Japan Arc Research Articles

Contrasting volcano spacing along SW Japan arc caused by difference in age of subducting lithosphere

The SW Japan arc built by subduction of the Philippine Sea (PHS) plate exhibits uneven distribution of volcanoes: thirteen Quaternary composite volcanoes form in the western half of this arc, Kyushu Island, while only two in the eastern half, Chugoku district. Reconstruction of the PHS plate back to 14 Ma, together with examinations based on thermal structure models constrained by high-density heat flow data and a petrological model for dehydration reactions suggest that fluids are discharged actively at depths of 90–100 km in the hydrous layer at the top of the old (> 50 Ma), hence, cold lithosphere sinking beneath Kyushu Island. In contrast, the young (15–25 Ma) oceanic crust downgoing beneath Chugoku district releases fluids largely at shallower depths, i.e. beneath the non-volcanic forearc, to cause characteristic tectonic tremors and low-frequency earthquakes (LFEs) and be the source of specific brine springs. Much larger amounts of fluids supplied to the magma source region in the western SW Japan arc could build more densely-distributed volcanoes.

Late Cenozoic Igneous Activity and Crustal Structure in the NE Japan Arc: Background of Inland Earthquake Activity

Late Cenozoic Igneous Activity and Crustal Structure in the NE Japan Arc: Background of Inland Earthquake Activity

Geological structures controlled the rupture process of the 2011 M9.0 Tohoku-Oki earthquake in the Northeast Japan Arc

By interpreting the 2D/3D seismic survey data acquired in the surrounding ocean areas of the Northeast (NE) Japan Arc, we clarified the detailed geological structure and demonstrated that the basic structure in the hanging-wall plate of the subduction system consists of many structural blocks (segments) separated by NW–SE trending large transcurrent faults (strike-slip faults). This structural configuration showed a close relationship with the distribution of foreshocks, mainshock, and aftershocks, coseismic slip models of the 2011 M9.0 Tohoku-Oki megathrust earthquake, coseismic slip area of M-7 class earthquakes, quasi-static slip rates, back slip rate, and seismic tomography images. In addition, the coseismic slip models revealed that the trenchward forearc of the structural blocks between the Offshore Hidaka tectonic line and the Honjo-Sendai tectonic line fitted well with the coseismic slip area of the 2011 Tohoku-Oki earthquake. These findings suggest that the structural blocks bounded by these two tectonic lines slipped rapidly trenchward when the mainshock occurred. The M7 earthquakes were also concentrated along these two tectonic lines, thereby suggesting a close relationship between seismic activity and the inherited geological structure of the overriding plate in the NE Japan forearc.

Oxidation during magma mixing recorded by symplectites at Kusatsu\u2013Shirane Volcano, Central Japan

Kusatsu–Shirane Volcano is an active Quaternary andesitic-to-dacitic volcano located in the Central Japan Arc. We conducted a detailed petrological investigation of orthopyroxene (opx)–magnetite (mt) symplectites associated with olivine in the Sessho lava, an andesitic lava flow from Kusatsu–Shirane. We concluded that the symplectites are pseudomorphs after olivine and were formed through the breakdown of olivine in a mafic magma as a result of oxidation during mixing with a felsic magma. Various olivines and opx–mt symplectites that show different stages of the progressive breakdown reaction of olivine coexist in a single lava flow. We suggest that basaltic recharge into the magma reservoir beneath Kusatsu–Shirane occurred repeatedly, leading to a hybrid andesite magma with different types of olivine and symplectite being erupted at Kusatsu–Shirane Volcano.

