NE Japan Research Articles

Textural and chemical variations in peridotite induced by hydrous melt migration in mantle under the back-arc spreading

This study presents the structural and petrological characteristics of mantle peridotites in relation to the rock-hydrous melt reaction found in the Hayachine ultramafic complex, NE Japan. We conducted sampling, microstructural observations, crystal-orientation analyses, and major element composition analyses of the major constituent minerals in the peridotites. We used 17 serpentinized peridotites that preserved better mantle textures. The peridotites are composed of lherzolite to harzburgite, consisting of olivine, orthopyroxene, clinopyroxene, amphibole, and spinel. The peridotites were classified into two textures according to olivine grain size: coarse-grained (ca. 2–3 mm) and fine-grained (ca. 0.3–0.5 mm). Fine-grained peridotites were newly found in this study and were characterized by aggregates of orthopyroxene and amphibole with a large number of spinel inclusions. Based on olivine crystal-preferred orientation (CPO), we found that the coarse-grained peridotites were further classified into Group 1 (A type CPO) and 2 (AG type CPO) and the fine-grained peridotites were accordingly classified into Group 3 (weak CPO). The systematic continuity in the chemical compositions of the minerals suggests that Group 1 peridotites partially melted to form Group 2 peridotites, followed by Group 3 peridotites, due to further reaction of Group 2 peridotites with hydrous melts. These textural and chemical variations in the peridotites could have resulted from rock-hydrous melt reactions under back-arc spreading and subsequent processes.

Exploring the Eastern Abukuma Mountains: A field guide to high-pressure metamorphic rocks and strike-slip crustal shear zones in the Southern Kitakami Belt, NE Japan

Exploring the Eastern Abukuma Mountains: A field guide to high-pressure metamorphic rocks and strike-slip crustal shear zones in the Southern Kitakami Belt, NE Japan

Geology and geography of the Matsushima Bay area: Geologic and topographic history of the fore-arc region of NE Japan from the Miocene opening of the Sea of Japan to the present

Geology and geography of the Matsushima Bay area: Geologic and topographic history of the fore-arc region of NE Japan from the Miocene opening of the Sea of Japan to the present

The role of continental heterogeneity on the evolution of continental plate margin topography at subduction zones

The nature of the overriding plate plays a major role for subduction zone processes. In particular, the highly heterogeneous continental lithosphere modulates intra-plate tectonics and the surface evolution of our planet. However, the role of continental heterogeneity is relatively under-explored for the dynamics of subduction models. We investigate the influence of rheological and density variations across the overriding plate on the evolution of continental lithosphere and slab dynamics in the upper mantle. We focus on the effects of variations in continental plate margin and keel properties on deformation, topographic signals, and basin formation. Our results show that the thickness, extent, and strength of the continental plate margin and subcontinental keel play a crucial role for the morphology and topography of the overriding plate, as well as the retreat of the subducting slab. We show that this lateral heterogeneity can directly influence the coupling between the subducting and overriding plate and determine the partitioning of plate velocities across the overriding plate. These findings suggest that back-arc extension and subsidence are not solely controlled by slab dynamics but are also influenced by continental plate margin and keel properties. Large extended back-arc regions, such as the Pannonian and Aegean basins, may result from fast slab rollback combined with a weak continental plate margin and a strong and extended continental keel. Narrow margins, like the Okinawa Trough in NE Japan, may indicate a comparatively stronger continental plate margin and weaker or smaller continental keel. Additionally, continental keel properties may affect the overall topography of the continental lithosphere, leading to uplift of the deformation front and the formation of intermontane basins.

Sulfur isotopic variations in the products of the 1895 CE eruption at Zao volcano (NE Japan): Implications for connecting eruption source and syn-eruptive magmatic-hydrothermal processes

