Yasuto Itoh

History of basin formation and tectonic evolution at the termination of a large transcurrent fault system: deformation mode of central Kyushu, Japana

Abstract The central part of the Kyushu Island is a locus of active arc-volcanism associated with subduction of the Philippine Sea plate. Being located at a bend in the boundary between the Philippine Sea and Eurasian plates, a complicated lateral displacement has generated half-grabens, a zone of strike-slip, and rhomboidal basins filled with volcanic material in central Kyushu. Initial northward subduction in latest Miocene time activated the N-S-trending Kokura-Tagawa Tectonic Line (KTL) as a left-lateral slip fault that bounds the western margin of the area of volcano-tectonic depression, the Hohi volcanic zone, which was initially formed about 6 Ma ago. The depocenter, which is now called Kuju basin, in the Hohi volcanic zone was at the corner of the KTL and the E-W-trending Median Tectonic Line (MTL). Relative convergence direction of the Philippine Sea plate shifted counterclockwise at about 1.5 Ma, and active motion on the KTL declined. At the same time, west-northwestward subduction enhanced right slip on the MTL. Geologic, gravimetric, and seismic data indicate that the MTL has shifted its active trace northward in central Kyushu as far as 10 km. As a result, the depocenter adjacent to the transcurrent fault migrated northeastward in the Hohi volcanic zone, specifically, from Kuju basin (Pliocene) via Shonai basin (early Quaternary) to Beppu Bay basin (late Quaternary). The latest depocenter of the Beppu Bay is surrounded by active faults that clearly delineate a rhomboidal basin on the MTL. Central Kyushu exemplifies the basin forming history and tectonic evolution at the termination of a large transcurrent fault system.

Confined ductile deformation in the Japan arc inferred from paleomagnetic studies

Abstract Miocene rocks collected from the Kanazawa and Daishoji areas in the western vicinity of the Itoigawa-Shizuoka tectonic line (ISTL) were investigated using paleomagnetic and biostratigraphic methods in order to clarify the deformation mode of a continuous granitic sliver in the southern part of the Japan arc which was bounded on east side by the ISTL. Seven sites in the Kanazawa area and nine sites in the Daishoji area were found to preserve stable characteristic remanent magnetization (ChRM) through progressive demagnetization by thermal and alternating field methods. Temporal and spatial changes in ChRM directions suggest that the areas were subjected to intra-arc tectonic deformation linked with the middle Miocene collision of the Izu-Bonin arc which succeeded to coherent clockwise rotation of the southern part of the Japan arc associated with the back-arc opening around 15 Ma. The data in this study, together with data previously obtained, indicate that the angle of relative counterclockwise rotation increases gradually toward the east, which was caused by the middle Miocene arc-arc collision. This result and the absence of identifiable faults which could compensate differential rotations, imply that the arc was ductilely deforming during the collision event. It is also suggested that significant rotation occurred in the eastern, confined, portion of the uniform sliver of continental crust. The confined ductile deformation of the crust observed in the Japan arc can be achieved by the irregular compressive force on the fore-arc side generated by landmasses on the subducting Philippine Sea plate and the plausible resisting force induced on the back-arc side.

Active right-lateral strike-slip fault zone along the southern margin of the Japan Sea

Abstract We describe an active right-lateral strike-slip fault zone along the southern margin of the Japan Sea, named the Southern Japan Sea Fault Zone (SJSFZ). Onshore segments of the fault zone are delineated on the basis of aerial photograph interpretations and field observations of tectonic geomorphic features, whereas the offshore parts are interpreted from single-/multichannel seismic data combined with borehole information. In an effort to evaluate late Quaternary activity along the fault zone, four active segments separated by uplifting structures are identified in this study. The east–northeast-trending SJSFZ constitutes paired arc-parallel strike-slip faults together with the Median Tectonic Line (MTL), both of which have been activated by oblique subduction of the Philippine Sea plate during the Quaternary. They act as the boundaries of three neotectonic stress domains around the eastern margin of the Eurasian plate: the near-trench Outer zone and NW–SE compressive Inner zone of southwest Japan arc, and the southern Japan Sea deformed under E–W compression from south to north.

