難 Bulletin o£the Geological Survey of Japan，vol，39（5），P。269－365，1988 551．242／．46／．782（265。54） 響

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Geologieal strueture of the Japan Seaεヒ簸〔l 馨

itste伽nieim：plieations 襲

窯 Kensaku TAMAKI＊

TAMAKI，K』． （1988） Geological structure of the Japan Sea and its tectonic 懸 implications．B翅．σεoZ．Sμrひ．」αpαη，vo1．39（5），p．269－365。

Abstract：：Extensive and new marine geolog・ical and geophysical data of the 藝 Japan Sea have been compiled into geolog・ical maps．The tectonic evolution of 華 the Japan Sea has been examined based on observation and interpretation of its geological structure． 襲 The ages of formation of the basins in the Japan Sea are estimated from sediment stratigraphy，basement depth，and heat flow data．They are from30to 15Ma for the Japan Basin，30to10Ma for the Yamato Basin，and nearly comparable age ranges for the Tartary Trough and the Tsushima Basin． Topographic highs in the Japan Sea are classified into four groups； continental fragments，rifted continental fragments，tectonic ridges，an（1volcanic seamounts．Continental fragments are large topographic features and composed of older rocks including those of Precambrian age。Rifted continental fragments are distributed at midwater depth，primarily along the basin margins because of their transitional structures between continental and oceanic crusts．Tectonic ridges exist along the eastem margin of the Japan Sea where they have been elevated by convergence since latest Pliocene．Volcanic seamounts are abundant in the basins． Two types of back－arc spread，ing are proposed l a single rift type and multi rift type。The multi rift type best fits the observed structures of the Japan Sea． Large tensional stresses over a broa（1volcanic zone in the former islan（l arc caused a multi rift system．Some of the rifts developed into multi back－arc spreading systems while others became aborted rifts．The broad volcanic zone of the Japanese islan（1arcs overprinte（l the activity of back－arc sprea（1ing，（iue to a shallow subduction angle．Arc volcanism during・the Japan Sea opening produced a thick accumulation of volcaniclastic sediments and abundant seamounts and knolls within the basins．This multi back－arc spreading system resulted in the fragmentation of the continental crust and isolation of continental fragments within the Japan Sea． The large tensional stresses necessary for the above evolution are possible only by retreat of the continental plate． The back－arc continental plate of the Japan Sea，the Amurian Block，is postulated to have retreated northward due to the India－Eurasia collision and associated lithospheric deformation in East Asia． The northward movement of the block generated the Stanovoy Range along its northem margin by collision with Siberia and a pull－apart basin of the Baikal Rift along its western margin． The Okushiri and Sado Ridges along the eastem margin of the Japan Sea are boun（1e（1 by thrust faults on either or both si（1es． These thrust faults have

＊Marine Geology Department（presently at Ocean Research Institute，University of Tokyo）

一269一 B翅ε伽oμんεGθoZo9‘oαZ8αrひθッ〇四⑫αη。VoZ．39，ノVo，5

been active since latest Pliocene time．The ridges are formed by thrust movements associated with the uplift of the edge of the hanging side over the footwal1． Lithospheric convergence is evi（ient along these thrust zones on the basis of the occurrence of thrust faults an（i their correspon（ling earth（luakes． The convergent stress is again inferre（1 to be （1ue to the India－Eurasia collision an（1associate（1 intra－plate or inter microplate movements in：East Asia。The eastward movement of the Amurian Plate have caused Baikal extension along its westem margin and Japan Sea convergence along its eastern margin。 The tectonic evolution of the Japan Sea is summarized in two significant teCtOniC StageS。 1） Divergent tectonics over the J’apanese island arcs caused back－arc spreading of the Japan Sea from30to10Ma．The spreading system was initiated from a multi rift system within a broad arc volcanic zone．Through the development of the spreading system，many continental fragments are left within the basins。 Overprinting of arc volcanism on the back－arc basin resulted in an abundance of volcanic seamounts and＆thick accumulation of volcaniclastics in the basins． 2） Lithospheric convergence along the eastern margin of the Japan Sea since latest Pliocene time produced the uplifted and thrust faulted Okushiri and S＆do Ridges。

1。 1聡trod．uct童o賂 Sea are weak and complicated（IsEzAKI andUYEDA，1973）． Most of the island arcs in the Westem The tectonic evolution of the Japan Pacific are associated，with marginal seas Sea has a close relation to the tectonics or back－arc basins． The Japan Sea is of the Japanese Islands，and its ambigu－ one of the back－arc basins of the West－ ity has caused divergent models of tec－ ern Pacific． Present geometry shows tonic evolution of the Japanese Islan（1s that the Japan Sea is situated．between and，ad．jacent areas． Although it is still the Eurasia Continent an（1the islands of di缶cult to constrain the age of the Ja－ Sakhalin，Hokkai（10，Honshu，an（i others． pan Sea from the data presently availa－ The back－arc basins of the Westem ble，a better un（1erstan（1ing of the geo－ Pacific hεしve been studie（1intensively since 10gy an（1 tectonics of the Japan Sea is the1960’s，an（1，in particular，results of in（1ispensable for the stu（1y of the geolo－ the Deep Sea Drilling Project and the gical evolution of the Japanese Islan（1s identification of geomagnetic anomaly and the Westem Paci且c． lineations have presente（1 effective con－ In this paper，the author presents new straints for the age determination of the and extensive marine geological an（1 back－arc basins such as the Shikoku geophysica1（1εlta of the Jεしpan Sea and Basin，the Parece Vela Basin，the West discuss its geological structure an（l tec－ Philippine Basin，an（l the South China tonics．The substantial improvement of Sea． The age of the Japan Sea，together this paper over previous works is the with the Kuril Basin，however，is still stratigraphic correlation of the se（limen－ enigma and．controversiaL It is because tary se（luence of the Japan Seεしon the four（leep sea（1rilling sites in the Japan basis of the synthetic analyses of seismic Sea could’not reached the basement pro且1es and．age estimated from bottom （：KARIG，INGLEαα1．，1973）and the mag－ sampling data．The stratigraphic con－ netic anomaly lineations of the Japan straints de（luce（1are use（1effectively，in

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this paper， for the （1iscussion of the continental slopeεし10ng the Eurasia Con－ 馨 tectonic evolution of the Japan Sea． tinent and that along the eastem margin of the Japan Sea are remarkable l that 1．1 ］Bathymetry of the Japan Sea is，the former is very simple and gener－ The Ja．pan Sea shows a d．epressional ally smooth without rugged．ness，while topographicfeature（Fig．1and2）．A the latter is very complicated with many 200mcontourlinesurroundingtheJa． ri（1ges an（i troughs tren（iing north－south． pan Sea shows the closed depressional The ridges along the eastem margin of 誕 feature of the Japan Sea． The Japan the Japan Sea are the Okushiri Ridge 議 Sea is characterized by a complicated and the Sado Ridge．Many troughs are sea noor topography with many to－ distribute（l in the area east of the pographic expressions of ri（lges，rises， Okushiri and Sado Ridges．Some of banks，and seamounts．It makes the them are，from the north，the Shiribeshi Japan Sea the most topographically Trough，the Okushiri Trough，the Nishi－ complicated marginal basin of the world。 tsugaru Trough，an（l the Mogami Trough． Physiographic nomenclatures ad．opted The Musashi Bank and the Oshima in this paper are basically those of Plateau also lie along the eastern margin LuDwIG θ6α1． （1975） and．GNIBIDENKo of the Japan Sea． This topographic con－ （1979）． The autho：r，however，used．ten－ trast between the eastem and western tative nomenclature in some cases such margins of the Japan Sea suggests neo－ as the Ullung Plateau and the Tsushima tectonic d．eformation along the eastem Deep Seεl ChanneL margin，which will be discussed later in There are four major topographic （letaiL d．epressions and two malor topographic highs in the Japan Sea． The four （1e－ 勘Pα轟挽s厩 pressions are the Japan Basin， the The Japan Basin is the1εlrgest（lepres． Yamato Basin，the Tsushima Basin，and sion in the Japan Sea． It is locate（i in the Tartary Trough．The two largest the central part of the Japan Sea an（1 highs are the Yamato Rise in the center trend．s ENE． The wid．th of the J’apan of the Japεしn Sea and the Korea Plateau Basin is200to300km and its length is in the western part of the Japan Sea． about700km．The eastem half of the Many other topographic highs and de． basin is the （leepest part of the Jεlpan pressions are （1istribute（1 in the Japan Sea．The wεlter depth of the basin Sea． ranges from3500to3700m．The great． In the southwestem Japan Sea，the est water depth of the Japanβea is topography is rather complicated．with 3742mand．isobservedintheeastem－ many topographic highs and．depres－ most area．The Japan Basin is for the sions． The topographic highs are the most part characterized by a smooth Yamato Rise，the Korea Plateau，the and且at sea且oor which forms the abyss一 Ullung Plateau，the Kita－Oki Bank，the ＆1Plain （1ue to the （1eposition of（1istal Oki Bank，and，the Oki Ridge． Seve：ral turbi（1ites． basins and troughs are developed among Several large （ieep sea channels an（i the topographic highs． They are the cεlnyons flow into the abyssal plain． Yamato Basin，the Tsushima Basin，the They are the Toyama Deep Sea Channel Genzan Trough，and the Oki Trough． from Honshu，the Tsushima Deep Sea Physiographic cont：rasts between the Channel from the Tsushima Strait

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（“Tsushima Deep Sea Channe1”is tenta－ two separated abyssal plains with a．flat tive nεming in this text），the Genzan and smooth sea且oor．The water depth Canyon from the：Korea Peninsula，and of the deeper abyssal plain in the west the Tartary Trough from Sikhote－Alin． central part of the basin is about3000m， Several smaller Channels are observed whichisabout500mlessthanthatof on the continental slopes off Sikhote－ the abyssal plain of the Japan Basin． Alin and Japanese Islands．One of them The shallower abyssal plain lies at a is the Mogami Channel o貸northeast depth of about2700m and is in the Honshu．The Jεlpan Basin is the present eastem part of the Yamato Basin．The depositionε11center of the Japan Sea． northeastem part of the Yamato Basin There are several seamounts in the is shallower than the central part of the Japan Basin．The larger ones are named． Basin，showing a water d．epth of2500m but smaller ones are unnamed．The an（i is characterize（1by the presence of largest one among them is the Siberia the meandering Toyama Deep Sea Chan－ Seamount off Nakhod．ka of Sikhote－Alin． neL The chamel comes from the Toya－ The seamount is about2000m high above ma Canyon in the Toyama Trough，runs the surroun（iing basin noor． The next across the northeastern part of the largest seamount is the Vityaz Seamount Yamato Basin，and extends into the which is located．just on the bound，ary Japan Basin． between the Japan Basin and the Ta：r－ A seamount chain with a ENE－WSW tary Trough．The seamountis2000m tren（i is locate（1 near the axis of the high above the basin noor an（1 higher Yamato Basin． The author tentatively than the Siberia Seamount． calls it“Yamato seamount chain”．The The other1＆rge seamounts in the Ja－ largest seamount in it is the Yamato pan Basin are the Bogorov Seamount Seamount which is located in the westem and the Gabass Seamounts．The Bogorov part．The Seamount is about2000m Seamount is isolated in the eastem J’a－ high above the basin Hoor．The Matsu pan Basin．The Seamount，2300m high Seamount with a height of about1000m above the basin且oor，is elongated and is in the eastern en（i of the chεlin． trend，s north－south with a length of70 km and a width of20km．The Gabass Tα融昭Tro膨φ Seamount is located in the southwestern The Tartary Trough，the secon（i larg－ part of the Japan Basin．The Seamount est bεlsin in the Japan Sea，apPears as a is about1500m high above the basin north－northeasterly elongate（1 （1epres－ floor． Several other unname（1seamounts sion between Sikhote－Alin and． the areobse：rved．intheJ’apanBasin．A Sakhalin－Hokkaid，o Islands． The water seamount north of the Oki Bank ls depth of the Tartary Trough is generally prominent among them． shallower than those of the othe：r th：ree major basins．The trough bottom deep． yα肌α彦oBαs加 ens from north to south and attains a The Yamato Basin is developed．in the maximum depth of3500m at the junction southern part of the Japan Sea， and with the Japan Sea． The Tarta：ry Can－ extends from off the Shimane Peninsula yon is （levelope（1 along the axis of the to off Oga Peninsula of Honshu． The Trough，as a（1iscontinuous feature。 Bεlsin trends northeasterly．Its width is 200to100km．The Yamato Basin has

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Figure2 3－D topographic ma．p of the whole Japan Sea。 The da．ta used for processing are GEBCO digital bathymetric data of JOCD and GSJ digital bathymetric data． Processing system is SIGMA of Geological Survey of Japan。

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Tsμs配1ηαBαs加 second stage is spreading of the Yamato The Tsushima Basin situated on the basin during about Early Miocene time． southwestern comer of the Japan Sea is Their d．iscussion on the age，however，as the smallest of the four malor basins． they have mentioned，is ambiguous due The dimension of the basin is200km EW to the lack of data on age assignment of ×150km NS．The basin floor deepens the marine sediments and rocks．They to the north，from1000m to2300m．The pointed out that the sediments of the basin opens northeastwards and joins to Japan Basin thicken from west to east the Japan Basin through a passage be－ an（i that there is rough，uneven base－ tween the Okli Bεlnk and the Ullung Pla－ ment along western margin which they teau．The Tsushima Deep Sea Channel supPose（i to be a fossil sprea（iing center． is（ievelope（i at the axis of this passage． LuDwIG αα1． （1975） P：resented．high Four seamounts are arranged in a quality seismic reflection data an（1sono－ WNW trend at the northem part of the buoy refraction record． They stud．ied． Tsushima Basin．Two of them are ele－ the crustal structure of the Jεlpan Basin， vated above sea level with a height great－ the Yamato Basin，and the Yamato Rise erthan2000mabovetheseaHoorand on the basis of sixty－five sono－buoy make Ullung Island and Takeshima refraCtiOn／reneCtiOn reCOrdS an（i exten－ Islan（1． sive seismic reflection coverage．They concluded that the Japan Basin an（l the 1．2 Previous works Yamato Basin are underlain by ocea．n－ Numerous studies on the marine geo1． ic crusts．It was emphasized that the ogy an（1 geophysics of the Japan Sea smooth basement of the Japan Basin have been previously done．Most of them and the Yamato Basin has a3．5km／sec have been made by Russian and Ja－ refrεlction velocity an（i that this velocity panese scientists． Several works have is unique to the Japεln Sea． been done by American scientists．The Two other synthetic works were done stu（iies cεln be largely（iivi（ied into three by MELANKHoLINA and KovYLIN（1977） categories＝geological structure，bottom and．GNIBIDENKo （1979），although they sampling，and geophysics． have not presente（i the originεll results of their s皿vey cruises．Melankholina oεoZo9♂cαZs孟rμc加rε and：Kovylin compiled a simple tectonic Synthetic works on the marine geo． province map of the Japan Sea．Similar 10gical structure of the entire Japan Sea works have been done by other Russian weredonebyHILDEandWAGEMAN（1973） scientists （e．9．，BERsENEvεlnd．LELIKov， an（1 LuDwIG αα1． （1975）． HILDE εlnd． 19791MARKovααZ．，1979；IvANovαα1．， WAGEMAN carried out regional seismic 1981）．GNIBIDENKo （1979）made a de－ reHection surveys of the Japan Sea for tailed sediment isopach map of the entire the first time and presented an outline Japan Sea，although the original data of the geologicε11structure． They sug－ were not presented，．Most of the Russian gested riftingεln（i sprea（iing as the ori－ scientists do not agree with the sea floor gin of the Japan Sea。 They propose（1 sprea（1ing origin of the Japan Sea，but two spreading stages of the Japan Sea； consi（ler that the oceanization of the the丘rst stage is sprea（iing of the Japan continental crust generate（1 the oceanic Basin and the Tsushima Basin during crust of the Japan Sea． Theεlges of the Mesozoic or early Tertiary time and the oceanic crust estimated by them are

一274一 GεoZo8’‘oαZs伽伽rθoμんeJiαPαηSeααηdあs孟θcカo胱伽頑c翻oηs（K．Tαmα勧 rather oldl e．g．，Paleozoic（M肌ANKHoLINA mation （MINAMI，19791TANAKA，19791 and KovYLIN，1977），Early Mesozoic SuzuKI，1979；TANAKA and OGusA，1981）． （GNIBIDENKo，1979），etc．KoBAYAsHI（1985） The Geological Survey of Japan car－ also（liscusse（i the general geological an（1 rie（10ut extensive marine geological sur－ geophysical aspect of the Japan Sea． veys in the late 1970’s which includ．e Many works on the geological struc． seismic profiling，Sono－buoy refraction tures of local areas of the Japan Sea by measurements，bottom sampling，geo－ seismic methods have been done by magnetiC meaSUrementS，and graVity Japanese scientists．HoTTA（1967）carrie（i measurements（HoNzAed．，1978ab；HoNzA out the first seismic reflection survey of ed．，1979）．The work of this paper is the Japan Sea at the Yamato Basin and． based mainly on the data obtained by the Sad．o Ridge． MuRAucHIαα1．（1970） these surveys． The surveye（1area covers an（1H：oTTA（1971），successively，presente（i most of the Japan Sea exclu（1ing the seismic profiles of the Oki Bank and the area along the coast of Sikhote－Alin． Tsushima Basin，and．the Okushiri Ri（lge The d．ata have been compiled into three and the Musashi Bank，respectively． marine geological maps（HoNzAαα1．， Although the seismic penetration of these 197g l TAMAKIε6α1．，1979a l TAMAKIαα1．， profiles was weak，the distribution of 1981a，），which have reveale（1the d．etailed． the sedimentary layer in the eastem and marine geology of the Japan Sea for the southern margins of the Japan Sea was 且rst time． studied．preced．ing the work of HILDE and． WAGEMAN（1973）．Many detailed seis－ ηo麓o禰8佛ρ伽σ mic profiling surveys at the continental Many bottom sampling surveys by shelves and slopes along the Jεlpanese dre（1ge hauls have been（ione（1uring this Islan（is have been carried out by the half of the century mainly by Japanese Hydrographic Department of Japan in and Russian scientists． In the early the1970’s（SATo，19711SAKuRAI an（1SATo， stages of investigation the （lata were 19711SAKuRAIθ6αZ．，1971；SAKuRAIαα1．， concentrated．on the Yamato Rise，the 19721 SATo ε6αZ．， 19731 KATsu頁A an（i Kita－Oki Bεlnk，εlnd the Oki Ridge（TsuYA， KITAHARA， 1977； TozAKI αα1．， 1978）． 19321NIINo，1933，1935，an（i1942；SATo These surveys presented．（1etaile（1data and，ONo，19641HosHINo an（1HoNMA， on the topography an（l geological struc－ 1966）． IwABucHI （1968） presente（1 d．e－ ture of the slopes along the Jεlpanese tailed bottom sampling data on the Sado Islands．INouE（1982）discussed the ma－ Ridge，the Yamato Rise，and their rine geological structure of the south－ SUrrOUn（iing areaS． westem comer of the Jεしpan Sea on the The Geological Su：rvey of Japan ca：r－ basis of the synthetic stu（1y of lan（1εln（i ried out extensive bottom sampling sur』 marinegeology． veys by dredge and coring methods The recent works of seve：ral Japanese （HoNzA ed．，1978ab l HoNzA e（i．，1979）． oil companies are outstanding．Although Bottom sampling by piston and gravity the stu（1ie（1 areas are restricte（l to the cores wεls carried out for stratigraphic continental shelf along the Japanese Is－ correlation of seismic profile data． 1an（1s，their high（luality an（1（leep pene－ KloIzuMI （1979） presented．age（lata for tration of seismic profiles，together with the numerous bottom sediment samples boring （1ata for oil prospecting，have including the d．ata of the Geological presented important stratigraphic infor一 Survey of J’apan on the basis of diatom

一275一 B砿ZZεむ‘ηqブむんεσ20Zo9∫oαZ SαrひθツoゾJd∫）αη、． γo Z．39，No．5 assemblage．The assigned ages by backl－arc basins．IsEzAKI and UYEDA Koizumi have presented an important （1973）suggested．the p：resence of weak information for the stratigraphic cor－ magnetic anomaly lineations trending relation of seismic profiles in this paper． N600E in the Japan Seε1，although they Other important data on bottom sam－ cou1（1not i（1entify the age of the linea－ pling are the results of the Deep Sea tions due to weak amplitudes less than Drilling Projects （：KARIG，INGLE αα1．， 300nT p－p an（i the lack of pronounce（1 1975）． Four sites we：re d．：rille（1in the 1inearity．IsEzAKI（1975）postulated．two Japan Sea during the Glomar Challenger sprea（1ing centers on the basis of the cruise Leg31． Site299is in the north－ correlation of the magnetic anomaly eastern part of the Yamato Basin，Sites peaks l one is in the Japan Bεlsin an（l 300an（i301are in the Japan Basin north the other is along eastern half of the of the Yamato Rise，and Site302is on Yamato Basin．IsEzAKI（1979）further the eastem margin of the Yamato Rise． suggeste（1 that the age of a fossil Sites299，300an（1301（ii（l not reach the sprea（1ing center of the Japan Basin basement rocks．Sites299and301，how－ might be30．6Ma with greαt ambiguity． ever，penetrεlte（1half to one thir（l of the Thus，the results strongly suggest the sedimentary layer and have presented sea floor sprea（1ing origin of the Japan important data for the stratigraphy of Sea．The age of the spread，ing，however， the basin sediments． is still controversia1． Thirty－five radiometric age determina－ G：ravity d．ata in the Japan Sea was tions of the sampled rocks by the：K－Ar compile（l by STRoYEv （1971），ToMoDA and Rb－Sr methods have been done by （1973），ToMoDA and．FuJIMoTo （1981）， Japanese and Russiεln scientists（SHIMAzu， and I sHIHARA （1983）． The Japan Sea is 1968；UENoθ6α1．，1971；OzIMAαα1．， considered to be isostatically compen－ 19721LELIKovan（iBERsENEv，19731SAHNo sated． and VAsILIEv， 19741 VAsILIEv，19751 Heat flow measurements of the Japan YuAsAαα1．，1978；HoNzA ed．，1978a； Sea have been done actively from the GNIBIDENKo，1979）．The age－determined 1960’s （YAsulααZ．1967；UYEDA and rocks are volcanic，intrusive，and met－ VAQuIER，19681YosHII，1972）．The heat amorphic rocks sampled from the Ya－ flow of the Japan Sea is remarkably mato Rise，the：Kita－Oki Bank，the：Korea high compared to that of the Westem Plateau，the Ullung Plateau，the Musashi Pacific Basin． Theεlve：rage heat flow of Bank，the Bogorov Seamount，the Gabass the Japan Basin is2．23±0．52HFU（YAsuI Seamount，and a few seamounts in the ααZ，1967）and that of the Yamato Yamato Basin．The data are referred Basin is2．5HFU．The high heat flow of to in（ietail in this paper． the Japan Sea has been （1iscusse（i by many authors （e．9．，KoBAYAsHI and （7εqρh〃sεcs NoMuRA，1972；KoNoand．AMANo，1977）． There have been many works on the Extensive crustal stu（1ies of the Japan geophysical surveys of the Japan Sea in Sea by seismic refraction methods have terms of geomagnetics，gravity，heat been（10ne by many Russian scientists flow， c：rustaユ study， and．earthquake （e．9．，TuEzov，196glBIKKENIAθ6α1．，19681 research． TuEzov，19711RoDNIKov and KHAIN， Geomagnetic anomaly lineations are 1971）． The：results have shown that the important for the age assignment of Japan Basin （10es not have a granitic

一276一 σ20Zo8‘cαZs惚伽reo∫孟んeJαPαηSεαα認あsむ2c古o肋漉p伽α孟εoηs（K。Tαmα観）

1ayer and that the Yamato Basin has a be un（lerlain by oceanic to suboceanic thin granitic layer．MuRAucHI（1972）， crust． however，suggested that both the Japan Distribution and focal mechanisms of Basin and the Yamato Basin have oce． the earthquakes present critical imfor－ anic crust an（i that the crust of the mation for recent teCtonics．FUKAO and Yamato Basin is thicke：r than that of the FuRuMoTo （1975） investigated． source Japan Basin，The Tsushima Basin also mechanisms of large earthquakes along apPears to have oceanic crust according the eastem margin of the Japan Sea to LuDwIGαα1．（1975）． BERsENEvααZ． an（i suggested that the lithosphere of （1970） postulate（i that suboceanic crust the Japan Sea is detached from the lies beneath the axis of the Tartary northern Japan Arc by large reverse Trough．Then，four major basins and faults．This suggestion is important for troughs of the Japan Sea are inferre（i to the neotectonics of the Japan Sea． The

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35。 6。響36 o 、4～鯵 、 く善3 Ll7 づ 『 L5 o L4 （〉 32。 127D 130。 1350 140D 144。E

Figure3 Ship7s track chart with line numbers of GH77－2，GH77－3，GH78－2and GH78－3 crulses。

一277一 BαZZeむ‘π01’乙h，ε Geo Zo9君cαZ S砿ruεy o∫」αPαη、． ・γo乙．39，1〉o．5

thickness of the lithosphere of the Japan mined by the analyses of diatom Sea was estimated to be30km by surface assemblages（KoIzuMI，1979）and pollen wave analyses of ea：rthq．uakes（ABE and． assemblages（HoNzA ed．，1979）．The age KANAMoRI，1970）． assignment data presented important constraints for the submarine stratigra－ 1．3D我t＆usedi龍掘sst翻y phic correlations． The（iata use（1in this paper obtεline（i Outlines of these surveys were pre－ by the Geologicε11Survey of Japan（1ur－ sented in three cruise reports （HoNzA ing four research cruise from1977to e（i．，1978ab l HoNzA e（1．，1979）． Synthetic 1978． The four research cruises are studies of seismic profiles and bottom GH77－2，GH77－3，GH78－2，and GH78－ sampling data were summarized into 3cruises of R／V Hakurei－maru．The three marine geological maps of the cruises were the first systematic marine Geological Survey of Japan，Marine geological an（1 geophysical surveys of Geology Map Series（HoNzAθεα1．，1979； the Japan Seε1． The surveys consist of TAMAKIαα1．，1979a；TAMAKIααZ．，1981 seismic profiling，sono－buoy refraction a）． meaSUrementS，geOmagnetiC meaSUre－ ments，gravity measurements，and bot－ 2．Desαi画o髄ofGeo亘09量ealS伽ct田e tom sampling． o£theJ即踊Se鼠 Track lines for geophysical surveys are shown in Figure3． The intervals of 2．1Co藍曲io聡ofse量smieμo飾聡9 the traverse lines are every 15nautical Seismic reflection pro丘1es are fun（la－ miles．The ship speed during the survery mental data source in this study．Seismic was maintained at10knots．Position promes，which present visual subbottom Hxing of the ship was done every30 sections，are most important for the minutes by a combination of satellite study of marine geological structures． navigation（NNSS）and Loran C． Along Seismic profiling data used in this study these tracks， seismic pro且1ing （lata， were obtained by the continuous seismic geomagnetic d．ata，and gravity d，ata we：re reflection survey method． Airguns were obtaine（i． Three sono－buoy refraction use（1 as the soun（l source． The e（luip－ measurements were carried out in the ment and conditions during the research Tsushima Basin，the Mogami Trough， cruises GH77－2，GH77－3，GH78－2，and and，the Tartary Trough． GH78－3are listed．in Table1． The seis－ The apparatus for bottom sampling mic recording system achieves high sen－ are the （1redge， gravity corer （core sitivity （high signal to noise ratio） by length：2m），and piston corer （core using hydrostreamers which are manu． length：8m）．The dredge was used for facture（i on boar（i （1uring the survey． recovering rocks from the outcrops at The high sensitivity of hydrostreamer the sea noor，the gravity corer for con－ was maintained by disassembling hydro． solidated sediments，and the piston corer streamer an（l replacing the hydrophone for soft sediments．The gravity corer elements frequently．A seismic pro丘1ing was effective for recovering consolidated system of high sensitivity makes it pos． sediments underlain by superficial soft sible to con（1uct a high spee（i survey sediments less than2m in thickness． （10－12knots）without suppressing q．uali． The ages of the sedimentary rocks and ty of the record．The towing system of the consolidated sediments were deter一 ai「9un soun（1sources is specially（lesign一

一278一 σθ・Z・9オcαZs伽伽re・∫むんeJαPαηSeααη疏s擁・痂伽pZ‘cαむオ・πs（K・Tαmαん‘）

Table1 Equipments and，conditions of seismic reflection survey during GH77－2，GH77－3， GH78－2，and GH，78－3cruises．

1） Equipment Air Gun Bolt Par Air Gun1900B×2 Compressor Norwalk APS－120（120S．C．F．M．） Receiver Hydrostreamer GS詣411－78（with78elements of Geo Space MP18－200） Ampli丘er Geo Space111Amplifier Recorder Raytheon UGR－196B 2） Con（iition Total volume of air gun 270to300in3（4425to4916cm3） Pressure 1500p．s．i．（105kg／cm3） Shot血terval 10sec． AGC of ampliGer off Filterrange 16 to 98Hz Record range 4sec． Ship spee（i 10knots Hydrostreamer towed150m behind the ship

e（1 for high spee（1 surveys． The high Japan Sea which includes these basins speed seismic profiling method improved an（i topographic highs． In the next sec－ e：缶ciency in the survey an（1 ma（1e it tion the author （1escribes the （ietail of possible to gather much more data in a each basin an（1 topographic high． The limited amount of time． overall view of the Japan Sea is d，escribed， The pro丘1es obtained by the seismic with some typical seimic sections，com－ reHection method are displayed with a piled geological m畿ps，sediment isopach vertical scale of two－way acoustic travel maps，and basement isodepth map as time in second．s．Two－way acoustic trav－ below． el time in secon（ls is use（l for the discussions of sediment thickness and 丁鰹》記αZ seε8灘紀pro泥ZεS Oヂ虚ゐe J㎎》α羅8εα basement depth on the profiles in this About70seismic re且ection transects paper． The vertical exaggeration of the we：re obtained．by GH77－2，GH77－3，GH seismic profiles is about20due to the 78－2and．GH78－3cruises． Twelve typ． SeiSmiC reCOrding SyStem． ica．1seismic promes with interpretative line d膿w加gs a：re shown in Figures4to 2．2 （）utl量ne of the geo且ogiea且s重r賦et眠re 15．丁賛ese seismic profiles show the gen． There are several sedimentary basins eral str竃1ctu：ral view of the Japan Sea and many topographic highs in the Ja－ from the south to the north． pan Sea． The geology of these basins Fi欝1res4and5show the structure of an（1topographic highs is closely relate（1 the Ts慧shima Basin and the continental to the tectonics of the Japan Sea． The she玉f off San－in．The acoustic basement un（lerstan（iing of these basins an（i highs o£the Tsushima Basin cannot be detected is fundamental for the discussion of the by the single channel seismic re且ection tectonics of the Japan Sea． In this sec－ system because of the thick and highly tion，the author introduces the overall reflective sediments of the basin．The view of the geological structure of the sediment thickness of the basin is great一

