circum-Pacific Orogenic Belts Research Articles

Cretaceous magmatic arc in Hainan and the peri-South China Sea as evidenced by geochemical fingerprinting of granitoids in the region

Mesozoic magmatic rocks occur widely in the South China Block and are generally interpreted as the manifestations of the subduction of the Paleo-Pacific oceanic lithosphere beneath Asia. Subduction-driven magmatism in southeast (SE) China continued from the Late Permian through the Late Cretaceous with an inferred lull between 125 Ma and 115 Ma that is known in the literature as the Cretaceous “magmatic quiescence”. We report in-situ zircon U–Pb ages, Hf–O and whole-rock Sr–Nd isotopes, and whole-rock geochemistry of Cretaceous granitoids on Hainan Island and discuss their magmatic evolution within the framework of the Late Mesozoic geodynamics of SE China. We recognize two main stages of the emplacement of Cretaceous granitoids on Hainan, first around 120 Ma and then around 100–95 Ma, displaying high-K calc-alkaline, I-type geochemical affinities. Granites in both age groups are enriched in LILE and LREE, but depleted in Nb, Ta, Ba, Sr, and Eu. The 120 Ma granites have zircon εHf(t) values of –2.6 to 2.3 corresponding to Hf crustal model ages, ranging from 0.79 Ga to 1.03 Ga, and δ18O values ranging from 6.9‰ to 7.7‰. Zircons from 100–95 Ma granites have εHf(t) values of –4.2 to 1.1 corresponding to Hf crustal model ages of 1.08 Ga to 1.42 Ga, and δ18O values ranging from 6.7‰ to 8.4‰. Increasing εHf(t) values of the Cretaceous Hainan granites with younger crystallization ages indicate addition of more juvenile components and reworking of crustal material into their melt evolution. The εNd(t) values of the 120 Ma and 100–95 Ma granitoids range between –4.1 to –0.4 and –7.7 to –4.0, respectively. The calculated two–stage model age of the 100–95 Ma granitoids clusters between 1.25 Ga and 1.53 Ga. These isotopic data suggest that magmas of the Cretaceous granitoids were produced by partial melting of Mesoproterozoic metabasaltic rocks, which make up much of the crystalline basement of the southern Cathaysia block. The geochemical and isotopic characteristics of the Cretaceous granitoids on Hainan resemble those of magmatic arcs in the Circum–Pacific orogenic belts and identical to those of nearly coeval granitoid intrusions in the continental fragments within the South China Sea basin. We interpret these Cretaceous granitoids in the Peri–South China Sea region as the remnants of a once contiguous Late Mesozoic magmatic arc system that bounded the southern margin of the entire continental Southeast Asia. Our findings do not support the existence of an episode of magmatic quiescence in the geological record of SE China during the Aptian.

Adaptive Approach to Time-Frequency Analysis of AE Signals of Rocks

The paper describes a new adaptive approach to the investigation of acoustic emission of rocks, the anomalies of which may serve as short-term precursors of strong earthquakes. The basis of the approach is complex methods for monitoring acoustic emission and for analysis of its time-frequency content. Piezoceramic hydrophones and vector receivers, installed at the bottom of natural and artificial water bodies, as well as in boreholes with water, are used as acoustic emission sensors. To perform a time-frequency analysis of geoacoustic signals, we use a sparse approximation based on the developed Adaptive Matching Pursuit algorithm. The application of this algorithm in the analysis makes it possible to adapt to the concrete characteristics of each geoacoustic pulse. Results of the application of the developed approach for the investigation of acoustic emission anomalies, occurring before earthquakes, are presented. We analyzed the earthquakes, that occurred from 2011 to 2016 in the seismically active region of the Kamchatka peninsula, which is a part of the circum-Pacific orogenic belt also known as the "Ring of Fire". It was discovered that geoacoustic pulse frequency content changes before a seismic event and returns to the initial state after an earthquake. That allows us to make a conclusion on the transformation of acoustic emission source scales before earthquakes. The obtained results may be useful for the development of the systems for environmental monitoring and detection of earthquake occurrences.

