The regional tectonic stress field, basin development, and crustal deformation of the NE Japan arc in the interval between 32 Ma to the Quaternary can be synthesized based on dike, vein, and fault orientation data, as well as on the compilation of the regional geology. An extensional stress field became prevalent from 32 Ma, and major normal faulting started at 25–20 Ma, which resulted from back arc rifting. Normal faulting, trending nearly parallel to the arc, propagated from the present Japan Sea coast to the forearc side, following the trenchward migration of the main volcanic field. From 20 to 15 Ma, normal faults with an oblique trend to the arc developed due to the counterclockwise rotation of NE Japan. The rapid clockwise rotation of SW Japan since 16 Ma produced a NW-SE directed transtensional stress regime in the NE Japan arc. Due to crustal stretching associated with this pull-apart movement, the back arc side of the NE Japan arc subsided rapidly to middle bathyal environments. After the termination of the opening of the Japan Sea at about 14 Ma, a neutral stress regime prevailed, which included phases of both weak extension and compression. Lithospheric cooling eventually led to thermal subsidence of the back arc region, and igneous underplating caused uplift of the axial zone of the volcanic arc. The increase in velocity of the westward motion of the Pacific plate at around 4 Ma produced strong compression across the arc, reactivating most of the Miocene normal faults, and uplifting the volcanic arc. The greatest crustal shortening occurred in areas that were stretched the most in the Miocene within the volcanic arc, which implies tectonic inversion.

The relationship between Late Cenozoic tectonic events and stress field and basin development in northeast Japan

The relationship between late Cenozoic tectonic events and stress field and basin development in northeast Japan

[1] Hf isotope compositions, coupled with Pb-Sr-Nd isotopes and trace element compositions, for the Miocene to the Quaternary volcanic rocks from the NE Japan arc have documented the geochemical evolution in the magma source compositions in association with back-arc opening (23–14 Ma). Clear temporal variation in Hf isotope ratios is demonstrated, from the lowest ɛHf (+9) in the early Miocene to the highest ɛHf (+14) in the late Miocene and the Quaternary, whereas there are little changes in Sr and Nd isotope composition. Hf isotope ratios are clearly correlated with some trace elemental ratios. The observed temporal variation is attributed to a change in metasomatic agents derived from the subducted slab, which could have been caused by cooling of the mantle wedge during the syn and post stages of the back-arc opening. In the early stage of the back-arc opening, when injection of hot asthenospheric mantle induced high-temperature conditions in the mantle wedge, slab melting contributed to low ɛHf coupled with low ɛNd, relatively low La/Nb, and high Th/Yb in volcanic rocks. On the other hand, the later stage volcanism, characterized by high ɛHf, decoupling of ɛHf and ɛNd, high and variable La/Nb and Ba/La, is best explained by addition of fluids from subducted sediments and oceanic crust to the mantle wedge.

Contribution of slab melting and slab dehydration to magmatism in the NE Japan arc for the last 25 Myr: Constraints from geochemistry

Geochemical secular variation of magma source during Early to Middle Miocene time in the Niigata area, NE Japan: Asthenospheric mantle upwelling during back-arc basin opening

Nd and Sr isotopic compositions were determined for the Tertiary volcanics from the back-arc side of the NE Japan arc. Sr isotopes show a linear trend through time from an enriched signature (87Sr86Sr = 0.705437) to a depleted signature (87Sr86Sr = 0.70270). In a complementary fashion, Nd isotopes start at low value (ϵNd = −0.80) and show a gradual increase (ϵNd=8.3) with decreasing age. Isotopic change of Nd and Sr from the enriched signature to the depleted one is synchronous with the opening of the Japan Sea at ∼ 15 Ma. This synchronism indicates that the opening of the Japan Sea was initiated by the injection of the asthenosphere. During the pre-opening stage, the mantle wedge was composed of a two-layered structure: the sub-continental lithosphere and the underlying asthenosphere. The volcanics of this stage characterized by an enriched isotopic signature were derived from a source with a higher proportion of sub-continental lithosphere. The sub-continental lithosphere of the back-arc side was thinned by the injection of the depleted asthenosphere, which accelerated the growth of the MORB source within the mantle wedge of the back-arc side and resulted in magma generation with the depleted isotopic signature of Nd and Sr at the post-opening stage.

