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1.
The Chilas Complex is a major lower crustal component of the Cretaceous Kohistan island arc and one of the largest exposed slices of arc magma chamber in the world. Covering more than 8000 km2, it reaches a current tectonic width of around 40 km. It was emplaced at 85 Ma during rifting of the arc soon after the collision of the arc with the Karakoram plate. Over 85% of the Complex comprises homogeneous, olivine‐free gabbronorite and subordinate orthopyroxene–quartz diorite association (MGNA), which contains bodies of up to 30 km2 of ultramafic–mafic–anorthositic association (UMAA) rocks. Primary cumulate textures, igneous layering, and sedimentary structures are well preserved in layered parts of the UMAA in spite of pervasive granulite facies metamorphism. Mineral analyses show that the UMAA is characterized by more magnesian and more aluminous pyroxene and more calcic plagioclase than those in the MGNA. High modal abundances of orthopyroxene, magnetite and ilmenite (in MGNA), general Mg–Fe–Al spatial variations, and an MFA plot of whole‐rock analyses suggest a calc‐alkaline origin for the Complex. Projection of the pyroxene compositions on the Wo–En–Fs face is akin to those of pyroxenes from island arcs gabbros. The presence of highly calcic plagioclase and hornblende in UMAA is indicative of hydrous parental arc magma. The complex may be a product of two‐stage partial melting of a rising mantle diaper. The MGNA rocks represent the earlier phase melting, whereas the UMAA magma resulted from the melting of the same source depleted by the extraction of the earlier melt phase. Some of the massive peridotites in the UMAA may either be cumulates or represent metasomatized and remobilized upper mantle. The Chilas Complex shows similarities with many other (supra)subduction‐related mafic–ultramafic complexes worldwide.  相似文献   

2.
The Niyasar plutonic complex, one of the Cenozoic magmatic assemblages in the Urumieh‐Dokhtar magmatic belt, was the subject of detailed petrographic and mineralogical investigations. The Niyasar magmatic complex is composed of Eocene to Oligocene mafic rocks and Miocene granitoids. Eleven samples, representing the major rock units in the Niyasar magmatic complex and contact aureole were chosen for mineral chemical studies and for estimation of the pressure, temperature, and oxygen fugacity conditions of mineral crystallization during emplacement of various magmatic bodies. The analyzed samples are composed of varying proportions of quartz, plagioclase, K‐feldspar, hornblende, biotite, titanite, magnetite, apatite, zircon, garnet, and clinopyroxene. Application of the Al‐in‐hornblende barometer indicates pressures of around 0.2 to 0.4 kbar for the Eocene–Oligocene mafic bodies and around 0.5 to 1.7 kbar for the Miocene granitoids. Hornblende‐plagioclase thermometry yields relatively low temperatures (661–780 °C), which probably reflect late stage re‐equilibration of these minerals. The assemblage titanite–magnetite–quartz as well as hornblende composition were used to constrain the oxygen fugacity and H2O content during the crystallization of the parent magmas in the Miocene plutons. The results show that the Miocene granitoids crystallized from magmas with relatively high oxygen fugacity and high H2O content (~5 wt% H2O). The Miocene granitoids show similar range of oxygen fugacity, H2O contents and mineral chemical compositions, which indicate a common source for their magmas. Although the crystallization pressures of the Miocene plutons discriminate various categories of plutonic bodies emplaced at depths of about 5.7–6.5 km (Marfioun pluton), about 4.2 km (Ghalhar pluton) and 1.9–2.3 km (Poudalg pluton), they were later uplifted to the same level by vertical displacement of faults. The emplacement depths of the Niyasar plutons suggest that the central part of the Urumieh‐Dokhtar magmatic belt has experienced an uplift rate of ca. 0.25–0.4 mm/yr from the Miocene onwards.  相似文献   

