Major-element petrochemistry of some extrusive rocks from the volcanically active Mariana Islands

Volcanic rocks from six of the currently or recently active volcances of the Mariana Island are show little variation in major element abundances. SiO₂ content averages 51.5 wt.%. The flows are high in Al₂O (mean 17.7 wt.%) and Fe oxides (mean 10.1 wt.% calculated as FeO only), and moderate in MgO content (mean 4.7 wt.%), Na₂O (mean 2.7 wt.%), and K₂O (mean 0.7 wt.%). Only the rocks from Farallon de Pajaros, the northernmost of the Mariana Islands, deviate slightly from the average of the analyses. Three analyses from this island are slightly higher in SiO₂ (about 54 wt.%) and Al₂O₃, and are lower in total Fe oxides and MgO. According to preferred classification, the lavas of the Mariana Islands can be termed mela-andesites, high-alumina basalts, or calc-alkaline (orogenic) basalts. The K₂O values (mean 0.7 wt.%) obtained from lavas of the Mariana Islands are significantly higher than the K₂O values (about 0.33 wt.%) from volcanics of the Izu chain to the north. Inasmuch as the substantial scatter in location of earthquake foci beneath both arcs prevents accurate delineation of the upper boundary of the Benioff zone, it presently cannot be determined whether this discrepancy in K₂O values reflects a difference in depth from the volcanic are to the dipping seismic zone or relates to other phenomena. The older volcanic islands within the Mariana-Bonin island chain apparently defined an island arc system during Eocene to Miocene time. This indicates that the present plane of convergence between the Pacific plate and the Philippine Sea plate has defined the convergence between these plates since Eocene time.     

Volcanoes of the southwestern extension of the active Mariana island arc: New swath-mapping and geochemical studies

Until recently it was thought that the volcanoes of the Mariana island arc of the western Pacific terminated at Tracey Seamount at ∼ 14°N immediately west of Guam. Sea floor mapping in 1995 shows a series of large volcanic seamounts stretching westward for nearly 300 km beyond that point. The morphology, spacing, and composition of those sampled are consistent with their having formed as a consequence of eruption of suprasubduction zone arc magmas. The relationships of the volcanoes to the tectonic processes of subduction of the Pacific plate beneath the southern portion of the Mariana convergent plate margin are becoming increasingly clear as new bathymetry and geochemical data are amassed. The volcanoes along this trend that lie closest to Guam are forming where the center of active extension in the back-arc basin intersects the line of arc volcanoes. They develop well-defined rifts that are parallel to rift structures along the extension center, whereas volcanoes of the spreading axis to the north are smaller than the frontal arc volcanoes and tend to form along lineaments. Compositions of lavas from these intersection volcanoes bear some similarities to back-arc basin basalt, but are on the whole well within the range of compositions for Mariana island arc lavas. The Pacific plate subducts nearly orthogonal to the strike of the trench along the southern part of the Mariana system and the distance to the arc line from the trench axis is only ∼ 150 km. Several deep fault-controlled canyons on the inner slope of the southern Mariana trench indicate an enhanced tectonic extension of this plate margin. The presence of these active arc volcanoes and the existence of the orthogonal normal faulting along the southern Mariana forearc supports a model of radial extension for formation of the Mariana Trough, a model previously dismissed because of the lack of evidence of these two major geological features.     

Quaternary volcanism of the Japanese Islands

Abstract Quaternary volcanism of the Japanese Islands is examined from the perspective of experimental petrology, geographic distribution of volcanoes and spatial geochemical variations. The dehydration of amphibole and chlorite at a 110 km depth and of phlogopite at ∼180 km in the downdragged hydrous mantle layer would result in the occurrence of two volcanic chains parallel to the trench axis. Long-term subduction of the old Pacific plate and recent subduction of the young Philippine Sea plate beneath East Japan and West Japan volcanic belts respectively, would be critical for the significant difference in intensity, style and geochemistry of Quaternary volcanism between the two volcanic belts. The geochemistry of volcanic rocks in Northeast Japan and those in the Ryukyu arc is typical of 'island-arcs' having low LIL/HFS element ratios, while alkalic basalts along the Japan Sea coast side in Southwest Japan have high LIL/HFS ratios similar to intra-continental or oceanic island basalts. Across-arc variations in eruptive volume and distributional density of volcanoes and in geochemistry are documented in Northeast Japan and are well explained by the decreasing degrees of partial melting toward back-arc side, and the difference in geochemistry of fluids supplied by the downdragged hydrous layer.     

