Japan Sea Opening Research Articles

日本海拡大時の中央および西南日本前弧域テクトニクス

Studies on geologic structures including some deformation microstructures in both areas along the Median Tectonic Line (MTL) in western Shikoku and of the South Fossa Magna indicate that N-S trending horizontal compression tectonics prevailed in these areas during the earliest Middle Miocene (ca. 15 Ma). Also, a review of structural development in Nankai province indicates that similar compression tectonics prevailed during the periods from the Early to earliest Middle Miocene (ca. 20-15 Ma). These periods are nearly coeval with the timing of the Japan Sea opening. Although there is uncertainty about the reactivation of the MTL around this time, south-dipping normal faults were formed in the Sambagawa belt truncating the MTL to produce the Kuma basin (18-16 Ma), which were later reactivated as thrusts (Tobe and Hanayama thrusts), indicating inversion tectonics. In the South Fossa Magna region, not only a large amount of shortening—as much as 40-50%—was caused in the Momonoki Subgroup (earliest Middle Miocence, 16-15 Ma) by folding strata, but also the constituent quartz grains were moderately plastically deformed and microcracked followed by healing (i.e. healed microcracks), indicating deformation under brittle-ductile transition conditions (ca. 300°C) at ca. 15 Ma. Furthermore, slaty cleavage caused by pressure solution developed in the strata constituting the Oligocene-Early Miocene (>20 Ma) Southern Shimanto belt, and forearc sediments (18-15 Ma) as thick as 4000 m overlying these sediments with angular unconformity (e.g. Tanabe and Kumano Groups in the Kii peninsula) were also deformed by folding with a shortening ratio of 10-20%. All these onland geological facts in the forearc region indicate that the forearc region was in a regime of compression, while the back-arc region was in a regime of extension at the same time during the Japan Sea opening. Similar tectonics also occurred in Italy, where the Tyrrhenian Sea (back-arc basin) has opened since the Pliocene (5 Ma), while shortening tectonics shown by the development of fold and thrust belts and deep (>7 km) basins occurred at the same time in the forearc region. Although these concurrent back-arc extension and forearc shortening are difficult to interpret, they may be caused by ridge push force (i.e. gravity tectonics) after back-arc spreading commences so that extensional force is no longer sustained in the arc.

新生代北部九州のテクトニクス史と火山活動史―背弧火山岩区研究における重要性―

Cenozoic western Pacific is characterized by numerous subduction volcanisms, opening of back arc basins, and within-plate volcanisms in backarc regions. These phenomena suggest that thermal disturbances induced by mantle flow caused mantle melting and eruption of magmas in back-arc regions, although diverse modes have been proposed for the mantle condition. To constrain the origin of the magmatism, it is important to clarify both the geodynamics and physicochemical conditions of the mantle in the arc-back arc system. Cenozoic back-arc volcanisms and tectonics in northern Kyushu are key objectives. This paper focuses on the dynamics of the upper mantle in relation to the tectonics of the area. Cenozoic volcanism of the northern Kyushu district is characterized by eruptions of a number of alkaline volcanisms. This volcanic province extends some one hundred kilometers along the Japan Sea coast and includes the Chugoku area. Although the volcanism has been regarded as a large single volcanic province, the northern Kyushu activity is distinguishable from that in Chugoku by geochemical characteristics. Cenozoic basalt volcanism in northern Kyushu occurred mainly in the Tertiary sedimentary basins, which is common to Cenozoic volcanism in the Japan Sea Basin. The sedimentary basin deposits underwent intensive folding with extensive inversion tectonics occurring mainly around the end of Miocene. Studies on dykes and thrust faults also indicate a compressional stress field dominated in northwestern Kyushu during the late Miocene. Initiation of the basaltic activity took place in Kita-Matsuura area and expanded outwards. Basaltic rocks from the Kita-Matsuura are mostly hypersthene-normative and sub-alkalic, whereas rocks from peripheries, such as North Kyushu and Goto Islands, are predominantly less normative-hypersthene and thus are alkalic. From spatial and temporal variations and genetic conditions of basalt magmas, a most plausible source was a small-scale mantle diapir with a potential temperature of more than 1400°C. Combined with an inversion tectonic background after the Japan Sea opening event, the origin of the back-arc magmatism was unrelated to large-scale mantle upwelling. Fluid-fluxed melting of the mantle is not supported by the chemistry. My proposal is active upwelling of a small-scale mantle diapir, probably from the depth of stagnant slab or mantle transition zone.