Sulfur isotopic ratio in sulfate and sulfide in subvolcanic hydrothermal systems is a valuable tracer to study the magmatic-hydrothermal processes from the magma source through to volcanic eruptions. Zao volcano is among the most active volcanoes in NE Japan, with historical explosive eruptions occurring during the last thousand years and unrest episodes since 2013. This necessitates a detailed assessment of the potential risk of future volcanic hazards. We investigated the magmatic-hydrothermal processes that occurred during the 1895 CE eruption sequence at Zao volcano by conducting mineralogical and sulfur isotope analyses in the exposed well: (i) six volcanic units (Layers 1–6) of the 1895 CE eruption products (clayish ash deposits with andesitic bombs, lapilli of scoria, and minor altered lithic fragments) deposited on the rim of Okama crater lake; and (ii) clay-altered and silicified rocks from the Nigorikawa alteration zone (NGA) surrounding the Goshikidake cone. Mineralogical data show that the samples mainly consist of alunite, pyrite, and gypsum. Alunite and pyrite occur as fine crystal mixtures associated with mineral assemblages of both advanced argillic alteration (i.e., those of cristobalite and kaolinite) and silicification (i.e., those of cristobalite, tridymite and native sulfur). Gypsum typically appears as isolated euhedral crystals of several millimeters in size. Samples of the 1895 CE eruption products have a narrow range of δ34S values from +3 ‰ to +5 ‰ for gypsum, from +9 ‰ to +13 ‰ for alunite, and approximately −10 ‰ for pyrite. For the NGA samples, the δ34Sgypsum, δ34Snative sulfur, and δ34Spyrite values range from −12 ‰ to −9 ‰, whereas for alunite, these range from +8 ‰ to +18 ‰. This indicates that alunite and pyrite in the 1895 CE eruption products were derived from the advanced argillic alteration and silicification zones that developed under Okama crater, which is exposed as the NGA. Estimated alteration temperatures based on the sulfur isotopic equilibrium between alunite and pyrite pairs are 200 °C–300 °C. By contrast, δ34Sgypsum values in the 1895 CE products are significantly higher than those in the NGA (which are derived from oxidation of pyrite or H2S, or both), ranging between an estimated parental fluid of δ34Sbulk-initial = ca. +1 ‰ and the Quaternary volcanic rocks of the Japan arc. This suggests that gypsum in the 1895 CE eruption products derived from magmatic vapor condensate (anhydrite) formed in the volcanic conduit during the eruption, thus becoming replacement of anhydrite by gypsum after or during the tephra deposition on the Zao summit surface. Our results on sulfur-bearing minerals provide new clues for better understanding (and monitoring) the syn-eruptive processes of volcanic eruptions focused on subvolcanic hydrothermal systems.

Reconstruction of the Cretaceous continental arc–trench system of the Japanese Islands: a basis for Cretaceous palaeoenvironmental studies

Abstract Spatiotemporal distributions of Cretaceous rocks differ markedly between the SW and NE Japan arcs. However, four parallel zonal arrangements of rocks are recognized broadly throughout both arcs: mostly non-marine sedimentary rocks in back-arc/intra-arc basins; granitic and volcanic rocks in magmatic arcs; predominantly marine and subordinately fluvial sedimentary rocks in forearc basins; and sedimentary rocks of turbiditic and mélange facies in accretionary complexes. These zones constituted a palaeo-Japan continental arc–trench system during the Cretaceous. We describe and correlate 71 Cretaceous back-arc/intra-arc and forearc basinal successions from Kyushu (south) to Hokkaido (north) islands, including a southern Sakhalin and two Kuril Arc (eastern Hokkaido) successions. Stratigraphic ranges and major sedimentary facies are generally similar between the SW and NE Japan arcs, except for the pre-Aptian Lower Cretaceous in Hokkaido of NE Japan, suggesting continuity throughout the two arcs during the Cretaceous. Although Cretaceous strata are sporadically exposed in northern Honshu, NE Japan, interpretation of seismic sections suggests that Cretaceous forearc sedimentary rock measuring several tens of kilometres laterally are developed offshore beneath the present Pacific forearc. In contrast, Cretaceous forearc strata in southern SW Japan are distributed along two narrow belts that may have been deformed by post-Early Miocene tectonism.

Short-period flow oscillation during eruptions of Onikobe geyser, NE Japan: Insights from thermal infrared observation and acoustic measurements

