bluuebunny/crossref_metadata_2025 huggingface dataset:
Intra‐arc opening and closure of a marginal sea: The case of the Guerrero Terrane (SW Mexico)
Authors: Monod Olivier , Faure Michel , Salinas Juan‐Carlos
Published: April 2006

Abstract
The pre‐Oligocene structure of southwest Mexico, south of the trans‐Mexico volcanic axis, is investigated from Taxco (Guerrero state, abbreviation: Gro) to the Pacific coast. Three volcano‐sedimentary units are recognized; from east to west the calc‐alkaline Teloloapan, tholeiitic Arcelia and calc‐alkaline Zihuatanejo suites. Structural and stratigraphic data show that the Teloloapan volcanic arc, active during ?Late Jurassic and early Cretaceous, was built upon continental basement. The Teloloapan lavas are overlain by the Albian–Cenomanian Morelos platform carbonates and followed by the Upper Cretaceous Mexcala flysch. In contrast, the Arcelia pillow lavas are associated with sandstones and cherts of Albian‐?Cenomanian age. The Zihuatanejo arc was also installed upon continental basement and its magmatic activity was in part coeval with Arcelia magmatism. Unlike the almost undeformed Zihuatanejo volcanic rocks, all the other volcanic units are involved in east‐vergent thrusting and recumbent folding associated with ductile tectonics, as well as the Late Cretaceous Mexcala flysch overlying the Morelos platform carbonates. Contrasting with previous views, the present results do not support a major mid‐Cretaceous thrusting event in the study area. The new geodynamic interpretation proposed here considers that the Arcelia rocks were formed in a marginal basin situated east of the Zihuatanejo arc. Closure of this basin in Paleocene times is responsible for the east vergent thrust tectonics in SW Mexico.
A Cretaceous to Paleocene‐Eocene South China sea basin origin for the Zambales Ophiolite Complex, Luzon, Philippines?
Authors: Yumul Graciano P.
Published: April 2006

Abstract
The Eocene Zambales Ophiolite Complex that exhibits transitional mid‐ocean ridge basalt‐island arc tholeiite (MORB‐IAT) characteristics was formed in a subduction‐related marginal basin. The different surrounding marginal basins of the Philippines, namely, the South China Sea, Sulu Sea Basin, Celebes Basin and the West Philippine Basin have all been modeled to be of probable provenance of this ophiolite complex. Certain information (e.g. age, rock geochemistry, paleomagnetic rotations) and limitations, nevertheless, are inconsistent with the ophiolite complex being generated in these regions. Recent geophysical evidence suggests that the southwest sub‐basin of the South China Sea Basin is probably Cretaceous to Paleocene‐Eocene in age. This makes it possible to speculate that the Zambales Ophiolite Complex could have come from this sub‐basin. The present day rifting of the southern Izu‐Mariana arc can be taken as a modern day analog of this type of ophiolite generation.
Tectonic evolution of the ultrahigh‐pressure (UHP) and high‐pressure (HP) metamorphic belts from central China
Authors: Maruyama Shigenori , Liou J. G. , Zhang Ruyuan
Published: April 2006

Abstract In the Triassic suture between the Sino‐Korean and Yangtze cratons, the Dabie metamorphic Complex in central China includes three tectonic units: the northern Dabie migmatitic terrane, the central ultrahigh‐P coesite‐ and diamond‐bearing eclogite belt, and the southern high‐P blueschist‐eclogite belt. This complex is bounded to the north by a north‐dipping normal fault with a Paleozoic accretionary complex and to the south by a north‐dipping reverse fault with Yangtze basement plus its foreland fold‐and‐thrust sequence. Great differences in metamorphic pressure suggests that these units reached different depths during metamorphism and their juxtaposition occurred by wedge extrusion of subducted old continental fragments. These units were subsequently subjected to (i) Barrovian type regional metamorphism and deformation at shallow depths; (ii) intrusion of Cretaceous granitic plutons; and (iii) doming and segmentation into several blocks by normal and strike‐slip faults. A new speculative model of tectonic exhumation of UHP rocks is proposed.
Ultra‐high pressure metamorphic rocks in the Dabie‐Su‐Lu region, China: Their formation and exhumation
Authors: Bolin Cong , Qingchen Wang , Mingguo Zhai , Ruyuan Zhang , Zhongyan Zhao , Kai Ye
Published: April 2006

Abstract Based on petrological, structural, geological and geochronological research, the authors summarize the progress of ultra‐high pressure (UHP) metamorphic rock study since 1989 by Chinese geoscientists and foreign geoscientists in the Dabie‐Su‐Lu region. The authors introduce and discuss a two‐stage exhumation process for the UHP metamorphic rocks that have various lithologies; eclogite, ultramafics, jadeitic quartzite, gneiss, schist and marble. The metamorphic history of UHP metamorphic rocks is divided into three stages, that is, the pre‐eclogite stage, coesite eclogite stage, and retrograde stage. Prior to UHP metamorphism, the ultramafics had a high temperature environment assemblage of mantle and others had blueschist facies assemblages. The granulite facies assemblages, which have recorded a temperature increase event with decompression, have developed locally in the Weihai basaltic rocks. Isotopic ages show a long range from > 700 Ma to 200 Ma. The diversity in protoliths of UHP metamorphic rocks may be related to the variation of isotopic ages older than 400 Ma. The Sm‐Nd dating of ~ 220 Ma could reflect the initial exhumation stage after the peak UHP metamorphism in relation to the collision between the Sino‐Korean and Yangtze blocks and subsequent events. Petrological and structural evidence imply a two‐stage exhumation process. During the initial exhumation, the UHP metamorphic rocks were sheared and squeezed up in a high P/T regime. In the second exhumation stage the UHP metamorphic rocks were uplifted and eventually exposed with middle crustal rocks.
White mica K‐Ar ages of blueschist‐facies rocks from the Piemonte ‘calc‐schists’ of the western Italian Alps
Authors: Takeshita Hiroyuki , Shimoya Hisamichi , Itaya Tetsumaru
Published: April 2006

