Non-volcanic deep low-frequency tremors accompanying slow slips in the southwest Japan subduction zone

Non-volcanic deep low-frequency tremors in southwest Japan exhibit a strong temporal and spatial correlation with slow slip detected by the dense seismic network. The tremor signal is characterized by a low-frequency vibration with a predominant frequency of 0.5–5 Hz without distinct P- or S-wave onset. The tremors are located using the coherent pattern of envelopes over many stations, and are estimated to occur near the transition zone on the plate boundary on the forearc side along the strike of the descending Philippine Sea plate. The belt-like distribution of tremors consists of many clusters. In western Shikoku, the major tremor activity has a recurrence interval of approximately six months, with each episode lasting over a week. The tremor source area migrates during each episode along the strike of the subducting plate with a migration velocity of about 10 km/day. Slow slip events occur contemporaneously with this tremor activity, with a coincident estimated source area that also migrates during each episode. The coupling of tremor and slow slip in western Shikoku is very similar to the episodic tremor and slip phenomenon reported for the Cascadia margin in northwest North America. The duration and recurrence interval of these episodes varies between tremor clusters even on the same subduction zone, attributable to regional difference in the frictional properties of the plate interface.     

Zenisu Ridge: mechanical model of formation

Seabeam, seismic and submersible surveys took place during the Kaiko Project and revealed significant compressive deformation at the northeastern end of the Philippine Sea plate, related to the recent collision of the Izu-Ogasawara Arc against Central Japan. Intraoceanic thrusting at the base of the Zenisu Ridge, a linear topographic high running a few tens of kilometers south of the Nankai Trough, is supported by tectonic, magnetic and gravimetric data. We investigate the formation of the Zenisu Ridge in terms of compressive mechanical failure of a thin elastic-perfectly plastic plate, subducting at a trench and subject to a regional compressive axial force. The rheological envelope concept is used throughout the numerical calculations. Based on a detailed study of flexure of the present-day bending far from the deformation zone, we evaluate the bending forces involved: the bulge is 120 to 150 m high and the compressive stress all along the Nankai Trough is about −100 MPa. In the Zenisu Ridge area, an additional compressive stress is superimposed due to the nearby collision at Izu-Peninsula. We compute the vertical distribution of the deviatoric stress before failure and find that the deviatoric stress is maximum at a depth of 20–25 km in the trench area, and again at the surface 60 to 100 km seaward, in the vicinity of the bulge. The development of a thrust joining these two maxima through the entire thickness of the lithosphere is discussed. The model predicts that the formation of the Zenisu Ridge did not occur before 4 Ma and is caused by progressive tectonic uplift due to the redistribution of bending stresses as the ridge approaches the Nankai Trough.     

Fluids in deep subduction zones: Features,compositions and related geochemical fractionations

<正>Plate subduction is the most magnificent process in the Earth.Subduction zones are important sites for proceeding matter-and energy-transports between the Earth’s surface and the interior,continental crust growth,and crust-mantle interactions.Besides,a number of geological processes in subduction zones are closely related to human beings’daily life,e.g.volcanic eruptions and earthquakes,formation of mineral deposits,etc.Subduction     

Geodynamic evolution of southern Costa Rica related to low-angle subduction of the Cocos Ridge: constraints from thermochronology

The Late Tertiary shallow subduction of the Cocos ridge under the Caribbean plate controlled the evolution of the Cordillera de Talamanca in southeast Costa Rica, which is a mountain range that consists mainly of granitoids formed in a volcanic arc setting. Fission track thermochronology using zircon and apatite, as well as ⁴⁰Ar–³⁹Ar and Rb–Sr age data of amphibole and biotite in granitoid rocks constrain the thermal history of the Cordillera de Talamanca and the age of onset of subduction of the Cocos ridge. Shallow intrusion of granitoid melts resulted in fast and isobaric cooling. A weighted mean zircon fission track age (13 analyses) and Rb–Sr biotite ages of about 10 Ma suggest rapid cooling and give minimum ages for granitoid emplacement. In some cases ⁴⁰Ar–³⁹Ar and Rb–Sr apparent ages of amphibole and biotite are younger than the zircon fission track ages, which can be attributed to partial resetting by hydrothermal alteration. Apatite fission track ages range from 4.8 to 1.7 Ma but show no correlation with the 3090-m elevation span over which they were sampled. The apatite ages seem to indicate rapid exhumation caused by tectonic and isostatic processes. The combination of the apatite fission track ages with subduction parameters of the Cocos plate such as subduction angle, plate convergence rate and distance of the Cordillera de Talamanca to the trench implies that the Cocos ridge entered the Middle America Trench between 5.5 and 3.5 Ma.     