Spatial variations of incoming sediments at the northeastern Japan arc and their implications for megathrust earthquakes

Abstract The nature of incoming sediments is a key controlling factor for the occurrence of megathrust earthquakes in subduction zones. In the 2011 Mw 9 Tohoku earthquake (offshore Japan), smectite-rich clay minerals transported by the subducting oceanic plate played a critical role in the development of giant interplate coseismic slip near the trench. Recently, we conducted intensive controlled-source seismic surveys at the northwestern part of the Pacific plate to investigate the nature of the incoming oceanic plate. Our seismic reflection data reveal that the thickness of the sediment layer between the seafloor and the acoustic basement is a few hundred meters in most areas, but there are a few areas where the sediments appear to be extremely thin. Our wide-angle seismic data suggest that the acoustic basement in these thin-sediment areas is not the top of the oceanic crust, but instead a magmatic intrusion within the sediments associated with recent volcanic activity. This means that the lower part of the sediments, including the smectite-rich pelagic red-brown clay layer, has been heavily disturbed and thermally metamorphosed in these places. The giant coseismic slip of the 2011 Tohoku earthquake stopped in the vicinity of a thin-sediment area that is just beginning to subduct. Based on these observations, we propose that post-spreading volcanic activity on the oceanic plate prior to subduction is a factor that can shape the size and distribution of interplate earthquakes after subduction through its disturbance and thermal metamorphism of the local sediment layer.

Three‐Dimensional Electrical Resistivity Structure Beneath a Volcanically and Seismically Active Island, Kyushu, Southwest Japan Arc

AbstractTo understand the subduction dynamics around Kyushu Island, Southwest Japan Arc, the distribution of fluids is imaged by resolving three‐dimensional (3‐D) electrical resistivity structure beneath a subregion the island, including volcanic and nonvolcanic areas. The nonvolcanic region in central Kyushu is situated between two regions with numerous Quaternary volcanic structures along a volcanic front aligned N30°E and related to subduction of the Philippine Sea Plate. Two source areas of slow slip events lie at depths of 20–40 km along the eastern coast of Kyushu, whereas earthquakes spread along the subducting Philippine Sea Plate plane over the eastern coast and toward the volcanic front at depths of 20 to >100 km. Our 3‐D resistivity model detects prominent conductors, indicating the presence of fluids, which might control volcanic and seismic activity. The most robust features of the model are three regions of high conductivity. A magma system and a fluid system are imaged as two vertical conductors extending from a depth of ~70 km, both related to slab‐derived fluid and located along the volcanic front, beneath the volcanic and nonvolcanic regions, respectively. The third conductor, which suggests the presence of slab‐derived fluid and hydrated sediments at the plate interface and the presence of serpentinite in the mantle wedge, appears in an area devoid of slow slip events at depths of <~30 km along the southeastern coast.

(U\u2013Th)/He thermochronometric mapping across the northeast Japan Arc: towards understanding mountain building in an island-arc setting

Topographic relief in arc-trench systems is thought to be formed by plate subduction; however, few quantitative investigations have so far been reported, with respect to the related mountain building process. This study applies apatite and zircon (U–Th)/He thermochronometry (AHe, ZHe, respectively) to Cretaceous granite rocks in the north part of the northeast (NE) Japan Arc to reveal its cooling/denudation history. Weighted mean AHe ages ranging from 88.6 to 0.9 Ma and ZHe ages from 83.9 to 7.4 Ma were determined for 10 rock samples. Using the AHe data, denudation rates were obtained for each sample. On the fore-arc side, denudation rates of < 0.05 mm/year were calculated, indicating a slow denudation process since the Paleogene. However, in the Ou Backbone Range and on the back-arc side, denudation rates at > 0.1–1.0 mm/year were computed, probably reflecting a recent uplift event since ~ 3–2 Ma. These data indicate a clear contrast in thermal and denudation histories between the tectonic units in this study area, similar to that previously reported from the southern part of NE Japan Arc. A comparison of the thermal/denudation histories between the N- and S- traverses, revealed the arc-parallel trend, the uplift model of the volcanic arc, and some minor variations of thermal/denudation histories in each tectonic unit. This study offers some further insights into the understanding of tectonic processes in an island-arc setting.

