bluuebunny/crossref_metadata_2025 huggingface dataset:
New radiolarian ages from the Troodos ophiolite and their tectonic implications
Authors: Osozawa Soichi , Okamura Makoto
Published: April 2006

Abstract
Newly obtained radiolarian biostratigraphic age combined with previous isotopic age of the Troodos ophiolite shows that the ophiolite becomes systematically younger from east to west: Turonian, early Campanian, and late Campanian. The youngest late Campanian part of the ophiolite is directly covered by the volcaniclastic sediment derived from an active island arc, whereas the older part is covered by pelagic radiolarite. These facts constitute evidence that the Troodos ophiolite was probably emplaced during the subduction of an active spreading ridge.
Two‐stage model of incorporation of seamount and oceanic blocks into sedimentary melange: Geochemical and biostratigraphic constraints in Jurassic Chichibu accretionary complex, Shikoku, Japan
Authors: Matsuda Seiji , Ogawa Yujiro
Published: April 2006

Abstract
The significance of timing and formation of mélange in accretionary prisms, particularly concerning basaltic and related rocks and pelagic sediments, is exemplified in the Sawadani area of the Jurassic Chichibu accretionary complex in Shikoku, southwest Japan. Major and trace element geochemistry of the basaltic and related rocks indicates that all are of a hot‐spot origin which produced a seamount. Most of the rocks have a trend of differentiation from an alkalic parental magma. The time relationship between the blocks and matrices of the mélange deduced from radiolarian fossil evidence and macro‐ to microscopic characteristics of contacts between different lithologies indicates two stages of mixing of materials in the seafloor. The first mixing occurred on the flank of the seamount in the pelagic environments in the Late Permian, and the second occurred on the trench floor or in the accretionary prism after the Early Jurassic. These two stages show respectively the geological phenomena of a seamount within the Izanagi‐Kula plate and its incorporation into the Asian continental margin.
Strain segmentation and lateral membrane deformation rate of the subducted Ryukyu slab
Authors: Chiao LingYun
Published: April 2006

Abstract
Focal mechanisms of intermediate‐depth earthquakes within the subducted Ryukyu slab indicate a systematic pattern of down‐dip extension in the northern part under Kyushu. These mechanisms switch rapidly around the Tokara channel to down‐dip compression in the southern part toward Taiwan. This intriguing pattern of strain segmentation, together with the differences between the slab geometry of the northern and southern parts, has raised the question of whether stress segmentation, as defined possibly by a tear fault, might exist in the slab. However the Ryukyu trench has a concave oceanward shape in the northern segment while it is convex in the southern oceanward part toward Taiwan. The inflection zone is located around the Tokara channel. The concept of Gaussian curvature of a curved surface suggests that the along‐arc variation of the geometric configuration of a subducted slab is related to the shape of the trench. This is in order to accommodate the lateral membrane deformation of the slab as the oceanic lithosphere subducts from a spherical shell to the geometry delineated by the Wadati‐Benioff zone. The membrane deformation regime of the subducted Ryukyu slab and its relation with the trench geometry was examined by assuming that the subduction be modelled by the flow field of a thin viscous sheet. A projection operator was utilized to compute the membrane strain‐rate tensor of an arbitrary non‐Euclidean surface. Numerical experiments indicated that the northern segment of the slab was dominated by lateral compression and down‐dip extension and the southern part by lateral extension and down‐dip compression. This transition is sharply located near the Tokara channel. These patterns were compatible with what had been observed from studies of focal mechanisms, suggesting that the strain segmentation might be controlled, at least in part, by the lateral membrane deformation within the slab due to the trench shape in this subduction zone. The slab geometry was predicted by minimizing the integrated total dissipation power; this revealed distinct features that were consistent with observations. This implies that the slab geometry may also be affected by the membrane deformation in a systematic fashion.
Tectonic and sedimentary processes at the submarine Antique Ridge and the accretionary wedge of Negros (Sulu Sea, Philippines): Results of an underwater television and photographic survey by RV ‘Sonne’
Authors: Beiersdorf Helmut
Published: April 2006

Abstract
Numerous Neogene/Quaternary marl outcrops of the submarine Antique Ridge and southern Negros accretionary complexes (Sulu Sea, Philippines) were formed by an oversteepen‐ing of the slope by the collision with the Cagayan Ridge and Cuyo Platform and also by erosion.

The outcrops exhibit distinct joint systems that were developed under compressional stress parallel to an east‐northeast subduction of the southeast Sulu Basin complex under the Panay‐Negros Fore‐Arc and Arc Complexes during the Late Miocene/earliest Pliocene. Typical bc‐(longitudinal) joints following the axial trend of the subduction zone, hkO (diagonal) shear joints, and ac‐(transverse) joints were formed. The regional stress in south‐southeast, which has changed to northeast since the Early Pliocene, has caused an uplift of the accretionary complexes and a clockwise rotation of the subduction/collision zone axis of the Antique Ridge complex from a more northern direction to NNE. Consequently the pre‐existing joint system has also rotated for 10° to 20°. A strike‐slip motion parallel to this axis as a consequence of the NE collision may have been accommodated within the accretionary complex by the bc‐joints.