Synchronous Japan Sea opening Miocene fore-arc volcanism in the Abukuma Mountains, NE Japan: An advancing hot asthenosphere flow versus Pacific slab melting

During Japan Sea back-arc opening between 21 and 15 Ma, low-K tholeiitic basalts erupted in the Abukuma Mountains on the trench side of NE Japan. The basaltic rocks in this region are divided into two units by a hiatus. The lower unit consists of the 20 Ma Tenmyosan volcanic rocks, including many monogenetic basaltic volcanoes whose vents occur throughout the district. The upper unit is the Ryozen Formation that occurred at about 16 Ma. This Formation is composed of a subaerial basaltic polygenetic volcanic edifice, with a diameter of ca. 30 km. Minor amounts of andesitic and dacitic gravels occur in the distal portion of the Ryozen edifice. Low-K tholeiitic basalts of the Tenmyosan volcanic rocks and the Ryozen Formation have similar in major and trace element compositions. Their MgO content decrease from 13.2 to 3.4 wt.% with increasing incompatible element abundances and more pronounced negative Nb and Ta anomalies. Nd, Sr and Pb isotopic compositions of the Tenmyosan basalt display a range of values for SrI (0.7040–0.7050), NdI (0.5127–0.5129) and 206Pb/ 204Pb (18.2–18.5). These features suggest that the Tenmyosan monogenetic volcanism at the early stage of the Japan Sea opening was generated by intermittent ascent of minor mafic melts from heterogeneous sources. Isotopic data for the Ryozen basalt define a binary mixing trend between a depleted and enriched crust-like source, varying with SiO 2 content. These chemical changes show that, as the opening progressed, the asthenospheric source of the Ryozen basalts became more homogeneous and depleted (SrI; 0.7043, Nd; 0.5129, 206Pb/ 204Pb; 18.2), and continuous ascent of the basaltic melt was able to make a large magma chamber that was associated with significant crustal assimilation. On the other hand, hornblende dacite in the Ryozen Formation has SiO 2 contents between 66 and 74 wt.%, high Mg# from 41.7 to 54.3. The dacite also shows high Sr (417–650 ppm) and Sr/Y (35–112), and high La/Yb (12–20). Compared with the basaltic rocks, the Ryozen dacite has relatively low SrI (0.70436) and high NdI (0.5128–0.5129). These features show that the dacite has no genetic relationship with the basaltic rocks and can be considered an adakite, traditionally viewed as product of eclogitic slab melting. The 14 Ma Nodegamiyama andesite has SiO 2 in 54 to 55 wt.%, high Mg# (59.0–62.3), high Cr and high Ni contents. The isotopic ratios of this andesite are enriched (SrI; 0.7054–0.7055, NdI; ~ 0.5127, 206Pb/ 204Pb; ~ 18.5). This high-Mg andesite may be generated by melting of subducting sediments and interaction of such melts with overlying mantle peridotites. Although the downgoing Pacific plate was Cretaceous in age and thus too cold to melt under normal conditions, incursion of the hot asthenosphere above the slab during the Japan Sea opening may have caused melting to occur.