3.
Yasuhiko Ohara 《Island Arc》2016,25(3):193-208
The Godzilla Megamullion is the largest known oceanic core complex (OCC) on the Earth, located in the Parece Vela Basin in the Philippine Sea. In this article, the history of Godzilla Megamullion study is reviewed for the first time, dividing it into three major phases: (i) the early studies done before Japan's extended continental shelf survey program; (ii) the studies during Japan's extended continental shelf survey program that discovered the OCC; and (iii) the studies by the post‐discovery cruises. The early studies included an interpretation of US nautical chart of the southwestern Pacific and the site surveys for Deep Sea Drilling Project cruises (DSDP Legs 6, 31 and 59). The early studies recognized the presence of the Parece Vela Rift, the extinct spreading axis of the Parece Vela Basin, and established the currently accepted model that the Philippine Sea evolved with eastward progression of backarc spreading and arc migration. The modern understanding of the Parece Vela Basin comes from Japan's extended continental shelf survey program. The program revealed the ultramafic petrology as well as a two‐stage evolution model of the basin. Following these results, the discovery of the Godzilla Megamullion was made in 2001. The studies by the post‐discovery cruises further revealed important characteristics of the OCC, such as the presence of abundant plagioclase‐bearing peridotite and the systematic temporal changes in both deformation microstructures and composition of plagioclase and amphibole in gabbroic mylonites and ultramylonites. Zircon U–Pb ages of gabboric and leucocratic rocks indicate that the terminal phase of Parece Vela Basin spreading was with a significant decline in spreading rate and asymmetry accompanying formation of the Godzilla Megamullion. The estimated denudation rate of the OCC was approximately 2.5 cm/yr; significantly slower than the previous estimate based on poorly constrained magnetic data.  相似文献   

4.
The new result of SHRIMP U–Pb zircon dating of the Kinshozan Quartz Diorite from the Kanto Mountains, Japan, provides 281.5 ± 1.8 Ma. The age is 30 m.y. older than the available age of the Kinshozan Quartz Diorite obtained by hornblende K–Ar method. The new U–Pb zircon age represents the time of crystallization of the Kinshozan Quartz Diorite. The hornblende K–Ar age indicates the time that the Kinshozan Quartz Diorite cooled down to 500 °C which is the closure temperature of the systematics. Permian granites are found in small exposures in Japan, and frequently referred to as 250 Ma granites. The Kinshozan Quartz Diorite is considered as a type of the 250 Ma granites, and the age was influential in establishing a model of Paleozoic tectonic evolution for the Japanese Islands. The new age of the Kinshozan Quartz Diorite provides the opportunity to re‐examine the model. The Kinshozan Quartz Diorite and other Permian granites in the south of the Median Tectonic Line of Japan were constituents of the Paleo‐Ryoke Belt. The geochemical characteristics of the granitic rocks in the Paleo‐Ryoke Belt indicated that the granitic rocks were formed in a primitive island arc environment, and the new trace element data also support this interpretation. Examination of the available data and results of the present study suggests the late Paleozoic granitic activity in Japan as follows. At about 310–290 Ma, arc magmatism generated adakitic granites and other granites in the South Kitakami Belt. Quartz diorite and tonalites of primitive characteristic, such as the Kinshozan Quartz Diorite and granites in the Maizuru Belt appear to have been formed at the immature island arc, and accreted to the Japanese Islands at the end of Paleozoic or early Mesozoic era. During 260–240 Ma, granitic activity took place in the Hida and Maizuru Belts as a part of the Asian continent.  相似文献   