/pdf/asthenospheric-injection-and-back-arc-opening-isotopic-4m8wh5z5i5.pdf

Asthenospheric injection and back-arc opening: Isotopic evidence from Northeast Japan

Based on Sr isotopic data for Tertiary and Quaternary basaltic rocks from the NE Japan arc, relationships are discussed between the temporal variation of magma source characteristics and the opening of the Japan Sea. The basaltic rocks from the trench side and from the transitional zone show initial87Sr/86Sr ratios (Sri ratios) in the range of 0.70411–0.70546 but no temporal variation in Sri ratios. The back-arc side basaltic rocks with ages of 29.8 to ≈ 15 Ma have Sri ratios similar to those of the trench side and the transitional zone, and these values also show no temporal change. In contrast, the basaltic rocks from the back-arc side, with ages younger than ≈ 15 Ma, show significantly lower Sri ratios (0.70396 to 0.70290), which are slightly higher than those of N-type MORB. These Sr isotopic features may imply that at least before ≈ 15 Ma the magma source regions (the sub-continental mantle) beneath the NE Japan arc had an enriched chemical character and that after ≈ 15 Ma, the magma sources for volcanic rocks from the back-arc side show a drastic change in Sr isotopic character, from an enriched nature to a depleted one. The depleted magmas may have been formed as a result of injection of depleted asthenosphere (or of a depleted mantle diapir) into the subcontinental mantle under the back-arc side of the NE Japan arc, during the spreading of the Japan Sea back-arc basin. The middle Miocene basaltic rocks from the back-arc side are characterized by lower contents of LIL elements such as K2O and Rb compared with those from the trench side, suggesting that during the middle Miocene (syn-opening stage of the Japan Sea) the degree of partial melting may have been higher in the back-arc side mantle than in the trench side mantle. High degree of partial melting in the back-arc side mantle can be attributed to an increasing geothermal gradient in the mantle due to the injection of hot asthenosphere. This injection might also have caused the melting of the lower crust from which the voluminous middle Miocene acidic volcanics in the back-arc side and transitional zone may have been produced.

The relationships between drastic changes in Sr isotope ratios of magma sources beneath the NE Japan arc and the spreading of the Japan sea back-arc basin

K-Ar dating was carried out on the Tertiary volcanic rocks from Okushiri Island in Japan Sea. We obtained K-Ar whole rock ages of 4.4±0.2, 20.4±0.5, 19.7±0.5 and 34.4±0.8 Ma for a basalt in the Kamuiyama Formation, an andesite in the Tsurikake Formation, an andesite in the Aonaegawa Formation and a basalt in the Matsue basalt Formation, respectively. The volcanic activity during Tertiary time in Okushiri Island are divided into main three stages, Oligocene (34-29 Ma), Early Miocene (23-20 Ma) and Pliocene (-4 Ma). The chemical compositions of representative 32 samples from the three stages were determined by XRF techniques, The Oligocene Matsue basalts (lower) are characterized by high contents of MgO, Ni and Cr whereas the upper Matsue basalts have high contents of HFS elements such as Ti, P, Zr and Nb, suggesting that the latter are different in chemical compositions from the typical island arc basalts. The Early Miocene volcanics composed mainly of calc-alkaline andesite and dacite, whose incompatible element contents are similar to those of the Quaternary volcanics from the back-arc side of the NE Japan arc. The Pliocene volcanics composed mainly of rock suites ranging from andesite to dacite of calc-alkaline series with subordinate amounts of basalt which have incompatible element contents comparable to those of the Pliocene alkali basalt from Kyuroku-shima Island, northeast Japan Sea. The NE Japan arc and the surrounding areas during the Oligocene to Early Miocene are characterized by occurrence of andesite and dacite of calc-alkaline series and tholeiitic and alkali basalts with high contents of HFS elements, and the calc-alkaline rocks were generally formed before basaltic volcanism took place.