Surface Wave Tomography of Lithospheric Structure in the Seas of East China

AbstractThis paper presents the S‐wave velocity structure of crust and upper mantle in the marginal seas of east China resulted from surface wave tomography, describes the structural features of various tectonic units in the region, and discusses their relation with present tectonic activity and past evolution history. The average velocity of the middle and lower crust has a remarkable relation with strong earthquakes, which took place within the low velocity zones or along the boundaries between low and high velocity. There are several NW‐trending low velocity zones in the crust east of the Taihang Mountains, among them the one beneath the Zhangjiakou– Bohai seismic belt is most conspicuous. The sea area can be divided into 4 tectonic units, i.e., the northern and southern Yellow Sea, the East China Sea, and the Okinawa Trough. The northern Yellow Sea has a thin and high velocity lithosphere similar to that of southern North China Basin, presumably resulted from Mesozoic lithosphere thinning caused by the northward subduction of Paleotethys. In contrast the northern North China has a low velocity crust and thicker lithosphere, the velocity of lithospheric mantle is rather low and homogeneous vertically, possibly indicating the result of long‐lasting Mesozoic collision along the northern suture zones. The southern Yellow Sea has a thicker lithosphere with the characteristics of lower Yangtze craton. In the suturing process of lower Yangtze and North China the subduction of oceanic lithosphere might have caused the low velocity features of the upper mantle in northern Yellow Sea and Su‐Wan region. However, such suturing relics were not found in Korea Peninsula east of 125°E. It may imply that the whole Korea Peninsula is a part of North China Craton, or the effect of Pacific subduction and Japan Sea opening might have totally reformed the lithospheric mantle beneath the peninsula and erased the sign of past suturing. In the East China Sea the crust, especially the lithosphere thins towards the Okinawa Trough, indicating that the subduction of Philippine Sea plate affects a vast back‐arc area. In the Okinawa Trough the subducting Philippine slab and the conspicuously low velocities above the slab are clearly seen, providing evidence for the back‐arc spreading mechanism of the Okinawa Trough formation.

Anomalous intra-plate high-Mg andesites in the Choshi area (Chiba, Central Japan) produced during early stages of Japan Sea opening?

Geochemical and isotopic data are reported for 20 Ma high-magnesian andesite (HMA) lavas erupted on the forearc side of Central Japan in the Choshi coastal area (Chiba). Rarely olivine-phyric (< 3% vol), these rocks have ca. 56 wt.% SiO 2, Na 2O ranging between 3.9 and 4.2 wt.%, and relatively low K 2O/Na 2O ratios (ca. 0.44). Their Mg numbers are relatively high (ca. 0.64) and matched by high Cr (> 345 ppm), high Sr (> 520 ppm), high Sr/Y (ca. 30), and high [La/Yb] N (> 8), conforming to the definition of low-silica adakites, transitional in character to HMA. However, despite slight negative Nb and Ta ‘troughs’ in a few samples, their incompatible element distribution patterns appear closer to those of intra-plate basalts, apparently confirming previous suggestions that terms such as HMA and adakite cover a diverse range of compositions. Enriched in radiogenic Sr and Nd (ca. 0.7040 to 0.7043 and 0.5128 to 0.51275, respectively), the Choshi HMA shows relatively low 206Pb/ 204Pb (18.25 to 18.35) and 208Pb/ 204Pb (38.37 to 38.48) ratios, resembling those of late Miocene intra-plate basalts in the Japan Sea and SW Japan. In this regard the Choshi HMA differs from those of the ‘classic’ Middle Miocene (14 to 12 Ma) HMA described from the Setouchi volcanic belt (e.g. Tatsumi, 1981), a short distance to the west which also shows relative high K contents. Although HMAs have been generally attributed to subduction-related processes, the lack of high field strength element (HFSE) depletions cf. large ionic lithophile elements (LILE) in most Choshi HMA suggests an absence of so-called ‘subduction signatures’. Accordingly, we suggest that the Choshi HMA probably tapped a fertile (asthenospheric) region of the convecting mantle wedge, primitive melt products having acquired HMA-like character as a result of shallow level modification by interaction with lithospheric components, aided by thermal input resulting from mantle extrusion, during the early stages of Japan Sea opening (23–15 Ma).