Tsunami propagation in rivers of the Japanese Islands

On 26 May 1983 the rivers of the Japanese Islands were invaded by a tsunami from the Sea of Japan. Water levels in five large rivers were measured and were highest midway towards the uppermost point of inundation. The level at the uppermost point was approximately the same as that at the river mouths. As a result of resonance, a standing wave develops in the rivers. The period of the invading tsunami was found to be 80 min. This continental shelf oscillation resulted in a long-wave propagation of the tsunami.The water levels showed another peak in the neighbourhood of estuarine inflows. The period of this wave was estimated to be 20 min; its formation was attributed to generation near the tsunami source.The cross-section and longitudinal profiles of the rivers, together with water flow, complicated the observed water level profiles.     

Cretaceous episodic growth of the Japanese Islands

Abstract The Japanese Islands formed rapidly in situ along the eastern Asian continental margin in the Cretaceous due to both tectonic and magmatic processes. In the Early Cretaceous, huge oceanic plateaus created by the mid-Panthalassa super plume accreted with the continental margin. This tectonic interaction of oceanic plateau with continental crust is one of the significant tectonic processes responsible for continental growth in subduction zones. In the Japanese Islands, Late Cretaceous-Early Paleogene continental growth is much more episodic and drastic. At this time the continental margin uplifted regionally, and intra-continent collision tectonics took place in the northern part of the Asian continent. The uplifting event appears to have been caused by the subduction of very young oceanic crust (i.e. the Izanagi-Kula Plate) along the continental margin. Magmatism was also very active, and melting of the young oceanic slab appears to have resulted in ubiquitous plutons in the continental margin. Regional uplift of the continental margin and intra-continent collision tectonics promoted erosion of the uplifted area, and a large amount of terrigenous sediment was abruptly supplied to the trench. As a result of the rapid supply of terrigenous detritus, the accretionary complexes (the Hidaka Belt in Hokkaido and the Shimanto Belt in Southwest Japan) grew rapidly in the subduction zone. The rapid growth of the accretionary complexes and the subduction of very young, buoyant oceanic crust caused the extrusion of a high-P/T metamorphic wedge from the deep levels of the subduction zone. Episodic growth of the Late Cretaceous Japanese Islands suggests that subduction of very young oceanic crust and/or ridge subduction are very significant for the formation of new continental crust in subduction zones.     

Regional metamorphic belts of the Japanese Islands

Abstract An overview of the regional metamorphic belts of Japan is given in the context of the tectonic evolution of the Japanese Islands. The Japanese Islands were situated on an active margin of the Eurasian continent or its constituent landmass before their assembly during the Phanerozoic. The Japanese Islands are composed mainly of metamorphosed and unmetamorphosed accretionary complexes, granitoids and their effusive equivalents that were formed by the Cordilleran-type orogeny. The metamorphic belts are regarded essentially as a deep-seated portion of an accretionary complex. In spite of continuous subduction of oceanic plates beneath the continents, these orogenic rocks were formed quite episodically, as evidenced by discontinuous matrix ages of the accretionary complexes and a striking concentration of isotopic ages of the granitoids. A systematic along-arc age shift of Cretaceous large-scaled granitic magmatism and regional metamorphism suggests a tectonic control such as ridge subduction, which triggered the episodic orogeny. A tectonic model based on the paired metamorphic belts, combined with the non-steady tectonic control, works well to explain this magmatism and metamorphism in a single arc-trench system as a continental margin process. However, the juxtapositional process of the paired metamorphic belts is still a problem. Two possible cases, namely transcurrent displacement and back-arc overthrusting are discussed.     