一279一 11

L24 0 5Ec， 畷一W E→ 醍 O opprox．20km 』《聯・窟・晦鬼ト 臨 ver管icolexq99，＝17x 1 1 ！ 馨 bd 、♂ミ毒＝・ ・瀞 蕊 轡 ．’蓋 騰．、欝鍵 簿 2．二 詳． τUSHI納A BASIN 鋤 ・難1 騨一ξ薫一一一4一一 …艶．・謬、 轡・ き 2 ㌧殺所ψ 昏’

亨： 2 9 カド 老 言 ・翼 罫 鐵一一 「甲一｝’曹’ 鞍・を・砿 Q 駿 翼箋灘．’ OSEC， Q ① o 3 －協 騰 へ 恥 O 繊饗rT ノ ヤ 詣勿雛 零’ ．izレ1 辮 ／ 匙 ．郷・購・踊 界森ぐ須 繋霧一1軒一’ 1 ／ でλ’ 4 鍵 鵠 勇’シグ，ノ鰍 、うヘハ ・欝一： ￥力 ロ ら ミ 錬7 》 ’．侮 ト 寒》》 ペンし o 遷 》 ’辱 一 玉 り」 ト… ／／〆7〆 ／ 髄 ＼ ＼＼． 2 ハノ 2 9 ノz伝． ノ 誉 、〃”” 1 ノ！ 寸！多 § ／ 獅 一＝づ二〆 ．雛／ oo り9 …ヨーコニニニ＞づ ノ／レ ！ P o 3 o o 3 ノ ぎ 1三髪多多一 ＼ラ こ 簿 ／ 4 さ

筋

009 5 ひ

Figure4Seismic re且ection pro且1e and interpretation of Line L24． L250 0瓢 ←NW SE－D・ O OpprOx．20km ver量lcdexogq，＝17x 董 1 諄 1 ｛ 』」L＿－．糞』 一＿．一＿＿＿．．＿一－↓． 褻 は一『

・讐一』麟、1邊’一 ①Q 十一 一 8看’ 脚†一一一 一一一亀一一 2 o i1盲 篇響 ○い 肥 糞 難謬 欝・＿ 晦 盤馨．墾転τ董虻 8’ 可一一再三鍾 騨1鱗欝．／ 巳 7…雰蕩舜／％， 象 鯉塾翻 、 1 ご二編 9 勤TH KOREA RISE Z ＼、／髭＼＼・／／！多 0 9け ・←一7 一・鯨？ 翻ノン、態瀬 ＼／・／／刀〃 レ ！ノ翫、・D222噌蔑欝縫… ￥ ド＼ ，／ ミ 2 2 TUSHl猷A 3ASIN ＼ ＼ 9 、 SB4 竺＿、うづ多峯三髪ン ∬コ ‘ 罫 ノ 3 ⑩ ～葛’憎一一一＝＝ニヌニニ》罪＝二一二二二丁．一一一一一一一一づニニニニ．一＿＿一／／ ！～ノ hトこ一・一一＿、一．！＿一一一一一一一一ゴ＞……・一一・・一一一・一一一…一一一一τこ二二一ニコー．．、憎．．、，．……て1ニレ，一一一一一一一一一一 ／ 誉

…… 4 4 留 Ω 5 Ω き Figure5 Seismic reflection profile an（1interpretation of Line L25， 儀 思’

05藍C． ⑦ 128 琳 0 Φ o 琳 ○ SOUτH KOREA RISE NQ捷宏騒 KOREA RISE 1 き 口」し．．」し． 、至L 一孕』，聾← 一議二 ぽ 1 ．銀．一．・ 貞【ll やNW SEゆ Ω． ．A 一＿一＿喪 灘 1 一一繰 ．油難 2 G匠NZAN τROUGH 撚 †｝’一／B ve『奮icロlexα99＝17x 毫 2 緊卜『 ．．＝丘 －Okm 卜一篭≧・」一♂耗 一鑑r鮮蕎 、響饗轟騨 書喩 罰 Ωo 雛厩 一薪ll擁耀 3 貸． 3＝ 源嘩一爵護一一一一翼・・ o 驚鰯齪響饗奪 ，製 管麟響逢欝響 き ’琴7”で 一韓『『皿一 べ難 騰．・ 鷺 醸 難一4 集 霞一旧＿『 脚 、1護』鰐ll｝一 4＿ 置☆二塾響響 民 ．』1』；難’．』二二 臼 一＿．＿止↓＿＿＿一一 5 Q 5・。 ．二夢≦一＝翼螺 ミ ．姦＝・一客一嬉誓論 霧÷ξ樺…貸蟹…… 罫 一一」一憎鋪 脂盤釜叢糞饗響 ＿曝二＿一＿． ．が弓瞬L｝ヂ鶴一一一驚戸 糠・P 6 o

璽＿二∫』二ニニニ1 腔 7 7 Figure6Seismicrenecti・npr・五1eandinterpretati・n・fLineL28・ l 1！

監．3で 0 05㏄ r一甲騨τ∵1τ‘．ン：㌦ 蜜ワ臼3 ナ’甲甲 P一一一惣τ甲Ψ一陶 ＝一．▼－帯一轟’㎜｝㎝糊｝鷹鶯輝驚τ署『署肉r 『 置 唇 1鑛 3 野 誓 卜喜i 一 ・ bu 1 藷犠 羅一 1 9 Nw㊥ 競覇 b 蔓 『紹彰 ド 。 『，二 ・≒ 蒙鑛 嵩 0 30km ゼ ’盤 ．甲一 鑓一ガ 1IF堰 、 磐．． v●r，icol・xo99。＝20．7 ミ’ R 2 2 ．一．．．腫．．箆’、’ 綴』1二 1τ”減 9 1 ． 罫 3 ①3 弩謎 一難、 讐 ① Q 一・ ・ 一 灘鞠ぬ 005㏄ い剛 糎 r P 』、 豫騨．蟹… 4 0 訟． OKI BANK o αq 8’ ＆ 15 鍵 鵠 ； の ミ 122 鷺 ㌦晒 ドミ珍汐＼ 遷 〆 一／ S ， ■ 3 9 3 zρ 誉 以の． 7畠． 鷺穿 へ溜一 ↑ON レ” 勘 一 一 § 4 4 ・て；く｝’ ノ＾、＿一ノ＾ ’嘱ソて §考嬉；擁 ぎ 5 5 ・r㌧ ・ト ｝ N 鳴儀ぜ ．岬 舞 斜O Ob ” 6 ω 6 3 魅

Figure7 Seismic reflection pro£le an（i interpretation of Line L3L L35 05K 0 誉 i呈 宕NW 讃評 ＿＿＿＿＿・＿，一．芦慧 鞍灘

’ 3 ．一ノわ 誌 喰x郵ζ縦峯 マ… ∫ 2自 謹 、、 f 4 9 4 ≡案 罫

甲’￥ の 2ミ 喜

…ii

留 Figure8Seismic re且ection proHle and interpretation of Line L35． ［ Ω Ω 鵠 き GQ 亀 L4005，⊂ 0 思’ 騒 Qり Φ肺 1 o ．．、1慧 oひ き ．購… ド 2 Ω． 2 lF

、 T O 5 A5 A s N ξ 3 Y A 納 A T O 響・1 05EC． 貸 τ討〒r」蔵4一1、・・諺論劉 o 7 騨 Ω 卿 ℃ 4 一ザ’・ 灘塵 1 貸． 房 o き 鰹 QD 5 ノ 2 ．継1饗締濾溺 、鷹℃， 聯、一、 ＿H ” 魯 6 轡’『’ 網無ゴ鞭欝 ％惚’ 3 爵 APAN 撫 麹一畢 一讐 4 隔置N4・ 虞 、暴 つト「 ト霧；吻 口 中NW 5【ゆ 鳶 ・饗繁 晃漏㍗ ） 5 ｝＝二騙甲｝f一 ゲ確 、罷コ 埠》ダ・・ γ 56 ” 9 6

7 獅 。 。 一 一 一 P 一 噸P 3 7 Figure9Seismi6renection proHle and interpretation of Line L40。 B認Ze琵几o∫むh，e GeoZo9εcαZ Sα，rひely oゾ」αPαη、。 ▽oZ．39，ヱVo．5

O F C『 n 一 “ 6 博 er than2．O seconds．A remarkable un． OD ・ ，藩 悪工 Z 5二 conformity is observed in the sedimenta－ な ぴめロ し 1 「 ハ やびざじ s、 ry se（luence of the continental shelf off 漣艦’窯1慧・・ 繋’、㌃、、 逡，醗盛1 ＝ O O艦 1 ，膨！ San－in． r－’

ト 一ミ O 』 Figu：re 6 shows the transect of the ’噌欝：．1’ 1鐸ダ 5．1 畿灘届・・轟⊥ 愚 涼傘 South Korea Rise（the Ullung Rise），the o芝呂≡ τ’灘∫ southwestern end，of the Japan Basin， ，一 ， ノ

the No：rth Korea Rise，and．the Genzan 難繍騨鰍．．卜i！ 二ちチ 9グ・ ，～ 語 Trough． High reflectivity of the se（li－ ト灘1’1』瀬 、 藻》、・ 隻ひ ments in the Genzan Trough is promi－ 練’ 雛隔 s 『懸イ・磨心，、 も 旨ド nent in association with the development り 灘 胃 赴・ 一 of the Genzan Cany・on．Both Korea Ri＄es 1騨 、 艦P『F 譲寡， have rough topography on the basement ゴづ一 幽， ■ ま弓．一 o

庇曳＝、．1

．， 梶 with sediment drape of thickness less 、』1 φ than O．5secon（1． T1 乞 轍輸マ瞳慮Fレ P！ 縣￥淫 融、、・ Φ 鞍越… 【、、騙、い Figure7shows the p：rofile of the Oki 縣繍、 蹴1隔 自 輯： Il緋 ぱ Bank． The profile suggests that the Oki ■ 聾キ 篤㍗． 、，翻7 獲 Z 唱 灘 ｝ Bank consists of an isolated basement ず．唱醸 O 瀞 q o high with a thin sedimentary cover．The ll繕 じぱ 蹄 1 タ 臨、． φ sedimentary cover makes the bank ap－ 令蕊、 ’ぎ噂’ 坪1 ρ 《 Φ 』レ．一 li！ ～ 旨 pear to be continuous to the continental 、1一 q 11N瞬） ， 角 》＾。二1鵬 ① 漂繋 静・一 V 一 P卜 shelf topographically． ，ち野 鰍，．’ り ‘ ’ 一 q ［ 難麩i癬一一 ♪メ〉1 じド 辮匹離脇1り刀 Figures8，9，an（110show the profile of ℃ 肋・暴1， ）R髪 q 醗、断7一 、1》’ o the Yamato Basin，the Yamato Rise， 1 ll‘・‘ ① プ and other topographic highs．Sed．iments 《 難罫 塑，、羅 蝦 rI伽、 ・。難 歎1 o 》》 角 with the thickness of1．5secon（ls are ≦ 、、’，、』 ・卜雛凹」 礁、 q 殉1』 『 ン く lllf q observed in the Yamato Basin．They o ，」， ■冒 醗、ヤ ・【→ 翻，一 are divid．ed into upPer and lower pεしrts． ト 欝 ρ 講＿4， 、窮 o ，q I Φ －穀、 焔 ，鶉：一， 唱 噌 茜冒 ’lj・㌧ The upper part is acoustically stratifie（1 ㊤ ，一 災膨’ ト ，ド1「 時器 ア and the lower part is acoustically t鍬ns． 呂 ．日 己 ・叫憩■ r！ 9 pa．rent． The Oki Ridge，the KitaだOki 》増 ．誓 ① Ridge，the Yamato Rise，and，the Takuyo 鰹一、汽『f 噴 の 墜 1 野 謬、 ε藤 〃 Bank are covere（i by a rather thin layer ，二 z チー 欝・・甲 彫憂㌘イ〆 〇 冨 一 「 Z く 鱒 ① of sediments with a thickness of less 舞 旨 麟醒 、§ づ than O．5seconds．The Yamato Bank is 錘、 bρ 多海 じバ free of sediments on the summit．A 国 冒，，欝“へ’ ン丹 膨叛い typical profile of the Toyama Deep Sea ドしゆ ロリ ふじノス洞’ 篇麟o ， r 》⇒ 1／1、 Channel is shown in Figu：re iO． The 、1整 へ’／シ Sεld，o Ri（lge and the Takuyo Bank are ll ど 庸 癒・撫 《 characterize（i by a blocked basements ？ 、 二 ト ・ろ （Fig．10）． The blocke（i feature of the 灘1、請 ．蔭㍗ 二・罪 1陣・熱．1．， 黙瀞 basements shou1（l be closely rei段te（1to 一込 1．ド∫ 5．讐 蕪、． 認確・ the origin of the both topographic highs． 史纂

「愚’厩9， Smaller basins such as the Mogami 脚砿 膿 〆〃’プ Trough are d．evelope（1along the eastern ．，，ド｛《 ロ0と

PZ《 《 甲§『 margin of the Japan Sea（Fig．10）． 轡1鯉L：p ■画≦1 蹴 、 o ≦ oo 懸 晒 ⊇ Z 卜 N》、・二 －284一 o の マ GeoZo9‘cαZs鋤伽rεo∫孟んeJαPαηSθααηゐ孟sむ2c孟oη‘c伽頑c誠oπs（K．Tαmぬ）

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8

Figure ll Seismic renection pro且le and interpretation of Line J18．

Figures ll an（112show the eastern of sediments which exceeds2．O seconds margin of the Japan Basin． The se（1i． on Line J2 （Fig．15）． In the southern ment thickness of the Japan Basin is part of the trough，the basement is much thicker than that of the Yamato blocked and the sea bottom is rough due Basin，The thickness excee（1s2．O second．s to the development of the Tartary Can－ on Line J18（Fig．11）．The sediments yon on Lines J6an（i J8 （Figs．13an（i of the Japan Basin are also acoustically 14）．The sediments of the Tartary Trough strati且e（1 in its upPer part and trans－ also show stratification in the upper parent in its lowe：r part as is in the part and transparency in the lower part． Yamato Basin and other basins．The The northern en（i of the Japan Basin is basement depth of the Japan Basin d．e－ observed on Line J8．The sediments in creases land．ward as shown in Line J10 the northern Japan Basin isεlcoustically （Fig．12）． The continental slope on Line more transparent and thinner，with the J10 north of Okushi：ri Island shows thickness of about l second，compared complicated topography．The compli－ to the southem main part of the Japan cate（i topography is the general feature Basin．Two topographic highs，the Vityaz of the eastern margin of the Japεしn Sea． Rise and the Okushiri Ridge，are ob－ Figures13，14，and15show the tran． served，on both sid．es of the Japan Basin sects of the northern part of the Jεlpan on Line J’8．The Vityaz Rise is free of Basin． The Tartary Trough along the sediments on its summit，but the Okushi－ Sikhote－Alin coast also has a thick layer ri Ridge is covered by sed．iments with a

一285一 1・

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1‘’∫，レ 1 3 05哩‘． 9 1．1 冒． 罫 ！．，羅頻繍’ ⑩ 1 、 ヤヤ 鵬、 、 、こ覧 Q 4 ぴ マ 1 o ∫Al P杢Nl BiAl・IN ！『 、去・ 琴 ■ 』 ’閉 o ’ 鷲欝尋へ1 例 No 偽 P・艶 、 4 ミ○ 5 F 、 、 「 2 糟．．働馨班・．騨・｝謬脇 蜂 蟹〃 匙 5難 騨難蹴惣1， 弊 糊 留 3 鵠 6 麟騨響． 欝 ’1’ さ ① 釈1 遷 珍一』 ・1 4 亀 7 ： 〆．黍く・、 ’ ゴi煎了．琴！1 ・，毒 書 ＝づ 才、勤 一3ヴ ノ § ・嬉

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Figure12Seismic re且ection pro丘le and interpretation of Line J’10。 」80 05匹． 1w E。 口PP臼診x．20km

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覗 冒一！ 留 Ω Figure13Seismic re且ection pro且1e and interpretation of Line J8． Q 1 嵩

鵠 05匿c． α 』60 く 思o 尋一W E一伽 覧 Qり 1 0 。PP厭．20km 瞬 P三瀞 軸 ver，k・Iexo99．＝ヨ％X ⑩ 1 ・鍔 o 要 へ ・雛晋 秘・ ＾ 1’轡’麟 o 臼亭1 ぎ き 誓！1’← ； いロ 了A臓7A腰Y TROUGH 2 ミ． 2 、，欝蟹籏噸 きき ヨロぞ 醗瀦㌧ i醸 暑 1 塗 塗 き ．ゴ締 OSEC． 黛 Ωo 『19 4 か 絶 ．一笏一’！ 暫． 2 f勢恥・ 静二 籏／ D252 o 4 き ．蛾羅 ．，’司 影雍i野 杉‘ 騰…ミ GD 難鰍一辮戯 慧講薄羅 淑麟羅墾、 ．三塾，…F、葬舞舞r＋・ 因 響 懸 ！・楚’偲 2 讃子r響驚瓶卜二1．1 へ甲ζ『甲 ／野／ ノニク → Ω 饗難劉戯騨 吟瞬繧婆・》 裂8 5 魂 尋 舜 譲｝撃’ ル、、タ膨鴛 ミ 門1｝1 け 3 3 〆 o 罫 懇薄舞．燐……考、麟…謬芦 o 一 ’』二’ 4 鯉、黛煽ジ丁ゴミミ∋ξ

￥亡iご＼三，ぐンニ，、v～ ＿ 誰蔀5 一 一 一 5 ・x 《魅 、ミー へ￥賀N 一二 ＼、 溜 、、づ’ －￥ 4、・ o ＝ 5 Figure14Seismic re且ectiQn pro丘1e and interpretation of Line J6。 』20 OSEC． 申W Eゆ O opprox．20km bu 饗． ver奮iこqlexq99．＝195x § 1 を 琴謹げ ．韓；蟹ギド・，．繋攣 簿 ‘ 糞 ． 計． ’準 ’『 き 2一 TARTARY TROUGH 2 9 ，欝購1 導 ①

3 瀬雛灘 あ 3 ぞ1鐙狸是無 懸羅響．響 穣． ㊤Q 際㎜ ．誓コ難舞繋嚢 ，’騨 粍 o い 藩 o 漣鰹駁 αq 8’ 4 、τ繋r 磯懸饗聯 SEC． ”』＿ぜ一．』r ！〆 一 ・塁二漉 騨 羅業潤．塗・ 匙 ・・磁・』〜． 宝 藝 ．．二f…翼 ＝ i ヤ ’ 尋 1驚 ！ 塗 謹讐 li 憂 留 5 1琶 婁 1 ＿無霧、藤猷醸、 覇繕糊聯ケ 鵠 粟：壌爵、ト マ 婚1脂ノ〆 ミ A 菅 1『ゴ v’く／ 遷 1 置 ン P， f多． 欝口 2 2 ・咋￥ト悩 9 書 N I 懸§謹… ．赫響 ドン「 …ii 3 3 嚇、、、 、 一r“ 1 ￥、 ＼ ＿ ぐ＝ミこ三ヨ！チタノ欝ヘ 『ミミぐ二よニニニニニニ：＝7∫二1夢1ヤ）．， 全、8 、 ＼ ぎ 4 置 一一 ま 4 、一・一曽一一P一’ ノ N ー 、一 曹一＿一 曽 臼 一 艶 宅 ’一『へ’ ！．＝ヨ ／、’を1 も 5 ！’8 弓 ≧ o 噺

Figure15Seismic reflection pro丘le and interpretation of Line J2。 GθoZo9‘oαZs伽伽rθoゾ乙んθ」αPαηSeαα認εカsεθc亡oπ乞cε即Z‘c翻oηs（K。Tαmα勧 thickness of 1．O secon（1． The（lifference basement rocks in the basin area are shoul（l be due to the （1ifference in the unknown with the lack of bottom sam－ history of each topographic high． Sev－ Pling （1ata of soli（i rocks an（i deep sea eral small basins are observed on the d：rilling d．ata which：reached．the base－ eastern slope area． The basement on ment． the eastern slope area shows a rather The Pliocene and the Miocene sedi． rugged topography． ments are prominent on the Sado Ridge． The outcrops are formed with some 0εoZogεeαZη3㎎｝0ノεhε」αpαπ8eα structural movement as is in the Okushi一 Three geological maps of the Japan ：ri Ridge．The faults observed are mostly Sea were compiled．by HoNzAαα1．（1979）， NNE－SSW trending and syncline and an－ TAMAKIααZ．（1979a），an（1TAMAKIααZ． ticline axes in the area have mostly the （1981a）based．on the compilation’of same trend．The Pliocene and the Mi． seismic pro且1es，bottom sampling data， ocene sediments are observed along the deep sea（1rilling results，an（l other pub－ topographic highs such as the Yamato lishe（1geological an（i geophysica1（1ata． Rise，the Kita－Oki Bank，the Oki Ridge， Figure16is a geological map of the an（1the Hakusanse Bank． J’apan Sea compiled by INouE and HoNzA Wide distribution of volcanic rocks （1982）from the three geological maps of are observe（l in the central part of the HoNzAααZ．（1979），TAMAK・αα1．（1979 Japan Sea．Age assignment of the vo1－ a），and，TAMAKIε6α1．（1981a）．rhe twelve canic rocks range from the Tertiary geological sections are shown in Figure to the Quatemary． Granitic rocks are 16also based on the above three maps． widely distributed on the：Kita－Yamato The geological sections on Figure16are Bank an（l the Kita－Oki Bank． The Ya－ almost identical with the seismic profiles mato Bank is mainly composed of vo1－ shown in Figures4－15．The sed，imentary canic rocks while the：Kita－Yamato Bank sequence is divi（1e（1into four units in the is composed of the gr＆nitic rocks．The basin an（i slope area． They are Quater－ pre－Tertiary sedimentary rocks are ob－ nary，Pliocene，Miocene sediments，and serve（1 0nly on the northern en（10f the Miocene volcanic sediments． Oki Bank．Some parts of the basement The Pliocene to Late Miocene sedi． ：rocks a：re described as unknown due to ments are observe（l on the Okushiri Very SCarCe infOrmatiOn． Ridge and．the continental slopeεしrea off The continental slope an（i basin areas Hokkaido．They are cut by many faults． are wholly covere（l by Quatemary sedi－ The faults are mostly NS trending．Syn－ ments in the southem part of the Japan cline and anticline axes are also mostly Sea．An outstanding unconformity on NS trending． The wid．e dist：ribution of the continental shelf （Figs．4an（i5）1ies Miocene volcanic sediments is observed un（ier the Quaternary or Plio－earliest in the eastern continental slope area． Pleistocene sediments．Many faults are The Neogene－Quaternary volcanic trace（10n the continental shelf，trending rocks are（1istribute（l on the continental parallel or subparallel to the coεlst line． shelf an（1 slope area in the northern Slumping structure of the Quatemary Japan Sea with minor distribution．The sediments is distinct in the southem distribution of pre－Neogene stratεl in the margin of the Tsushima Basin． submarine area is observed on the Mu． sashi Bank and．the Soya Strait． The

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1sqραeh肌αps oノ孟ゐε」¢匹》αη8eα presented isopach maps of the continen－ Several isopach maps of sediments of ta1 ．shelf an（1 slope area along the the Japan Sea have been publishe（1．HILDE Japanese Isalan（1s．IsHlwADAε6α1．（1984） andWAGEMAN（1973），LuDwIGθ6αZ．（1975） compiled all these isopach maps wlth and MoRozowsKI and H：AYEs（1978）pre． some of unpublished data of the sented．isopach map of the deep sea basin Geological Survey of Japan （Fig17）． a：rea． TAMAKI e6α1．（1979a）εln（1TAMAKI GNIBIDENKo （1979） also showed． the θ6α1． （1981a） also presente（i a（1etaile（l overall isopach map of the Japan Sea． isopach map of the whole Yamato Basin Figure 17 shows the d，istribution of an（l a part of the Japan Basin．IsHlwADA sediments in the Japan Sea．The major and OGAwA （1976） an（1 SuzuKI （1979） deep sea sedimentary basins are the

129 135 140 14347・N

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34 129 135 140 1430E

Figure17 Isopach map of the whole Japan Sea after IsHlwADAααZ． （1984）。 Contours are in second of two－way acoustic travel time．

一290一 GεoZo9ごoαZs偬c加reoノ伽JiαPαηSεααηd‘孟s古2e孟o痂伽pZεcαむ‘oπs（K．Tα肌α勧

Japan Basin，the Yamato Basin，the tion in the Tartary Trough apPears to Tartary Trough，and the Tsushima Ba－ be saturated and the excess sediments sin，which are presently under εlctive are fe（i into the Japan Basin through deposition．The thickest sedimentary fill the Tartary Canyon． in the Japan Sea，however，is not ob． There are many srdall sedimenta－ serv6（1 in these deep sea basins but ry basins observe（1 0n the continental Place（l on the continental shelf of the shelves along the west coast of the Ja－ Northeast Japan （Tohoku） Arc． The panese Islands，The sed．iment thickness sediment thickness off Niigata exceeds is generε111y larger thεln that of the（1eep 5．O seconds which is the largest of the sea basin and at maximum exceeds5．O Japan Sea in our available（1ata． secon（1s． These basins are continuous The sediments are generally thicker in onto the shore area． The litho－ an（1 the central part of the Japan Basin an（i biostratigraphy of the basins shows that an isopach contour of2．O seconds shows the sedimentary basins were formed in E－W extension in the north of the Ya－ the deep sea since early Middle Miocene mato Bank．The thickest sediment accu． （IsHlwADAε6αZ．，1984）． mulation of 2．3 seconds is observe（1 in The sediments on the topographic highs the center of the basin south of the are：rather thin with a thickness of less Bogorov Seamount（MoRozowsKI and than O．5 secon〔1s． The shin （1eposition HAYES，1978）。 on the topog：raphic highs show that the The sed，iment thickness of the Yamato main part of the sedimentation in the Basin is less than that of the Japan J’apan Sea is（1ue to the turbid．ites which Basin．The maximum sediment thickness are （1eposite（i only in the basin area． observed in the Yamato Basin is1．6 Some part on the Okushiri Ridge，how－ seconds（TAMAKIαα1．，1979b），which is ever，is overlain by sediments with a εlbout 1．O secon（i less than that of the thickness of1．O secon（1． The thick se（1i－ Japan Basin． Isopach contours of the mentation on the Okushiri Ridge is due Yamato Basin complicated with many to the recent uplift of the ri（1ge by the seamounts and subbottom basement re－ compressional tectonics since latest Pli－ lieves．The Yamato Basin is generally ocene as（iiscusse（10n（1etail in Chapter separated into two sedimentary basins； 3．The Okushiri Ridge was uplifted with the northeastern basin and the south－ the basin turbi（1ites on it． westem one． The Tsushima Basin has a sediment 1soδαsεm⑭o㍑he勘ρ伽8eα thickness over than2．O secon（1s in thick－ Figure18shows a contour map of the ness．The sedimentary structure and depth of the acoustic b＆sement of the high reflectivity of the sediments show Japan Sea by TAMAKI αα1． （1981εし）． that slumping and sliding deposits are The map covers a part of the Japan Sea main constituents of the sediments in including the whole Yamato Basin and a the basin．The prevalence of slumping part of the Japan Basin． This is the in the Tsushima Basin may suggest rapid first isobase map on the Japan Sea． subsi（lence of the basin． The isobase map clearly shows the The sediment thickness of the Tartary configuration of the Yamato Basin．The Trough exceeds2．O secon（ls． The south－ Yamato Basin trends NE with a general ern part of the Tartary Trough is（lis－ width of about110km and a maximum sected by the Tartary Canyon．Deposi． width of150km in its southwestem part．

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Figufe18 Isobase map of the Japan Sea after TAMAKI e6αZ。（1981a）。 Contours are in second of two－way acoustic travel time．

The southwestern part of the basin depth of the both basins is discusse（1in shows deep basement depth greater than detail in Chapter3． 5．O secon（is． Another characteristics of the base－ The basement depth of the Japan Sea ment depth of the Japan Sea is that the excee（1s 7．O secon（1s in the central part． central part of the Japan Basin is gen－ The difference of the basement depth erally deeper than its marginal area． between the Japan Basin and the Yama－ This feature is also shown on the sedi． to Basin is remarkable．If the acoustic ment isopach map．The sediment thick－ basement represents oceanic crust，the ness of the Japan Bεlsin is also greater greater（1epth of the Japan Basin shou1（i in the central part than in the marginal in（1icate that the Japan Basin is older part． Simple application of the、age－ than the Yamato Basin．The relation－ depth relationship of the oceanic crust ship between the age and，the bεlsement suggests that the central part of the

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Japan Basin is older than the marginal 1）arent layer （Fig．20）． This unconfor－ part of the bεlsin． If this is the case， mity suggests a different depositional such feature，younger in the margin and manner of both sedimentary layers．The o1（1er in the center，is unrealistic in the sea bottom of the Japan Basin，corre－ back－arc basin which gene：rally hasεl sponding to t紅e upPer surface of the sprea（1ing center in the center of the stratified layer，is generally smooth and bεlsin．This discrepancy is also d．iscusse（1 且at．Such sea bottom topography is in Chapter3． general features of the abyssal Plain． The stratified layer is （ieposite（i in the 2．3Geologic＆1struc撫reofb撹s沁s離d topo9：raphic 更ow and onlaps to the troughs slopes．The sedimentary manner hke this also suggests that the strati且e（1 」姻伽Bσ伽 1ayer is a turbi（iity layer． The strati且ca－ The Japan Bεlsin is the largest basin tion of the stratifie（11ayer represents the in the Japan Sea an（1 0ccupies about distal turbidites facies of the submarine two－thi：rd of the whole J’apan Sea． Two fan．The tu：rbid，ities we：re fed，to d．eep of the typical seismic profiles of the sea basins through sever＆11arge subma． Japan Seaεlre shown in Figures ll and rine canyons an（1 channels such as the 19． Toyama Deep Sea ChanneL The maximum sediment thickness of The sedimentary structure of the trans． the Japεln Basin reaches2．2secon（ls on parent layer is（1ifferent from that of the the seismic pro且les（Fig．19）．The sedi－ turbi（lity layer． The transparent layer ment thickness generεし11y excee（ls1．5sec－ drapes the basement topography．Acous－ on（is on the central part of the Japan tic transpare甑）y of the layer in（iicates Basin （Fig．17）．The sedimentary se． the uniform nature of the sedimentary quence of the Japan Basin is （1ivi（1e（i layer．Such sedimentary manner ap－ into two acoustic units l the upPe：r strat－ pears to be the results of the sedimenta－ ified．layer and，the lowe：r tra，nspa：rent tion of pelagic sed．iments rather than layer．The st：ratified」ayer shows a rath－ terrigenous turbi（iites． er uniform thickness of1．O second over The sedimentary layers onlap to the the entire basinε1：rea while the thickness continental slope of the Japanese Islands of the transparent layer is variable from and no major unconformable relation is less than1．O to1．5secon（1s． The varia－ observed，there（Fig．19）while the uncon． tion of sediment thickness is due to the formity between the strati且ed layer and vεlriation of the lower transpεし：rent layer． the t：ransparent laye：r is well obse：rved The sedimentary layers onlap to the around the Yamato Rise（Fig．20）．The continental slope of the Japanese Islands unique unconformity around the Yamato （Fig．11）． Rise suggests the tectonic movement of The stratifie（i layer an（i the transpar－ the Yamato Rise，in terms of uplift or ent layer show conformable relation in subsidence，which is different from that the basin area，whereas they show dis－ of the Japanese Islan（1s． conformable relation around the Yamato A（leepseadrillingsite，Site301，is Rise．The disconformity is shown in locate（1in the Japan Basin north of the the manner that the transparent layer is Kita－Yamato Bank．The hole stopped continuous on to the Yamato Rise while at the depth of497m below the sea the stratified layer onlaps to the trans一 bottom，the mid part of the entire sedi．