Geysers are episodic features with variable eruption intervals that range from minutes to years. Although many previous studies on geysers have focused on subsurface properties and processes such as plumbing geometry and recharge process, it is known that erupting fluid column exhibits short-term behaviors such as individual water jets from the vent. In the present study, we conducted observations at Onikobe geyser, NE Japan, a relatively small geyser (geysering well) erupting water up to ∼6–8 m every ∼10 min and observed the erupting fluid column using a thermal infrared camera and an acoustic sensor. We succeed in tracking the water jets by analyzing spatio-temporal temperature map obtained from the thermal infrared observation, which clearly shows the eruptions at Onikobe geyser are not completely stationary but rather a series of intermittent jets with a short period interval of <1 s. We estimate the exit velocity by fitting a ballistic model under the air drag condition of inertial resistance to the jet trajectory. The exit velocity and the averaged gas volume fraction of the erupting fluid are estimated to be ∼4–79 m/s and ∼0.93–0.96, respectively. The exit velocity is ∼30 m/s during the first bursts, then it rapidly increases to ∼50–80 m/s for ∼15 s, and then decreases to ∼20–30 m/s until ∼10 s before the eruption ends. Time series analyses of the thermal infrared and acoustic signals during an eruption indicate harmonic spectra with integer multiple peaks. The fundamental frequency showing ∼4 Hz at the beginning gradually decreases to ∼2 Hz for ∼15 s, keeps almost constant in the following 30 s, and then slightly increases near the end of the eruption. These harmonic spectra may be caused by a resonance mechanism, non-linear fluid motion, and/or subsurface two-phase flow. We discuss the case of a closed organ-pipe resonance in a subsurface crack and attribute the frequency decrease to a decrease in the water level in the crack (an increase in the length of the resonating liquid-vapor column) and/or a decrease in the gas volume fraction of the liquid-vapor mixture in the crack. Since such harmonic characteristics are observed at other geysers, further simultaneous observation of the short-period oscillations on the erupting column proposed in this study with seismic observations of harmonic tremor may provide an improved understanding of geyser subsurface phenomena.

Olivine-Hosted Melt Inclusions Track Progressive Dehydration Reactions in Subducting Slabs Across Volcanic Arcs

Abstract The stability and breakdown of mineral phases in subducting slabs control the cycling of trace elements through subduction zones. Stability of key minerals and the partitioning of trace elements between these minerals and liquid phases of interests have been charted by natural sample analysis and experimental constraints. However, systematic study from arc front to far back arc has rarely shown that the expected geochemical variations of the slab liquid are actually recorded by natural samples. Complexities arise by uncertainties on the nature of the slab component (melts, fluids and supercritical liquids), source heterogeneities and transport processes. Using data from olivine-hosted melt inclusions sampled along and across the NE Japan and southern Kurile arcs, we demonstrate that experimentally and thermodynamically constrained phase stabilities in subducted materials indeed control the trace element signatures as predicted by these models and experiments. The main reactions that can be traced across arc are progressive breakdown of light rare earth element-rich accessory phases (e.g. allanite), enhanced dehydration of the lithospheric mantle (serpentine breakdown) and changes in the nature of the slab component. This work elucidates subduction zone elemental cycling in a well-characterized petrogenetic setting and provides important constraints on the interpretation of trace element ratios in arc magmas in terms of the prograde metamorphic reactions within the subducting slab.

Ontake-Katamachi tephra: Marine-terrestrial correlation of a time marker of marine isotopic stage 5b in NE Japan, the Japan Sea, and the NW Pacific

We characterized Ontake volcano-derived tephras on the basis of their shard major- and trace-element compositions to distinguish between tephras by different multiple large eruptions of Ontake volcano during marine isotopic stage (MIS) 5. Then we reconsidered the ages of the tephras and their correlations between proximal and distal areas in light of their stratigraphic relationships with other widespread tephras and the δ18O stratigraphy of deep-sea sediments of the Pacific Ocean. As a result, we were able to discriminate Ontake-Katamachi tephra (On-Kt), a previously identified Ontake volcano-derived tephra in deep-sea sediments of Holes C9001C and ODP1150A off NE Japan, from other tephras in the proximal area. In addition, we suggest that previous studies may have miscorrelated On-Kt in terrestrial sequences at some locations in NE Japan to proximal Ontake-Pumice 1 (On-Pm1). Therefore, On-Kt is recognized here for the first time as a widespread tephra covering NE Japan and parts of the Japan Sea and the Pacific Ocean. The On-Kt horizon in Pacific Ocean deep-sea sediments corresponds to MIS 5b in the reported δ18O stratigraphy, and it occurs below Aso-4 (ca. 87 ka), a widespread tephra derived from Aso caldera in the Kyushu arc. As a widespread tephra covering a wider area than previously thought, On-Kt is expected to be useful for constraining the ages of regional tephras in central Japan, NE Japan, the Japan Sea, and the NW Pacific.