Kinematic reconstruction of the Raohe accretionary complex, Northeast China: Integration of onshore geologic evidence and global plate model

The Raohe accretionary complex (RHC) is located at the eastern of northeast China and adjacent to Russian Far East. As a part of the Circum-Pacific Orogenic Belt, it is the unique region of the accretionary orogenic belt, which is associated with the subduction process of the Panthalassic-Pacific Plate (PPP). We synthesize the detrital zircon ages of terrigenous clastic rocks of the RHC and tectonic units along the East Asian Continental Margin (EACM) to clarify its provenance. Then we place the docking position of the RHC adjacent to the South China Block, and determine that the final accretion of the RHC occur during the later Late Jurassic (~150 Ma) according to combination for ages of stitching plutons and terrigenous clastic rock. Integrating with the published global-scale plate kinematic frame, we restored the pre-docking motion path of the RHC using the Gplates software. The reconstructed scenario shows that it is a long distance of at least 1000 km between the proto-RHC and continent margin when the basaltic volcanism occurred subaqueously within the abyssal basin of the PPP. This model also provides a probability that the proto-RHC and the proto-Yuejingshan accretionary complexes have the same drift history, before their simultaneous emplacement into the continental margin.

Magmatism: A crustal and geodynamic perspective

The Earth's continental crust constitutes a major interface between the inner and outer envelops of the planet, controlling the differentiation of magmas produced in the mantle and their transfer to the surface. This close link facilitates the use of different chemical proxies to qualitatively unravel the crustal thickness related to fossil magmatic systems based on the message carried by magmas. This paper aims to bridge different results of statistical petrology, recently obtained at different scales of observation, in a global geodynamic model. Statistical analyses applied to a large multidimensional database of magmatic rocks show that crustal thickness could actually exert a first-order control on the composition of magmas, which become more calc-alkaline and comparatively less tholeiitic with increasing crustal thickness. Using this correlation, we document the progressive build-up of a thick (>40 km) Jurassic to Cretaceous accretionary belt along the Circum-Pacific Orogenic Belts (CPOB) that bounded the Panthalassa Ocean. The destruction of this thick belt started at ca. 125 Ma and was initially recorded by the thinnest magmatic systems hosting amphibole-bearing magma. Thinning of the CPOB became widespread in the northern regions of western America and in the western Pacific after ca. 75 Ma, possibly in response to oceanic plate segmentation, which triggered slab rollback and overriding plate extension. This chemical evolution is superimposed on a more global evolution of magma controlled by the temperature of the mantle that has gradually decreased since 200 Ma. Although the relative contribution of crust vs mantle cooling in the chemical signature of magmatic rocks should be further explored in the future, our results offer a new global perspective of the magmatic history of Pangea, the last supercontinent.

The Mo deposits of Northeast China: A powerful indicator of tectonic settings and associated evolutionary trends

