Bulletin of the Geological Survey of Japah，vol．43（1／2），p。1－30，1992 FOSSA MAGNA－A masked borαer region separating southwest and，no並heast Japan Hirokazu KIATO＊ KATo，Hirokazu（1991）FOSSA MAGNA－A masked border region separating southwest and northeast J’apan．B認ム GeoZ．Sμrひ．」αpαη，vol．43（1／2）， p．1－30，10且g． Abstract：The deGnition，sedimentation and tectonic events of Fossa Magna are reviewed．Fossa Magna as a．border region separating southwest Japan and northeast Japan initiated during the late Early to early Middle Miocene opening of the Japan Se＆．In middle Miocene，marine transgression was at a maximum and the whole area of Fossa Magna was submerged．The Fossa Magna sedimentary basin was a large half graben－1ike depression whose westem margin was bordered with a major fault．Subsequently，the Central Upheval Zone emerged between the northem and southem Fossa Magna regions and the Kanto Syntaxis，the northward cUrvature of the basement rocks formed，by an increas－ ing stress imparted from the collision with the Phillipine Sea Plate．Subsequent geologic development of the two regions became increasingly di任erent。Hence it is oversimpliHcation to treat the whole area of Fossa Magna as a single structural domain．The Itoigaw＆一Shizuoka Tectonic Line has been regarded as the westem boundary of Fossa Magna，but is not the westem boundary of the Miocene sedimentary basin of Fossa Magna．It is the Pliocene deformation boundary．During・and after Pliocene the block on the western side of the tectonic line upheved．with little strike－slip displacement． the structural and sedimentary history of the 1．亘ntro雌uction area．For example，Paleogene and early Mio－ cene behavior of the Itoigawa－Shizuoka Tecto－ Fossa Magna region is a vaguely （1efine（1 nic Line （ISTL），westem boundary of Fossa transition zone between southwest Japan Magna is poorly understood partly because of． （Seinan Nihon）and northeast Japan（Tohoku the scarcity of deposits of those times，and Nihon）micro continents．A lot of geological partly because Quaternary （1eposits cover the and geophysical data have been accumulated ISTL especially in the northem Fossa Magna an（1 interpretations of these （1ata are eagerly region． This paper reviews，the geology of debated because of the limitation of these data Fossa Magnεl and argues unsolved tectonic an（i their inherent nonuniqueness since Nau－ problems conceming the structural history of mann proposed the Fossa Magna in1885．The FossaMagna． definition of Fossa Magna is also confuse（1as the area （1evelope〔l as the result of several 2。Geologicsett豊ng tectonic events． The deeper structural charac－ teristics of the area were likely produced pri－ 2．1De伽ition of Fossa Magna marily during the opening of the Japan Sea， One of the most important tectonic evcnts in but subsequent tectonic events have complicated Keywor（ls：Fossa Magna，Itoigawa－Shizuoka Tectonic Line， ＊Geology Department Central Zone，Kanto Syntaxes，Nishikubiki－Omine Block 1 B乙¢Zθ古‘ηo∫6んεGeoZo8’εcαZ Sαrひ（3ツo∫e／1αPαη， VoZ．43フ．〈ro．1／2 the Cenozoic geohistory of eastern Asia is the opening of Japan Sea during early to middle Miocene age． This event is（iirectly relate（i to the formation of Fossa Magna．Therefore，it 38。N38 should be distinguished from the geologic 嫡 丁6hoku Region features which（leveloped prior to and after the 試マ Niigata opening of the Japan Sea． In this paper the 、 author mainly focuses on the post－opening τochigi stratigraphy an（i tectonics of the region． In Gunma the post－opening stage there is general agree－ Nagano lbaraki Sai一 336。N ment that the westem margin of Fossa Magna tama Y引閣ξhi T。ky・ Chiba coincides with the Itoigawa－Shizuoka Tectonic Kana一 gawa Line （the ISTL）． In general the ISTL is the 〆 boundary between the w6stwar（l pre－Tertiary Sh脚㎞N逮 rocks and the eastward Tertia、ry rocks． How－ ever，someNeogenerocksofFossaMagnaare distributed to the west of the ISTL，which 34。N implies that in some places，the ISTL was not 138。E 140。E 142。E the westemmost boundary of Fossa Magna basin during Neogene time．The ISTL was a Figure l Index map・ mεしjor growth fault in the basement and had a significant influence on sedimentation．The’ formation along the ISTL in this region change ISTL as surface expression of the westem signi£cantly。 