A Miocene pull-apart deformation zone at the western margin of the Japan Sea back-arc basin : implications for the back-arc opening mode

Abstract Opening mode of the Japan Sea is studied from the viewpoint of geologic structure around the western marginal fault zone of the back-arc basin. Latest reflection seismic survey in the Fukue Basin, located at the western end of the rifted sliver of southwest Japan, delineates an NNE–SSW transcurrent fault that is an extension of the Yangsan Fault running along the eastern coast of the Korean Peninsula. As suggested by paleomagnetic studies in Korea, the Yangsan Fault was activated with dextral motions during the early Miocene rifting event of the Japan Sea. Our offshore seismic study indicates that the NNE–SSW fault zone was simultaneously active with dextral sense because it constitutes a pull-apart depression on a right stepping of the fault. The Fukue Basin was under continuous transtensional stress regime during the Japan Sea rifting accompanied with a clockwise rotation of southwest Japan. It is discordant from the previous tectonic model of two-stage (parallel and fan-shaped) back-arc opening, which requires compressive regime with sinistral fault motions to the west of rotation pivot of southwest Japan in the second stage. The southward transportation of the rifted sliver with formation of pull-apart basins was the dominant mode along the western margin of the Japan Sea throughout the opening event. The southern part of the marginal fault zone has been immune from post-opening tectonic events, such as the late Miocene strong inversion on the back-arc margin of southwest Japan or the Quaternary E–W tectonic stress in the Japan Sea reactivating the northern sector of the fault zone, Yangsan Fault.

Neogene deformation of the back-arc shelf of Southwest Japan and its impact on the palaeoenvironments of the Japan Sea

Abstract The evolutionary process of the back-arc basins of Southwest Japan is presented on the basis of geological and geophysical data-sets. Surface geology and borehole information demonstrate rapid subsidence throughout the back-arc margin in the Early Miocene as a result of continental rifting. Uplift in the Middle Miocene and extensive truncation in the Late Miocene on the back-arc margin tend to become more intense eastward and westward, respectively. Palaeomagnetic studies show that the Middle Miocene uplift is linked to collision of the Izu-Bonin Arc with the eastern part of Southwest Japan and confined deformation of the island arc. Offshore seismic data illustrate that the Late Miocene unconformity event was caused by intra-arc folding under N-S compression. Coeval resumption of subduction of the Philippine Sea Plate is the probable cause of this episode. Seismic profiles also indicate that the folds converge into the Tsushima Strait. The crustal contraction brought about closure of the Tsushima Strait at the western end of Southwest Japan, blocking the warm Tsushima current, and so impacting strongly on the Neogene palaeoenvironments of the Japan Sea.

Quaternary geomorphic trends within Southwest Japan: Extensive wrench deformation related to transcurrent motions of the Median Tectonic Line

Abstract Large-scale transcurrent faults contribute to anelastic deformations along convergent plate boundaries. The Inner zone of Southwest Japan has been subjected to shearing deformations during the Quaternary, associated with dextral motions of the Median Tectonic Line, which was activated as a right-lateral fault by oblique subduction of the Philippine Sea plate. Considering the neotectonic framework, the continental crust of the Chugoku block in the western part of the Inner zone is simplified as a granitic test-piece under a simple shear. Geological and geographical data in the block are incorporated to produce a quantitative analysis of the deformation mode. Together with an empirical model of the rheology of an orogenic crust, the present results indicate that the Chugoku block behaves as a Newtonian fluid, and flows with a strain rate of about 10 −14 s −1 under a tectonic stress of 10 MPa. The mode of wrenching of the study area is compared with confined deformations of other plate margins in the same tectonic regime. Unique extensive wrench deformation (as wide as 150 km) along the Median Tectonic Line is attributed to the absence of weak zones in the continental crust, such as ductile melange belt or arc volcanism which is a common phenomenon in the area of active convergence. The Inner zone of Southwest Japan is immune from these major factors reducing the shearing strength of the crust because of the intensive intrusions of Cretaceous granites and the regional Quaternary thermal event.