Abstract Fifty calc‐schists have been systematically collected from the Piemonte zone of the western Italian Alps and examined in terms of petrology, X‐ray powder diffractometer (XRD) analysis of carbonaceous materials, and K‐Ar ages of white mica separates. The petrological study and XRD analysis of carbonaceous materials have shown that calc‐schists have suffered blueschist‐facies metamorphism in the subduction zone of the convergent margin between the Apulian (African) continental and Tethyan oceanic plates. The metamorphic sequence is divided into three mineral zones based on increasing metamorphic temperature: chlorite (lower than 300°C), chloritoid, and rutile (higher than 450°C). The chlorite zone has dispersed ages of white mica separates, ranging from 115 to 44 Ma, whereas the rutile zone has a comparatively uniform age distribution from 60 to 40 Ma. The chloritoid zone has an intermediate age variation. The large variation in the chlorite zone is attributed to mixing of variable amounts of detrital mica derived from older high temperature metamorphic rocks in the separates, which have not been completely reset during Alpine metamorphism. The uniform age (average ca 50 Ma) in the rutile zone is the cooling age of blueschist‐facies calc‐schists, which have been episodically exhumed at the collision event of the European and Apulian continents in the Paleocene‐Eocene.
Mineral composition variation in Alpine Schist, Southern Alps, New Zealand: Implications for recrystallization and exhumation
Authors: Grapes RodneyH. , Watanabe Teruo
Published: April 2006

Abstract Compositional variation of silicates (plagioclase, K‐feldspar, epidote, titanite, garnet, white mica, biotite, chlorite), ilmenite, carbonates (calcite, ankerite) and apatite, in quartzofeldspathic lithologies of the Alpine Schist, New Zealand, is discussed in terms of increasing metamorphic grade and possible isograd‐producing reactions. The mineral data, in conjunction with geological considerations, are used to determine polychronous P‐T arrays of an early high P/T event (c. 16°C/kb; 5°C/km) overprinted by a lower P/T event (c. 50°C/kb; 15°C/km) that provides an estimation of Mesozoic and Cenozoic exhumation of schist of 11 to 13 km and 19 to 22 km respectively. The effects of possible shear heating and recrystallization to form K‐feldspar zone schist near the Alpine Fault is consistent with movement along a mid to lower crustal detachment surface during Cenozoic shortening, and near isothermal exhumation of the schists to form the Southern Alps.
Tectonic evolution of lower crustal rocks in an exposed magmatic arc section in the Hidaka metamorphic belt, Hokkaido, northern Japan
Authors: Toyoshima Tsuyoshi , Komatsu Masayuki , Shimura Toshiaki
Published: April 2006

Abstract : The Hidaka metamorphic belt consists of an island‐arc assembly of lower to upper crustal rocks formed during early to middle Paleogene time and exhumed during middle Paleogene to Miocene time. The tectonic evolution of the belt is divided into four stages, D0rs, D1, D2rs, and D3, based on their characteristic deformation, metamorphism, and igneous activity. The premetamorphic and igneous stage (D0) involves tectonic thickening of an uppermost Cretaceous and earliest Tertiary accretionary complex, including oceanic materials in the lower part of the complex. D1 is the stage of prograde metamorphism with increasing temperatures at a constant pressure during an early phase, and with a slight decrease of pressure at the peak metamorphic phase, accompanying flattening of metamorphic rocks and intrusions of mafic to intermediate igneous rocks. At the peak, incipient partial melting of pelitic and psammitic gneisses took place in the amphibolite–granulite facies transition zone, the melt and residuals cutting the foliations formed by flattening. In the deep crust, large amounts of S‐type tonalite magma formed by crustal anatexis, intruded into the granulite facies gneiss zone and also into the upper levels of the metamorphic sequence during the subsequent stage. During D1 stage, mafic and intermediate magmas supplied and transported heat to form the arc‐type crust and at the same time, the magmatic underplating caused extensional doming of the crust, giving rise to flattening and vertical uplifting of the crustal rocks. D2 stage is characterized by subhorizontal top‐to‐the‐south displacement and thrusting of lower to upper crustal rocks, forming a basal detachment surface (décollement) and duplex structures associated with intrusions of S‐type tonalite. Deformation structures and textures of high‐temperature mylonites formed along the décollement, as well as the duplex structures, show that the D2 stage movement occurred under a N‐S trending compressional tectonic regime. The depth of intra‐crustal décollement in the Hidaka belt was defined by the effect of multiplication of two factors, the fraction of partial melt which increases downward, and the fluid flux which decreases downward. The crustal décollement, however, might have extended to the crust‐mantle boundary and/or to the lithosphere and asthenosphere boundary. The subhorizontal movement was transitional to a dextral‐reverse‐slip (dextral transpression) movement accompanied by low‐temperature mylonitization with retrograde metamorphism, the stage defined as D3. The crustal rocks from the basal décollement to the upper were tilted eastward on the N–S axis and exhumed during the D3 stage. During D2 and D3 stages, the intrusion of crustal acidic magmas enhanced the crustal deformation and exhumation in the compressional and subsequent transpressional tectonic regime.
Mesozoic terranes of the northwest Pacific continental margin (Russia): Radiolarian ages and sedimentary environments
Authors: Vishnevskaya V. , Filatova N.
Published: April 2006