5.
Plutonic rocks in the southern Abukuma Mountains include gabbro and diorite, fine‐grained diorite, hornblende–biotite granodiorite (Ishikawa, Samegawa, main part of Miyamoto and Tabito, Kamikimita and Irishiken Plutons), biotite granodiorite (the main part of Hanawa Pluton and the Torisone Pluton), medium‐ to coarse‐grained biotite granodiorite and leucogranite, based on the lithologies and geological relations. Zircon U–Pb ages of gabbroic rocks are 112.4 ±1.0 Ma (hornblende gabbro, Miyamoto Pluton), 109.0 ±1.1 Ma (hornblende gabbro, the Hanawa Pluton), 102.7 ±0.8 Ma (gabbronorite, Tabito Pluton) and 101.0 ±0.6 Ma (fine‐grained diorite). As for the hornblende–biotite granodiorite, zircon U–Pb ages are 104.2 ±0.7 Ma (Ishikawa Pluton), 112.6 ±1.0 Ma (Tabito Pluton), 105.2 ±0.8 Ma (Kamikimita Pluton) and 105.3±0.8 Ma (Irishiken Pluton). Also for the medium‐ to fine‐grained biotite granodiorite, zircon U–Pb ages are 106.5±0.9 Ma (Miyamoto Pluton), 105.1 ±1.0 Ma (Hanawa Pluton) and the medium‐ to coarse‐grained biotite granodiorite has zircon U–Pb age of 104.5 ±0.8 Ma. In the case of the leucogranite, U–Pb age of zircon is 100.6 ±0.9 Ma. These data indicate that the intrusion ages of gabbroic rocks and surrounding granitic rocks ranges from 113 to 101 Ma. Furthermore, K–Ar ages of biotite and or hornblende in the same rock samples were dated. Accordingly, it is clear that these rocks cooled down rapidly to 300 °C (Ar blocking temperature of biotite for K–Ar system) after their intrusion. These chronological data suggest that the Abukuma plutonic rocks in the southern Abukuma Mountains region uplifted rapidly around 107 to 100 Ma after their intrusion.  相似文献   

6.
The troctolites and olivine‐gabbros from the Dive 6 K‐1147 represent the most primitive gabbroic rocks collected at the Godzilla Megamullion, a giant oceanic core complex formed at an extinct spreading segment of the Parece Vela back‐arc basin (Philippine Sea). Previous investigations have shown that these rocks have textural and major elements mineral compositions consistent with a formation through multistage interaction between mantle‐derived melts and a pre‐existing ultramafic matrix. New investigations on trace element mineral compositions basically agree with this hypothesis. Clinopyroxenes and plagioclase have incompatible element signatures similar to that of typical‐MORB. However, the clinopyroxenes show very high Cr contents (similar to those of mantle clinopyroxene) and rim having sharply higher Zr/REE ratios with respect to the core. These features are in contrast with an evolution constrained by fractional crystallization processes, and suggest that the clinopyroxene compositions are controlled by melt‐rock interaction processes. The plagioclase anorthite versus clinopyroxene Mg#[Mg/(Mg + FeTot)] correlation of the Dive 6 K‐1147 rocks shows a trend much steeper than those depicted by other oceanic gabbroic sections. Using a thermodynamic model, we show that this trend is reproducible by fractionation of melts assimilating 1 g of mantle peridotite per 1 °C of cooling. This model predicts the early crystallization of high Mg# clinopyroxene, consistent with our petrological observation. The melt‐peridotite interaction process produces Na‐rich melts causing the crystallization of plagioclase with low anorthite component, typically characterizing the evolved gabbros from Godzilla Megamullion.  相似文献   

