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1.
Yukio  Isozaki 《Island Arc》1996,5(3):289-320
Abstract The Japanese Islands represent a segment of a 450 million year old subduction-related orogen developed along the western Pacific convergent margin. The geotectonic subdivision of the Japanese Islands is newly revised on the basis of recent progress in the 1980s utilizing microfossil and chronometric mapping methods for ancient accretionary complexes and their high-P/T metamorphic equivalents. This new subdivision is based on accretion tectonics, and it contrasts strikingly with previous schemes based on‘geosyncline’tectonics, continent-continent collision-related tectonics, or terrane tectonics. Most of the geotectonic units in Japan are composed of Late Paleozoic to Cenozoic accretionary complexes and their high-PIT metamorphic equivalents, except for two units representing fragments of Precambrian cratons, which were detached from mainland Asia in the Tertiary. These ancient accretionary complexes are identified using the method of oceanic plate stratigraphy. The Japanese Islands are comprised of 12 geotectonic units, all noted in southwest Japan, five of which have along-arc equivalents in the Ryukyus. Northeast Japan has nine of these 12 geotectonic units, and East Hokkaido has three of these units. Recent field observations have shown that most of the primary geotectonic boundaries are demarcated by low-angle faults, and sometimes modified by secondary vertical normal and/or strike-slip faults. On the basis of these new observations, the tectonic evolution of the Japanese Islands is summarized in the following stages: (i) birth at a rifted Yangtze continental margin at ca 750–700 Ma; (ii) tectonic inversion from passive margin to active margin around 500 Ma; (iii) successive oceanic subduction beginning at 450 Ma and continuing to the present time; and (iv) isolation from mainland Asia by back-arc spreading at ca 20 Ma. In addition, a continent-continent collision occurred between the Yangtze and Sino-Korean cratons at 250 Ma during stage three. Five characteristic features of the 450 Ma subduction-related orogen are newly recognized here: (i) step-wise (not steady-state) growth of ancient accretionary complexes; (ii) subhorizontal piled nappe structure; (iii) tectonically downward-younging polarity; (iv) intermittent exhumation of high-P/T metamorphosed accretionary complex; and (v) microplate-induced modification. These features suggest that the subduction-related orogenic growth in Japan resulted from highly episodic processes. The episodic exhumation of high-P/T units and the formation of associated granitic batholith (i.e. formation of paired metamorphic belts) occurred approximately every 100 million years, and the timing of such orogenic culmination apparently coincides with episodic ridge subduction beneath Asia.  相似文献   

2.
Abstract The Jurassic Tamba accretionary complex is divided into two tectono‐stratigraphic suites (Type I and II nappe groups), which are further divided into six complexes (nappes) each of which is characterized by a rock sequence of Late Paleozoic greenstone/limestone, Permian to Jurassic chert and Jurassic terrigenous clastic rocks. The mode of occurrence of the greenstone is divided into two types. The major basal type occurs as a large coherent slab associated with Permian chert and limestone, constituting the basal part of each complex, and the minor mixed type occurs as fragmented allochthonous greenstone blocks and lenses mixed with chert, limestone and sandstone in the Jurassic mudstone matrix. Most of the basal greenstones have uniform geochemical characteristics, which indicate enriched‐mid‐oceanic ridge basalt (MORB) affinity. Their geochemical compositions are akin to the reported Permo‐Carboniferous and Triassic oceanic plateau basalts. Mixed greenstones are divided into two petrochemical types: (i) tholeiitic basalt with normal‐MORB affinity, which is predominant in the uppermost complex of the Type II suite (upper nappe group); and (ii) tholeiitic and alkalic basalts of oceanic island or seamount origin, which are common in all complexes of the Tamba Belt. Geochemical characteristics of the greenstones thus vary in accordance with their occurrences and the structural units to which they belong. This relationship reflects the difference in topographic relief and crustal thickness of the accreted oceanic edifices – the remnants of thick oceanic plateau crust tended to accrete to the continental margin as a large basal greenstone body, whereas thin normal oceanic crust with small seamounts or oceanic islands accreted as mixed greenstones because of their mechanical weakness. The Type II suite (upper nappe group) contains the basal and mixed greenstones, whereas the Type I suite (lower nappe group) includes only mixed greenstones. This distinction may reflect the temporal change of subducting edifices from a thick oceanic plateau to a thin normal oceanic crust, and suggests that the accretion of a large oceanic plateau may be responsible for building accretionary complexes with thick basal greenstones slabs.  相似文献   