Northeast (NE) China lies in the eastern sector of the Central Asian Orogenic Belt (CAOB) that connects with the Circum-Pacific Orogenic Belt in the east, and accommodates multistage magmatism, crustal growth and mineralization. In this region 69 Mo-only or Mo-dominated and 9 Cu-Mo deposits have been discovered, including 65 deposits ranking medium-size (>10Kt Mo metal) or larger in tonnage and containing a total resource of 10.5Mt Mo metal. Six giant and seventeen large deposits have total reserves of 6.7 and 3.1Mt Mo, respectively. These deposits occur in the areas surrounding the Songliao Basin, including the northern margin of the North China Craton, the Great Hingan Range and the Ji-Hei Fold Belt, and have been formed during Paleozoic and Mesozoic tectono-magmatic events. All the Mo-only or Mo-dominated deposits were formed in the Mesozoic, postdating the closure of the Paleo-Asia Ocean, and in a series of pulses around 250–200Ma, 200–160Ma, 160–130 and <130Ma (130–100Ma), suggesting that the Paleozoic crust, compared with the Mesozoic crust, was less sialic and unfavorable for Mo mineralization. This is supported by the variation of Re contents in molybdenites from the deposits, i.e., increasing with the Cu/Mo ratios and the ore-forming ages. Mineralization was generally associated with granitic rocks mainly of crust-sourced and high-K calc-alkaline to shoshonite series. The majority of the deposits are porphyry (including breccia pipes) type, followed by the skarn and quartz vein types. The porphyry Mo deposits can be further subdivided into three subtypes, i.e., collision- or Dabie-, rift- or Climax-, and subduction- or Endako-types. The Mo deposits aged 250–200-Ma and 200–160-Ma belong to the collision-type and have been formed in syn- to post-collisional tectonic setting. The 160–130-Ma Mo mineralization mainly occurs in the Great Hingan Range and northern margin of the North China Craton, and includes Endako- and Climax-types of porphyry Mo systems, which resulted from southward subduction of the Mongol-Okhotsk oceanic plate. The 130–100-Ma deposits belong to the Endako-type and are only located in the eastern Ji-Hei Fold Belt, which must be related to the westward subduction of the Paleo-Pacific oceanic plate. Therefore, as shown by the porphyry Mo deposits in NE China, these mineral systems are a powerful indicator of tectonic settings and associated evolutionary trends.

Origin and accretion history of the circum-Pacific orogenic belts in NE Russia

Origin and accretion history of the circum-Pacific orogenic belts in NE Russia

Ores and Orogenesis: Circum-Pacific Tectonics, Geologic Evolution, and Ore Deposits.: JON E. SPENCER and SPENCER R. TITLEY, Editors. Pp. 618. 2008. Arizona Geological Society, Digest 22. ISBN-10: 978-1-891924-10-1. Price US$75.00.

This publication was born out of an Arizona Geological Society meeting with the same title, held in October 2007, and consists of 45 papers representing a fraction of the material presented at the meeting. The compilation is dedicated to William R. (Bill) Dickinson, emeritus professor of geology at the University of Arizona, who was one of the first geologists to recognize and promote the inter-relationship between tectonics and ore deposit formation. The editors proclaim the volume to be a measure of the current level of cross-pollination between and advancement within the fields of tectonics and economic geology. As the title suggests, the papers within the volume cover a broad range of tectonics and ore deposits studies, from the global to deposit scale. Thus, this 618-page-long book is divided into six sections, with a large-scale Pacific margins, orogenesis, and metallogeny section; an exploration, mining and modeling section; and four other sections, each of which contains papers on one of the four quadrants of the circum-Pacific orogenic belts. Even within the individual sections, as outlined below, the range of topics is exceptionally broad. Five papers make up the first section, which opens with a paper by Dickinson that outlines basic observations of the current configuration of circum-Pacific orogenic belts and sets the stage for reconstructing older, dissected belts based on these observations. Kerrich et al. present a summary paper of metallogenic provinces of North America through super-continent assembly and breakup, and Sengor et al. propose potential for strata-bound …