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賑 竈

Figure19 Seismic reflection profile of the eastern part of the Japan Basin on Line J17． GeoZO9乞0αZS伽伽r20ノオんε」⑫αηSeααη引孟Sむecカ0痂伽頑0αε‘0πS（K。Tαmαんε）

mentary layer and did not reach the an（i the transparent layer is generally basement（KARIG and INGLE，Jr．αα1．， correlated to4Ma in the Early Pliocene 1975）． The （irilling （iata at this site， accor（1ing to the results of Site301． however，present important information for㌻he sediment stratigraphy of the yαm碗oBαs加 Japan Basin．The drilling hole almost The sediment thickness of the Yamato reached．the bound．ary between the strat－ Basin is less than that of the Japan ifie（1 1ayer and the transpεlrent layer Basin．The sedimentary sequence of the which is not generally sharp．The uppe：r bεlsin is also divid．ed．into two units in its 250meters of the drilling sample is silty central part as is in the Japan Basin clay and．the lower half of the hole is （Fig．21）． The upper layer is stratifie（1 diatomaceous clay・ The ag60f the bpt－ and the lower layer is transparent．The tom of the hole is4Ma according to the upper stratifie（11ayer is generally thicker analysis of the diatom fossil by KolzuMI than the lower transparent layer in the （1979）． The upper stratifie（11ayer gen－ Yamato Basin while，in the Japan Basin， erally shows a uniform thickness in the the stratiHed layer is much thinner than main part of the basin area．Thus，the the transparent layer． The thickness of boundary between the stratified．1ayer the stratified layer is variable from O．6

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無子賠尋一懸灘 善 野舞圭． ＆’議こ ．・1ミ｝㌧ 、 髄＝＝、1 魂．｝∴1

Figure20 Seismic reHection proHle on the Yamato Rise and the Japan Basin。An arrow shows the DSDP drilling Site301．The profile data is of DSDP cruise Leg・31．

一295一 0 中鼎 錐r390SEC YAMATO BANK 騒 切 蕊 簿 1 租 讐． き YAMATO BAS圓N 9 罫 ⑩ 2 ’一2 ⑩Q o 籔騨 oへ 晦 8’ 3一 v．e．＝22 一一20㎞＿ Yamato Seamount Chain 3 巳 し 慈 留 ①8 桂 縫 き 糠 褻 ・去ミ 骸議諺 遷 4＿鐘 な 4 ヨ 9 喜 妻 § 一5 ご N ，暫 腰 ヌ 恥 ’諺 讐 一6 も ≧ o 斬

Figure21Seismic pro且1e of the Y’amato Basin and the Yamato Seamount Chain on Line L39。 GθoZo9‘oαZs施伽reo∫オんεJiαPαηSεααη面εsεεc孟o痂ε即Z‘oα孟乞oηs（K．Tαmぬ）

領一r l 睡NW

ぐ＝YAMATO BANK 2「愚

重レ爆・鐸 r 縄 ・ L38

1＿6

Figure22 Seismic pro丘1e of the westem margin of the Yamato Basin Line L38。 to1．O second．． The variable thickness of stratified layer is correlated to the low－ the upper strati且ed layer of the Yamato ermost part of the transparent layer in Basin is due to the development of a the central part of the basin． It shou1（1 deep sea submarine fan and the Toyama be note（1 that the （1istribution of the Deep Sea Channel in the northeastern lower stratified layer is limited only in part of the basin （Fig．10）． the marginal area of the basin．The Another stratified laye：r which un（ler． lower st：ratified，1aye：r is also obse：rve（l at lies the transparent layer is（1istribute（l the northem end of the Yamato Basin in the margin of the basins（Fig．22）． as shown in Figure23． The author calls this layer the lower The sea bottom topography of the stratified layer in this paper．The lower Yamato Basin shows a typical abyssal stratifie（11ayer is observe（i in the basin plain with a，且at and．smooth su：rface in margin and it is traced onto the slopes t｝le southern half of the basin （Figs．9 of the Yamato Rise and the Sado Ridge， and21）．The fea，ture shows the deposi－ The thickness of the layer is less than tional environment of （1istal turbi（1ites O．3second．s． The lower stratified．1ayer as is in the Japan Basin．The Yamato seems to change to transparent towards Basin has two abyssal plains．The larger the centrε11part of the basin．This change one is located in the southwestern part of facies from the margin to the center of the Yamato Ba．sin and．has the water of the basin suggests that the lower depth of about3000m．The smaller one

一297一 ⑭餓喰 i1 醐 中騨es曾 fault for Japan Sea Earthquake（1983，輔7．7》 』232sEC． bJ 2「 蕊 ▽ 簿 讐． サ き 【 E 鼻 YA躍ATO 8ASl髄 ．、レ3 罫 3r ⑩ ▽oy畠m翻0⑧轡審》S㊥翻C恥a碗隔㊥猛

①Q 母 o oい 閃 8一 4 ，一4 匙 GQ 1 9 器 、 ご 1 遷 亀 5 書 萎 驚籍 …ii u

』JP “ く 9 匪 働 も ≧ o 噺 Figure23 Seismic profile of the northern end of the Yamato Basin on Line J23・ GεoZo9‘cαZs惚伽rθo∫乙んε」αPαηSεαα認‘乙s孟θc孟o漉漉頑cαεεoηs（K．Tαm磁） is locate（1 in the central part of the eastem part of the Yamato Basin．The basin and shows a shallower water depth hole did not reach the basement layer of about2700m．These two separated andstoppedatthedepthof532mbelow abyssal plains are inferre（i to have（1if－ the sea bottom，the mid part of the ferentprOVenanCeS． entire sedimentary layer （KARIG and The northeastem part of the Yamato INGLE，Jr．θ6α1．，1975）． The（irilling hole Basin is characterize（1by the（ievelop－ almost reached the boundary between ment of a恵ubmarine fan（Fig．10）．The the upper stratified．1ayer and．the lower submarine fan is developed just off the transparent layer which is not generally Toyama Trough．The Toyama Deep Sea sharp．The drilling samples are c1段y， Channel cuts and meanders through the silty clay an（1 san（1y clay． The age of deep sea fanl The Toyama Deep Sea the bottom of the hole is3．5Ma accord－ Channel extend，s into the Japan Basin． ing to the analysis of the diatom fossil These features suggest the following by KolzuMI（1979）． Site302is locεlte（l sedimentary history in the area．The at the northem margin of the Yamato deep sea fan was formed off the Toyama Basin． The hole penetrated．the lowe：r Trough by sediment transportation臼from strati且e（i layer an（1 reεlche（1 the base－ the Toyama submarine canyon at the ment at a depth of531．5m below the sea first stage．After the saturation of the bottom。The upper two－thirds of the submarine fan，the fan was eroded by core，which is correlated．to the t：rans－ the Toyama Deep Sea ChanneL Now parent layer，is composed of mostly the （lepression of the channel reaches diatomaceous clay with the age of post 300mwithawidthofabout10km，and Pliocene． The lower st：ratified．1aye：r is the mεlin depositional arga is shifted composed of clay of upPer Miocene age． into the Japan Basin．Some of over－ The basement rocks appear to be corre－ flowed sediments from the channel may 1ate（l to the Green Tuff（SHIMAzu，1979）． be d．eposited in the shallowe：r abyssal The composite analysis of seismic pro－ plain of the Yamato Basin． files with these two DSDP holes presents The Yamato Basin is characterize（l by an important information for the strati－ the abundant distribution of seamounts graphy of the Yamato Basin as discussed and．knolls in contrast with the other in Chapter3．1． basins in the Japan Sea．The seamounts an（1knolls are concentrate（1in the axial Tα吻彫丁7側φ central part of the basin． There are The sediments of the Tartary Trough two seamount chains．One is just along are highly stratifie（i in the upper part the central line of the Yamato Basin an（1transpa：rent in the lower part． The t：rend．ing NE． The other c：rosses the ba－ maximum se（1iment thickness exceeds2．3 sin trend．ing ENE，Both seamount chains second．s at the west of the Soya St：rait cross each other at the north of the （Fig．15），where the basement could not ］日：akusanse Bank． The author calls the be detected on our seismic profiler rec－ former one the Central Yamato Seamo－ ords．The upper stratified sediments unt Chain （Fig．21） an（1 the latter one increase their reHectivity to the north－ the Yamato Seamount Chain． and to the west．This phenomena sug－ Two deep sea d．rilling sites，Sites299 gest that the sediments are supPlied and 302，are located in the Yamato from the northern end of the trough and Basin． Site299is locate（1in the north一 the continent to the west of the trough．

一299一 B棚ε伽o∫伽GθoZo9‘cαZ翫rひεッo∫」αPαη．VoZ。39，1〉o．5

An e：rosional featu：re by the Tartary This feature suggests that the subsidence Trough is prevaile（1in the southern area of the basement highs has occurred after of the trough （Figs．13 an（1 14）． The the deposition of the lower sediments． Ta：rtary Canyon is shown on a typical The age of the se（limentary sequence p：rofile on Line J8（Fig．13）with a can－ of the trough can be estimated from the yon（1issecte（1to the depth of500m． The outcrops on the Okushiri Ridge as shown topographic corre1εltion from profile to on section JS20f Figu：re16． The lowe：r profile，however，is poor，which indicates part of the sediments is unknown be－ a complicated depositional environment cause of the1εlck of any boring data an（i in the southern area of the trough． The outcrOPS． thickness of the upPer strεltifie（i se（1i－ ments is generally1．O second with a T8麗8配隅αBαs加 variable range of O．8to1．2secon（1s． The The Tsushima Basin has a smooth thickness of the lower transparent sedi－ noor gently tilting upwa：rd．to the south。 ments is variable from O to more than The basement cannot be（letecte（1except 1．4seconds．The sediments are thicker around the Ullung Rise．The d．eepest in the（iepression． The transparent se（li－ reflector bbserved in the sediments is2．2 ments are intercalated by a stratified second．s below the sea floor on the foot horizon in some places（Fig．14）． of the continental slope off San－in． The basement morphology is rugged The sedimentary sequence is divided with a d．epth of4to5．6seconds where into three units． The upPer unit is a the basement is detected．The basement non－deformed stratified layer with high at an axial d．eep is obse：rved，with the frequency reflection and it forms a small depth excee（ling5sec． The rugge（1base－ abyssal plain south of Ullung Island． ment morphology may be due to or This unit is presumably composed of show a relic of the initial rifting of the turbidites．Its maximum thickness is O．3 Tεlrtary Trough． second． A sono－buoy refraction measurement The midd．1e unit is a stratifie（l and at Station SB7was carried out in the highly reverberant layer which is weaklly trough at the west of Rishiri Island d．eformed in the basin and strongly de－ （HoNzAααZ．，1978）． The results show formed on the foot of the continental that the sonic velocity of the basement slope．The middle unit is overlain slight． is3．6km／sec with a thickness of1．72km． 1y unconformably by the upper unit。 The basement velocity of about3．5km／ The thickness of the middle unit in－ sec，which is slower than4．5km／sec of creases southward and the maximum the normal oceanic basement，is also thickness is 1．4 secon（1s on the foot of observe（1 in the Japan Basin an（l the the continental slope ofr Sεしn－in． This Yamato Basin． southward thickening of the middle unit Broad basement highs are observed is due to the southward．tilting of the sea along the eastem margin of the Trough floor．The middle unit is correlated to as typically shown on Line J2（Fig．15）． the upPer sedimentary unit of the conti－ 嚢 The upper stratified sediments of the nental shelf ofr San－in whereas，north－ trough continue eastwa：rd．onto the base－ ward．，it abuts onto the basement of the ment high，while the thin lower tran－ Ullung Rise．A reverberant layer similar 馨

き sparent sediments are observed only in to this unit is also observe（1 in the

き the depressions on the basement highs． H：i（iaka Trough off the Pacific coast of 蓑 義

き 一300一 華 護

嚢 GεoZo9‘cαZs施c齢θo∫孟んεJiαPαηSθα侃ぬ古sむθc孟o痂オ1π抑oα伽ηs（K．Tαmα勧 southwestern Hokkaid，o （TAMAKIθ6αZ． in cont：rast with that in the southern 1977）． part，and d．ecreases in depth toward． The lower unit is a transparent laye：r north（Fig．6）．The northem sedimenta－ with weak reflectors which are almost ry sequence is highly reflective in com－ evenly deposited．．The middle unit lies parison with the southern sedimentary slightly unconformably on the lower unit． sequence． These features suggest that The thickness of the lower unit is sup－ the terrigenous sediments derived from pose〔1to excee（11．O second although the the north form the northem reflective bottom of the unit cannot be （ietecte（1． Sedimentary SeqUenCe． The lower unit might also be continuous A prominent chamel is observed on with the lower sed，imentary unit on the the foot of the continental slope an（i the continental shelf whereas it abuts onto features suggests its activity．The chan－ the Ullung Rise． nel has the width of5km and a relative Submarine sliding and slumping struc－ depthof150monLineL28（Fig．6）． tures are common in the southem mar． The upper half of the sedimentary gin of the bεlsin an（l appear as small layer of the Genzan Trough is stratified， rid．ges or swells on the profiles L24and and．the lowe：r part is t：rεlnsparent as is 25（Figs．4and5）．The rapid accumula． in other bεlsins． The stratification of tion of coarse materials associated with the upPer strati且e（11ayer is complicate（l such active slumping or sliding compose suggesting the change of the（iepositional the highly renective upper se（limentary environment through time． unit．The rapid accumulation of the sediments in the Tsushima Basin may ノ睡09αη諺Tro配gh in（iicate the rapid subsidence or excess The Mogami Trough lies alnog the sediment feed to the basin from the eastem margin of the Japan Sea from San－in coast． The（leposition of the re－ off Niigata to the Oga Peninsula． The verberant layer in the Tsushima Basin width of the trough is40km and its may be related to some kind of tectonic length exceed，s200km with the trend．of movement of the basin and the sur－ NNE－SSW．The trough is developed be－ roun（iingεしrea． tween the Sado Ridge and．the northern Honshu． θcη2απTroz忍gh A pronle of the Mogami Trough is The Genzεln Trough，one of large sed．i－ shown on Line N8（Fig．10）．The bottom mentary basins in the Japan Sea，is of the Mogami trough is not simple due located，in the western part of the Japan to the presence of several banks an（i a Sea． The t：rough lies between the：Korea channe1．The sedimentary layer in the Rise and the continent． The trough trough is wholly strati丘ed with a maxi－ trend．s NE． There has been very little mum thickness exceeding1．5seconds． description and discussion of the Genzan Several banks in the trough are overlain Trough in previous works．Two seismic by thick sediments with a thickness of profiles of the trough were obtained． over O．5second，．The NNE trend．ing faults during the GH77－2cruise． occur on either si（le of the banks． Such The thickness of the sedimentary se－ thick accumulation of the sed．iments on q．uence ranges from O。5to1．4seconds in the banks in（licates their recent uplift the Genzan Trough．The basement to． along the faults with the ove：rlying sed，i－ pography is rugge（i in the northern part ments on them．The lower half of the

一301一 B砿ZZε古‘ηo∫古んe G（30Zo9‘oαZ S砿rひθツoゾ’eZαPαπ． γo Z．39，ノ〉o．5 se（limentεしry layer in the trough is con－ 1．O second and in some basins reaches sidered to be Miocene age based on 2．O seconds or more．The sediments are comparison with the shore geology＆s is stratifie（1in the upper part． The basins shown in geological section JS60f Fig－ are characterized by a且at a．nd smooth ure16． sea floor suggesting the （1eposition of the（1istal turbidites．The origin of these 0膨Tro麗φ basins is（iiscusse（1in Chapter3．4． The Oki Trough is locate（1in the south－ ern part of the Japan Sea east of the 2．4Geo且ogiea亘struetureof撮ges， Oki Island．s off San－in． The trough has ba曲s，anαseamO憾ts the width of about35km and the length of200km with the trend of NE．The yαη協彦o観8ε Oki Trough lies along the southern The Yamato Rise is the la：rgest topo－ margin of the Oki Ridge． graphic high in the Japan Sea． The rise The Oki Trough is filled．by rather thick is（1ivi（1e（1into three topographic units； sediments with a thickness exceeding2．O the Yamato Bank，the ：Kita－Yamato secon（is（LineL35，Fig．8）． Theupper Bank，and．the Takuyo Bank （Fig．1）． sequence of the sediments is stratined The topog：raphic d．ep：ression between the with a thickness of about1．O second．． Yamato Bank and the Kita－Yamato The lower sequence of sediments is trans－ Bank is called the：Kita－Yamato Trough． parent．The whole sediments are undis． The width of the Yamato Rise is180km turbed．The sediments in the trough and．its length is300km．The height of about the Oki Ridge in the northwest， the rise above the surroun（ling sea Hoor while to the southeast the feature is not exceeds 2500m． The rise tren（1s NE to simple．The sed．iments generally con． NEE． tinue to the sedimentary layer on the The Yamato Bank forms an NEE continental slopes，but the sediments t：rend．ing，：ridge－1ike feature with a flat abut to the basement or lower sedi－ summit，on which basement is widely mentary layer on the continental slope exposed （Fig．9）．This flat summit is in some areas． The Sea Hoor of the Oki interpreted as a wave cut terrace formed Trough is smooth and flat and appears when the upper pεlrts of the Bank were to show the deposition of d．istal turbi． near the sea leve1． The feature suggests dites．The age of sed．imentary layer of that the subsi（1ence of the ri（1ge occurre（l the oki Trough is Pliocene to Quaternary in associa．tion with the subsid．ence of the accor（lingto TANAKAan（l OGusA（1981）． surrounding basin floor with time．The Y＆mato Bank is composed mainly of 8ゐか♂わε8hε， 0ん麗shかレε， απα ハだ8ん泥sz69αr麗 volcanic rocks （Fig．16）． The volcanic Tm勘ghs rocks are basalts and andesites．Many The Shiribeshi，the Okushiri，an（i the basalts an（l an（lesites are （1ate（1by the Nishitsugaru Troughs are （1evelope（1 K－Ar age determination method（Tables along the eastern si（1e of the Okushiri 2and3）．Most of them are of the Ridge in the eastem J’apan Sea． The Eocene－01igocene to the Early Miocene basins show a north－south elongated in age（20to46Ma）．An age of76Ma structure． Each basin hεls a wid．th of20 forbasaltisreportedbyVAsILIEv（1975）． km and a length of70to80km．The Some of volcanic rocks on the Yamato sediment thickness of the basins exceeds Bank appear to be Late Cretaceous．

一302一 σ2・Z・9‘・αZS施C雄ε・∫伽如αη8θααη疏SεθC孟・痂‘脚Z‘・αむ‘・πS（K・Tαmα勧

Table2 Absolute age of the rocks recovered by the bottom sampling。 Age determination by Japanese scientists．

10cality site no。 rock age（Ma） method reference Hakusanse HSA an（iesite 007．7 ±0．81 K－Ar UENOθ6α♂．（1971） Klita Oki Bank D266 grεmite 141．6 K－Ar GSJ unpublished KitεしYama．to Bank granodiorite 197 K－Ar SHIMAzu（1968，MS） Korea Plateau D223－7 granite 126．8 ±6．3 K－Ar HONZA ed．（1978） Matsu Seamount MASA an（iesite 004．16 ±0。16 K－Ar UENOαα1．（1971） Meiyou No．2Smt MESA 013 K－Ar OZIMAθ乙α1． （1972） Musashi Bank D248 welded tu任 077．8 土3．9 K－Ar YUASAθ6（zl． （1978） Nishitakuyou Bank grano（iiorite 227 Rb／Sr OZIMA θ診α」． （1972） Takuyou Bank granite 220 K－Ar SHIMAzu（1968，MS） Yamato Bank YS7－l andlesite 019．3 ±0．5 K－Ar UENO θ孟α1．（1971） Yama．to Bank basalt 021．6 ±0．5 UENO θオα」．（1971） YS1－2 K－Ar 灘

Table3 Absolute age of the rocks recovered by the bottom sampling． Age determination by Russian scientists． locality site no． rock age（Ma） method reference Bogorov Seamount basalt 18 K－Ar SAHNo＆VAsILIEv（1974） Gabass Seamount granite 110 K－Ar LELIKov eヵα」． （1975） Kita Yamato Bank granite 136 K－Ar VAsILIEvαα」．（1975） Kita Yamato Bank granite 128 K－Ar VAsILIEvθ孟α♂．（1975） Kita Yamato Bank granite 110 K－Ar VAsILIEvθ孟α1．（1975） Kita Yamato Bank granite 156 K－Ar VAsILIEvθ診αZ．（1975） Klorea Plateau gne1SS 2231 Rb／Sr LELIKovθ孟α」． （1975） Klorea Plateau gnelSS 2097 Rb／Sr LELIKovαα1． （1975） Korea Plateau gne1SS 1983 Rb／Sr LELIKov eむαZ． （1975） Korea Plateau gne1SS 2729 Rb／Sr LELIKov eむαZ．（1975） Korea Plateau gne1SS 2139 Rb／Sr LELIKovαα」。 （1975） Slope o任Primore granite 60 K－Ar LELIKov eオα1． （1975） Slope o任Primore granite 76 K－Ar LELIKov θ乙α」． （1975） Slope o仔Primore granite 90 K－Ar LELIKovθむα1． （1975） Ullung Plateau granitoids 102 K－Ar LELIKov （3孟α♂． （1975） Ullung Plateau granitoi（IS 110 K－Ar LELIKov（覚αZ． （1975） Yamato Bank basalt 76 K－Ar VAsILIEv（1975） Yamato Bank basalt 28．5 to 35．6 K－Ar GNIBIDENKo（1979） Yamato Bank basalt＆an（1esite 23to46 K－Ar GNIBIDENKo（1979） Yamato Rise granitoids 270 K－Ar LELIKov αα1。 （1975） Yamato Rise granitoids 310 K－Ar LELIKovθ孟α1． （1975） Yamato Rise granitoids 220 K－Ar LELIKovθ診αZ． （1975） Yamato Rise granitoids 178 K－Ar LELIKovε孟α♂。 （1975） Yamato Rise granitoids 194 K－Ar LELIKov e6α♂。 （1975）

The Kita－Yamato Bank forms a pla． ged on the summit（Fig。9）．The base－ teau like feature with a weak trend，of ment is overlain by densely stratified NE．The basement morphology is rug． sedimentary layer with the thickness of

一303一 挽ZZθ魏cゾ6hθ0εoZo8εoαZ磁rびθッoゾ」αPαη．γoZ。39，1▽o．5

around O．5second．The sedimentary lay． Basin． er abuts the basement． The boundary The Oki Bank shows spu：r feature ex． between the sedimentary layer and the tend．ing northward．off Oki Islands．Seis－ basement is di伍cult to recognize in mic profiles of the Oki Bank （Fig。7） places because of the（iense stratification show a plateau like feature of the base－ of the sed．imentary layer．The：Kita－ ment．The basement plateau is isolated Yamato Bank is mostly composed of from the continental shelf area．Well 9「anitic rocks （Fig．16）， Several gran－ stratified．sediments with a thickness of ites an（i grano（1iorites are also（1ate（i by O．5 secon（l are （1eposite（1 0n the base－ the K－Ar Age determination method ment，The sediments buried the base－ （Tables2εlnd，3）． The（1ate（l age ranges ment depression between the basement from110to197Ma（Jurassic to Early high of the Oki Bank an（i the continental Cretaceous）． shelf with a thickness of1．5 second．． The Takuyo Bank shows a』rugged， Such sediment accumulation has made basement morphology （Fig．10）．Sedi－ the topogrεlphic connection of the Oki ments with a thickness of1．O second are Bank an（1 the continental shelf εlrea． deposited in the depressions among the Phyllite is sampled from the basement basement highs． The northern part of outcrops of the northern cliff of the Oki the bank is composed of granitic rocks Bank． while the southem part is composed of The Oki Ridge is clearly NE trend．ing． volcεしnic rocks （Fig．16）．It has the same The wid．th of the Oki Ridge is about characteristics as the Yamato Rise as 50km and its length is about150km． the northem area is composed of gra－ Oki Islan（l are locate（1 just southern nitic：rocks while the southem part is extension of the Oki ri（1ge． The Oki characterized by the wid．e distribution of Trough lies just south of the Oki Ridge the volcanic rocks．Radiometric age of as a trεlppe（i se（1imentary basin behin（i granite from the Takuyo Bank is report－ the ri（lge． ed to be220Ma （Late Triassic）by The mid part of the Oki Ridge has a SHIMAzu（1968）（Table2）． prominent depression of the basement The Kita－Yamoto Trough shows an as is well shown on Line L35（Fig．8）． elongated basement depression trending The depression has the same trend with NEl The feature on the seismic profile the ridge．The strati丘ed sed．iments with （Fig．9） suggests that the （iepression the thickness of O．7 to O．8 secon（1s are was formed by the graben－1ike move－ accumulated in the basement depression． ment bounded by normal faults on both The basement depression is bounded by sides．The faults are well traced from fault scarps． The structural feature on pro且1e to profile． The se（1iment thick－ the seismic profile suggests that the ness in the trough excee（1s1．O second in depression is a graben boun（le（i by nor－ the central graben． mal faults on both sides．The deforma－ tion of the sediment丘11in the depression 0んε．Bα醜，0ん辺吻ε，α認Kぬ一〇師・Bα醜 is weak and．it does not suggest any The Oki Bank，the Oki Ridge，an（l the recent activity of the faults． Kita－Oki Bank are locate（l off San－in in Northern part and southern part of the southern part of the Jεlpan Sea． the Oki Ridge have exposed basement． They are isolated topographic highs at Many volcanic rocks were dredged from the southwestem end of the Yamato the：rid．ge du：ring GH78－2cruise． They

一304一 σθoZO9ε0αZS伽伽rθ0∫むんθ」1αPαηS2ααηの乙S孟eo孟0π乞C伽頑0αεε0ηS（K．Tαmα勧

are weld，e（i tuff，tuff b：reccia，an（lesite， highs are shown in Line L28（Fig．6）． bεしsalt，an（i（iolerite （YuAsAαα1．，1979）， A sedimentary seq．uence with a maxi－ There is no radiometric age data on the mum thickness of1．1seconds is observed ridge． on the Korea Plateau and．the Ullung The Kita－Oki bank is located．between Rise．The sedimentary sequence is di－ the Yamato Rise and the Oki Bank．Its vi（ie（i into three units boun（ie（l by （iis－ wi（ith is80km and its length is130km conformities．The upper unit（A on Fig－ with a trend of NE． The：Kita－Oki Bank u：re6）is a transparent or weakly strεlt． shows a rough basement morphology ified」ayer with a，thickness less than O．1 with highs and d，epressions（Fig．8）．The second．an（1is occasionally absent． The ba．sement highs are almost free of sedi－ middle unit（B on Figure6）with a max－ ments while the basement depressions imum thickness less than O．6second are filled with strati丘ed sediments with a forms the main part of the sedimentary thickness mo：re than 1．O second，which sequence． The upPer part of the unit is are less opaque than the sediments of stratified，and．its lower part is t：ranspa：r－ the Kita－Yamato and Oki Banks．The ent of very weakly stratified．．The lower thickness of the sediments on the bank unit of the sedimentary sequence（C on is generally less than O．5secon（i． Fault Figure6）is observed in the basement like structures are observed in some depressions on the rise and plateaus． places in the basement of the bank．The The unit is compose（l of an upPer strati－ northem part of the bank is composed fied laye：r and a lowe：r transparent layer of granitic rocks while the southern part which is absent where the unit is thin． is characterized by the （1istribution of The maximum thickness of the unit is volcanic rocks （Fig．16）． The separate（1 O．6secOnd，． distribution of the grεしnitic rocks an（1 丁紅e b歌sements of the plateau an（i rise the volcanic rocks in the northern part are comPQse（i of granitic rocks εln（1 and the southem part are the same as gneiss． 丁捻融 gneiss is restricte（1to the in the Yamato Rise．An radiometric Korea Pl＆teau． The absolute age of the age of141．6Ma（Early cretaceous）for gneiss by the Rb／Sr dating method is the granite is known on the Kita－Oki 1，983to2，729M＆（Precambrian）（Table Bank（Table2）．This age is馨ompar餓bl轄 3）． Such ol（i Pre¢＆mbr搬n roeks suggest to the age of むhe granitic rocks of t葺e the Korea Rise i＄dosely related to the Kita－Yamato Bank． shield of the Kore＆n Peninsula．

κoreαP観ε側侃αu伽ησ捌se 0んzεshかゼ石琵dge The ：Korea Plateau and the UUun募 The Okushiri Ridge is an outstanding Rise are located．in the southwestem topographic feature in the northeastern part of the Japεしn Sea just east of the margin of the Japan Sea．The ridge Korea Peninsula． The KoreεしPlateau is shows nearly NS trend，ing． The ridge is the secon（1 1argest topographic high in not a single continuous ri（ige but is the J’apan Sea． The width of the Korea composed of en－echelon arrangement of Plateau is100km and the length is250 several small ridges．The ridge is traced km with the trend of NE．The Ullung from the west of Rishiri Island to off the Rise hasε1：round，configuration with a Oga Peninsula． Its overall length reach－ diameter of about100km．The typical es450km．Its width is not greater than seismic profiles of both topographic 50km．The maximum relative height over

