Accretionary Complex (accretionary + complex)

Distribution by Scientific Domains


Selected Abstracts


Parentage of low-grade metasediments in the Sanbagawa belt, eastern Shikoku, Southwest Japan, and its geotectonic implications

ISLAND ARC, Issue 3 2010
Kazuo KiminamiArticle first published online: 19 AUG 2010
Abstract This study examines the geology of low-grade (chlorite zone) metamorphic rocks in the Sanbagawa belt and of a Jurassic accretionary complex in the Northern Chichibu belt, eastern Shikoku, Japan. The bulk chemistries of metasandstones and metapelites in the Sanbagawa belt of eastern Shikoku are examined in order to determine their parentage. The Sanbagawa belt can be divided into northern and southern parts based on lithology and geologic structure. Geochemical data indicate that metasediments in the northern and southern parts are the metamorphic equivalents of the KS-II (Coniacian,Campanian) and KS-I (late Albian,early Coniacian) units of the Shimanto belt, respectively. The depositional ages of the parent sediments of low-grade metamorphic rocks found in the Sanbagawa belt and the Jurassic Northern Chichibu belt, indicate a north-younging polarity. In contrast, sedimentological evidence indicates younging to the south. These observations suggest that a tectonic event has resulted in a change from a northerly to southerly dip direction for schistosity and bedding in the Sanbagawa and Northern Chichibu belts of eastern Shikoku. The younging polarity observed in the Sanbagawa and Northern Chichibu belts, together with previously reported data on vitrinite reflectance and geological structure, indicate that the Northern Chichibu belt was part of the overburden formerly lying on top of the Sanbagawa low-grade metamorphic rocks. [source]


Metamorphic and cooling history of the Shimanto accretionary complex, Kyushu, Southwest Japan: Implications for the timing of out-of-sequence thrusting

ISLAND ARC, Issue 4 2008
Hidetoshi Hara
Abstract Illite crystallinity, K,Ar dating of illite, and fission-track dating of zircon are analyzed in the hanging wall (Sampodake unit) and footwall (Mikado unit) of a seismogenic out-of-sequence thrust (Nobeoka thrust) within the Shimanto accretionary complex of central Kyushu, southwest Japan. The obtained metamorphic temperatures, and timing of metamorphism and cooling, reveal the tectono-metamorphic evolution of the complex, and related development of the Nobeoka thrust. Illite crystallinity data indicate that the Late Cretaceous Sampodake unit was metamorphosed at temperatures of around 300 to 310°C, while the Middle Eocene Mikado unit was metamorphosed at 260 to 300°C. Illite K,Ar ages and zircon fission-track ages constrain the timing of metamorphism of the Sampodake unit to the early Middle Eocene (46 to 50 Ma, mean = 48 Ma). Metamorphism of the Mikado unit occurred no earlier than 40 Ma, which is the youngest depositional age of the unit. The Nobeoka thrust is inferred to have been active during about 40 to 48 Ma, as the Sampodake unit started its post metamorphic cooling after 48 Ma and was thrust over the Mikado unit at about 40 Ma along the Nobeoka thrust. These results indicate that the Nobeoka thrust was active for more than 10 million years. [source]


Olivine-spinifex basalt from the Tamba Belt, southwest Japan: Evidence for Fe- and high field strength element-rich ultramafic volcanism in Permian Ocean

ISLAND ARC, Issue 3 2007
Yuji Ichiyama
Abstract Permian basalt showing typical spinifex texture with >10 cm-long olivine pseudomorphs was discovered from the Jurassic Tamba accretionary complex in southwest Japan. The spinifex basalt occurs as a river boulder accompanied by many ferropicritic boulders in a Permian chert-greenstone unit. Groundmass of this rock is holocrystalline, suggesting a thick lava or sill for its provenance. Minor kaersutite in the groundmass indicates a hydrous magma. The spinifex basalt, in common with the associated ferropicritic rocks, is characterized by high high field strength element (HFSE) contents (e.g. Nb = 62 ppm and Zr = 254 ppm) and high-HFSE ratios (Al2O3/TiO2 = 3.9, Nb/Zr = 0.24 and Zr/Y = 6.4) unlike typical komatiites. The spinifex basalt and ferropicrite might represent the upper fractionated melt and the lower olivine-rich cumulate, respectively, of a single ultramafic sill (or lava) as reported from the early Proterozoic Pechenga Series in Kola Peninsula. Their parental magma might have been produced by hydrous melting of a mantle plume that was dosed with Fe- and HFSE-rich garnet pyroxenite. The spinifex basalt is an evidence for the Pechenga-type ferropicritic volcanism taken place in a Permian oceanic plateau, which accreted to the Asian continental margin as greenstone slices in Jurassic time. [source]