Some bedding‐plane parallel white veins or layers may be related to calcium carbonate precipitation via oxidation of methane which was probably carried by migrating fluids along shear zones.

Downslope, sediment transport as well as trench‐parallel sediment transport in southerly directions is still going on, indicating active tectonic oversteepening of the slopes of the accretionary complexes as well as flowing water, possibly of intermediate water from the Northwest Sulu Basin into the Southeast Sulu Basin via the Panay Canyon.
Normal faults in an accretionary complex formed at trench‐trench‐ridge triple junction, as an indicator of angle between the trench and subducted ridge
Authors: Osozawa Soichi
Published: April 2006

Abstract
Normal faults parallel to the trend of an active ridge are formed in the accretionary prism at trench‐trench‐ridge triple junction, due to continuous spreading of the subducted ridge. Normal faults are observed in the Nabae and Mugi sub‐belts, accretionary zones formed by ridge subduction in the Shimanto Belt. Igneous and sedimentary dykes intrude through the previous normal faults. Using these fault and dyke data, intermediate principal axis of stress relating to the normal faulting is determined, and is fitted to the trend of the subducted ridge. Normal faults formed by ridge subduction are useful for plate reconstruction.
Origin of cyclicity in Triassic‐Jurassic radiolarian bedded cherts of the Mino accretionary complex from Japan
Authors: Hori Rie S. , Cho Chang‐Fee , Umeda Hiroyuki
Published: April 2006

Abstract
The abundance of magnetic microspherules in a Triassic‐Jurassic continuous sequence of alternating chert and shale beds in the Mino accretionary complex, central Japan, was measured systematically. Depending on time, the magnetic microspherules extracted from shale beds change in abundance considerably from the minimum 0.9ppm/cm3 at latest Triassic (ca 208Ma) and the maximum 75ppm/cm3 at late Early Jurassic (ca 187Ma); however, the abundance is always higher approximately 10–100 (average 70) times than those from adjacent chert bed at any stratigraphic horizon. Such systematic difference reveals the origin of radiolarian bedded chert as cyclic‐rapid accumulation of biogenic SiO2 under extremely slow accumulative environments of shale with probable aeolian dust in origin. The accumulation data for individual shale and chert beds were obtained based on the microspherule abundance and radiolarian biostratigraphy, i.e., ca 0.018g/cm2Ka for lower Jurassic shale beds and ca 1.9g/cm2Ka for adjacent chert beds.

Duration time to make a chert‐shale couplet corresponds to a dominantly 15–20Ka interval (average 23 Ka) in Upper Triassic bedded cherts with a low paleolatitude, whereas a 40–45 Ka interval (average 42 Ka) in Lower Jurassic ones which may been formed in higher latitude than Triassics before the final accretion to the Asian continental margin. Depending on paleolatitude, the cyclicity of 23 and 42 Ka may correspond to Milankovitch cycles which have been well documented in deep‐sea sediments.
Regional metamorphism and hydrothermal alteration related to Miocene submarine volcanism (‘green tuff’) in the Yurihara oil and gas field, northwest Honshu Island, Japan
Authors: Yagi Masahiko
Published: April 2006

Abstract
Alteration of reservoir rocks in the Yurihara Oil and Gas Field, hereafter referred to as the ‘Yurihara field’, have been examined by using samples from six wells. These rocks are basalts in the lowermost part of the basin‐fills (‘green tuff’ Formation). These basalts were produced in many eruptions in a submarine environment during the early to middle Miocene, and they underwent continuous intensive alteration genetically associated with Miocene submarine volcanism. The alteration of the basalts is of two types: low grade metamorphism and hydrothermal. The former belongs to the type of ocean floor metamorphism and comprises two subgroups: zeolite (zone I) and prehnite‐pumpellyite (zones IIa: vein and amygdule occurrence, and IIb: replacing plagioclase). The latter is characterized by potassic metasomatism accompanied by adularia, quartz and calcite veins (zones IIIa: center and IIIb: margin of the metasomatism). This overprints the low grade metamorphic alteration. The central zone of hydrothermal alteration coincides with a major estimated fault, so that fluids probably assent along the fault. The basalts erupted during 16.5‐15.5 Ma, determined by planktonic foraminifera assemblages of inter‐bedded shales, then underwent successive low grade metamorphism. In time, the hydrothermal alteration that overprints low grade metamorphism occurred. Adularia veins of the altered rocks located in the hydrothermal alteration zones (zone IIIa and IIIb) have been dated as 9 Ma determined by the K‐Ar method. This fact indicates that the activity of low grade metamorphism had already crossed the peak before hydrothermal alteration occurred at 9 Ma. The shape of isotherms of fluid inclusion homogenization temperatures (Th) and that of isolines of apparent salinity (Tm) almost coincide with each other, and these also coincide with the distribution of hydrothermal alteration (zones IIIa and IIIb). This indicates that the fluid inclusions formed at the same time as ascending fluids produced the potassic metasomatism. The maximum Th of the fluid inclusions is 222°C and Tm indicates trapped fluids of up to 3.3 wt% equivalent NaCl (i.e. almost the same as seawater). A Th versus Tm plot indicates mixing occurred between hydrothermal fluids and formation water that has low salinity. Corrensite and chlorite form veins, and the temperatures of their formation, estimated by the extent of aluminium substitution into the tetrahedral site of chlorite, ranges between 165 and 245°C in the centre of the hydrothermal alteration zone (zone IIIa). This is consistent with the result of Th analyses. The deposition temperature of chlorite associated with prehnite in veins ranges between 190 and 215°C in zones IIa and IIb.
A discussion on the boundary between diagenesis and metamorphism with reference to zeolite facies
Authors: Liu Yiqun , Zhou Dingwu , Li Taohong
Published: April 2006