7.
Mesozoic volcanic rocks are widespread throughout the Great Xing'an Range of northeastern China. However, there has been limited investigation into the age and petrogenesis of the Mesozoic volcanics in the eastern Great Xing'an Range. According to our research, the volcanic rocks of the Dayangshu Basin, eastern Great Xing'an Range are composed mainly of trachybasalt, basaltic andesite, and basaltic trachyandesite, with minor intermediate–basic pyroclastic rocks. In this study, the geochemistry and geochronology of the Mesozoic volcanic rocks are presented in order to discuss the petrogenesis and tectonic setting of the Ganhe Formation in the Dayangshu Basin. Zircon U–Pb dating by laser ablation inductively coupled plasma–mass spectrometry indicates that the Mesozoic lavas formed during the late Early Cretaceous (114.3–108.8 Ma). This suite of rocks exhibits a range of geochemical signatures indicating subduction‐related genesis, including: (i) calc‐alkaline to high‐K calc‐alkaline major element compositions; (ii) enrichment of large ion lithophile elements (e.g. Rb, Ba, K) and light rare earth elements (LREEs/HREEs =7.33–9.85); and (iii) weak depletion in high field strength elements (e.g. Nb, Ta, Ti). Furthermore, Sr–Nd–Pb isotopic data yield initial 87Sr/86Sr values of 0.70450–0.70463, positive εNd(t) values of +1.8 to +3.3, and a mantle‐derived lead isotope composition. Combined with the regional tectonic evolution, the results of this study suggest that the Ganhe Group lavas are derived from decompression melting of a metasomatized (enriched) lithospheric mantle, related to asthenospheric upwelling, which resulted from lithospheric mantle delamination and produced extension of the continental margin following the subduction of the Paleo‐Pacific Plate.  相似文献   

8.
The Permian–Triassic high pressure metamorphism and potassic magmatism in central Korea attest to the extension of the Dabie‐Sulu collision belt in central‐eastern China towards the Korean Peninsula and possibly the Japanese Islands. We present major and trace element and Sr–Nd isotope data for a ca. 230 Ma monzodiorite pluton emplaced in the Goesan area, central Okcheon belt, Korea. This pluton shows geochemical features comparable with those of the coeval monzonite–syenite–gabbro–mangerite suite documented recently in the Gyeonggi massif. The metaluminous and alkali–calcic signatures of the Goesan intrusives correspond to the Caledonian‐type post‐orogenic granitoids. The K2O/Na2O ratios of all analyzed samples are greater than 1, and are not correlative with their SiO2 contents. The enrichment of both large‐ion‐lithophile elements and highly compatible elements in the Goesan pluton is probably indicative of metasomatized mantle origin. The elemental fractionation in the source region must have occurred in the distant past, possibly the Paleoproterozoic, to generate significantly negative εNd(t) values (< –16). Chondrite‐normalized rare earth element patterns as well as Rb/Sr and Ba/Rb ranges suggest that the source consists of amphibole‐bearing rocks. Progressive decreases in negative Eu anomaly and Ba, Sr, Ni, Cr and V contents with increasing SiO2 contents reflect an important role of plagioclase, biotite and hornblende for the fractionation process. Zr is undersaturated in the potassic, metaluminous melt. The initial Sr–Nd isotopic compositions of the samples are correlated with their SiO2 contents, substantiating a role of crustal assimilation during the magmatic differentiation. The Sr–Nd elemental and isotopic modeling suggests that the Goesan pluton was initially slightly heterogeneous in its isotopic composition, and underwent concurrent assimilation and fractional crystallization. The occurrence of the Goesan pluton provides further evidence corroborating the amalgamation of allochthonous terranes within the Okcheon belt during the Permian–Triassic collisional orogeny.  相似文献   