一305一 照撚㏄▽冒

B副ε伽oμんεGθoZo9εcαZ翫rひeッo∫」αPαη．▽oZ．39，No。5

the sea floor of the Japan Basin exceeds ments of the ridge are mainly composed 2500m．The width and height of the of Late Miocene to Pliocene sediments ridge increases to the north． with a thin or no Quatemary sediments． The Okushiri Ridge is covered by rath－ The sampling data are restricted in the er thick sediments which reach1．O sec－ northern part of the ri（1ge． The acoustic ond．The sediment thickness of1．O sec－ basement of the northern Okushi：ri Ridge on（10n the ri（1ge is extraor（1inarily thick is composed of Early－Middle Miocene for such narrow ridges．The sediments， siltstones an（i san（1stones． The age d．e－ at the middle part of the ridge，are termination of the siltstone is based on continuous with the sediments of the diatom（SAwAMuRA1978MS）and pollen Japan Basin with appreciable thinning assemblages of the samples of GH77－3 of the upper most part．The rid．ge is cruise． The geological structure of the commonly associated with fault scarps Oklushi：ri Rid．ge is（liscussed．in detail in on either or both sides．The sedimen－ Chapter3．4． tary sequence crops out at the scarps of the Okushiri Ridge．Bottom samples 8磁o羅吻e from the scarps indicate that the sedi一 The Sado Ridge 1ies ln the eastern

駐◎9◎r◎v Seαm◎u爵曾 3一

」租04 v．e．器20 SEφ 4 9km 45ec

5 5

’鐵姦

9顯 6一∫ 6

7 7 tl驚

Figure24 Seismic pro丘1e of the Bogorov Seamount in the Japan Basin on Line J104．

一306一 σεOZO9乞CαZS施C伽θ0ノ伽JiαPαηSeαα記εむSカθCホ0痂εm頑C翻0ηS（K．Tαmα勧 margin of the Japan Sea parallel to Northeast Japan． The Sa（10 ri（1ge is 3． D量SCuss亘o聡 not a single ri（ige，but is compose（1 0f many small ridges．The small ridges The origin of the Japan Sea in contro－ are in a：row with the wid．th of80km． versia1（iespite of the abun（1ant geolo9－ Each ridge has a width of about10km ica1εln（i geophysica1（1ata on the area． and a length less than50km with the The author （lescribe（i the geological general trend of NNE．These small structure of the Japan Sea in Chapter2． ri（iges stand on a broa（l topographic The geological structure is the most rise （Fig．10）．The small ridges are essential data for（iiscussing the tecton－ boun（ie（玉 by faults on their either or ics of the area． The author discusses both sides．The ridges are commonly several principal problems of the Japan boun（1e（i by faults on its eastern si（le Sea， base（l on the observation of the but occasionally on both si（1es． The geological structure． The principal pro－ faults are（1iscusse（1in（1etail in Chapter blems are the age of the basins（when 3．4． was the Japan Sea formed？），the origin The sediment thickness on the small of the topographic highs which are close－ ri（ige is generally less than O．5 secon（1． 1y related to the sprea（1ing tectonics of The sediments are mostly composed of the Japan Sea，the tectonics of back－arc Pliocene and．Miocene sed．iments without spread．ing （review and，the case of the Quεlternary d，eposits on the ridges． The Japan Sea），and the recent crustal move－ basement is composed generally of vo1． ment along eastem margin of the Japan canic rocks．No radiometric ages are Sea． Through these（iiscussions，the au－ reporte（i on the Sa（10Ri（1ge． thor summarizes the tectonic evolution of the Japan Sea since its earliest stage ，BogorooSεαmoμ磁 ofdevelopment． The Bogorov Seamount is one of the largest topographic high in the Japan 3．1 Age o實he basi聡s Basin．The Bogorov Seamount is iso－ The age of formation of the Japan Sea 1ate（1 in the central part of the Japεln is critical for the tectonics of Japanese Basin．The basement of the Bogorov Island．s．However，until recently it was Seamount is not covered by an apPre－ a controversial subject．Usually，the ciable amount of sediments in the subsea most important information for the age area （Fig．24）．The sediments of the of back－arc basins are gotten from the Japan Basin abut to the basement of the deep sea boring an（1the i（1enti且cation of Bogorov Seamount．Some Russian sci－ magnetic anomaly lineations．As d．is－ entists identified the Bogorov Seamount cussed in Chapter1．1，these information as a Cretaceous basement（M肌ANKHoLINA is very weak in the J’apan Sea． and．KovYLIN，1977；GNIBIDENKo1979）． The plate tectonics show that the for． SAHNo and VAsILIEv （1974），however， mation of oceanic basins such as the reported，the occurrence of basalt with Japan Basin and，the Yamato Basin is the K－Ar age of18Ma（Tεlble3）． The caused by the sea且oor spreading Pro－ volcanic activity of such age in the cen－ cess which is analogous to the process trεll part of the Japan Sea shou1（i be along the mid－oceanic ridges．This means note（1in discussing the sprea（iing tecton－ that the formation age of the Japan Sea 1CS． is represente（1by the age of the oceanic

一307一 B沼ε漉o∫伽GeoZo8∫oαZ翫ro召ッo∫JiαPαπ．Vo乙39，1▽o。5

を γA棚70θ’sε 9ぜ。爵！ o“らγメ竃解ハ70 8 鴻 S ’ ～

upper stratified iayer （middl8Pl‘ocene》一RecenO 傭99 ’tran sparentlayer（。arlyP踊。cene》 ’ ＆ 『！クsハD・ 漁 『 SハDO R’Sε ＼ ～ ｝ 輔… 舗 而i評

lower stratified Iayer（寮ate剛ocene》雪 1 1

Figure25Summary cartoon of stratigraphy of the Yamato Basin． basement of the Japan Basin，the Yama． ent layer，but outcrops of the transpar－ to Basin，and，along with the othe：r oce－ ent layer，are common on the southem anic basins in the Japan Sea． Then，in slope of the Yamato Rise．Piston＆nd this section， the author （1iscusses the gravity cores from the southem slope of age of the oceanic basement of the Ja． the Yamato Rise such as sites RC24and pan，the Yεlmato，and．other significant St．656recovered older sediments with oceεmic b＆sins in the Japε皿Sea． the＆ge of4to5Ma．Site302penetrated the transparent layer and，the lowe：r ノ1geofyα肌α孟oBα伽 stratifie（11ayer． Accord，ing to these re－ There are two deep sea（lrilling sites sults，the age of the lower strεltified． in the Yamato Basin；Site299in the 1ayer is older than5Ma，presumably center of the basin an（i Site 302 in the extending to10Ma．Site302reached margin．There are many outcrops of the acoustic basement．SHIMAzu（1979） the lower sedimentary sequence of the identified the basement rock at Site302 Yamato Basin along the surrounding as possibly Green Tuff，but the rad．i－ topographic highs such as the Yamato OmetriC age WaS nOt giVen． Rise and the Sado Rid．ge． The：results The lower stratified．1ayer of Late of two deep sea d．rilling sites together Miocene age（5Ma to10Ma）is not dis－ with the bottom sampling data from the tribute（1in the central part of the bεlsin， surrounding outcrops present informa－ but is distributed only in the marginal tion effective for estim＆ting the age of area of the Yamato Basin．It is not the acoustic basement of the Yamato easy to trace the lower stratified layer Basin，even though the hole of Site299in into the central basin area． The layer the basin did not reach the basement． apPears to change into the transparent The principle of the age estimation of laye：r towa：rds the centrε11part of the tねe Yamato Basin based on such（iata basin．The age of the bottom．of the are summarized in a drawing shown in lower stratified layer is not determined Figure25． at this time，but it appears not to be The hole of Site 299 stoppe（l at the older than10Ma．The age of the bot－ bottom of the upPer strati且e（i layer an（i tom of the transparent layer at the the age of the bottom of t亘e hole was central part of the basin is estimated．to estimated to be3．5Ma by diatom analy． have almost the same age as that of the ses as d．escribed．in Chapter 2．3． The lower stratified layer at the margin。 hole di（1not penetrate into the transpar一 Then，the author estimates that the age

一308一 G20ZO8乞CαZS伽0傭eo∫伽」αPαηSεααη面孟SむεC古0η‘C伽頑Cα孟乞0ηS（K．Tαmα勧

γ鴻摺渦7’08ハ3’四 一3SEC．

」ハρ鴻」》B論sダ鯉 4一 冒 ＿4 』イ そ 1” ／ 心ρ♂ t

一レ ⑳ りo ／ 5 甑 餐1繋5

ヂ 6」 一6 ヂ ’ ／ 7」，，欝 鐘，

Figure26 Typical pro且les of the Japan Sea and the Yamato Basin． Both profiles represent the area of the thickest sediments of each basin．

of the acoustic basement or the age of 1ess．The sediment thickness of the Ja． the bottom of the sedimentary sequence pan Basin is2．1second，s while that of of the Yamato Basin is around10Ma the Yamato Basin is1．5seconds or less． （Middle－Late Miocene）． Thewaterdepthisabout3650minthe Japan Basin and about3000m in the ・49εεs6加αオεo功α8edoπわα8e肥漉 Yamato Basin． 砲ρ痂 The greater basement depth of the Figure26shows a comparison between oceanic crust is associated with the older seismic profiles of the Japan Basin and age of the crust （PARsoNs an（i ScLATER， the Yεしmato Basin． The figure ＄hows 1977）．The J’apan Basin and the Yamato the prominent difference on both basins． Basin are un（ierlain by oceanic crusts The Japεしn Basin is（ieeper in（1epth and． （BERsENEvθ孟αZ。，19701MuRAucHI，19721 greater in sediment thickness，and lies in LuDwIGααZ．，1975）．Then if the acoustic deeper water．The ba．sement depth of basement of the both basins represents the J’apan basin is7．O seconds while that an oceεlnic basaltic layer， the （1eeper of the Yamato Basin is5．5seconds or basement of the Japan Basin indicates

一309一 灘難灘馨馨

一B沼θ伽oμんθGeoZo9εcαZS那rひεッ・ノ」αPαπ。γoZ．39，No・5

Table4 Theoretical depth of the basement（“Depth1”and“Depth2”），thickness of the lithospheマe（“Thickness”），and heat flow value（“Heatflow1”and“Heatflow2”） aCCOrdingtOageVariatiOn． Age（Ma）Depth1（m） Depth2（m） Thickness（km） Heatflow1（HFU） Heatnow2（HFU）

5 3504 3283 16．75 5．05 5．37 6 3561 3357 18．35 4．61 4．90 7 3614 3426 19．82 4．27 4．54 8 3664 3490 21．18 4．00 4．24 9 3710 3550 22．47 3．77 4．00 10 3754 3607 23．69 3．57 3．79 11 3795 3661 24．84 3．41 3．62 12 3835 3712 25．95 3．26 3．46 13 3873 3762 27．01 3．13 3．33 14 3910 3810 28．03 3．02 3．21 15 3946 3856 29．01 2．92 3．10 16 3980 3900 29．96 2．83 3．00 17 4013 3943 30．88 2．74 2．91 18 4046 3985 31．78 2．66 2．83 19 4077 4026 32．65 2．59 2．75 20 4107 4065 33．50 2．53 2．68 § 21 4137 4104 34．32 2．47 2．62 22 4166 4142 35．13 2．41 2．56 ＄ 23 4195 4179 35．92 2．36 2．50 24 4223 4215 36．69 2．31 2．45 4250 4250 に 25 37．45 2．26 2．40 蒙 窪 26 4277 4285 38．19 2．22 2．35 購 27 4303 4319 38．92 2．17 2。31 黙 翼 28 4329 4352 39、63 2．14 2．27 29 4354 4385 40．33 2。10 2．23 嚢 4417 嚢 30 4379 41．02 2．06 2．19 31 4403 4449 41．70 2．03 2．16

暴 32 4427 4480 42．37 2．00 2．12 33 4451 4511 43．03 1．97 2．09 軽 34 4474 4541 43．67 1．94 2．06 35 4497 4571 44．31 1．91 2．03 36 4520 4600 44．94 1．88 2．00 37 4542 4629 45．56 1．86 1．97 鐸 38 4564 4658 46．17 1．83 1．95 纂 39 4586 4686 46．78 1．81 1．92 40 4608 4714 47．37 1．79 1．90 41 4629 4741 47．96 1．76 1．87 42 4650 4768 48．54 1．74 1．85 難 43 4671 4795 49．12 1．72 1．83 サぞ 44 4691 4822 49．68 1．70 1．81 繋 45 4711 4848 50．24 1．68 1．79 Calculation formula is as follows．“Age”is in Ma． Depth1ニ2900＋270＊SQRT（Age）…HAYEs（1983）， Depth2＝2500＋350＊SQRT（Age）・・・PARsoN＆ScLATER （1977）， Thickness＝7。49＊SQRT（Age）…YosHIIθ6α乙（1976）， Heatflow1＝1L3／SQRT（Age ”。PARsoN＆ScLATER（1977）， Heatflow2；12／SQRT（Agle）…DAvls＆LlsTER（1977） 一310一 σ20ZO8’ε0αZS施e卿eoμんε」1αPαηSθααη引古S乙ε0む0ηεC‘m頑Cα古‘0ηS（K．Tαmα勧

an older age than that of the Yamato 26，thewater depthis3630mandthe Basin．The thicker sediments of the two－way sedim，ent thickness is2。1sec－ J’apan Basin also supports this compar－ onds．Then the basement d．epth after ative age estimation．The thicker sedi－ the sediment loading correction is ob－ ments in the Japan Basin correspond to tained．as4890masfollows． an ol（1er age of the basin， although 3630＋6000×2．1＝4890m there is a problem with sedimentation For example，the basement depth of4890 rate．Both basins have a similar acous－ mcorrespondstoanageolderthan45 tic sedimenta「y sequence，which may Ma according to Table4．Depth l and suggest that the sedimentation rate in Depth20n the table，however，show the the both basins is similar．Thus，it is theoretica1（iepths of an open ocean such p：robable that the Japan Basin is old．er as the Pacific and．Atlantic Ocean． than the Yamato Basin，if the acou怠tic The basement depth of back－arc ba－ basements of the both basins are oce． sins is definitely greater than the theo－ anic．Seismic refraction d．ata，however， retical depth，where the age determina－ suggest that the acoustic basement of tion is well establishe（1（ScLATER，1972）． the Yamato Basin is different from that The Shikoku Basin，which is documented of the oceanic crust．The problem will to have been active during30Ma to15 be discussed in later in this section． Ma based on the deep sea drilling results Table4shows the relation among the and the identification of magnetic anom－ age of the oceanic crust，the theoretical aly lineations （KLEIN an（l KoBAYAsHI， water depths，lithospheric thickness，and 1980），has a basement depth of4500to heat flow values．Theo：retical depths and． 5000m．The range of age and basement heat flow values are shown for two d．epth of the Shikoku Basin shows a cases． It shoul（1be note（i that the theo－ discrepancy with the theoretical age of retical water d．epth is the water d，epth Depth l and，Depth2in the tεlble． The without sediments．So，a sediment load－ basement（1epths of back－arc basins ap－ ing correction is nee（1e（1 in the case of peartobe1000mgreaterthanthoseof the basins which have a thick sedimen． normal oceanic basins（KoBAYAsHI，1984）． tary cover such as’the basins in the But no definite theoretical age一（iepth Japan Sea．If the sedimentary cover is re王訊tion has been establishe（l in back－arc removed from the basin，the basement basins． Therefore，the author trie（i to of the basin wil1：rebound．isostεしtically． compare the basement depths of the The basement depth of isostatic compen－ Japan and Yamato Basins with other sation after removal of the sediments back－arc basins where the age of the should be that estimated．by the theoreti－ basins are well documented．The results cal age． are shown in Figure27． CRouGH（1983）presented．a simple for－ Figu：re27shows that the age range of mula for the sediment loading correction the Japan Basin is comparable with of an oceanic bεlsin． The correction for－ those of the Shikoku Basin an（l South mula乳sasfollows． China Sea Basin． The Shikoku Basin Basement depth after sediment has an age range of30to15Ma and the loading correction（m） South China Sea Basin has an age range ＝（water depth（m））＋600 of 32 to 15Ma． Then，it is probable ×（two－way sediment thickness（sec）） that the Japan Basin hεls an age range In the case of the Japan Basin on Figure of about30to15Ma．The depth range

一311一 B沼e伽oゾ伽σθ・Z・9オoαZ翫r∂εッoμαPαπ．V・乙39，〈「・・5

5000m 4200m 3600m Japa鴎Basln←一一一一一一・ ？ ゆ

3900m 3300m Ya鵬ato Basi『1《一

4100m 3600m Marla廟a Troug㎞磯一6Ma OMa

5000m 4500m SMkokuBasln←一一30Ma 15Ma

5000m 4500m Sou量hChlnaBasin←一一一一一一一一一㊥ 32Ma 17Ma

Figure27 Basement depths after sediment loading correction of the Japan and Yamato Basins with those of other several back－arc basin whose ages are well established by DSDP holes＆nd／or identi且cation of magnetic anomaly lineations． of the Kuril Basin is also comparable of YosHII and，YAMANo（1983）． with that of the Japan Basin although The heat flow data of the basin area the age is not established．The compar－ are selected from about100sites of the ison between the both basins is discussed． heat且ow measurements in the Japan in Chapter3．5． Sea． The sites were selected，from the Figure27 also shows that the age areas where the sediment thickness is range of the Yamato Basin is very young greaterthan300mandwherethereare less than6Ma．This estimation shows no seamounts in a range of10km。・The a large discrepancy with the former other sites were rejected． The results stratigraphic age estimation of around are shown in Table5and．Figure28． 10Ma．The problem will be discussed Theεlverage heat flow value of the later in this section． Japan Basin is2．26H：FU。The value of 2．26HFU correspond．s to an age of25to 過gee8伽αオε0舶α8e面πんeα薦0脚α伽 28Ma． The average heat flow vaユue of The theoretical age－heat flow relation the Yamato Basin is2．34HFU．The value is applicεlble to back－arc basins， al－ of2．34HFU corresponds toεln age of23 though the theoreticεし1age－depth re1εltion to26Ma．It should be noted that the does not match in the back－arc basins． average heat flow value of the Japan TAYLoR and H：AYEs（1983）con且rmed that and Yamato Basin are very close and the theoretical age－heat flow relation in that the age estimation based．on the the open ocean is vali（1 in the South average heat How value of the Yamato China Sea Basin．The author examined Basin show a large d．iscrepancy with the the heat flow dεlta in the Japεln Sea age estimation based on other techniques bεlsed on the d，igital heat flow dεlta file such as stratigraphy an（1 basement

一312一 GεoZO9‘0αZS伽伽rθ0∫古んε」αPαπSθααπ面むSεθ0む0耽伽頑0面0ηS（KTαmα勧

Table5 Heat flow values with the average of the basins in the J’apan Sea and Okhotsk Sea． The血eat且ow values are selected from YosHII and YAMANo’s（1983）heat flow digital data丘1e according to the manner that the sites of the sediment thickness less than 300meters are relected and that the sites near to topographic highs with the range of10kmarealsorejected． JAPAN BASIN 2．88 2．69 2．66 2．65 2．64 average hf of 2．64 2．62 2．62 2．60 2．60 Japan Basin＝2．26 2．58 2．57 2．53 2．51 2．50 2．50 2．48 2．48 2．44 2．44 2．42 2．39 2．38 2．37 2．32 2．28 2．28 2．25 2．24 2．22 2．20 2．20 2．19 2．18 2．15 2．14 2．14 2．13 2。13 2．13 2．12 2．12 2．11 2．08 2．07 2．07 2．07 2．05 2．05 2．03 2．02 2．02 2．01 2．00 1．99 1．98 1．95 1．91 1．89 1．87 1．70 1．40 YAMATO BASIN 3．46 2．77 2．66 2．58 2．56 average hf of 2．45 2．44 2。43 2．43 2．39’ Yamato Basin二2．34 2．36 2．35 2．34 2．33 2．32 2．32 2．31 2。21 2．16 2。16 2．15 2．13 2．12 2．12 2．07 2．05 2．02 1．95 TSUSHIMA BASIN 2．46’ 2．40 2．36 2．25 2．16 averagehfof 2．12 Tsushima Basin＝2．29 TARTARY TROUGH 3．28 2．40 2．32 2。29 2．18 average hf of 1．99 Tartary Troughニ2．41 MOGAMI TROUGH 1．28 1．39 1。40 1．61 averagehfof Mogami Trough＝1．42 OKI TROUGH 1．99 1．89 2．08 1．73 average hf of Oki Trough＝1．92 KITAYAMATO T． 1．80 1．80 average hf of Kitayamato Troughニ1．80 KURIL BASIN 2．82 2．58 2．57 2．53 2．41 average hf of 2．33 2．28 2．27 2．22 2，21 Kuril Basin＝2．33 2．20 2．1：L 2．11 1．91

depth． The average heat flow values of tion in the sediments and the basement the Tsushima Basin（2．29HFU）and．the ：rocks，while they rεしrely appear larger Tartary Trough （2．41HFU） are also than the t：rue hea，t flow value． Thus，the close to those of the Japan and．Yamato discussion of the highest heat flow value Basins． is valid．． The heat flow data tend．to appea：r less The highest heat flow value in the than the true heat flow value bacause of Japan Basin is2．88HFU which corres－ the mass heat transfer by water circula一 ponds to an age of15to18Ma．The

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HeatFlo㈹ftheJapanSea VaユUe diStribUt10n

Datapo工nts Ya偲to Basin 轟0 □ 論pan Basin 鯉 30

20

10

‘蓑 ●釜 0 3．25 3零00 2。75 2．50 2唇25 2．00 1．75 1．50 1．25

Heat F10U （HFU）

Figure28 Distribution of heat且ow values in the Japan Basin and the Yamato Basin。The heat flow data a．re after YosHII and YAMANoヲs（1983）d，igital hea．t now data file．

highest heat How value in the Yamato cussion about the age of the Japan an（1 Basin is3．46HFU which corresponds to Yamato Basins．The results of the age an age of10to12Ma．An age of10to estimation based on basement depth and 12Ma for the Yamato Basin is compa． heat flow of the Japan Basin show the rable to the stratigraphic age estima． range of30to15Ma．The true age（1e－ tion．The comparison of the highest termination will be made by further heat flow value in the Japan and Yamato deep sea drilling and magnetic anomaly Basins shows the younge：r age of the surveys・ Yamato Basin．The sites of the highest On the contrary，the results on the heat flow of each basin a：re located．at Yamato Basin show－a large discrepancy the central part of the Japan Basin an（1 among the three age estimations of at the marginal part of the Yamato stratigraphy，basement depth，and heat Basin．The highest value in the Yamato 且ow．Heat flow values show the age Basin is isolated on the （iistribution range from300r more to10Ma．The g：raph of Figu：re28． The heat flow（1ata strεしtigrεlphy shows the age to be around present a valuable information for the 10Ma．The basement depth shows the age estimation of the Japan Sea but age to be less than6Ma．The Yamato they apPear to be insuf丑cient for a（1e－ Basin is overlain by a thick accumula－ taile（1discussion． tion of sediments which are not strongly deformed。The structural features do 翫㎜㎜α昭oノ痂eα9εoκheゐα伽8 not show any active spread．ing in the Figure29summarizes the above dis． basin．Therefore，the age estimation by

一314一 GθoZo9εoαZs伽伽rεo∫孟んeJαPαπSθα侃面εsホθcむoη‘c‘脚Z‘cα加ηs（K．Tαmα勧

30朋a 2？馳 7？麗θ OI所a

stratigraphy？？1

JAPAN BAS圓N ←basement depth→ ？ 一③1 一悼heat flow－ I I

1 stratigraphy←→ i YAMATO BASIN basement depth磯＿吋》i 岡中一heat flow

Figure29 Summary sheet of the age of the Japan Basin and，the Yamato Basin base（10n several estimation methods． basement depth of6to O Ma is unreali－ er basement in the central part of the stic． basin．The back－arc basins commonly Apossibleexplanationfo：rthisd．is． have a more shallow basement in the crepancy is that the acoustic basement central pεlrt，because the back－arc basin of the Yamato Bεlsin d．oes not rep：resent was formed symmetrically by a spread－ the oceanic crust an（l that the true oce－ ing center as the younger crusts repre－ anic crust un（ierlies the acoustic base－ sent more shallow basement depth．The ment．According to the seismic refrac－ structure of出e Yamato Basin with tion study of the Japan Sea by LuDwIG deeper basement in thG central part is e6α1． （1975），the acoustic basement of incompatible with b農ck－arc spreading． the Yamato Basin has slower sonic ve． Such structure，how鯵vむr，is not unrea－ locity of3．5km／sec than the normal sonable on the assumption that the oceanic basalt layer of4．5km／sec．If a acoustic basement o董毎the Yamato Basin deeper true basement is under辱the Ya． represents volcεmic1＆stics such as Green mato Basin，the heat now value of the Tuff and that the m鍵ginal more shallow basin is probably comparable with that acoustic basement鐙due to thicker ac－ of the true basement．The stratigraphic cumulation of the volcaniclastics．The estimation of the age of the Yamato thicker accumul＆tion of the volcano－ Basin is base（i only on the thickness of clastics in the margin of the basin will se（1imentary layer． The rather young be（liscusse（i in Ch縦pter3．3． age from the stratigraphic estimation is The mo艶shal／ow acoustic basement reasonable if the3．5km／sec layer inter－ in the marginal p畿rt of the basin is also venes between the sedimentary layer and thec＆seintheJ＆p鋤Basin．Abroken the true basement． line ar：row in Fi諾臆驚29shows this fea－ LuDwIG θ診α1． （1975） pointe（l out the ture．Figure30show＄a continuous pro一 possibility that the 3．5km／sec layer is 且1e through the J慕pan Basin and the correlated to Green Tuff．Some profiles Yamato Basin．丁負G decreasing basement of the Yamato Basin such as Line N80f depth toward t姓e Yamato Basin is Figure10show more shallow acoustic observed on the proHle．The profile may basement in the marginal part and deep一 indicate thicker volcaniclastic accumula一

一315一 B沼θ伽oμんeGθoZo9‘oαZS群ひ2ッo／JiαPαη．γoZ．39，ノ〉o．5

tions toward the margin of the Japan Basin and the Yamato Basin． り 幽 In conclusion，the Japan Basin has an の の マ 嶋 age range of about30to15Ma and the 噌 ＝T『戸甲一 ◎ Yamato Basin has that of30to10Ma． 鍔 ！ The Tartary and the Tsushima Basins are estimated．to have similar age range き で一葛識 難轍†廟 of the Japan and，the Yamato Basins． ｛・ 》 ，』－臨， 目o ㌧ 3．2 （）rigin oぜthe topogr乱1｝hie h畳ghs of Φ 自 o じ theぬpa取Sea 卜 一 ．ユ」． There are many topographic highs in 《 q 署 O 一一r・1 the Japan Sea （Fig．2）． The Japan Sea q 《 ・一 の has an（l extraor（iinary mεlny topograph－ 》 o 口q ic highs in comparison to other back－arc の 捻 basins． The South China Sea Basin also d9 has abundant of topographic highs，but ＞

① the outstanding feature of the Yamato 調 妻L， 一 Rise in the center of the Japan Sea 〇 り shou1（l be note（i． The southern part of

・Fくq の the Japan Sea is occupie（1by an especia1－ ¢ m ly large number of topographic highs の 娼 巨 邸 （Fig．31）． The origin of the topographic 邸q 隷！聡 ㌧ highs shou1（1 be closely relate（i to the ゆ tectonics of the Japan Sea． 番・》 曇 o 田 The author divided the topographic ㊦ 一 9 highs into four groups base（i on the ＞ o ト 遷 竈 geological structure． They are continen－ tal fragments，rifted continental frag－ q2 ．2 ments， tectonic ridges， and volcanic 9 ．曽 seamounts．Their distribution is sum． 8 marized in Figure32excluding the tec． 撃 tonic ri（1ges． The detail of each group

．日2 is as follows． 揖 （1） Continental fragments O Q The topographic highs composed of continentε11 crust are εlssigne（i to this r oつo Φ 旨 group．Many granitic rocks are recov－ ⇒ bρ プ ered from the topographic highs in the h Japan Sea．The Yamato Rise，the Korea Plateau，the Ullung Plateau，the Kita－Oki Bank，the Oki Bank，and the Musashi

玄 Bank are the typical example of conti－ 置 母 nental fragments．The Bogorov Sea－ ；’撃一一 噌 輪 鱒 臥 mount is also included in this group accord．ing to the Russian scientists（e．g．，

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①琳 o o軸 き ぽ コ． § 罰 Ωo 営． ○ さ 集 ．因 醤 iii

謹 ）

Figure31 3－D topographic view of the southern part of the Japan Sea． The data used for processing are GEBCO digital bathymetric data of JODC and GSJ digital bathymetric data。Processing system is SIGMA of Geological Survey of Japan． B観e伽oμんεGθoZo9‘oαZSμ脱ッo∫」αPαη．．VbZ．39，No．5

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Figure32 ClassiHcation of the topographic highs of the Japan Sea．

一318一 GθoZo9‘cαZs伽伽rεo∫カんε」⑫αηSεααη霞古sむ20むoη‘c伽pZ‘o面oηs（KTα肌αん‘）

MELANKHoLINA and KovYLIN，19771GNI－ thisgroup．Arifted，continentalfragment BIDENKo， 1979）． 01der granitic rocks is inferre（1to be stretche（1from its orig－ from Precambrian to Cretaceous are inal configuration． The i（ientification of distribute（i on the topogrεしphic highs as the（iistribution of the rifte（l continental described in Chapter2．4．Most of the fragments is important for the recon－ large topographic highs in the Japan struction of the sprea（iing history of the Sea are inclu（1e（1 in this group． The Japan Sea． scattered （listribution of continentεll （3）Tectonicridges fragmentS makeS an effeCtiVe COnStraint The Okushiri Rid．ge and．the Sad．o Rid．ge for discussing the spreading system of belong to this group．Recent EW con－ the Japan Sea．The mechanisms that vergent tectonics is evi（lent along these cause（i such a scattere（i （iistribution of ridges accompanying the active thrust the numerous continental fragments is a faults and compressional type earth． principal problem for （iiscussion． This quakes． The ridges are boun（le（i by discussion is in Chapter3．3． thrust faults on either si（ie or both （2） Rifted continental fragments si（1es．The thrust faults have been active A rifted continental fragment is a since latest Phocene．Several compres－ continental f：ragment which was thinned． sional type earthquakes with magnitudes an（1 stretche（1 by the rifting activity。 ofM6．9toM7．70ccurredalongthese This group was identified．as d．eeply thrust faults． The topographic highs of submerged basement highs with very this group are formed by the thrust rugged morphology．One typical exam－ movements associated with the uplift of Ple is the northern extension of the the hanging si（le over the footwa11． A Musashi Bank which is shown in Figure detailed （liscussion about the Okushiri 15． Horst an（l graben structures are and the Sado Rid．ges is made in Chapter prominent there．This structure is ana1－ 3．4． ogous to that of the lower continental （4） Volcanic seamounts slope of the Atlantic continental ma：rgin Many seamounts and knolls are distri． （DINGLE an（l ScHuRuTToN，1977）． Such bute（i throughout the Japan Sea besi（les features are inferre（1 to be foun（iere（1 the large topographic highs such as continental crust which was rifted by rises， plateaus， an（1 banks． They are the initial generation of oceanic crust almost all volcanic in origin．There are within the continental crust（MoNTADERT two groups of volcanic seamounts．The θ6α1．，1977）． The Takuyo Bεlnk an（l the one group was formed by the recent day Sa（io Rise are inferre（i to be the other islan（1arc activity．The Northeast Japan examples of rifte（i continental frag－ Arc and the Southwest Japan Arc have ments．The Sado Rise is the found．ation a broad．volcanic zone which exten（1s rise of the Sado Ridge which is shown in into the offshore area of the Japan Sea． Figure10． The Sado Rise is also includ．一 Takeshima Island，Ullung Island，and ed in the rifted continental fragments． possibly the Shiribeshi Seamount west The Sa（lo Ridge itself is classified，asεl of Hokkai（loεlre inclu（1e（1in this group． tectonic ri（ige． The small rises aroun（1 Takeshima and Ullung Islands are cov． the Yamato Rise，especially the north－ ered by alkali basalt which is re1εしted，to eastern en（10f the rise，are also inclu（1e（l the recent volcanism of the Southwest in this group． The Hakusanse Bank off Japan Arc． The other group consists the Note Peninsula may also belong to of older seamounts and knolls whose