東北日本弧南端部，鹿沼地域に分布する高Sr・低Nd同位体比を有する中新世玄武岩

東北日本弧南端部，鹿沼地域に分布する高Sr・低Nd同位体比を有する中新世玄武岩

Termination of intra‐arc rifting at ca 16 Ma in the Southwest Japan arc: The tectonostratigraphy of the Hokutan Group

AbstractMiocene intra‐arc rifting associated with the opening of the Japan Sea formed grabens in several areas in Southwest (SW) Japan, but the extensional tectonics of the arc are still not well understood. In this study, we first document the tectonostratigraphy of the Hokutan Group in the northwestern part of the Kinki district, and demonstrate the termination of extensional tectonics at ca 16.5 Ma, as inferred from grabens in the lower part of the group being unconformably overlain by sediments of the upper part. Second, we review early Miocene grabens in SW Japan to suggest that intra‐arc rifting was abandoned at ca 16 Ma, essentially simultaneously with the end of rotation of the SW Japan arc as evidenced by paleomagnetic studies. The lesser numbers of grabens and reduced thicknesses of graben fills suggest that extensional deformation of the SW Japan arc was significantly weaker than that of the Northeast (NE) Japan arc, which was broken into blocks, indicating various degrees of paleomagnetic rotation within NE Japan. The weak deformation has allowed paleomagnetic studies to infer the coherent rotation of the SW Japan arc.

Control of Volcanic Activity by Crustal Structure: Inference from the Izu‐Bonin‐Mariana and Northeast Japan Arcs

AbstractVolcanic activities in the Izu‐Bonin‐Mariana (IBM) oceanic arc and the Northeast Japan (NEJ) continental arc are different in spite of rather constant subduction rates of the same Pacific plate. IBM volcanoes are much more voluminous than NEJ volcanoes; IBM magmas are predominantly basaltic whereas NEJ magmas are andesitic. We propose that the crustal density structures play a key role in those differences. The crustal density structures were estimated based on seismic P‐wave velocity structure and thermodynamic calculation, suggesting that the neutral buoyancy level of a primitive, mantle‐derived basalt, where a magma chamber forms, is shallower in the IBM arc than the NEJ arc. If a magma that differentiates and attains water oversaturation could erupt at the surface, a larger amount of crystallization should occur for the magma in a deeper chamber. This idea explains that erupting magma in the IBM arc is more voluminous and more mafic than those in the NEJ magma.

U–Pb ages of Miocene near-trench granitic rocks of the Southwest Japan arc: implications for magmatism related to hot subduction

AbstractWe present a new dataset of zircon U–Pb ages that document igneous activity in the SW Japan arc during middle Miocene time and discuss its relationship with the opening of the Japan Sea, Philippine Sea plate migration, and subduction of the young hot lithosphere of the Shikoku Basin. Precursory magmatism, characterized by dike and stock intrusions, startedc. 15.6 Ma in both Kyushu and the Kii Peninsula. Most plutonism occurred between 15.5 and 13.5 Ma in an area 600 km long and 150 km wide. No along-arc trend was recognized in the U–Pb ages of igneous activity near the trench. Our data indicate that all near-trench middle Miocene igneous activity occurred immediately after the opening of the Japan Sea ceased, i.e. after 16 Ma, implying that melt extraction and the emplacement of granites in the near-trench region had some influence on the back-arc opening. Our data also imply that the trench–trench–trench-type triple junction between the Japan arc and the Izu–Bonin–Mariana arc must have reached the east side of the Kii Peninsula by 15.6 Ma. The wide distribution of contemporaneous magmatic activity along the arc requires a trench-parallel heat source, such as the subduction of a trench-parallel ridge or a young and highly segmented ridge–fracture zone system in addition to the hot wedge mantle condition related to the opening of Japan Sea.

Geochemical variations of the Quaternary Daisen adakites, Southwest Japan, controlled by magma production rate

The late Cenozoic Southwest Japan arc was formed by hot subduction of the young Shikoku Basin slab, and Daisen Volcano has erupted exclusively adakitic products since ca.1.0 Ma under this subduction regime. Although previous studies have revealed that the adakites of this arc were generated by melting of the subducted basaltic crust and incorporation of slab sediments, this model cannot simply explain the observed geochemical variations of the Daisen adakites. We determined chronological variations in the major and trace element and Sr-Nd-Pb isotopic compositions of the Younger stage products of Daisen (<210 ka) based on a newly reported stratigraphy. Our result shows that the Daisen adakites are divided into higher-K and lower-K groups; the former having higher in TiO2, Sr, Ba, Nb and La contents and more depleted in Sr-Nd-Pb isotopic compositions than the latter. Mass balance estimation suggests that the lower-K adakites represent a higher degree of lower crustal assimilation than the higher-K adakites. Furthermore, the lower-K adakites erupted during a period of increased eruptive activity (100–27.8 ka), whereas the higher-K adakites were erupted before and after that period. We attribute this correlation to an increased degree of lower crustal assimilation resulting from an increased rate of magma intrusion into the crust during the period of increased activity. Following this flare-up period, the eruption rate and degree of assimilation declined, and eruptive activity ceased at 20.8 ka.