3.
The Anyui Metamorphic Complex (AMC) of Cretaceous age is composed of metachert, schist, gneiss, migmatite and ultramafic rocks, and forms a dome structure within the northernmost part of the Jurassic accretionary complex of the Samarka terrane. The two adjacent geological units are bounded by a fault, but the gradual changes of grain size and crystallinity index of quartz in chert and metachert of the Samarka terrane and the AMC, together with the gradual lithological change, indicate that at least parts of the AMC are metamorphic equivalents of the Samarka rocks. Radiolarian fossils from siliceous mudstone of the Samarka terrane indicates Tithonian age (uppermost Jurassic), and hence, form a slightly later accretion. This signifies that the accretionary complex in the study area is one of the youngest tectonostratigraphic units of the Samarka terrane. The relationship between the Samarka terrane and AMC, as well as their ages and lithologies, are similar to those of the Tamba–Mino–Ashio terrane and Ryoke Metamorphic Complex in southwest Japan. In both areas the lower (younger) part of the Jurassic accretionary complexes were intruded and metamorphosed by Late Cretaceous granitic magma. Crustal development of the Pacific‐type orogen has been achieved by the cycle of: (i) accretion of oceanic materials and turbidites derived from the continent; and (ii) granitic intrusion by the next subduction and accretion events, accompanied by formation of high T/P metamorphic complexes.  相似文献   

4.
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.  相似文献   

5.
Makoto  Saito 《Island Arc》2008,17(2):242-260
Abstract   Detailed geologic examination of the Eocene accretionary complex (Hyuga Group) of the Shimanto terrane in southeastern Kyushu revealed that the oceanic plate was composed of Paleocene to Lower Eocene mudstone and siliceous mudstone, lower Middle Eocene red mudstone, and mid-Middle Eocene trench-fill turbidite with siltstone breccia, successively overlying the pre-Eocene oceanic plate. This oceanic plate sequence was overlain by Upper Eocene siltstone. Deposition of the lower Middle Eocene red mudstone was accompanied by basalt flows and it is interbedded with continental felsic tuff, which indicates that the basalt and red mudstone were deposited near the trench just before accretion. The Hyuga Group has very similar geological structure to that of the chert–clastic complexes found in the Jurassic accretionary complexes in Japan: that is, a decollement fault formed in the middle of an oceanic plate sequence, and an imbricate structure formed only in the upper part of the sequence. Thus, it appears that the Hyuga Group was formed by the same accretionary process as the Jurassic accretionary complexes. No accretion occurred before the Middle Eocene, and the rapid accretion of the Hyuga Group was commenced by the supply of coarse terrigenous sediments in the mid-Middle Eocene, when the direction of movement of the Pacific Plate changed. The pre-Eocene oceanic basement and lower Middle Eocene volcanic activity suggest that the oceanic plate partly preserved in the Hyuga Group was very similar to the northern part of the present West Philippine Sea Plate.  相似文献   