Geodynamic significance of S-type granites in circum-Pacific orogens

Research Article| July 01, 2008 Geodynamic significance of S-type granites in circum-Pacific orogens W.J. Collins; W.J. Collins 1School of Earth and Environmental Sciences, James Cook University, Townsville, Queensland 4814, Australia Search for other works by this author on: GSW Google Scholar S.W. Richards S.W. Richards 2Research School of Earth Sciences, Australian National University, Canberra 0200, Australia Search for other works by this author on: GSW Google Scholar Author and Article Information W.J. Collins 1School of Earth and Environmental Sciences, James Cook University, Townsville, Queensland 4814, Australia S.W. Richards 2Research School of Earth Sciences, Australian National University, Canberra 0200, Australia Publisher: Geological Society of America Received: 29 Nov 2007 Revision Received: 04 Apr 2008 Accepted: 09 Apr 2008 First Online: 02 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 © 2008 Geological Society of America Geology (2008) 36 (7): 559–562. https://doi.org/10.1130/G24658A.1 Article history Received: 29 Nov 2007 Revision Received: 04 Apr 2008 Accepted: 09 Apr 2008 First Online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation W.J. Collins, S.W. Richards; Geodynamic significance of S-type granites in circum-Pacific orogens. Geology 2008;; 36 (7): 559–562. doi: https://doi.org/10.1130/G24658A.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract In Phanerozoic circum-Pacific orogenic belts, most “post-collisional” S-type granites and associated high-temperature, low-pressure metamorphic complexes formed during early arc extension. The granites are part of a tripartite association consisting of (1) inboard S-type granite, (2) outboard oceanic arc, and (3) intervening, turbidite-filled backarc basin. S-type granites herald the formation of new outboard oceanic arc and extensional backarc systems, but thickening of a preexisting, sediment-dominated back-arc basin is a prerequisite for their generation. In these environments, S-type plutonism is triggered by renewal of arc magmatism following thickening, when hot basaltic magmas are intruded into the thickened backarc crust once slab retreat is reestablished. With ongoing extension during retreat, the crust becomes progressively thinned, the sedimentary contribution is diminished, and the granites lose their S-type character. Such tripartite associations involving S-type granite are probably diagnostic of repeated slab-retreat episodes, and the Jurassic U.S. cordillera might be an example. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Geology of the Gorny Altai subduction–accretion complex, southern Siberia: Tectonic evolution of an Ediacaran–Cambrian intra-oceanic arc-trench system

The Gorny Altai region in southern Siberia is one of the key areas in reconstructing the tectonic evolution of the western segment of the Central Asian Orogenic Belt (CAOB). This region features various orogenic elements of Late Neoproterozoic–Early Paleozoic age, such as an accretionary complex (AC), high- P/ T metamorphic (HP) rocks, and ophiolite (OP), all formed by ancient subduction–accretion processes. This study investigated the detailed geology of the Upper Neoproterozoic to Lower Paleozoic rocks in a traverse between Gorno-Altaisk city and Lake Teletskoy in the northern part of the region, and in the Kurai to Chagan-Uzun area in the southern part. The tectonic units of the studied areas consist of (1) the Ediacaran (=Vendian)–Early Cambrian AC, (2) ca. 630 Ma HP complex, (3) the Ediacaran–Early Cambrian OP complex, (4) the Cryogenian–Cambrian island arc complex, and (5) the Middle Paleozoic fore-arc sedimentary rocks. The AC consists mostly of paleo-atoll limestone and underlying oceanic island basalt with minor amount of chert and serpentinite. The basaltic lavas show petrochemistry similar to modern oceanic plateau basalt. The 630 Ma HP complex records a maximum peak metamorphism at 660 °C and 2.0 GPa that corresponds to 60 km-deep burial in a subduction zone, and exhumation at ca. 570 Ma. The Cryogenian island arc complex includes boninitic rocks that suggest an incipient stage of arc development. The Upper Neoproterozoic–Lower Paleozoic complexes in the Gorno-Altaisk city to Lake Teletskoy and the Kurai to Chagan-Uzun areas are totally involved in a subhorizontal piled-nappe structure, and overprinted by Late Paleozoic strike-slip faulting. The HP complex occurs as a nappe tectonically sandwiched between the non- to weakly metamorphosed AC and the OP complex. These lithologic assemblages and geologic structure newly documented in the Gorny Altai region are essentially similar to those of the circum-Pacific (Miyashiro-type) orogenic belts, such as the Japan Islands in East Asia and the Cordillera in western North America. The Cryogenian boninite-bearing arc volcanism indicates that the initial stage of arc development occurred in a transient setting from a transform zone to an incipient subduction zone. The less abundant of terrigenous clastics from mature continental crust and thick deep-sea chert in the Ediacaran–Early Cambrian AC may suggest that the southern Gorny Altai region evolved in an intra-oceanic arc-trench setting like the modern Mariana arc, rather than along the continental arc of a major continental margin. Based on geological, petrochemical, and geochronological data, we synthesize the Late Neoproterozoic to Early Paleozoic tectonic history of the Gorny Altai region in the western CAOB.