n-Alkane parameters from a deep sea sediment transect off southwest Africa reflect continental vegetation and climate conditions

An isobathic transect of marine surface sediments from 1°N to 28°S off southwest Africa was used to further evaluate the potential of the chain length distribution and carbon stable isotope composition of higher plant n-alkanes as proxies for continental vegetation and climate conditions. We found a strong increase in the n-C_29–33 weighted mean average δ¹³C values from −33‰ near the equator to around −26‰ further south. Additionally, C_25–35 n-alkanes reveal a southward trend of increasing average chain length from 30.0 to 30.5. The data reflect the changing contribution of plants employing different photosynthetic pathways (C₃ and C₄) and/or being differently influenced by the environmental conditions of their habitat. The C₄ plant proportions calculated from the data (ca. 20% for rivers draining the rainforest, to ca. 70% at higher latitude) correspond to the C₄ plant abundance in continental catchment areas postulated by considering prevailing wind systems and river outflows. Furthermore, the C₄ plant contribution to the sediments correlates with the mean annual precipitation and aridity at selected continental locations in the postulated catchment areas, suggesting that the C₄ plant fraction in marine sediments can be used to assess these environmental parameters.     

Diamond in metasedimentary crustal rocks from Pohorje,Eastern Alps: a window to deep continental subduction

We report the first finding of diamond and moissanite in metasedimentary crustal rocks of Pohorje Mountains (Slovenia) in the Austroalpine ultrahigh‐pressure (UHP) metamorphic terrane of the Eastern Alps. Microscopic observations and Raman spectroscopy show that diamond occurs in situ as inclusions in garnet, being heterogeneously distributed. Under the optical microscope, diamond‐bearing inclusions are of cuboidal to rounded shape and of pinkish, yellow to brownish colour. The Raman spectra of the investigated diamond show a sharp, first order peak of sp³‐bonded carbon, in most cases centred between 1332 and 1330 cm^?1, with a full width at half maximum between 3 and 5 cm^?1. Several spectra show Raman bands typical for disordered graphitic (sp²‐bonded) carbon. Detailed observations show that diamond occurs either as a monomineralic, single‐crystal inclusion or it is associated with SiC (moissanite), CO₂ and CH₄ in polyphase inclusions_. This rare record of diamond occurring with moissanite as fluid‐inclusion daughter minerals implies the crystallization of diamond and moissanite from a supercritical fluid at reducing conditions. Thermodynamic modelling suggests that diamond‐bearing gneisses attained P–T conditions of ≥3.5 GPa and 800–850 °C, similar to eclogites and garnet peridotites. We argue that diamond formed when carbonaceous sediment underwent UHP metamorphism at mantle depth exceeding 100 km during continental subduction in the Late Cretaceous (c. 95–92 Ma). The finding of diamond confirms UHP metamorphism in the Pohorje Mountains, the most deeply subducted part of Austroalpine units.     

Anatomy of a continental subduction zone: The main mantle thrust in Northern Pakistan