Abstract
A Triassic formation in the Turpan‐Hami and Ordos basin of China gives two typical examples of occurrence of laumontite belonging to diagenetic facies. Sandstones of the Upper Triassic Yanchang Formation in the eastern sector of the Ordos basin are formed at the mature stage of mesodiagenesis at temperatures in the range of 71‐120°C, while the temperatures of the oil‐bearing beds Chang‐6 and Chang‐7 where laumontite is present are about 81‐88°C. The laumontite‐bearing beds of the Middle Triassic in the Hami depression are presently at the supermature stage of organic matter, the corresponding diagenetic temperature being about 140 °C. The term zeolite facies should be rejected, and the features of metamorphic stage should be clearly denned, such as closely packed grains, absence of pores and cements, and characteristic mineral assemblages including sericite, epidote, muscovite, illite (2M1), chlorite (IIb, β= 97°), dickite, pyrophyllite, graphite, chlorozeolite formed at temperatures of 200°C and Ro values of 2.5%; the stage is also characterized by schistosity and illite crystallinity of IC = 0.42°Δ 2θ. The anchimetamorphic zone is characterized by some characteristic minerals such as paragonite, rectorite, albite, laumontite, illite (1M), chlorite (Ib, P = 90°+ IIb, β= 97°) formed at temperatures of 130‐200°C and Ro values of 1.3‐2.5% as well as other quantitative parameters of organic matter.
An introduction to ultrahigh‐pressure metamorphism
Authors: Liou J. G. , Zhang Ruyuan , Ernst W. G.
Published: April 2006

Abstract
Ultrahigh‐pressure (UHP) metamorphism refers to mineralogical and structural readjustment of supracrustal protoliths and associated mafic‐ultramafic rocks at mantle pressures greater than ∼ 25 kbar (80‐90 km). Typical products include metapelite, quartzite, marble, granulite, eclogite, paragneiss and orthogneiss; minor mafic and ultramafic rocks occur as eclogitic‐ultramafic layers or blocks of various dimensions within the supracrustal rocks. For appropriate bulk compositions, metamorphism at great depths produces coesite, microdiamond and other characteristic UHP minerals with unusual compositions. Thus far, at least seven coesite‐bearing eclogitic terranes and three diamond‐bearing UHP regions have been documented. All lie within major continental collision belts in Eurasia, have similar supracrustal protoliths and metamorphic assemblages, occur in long, discontinuous belts that may extend several hundred kilometers or more, and typically are associated with contemporaneous high‐P blueschist belts. This paper defines the P‐T regimes of UHP metamorphism and describes mineralogical, petrological and tectonic characteristics for a few representative UHP terranes including the western gneiss region of Norway, the Dora Maira massif of the western Alps, the Dabie Mountains and the Su‐Lu region of east‐central China, and the Kokchetav massif of the former USSR. Prograde P‐T paths for coesite‐bearing eclogites require abnormally low geothermal gradients (approximately 7°C/km) that can be accomplished only by subduction of cold, oceanic crust‐capped lithosphere ± pelagic sediments or an old, cold continent. The preservation of coesite inclusions in garnet, zircon, omphacite, kyanite and epidote, and microdiamond inclusions in garnet and zircon during exhumation of an UHP terrane requires either an extraordinarily fast rate of denudation (up to 10 cm/year) or continuous refrigeration in an extensional regime (retreating subduction zone).
Hydrothermal alteration of green tuff belt, Japan: Implications for the influence of seawater/volcanic rock interaction on the seawater chemistry at a back‐arc basin
Authors: Shikazono Naotatsu
Published: April 2006

Abstract
Chemical data on hydrothermally altered volcanic rocks from a green tuff belt in Japan indicate that the average rate of Mg removal from seawater due to seawater cycling through back‐arc basins in the circum‐Pacific region during the early to middle Miocene (25–15 Ma) is estimated to be 2.6±1 × 1013 g/year. This is similar to that through present‐day mid‐ocean ridges (2.4 × 1013 g/year). Hydrothermal fluxes of K, Ca and Si are calculated to be 4.2±1.6 × 1013 g/year, 4.3±1.7×1013 g/year and 1.0±0.4 × 1014 g/year, respectively. These calculated results indicate that the seawater/volcanic rocks interaction at subduction‐related tectonic settings have to be taken into account in considering the geochemical mass balance of seawater over geologic time.
Discovery of an ancient tsunami deposit in coastal sequences of southwest Japan: Verification of a large historic tsunami
Authors: Minoura Koji , Nakata Takashi
Published: April 2006

Abstract
A tsunamigenic sand layer is present in coastal sequences of the Masuda Plain, southwest Japan. The radiometric age of the layer has been estimated at 930 ± 80 years BP. It is proposed that the deposit is the product of a large historic tsunami believed to have occurred in the Japan Sea on 16 June 1026 AD.
Origin of the Kunlun Mountains by arc‐arc and arc‐continent collisions
Authors: Youngun Yao , Hsü KennethJ.
Published: April 2006