一319一 B捌2伽oゾ伽GεoZo9‘cαZSrひθッo∫」αPαη．VoZ．39，ノ〉o。5

activities we：re presumably synchronous back－arc sprea（1ing． The convection is with the sp：readingεlctivity of the basin generate（1by the（1rag force of the sub－ area．Such seamounts and knolls are ducting oceanic plate （e．9．， SLEEP an（i ・commonly observed in the J’apan Basin TOKsoz，1971）． and the Yamato Basin．The Yamato 3） Divergent movement between the Seamount Chain and the Central Yamato Plates causes the generation of new oce－ Seamount chain are the typical examples anic crust in the gap．In this case，back－ of this group．Both seamount chains arc sprea（1ing occurs passivlely． There apPear to be composed of andesites． a：re two ideas on the d．ivergent vectors Andesites from the Matsu Seamount of over island arcs．MoLNAR and ATwATER the Yamato Seamount Chain are dated （1978）consid．e：red that oceanwεしrd．retreεしt to be4．16±0．16Ma（UENoααZ．，1971）． of trench is significant for back－arc Unknown rocks from Meiyo No．2Sea． sprea（1ing．They consi（1ered，that the o1（l mount are dated to be about13Ma an（i cold lithosphere sinks into the as－ （OzIMA ααZ．，1972）． The age of these thenosphere and，causes the oceanwa：rd seamounts is possibly synchronous with retreat of trench． On the contrary， the basin formation and presents infor－ UYEDA and KANAMoRI（1979）considered mation for the formation age of the that the retreat of the back－arc plate is basin area． significant for the generation of the Aborted rifts are annota，ted in Figure back－arc basin． 32。 Some possible major aborted rifts There is a significant （iifference be－ are common on and in between the tween the former two models and the continental fragments in the Japan Sea． last one mentioned above．The former Figu：re33shows a profile of the Kita－ two models are based on the active Yamato Trough in the Yamato Rise extension or sprea（1ing force in the back－ which is the largest possible abo：rted，rift arc basin which is caused by mass up－ in the Japan Sea． welling or secondary induced mantle The tectonic origins of topographic convection． In these two cases，the highs mentioned．above are discussed in spreading center in the back－arc basin Chapters3．3an（13。4． will cause compressional stresses in the surroun（ling area inclu（1ing the arc re－ 3．3Tecton量esofback一鍵esprea伽9 gion an（1 also the sprea（1ing center The tectonics of back－arc sprea（1ing should be fixed in the reference frame of have been controversial since KARIG the trench axis．The study of the Mari－ （1981） originally propose（i the concept ana Arc，which is associated with a of back－arc sprea（ling． There are sev－ typical active bεlck－arc basin，however， eral i（1eas about the origin of the back－ shows tha，t the Ma：riana A：rc is entirely arc sprea（1ing． They are classifie（1into un（ier tensional stresses including the three g：roups as follows． forearcarea（HussoNGand．UYEDA，19821 1） Excess mass upwelling，whic赴is FRYER and HussoNG and．FRYER，1982）． generated．in association with subduction Almost all the back－arc basins are gen－ of the oceanic plate beneath the islan（l erated．symmetrically from a spreading arc， causes the back－arc sprea（ling center which migrates land．ward from （KARIG，1971）． the trench axis． These observations in－ 2） Secondary induced mantle convec－ dicate that there is no active force from tion beneath the island arc causes the the sprea（1ing ri（1ge．

一320一 瞬d 0 O5＠c．

一 1

鳴Q 1 ○ 孕 娘葺 ○へ 一謬1 晦 謡『 贈 緊呼 騨鞭 8り 愛 、τ一 蟹慧騨． ・二 伊 ．葉萎 1も 巳 2 灘 套1 ．置掩一 丁 象 、 璽畠樺 ．鷺 ｛ 一．』回 ～ 欝 9 鐸・ 芋胃響欝 1直’ 0 鞭、・鰭 美 譲 ‘麟劉 9け ・善 踊叢・・髄 置 灘 禰欝i，難i置 ・舜マ裟．・亀 ミ ■ 3 、灘 3 盗 ㎡ D277 野」』鴛噌 『唱 ・ミ鐸重 YA納ATo／ 義 9 璽 縫 、響 一 瓢ξ 溶津 よ』紅7 ’気 と 罫 騰 ’ 々シ㌧ ・難 ・藁 ㌻こ糠・紫 心 ぎ．・霧騨1 響 ノ！ノーり ① 〆’1 ■’ 義 禽も 驚響 ぎ ”！ ＾ 、 〆 議 ＿4 BANK 一 4 乳噌， A－YA納ATO ハ 喜 獲酵・ 鮮∵ 欝遵 ！ 鯵 、！，こ 巽 § ’11ノ’ 騰 饗叢 驚鞠’ 、ll・． ，r．1 ’、 一奨 野1 該、 、 ；1 ε ／ 留 艶輯 の一自 禦酵霧響餓 、幻TA－YAハ《ATO 躁 9 ’ 5一 、 、’ 、一 へ 5 聖、ヤ・ン ・〆ノ ℃〃第・ ￥ 、＼ TROUG闇 Ω 認 嵐｝鑛讐鐘警 男い’＼v ！’ 、＝箔、／ ・、♪二！、～、、、 1『マ 灘 § 1￥ 、、 繕 難鑓 l’ 、 、／ ノノ’ 、r ＼、 ，一矧蘂薇 H 一 謙’ 泥．糞’囎’・ 、㌧、 、 劣ノ I 、 QD 費漁P． 、 尽醗、荻灘 凝い鎗塗 ・・料 尊 3 Φ 6 ’ o ！へ、、熱 幹 ．T『； ， o 醤 母鵠欝 ド 』AP《N 《 ♀． 国 BAS悩 群 、 N 4 ガ‘るク 1 1 ・灘§ ξ 1 1 嘩囲W 籠瞼 、 、 黛 30km 0 O 9 薬 き． ”・・一一一 ぐ㌧、 ver曾icol exo99。＝20．7 吻撫4 ＼ O 5 蕎

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Figure33 Seismic profile an（1 its interpretation of the ：Kita－Yamato Trough．Graben structure is remarkable in the trough。

隔蹴撒一徽鮒蹄織脳鰍儲麗㎜鋤照蹴難撒罵脳配獺㈱ 繍襯騰配囎繍磯購 陛

一B詔θ伽oμhθ（7εoZogZcαZSαrひeyo∫♂αPαπ。γoZ．39，No。5

DEwEY（1980）summarized the tectonics together to make a single rift system of island arcs in the reference frame of parallel with the trench． If the exten－ three significant plates l the back－arc sional stress is sustained，the rift system Plate，the forearc sliver （forearc plate）， will cause a spread．ing center of a new and the oceεlnic p1εlte．DEwEYexplained back－arc basin． The rifting feature of th6tect・nic・rigin・fback－arcspreading the Bonin Arc shows thεlt，when the arc very reasonably． His mo（iel is a re丘ne（1 is extension＆1，the break－up of the arc version of the thir（1mo（1el，the passive occurs along the volcanic f：ront where spreading mode1．The tectonics of island the arc is weakest． arcs are well described．by three plates， It should be noted thatむhe moving the sub（iucting oceanic plate，the back－ vector of the sub（1ucting oceanic plate arc plate，an（1the forearc plate（Fig．34）． scarcely affects the movement of the The arc volcanic zone marks a boundary forearc plate an（i that the vector is not between the back－arc plate and the fore－ signiHcant for back－arc sprea（1ing． The arc plate，because the lithosphere of the signi且cant factor for constraining the island a：rc is hot，thin，weak，and．then movement of the forearc plate is the ductile along the volcanic zone． In other migration of the trench．DEwEY（1980） word，s，the arc is d．ecoupled．along the volcanic zone and．forms two’plates，the forearc plate and the bεlck－arc plate． 7．comρ『eSS’の遡aヂ

The sub（1ucting oceanic plate，the forearc 〃 plate，an（i the back－arc plate have their 争築蓑ダ own moving vectors．The significant vec－ 犠、 tors for back－arc sprea（ling are the moving vectors of the back－arc and the

forearc plates． If the back－arc plate 響， an（i the forearc plate are （iiverging to eεlch other，the volcanic zone is place（1 2．r’奮血g sfθge under tensional stress an（i the tensional

stress produces a rift system along the プク F一勢‘ volcanic zone．Such a rift system is observed．in the present day Bonin Arc ＼麓疑，、、／ （TAMAKI e6α1．，1981b l TAMAKI and，MIYA－ ZAKI，1984）． The Bonin Arc is presently und．ergoing incipient back一εlrc sp：read．ing．Rifting of

the arc is in progress just along the 3．bacκ8arc sρfead㎞9 volcanic front．The volcanism of the 、多 ’養 Bonin Arc is concentrated．in the volcanic ＆

マニノプ ィフノ front，forming an active volcanic ridge 筆・象 秘．6熱，実 ／ （the Shichito Rid．ge），d．ue to the high dip ＼ angle （60。 to 75。） of the sub（iucting slab． The rifts are not continuous but segmented （Fig。35）．Figure36shows one of the typical profiles of the rift near Sumisu Island in the Bonin Arc． Figure34 Rifting and back－arc spreading of The rifts are now extending and．will join the arc with single volcanic chain．

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propose（1 that the trench shou1（1 sub－ is1εlnd．arc is arrange（10n a line． The stantially ret：reat oceanwa：rds or roll single volcanic chain is due to the con－ bεlck．This roll back of trench is accord． centration of volcanic翫ctivity along the ing to the idea of MoLNAR an可ATwATER volcanic front which is fu：rther d．ue to （1978）． The g：reater is the age of the the steep （1ip angle of the sub（1uction oceanic plate，the larger is the roll back slab．A spreading system developed from vector of the trench will be． Thus，the the single rift system usually causes a roll back vector of the trench is signi丘一 single lineate（l remanent arc such as the cant for constraining the movement of ：Kyushu－Palau Ridge and the west Mari－ the foreal℃ plate． The migration of ana Ridge in the Philippine Sea． The trench，howeve：r，is not restricted to roll spreading of a single rift system lik：e back movement．CARLsoN and MELIA this would not cause scattered．continen． （1984）P・inted・utthep・ssibility・flahd－ tal fragments in the oceanic basins as ward migration or roll forward．of the shown in the Japan Sea．Spreading with trench in the case of the Bonin Trench． a single rift system is not the case for The moving vector of the forearc plate the Japan Sea． The plausible spreading is also constrained by drag forces which causing the scattered continental frag－ are cause（I by the oblique subduction of ments is as follows．Figure37summa－ the oceanic plate when the arc is und．e：r rizes this concept in3－D cartoons． compressional stresses （KIMuRA and In the case of the Japan Sea，many TAMAKI，1985）． The d，rag force vector parallel rifts appear to have been formed parallels to the trench． The magnitu（1e in a wide volcanic zone． The wid，e vol－ of the drag force vector is less than the canic zone of the islan（1arc correspon（ls trench－parallel component of the moving to the low dip angle of the subd．ucting vector of the sub（iucting oceanic plate slab． Large tensional forces over the which is parallel to the t：rench． The wid．e volcanic zone cause（l many rifts moving卜vector，of the forearc plate is accompanying the volcanoes as multi calculated by the summation of the rift system （Fig．37，2）．Some of the trench roll back vector an（i the （1rag ：rifts were failed．an（11eft as remnant force vector．The trench roll back vector rifts such as the Kita－Yamato Trough is more signi且cant for back－arc spread．一 and．the Oki Trough． Othe：r：rifts were ing than the（1rag force vector． joine（i together an（1 （1evelope（1 into Thus，the two vectors，the trench roll spreading centers of the back－arc basin． back vector and the moving vector of In the course of the development，many back－arc plate，are signi丘cant for back－ COntinental fragmentS Were fOrmed and arc spreading． If there is some diver－ trapPed in the Japan Sea as remnant gent component between the two vectors， arcs such as the Yamato Rise，the Korea rifting of the a：rc is initiated．and．it will Rise，the Bogo：rov Seamount，and．so on． be （ieveloped into back－arc sprea（iing Stretching of the continental lithosphere （Fig．34）． Then，when we d．iscuss the may also have occurred at some places tectonics of the back－arc sprea（iing of due to the activity of multi rift system the Japan Sea，we have to estimate the and cεluse（I the rifte（l continental frag－ trend and the magnitude of both vectors． ment such as the Sado Rise． The spreading of the Mariana and The low dip angle subduction causes a Bonin Arcs causes simple single rift wid．e volcanic zone of island，arc． Arc system，as the volcanic chain along the volcanism should．still be active during

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200

Figure35 Rift system of the Bonin Arc（TAMAKIααZ。，1981）．

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1 ミ、、、、 1騙 曇 蕊

Figure36 Seismic pro丘le of the Sumisu Depression，one of the rift of the Bonin Arc。

畢 匹㎜－

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7，co㎎esε㎞’ occurs in the arc volcanic zone． Thus，

〆多ク the（1istribution of such overprinting fea－ 下ズ 瓜 弓“ tures is prominent in the marginal area 《 〃 F of the bεlck－arc basin and back－arc si（1e 8／ 衆 瓜《 シ ’為 殊 《 、β O£the Japanese Islan（1s． Thus the back－arc sprea（1ing of the ねpan Sea is different from that of the Mariana－Bonin Arc．The author calls the case of the Mariana－Bonin Arc as a 2・欣動g s拍ge single rift type back－arc sprea（iing an（1 〃 〃 the case of the Japan Sea a multi rift がドモ 喝￥一警．影！ ＼ 諺 4 type back－arc sprea（iing （Fig．38）． The ／ 〃． back－arc spreading systems and their そ《・ 〃 resultant geological an（1geophysical fea－ tures are quite different each other （Table6）．Remnant arcs are single and continuous in the single rift type but

3．back－afc sp〆eθd冨”9 segmented in the multi rift type．Sea－ mounts are rare in the single rift type “多 一’¢弓一’『・‘ゑノ but abundant ill the multi rift type．The ＼一＝’ 塗 ，“ crust is pure oceanic in the single rift ／ type but overprinte（1 by islan（i arc vo1－ 蒸．．． 6’』〃 canism in the multi rift type．Aborted． 鰹、 撒 rifts are rare in the single rift type but common in the multi rift type．As a result，the spreading system is simple in Figure37 Rifting and back－arc spreading of the single rift type an（i complicate（i in the arc with broad volca．nic zone． the muiti rift type．The multi type causes a complicated geological and geomor－ the （levelopment of rifts an（i sprea（1ing phoiogic＆1feature of the back－arc basin． systems．Large tensional stress in the The two types are end members and back arc area improves the arc volcan． there are probably many gradual varia－ ism resulting in an accumulation of thick tions betw奪e蓑 the two types． Typical volcanic materials on the basaltic layer multi rift type back－arc spreading like of the bεlsement along with the occur． the Japan Se乱，however，is very sca：rce． rence of many seamounts and knO玉1s． The South China Sea may be rare anoth－ The accumulated volcanic materials are ercaseoft亘emultirifttype． correlate（1 to Green Tuff．The over－ In cond慧sion，back－arc sprea（1ing is printing of arc volcanism and back－arc initiated．with the rifting along a volcanic sprea（1ing activity is the case of the zone of the就c when the a：rc is under Yamato Basin，where thickl accumula． extension縦！tectonics，because the arc is tions of volcanic materials on the oceanic hottest，thin簑est，and，weakest along the basaltic layer are presumed．and abun－ volcanic zone（DEwEY，1980）． Then，the dant an（iesitic seamounts an（i knolls are wid．th of vo豆canic zone of the arc，which observe（1．The overprint of arc volcanism is closely relate（1 to the （iip angle of over the back－arc sprea（1ing activity sub（iucting slεlb，shoul（1cause a variation

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＼ 灘轟4 b70adμo’caπ’c zoηe 〃・ 傷 ／ を《 ．〃 40暁1aη9’e s’ab

Figure38 Two modes of arc rifting and back－arc spreading： multi rift type and single rift type．

Table6 Summary of characteristics of the single rift type and the multi rift type．

Single Rift Type Multi Rift Type

remnant arc Single／COntinUOUS segmented seamount rare abun（iant crust oceanic overprinted by island arc volcanism aborted rift rare common spreading system simple complicated in the initial rifting an（1succee（1ing back－ Sea shows an outstandi血g cont：rast bet－ arc spreading l the multi rift type and ween the western sid．e（continental sid．e） single rift type．The multi rift type and the eastern si（ie （Japan side） as is back－arc sprea（1ing is the case of the well shown in Figure2． The continental J’apan Sea with the resultant complicated slope of the westem sid．e of the Japan geological and geomorphological fea－ Sea shows a simple slope，while the ture． The origin of the large tensional continental slope of the eastern si（ie stress over the broad volcanic zone may shows a complicated．feature with many be possible when the continent behind ri（1ges an（1troughs parallel to the coast the Japan Sea retreats． line（Fig．39）．This topographic contrast indicates that the continentε11 si（ie an（1 3．4Reee醜t e臓st＆亘movement alo簸gthe the Japanese side of the Japan Sea have easter簸marg量n of the Japa簸Sea a（1ifferent geological history． Submarine topography of the Japan The complicated topography of the

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eastern side of the Jεlpan Sea is shown the hanging si（1es of the faults． from the north of the Toyama Bay to The interpretation of the seismic pro－ the west of the Soya St：rait． The com－ files across the Okushiri an（1 Sa（10 Plicate（i topography consists of the Oku－ Ridges，according to the above methods， shiri Ridge and，the Sεld．o Ridge． There shows that the ridges are formed by the are several basins an（1troughs develope（1 activity of the thrust faults．It means between the Okushi：ri／Sad，o Ridges and． that施e ridges are formed by the thrust－ the Japanese Islands，such as the Moga． ing of the hanging side crust over the mi Trough，the Nishitsugaru Basin，the footwall crust． Okushiri Basin，the Shiribeshi Trough， Three types of ri（1ges are（listinguished． an（1 the Futago Basins． The Okushiri in terms of the mechanism of their for－ Ridge is developed．north of the Oga mation（Fig．41）． Peninsula and，is nearly NS trending． 1） Type E：The rid．ges which are The Sad．o Ridge is d，eveloped south of formedεlssociated with eastward dipPing the Oga Peninsula and tren（is NNE． thrust fault along their west sides． 2）TypeWl Theridgeswhichare Orδ9加 oノ 重he rたZge8 απδ ひαs加s αZoπ9 formed．associated with westward dip－ オんeεαs陀rπη乞αrg加oノεhe Jα界》απ8eα ping thrust f歌ults along their east si（1es． It is rather eεlsy to i（ientify the faults 3）Type EW：The ridges which are on the seismic profiles．The identi丘cation formed associated with eastward dip－ of their nature in terms of normal or ping thrust fault and westward，d．ipping reverse（th：rust），howeve：r，is di伍cult on thrust faults along both si（1es of the the single channel seismic pro且1es．This ridges． is because the vertical exagger＆tion on An example of Type E is shown in the recor（i is high， such as 10 to 20， Figure 42 across the southern Okushiri usually．For example，a fault with a Ridge．The sedimentary seq．uence on dip angle of30d，egrees shows an appa：r． the eastern slope of the ri（1ge is the ent（1ip angle of85（1egrees on a recor（i lower sed．imentary layer in the Okushiri with a vertical exaggeration of20．Thus， Basin and，is correlated to Pliocene age． almost all faults show a nearly vertical Thick Quεlternary d．eposits are trapPe（1 dip angle on the single channel seismic in the Okushiri Basin，which is caused． records．So，it is important to identify by the uplift of the Okushiri Ridge afte：r the nature of the faults accor（iing to the deposition of the Pliocene sediments． basement morphology and sedimentary A large eastward dipPing thrust fault， structure across the faults． In the case which cεlused．the uplift of the：ridge，is of thrust faults，the hanging si（1e shou1（l assumed along the westem side of the make an uphfted peak along the fault． Ok皿shiri Ridge on this p：rofile． Such feature shoul（1not be observe（i，in Figure43shows a seismic pro且le cross－ the case of normεll faults． The uplifted． ing the Okushi：ri Ridge along430N。 A peak along the fault is consi（1ere（1to be typicalcaseofTypeWisshownonthis an in（lication of a thrust fault． Figure profile．A westward d．ipping thrust fault 40 shows the case of thrust faulting． is observed along the eastem margin of Two eastwa：rd，dipping th：rust faults are the ri（ige． It is suggeste（1that the oce－ identified on the foot of the Oshima anic crust of the Japan Sea thrusted up Plateau，which are evidenced by the ob－ to the east．The d．epth of the basement servation of the typical uplifte（i ri（lge on of the Shiribeshi Trough is evid．ently

一328一

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Figure39 3－D topographic view of eastem margin of the Japan Sea．The data used for processing are GSJ digital bathymetric data after Bathymetric Maps6311and6312 （Maritime Safety Agency of Japan，1980ab）．Processing system is SIGMA of Geological Survey of Japan．

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Figure40 Typical example of thrust faults on the foot of the Oshima Plateau in the e＆stem ma．rgin of the Japan Sea on Line J19，An arrow with annotation“E”shows a fault dipPing eastward． greater than that of the Japan Basin thrusted up over the eastern oceanic although the basement cannot be detect． crust． This interpretation is supPorte（l e（10n this seismic profile． This feature by a geophysical observation that there suggests that the basement of the Shiri－ is a large low gravity anomaly of－47 beshi Trough was forced．d，own by the milligals in the Shiribeshi Trough。A thruste（i－up oceanic crust of the Japan low gravity anomaly is common along Basin aユong the Okushiri Ri（ige． The the ocean－continent boun（iary of the ocean－continent boun（iary on this profile J’apan Sea． The g：ravity anomaly along is consi（1ere（i to be locate（i at the foot of the ocean－continent boun（1ary ranges the basement of the continental slope from－20to－30milligals in the Japan along the eastem margin of the Shiri－ Sea．The gravity low of－47milligals is beshi T：rough． Then，from this profile， de丘nitely anomalous． So， the grεlvity it is interpreted that a thrust fault was anomaly of－47milligal＄at the Shiribeshi forme（l in the oceanic crust of the Japan Trough should include the inequilibrium Basin and．that the western oceanic crust effect of isostasy． The gTavity anomaly

一330一 GeO乙09‘Cα乙S偬伽rε0∫伽」αPαnSeαα認εむSむeCε0πεC舌m頑C翻0ηS（K。Tαmα勧 ridgesofTypeWnorthoftheOkushiri 鐙妙ρ㊧ Island． A Typical example of Type EW is obse：rved．in the northem Okushiri Ri（1ge as shown in Figure45．The sedimentary sequence on the Ridge is Miocene and 憶 Pliocene in age．The bottom sampling data of GH77－3 and GH78－2 c：ruises show that deposition of the Quatemary 膨妙ρ磯 is thin on the ri（ige． This （iepositional feature on the ridge shows a marked contrast with the thick d．eposition of the Quatemary sediments in the Musashi Basin to the e＆st．丁紅e contrast suggests that the Ok薦hi：ri Ridge initiate（1its up－ lifting after or （iuring the（leposition of 〃 the Pliocene！＆yer訊n（I that the Musashi 置膨妙ρ㊧ Basin was formed．by trapPing the Qua－ t6rna：ry deposits eastw＆rd．behind．of the Okushiri Ridge． Several basins along th．e eastem side of the Okushiri Rid．ge， such as thg F瓢ago Basins，the Shribeshi Trough， the Okushiri Basin，and．the Nishitsugaru Basin，are formed by the 微 same mech＆nism．Uplift of the Okushiri Ridge trapped the sediments which were derived from the Japanese Islands and Figure41Three types of ridge formation mechanism observed in the eastem formed the many sedimentary basins margin of the Japan Sea． along the eastern side of the Okushiri－ Sado Ridges． is concord，ant with the interp：retation thεlt the oceanic crust of the Shiribeshi ・勉髭如翻oπoノ虚he孟hr撹sεαcお魂なノαloπ9診hε Trough is forced down by the thrust up εα8陀r轟配αrg加oノオhε」¢ραπ8εα oftheOkushiriRidge．AgravityHighof The pro且1e in Figure43presents goo（i 37milligals over the Okushiri Ridge is information for identifying the age of consi（ierably higher than the gravity the initiation of thrust up of the Okushiri anomaly of O milligal over the continen． Ridge，bacause the sedimentary sequence tal slope at the same depth with the is well preserved on the Okushiri Ridge． summit of the Okushiri Ridge on the It is possible to trace the sedimentary profile． The gravity high over the Oku－ sequence from the J’apan Basin to the shiri Ridge is inferred to be due to the Okushiri Ridge． uplift of the high（iensity of the oceanic The sed．imentation of the J’apan Sea crust．This tectonic feature Suggests the since Pliocene is mainly controlle（i by obduction tectonics of the oceanic crust distal turbi（1ity activity． The turbi（1ity of the Japan Basin along the Okushiri activity causes the present （lay config－ Rid．ge． Figure44 also shows several uration of abyssal Plain of the Japan

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3 3

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Figure42 AseismicprofileacrossthesouthempartoftheOkushiriRid．geonLine J17。Example of Type E is observed．、An arrow with annotation“E”shows a fault dipPing．eastward・

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Figure43 Aseismicpf。且1eacr。sstheOkushiriRidge・nLine」12withfree－airgravityan・malypr・Hle．Example・f Type w is observed．An arrow with amotation“w”shows a fault dipPing westward。“Q／P”shows boundary of Quatemary and Pliocene deduced from DSDP Site30L 一翫ZZθ伽o∫伽GεoZo9‘oαZSμrひθッo∫」αPαη．Vo乙39，ノ〉o。5

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Figure44 Seismic profile of the Okushiri Ridge north of Okushiri Island on Line J14．

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灘 垂W 3 演 恥 薪 3 19km － ．“晒砦・6 鞭・ 轟 v．e．＝20 、蓼1繋鍵、野』一畷灘 J A P A N B A S l N 4 4

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Figure45 A seismic pro且1e across northern part of the Okushiri Ridge on Line J8． Example of Type EW is observed。

Basin． The turbi（iites are not（1eposited Pliocene／Miocene boundary according to on the topographic highs． So，after the the results of DSDP Site 301 in th，e Okushiri Rid．ge we：re uplifted．by the Japan Basin，the change in the sedimen－ thrust fault activity， there shou1（i be tation rate on the Rid．ge is infe：rred．to little sedimentation on the ridge．This have occurred some time after Pliocene． results in the reduction of the sedimenta． Reflecto：r Q／P on the profile of Figure43 tion rate or thinning of the sedimentary is an assumed．boundary between the layer after the initiation of activity of Quatemary and．the Pliocene which is the thrust faults． interpolated from deep sea drilling re－ The thickness of the upPer stratifie（1 sults at Site 301． The tracing of the 1ayer in the Japan Basin of O．8 sec renector onto the Okushiri Ri（ige gives decreεlses to O．6 sec on the Okushiri an important information for the initial Ridge on the seismic pro且1e of Figure ti卑ing age of the uplift of the ri（1ge・The 43．As the bottom of the upper strat－ reflector Q／P is situate（1 at o．25 sec ifie（i layer is nearly correlated to the below the sea floor in the Japan Basin．

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137。 Although it is d．ifHcult to t：race the re－ 139。 141。E flector onto the Okushiri Ridge d．ue to 夢 the limit of the resolution of record and 鰐 46。N ♂ ’ the structu：ral d，isturbance at the westem ’ →l foot of the ri（ige，the reflector aPPears I to be at most less than O．1sec below the 5 つ 2・ ノ o seεしfloQr on the Okushiri Ridge． So，a ・郭，1一 differe盤c倉i慧t敷e se（iiment thickness bet－ ween the Jap細Basin and，the Okushiri

Ridge of O．2sec appears to be mainly ク「 440 due to the鐙d慧ction oぞthe sedimentation イ プ ：rate in the Quaternary． Accord．ing to も．店 the above discussion，it is assumed that 、、p繍 the uplift of the Okushiri Rid．ge，or the activity of the thrust faults，was initiated aroun（11atest Pliocene． S e a 趣君

42D σゐαrαc陀冠s翻cs of診hεd薩sむ・め乱甜oπoノが飽 羅 痴 窃c伽ε 痂遡8ε ブ側Zεs επ εh尼εαs陀m 1郵》 η凝r4ηoノ痂ε」¢ραπSeα The （iistribution of thrust faults is ．隷 summarized in Figure46based on the seismic reflection data and recently 疹 publishe（1 submarine topographic maps Q ～ 40。 （Maritime Safety Agency of Japan，1980 。 〆 ab）．Active synclines are also sum－ 。 夕 marized in the figure according to the ゆ7曜 o 勺．㌦／ o identi且cation of a thick accumulation of 夕 〆 q the Quaternary（1eposits． ぬ ｛ o 移 、 There are two types of thrusts faults ’ E： ♂ ん observed．l eastward．（1andwar（1）d，ipping 38。 、 thrust faults and．westward，（oceanward，） ノ ／ dipping th：rust fε皿1ts．Eastward，dipping ヅ thrust faults are predominant north of the Musashi Bank at the northemmost part of the eastem margin of the Japan Sea． The northern en（l of the Okushiri 137。 139。 141。E Ridge is bounded by eεlstward d．ipping Figure46 Distribution of active thrusts faults th：rust faults on its west sid．e and by an（1 active synclines． Several pre－ westward，d，ipping thrust faults on its vious large ea．rthquakes are shown with paired arrows．The direction east side as is well shown in Figu：re of arrows shows compressional 45．The middle part of the Okushiri axes based on focal mechanism Rid．ge，between42。N and．43。N，is char－ solution．Theearthqua．kesareA： acte：rized by well developed．westward． O任Sakhalin（1971，M7。1），B：O任 Shakotan（1940，M7．0），C：0賃Oga dipping thrust faults as typically shown （1964，M6．9），D：Centra1JapanSea in Figure43and Figure47．Eastward （1983，M7．7），andE：Niigata（1964， M7．5）． 一336一 σθ・Zo9‘cαZs偬伽rθo∫伽」αPαηSεααηの醜εo孟oη‘・‘㈱Zεcα古‘oηs（K．Tαmα勧