The Himalayas form an ideal natural laboratory to study the deformation processes of continental crust during collision orogeny. New information is presented concerning the structural evolution of the Main Mantle thrust zone in the Himalayas of N Pakistan, in the region around Nanga Parbat. The hanging-wall lies at relatively high levels within the Kohistan arc terrane which has been emplaced onto Indian continental rocks. This thrust probably originated as a breakback structure in the hanging-wall to the pre-collisional (oceanic) subduction zone. The present hanging-wall contains a shear zone of about 1 km width developed under amphibolite facies conditions. Simple shear dominant strains have developed new fabrics parallel to the main thrust zone. However, these structures are redeformed by discrete extensional and compressional shears within about 100 m of the thrust contact, again developed under amphibolite facies conditions. The footwall consists of an old basement complex (the Nanga Parbat gneisses) overlain by a distinct suite of metasediments now at amphibolite facies. This cover assemblage of psammites, pelites and marbles with local metabasites consistently lies directly against rocks derived from the Kohistan arc along the MMT. The absence of structures suggestive of consistent rheological contrasts within the cover assemblages suggests that the vast majority of the deformation features they contain were developed only once they experienced substantial tectonic overburdens. Prior to this the Indian cover rocks will have been »passively« subducted beneath the Kohistan arc until into amphibolite facies. We discuss these inferences in terms of evolving shear zone width with time and comment on the implications for predicting the character of mid-deep crustal shear zones, particularly from seismic reflection profiles.

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Zusammenfassung Der Himalaya bildet ein ideales, natrliches Laboratorium für Untersuchungen von Deformationsprozessen in kontinentalen Krustengesteinen während der Kollision bzw. Orogenese. Hier werden neue Daten vorgelegt, die sich mit der strukturellen Entwicklung der Hauptmantelüberschiebung im Himalaya von Nordpakistan im Gebiet um den Nanga Parbat befassen. Die Hangendeinheiten oberhalb der Störung liegen in einem relativ hohen Niveau innerhalb des »Kohistan arc terrane«, das auf die indischen Kontinentalgesteine überschoben wurde. Diese Überschiebung entstand wahrscheinlich als Rücküberschiebungsstruktur im Hangenden der Subduktionszone vor der Kollision. Im Hangenden befindet sich eine ca. 1 km breite Scherzone, die sich unter amphibolitfaziellen Bedingungen gebildet hat. Die durch »simple shear« erzeugten Deformationen sind mit ihren neuen Gefügen parallel zur Hauptüberschiebungszone ausgerichtet. Die Gefüge wurden nachfolgend von Extensionsund Kompressionsbewegungen im Bereich von ca. 100 m um den Überschiebungskontakt erneut unter amphibolitfaziellen Bedingungen erfaßt und deformiert. Das Liegende der Hauptüberschiebung besteht aus einem alten Basementkomplex (den Nanga Parbat Gneisen), die von deutlich abgesetzten, amphibolitfaziellen Metasedimenten überlagert werden. Diese Sedimenthülle bestehend aus Psammiten, Peliten und Marmoren mit lokalen Metabasiten stößt entlang der MMT direkt gegen die Gesteine des Kohistanbogens. Das Fehlen von Strukturen, die auf gleichbleibende rheologische Unterschiede hinweisen würde, läßt vermuten, daß der Großteil der in ihnen enthaltenen Deformationsgefüge auf einmal während beträchtlicher tektonischer Auflast entstanden ist. Vorher wurden die indischen Hüllgesteine »passiv« unter den Kohistanbogen bis in den Bereich der Amphibolitfazies subduziert. Die Folgerungen aus der sich über die Zeit entwickelnden Breite dieser Scherzone werden diskutiert und die Bedeutung für die Vorhersage der Charakteristik von mitteltiefen krustalen Scherzonen, insbesondere in Verbindung mit seismischen Reflektionsprofilen betont.

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Résumé L'Himalaya constitue un laboratoire naturel idéal pour l'étude des processus de déformation de la croûte continentale au cours d'une orogenèse de collision. Les auteurs présentent des informations nouvelles relatives à l'évolution structurale de la zone du Main Mantle Thrust dans la région du Nanga Parbat au nord du Pakistan. Le toit de cet accident occupe un niveau assez élevé dans le «Kohistan arc Terrane» qui a été charrié sur les roches du continent indien. Le charriage doit probablement son origine à une structure en retour apparue au-dessus de la zone de subduction pré-collisionnelle (océanique). Le toit actuel de l'accident contient une zone de glissement (shear zone) épaisse d'environ 1 km et formée dans les conditions du facies des amphibolites. Les déformations engendrées par glissement simple (simple shear) ont développé de nouvelles fabriques parallèles à la surface de charriage. Cependant, dans une tranche d'une centaine de mètres à partir du contact du charriage, ces structures ont été reprises par des cisaillements extensionnels ou compressionnels, toujours dans les conditions du facies des amphibolites.Le mur de l'accident est formé d'un complexe ancien (le gneiss du Nanga Parbat) surmonté d'une série de métasédiments distincts qui présentent aujourd'hui le facies des amphibolites. Cette couverture de psammites, de pélites, de marbres et de métabasites locales est directement en contact le long du MMT avec l'arc du Kohistan. L'absence, dans cette couverture, de structures témoignant de contrastes rhéologiques marqués, suggère que la grande majorité des structures déformatives n'y ont été développées qu'après un enfouissement tectonique important. Avant cela, les roches de la couverture indienne ont dû être subductées passivement sous l'arc du Kohistan, jusqu'au facies des amphibolites. Les auteurs discutent ces conclusions en termes d'évolution temporelle d'une shear zone et en commentant les implications dans le domaine de la prédiction du caractère des shear zones de profondeur crustale moyenne, en particulier à partir des profils de sismique réflexion.