6.
Accretionary wedge is the typical product of subduction-zone processes at shallow depths.Determining the location,composition and mechanism of accretionary wedge has important implications for understanding the tectonism of plate subduction.The Central Asian Orogenic Belt(CAOB) is one of the world's largest accretionary orogenic belts,and records the bulk evolution of Paleo-Asian Ocean from opening to closure,with multi-stages and multi-types of crust-mantle interaction in the Paleozoic.West Junggar(western part of Junggar Basin),located in the core area of CAOB,is characterized by a multiple intra-oceanic subduction system during the Paleozoic.In its eastern part crop out Devonian-Carboniferous marine sedimentary rocks,Darbut and Karamay ophiolitic melanges,alkali oceanic island basalts,island arc volcanic rocks and thrusted nappe structure.Such lithotectonic associations indicate the occurrence of accretionary wedge at Karamay.In order to decipher its formation mechanism,this paper presents a synthesis of petrography,structural geology and geochemistry of volcanic rocks.In combination with oceanic subduction channel processes,itis suggested that the accretionary wedge is acomposite melange with multiple stages of formation.The application of oceanic subduction channel model to the Karamay accretionary wedge provides new insights into the accretion and orogenesis of CAOB.  相似文献   

7.
Abstract Mesozoic accretionary complexes of the southern Chichibu and the northern Shimanto Belts, widely exposed in the Kanto Mountains, consist of 15 tectonostratigraphic units according to radiolarian biochronologic data. The units show a zonal arrangement of imbricate structure and the age of the terrigenous clastics of each unit indicates successive and systematic southwestward younging. Although rocks in these complexes range in age from Carboniferous to Cretaceous, the trench-fill deposits corresponding to the Hauterivian, the Aptian to Middle Albian and the Turonian are missing. A close relationship between the missing accretionary complexes and the development of strike-slip basins is recognizable. The tectonic nature of the continental margin might have resulted from a change from a convergent into a transform or oblique-slip condition, so that strike-slip basins were formed along the mobile zones on the ancient accretionary complexes. Most terrigenous materials were probably trapped by the strike-slip basins. Then, the accretion of the clastic rock sequence occurred, probably as a result of the small supply of terrigenous materials in the trench. However, in the case of right-angle subduction, terrigenous materials might have been transported to the trench through submarine canyons and deposited there. Thus, the accretionary complexes grew rapidly and thickened. Changes both in oceanic plate motion and in the fluctuation of terrigenous supply due to the sedimentary trap caused pulses of accretionary complex growth during Jurassic and Cretaceous times. In the Kanto Mountains, three tectonic phases are recognized, reflecting the changes of the consuming direction of the oceanic plates along the eastern margin of the Asian continent. These are the Early Jurassic to early Early Cretaceous right-angle subduction of the Izanagi Plate, the Early to early Late Cretaceous strike-slip movement of the Izanagi and Kula Plates, and the late Late Cretaceous right-angle subduction of the Kula Plate.  相似文献   

8.
Abstract In this paper, a summary of the tectonic history of the Mirdita ophiolitic nappe, northern Albania, is proposed by geological and structural data. The Mirdita ophiolitic nappe includes a subophiolite mélange, the Rubik complex, overlain by two ophiolite units, referred to as the Western and Eastern units. Its history started in the Early Triassic with a rifting stage followed by a Middle to Late Triassic oceanic opening between the Adria and Eurasia continental margins. Subsequently, in Early Jurassic time, the oceanic basin was affected by convergence with the development of a subduction zone. The existence of this subduction zone is provided by the occurrence of the supra‐subduction‐zone‐related magmatic sequences found in both the Western and Eastern units of the Mirdita ophiolitic nappe. During the Middle Jurassic, continuous convergence resulted in the obduction of the oceanic lithosphere, in two different stages – the intraoceanic and marginal stages. The intraoceanic stage is characterized by the westward thrusting of a young and still hot section of oceanic lithosphere leading to the development of a metamorphic sole. In the Late Jurassic, the marginal stage developed by the emplacement of the ophiolitic nappe onto the continental margin. During this second stage, the emplacement of the ophiolites resulted in the development of the Rubik complex. In the Early Cretaceous, the final emplacement of the ophiolites was followed by the unconformable sedimentation of the Barremian–Senonian platform carbonate. From the Late Cretaceous to the Middle Miocene, the Mirdita ophiolitic nappe was translated westward during the progressive migration of the deformation front toward the Adria Plate. In the Middle to Late Miocene, a thinning of the whole nappe pile was achieved by extensional tectonics, while the compression was still active in the westernmost areas of the Adria Plate. On the whole, the Miocene deformations resulted in the uplift and exposition of the Mirdita ophiolites as observed today.  相似文献   