Abstract The Kunlun Mountains were formed by early Mesozoic arc‐arc and arc‐continent collisions. The Middle Kunlun Are was the outer volcanic arc of the Paleozoic Asiatic continent, and the arc‐related magmatic activities from the Proterozoic to Mesozoic are recorded by numerous volcanic and plutonic rocks of the area. Several back‐arc basins and relic arcs exist north of the arc and the north Kunlun arc is one of these. The Kudi mélange of Kunlun was formed in a south‐dipping subduction zone when the basin between the north and middle Kunlun arcs was consumed by the process of back‐arc basin collapse, and the ophiolite mélange marked the suture zone where the two arcs collided. The Mazar mélange was formed in the north‐dipping subduction zone under the middle Kunlun arc, and the mélange marks the main Paleotethys suture where the Qogir‐Karamilan rocks of the Qangtang block (a fragment of Gondwanaland) is sutured on to Laurentia. The geology of Kunlun emphasizes the importance of arc‐arc and arc‐continent collisions in mountain‐building processes.
Sulfur isotopic characteristics of late Cenozoic ore deposits at the arc junction of Hokkaido, Japan
Authors: Ishihara Shunso , Sasaki Akira
Published: April 2006

Abstract Sulfide minerals of late Cenozoic vein‐type deposits of southwest Hokkaido and Kuril Islands yielded δ34CDT S values of 2 to 8 permil, which are typical green‐tuff values of magnetite‐series igneous terrane. Sulfides of the Kitami district of northeast Hokkaido, on the other hand, are characterized by negative δ34SCDT values, ranging from 0 to ‐ 7 permil. This unique value among ore deposits in the late Cenozoic back‐arc terranes in the Japanese Islands is considered to have resulted from extraction of 32S enriched sulfur from the basement rocks, because of well‐developed N‐S fracturing in the basement, which is characteristic of the axial belt and Kitami district of Hokkaido.
Summary of taxa and distribution of Sirenia in the North Pacific Ocean
Authors: Domning Daryl P. , Furusawa Hitoshi
Published: April 2006

Abstract North Pacific fossil sirenians comprise representatives of three subfamilies of the Dugongidae: Halitheriinae (Metaxytherium arctodites, Middle Miocene, North America), Hy‐drodamalinae (Dusisiren spp., Early‐Late Miocene, and Hydrodamalis spp., Late Miocene‐Pleistocene, North America and Japan), and Dugonginae (Dioplotherium allisoni, Early‐Middle Miocene, North America). Indeterminate dugongid remains are also known from the Late Oligocene of Japan, and the discovery of additional taxa in the western Pacific, especially in Paleogene rocks, can be anticipated. The known North Pacific Neogene taxa apparently dispersed into the Pacific from the Caribbean. Metaxytherium gave rise in the Pacific to Dusisiren; a series of chronospecies of the latter genus eventually culminated in Hydrodamalis, which was exterminated by humans circa AD 1768. Dioplotherium left no known descendants in the Pacific. The Recent Dugong probably entered the Pacific from the Indian Ocean. The presence in the North Pacific Miocene of at least three sympatric dugongid lineages, together with desmostylians, is evidence for a diversity of marine plants that was reduced by subsequent climatic cooling.
Early deformation fabrics of melange in the Mesozoic Waipapa Terrane, North Island, New Zealand
Authors: Kano Ken–ichi
Published: April 2006

Abstract The Waipapa Terrane in northern North Island, New Zealand, is a Mesozoic accretion‐ary complex formed along the Gondwana margin. It contains abundant melange rocks with distinctive characteristics. Precise analyses of their mesoscopic fabrics in Waiheke Island near Auckland have revealed the following sequence of deformation. The earliest phase of deformation of the sandstone/mudstone association, which is the main constituent of this terrane, originated by chaotic mixing of sand and mud due to liquidization of water‐saturated, poorly consolidated sediments. The second phase was characterized by hydrofracturing and subsequent forceful injection of ductile mud into rather brittle sand. Local intrusions of sand forming dykes and sills followed these events, as well as intrusions of pelagic/hemipelagic green argillite originally underlying the sandstone/mudstone association. An abundant occurrence of these mixing and multi‐stage injection/intrusion fabrics strongly suggest that the Waipapa Terrane around the study area was a site of high pore‐fluid pressure. Scaly‐foliated melange fabrics with monoclinic symmetry, originating from layer‐parallel shearing, were then locally superimposed on the pre‐existing melange fabrics. Similar scaly‐foliated fabrics also developed in the chert beds originally intercalated between the green argillite and the uppermost part of the oceanic crust. These scaly fabrics might have been related to the regional stacking and juxtaposition of the accreted sediments. The sequence and variation in style of deformation forming the melange fabrics presumably reflected changes in porosity and state of compaction of accreted sediments in a shallow tectonic level.
Slab weakening: Mechanical and thermal‐mechanical consequences for slab detachment
Authors: Yoshioka Shoichi , Yuen David A. , Larsen Tine B.
Published: April 2006