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Mantle peridotite in newly discovered far-inland subduction complex,southwest Arizona: initial report

The latest Cretaceous to early Palaeogene Orocopia Schist and related units are generally considered a low-angle subduction complex that underlies much of southern California and Arizona. A recently discovered exposure of Orocopia Schist at Cemetery Ridge west of Phoenix, Arizona, lies exceptionally far inland from the continental margin. Unexpectedly, this body of Orocopia Schist contains numerous blocks, as large as ~300 m, of variably serpentinized mantle peridotite. These are unique; elsewhere in the Orocopia and related schists, peridotite is rare and completely serpentinized. Peridotite and metaperidotite at Cemetery Ridge are of three principal types: (1) serpentinite and tremolite serpentinite, derived from dunite; (2) partially serpentinized harzburgite and olivine orthopyroxenite (collectively, harzburgite); and (3) granoblastic or schistose metasomatic rocks, derived from serpentinite, made largely of actinolite, calcic plagioclase, hercynite, and chlorite. In the serpentinite, paucity of relict olivine, relatively abundant magnetite (5%), and elevated Fe³⁺/Fe indicate advanced serpentinization. Harzburgite contains abundant orthopyroxene, only slightly serpentinized, and minor to moderate (1–15%) relict olivine. Mantle tectonite fabric is locally preserved. Several petrographic and geochemical characteristics of the peridotite at Cemetery Ridge are ambiguously similar to either abyssal or mantle-wedge (suprasubduction) peridotites and serpentinites. Least ambiguous are orthopyroxene compositions. Orthopyroxene is distinctively depleted in Al₂O₃, Cr₂O₃, and CaO, indicating mantle-wedge affinities. Initial interpretation of field and petrologic data suggests that the peridotite blocks in the Orocopia Schist subduction complex at Cemetery Ridge may be derived from the leading corner or edge of a mantle wedge, presumably in (pre-San Andreas fault) southwest California. However, derivation from a subducting plate is not precluded.     

Deep structure of a major subduction back thrust: Magneto-telluric investigations of the Taranaki Fault,New Zealand

A magneto-telluric (MT) pilot study is used to investigate the Taranaki Fault, a major thrust along the eastern margin of the Taranaki Basin, New Zealand. Analysis of the survey data (MT phase tensor analysis and 2D models), along a 25 km east–west oriented line perpendicular the fault, indicates the Taranaki Fault is the tectonic boundary between major Permian and Mesozoic basement terranes with differing resistivities. Models show the resistivity boundary at the fault dips eastward at about 45° to a depth of at least 12 km. These results suggest that the fault is a major long lived tectonic feature that formed in Mesozoic time during the terrane accretionary phase and has subsequently reactivated during the Cretaceous and Tertiary. At depths less than 5 km, MT models that use constraints from seismic and borehole data show low resistivity sediments extend about 4 km eastward, beneath the tip of the fault. The sedimentary succession beneath the thrust wedge is a petroleum exploration target and with a higher density of MT measurements, a better image of this region may be achieved.     