Abstract Geological mapping using detailed tectonic and complex radiolarian analysis revealed significant northward displacement of a number of Russian Far and Northeast Asia terranes. It was recorded that some terranes possibly crossed the equator. Terranes of north‐east Russia were composed of different allochthonous formations, ranging in age from Middle Triassic to Maestrichtian‐Paleocene and accumulated from the margin to oceanic basins. The Middle to Upper Triassic interval included two formations: (i) volcanogenic, consisting of typical volcanic rocks of the island arcs (up to 800 m thick); and (ii) a chert‐limestone‐terrigenous one composed of marginal sandstone, siltstone, limestone and tuffic chert (about 400 m). Lower Jurassic allochthonous formations are represented by chert‐terrigenous (about 300 m) and jasper‐alkaline‐basaltic (WPB‐type) seamount deposits (about 100 m). Middle Jurassic to Hauterivian allochthonous terranes from the northern part of the Koryak‐Kamchatka region include five formations: jasper (bedding jaspers with condensed limestone lenses with Buchias, 80 m), jasper‐basalt (with MORB, 100‐150 m), ferrotitanic basalt (WPB with lenses of jasper mainly composed of genus Parvicingula, about 75%, 150 m), terrigenous‐volcanic (with MORB, IAT, CA basalts and olistostrome, 600 m), tuffic‐jasper‐basalt (MORB and deposits of arc‐trench system, about 500 m) with the same age according to radiolarian data. Aptian? Albian‐Maestrichtian ones are predominantly terrigenous‐tuffaceous‐siliceous. Moreover, the Early and Middle Jurassic faunas of the northwest Pacific margin contain many boreal elements similar to those of New Zealand (Southern Hemisphere), Japan, ODP Site 801. The Late Jurassic faunas of the Koryak and Kamchatka region are mainly North Tethyan and seldom Central Tethyan and are very closely related to those of the Americas. The Tithonian to Early Cretaceous radiolarian are predominantly Central Tethyan and Equatorial in contrast to Boreal Late Cretaceous. The combining in the same region at 60°N Pacific margin of the formations accumulated in different tectonic paleoenvironments and paleoclimatic provinces, is good evidence for the possible significant northward displacement of some terranes in the northwestern Pacific.
Noble gas isotopic composition of deep underground water in Osaka plain, central Japan: Evidence for mantle He and model for new volcanism
Authors: Okada Toshinori , Itaya Tetsumaru , Sato Mitsuo , Nagao Keisuke
Published: April 2006

Abstract Elemental and isotopic compositions of noble gases extracted from the bore hole water in Osaka plain, central Japan were examined. The water samples were collected from four shallow bore holes (180‐450 m) and seven deep bore holes (600‐1370 m) which have been used for an urban resort hot spring zone. The water temperatures of the deep bore holes were 22‐50°C and that of the shallow bore holes, 13‐23°C. The elemental abundance patterns show the progressive enrichment of the heavier noble gases compared with the atmospheric noble gas composition except for He, which is heavily enriched in deep bore hole water samples. 3He/4He ratios from the bore holes reaching the Ryoke granitic basement were higher than the atmospheric value (1.4 × 10−6), indicating a release of mantle He through the basement. The highest value of 8.2 × 10−6 is in the range of arc volcanism. On the other hand, the bore holes in sedimentary rocks overlying the basement release He enriched in radiogenic 4He, resulted in a low 3He/4He ratio of 0.5 × 10−6. 4He/20Ne and 40Ar/36Ar ratios indicate that the air contamination is generally larger in shallow bore holes than in deep ones from each site. The helium enriched in mantle He is compatible with the previous work which suggested up‐rising magma in ‘Kinki Spot’, the area of Osaka and western Wakayama, in spite of no volcanic activity in the area. A model to explain an initiation of magma generation beneath this area is presented.
Miocene pinnipeds of the otariid subfamily Allodesminae in the North Pacific Ocean: Systematics and relationships
Authors: Barnes Lawrence G. , Hirota Kiyoharu
Published: April 2006

Abstract Fossil pinnipeds in the extinct otariid subfamily Allodesminae are large, relatively highly evolved marine carnivores that became abundant and diverse in Middle Miocene time and were restricted to the North Pacific Ocean. Their record extends from early Middle Miocene through Late Miocene, with records from California, Oregon, Washington, Baja California and Japan. Allodesmines are characterized by extreme sexual dimorphism, a large orbit, retracted orbital margin of the zygomatic arch, a deeply mortised jugal‐squamosal junction, wide palate, bulbous cheek tooth crowns, nearly flat tympanic bulla with wrinkled ventral surface, a large tympanohyal fossa, large ear ossicles and deep mandible.

Eleven allodesmine species are known (eight of which are named), in at least four genera, and most belong to the typical genus Allodesmus Kellogg, 1922. The earliest and most generalized allodesmine known is from the early Middle Miocene (ca 16 Ma) Astoria Formation in coastal Oregon. The last known records are from Late Miocene rocks (ca 10 Ma) in California and Washington. New taxa proposed here are: the genus Brachyallodesmus Barnes and Hirota, to contain Allodesmus packardi Barnes, 1972; the genus Megagomphos Hirota and Barnes, to contain Allodesmus sinanoensis (Nagao, 1941); the species Allodesmus sadoensis Hirota, (Middle Miocene, Japan); the species Allodesmus megallos Hirota (Middle Miocene, Japan); and the species Allodesmus gracilis Barnes (Middle Miocene, California). Additionally, the genus Atopotarus Downs, 1956, and the species Allodesmus kelloggi Mitchell, 1966, are resurrected.