Arc building in the Abukuma Mountains of NE Japan: Insights from petrology, geochemistry and zircon U-Pb-Hf isotope data

Subduction zones along convergent plate margins are the major regions of continental crust building on Earth. The petrological and geochemical features of arc crust are therefore critical in understanding the formation and evolution of continental crust. Here we investigate the magmatic suites in the Abukuma Mountains of NE Japan, which document episodic accretionary events during the Cretaceous. The rock suite in the Abukuma Mountains mainly comprises hornblende (Hbl)-gabbro, diorite, granodiorite and monzogranite. Zircon UPb dating suggests that dioritic to granitic rocks in the Abukuma Mountains were formed during 110 to 115 Ma, followed by gabbroic intrusions (100 to 110 Ma) and large-scale high-T/low-P metamorphism. But combined with literature geochronological results, the gabbroic rocks in Abukuma are coeval with the formation of intermediate to felsic rocks with a peak age of ∼108 Ma. Major and trace elements characteristics of these rocks including their calc-alkaline nature and negative Nb, Ta anomalies suggest their arc-related origin linked with the subduction of the Paleo Pacific (Izanagi) plate. About 200 km north of the Abukuma Mountains, the Kitakami magmatic rocks were also formed during the subduction of Izanagi plate, but previous studies show that parts of the Kitakami intermediate-felsic rocks have distinct adakitic features and more juvenile Hf signatures (ƐHf(t) = 7.9 to 10.6) than those in the Abukuma suite (ƐHf(t) = 1.5 to 8.8, average at 6.1 ± 1.8). The geochemical features of the magmatic rocks in NE Japan are consistent with slab-break off, mantle upwelling and partial melting resulting in the formation of adakitic rocks in the Kitakami region. However, subsequent to this event, the slab subduction was reestablished, leading to the formation of normal island arc rocks in the Abukuma area. Our study provides new insights into the geodynamic evolution of NE Japan.

Low-temperature thermochronology of Ryoke belt granitoids, SW Japan: New insights into the recent cooling history from monazite fission-track dating

Traditional low-temperature thermochronology techniques such as apatite fission track (AFT) and apatite (U-Th-Sm)/He (AHe) have so far produced limited information on the neotectonic (late Miocene and younger) evolution of Japan. However, the development of monazite fission track (MFT) dating, which has an ultra-low temperature sensitivity (<∼25 – 46°C), provides a fresh opportunity to directly analyse recent denudation histories. Low-temperature thermochronology methods have been applied to granitoid samples from the Ryoke belt, located in eastern Yamaguchi and Nara Prefectures, SW Japan. Zircon (U-Th)/He (ZHe), AFT and AHe data and modelled thermal histories reveal Late Cretaceous - Pliocene cooling related to granitoid thermal relaxation combined with paleo-Izanagi and Pacific plate subduction along the eastern Eurasian continental margin. MFT dating reveals Plio – Pleistocene cooling ages, coincident with elastic loading caused by Philippine Sea plate subduction since the Middle - Late Miocene, along with the Quaternary collision of NE and SW Japan at the Itoigawa-Shizuoka Tectonic Line. Silica concentration, non-thermal “radiation assisted” annealing, and changing climatic conditions are proposed as possible caveats on MFT annealing kinetics since the samples are residing in an “open” system. Taking all these factors into consideration, the apparent MFT ages are interpreted as minimum cooling ages, being slightly younger than the thermal events that cooled the samples through the MFT nominal closure temperature. Estimated denudation rates based on MFT dating are higher in Nara relative to eastern Yamaguchi Prefecture, with the difference likely reflecting variations in the tectonic regime, timing of uplift and uplift mechanisms of the two regions.

Geochemical modeling of a volcano-centered acidic geothermal fluid: Case study at Tamagawa hot spring, NE Japan

Using chemical data of highly acidic Tamagawa Hot Spring, Akita Prefecture, NE Japan, upwelling and geochemical processes of geothermal fluid in the central region of volcanic–hydrothermal system were examined through geochemical calculation. The Tamagawa hot springs include Cl-rich acid SO4–Cl liquid discharges with a slight temporal variation in S concentration. The calculation indicated deep solutions at 300 °C having Cl concentrations of 98–100 mmolal and S concentrations of 59–64 mmolal at SO2/H2S log gas ratios of -0.86 to -0.46 under an assumption of fluid ascent after boiling and mixing twice. The temporal variation in the S of the discharges might be attributed to the input of S from the magmatic vapor to the hydrothermal system. The calculation tentatively required a double-fluid flow through the conduit and surrounding wall rock and sequential mixing of the fluids. This study revealed the upflow condition coupled with geochemical evolution of geothermal fluids in the central region of volcanic–hydrothermal systems as the geothermal exploitation.