Abstract Slab detachment is a geophysical instability whose manifestation can be revealed by seismic tomography. Evidence of this phenomenon is in the Dinarides/Hellenic and the New Hebrides subduction zones. Subducted slabs in these regions are torn horizontally at depths ranging from 100 to 300 km. We constructed a viscoelastic three‐dimensional finite element model and investigated the state of stress. We found that an area with high stress concentration of the order of several hundred MPa is formed near the tip of the tear inside the slab, which can cause lateral migration of the tear. Favorable conditions for slab detachment are characterized by large interplate frictional force at a subduction zone and small slab resistance force deeper down. Stress concentration increases with the down‐dip tension inside the slab. The phenomenon of slab weakening has also been studied from a thermal‐mechanical standpoint, using a two‐dimensional convection model with non‐Newtonian, temperature‐dependent rheology. The stress‐dependent rheology plays an important role in causing local weakening of the descending slab. In strongly time‐dependent situations the fast descending slab is not strong everywhere but has a weak region in the middle, making it vulnerable to slab detachment. The presence of viscous heating will enhance slab detachment tendency by further weakening the interior by the frictional heating. Besides these effects, there are other mechanisms which can also weaken the slab interior and help to make slabs more pliable and susceptible to detachment.
The history of the Cretaceous environments of the northeastern Asian continental margin, Russia
Authors: Filatova Nadezhda I.
Published: April 2006

Abstract The Cretaceous geodynamic and paleogeographic evolution of the northeastern Asian continental margin is summarized in five maps for time‐intervals of 115‐110, 100‐90, 90‐85, 85‐74 and 70‐50 Ma. Four major evolutionary stages are recognized: (i) the moderate extensional tectonic regime and origination of a system of island arcs in the Jurassic‐Neocomian stage; (ii) Middle Cretaceous strong collisional and accretional processes resulting in the Asian continent formation in the Barremian‐Early Albian stage; (iii) the origination of the subduction‐related Okhotsk‐Chukotka continental marginal volcanic belt in relation to the newly formed convergent plate boundary in the Late Albian‐Senonian stage; and (iv) the next collision accompanied by the extinction of the subduction‐related volcanism in the Late Cretaceous‐Early Paleocene stage (Laramian orogeny) and displayed rift‐related processes with the intraplate‐type volcanism on the Asian continental margin. Those stages had been established through the whole Pacific ring and had close genetic relationships with mid‐ocean ridges tectonic activity.
Geomorphological study on a clastic accretionary prism: The Nankai Trough
Authors: Okino Kyoko , Kato Yukihiro
Published: April 2006

Abstract The Nankai Trough, off southwest Japan, is one of the best sites for the study of geomorphic characteristics of a clastic accretionary prism. A recent multibeam survey over the central and eastern parts of the Nankai accretionary prism has revealed a large variation of the topography along the trough axis. Analysis of the bathymetric data suggests the existence of prism deformational features of different scales, such as depressions, embayment structures and cusps. These structures are the results of slope instability caused by basement relief of subducted oceanic plate. Unstable slopes recover by new accretion and development of a low angle thrust. Small‐scale deformation due to the subduction of a small isolated seamount is then adjusted to the regional trend. By contrast, a 30 km indentation of the wedge observed in the eastern part of the Nankai Trough, the Tenryu Cusp, has seemed to retain its geometry. The subducted Philippine Sea plate has deformed greatly near the eastern end of the Nankai Trough, because of the collision between the Izu‐Ogasawara (Bonin) arc and central Japan. Therefore, the indentation may be the result of the continuous subduction of a basement high, such as the Zenisu Ridge, which has been formed under north‐south compression due to the arc‐arc collision.
Fossil worm tubes from the presumed cold‐seep carbonates of the Miocene Hayama Group, Central Miura Peninsula, Japan
Authors: Naganuma Takeshi , Okayama Yuichi , Hattori Mutsuo , Kanie Yasumitsu
Published: April 2006

Abstract Fossil worm tubes were collected from the Hayama Group, Miura Peninsula, Japan, together with abundant fossils of Calyptogena‐Acharax clams. The fossil worm tubes were well preserved and coated with milky white amorphous silica. Most of the tubes were 1‐3 mm in diameter, and up to 10 cm in length. Worm tubes were found in siltstone and limestone, and formed network‐like assemblages. Elemental mapping on the tube cross‐sections revealed the localization of sulfur, zinc and iron at the worm tubes, which suggests that sulfur‐related metabolism and deposition occurred in association with the worm tubes. High resolution analysis revealed the localization of zinc‐sulfur (sphalerite, ZnS) on the tubes, while iron‐sulfur (pyrite, FeS2) was localized at the center of the tubes. The spatially separate sphaleritization and pyritization imply that epiphytic and endosymbiotic microorganisms perform different sulfur metabolisms, such as sulfate‐reduction and sulfide‐oxidation.
Petrogenetic grid for ultrahigh‐pressure metamorphism in the model system CaO‐MgO‐SiO2‐CO2‐H2O
Authors: Ogasawara Y. , Liou J. G. , Zhang R. Y.
Published: April 2006

