Northeast Honshu Arc Research Articles

Petrogenesis of two types of gabbro from Neoproterozoic Fuchuan ophiolite complex, Jiangnan Orogen: Implication for a Japan Sea‐like back‐arc basin

AbstractThe Neoproterozoic Fuchuan ophiolite complex (FCO) represents the lithospheric remnant of the back‐arc basin in the southeastern margin of the Yangtze Craton. However, the nature and development of this back‐arc system are still confusing. This study focuses on the tholeiitic gabbros in the FCO, which can be divided into enriched and depleted types according to their geochemical characteristics. Enriched gabbros (843 ± 5 Ma) in the FCO are characterized by left‐sloping rare earth element (REE) patterns and relatively low εHf(t) (mostly in the range of 4.6–7.8). In contrast, depleted gabbros are slightly younger (838 ± 5 Ma) and isotopically more depleted (εHf(t) mostly in 7.7–11.8) than enriched gabbros, exhibiting flat REE patterns. Based on geochemical variations and numerical modeling, the primary magmas of enriched gabbros were generated by a low degree (~14 %) partial melting of the melt‐modified depleted mantle. In contrast, the more depleted isotopic composition and variable Ba/Th and V/Yb ratios of depleted gabbros suggest that their magmas have originated from a higher degree (~14–26 %) partial melting of the fluid‐modified residual depleted mantle. Considering that the most ~860–830 Ma magmatism in the eastern Jiangnan Orogen is calc‐alkaline, the Neoproterozoic back‐arc basin where the FCO formed could be sialic and built on the thinning continental crust, resembling the Japan Sea. The occurrences of tholeiitic gabbros in the FCO thus provide crucial insights into the opening of the back‐arc basin in the southeastern margin of the Yangtze Craton. The changes of mantle source nature and metasomatic agent reflect the upwelling of the depleted asthenosphere and oceanward retreading of the continental arc during the back‐arc opening. Consequently, the early continental arc in the southeastern Yangtze Craton may have been transformed into a new island arc, resembling the formation of the northeast Honshu arc during the Miocene.

U–Pb ages of granitoids around the Kofu basin: Implications for the Neogene geotectonic evolution of the South Fossa Magna region, central Japan

AbstractThe Japanese archipelago underwent two arc–arc collisions during the Neogene. Southwest Honshu arc collided with the Izu‐Bonin‐Mariana arc and the northeast Honshu arc collided with the Chishima arc. The complicated geological structure of the South Fossa Magna region has been attributed to the collision between the Izu‐Bonin‐Mariana arc and the southwest Honshu arc. Understanding the geotectonic evolution of this tectonically active region is crucial for delineating the Neogene tectonics of the Japanese archipelago. Many intrusive granitoids occur around the Kofu basin, in the South Fossa Magna region. Although the igneous ages of these granitoids have been mainly estimated through biotite and hornblende K–Ar dating, here, we perform U–Pb dating of zircon to determine the igneous ages more precisely. In most cases, the secondary post‐magmatic overprint on the zircon U–Pb system was minor. Based on our results, we identify four groups of U–Pb ages: ca 15.5 Ma, ca 13 Ma, ca 10.5 Ma, and ca 4 Ma. The Tsuburai pluton belongs to the first group, and its age suggests that the granite formation within the Izu‐Bonin‐Mariana arc dates back to at least 15.5 Ma. The granitoids of the second group intruded into the boundary between the Honshu arc and the ancient Izu‐Bonin‐Mariana arc, suggesting that the arc–arc collision started by ca 13 Ma. As in the case of the Kaikomagatake pluton, the Chino pluton likely corresponds to a granodiorite formed in a rear‐arc setting in parallel with the other granodiorites of the third group. The U–Pb age of the Kogarasu pluton, which belongs to the fourth group, is the same as those of the Tanzawa tonalitic plutons. This might support a syncollisional rapid granitic magma formation in the South Fossa Magna region.