Allodesmines were apparently a rapidly evolving group, and most appear to have been adapted to roles later filled by otariine, dusignathine and imagotariine otariids, and the Phocidae (true seals). They became extinct in Late Miocene time and left no living descendants. Although some of their characters evolved convergently with various living species of the pinniped family Phocidae, Allodesminae are an otariid group and not part of the evolutionary history of Phocidae.
A new type of primitive baleen whale (Cetacea; Mysticeti) from Kyushu, Japan
Authors: Okazaki Yoshihko
Published: April 2006

Abstract An important and relatively complete fossil mysticete cetacean specimen was collected from the Oligocene age Ashiya Group in Kyushu, Japan. The specimen includes a complete skull, with the anterior part of the vertebral column, several ribs, and the left pectoral limb. It provides significant information about the early evolution of cetotheres and transitional steps from the Archaeoceti to Mysticeti. Several features of the specimen, such as the anterior position of the nasal opening and large mandibular foramen, are primitive, but others are derived, such as the wide maxilla, broad supraorbital process of the frontal, occipital shield overhanging the temporal fossae, and the flat articulation of the distal end of the humerus. This animal, therefore, has a mixture of primitive and derived characters. It represents a stage of evolution in mysticetes that has been previously unrecognized.
Diversity of Pliocene mysticetes from eastern Japan
Authors: Oishi Masayuki , Hasegawa Yoshikazu
Published: April 2006

Abstract Assemblages of Early Pliocene Mysticeti (Cetacea) from the Sendai‐Iwate and Choshi areas in eastern Japan were examined. The early Early Pliocene Tatsunokuchi and Yushima formations of the Sendai‐Iwate area have yielded many cetotheres assigned to Herpetocetus (Cetotheriidae) and some extinct rorquals assigned to Burtinopsis (Balaenopteridae) and other genera. The late Early Pliocene strata of the Na‐arai Formation of the Choshi area has yielded more than 300 mysticete ear bones which fall into seven morphotypes. Assemblages from the two areas differ greatly in taxonomic composition. Archaic mysticetes are dominant in the Sendai‐Iwate area, while modern mysticetes are dominant and diverse in the Choshi area. Based on the results of this study we conclude that the diversity of the Early Pliocene mysticetes in the western North Pacific is comparable to that of the Recent.
A new fossil kentriodontid dolphin (Cetacea; Kentriodontidae) from the Middle Miocene Takinoue Formation, Hokkaido, Japan
Authors: Ichishima Hiroto
Published: April 2006

Abstract Kentriodon hobetsu, a new species, is described from a partial skull from the Middle Miocene Takinoue Formation, in Hokkaido, Japan. The holotype skull includes the base of the rostrum and a fairly well preserved braincase. The new species is distinguished clearly from previously described species of Kentriodon Kellogg, 1927, by the shape of the anterior margin of the pterygoid sinus fossa, the large premaxilla, rounded facial margin, and lesser development of rugosities and processes. This new record of Kentriodon, the first named kentriodontid dolphin from the western North Pacific, extends the geographic range of the genus significantly.
A review of kentriodontine dolphins (Cetacea; Deiphinoidea; Kentriodontidae): Systematics and biogeography
Authors: Ichishima Hiroto , Barnes Lawrence G. , Fordyce R. Ewan , Kimura Masaichi , Bohaska David J.
Published: April 2006

Abstract Fossil dolphins belonging to the extinct family Kentriodontidae are small to medium‐sized toothed cetaceans, which probably include the ancestors of some living species. Kentriodontids are known from rocks of Late Oligocene to Late Miocene age in various parts of the world. Among kentriodontids, species in the subfamily Kentriodontinae (e.g. species of Kentriodon Kellogg, 1927) are the most ubiquitous and generalized; these are now known from latest Oligocene to earliest Miocene strata in New Zealand and Patagonia, and Middle Miocene deposits in Maryland, Virginia, California and Japan. The diversity, morphologies and distributions of Miocene species of Kentriodontinae seem to parallel those of the living species of mostly pelagic delphinids in the subfamily Delphininae, and the fossil group may have been an ecological or behavioral/functional counterpart of the latter. Kentriodontines are inferred to have been wide‐ranging neritic to pelagic animals that ate small fish and other nectonic organisms; they were probably active echolocators, and might have formed large schools. They are relatively common as fossils and, therefore, are potentially useful for intercontinental correlations of marine deposits.
A list of fossil cetaceans in Japan
Authors: Oishi Masayuki , Hasegawa Yoshikazu
Published: April 2006

Abstract Fossil cetacea of Japan are abundant and diverse. Despite several summary articles, all the known taxa have not previously been compiled in one place. Here we present a list of the published fossil cetacean records from the Japanese archipelago. The record extends from Middle or Late Oligocene to the Holocene, and includes representatives of both the suborders Odontoceti and Mysticeti. No Eocene Cetacea or Archaeoceti are recorded from Japan. As is typical of the eastern North Pacific and the South Pacific, the Oligocene and Early Miocene assemblages are comprised of archaic and usually extinct families, and the younger assemblages are represented by modern groups comprised of extant genera and families.
On sexual dimorphism in Paleoparadoxia tabatai
Authors: Hasegawa Yoshikazu , Taketani Yojiro , Taru Hajime , Sakamoto Osamu , Manabe Makoto
Published: April 2006