Abstract Petrogenetic grids for ultrahigh‐pressure (UHP) metamorphism were calculated at different Xco2 conditions in the model system CaO‐MgO‐SiO2‐CO2‐H2O involving coesite (Co), diopside (Di), dolomite (Do), enstatite (En), forsterite (Fo), magnesite (Ms), quartz (Qz), talc (Tc), tremolite (Tr) using a published internally consistent thermodynamic data set. Two P‐T grids at Xco2= 0.01 and 0.5 are described. In the calculated P‐T grid at Xco2= 0.01, four out of 10 stable invariant points, Co‐En‐Ms‐Tc, Co‐Di‐En‐Tc‐Tr, Co‐Di‐Ms‐Tc‐Tr and Di‐En‐Ms‐Tc‐Tr lie within the stability field of coesite. If the fluid phase has Xco2= 0.5, no invariant point is stable under UHP conditions. Some magnesite‐bearing assemblages are stabilized by the following three reactions: Di + Ms = Do + Fo + CO2, Ms + Tr = Do + Fo + CO2+ H2O and Ms + Tc = Fo+ CO2+ H2O at Xco2= 0.01 and by reaction Ms + Tc = Fo + CO2+ H2O together with these three at Xco2= 0.5. Ten possible UHP assemblages for mafic and ultramafic compositions at very low Xco2 conditions include the following: Co‐Do‐Ms, Co‐Di‐Ms, Co‐Di‐Tc, Di‐Ms‐Tc, Di‐En‐Tc‐, Di‐En‐Ms, Co‐Di‐En, Di‐En‐Fo, Di‐Fo‐Ms, Di‐Do‐Fo. Among them, talc‐bearing assemblages are restricted to Xco2 < 0.02 and their high‐P limit is 31.7 kb (749°C) at Xco2= 0.01. Dolomite‐magnesite‐silica assemblages have large P‐T stability fields even if Xco2 is as low as 0.1, and could occur in cold subduction zones with very low geothermal gradients. Reported UHP coesite‐dolomite assemblage is restricted only to a calc‐silicate rock interlayered with marble where Xco2 is relatively higher; no such assemblage appears for mafic and ultramafic rocks with low Xco2 evidenced by the occurrence of diopside (or omphacite) at the expense of dolomite + coesite. The effect of Xco2 on the stability of coesite‐dolomite‐magnesite, diopside‐enstatite‐magnesite, diopside‐talc assemblages is examined and the occurrence of coesite‐dolomite, magnesite‐bearing and talc‐bearing assemblages in the Dabie UHP rocks are interpreted by employing the calculated P‐T grids.
Tectonic significance of Eocene and Cretaceous radiolaria from South Andaman Island, northeast Indian Ocean
Authors: Ling Hsin Yi , Chandra Ram , Karkare S. G.
Published: April 2006

Abstract
Eocene (Middle) and Cretaceous (Campanian) radiolarian faunas from the basement rocks of the southern part of South Andaman Island in the northeastern Indian Ocean affirm the sedimentological hiatus that encompassed part of the Paleocene to Early Eocene ages in these islands, and its extension northward to Indoburma region and south to the outer islands of the Sunda Arc.
Collision orogeny at arc‐arc junctions in the Japanese Islands
Authors: Kimura Gaku
Published: April 2006

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.
Anatomy and genesis of a subduction‐related orogen: A new view of geotectonic subdivision and evolution of the Japanese Islands
Authors: Isozaki Yukio
Published: April 2006

Abstract
The Japanese Islands represent a segment of a 450 million year old subduction‐related orogen developed along the western Pacific convergent margin. The geotectonic subdivision of the Japanese Islands is newly revised on the basis of recent progress in the 1980s utilizing microfossil and chronometric mapping methods for ancient accretionary complexes and their high‐P/T metamorphic equivalents. This new subdivision is based on accretion tectonics, and it contrasts strikingly with previous schemes based on‘geosyncline’tectonics, continent‐continent collision‐related tectonics, or terrane tectonics. Most of the geotectonic units in Japan are composed of Late Paleozoic to Cenozoic accretionary complexes and their high‐PIT metamorphic equivalents, except for two units representing fragments of Precambrian cratons, which were detached from mainland Asia in the Tertiary. These ancient accretionary complexes are identified using the method of oceanic plate stratigraphy. The Japanese Islands are comprised of 12 geotectonic units, all noted in southwest Japan, five of which have along‐arc equivalents in the Ryukyus. Northeast Japan has nine of these 12 geotectonic units, and East Hokkaido has three of these units. Recent field observations have shown that most of the primary geotectonic boundaries are demarcated by low‐angle faults, and sometimes modified by secondary vertical normal and/or strike‐slip faults. On the basis of these new observations, the tectonic evolution of the Japanese Islands is summarized in the following stages: (i) birth at a rifted Yangtze continental margin at ca 750–700 Ma; (ii) tectonic inversion from passive margin to active margin around 500 Ma; (iii) successive oceanic subduction beginning at 450 Ma and continuing to the present time; and (iv) isolation from mainland Asia by back‐arc spreading at ca 20 Ma. In addition, a continent‐continent collision occurred between the Yangtze and Sino‐Korean cratons at 250 Ma during stage three. Five characteristic features of the 450 Ma subduction‐related orogen are newly recognized here: (i) step‐wise (not steady‐state) growth of ancient accretionary complexes; (ii) subhorizontal piled nappe structure; (iii) tectonically downward‐younging polarity; (iv) intermittent exhumation of high‐P/T metamorphosed accretionary complex; and (v) microplate‐induced modification. These features suggest that the subduction‐related orogenic growth in Japan resulted from highly episodic processes. The episodic exhumation of high‐P/T units and the formation of associated granitic batholith (i.e. formation of paired metamorphic belts) occurred approximately every 100 million years, and the timing of such orogenic culmination apparently coincides with episodic ridge subduction beneath Asia.
The relation between ore‐deposit formation and subduction: Mass balance considerations
Authors: Glasby G. P.
Published: April 2006