Geochemical mapping in Shikoku, southwest Japan

Geochemical mapping of Shikoku in the SW Japan Arc was conducted using stream sediments (<0.18 mm fraction) with sample densities of 1 site per 106 km². Concentrations of 51 elements in 173 stream sediments were determined. Areas with especially high Cr, Ni, Mg and Co concentrations run intermittently and linearly through Shikoku, and coincide with exposures of the Mikabu greenstone complex composed of oceanic basaltic and gabbroic rocks with minor ultramafic bodies. Areas with higher concentrations of Fe, V, Sc, Ti, Mn, Cr, Ni, Mg, Co and Eu are distributed widely along the Shikoku Island axis; they mainly overlap with the zones of the high-pressure type metamorphic rocks (Sanbagawa Belt) and of the Jurassic accretionary complexes with sedimentary rocks (Chichibu Belt): in contrast, areas with lower concentrations spread over the northern coastal zone of Cretaceous granitoids (Ryoke Belt) and in the southern coastal zone of Cretaceous-Paleogene accretionary complexes (Shimanto Belt). Distribution patterns of these elements reflect that various amounts of mafic and ultramafic materials, possibly of oceanic origin, are associated with the rocks in the Sanbagawa and Chichibu Belts, although such components occur sparsely in the Shimanto and Ryoke Belts. Calcium, Sr, Na, Nb, Sm and Gd contents are lower along the southern coastal zone. High values of Th, U, La, Ce and Ba are associated with granitic rocks. The positive anomalies of Cu and Cd coincide with the distribution of stratabound Cu deposits in the Sanbagawa Belt. Positive anomalies of Sb in the northern area are associated with Sb deposits near the large Median Tectonic Line fault zone. A high-Hg zone is present in the southern fore-arc area. The median concentrations for stream sediments in Shikoku are higher in Hg, Cu, Ni, Cr, Rb, Li, Cs, K, slightly higher in Nb, Ta, La, Ce, Sn and lower in Ca than those for average Japanese stream sediments. The median concentrations for the four geologic zones show systematic wide variations of mafic associated elements, and narrow variations of lithophile elements generally more abundant in felsic rocks. The chemical compositions of the stream sediments in Shikoku largely reflect the concentrations in the pre-Neogene accretionary complexes and in the high-P metamorphic rocks mainly formed from clastic materials derived from continental margins, and ratios of mafic and ultramafic materials within surface rocks; they are partly influenced by sulfide mineralization and fluid migration.     

Subduction related antigorite CPO patterns from forearc mantle in the Sanbagawa belt,southwest Japan

Antigorite (Atg) is stable throughout large parts of the wedge mantle of most subduction zones. Atg shows strong acoustic anisotropy and crystallographic preferred orientation (CPO) patterns of this mineral may contribute significantly to seismic anisotropy in convergent margins. Atg CPO patterns from the Higashi-Akaishi (HA) forearc mantle body of southwest Japan adds to the data set suggesting the most common Atg CPO pattern has a c-axis perpendicular to the foliation and a b-axis parallel to the stretching lineation. Statistical analysis using the eigenvector method of Atg CPO from two mutually perpendicular directions in the same sample (YZ-section and XZ-section) shows no significant differences implying sample preparation has no significant affect on the resulting Atg CPO. Reuss (uniform stress) averages of anisotropy for the Higashi-Akaishi samples are approximately treble the values for Voigt (uniform strain) averages. When comparing calculated anisotropy of hydrated mantle peridotite samples—such as the Higashi-Akaishi unit—with observed S-wave delay times in convergent margins, the appropriate averaging method needs to be considered.     

