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431.
The notion that the Yakuno ophiolite and overlying Maizuru Group represents an accretionary prism formed during the Permian evolution of Japan on the Yakuno eruptive sequence, association of hemipelagic mudstone with silicic tuff, exotic fossiliferous limestones derived from previously accreted sea-mounts, upward coarsening of sequences terrigenous sandstone and conglomerate, and mildly deformed Permian and Triassic forearc basin formations. The most important indicator, however, is the seaward imbrication and repetition observed in both the Maizuru Group and the ophiolite itself. D1 deformation structures include axial–planar foliations (pressure-solution cleavage for the Maizuru Group and granulite–amphibolite metamorphic layering in the ophiolite), flattening type strain, symmetric pressure shadows and fringes, and isoclinal folds showing axial–planar foliations and thrust faulting at their overturned limb. The exceptional asymmetry observed indicates seaward-directed shearing near the thrust, while D1 structures in the Maizuru zone are explained by off-scraping, above the basal decollement. The later Jurassic D2 kink fold structure includes a first-order asymmetric kink with a brittle thrust at its overturned limb, more-or-less coeval with M2 retrograde metamorphism. Medium-pressure M1 prograde metamorphism in the Yakuno ophiolite produced layering of granulite and amphibolite, and in the Maizuru Group, formation of illite along pressure-solution cleavage of mudstones. The metamorphic grade is controlled by the stratigraphic relationships and appears typical of that in ocean floor regions. However, there was only one episode of M1 prograde metamorphism which occurred contemporaneously with D1 off-scraping. Given that subduction zones are normally characterized by high P/T metamorphic regimes, the observed P/T history appears to reflect relatively unusual conditions. Such high thermal gradients may plausibly reflect the approach of a young, hot oceanic plate which continued subducting beneath the Japanese arc. Accordingly, the Yakuno ophiolite was probably formed at the trench–trench–ridge triple junction. 相似文献
432.
Shengyi Mao Hongxiang Guan Lihua Liu Xiqiu Han Xueping Chen Juan Yu Yongge Sun Yejian Wang 《海洋学报(英文版)》2021,40(8):53-64
Hydrothermal venting has a profound effect on the chemical and biological properties of local and distal seawater and sediments. In this study, lipid biomarkers were analyzed to examine the potential influence of hydrothermal activity on the fate of organic matter(OM) in surface sediments around Tianxiu Hydrothermal Field in the Carlsberg Ridge(CR), Northwest Indian Ocean. By comparing the biomarker distributions of the samples with that of other typical hydrothermal sediments in the mid ocean ridge, it is shown that the location of the samples is not affected by the hydrothermal activity. The relatively low abundances of terrestrial n-alkyl lipids and riverine1,15-C32 diol suggested a minor contribution of terrigenous OM to the study area. The bacteria contributed predominantly to sedimentary marine OM; however, other marine source organisms, e.g., eukaryotes(i.e.,phytoplankton and fungi) could not be completely neglected. The marine-originated biomarkers showed significantly variable distributions between the two sediments, suggesting different dynamic physical and biogeochemical processes controlling the fate of marine OM. This study identified various diagnostic biomarkers(5,5-diethyl alkanes, diols and β-OH FAs), which may have significant environmental implications for future works in this region. 相似文献
433.
M. J. Bartholomew 《Geological Journal》1983,18(3):241-253
Examinations of Grenville massifs in the Blue Ridge Geologic Province of Virginia and North Carolina indicate that the country rocks (∼ 1100–1450 Ma) are layered gneisses that were metamorphosed during Grenville orogenesis (∼ 1000–1100 Ma) to amphibolite to granulite facies and intruded by plutonic suites. Subsequently, the Grenville terrane was intruded by a suite of peralkaline granitic plutons (∼ 700 Ma) and progressively overlapped westward by Upper Precambrian to Cambrian sedimentary and volcanic rocks. Following deposition of Upper Precambrian and Palaeozoic rocks, the Blue Ridge Geologic Province was subjected to Taconic metamorphism (∼ 450–480 Ma) which generally increased in intensity southeastward from greenschist (chlorite grade) to upper amphibolite (sillimanite grade) facies. Large-scale late Devonian thrusting (∼ 350 Ma) along the Fries fault system and the Brevard zone-Yadkin fault system produced the present day distribution of juxtaposed Grenville massifs and Palaeozoic metamorphic zones in the Blue Ridge Geologic Province. Palinspastic restoration of the Taconic metamorphic zones to their pre-late Devonian relative positions yields an ∼ 50 km displacement on the Fries fault system near the Grandfather Mountain window and and an ∼ 80 km displacement on the Smith River allochthon farther east. Restoration of the Grenville massifs to this same palinspastic base shows that Grenville metamorphic grade decreased southeastward from the deeper granulite facies (opx + gar) to the shallower granulite facies (opx ± amp) to amphibolite facies. 相似文献
434.