Tectonic accretion of a subducted intraoceanic remnant arc in Cretaceous Hokkaido, Japan, and implications for evolution of the Pacific northwest

ISLAND ARC, Issue 4 2005
Hayato Ueda
Abstract An accretionary complex, which contains fragments of a remnant island arc, was newly recognized in the Cretaceous accretionary terranes in Hokkaido, Japan. It consists of volcanics, volcanic conglomerate, intermediate to ultramafic intrusive rocks with island-arc affinity including boninitic rocks, accompanied by chert and deformed terrigenous turbidites. Compared with the results of modern oceanic surveys, the preserved sequence from island-arc volcanics to chert, via reworked volcanics, is indicative of intraoceanic remnant arc, because the sequence suggests an inactive arc isolated within a pelagic environment before its accretion. The age of a subducting oceanic crust can be discontinuous before and after a remnant-arc subduction, resulting in abrupt changes in accretion style and metamorphism, as seen in Cretaceous Hokkaido. Subduction of such an intraoceanic remnant arc suggests that the subducted oceanic plate in the Cretaceous was not an extensive oceanic plate like the Izanagi and/or Kula Plates as previously believed by many authors, but a marginal basin plate having an arc,back-arc system like the present-day Philippine Sea Plate. [source]


Metamorphism and metamorphic K,Ar ages of the Mesozoic accretionary complex in Northland, New Zealand

ISLAND ARC, Issue 3 2004
Yujiro Nishimura
Abstract A southwest dipping Mesozoic accretionary complex, which consists of tectonically imbricated turbiditic mudstone and sandstone, hemipelagic siliceous mudstone, and bedded cherts and basaltic rocks of pelagic origin, is exposed in northern North Island, New Zealand. Interpillow limestone is sometimes contained in the basaltic rocks. The grade of subduction-related metamorphism increases from northeast to southwest, indicating an inverted metamorphic gradient dip. Three metamorphic facies are recognized largely on the basis of mineral parageneses in sedimentary and basaltic rocks: zeolite, prehnite-pumpellyite and pumpellyite-actinolite. From the apparent interplanar spacing d002 data for carbonaceous material, which range from 3.642 to 3.564 Ĺ, the highest grade of metamorphism is considered to have attained only the lowermost grade of the pumpellyite-actinolite facies for which the highest temperature may be approximately 300°C. Metamorphic white mica K,Ar ages are reported for magnetic separates and <2 µm hydraulic elutriation separates from 27 pelitic and semipelitic samples. The age data obtained from elutriation separates are approximately 8 m.y. younger, on average, than those from magnetic separates. The age difference is attributed to the possible admixture of nonequilibrated detrital white mica in the magnetic separates, and the age of the elutriation separates is considered to be the age of metamorphism. If the concept, based on fossil evidence, of the subdivision of the Northland accretionary complex into north and south units is accepted, then the peak age of metamorphism in the north unit is likely to be 180,130 Ma; that is, earliest Middle Jurassic to early Early Cretaceous, whereas that in the south unit is 150,130 Ma; that is, late Late Jurassic to early Early Cretaceous. The age cluster for the north unit correlates with that of the Chrystalls Beach,Taieri Mouth section (uncertain terrane), while the age cluster for the south unit is older than that of the Younger Torlesse Subterrane in the Wellington area, and may be comparable with that of the Nelson and Marlborough areas (Caples and Waipapa terranes). [source]


Evolution of an accretionary complex along the north arm of the Island of Sulawesi, Indonesia