Paleomagnetism, rock magnetism, and fission track dating of the Kurihashi Granodiorite in northeast Japan: Evidence for a Cretaceous hydrothermal event and differential rotation along the eastern Eurasian margin

Samples from the Late Cretaceous Kurihashi Granodiorite on the Pacific coast of northeast Japan are divided into two groups based on paleomagnetic data. One group (A) preserves stable single thermoremanent magnetizations (TRM) with large westerly deflected declinations. The other group (B) has two or three distinct components of natural remanent magnetization (NRM): components with high, middle, and low unblocking temperatures show westerly declinations (nearly identical with the stable TRM directions of group A), as well as northerly and easterly declinations. Rock magnetic experiments indicate that the dominant magnetic mineral of both groups is Ti-poor titanomagnetite. Group A samples contain superparamagnetic (SP) titanomagnetite because they show a frequency dependence with the initial susceptibility measurements: Hysteresis parameters of group B samples are consistent with a mixture of single-domain (SD) and multidomain (MD) grains. Electron microscopy confirms the presence of MD size magnetite in the group B samples. Fission track thermochronology indicates that the Kurihashi Granodiorite underwent a post cooling hydrothermal event during the Late Cretaceous; thus the northerly and easterly NRM components seem to have a thermoviscous origin. Many samples have anomalously high unblocking temperatures (up to 560°C) compared to predictions of SD theory and probably result from long thermal demagnetization “tails” of thermoviscous remanent magnetizations (TVRM) carried by MD magnetite. A strong TVRM was acquired during the hydrothermal event and is correlated with the long normal-polarity superchron of the geomagnetic field. Because highly oblique (left-lateral) subduction was active along the eastern Eurasian margin during the same period, the primary TRM and TVRM components record counterclockwise rotation (∼120°) of the Kurihashi Granodiorite and remarkable wrench deformation along the continental margin.

Geomagnetic and gravity modeling of the fore-arc shelf of North-east Japan

Abstract Northeast Japan is one of the continental slivers of the Eurasian Continent rifted by the Miocene fan-shaped genesis of the Japan Sea. A pair of longitudinal geomagnetic anomalies are traced along the Pacific coast of Northeast Japan. Basement core samples obtained from MITI Kesennuma-Oki well drilled on the southernmost part of anomaly belt are examined. The basement, which is identified as biotite granite and characterized by sheared fabric, shows early Cretaceous radiometric ages. Through rock-magnetic experiments, it was confirmed that the granitic basement holds thermoremanent magnetization (TRM) strong enough to cause the observed anomaly with a minor contribution from induced magnetization. The carrier of TRM is multi-domain titanomagnetite. On the basis of experimental results, two-dimensional geomagnetic and gravity modeling for semi-infinite magnetized polygons have been constructed along two E–W seismic lines on the fore-arc shelf of Northeast Japan. Vector components of magnetization are tuned through fitting anomaly peaks caused by basement topography, delineated by seismic survey. As for the northern part of study area, the inclination of westerly deflected magnetization vector should be 30° or smaller, the amount required to restore the Cretaceous Northeast Japan to the south of Southwest Japan. The gravity model for the same lines as the geomagnetic modeling suggests that fore-arc basement off the northern Kitakami Terrane consists of granitic rocks, except for a conspicuous basalt, whereas the non-magnetic basement off the southern Abukuma Terrane consists of a variety of blocks of different density. The anomalous direction of remanent magnetization and variety in gravity data are linked to NNW–SSE sinistral transcurrent faults activated throughout the Paleogene.

Numerical Modeling of Sedimentary Basin Formation at the Termination of Lateral Faults in a Tectonic Region where Fault Propagation has Occurred