Carbonates and intermediate-depth seismicity: Stable and unstable shear in altered subducting plates and overlying mantle

A model for intermediate-depth earthquakes of subduction zones is evaluated based on shear localization, shear heating, and runaway creep within thin carbonate layers in an altered downgoing oceanic plate and the overlying mantle wedge. Thermal shear instabilities in carbonate lenses add to potential mechanisms for intermediate-depth seismicity, which are based on serpentine dehydration and embrittlement of altered slabs or viscous shear instabilities in narrow fine-grained olivine shear zones. Peridotites in subducting plates and the overlying mantle wedge may be altered by reactions with CO2-bearing fluids sourced from seawater or the deep mantle, to form carbonate minerals, in addition to hydrous silicates. Effective viscosities of magnesian carbonates are higher than those for antigorite serpentine and they are markedly lower than those for H2O-saturated olivine. However, magnesian carbonates may extend to greater mantle depths than hydrous silicates at temperatures and pressures of subduction zones. Strain rates within altered downgoing mantle peridotites may be localized within carbonated layers following slab dehydration. A simple model of shear heating and temperature-sensitive creep of carbonate horizons, based on experimentally determined creep laws, predicts conditions of stable and unstable shear with strain rates up to 10/s, comparable to seismic velocities of frictional fault surfaces. Applied to intermediate-depth earthquakes of the Tonga subduction zone and the double Wadati-Benioff zone of NE Japan, this mechanism provides an alternative to the generation of earthquakes by dehydration embrittlement, beyond the stability of antigorite serpentine in subduction zones.

Fore-arc volcanism in NE Japan during the Miocene opening of the Japan Sea

茨城県城里町塩子に分布するアルミナに乏しく，鉄に富む無斑晶状安山岩の貫入岩から，新たに16.40±0.10 MaのK‒Ar年代値を得た．また微量成分組成とSr‒Nd同位体比から，この火山岩が近傍の元古沢玄武岩に近い組成の苦鉄質マグマの結晶分化で生成された残液マグマ起源であることを明らかにし，元古沢玄武岩については日本海拡大時の高温アセノスフェアの注入により溶融した島弧リソスフェア性マントルに由来する可能性を示した．

Zircon U–Pb–Hf Isotopes and Whole-rock Geochemistry of Rhyolite and Tuff from the Harachiyama Formation, North Kitakami Mountains, NE Japan

The Harachiyama Formation comprises Lower Cretaceous volcanic rocks, which are distributed in the eastern margin of the Kitakami Mountains in northeastern Japan. Whole-rock chemical analysis, U–Pb zircon dating and Hf isotope analysis of the Harachiyama Formation were performed to constrain the formation age and the magmatic origin was discussed. The lava and tuff samples of the Harachiyama Formation from the Omoto and Tsukue areas supported island-arc rhyolitic rocks (SiO2 content > 70%), and yielded the weighted mean U–Pb ages of 127.8 ± 3.4 Ma and 129.2 ± 2.6 Ma (2σ), respectively. Eighteen zircon grains from two samples, dated between 141.6 Ma and 123.9 Ma, yielded positive εHf(t) values between +5.0 and +8.7. The U–Pb ages and εHf(t) values are consistent with those of the Kitakami Granitoid reported in previous studies. Therefore, it is suggested that the Early Cretaceous volcanic rocks in northeastern Japan, such as the Harachiyama Formation, have the same magmatic origin as the Kitakami Granitoid.

Ordovician Japan: geotectonic setting and palaeogeography

Abstract Ordovician Japan formed a mature arc-trench system developed along the palaeo-Pacific (Panthalassa) margin of the Greater South China (GSC) continental block. GSC consists of South China, East China Sea, SW–NE Japan and the Khanka–Jiamusi–Bureya megablock in the Far East; Paleozoic GSC was thus, in total, twice as large as the South China components by themselves (Yangtze and Cathaysia). The Ordovician crust of Proto-Japan comprised coeval arc-related rocks, such as granitoids, supra-subduction zone ophiolites and fore-arc basin strata, although most of them were considerably fragmented. The Ordovician and middle–late Paleozoic fossils from Japan are highly limited but suggest that Proto-Japan was positioned in the low-latitude domains probably of the palaeo-Pacific Ocean in connection to Paleo-Tethys. GSC became separated from Rodinia in the Neoproterozoic, and its Proto-Japan segment evolved as a collision-free subduction margin for nearly 500 myr since the mid-Cambrian. The GSC framework provides critical constraints to the palaeogeographical reconstruction of circum-Pacific continental blocks. First, the Cambro-Ordovician GSC should have been isolated from Australia/India/East Antarctica that formed East Gondwana by a relatively wide ocean domain for keeping ‘subduction potential’. Second, the Cathaysian margin of GSC should have faced to an extensive ocean without major continents since the Cambrian. The palaeo-Pacific is the only possible candidate for this.