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Figure47 Westward dipping thrust faults along the Okushiri Ridge on Line J13． dipPing thrust faults are predominant in is composed of the rifted basin margin the southern part of the Okushiri Ridge which inclu（1es both of continental and as shown in figure42． Thus，the dist：ri． oceanic crust， as discusse（i in Chapter bution of the thrust fault types is varia－ 3．2．The basement of the middle part of ble εllong the Okushiri Ridge，but it the Okushi：ri Ridge has the oceanic crust， shou1（i be noted that the general tren（i while that of the southem part ha．s the of the th：rust faults associated．with the continental crust and is exposed as Mes－ ：rid．ge is quite linea：r with st：rike of N12。 ozoic granitic rocks on Okushiri Islan（i． Wshowinganen－echelonarrangement． Thus，the Okushiri Ridge is d，eveloped． One of the other characteristics of the with q．uite a line：r trend．through the Okushiri Ridge is the va：riation in the rifted continental fragments，the oceanic basement geology． The basement of the crusts，an（i the continental crusts． northernmost part of the Okushiri Ri（ige The Sad．o Ridge shows a d．iffe：rent fea一

一337一 BひZZεヵ‘η，oゾじオんθ Gθo Zo9‘cαZ Sωrひ2ツo∫」αPαη． Vo Z．39，1〉o。5 ture from that of the Okushiri Ridge． dip．According to the focal mechanism While the Okushiri Ridge shows a linear solution by FuKAo an（1FuRuMoTo（1975）， distribution，the Sado Ridge shows a the fault is a thrust type an（i has dip distribution within a broad zone100km angle of38。with a strike of N160E if in width，trending N300E．Many ridges the eastward，dipping nod．al plane is lie within the wide zone，showing，the adopted as a fault plane．There is some same trend of N30。E．Each ridge shows discrepancy of the dips an（i strikes be－ mostlyTypeWaswellshowninFigure tween the thrust fault and mechanism 48．The basement of the Sado Ridge is solution of earthquake． Howeve：r，as composed of rifted continental crust as there is no other correspon（1ing active discusse（i in Chapter3．2． thrust fault near the earthquake epicen－ Eastwa：rd，dipping th：rust faults are ter，the thrust fault along the Rishi：ri again（levelope（i at the Toyama Trough Trough is possibly relate（l to this earth－ of the southem end of the eastem mar． quake． gin of the Japan Sea． Large eastwa：rd． （2） Earthquake off Shakotan dipping thrust faults a：re observed，west The epicenter of this earth（luake is of the Sa（io Island． located at the northemmost part of the Okushi：ri Ridge． The focal depth is33 丁距rμs孟 ブα麗麗s α鷺d ブocαど η乙εchαη謬8那 km．The eastward dipping nodal plane so観‘0π80fεα融9麗んes of FuKAo and FuRuMoTo’s（1975）mecha－ There is a concor（1＆nt relationship be－ nism solution，（1ip of 45。 and strike of tween the dist：ribution of the thrust NS，shows q．uite a good．coincidence with faults and focal mechanism solution of a large thrust fault with the st：rike of the earthquakes which occurre（l in this N50E along the westem margin of the century． There are five earth（luakes of northern Okushi：ri Ridge． If the la：rge which focal mechanism solutions are thrust fault is assumed to have dip publishe（1．They are all thrust－type earth－ angle of 45。，the earthquake is locate（1 quakes． The distribution of the thrust just on the fault p1εしne． Active th：rust faults is shown in Figure46and，the faults which correlate to the westward parameters for the thrust faults and dipPing no（1al Plane of the solution are earthquakes are summarized in Table7． not obse：rved． As the thickness of the The ：relationship between the thrust lithosphere of the Japan Sea is30km faults an（1the earthquakes is （iiscusse（i （ABE and．KANAMoRI，1970），the focal below for each earthquake． depth of33km suggests that the thrust （1） Earthquake off Sakhalin fault at the northern Okushi：ri Ridge Ala：rgethrustfaultisd．evelopedalong cuts the entire lithosphere （FuKAo an（1 the eastern margin of the Rishiri Trough FuRuMoTo，1975）．Vertical displacement from west of Rebun Island and to west of the thrust fault is3km．The displace－ of southem Sakhalin as shown in Figure ment of3km may be the accumulated 46． The thrust fault is eastwa：rd．dipping displacements since latest Pliocene．Some with a strike of N5。E． The Earthquake extent of the initial displacement before off Sakhalin occurred at the （1epth of latest Pliocene，however，is plausible be． 22km and10km east of the thrust fault． cause the no：rthem Okushi：ri Ri（lge is If the earth（luake occurred on the thrust Originally composed of rifted continental fault，the（1ip angle of the thrust fault is crust an（1is supPose（1to have originally calculated to be65。with an eastward． been a rise．A displacement of3km for

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BαZZθあη oゾ6h（3（7θo Zo9茜oαZ S膨rびεツ oゾ’♂iαPαπ． γo Z．39，No。5

Table7 Dips and strikes of the submarine thrusts faults and their correlative earthquake faults。 “Dip angle”and“Strike”show the dip angles and strikes of the nodal plane of mechanism solution a、fter FuKAo and．FuRuMoTo（1975）．“Fault dip”show the dip angle of the fault on assumption that the submarine fault and hypocenter are located on a same fault plane。“Fault strike”shows the strike of the submarine fault．

Earthquake Date Magnitude Dip angle Strike Fault（iip Fault strike Central JS May26，1983 M7．7 36 N20E 40 N18E Niigata Jun． 16， 1964 M7．5 56 N9E NS Off Oga May7，1964 M6．9 50 N31E 57 Nま8E Off Sakhalin Sep． 5， 1971 M7．1 38 N16E 65 N5E Ofr Shakotan Augレ．1，1940 M7．0 45 NS 45 NS

this thrust fault may be over－estimated． The dip 蔽ngめoぞths fault is calculate（1 It should be emphasized that the focal to be abQ購4伊on the assumption that mechanism of the Earthquake off Shako－ this eart難qu農keぐ）ccurre（10n the plane of tan an（i the active thrust fault on the this fault。 Th鷺 奪盆stwar（1（1ipPing no（lal seεl floo：r showεln excellent coincid，ence p！＆ne by the m¢漁鋤ism solution of the an（1thεlt the thrust fault is the largest in pre－event Qf轍奪m．ai蓑shock by IsHIKAwA the eastern JaPan Sea， cutting entire ααZ．（1984）shOw＄＆strikeofN20。E lithosphere． and，dipεlng1馨く）f36⇔which are in good （3） Earth（luake off Oga coinci（lence with t魚e strike of N180E This earthq．uake occurred at a focal an（1 the （1ip a論91G of 40。 of the corre－ depth of19km just10km east of a large spon（1ing fault o鷺むhe sea floor． There fault． The（1ip angle of this fault is57。 apPears to be some problem about main on the assumption that this earthquake event of the m農沁＄hoek（lsHIKAwAααZ．， occurre（10n the plane of this fault． The 1984）．The maximum displacement of mechanism solution by IcHIKAwA（1971） the bεしsement by t五e£＆ult activity is1400 shows d，ipεlngle of50。with a st：rike of m （HoNzA，1983）． 丁鼓e （iisp魚cement of N31。E if the eastward．dipping nod．al 1400m shou1（i be t鼓e＄鷲mmε沈ioll of the plane is a（lopte（1as the fault plane， The displacements since／縦test Piiocene． strike of the correspon（1ing fault is （5）Ea緬qu磁難o鐸Niigata N18。E． There is（iiscrepancy of strikes There are sever＆1westward．dipping between the submarine topographic fault thrust faults 痕rou訟d the epicenter of an（l the earth（luake fault．The（lifference this earthquake （HoNzA ααZ．， 19771 may be due to the ambiguous mechanism HoNzA，1983）．A躍（1975）：reported．the solution of IcHIKAwA．The slight（1iscrep－ mechanism sdぬo蓑of this thrust－type ancy between both dip angels is explain－ earthquake as dip ＆ngle of 56。 an（1 a ed．by the steep dip angle in the shallow strike of N9伊E wit｝l westward．d．ipping part of the fault plane． feature。 It is（1i缶cult to i（1entify a single （4） Central Japan Sea Earthquake cor欝espon（iing thrust f』ault on the sea The epicenter of this earthq，uεしke（400 HOor圭》rom the available data because 21．4’N，139。04．6’E）islocated13kme＆st there are several westward dipPing of the correspond．ing fεlult（F孟経．23）with thrust faults aroun（i the epicenter． SA－ a focal depth of14km．The eorrespon（i． TAKE and．ABE（1983）d．iscusse（l a revised． ing fault of this earthquake is i（1entical mechanism solution of an eastward dip－ to the fault of the Earthquake of Oga。 ping fault type．The geological structure，

一340一 GεoZO9εCα乙S飽伽rcO∫伽」αPαηSeαα掘琵SホθCむ0η‘0漉頑0αむ‘0ηS（K。Tαmα爾 however，d．oes not show a cor：respond．ing ments scattered in the basins，the occur． eastward dipping thrust fault． There rence of rifted continental fragments， are an abundant distribution of west－ the several prominent aborte（1rifts，the ward．d．ipping th：rust faults in the south． abundant occurrence of seamounts and em part of the eastem margin of the knolls，and the thick accumulation of Japan Seεし （Fig．46）． This earth（luake volcanic clastics on the oceanic basalt is inferred to have occurre（i on one of layer． In this section，the author （1is－ the westward．dipping th：rust faults。 cusses the tectonics of back－arc sprea（1－ The d．istribution of active thrust faults ing by applying this model to the history along the eastern margin of the Ja－ of the Japan Sea． pan Sea，including severε111arge thrust－ Large tensional stress is necessary type earthquakes withEW compressiopal for the initiation of the back－arc sprea（1－ axes，shows an evident EW compression－ ing． Accord．ing to the DEwEYフs （1980） al stress 且e1（1in the area as （1escribe（i concept，the：re are two possible causes above． The tectonic significance of the for the tensional stress． One is trench EW compressional stress will be dis－ roll back an（l the other is retreat of the cusse（1in Chapter3．5． back－arc plate．A combination of both causes is also possible． 3．5 Tectonic evo互ut量on o£ the J＆1）我聡 SENo and MARuYAMA（1984）proposed Se＆ the trench roll back origin of the Japan Two significant phases of the tectonic Sea with the simultaneous spreading of evolution of the Japan Sea are intro－ the Kuril Basin in the Okhotsk Sea an（1 duce（1 through the above （1iscussions． the Shikoku Basin． Trench roll back One is the back－arc sprea（1ing tectonics occurs accor（1ing to the age of the sub－ of the Japan Sea． The other is the ducting oceanic plate （MoLNAR and neotectonics of the Jεlpan Sea along its ATwATER，1978；SENo，1983）．An older eastem margin．In this section，the au－ Plate has the ten（iency of roll bεlck． The thor discusses and summarizes the two time range of the back－arc spreading of phases of the tectonic evolution of the the Japan Sea is suggeste（i to be30to Japan Sea respectively． The back－arc 10Ma in Chapter3．L The Shikoku Basin spreading of the Japan Sea is summa－ had been active simultaneously during rize（1 0n the basis of the （1iscussion in 30to15Ma just south of the Southwest Chapters 3．1，3．2，and 3．3． The neotec－ Japεln Arc．The young lithosphere south tonics of the Japan Sea is summarized of the southwestem J’apanese Island．s on the basis of the（1iscussion in Chapter rejects the possibility of trench roll back 3．4． at that time．Trench roll back along the Japan Trench may be possible be－ Bαcん一αrc＄prεα伽goκんeJ復Pαπ8eα cause of the subduction of the o1（ier Th：rough the d．iscussion in Chapter3．3， Pacific Plate．Geog：raphic reconst：ruction the author presented a model of back－ during the back－arc sprea（iing of the arc sprea（1ing for the case of the Japan Japan Sea，however，is di伍cult to be Sea．The model is a multi rift type accommodated only by the roll back of back－arc spreading system （Fig．37）． the Japan Trench． The model reasonably explains the pres－ The other possibility is the retreat of ent geologicεll structure of the Japan the back－arc plate from the trench line． Sea with numerous continental frag一 The back－arc plate of the Japan Sea is

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the Eurasia Plate．The back－arc sprea（1－ Plate and the South America Plate．The ing of the Japan Sea is possible by the deformation or the movement of micro－ retreat of the Eurasia Plate during30to plates in East Asia may be large enough 10Ma．The movement of the Eurasia to influence the tectonics of its marginal Plate at that time is calculated by the area inclu（iing the Japanese Islan（1s． model of ENGEBRETsoN（1982）．The mov． ZoNENsHAIN an（i SAvosTIN （1981） pos－ ing vector of the Eurasia Plate is calcu－ tulate（l that the India－Eurasia collision 1ated to be1．25cm／yr with a direction of fragmented East Asia into several micro－ S780E at the central part of the present Plates and that compressional tectonics Japan Sea． This calculation （10es not an（i extensional tectonics have occurred supPort the retreating sense of the Eur－ along the boundaries of the microplates． asian Plate as a back－arc plate of the The compressional tectonics caused Japan Sea（1uring30to10Ma．It appears mountain ranges such as the Tien Shan to be （ii伍cult to （1e（iuce the retreat of Range and the Altai Range．The ex－ the back－arc plate in the reference frame tensional tectonics cause（i the rift or of the major plates． graben system such as the Baikal Rift KIMuRA and．TAMAKI（1986）presente（1 and，Shεlnsi Grabens．KIMuRA and．TAMAKI an alternative origin of the back－arc （1986） suggested that relative move－ spread．ing of the J’apan Sea with the ments of the microplates could be the sim｝1taneous spreε1（iing of the Kuril cause of the back－arc sprea（iing an（l Basin in the Okhotsk Sea． They have thatεl microPlate behin（l the Japan Sea presente（i the concept that the retreat of may retreat from the trench in spite of the back－arc plates of the Japan Sea the immobility of the Eurasia Plate． an（1the Kuril Basin is explaine（1by the KIMuRA and TAMAKI further considere（1 defo：rmation of the Asian Continent d，ue that the microplates in East Asia are to the India－Eurasia collision．MoLNAR corre1εlte（1to each of the allochthonous and TAppoNNIER（1975）documented the terranes which were accreted．to the intra－plate deformation of the Eurasia Eurasia Continent prior to the In（iia－ Plate in East Asia d，ue to the Ind．ia－Eu：r． Eurasia collision．The movement of mi－ asiεl collision． ZoNENsHAIN an（1SAvosTIN croPlates occurred in a mamer of rear－ （1981）also interpreted the deformation rangement of accreted allochthonous as relative movements of microPlates in terraneS dUe tO a neW large aCCretiOn Of East Asia due to the Ind．ia－Eurasia colli－ the In（1ia terrane to Eurasiε1． They also sion． The collision began prior to 40 stressed on simultaneous spreading of Ma．（TAppoNNIERε6αZ．，1982）．MoLNAR the Japan Seεl an（1the Kuril Bεlsin． an（i QIDoNG （1984） calculated the move－ The Kuril Basin is the back－arc basin ment velocity of the Southeast China of the Kuril Arc．The basin shows pro－ block an（1conclu（ie（i that the block has minent a．fan－shaped geographic config－ moved with velocity of2．3cm／yr to the u：ration which opens southwestward（Fig． east－southeast with respect to the Eur－ 49）．The basin narrows to the no：rtheast asia Plate fo：r the last80years． As the and disappears just south of the Kam－ rate of movement of the Eurasia Plate chatka Peninsula． The Kuril Basin has in East Asia is O．3to O．4cm／yr，the rate similar characteristics with the Japan of2．3cm／yr is quite large and fairly Basin．Basement depth after the sedi－ comparable to the movement of the ma． ment loa（iing correction an（i the acoustic jor plates such as the North America stratigraphy are similar to those of the

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Figure49 Physiography of the Japan Sea and the Kuril Basin．

Japan Basin （Fig．50）． The upper part pa：rable with that of the Japan Basin of the sedimentary sequence of the：Kuril （Fig．27）． The average heat flow value Basin is also stratified，whi！e the lower of the Kuril Basin is2．33HFU． The part is transparent．The maximum sedi－ value is slightly higher than average ment thickness of the Kuril Basin is2．8 heat flow value of the J’apan Basin of seconds which is much greater than that 2．26HFU and almost comparable to that of the Japan Basin．The basement depth of the Yamato Basin of2．34HFU（Table of7．2seconds is also（ieeper than of the 5）．A rough estimate of the age range Japan Basin．The basement depth after of back－arc spread．ing of the Kuril Basin， the sediment loading correction，howev－ on the basis of basement depth and heat er，is5000mwhichisalmostcomparable How value，is30to15Ma or little more to that of the Japan Basin． The range younger・ of the corrected basement depth of the The age range of the back－arc sprea（i－ Kuril Basin，5000to4000m，is also com一 ing of the Kuril Basin is almost the

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Figure50 Typical Seismic proHles of the Japan Basin and the：Kuril Basin．The sediment stratigraphy and the basement depth after sediment loading correction are comparable to each other．

same as that of the Japan Sea．The its eastern si（ie． The back－arc plate of simultaneous formation of the Kuril Ba． the Kuril Basin is the Okhotsk Block sin and the Japan Sea is implied by their which occupies most part of the Okhotsk physiographic similarities as shown in Sea．The simultaneous spreading of the Figure49．If it is assumed that both of both basins’also suggests that the both the back－arc basins are generate（1 by blocks moved simultaneously． the retreat of their back－arc plates，the KIMuRA an（l TAMAKI （1986） stressed． separated feature of both basins implies on another event which occurred simu1－ that the back－arc plates are not the taneously with the back－arc sprea（lings same，but just tectonically related each of the Japan an（1 Kuril Basins。 The other． The back－arc plate of the Japan event is the dextral collision between the Sea is the Amurian Block，one of the Okhotsk Block and the Amurian Block． microplates of Easta Asia，which is The （lextrε11collision is evi（1ent in the bound．ed，by the Baikal Rift on its en－echelon arrangement of the folding western sid，e and．by the Japan Sea on of the Paleogene formations on the

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Sakhalin－Hokkai（10 islan（1s （KlIMuRA KIMuRA and TAMAKI presented，an i（1ea θ6αZ．，1983）．KIMuRAθ孟αZ．，（1983）have which reasonably explains the simulta一一 indicated that the dextrεし1collision may neous events of back－arc spreadings in be an expression of the collision between the Japan and．Okhotsk Seas and．the the Eurasia and the North America dextral collision along the Hokkaido－ Plates．The relative motion of the two Sakhalin islands．The concept is sum． plates at that time，however，dose not marized in Figure51and its details are show a dextral sense． According to as follows． ENGEBRETsoN’s（1982）reconstruction，the The retreat of．th．e Amurian and collision between the Eurasia and North Okhotsk Blocks occurred simultaneously America Plates should show a perpen－ and their movement caused the back－arc dicular sense along the Hokkai（10－Sakha－ sprea（ling of the Japan Sea an（1 the lin islands． Kuril Basin．The motion of both retreat．

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Figure51Movement of microplates in relation to Indo＿Eurasia collision and spreading of the Japan Sea and：Kuril Basin．This且gure is modified from TAppoNNIERθ孟αZ． （1982）． 一345一 BαZZε孟εηo∫孟hεGleoZo8・‘oαZ Sαrひεツoゾ’・ZαPαη． VbZ．39，1〉o．5 ing back－arc plates was d．ifferent to tion of the Japan Sea on the basis of each other．The movement of the A－ their paleomagnetic datεl in Southwest murian Block was to the northeast direc－ Japan． They suggeste（i that the forma－ tion causing the pull－apart opening of tion of the Japan Seεl occurre（1rapi（11y the Baikal Rift along the transform－1ike over a period of only a few million years bound．a：ry between the Sibe：ria and the around15Ma．Their model is inconsis． Amurian Block，A sinistral collision is tent with that of KIMuRA and．TAMAKI． expected．along the Stanovoy Range at The spreading tectonics of the Japan the northem end of the Amurian Block． Sea is still controversiεし1． The northeastward movement of the Amurian Block simultaneously caused ノVeo彦εe診0漉80κheJ縄ρ伽8εα the clockwise rotation of the Okhotsk NAKAMuRA（1983）and．KoBAYAsHI（1983） Block．The movement of the Okhotsk： have propose（1a hypothesis of ongoing Block was caused，by the d』rag force of eεlstwa：rd，subd．uction along the eastern the d．extral collision between the Amurian margin of the Japan Sea，on the basis of Block an（1the Okhotsk Block along the the general tectonic view a：round the Sakhalin－Hokkaido line． The clockwise Japanese Islan（1s． If their hypothesis is rotation of the Okhotsk Block is sug－ the case，it w皿have a significant influ－ geste（l by the fan－shape configuration of ence on the discussion of neotectonics in the Kuril Basin which opens southwest－ the Japanese Islands． The author（1is－ ward，． The Kuril Basin closes towa：rd cusses the possibility of sub（iuction in the northeast and diminishes at the the Japan Sea， on the basis of the south of the Kamchatka Peninsula，where interpretation of seismic reHection（iata， the relative rotation pole of the Okhotsk for verifying the hypothesis， Block is inferred to be located． They To i（lentify an ongoing sub（1uction pro－ propose（1that the Kamchatka Peninsula cess， it shou1（1 be con且rme（i that the was the zone of collision between the basement of the oceanic crust is down－ Okhotsk Block an（i the forearc plate of going beneath the landward。trench walL the Kuril Arc during the spreading of This feature is commonly observed on the Kuril Basin． The Sredinny Range in seismic profiles of the sub（iuction zone． the Kamchatka Peninsula is a possible There is no such feature observe（l in the result of the collision． The Sre（1inny eastem margin of the Japan Sea．There Range was active during Late Oligocene are thick accumulations of sediments， （FuJITA and WATsoN，1983），and the reaching2．O seconds，in the Japan Basin． duration of activity is consistent with If the oceanic basement overlain by such the above idea． a thick accumulation of sediments is The above i（1ea is rough an（i hypo－ subducted，then，dynamic sediment ac－ theticaL There are many problems to be c：retion will occur along the landwa：rd， resolved for the documentation of this slope and it will make an accretionary hypothesis． HoNzA （1979） presente（i a prism，as is weil observed in the Nankai fan shape spreading model of the Japan Trough in the Philippine Sea．An accre． Sea associated with the clockwise rota． tionary prism is characterized by many tion of Southwest Japan based on geo． imbricated thrust faults dipping land． logical and geophysical consi（lerations． wa：rd．and．by a rough topography with OToFuJI and MATsuDA（1983）proposed many ri（1ges an（i troughs．Such feεltures almost the same model for the genera． are also not observe（i in the eastern

一346一 σeoZo9‘oαZs伽伽reo∫古んθJiαPαηSeααη面孟s古εo古o痂εm頑oα孟εoηs（K．Tα肌αん‘）

margin of the Japan Sea．Thus，it is The other possibility is the conversion of di伍cult to i（ientify ongoing sub（1uction the fault into a sub（iuction zone after in the eεlstem Japan Sea． Howeve：r， the fault reaches the maximum displace－ there is a possibility of incipient sub（1uc－ ment．After subduction is initiated，fur－ tion as discussed，below． ther lithospheric convergence will not Many active thrust faults are distrib－ result to increase the displacement over uted along the eastem margin of the the critical displacement。 Japan Sea． They in（1icate the present It shou1（1be note（1that subduction is compressional stress in the e＆stem mar－ different from thrust fault activity and gin of the Japan Sea．Some of them cut that it means the overlapping of the two the entire oceanic lithosp簸ere of the lithospheres． Sub（luction is（1efine（1as a Japan Seε1農s （1iscusse（i in Chapter3。4． P：rocess whe：re the upPer surface of one The existence of l翫rge thrust £aults ／ithosphere sub（1ucts beneath the other which cut th．e entire lithosphere ind．icate lithosphere．When the subduction is that converge煎 tectonics between the ongoing， the convergence between the lithospheres Qr plat¢s is occurring in the twO lithospheres does not cause a com－ eastem margin of the Japan Sea． pressional stress proportional to the The largest displacement along the convergence rate， because the conver－ thrust faults reaches3km at the north－ gence is consumed by the subduction of ern end．of the Okushiri Ridge． As the the footwa111ithosphere． On the con－ thickness of the lithosphere of the Japan t：rary，when the converging hthospheres Sea is30km，the displacement of3km are bound．ed by thrust faults，the com－ correlates to 10％ of the entire litho－ pressional stress shou1（1be proportional sphere．This displacement of3km is a to the convergence rate an（1the（1isplace－ cumulative displacement for the past2 ment of the thrust fault increases ac－ 0r3Ma．If the compressional stress is cor（1ing to the converg曝灘ce． maintained in the future，will the dis－ If there is a critic＆！displacement for placement increase further？ It is im－ the conversion from trust fault to the possible for the displacement to reach initiation of subd，uction，how large is the the entire thickness of the lithosphere， critical displacement？ An assumption because there are no such large dis－ shou1（i be intro（luce（i to （liscuss this placements of30km observed on the problem．The assumption is that the earth． It is reasonable to consider that Okushi：ri Ridge is co：rre王ated．to the initial there is a maximum values of displace－ stage of the basement high at the trench ment along thrust faults which cut the slope break and that the Musashi Basin entire lithosphere． After the d．isplace． on Figure 52 is also correlate（i to the ment of the thrust fault reaches some initial stage of the forearc basin． The maximum value，what will happen with forearc basin and basement high at the continuing convergence between the trench slope are commonly observed in hanging side and．footwa111ithospheres？ the present islan（i arcs． So，the present There may be two possibilities in such day height of the trench slope break a case．One is the formation of another basement high above the subducting thrust fault in the（lifferent place on the ocean且oor may be an expression of the same transect．The convergence between critical displacement．There are a varie． the two lithospheres is compensated by ty of the origins of the basement high at the displacement of the new thrust fault． the trench slope break．For example，

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7一・ ・捲響 一一む離、滅蕪、‘紙さ．， Figure52 Seismic reflection pro且1e with free－air gravity anomaly pro丘le on Line J7。 The free－air gravity low of－57mgals is the lowest one in the Japan Sea。 the basement high at the trench slope present （iiscussion， because young break of the Sunda Trench is composed trenches εlre consi（1ered to represent a of an ancient accretiona：ry wedge（KARIG more original configuration of the base－ an（1SHERMAN III，1975），and．that of the ment high at the trench slope break． Japan Trench is composed of the ocean－ The South Chin＆Sea is a marginal ward edge of the continental crust（von basin whose water depth，sediment thick－ HuENE θ6αZ．， 1980）． Since the original ness，crustal structure，an（1 age （32－17 configuration of the trench slope break Ma）are quite similar to those of the basement high may・be changed by the J’apan Sea． The Manila T：rench，which active sub（luction process，an（i analogy was formed during Miocene，represents with a young trench is better for the an active eastward subd．uction zone along

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the eastem margin of the South China of subduction is only based on some Sea（TAYLoRand．HAYEs，1983）．Abase． analogies． The thickness of the litho－ ment high at the trench slope break is sphere may also have to be taken into well developed and makes a ridge along consi（ierεltion for such discussion，an（i the Manila Trench．The west Luzon the ben（1ing effect of the’sub（iuction lith－ Trough is traPPe（1just east of the ri（1ge． osphere is also significant problem．A The relative height of the basement high detaile（l geophysical an（1geological mod－ of the Manila Trench above the base－ el for the initiation of subduction from a ment of the basin area of the South thrust fault is significant for future China Sea is2．2to2．8km according to stud．y． the seismic proHles of：LuDwIG αα1． The possibility of incipient sub（1uction （1967）．If the value，2．2to2．8km，is is also supPorted by the free－air gravity consi（1ere（1 to be the critica1 （iisplace－ anomaly data．One of the largest free－ ment for the initiation of subduction，the air gravity anomalies in the Japan Sea， no：rthern Okushiri Ridge with the d，is－ 一57milligals，is obse：rved．along the west． placement of3km may be the site of em margin of the northem Okushiri incipient subduction．The Manila Trench， Ridge （Fig．52）．The normal free－air however，is not very young sub（1uction gravity anomaly along an ocean－conti－ zone．The Trench was formed before10 nent boun（1ary is －20 to －30milligεしls， Ma．It should be noted that there is a then，anomaly of－57milligals may indi－ possibility that the basement high at the cate that the crust of the Japan Basin trench slope break uplifte（i or subsi（ied along the no：rthem Okushiri Ridge is after the initiation of subduction． being forced down．Such a fea．ture may The Mussau trench along the Caroline be explained．by a simple thrust fault and．Pacific Plate boundaries in the west． mode1．The inequilibrium of isostasy， ern Pacific is one of the youngest trench－ however，is evident along the northern es on the earth （HEGARTY ααZ．，1982）． Okushiri Rid．ge． The Mussau Trench initiated eastward The seismic pro丘le on Figure52shows subduction at l Ma and the Caroline the eastwεlrd thickening of the upper Plate has been sub（1ucte（l to an extent of part of the sed．imentary sequence of the 10km beneath the Pacific Plate．The Japan Basin． This feature suggests Mussau Ridge is a hnear feature on the ongoing subsi（ience of the crust of the western edge of the Pacific Plate along Japan Basin．Ponded sediments are ob－ the Mussau Trough．The elevation range served along the foot of the Okushiri of the Mussau Ridge above the basement Ridge．The presence of the ponded，sed．i－ of the East Caroline Basin is 1．8 to ments also suggests the recent subsi－ 3．8km．The analogy of the Ok：ushiri dence of the Japan Basin along the Ridge with the Mussau Ridge，also，does Okushi：ri Ridge． The subsid．ence is also not reject the possibility of the incipient evident in the eastward．declining sea sub（1uction at the northern Okushiri floor surface of the ponded sediments． Ridge．The thmst fault at eastern mar． The （1istribution of the pon（1e（1 se（ii－ gin of the Rishiri Trough（Fig．15），which ments，howeve：r，is not commonεし10ng has the displacement of2．5km，also has the no：rthern Okushiri Ridge（Fig．46）． the possibility of being an incipient sub－ There is possibility of incipient subduc－ duction zone． tion in the eastem margin of the Japan The above（1iscussion on the initiation Sea as（liscussed，above． The sub（1uction