9.
Pacific-type orogeny revisited: Miyashiro-type orogeny proposed   总被引：30，自引：0，他引：30  
Shigenori  Maruyama 《Island Arc》1997,6(1):91-120
Abstract The concept of Pacific-type orogeny is revised, based on an assessment of geologic data collected from the Japanese Islands during the past 25 years. The formation of a passive continental margin after the birth of the Pacific Ocean at 600 Ma was followed by the initiation of oceanic plate subduction at 450 Ma. Since then, four episodes of Pacific-type orogeny have occurred to create an orogenic belt 400 km wide that gradually grew both oceanward and downward. The orogenic belt consists mainly of an accretionary complex tectonically interlayered with thin (<2 km thick), subhorizontal, high-P/T regional metamorphic belts. Both the accretionary complex and the high-P/T rocks were intruded by granitoids ～100 million years after the formation of the accretionary complex. The intrusion of calc-alkaline (CA) plutons was synchronous with the exhumation of high-P/T schist belts. Ages from microfossils and K-Ar analysis suggest that the orogenic climax happened at a time of mid-oceanic ridge subduction. The orogenic climax was characterized by the formation of major subhorizontal orogenic structures, the exhumation of high-P/T schist belts by wedge extrusion and subsequent domed uplift, and the intrusion-extrusion of CA magma dominantly produced by slab melting. The orogenic climax ended soon after ridge subduction, and thereafter a new Pacific-type orogeny began. A single Pacific-type orogenic cycle may correspond to the interaction of the Asian continental margin with one major Pacific oceanic plate. Ophiolites in Japan occur as accreted material and are not of island-arc but of plume origin. They presumably formed after the birth of the southern Pacific superplume at 600 Ma, and did not modify the cordilleran-type orogeny in a major way. Microplates, fore-arc slivers, intra-oceanic arc collisions and the opening of back-arc basins clearly contributed to cordilleran orogenesis. However, they were of secondary importance and served only to modify pre-existing major orogenic components. The most important cause of cordilleran-type orogeny is the subduction of a mid-oceanic ridge, by which the volume of continental crust increases through the transfer of granitic melt from the subducting oceanic crust to an orogenic welt. Accretionary complexes are composed mainly of recycled granitic sediments with minor amounts of oceanic material, which indicate that the accretion of oceanic material, including huge oceanic plateaus, was not significant for orogenic growth. Instead, the formation and intrusion of granitoids are the keys to continental growth, which is the most important process in Pacific-type orogeny. Collision-type orogeny does not increase the volume of continental crust. The name ‘Miyashiro-type orogeny’ is proposed for this revised concept of Pacific-type or cordilleran-type orogeny, in order to commemorate Professor A. Miyashiro's many contributions to a better understanding of orogenesis.  相似文献   

10.
Hayato  Ueda  Sumio  Miyashita 《Island Arc》2005,14(4):582-598
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.  相似文献   