New model for the Early Cretaceous development of SW Japan based on basic rocks of the Chichibu Composite Terrane

The Kurosegawa Terrane in SW Japan is a fault zone that divides the Chichibu Composite Terrane into the Northern Chichibu and Southern Chichibu Terranes. It contains exotic tectonic blocks including granitoids, variously metamorphosed rocks and Siluro-Devonian deposits together with tectonic slices of the Jurassic-Early Cretaceous accretionary complexes. It is also intimately associated with Cretaceous forearc basin deposits. Understanding of the Korosegawa Terrane is regarded as a key to clarify the Mesozoic geotectonic history of the western circum-Pacific orogenic belts. Detailed studies of the geochemistry of basic rocks of the Chichibu Composite Terrane in eastern Kii Peninsula revealed that HIMU basalts are included in basic rocks. The presence of HIMU basalts provides further evidence for the existence of the Kurosegawa Terrane in this area and supports our model on the tectonic evolution of the Kurosegawa Terrane as a transform fault zone formed due to oceanic ridge subduction in the late Early Cretaceous. This model can comprehensively describe not only the features of the Kurosegawa Terrane but also the coeval key situations of the western circum-Pacific orogenic belts.

Plate kinematics, origin and tectonic emplacement of supra-subduction ophiolites in SE Asia

A unique feature of the Circum Pacific orogenic belts is the occurrence of ophiolitic bodies of various sizes, most of which display petrological and geochemical characteristics typical of supra-subduction zone oceanic crust. In SE Asia, a majority of the ophiolites appear to have originated at convergent margins, and specifically in backarc or island arc settings, which evolved either along the edge of the Sunda (Eurasia) and Australian cratons, or within the Philippine Sea Plate. These ophiolites were later accreted to continental margins during the Tertiary. Because of fast relative plate velocities, tectonic regimes at the active margins of these three plates also changed rapidly. Strain partitioning associated with oblique convergence caused arc-trench systems to move further away from the locus of their accretion. We distinguish “relatively autochthonous ophiolites” resulting from the shortening of marginal basins such as the present-day South China Sea or the Coral Sea, and “highly displaced ophiolites” developed in oblique convergent margins, where they were dismantled, transported and locally severely sheared during final docking. In peri-cratonic mobile belts (i.e. the Philippine Mobile Belt) we find a series of oceanic basins which have been slightly deformed and uplifted. Varying lithologies and geochemical compositions of tectonic units in these basins, as well as their age discrepancies, suggest important displacements along major wrench faults. We have used plate tectonic reconstructions to restore the former backarc basins and island arcs characterized by known petro-geochemical data to their original location and their former tectonic settings. Some of the ophiolites occurring in front of the Sunda plate represent supra-subduction zone basins formed along the Australian Craton margin during the Mesozoic. The Philippine Sea Basin, the Huatung basin south of Taiwan, and composite ophiolitic basements of the Philippines and Halmahera may represent remnants of such marginal basins. The portion of the Philippine Sea Plate carrying the Taiwan–Philippine arc and its composite ophiolitic/continental crustal basement might have actually originated in a different setting, closer to that of the Papua New Guinea Ophiolite, and then have been displaced rapidly as a result of shearing associated with fast oblique convergence.

EVOLUTION OF THE NORTH AMERICAN CORDILLERA

▪ Abstract The Cordilleran orogen of western North America is a segment of the Circum-Pacific orogenic belt where subduction of oceanic lithosphere has been underway along a great circle of the globe since breakup of the supercontinent Pangea began in Triassic time. Early stages of Cordilleran evolution involved Neoproterozoic rifting of the supercontinent Rodinia to trigger miogeoclinal sedimentation along a passive continental margin until Late Devonian time, and overthrusting of oceanic allochthons across the miogeoclinal belt from Late Devonian to Early Triassic time. Subsequent evolution of the Cordilleran arc-trench system was punctuated by tectonic accretion of intraoceanic island arcs that further expanded the Cordilleran continental margin during mid-Mesozoic time, and later produced a Cretaceous batholith belt along the Cordilleran trend. Cenozoic interaction with intra-Pacific seafloor spreading systems fostered transform faulting along the Cordilleran continental margin and promoted incipient rupture of continental crust within the adjacent continental block.