Abstract Upon studying the well preserved skulls referred to Paleoparadoxia tabatai from Japan, the short and low skull of the Yanagawa specimen requires an explanation. Six cranial and dental characters lead us to suggest that the Yanagawa individual is a female while the Izumi, Ohnohara, and Itsukaichi skulls are male. The important characters in the Yanagawa skull are its shortness and shallow depth, less pronounced sagittal and nuchal crests, smaller zygoma, and the small dentition with especially poorly erupted canines, most of which are indicative of weak masticatory musculature.
Possible sub‐arc origin of podiform chromitites
Authors: Arai Shoji , Yurimoto Hisayoshi
Published: April 2006

Abstract The sub‐arc mantle condition possibly favors the formation of podiform chromitites. The Cr/(Cr + Al) atomic ratio (= Cr#) of their chromian spinel frequently is higher than 0.7, which is comparable with the range for arc‐related primitive magmas. This almost excludes the possibility of their sub‐oceanic origin, because both oceanic peridotites and MORB have chromian spinel with the Cr# < 0.6. Precipitation of chromitite and associated dunite enhances a relative depletion of high‐field strength elements (HFSE) to large‐ion lithophile elements (LILE), one of chemical characteristics of arc magmas, for the involved magma. This cannot alter completely, however, the MORB to the arc‐type magma, especially for Ti and Zr. The presence of chromitite xenoliths, similar both in texture and in chemistry to podiform chromitites of some ophiolitic complexes, in some Cenozoic alkali basalts from the southwest Japan arc indicates directly that the upper mantle beneath the Japan arcs has chromitites.
Comparison and interpretation of graphitization in contact and regional metamorphic rocks
Authors: Nakamura Daisuke
Published: April 2006

Abstract X‐ray diffraction (XRD) analyses of carbonaceous materials were carried out in conjunction with petrological studies for selected metamorphic rocks in order to compare the structural state of carbonaceous materials between contact and regional metamorphic rocks. The most extensive study was done for the Daimonji contact aureole in the eastern part of Kyoto city, Japan. The Daimonji contact aureole can be divided into three mineral zones using mineral parageneses of pelitic rocks: chlorite, biotite and cordierite zones. The cordierite zone can be further subdivided into lower‐ and higher‐grade subzones. Petrological considerations allow the two isograd reactions that define the lower‐ and higher‐grade cordierite subzones to be determined and suggest these reactions occurred at 510–560°C and 560–590°C per 2.0‐2.3 kbar, respectively. A combination of the petrological studies and the XRD data of carbonaceous materials suggest that fully ordered graphite (FG; defined by d(002) ≤ 3.360 Å following the convention used by many workers), appears around 560–590°C in the Daimonji contact aureole. This data and refinement of geothermometer for published data confirmed that the FG appears at 400–500°C in regional metamorphic rocks, but at higher than 530°C in contact aureoles. One possible explanation for such a temperature difference is the duration of heating. However, the width at half height (WH) of the graphite peak attains a similar value of 0.30° at around 500°C both in contact and regional metamorphic rocks, suggesting that WH value is a more reliable indicator of metamorphic grade than the change of d(002) value. Furthermore, the depressed d(002) data of graphite was observed locally in the higher grade part (≥ 500°C) of the Ryoke regional metamorphic belt, where granitic intrusions exist within a few km distance. These facts indicate that the duration of heating is not an important factor controlling the change of d(002) value. It is possible that interlayered impurities, such as chlorine, which was derived from igneous intrusions, may be an important factor in suppressing the reduction in d(002) at temperatures greater than 500°C.
Jurassic and Lower Cretaceous radiolarian zonation in Japan and in the western Pacific
Authors: Matsuoka Atsushi
Published: April 2006

Abstract A radiolarian zonal scheme for the entire Jurassic and Lower Cretaceous using biostratigraphic data from both Japanese Island sections and the western Pacific seafloor is documented. The zonation is applicable to low and middle paleolatitude portions of the Paleo‐Pacific ocean. Radiolarian bio‐events such as the evolutionary first appearance biohorizon, first occurrence biohorizon, and last occurrence biohorizon were used to define zones. The 11 zones proposed are, in ascending order, Parahsuum simplum, Trillus elkhornensis, Laxtorum(?) jurassicum, Tricolocapsa plicarum, Tricolocapsa conexa, Stylocapsa(?) spiralis, Hsuum maxwelli, Pseudodictyomitra primitiva, Pseudodictyomitra carpatica, Cecrops sep‐temporatus, and Acanthocircus carinatus zones. Preliminary age assignments for these zones are presented.
A seismic gap in the eastern margin of the Sea of Japan as inferred from the time‐space distribution of past seismicity
Authors: Ohtake Masakazu
Published: April 2006

Abstract Based on the spatial distribution of the focal areas from previous large earthquakes, a significant seismic gap was found off the coast of the Akita Prefecture at the eastern margin of the Sea of Japan. We forecast that the 75 km segment of plate boundary may be ruptured by a large thrust earthquake with a magnitude of 7.5 in the near future. The time of occurrence is uncertain, but the recurrence interval of large earthquakes in this region suggests that it will be no later than the end of next century. The time sequence of large earthquakes in the eastern margin of the Sea of Japan shows a systematic shortening of the earthquake interval for the past 250 years. This feature is simulated successfully by a simple mechanical model that represents a plate boundary by segmented blocks, each of which is composed of a spring friction slider system. We further propose to revise the conventionally accepted plate boundary between the Eurasian and Okhotsk plates in the eastern margin of the Sea of Japan; the observed data of seismic activity and recent crustal deformation indicate that the southern part of the plate boundary traverses the land region from Niigata to Otari along the Shinanogawa seismic belt. The fault geometries of previous large earthquakes show that the mode of plate convergence in the eastern margin of the Sea of Japan is a collision type rather than a simple eastward subduction as was postulated in early studies.
Episodic exhumation of accretionary complexes: Fission‐track thermochronologic evidence from the Shimanto Belt and its vicinities, southwest Japan
Authors: Tagami Takahiro , Hasebe Noriko , Shimada Chica
Published: April 2006