11.
The Cretaceous accretionary complexes of the Idonnappu Zone in the Urakawa area are divided into five lithological units, four of which contain greenstone bodies. The Lower Cretaceous Naizawa Complex consists of two lithologic units. The Basaltic Unit (B‐Unit) is a large‐scale tectonic slab of greenstone, consisting of depleted tholeiite similar to that of the Lower Sorachi Ophiolite (basal forearc basin ophiolite) in the Sorachi‐Yezo Belt. The Mixed Unit of Naizawa Complex (MN‐Unit) contains oceanic island‐type alkaline greenstones which occur as slab‐like bodies and faulted blocks with tectonically dismembered trench‐fill sediments. Repeated alternations of the two units in the Naizawa Complex may have been formed by the collision of seamounts with forearc ophiolitic body (Lower Sorachi Ophiolite) in the trench. The Upper Cretaceous Horobetsugawa Complex structurally underlies the Naizawa Complex in its original configuration, and it also contains greenstone bodies. Greenstones in the MH‐Unit occur as blocks and sedimentary clasts in a clastic matrix, and exhibit depleted tholeiite and oceanic‐island alkaline basalt/tholeiite chemistry. This unit is interpreted as submarine slide and debris flow deposits. Greenstones in the PT‐Unit occur at the base of several chert‐clastic successions. Most of the greenstones are severely sheared and show normal‐type mid‐ocean ridge basalt composition. The PT‐Unit greenstones are considered to have been derived from abyssal basement peeled off during accretion. The different accretion mechanism of the greenstones in the Naizawa and Horobetsugawa complexes reflects temporal changes in subduction zone conditions. Seamount accretion and tectonic erosion were dominant in the Early Cretaceous, due to highly oblique subduction of the old oceanic crust and minimal sediment supply. Whereas, thick sediments with minor mid‐ocean ridge basalt and olistostrome accreted in the Late Cretaceous, due to near‐orthogonal subduction of young oceanic crust with voluminous sediment supply.  相似文献   

12.
Tetsuji  Onoue  Hiroyoshi  Sano 《Island Arc》2007,16(1):173-190
Abstract   The Sambosan accretionary complex of southwest Japan was formed during the uppermost Jurassic to lowermost Cretaceous and consists of basaltic rocks, carbonates and siliceous rocks. The Sambosan oceanic rocks were grouped into four stratigraphic successions: (i) Middle Upper Triassic basaltic rock; (ii) Upper Triassic shallow-water limestone; (iii) limestone breccia; and (iv) Middle Middle Triassic to lower Upper Jurassic siliceous rock successions. The basaltic rocks have a geochemical affinity with oceanic island basalt of a normal hotspot origin. The shallow-water limestone, limestone breccia, and siliceous rock successions are interpreted to be sediments on the seamount-top, upper seamount-flank and surrounding ocean floor, respectively. Deposition of the radiolarian chert of the siliceous rock succession took place on the ocean floor in Late Anisian and continued until Middle Jurassic. Oceanic island basalt was erupted to form a seamount by an intraplate volcanism in Late Carnian. Late Triassic shallow-water carbonate sedimentation occurred at the top of this seamount. Accumulation of the radiolarian chert was temporally replaced by Late Carnian to Early Norian deep-water pelagic carbonate sedimentation. Biotic association and lithologic properties of the pelagic carbonates suggest that an enormous production and accumulation of calcareous planktonic biotas occurred in an open-ocean realm of the Panthalassa Ocean in Late Carnian through Early Norian. Upper Norian ribbon chert of the siliceous rock succession contains thin beds of limestone breccia displaced from the shallow-water buildup resting upon the seamount. The shallow-water limestone and siliceous rock successions are nearly coeval with one another and are laterally linked by displaced carbonates in the siliceous rock succession.  相似文献   

13.
The Precambrian and lower Paleozoic units of the Japanese basement such as the Hida Oki and South Kitakami terranes have geological affinities with the eastern Asia continent and particularly strong correlation with units of the South China block. There are also indications from units such as the Hitachi metamorphics of the Abukuma terrane and blocks in the Maizuru terrane that some material may have been derived from the North China block. In addition to magmatism, the Japanese region has seen substantial growth due to tectonic accretion. The accreted units dominantly consist of mudstone and sandstone derived from the continental margin with lesser amounts of basaltic rocks associated with siliceous deep ocean sediments and local limestone. Two main phases of accretionary activity and related metamorphism are recorded in the Jurassic Mino–Tanba–Ashio, Chichibu, and North Kitakami terranes and in the Cretaceous to Neogene Shimanto and Sanbagawa terranes. Other accreted material includes ophiolitic sequences, e.g. the Yakuno ophiolite of the Maizuru terrane, the Oeyama ophiolite of the Sangun terrane, and the Hayachine–Miyamori ophiolite of the South Kitakami terrane, and limestone‐capped ocean plateaus such as the Akiyoshi terrane. The ophiolitic units are likely derived from arc and back‐arc basin settings. There has been no continental collision in Japan, meaning the oceanic subduction record is more complete than in convergent orogens seen in intracontinental settings making this a good place to study the geological record of accretion. Hokkaido lacks most of the Paleozoic history recognized in Honshu, Shikoku, Kyushu, and the Ryukyu Islands to the south and its geology reflects the Cenozoic development of two convergent domains with volcanic arcs, their approach, and eventual collision. The Hidaka terrane reveals a cross section through a volcanic arc and the main accretionary complex of the convergent system is represented by the Sorachi–Yezo terrane.  相似文献   