Late Eocene shoreline volcanism along the continental margin:the volcanic succession at Kabuki Iwa, Oga Peninsula, NE Japan

The Oga Peninsula is situated on the Japan Sea side of the NE Japan Arc (Fig. 1). A Cenozoic succession is well exposed along the coast and has been referred to as a type of Cenozoic succession of northern Japan since Huzioka (1959) established the stratigraphic framework of the succession. Volcanic rocks are extensively exposed in the western part of the peninsula and occupy the lower part of the Cenozoic succession (Huzioka, 1959, 1973). They are believed to represent the initial process of the Japan Sea opening that prevailed during the Early Miocene time, but their volcanological and tectonic aspects remain poorly understood (e.g. Kano et al., 2007a,b, 2008; Kobayashi et al., 2008). This paper describes the occurrence of the volcanic rocks which are exposed at Kabuki Iwa (Rock) near the northern tip of the peninsula (Fig.1), in order to reveal volcanic and associated tectonic activities at that time. The lower volcanic succession in the western Oga Peninsula is divided into the Akashima Formation, Monzen Formation, and Nomuragawa Formation in ascending stratigraphic order and is unconformably overlain by the Middle to Late Miocene marine sediments of the Nishikurosawa and Onnagawa Formations (Figs. 1 and 2). The Nomuragawa Formation is a stratigraphic unit newly identified by Kobayashi et al. (2004). This formation comprises mainly dacite pyroclastic flows, basaltic andesite lava flows and scoriaceous deposits and is correlative with the main part of the Daijima Formation distributed on the immediate eastern side (Fig. 2). The Daijima Formation is accompanied by non-marine clastic sediments at its uppermost part, but radiometric dates indicate both the Nomuragawa and Daijima Formations are Early Miocene in age and mutually correlative (Kobayashi et al., 2004). The Monzen Formation is a mafic to felsic volcanic complex emplaced in a transition area from land to shallow water (Kobayashi et al., 2008) and can be divided into eight units (Fig. 2). Radiometric ages reported for this volcanic succession have been diverse but could span a few million years of Late Eocene to Early Oligocene time (Kano et al., 2007b, 2008). There are many contemporaneous dikes extending mostly in a NE-SW direction, as shown by the geological maps (Huzioka, 1959, 1973), perhaps reflecting an NW-SE extensional stress field of that time (Yamamoto, 1991). The upper most subaqueous rhyolite lava dome of Shinzan Rhyolite is over hundreds of meters thick and suggests the increase of crustal rifting. On the other hand, the overlying Nomuragawa Formation and Daijima Formation are characterized by bimodal volcanism and on-land eruptions. Shallow-water sediments of the uppermost Daijima Formation are likely to record the succeeding subsidence of this area in association with Late Eocene shoreline volcanism along the continental margin: the volcanic succession at Kabuki Iwa, Oga Peninsula, NE Japan

日本海竹島海山 (仮称) のかんらん岩捕獲岩の成因とその意義

The Sea of Japan is one of the back-arc basins distributed around the western rim of the Pacific. Nature of the plume for back-arc basin (= BAB) opening is unknown partly because petrological characteristics of the BAB mantle are not well known compared with mid-ocean ridge (= MOR) mantle. Peridotite xenoliths in an alkali basalt dredged from the Takeshima seamount (tentative name), ca. 230 km to the north of Oki islands, are examined to clarify the petrological feature of the Japan-Sea mantle. They are spinel-bearing lherzolite to harzburgite with equigranular to porphyroclastic textures. They can be classified into two types (Type 1 and Type 2 peridotites) in terms of REE (= rare earth elements) patterns of clinopyroxene (= cpx). Type 1 peridotites (Cr# of spinels, 0.4-0.5) are similar to abyssal peridotites in terms of major element chemistry and middle- to heavy-REE concentrations of cpx. The cpx in the former shows slight enrichment of LREE (= light REE), which is apparently due to metasomatism by slab-derived fluids/melts. Type 2 peridotites have high-REE cpx with flat to slightly LREE-enriched patterns, and are quite similar to subcontinental peridotites obtained as xenoliths from China. The southern part of the Sea of Japan was formed by mainly thinning and rifting of the continental lithosphere. Type 2 peridotites are a remnant of the continental lithospheric mantle. Chemical characteristics of BAB basalts and Type 1 peridotites indicate that open-system melting of peridotite affected by a flux from the downgoing slab was involved in the Japan-Sea opening.