地震や火山活動で生じた地殻変動の定量的解釈によく用いられるディス ロケーション・モデリング手法に,断層の活動履歴を取り込めるよう解の 重ね合わせを行い,堆積盆地形成のプロセスについての議論を試みた. 一般に,ディスロケーション・モデリングとは,半無限弾性体中に仮定 された断層面に食い違い量(ディスロケーション)を与え,地表面や地中の 変形やひずみなどを計算する手法の一つである.Steketee (1958)にはじまる ディスロケーションの研究は,半無限均質弾性体中の任意の傾斜角をもつ 断層面に対応した媒質表面と内部での変位やひずみに対する解が,解析的 に閉じた形で導かれるという格好で,Okada (1985)あるいは Okada(1992)に より一応の完成をみている.不均質構造を取り入れたモデルや,粘弾性媒 質を仮定したモデル(例えば,Matu’ura et al., 1981; Du et al., 1994)の解も与え られているが,実際の地殻変動データの解釈やモデリングでは, Okada(1985)や Okada(1992)で与えられている均質媒質中での解析解が多く 用いられ,またその解が実際の地殻変動データをよく説明している.本研 究でも,横ずれ断層端に形成されるプルアパートベイスンの形成モデリン グに,Okada(1985)の式を用いた. ディスロケーション・モデリングを用いたこれまでのプルアパートベイ スン形成シミュレーションでは,地殻中に仮定した断層に,一度だけの断 層運動を仮定している(例えば,Kusumoto et al., 1999).実際には,地質学的 時間スケールで何度も運動している.そこで,ディスロケーション理論が 線形弾性論に基礎をおいていることに着目し,同じ断層の解の重ね合わせ を行うにより,断層の活動履歴をモデルに取り込むことを試みた.例えば, 断層の活動域が変化する(進展あるいは後退する)ことが地質学的に明らか にされている場合,それらの条件に従って重ね合わせる断層の長さを変化 させる.このモデリング手法では,断層運動の様式をこまめに変化させる ことができるため,地質学的な証拠に基づいた断層運動の活動履歴をモデ ルに組み込むことができ,詳細なテクトニクスの議論を可能にする.実際, Itoh et al. (2008),Itoh et al (2009),Tamaki et al (2010)らは,このモデリング 手法により,断層の活動履歴や運動量の変化についての議論を行っている. 北海道中部に分布し,約 48Ma から 12Ma 頃にかけて形成された堆積盆 地群は,現在地表で観測されている断層の逆断層運動によるものなのか, 或いは,横ずれ運動によるものなのか未だ決定的な結論が出されていない. 本研究では,これらの堆積盆地の分布や形状を,上述のディスロケーショ ン・モデリング手法により再現することを試みた.その結果,北海道中部 地域の堆積盆地は,基本的に地表面で観測されている断層の右横ずれ運動 により形成されるプルアパートベイスン群として再現できることが明らか にされた.顕著な逆断層運動は,川端期(15Ma-12Ma)後期の日高山脈形成 時だけであった.48Ma 頃から川端期前期までは右横ずれ断層が卓越して いたが,川端期後期から突然,逆断層運動が卓越するようになる.現段階 では原因(力源)は特定できていないが,この時期に広域応力場の大きな転 換があったと考えられる.

Integrated study of high resolution geophysical and geological information of Osaka Bay, Southwest Japan

Abstract The stratigraphic framework of Quaternary sediments in the Osaka Basin, Southwest Japan was revealed by high resolution geophysical and geological surveys acquired after the 1995 Kobe Earthquake. Osaka Bay is located in the central part of the Osaka Basin and is underlaid with Pre-Neogene basement rocks covered by an unconsolidated sequence of Plio-Pleistocene marine, fluvial and lacustrine sediments. Fifteen laterally continuous marine clays (from Ma-1 to Ma13, in ascending order) have been identified throughout the Osaka Basin that have been correlated with the marine eustatic record. Deep borehole and high resolution seismic data were acquired in Kobe (northern part of the basin) and Kansai International Airport (southern part of the basin). Sequence stratigraphy defined by seismic reflectors was used to reveal the stratigraphic differences between the two areas. By identifying reflectors as marine clay layers throughout the basin, we were able to divide the study area into three parts (northern, middle and southern parts) and to estimate the sedimentation rate in each location. The sedimentation rate increases from the northern and southern margins of the basin towards central Osaka Bay in the middle of the basin. In the southern parts, the sharp decline of sedimentation rate between Ma2 and Ma4 and thinning or complete lack of the reflectors corresponding to Ma5 and Ma6 layers result from tilting in this region.