14.
Detrital zircon multi‐chronology combined with provenance and low‐grade metamorphism analyses enables the reinterpretation of the tectonic evolution of the Cretaceous Shimanto accretionary complex in Southwest Japan. Detrital zircon U–Pb ages and provenance analysis defines the depositional age of trench‐fill turbidites associated with igneous activity in provenance. Periods of low igneous activity are recorded by youngest single grain zircon U–Pb ages (YSG) that approximate or are older than the depositional ages obtained from radiolarian fossil‐bearing mudstone. Periods of intensive igneous activity recorded by youngest cluster U–Pb ages (YC1σ) that correspond to the younger limits of radiolarian ages. The YC1σ U–Pb ages obtained from sandstones within mélange units provide more accurate younger depositional ages than radiolarian ages derived from mudstone. Determining true depositional ages requires a combination of fossil data, detrital zircon ages, and provenance information. Fission‐track ages using zircons estimated YC1σ U–Pb ages are useful for assessing depositional and annealing ages for the low‐grade metamorphosed accretionary complex. These new dating presented here indicates the following tectonic history of the accretionary wedge. Evolution of the Shimanto accretionary complex from the Albian to the Turonian was caused by the subduction of the Izanagi plate, a process that supplied sediments via the erosion of Permian and Triassic to Early Jurassic granitic rocks and the eruption of minor amounts of Early Cretaceous intermediate volcanic rocks. The complex subsequently underwent intensive igneous activity from the Coniacian to the early Paleocene as a result of the subduction of a hot and young oceanic slab, such as the Kula–Pacific plate. Finally, the major out‐of‐sequence thrusts of the Fukase Fault and the Aki Tectonic Line formed after the middle Eocene, and this reactivation of the Shimanto accretionary complex as a result of the subduction of the Pacific plate.  相似文献   

15.
Abstract The Shimanto accretionary complex on the Muroto Peninsula of Shikoku comprises two major units of Tertiary strata: the Murotohanto Sub-belt (Eocene-Oligocene) and the Nabae Sub-belt (Oligocene-Miocene). Both sub-belts have been affected by thermal overprints following the peak of accretion-related deformation. Palaeotemperatures for the entire Tertiary section range from ～ 140 to 315°C, based upon mean vitrinite reflectance values of 0.9–5.0%Rm. Values of illite crystallinity index are consistent with conditions of advanced diagenesis and anchimetamorphism. Illite/mica b0 lattice dimensions indicate that burial pressures were probably no greater than 2.5kbar. In general, levels of thermal maturity are higher for the Murotohanto Sub-belt than for the Nabae Sub-belt. The Eocene-Oligocene strata also display a spatial decrease in thermal maturity from south to north and this pattern probably was caused by regional-scale differential uplift following peak heating. Conversely, the palaeothermal structure within the Nabae Sub-belt is fairly uniform, except for the local effects of mafic intrusions at the tip of Cape Muroto. There is a paleotemperature difference of ～ 90°C across the boundary between the Murotohanto and Nabae Sub-belts (Shiina-Narashi fault), and this contrast is consistent with approximately 1200 m of post-metamorphic vertical offset. Subduction prior to Middle Miocene probably involved the Kula or fused Kula-Pacific plate and the background geothermal gradient during the Eocene-Oligocene phase of accretion was ～ 30–35°C/km. Rapid heating of the Shimanto Belt evidently occurred immediately after a Middle Miocene reorganization of the subduction boundary. Hot oceanic lithosphere from the Shikoku Basin first entered the subduction zone at ～ 15 Ma; this event also coincided with the opening of the Sea of Japan and the rapid clockwise rotation of southwest Japan. The background geothermal gradient at that time was ～ 70°C/km. Whether or not all portions of the inherited (Eocene-Oligocene) palaeothermal structure were overprinted during the Middle Miocene remains controversial.  相似文献   