Last stage of the Japan Sea back‐arc opening deduced from the seismic velocity structure using wide‐angle data

The Japan Sea is one of the most well studied back‐arc basins in the northwestern Pacific. The seismic crustal model, however, has been inadequate to elucidate the detailed opening model of the Japan Sea. In 2002, to clarify the late stage of the formation style of the Japan Sea opening, a seismic experiment using 35 ocean bottom seismographs (OBSs), an air gun array, and a multichannel hydrophone streamer was undertaken in the areas from the southwestern Yamato Basin, the Oki Ridge, and the southwestern Oki Trough to the coast of the southwestern Japan Island Arc. The crusts beneath the southwestern Yamato Basin and the Oki Ridge are estimated as having approximately 13 km and 19.5 km, respectively. The upper and lower crusts of the southwestern Yamato Basin are approximately 3.2 km and 8 km thick, respectively. Those of the Oki Ridge are approximately 8.2 km and 10.5 km thick, respectively. The upper crust of the Oki Ridge thickens more steeply than that of the southwestern Yamato Basin; however, the lower crust thickens more gently. The crustal structure of the southwestern Yamato Basin shows the extended continental crust accompanied with the opening of the Japan Sea. A remarkable structural characteristic, the upper crust being thinner than the lower crust, caused by listric or complicated normal faults developed in the upper crust of the southwestern Yamato Basin. This deformed upper crust is a common structural characteristic in the southern Japan Sea, which includes the Yamato Basin. The southern Yamato Basin, including the southwestern Yamato Basin, has the thinnest upper and lower crusts in the Japan Sea. For that reason, it is suggested that the southern Yamato Basin had the strongest deformation by a back‐arc opening and that the period of the opening in the southern Yamato Basin had been longest in the southern Japan Sea. The formation process of the southern Yamato Basin is inferred to have two stages: rifting and extension of continental crust separating the northeastern and southwestern Japan Island Arcs from the Asian continent and, further, the extension affected by the rotation of the southwestern Japan Island Arc.

富山県八尾地域に分布する下部‐中部中新統の古地磁気学的研究

Machiko Tamaki, Yasuto Itoh and Mahito Watanabe (2006) Paleomagnetism of the Lower to Middle Miocene Series in the Yatsuo area, eastern part of southwest Japan: clockwise rotation and marine transgression during a short period. Bull. Geol. Surv. Japan, vol. 57 (3/4), 73-88, 8 figs, 1 table. Abstract: Process of the tectonic rotation related to Japan Sea opening is viewed from tilt-corrected paleomagnetic data obtained from the Early to Middle Miocene sequence in the Yatsuo area on the Japan Sea coast, eastern part of southwest Japan. Progressive demagnetization experiments have been executed on samples obtained from two routes along the Yamada and Wada Rivers, and success- fully isolated stable primary magnetic components in 16 sites. Together with data of previous studies, tilt-corrected reliable paleomagnetic directions comprising 10 normal and 27 reversed polarity sites of the Yatsuo Group on accurate stratigraphic positions show systematic temporal variation of east- erly deflection in declinations. On the basis of our presented magnetostratigraphy, Yatsuo Group is assigned from the late Early to early Middle Miocene. Significant stage of rotation as much as 30° is specified during late Early Miocene between a period of active volcanism and subsequent period of marine transgression. We suggest that the Japan Sea opening event had a final phase of rapid rotation around the late Early Miocene, which seems to have caused marine transgression in relation to re- markable subsidence along the Japan Sea coast.

日本海拡大時の東北日本弧と西南日本弧の境界

日本海拡大時の東北日本弧と西南日本弧の境界

Slip sense inversion on active strike-slip faults in southwest Japan and its implications for Cenozoic tectonic evolution