AbstractMass balance calculations indicate that the potential supply of metals resulting from subduction of oceanic crust and sediment far exceeds that required for mineralization in island ares. Subduction of oceanic crust is more important than that of sediment in supplying these metals and the subduction of manganese nodules makes only a negligible contribution. The nature and extent of mineralization in island ares therefore depend on the nature, geometry and rate of subduction. In the Pacific, two types of subduction system occur: the high‐stress (Chilean) type, characterized by shallow subduction and the formation of porphyry copper deposits in andesitic ares, and the low‐stress (Mariana) type, characterized by deep subduction and the formation of Kuroko deposits in the back‐arc basin, and porphyry copper and epithermal gold deposits in the island are. Kuroko deposits are analogous to the epithermal deposits but are formed from saline hydrothermal fluids under much higher hydrostatic pressure.
Paleogeographic maps of the Japanese Islands: Plate tectonic synthesis from 750 Ma to the present
Authors: Maruyama Shigenori , Isozaki Yukio , Kimura Gaku , Terabayashi Masaru
Published: April 2006

Abstract
A series of paleogeographic maps of the Japanese Islands, from their birth at ca 750–700 Ma to the present, is newly compiled from the viewpoint of plate tectonics. This series consists of 20 maps that cover all of the major events in the geotectonic evolution of Japan. These include the birth of Japan at the rifted continental margin of the Yangtze craton (ca 750‐700 Ma), the tectonic inversion of the continental margin from passive to active (ca 500 Ma), the Paleozoic accretionary growth incorporating fragments from seamounts and oceanic plateaux (ca 480‐250 Ma), the collision between Sino‐Korea and Yangtze (250–210 Ma), the Mesozoic to Cenozoic accretionary growth (210 Ma‐present) including the formation of the Cretaceous paired metamorphic belts (90 Ma), and the Miocene back‐arc opening of the Japan Sea that separated Japan as an island arc (25‐15 Ma).
Paleogeographic changes in central Honshu, Japan, during the late Cenozoic in relation to the collision of the Izu‐Ogasawara Arc with the Honshu Arc
Authors: Kitazato Hiroshi
Published: April 2006

Abstract The northern tip of the Izu‐Ogasawara Arc on the Philippine Sea plate collided with the central part of the Honshu Arc in the early Quaternary. The collision history is recorded in late Cenozoic strata that are distributed widely in central Japan. To reconstruct paleotopography during the collision process, paleogeographic maps of central Japan were drawn at six time slices during the late Cenozoic. These maps were made from paleodepth data that were inferred from benthic foraminiferal fossil assemblages. Sedimentological information was also added to the maps. The paleogeographic maps show several distinctive geological features. The paleodepth of the area between the Izu‐Ogasawara Arc and the Honshu Arc changed quickly from deep‐sea to shallow marine during the Quaternary by means of rapid deposition of large amounts of coarse‐grained detritus. The conglomerate was first deposited in a trough as deep‐sea fan deposits, and filled the trough until an alluvial fan was formed. Forearc basins of the Honshu Arc facing the collision area subsided from 3 to 1 Ma. Vertical movement of the basin was inferred from a strata thickness/paleodepth correlation graph. It is thought that the tectonic inversion seen in sedimentary basins in the Kanto and Tokai regions might be closely related to the change of motion of the Philippine Sea plate. However, a basin of the Ashigara area sunk continuously without interruption until 0.7 Ma. The collision event affected strongly distribution of deep‐sea benthic foraminifera. Paleogeographic maps show that a deep trough appeared in the Ashigara area between 3 and 1.5 Ma. This trough may have served as a passage for the migration of deep‐sea benthic foraminifera between the Pacific and the Philippine Sea. Cold water benthic foraminiferal species occur west of the Izu‐Ogasawara ridge deposited in strata during the existence of the passage.
K‐Ar dating studies of Ashigawa and Tokuwa granodiorite bodies and plutonic geochronology in the South Fossa Magna, central Japan
Authors: Salto Kazuo , Kato Koichi , Sugi Shunji
Published: April 2006

Abstract K‐Ar age studies in the Ashigawa and the southern part of the Tokuwa granodioritic bodies, which consist of the southern part of the Kofu plutonic complex, revealed that they formed between 12 and 9 Ma. Quite a narrow range of ages obtained from the Ashigawa southernmost part of the Tokuwa pluton implies that they cooled rapidly. The southern part of the Tokuwa pluton, as a whole, shows a systematic age distribution with a decrease in age to the north. Compilation of currently available plutonic ages in the South Fossa Magna suggests that the plutonic activities occurred three times in this region. Episodic activity like this could be argued in relation to the tectonic development of this region.
Miocene intra‐arc bending at an arc‐arc collision zone, central Japan
Authors: Takahashi Masaki , Saito Kazuo
Published: April 2006