Subduction polarity of the Ailaoshan Ocean (eastern Paleotethys): Constraints from detrital zircon U-Pb and Hf-O isotopes for the Longtan Formation

Abstract The Paleotethys Ailaoshan Ocean separated the South China and Indochina blocks during the late Paleozoic. Uncertainty remains regarding subduction of this ocean—whether it was subducted eastward beneath the South China block or westward beneath the Indochina block. In this study, we present new detrital zircon U-Pb age, and Hf and O isotope data from the Longtan Formation, which was recognized to be deposited before the ocean closed. Our results show that the formation can be divided into three units: Unit 1 is distributed west of the suture and dominates the area; it contains major age peaks at 290–250 Ma and minor multiple old age peaks. Unit 2 consists of a minor distribution west of the suture, and it shows a dominant 250 Ma age peak; old zircons are very few or not present. Their Hf and O isotopic signatures are similar to those of unit 1. Unit 3 is distributed east of the suture and is characterized by a single distinct ca. 240 Ma age peak with almost no Precambrian zircons. We interpret that units 1 and 2 were likely deposited in a back-arc and forearc basin, respectively, and a volcanic arc developed on the eastern margin of the Indochina block, similar to the present-day northeastern Japan arc. Meanwhile, unit 3 was likely deposited in a forearc basin on the western margin of the South China block. Therefore, the Ailaoshan Ocean may undergone bipolar subduction both westward and eastward beneath the Indochina and South China blocks, respectively.

Arc-arc collision caused the 2018 Eastern Iburi earthquake (M 6.7) in Hokkaido, Japan

Inland crustal earthquakes usually occur in the brittle upper crust (0–20 km depths), but the 6 September 2018 Eastern Iburi earthquake (M 6.7) took place in southern Hokkaido with a focal depth of ~37 km, causing 41 fatalities and serious damage to the local infrastructure. The reason why this event was so deep and its causal mechanism are still unclear. In this work we study the three-dimensional P and S wave seismic attenuation (1/Q) structure in the source zone of the 2018 Iburi earthquake. Our results show that this event occurred at the boundary between the Sorachi-Yezo belt (low Q) and the dipping Northeastern (NE) Japan arc (high Q) that is descending beneath the Kuril arc. The collision between the NE Japan and Kuril arcs as well as fluids from dehydration of the subducting Pacific plate caused this big event and its unusual focal depth. Similar attenuation structures are revealed in source zones of the 1970 Hidaka earthquake (M 6.7) and the 1982 Urakawa-oki earthquake (M 7.1), suggesting that they were caused by similar processes. We think that large earthquakes will take place again on the active thrust faults in southern Hokkaido in the coming decades. Hence, we should pay much attention to the seismic risk and prepare for reduction of earthquake hazards there.

Magma generation conditions at the Akita-Komagatake volcano, Northeast Japan arc: Implications of across-arc variations in mantle melting parameters