Analyses of deflected river channels, offset of basement rocks, and fault rock structures reveal that slip sense inversion occurred on major active strike-slip faults in southwest Japan such as the Yamasaki and Mitoke fault zones and the Median Tectonic Line (MTL). Along the Yamasaki and Mitoke fault zones, small-size rivers cutting shallowly mountain slopes and Quaternary terraces have been deflected sinistrally, whereas large-size rivers which deeply incised into the Mio-Pliocene elevated peneplains show no systematically sinistral offset or complicated hairpin-shaped deflection. When the sinistral offsets accumulated on the small-size rivers are restored, the large-size rivers show residual dextral deflections. This dextral offset sense is consistent with that recorded in the pre-Cenozoic basement rocks. S–C fabrics of fault gouge and breccia zone developed in the active fault zones show sinistral shear sense compatible with earthquake focal mechanisms, whereas those of the foliated cataclasite indicate a dextral shear sense. These observations show that the sinistral strike-slip shear fabrics were overprinted on dextral ones which formed during a previous deformation phase. Similar topographic and geologic features are observed along the MTL in the central-eastern part of the Kii Peninsula. Based on these geomorphological and geological data, we infer that the slip sense inversion occurred in the period between the late Tertiary and mid-Quaternary period. This strike-slip inversion might result from the plate rearrangement consequent to the mid-Miocene Japan Sea opening event. This multidisciplinary study gives insight into how active strike-slip fault might evolves with time.

Japan Trench and tectonics of the Japanese Island Arcs

Abstract Seven chronostratigraphic stages were established based on the correlation of magneto‐biostratigraphic marker horizons within the Deep Sea Drilling Project and Ocean Drilling Program cores from the forearc of the Japan Trench. Because the stages are coeval with changes in the rate of sedimentation, lithofacies, magnetic intensity and composition of fossil assemblages, they probably reflect the tectonic situation in the Japan Trench forearc and also in the arc–trench system. The stages correlate to the tectonic events of the Japanese Island Arcs; for example, evolution of the Boso triple junction, initiation of Philippine Sea Plate subduction and the Japan Sea opening.

瀬戸内高マグネシア安山岩及び玄武岩の希土類元素組成：日本海拡大時におけるマントルウエッジの組成変化に関する示唆

瀬戸内高マグネシア安山岩及び玄武岩の希土類元素組成：日本海拡大時におけるマントルウエッジの組成変化に関する示唆

Early Miocene rotational process in the eastern part of south‐west Japan inferred from paleomagnetic studies

Abstract This paper provides untilted paleomagnetic data obtained from the early Miocene strata around the Kanazawa‐Iozen area, in the eastern part of south‐west Japan. A thick pile of volcaniclastics and marine transgressive sediments underlie the area; they were deposited in the early stage of the Japan Sea opening event. Progressive thermal demagnetization tests isolated stable primary magnetic vectors from eight sites in the upper part of the Iozen Formation. Overall, the tilt‐corrected mean direction of this unit is D = 36.4°, I = 51.6° and α95 = 12.1. Together with a published paleomagnetic and chronological database, the present results suggest that clockwise rotation of south‐west Japan, linked to the back‐arc opening, commenced in the early Miocene and accelerated at the same time as rapid subsidence along the Japan Sea coast. Post‐opening, differential rotation within the eastern part of south‐west Japan is assumed, based on selected paleomagnetic data from the latest Early Miocene.

Cenozoic deformation history of the Tancheng–Lujiang Fault Zone, north China, and dynamic implications

Abstract Based on a field analysis of slip vectors from Cretaceous and Tertiary rocks and coupled with rift basin analysis in north China, the Cenozoic deformation history of the Tancheng–Lujiang (Tan–Lu) Fault Zone can be divided into three main phases: early Tertiary normal faulting and northwest–southeast extension; Miocene normal faulting and northeast–southwest to north‐northeast–south‐southwest extension; and Quaternary dextral strike–slip faulting and east‐northeast transpression. The early Tertiary extension, which was responsible for rift basin formation in north China, originated from back‐roll mantle convection induced by westward subduction of the Pacific Plate beneath the Asia continent. The Miocene extension occurred possibly in association with the process of the Japan Sea opening. The Quaternary dextral slip was mainly localized along the middle part of the Tan–Lu Fault Zone and resulted from the far‐field effect of the late‐stage India–Eurasia convergence.