Abstract Recent paleomagnetic studies are reviewed in an effort to clarify the relationship between the intra‐arc deformation of central Japan and the collision tectonics of the Izu‐Bonin Arc. The cusp structure of the pre‐Neogene terranes of central Japan, called the Kanto Syntaxis, suggests a collisional origin with the Izu‐Bonin Arc. The paleomagnetic results and newly obtained radiometric ages of the Kanto Mountains revealed the Miocene rotational history of the east wing of the Kanto Syntaxis. More than 90° clockwise rotation of the Kanto Mountains took place after deposition of the Miocene Chichibu Basin (planktonic foraminiferal zone of N.8: 16.6–15.2 Ma). After synthesizing the paleomagnetic data of the Japanese Islands and collision tectonics of central Japan, it appears that approximately a half rotation (40–50°) probably occurred at ca 15 Ma in association with the rapid rotation of Southwest Japan. The remainder (50‐40°) continued until 6 Ma, resulting in the sharp bent structure of the pre‐Neogene accretionary complexes (Kanto Syntaxis). The latter rotation seems to have been caused by the collision of the Izu‐Bonin Arc on the northwestward migrating Philippine Sea Plate.
Arc‐type and intraplate‐type ridge basalts formed at the trench‐trench‐ridge tripIe junction: Implication for the extensive sub‐ridge mantle heterogeneity
Authors: Osozawa Soichi , Yoshida Takeyoshi
Published: April 2006

Abstract ‘In situ basalts’ represent the ridge magmatism at and close to the ancient trench‐trench‐ridge triple junction. Such basalts in the Amami, Mugi, and Setogawa accretionary complexes, Southwest Japan, were described and analysed. The geochemical data show that the ‘in situ basalts’ include all the types of basalts, ranging from alkali basalts and high‐alumina basalts to tholeiites, and the compositions tend towards intermediate and silicic rocks. The data also reveal that the ridge basalts are indistinguishable both from the island arc and intraplate basalts, no affinities with mid‐ocean‐ridge basalts. The sub‐ridge mantle adjacent to the triple junction had a component of sub‐arc mantle, and this mantle heterogeneity can be generated by the formation of a slab window.
Geochemical evolution within a Devonian intra‐oceanic island arc: The Gamilaroi terrane, southern New England orogen, Australia
Authors: Stratford James M. C. , Aitchison Jonathan C.
Published: April 2006

Abstract Geochemical analyses of volcanic rocks in the Gamilaroi terrane reveal several phases of arc activity within an intra‐oceanic island‐arc terrane. Felsic volcanic rocks at the base of the section have rare earth element (REE) and trace element compositions which indicate that they were derived from an island‐arc source. Basalts immediately overlying the felsic volcanic rocks have a distinctive geochemical signature with low levels of Ti and Y and high levels of Ni, Cr and Mg. Low concentrations of REE and trace elements relative to mid‐ocean‐ridge basalts (MORB) indicate that they were also derived from an intra‐oceanic island‐arc source. Extensive basalts and basaltic andesites among the youngest rocks of the terrane have typically flat to enriched REE and trace element compositions, indicating a transitional arc‐back‐arc source. The change in basalt compositions indicates that rifting had occurred by this stage in the evolution of the arc. Confirmation of an intra‐oceanic setting for this terrane enables a more detailed comparison with similar intra‐oceanic rocks in the northern New England orogen. This study of the Gamilaroi terrane is an example of the potential use of geochemical data to identify other ancient intra‐oceanic island‐arc‐rift suites.
Curvature of convergent boundaries
Authors: Streltsov M. I.
Published: April 2006

Abstract It is well known that knowledge about convergent boundary geometry is crucial for global geodynamical reconstructions. At the same time, views related to the subject are controversial and speculative. For instance, island arcs, which are considered to be the most important convergent boundary structural elements, are thought to be either the parts of the small circles on the Earth's surface, or adjacent to each other as straight‐line segments. Accordingly, one view says that island arcs are rectilinear rather than arched, and there are no reasons to consider their arched shape to be a rule. Is the island arc curvature invariant in time and space? General considerations, palaeomagnetic data and structural features such as strike‐slip displacements of the opposite sign along the longitudinal faults on the island arc flanks, prompt the author to have a similar view to others, that is, that island arc curvature varies with time. It is worthy to note, nevertheless, that this statement could hardly be referred to as generally adopted, and it is not necessarily true. One can try to answer the second part of the question by means of mathematical statistics. Points of an island arc characterizing the geometry, such as volcanic centres or deep‐trench axial line points, were led to the stereographical projection centre, where distortions are minimal. By the least squares method these points have been approximated with straight lines, circumferences and lines with variable curvature. Then by means of variance analysis the competing hypotheses have been compared. The results of the present study show that the curvature of the island arcs in the north‐western Pacific varies with location. It is possible to search for relationships between island arc curvature as a measure of its bending, various physical parameters of island arc lithosphere and such recent island arc processes as volcanism, seismicity etc. Progress in this direction has already been made; namely a direct relationship between trench depth and island arc curvature has been established.