9.
The Hakusan volcano, central Japan, is located in a region where two subducting plates (the Pacific Plate and the Philippine Sea Plate) overlap near the junction of four plates adjacent to the Japanese Islands (the Pacific Plate, the Philippine Sea Plate, the Eurasia Plate, and the North American Plate). The Hakusan volcano consists of products from four major volcanic episodes: Kagamuro, Ko‐hakusan, and Shin‐Hakusan I and II. To date the eruption events of the Hakusan volcano we applied thermoluminescence and fission track methods. 238U(234U)–230Th disequilibrium and 206Pb/238U methods were applied to date the zircon crystallization ages for estimating the magma residence time before the eruptions. The eruption ages we obtained are ca 250 ka for Kagamuro, ca 100 ka and ca 60 ka for Ko‐Hakusan, ca 50 ka for Shin‐Hakusan I, and <10 ka for Shin‐Hakusan II. They are concordant with previous reports based on K–Ar dating. Some of the pyroclastic rocks, possibly originating from Shin‐Hakusan II activities, are dated to be ca 36 ka or 50 ka, and belong to the Shin‐Hakusan I activity. The zircon crystallization ages show several clusters prior to eruption. The magma residence time was estimated for each volcanic activity by comparing the major crystallization events and eruption ages, and we found a gradual decrease from ca. 500 ky for the Kagamuro activity to ca. 5 ky for the Shin‐Hakusan II activity. This decrease in residence time may be responsible for the decrease in volume of erupted material estimated from the current topography of the region. The scale of volcanic activity, which was deduced from the number of crystallized zircons, is more or less constant throughout the Hakusan volcanic activity. Therefore, the decrease in magma residence time is most likely the result of stress field change.  相似文献   

10.
The tectonic setting of the late mesozoic of South China is in a debate between two schools of thought: an intra‐continental rift zone along a passive continental margin or active rifting associated with subduction of the paleo‐Pacific Plate. In this study, we present new sensitive high‐resolution ion microprobe (SHRIMP) U‐Pb zircon ages, along with geochemical data of three basic dikes that cross‐cut the Dexing porphyry copper deposit. The deposit is the largest of its kind in eastern China and part of large scale mineralization associated with Mesozoic magmatic activity in the area. Our results indicate that the dikes were emplaced in the Late Jurassic with an average U‐Pb age of 153.5 ± 2.4 Ma. The intrusions have bulk εNd(t) of ca +0.7 and zircon εHf(t) value of +1.54 to +6.92. Based on relatively enriched light rare earth elements (LREE) and depleted high‐field‐strength elements (HFSE) abundances with pronounced negative Ta–Nb, Hf–Zr and Ti anomalies in multi‐element diagrams, we propose that these dikes were derived from a subduction‐modified lithospheric mantle source. The variability in Hf isotopes identifies some degree of crustal contaminations. Our data support a scenario with a back‐arc extensional setting or an intra‐arc rift environment associated with the westward subduction of the paleo‐Pacific Plate at or prior to the late Jurassic as the most likely cause for these subduction signatures.  相似文献   

11.
Late Quaternary foraminifera assemblages have been examined in two sediment cores (MD179‐3296 and MD179‐3317) from cold seep areas in the eastern margin of the Japan Sea, off Joetsu, Niigata Prefecture. Foraminifera assemblages in core MD179‐3296, which was located at the center of a pockmark on the Umitake Spur, show no evidence of methane flux and, especially in its upper portion, share the same paleo‐environmental history as other free gas hydrate areas of the Japan Sea. In comparison, in the core MD179‐3317 at the center of a pockmark at Joetsu Knoll, foraminiferal distributions were strongly affected by methane activities and, in the main part of the core, were deposited under local conditions. Three horizons were identified in this core, which are characterized by the high abundance value of Thalmannammina parkerae and might be related to methane flux due to sea level fall especially through late marine isotope stage (MIS) 3 and MIS 2.  相似文献   