16.
Abstract Greenstones, representing remnants of paleo-oceanic crust, occur in Permian and Jurassic accretionary complexes of the Inner Zone in the Southwestern Japan arc. The formation age of most of the greenstones is early Carboniferous, based on fossil ages for overlying limestones and Sm-Nd isotope ages of the greenstones themselves. The geochemistry of such greenstones is similar to those of present-day oceanic islands. Greenstones of the Permian accretionary complex (Akiyoshi belt) are alkalic and tholeiitic in composition. Some alkali basalts show peculiar features from an EM-1 mantle source, such as the Gough Island and Tristan da Chunha basalts in the South Atlantic. Greenstones of the Jurassic accretionary complex (Tamba belt) are also alkali and tholeiitic basalts with both basalt types in the northern part of the Tamba belt coming from strongly depleted characters similar to a mid-ocean ridge basalt source mantle. The variable geochemistry of the oceanic basalts is explained by hypothesis on existence of a Carboniferous mantle plume below the spreading ridge which divides the Farallon and Izanagi plates. The Akiyoshi belt seamounts and/or oceanic islands of the Farallon plate and Tamba belt seamounts and/or oceanic islands of the Izanagi plate formed simultaneously by the upwelling of the thermal plume. Some part of the Akiyoshi belt basalts originated locally from an EM-1 mantle source, while basalts from the northern parts of the Tamba belt have a normal-type mid-ocean ridge basalt (N-MORB) source component. Existence of an N-MORB signature is consistent with the presence of a spreading center in a Carboniferous 'Pacific Ocean' that caused separation of the Farallon and Izanagi plates. Disparity in accretion ages of the basaltic rocks in the Permian and Jurassic may have been caused by differences in the relative motion of the two plates.  相似文献   

17.
Ladakh (India) provides a complete geological section through the northwestern part of the Himalayas from Kashmir to Tibet. Within this section the magmatic, metamorphic and geotectonic evolution of the northern Himalayan orogeny has been studied using petrographic, geochemical and isotope analytical techniques.The beginning of the Himalayan cycle was marked by large basaltic extrusions (Panjal Trap) of Permian to Lower Triassic age at the “northern” margin of the Gondwana continent (Indian Shield). These continental type tholeiitic basalts were followed by a more alkaline volcanism within the Triassic to Jurassic Lamayuru unit of the Gondwana continental margin.Lower Jurassic to Cretaceous oceanic crust and sediments (ophiolitic mélange s.s.) accompany the Triassic to Cretaceous flysch deposits within the Indus-Tsangpo suture zone, the major structural divide between the Indian Shield (High Himalaya) and the Tibetan Platform. So far, no relic of Paleozoic oceanic crust has been found.Subduction of the Tethyan oceanic crust during Upper Jurassic and Cretaceous time produced an island arc represented by tholeiitic and calc-alkaline volcanic rock series (Dras volcanics) and related intrusives accompanied by volcaniclastic flysch deposits towards the Tibetan continental margin.Subsequent to the subduction of oceanic crust, large volumes of calc-alkaline plutons (Trans-Himalayan or Kangdese plutons) intruded the Tibetan continental margin over a distance of 2000 km and partly the Dras island arc in the Ladakh region.The collision of the Indian Shield and Tibetan Platform started during the middle to upper Eocene and caused large-scale, still active intracrustal thrusting as well as the piling up of the Himalayan nappes. The tectonically highest of these nappes is built up of oceanic crust and huge slices of peridotitic oceanic mantle (Spongtang klippe).In the High Himalayas the tectonic activity was accompanied and outlasted by a Barrovian-type metamorphism that affected Triassic sediments of the Kashmir-Nun-Kun synclinorium up to kyanite/staurolite grade and the deeper-seated units up to sillimanite grade. Cooling ages of micas are around 20 m.y. (muscovite) and 13 m.y. (biotite). Towards the Indus-Tsangpo suture zone metamorphism decreases with no obvious discontinuity through greenschist, prehnite-pumpellyite to zeolite grade. Remnants of possibly an Eo-Himalayan blueschist metamorphism have been found within thrust zones accompanying ophiolitic mélange in the suture zone.  相似文献   