Chemical evolution of a gas-capped deep aquifer,southwest of Iran

The Kangan Aquifer (KA) is located below a gas reservoir in the crest of the Kangan Anticline, southwest of Iran. This aquifer is composed of Permo-Triassic limestone, dolomite, sandstone, anhydrite and shale. It is characterized by a total dissolved solid of about 332,000 mg/L and Na–Ca–Cl-type water. A previous study showed that the source of the KA waters is evaporated seawater. Chemical evolution of the KA is the main objective of this study. The major, minor and trace element concentrations of the KA waters were measured. The chemical evolution of KA waters occurred by three different processes: evaporation of seawater, water–rock and water–gas interactions. Due to the seawater evaporation process, the concentration of all ions in the KA waters increased up to saturation levels. In comparison to the evaporated seawater, the higher concentrations of Ca, Li, Sr, I, Mn and B and lower concentrations of Mg, SO₄ and Na and no changes in concentrations of Cl and K ions are observed in the KA waters. Based on the chemical evolution after seawater evaporation, the KA waters are classified into four groups: (1) no evolution (Cl, K ions), (2) water–rock interaction (Na, Ca, Mg, Li and Sr ions), (3) water–gas interaction (SO₄ and I ions) and (4) both water–rock and water–gas interactions (Mn and B ions). The chemical evolution processes of the KA waters include dolomitization, precipitation, ion exchange and recrystallization in water–rock interaction. Bacterial reduction and diagenesis of organic material in water–gas interaction also occur. A new type of chart, Ca_excess versus Mg_deficit, is proposed to evaluate the dolomitization process.     

Kenn Plateau off northeast Australia: a continental fragment in the southwest Pacific jigsaw

The submarine Kenn Plateau, with an area of about 140 000 km², lies some 400 km east of central Queensland beyond the Marion Plateau. It is one of several thinned continental fragments east of Australia that were once part of Australia, and it originally fitted south of the Marion Plateau and as far south as Brisbane. It is cut into smaller blocks by east- and northeast-trending faults, with thinly sedimented basement highs separated by basins containing several kilometres of sediment. In the Cretaceous precursor of the Kenn Plateau, Late Triassic to Late Cretaceous basins probably rested unconformably on Palaeozoic to Triassic rocks of the New England Fold Belt. Rift volcanism was common on the northern plateau and was probably of Early Cretaceous age. Late Cretaceous extension and breakup were followed by Paleocene drifting, and the Kenn Plateau moved to the northeast, rotated 30° anticlockwise and left space that was filled by Tasman Basin oceanic basalts. During these events, siliciclastic sediments poured into the basins from the continental mainland and from locally eroding highs. After a regional Late Paleocene to Early Eocene unconformity, siliciclastic sedimentation resumed in proximal areas. In deep water, radiolarian chalks were widely deposited until biosiliceous sediment accumulation ended at the regional Late Eocene to Early Oligocene unconformity, and warming surface waters led to accumulation of pure biogenic carbonates. Calcarenite formed in shallow water on the margins of the subsiding plateau from the Middle Eocene onward. Some seismic profiles show Middle to Late Eocene compression related to New Caledonian obduction to the east. Hotspots formed parts of two volcanic chains on or near the plateau as it moved northward: Late Eocene and younger volcanics of the Tasmantid chain in the west, and Late Oligocene and younger volcanics of the Lord Howe chain in the east. As the volcanoes subsided, they were fringed by reefs, some of which have persisted until the present day. Other reefs have not kept up with subsidence, so guyots formed. The plateau has subsided 2000 m or more since breakup and is now subject solely to pelagic carbonate sedimentation.     

Subsidence and extension at a convergent plate margin: evidence for subduction erosion off Costa Rica

Subduction erosion rather than subduction accretion is proposed as the dominant process currently occurring at the Nicoya segment of the convergent plate margin off Costa Rica. Based on new results from ODP drilling cores and our interpretations of published seismic data we present a tectonic model of subsidence and extension due to tectonic erosion of the forearc wedge and landward migration of the Nicoya coastline. High seismic velocities in the outermost part of the forearc wedge off the Nicoya Peninsula below the BOSS (bottom-of-slope-sediment) reflector indicate the seaward continuation of the Nicoya ophiolite complex into the basement of the forearc wedge. ODP Site 1042 revealed neritic sediment that points to strong subsidence of the forearc basement, progradation of the sedimentary succession, and landward migration of the coastline. Tilted block structures are explained by substantial fore-arc extension. In our interpretation, the removal of material from the base of the forearc wedge by the process of basal subduction erosion leads to progressive subsidence and thus to landward migration of the coastline. Landward shift of the active volcano chain in the same order of magnitude as the coastline migration is consistent with this interpretation.     