12.
The South Kitakami Massif is one of the oldest geological domains in Japan having Silurian strata with acidic pyroclastic rocks and Ordovician–Silurian granodiorite–tonalite basement, suggesting that it was matured enough to develop acidic volcanisms in the Silurian period. On the northern and western margin of the South Kitakami Massif, an Ordovician arc ophiolite (Hayachine–Miyamori Ophiolite) and high‐pressure and low‐temperature metamorphic rocks (Motai metamorphic rocks) exhumed sometime in the Ordovician–Devonian periods are distributed. Chronological, geological, and petrochemical studies on the Hayachine–Miyamori Ophiolite, Motai metamorphic rocks, and other early Paleozoic geological units of the South Kitakami Massif are reviewed for reconstruction of the South Kitakami arc system during Ordovician to Devonian times with supplementary new data. The reconstruction suggests a change in the convergence polarity from eastward‐ to westward‐dipping subduction sometime before the Late Devonian period. The Hayachine–Miyamori Ophiolite was developed above the eastward‐dipping subduction through three distinctive stages. Two separate stages of overriding plate extension inducing decompressional melting with minor involvement of slab‐derived fluid occurred before and after a stage of melting under strong influence of slab‐derived fluids. The first overriding plate extension took place in the back‐arc side forming a back‐arc basin. The second one took place immediately before the ophiolite exhumation and near the fore‐arc region. We postulate that the second decompressional melting was triggered by slab breakoff, which was preceded by slab rollback inducing trench‐parallel wedge mantle flow and non‐steady fluid and heat transport leaving exceptionally hydrous residual mantle. The formation history of the Hayachine–Miyamori Ophiolite implies that weaker plate coupling may provide preferential conditions for exhumation of very hydrous mantle. Very hydrous peridotites involved in arc magmatism have not yet been discovered except for in the Cambrian–Ordovician periods, suggesting its implications for global geodynamics, such as the thermal state and water circulation in the mantle.  相似文献   

13.
The origin of active faults in the Inner zone of the western part of Southwest Japan was explained by a decrease of the minimum principal stress and reactivation of ancient geologic structures. Although the E–W maximum principal stress in Southwest Japan due to the collision of the Southwest and Northeast Japan arcs along the Itoigawa–Shizuoka Tectonic Line is assumed to decrease westward, the density of active strike‐slip faults increases in the western margin of the Southwest Japan Arc (western Chugoku and northern Kyushu) where the subducting Philippine Sea Plate dips steeply. The E–W maximum compressional stress is predominant throughout Southwest Japan, while the N–S minimum principal stress that is presumably caused by coupling between Southwest Japan arc and Philippine Sea Plate decreases due to the weak plate coupling as the plate inclination increases under the western margin of Southwest Japan. The increase of the fault density in the western margin of the arc is attributed to a decrease of the minimum principal stress and consequent increase of shear stress. Low slip rates of the active faults in this region support the view that the westward increase of fault density is not a response to increasing maximum stress. These faults of onshore and offshore lie in three distinct domains defined on the basis of fault strike. They are defined domains I, II, and III which are composed of active faults striking ENE–WSW, NW–SE, and NE–SW, respectively. Faulting in domains I, II, and III is related to Miocene rift basins, Eocene normal faults, and Mesozoic strike‐slip faults, respectively. Although these active faults are strike‐slip faults due to E–W maximum stress, it is unclear whether their fault planes are the same as those of pre‐Quaternary dip‐slip faults.  相似文献   

14.
Ian Metcalfe 《Island Arc》2016,25(2):126-136
Limestones exposed north of Raub, Pahang, Malaysia, and sandwiched between the Bentong‐Raub Suture Zone and the westernmost margin of the Sukhothai Arc terrane, yield a late Dienerian (late Induan) conodont fauna. The co‐occurrence of Neospathodus dieneri Sweet (morphotypes 1, 2 and 3) and Neospathodus pakistanensis Sweet represents the Neospathodus dieneri morphotype 3 sub‐zone of the Neospathodus dieneri Zone. The sampled limestones are interpreted as the northwards extension of the Jerus Limestone which crops out near Cheroh and Jerus villages, significantly extending the known outcrop of the Jerus Limestone northwards. The Jerus Limestone is interpreted as hemipelagic and formed in a foredeep or forearc setting on top of the accretionary complex formed by eastwards subduction of the Palaeo‐Tethys during the Lower to Middle Triassic.  相似文献   