Kurosegawa Terrane as a Transform Fault Zone in Southwest Japan

The Kurosegawa Terrane intervening in the Jurassic-Early Cretaceous accretionary complexes along the Pacific side of the SW Japanese Islands is a serpentinite mélange zone. It contains various kinds of exotic rocks, for example, granitoids, metamorphic rocks, Siluro-Devonian deposits and is intimately associated with Cretaceous forearc basin deposits. The terrane is regarded as a key to clarify the Mesozoic geotectonic history of the western circum-Pacific orogenic belts. The current model, in which the formation of the Kurosegawa Terrane is attributed to nappe-movement or sinistral strike-slip faulting, can explain neither the mode of occurrence of the Kurosegawa Terrane we observed in eastern Kii Peninsula nor the array of evidence obtained from the Ryoke Terrane southward to the Shimanto Terrane. We suggest a new hypothesis in which the Kurosegawa Terrane was a transform fault zone that originated because of oceanic ridge subduction along the southern margin of the coeval accretionary prism (Butsuzo T.L.) in the late Early Cretaceous. Our model is mainly based on new geological evidence from the Kurosegawa Terrane in eastern Kii Peninsula where the deepest erosion level is exposed due to neotectonic uplift.

Granulite Facies Relics in the Early Paleozoic Kyanite-bearing Ultrabasic Metacumulate in the Oeyama Belt, the Inner Zone of Southwestern Japan

Granulite facies relics are found in the early Paleozoic kyanite-bearing melanocratic metagabbro from the Fuko Pass metacumulate mass of the Oeyama belt, southwestern Japan. The granulite facies assemblage consists of relict Al-rich clinopyroxene (up to 8.5 wt.% Al 2O 3) and pseudomorphs of spinel and plagioclase. The spinel pseudomorphs consist mainly of symplectic intergrowth of corundum and magnetite with minor gahnitic spinel. The plagioclase pseudomorphs are composed mainly of clinozoisite with minor kyanite. The symplectite suggests oxidation and Mg-depletion of the original spinel: hercynite (3FeAl 2O 4)+(O) = corundum (3Al 2O 3)+magnetite (Fe 3O 4). This oxidation reaction may have taken place at 700-900°C temperature. The melanocratic metagabbro has later been hydrated to form the epidote amphibolite assemblage represented by clinozoisite+kyanite+paragonite. The clinozoisite+kyanite assemblage has further reacted to form margarite at a lower temperature. The first granulite facies assemblage implies that the metacumulate has originally constituted a basal part of thick oceanic crust, and then has experienced the high-P/T type metamorphism in a subduction zone. This indicates that the thick oceanic crust has been formed and accreted to the Circum-Pacific orogenic belt in the early Paleozoic time.

Blueschists and Eclogites of the World and Their Exhumation

High-P/T metamorphic belts were classified into types A and B according to their protoliths. The A-type (collision-type) blueschists possess passive-margin protoliths characterized by platform-type carbonates, bimodal volcanics, and peraluminous sediments. B-type (Cordilleran-type) blueschists consist of active continental-margin protoliths in an accretionary complex characterized by bedded chert, MORB and ocean-island basalts, reef limestones, and graywackes. The spatiotemporal distribution of blueschists and eclogites of the world was compiled; among 250 recognized high-P/T belts, about 20% belong to the A type and the rest to the B type. Most A-type zones lie in Europe and the Tethyan domain, include ultrahigh-pressure metamorphic terranes, and have metamorphic pressure up to 45 kbar. B-type zones occur mainly in the circum-Pacific orogenic belts and intracontinental orogens in Asia, and were recrystallized at P <12 kbar. Associated peridotites include garnet peridotite in the A type and strongly serpe...