ISLAND ARC, Issue 1 2004
Yusuf Surachman Djajadihardja
Abstract Seismic reflections across the accretionary prism of the North Sulawesi provide excellent images of the various structural domains landward of the frontal thrust. The structural domain in the accretionary prism area of the North Sulawesi Trench can be divided into four zones: (i) trench area; (ii) Zone A; (iii) Zone B; and (iv) Zone C. Zone A is an active imbrication zone where a decollement is well imaged. Zone B is dominated by out-of-sequence thrusts and small slope basins. Zone C is structurally high in the forearc basin, overlain by a thick sedimentary sequence. The subducted and accreted sedimentary packages are separated by the decollement. Topography of the oceanic basement is rough, both in the basin and beneath the wedge. The accretionary prism along the North Sulawesi Trench grew because of the collision between eastern Sulawesi and the Bangai,Sula microcontinent along the Sorong Fault in the middle Miocene. This collision produced a large rotation of the north arm of Sulawesi Island. Rotation and northward movement of the north arm of Sulawesi may have resulted in southward subduction and development of the accretionary wedge along North Sulawesi. Lateral variations are wider in the western areas relative to the eastern areas. This is due to greater convergence rates in the western area: 5 km/My for the west and 1.5 km/My for the east. An accretionary prism model indicates that the initiation of growth of the accretionary prism in the North Sulawesi Trench occurred approximately 5 Ma. A comparison between the North Sulawesi accretionary prism and the Nankai accretionary prism of Japan reveals similar internal structures, suggesting similar mechanical processes and structural evolution. [source]


Middle Archean ocean ridge hydrothermal metamorphism and alteration recorded in the Cleaverville area, Pilbara Craton, Western Australia

JOURNAL OF METAMORPHIC GEOLOGY, Issue 7 2007
T. SHIBUYA
Abstract A hydrothermally metamorphosed greenstone complex, capped by bedded cherts and banded iron formations (BIFs), is exposed in the Cleaverville area, Pilbara Craton, Western Australia. It has been interpreted as an accretionary complex characterized by both a duplex structure and an oceanic plate stratigraphy, and is shown to represent a 3.2 Ga upper oceanic crust. Three metamorphic zones are identified in the basaltic greenstones. The metamorphic grade increases from sub-greenschist facies (zones A and B) to greenschist facies (zone C) under low-pressure conditions. The boundaries between three mineral zones are subparallel to the bedding plane of overlying chert/BIF, and metamorphic temperature increases stratigraphically downward. The zones correspond to the thermal structure of ocean-floor metamorphism, at a mid-ocean ridge. The uppermost greenstone in the study area is more pervasively altered and carbonatized than the modern upper oceanic crust. This indicates the enrichment of CO2 in the metamorphic fluid by which widespread formation of carbonate occurred, compared with a narrow stability region of Ca-Al silicates. It is, therefore, suggested that the Archean hydrothermal alteration played a more important role in fixation of CO2 than present-day ocean-ridge hydrothermal alteration, as an interaction between sea water and oceanic crust. [source]


Neoproterozoic high-pressure/low-temperature metamorphic rocks in the Avalon terrane, southern New Brunswick, Canada

JOURNAL OF METAMORPHIC GEOLOGY, Issue 5 2001
C. E. White
Abstract High -P/low -T metamorphic rocks of the Hammondvale metamorphic suite (HMS) are exposed in an area of 10 km2 on the NW margin of the Caledonian (Avalon) terrane in southern New Brunswick, Canada. The HMS is in faulted contact on the SE with c. 560,550 Ma volcanic and sedimentary rocks and co-magmatic plutonic units of the Caledonian terrane. The HMS consists of albite- and garnet-porphyroblastic mica schist, with minor marble, calc-silicate rocks and quartzite. Pressure and temperature estimates from metamorphic assemblages in the mica schist and calc-silicate rocks using TWQ indicate that peak pressure conditions were 12.4 kbar at 430 °C. Peak temperature conditions were 580 °C at 9.0 kbar. 40Ar/39Ar muscovite ages from three samples range up to 618,615 Ma, a minimum age for high -P/low- T metamorphism in this unit. These ages indicate that the HMS is related to the c. 625,600 Ma subduction-generated volcanic and plutonic units exposed to the SE in the Caledonian terrane. The ages are also similar to those obtained from detrital muscovite in a Neoproterozoic-Cambrian sedimentary sequence in the Caledonian terrane, suggesting that the HMS was exposed by latest Neoproterozoic time and supplied detritus to the sedimentary units. The HMS is interpreted to represent a fragment of an accretionary complex, similar to the Sanbagawa Belt in Japan. It confirms the presence of a major cryptic suture between the Avalon terrane sensu stricto and the now-adjacent Brookville terrane. [source]