15.
Recent field prospecting in the Cretaceous sequences of the lower Narmada valley has led to the discovery of three isolated archosaur teeth from the upper part of marine Cretaceous rocks of the Bagh Group. The specimens were recovered by surface prospecting from an oyster‐bearing green sandstone bed occurring at the top of the Coralline Limestone (Coniacian) from a site near Phutibawri village, Dhar District, Madhya Pradesh, India. Of the three teeth recovered from this horizon, two are identified with abelisaurid dinosaurs and the third one with an indeterminate crocodile. The abelisaurid teeth conform to the premaxillary and maxillary tooth morphology of Majungasaurus and Indosuchus. Earlier reports of abelisaurid dinosaurs from India are from the Upper Cretaceous (Maastrichtian) Lameta Group of Jabalpur, Pisdura (Central India) and Balasinor (Western India) and Upper Cretaceous (Late Maastrichtian) Kallamedu Formation (South India). As no associated age diagnostic fossils are found, the specimens described here are considered to represent pre‐Late to Late Maastrichtian age based on the known ages of the underlying and overlying formations. The new finds, therefore, document stratigraphically the oldest occurrence of abelisaurid dinosaurs known from the Indian subcontinent.  相似文献   

16.
A 4.3 m‐long peat sequence from the shore of Lake Tiriara, Mangaia, Cook Islands, was analyzed using an ITRAX core scanner equipped with a magnetic susceptibility meter. Variations in the elemental profiles, providing insights into long‐ and short‐term environmental changes over the last 3500 years, are supported by grain size data and diatom assemblages. The scattering ratio (Mo Inc/Mo Coh) was evaluated and found to represent a good proxy for organic matter in peat. X‐Ray Fluorescence (XRF) data were processed by principal component analysis that confirmed the distinction of biogenic and detrital phases, organic matter and elements of marine origin. The record preserved in the peat sequence includes a peatland infilling stage followed by alternating drier and wetter periods. A notable steady increase in clay associated with high counts of detrital elements from 2000–1700 cal yr BP is attributed to increased erosion, which is most probably linked with human colonization and/or more intense chemical weathering linked with a wetter climate. Freshwater gastropods (Melanoides sp.), which were possibly introduced by humans, or are native, occupied the wetland during a period of lower water level about 1000–1100 cal yr BP. Short‐term changes in the elemental profiles are often linked with slight coarsening of the inorganic fraction that is, however, only revealed after grain size analysis. Peaks in marine indicators (Br, Cl, S, and/or Ca) associated with marine‐dominated diatom assemblages most probably represent marine incursions through the underground tunnel in the makatea, a fossilized, uplifted coral limestone rim. While none of the marine event units present characteristics typical of cyclone or tsunami deposits, the concurrent or absent peak of detrital elements (Fe, Si, Rb, Ti, K) attributed to increased erosion of the volcanic cone associated with a cyclone is used to distinguish both types of events, as also suggested by principal component analysis.  相似文献   

17.
We examined the geochemical characteristics and temporal changes of deposits associated with the 2011 Tohoku‐oki tsunami. Stable carbon isotope ratios, biomarkers, and water‐leachable ions were measured in a sandy tsunami deposit and associated soils sampled at Hasunuma, Kujukuri coastal plain, Japan, in 2011 and 2014. At this site, the 2011 tsunami formed a 10–30 cm ‐thick layer of very fine to medium sand. The tsunami deposit was organic‐poor, and no samples contained any detectable biomarkers of either terrigenous or marine origin. In the underlying soil, we identified hydrocarbons and sterols derived from terrestrial plants, but detected no biomarkers of marine origin. In the samples collected in 2011, concentrations of tsunami‐derived water‐leachable ions were highest in the soil immediately beneath the tsunami deposit and then decreased gradually with depth. Because of its finer texture and higher organic content, the soil has a higher water‐holding capacity than the sandy tsunami deposit. This distribution suggests that ions derived from the tsunami quickly penetrated the sand layer and became concentrated in the underlying soil. In the samples collected in 2014, concentrations of water‐leachable ions were very low in both soil and sand. We attribute the decrease in ion concentrations to post‐tsunami rainfall, seepage, and seasonal changes in groundwater level. Although water‐leachable ions derived from seawater were concentrated in the soil beneath the tsunami deposit following the tsunami inundation, they were not retained for more than a few years. To elucidate the behavior of geochemical characteristics associated with tsunamis, further research on organic‐rich muddy deposits (muddy tsunami deposits and soils beneath sandy tsunami deposits) as well as sandy tsunami deposits is required.  相似文献   