Akita-Komagatake is an active volcano in the Northeast Japan arc with a slab depth (H) of ∼110 km. In this study, magma generation conditions were estimated for basaltic products from the volcano to examine the thermal structure of the sub-arc mantle and the across-arc variations in the parameters related to magma genesis. The studied basalts have the highest MgO content (∼6 wt%) among the products from the volcano and contain ∼15% plagioclase phenocrysts, ∼5% olivine phenocrysts, and <1% pyroxene phenocrysts. The water content of the melt was estimated to be ∼4 wt% by melt inclusions in the phenocrysts using plagioclase–melt hygrometry. Multi-component thermodynamic analyses suggest that the primary magma was generated by ∼14% melting of the source mantle with ∼0.5 wt% H2O at ∼1275 °C and ∼1.3 GPa. The estimated pressure condition of ∼1.3 GPa coincides well with the low-velocity anomaly observed in the mantle at ∼40 km depth beneath the volcano. By comparing the results from Akita-Komagatake to the published results of the frontal-arc Iwate volcano (H = ∼90 km) and the rear-arc Sannome-gata volcano (H = ∼150 km), which are arranged along a cross-arc line in the Northeast Japan arc, we found that the pressure conditions of the magma generation are ∼1.3 GPa at both Iwate and Akita-Komagatake, which are significantly lower than the ∼1.8 GPa at the rear-arc Sannome-gata volcano. The source mantle temperature of ∼1275 °C at ∼1.3 GPa beneath Akita-Komagatake is higher than the ∼1250 °C at ∼1.3 GPa beneath Iwate, which suggests that the ascending flow of the mantle wedge corner flow may reach the shallowest levels beneath Akita-Komagatake. It was also demonstrated that the depth at which fluid was released from the subducting slab tends to increase for the volcanoes with increasing H.

Causes of the N\u2013S compressional aftershocks of the E\u2013W compressional 2008 Iwate\u2013Miyagi Nairiku earthquake (M7.2) in the northeastern Japan arc

In this study, we investigated the influence of local structural heterogeneities on aftershocks of the 2008 Iwate–Miyagi Nairiku earthquake (M 7.2) that occurred in the northeastern Japan arc. Although this area is characterized by an E–W compressional thrust faulting stress regime, many N–S compressional thrust-type and strike-slip-type earthquakes were also observed in the aftershock activity of the 2008 event. The occurrence of such N–S compressional aftershocks indicates that the differential stress was very low before the main shock. We investigated Vp/Vs structure in the aftershock area in detail to find most of the area showed Vp/Vs as low as less than 1.70, which is consistent with the previous studies. Our result shows, however, the aftershock area was dotted with high Vp/Vs small areas, which suggests the locations of abundant fluid-filled cracks. The comparison of the distributions of Vp/Vs, the stress changes due to the mainshock, and focal mechanisms of the aftershocks indicate that the N–S compressional aftershocks are concentrated in regions with high Vp/Vs and large N–S compressional stress change. From these results, we concluded that the N–S compressional aftershocks occurred in regions with low strength due to the high pore pressure and with N–S compressional stress caused by the stress change due to the mainshock in a low differential stress regime.

Reconstruction of the lithosphere-asthenosphere boundary zone beneath Ichinomegata maar, Northeast Japan, by geobarometry of spinel peridotite xenoliths

Abstract Accurate estimation of the depths of spinel peridotite xenoliths for which reliable geobarometers are not available is imperative to be able to reconstruct the precise structures of the lithosphereasthenosphere boundary (LAB). The LAB can be defined based on thermal, chemical, rheological, and petrological contrasts, and knowing its depth is crucial to understanding mantle dynamics. We attack this problem by examining spinel peridotite xenoliths from Ichinomegata maar in the back-arc side of Northeast Japan Arc. Extensive mineral compositions of nine xenolith samples revealed various patterns of chemical zoning in pyroxenes, suggesting diverse thermal histories. We examined the timescales of development of each zoning pattern and identified minerals, grain portions, and components closely approached equilibrium just before xenolith extraction as orthopyroxene and clinopyroxene, the outermost rims, and Ca-Mg-Fe components, respectively. Applying the best pair of geothermobarometers to the chosen analyses, plausible derivation depths of eight samples were obtained. They range from 0.72–1.6 GPa in pressure and from 830–1080 °C in temperature, which defines a high thermal gradient of 10 K/km or 290 K/GPa. There is an intimate correlation between the zoning patterns of pyroxenes and the depth estimates: pyroxenes in the deeper samples have zoning indicating cooling followed by heating just before xenolith extraction, and those of the shallower samples have zoning indicating monotonic cooling. Depth variations of rock microstructures, grain size of olivine, chemical compositions of minerals, and phase assemblage, including the presence or absence of glass or fluid phase, show that the mantle beneath Ichinomegata consists of two distinct layers. The shallower (28–32 km) layer is granular, less oxidized, amphibole- and plagioclase-bearing, and subsolidus, whereas the deeper (41–55 km) layer is porphyroclastic, amphibole- and plagioclase-free, oxidized, and partially molten. The contrasts between the two layers suggest that the upper layer represents a lithospheric mantle and the lower layer a LAB zone. These layers are similar to those reported from the bottom of subcontinental lithospheric mantle in various aspects, but the LAB beneath Ichinomegata is much shallower (40–60 km) and cooler (~1100 °C). The coincidence of (1) the depth of a rheological transition, marked granular to porphyroclastic textures, and (2) the depth of a phase transition, from subsolidus hydrous peridotite to a hydrous mantle with melt in localized pockets, is the remarkable feature of the LAB beneath Ichinomegata. This suggests that a rheological boundary zone in arc settings is governed by melting of the hydrous mantle and that the underlying asthenosphere is partially molten. The depth-dependent thermal history shown by chemical zoning in pyroxenes and the presence of melt as pockets suggest that the LAB beneath Ichinomegata was in a transient state that was affected by thermal and material transport.