Abstract Apatite and zircon fission‐track (FT) analyses of the Shimanto accretionary complex and its vicinities, southwest Japan, unraveled the episodic material migration of the deep interiors of the accretionary complex. Apatite data with 100°C closure temperature (Te) generally indicate ~10 Ma cooling throughout the Shimanto complex. In contrast, zircon data with 260°C Te exhibit a wide range of apparent ages as a consequence of paleotemperature increase to the zircon partial annealing zone. In the Muroto and Kyushu regions, maximum temperatures tend to have been higher in the northern, older part of the complex, with indistinguishable temperature differences between coherent and melange units adjacent to each other. It thus suggests, along with vitrinite reflectance data, that older accretionary units occurring to the north sustain greater maximum burial during the accretion‐burial‐exhumation process. Zircon data suggest two cooling episodes: ~70 Ma cooling at widespread localities in the Cretaceous Shimanto Belt and Sambagawa Belt, and ~15 Ma cooling in the central Kii Peninsula. The former is consistent with 40Ar/39Ar cooling ages from the Sambagawa Belt, whereas the latter slightly predates the widespread 10 Ma apatite cooling ages. These data imply that the extensive material migration and exhumation took place in and around the Shimanto complex in Late Cretaceous as well as in Middle Miocene. Considering tectonic factors to control evolution of accretionary complexes, the episodic migration is best explained by accelerated accretion of sediments due to increased sediment influx at the ancient Shimanto trench, probably derived from massive volcano‐plutonic complexes contemporaneously placed inland. Available geo‐ and thermochronologic data suggest that extensive magmatism triggered regional exhumation twice in the past 100 Ma, shedding new light on the cordilleran orogeny and paired metamorphism concepts.
Kyanite‐anthophyllite schist and southwest extension of the Dabie Mountains ultrahigh‐ to high‐pressure belt
Authors: Liu J. , Liou J. G.
Published: April 2006

Abstract Kyanite‐anthophyllite schist preserves the first record of high pressure in the amphibolite‐facies unit of the SW Dabie Mountains, whereas ultrahigh‐ and high‐pressure (UHP and HP) metamorphism has been well documented by the occurrence of coesite, diamond and mafic eclogite in the SE Dabie Mountains. Textural evidence indicates that minerals of the kyanite‐anthophyllite schist formed mainly in two stages: (i) garnet + kyanite + antho‐phyllite + rutile formed at pressure in excess of 1.2 GPa at T < 650°C; (ii) cordierite±staurolite formed by reaction of anthophyllite + kyanite at P < 0.5 GPa, T∼530°C. Plagioclase and ilmenite replaced garnet and rutile respectively during decompression. In a still later stage, secondary biotite recrystallized, accompanied by sillimanite replacing kyanite, and spinel replacing staurolite. The P‐T information suggests that the amphibolite unit in the SW Dabie Mountains is part of the Triassic collision belt between the Sino‐Korean and Yangtze cratons. The P‐T paths of the UHP eclogite in the eastern Dabie Mountains and the HP kyanite‐anthophyllite schist in the SW Dabie Mountains show similar decompression and equivalent late stage Barrovian‐style metamorphism. Emplacement of voluminous granitoid at middle crustal levels between 134–118 Ma contributed to the development of the Barrovian‐type metamorphism in the Dabie Mountains.
Ultrahigh‐pressure metamorphic rocks and the thermal evolution of continent collision belts
Authors: Peacock S. M.
Published: April 2006

Abstract Coesite‐bearing eclogites exposed in the Alpine, Qinling‐Dabie (China), Caledonian, and Ural orogenic belts provide insight into the time‐dependent thermal structure of continent collision belts. Coesite‐bearing eclogites record peak metamorphic temperatures of 550‐900°C at pressures ≥ 2.5 GPa reflecting anomalously cool conditions at depths of 90 km or more. The low temperatures recorded by coesite‐bearing eclogites strongly suggest formation in a convergent plate margin where the downward advection of cool lithosphere depresses isotherms on a regional scale. Subduction zone pressure‐temperature (P‐T) paths calculated using a two‐dimensional finite‐difference model predict steady‐state temperatures of 450‐650°C at 100 km depth at the slab‐mantle interface for convergence rates of 10 to 100 mm/yr. Coesite‐bearing eclogites record peak temperatures ~100‐250°C higher, possibly reflecting (i) formation during the early stages of convergence prior to the achievement of thermal steady state; (ii) attainment of peak metamorphic temperatures during decompression (exhumation); (iii) formation during slow, <10 mm/yr, convergence; or (iv) uncertainties in the modeling parameters. Retrograde P‐T paths determined for coesite‐bearing eclogites from the western Alps and China indicate cooling during decompression from depths of ~100 km. Cooling of eclogite terrains during exhumation requires loss of heat downward into lithosphere that continues to subduct beneath the eclogites, loss of heat upward into the cooler hanging wall of a large‐scale normal fault/shear zone, or a combination of the two scenarios.
CHIME age dating of monazites from metamorphic rocks and granitic rocks of the Ryoke belt in the Iwakuni area, Southwest Japan
Authors: Suzuki Kazuhiro , Adachi Mamoru , Nureki Terukazu
Published: April 2006