Mid,Cretaceous Episodic Magmatism and Tin Mineralization in Khingan-Okhotsk Volcano,Plutonic Belt, Far East Russia

RESOURCE GEOLOGY, Issue 1 2002
Kohei SATO
Abstract: Age of magmatism and tin mineralization in the Khingan-Okhotsk volcano,plutonic belt, including the Khingan, Badzhal and Komsomolsk tin fields, were reviewed in terms of tectonic history of the continental margin of East Asia. This belt consists mainly of felsic volcanic rocks and granitoids of the reduced type, being free of remarkable geomagnetic anomaly, in contrast with the northern Sikhote-Alin volcano,plutonic belt dominated by oxidized-type rocks and gold mineralization. The northern end of the Khingan-Okhotsk belt near the Sea of Okhotsk, accompanied by positive geomagnetic anomalies, may have been overprinted by magmatism of the Sikhote-Alin belt. Tin,associated magmatism in the Khingan-Okhotsk belt extending over 400 km occurred episodically in a short period (9510 Ma) in the middle Cretaceous time, which is coeval with the accretion of the Kiselevka-Manoma complex, the youngest accretionary wedge in the eastern margin of the Khingan-Okhotsk accretionary terranes. The episodic magmatism is in contrast with the Cretaceous-Paleogene long,lasted magmatism in Sikhote,Alin, indicating the two belts are essentially different arcs, rather than juxtaposed arcs derived from a single arc. The tin-associated magmatism may have been caused by the subduction of a young and hot back-arc basin, which is inferred from oceanic plate stratigraphy of the coeval accre-tionary complex and its heavy mineral assemblage of immature volcanic arc provenance. The subduction of the young basin may have resulted in dominance of the reduced-type felsic magmas due to incorporation of carbonaceous sediments within the accretionary complex near the trench. Subsequently, the back-arc basin may have been closed by the oblique collision of the accretionary terranes in Sikhote,Alin, which was subjected to the Late Cretaceous to Paleogene magmatism related to another younger subduction system. These processes could have proceeded under transpressional tectonic regime due to oblique subduction of the paleo-Pacific plates under Eurasian continent. [source]


Present-day stresses in Brunei, NW Borneo: superposition of deltaic and active margin tectonics

BASIN RESEARCH, Issue 2 2010
R. C. King
ABSTRACT The Baram Delta System, Brunei, NW Borneo, is a Tertiary delta system located on an active continental margin. Delta top regions in many Tertiary delta systems (e.g. Niger Delta) are thought to exhibit a normal-fault stress regime and margin-parallel maximum horizontal stress orientations. However, unlike in passive margin Tertiary delta systems, two present-day stress provinces have been previously identified across the Baram Delta System: an inner shelf inverted province with a margin-normal (NW,SE) maximum horizontal stress orientation and an outer shelf extension province with a margin-parallel (NE,SW) maximum horizontal stress orientation. Before this study, there were few data constraining the inverted province other than in the vicinity of the Champion Fields. New data from 12 petroleum wells in the western inner shelf and onshore west Brunei presented herein confirm the margin-normal maximum horizontal stress orientations of the inverted province. A total of 117 borehole breakouts, all documented in shale units, and one drilling-induced tensile fracture (in a sandstone interval) reveal a mean maximum horizontal stress orientation of 117 with a standard deviation of 19°. This orientation is consistent with contemporary margin-normal maximum horizontal stress orientations of the inverted province described previously in the vicinity of the Champion Fields that have been linked to basement tectonics of the Crocker,Rajang accretionary complex and associated active margin. However, stress magnitudes calculated using data from these 12 petroleum wells indicate a borderline strike,slip fault to normal fault stress regime for the present day; combined with the absence of seismicity, this suggests that the studied part of the NW Borneo continental margin is currently tectonically quiescent. [source]


Jurassic synorogenic basin filling in western Korea: sedimentary response to inception of the western Circum-Pacific orogeny