A middle Miocene post‐rift stress regime revealed by dikes and mesoscale faults in the Kakunodate area, NE Japan

AbstractSignificant advances were made in the last century in the investigations of the Neogene stress history of the NE Japan arc. However, previous studies have failed to fully resolve middle Miocene post‐rift stress conditions owing to their assumption of Andersonian faulting and an inability to determine maximum and intermediate stress axes from dike orientations. We applied the latest methods of paleostress analysis in this study to igneous dikes and mesoscale faults in the Kakunodate area of the NE Japan arc to elucidate post‐rift stress conditions. Stratigraphic constraints and U–Pb dating indicate that the doleritic and dacitic dikes were formed at 16–12 Ma and 15–12 Ma, respectively. Dolerite and dacite dikes yielded NW–SE extensional stresses with intermediate and low stress ratios, respectively. Mesoscale faults in the middle Miocene formations of the studied area indicated similar stresses. We suggest the sluggish deformations resulting in the dike intrusion and faulting in the normal‐faulting stress regime after the termination of intra‐arc rifting at ca. 15 Ma.

Magma Transfer Processes in the NE Japan Arc: Insights From Crustal Ambient Noise Tomography Combined With Volcanic Eruption Records

Ambient noise tomography data from northeast Japan reveals upper crustal low-VS bodies beneath most sites of active arc-front volcanoes, with the exception of Zao volcano. Strikingly, when small eruptions (VEI 1 to VEI 3) from arc-front volcanoes in the last 2 kyr are considered, Zao volcano has erupted most frequently and has produced a more than three times greater estimated tephra volume than any of the other volcanoes in this region. We propose that the frequent low-volume volcanism at Zao is fed by dikes that traverse the crustal section rapidly, fracturing through brittle crust. Beneath the other volcanoes, hotter and more ductile plutonic bodies have developed through repeated intrusive activity, hindering magma transport to the surface. This positive feedback mechanism ultimately results in cataclysmic caldera-forming events. We show that the number of late Cenozoic calderas is higher above the imaged low-VS crust, consistent with this interpretation. We propose that generation of plutonic bodies is facilitated by volcaniclastic materials buried in a Miocene rift graben, which dampen fracture propagation and promote repeated sill intrusion. In contrast, transcurrent faulting has moved cold Cretaceous granitic basement of the fore-arc range into the arc-front beneath Zao. These brittle rocks instead provide fracture pathways explored by magmas rapidly rising to the surface. The combined data imply that millions of years of crustal growth and tectonic history have a direct control on present-day volcanic eruption style.

Compositional heterogeneity of a websterite xenolith from Kurose, southwest Japan: insights into the evolution of lower crust beneath the Japan Arc

Compositional heterogeneity of a websterite xenolith from Kurose, southwest Japan: insights into the evolution of lower crust beneath the Japan Arc