Abstract
CHIME (chemical Th‐U‐total Pb isochron method) ages were determined for monazite from gneisses and granitoids of the Ryoke belt in the Iwakuni area. The CHIME monazite ages are 99.6 ± 2.4, 98.9 ± 2.1 and 98.2 ± 5.7 Ma for the Ryoke gneiss, 90.7 ± 2.2, 89.7 ± 2.0 and 89.3 ± 2.2 Ma for the Tajiri Granite, 91.0 ± 3.2, 90.6 ± 3.2 and 89.9 ± 3.2 Ma for the Namera Granite, 89.3 ±3.3 and 88.6 ± 5.6 Ma for a small stock at Shimizu, and 87.3 ± 1.6 and 86.6 ± 2.1 Ma for the post‐tectonic Shimokuhara Granite. The CHIME monazite ages, interpreted as the time of the first attainment at the amphibolite facies conditions for the gneisses and as the time of emplacement for the granites, respectively, agree with the field intrusive sequence. The present dating documented that the Ryoke metamorphism in the Iwakuni area reached the amphibolite facies conditions at ∼98 Ma, was complete at ‐87 Ma, the time of emplacement of the post‐tectonic Shimokuhara Granite.
Illite crystallinity study of the Cretaceous Shimanto Belt in the Akaishi Mountains, eastern Southwest Japan
Authors: Tanabe Hirotaka , Kano Ken‐Ichi
Published: April 2006

Abstract
Illite crystallinity (IC) analyses in the Upper Cretaceous Shimanto accretionary complex of the southern Akaishi Mountains, eastern Southwest Japan confirm the applicability of this technique for evaluating the grade of diagenesis/metamorphism in a sediment‐dominated accretionary complex. Reproducibility analyses of IC values show a variance of about 15% from the mean. Data from three traverses, which transect across‐strike sections of ∼25 km, clearly demonstrate that the IC distributions have specific overall trends. The IC values belong to the lQwer anchizone and the zone of diagenesis. The IC distributions may be controlled locally by structural features, but there are no distinct relationships with regional‐scale geological structures. This may indicate that the heterogeneous geothermal rise affected the pre‐existing structural and diagenetic/metamorphic framework of the accretionary sequence. Along‐strike variations of grade tend to increase toward the northeast where a Middle Miocene granitoid occurs. Hence, the original diagenetic/metamorphic framework of this part of the Shimanto Belt was presumably overprinted during the Middle Miocene.
Cretaceous radiolarian fauna from the Kiselyovsky subterrane, the youngest accretionary complex of the Russian continental far east: Paleotectonic and paleogeographic implications
Authors: Zyabrev Sergei V.
Published: April 2006

Abstract
The Kiselyovsky subterrane is the northeastern section of the Kiselyovsko‐Manominsky terrane, a distinguishable tectonic unit in the north of the Sikhote‐Alin Range. The terrane has been treated as part of the accretionary wedge belonging to the Khingan‐Okhotsk active continental margin, but its structure and stratigraphy have been poorly understood. This paper presents new data on the subterrane structure, lithology and radiolarian biostratigraphy. The following lithostratigraphic units are established in the terrane: a ribbon chert unit, a siliceous mudstone unit and a elastics unit. Abundant Valanginian to late Hauterivian‐early Barremian radiolarian assemblages are obtained from the upper part of the chert unit in addition to the known Jurassic radiolarians. The radiolarian age of the lower part of the siliceous mudstone unit (red siliceous mudstone) is determined as early Hauterivian‐early Aptian. The unit's upper part (greenish‐gray siliceous mudstone and dark‐gray silicified mudstone) and the clastics unit contain Albian‐Cenomanian assemblages. The arrangement of the units is treated as a chert‐elastics sequence, whose vertical lithologic variations indicate environmental changes from a remote ocean to a convergent margin, reflecting an oceanic plate motion towards a subduction zone. The subterrane structure is a stack of imbricated slabs composed of various lithostratigraphic units, and is complicated by folding. The structure's origin is related to subduction‐accretion, which occurred in the Albian‐Cenomanian. The data presented provide a unique basis for accretionary wedge terranes correlation in the circum‐Japan Sea Region, and the Kiselyovsky subterrane is correlated in this study with the synchronous parts of the East Sakhalin, Hidaka and Shimanto terranes. The Albian‐Cenomanian radiolarian assemblages were deposited in the Boreal realm, while Valanginian ones are Tethyan; this indicates a long oceanic plate travelling to the north. The former assemblages contain an admixture of older species, redeposited by bottom traction currents and turbidite flows in trench environments.
Correlation between the bedded chert sequence of southwest Japan and δ13C excursion of carbonate sequence, and its significance to the Permian‐Triassic mass extinction
Authors: Kakuwa Yoshitaka
Published: April 2006

Abstract
Stratigraphic productivity variations of radiolarians below the Permian‐Triassic boundary are examined with Ishiga Diagrams in bedded chert sequences of southwest Japan. The diagrams of two different outcrops, drawn from the thickness variation of chert beds, show common stratigraphic variation, which indicates the diagram is a useful tool for correlation of bedded chert sequence. The common stratigraphic productivity variation is also well correlated to a compiled δ13C excursion of shallow carbonate sequences. Bedded chert records a dramatic extinction event in a shallow surface zone of oceans below the Permian‐Triassic boundary. The Permian‐Triassic mass extinction is divided into three intervals based on the Ishiga Diagrams, the stratigraphic lithological variation of bedded chert sequences, and the δ13C curve. The preceding extinction interval in the late Djulfian stage was not as serious an event and the biosphere soon recovered. The event of the main extinction interval commenced in the Dorashamian stage and caused a serious destruction of the biosphere. An event of the aftermath interval during the Early Triassic caused a delay in the recovery from the main extinction interval.
Pre‐accretionary mineralization of Japan
Authors: Sato Kohei , Kase Katsuo
Published: April 2006