This author prefers to empha－ boundary of deformed Fossa Magna has been size that the Central Upheaval Zone，a major after Pliocene． On the other hand the position NE－SW trending horst block，which has acti－ of the eastem margin is less well defined and is vely developed since the middle Miocene， the focus of considerable （1ebate． In many provides a Neogene age structural sub（livision ways，it is almost meaningless to debate the of Fossa Magna into northem and southem exact loca、tion of the eastem margin of Fossa parts （Figure2）． In particular， the tectonic Magna． Fossa Magna is thought to be a kind activity of southem Fossa Magna is controlled of complex half－graben and there is no firm by the relatively dynamic processes of micro evidence of major faulting in the eastern part block collision and subduction of the Philippine 〔luring Neogene． There is，however，one excep－ Sea Plate． Divisions of Fossa，Magna a、nd tion l Neogene deformation is observed along surroun（1ing regions（liscusse（1in this paper are the NNE－SSW trending Shibata－Koide Tectonic shown in Figure2． Line （SKTL） along the northeastern margin of deformed Fossa Magna（See Figure2）．The 2．2Pre－Tertiary basement童n Eoss＆Magna distinction of tectonic events occurring in the In the Kanto Mountains of the southern southern and northern Fossa Magna are・of Fossa Magna，the distribution of pre－Neogene generally considered more import＆nt to the tectoni？belts is very similar to that found in development of the F6ssa Magna region during the Outer Zone of southwest Japan． Tectonic middle Miocene． belts foun〔i in both areas inclu（ie the Sanba－ The northern and southern parts of the Fossa gawa Belt and the Chichibu Belt，which can be Magna region are separate（l geologically by the exten（1e（l through the Fossa Magna region on concealed Median Tectonic Line．Many geo－ the basis of geological data，volc＆nic xenoliths， logists regards the Suwa region of Nagano deep－drilling data and geophysical data． As Prefecture to be regar（1e（1as the boun（1ary zone another example，the Ookitano－lwamura Line between the northem and soutぬem parts of （FulimotoθεαZ．，1953）is interpreted to be the Fossa Magnεし，since the characteristics of de一 extension of the Median Tectonic Line（the 2 一FOSSAMIAα〉IA一．Amαsんθ面ordεrrθ8’♂oηsεPαrα伽gso痂ωεs孟α屈ηor孟んθαs偏αPαη但．Kαεoク 罐1、蟻 1380E 臨 38。N Sado Island プ◎熱 薫 廟、 Japan Sea kりma Mountains ド ’ p p 3 一 ミ P 』 ／凋＼．茜塊 P P 』 P 自 P ド 、 唱 蔚Ya轟譲・・ P M。u噸ns弧、、 ，つ しの 一 一 ＼ 甲 『難 ￥ ＼ 働 ㊥ Mito 無無 臨 360N Urawa’紬継． ＼＼鱒 曲i To壼yo Tokyo Bay Boso Pen玉nsula Pacific Ocean・ 態翻夢 懲Sur㎎aBay O o ρ c 團 Tertiary gτanitic rocks ひ 0 1400E 34。N 懸 Pre－Neogene Basement rocks 0 王00km Figure21ndex map of Fossa Magna， MTL）across the northem margin of the Kanto belonging to the Ryoke Belt，one of tectonic Mountains，In addition，the pre－Tertiary base－ belts of the Inner Zone， are scattere（i to the ment rocks of the southern Fossa Magna show northern side of the MTL． So，the geology of zonal arrangements towards the Pacific Ocean pre－Tertiary basement rocks in the southern very similar to these of southwest Japan．For Fossa Magna is fundamentally the same as instance granitic rocks and metamorphic rocks that of southwest Japan． 3 Bα，乙乙e麗πo∫むh，e GeoZo9乞cα乙．SUlruely o∫」αPαη，，VbZ 43，ハ70．1／2 On the other han（i， in the northern Fossa tributed in a narrow belt（See Figures l and2）． Magna the pre－Tertiary geology and structura1 These are mainly composed of dacite and alkali 丘amework is largely unknown since there are basalt of late Early Miocene age and are few pre－Tertiary basement rocks exposed at considered to be part of accretionary prisms the surface．Rock fragments such as gabbro， formed in middle Early Miocene time．To the diabase，clionopyroxenite，san（lstone an（i shale east of this thrust sheet the Shizuoka Group of εしre inclu（ie（1 in the volcanic ash an（1 ejecta la、te Miocene to Pliocene age is distributed exposed from Yak“yaka volcano in 1974 between the ISTL and the west－dipping Ta．一 （Komatsu and Chihara，1976；Chihara，1976）． shiro－toge Thrust． The Shizuoka Group con－ Deep（1rillingl data to the east of Kashiwazaki sists of san（istone and alternating be（ls of City show the existence of Cretaceous granite， sandstone and mudstone of marine origin。It clionopyroxeniteandserpentiniteabout3，000m is folded and its strikes swings from a N－S to deep from the surface（lnoma，1971；Ishiwada E－W direction forming a conscipious bending and Igi，1971）． Xenolith inclu（le（l in Pliocene structure． To the east the Tashiro－toge Thrust， volcanic rocks contain hornblen（ie－gabbro． the Pliocene Hamaishidake Group is distribut－ These facts suggest that the zonal εしrrange－ ed．It is composed of clastic rocks with a ments observed in the Imer Zone of southwest small amount of andesite to dacite volcani－ Japan exten（l beneath Tertiary rocks in the clastic rocks an（1is very thick． This group is northern Fossa Magna． This structural chara－ deformed into N－S to NNW－SSE trending cteristics is significantly different from that of structures an（玉 is thrust over the Quaternary northeast Japan which is to the Kashiwazaki－ deposits． The Shizuoka and the Hamaishi（1ake Choshi Line or the Tanakura Tectonic Line， Groups are trench－fill deposits（Sugiyama and there is not complete agreement conceming Shimokawa，1990）。 this。However，since the principal emphasis of To the north of the Shizuoka area， the this paper is on