東北本州弧における後期新生代の火成活動史( 島弧の深部構造-地質・地震・地震波トモグラフィによる解析(II))

東北本州弧における後期新生代の火成活動史( 島弧の深部構造-地質・地震・地震波トモグラフィによる解析(II))

B61 東北本州弧、火山フロント側における新第三紀火山岩組成の時間的変遷

B61 東北本州弧、火山フロント側における新第三紀火山岩組成の時間的変遷

Self-affinities of landforms and folds in the Northeast Honshu Arc, Japan

A method to analyze self-affinities is introduced and applied to the large scale fold geometries of Quaternary and Tertiary sediments or geographical topographies in the inner belt of the Northeast Honshu Arc, Japan. Based on this analysis, their geometries are self-affine and can be differently scaled in different directions. We recognize a crossover from local to global altitude (vertical) variation of the geometries of folds and topographies. The characteristic length for the crossover of topographies (landforms) is about 25 km and is related to the half wavelength of the crustal buckling folds or possible maximum magnitude of inland earthquakes in the Northeast Honshu Arc. Moreover, self-affinity of the folds and topographies can be connected with the b-value in Gutenberg-Richter℉s law. We obtain two average Hurst exponents obtained from the self-affinities of folds in the Northeast Honshu Arc. This indicates that there are two possible seismic modes for the smaller and larger ranges in the focal regions in the Northeast Honshu Arc.

Relationships between Kuroko volcanogenic massive sulfide (VMS) deposits, felsic volcanism, and island arc development in the northeast Honshu arc, Japan

The northeast (NE) Honshu arc was formed by three major volcano-tectonic events resulting from Late Cenozoic orogenic movement: continental margin volcanism (before 21 Ma), seafloor basaltic lava flows and subsequent bimodal volcanism accompanied by back-arc rifting (21 to 14 Ma), and felsic volcanism related to island arc uplift (12 to 2 Ma). Eight petrotectonic domains, parallel to the NE Honshu arc, were formed as a result of the eastward migration of volcanic activity with time. Major Kuroko volcanogenic massive sulfide (VMS) deposits are located within the eastern marginal rift zone (Kuroko rift) that formed in the final period of back-arc rifting (16 to 14 Ma). Volcanic activity in the NE Honshu arc is divided into six volcanic stages. The eruption volumes of volcanic rocks have gradually decreased from 4,600 km3 (per 1 my for a 200-km-long section along the arc) of basaltic lava flows in the back-arc spreading stage to 1,000–2,000 km3 of bimodal hyaloclastites in the back-arc rift stage, and about 200 km3 of felsic pumice eruptions in the island arc stage. The Kuroko VMS deposits were formed at the time of abrupt decrease in the eruption volume and change in the mode of occurrence of the volcanic rocks during the final period of back-arc rifting. In the area of the Kuroko rift, felsic volcanism changed from aphyric or weakly plagioclase phyric (before 14 Ma), to quartz and plagioclase phyric with minor clinopyroxene (12 to 8 Ma), to hornblende phyric (after 8 Ma), and hornblende and biotite phyric (after 4 Ma). The Kuroko VMS deposits are closely related to the aphyric rhyolitic activity before 14 Ma. The rhyolite was generated at a relatively high temperature from a highly differentiated part of felsic magma seated at a relatively great depth and contains higher Nb, Ce, and Y contents than the post-Kuroko felsic volcanism. The Kuroko VMS deposits were formed within a specific tectonic setting, at a specific period, and associated with a particular volcanism of the arc evolution process. Therefore, detailed study of the evolutional process from rift opening to island arc tectonics is very important for the exploration of Kuroko-type VMS deposits.

Chronological data for the Middle Miocene to Pliocene sequence around the southwestern Sendai Plain, with special reference to the uplift history of the Ou Backbone Range

Middle Miocene to Pliocene chronostratigraphic data were obtained from three sections on the southwestern margin of the Sendai Plain, northeast Japan. In this tectonically active area, we newly infer the numerical age of the coastal sedimentary sequence on the basis of 27 fission track ages in combination with planktonic foraminiferal and diatom analyses. The Hatatate Formation cropping out along the Natori-gawa Section approximately ranges from 13 Ma to 11.5 Ma in age. The Tsunaki Formation ranges from 10 Ma to 8.3 Ma, and the basal part of the Nashino Formation is assigned to 6.4 Ma. Four major unconformities were recognized in the Middle Miocene to Pliocene sequence in the study area from a compilation of chronological data: ca. 9 Ma, 6.4Ma, latest Miocene, and 3.5Ma, respectively. The oldest unconformity was newly identified in this study. Formative timings of these unconformities correlate well with the uplift history of the Ou Backbone range reconstructed by Nakajima et al. (2006). These data provide a restriction to reconstruct the uplift history of the Northeast Honshu Arc.

Spatial and temporal evolution of arc volcanism in the northeast Honshu and Izu‐Bonin Arcs: Evidence of small‐scale convection under the island arc?