Collision orogeny at arc-arc junctions in the Japanese Islands

Abstract In the Japanese Islands, collision tectonics are operating at arc-arc junctions in three regions: Hokkaido, Central Japan and Kyushu. Hokkaido is situated at the junction of the Kuril and Northeast Japan Arcs. The Kuril fore arc sliver collides with the Northeast Japan Arc, and the lower crust of the Kuril Arc thrusts upon the fore arc of the Northeast Japan Arc in Hokkaido. Outcrops of the lower crust are observed in the Hidaka Mountains in the fore arc of the junction area. Central Japan is in the juncture area among the Northeast Japan, Izu-Bonin, and Southwest Japan Arcs. The Izu-Bonin arc is colliding against the Honshu mainland, which has been bent by the collision. Kyushu is a juvenile collision area between the Southwest Japan and Ryukyu Arcs. The fore arc of the Southwest Japan Arc is starting to underthrust beneath the Kyushu islands along the Bungo Strait, where shallow seismicity within the crust is active in terms of the collision. Collision tectonics are observed at most of the arc-arc junctions in the circum-Pacific orogenic belts and may be an important process contributing to the relatively rapid growth of new continental crust in subduction zones.     

Statistical analysis for thermal data in the Japanese Islands

We calculated statistical average of thermal data to speculate regional thermal structure of the forearc area of the Japanese Islands. The three thermal statistical averages show a difference of a high thermal regime in the western part of forearc inner zone and a low in the Kanto forearc outer zone. The Kanto zone marks 18 K km⁻¹ for mean geothermal gradient, 44 mW m⁻² for mean heat flow, while the western inner zone shows 27 K km⁻¹ for mean geothermal gradient, 63 mW m⁻² for mean heat flow. The geothermal gradients of the Nobi Plain and the Osaka Plain in the western inner zone are 29 and 36 K km⁻¹, respectively, while the value of the Kanto Plain in the Kanto zone is 21 K km⁻¹. Taking account of the effect of accumulation of sediments, we see the difference in the thermal regime between the plains and conclude that the difference is significant. Heat flux in the crust depends on the volume of granite rich in radioactive elements. There are few granitic rocks in the Kanto zone, while granitic rocks are dominant in the western inner zone. The heat flow of 20 mW m⁻² is attributed to the granitic rocks of about 8 km in thickness. There are two oceanic plate subductions of the Pacific plate and the Philippine Sea plate under the Kanto zone, while only the Philippine Sea plate has been subducting under the western inner zone. The model simulation based on thermal and subduction model shows a heat flow ranging 50-60 mW m⁻² in the southwest Japan forarc area and a low value of about 20 mW m⁻² in the northeast Japan forearc area. The heat flux from the cooling oceanic lithosphere depends on the age of plate. The Shikoku Basin, a part of the Philippine Sea plate, off the western inner zone is 15-30 Ma, while the Pacific plate off the Kanto zone is 122-132 Ma. Theoretically, heat flux values of 15 and 50 Ma oceanic plates range 60-120 mW m⁻² and those of 122-132 Ma could be about 10 mW m⁻². If the heat flux contribution from the Philippine Sea plate under the Kanto zone is smaller than the plate under the western inner zone, there could be a thermal regime difference in order of several tens of mW m⁻². Conclusively, the cause of the difference of heat flux could be the uneven granitic rocks distribution and/or the difference of heat flux between the two subducting plate.     

Origin of some calc-alkalic andesites in the Japanese Islands

Origin of calc-alkalic andesite in the Japanese Islands is reviewed on the basis of the recent trace element data and new experimental results. It is suggested that calc-alkalic andesites in the Japanese Islands have at least four different origins; (1) fractional crystallization with separation of magnetite of high-alumina basalt magma, (2) partial melting of hydrous upper mantle peridotite (for magnesian andesite), (3) fractional crystallization with separation of olivine and/or orthopyroxene of magnesian andesite magma and (4) mixing of dacitic and basaltic magmas. Emphasis is placed on the possible generation of primary magnesian calc-alkalic andesite magmas by direct partial melting of the upper mantle peridotite under hydrous conditions at depths between 40 and 60 km.     