Late Jurassic tropical Radiolaria: Vallupus and its related forms

Abstract The Vallupinae is a radiolarian subfamily whose range is restricted to the Late Jurassic and the acme in the latest Jurassic (Tithonian). Species of the subfamily ( Vallupus group) are found in low- and middle-latitudes of the western Pacific, East Asia, North-Central-South America, and Mediterranean regions. Available paleomagnetic data suggest that Vallupus group-bearing rocks in these localities accumulated within 25° of the Jurassic paleoequator. Seawater temperature is considered to be the major factor controlling the distribution of the Vallupus group. Warm seawater temperatures could be favourable to the propagation of the group. The Vallupus territory is proposed as a water mass inhabited by the Vallupus group. It occupied tropical surface waters from the low-latitude part of the Panthalassa to Tethys. Some Vallupus group-bearing rocks in the Circum-Pacific orogenic belt have been translated from low-latitude depositional sites to higher locations by lateral plate movements. Most Upper Jurassic strata in Japan accumulated outside the Vallupus territory. This means that the Vallupus group cannot be utilized for Upper Jurassic radiolarian zonal correlations between Japan and other regions. However, the rapid speciation of the Vallupus group has a high potential to produce a fine biostratigraphic framework for the Upper Jurassic within the Vallupus territory.

METAMORPHIC BELTS OF JAPANESE ISLANDS

In 1961 Miyashiro presented the concept of paired metamorphic belts based on the mode of occurrences of high-pressure metamorphic belts along the Circum-Pacific orogenic belts, particularly in Japan. According to him the Japanese islands are composed essentially of three paired metamorphic belts: the late Paleozoic Hida-Sangun and late Mesozoic Ryoke-Sanbagawa pairs in Honshu, and the late Mesozoic Kamuikotan­ Hidaka in Hokkaido. At that time he could use only 11 K-Ar mica ages. Detailed petrographic work has continued since that time, and many radiometric ages are now available in the literature. From the late 1970s to the early 1980s, two major breakthroughs were made in Japanese basement geology; the first was in conodont-biostratigraphy followed by radiolaria-biostratigraphy, and the second was due to the increased pro­ ductivity ofK-Ar dating. Finally, the geosyncline concept was abandoned, and has been replaced by plate-tectonics approaches for explaining why particular metamorphic belts are located where they are today. Uyeda & Miyashiro (1974), Saito & Hashimoto (1982), and Maruyama & Seno (1986) pioneered this research. These and later works revealed that many metamorphic areas actually consist of several units.

Late Palaeozoic to early Mesozoic geodynamic evolution of the Circum-Pacific orogenic belt in South Korea and Southwest Japan

A review of the overall features of the late Palaeozoic-early Mesozoic orogen of Southwest Japan and its Korean hinterland allows a new geodynamic model for the building of the east Asian margin to be proposed. During middle Carboniferous to early Permian times, an oceanic basin formed in a passive margin environment at a moderate spreading rate as the Honshu continental block drifted away from the Sino-Korean continent. During that period, paralic platform conditions and extensional synsedimentary tectonics prevailed in Korea. In middle Permian times, the closure of the oceanic area was responsible for Yakuno ophiolite obduction and high pressure metamorphism while highly subsiding coal basins developed in the emerged Korean hinterland. Thereafter, in late Permian times, when the Honshu block collided with the Asian margin (Akiyoshi orogeny), locally derived terrestrial clastic sedimentation in Korea has succeeded coal measure deposition with local unconformity. In early Triassic times, the collision was completed and regional uplift of the thickened crust resulted in the deposition of thick distal siliciclastic rocks in rapidly subsiding hinterland troughs (Korea) and flyschoid deposition in the Tamba foreland basin (outer zone of Southwest Japan). Middle to late Triassic medium- to low-pressure metamorphism, large-scale transcurrent tectonics and associated calc-alkaline synkinematic plutonism in the Hida and Ogcheon belts represent a post-collisional feature of the Akiyoshi orogenic event. Post-tectonic intrusives and terrestrial deposits post-date intracratonic tectonics at ca. 200 Ma. Thus, the Akiyoshi orogeny of Southwest Japan is a result of continental drift and ensuing collisional rewelding of a fragment of Sino-Korea: the Honshu microcontinent.