18.
The clustering of fracture orientations is important for tectonic studies and for geotechnical engineering. In this study, a real‐coded genetic algorithm was adopted to fitting a mixed Bingham distribution to orientation data by maximizing the log‐likelihood function of the distribution. The maximization is a difficult problem, because the function has multimodality and singularity. It was found that the algorithm was effective for this problem. Given the orientations of dilational fractures, the present method determines not only the stress axes and stress ratio of each of the fracture groups but also the maximum non‐dimensionalized fluid pressure at the time of their formation. In addition, the software calculates the 95 % error ellipses of the concentration axes. The present method found that the orientations of ore veins of the Akenobe Mine, SW Japan, should be partitioned into three clusters. It is shown that two of the groups had distinctive Zn and Sn contents, and that the ore fluids had overpressures only slightly greater than the minimum principal stress at the time of the deposition of Zn‐ and Sn‐rich veins.  相似文献   

19.
This study presents new major and trace element, mineral, and Sr, Nd, and noble gas isotope geochemical analyses of basalts, gabbro, and clinopyroxenite from the Mariana Arc (Central Islands and Southern Seamount provinces) including the forearc, and the Mariana Trough (Central Graben and Spreading Ridge). Mantle source compositions beneath the Mariana Arc and the Mariana Trough indicate a mantle source that is depleted in high field strength elements relative to MORB (mid‐oceanic ridge basalt). Samples from the Mariana Arc, characterized by high ratios of Ba/Th, U/Th, 84Kr/4He and 132Xe/4He, are explained by addition of fluid from the subducted slab to the mantle wedge. Correlations of noble gas data, as well as large ion lithophile elements, indicate that heavy noble gases (Ar, Kr, and Xe) provide evidence for fluid fluxing into the mantle wedge. On the other hand, major elements and Sr, Nd, He, and Ne isotopic data of basalts from the Mariana Trough are geochemically indistinguishable from MORB. Correlations of 3He/4He and 40Ar/36Ar in the Mariana Trough samples are explained by mixing between MORB and atmosphere. One sample from the Central Graben indicates extreme enrichment in 20Ne/22Ne and 21Ne/22Ne, suggesting incorporation of solar‐type Ne in the magma source. Excess 129Xe is also observed in this sample suggesting primordial noble gases in the mantle source. The Mariana Trough basalts indicate that both fluid and sediment components contributed to the basalts, with slab‐derived fluids dominating beneath the Spreading Ridge, and that sediment melts, characterized by high La/Sm and relatively low U/Th and Zr/Nb, dominate in the source region of basalts from the Central Graben.  相似文献   

20.
Given the unquestioned impacts of recent/future environmental changes on human societies, it is reasonable to posit that past societal responses could be used as proxies of contemporaneous environmental forcing and might help identify the drivers of these, particularly where independent evidence of the effects of these events is inadequate. Such areas include Pacific oceanic islands for most of which there is evidence of a societal perturbation in the 14th and 15th centuries that saw the outbreak of region‐wide conflict, abandonment of coastal settlements and the abrupt end of long‐distance cross‐ocean voyaging networks. The contemporaneity of these effects across a vast region requires a driver that is external to particular island groups with the only possibility that of oceanic origin, most likely large waves (tsunamis or storm surges) or rapid sea‐level change. The explanation is less important than the principle that societal changes can be used as proxies for environmental activity.  相似文献   

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