Cenozoic episodic volcanism and continental rifting in northeast China and possible link to Japan Sea development as revealed from K–Ar geochronology

Cenozoic volcanic rocks in northeast China comprise part of the volcanic belt of the western circum-Pacific rim and represent one of the presently active tectono-magmatic regions of the world. Over 590 volcanoes with erupted basalts covering an area of ∼50,000 km 2 are exposed in the Songliao graben and particularly on its flanks. K–Ar dating of the volcanic rocks sampled from a selection of eruption sites over the region reveals that the volcanism occurred episodically through the entire Cenozoic corresponding to the tectono-magmatic interactions particularly between the India, Eurasia and Pacific Plates. The volcanism initiated in the Songliao graben in late Cretaceous (∼86 Ma) and migrated flankward since ∼28 Ma. The magmatic propagation shows three major stages intimately associated with the evolution of Japan Sea and related to the geodynamic forcing of the India–Eurasia collision. Prior to Japan Sea rifting, volcanic eruptions were restricted to the Songliao basin. During Japan Sea opening, volcanism migrated into the lateral belts. After cessation of Japan Sea opening at ∼15 Ma, volcanism with greatest intensity occurred on both flanks particularly along the Yi–Yi fault system around 14–13 Ma and subsequently the Fu–Mi fault between 11 and 7 Ma. Continuing this trend of migration, eruptions scattered further away from the Songliao basin in the last 4.5 Ma. We suggest that the Cenozoic volcanic belt in northeast China and the Japan Sea backarc basin are competitive parts in the eastern-marginal magmatic system of the Eurasian Plate. The early stage of the volcanism in the Songliao basin may have resulted from mantle upwelling beneath the Songliao–Bohai basin belt. Rifting of the NE faults and relatively weak intensity of volcanism in northeast China during Japan Sea opening could be attributable to the lithospheric breakup in the Japan Sea and spreading of Japan Sea. The subsequent strong volcanism in northeast China after cessation of the Japan Sea opening implicates the easiest pathway for the melts of the magmatic system in the plate margin. The diversified volcanism in scattered localities during the Quaternary may indicate localized melting processes or notable modifications in smaller sized individual domains and possibly, a magmatically waning phase of the limited source in eastern China.

The block rotation in the Uetsu area, Northern Part of Niigata Prefecture, Japan

Paleomagnetic and geologic studies were made in the Uetsu area, NE Japan, to clarify the Miocene intra-arc deformation associated with the Japan Sea opening. We obtained paleomagnetic directions from the Early to Middle Miocene pyroclastic rocks and lavas in the area. The average paleomagnetic direction is D = 28.0°, I = 48.7° and α95 = 9.6°. It suggests that the clockwise rotation occurred in this area, which is opposite to the counter-clockwise rotation of NE Japan. A fault was discovered which could be the boundary of this rotating domain. The dextral movement is recognized for this fault, concordant with the clockwise rotation in the area. These paleomagnetic and structural data suggest that the clockwise block rotations occurred in the Uetsu area, accompanied by the dextral deformation.

Paleomagnetism and fission-track ages of Oki-Dogo Island in Southwest Japan

Zircon fission-track (F.T.) ages and paleomagnetic directions were measured on the Neogene series of the Oki-Dogo Island. The results of F.T. dating on the the Tokibariyama Formation show ages of 23.7 ± 1.3 and 20.0 ± 2.3 Ma, indicating the early Early Miocene. Paleomagnetic measurements of the Tokibariyama, Kori Formations (24–17 Ma) and the Oki Trachyte-Rhyolite (6.8 Ma) were done to reveal the movement of Oki-Dogo Island during the Japan Sea opening. The Tokibariyama and Kori Formations show an average declination of 39.9°. The Oki Trachyte-Rhyolite has the same average geomagnetic direction as the present day value. These results show that a clockwise rotation of 40° ± 15° of the Oki-Dogo Island occurred between 17 and 6.8 Ma which is similar to the main land of southwest Japan.