Abstract Arc volcanism of the past 10 my in the northeast Honshu and Izu‐Bonin Arcs shows several notable features. In the northeast Honshu Arc, the spatial distribution of volcanism exhibits several clusters elongated nearly perpendicular to the arc and the possible migration of volcanism from the back‐arc side to the volcanic front side, at least, during the past 5 my. The pattern of clusters seems to have flip‐flopped around 5 Ma. In the Izu‐Bonin Arc, there are a series of across‐arc seamount chains, in which volcanic activity occurred from ca 17 Ma to ca 3 Ma, similar to the clusters of the northeast Honshu Arc, although the recent active rifting occurs almost parallel to the arc. On the basis of studies of numerical modeling, these features might be explained, at least qualitatively, by the small‐scale convection under the island arc. Several inferences can be made from our modeling results for the tectonics of the Izu‐Bonin Arc. The angle of dip of subducting plate in the Izu‐Bonin Arc might have increased. This can explain the disappearance of volcanism along the seamount chains and the recent along‐arc volcanism with narrow rifting. The trend of seamount chains, which is oblique to the arc, might not be their intrinsic feature but rather a result of the lateral movement of the back‐arc region after their formation. These inferences can be tested by the future detailed morphological and chronological studies of the Izu‐Bonin Arc.

Supervised landform classification of Northeast Honshu from DEM-derived thematic maps

This paper proposes a quantitative method to classify landforms using four morphometric parameters from DEM-derived thematic raster maps of slope and topographic openness. Because the different surficial processes and stages in the evolution of slopes create landscapes with different shapes, these parameters may lead to a genetic interpretation of topography. The raster maps of slope and topographic openness were constructed for Northeast Honshu, Japan, from 50-m DEMs. The mean and standard deviation of morphometric parameters within a 3050 m by 3050 m moving window on the raster maps were calculated. The results for some training areas show that constructional/depositional and erosional landforms with different relief have different morphometric characteristics. A supervised landform classification for Northeast Honshu using the knowledge from the training areas revealed a ladder geomorphological structure composed of high mountains, ranges and volcanoes. The close relationship between the ladder geomorphological structure and volcano distribution indicates that the structure reflects the magmatic plumbing system from the upper mantle to the crust of the Northeast Honshu arc.

東北本州弧，磐司岩火山岩類の地質と岩石

東北本州弧，磐司岩火山岩類の地質と岩石

Laboratory measurement of P-wave velocity in crustal and upper mantle xenoliths from Ichino-megata, NE Japan: ultrabasic hydrous lower crust beneath the NE Honshu arc

Ultrasonic laboratory measurements of P-wave velocity (Vp) were carried out up to 1.0 GPa in a temperature range of 25–400 °C for crustal and mantle xenoliths of Ichino-megata, northeast Japan. The rocks used in the present study cover a nearly entire range of lithological variation of the Ichino-megata xenoliths and are considered as representative rock samples of the lower crust and upper mantle of the back arc side of the northeast (NE) Honshu arc. The Vp values measured at 25 °C and 1.0 GPa are 6.7–7.2 km/s for the hornblende gabbros (38.6–46.9 wt.% SiO 2), 7.2 km/s for the hornblende-pyroxene gabbro (43.8 wt.% SiO 2), 6.9–7.3 km/s for the amphibolites (36.1–44.3 wt.% SiO 2), 8.0–8.1 km/s for the spinel lherzolites (46.2–47.2 wt.% SiO 2) and 6.30 km/s for the biotite granite (72.1 wt.% SiO 2). Combining the present data with the Vp profile of the NE Honshu arc [Iwasaki, T., Kato, W., Moriya, T., Hasemi, A., Umino, N., Okada, T., Miyashita, K., Mizogami, T., Takeda, T., Sekine, S., Matsushima, T., Tashiro, K., Miyamachi, H. 2001. Extensional structure in northern Honshu Arc as inferred from seismic refraction/wide-angle reflection profiling. Geophys. Res. Lett. 28 (12), 2329–2332], we infer that the 15 km thick lower crust of the NE Honshu arc is composed of amphibolite and/or hornblende (±pyroxene) gabbro with ultrabasic composition. The present study suggests that the Vp range of the lower crustal layer (6.6–7.0 km/s) in the NE Honshu arc, which is significantly lower than that obtained from various seismic measurements (e.g. the northern Izu-Bonin-Mariana arc: 7.1–7.3 km/s), is due to the thick hydrous lower crustal layer where hornblende, plagioclase and magnetite are dominant.