BASIN RESEARCH, Issue 4 2009
Kosuke Egawa
ABSTRACT This is the first sedimentologic and stratigraphic attempt to demonstrate Jurassic subduction-induced basin-filling processes in the early stage of the western Circum-Pacific orogeny. The Chungnam Basin in western Korea was filled with a Lower to Middle Jurassic nonmarine succession, the Nampo Group, whose deposition postdated the Triassic final assembly of Chinese continental blocks. The Nampo Group consists of two repeated, fining- to coarsening-upward alluvio-lacustrine sequences, separated by an interval of thick breccia,gravel progradation deposits and its related strong proximal unconformities. No temporal variation in the degree of chemical weathering, along with the predominance of coals and a tropic to subtropic paleoflora, reveals little or no climate fluctuations during deposition of the Nampo Group. The observed relationships provide a record of sedimentation most likely controlled by temporal variations of tectonically driven sediment flux. Such syntectonic sedimentation of the Chungnam Basin occurred at a convergent margin of continental-arc setting during the Daebo orogeny, synchronous with the early subduction of the western paleo-Pacific ocean that resulted in formation of an accretionary complex along the East Asian continental margin during Jurassic time. Hence, synorogenic deposition in the Chungnam Basin is interpreted as sedimentary response to subduction,accretion of the western paleo-Pacific plate. [source]


The processes of underthrusting and underplating in the geologic record: structural diversity between the Franciscan Complex (California), the Kodiak Complex (Alaska) and the Internal Ligurian Units (Italy)

GEOLOGICAL JOURNAL, Issue 2 2009
F. Meneghini
Abstract Existing studies on active subduction margins have documented the wide diversity in structural style between accretionary prisms, both in space and time. Together with physical boundary conditions of the margins, the thickness of sedimentary successions carried by the lower plate seems to play a key role in controlling the deformation and fluid flow during accretion. We have tested the influence of the subducting sedimentary section by comparing the structural style and fluid-related structures of four units from three fossil accretionary complexes characterized by similar physical conditions but different subducting sediment thicknesses: (1) the Franciscan Complex of California, (2) the Internal Ligurian Units of Italy and (3) the Kodiak Complex, Alaska. Subducting plates bearing a thick sedimentary cover generally result in coherent accretion through polyphase deformation represented by folding and thin thrusting events, while underplating of sediment-starved oceanic sections results in diffuse deformation and mélange formation. These two structural styles can alternate through time in a single complex with a long record of accretion such as Kodiak. The parallel analysis of the selected analogues show that although the volume of sediments carried by the lower plate determines different structural styles, deformation is strongly controlled by injection of overpressured fluids during underthrusting and accretion. Transient hydrofracturing occurs through the development of a system of dilatant fractures grossly parallel to the décollement zone. Copyright © 2009 John Wiley & Sons, Ltd. [source]


Continental basalts in the accretionary complexes of the South-west Japan Arc: Constraints from geochemical and Sr and Nd isotopic data of metadiabase

ISLAND ARC, Issue 1 2000
Hiroo Kagami
Abstract The Ryoke Belt is one of the important terranes in the South-west Japan Arc (SJA). It consists mainly of late Cretaceous granitoid rocks, meta-sedimentary rocks (Jurassic accretionary complexes) and mafic rocks (gabbros, metadiabases; late Permian,early Jurassic). Initial ,Sr (+ 25, + 59) and ,Nd (, 2.1,,5.9) values of the metadiabases cannot be explained by crustal contamination but reflect the values of the source material. These values coincide with those of island arc basalt (IAB), active continental margin basalt (ACMB) and continental flood basalt (CFB). Spiderdiagrams and trace element chemistries of the metadiabases have CFB-signature, rather than those of either IAB or ACMB. The Sr,Nd isotope data, trace element and rare earth element chemistries of the metadiabases indicate that they result from partial melting of continental-type lithospheric mantle. Mafic granulite xenoliths in middle Miocene volcanic rocks distributed throughout the Ryoke Belt were probably derived from relatively deep crust. Their geochemical and Sr,Nd isotopic characteristics are similar to the metadiabases. This suggests that rocks, equivalent geochemically to the metadiabases, must be widely distributed at relatively deep crustal levels beneath a part of the Ryoke Belt. The geochemical and isotopic features of the metadiabases and mafic granulites from the Ryoke Belt are quite different from those of mafic rocks from other terranes in the SJA. These results imply that the Ryoke mafic rocks (metadiabase, mafic granulite) were not transported from other terranes by crustal movement but formed in situ. Sr,Nd isotopic features of late Cretaceous granitoid rocks occurring in the western part of the Japanese Islands are coincident with those of the Ryoke mafic rocks. Such an isotopic relation between these two rocks suggests that a continental-type lithosphere is widely represented beneath the western part of the Japanese Islands. [source]