https://www.jstage.jst.go.jp/article/ganko1988/86/11/86_11_507/_pdf

K-Ar ages of the Tertiary volcanic rocks from Okushiri Island and the petrological characters of the Oligocene to Early Miocene volcanic rocks from the Northeast Japan arc and the surrounding areas

Sr isotopic compositions have been determined for the Tertiary volcanic rocks from Okushiri Island, Northeast Japan Sea, together with the temporal variation of compositions of magma sources beneath Okushiri Island. The Cretaceous to Tertiary volcanics from Okushiri Island have the following SrI (initial 87Sr/86Sr) ratios; Cretaceous Monai volcanics: 0.70532 (Shuto et al., in preparation), Oligocene, Lower Matsue basalts: 0.70344-0.70346, Upper Matsue basalts: 0.70378-0.70419, Early Miocene andesites in the Aonaegawa Formation: 0.70448-0.70475, Early Miocene andesites in the Tsurikake Formation: 0.70432-0.70442, Middle Miocene dolerite: 0.70325, Pliocene volcanics: 0.70294-0.70331. Significant Sr isotopic variations recognized in the Upper Matsue basalts may be explained by assimilation of the Cretaceous granitic rocks from Okushiri Island. Thus the uncontaminated Upper Matsue basalts may yield SrI ratio around ?? 0.7037. Comparative examinations of SrI ratios reported by the previous and present workers for the volcanic rocks after Cretaceous from the back-arc side in the NE Japan arc and the Japan Sea suggest that the primary magmas for the Cretaceous and the early Miocene volcanics from Okushiri Island were generated from the sub-continental mantle enriched in Sr isotopic ratio, whereas those for the middle Miocene and Pliocene volcanics from the same island were derived from the depleted asthenosphere, probably injected into the sub-continental mantle during the opening of the Japan Sea. The data on the Matsue basalts indicate a possibility of the generation of basalt magmas from the depleted asthenosphere before the spreading of the Japan Sea.

Temporal variation of Sr isotopic compositions of the Cretaceous to Tertiary volcanic rocks from Okushiri Island, Northeast Japan Sea

Rb-Sr whole rock isochron age has been determined on the Early Cretaceous Monai volcanic rocks occurring in Okushiri Island, Northeast Japan Sea. Five samples of dolerite, andesite and rhyolitic welded tuff give a well defined whole rock isochron with an age of 101.3±4.8 Ma and an initial 87Sr/86Sr ratio of 0.70532±0.00007 (2σ). These results almost agree with the mineral and whole rock isochron ages and the initial Sr isotopic ratios determined by the previous workers for the pre-Tertiary granitoids occurring in Okushiri Island and the western margin of southwest Hokkaido. This facts indicate the concurrence of the pre-Tertiary volcanism and plutonism in these areas and the similarity in isotopic compositions of the source materials for the Monai volcanic rocks and these granitoids. The initial Sr isotopic ratio of the Monai volcanic rocks are much higher than those of the Early Cretaceous volcanic rocks from the region to the west of Hidaka Mountains and the eastern margin of the Kitakami Mountains. Comparative investigations of initial Sr isotopic ratios reported by the previous and present workers for the Early Cretaceous volcanic rocks and granitoids from the NE Japan arc and Hokkaido indicate that initial Sr isotopic ratios for these igneous rocks increase from the eastern tholeiitic volcanic zone to the western calc-alkaline volcanic-plutonic zone, in contrast to the across-arc variation of the Sr ratios of the Quaternary volcanic rocks from the NE Japan arc.

A Rb-Sr geochronological study on the Early Cretaceous Monai volcanic rocks from Okushiri Island, Northeast Japan Sea

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The relationships between drastic changes in Sr isotope ratios of magma sources beneath the NE Japan arc and the spreading of the Japan sea back-arc basin

K-Ar ages of the Tertiary volcanic rocks from Okushiri Island and the petrological characters of the Oligocene to Early Miocene volcanic rocks from the Northeast Japan arc and the surrounding areas

Temporal variation of Sr isotopic compositions of the Cretaceous to Tertiary volcanic rocks from Okushiri Island, Northeast Japan Sea

A Rb-Sr geochronological study on the Early Cretaceous Monai volcanic rocks from Okushiri Island, Northeast Japan Sea