Abstract
The metallogeny of Japan can be grouped into four environments: (1) Paleozoic‐Mesozoic stratiform Cu and Mn deposits within accretionary complexes, (2) Cretaceous‐Paleogene post‐accretionary deposits related to felsic magmatism in a continental‐margin are environment, (3) Miocene epigenetic and syngenetic deposits related to felsic magmatism during back‐arc opening, and (4) late Miocene‐Quaternary volcanogenic deposits in an island‐are environment. Group (1) deposits were a major source of Cu and Mn for the Japanese mining industry, and this style of mineralization is reviewed here. The stratiform Cu and Mn deposits were formed on the sea floor during the late Paleozoic to Mesozoic, and were subsequently accreted to active continental margins mainly in Jurassic to Cretaceous age. The Cu sulfide deposits, termed Besshi type, are classified into two subtypes: the Besshi‐subtype deposit is related to basaltic volcanism, probably at a mid‐oceanic ridge or rise; the Hitachi subtype is related to bimodal volcanism, probably in a back‐arc or continental rift. Most of the Besshisubtype deposits occur in the Sanbagawa metamorphic belt, with some occurrences in weakly metamorphosed Jurassic and Cretaceous accretionary terrains. This subtype is divided into two groups: the sediment‐barren group is hosted by basalt‐chert sequences; whereas the sedimentcovered group is hosted by basalt‐shale sequences. Both subtypes are characterized by S isotope trends similar to those of sea‐floor sulfide deposits now forming at mid‐oceanic ridges. The Hitachi‐subtype deposits occur in late Paleozoic volcanic‐sedimentary sequences and lack pelagic sediments. These deposits are characterized by association of sphalerite‐ and barite‐rich ores. The Mn deposits occur mainly in Middle Jurassic to Early Cretaceous accretionary complexes containing abundant chert beds of Triassic to Jurassic age. Their locations are well separated from those of the Cu sulfide deposits. The Mn deposits are divided into two types: the Mn type, hosted by chert, and the Fe‐Mn type, sandwiched between chert and basaltic volcanic rocks. The Mn‐type ores appear to have deposited on the deep‐sea floor further from the site of hydrothermal activity than the Fe‐Mn type. Primary Mn precipitates may have been transformed to rhodochrosite and other Mn‐minerals during diagenesis. Many of the Mn deposits were significantly metamorphosed during intrusion of Cretaceous granitoids, resulting in a very complex mineralogy.
Episodic accretion and metamorphism of Jurassic accretionary complex based on biostratigraphy and K‐Ar geochronology in the western part of the Mino‐Tanba Belt, Southwest Japan
Authors: Takami Michio , Itaya Tetsumaru
Published: April 2006

Abstract
The low grade metamorphic Jurassic accretionary complex in the western part of the Mino‐Tanba Belt, Southwest Japan, is a chaotic sedimentary complex which consists of argillaceous matrices with allochthonous blocks of chert, greenstone, siliceous mudstone, terrigenous sandstone and mudstone. The complex is divided into three distinct geologic units, Units I, II and III, with a tectonic boundary (thrust) between them, forming a pile‐nappe structure. They have different features for lithologies, fossil age, metamorphic condition and K‐Ar age. Microfossil researches revealed that their timings of accretion were in the early Early Jurassic (ca 195 Ma) for Unit III, in the early Middle Jurassic (ca 175 Ma) for Unit II and in the latest Late Jurassic (ca 147 Ma) for Unit I. On the other hand, K‐Ar age determinations of white mica separated from pelitic rocks of the three units clarified that the subsequent subduction‐related metamorphism was 23 million years after the accretion of each unit. These results strongly suggest that the accretionary and metamorphic process had taken place episodically with an interval of 20 to 28 million years during Mesozoic time in the western part of the Mino‐Tanba Belt, Southwest Japan.
Crustal shortening of Southwest Japan in the Late Miocene
Authors: Itoh Yasuto , Nagasaki Yasuhiko
Published: April 2006

Abstract
Tectonic deformation of an island arc is interpreted on the basis of geophysical data. Extensive reflection seismic, gravity, geomagnetic data around the back‐arc region of Southwest Japan delineate east‐west to northeast‐southwest folding, and imply conspicuous compression on the southern margin of the Sea of Japan. Because geological data of exploration boreholes indicate that the coinpressive regime was dominant in the late Miocene, the tectonic event seems to be linked with coeval resumption of subduction of the Philippine Sea Plate. Strong coupling of the young buoyant oceanic plate brought about north‐south shortening of the overriding continental lithosphere, and left wrench deformation at the southwestern corner of the Sea of Japan. Amount of shortening for the back‐arc shelf and mountainous ranges of Southwest Japan is estimated to be ca 10 km, adopting a uniform ratio of shortening (0.944) since the Miocene determined on the shelf from depth‐converted seismic profiles. Along the western side of a bend of boundary between the Eurasian Plate and Philippine Sea Plate, the middle Miocene and younger sediments upon the back‐arc shelf are much less deformed than the northern equivalents, and the fore‐arc Miocene strata are deformed by left wrenching, facts which are indicative of northerly initial convergence of the Philippine Sea Plate at the end of Miocene and crustal decoupling on the west of Kyushu Island.