Lithofacies and eruption ages of Late Cretaceous caldera volcanoes in the Himeji–Yamasaki district, southwest Japan: Implications for ancient large‐scale felsic arc volcanism

Abstract The Himeji–Yamasaki region in the Inner Zone of southwest Japan is underlain mainly by Late Cretaceous volcanic rocks called the Ikuno Group or the Hiromine and Aioi Groups. A new stratigraphic and geochronological study shows that the volcanic rocks in this area consist of 15 eroded caldera volcanoes between 82 and 65 Ma; they are, in order of decreasing age, the Hiromine, Hoden, Ibo, Okawachi, Seppikosan, Hayashida, Shinokubi, Fukusaki, Kurooyama, Ise, Fukadanigawa, Nagusayama, Matobayama, Yumesaki and Mineyama Formations. These calderas vary in diameter from 1 to 20 km and are bounded by steep unconformities; they coalesce and overlap each other. The individual caldera fills are composed mainly of single voluminous pyroclastic flow deposits, which are often interleaved with debris avalanche deposits and occasionally underlie lacustrine deposits. The intracaldera pyroclastic flow deposits are made up of massive, welded or non‐welded tuff breccia to lapilli tuff, and are characterized by their great thickness. The debris avalanche deposits are ill‐sorted breccia, generated by the collapse of the caldera wall toward the caldera floor during the pyroclastic‐flow eruption. The large calderas that are more than 10 km in diameter contain original values of approximately 100 km3 of intracaldera pyroclastic flow deposits. These large calderas are similar to the well‐known Valles‐type calderas in their dimensions, although it is uncertain whether their caldera floors are coherent plates or incoherent pieces. Conversely, the small calderas have diatreme‐like subsurface structures. The variety of the caldera volcanoes in this area is caused by the difference in the volume of caldera‐forming pyroclastic eruptions, as the large and small calderas coexisted. The caldera‐forming eruption rates in Late Cretaceous southwest Japan, including the studied area, were similar to those in late Cenozoic central Andes and northeast Honshu arc, Japan, but obviously smaller than those of late Cenozoic intracratonic caldera clusters in western North America and the Quaternary extensional volcanic arcs in Taupo, New Zealand. The widespread Late Cretaceous felsic igneous rocks in southwest Japan were generated by a long‐term accumulation of low‐rate granitic magmatism at the eastern margin of the Eurasian Plate.

日本列島と周辺海域の地震地体構造区分

A new seismotectonic province map of the Japanese Islands and the adjacent areas, which carries maximum magnitudes of earthquake (Mmax) expected for the individual provinces, has been prepared as a revised edition of Kakimi et al. (1994). The major part of the mapped region constitutes an island arc-trench system, which is surrounded by Northwest Pacific Basin (1), Shikoku Basin (2), Philippine Basin (3), Kurile Basin (4), Japan Sea Basins (5), and Korean Peninsula and Tonhai Continental Shelf (6). All of the peripheral provinces have too low seismicity to be given Mmax. The island arc-trench system is subdivided into the following constituent arcs: Kurile Arc (7), Northeast Honshu Arc (8), Izu-Bonin Arc (9), Southwest Honshu Arc (10), Ryukyu Arc (11), Sakhalin Arc (12), and the Tectonic Belt along the Eastern Margin of Japan Sea (13). While the constituent arcs 7 to 11 are divided into three tectonic belts, which remarkably differ from each other in tectonic, seismic, and volcanic activities, from the trench to the inland: Continental Slope on the Trench Side (A), Non-volcanic Outer Belt (B), and Volcanic Inner Belt (C), the constituent arc 10 alone has additionally the Continental Slope on the Marginal Sea Side (D). Province 12 started developing in Late Mesozoic and functioned as a collision belt between the North American Plate (NA) and the Eurasian Plate (EUR) in Late Cenozoic, whereas province 13 is considered to form a current collision belt between NA and EUR plates. Province 11X, Okinawa Trough, is defined as a current rift zone developing between the Tonhai Continental Shelf and the Ryukyu Arc. Some of the provinces are further divided into subprovinces in response to local differences in active faults, seismicity, Mmax etc.All the active faults on land are grouped into seismogenic faults (Matsuda, 1990), which are considered to generate characteristic earthquakes. The magnitudes of earthquake expected for the seismogenic faults (MLmax) are estimated by the equation: log L=0.6ML-2.9 (Matsuda, 1975), where L is the length of the faults in kilometers. The maximum magnitude of earthquake expected for seismogenic faults (MLmax) and the maximum one for historical shallow earthquakes (Mhmax) are compared in each province to choose the larger one as the expected maximum earthquake magnitude (Mmax) for the province. Since no method to decide a seismogenic unit from offshore active faults has been established, Mhmax is tentatively adopted as the Mmax representing the province. Extraordinarily long faults found in inland provinces, which are called the designated faults (Matsuda, 1990), are excluded from estimation of the Mmax. None of the magnitudes of earthquake expected for the designated faults is shown here, because they should be individually estimated. All of the information, such as tectonic geomorphology and geology, characteristics of active (seismogenic) faults, historical earthquakes, modern seismicity, and other, is put into a table to facilitate the identification of a seismotectonic province and the determination of the Mmax and the designated faults. The details of the boundaries between seismotectonic provinces are shown in another table.