Accreted oceanic materials in Japan

The Phanerozoic circum-Pacific orogenic belts contain numerous ocean-derived materials accreted through plate converging processes. Japanese Islands, in particular, display various kinds of oceanic materials of different origins including fragments of seamounts, oceanic reef limestone, MORB-like rocks and oceanic mantle, and pelagic sediments. The compilation of these rocks in many subduction complexes of Late Permian to the present, led to following conclusions. Accretion processes work effectively only for materials primarily composing the upper portion of subducting oceanic crust, i.e. Layer 1 and Layer 2. Many fragments of seamount with alkali basalt (600), hot-spot seamount (26), oceanic reef limestone (291), MORB-like basalt (200), and numerous cherts (more than 1000) are recognized as ancient oceanic materials accreted to the Japanese Islands. However, gabbros and mantle materials of Layer 3 and lower parts of the oceanic lithosphere, scarcely occur in subduction-accretion complexes except for a few examples of back-arc basin or fore-arc origin. Accretion occurs episodically. In Southwest Japan, oceanic materials were accreted intermittently in 1. (a) end-Permian, 2. (b) Middle-Late Jurassic, 3. (c) Late Cretaceous times, 4. (d) at ca. 50 Ma, and 5. (e) in Miocene times, while in Northeast Japan and Hokkaido this occurred in (b) Middle-Late Jurassic, (c) Late Cretaceous, and (f) Early Cretaceous times. In contrast to the general belief on accretion of younger oceanic plates, the majority of Japanese subduction-accretion complexes were formed during the subduction of plates, up to 160 Ma old. The accretionary events in end-Permian and Middle-Late Jurassic times coincide with northward collision of ancient island arcs, oceanic rises or seamount chains (of hot-spot origin) with the Asian continent. Accretion relevant to subduction of older plates may be controlled by the collision-subduction process of these topographic reliefs on an oceanic plate. In addition, the chronological coincidence with the continent collision-amalgamation between the Sino-Korean and Siberian platforms and between the Sino-Korean and Yangtze blocks, also implies collision-induced voluminous supply of elastics from back-arc regions and its contribution to the formation of huge accretionary complexes. Accreted fragments of ancient seamounts are much smaller than the average size of modern seamounts. This implies that most parts of a colliding seamount are not accreted but subducted together with the underlying oceanic crust to much deeper levels. With respect to the metamorphic grades for Japanese subduction complexes, oceanic materials have less than 1 vol.% in the zeolite facies, 15–20% in the prehnite-pumpellyite metagraywacke facies, and ca. 30% in the greenschist/glaucophane schist facies and albite-epidote amphibolite fades. This relationship indicates that the major process for landward accretion of oceanic materials is not off-scraping or sedimentary mixing at the trench, but underplating (subcretion) at much deeper levels of a subduction zone.

Age and nature of the Canadian part of the circum-pacific orogenic belt

The Canadian Cordilleran orogen evolved from a miogeosyncline which existed from Proterozoic to mid-Jurassic time in the Rocky Mountains and part of the Yukon, and a more or less contemporaneous, complex eugeosyncline on the west. The history of the eugeosynclinal belt since the mid-Palaeozoic, was influenced by the more or less persistent Coast and Omineca crystalline geanticlines and at times, by the non-metamorphic Pinchi geanticline between them. Since mid-Triassic time the crystalline geanticlines have been the principal sites of frequent uplift, recurrent granitic intrusion, metamorphism and deformation. Orogeny was rarely widespread at any time. The crystalline geanticlines were repeatedly deformed but not always synchronously, whereas adjoining troughs were rarely deformed more than once. Orogenic pulses took place at the following times: Proterozoic, Early and mid-Palaeozoic, Middle and locally Early Triassic, Middle Jurassic, latest Jurassic and earliest Cretaceous, mid-Cretaceous, Early and Late Tertiary.