18.
Lawrence R.  Zamoras  Atsushi  Matsuoka 《Island Arc》2004,13(4):506-519
Abstract   Upper Paleozoic to Mesozoic sedimentary sequences of chert (Liminangcong Formation), clastics (Guinlo Formation) and a number of limestone units (Coron Formation, Minilog Formation and Malajon Limestone) constitute the accretionary complex of the North Palawan block, Philippines. Based on chert-to-clastic transitions from different stratigraphic sequences around the Calamian Islands, three accretionary belts are delineated: the Northern Busuanga Belt (NBB), the Middle Busuanga Belt (MBB) and the Southern Busuanga Belt (SBB). The accretion events of these belts along the East Asian accretionary complex, indicated by their sedimentary transitions, began with the Middle Jurassic NBB accretion, followed by the Late Jurassic MBB accretion and the Early Cretaceous SBB accretion. Several limestone blocks that formed over the seamounts became juxtaposed with chert–clastic sequences during accretion. During the Late Cretaceous, accretion-subduction along the East Asian margin subsided bringing tectonic stability to the region. The seafloor spreading during the mid-Oligocene disconnected the entire North Palawan block from the Asian mainland and then migrated southward. The collision between the North Palawan block and the Philippine Island Arc system in the middle Miocene generated a megafold structure in the Calamian Islands as a result of the clockwise turn of the accretionary belts in the eastern Calamian from originally northeast–southwest to northwest–southeast.  相似文献   

19.
Yukio  Isozaki 《Island Arc》1997,6(1):2-24
Abstract Proto-Japan originated from a continental margin of the Neoproterozoic Yangtze (South China) craton. It represents a unique Permo-Triassic tectonic setting in western Panthalassa, where two distinct types of orogenic belt occurred side by side. There was an accretionary orogen between the Yangtze craton and the Proto-Pacific (Farallon) Plate and a collisional orogen between the Sino-Korean (North China) and Yangtze cratons. This article reviews results of the latest on-land geological studies concerning Permo-Triassic tectonics in Japan and proposes a new plate tectonic interpretation as well as a paleogeographic reconstruction of this particularly unique geotectonic regime. Special emphases are given to (i) the accretion processes and products derived by collision-subduction of the Permian Akiyoshi paleoseamount and Maizuru paleo-oceanic plateau; (ii) the field occurrence of 220-Ma Sangun high-P/T schists and its implication for the exhumation process and 'tectonic sandwich' structure; (iii) the extensive development of a subhorizontal nappe of the pre-Jurassic rocks and their bearing on the orogenic edifice; and (iv) the restricted occurrence of the 250-Ma collision complex in the Hida and Oki belts and the relevant connection to the Precambrian cratons and collision suture in East Asia. The newly proposed paleogeographic reconstruction is also tested by faunal provinciality of Permo-Triassic fossils from shallow-water sediments.  相似文献   

20.
Sergei V.  Zyabrev 《Island Arc》1996,5(2):140-155
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.  相似文献   

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