Recharge and aquifer response: Northern Guam Lens Aquifer, Guam, Mariana Islands

The Northern Guam Lens Aquifer is an island karst aquifer in uplifted young, highly conductive limestone. Calculations of recharge based on differences between daily rainfall and daily pan evaporation suggest that the maximum annual mass of water delivered to the freshwater lens is about 67% of mean annual rainfall. Hydrographs of daily well-level responses plotted against daily rainfall indicate that the rate at which water is delivered to the lens is a function of rainfall intensity and the relative saturation of the vadose zone. Together, these variables determine the degree to which stormwater is shunted into fast flow through preferred pathways that bypass the bedrock matrix, rather than percolating slowly through the bedrock matrix.

Data from the 40-year interval from 1956 to 1995 show that some 17% of rainfall on northern Guam arrives in small amounts (<0.6 cm/day). Most of this light rainfall is probably lost to evapotranspiration. At least another 20% of total rainfall on Guam arrives at very high intensities (>5.0 cm/day), which tend to promote fast flow at the expense of percolation. Rapid recovery of the water table from rapid recharge suggests that the lens either takes such recharge into storage very rapidly, discharges it rapidly without taking it into storage, or some combination of both. Significant vadose buffering of recharge to the lens is indicated by the fact that simulations assuming that the recharge from precipitation received in any given month is transmitted to the lens during the same month consistently over-predict observed peak mean monthly water levels and under-predict the minima.     

Open and coastal seas interactions south of Japan represented by an ensemble Kalman filter

We investigated the feasibility of the ensemble Kalman filter (EnKF) to reproduce oceanic conditions south of Japan. We have adopted the local ensemble transformation Kalman filter algorithm based on 20 members’ ensemble simulations of the parallelized Princeton Ocean Model (the Stony Brook Parallel Ocean Model) with horizontal resolution of 1/36°. By assimilating satellite sea surface height anomaly, satellite sea surface temperature, and in situ temperature and salinity profiles, we reproduced the Kuroshio variation south of Japan for the period from 8 to 28 February 2010. EnKF successfully reproduced the Kuroshio path positions and the water mass property of the Kuroshio waters as observed. It also detected the variation of the steep thermohaline front in the Kii Channel due to the intrusion of the Kuroshio water based on the observation, suggesting efficiency of EnKF for detection of open and coastal seas interactions with highly complicated spatiotemporal variability.     

Palaeozoic and early Mesozoic fish faunas of the Japanese Islands

Abstract In recent years, many fish teeth and scales have been found from the Palaeozoic and Mesozoic age strata of the Japanese Islands. This study is a compilation of the Japanese fish record from the Palaeozoic and early Mesozoic age deposits. Based on the published and unpublished data, the fossil fishes from the Palaeozoic and Early Mesozoic of Japan can be classified into 27 genera and 33 species, that is, one species of Devonian placoderms, 19 species of Permian to Jurassic elasmobranchs, three species of Permian cochliodonts, seven species of Carboniferous to Permian petalodonts, and three species of Triassic to Jurassic osteichthyans.     

Low‐temperature thermochronological database of bedrock in the Japanese Islands

To provide better access to thermochronological data and understand the long‐term denudation history of the Japanese Islands, we compiled a low‐temperature thermochronological dataset of fission‐track (FT) and (U–Th–Sm)/He (He) ages for apatite and zircon in bedrocks. These thermochronometric ages are compiled from 90 literature sources and 1,096 localities, and include 418 apatite FT ages, 851 zircon FT ages, 42 apatite He ages, and 30 zircon He ages. Many FT ages have been reported previously; however, the number of He ages is limited in the Japanese Islands. The compiled data are spatially biased; for instance, more data are reported for the Chubu and Kinki districts and the Pacific coast of the Shikoku Island, whereas less data were available for the Tohoku and Chugoku districts. For better understanding arc‐scale uplift‐denudation history, further thermochronological research in the lesser‐studied regions and more He thermochronometric measurements are desired. This compilation will be updated and provided on the website of the Fission‐Track Research Group in Japan ( http://ftrgj.org/index.html ).     

