relationship between Japan arc system and KPR. It is noted that rocks reflecting continental crust basement feature have been collected on the northern tip of KPR, which may be related to the process of KPR accreting on Japan arc, but the arc-continent accretion process are still at initial stage of modern continental crust accretion model. However, due to the scarcity of data of the northern tip of KPR, crustal structure of this location and its adjacent Nankai trough need to be further constrained by geophysical studies in the future.

Geochemical mapping in Shikoku, southwest Japan

Abstract Geochemical mapping of Shikoku in the SW Japan Arc was conducted using stream sediments (