東北本州弧，朝日山地南縁に分布する中新世火山岩類のフィッション・トラック年代測定

東北本州弧，朝日山地南縁に分布する中新世火山岩類のフィッション・トラック年代測定

なぜ東北本州弧の地殻短縮は火山フロントより背弧で大きいか？

なぜ東北本州弧の地殻短縮は火山フロントより背弧で大きいか？

G19 東北本州弧、月山火山の地質と K-Ar 年代

G19 東北本州弧、月山火山の地質と K-Ar 年代

B22 東北本州弧南部, 会津地域の第四紀大規模火砕流堆積物

B22 東北本州弧南部, 会津地域の第四紀大規模火砕流堆積物

山形県小国町付近の地質 東北本州弧前期中新世ハーフ・グラーベンの例

山形県小国町付近の地質 東北本州弧前期中新世ハーフ・グラーベンの例

Partial melting of mantle wedge and evolution of island arc crust

Evolution of the crust in the northeast Honshu arc is discussed on the basis of the melting experiments of a peridotite under hydrous conditions and the crustal compositions estimated from deep crustal xenoliths and seismic velocity data. The chemical compositions of melts formed by partial melting of a hydrous peridotite within the pressure‐temperature conditions for shallow parts of the mantle wedge (1.2–2.0 GPa) have been experimentally determined with the “sandwich experiment” technique. The melts range from olivine tholeiitic to magnesian andesitic compositions (SiO2 48–56 wt %; MgO 9–14 wt %) beneath the volcanic front in the northeast Honshu arc and are slightly less silicic beneath the coast of the Sea of Japan. The crustal compositions of the volcanic zone in the northeast Honshu arc estimated from deep‐seated crustal inclusions in volcanic rocks and the velocity structures of the crust are andesitic in the central zone and high‐alumina basaltic near the coast of the Sea of Japan. The compositions of possible primary melts formed in the mantle wedge are significantly more mafic than these crustal compositions. Consequently, if the major part of the crustal materials in the volcanic zone was formed by magmas generated in the mantle wedge, thick (up to 30 km) ultramafic cumulates including olivine pyroxenite must have been formed beneath Moho during fractional crystallization and would constitute the upper part of the mantle wedge. The volume of the mantle wedge consisting entirely of even fertile peridotite was not sufficient to produce crustal materials and cumulates in the volcanic zone by such processes in the northeast Honshu arc. A static mantle wedge model is, therefore, not satisfactory for the growth of the crust in northeast Honshu arc. A dynamic model is suggested in which the mantle materials have been supplied into the mantle wedge from the back arc side by a convective flow.

Neogene tectonic stress fields of northeast Honshu Arc and implications for plate boundary conditions

More than 3300 metalliferous veins were analyzed to reconstruct the Neogene tectonic stress field of the northeast Honshu Arc using basic fracture mechanics. The veins are grouped into a dominant NE system and subordinate E-W, N-S and NW systems. The NE system is associated with assemblages of conjugate strike-slip faults and extension joints or normal faults which were formed during a period from 15 to 5 Ma. The fractures suggest that σ 1 or σ 2 was oriented ENE and σ 3 was oriented NNW. σ 3 is thought to have been tensional, because no veins are associated with thrust faults. This stress orientation is neither perpendicular nor parallel to the axis of the Japan Trench where the Pacific plate was moving west-northwest and was subducted under the northeast Honshu Arc. The inconsistency between the stress orientation and the plate kinematics can be explained by the dynamic effects related to other plate boundaries surrounding the northeast Honshu Arc, namely compression on the Hokkaido Axial Zone and the Itoigawa-Shizuoka Tectonic Line. In addition, the triple junction of the Japan Trench, Izu-Bonnin Trench and the Sagami Trough was probably located 300–400 km northeast from its present position and the subduction zone of Nankai Trough-Sagami Trough is assumed to have been a tensional plate boundary. The reconstructed stress field is consistent with transtensional backarc rifting at about 15 Ma.