Water-weakened lower crust and its role in the concentrated deformation in the Japanese Islands

The nature and origin of the concentrated deformation zone along the Japan Sea coast (NKTZ: Niigata-Kobe tectonic zone) was investigated by carefully analyzing the GPS data and qualitatively modeling the lower crust in NKTZ. It was concluded that this deformation zone is not a plate boundary between the Amurian plate (AMU) and the North America plate but is rather an internal deformation zone near the eastern margin of AMU. The data previously obtained on the conductivity anomalies in the lower crust and the ³He/⁴He ratios suggest that the concentrated deformation in NKTZ results from the lower crust in NKTZ being weakened by a high water content. The high water content is thought to result from the dehydration of subducting slabs. NKTZ has a higher water content in the lower crust than other regions do because there is no Philippine Sea plate (PHS) seismic slab beneath NKTZ. In other regions, it is estimated that the mantle wedge above the seismic Philippine Sea slab prevents the water dehydrated from the slab from rising to the lower crust, and that the lithosphere within PHS itself prevents the water dehydrated from the Pacific plate from rising up through it.     

Geochemistry of basaltic and gabbroic rocks from the West Mariana basin and the Mariana trench

This paper describes the chemistry of 33 basaltic rocks dredged from the West Mariana basin and from the Mariana trench during the R/V “Dmitry Mendeleev” 1976 cruise in the western Pacific.The shipboard investigations were carried out by an international working group of 66 earth scientists under the IGCP Project “Ophiolites” and sponsored by the U.S.S.R. Academy of Sciences, Moscow. The purpose of the expedition was to investigate the structure and composition of the oceanic crust of marginal basins, remnant island arcs and deep-sea trenches. Tholeiitic basalts and gabbros as well as ultramafic rocks in various stages of alteration were dredged from the central part of the West Mariana basin demonstrating the presence of oceanic crust.The Pacific slope of the Mariana trench yielded altered basaltic rocks of tholeiitic and alkalic (? trachybasaltic to shoshonitic) composition. The lower part of the island arc slope contains typical tholeiitic basalts, dolerites and gabbros as well as ultramafics associated with flysch-type sediments. This is strong evidence for the formation of an “ophiolite-schuppenzone”, probably due to subduction of Pacific oceanic crust.Associated with these rocks are amygdaloidal, highly magnesian lavas (similar to boninites), which have not been recognized previously in oceanic ridge basalts.These rocks (together with the dolerites) are interpreted as parts of the Mariana island arc and are thought to be the first stage of island arc development (an immature island arc).     

Ar and Sr isotopes of mantle-derived rocks from the Japanese Islands

⁴⁰Ar-³⁰Ar and Sr isotope analyses have been made for ultramatic nodules in basaltic rocks from Itinome-gata and the Oki-Dōgo Island, northeast and southwest of Japan respectively, in order to examine the state of the upper mantle under the Japanese Islands. ⁴⁰Ar-³⁰Ar analyses have revealed that ultramafic nodules from Oki-Dōgo Island were involved in events which caused Ar redistribution in these rocks and probably were related to their formation not older than several tens of millions of years ago. These nodules are relatively depleted in both Rb and Sr contents together with some other incompatible elements compared to similar nodules from the other regions in the world. This may be typical for ultramafic nodules from the Japanese Islands. Ultramafic nodules show mostly a little higher⁸⁷Sr/⁸⁶Sr ratios (0.704–0.706) than those of host basaltic rocks (0.703–0.705) indicating the accidental origin for these nodules. Present results also suggest both horizontal and vertical inhomogeneities of the uppermost part of the mantle with respect to Sr isotopes on a regional scale. Hence, the ultramafic nodules in volcanic rocks may not always directly represent the upper mantle materials from which the magma is produced at present. Such ultramafic nodules might be relatively old material from the uppermost part of the mantle under the Japanese Islands.