一349一 B認Ze伽o∫抗eGeoZo9‘eαZ翫ruεッo∫」αPαη。▽oZ．39，No。5

sense，however，d，oes not prevail over the trast between the two convergent litho． entire eastern Japan Sea． Figure53 spheres may control the polarity of the shows a composite distribution of the thrust faults in the mamer that the eastward．dipping thrust faults and west－ heavier side makes footwall and the ward，（1ipping thrust faults in the eastern

margin of the Japan Sea． Zone of east－ 137。 139。 141DE ward，d，ipping th：rust fa，ults a：re observed， A to the west of southern Sakhεし1in and O 46。N northern Hokkai（10，at the northern en（i ♂ of the Okushiri Ridge，to the west of the Oga Peninsula，and to the west of Sad，o Islan（i． These zones have the possibility 2・ O メ of incipient subd．uction． Zones of west－ ward．d．ipping thrust faults are observed B一一／ north of Okushiri Islan（1an（l along the 1 づ／／甘1 44。 Sad．o Rid．ge as d．iscussed，in Chapter3．4． ク The westward．dipping thrust faults ind．i－ “．1 cate an opposite sense against the sub＿ o郊

duction sense． 』12一一一 Thus，the sub（luction sense （zone of J a p a n

eastward．dipping thrust faults）and．the S e a →納

obd．uction sense（zone of westwa：rd．dip－ ’羅 ping thrust faults）show a composite 42。 distribution throughout the eastem mar． ／ 1翼 gin of the Japan Sea。 The change of 卜卜 polarity of the thrust faults occurs in a レ short （1istance． The cause of such a polarity change is ambiguous．HEGARTY 疹 、 ααZ． （1982） also observe（1 a polarity Il Q ノ change of sub（1uction sense to ob（iuction 40。 sense at the Mussau Trench．The Mus－ ら 〆 蕪 ⊂） 盃 sau Trench shows a prominent eastward o sub（luction structure in its southern part o ヨン Q but，in the northern part，it changes to ’チ o 、 an eastward obduction sense with a ソ 1 ’ E swarm of thrust faults．They presented ’ブ an explanation that the polarity change 38“

may be due to the change in convergence ブ rate．The convergence rate in the south－ 1111111ぞ脚eastwa「ddipPing em part of the Mussau Trench is more … zone of westward dipPing than1．5cm／yr while that of the northern ； thrust

part is less than L5cm／yr．Such expla－ 137。 139。 141。E nation is difHcult for the composite dis－ tribution of subduction an（1 0bduction Figure53 Composite distribution of eastward sense in the eastern Japan Sea． One dipping thrust faults and the west－ ward dipPing thrust faults in the possible inference is thεlt a（lensity con一 eastern margin of the Japan Sea．

一350一 GθoZO9‘0αZS施伽rθ0∫孟んθJiαPαηSεααηd‘孟SホεCむ0疵伽頑0α孟ε0ηS（K．Tαmα勧 lighter side makes the hanging wall of Northeast Japan Arc is on the North the faults． America Plate， NAKAMuRA（1983）an（1KoBAYAsHI（1983） FuKAo and YAMAoKA（1983）presented have presente（1 a hypothesis thεしt the the trends of the axes of maximum eastem margin of the J’apan Sea is a compression along the Itoikawa－Shizu． new Eurasia－North America plate bound，． oka Tectonic Line on the basis of focal ary． According to thei：r idea，the plate mechanism solutions of several small boundary in the eastem margin of the earthquakes．The P axes show NW－SE Japan Sea extends to the Itoikawa－Shi－ to WNW－ESE．The trend of convergence zuoka Tectonic Line in the central Japan should．beN500Eifthetectoniclineis and reaches to the Nankai Trough in the the bounda』ry between the Eu：rasia and． Philippine Sea． The new plate bound，ary North America Plates （MINsTER and is considered to have jumped from the JQRDAN， 1978）． This （iiscrepancy also former plate boundary along the central does not support the new plate bound．ary axis of Sakhalin－Hokkai（10in eεlrly Qua－ hypothesis． ternary．This attrεlctive hypothesis，how－ Thus，the new plate bound．a：ry hypoth－ ever，has some problems． esis is ambiguous．The author presents KELLER （1980） an（1 von HuENE θ6α1． another possible explanation， that the （1980） documented that the vertical convergent stress in the Japan Sea is movement of the basement in the forearc εllso due to the In（iia－Eurasia collision basin of the No：rtheast J’apan Arc has an（1 its associate（1 intra－Plate or inter changed from subsidence to an uplift microplate movement in East Asia．The sense since early Quaternary， on the Baikal Rift has：reactivated its extension basis of （ieep sea （irilling results． The at some time in the Pliocene（ZoNENsHAIN change in vertical movement suggests an（1 SAvosTIN， 1981）． The reactivated． the prevalence of compressional stress Baikal extension隔at some time in the since early Quaternεlry in the forearc Pliocene is roughly compara，ble with the area．The compressional stress且eld in initiation of convergence along the east－ the forearc area of the Northeεしst Japan ern Japan Sea at about2Ma．The Baikal Arc in the Quatemary is comparable Rift and the eastem margin of the Japan with the compressional tectonics in the Sea bound the Amurian Plate on its eastern Japan Sea since latest Pliocene． western and．eastern sid．es：respectively． The compressional stress且eld in the The recent tectonics of the eastem mar－ forearc area，however，is inconsistent gin of the Japan Sea is well und．e：rstood． with the new plate bound，ary hypothesis． in the reference frame of the eastward Accord．ing to the hy’pothesis，the No：rth－ movement of the Amurian Plate．The east Japan Arc is includ．ed．in the North eastwardmovement of the AmurianPlate America Plate which moves westward causes the Baikal extension along its with a rate of greate：r than1．O cm／yr at westem margin and Japan Sea conver－ the Jεlpan Trench（MINsTER an（i JoRDAN， gence along its eastern margin（Fig．54）． 1978）．Such movement should produce If this is the case，the con（1ition shou1（1 a tensional stress fie1（i over the forearc be id．entical with the initiation of the area if the trench axis is fixe（1． The Manila Trench along the eastern margin compressional stress over the forearc of the South China Sea in Miocene which area of the Northeast Japan Arc in the is possibly due to the eastward move－ Quaternary does not supPort that the ment of the South China Block in relation

一351一 B沼2伽・∫伽Gθ・Zo9ε・αZ翫rひ2ッ・μαPαη。．Vb乙39，No・5

7◎o 9◎。1葛◎o 13◎。 15◎o 0 50。 40 、 、 、、 、 、 馳 ノ 、 ヤ ヤ ヤ も 、、 、、 略’！・、、一．遷）＼、

e ㌔ 、 ㌔ 、り一、、 s 亙 趨 薦 露 4 幽 、幽 6◎o 之幻1 ’｝’ぜ’覇 441二 1 」 ‘、、 ‘ 、聾 ＼禰馨蟹鯉餐 、、 ㌧ ＼ 、、 1 150。 、 、 、 覧 、 、 3◎。 、、 ノ 、、 覧 、 5げ 婁 ’ 、 、 1 、 ’ ヤ 4 1 ノ ヤ 陰 ’ ． 、 1 ρ o 認 思4◎。 ノ’、（

鴻◎。 帰o 200 μ一ゲ ノ，つ ノ’ノ・ノ7＾』 もタノ ノ ＼く～＿ 嶺 30。 罫 ヨ0。 ∂ 一一『』 、 麩 ￥ 恐 130。 ノ、、＿一噛 蕊 、 、 ＼ き 2◎。 、 ’ 1 、 ”吻網 、 1 ＼＼ 護 ノ 、 ！ 、 ’ ノ ， ぐ 義 、 ノ r”げ‘ ノノ ’ 、！ ノ 馨 ＼ ■ 、覧ノ ノ ノ 1 ’ oo ～ ￥ ， も 一 ノ 、 ’ き 、 、 蓑 1 、 1 ヤ し

、 、、 難 f 思0。 、 》慧 、 e鴨心 ￥一，ノ 評 ノ夢毎ノ 、、 1◎。 嚢 、 会》o 、 嚢 、 グ，灌露） ＼ミマr、、有一ψ9ノ’、

◎o 暦200 700 800 goo 1000 11

Figure54Movement of microplate，the A卑urian Plate，in relation to Indo－Eurasia collision and convergence along』the eastem margin of the Japan Sea．This figure is modified from TAppoNNIERεむαZ．（1982）．

欝

一352一

羅 σθoZO9‘0αZS傭伽reo∫孟んθ」αPαπSεαα認‘6SむθCカ0肋伽頑0α孟‘0ηS（KTαmαんε） to the In（1ia－Eurasia collision． This ex－ tures on these maps，their structural planation，however，also has problems． characteristics，an（1 the age一（1epth－heat ZoNENsHAIN an（i SAvosTIN （1981） calcu－ flow ：relationships of the basins have 1ated the relative movement of the been combined to interpret the tectonic Amurian Plate against the Eurasia Plate evolution of the Japan Sea． （Siberia Block）． The rotation pole of The age of the basins were examined the Amurian Block is located just north by a comparative study of sediment of the Baikal Rift． This：results do not stratigraphy，basement（1epth，and heat supPort the eastward movement of the now data．Presented age estimation is Amurian Plate． about30to15Ma for the Japan Basin， The origin of compressional stress in 30to10Ma for the Yamato Basin，and the eastern margin of the Jεlpan Sea is comparable age ranges for the Tartary still uncertεlin． The incipient sub（iuction Trough and the Tsushima Basin．The an（10b（iuction in the eastern Japan Sea， acoustic basement of the Japan Basin is howeve：r，εlre ra．ther evident． Further mostly correlated to a oceanic basalt detaile（1 stu（1ies an（1 discussion about layer，exclud．ing its margina1εしrea． The the tectonics of the Japan Sea will pro． acoustic basement of the Yamato Basin duce general mo（lels for tectonics of is inferred，to be volcaniclastics which lithospheric convergence， especially on are correlated to the Green Tuff forma． itS initial Stage． tion in Japan．The volcεlniclastics overlie a deeper oceanic basement．When these 4．Summ＆ryandCo聡elus量o聡s deposits are considere（1， the greater oceanic basement depth of the Yamato The tectonic evolution of the Japan Basin is consistent with its estimated Sea for a long time has been a contro－ age range of30to10Ma． versial subject and models for its deve1． The topographic highs of the Japan opment have often been ambiguous．its Sea are classifie（i into four groups；con－ evolution has a close relation with the tinental fragments，rifted continental tectonics of the Japanese Islan（is，but fragments，tectonic ridges，and volcanic the lack of a well constrained model has seamounts．Continental fragments are been an obstacle for studying the tec－ compose（10f o1（ier rocks inclu（1ing rocks tonic evolution of the Japanese Islan（ls of Precambrian age．These fragments an（1the adjacent area． This study pro－ are clearly of a pre－rift age． They con－ vides new constraints on the age and sist of large an（l elevate（i topographic nature of the tectonic evolution of the highs such as the Yamato Ridge and the Japan Seεl through new and，detailed， Korea Plateau．Rifted．continental frag－ discussion of its geological structure， ments represent a low topographic rise base（i in large part on marine geological because of its transitional nature from and geophysica1 （iεlta obtaine（1 （luring continentεll to oceanic．The Takuyo Bank the research c：ruises GH77－2，GH77－3， and the Sado Rise are examples of this GH78－2，and．GH78－3con（lucte（1by the group．The tectonic ri（1ges are observed． Geological Survey of Japan．Marine geo－ along the eastern margin of the Japan 10gical maps were compiled on the basis Sea． They were elevated by the conver－ of the stratigraphic correlation of seis－ gent tectonics since about latest Plio－ mic profiles and the bottom sampling cene． The Okushiri an（i Sa（io Ri（1ges results．The（1istribution of geologic fea一 are the typical cases of this group．Vol．

一353一 匿鼠濫『

B沼e伽oμんθGeoZo9茜oαZS砿roeッoノ」伽αη．γoZ．39，1〉o．5

canic seamounts which were caused．by Amurian Block generated the Stanovoy arc volcanism since the initiation of the Range along its northem margin by the sprea（1ing of the Japan Sea are abun－ collision with Siberia，and．the pu11－apart dantly observe（1． The （1istribution of basin of Baikal Rift along its western these topogfaphic highs are closely re－ transform bound．ary with Siberia．The late（l to the tectonic evolution of the movement also caused clockwise rota． Japan Sea． tion of the Okhotsk Block by（1rag forces Two types of back－arc spreading were along Sakhalin－Hokkai（101ine．The clock－ proposedlasinglerifttypeandamulti wise rotation of the Okhotsk Block is rift type．The multi rift type is the evi（1ence（1by opening of the fan－shape（i case for the Japan Sea．Large tensional Kuril Basin an（i the Sre（1inny collision stresses over the former islan（i arc on the Kamuchatka Peninsu1ε1． caused a multi rift system in its broad The Oklushi：ri and．Sad．o Ridges along volcanic zone along which the lithosphere the eastem margin of the Japan Sea are of the Island．a：rc is weakest． Some of boun（1e（l by thrust faults． The thrust the rifts developed into multi back－arc faults have been active since latest Plio－ spreading ridge system and others were cene．The ridges are formed by the left as aborte（1 rifts such as the Kita－ thrust movements a，ssociated with the Yamato Trough．The broad volcanic uplift of the e（ige of hεしnging si（1e over zone of the former Japanese islan（1arc the footwa11．The zone of eastward．d．ip－ overprinted the volcanism of back－arc ping thrust faults and the zone of west－ spreading due to a shallow subduction ward dipping thrust faults are discrimi－ angle．Arc volcanism under the tension－ nated．They are d．istributed composite－ al tectonics caused a thick accumulation 1y．Incipient subduction may be the case of volcaniclastics an（i the abun（1ant oc－ in some parts of the zone of eastward currence of seamounts and knolls in the dipPing thrust faults． Incipient ob（luc－ basins． The multi back－arc spread．ing tion of the oceanic crust of the Japan system resulted．in the fragmentation of Sea also is taking Place in the zone of the continental c：rust which generated westward．d．ipping th：rust faults． Litho－ topographic highs classi且ed as continen－ spheric convergence is evi（lent along tal fragments． these thrust zones on the basis of the The large tensional stress in the over－ synthetic stu（1y of the thrust faults an（1 ri（ling Plate is significant for initiating their corresponding earth（luakes． The the above tectonics． It is inferre（1to be origin of the convergent stress is infer－ possible only by retreat，of the continen－ re（1also to be（1ue to the In（1ia－Eurasia tal block from the trench line． The collision an（1 its associated intra－Plate Eurasia Plate itself（1i（1not retreat as a or inter microPlate movements in East whole from the westem Paci且c trench Asia． The recent tectonics of the a，rea systems at the corresponding time．The are well und．e：rstood．in the reference back一εlrc continental block of the Japan frame of the eastward movement of the Sea，the Amurian Block，however，is Amurian Block relative to the Japanese postulated．to hεlve：retreated，no：rthward． Islands．The eastward movement of the due to the In（iia－Eurasia collision an（1 Amurian Block caused the Baikal exten－ associated lithospheric deformation in sion along its western margin and the East Asia which initiated．at about40 eastern Japan Sea convergence along its Ma．The northward movement of the eastern margin，

一354一 韓

護 GεoZo8‘oαZs施c傭θo∫孟んθ」1αPαηSθααη面むsむεe孟o耽ε即Z‘oα6εoηs（KTαmαん‘）

In conclusion，the two malor stages of （1iscussion． The autho：r would．1ike to tectonic evolution are （1e（1uce（i for the acknowledge the critical reading of the JapanSea manuscript and encouragement by Dr． 1） Dive：rgent tectonics over the Japa． Eiji INouE and．D：r．Eiichi HoNzA of the nese islan（i arcs cause（1back－arc sprea（1－ Geological Survey of Japan and．P：rof． ing of the Japan Sea from30to10Ma． Atsuyuki MIzuNo of Yamaguchi Univer－ The spreading system was initiated from sity． The autho：r also would，1ike to a multi rift system over a broad arc express his thanks to Dr．Hideki IMAI volcanic zone．Through the development for his constant guidance an（i encour－ of the spreading system，many continen－ agement．The author should extend．his tal fragments were left in the basins． thanks to the captains and crews of the Overprinting of the arc volcanism on㌻he research vessel Hakurei－maru for the back一εlrc sprea（1ing activity resulte（1 in cruises of GH77－2，77－3，78－2，and，78－3， the occurrence of abun（1ant volcanic sea－ aboard．which he had．the oppo：rtunity to mounts and knolls and a thick accumula－ collect（iata for this stu（1y． The author tion of volcanic1εlstics in the basins． apPreciate the help of all the scienti丘c The tensional stress for generating such stε1鉦 an（l assistant staff of the cruises tectonics is considere（i to be due to the an（1his previous colleagues of the Geo－ India－Eurasia collision an（i associate（1 10gical Survey of Japan for their con－ 1ithospheric deformation in East Asia． stant discussion． Discussion with the 2） Lithospheric convergence has been late Prof．：Kazuaki NAKAMuRA improved concentrated along the eastem margin this ：resea：rch extensively． Discussions of the Japan Sea since latest Pliocene． with Profs． Seiya UYEDA and Kazuo The Okushiri and Sad．o Ridges，bounded KoBAYAsHI and．Drs．Tetsuzo SENo and by many active thrust faults，are conse－ Gaku KIMuRA are gratefully acknowl－ quences of the convergence along this edged．The author thanks Profs．Thomas new lithospheric bound．ary．This tecton－ W。C．HILDE and Roger L．LARsoN for ic stage is also due to the In（1ia－Eurasia their partial critical reading of the man． collision．The eastward movement of uscript．The author must acknowledge the Amurian Block has caused extension the English cor：rection by Mr．Dεlvid．B． along the Baikal Rift at its western SHEpARD and proof reading of final man－ margin and convergence in the Japan uscript by Mr，Manabu TANAHAsHI． Sea along its eastern margin． Last of all the author would like to thank Dr．Yasumoto SuzuKI of the Geo－ Ak麹owledgements：This research was 10gical Survey of Japan for critical re－ done when the author was a research view of the manuscript and Dr．Teruo scientist of the Geological Survey of SHIRAHAsE of the Geological Survey of Japan from1974to1986．The original Japan for editing the manuscript．With－ version of this paper was submitted to out their patient efforts， this paper Kyushu University as a doctoral thesis would．not have been published．． in1985． The author would like to ex． press his sincere thεlnks to Prof．Ryohei Refere聡ces TAKAHAsHI，Prof．Tsugio SHuTo，Prof． ：Kametoshi KANMERA，and Assoc．Pro． ABE，K （1975）Re－examination of the fault fessor Yujiro OGAwA of Kyushu Univer－ model for the Niigata earthquake of sity for their expert gui（1＆nce an（1 1964．♂．．P妙s．Eα航，voL23，p．349一

一355一 B4ZZθ翻7z oゾ』オhθσθoZo8オoαZ Sμr∂θツoゾJiαPαη． VroZ．39，．〈「o．5

366． style at convergent plate boundaries。 ABE，K．and：KANAMoRI，H．（1970a） Mantle σ20Z．Soc。Cαηαdαβ加cεαZ－PαPεr，voL structure beneath the Japan Sea as 20，p．553－573。 revealed by surface wave．B認Z．Eαr地9． DINGLE，R．V．and ScHuRuTToN，R。A．（1977） Rεs．Zηβヵ．Uη，加．To凝yo，vo1．48，p．1011－ Continental margin fault pattem map－ 1021． pedsouthwestoflreland．ノ〉伽rε，voL BERsENEv，A．F．，GAINANov，A．G．，：KovYLIN， 268，P．720－722． V．M．and STRoEv，P。A．（1970）Struc－ ENGEBRETsoN，D。C．and Cox，A．（1984） Rela－ ture of the earthys crust and upPer tive motions between oceanic plates of mantle of the Japanese Sea an（l the the Pacific Basin．Jl．σθo∫）んlys．Rεs．， zone of the Paci丘c Ocean a（ljoining vo1．89，P．10291－10310． Japan．ln．ProbZθ耽SOゾS傭伽rθ0直んθ FuJITA，K．and WATsoN，B．F．（1983） Tectonic ．Eα励’sσ辺sオαπ霞加Pεrハ4α㍊θ，Nauk， evolution of Kamchatka and adlacent Moscow，p．50－61 （translated from regions．EOS Trαηs．AGU，vo1．64，p。 Russian to Japanese，Nihonkai（Japan 871． Sea），no．6，p．53－61）， FuKAo，Y．and FuRuMoTo，M．（1975）Mecha－ and LELIKov，E．P．（1979） Geological nism of large earthquakes along the map of the Japan Sea．jprかodα，voL， eastemmarginoftheJapanSea．Tθc－ p．74－79 （in Russian，transla．ted by 60ηo加ys．，voL25，P．247－266。 KIAsENo， Y。and．KuwANo，Y．，1980， and．YAMAoKA，K．（1983） Stress esti－ Bull．Japan Sea Res．Inst。，Kanazawa mate for the highest mountain system Univ．，no．12，p．97－103．）． in Japan．Tθo乙oη‘（〕s，voL2，P．453－471。 BIKKEMA，S．K．，ZH皿sov，E．G．，LlvsHITz，M． GNIBIDENKo，H．（1979） The tectonics of the ：KH．，NIKoNovA，N．A。，SuvoRov，A，A． Japan Sea．ハ4αrεπe Gセo乙，voL32，P。71－ and TREsKovA，YU．A． （1968） Deep 87． seismic soundings in the vicinity of HAYEs，D．E．（1983） Global studies of ocean Sakhalin．Tんεσrαs孟απdのperハ4απむZθ crustal （iepth－age relationships． EOS 0∫伽PαC4βcArθα．σθOPん』ys．Moηogr。 Trαηs。AσU，voL64，p．760． Sεr．，vo1．12，P．349－366． HEGARTY，K．A．，WElssEL，」．K and HAYEs， CARLsoN，R．L．and MELIA，P．」．（1984）Sub－ D．E．（1982） Convergence at the Caro－ duction hinge migration．TεcカoπoPんツs．， line－Pacific plate boundary：collision vol．102，p．399－411， an（i sub（iuction． In Tんe Tee60η‘c αηd CHAPMAN， M．E． and．SoLoMoN， S。C． （1976） GθoZo9‘oEひoZ画oηoブSo痂θαsカ．Asεαη North－American－Eurasianplatebound－ SθαsαηdZsZαπdsPαr孟2．∠4GUσεOlρんlys ary in northeast Asia。」．GeoPんlys。 Moηogr Sθr。，ed，ited．by D。E HAYEs， Rεs．，vol．81，p．921－930． AGU，Washington，D。C。，voL27，p。 CRouGE，S．T．（1983） The correction for se（1i－ 326－348． ment loading on the seaHoor。♂．Gθo－ HILDE，T．W．C。and WAGEMAN，」。M。（1973） Pんツs．Rεs．．voL88，P。6449－6454． Structure and origin of the Japan Sea． DAvls，E。E．and LlsTER，C．R．B．（1977） Heat In Tんθ 四θsオθrηPα040， edited by P．」。 now measured over the Juan de Fuca CoLEMAN，Univ。Westem Australia Ridge：Evidence for widespread hydro－ Press，p．415－434． thermεしl circulation in a highly heat HoNzA，E．ed．（1978a） Geological Investiga－ transportive crust， 」． σεo』ρんツs． Rεs．， tion in the Northern Margin of the vo1。82，P．4845－4860． Okinawa Trough and the Westem Mar－ DEwEY，」．F．（1980） Episod．icity，sequence and， gin of the Japεしn Sea。σθo乙Sαro．」αPαη

一356一 GεoZo9オcαZs伽伽rθo∫伽。吻αηSθα侃伽力s孟θc孟o耽伽頑oα孟‘o几s（K．Tαmαん茜）

C漉sθR師．，vol．10，P．79。 abstract）． HoNzA，E。ed。（1978b）Geological Investigation HussoNG，D．M．and FRYER，D．（1982）Struc－ of the Okhotsk and Japan Seas off t皿e and Tectonics of the Mariana arc Hokkaido．Gθoz．翫rひ．JlαPαησ臨sθ an（1 fore－arc： （irilling selection sur－ RεP孟。，no．11，P．72． veys．ln翻ぬZRεporεsoゾむんθDθεpSeα ed．．（1979）Geologlical Investigation of 1）r♂Z伽8’Proゾεoむ，edited by D．M．Hus－ the Japan Sea． σεoZ． Sωru． 」4Pαη soNG and S．UYEDA，ααZ．，U．S．Govt． C臨sθR餌．，no．13，P．99。 Printing O伍ce，Washington，D。C。，vol． （1979） Se（1iments，structure an（1 0ri－ 60，p．33－44． gin ofJapan Sea－concluding remarks一． and UYEDA，S。（1982） Tectonic pro－ InσθoZO9ε0αZ伽θS古♂9α古‘0η0∫伽」α一 cess and the history of the Mariana pαη，Sεα，edited by E．HoNzA，Geo1． Arc：a synthesis of the results of Deep Surv．Ja．pan Cruise Rept．，no．13，p』89－ Sea Drilling Project Leg60．In1η痂αZ 93． Rθpor孟soμんεDθεpS2αDrεZ伽gPro一 （1983） Central Japan Sea earthquake ノec古，edited by D．M．HussoNG，and S． and submarine faults。Kα9醜α（So‘一 UYEDA，ααZ．，U．S．Govemment Print－ θηcε），vol．53，P．510－514（in Japanese）． ing O伍ce，Washington D．C．，voL60， ，KAGAMI，H．and NAsu，N。（1977） P．909－929． Neogene geological history of the To－ Ic田KAwA，M．（1971）Reanalysis of mechanism hoku arc system．」1． Ocθαη08ア・αメ）ん‘oαZ of earthquakes which occurred in and Soo。JiαPαη，voL33，P．297－310． near Japan，an（i StatiStiCal StU（lieS On ，TAMAKI，K．andNls田MuRA，K。（1978） the no（ial plane solutions obtained， Sono－buoy refraction measurement．In 1929－1968．020Pんツs．ハ4α9．，voL35，P、 σεoZo9♂oαZ伽θs孟‘9α翻oηoμんθ0肋o醜 207－273． α認」αPαηSθαsoガHo娩α乞do，edited INouE，E． （1982）Geological problems on by E。HoNzA，GeoL Surv．Japan Cruise Cretaceous an（i Tertiary rocks in an（1 Rept．，no．11，p。46－49． ＆round Tsushima－Korea straits．CσOP ， ，YuAsA，M．and MuRAKAMI， Tec勧‘cα詔躍．，voL15，p．85－121。 F．（1979）Geological map ofthe south－ and HoNzA，E．（1982）Marine Geolo－ em Japan Sea and Tsushima Straits。 gical Map around Japanese Islands， ハ4αr加εGεoZ．Mαp Sεr．，no．13，Geo1． Marine GeoL Map。Ser。，no．23，Geo1。 Surv．Japεしn， Surv．Japan． HosHINo，M．and HoNMA，H。（1966）Geology IsEzAKI，N．（1975） Possible sprea（iing centers of submarine banks in the Japan in the Japan Sea． ハ4αr‘ηθ Gθophツs。 Sea。 ρん‘んツω Kα9αんα， voL82，P．10－16 Rεs。，vol．2，p．265－277。 （in Japanese with English abstract）． （1979） On the age of opening of the HoTTA，H． （1967） The structure of se（1imenta－ Japan Basin inferred from the mag． ry layer in the Japan Sea． σθoPんツs． netiC lineatiOnS．〈砿ん0ηんα‘（7iαPαηSθαク， B認Z．Ho娩α‘do U漉ひ。，voL18，p．111－ vol．19，P．111－119 （in Japanese）． 131． and UYEDA，S，（1973） Geomagnetic （1971）Sediment distribution in the anomaly p＆ttem of the Japan Sea。 continental bor（lerlan（1 0任 Japanese Mαr‘ηθσεOPんツs．Rεs．，voL2，P．51－59． Islands。lnlsZα屈ルoα屈Mαr8伽Z IsHIHARA，T．（1983） Gravity field．around，Ja－ Sθαs，ed，ited．by S。AsANo and．G。B． pan－sea gravimetry by the geological UDINTsEv，Tokai University Press，P。 surveyofJapan。ハ4αr‘ηεG20dθsツ。，voL 147－153 （in Japa．nese with English 7，P．227－256．

一357一 B砿ZZε孟‘ηo∫εんθGeoZo8♂（｝αZ S砿rひθツoゾ’JdPα7z． VbZ．39，1〉o．5

IsHIKAwA，Y，，TAKEo，M．，HAMADA，N．， area．，Leg．57，Deep Sea Drilling Pro－ KATsuMATA，M．，SATAKE，K．，ABE，K．， ject。ln翻倣ZRepor孟soゾ伽1）εεpSεα ：KIKucHI，M，，SuD6，：K．，TAKAHAsHI， 1）r泥麗ηg Proブeoカ， e（iite〔1 by Scienti丘c M．，K』AsHlwABARA，S．and MIKAMI，N． Party， U．S． Govt． Printing O伍ce， （1984）Mechanism of the Central Ja－ Washington，D。C。，vo1．56，p。835－865． pan Sea Earthquake，1983．Cんε為ッω（Tんε KIMuRA，G．，MIYAsHITA，S．and MIYAsAKA，S． Eα融Mo説配y），voL6，P．11－17（in （1983）CollisiontectonicsinHQkkaido Japanese）． an（1 Sakhalin． In Aocr2麗oη Tθ（〕むoπεos IsHlwADA，Y．and OGAwA，：K．（1976）Petroleum 漉伽αroμm－Pαo玩cRε9‘oηs，edited geologyofo登shoreareas＆roundthe by M。HAsHIMoTo and S．UYEDA，Terra Japanese Islands．σ00P Te（〕んηεcαZ BμZ－ Pub．，Tokyo，P．69－88． Zε伽，vol。10，p．23－34． an（l TAMAKI， Kl． （1985） Tectonic ，HoNzA， E。 and TAMAKI，K。（1984） framework of the：Kuril Arc since its Se（limentary Basins of the Japan Sea． initiation．In Forητα翻oηoゾAo擁uθOceαη Proo． 27乙h，五η孟θrη． （｝20Z． Con8r。， vo1． Mαr8εηs， e（lited by N． NAsu εむαZ．， 23，P．43－65． Terra Pub．，Tokyo，p．641－676． IvANov，V．V．，PucHARovsKIY，YU．M．and an（1 （1986） Collision，rotation， TIゴMAN，S，M．（1981）Tectonicposition an（1 back－arc sprea（ling： the case of and structure of sedimentary basins on the Okhotsk an（1Japan Seas。Tθe孟oη‘os， the northwestem margin of the Paci丘c voL5，P．389－401． Ocean。G20カθc古oη‘（｝s，vo1。15，P．291－298 KLEIN，G．deV．and KoBAYAsHI，K．（1980） （translated in English from Russian）． Geological summary of the north Phi－ IwABucHI，Y．（1968） Submarine geology of the 1ippine Sea，bεしsed on Deep Sea Drilling southeastern part of the Japan Sea． Project Leg 58 results．In 五ηあ‘αZ Rθ一 Toんoん砿U漉ひ．玩s診．GθoZo9ツαη，d PαZε一 por孟soゾ伽DeεpSθαDr‘Z伽8’Proゾec孟， 〇ηεoZo9ツ σoπ診r．， vol．66， P．1－76 （in edited by G。deV．KLEIN，K．KoBAYAsHI， Japanese）． θ6αZ．，U．S．Govemment PrintingO伍ce， KARIG，D．E．（1971） Origin and development Washing・ton，D．C．，vo1．58，p。951－961． of marginal basins in the westem Pa－ KoBAYAsHI，K．（1983） Sprea（iing of the Sea of ci且c．♂．σθoPんツs。Rθs．，vol．84，P．6796－ Japan an（i （irift of Japanese Islan（l 6802． Arc：A synthesis and speculation．In and，SHARMANIII，G。F。（1975） Subd．uc－ ls乙α掘．Ares，Mαrg‘nαZ Seαs，α几d tion εしnd accretion in trenches。 σεoZ． 面roんo DεPOS‘孟S，ハ4謡η8’GεoZO9ツ Soc．．Amεr‘oα．翫ZZ．，voL86，P．377－ 助80εαZ lssωe，e（1ited by E．HoRIKosHI， 389． The Society of Mining Geologists of and INGLE Jr．，」．C．M．eds．（1975）甜一 Japan， Tbkyo， vo1．11， P．23－36 （in 伽ZRθpor孟soμんθDθθpSεαPr淵π9 Japanese with English abstract）． Proブθ06，U，S．Govt。Printing Office， （1984） Subsidence of the Shikoku Washington，D．C．，vol．31，p．927． back－arc basin．TθαoηoPんツs．，vol．102， KATsuRA，T．and KITAHARA，S．（1977） Geolo－ P。105－117． gical structure of the marginal Plateau （1985） Sea of J’apan and Okinawa off Tottori，south－west of Honshu，Ja－ Trough．In TんθOcθαηBαs腕sαπdハ4αr－ pan．Jl．OoθαηogrαPん．Soo。」αPαπ，vol。 g謡s，edited by A。E。M。NAIRN，F．S． 33，P．259－266． STEHLI，and S．UYEDA，vo1．7A，p．419－ KELLER，G．（1980）Benthic foraminifers and 458． paleobathymetry of the J’apan Trench and NoMuRA，M、（1972）Ironsulfidein