Metallogenic epochs in the Inner Zone of southwest Japan

The principal metallogenic epochs in the Inner Zone of southwestern Japan are classified by considering mainly radiometric and microfossil age data, as follows—(1) Early Cambrian Epoch: Characterized by podiform-type chromite deposits hosted in ophiolite complexes; these are the earliest-formed mineral deposits in the Japanese Islands, and probably formed in the shallow mantle of island-arc settings; (2) Carboniferous to Permian Epoch: Characterized by Besshi-type base metal sulfide deposits mainly associated with submarine lavas and tuffs of basaltic composition including their metamorphosed equivalents, and by bedded cupriferous pyrite deposits associated with submarine acidic tuffs. Both types of deposits would have been formed in a back-arc basin environment, geochemically influenced by an island-arc; (3) Middle Triassic to Jurassic Epoch: Characterized by most of the bedded manganese deposits associated with bedded cherts, including their metamorphosed equivalents, which would have formed by hot spring activity in an ocean basin environment with a limited current circulation. The manganese deposits and their host rocks are the result of seafloor accretion; (4) Late Cretaceous to Paleogene Epoch: Characterized by extensive continental-arc magmatism (volcanic and plutonic), resulting in formation of various mineral deposit types, related mainly to ilmenite series (Late Cretaceous) or magnetite series (Paleogene) granitic rocks. Related to the former are polymetallic skarns and veins containing Cu, Sn and W; also related to this metallogenic activity are stratabound hydrothermal clay deposits of kaolin, pyrophyllite, and sericite associated with pre-ilmenite series granitoids subaerial felsic pyroclastic rocks. Molybdenite-bearing veins and replacement-type sericite deposits are characteristics of the magnetite series terranes, with local skarns and vein occurrences. As with ilmenite — series granitoids occurring in southwestern Japan, eastward younging is also recognized in these mineral deposits, regardless of ore type; (5) Neogene Epoch: Characterized by intense submarine volcanism of acidic composition and associated Kuroko deposits in the Green Tuff region, a major tectonic division and a major metallogenic province of the Neogene Tertiary of Japan. These mineral deposits would be related to Japan Sea opening. In the `Non-Green Tuff region,' important mineralization events are represented by sedimentary uranium deposits, and veins characterized by Au, Sb or Hg occurrences; (6) Plio-Pleistocene Epoch: Characterized by epithermal Au–Ag and Au–Ag–Cu veins associated with subaerial andesitic to dacitic volcanic rocks of Plio-Pleistocene age. Volcanism of this and following epochs is characteristic of an island-arc setting; and (7) Quaternary Epoch: Characterized by volcanic sulfur deposits and minor iron sand deposits.

K–Ar ages of the basaltic rocks from Far East Russia: Constraints on the tectono‐magmatism associated with the Japan Sea opening

K–Ar ages of the Cenozoic basaltic rocks from the Far East region of Russia (comprising Sikhote‐Alin and Sakhalin) are determined to obtain constraints on the tectono‐magmatic evolution of the Eurasian margin by comparison with the Japanese Islands, Northeast China, and the formation of the back‐arc basin. In the early Tertiary stage (54–26 Ma), the northwestward subduction of the Pacific Plate produced the active continental margin volcanism of Sikhote‐Alin and Sakhalin, whereas the rift‐type volcanism of Northeast China, inland part of the continent began to develop under a northeast–southwest‐trending deep fault system. In the early Neogene (24–17 Ma), a large number of subduction‐related volcanic rocks were erupted in connection with the Japan Sea opening. After an inactive interval of the volcanism ∼ 20–13 Ma ago, the late Neogene (12–5 Ma) volcanism of Sikhote‐Alin and Sakhalin became distinct from those of the preceding stages and indicated within‐plate geochemical features similar to those of Northeast China, in contrast to the Japan Arc which produces island arc volcanism. During the Japan Sea opening, the northeastern Eurasian margin detached and became a continental island arc system, and an integral part of continental eastern Asia comprising Sikhote‐Alin, Sakhalin and Northeast China, and the Japan Arc with a back‐arc basin. The convergence between the Eurasian Plate, the Pacific Plate and the Indian Plate may have contributed to the Cenozoic tectono‐magmatism of the northeastern Eurasian continent.