Volatiles H2O,CO2, and Cl in a subduction related basalt

The products of the 1974 eruption of Fuego, a subduction zone volcano in Guatemala, have been investigated through study of silicate melt inclusions in olivine. The melt inclusions sampled liquids in regions where olivine, plagioclase, magnetite, and augite were precipitating. Comparisons of the erupted ash, groundmass, and melt inclusion compositions suggest that the inclusions represent samples of liquids present in a thermal boundary layer of the magma body. The concentrations of H₂O and CO₂ in glass inclusions were determined by a vacuum fusion manometric technique using individual olivine crystals (Fo77 to Fo71) with glass inclusion compositions that ranged from high-alumina basalt to basaltic andesite. Water, Cl, and K₂O concentrations increased by a factor of two as the olivine crystals became more iron-rich (Fo77 to Fo71) and as the glass inclusions increased in SiO₂ from 51 to 54 wt.% SiO₂. The concentration of H₂O in the melt increased from 1.6 wt.% in the least differentiated liquid to about 3.5% in a more differentiated liquid. Carbon dioxide is about an order of magnitude less abundant than H₂O in these inclusions. The gas saturation pressures for pure H₂O in equilibrium with the melt inclusions, which were calculated from the glass inclusion compositions using the solubility model of Burnham (1979), are given approximately by P(H₂O)(Pa)=(SiO₂−48.5 wt.%) × 1.45 × 10⁷. The concentrations of water in the melt and the gas saturation pressures increased from about 1.5% to 3.5% and from 300 to 850 bars, respectively, during pre-eruption crystallization.     

Strain partitioning between the Suruga Trough and the Zenisu Thrust: Neogene to present tectonic evolution in the onland and offshore Tokai district, Japan

The Tokai district in central Japan is located close to the convergent boundary between the Philippine Sea and Eurasian plates, and has experienced not only repeated large interplate earthquakes but also intense aseismic movement. In this paper, the spatial and temporal tectonic evolution of the Tokai district, particularly around the Omaezaki area, is discussed to assess whether the district has been and will be active or inactive. According to a geological survey, the horizontal crustal shortening strain can imply the hypothetical tectonic model that the area has been getting less active and the strain rate since the Neogene can be calculated as 12% and 2×10⁻⁶%/year, respectively. The present interseismic horizontal crustal strain and strain rate around the Omaezaki area are approximately 4×10⁻⁷% and 4×10⁻⁹%/year. By comparing these rates, the decrease since Neogene can imply the hypothetical tectonic model that the area has been getting less active influenced by the strain partitioning between the Suruga Trough and the Zenisu Thrust.     

High-Fe Layer Silicates of the Sado Ridge,Japan Sea

Detailed mineralogical investigations of high-Fe layer silicates from loose sediments (glauconite sands) of the Sado Ridge revealed that green aggregates found on submarine rises of the Japan Sea floor have a different genesis. It was demonstrated that round dark green grains approximate micas in composition. Primary volcanic rocks presumably experienced extensive secondary alterations and then were disintegrated. Their disintegration products (protoceladonite) filling the pores were redeposited and buried in the sediment for a long time. Angular green grains, mainly represented by smectite, were also formed at lower temperatures during the disintegration of altered volcanosedimentary rocks. These younger grains did not experience prolonged exposure. Pseudomorphs of siliceous microplankton consist of both hydromica and smectites. They are presumably authigenic products formed with the participation of microorganisms or electrostatic processes (spherical shape), or their combination. The formation mechanism of minerals filling the cavities in pyroclastics is not entirely clear.     