Quaternary caldera-forming eruptions at the Sanzugawa caldera, NE Japan, revealed by zircon U-Pb geochronology

Recent application of zircon U-Pb geochronology has contributed to deciphering the evolution of caldera systems worldwide. However, in the Tohoku region, NE Japan, this is not the case, although it is well studied as a typical island arc subduction system. Here, U-Pb dating of zircon suggests that the Sanzugawa caldera, the largest caldera in Tohoku since late Miocene, initiated its caldera-forming volcanic activity at ∼7 Ma and culminated its activity in late Pliocene to Quaternary (3.0–1.5 Ma). This is contrary to a previous notion that the caldera developed in late Miocene to Pliocene (6–3 Ma) based on mainly whole-rock K-Ar dating results. This finding may also question a current hypothesis of relatively subdued volcanism at 3.5–1.5 Ma in NE Japan and thus shows the need to date other calderas in the Tohoku region with zircon U-Pb and/or Ar-Ar methods to better understand the magmatic history of this region.

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

AbstractLocated 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.

The Origin of the Paleo‐Kuril Arc, NE Japan: Sediment Provenance Change and Its Implications for Plate Configuration in the NW Pacific Region Since the Late Cretaceous

AbstractThe Nemuro Belt is a tectonic belt in the Paleo‐Kuril Arc system, the NW Pacific region. The Paleo‐Kuril Arc has been interpreted as an intraoceanic arc system between the Izanagi and Pacific Plates during the Late Cretaceous, suggesting that the boundary between these plates was a trench. However, this study shows it was a volcanic arc that developed atop a continental margin. To determine the nature of this arc during the Late Cretaceous, U‐Pb ages of detrital zircons from the Nemuro and Urahoro Groups in the Paleo‐Kuril Arc were analyzed. Our results identify two distinct types of detrital zircon U‐Pb age distributions. The first (Type 1) is characterized by multimodal age distributions with peaks ranging from ca. 1.8 Ga to 78 Ma. The other (Type 2) exhibits a unimodal age distribution with a peak at 60‐52 Ma. These different age distributions indicate a provenance transition occurred between the Late Cretaceous and Paleogene. Precambrian zircons in Type 1 sandstones indicate that the Paleo‐Kuril Arc was part of a continental plate, presumably the Okhotsk Block, in NE Asia during the Late Cretaceous. In contrast, Type 2 sandstones were supplied only from the magmatic arc region that the Izanagi‐Pacific Ridge subduction could have activated. This provenance transition suggests the initially continental arc became separated from its continental source at the beginning of the Paleogene.

Waveform inversion of large data sets for radially anisotropic Earth structure

SUMMARY The amount of high-quality seismic data is expanding rapidly, and there is a need for algorithms that take advantage of classical methods to achieve high efficiency using widely available computing power. In this study, we develop a novel waveform inversion method to retrieve radially anisotropic Earth models that can be used to investigate deformation and flow in the mantle. Our method is comprised of two parts: (1) extraction and fitting of the fundamental mode and (2) fitting of the full synthetic waveform. The waveform inversion method results in path average model constraints with uniquely determined independent uncertainties. We demonstrate through synthetic testing that the method is able to retrieve radially anisotropic perturbations down to the mantle transition zone, and leakage effects due to ignoring P-wave anisotropy are minimal. We apply the method to ∼16 000 waveforms generated by earthquakes occurring in the East Sea (Sea of Japan) region, and we demonstrate that the subsequent linear inversion of radially anisotropic path constraints produces models that are similar to those resulting from full waveform adjoint tomography methods. We validate our model by predicting waveforms for earthquakes not included in our inversion, and we show that our method is able to extract structural information. Our results indicate low-velocity anomalies and weak radial anisotropy in NE Japan, which may be due to competing influences from ascending fluids and/or melts and horizontal flow in the lower crust and upper mantle. In the southern East Sea, we image low velocities and relatively high radial anisotropy, which may reflect high temperatures, shallow dehydration and olivine LPO in the upper mantle.