一358一 嚢

GθoZo9εoαZs傭c加εo∫ホんθ」αPαηSεαα記髭s孟εoカo耽‘m頑cα古‘oηs（K．Tα肌ぬ）

the sediments cores from the Sea of η‘cs， vol．10，P．273－281 （translate（1 in Japan and their geophysical implica－ English from Russian）． tions．Eαr疏PZαπθむ．Seε．一L観．，vol．16， MINAMI，A．（1979） Distribution and character－ p．200－206． istics of the sedimentary basin offshore KoBAYAsEI， Y． （1983） Initiation of “sub（iuc－ San－in to Tsushima strait．♂．」αPαπεsε tion’7 0f plates． σんオ歳y乱 （Tんθ Eαrむん z4ssoo．一Pθ孟roZ．TθoんηoZo9‘s古s，vo1．44， ハ40η孟んZ二y，）， voL3， P．510－518 （in Japa－ p．407－414 （in Japanese with English nese）． abstract）． KolzuMI，1． （1979） The geological history of MINsTER，」．B．and JoRDAN，T．H．（1978）Pres－ the Sea of Japan －base（1 upon sedi－ ent－day plate motions．♂．GθoPんツs． ments and microfossils一．1〉漉oηんαε Rεs．，vol．83，p．5331－5354． 6」伽αηSθαク，voL10，P。69－90（inJapa－ MoLNAR，P．and ATwATER，T．（1978）Interarc nese）． sprea（ling an（l Cor（iilleran tectonics as KoNo，Y．and AMANo，M．（1977）Thickening altematesasrelatedtotheageof plate model with sedimentation．♂． sub（lucte（i oceanic lithosphere。 Eαrむん SεεsmoZ。Soc．」αPαπ，vol．30，P．163－178 PZαηθε．Soε．Lε甜．，vol。41，P．330－340． （in Japanese with English Abstract）． and QuIDoNG， D． （1984） Faulting LELIKov，Ye．P．and BERsENEv，1．1．（1973） Early associated，with large earthquαkes and Proterozoic gneiss－magmatite complex the average rate of deformation in in the southwestem part of the Sea of Central and Eastem Asia．♂．σεoPんlys． Japan。DoんZα⑳Aんαd．1▽α雄SSSR， Res．，vol。89，p．6203－6228． vol．223，p．74－76．（translate（i in English an（1TAPPoNNIER，P．（1975） Cenozoic from Russian）． tectonics of Asia：effects of a continen－ LuDwIG，W．」．，HAYEs，D．E．and EwING，」．1． tal collision． So‘εη（）θ， vol．189，P．419－ （1967） The Manila Trench and West 426。 Luzon Trough，1，bathymetry and sedi． MoNTADERT，L．，RoBERTs，D．G．，AuFFRET，G．A．， ment distribution．1）θθp SθαRεs．，voL BocK，W．，DuPEuBLE，P。A．，HAILwooD， 14，P．533－544． E．A．，HARRlsoN，W．，：KAGAMI，H．， ，MuRAucHI，S．and HouTz，R．E．（1975） LuMsDEN，D．N．，MuLLER，C．，ScHNITKER， Sediments and structure of the Japan D，，THoMpsoN，R．W．，THoMpsoN，T．L． Sea．σθoZ．Soo．Am．B沼．，vol．86， and TIMoFEEv， P．P． （1977） Rifting P．651－664． an（l subsi（ience on passive continental Maritime Safety Agency of Japan （1980a） margins in the North：East Atlantic． Bathymetric map of Hokkaido，1： 2Vα伽θ，vo1．268，p．305－309． 1，000，000，no．6311． MoRozowsKI，C．L．and H：AYEs，E。H．（1978） Maritime Safety Agency of Japan （1980b） Sediment isopachs．In AσεoPんツs∫cαZ Bathymetric map of north－east Japan， ．AぬSOプ伽EαS孟αηdSOμ孟んθαSたAs‘αη 1：1，000，000，no。6312． Seαs，edited by HAYEs，E．，Geo1．Soc。 MARKov， M．S。， PusHcHARovsKIY， Yu。M。， Amer。vo1。MC－25． ， TIL’MAN，S．M．，FEDoRovsKIY，V，S．and MuRAucHI，S．（1972） Crustal structure of the SHILo，N．A．（1979） Tectonics of East Japan Sea．Kα9αん撹 （Soεθηoε），vo1．38， Asia and Far Eastem Seas．Gθoむεc孟〇一 p．367－375 （in Japanese）． η‘cs，vol．13，P．1－11 （translate（i in En－ ，AsANuMA，T．and HAGlwARA，K．（1970） glish from Russian）． Geological studies on the area，off MELANKoLINA，YE．N．and KovYLIN，V．M．（1977） San’in （listrict in the Japan Sea by Tectonics of the Japan Sea．Gεo孟εo孟o一 means of seismic profiler．B沼。1〉α古．

一359一 犀＝ニユ．一

BωZZε亡‘η、0プ’むんε（｝θo Zo9‘cαZ SμroθツqブJdPαη． γo Z．39，1〉o。5

So‘．ハ4αsεωm．vol．13，P．83－93（in Japa－ sian）。 nese with English＆bstract）． SAKuRAI，M．and SATo，T．（1971）Submarine NAKAMuRA，K．（1983） Possible nascent trench geological structures an（i history of along the eastern Japan Sea as the the Mogami Trough．」。Gθo乙Soα convergent bound，ary between Eurasian 」1α∫）αη，vol．77，p．489－496 （in Japanese and North American Plates．B認Z． with English abstract）． Eαr抗α．Rεs．Zπsε．Uη加．Toんッo，vol．58， ， ，TAGucHI，H，，NAGANo，M． p．711－722 （in Japanese with English and UcHIDA，M．．（1971）Submarine abstrεしct）． topography an（i geological structure of NIINo，H．（1933） On the bottom materials of the continental shelf west of the Noto the Yamato Bank in the Japan Sea． peninsula。Ji．GθoZ．Soo。」αPαη，vol．77， 」．GeoZ．Soo．。瓦αPαη，voL40，P．86－100 p。645－654 （in Japanese with English （in Japanese）． abstract）．

（1935） On the newly discovered bot－ ， シ ， 7 tom materials of the“Kita－Yamato and HAMAMoTo，F．（1972） Submarine Bank”。」．σεoZ．Soc．Jdpαη，vo1．42，p． geolo9』ical structures and submarine 676－684（in Japanese）． cany・nsin，thearean・rth・fT・蜘a （1942）Bottom materials of the Oki Bay．Jl．GeoZ。Soc．Jiαpαη，，voL78，p． Bank in the Japan Sea．♂。GεoZ．Soo． 475－484（in Japanese with English ab－ 」αpαη，vol．49，p。232－233（in Japanese）． stract）． OToFuJI，Y．and MATsuDA，T．（1983） Paleo－ SATAKE，T．and ABE，：K．（1983） A fault model magnetic evidence for the clockwise for the ］N’iigata，Japan，earthquake of rotation of Southwest Japan．Eαπh！ June l6，1964．⊂Z．Pんys．五7αr古ん，voL31， PZα惚．So‘。Lε甜．，vol．62，P．349－359． P．217－223． OzIMA，M．，KANEoKA，1．and UENo，N’． SATo，T．（1971）Sea bottom survey in west－ （1972） Sprea（1ing Jεしpan Sea？．Kα8αんω ward of the Northeast J’apan．Cん乞gαんα （Sc‘eηcθ），vol．42，p。350－352 （in Japa－ Zαssんε，vo1．80，P．285－301 （in Japanese

nese）． with English abstract）。 PARsoNs，B．and ScLATER，」，G．（1977） An and ONo，：K．（1964）The submarine analysis of the variation of ocean且oor geology off San’in （listrict， southern bathymetry and hea．t flow with age．♂． Japan Sea．」． σθoZ．So（｝．」1αPαπ， vo1． σεop妙s．Res．，voL82，p．803－827． 70，P．434－445． RoDNIKov，A．G．and KHAIN，V．B．（1971） On ，SAKuRAI，M．，TAGucHI，H．，NAGANo， the tren（1 in evolution of the earth’s M．，UcHIDA，M．and OMoRI，T．（1973） crust in the north－westem part Submarine geology of the continental of the PaciHc mobile belt．In lsZα記 borderland west of Hokkaido．Rθμ。 Arc αアL（メ ハ4αr9εηαZ Sθα， e（lite（1 by S． 恥dlrogrαPん． Rθs．， no．8， P。1－49 （in AsANo and G．B． UDINTsEv， Tokai Japanese with English abstract）． University Press，p．65－76 （translated． SAwAMuRA，M．（1978MS）Diatom analysis of in Japanese with English abstract）． the bottom sample data of GH77－3 SAHNo，B．G。and VAsILIEv，B．1．（1974） Basalt－ cruise． oids of the Japan Sea bottom．In．Pro一 ScLATER，」．G．（1972） Heat且ow and elevation わZeητs oノσeoZogyαηdl GeOPんlys‘cs o∫ of the marginal basins of the westem 孟んeハ4αr8‘砿ZSθαsoμんεノVWPαo雛o， PaciHc。」．GεoP妙s．Rεs．，vol。77，P． edited by N．P．BAsILKovsKY and B．Ya． 5705－5719． KARP，Far East Sci．，P．52－55 （in Rus一 SENo，T．（1983） Age of subducting・1ithosphere

一360一 σθoZo9‘cαZs施伽rεo∫孟んθJiαPαπSθααηの古s亡θoεo痂伽画c翻oηs（K。Tαmα勧

and back－arc tectonics： evolution of Progrαητ）．」4η．04－05．1981．Toんニゾo． the westem Pacific since the early TAMAKI，K and MIYAzAKI，T．（1984）Rifting Tertiary．肋s惚o孟s．OJ互漉mα孟‘oηαZ of the Bonin Arc．Aわs加ααs．∫or疏θ Sθ而παroπ伽FormαあoηoゾOoθαη 傭erη翻0παZ貫y肌POS‘αm oηjRθ0θn，カ Mαr8‘ηs，P．50。 C配sホαZ1レfoo餓傭soゾむんεPα磁oRε一 SENo，T．and MARuYAMA，S。（1984） Paleogeo－ 9‘oη，．Fεわ．1984，P．57． graphic reconstruction・an（i origin of ，MuRAKAMI，F。，NlsHIMuRA，K．＆nd the Philippine Seεし．Tθc60πopんッ3。，voL HoNzA，E．（1979b）Continuous seismic 102，p．53－84． reHection proHling survey．In σθoZo9ε一 SHIMAzu，M．（1968）Absolute age determina． 〇αZZηひεs廊9α6‘oπo∫地θJiαPαηSεα， tion of granite in Yamato Bank． edited by E．HoNzA，Geo1．Surv．Japan Mんo漉α茜（」αPαπSθα），P．55－56． Cruise Rept。，no．13，p．48－51． （1979） “Green tuff”in the core from ，YuAsA，M．，NlsHIMuRA，：K．and DSDP Leg31Site302：：Kita－Yamato HoNzA，E．（1979a） Geological map of tai．」。σθoZ．Soo。」αPαη，vol．85，P， the Japan an（1 0khotsk Seas aroun（i 655－656． Hokkaido．ハ4αr加θGθoZ．ハ4αp Ser．，no． SLEEp，N’．H．and ToKsoz，M．N，（1971）Evolu－ 14，Geo1．Surv．Japan． tion of marginal basins．一くπα孟㏄rε， vol． TANAKA，T．（1979） Distribution and．nature of 33，p．548－550。 the sedimentary basins off Hokuriku STRoYEv，P．A。（1971）・Gravity anomalies in an（i San－in，Japan．」．JiαPαηθsεAssoc． the Sea of Japan。1）oんZαdツAんαdl．．八τα砿ん PθむroZ．Teo勧oZo8‘sオs，voL44，P。308－320 SS肌， vo1．198，P．18－21 （translated in （in Japanese with English abstract）． Japanese with English abstract）． and OGusA，：K．（1981）Structural mo－ SuzuKI，U。（1979） Petroleum geology of the vement since middle Miocene in the Sea of Japan， northern Honshu． Jl． Offshore San－in sedimentary ba．sin，the 」；αPαηεSε．Assoe。Peケo乙丁θCんηoZO8諺SむS， Sea of Japan。 Jl． GθoZ． Soc． 」1αPα7z， vol．44，p．291－307． voL87，p．725－736（in J’apanese with TAMAKI，K』．，HoNzA，E。，YuAsA，M．，NlsHIMuRA， English abstract）． K and MuRAKAMI，F。（1981a） Geolo－ TAppoNNIER，P。，PELTzER，G．，LeDAIN，A．Y。， gical map of the central J’apan Sea． ARMIJo，R．and CoBBoLD，P．（1982） ハ4αr‘η2（弛oZ．Mαp Ser．，no．15，Geo1． Propagating extrusion tectonics in Surv．Japan． Asia：new insights fro卑simple experi－ ，INoucHI，Y．，MuRAKAMI，F．and HoNzA， ments with plasticine。GεoZogy，voL E．（1977）Continuous seismic re且ec－ 10，p．611－616． tion profiling survey． In GεoZo8∫cαZ TAYLoR，B。and HAYEs，D。E。（1980） The tec－ 伽εsあ8αεεoπo∫」αPα几α記So碗んerη tonic evolution of the South China K副ZθTrεηoんαηdSZope・4rεαs，edited Basin．In TんεTθoむo痂osαηdσεoZo9‘c by E．HoNzA，Geo1．Surv．Japan Cruise Eoo臨‘0η0ゾ・4SεαπSeαSαπdZsZα記s． Rept．，no．7，p．50－71． σθopんlys。ハ40nρgr。Sεr．，edited，byD。E． ，INouE，E．，YuAsA，M．，TANAHAsHI，M． HAYEs，AGU，Washington，D．C．，voL an（i HoNzA，E．（1981b） Possible active 23，P．89－104。 back－arc spreading of the Bonin Arc， an（i （1983） Origin an（1history 肋s伽C孟0ノ融mpOS認moη‘‘Gθ0孟2C孟0ル of the South China Sea Basin．In Tんθ ‘eso／Sα8α禰Troα8ん一Sωr砿9αTroω9ん Tε0孟0ηεCSα認σεoZO9εcEuo臨めπ0∫ ♂槻めoπArεα”（FかsむF’rεηoん一」αPαπεsε So乙6乙んεαs孟As乞αηSeαsαηd lsZαη（ノs Pαrむ

助mpos彪m oπ」⑫απθsθS励ぬoε‘oη 2。 σθoPんys． ハ40n、ogr． Sθr。， e（lited by

一361一 B沼ε伽oμんeσθoZo9εcαZS脚uεッo∫JiαPαπ．VoZ．39，ノVo．5

D．E。HAYEs，AGU，Washington，D．C．， Washing・ton，D．C。，vol』12，p．349－366． vo1．27，P。23－56． VAsILIEv，B．1．（1975） New data on age and ToMoDA，Y．（1873）Maps of free air and Bou－ mechanism of formation of margina1－ gUer graVity・anOmalieS in and arOUnd sea basins and（1eep－sea trenches in the Japan．Tokyo Univ．Press． northwestern Pacific．一DoんZαdツAκαdl． and FuJIMoTo，H．（1981）Maps of ノ〉α醜USSR，vo1．225，p．60－62． gravity anomalies and bottom topo－ von HuENE，R．，LANGsETH，M．，NAsu，N．and graphy in the westem Paci且c and refer－ OKADA，H．（1980）Summary，Japan ence book for gTavity an（1bathymetric Trench transect．In ZηあεαZ Rεpor孟s o∫ data．。翫ZZ。Ocεαη．Rεs．Zηs古．，P．1－158． 伽1）εθpS2αDr‘Z伽9．Proゾεoむ，edited TozAKI，T．，KIATo，S．and．KITAHARA，S．（1978） byScientificParty，U。S．Govt。Printing Submarine geology off San－in．Rεμ． O伍ce，Washington，D．C。，vol．56－57， 丑ソdrogrαpん．Rεs．，p．1－36（in Japanese p．473－503． with English abstract）． YAsul，M．，：KlsHII，T．，WATANABE，T．and TsuYA，H．（1932）On some pebbles collected UYEDA，S。（1967） Studies of the ther－ from the且oor of the Japan Sea．一B沼． mal state of the earth，the18th paper； Eαr疏α．Res．1η，S乙．Toんlyo IηLP。Uη、‘U．， terrestrial heat flow in the Japan Sea voL40，p．864－875． （2）．B副．Eαr吻．R2s。1ηs乙，voL44，P。 TuEzov，1。Kl．（1969） Geophysical study of far 1501－1518． eastern part of the circum Paciflc．B砿ZZ． YosHII，T．（1972） Terrestrial heat flow and Sα航α伽Comμe劣S漉ηoε加s痂鷹， features of the upPer mantle beneath voL20，P．5－26。 the Paciflc an（1Sea of Japan．Jl．Phッs。 （1971） Crustal structure of the Okhotsk Eα肋，vol．20，P．271－285． and Japanese area from regional ，KoNo，Y．an（11To，K．（1976） Thicken－ seismic prospecting （latεし． In lsZαηd ing of the oceanic lithosphere．In Tんθ Arc αηdハ4αrg‘ηαZ Sθα， e（1ite（i by S。 GεOPん』ys‘os oゾ地θPαoφ000eαηBαs‘η AsANo and G．B．UDINTsEv，Tokai Univ． α記16sMαr8εηs．GeOPんツs．Moηogr． Press，P．121－135． Sεr．，edited by G．H．SuTToN θ孟α乙． UENo，N．，KANEoKA，」．，OzIMA，M。，ZAsHu，S．， AGU，Washington，D．C．，voL19，p。 SATo，T．and，IwABucHI，Y．（1971） Kl－ 423－430． Ar age，Sr isotopic ratio and K／Rb and YAMANo，M．（1983）Digital heat ratio ofthevolcanicrocks’dredged 且owdata丘learoundJapaneselslands。 from the Japan Sea．In lsZα認αroα掘 Y’uAsA，M．，KANAYA，H．and TERAsHIMA，S． mαr9‘ηαZ sεα，edited by S．AsANo and （1979） Rocks and sediments．InσεoZ－ G．B．UDINTsEv，Tokai Univ．Press，p。 09‘CαZ伽εSむ‘8α乙‘0η0∫伽」伽αηSθα， 305－309 （in Japanese）． edited by E．HoNzA，Geo1．Surv．Japan UYEDA，S．and：KANAMoRI，H．（1979） Back－arc Cruise Rept．，no．13，p．54－60． opening and the mo（ie of sub（iuction． ，TAMAKI，K．，NIsHIMuRA，K．and 」．σεopんys、Rεs．，voL84，p．1049－1062． HoNzA，E．（1978） Welded tuff dredged and VAQuIER，V．（1968） Geothermal from Musashi Bank，northem Japan an（1 9’eOmεしgnetiC data in an（1 arOUn（i Sea and its：K－Ar age．」．GθoZ。Soo。 the islan（i of Japan，in the crust an（i 」αPαπ，vo1。84，P．375－377． upPer mantle of the Paci丘c area．In ZoNENsHAIN，L．P．and．SAvosTIN，L．A．（1981） Tんεor臨α記ゆpermα碗zθo∫伽 Geodynamics of the Baikal rift zone Pα卵oαreα．GεOP妙s．ハ40η08γ．Sθr．， and plate tectonics of Asia．．Tεc古oη一 edited by L．KNopoFF ε6αZ．，AGU， 〇Pんツs．，voL76，P。1－45．

一362一 GεoZo9オcαZs伽伽rθoμんeJlαPαπSθαα掘‘むsむeo孟o耽加LpZ‘oαε‘oηs（KTαmα爾

日本海の海底地質構造とテクトニクス

玉木賢策

要 旨

西太平洋に発達する多くの縁海の一つである日本海のテクトニクスは日本列島のテクトニクスと密接 な関係を有しており，日本海の正しい理解は日本列島のテクトニクスの研究にとって不可欠である．縁 海のテクトニクスにおいて最も重要なその形成時期は，地磁気縞状異常の年代同定と，深海掘削による 基盤岩層の年代決定により求められるが，残念ながら，日本海では地磁気縞状異常の発達は微弱かっ複 雑で既存のデータでは年代同定は困難であり，またDSDPによる深海掘削も基盤岩まで達していない． このため他の西太平洋の大部分の縁海の形成時期が確定しているにもかかわらず，日本海の形成時期に っいては，新第三紀から白亜紀までの多くの意見に分かれ，まだ定説といえるものはなく，日本列島周 辺のテクトニクス研究において大きな足かせとなっている． 本研究は，1977年から1978年にかけて地質調査所によって行われた，日本海においては初めての広 域的かっ系統的海洋地質・地球物理調査データと過去に数多くなされている日本海の調査研究結果を統 合することにより，日本海の海底地質層序構造を検討し，その結果にもとづいて日本海の海盆部と海膨， 海嶺，海山部の形成，および，現在のテクトニクスについて論じたものである．海底地質層序構造は音 波探査記録の解析と海底サンプリング結果の対比により検討された。従来，日本海のテクトニクスは周 辺陸域の地質との関連で論じられることが多かったが，本研究では全面的に海域のデータにもとづいて 議論しているのが特徴である． 日本海には大陸側の日本海盆，タータリー舟状海盆と，日本列島側の大和海盆，対馬海盆の4っの大 きな海盆が存在し，それぞれに特徴を有する．日本海盆では堆積層の厚さは2500m程度，音響的基盤 の深度は6000m程度である．一方，大和海盆は堆積層の最大層厚1500m，音響的基盤深度4000m 程度で，日本海盆とは大きく異なる．基盤深度の差異は，日本海盆の音響的基盤が大洋性玄武岩層と一 致するのに比べて，大和海盆の音響的基盤はグリーンタフ層に対比される火山性砕屑物に相当し，真の 大洋性玄武岩層はさらに下位にあることにより生じるものと解釈される．日本海盆と大和海盆の地殻熱 流量の平均値はそれぞれ2．3HFU前後で，このことは両海盆がほぼ同等の生成年代を有することを意 味する．日本海盆の基盤深度，地殻熱流量と，大和海盆の地殻熱流量をそれぞれ検討すると，日本海盆 の生成年代は30～15Ma，大和海盆の生成年代は30～10Ma程度と推定される．タータリー舟状海盆， 対馬海盆も同様の検討をすると，その生成年代はやはり30～10Maの問に入るものと考えられる． 日本海の海嶺，海山，堆などの地形的高まりはその成因からみて次の4っのグループに分けられる． 1．大陸地殻の断片よりなるグループ 海底拡大による日本海の形成の際，海洋地殻中に取り込まれてしまった大陸地殻の断片で，大和 堆，隠岐堆，北隠岐堆，隠岐海嶺，朝鮮海台，武蔵堆が主な例である．このグループの基盤岩の地 質は中生代以前のものより構成され，古いものではプレカンブリアンのものも含まれる．このグル ープは岩相によってさらに細分され，火山性のもの，深成岩性のもの，堆積岩性のものがある．こ れらの地形的高まりは，その形成以降沈降運動を行っており，これは周辺海洋地殻の年代経過にと もない沈降したものと考えられる． 2。 断裂化した大陸地殻の断片 日本海の形成初期に大陸地殻の間に起こった断裂運動が海洋底拡大活動に発達する前に中絶した ため生じた，半陸・半海洋的な地殻よりなる高まりで，水深の深いのが特徴である．佐渡海膨，北 海道西方の大陸斜面部がこの例と考えられる．

一363一 ．B沼θ伽qμんθGeoZo8‘oαZ翫rひcッo∫」αPαπ．．γoZ．39，No．5

3． 構造性海嶺群 富山トラフより北海道西方まで南北に発達する佐渡海嶺，奥尻海嶺よりなる．これらの海嶺群は， 鮮新世後期以降の日本海の東西性収縮テクトニクスによってスラストアップすることにより形成さ れたものである． 4．火山性の海山，堆群 大和海盆の中の海山列などの，海底拡大活動と同時期の海底火山群，および，ウツリョウ島，竹 島などの，島弧火成活動によるアルカリ玄武岩性の海山とからなる． 以上のような日本海の海底地質構造の特徴と，マリアナ弧のマリアナトラフ，小笠原弧の背弧凹地な どの活動的背弧海盆の拡大テクトニクスとの比較検討を行うと，日本海は，単一リフト系によって生成 されたマリアナー小笠原弧の背弧海盆とは異なり，多重リフト系から生成されてきたものと考えること ができる．多重リフト系は，そもそも緩傾斜でスラブがもぐり込みにより形成された巾の広い島弧火山 帯が巨大な引張応力場のもとにおかれたため，島弧における弱線としての島弧火山帯上にリフトが並列 して多数発生して生じたものと推定される．このような多重リフト系から多重の海底拡大系が生じ，こ れによって多くの大陸地殻の断片が海洋性地殻中にとりこまれたものであろう．さらに，リフト群の中 で海底拡大系にまで発達するものはわずかで，多くのリフトは未発達のまま活動を停止し，断裂化した 地殻の断片を形成するであろう．活動を停止したリフトの典型的な例が大和海嶺中の北大和トラフであ る．また，緩い傾斜のもぐり込みによって生じた巾の広い島弧火山帯は海底拡大活動と複合され，巨大 な引張応力場のもとで活発な火山活動を行い，海洋性地殻上に多くの海山や，グリーンタフのような厚 い火山性砕屑物を生ぜしめることになるものと考えられる． 上記のようなテクトニクスを発生させるには大きな引張応力場が必要であるが，このような引張応力 は大陸の島弧からの後退によってのみ可能であろう．日本海の後背のユーラシアプレートの動きは新生 代を通じてほぼ不動であるかわずかに東方へ移動している程度であり，日本海の形成時期に後退した様 子はない．しかし，40Ma前に起こったインド亜大陸のユーラシァプレートヘの衝突は，その後，東 アジア内に大規模なプレート内変形あるいはマイクロプレート化をもたらしており，日本海後背大陸の 後退はそのようなマイクロプレートの動きによって説明可能であるかも知れない． 日本海東縁部の海陸境界部の地形は複雑な様相を呈し，大陸側の単純な大陸斜面とは対照的である． この複雑な地形は一連の海嶺・海盆群により構成されており，富山湾より北方にのみ見られ，西南日本 の日本海側には見られない．海嶺群は男鹿半島より北方のNS系の走向を持っ奥尻海嶺と，南方の NNE－SSW系の走向を有する佐渡海嶺からなる．この海嶺・海盆群が鮮新世後期以降（約3Ma～） に形成されており，その運動が現在も継続中であることが鮮新世堆積層の変形からわかる．すなわち， 海嶺群は鮮新世堆積層堆積中あるいは堆積後に，その堆積層を持ち上げ上昇することによって形成され， その東方にそれ以降の新しい堆積層がトラップされて海盆群が発達している．これらの隆起構造は，プ レート収れん域に見られるスラスト性隆起構造と同様のものであり，鮮新世後期以降日本海東縁部が東 西性の圧縮応力場におかれていることを示す．日本海東縁部に発生するスラスト性地震と海底断層の関 係を検討すると，このような圧縮応力は，日本海東縁部においてリソスフェア間の収束をもたらしてい ることがわかる．バイカルリフトにおける東西系の引張力が鮮新世以降活発化していることを考えると， このような収束運動もまた，インド亜大陸のユーラシアプレートヘの衝突に起因する，アムリアンマイ クロプレートの東方移動の結果として説明できる． 以上の考察にもとづき日本海の主なテクトニクスをまとめると，次の2っのイベントに分けることが できる． 1．30Maから10Maにかけて多重リフトから発達した背弧拡大活動による日本海盆，大和海盆，対 馬海盆 タータリー舟状海盆などの海盆群の形成 それにともなう大和海嶺，朝鮮海台，北隠岐堆， 隠岐海嶺，ボゴロフ海山などの大陸地殻の断片よりなる堆・海嶺群の形成さらに，拡大中の背弧 海盆内における活発な島弧火成活動による厚い火山性砕屑物の堆積，および海山群の形成． 2．3～2Ma以降現在までに至る日本海東縁部における東西性圧縮テクトニクス．それにともなう奥

一364一 GεoZo9εoαZs伽o卿θo∫孟んθ。瓦αPαηSθααη面6sカεoむo耽εm頑oαむε侃s（KITαmαんε）

尻海嶺，佐渡海嶺のスラストアップによる形成，および，その東方の後志海盆奥尻海盆，西津軽 海盆，最上海盆のトラップによる形成 以上の日本海形成以降のテクトニクスには，40Ma以降現在まで続いているインド亜大陸のユーラ シアプレートヘの衝突によって引き起こされた，東アジア付加テレーン群の再配列にともなうユーラシ アプレート内の変形運動が大きく寄与しているものと考えられる。

（受付：1985年11月16日；受理1988年4月28日）

一365一