柴北缘东段奥陶纪埃达克岩-富Nb玄武岩:对大陆深俯冲之前大洋俯冲及地壳增生的启示

在柴北缘东段识别出早古生代埃达克岩-富Nb玄武岩的火山岩组合。埃达克岩富Na₂O、贫K₂O,K₂O/Na₂O比值介于0.14~0.43之间;高Sr（614×10^-6~1043×10^-6）,但亏损Y（3.26×10^-6~14.1×10^-6）和Yb（0.33×10^-6~1.46×10^-6）,具有高的Sr/Y比值（44~282）;富集Sr、Ba等大离子亲石元素,亏损Nb、Ta、Ti等高场强元素及Cr、Ni、Co、V等相容元素。富Nb玄武岩富Na₂O、贫K₂O、高TiO₂,其Nb含量较高,介于16.9×10^-6~17.9×10^-6之间,具有高的Nb/Ta、Nb/U、（Nb/La）_N比值,同时富集高场强元素。埃达克岩锆石U-Pb定年得到453±4Ma和457±4Ma的结晶年龄。锆石ε_Hf（t）范围较大,介于3.40~13.23之间,对应的二阶段模式年龄t_DM2介于1059~566Ma之间,显示以新生物质为主的特征。综合研究表明柴北缘东段埃达克岩可能为岛弧环境下俯冲的南祁连大洋板片部分熔融的产物。板片来源的埃达克质熔体交代或与上覆地幔楔橄榄岩反应,导致被交代的地幔橄榄岩部分熔融而形成富Nb玄武质岩浆。柴北缘东段埃达克岩-富Nb玄武岩火山岩组合的厘定表明南祁连洋可能直到~455Ma之前并未完全闭合,同时表明俯冲大洋板片的部分熔融可能是柴北缘早古生代地壳增生的一种重要方式。     

Propagation history of the Osaka-wan blind thrust,Japan, from trishear modeling

Mapping the nucleation and 3D fault tip growth of the active Osaka-wan blind thrust provides an opportunity to asses how reactivated thrusts build slip from preexisting faults and the threat they pose as sources of large earthquakes. Analysis of folded growth strata, based on 2D trishear inverse modeling allows a range of best-fit models of the evolution of slip and propagation of the fault to be defined. The depth of the fault tip at 1200 ka varies between ∼1.5–4.5 km, suggesting the fault grew upward from high in the crust, and that it is reactivated. From its onset at ∼1500 ka, the fault grew rapidly along strike in ∼300 ky, and upwards with a P/S ratio of 2.5–3.0, but variable fault slip in space and time. Shallower depths of the fault tip at initiation and thinner basin fill correlates with slower propagation with time, contradicting models that argue for sediments as inhibitors of fault growth. Results also suggest the displacement profile of the currently active thrust is offset from its predecessor, assuming shallower depths to the original fault correlate with greater displacement in its prior history. These results suggest reactivated faults may accrue slip differently than newly developed ones, based on the history of upward fault propagation.     

The anatomy of a deep water mud-mound complex to the southwest of the Dinantian platform in Derbyshire, UK

ABSTRACT An elongate Waulsortian mud-mound complex developed at Dovedale on a ramp to the southwest of a developing carbonate platform in Derbyshire during Chadian (early Viséan) times. The complex occupied an area of approximately 6 km² and grew to a maximum relief of 80 m with longitudinal and transverse valleys developed near the southern margin. Five mound associated facies have been identified: mound core, mound flank (fine), mound flank (coarse), intermound (fine) and intermound (coarse). The mound core facies is a massive skeletal wackestone with comminuted sponge debris, foraminifera, ostracodes and crinoid debris set in a matrix of clotted micrite. The mound flank sediments display moderately inclined bedding surfaces. While the mound flank (fine) contains sponge debris, the mound flank (coarse) is dominated by articulated crinoid columnals, and includes algal-encrusted micritized intraclasts and coarse peloids. The well-bedded intermound (fine) facies is bituminous and micritic while the intermound (coarse) facies is composed of skeletal-peloidal-intraclast grainstones which locally contain calcified algae. Although the fauna is diverse, the density of colonization by metazoans was low and the supply of macrofossil debris modest. The clotted micrite texture is interpreted as the product of micro-organisms which precipitated and trapped fine-grained sediment. The mud-mound complex is dominated by the bathymetric assemblages B and C proposed by Lees, Hallet & Hibo which on their model of the Belgian Waulsortian, indicate depths of between 220 and 280 m. Intercalation of assemblages B/C and C/D on the northern margin of the complex is interpreted as the result of local storm disturbance. A deep water drift is postulated to explain the NW-SE alignment of the complex which probably fitted the ‘export model’ of Bosence, Rowlands & Quine. Beneath the sediment surface, phreatic flow eroded unlithified sediments and developed interconnected cavities which were filled by cement and sediment relatively eariy. Mound instability triggered the opening of fissures which filled with crinoid debris, peloids, indurated lithoclasts and micrite.